Faculty of engineering "Mechanical engineering department"
Solar cooling
Advised by: Pro. Afif Hasan.
A graduation project submitted to The Mechanical Engineering Department in partial fulfillment Of the requirements for the degree of B.Sc. in mechanical engineering
Birzeit May 2010
Contents
Page
Abstract Abstract in Arabic Acknowledgment
I III IV
Chapter One:
Introduction
Chapter Two:
Solar energy
1 5
2.1Solar Radiation
6
2.1.1 The solar constant
6
2.1.2 Measurement of solar radiation 2.1.3 Solar radiation angles 2.2 Solar collectors
9 18
2.2.1 Flat-plate collectors
18
2.2.2 Evacuated tube collectors
23
2.2.3 Concentrating Solar Collector
31
Chapter Three:
Solar refrigeration
3.1Absorption cycle
39 40
3.1.1 The structure of absorption chiller
40
3.1.2 The principle of absorption chiller
41
3.1.3 The coefficient performance of the ideal absorption cycle
43
3.1.4 Market available chilled water systems
44
3.1.5 Cost analysis
46
3.2 Adsorption cycle 3.2.1 The cycle consists of four periods 3.2.2 Advantages of the adsorption cycle 3.2.3 Disadvantages of the adsorption cycle 3.3 Electricity Electricity (Photovoltaic) Driven systems 3.4 Desiccant cooling cycles 3.4.1 How desiccants work
47 47 47 48 48 49 49
3.4.2 Advantages of desiccant cooling system 3.4.3 Applications 3.4.4 Types of desiccant cycle Chapter Four:
Solar energy cooling case study
50 50 50 55
4.1 Case description
56
4.2 Construction elements description
56
4.3 Thermal resistance for walls, windows and doors
57
4.4 Load calculation
60
4.4.1 Sample Sample calculation for for the Multipurpose Multipurpose Hall (120 seat hall) hall) 60 4.4.2 The load results for the ground and 1st. floors 4.5 Solar system design
62 65
4.5.1Collector Calculation
65
4.5.2 Collector installation installation
67
4.5.3 Double jacket Storage tank
69
4.5.4 Boiler (back up) Calculation
69
4.5.5 Solar system pump selection
70
4.5.6 Expansion Tank selection
73
4.6 Duct design
74
4.7 Chilled water distribution
77
4.7.1 Fan coil selection
77
4.7.2 Chilled water pump selection
77
4.8 Economic analysis for solar energy cooling case study
80
4.9 Conclusions and recommendations
84
Chapter Five
Adsorption refrigeration
86
5.1 Introduction
87
5.2Adsorption refrigeration
87
5.3Working pair¶s selection
91
5.4 Lab .Scale adsorption ice maker
94
5.4.1 Adsorption ice maker model components
94
5.4.2 Components description
95
5.4.3 Lab scale experiment
96
5.4.4 Conclusions and recommendations
98
eferences R eferences Appendices
Appendix A.1 Arch plane for case study law building in Birzeit University Appendix A.2 Wall section Appendix A.3 dimension of doors and windows of the case study building Appendix B.1 Absorption chiller catalogue Appendix B.2 CPC solar collector catalogue Appendix B.3 Boiler catalogue Appendix B.4 Pumps catalogue Appendix B.5 Expansion tank catalogue Appendix C.1 Palestine climatologically Appendix C.2 Cooling design condition Appendix D.1 Pressure drop figure to steel duct Appendix D.2 Duct sizing s izing Appendix D.3 Fan coils catalogue Appendix D.4 pill of quantity
100
Table of Figures Figures #No 2.1 2.2 2.3 2.4 2.5 1.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 2.24 2.25 2.26 2.27 2.28 2.29 2.30 2.32
Name Pyranometer Pyheliometer device Diversity of season Angle of sun radiation Tilt angle and season The zenith angle The declination angle The latitude angle The azimuth angle The hour angle the length of day at different latitude angle with variation of season The solar radiation the variation of solar Insolation at 21 June and 21 December at different different latitude latitude The variation of solar radiation at different latitude during full year A typical liquid Flat Plate Collector the heat flow through a Flat Plate solar collector Typical solar energy collection system efficiency versus T/I evacuated tube collector& hot water storage Insulation of evacuated tube The construction of evacuated tube direct flow model and its efficiency indirect flow model and its efficiency glass-to-metal seal model and its efficiency thermosyphoning principle flat plate fins V-shaped fins the efficiency for flat plate collector and for evacuated tube collector Cylindrical concentrator Absorber Parabolic concentrator
page 8 8 9 10 11 11 12 13 13 14 14 15 17 18 19 20 20 23 24 25 25 26 26 27 28 29 29 30 32 32 33
2.33 2.34 2.35 2.36 2.37 2.38 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 4.1 4.2 4.3 4.4 4.5 5.1 5.2
Cross Section of Cylindrical parabolic Concentrating Collector Parabolic Trough Solar Field Technology Field of Compact Linear Fresnel Reflectors Field of Compact Linear Fresnel Reflectors Field of Solar Furnace Parabolic Dish absorption chiller component of absorption refrigeration cycle pressure versus temperature working principle of absorption chiller schematic for the absorption cycle Single effect absorption chiller double effect absorption chiller annual total cost versus cooling capacity for both chiller compression and absorption Working principle of desiccant cycle. solid desiccant cooling system desiccant wheel liquid desiccant cycle Installation of CPC-18 OEM collector dimension of CPC-18 OEM collector installation angles of the CPC-18 OEM collector The case study solar system component The supply duct of offices (1,2and3) in1st floor Adsorption cycle represented in a Clapeyron-Clausius diagram Lab . scale adsorption ice maker
33 35 35 36 36 37 40 41 42 42 43 44 45 46 49 51 52 53 67 67 68 72 74 88 94
List of Tables
Table #No 2.1 2.2 3.1 3.2 4.1 4.2 4.3 4.4 4.5 4.6 4.7 5.1 5.2
Name comparison between flat plate and evacuated tube Temperature range of solar collector Advantage and disadvantage for the solar vapor compression refrigerator Comparison between solid and liquid desiccant cycle Specifications of an 40 ton absorption chiller The dimensions of the CPC -18 OEM collector Selected fan coil specification for ground floor Selected fan coil specification for first floor Fixed cost for absorption chiller working system Fixed cost for absorption chiller working system Life cycle cost absorption chiller compare with electrical chiller charcoal/methanol pair experiment data silica-gel/water pair experiment data
page 29 38 48 54 64 68 77 77 80 81 84 96 97
Nomenclature
FR I TC Ti Ta Ul Qi Qu Qo
Awindow
(T SHGF SC CLF y
V inf (w
Collector heat removal factor Intensity of solar radiation(W/m2) Collector average temperature(Co) Inlet fluid temperature(Co) Ambient temperature(Co) Collector overall heat loss coefficient(W/m2Co) Collector heat input(W) Useful energy gain(W) Heat loss(W) Transmission coefficient of glazing Absorption coefficient of plate Mass flow rate of fluid through the collector(Kg/s) Thermal resistance (m2Co/W) Area of the windows( m 2 ) The difference in temperatures(Co) Solar heat gain factor W/ m 2 Shading coefficient Cooling load factor Volumetric flow rate of infiltration air(L/s)
fu
The difference in moisture content between two regions (moisture continents Kg/Kg Dry Air ) Cooling load temperature difference Usage factor
fb
Ballast factor
CLTD
^ collect or or qu Q s
Constant pressure specific heat(KJ/Kg.Co) Efficiency Solar irradiance(W/m2) Optical efficiency Collector efficiency
Useful heat required(W) Heat stored
V
Density of water Kg/ m
Re
Reynolds number
3
Abstract
In light of the global struggle for energy and because of the high prices of oil and its negative impact on the environment intensives the approach to renewable energy sources, especially solar energy , innovations and development of many systems in the various parts of the world. In order to take the advantage of solar energy in several areas, including electrical energy production, heating and cooling, as a result of the mentioned factors this work has been selected, which looks at ways to harness solar energy for cooling.
In this project a range of cooling systems that takes the advantage of solar energy had been offered, in terms of the principle of work and of the thermal analysis of these systems. In addition a cooling system for an existing building by using the absorption cooling system had been designed, also an economic comparison between absorption chiller used in the design and an electric chiller with the same cooling capacity, In addition a model for an adsorption ice maker had been built to investigate the working conditions of the adsorption cycle.
It had been founded at the end of the working in this project that to cool two floors of the annex-of the law building in Birzeit University with an approximate 800 m 2 area an adsorption chiller of capacity 40 tons refrigeration is needed, a 15 solar collectors of type CPC OEM-18 and a 34KW water boiler had to be adopted to run the chiller.
The economic study reveals that the life cycle cost for operating the absorption chiller 25 years equals (776500 $ ) and (1094800 $ ) for the electrical chiller so the installation of the absorption chiller instead the electrical chiller is justified , the study also shows that the payback period for the absorption chiller equals 4.7 years.
The last part of the project shows that for the adsorption ice maker to work efficiently the pressure inside the system must be lower than (9 cm Hg), also at the same working conditions the performance for the system that uses charcoal/methanol pair is better than that uses silica-gel/water pair, also it had been found that the distance between the generator and the evaporator and the relative position of them plays a very important role in the operation of the adsorption ice maker.
, . , , . 15 40 2 800 . 34 CPC OEM-18 ( 1094800) ( 776500) 25 . 4.7 . ( 9) silica- /
, / gel/water . .
. ....... ........
( ) .
Chapter
One
Introduction
Introduction:
Now a day the governments all over the world are concerned with how to expand the usefulness of the renewable energy sources. Scientists all over the world are now trying to find new methods to extract the power from the alternative energy sources and to increase the efficiency of the available methods. All these efforts have been made so as to reduce the dependence on the energy come from fossil fuels mainly as the world generates 85% of its energy from fossil fuels. But this source is non-permanent source of energy and it can be vanish any time and its price depends on the political situation between countries as its price increase whenever there are wars. If we know that only 0.54% of the energy consumed in the world is generated from solar power then we must work seriously on developing the methods used to extract this power from the sun especially in the regions that have a good solar potential, and Palestine is one of them. t hem. The interest in solar cooling systems first started to increase due to the oil crisis in the 1970s, and then later, in the 1990s, because of ecological problems related to the use of CFCs and HCFCs as refrigerants. Such refrigerants, when released into the atmosphere, deplete the ozone layer and contribute to the greenhouse effect. Furthermore, with the increase in energy consumption worldwide, it is becoming even more urgent to find ways to use the energy resources efficiently as possible. Thus, machines that can recover waste heat at low temperature levels such as absorption and adsorption machines can be an interesting alternative for wiser energy management. The conventional adsorption cycle had been presented extensively in the literature and it mainly includes two phases: A- Adsorbent cooling with adsorption process, which results in refrigerant evaporation inside the evaporator and, thus, in the desired refrigeration effect. At this phase, the sensible heat and the adsorption heat are consumed by a cooling medium, which is usually water or air.
B- Adsorbent heating with desorption process (also called generation), which results in refrigerant condensation at the condenser and heat release into the environment. The heat necessary for the generation process can be supplied by a low-grade heat source, such as solar energy, waste heat, etc. In comparison with mechanical vapor compression systems, adsorption systems have the benefit of saving energy, if powered by waste heat or solar energy, simpler control, no vibration and lower operation costs. In comparison with liquid absorption systems, adsorption systems can be powered by a large range of heat source temperatures, starting at 50 and going up to 600 or even higher. Moreover, the latter system does not need a liquid pump or rectifier for the refrigerant, does not present corrosion problems due to the working pairs normally used, and it is less sensitive to shocks and to the installation position. These last two features make it suitable for applications in locomotives, busses, boats and spacecrafts. Although adsorption systems offer all the benefits listed above, they usually also have the drawbacks of low coefficient of performance (COP) and low specific cooling power (SCP).
This project has been divided into five chapters: the first chapter which contains an introduction to the project. The second chapter under the title of solar energy discusses the solar radiation and changes throughout the year through the study of angles and mathematical formulas as well as a collection of graphs and tables. Then it will show a range of solar collector types and discuss its temperature characteristics and cost to compare between them. The chapter has also reviewing a series of graphs and tables that shows the properties of these collectors.
Chapter three, under the title of solar cooling discusses how we can use solar energy for cooling purposes through a review of a range of cooling techniques those included absorption, adsorption, desiccant cooling in terms of the principle of their respective work and thermal analysis of these techniques.
Chapter four, this chapter contains a design for a cooling system using absorption cycle to cool the Annex Building of the Faculty of Law at Birzeit University. Calculations using mathematical equations for the design and selection of the appropriate absorption air conditioning chiller had been shown in this chapter, also solar collectors had been selected to obtain the energy needed to run the chiller. This chapter also contains a presentation of an economic study done on the predesigned cooling system for the law building to compare between two systems the first on uses absorption chiller and the second one uses electrical chiller by evaluating the life cycle cost for both of them during 25 years, the study shows the number of years required to recover the overall cost of the system that uses the absorption chiller. Chapter five, this chapter contains a brief description of the adsorption cycle; also it shows the principles of choosing working pairs for the system. This chapter contains the details of the construction of an adsorption ice maker lab. scale model , the data gained from operating of the lab. Scale model using charcoal/methanol and silica-gel/water pairs were shown in this chapter.
Chapter Solar
Two
energy
2.1Solar R adiation adiation The sun is a sphere of diameter 1.39 v 10 9 m and its average distance from the earth is 1.49 v 1011 m . The interior of the sun is extremely hot, with temperature of many millions of degree. The surface temperature is approximately 6000 K, one can define the effective black body temperature of the sun as the temperature of black body radiating the same amount of energy per unit surface area as the sun. The effective black body temperature of the sun is 5762 K other effective temperature could also be define for example, the temperature of black body with the same wave length of maximum radiation. as the sun is approximately 6300K Two type of solar radiation reach us, the first one is director beam radiation which is the solar radiation received from the sun without having been scattered by the atmosphere. Diffuse radiation on the other hand, is the solar radiation received from the sun after its direction has been changed by scattering is the atmosphere. Diffuse radiation is sometimes referred to as sky radiation and diffuse radiation on a surface is termed the total or global radiation. Irradiance is the term given to rate at which radiant energy is incidence on a surface per unit area of the surface. Using SI unit¶s irradiance is measured in W
m
2
. Integrating
irradiance over a period of time gives the irradiance or isolation with J m2 units.
2.1.1 The solar constant The radiation emitted by the sun is nearly constant. The intensity of this radiation can be characterized by the solar constant I 0 .The solar constant is defined as the solar irradiance at normal incidence incidence just outside the earth atmosphere , When the sun ± earth distance is at its mean value of 1.49 v 1011 m (Due to eccentricity of the earth¶s orbit the radiation incident on the earth varies with season by s 3.3 % ) . Measuring solar constant is very difficult due to atmosphere effect, but satellite technologies helped in measuring it to have a value of 1372.7
W m
2
.
The actual solar power that reaches the earth is obviously less than the solar constant due to many factors including the fact that sun rays must penetrate 150 Km thick
atmosphere before reaching the earth, so much of the radiation is absorbed or scattered or reflected as a result. Clouds and smog also limited the portion of solar radiation that reaches the earth. [1]
2.1.2 Measurement Mea surement of of solar radiation ra diation The two common methods which characterize solar radiation are the solar radiance (or radiation) and solar insolation. The solar radiance is an instantaneous power density in units of kW/m2. The solar radiance varies throughout the day from 0 kW/m2 at night to a maximum of about 1 kW/m2. The solar radiance is strongly dependant on location and local weather. The solar insolation is the total amount of solar energy received at a particular location during a specified time period, often in units of kWh/(m2 day). While the units of solar insolation and solar irradiance are both a power density (for solar insolation the "hours" in the numerator are a time measurement as is the "day" in the denominator), solar insolation is quite different than the solar irradiance as the solar insolation is the instantaneous solar irradiance averaged over a given time period. There are two basic types of instruments used to measure solar radiation, pyranometer and pyheliometer . y Pyranometer: has a hemispherical view of surroundings and is used to measure total, direct and diffuse solar radiation on a surface, also known as solar meter. See fig 2.1. o y Pyheliometer: has a restricted view, about 5 and is used to measure direct or beam solar radiation it follows the sun with two axis tracking see fig 2.2. y Pyranometer is used also to measure diffuse radiation by using a shadow band to black the direct sun view. Sunshine duration: Campball-Stokes sunshine recorder is used to measure sunshine duration. It uses a solid clear glass sphere as a lens to concentrate the solar beam on the opposite side of the sphere. A strip of heated paper marked with time graduations is mounted on opposite side of sphere where the beam is concentrated, and it burns the paper, the length of the burned part of strip gives duration of bright sunshine. [2]
Figure(2.1) : Pyranometer[2] Pyranometer[2]
Figure (2.2): Pyheliometer[2] Pyheliometer[2]
2.1.3 Solar radiation angles: The earth rotates at its its axis and complete one rotation every day. And And it revolute about the sun every year once, this revolution creates the season.
Figure (2.3): shows creational of season due to revolution [3]
Misconception: The people think that it is summer when the sun is close to earth and its winter when it is furthest from the earth. This wrong see figure (2.3). The main reason for change of season is not the distance it is the tilt angle of earth. How the tilt change the season:
The earth is tilted at 23.45 o toward the Polaris:When the earth is tilted toward the sun it is summer y When the earth in not tilted its equinox y When the earth is tilted in the opposite direction its winter. y
Figure (2.4): solar irradiance angle
From figure (2.4) we see that when the angle is increasing its projection (cosine the angle) is decreasing Cos90
Figure (2.5) shows how the tilt angle creates the summer and winter seasons [4]
We see that the when it is summer above horizontal it is winter below it and conversely when it is winter above horizontal it is summer below and this is due to the tilt angle of the earth. During one year the position of earth with respect to the sun is changed and this affected the solar radiation on the earth .there are several angle affect the solar radiation:y
The zenith angle: the angle between vertical and sun light. Its symbol is
Figure (2.6): the zenith angle [4]
We see that when it is summer the zenith angle is bigger than it is winter so the sun is high in summer and low in winter (height of sun depend on the sine of zenith angle) .this difference of height is due to the inclination of earth¶s axis.
The elevation angle (altitude): angle between horizontal and sunlight. y Declination angle: the angular position of the sun at solar noon with respect to the y
plane of equator. =23.45
Figure (2.7): The declination angle [4]
y
Latitude: the angle between the horizontal line and the line to the center of earth««.
Figure (2.8): The latitude angle [4] y
Azimuth: the angle between south meridian and sun light projection.
A=
Figure (2.9): The azimuth angle [4]
y
=hour angle that it is the angular displacement of the sun east or west of local meridian. That it is 15 for each hour from solar noon. For example . o
B: tilted angle of the collector. y Incidence angle :the angle between sun light line and the normal line on the collector. y
Figure (2.10): (2.10): The hour angle angle () [4]
Day length:
Figure (2.11): The length of day at different different latitude angle with with variation of season [5]
Day light hours =
Sidereal day: the time that it takes for the earth to rotate with respect to the stars =23 hour and 56 minutes and 4.091 second. Solar day: the time it takes for earth to rotate with respect to the sun=24 hours. The solar radiation:
Figure (2.12) the solar radiation [6]
Diffuse radiation results from the scattering of sun rays by clouds and other atmosphere gases. Total energy reaching a collector surface is equal to the sum of the direct beam radiation and the diffuse radiation. On bright sunny day diffuse radiation is 10% of total radiation while on a partly cloudy diffuse radiation is 50% of total. But on a completely overcast day diffuse radiation is 100%. The collecting surface will receive the same diffuse radiation for any orientation of the surface because the diffuse radiation is assumed to be uniformly over the sky. The direct radiation En= Eo «..The collector surface is normal to the sun radiation Eo= solar constant =transmission coefficient
=0.1 ««overcast day =0.8«... clear day
m=air factor mass
m=
When the collector surface is aligned at angle the equation become:
Ei= En
The general equation that include diffuse radiation:
+F E +F (E +E )
Et=En
I d
2
n
d
Et=total solar radiation =ground reflectivity of diffuse radiation
) «««««Sky to collecting surface F2= ( )«««««..ground to surface F1= (
F1+F2=1 South-facing tilted surface:
When the collectors face is toward the south so the best tilt angle is that the angle make the incidence angle =0 at solar noon
at solar noon 0
At
This lead to:-
hence
1=
Non-south facing tilted surface:
This lead to
So A=
This lead to
So the surface is rotated about a vertical axis holding A= and tilt angle
Figure (2.13): The variation of solar Insolation at 21 June and 21 December at different latitude [6].
The last figure shows the variation of solar Insulation at 21 June and 21 December at different latitude.
Figure (2.14) :The variation of solar radiation at different latitude during full year [6]
The solar radiation and climate information in Palestine is attached to the appendix C.1
2.2Solar 2.2 Solar collectors: 2.2.1 Flat-plate collectors: A flat plate is the most common type of solar thermal collector, and is usually used as a solar hot water panel to generate solar hot water. A weatherproofed, insulated box containing a black metal absorber sheet with built in pipes is placed in the path of sunlight. Solar energy heats up water in the pipes causing it to circulate through the system by natural convection (thermosyphon). The water is usually passed to a storage tank located above the collector. This passive solar water heating system is generally used in hotels and homes in sunny climates such as those found in southern Europe. These collectors heat liquid or air at temperatures less than 80°C. For these purposes, the general practice is to use flat-plate solar energy or evacuated tube collectors with a fixed orientation (position). The highest efficiency with a fixed flat plate collector or evacuated tube collector is obtained if it faces toward the sun and slopes
at an angle to the horizon equal to the latitude plus about 10 degrees. Solar collectors fall into two general categories: non-concentrating and concentrating. There are many flat-plate collector designs but generally all consist of (1) a flat-plate absorber, which intercepts and absorbs the solar energy, (2) a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber, (3) a heattransport fluid (air, antifreeze or water) flowing through tubes to remove heat from the absorber, and (4) a heat insulating backing. One flat plate collector is designed to be evacuated, to prevent heat loss. The first accurate model of flat plate solar collectors were developed by Hottel and Whillier in the 1950s
Figure (2.15) a typical liquid Flat Plate Collector [7]
Thermal analysis of flat plate collector:
Figure (2.16): shows the heat flow through a Flat Plate solar collector. [7]
Figure (2.17) shows the schematic of a typical solar system employing a flat plate solar collector and a storage tank.
Figure (2.17): Typical solar energy collection system [1]
If I is the intensity of solar radiation, in W/m2, incident on the aperture plane of the solar collector having a collector surface area of A in m2, then the amount of solar radiation received by the collector is:
However, as it is shown Figure (2.16), a part of this radiation is reflected back to the sky, another component is absorbed by the glazing and the rest is transmitted through the glazing and reaches the absorber plate as short wave radiation. Therefore the conversion factor indicates the percentage of the solar rays penetrating the transparent cover of the collector (transmission) and the percentage being absorbed. Basically, it is the product of the rate of transmission of the cover and the absorption rate of the absorber. Thus,
As the collector absorbs heat its temperature is getting higher than that of the surrounding and heat is lost to the atmosphere by convection and radiation. The rate of heat loss (Qo) depends on the collector overall heat transfer coefficient (UL) and the collector temperature. Thus, the rate of useful energy extracted by the collector (Qu), expressed as a rate of extraction under steady state conditions, is proportional to the rate of useful energy absorbed by the collector, less the amount lost by the collector to its surroundings. This is expressed as follows:
It is also known that the rate of extraction of heat from the collector may be measured by means of the amount of heat carried away in the fluid passed through it, that is:
Equation 4 proves to be somewhat inconvenient because of the difficulty in defining the collector average temperature. It is convenient to define a quantity that relates the actual useful energy gain of a collector to the useful gain if the whole collector surface were at the fluid inlet temperature. This quantity is known as ³the collector heat removal factor (FR)´ and is expressed as:
The maximum possible useful energy gain in a solar collector occurs when the whole collector is at the inlet fluid temperature. The actual useful energy gain (Qu), is found by multiplying the collector heat removal factor (FR) by the maximum possible useful energy gain. This allows the rewriting of equation (4):
Equation (7) is a widely used relationship for measuring collector energy gain and is generally known as the ³Hottel-Whillier-Bliss equation´. A measure of a flat plate collector performance is the collector efficiency () defined as the ratio of the useful energy gain (Qu) to the incident solar energy over a particular time period:
The instantaneous thermal efficiency of the collector is:
If it is assumed that FR, , , UL are constants for a given collector and flow rate, then the efficiency is a linear function of the three parameters defining the operating condition: Solar irradiance (I), Fluid inlet temperature (Ti) and Ambient air temperature (Ta). Thus, the efficiency of a Flat-Plate Collector can be approximated by measuring these three parameters in experiments. The result is a single line (T/I ± Curve) shown in Figure (2.18)
Figure (2.18) efficiency versus T/I [7]
The collector efficiency is plotted against (Ti ± Ta)/I. The slope of this line (- FR UL) represents the rate of heat loss from the collector. For example, collectors with cover sheets will have less of a slope than those without cover sheets. There are two interesting operating points on Figure (2.18). 1) The first is the maximum collection efficiency, called the optical efficiency. This occurs when the fluid inlet temperature equals ambient temperature (Ti= Ta). For this condition, the T/I value is zero and the intercept is FR ( ). 2) The other point of interest is the intercept with the T/I axis. This point of operation can be reached when useful energy is no longer removed from the collector, a condition that can happen if fluid flow through the collector stops (power failure). In this case, the optical energy coming in must equal the heat loss, requiring that the temperature of the absorber increase until this balance occurs. This maximum temperature difference or ³stagnation temperature´ is defined by this point. For well-insulated collectors or concentrating collectors the stagnation temperature can reach very high levels causing fluid boiling and, in the case of concentrating collectors, the absorber surface can melt [7]
2.2.2 Evacuated tube collectors: it is solar panel was built to reduce convective and heat conduction loss (vacuum is heat insulator).It is one of solar radiation collector, which uses the solar energy to heat water for different applications such as heating and cooling at high temperature range (77 Co 177 Co) better than flat plate .now evacuated tubes are using in domestic application instead of flat plate specially in cloudy region.
Figure :( 2.19) Evacuated tube collector& hot water storage [8]
Structure:
The evacuated tube collector may contain 6,8or16«.. Tubes dependent on the application, each tube consist of two glass tubes. The outer tube is made from strong transparent borosilicate glass that is able to resist impact from hail. The inner tube is also made of borosilicate glass, but coated with a special selective coating, which has an advantages such that it is is excellent solar heat heat absorption and smaller heat heat reflection properties .It is also good antifreeze collector because its manifold is insulated with rock wool so the temperature of header is rarely to fall below 10 C o . The cylindrical design of tubes ensures effective collection of solar energy throughout entire day because the incidence angle is always equals 0o.
Figure (2.20): Shows how the manifold is insulated with thickness of rock wool [9]
Figure (2.21): The construction of evacuated tube [9]
From last figure the heat pipe condenser area is large as possible to increase the heat transfer between the tube and the fluid inside the manifold.
Construction type of evacuated tube:
y
Direct flow (heat pipes)
Figure (2.22): The construction of direct flow model at left a graph shows the collector efficiency versus collector inlet temperature at different values e= (0.05, 0.01), e = emissivity [10] y
Indirect flow
Figure (2.23): The indirect flow model with a graph of collector efficiency versus collector inlet temperature at different value of the emissivity. [10]
y
Metal absorber with glass-to-metal seal
Figure (2.24) A Metal absorber with absorber with glass-to-metal seal model beside a graph observes the relation between the collector efficiency versus collector inlet temperature at different value of the emissivity. [10]
Performance of evacuated tube collector:
Collector efficiency: as useful heat divided by solar radiation on the collector surface.
= qu=useful heat
So = qu=AFR
FR=heat removed factor. = absorptivity of absorber plate.
t ransmittivity. ty. =cover transmittivi U=overall conductance heat transfer coefficient between the plate and the ambient air.
Ti=fluid inlet temperature. Ta=ambient air temperature. A=collector area I=solar radiation falling into the per unit area. How does the water circulates in the tubes? 1. Active system: this system uses the pump to circulate the water in tubes which are opened to the manifold and this type of tubes is called pressurized tube. 2. Passive system: there is no need for pump to circulate the water; it circulates on thermo siphon principle. Thermosyphon system: - use sunlight to circulate water or heat absorbing fluid through
the solar collectors to the storage tank using the thermosyphon principle that hot water or fluid rises and that dark surface absorb heat.
Figure (2.25): Circulation of water depending on thermosyphoning principle [11] Installation of evacuated tube:
1. Open loop system: the water is circulated in collector and heated directly from sun, then it goes to the storage tank .this system is installed when the climate is frost ±free and the quality of water is good. 2. Closed system: use a second fluid which is heated from sun then it flows directly to water storage tank in order to heat water. This system is useful when the quality of water is is not good good and when the climate is is frost prone. [11]
eflector types: R eflector y
Flat sheet
Figure (2.26): Flat plate fins [12] y
V-shaped fins
Figure (2.27): V-shaped fins[12] Table (2.1): Comparison between between flat plate and evacuated tube:-
performance Cost cleaning Area
Evacuated tube Flat plate Best performance in cold, cloudy day Best performance in warm sunny days More expensive Less expensive Difficult to clean Easier to clean Have less collector area May have larger collector area More efficient when ¨t>45Co Historically more reliable
Evacuated tubes are better than flat plate because the incidence angle =0o throughout entire day .but flat plate angle = 0o just at noon.
Figure (2.28): Shows that the efficiency for flat plate collector and for evacuated tube collector [10]
Figure (2.28) shows that the efficiency of flat plate becomes =0 when the collector inlet temperature is >75 Co but the efficiency of vacuum tube is still >60% in winter solstice where both collectors efficiency will not reach zero value in summer.
Orientation of evacuated tube collector:
Evacuated tube collectors are customarily installed in a fixed position .tilted toward the south with the angle such that:
lue=-12 , and so
=
o
=44o
During summer an average value =12 and in Palestine so =20 During o
o
[10] 2.2.3 Concentrating Solar Collector:
In the last decade, much attention has been given to concentrating solar collectors, which are capable of reaching higher temperatures compared with other collectors. Several designs of solar collectors have been developed over the years in order to improve their performance, a simple design that combines a static parabolic or cylindrical reflector with a tracking absorber positioned along a path, the static reflector concentrate sun light onto a linear thermal absorber. For any change in the incident angle, the reflected rays will always intersect in different points of the tracking path, therefore, it would be possible to track the sun movement simply by positioning the collector on the correct position of the tracking path, without moving the reflector. Usually a parabolic Trough concentrator is installed east- west aligned with one degree of freedom for tracking, about its axis at latitude of 40° N for the summer Season.
1- Cylindrical Reflector Tracking Path: It can be easily demonstrated that, for cylindrical reflector the tracking path is a circle. As shown in Fig (2.29)
Figure (2.29): Cylindrical concentrator [13]
Figure (2.30): Absorber. [13]
2- Parabolic Reflector Tracking Path: It is well known that a parabola has a point focus at a normal incidence; nevertheless, it can be shown that for any other incidence angle, all rays are concentrated into an area. Therefore the smallest possible region intersecting all of the reflected radiation will be the absorber position as shown in Fig (2. 30) the analysis consisted in a ray tracing
computation of the average concentration ratio versus sun angle. The concentration (intersection) ratio C of the collectors defined as the ratio of the effective aperture area R
and absorber tube area.
Figure (2.31): Parabolic concentrator. [13]
Figure (2.33) Cross Section of Cylindrical parabolic Concentrating Collector Collector [14]
The instantaneous efficiency of parabolic trough is found by: L c ! L Q
Where
T a : U c : I c :
Temperature of absorber. Temperature of ambient air. air. Overall heat transfer coefficient of the collector.
Solar radiation on collector.
C R
: Concentration ratio. C R
Where
I c C R
LQ: Optical efficiency. T r :
And
U c (T r T a )
!
Ar :
Area of absorber.
Aa :
Area of aperture.
Aa Ar
Losses from absorber in the parabolic trough are smaller because of smaller area comparing to the flat plate, but losses due to radiation are higher. A disadvantage of the parabolic trough is that it requires tracking of the sun such that the aperture area is normal to suns direction. [14]
Primary Types of Solar Collectors:
1. Parabolic Trough.
Figure (2.34): Parabolic Trough Solar Field Technology [15]
2. Compact Linear Fresnel Reflector.
Figure (2.35): Field of Compact Linear Fresnel Reflectors.[15]
A series of long, shallow-curvature mirrors
Focus light on to linear receivers located above the mirrors, which is appearing in the Fig (2.35) bellow.
Figure (2.36): Schematic Field of Compact Linear Fresnel Reflectors. [15]
Lower costs compared to parabolic troughs
Several mirrors share the same receiver ± Reduced tracking mechanism complexity Stationary absorber ± No fluid couplings required ± Mirrors do not support the receiver Denser packing of mirrors possible ± Half the land area
3. Solar Furnace.
Figure (2.37): Field of Solar of Solar Furnace. [15]
Solar furnaces are used for: - High temperature processes processes ³Solar Chemistry´ - Materials testing A field of heliostats tracks the sun and focuses energy on to a stationary parabolic concentrator which refocuses energy to the receiver. Receivers vary in design depending on process:
Batch or continuous process
Controlled temperature and pressure
Collection of product (gas, solid, etc.)
4. Parabolic Dish & Engine.
Figure (2. 37): Parabolic Dish. [15]
Mature and Cost Effective Technology: Large utility projects using parabolic dishes are now under development. Technical Challenges Have Been:
Development of solar materials and components
Advantage: High Efficiency
Demonstrated highest solar-to-electric conversion efficiency Potential to become one of least expensive sources of renewable energy. (Still true with development of Fresnel reflectors?) Advantage: Flexibility
Modular - May be deployed individually for remote applications or grouped together for small-grid (village power) systems.
5. Solar Central Receiver (Solar Power Tower) 6. Lens Concentrators. Table (2.2): Temperature range of solar collector [15]
Motion
Collector type Flat plate collector (FPC)
Stationary
Evacuated tube collector (ETC)
Compound parabolic collector (CPC) Singleaxis tracking
Two-axes tracking
Absorber type
Concentration ratio
Indicative temperature range (°C)
Flat
1
30-80
Flat
1
50-200
1-5
60-240
5-15
60-300
Tubular
Linear Fresnel reflector (LFR)
Tubular
10-40
60-250
Parabolic trough collector (PTC)
Tubular
15-45
60-300
Cylindrical trough collector (CTC)
Tubular
10-50
60-300
Parabolic dish reflector (PDR)
Point
100-1000
100-500
Heliostat field collector (HFC)
Point
100-1500
150-2000
Chapter
Three
Solar refrigeration
3.1Absorption cycle: As stress increased on generation and distribution of electricity in the world, there is a need to look for solution .one of these solutions is absorption technology which is operated by environmentally friendly substances, and utilizes waste heat from different sources. 3.1.1 The structure of absorption abso rption chiller:
Figure (3.1) Schematic for absorption chiller [16]
The absorption chiller consists of: y y y y y y
Condenser Evaporator Absorber Generator Throttling valve Pump
3.1.2 The principle of absorption chiller:
The main reason for inventing is to reduce consumption of electricity, so the researcher wanted to replace the compressor which is the element that consume the electricity with other elements could achieve the same process. These elements from fig (3.1) are generator, absorber and pump. The process:
The vapor exist from evaporator with low pressure so to increase its pressure using pump it must absorbed by a liquid solution to form liquid phase then this solution is pumped using the pump and is delivered to the generator with high pressure (the pump couldn¶t pressurize vapor) .then the generator- which is operated by solar energy ± separates the vapor from the solution by adding heat to the vapor-liquid solution .then the vapor moves to the condenser in order to reject the heat then it passes through the throttling valve whose purpose is to provide a pressure drop to maintain pressure difference between generator and absorber. After that the water with low pressure with low pressure enters the evaporator here the water evaporates at low pressure and takes heat from surroundings causing cooling effect. Then the vapor repeats the cycle. [17]
Figure (3.2) the main component of absorption refrigeration cycle [18]
Figure (3.3): The basic process- close cycle on pressure versus temperature. [19]
Figure (3.4) working principle of absorption chiller on pressure versus temperature [19]
Why using pump if it also consume electricity? The function of pump is to pressurize the liquid which its density (lower specific volume) is higher than the density of vapor (higher specific volume) so the pumping of liquid consumes less electricity than compressing vapor according to the following equation:
w=work done on liquid or vapor. P=pressure of the fluid. dv =differential of specific volume of the fluid. 3.1.3 The coefficient performance of the ideal absorption cycle:
COPabs=
In certain respects applying the term COP to the absorption system is unfortunate because the value is appreciably lower than that of the vapor-compression cycle (0.6-versus 3 for example). The comparatively low value of COPabs should not be considered prejudicial to the absorption system to; because the COPs of the two cycles are defined differently .the COP of the vapor-compression cycle is the ratio of the refrigeration rate to the power in the form of work supplied to operate the cycle. Energy in form of work is normally much more valuable and expensive than energy in form of heat. The absorption cycle can be thought of as a combination of a power cycle and refrigeration cycle.
Figure (3.5) a schematic for the absorption cycle [20]
Ts source temperature, Ta ambient temperature, Tr refrigerating temperature. For an ideal power cycle.
For an ideal refrigerator
Using the definition of COPabs for an absorption cycle above,
[20]
3.1.4 Market available chilled water systems: y
Single effect absorption : many products for operation with hot water steam in the capacity range>100KW .typical opeation temperature 80 Co-110Co. Figure (45)
Figure (3.6) Single effect absorption chiller [21]
y
Double effect absorption:often directly fired system .operating temperature 130Co-160Co. Figure(3.7)
Figure (3.7) Double effect absorption chiller
Solution types: Lithium bromide y y y y y
y
Water is the refrigerant and the aqueous lithium-bromide solution represents the solvent, the refrigerant 'water' avoids an application below 0 Co Lithium-bromide salt in aqueous solution is non-caustic, nearly non-toxic, noncombustible and odorless. Are used almost exclusively for the generation of cold water in the field of air conditioning. Cold-water outlet temperatures up to +5°C may be achieved, Working pressures in the evaporator and in the condenser are in the deep vacuum range. They are designed as compact sets and are manufactured serially and economically in large quantities. quant ities. 1. Ammonia-water
Ammonia is the refrigerant and water represents the solvent. y Ammonia is caustic, has a pungent smell and is toxic, but on the other side y
The pungent smell alerts in time thus avoiding damages to health in general. y
Ammonia/Air mixtures are barely inflammable but may be explosive in the
y y y y
Case of high percentages of ammonia between 15.5 and 27 % by volume. Ammonia dissolved in water is caustic. Ammonia is considerably lighter than air. At atmospheric pressure above - 33.4 deg.C, ammonia is gaseous, hence Plants with evaporation temperatures < - 33 deg.C are working in the vacuum. Deep temperatures down to -60 deg.C (-65 deg.C) may be achieved.
[22] 3.1.5 Cost analysis:
Figure (3.8) Relation between annual total cost versus cooling capacity for both chiller compression compression and absorption. [23]
Figure (3.9) shows the relation between annual total cost versus cooling capacity for both chiller compression and absorption. This graph shows that the total cost of absorption chiller is higher than the compression chiller.
3.2 Adsorption cycle: The interest in adsorption systems first started to increase due to the oil crisis in the 1970s, and then later, in the 1990s, because of ecological problems related to the use of CFCs and HCFCs as refrigerants. Such refrigerants, when released into the atmosphere, deplete the ozone layer and contribute to the greenhouse effect. Furthermore, with the increase in energy consumption worldwide, it is becoming even more urgent to find ways to use the energy resources as efficiently as possible. Thus, machines that can recover waste heat at low temperature levels²such as adsorption machines² can be an interesting alternative for wiser energy management. The heat necessary for the generation process can be supplied by a low grade heat source, such as solar energy, waste heat, etc. [24]
3.2.1 The cycle consists of four periods: pe riods:
1: HEATING AND PRESSURISATION: During this t his period, the adsorber receives heat while being closed. The adsorbent temperature increases, which induces a pressure increase, from the evaporation pressure up to the condensation pressure. This period is equivalent to the "compression" in compression cycles. 2 : HEATING AND DESORPTION + CONDENSATION: During this period, the adsorber continues receiving heat while being connected to the condenser, which now superimposes its pressure. p ressure. The adsorbent temperature continues increasing, which induces desorption of vapor. This desorbed vapor is liquefied in the condenser. The condensation condensat ion heat is released to the second seco nd heat sink at intermediate temperature. This period is equivalent to the "condensation" in compression cycles. 3 : COOLING AND DEPRESSURISATION: During this period, the adsorber releases heat while being closed. The adsorbent temperatur decreases, which induces the pressure decrease from the condensation pressure down to the evaporation pressure. This period is equivalent to the "expansion". 3.2.2 Advantages of the adsorption cycle:
In comparison with mechanical vapor compression systems, adsorption systems have the benefit of saving energy, if powered by waste heat or solar energy, simpler control, no vibration and lower operation costs. In comparison with liquid absorption systems, adsorption systems can be powered by a large range of heat source temperatures, starting at 50 C and and going up to 600 C or even even higher. Moreover, the latter latter system does not need a liquid pump or rectifier for the refrigerant, does not present corrosion problems due to the working pairs normally used, and it is less sensitive to shocks and to the installation position. These last two features make it suitable for applications in locomotives, busses, boats and spacecrafts.
3.2.3 Disadvantages of the adsorption cycle:
Although adsorption systems offer all the benefits listed above, they usually also have the drawbacks of low coefficient of performance (COP) and low specific cooling power (SCP). However, these inconveniences can be overcome by enhancing of the heat and mass transfer properties in the adsorber, by increasing the adsorption properties of the working pairs and by better heat management during the adsorption cycle. [24]
3.3 Electricity (Photovoltaic) Driven systems: A vapor compression refrigeration system is the most widely used cooling system because of high efficiency and reliability. Electricity, as the main energy source, is used as the driven energy for almost vapor compression system. Solar energy can be integrated with vapor compression cooling system by both photovoltaic cells and solar thermal collectors with Rankin engines. The main component of the vapor compression refrigeration system is a compressor. The compressor for the solar driven system is a direct current (DC 12 or 24 volts) compressor since the electricity output from the PV cell is the direct current. Inverter is needed to convert DC electricity to Ac electricity when using AC compressor. Battery is needed to prolong the cooling period when there is lack of sunlight. Battery¶s capacity is generally 340 Amp-hour. The size of the PV array depends on the available insulation of each area. The small application such as vaccine box or a cooling box is more economic than the large one. The power-driven compressor requires Rankin engine to convert heat from the solar thermal collectors into a useful work for the collector. [25] Table (3.1): Advantage and disadvantage for the solar vapor compression refrigerator refrigerator [33]
Advantage High COP Simplicity Simplicity for the refrigeration system Long term experience that is easy to maintenance when the problem happens happens Low price Required little maintenance
Disadvantage For a PV system, installation cost is high and it requires battery for energy backup. Noisy from compressor Required high technical Knowledge for PV system Refrigerant can be leaked
3.4 Desiccant cooling cycles Desiccants are materials which can attract and hold moisture. Nearly any material is a desiccant-even glass can collect a small amount of moisture. But desiccants used in commercial equipment are selected for their ability to hold large amounts of moisture .for example the silica gel packets often sealed into vitamin bottles can hold moisture equal to about 20% of their dry weight. Liquid desiccant materials can hold even more moisture. 3.4.1 How desiccants work:
Desiccants remove water vapor by chemical attraction caused by differences in vapor pressure. When air is humid, it has a high water vapor pressure. In contrast, there are very few water molecules on a dry desiccant surface, so the water vapor pressure at the desiccant surface is very low. Water molecules move from the humid air to the dry desiccant in order to equalize this pressure differential. With desiccants as shown in Fig (3.9), moisture removal occurs in the vapor phase. There is no liquid condensate. Consequently, desiccant dehumidification can continue even when the dew point of the air is below freezing. This is different from cooling-based dehumidification, in which the moisture freezes and halts the process if part of the coil surface is below 32°F.
Figure (3.9) working principle of basic cycle.
Desiccant change the vapor to heat
One aspect of desiccant wheel behavior can be confusing to the first-time user of the technology; air leaves a desiccant wheel dry, but warmer than when it entered the wheel. For example, if air enters a desiccant wheel at 70°F and 50%Rh, it will leave the wheel at about 100°F and 4% Rh. This non-intuitive behavior becomes easier to understand the reverse of evaporative cooling. When water is sprayed into air, it evaporates by using part of the sensible heat in the air²so the dry bulb temperature falls as water vapor is added to the air. Desiccants produce the opposite phenomenon. phenomenon. As water vapor is removed from air, the dry bulb temperature of the air rises. The amount of temperature rise depends on the amount of water removed. More water removal produces a greater temperature rise as shown in figure figure (3.14). The initial initial user naturally asks: how can desiccant systems save cooling energy if dehumidification adds sensible heat to the air? Part of the answer is that some heat is moved to reactivation by a heat exchanger. The rest of the answer depends on the application. For example, if air is dry, it may not be necessary to cool it if the space is already overcooled²as in a supermarket. Alternatively, dry air can be cooled using low-cost indirect evaporative cooling such as cooling towers, or with highly efficient vapor compression systems operating at high evaporator temperatures. In such cases, desiccants can save energy and energy cost.
3.4.2 Advantages of desiccant d esiccant cooling system:
* Since only air and water are used as working fluids and no fluorocarbons are required thus there is no danger to ozone layer depletion. * Significant potential for energy savings and reduced consumption of fossil fuels achieved. Electrical energy requirements are 25% less than the conventional V-C refrigeration system. Source of input thermal energy are solar, waste heat and natural gas. * Since Desiccant systems operate at near atmospheric pressure, their construction and maintenance is simple. 3.4.3 Applications:
Large latent loads and low humidity requirements e.g. Hotels, supermarkets, auditoriums, ice rinks, pools, Ventilation air etc. 3.4.4 Types of desiccant cycle:
1. Solid desiccant cycle. 2. Liquid desiccant cycle.
1- Solid desiccant cycle:
Figure (3.10) solid desiccant cooling system
Dry desiccant systems continuously remove moisture from the air using a corrugated ceramic composite, impregnated with desiccant, formed into a wheel. Generally, silica gel is used as the composite. Moist process air, which has a high vapor pressure, passes through the upper portion of the rotating desiccant wheel. The desiccant, which has a low vapor pressure, absorbs the moisture. The dry process airstream then passes through the conventional refrigeration coil, where the temperature is lowered to design conditions. The dry desiccant wheel is effective only until it is saturated. Once it is, a scavenger hot airstream is forced through the desiccant wheel to remove moisture. After it is regenerated, the desiccant is cooled to lower its vapor pressure, and then rotated back into the moist airstream, where the dehumidification cycle repeats. As shown in Fig (3.10). Descent wheel
The Figure (3.11) shows the basic desiccant component²the wheel. The desiccant material, usually a silica gel or some type of zeolite, is impregnated into a support structure. This looks like a honeycomb which is open on both ends. Air passes through the honeycomb passages, giving up moisture to the desiccant contained in the walls of
the honeycomb cells. The desiccant structure is formed into the shape of a wheel. The wheel constantly rotates through two separate airstreams. The first air stream, called the process air, is dried by the desiccant. The second air stream, called reactivation or regeneration air, is heated. It dries the desiccant.
Figure (3.11) desiccant wheel
Dry Desiccants material: 1. Silica Gel 2. Titanium Gel 3. Dry Lithium Chloride 4. Natural Zeolites 5. Activated Alumina Advantage of silica gel as desiccants:Silica gel has many other properties that recommend it as a desiccant. -It will adsorb up to 40% of its own weight in water vapor. This adsorption efficiency is approximately 35% greater than typical desiccant clays, making silica gel the preferred choice where weight or efficiency are important factors.
- It has an almost indefinite shelf life if stored in airtight conditions. - It can be regenerated and reused if required. - It is a very inert material; it will not normally attack or corrode other materials. - It is non-flammable. 2- Liquid desiccant cycle:
Figure (3.12) liquid desiccant cycle
Liquid desiccant technology has been in use for many years, primarily in process applications requiring dehumidification and humidity control. In some applications, it may be more energy efficient than traditional defrost or dry desiccant systems. The air to be dehumidified is passed through a desiccant solution spray. The solution has a lower water vapor pressure than the air and the air is dehumidified. The water extracted from the air will dilute the concentration of the liquid desiccant and over time, subsequently reduce its effectiveness. To maintain the desiccant at a fixed concentration, it is fed to a regenerator section. At the regenerator the liquid desiccant is heated, which raises the vapors pressure in the air causing moisture to transfer to the desiccant. The heated desiccant is sprayed into an air stream of outdoor air and the moisture is released and exhausted to the outdoors. The regenerated desiccant is then cooled and reused. As shown in Fig (3.12) Liquid desiccant systems can be used with any type of refrigerant or refrigeration system. Mediums include salt (sodium chloride, calcium chloride or lithium chloride) and propylene glycol. Liquid desiccants solution:1) Calcium and lithium chloride use is not permitted with edible food product.
2) Sodium chloride, temperatures are limited to approximately -6F (-21.1C), but its use is permissible around edible food product. The biggest problem with inorganic salt solutions is that they are highly corrosive. Thus, there is the potential for increased maintenance costs. Propylene glycol is well-suited for liquid desiccant systems used in food processing operations. y y y y
Does not have corrosive characteristics. Is antimicrobial. Is effective as a food preservative. Is more effective than the inorganic salts at lower temperatures.
Application of liquid desiccant system:The liquid desiccant is its ability to supply biologically uncontaminated air. The solutions used kill bacteria on contact. Additionally, there are no wet surfaces, such as those found on cooling coils, to promote bacterial growth. This makes liquid desiccants ideal for use in healthcare facilities and in sensitive industrial applications such as pharmaceuticals. The table bellow explains Comparison between solid and liquid desiccant cycle: Table (3.2) Comparison between solid and liquid desiccant cycle cycle
SOLID DESICCANT
LIQUID DES ICCANT
Less degree of dehumidification Inexpensive materials like Silica gel, , alumina
More drying capability Costly materials like LiBr,LiCl,Cacl2 Glycols with water Pressure drop is lower Adsorption ± desorption is continuous Modifications are necessary for coupling
Pressure drop is higher Adsorption ± desorption is not Continuous Easily coupled with conventional VC& AC system
Chapter
Four
Solar energy cooling case study
4.1 Case description: In this chapter we want to design an air conditioning system using absorption chillers instead of using electric chillers. For this design the extension of the law building in Birzeit University was taken as our case, this building consists of three floors, Basement floor which is used mainly as archive and small store, ground floor which has a large hall that contains 120 seat so we can consider it as a lectures room and the first floor which contains the teachers offices and two multipurpose rooms. The first thing that would be done is to calculate the cooling load, after that the required equipment should be chosen such as chillers, fan coil units, and solar system component. After that the ducts design should be done, finally economic analysis and bay back period to absorption working system compare with electrical working system could be calculated.
4.2 Construction elements description: 1- Walls : y
External walls: which consists of five layers that
. are:
y
Internal walls: which consists of 3 layers that are :
. y
Exposed ceiling : which consists of 4 layers that are :
.
y
Non-exposed ceiling : which consists of 5 layers that are :
+
.
y
Floor : which consists of 4 layers that are:
The wall section is shown in the wall section plane in the appendix A2. y In this building the glass used is double glass and the external doors are from iron and the internal ones are from wood. y The architecture planes of the building can be found in the appendix A1. y
4.3 Thermal resistance for walls, windows and doors: y
For non-exposed ceiling:
For
exposed ceiling :
For exposed ceiling :
For floor :
The room¶s height is 4 meters. The windows and the doors dimensions are shown in table 1and 2 respectively in appendixes A3.also in Appendix C.2 the temperature and relative humidity indoor and outdoor can be found.
4.4 Load calculation: 4.4.1 Sample calculation for the Multipurpose Hall (120 seat hall) in the ground floor:
1) Windows glass load: q g l la ss ! qt r ra n s
q s
ol ar
! u g Awindow (T A * SHGF * SC * CLF
Taking into account the orientation of windows to the sun the following data for the load gained: Space
q g l la ss (W )
120 seat hall
3955
2) Infiltration load: Qinf ! q s
q
y
y
! 1.23 * V inf * (T 3000 * V inf * (w l
The following data were obtained for the load: Space
q (W )
120 seat hall
6048
3) Transmission through walls: For external walls: q S , wall s ! u wall * Awall * CLTD wall
For internal walls: qint . wall s roo f &ceilin g ! u wall
(T
Awall
Taking into account the orientation of the walls to the sun and the internal walls and the floor were included the following data were obtained for the load: space
q wall s t otal
120 seat hall
3107
q f loor ceilin g
(W )
5169
4) Lights radiation load q li ght s ! ( watt r at in g ) v fu v fb v CLF at in
fu v fb ! 0.79 qli ght s ! (area v 20) v 0.79 v 0.85
Thus, the load obtained is: 120 seat hall
Space
q li ght
(W )
3223
5) Appliances load Assuming proper equipments in hall (computers, speakers and displaying machine) Thus, the load obtained: Space
120 seat hall
q
(W ) 3936
6) People & ventilation load: Assuming the number of people in each room and the time of occupancy and the activity type as foll fo llows: ows: 1) Assuming to have 120 persons, and assumed as a lecture attendance, then the load equals the value follows.
Space
q (W )
120 seat hall
12765
Total load for the 120 seat hall: space
Ql at ent at en
120 seat hall
(KW)
7.3
Q sen sible
Qt ot
(KW)
32.9
(KW)
40.2
TR
11.4
4.4.2 The load results for the ground and 1st. floors: For the ground floor: 1- Multipurpose hall (120 seat hall)
KW
BT U/H
TOTAL COOLING LOAD (KW)
40.20039079
137167.601
2- Sound rooms
KW
TOTAL COOLING LOAD (KW)
1.855163348
BT U/H
6329.99581
3- Entrance lobby
KW
BT U/H
TOTAL COOLING LOAD (KW)
29.80593872
101700.73
For the 1st floor: 1- Corridor TOTAL COOLING LOAD (KW)
2- Office # 5 TOTAL COOLING LOAD (KW)
KW
29.23144023
KW
4.615515726
BTU/H 99740.4861
BT U/H 15748.5837
3- Office # 4 TOTAL COOLING LOAD (KW)
KW
3.630827407
4- Office # 3
KW
TOTAL COOLING LOAD (KW)
4.160639132
5- Office # 2
KW
TOTAL COOLING LOAD (KW)
6- Office # 1 TOTAL COOLING LOAD (KW)
7- For multipurpose room #1
4.658028829
KW
4.88681095
KW
TOTAL COOLING LOAD (KW)
11.93323654
8- For multipurpose room #2
KW
TOTAL COOLING LOAD (KW)
12.1538647
BT U/H 12388.7324
BT U/H 14196.501 BT U/H 15893.6425
BT U/H 16674.2691
BT U/H 40717.351
BT U/H 41470.1556
The total load needed for ground and 1st. floors = 144 kilowatt = 40 RT. So we need an absorption chiller of capacity 40 ton refrigeration , we have select a hot water fired absorption chiller manufactured by Sanyo company the specification of the chiller selected is shown in table 4.1
Table 4.1 Specifications of a 40 ton absorption chiller:
Item
Scope of Works
To supply Sanyo Hot Water LiBr Absorption AA Chiller H Series, model: mode l: LCC-E02 with total cooling capacity of 40 USR T inclusive of the following features: - Chilled water, Temp. out: 8ºC, Temp.in:13ºC - Cooling water, Temp. out: 37ºC, Temp.in:31ºC - Hot Water, Temp.out: 83ºC, Temp.in:88ºC - Power Supply : 415V/ 3phase 3phase / 50HZ - Single effect - High efficiency part load operation - Digital intelligent microprocessor integrated control - Four crystallization prevention safety controls - Patented Li-Br solution - One year equipment warranty
Qty
1 Nos.
Unit Price
Total Price
(USD)
(USD)
69,000.00
69,000.00
(1) (2)
This quotation is based on CFR - Ramallah, Palestine. Solar District Cooling (M) Sdn. Bhd. is the sole-distributor for Dalian Sanyo absorption chiller.
(3)
Exclude all mechanical, electrical and structural works.
4.5 Solar system design: 4.5.1Collector Calculation: A- Case one :
This case related with Initial load to heat the water from 18C Qto 88 C Q which is request as Heat source for the chiller. And this case neglected because large number of solar collector. (Also in Appendix C.1 the climatologically average temperature) Depending of the collectors catalogue CPC ³Evacuated tube collector´ in appendix B2 ^ ! 60 With T u =88 r and T =18 o t collect or or
in collect or or
Flow rate request 0.947 k g s (from chiller catalogue) in Appendix B.1 y
Q ! mC p (T
= 0.947 v 4.17 (88 18) = 276.4293 KW (Initial load) * The area of collectors request to cover this load ! ^ collect or or
qu AG
Where: : Efficiency of the collector. ^ collect or or qu :
Useful heat required (Watt). A: Area of the collectors (meter). G: Average Summer Insolation on A Horizontal Surface in Ramallah ( W And G= 740 W A
276429 .3 740 v 0.6 = 622.5885 m 2
!
m
2
(from solar data in Appendix C1).
m
2
)
* The area of each collector from (collector catalogue in Appendix B.2) is 3.41 m 2 * Number of collectors is 183 B- Case Two:
This case related with steady load for the chiller heat source, which is used in our solar system. Where: Q T : Hot water enter the chiller is 88 C . in
Hot water out flow from the chiller is 83 C Q. (From the chiller specification) * The total heat required is T out :
y
Q ! mC p (T
= 0.947 v 4.17(88 83) =19.7449 KW
* The efficiency of the collectors in this case 58% depending on the collector average temperature. (Appendix B.2) *The area of the collectors request is 19744 .9 A ! 740 v 0.58 = 46
m
2
* The number of collector is 14
For the safety due to thermal losses in system we added another collector to become 15 collectors.
4.5.2 Collector installation:
This type of collectors is installed in the way shown in figure (3.1)
Figure 4.1 shows the way to install CPC-18 OEM collectors
The dimensions of each collector is shown in figure (4.2) and table (4.2)
Figure 4.2 shows the dimension of CPC-18 OEM collector
Table (4.2) the dimensions of the CPC -18 OEM collectors
The installation angles of the collectors and the free space between them is shown in figure (4.3)
Figure (4.3) shows the installation angles of the CPC-18 OEM collector and the free space between the collectors.
The catalogues from where we took the information about the chiller and the collector can be found respectively in appendix B.1 and B.2
4.5.3 Double jacket Storage tank:
The double jacket jacket storage tank designed to store 60% of the evacuated tube heat heat collection during 8 hour. Q s
! VVc (t s t ) m
Q s !
Heat stored, KJ
V ! Density of water, K g / m
3
.
V ! Volume, m 3 . c p
= specific heat, K J / K g K
t s
! Storage temperature,
t m
! Minimum useful temperature,
V
!
Q
C Q
C
(19.7449 KW v 8 v 60 v 60) v 0.6 980 v 4.17 v 30
! 2.74m 3 The volume of chosen storage tank is 2.5 boiler.
m
3
to decrease initial heating load on the
4.5.4 Boiler (back up) Calculation:
A 34KW boiler can cover the entire initial load to increase water temperature from 18C Qto 88 C Q in storage tank request as Heat source for the chiller, but the time for this process take 5.96 hour, after this time chiller turn on. Q
34 KW !
!
m t ime
2500 t ime
C p
(T
v 4.17 v (88 18)
Time=5.96 h
If the efficiency of the boiler had been taken 90%, then the time required to cover all the heating load would be 6.65 hour. The selected boiler is Buderus G115E/G115 S with heating capacity of 34 KW, the boiler specification shown in (Appendix B.3).
4.5.5 Solar system pump selection
In order to select a suitable pump, we must cover the largest head to be sure that the pump will cover all of our system. The optimal velocity should be chosen in closed loop heating system not exceed 10 ft/s the velocity taken to be 8 ft/s which equal 2.5 m/s. In order to calculate the total head of the circulated pump, that is on the line between double jacket storage tank and solar collectors with flow 0.947 Kg/s. total pipe length estimated by 44 meter. Pump head and losses calculation: y
The diameter of the pipe had been calculated to be 1 in =0.0254m Velocity assumed not exceed 8ft/sec = 2.5 m/sec for not noisy system and less friction losses, and volumetric flow rate 0.978 L/sec with = 968 kg/m3 at 85o for water L= 44 m = 968 kg/m3 at 85o for water = 451x10-6 N.s/m2 (at 85o) from heat transfer tables. D=1´ =0.0254 m V=2.5 m/s VVD 968 v 2.5 v 0.0254 ! ! 136.3 v 10 3 Re ! 6 Q 451 v 10 Turbulent lo y
Frictional losses in pipes = 0.05 mm
0.05 ! 1.97 v 10 3 0.0254 d From moody chart f ! 0.025 I
!
2
y
¨ L ¸ V ( H f ! f v © ¹ v 1 ª D º 2 g 44 ¸ 2.5 2 ¨ ! 13.8 m ( H f ! 0.025 v © ¹v ª 0.0254 º 2 v 9.81 Frictional losses in elbows: # of elbows = 6X (1 Tee+ 5 elbows 90 degree) ( H ! K v
V 1
2
2 g
Where; K: is the factor of fittings fittings which which is for elbows=0.95 2.5 2 ) ! 2m ( H ! 6 v (0.95 v 2 v 9.81 y Frictional losses in valves: 3 valve (gate valve, fully open) K=0.3 ( H l ! K v
( H l
y
V 1
2
2 g
2.5 2 ! 3 v (0.3 v ) ! 0.3m 2 v 9.81
pressure drop in CPC 18 OEM evacuated tube collectors From a catalogue in (Appendix B.2) is 100 mbar = 1 m for each one. *The 15 CPC evacuated tube collector connected as shown in figure 4.1, five rows in parallel each one has three CPC 18 OEM evacuated tube collectors. And the largest length of the pipe go through 3 collector. For 3 collectors = 3 m at flow rate 11.7 L/min. Total Head =13.8+2+0.3+3 =19 m
The selected pump (1) with flow rate 3.5 m 3 h and head 19 m is circulated Salmson LRL 203-13/1.1 pump. Other selected pumps. Pump (3) 3.5 m 3 h flow rate and total head 12 m is circulated Salmson LRL 203-10/0.55 pump. And pump (2) 0.43 m 3 h flow rate
and total head 5 m. is circulated Salmson LRL 404-13/0.25 pump. The specification of the pump selected shown in (Appendix B.4). The figure (4.4) bellow show the case study solar system component
Figure (4.4): The case study solar system component.
4.5.6 Expansion Tank selection:
It is considered as an important unit in heating system. It is necessary to contain the extra volume of water when its temperate rises to high degree, so this tank designed so that its active volume will contain the extra volume of water. * The system volume Pipe volume= total length v T 4 v (0.0254) = 0.0223
m
2
3
Collectors manifold volume each one 10 liter total volume = 150 liter liter V syst em
V t ! V s
! 0.1723m 3
[(v2 v1 ) 1] 3E (t 2 t 1 ) ( P P 1 ) ( P P 2 ) a
a
Where : t 1
= lower temperate ( 18 QC ).
t 2
= higher temperate (100 QC ).
v1
= specific volume of water at t 1 =0.001 m 3 k g .
v2
= specific volume of water at
E
= liner coefficient of thermal expansion ( 11.7 v 10 6 m m.k ).
t 2
=0.001044 m 3 k g .
P a =
atmospheric pressure 101.3 Kpa.
P 1 =
pressure at
P 2 =
maximum pressure of the system = 270 Kpa
t 1
= V v g v H pum (181 Kpa) , H pum = 19 m. pum p pum p
[(0.001044 0.001) 1] 3 v 11.7 v 10 6 (100 18) V ! 0.1723 (101 .3 181) (101.3 270) t
Volume of expansion tank (2) = 0.0383 After sample calculation the volume of expansion tank (1) is 25 liter.
So according to this values (ZILMET) expansion tank type will be selected which about 35 Liter for expansion tank (2) , and 25 liter expansion tank(1) respectively. And the specification of (ZILMET) expansion tank type shown in the catalogue in (Appendix B.5).
4.6 Duct design:Depending on equal friction method there is a sample calculation for duct sizing, which is supply three offices (1,2and3) in1st floor as shown in Appendix A.1, and in figure (4.5).
Figure (4.5): The supply duct of offices (1,2and3) in1st floor.
For the comfort condition the air flow velocity in main duct supply assumed to be 5 m s
y
In main duct (section one upstream) Q0=1367.6 CFM=0.65m3\s & air velocity 5 m s By using pressure drop loss chart (Appendix D.1)
V=990 ft\min
So«« But
And aspect ratio
=2:1
b =2a
So
a=0.203 m b=0.405 m y
Section two (downstream1)
Remain the same value for all following cases
Q1 =1368-413=955 CFM=0.45 m3\s &
And From pressure drop loss chart
m
So
a=0.18m b=0.36m y
For section three of the duct(downstream two)
Q2=955-425=530 CFM =0.25 m3\s From pressure drop loss chart So
m
a=0.14m b=0.28m Branches:-
The Fig (4.5) show the distribution of branches A) Q1=413CFM =0.195 m3\s From pressure drop loss chart
So
m
a=0.13m
B) Q2=425CFM =0.2 m3\s From pressure drop loss chart So
m
a=0.14m
b=0.28m
Static pressure
Static pressure drop = duct length *
( P L
= 0.65*15 = 9.75 Pa Dynamic pressure drop
Assume a dynamic loss coefficient of 0.3 for upstream to downstream For (A)
=4.4 m/s u-d =
V=
V= =3.7 m\s =
u-d
l +
FTP=
i
=0.65(2+5+8) +3.5+2.5=15.75 Pa All duct distribution and sizing for ground and first floor explained respectively in Appendixes A1, D2.
4.7 Chilled water distribution 4.7.1 Fan coil selection:
After the calculation of the cooling load and the CFM request for each duct design as shown in Appendix A.1. A Petra DC type fan coil ( chilled water medium static) were selected to cover all the cooling load for the ground and first first floor with Catalogue specification in Appendix Appendix D.3 , the table table (4.3)and (4.4) bellow show the selected selected one . Ground
floor
Table (4.3): Selected fan coil specification for ground floor. Fan coil number (As shown in Appendix A.1)
Fan coil type
Fan coil air flow (CFM)
Fan coil chilled water flow(GPM)
1 2 3 4
DC 30 DC 30 DC 24 DC 24
2286 2286 1908 1908
15.3 15.3 11.47 11.47
Fan coil type
Fan coil air flow (CFM)
Fan coil chilled water flow(GPM)
DC 18 DC 16 DC 18 DC 14 DC 18
1507 1312 1507 1200 1507
8.98 7.72 8.98 7.32 8.98
First floor Table (4.4): Selected fan coil specification for first floor . Fan coil number (As shown in Appendix A.1) 1 2 3 4 5
4.7.2 Chilled water pump selection:
All the fan coil need 95.6 GPM chilled water, which is 6.03 v 10 3 m 3 s . 3.38 v 10 3 m 3 s For the ground floor and 2.65 v 10 3 m 3 s for the first floor. by
assuming the optimal velocity 2.5 m s of chil hilled water ter for not nosey osey cool ooling sys system tem. A cross diameters were calculated for for each length as flow flow rate distribution and the largest largest length which selection pump depend on is 8 meter with pipe diameter 2``, 4 meter with 3 `` 1 `` diameter 1 and 20 meter with diameter . 4 2 L= 8 m = 980 kg/m3 at 10o for water = 451x10-6 N.s/m2 (at 10o) from heat transfer tables. D=2``=0.0508 m V=2.5 m/s VVD 980 v 2.5 v 0.0508 ! ! 275.9 v 10 3 Re ! 6 Q 451 v 10 Turbulent lo y
Frictional losses in pipes = 0.05 mm 0.05 I ! ! 9 v 10 4 d 0.0508 From moody chart ! 0.021 2
¨ L ¸ V ( H f ! f v © ¹ v 1 ª D º 2 g 8 ¸ 2.5 2 ¨ ! 0.97 m ( H f ! 0.021 v © ¹v ª 0.0508 º 2 v 9.81 y
For pipe L=4m 1 `` D= 1 =0.0381 m 2 ( H f =0.67m
y
For pipe L=20m 3` D= =0.01905m 4 ( H f =8.7m y
Frictional losses in elbows:
The equivalent length calculation
Leq
!
KD
f
Where; K: is the head loss coefficient of fittings fittings which is for elbows=0.95 and for Tee =1.8.
For elbow: Leq
= 4.76 m
For tee: L q ! 10 m e
( H ( f itt inin g ) ! 1.8m y
The head loss for water collector= 0.8m. ( H ( ) ! 1.8 0.8 ! 2.6m t ot al al
y
pressure drop in fan coil as shown in fan coil (1) catalogue in Appendix D.3 as average is 5ft of water =1.55 m.
All calculation above for supply line, which nearly same of return line. Total head = (supply line*2) +fan coil head loss ! 2(0.97 0.67 8.7 2.6) 1.55 ! 27.5m As shown above from the calculation, the pump required to supply a fan coil with chilled water 22 cubic meter per hour and total head 27.5 m, depending on this calculation Salmson LRL 203-16/1.1 pump was selected with specification ( appendix B.4)
4.8 Economic analysis for solar energy cooling case study: The case study economic analysis period time is 25 year (absorption chiller working system), compare with (vapor compression chiller working system). Two systems operate for 3 months, 8 hour per day
Assuming Interest rate (d) =8% Prices inflation (f) =10%
Investments: Absorption Chiller working system: Table (4.5): fixed cost for absorption chiller working system
Equipment (#No) Absorption chiller (1) Solar Collectors(15),1500$/collector Boiler (1)
Cost($) 69000 22500
2160 810 670 Pumps (4) 620 350 Fan coils (9) 950$/fan coil 8500 Storage tank(double jacket 2430 insulation)(1) Expansion tank (2) 67$/tank 130 Pipes, fittings and insulation 670 Duct+ insulation 1900 installation 10000 Total cost 119740
Electrical Chiller working system: Table (4.6): fixed cost for absorption chiller working system
Equipment(#No) Cost($) Electrical chiller (1) 40000 Fan coil(9),950$/fan coil 8500 Pipes, pump and fittings 1240 Duct+ insulation 1900 installation 5000 Total cost 56640 The fixed cost shown in table (5, 6) respectively is depending on pill quantity in Appendix D.4 Operating Cost: Absorption chiller working system:
For boiler Assume that the boiler is operated 3 times 6.65 hour per year (to increase the water temperature from18C Qto 88 C Qwhich is request as Heat source for the chiller (3 months).
y
Cost of diesel= Where: y
m f : foil consumption of boiler=0.014 Kg/s (Boiler catalogue in Appendix B.3)
V
: Diesel density=0.85.
Diesel price per liter=1.6$ Time: 3*6.65*60*60=
(sec)
Cost diesel/year =
=1890$/year Each pump consumes 3 KW electrical powers. y Cost of Electricity =Electrical consumption for four pumps= y
=
=1750$/year y
Each fan coil consumes 12 KW electrical powers. Electrical consumption of fan coils (9 fan coils)/year=
=15780$/year y
Absorption chiller consumes 2KW electrical powers.
Electrical consumption for the chiller/year=
=290$/year Total consumption of electricity=1750$/year +15780$/year +290$/year =17800$/year Maintenance = 3000$/year Total operating cost= Total consumption of electricity/year + Maintenance+ Cost diesel/year =17800$/year+3000$/year+1890$/year =22690$/year Annual cost=annual operating cost+ capital cost*FCR FCR=interest +depreciation
=
FCR=
=0.092 =22690$/year +119740*0.092 =33700$/year Electrical chiller working system:
For 40 TR COP=1.2 y
Electrical chiller consumes120KW electrical power.
Electrical power consumption of electrical chiller/year=
=17500$/year Fan coil electricity consumption (9 fan coils) =15780$/year y
In this working system just one pump has been working to distribute chilled water.
Pump consumption=
=440$/year Total electricity consumption/year= Electrical power consumption of electrical chiller/year+ Fan coil electricity consumption/year+ Pump consumption/year =17500$/year+15780$/year+440$/year = 33700$/year Maintenance/year =2000$/year Total operating cost/year = Total electricity consumption/year+ Maintenance/year =33700$/year+2000$/year =35700$/year Annual cost=annual operating cost+ capital cost*FCR Annual cost=35700+56640*0.092 =40900$/year Life cycle cost
PWF 1. For future PWF=
=
=0.146 2. For annual payment : PWF=
=
=29.1
Table (4.6): Life cycle cost absorption chiller working system (A), compare with electrical chiller working system (B) during 25 years.
Cost item
Cost option A,
Cost option B,
First cost ( investment) investment)
119740$
56640$
PWF
1
1
Present worth
119740$
56640$
22690$/year
35700$/year
PWF
29.1
29.1
Present worth
660000$
1038900$
22000$
5000$
PWF
0.146
0.146
Present worth
- 3212
-730
776500$
1094800$
Annual Ann ual operating cost
Salvage value
Life Life cycle cycle cost
From table above (4.6) it¶s easy to note that life cycle cost for absorption chiller working system less than electrical chiller working system, this lead cooling with absorption chiller is better. Payback period: =
=4.7 years
4.9 Conclusions and recommendations: y
It¶s clearly appear that the initial cost of instillation of absorption chiller working system costly more than electrical chiller installation for the same cooling
capacity, but if you look father on time with operate system ,the life cycle cost for the electrical chiller is being more costly than absorption chiller. y
The
economic study reveals that t he life cycle cost f or operating the absorption
chiller 25 years equals (776500
$ ) and (1094800 $ ) for the electrical chiller so
the installation of the absorption chiller instead the electrical chiller is justified , the study also shows that the payback period for the absorption chiller equals 4.7 years.
Chapter
five
Adsorption refrigeration
5.1 Introduction: The need for energy is constantly increasing and is leading to an increase in the price of energy; however energy sources are getting scarce .Therefore, the search for an efficient technology has become a necessity .Heating and cooling systems are technologies that consume energy; and the demand for these systems is increasing in every aspect .Therefore, the development of anew refrigeration and heat pump systems that is driven by cogeneration of waste heat or renewable energy sources is highly desirable . Since the adsorption chillers are usually driven by heat, they can contribute to reducing the energy consumption by utilizing non-fossil fuels, such as waste heat from the cogeneration process or alternative renewable energy resources, besides that adsorption chillers contributes in minimizing the depletion in the ozone layer and adsorption chillers are CFCs free this makes those chiller a strong competitive to the electrical refrigeration systems . The correct selection of the adsorbent-adsorbate pair is the first step to increase the performance of the system, Activated carbon-methanol adsorbent±adsorbate pair is widely used in adsorption refrigeration systems that needs very low temperatures such as ice making applications in literature activated carbon-methanol adsorbent±adsorbate pair has the highest coefficient of performance among the adsorbent-adsorbate pairs.
5.2Adsorption refrigeration: Adsorption in literature is the process of absorbing liquids and gases by using solid materials, and the adsorption cooling cycle is one of the refrigeration cycles that utilize heat to drive the refrigerant throughout the cycle instead of using a mechanical compressor .
When a porous solid is exposed to a gas for which it has an affinity, forces of attraction act between the individual gas molecules and the atoms or ions composing the solid, at the interface of the two phases .The unbalanced forces at the phase boundary result in the adsorption of the gas by the solid .The solid is referred to as the adsorbent while the gas is referred to as the adsorbate. The evaporation of the adsorbate takes its energy from the environment which gives us the cooling effect and the regeneration of the adsorbate adsorbed by the adsorbent needs a heat which is taken from the low temperature heat sources, the evaporation and the regeneration of the adsorbate through the system completes one cycle. One operating cycle of the system will consist of four distinct steps, described and presented graphically in the Clapeyron-Clausius diagram of Figure (5.1). Assumption :The system evaporator is named as vessel B and the generator is named as vessel A and those vessels each have a control valves .
Figure [5.1] an adsorption cycle represented in a Clapeyron-Clausius Clapeyron-Clausius diagram
Step 1 :Isosteric heating of the wet adsorbent (charging stage): At the beginning of the cycle, point 1 in Figure 5.1, the adsorbent of vessel A contains the maximum amount of adsorbate within its pores, i.e .the adsorbent is wet .The ratio X of adsorbate mass to the dry mass of adsorbent at this point is Xmax .The valve between the adsorbent vessel A and the condenser/evaporator (vessel ( vessel B)is initially initially closed . As heat Qd at temperature Td is applied to vessel A containing the wet adsorbent, the pressure of the adsorbate in vessel A will increase to the condenser pressure, Pc, with no change in X.Therefore, the ratio of mass of adsorbate to mass of adsorbent is Xmax (point 2 in Figure5.1). The process between points 1 and 2 is referred to as the isosteric (constant mass ) heating of the wet adsorbent. Step 2 :Desorption and condensation of adsorbate (charging stage): At point 2 (Figure 5.1) the valve connecting the adsorbent vessel A to the condenser (vessel B) is opened .Heat Qd continues to be supplied to the adssorbent in vessel A and desorption of the adsorbate from the adsorbent occurs until the adsorbent vessel A and the condenser (vessel B) reach the equilibrium pressure .The adsorbate gas from vessel A flows to the condenser (vessel B) , Where it is condensed to its liquid state.Heat of condensation Qc is rejected at temperature Tc .The desorption of the adsorbent in vessel A At a constant pressure Pc continues until the adsorbent reaches the temperature Td and the ratio of mass of the adsorbate to mass of adsorbent is at the minimum value Xmin. i.e .the adsorbent is dry (point 3 in Figure 5.1). The valve connecting the adsorbent vessel A to the condenser/evaporator (vessel B) is now closed. Step 3 :Isosteric cooling of the dry adsorbent (discharge stage): At point 3 (Figure 5.1) the adsorbent within the adsorbent vessel A has a minimum amount of adsorbate within its pores, Xmin .The adsorbent is at a temperature Td, and is separated from the adsorbate in liquid form that is contained within the condenser/evaporator (vessel B). The valve is kept closed .As the adsorbent vessel A cools, the pressure of the adsorbate contained within the pores of the adsorbent decreases
to the evaporator pressure Pe with no change in the ratio of mass of the adsorbate to mass of the adsorbent, Xmin (point 4 in Figure 5.1). The process between point 3 and 4 is referred to as the isosteric (constant mass) cooling of the adsorbent. Step 4 :Evaporation of the adsorbate and adsorption (discharge stage): At point 4 (Figure 5.1) the valve connecting the adsorbent vessel A to the evaporator (vessel B) is now opened .Heat Qe is supplied to the evaporator so that the liquid adsorbate within the evaporator returns to its gaseous state .Heat continues to be supplied to the evaporator B and the equilibrium pressure is achieved as the adsorbate gas flows from vessel B to the adsorbent vessel A .Adsorption at a constant pressure Pe continues until the adsorbent in vessel A reaches the temperature Ta .At this point, the ratio of mass of adsorbate to mass of adsorbent that can be contained within the pores of the adsorbent is at the maximum value Xmax. (Point 1 in Figure 5.1). Heat of adsorption Qa is released during this process .This last process completes one operating cycle. If the adsorption system is used for storage applications, the operating cycle described above is discontinuous .Assuming an ideal cycle, the charging phase consists of steps 1and 2, where the temperature required to charge the system is Td and the heat to be stored is Qd .Heat can be stored indefinitely as long as the valve connecting the adsorbent vessel A and the condenser/evaporator (vessel B which contains the liquid adsorbate) remains closed at the end of step 2 . Adsorption system performance:
Adsorption cycles performance parameters are usually measured in terms of the cycle coefficient of performance (COP) and its specific cooling power (SCP) COP is defined as the ratio of the useful thermal energy moved in or out of the cycle (Qev) to that of the high temperature thermal energy used (Qd ) it can be expressed as:
(1)
The performance for solar powered adsorption cycles can be measured by its solar coefficient of performance (SCOP) which is the ratio between the useful energy output to the total solar energy insolation on the collector surface ( I )
(2)
Another term that is useful in showing the cycle performance is specific cooling power (SCP) which is expressed as:
M e
(3)
is the total mass of adsorbent and
c ycle
is the cycle time.
[26] 5.3Working pairs selection: There are several working pairs for solid adsorption .For the successful operation of a solid adsorption system, careful selection of the working medium is essential .It is because; the performance of the system varies over a wide range using different working pairs at different temperatures. The advantages and disadvantages of different working media and their properties are listed and discussed in this section .For any refrigerating application, the adsorbent must have high adsorptive capacity at ambient temperatures and low pressures but less adsorptive capacity at high temperatures and high pressures .Thus, adsorbents are first characterized by surface properties such as surface area and polarity .A large specific surface area is preferable for providing large adsorption capacity, and hence an increase in internal surface area in a limited volume inevitably gives rise to large number of small sized pores between adsorption surfaces .The size of the micro-pores determines the effectiveness of adsorptivity and therefore distribution of micro-pores is yet another important property for characterizing adsorptivity of adsorbents.
In order to select an adsorbent for refrigeration applications we must look at the following properties:
High adsorption and desorption capacity, to attain high cooling effect.
Good thermal conductivity, in order to shorten the cycle time.
Low specific heat capacity.
Chemically compatible with the chosen refrigerant.
Low cost and widely available.
In order to select an adsorbate for refrigeration applications we must look at the following properties:
High latent heat per unit volume;
Molecular dimensions should be small enough to allow easy adsorption;
High thermal conductivity;
Good thermal stability;
Low viscosity;
Low specific heat;
Non-toxic, non-inflammable, non-corrosive; and Chemically stable in the working temperature range.
A survey of the favored working adsorbate shows that methanol and water operates at sub atmospheric saturation pressure at the operating temperatures needed and ingress of air immediately results in system malfunction .Ammonia doesn't have this problem because its outward leak could be tolerated for some time, but its saturation pressure of 13 bar at 35 condensing temperature is quite high .In the case of methanol with a normal boiling point of 65 , the law saturation pressures could be exploited advantageously to detect leakage, since it most necessarily result in abnormal increases in pressure and poor performance.
Ammonia, methanol and water, all have relatively high latent heat values of 1368, 1102 and 2258 KJ/Kg. respectively and their specific volumes are low, on the order of about 10-3m3/Kg. Ammonia is toxic and corrosive, while water and methanol are not, but the problem with alcohols is that they are flammable . Water is the most thermally stable with adsorbents, closely followed by methanol and ammonia in that order .However, water cannot be used for freezing purpose because of its freezing temperature is 0 this makes methanol a favored adsorbate for pairing with a stable adsorbent. Various kinds of working pairs for adsorption refrigeration have been studied, and they include both physical and chemical adsorption working pairs .The main physical adsorbents are activated carbon, zeolite and silica gel, and accordingly, the physical adsorption working pairs are mainly activated carbon-methanol, activated carbonammonia, zeolite ±water and silica gel water. In recent years, the working pairs activated carbon (HFC-134A) and activated carbon dimethyl ether were also investigated. As water is the refrigerant normally used with zeolite or silica gel, the evaporating temperature is never lower than 0 .Compared to other physical adsorption working pairs, the main advantage of the utilization of activated carbon as adsorbent is the low evaporating temperature that can be reached, as the refrigerants most employed are ammonia or methanol .Due to the low evaporation temperature of these refrigerants, these pairs are more suitable for ice making technology . A study published by the ''Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China'' titled '' The performance of two adsorption ice making test units using activated carbon and a carbon composite as adsorbents ''that constrains on studying the best ''activated carbon/adsorbate'' pair reveals that the best choice for a refrigeration pair is the ''activated carbon/methanol'' pair which has adsorption quantity of 59 %larger than that of ''activated carbon/ammonia''.
Another study for ''Meunier F, Douss N "titled '' Performance of adsorption heat pumps :activated carbon/methanol and zeolite/water pairs'' shows that the C.O.P of the heat pump used in the test which uses activated carbon/methanol pair is 0.4-0.5 but the C.O.P of the same heat pump undergoes the same operating conditions is 0.3. For all the purposes listed above and because we want to build an adsorption ice maker model which means very low temperature application we have selected activated carbon-methanol pair as working pair. [27]
5.4 Lab .Scale adsorption ice maker: In order to investigate the adsorption cycle behavior we built an adsorption ice maker model, the model were built by using glass ware components. 5.4.1 Adsorption ice maker model components:
Figure [5.2] Lab . scale adsorption ice maker.
1- Vacuum pump : which is used to reduce pressure inside the system.
2- Evaporator : which contains the adsorbate and the refrigeration effect is extracted there. 3- Heat exchanger) Condenser) to reduce the temperature of the adsorbate when it is desorbed from the generator. 4- Generator :which contains the adsorbent where the heat is added to the cycle.
5.4.2 Components description: A- Evaporator:
The evaporator is the part of the model from where the cooling effect is extracted so the construction of the evaporator depends on the way we want to utilize this effect in our case we want only to investigate the process working conditions so we want to measure the temperature of a known amount of water surrounds the evaporator so we can choose the any shape of the evaporator (cubic box or a spherical shape) so we choose the spherical shape as the glass sphere can sustain larger pressure on it . B- Generator (Adsorber):
The generator is the part of the model to be heated and its shaped is determined according to the way we want to heat the adsorbent material some shapes are made like a flat plate water collector as the adsorbent material is arranged so that the water passages passes through the material which is the most common type but in our model we use a spherical container to but the adsorbent inside it and we immerse it inside paraffin oil container and we use to heat the oil as it has a boiling temperature 200 rather than water .
C- Condenser:
The most common types of condensers are: 1- Finned type heat exchangers: This type of heat exchangers are used in the small adsorption units as the heat to be rejected is small and this type of heat exchangers depends on the natural convection which is a poor process , when one decide to use this type of heat exchangers then there must be another pipe that is not finned which connects the generator with the evaporator so as to make sure that the
adsorption process happens and the adsorbate not to condense and return back to the evaporator. 2- Shell and tube heat exchanger: this type can be used with the small and large units and this type can be used alone as the connection between the evaporator and the generator this means more simple construction and large heat rejection capacity, For all reasons above and because the glass Finns gives no effect as the thermal conductivity for the glass is very low then we use the shell and tube heat exchanger as the condenser of the unit [28]. 5.4.3 Lab scale experiment:
In this section we show the results obtained from the experiments done using the model. Table (5.1): charcoal/methanol pair experiment data.
Time
Q
Q
Q
Q
Pressure
Tevap. ( C )
Tcond. ( C )
Tgen. ( C )
Tamb. (( C )
11:20
23
23
22
23
67
11:50
22.8
23
22
23
66
12:20
22.5
23
22
23
65
12:50
22
23
22.5
23
64
01:30
21.5
23
22.7
23
63.5
01:40
21
23
22.8
23
63
01:50
20.3
23
23
23
62
02:10
20
23
25
23
60
02:30
20
23
29
23
58
02:50
20.2
19
39
23
56
03:00
20.6
19
50
23
50
03:20
21
19
60
23
48
03:50
21
19
71
23
43
04:10
21
19
83
23
39
04:30
21
19
92
23
38
(cm Hg)
Comments on data:
From 11:20 we evacuate the system and then inject the methanol into it, Adsorption process ends at 01:50, so the adsorption cycle takes 2.5 hours.
The desorption process starts at 01:50 till 04:30, so the desorption process takes 2 hours and 40 minutes.
The amount of methanol adsorbed by the charcoal is 25 ml water.
Table (5.2): silica-gel/water pair experiment data
Time
Q
Q
Q
Q
Pressure
Tevap. ( C )
Tcond. ( C )
Tgen. ( C )
Tamb. ( C )
11:10
20
19
21
23
68.3
11:28
20
19
21
23
68.2
11:36
20
19
21
23
67.8
11:50
19.5
19
21.4
23
67.6
12:00
19.5
19.5
21.8
23
67.5
12:15
19.3
19.5
21.8
23
66.9
12:30
19
19.6
22
23
66.8
12:50
19
18
22
23
66.5
01:20
19
18
23
23
65
01:50
19.3
18
36
23
64
02:30
19.5
18
48
23
60
03:00
19.6
18
60
23
58
03:30
19.6
18
76
23
55
04:00
19.6
18
85
23
53
04:30
19.6
18
95
23
50
(cm Hg)
Comments on data:
From 11:10 we evacuate the system and then inject the water into it, Adsorption process ends at 12:50, So the adsorption cycle takes 100 minutes.
The desorption process starts at 01:50 till 04:30, So the desorption process takes 2 hours and 40 minutes.
The amount of water adsorbed by the silica-gel is 14 ml water.
The following experiment was done in order to calculate the latent heat of the methanol.
We put an 40 ml methanol in the evaporator and then we open the system on the vacuum pump and the result was that the 40 ml evaporated in 15 minutes and this evaporation decreases the temperature of a liter water 6 degrees Celsius.
Cooling amount Q equals eq uals Q=
=
Latent heat of methanol equals: Latent heat =
5.4.4 Conclusions and recommendations : y
The adsorption pairs doesn't work on the same working conditions that the charcoal starts the desorption process at 71 but the silica-gel starts the desorption process at 76 .
y
Silica-gel/water system needs lower pressure inside the system than that is needed by the systems that uses activated-carbon/methanol pair.
y
The model built has two drawback the first the pipes weren't lubricated so that the refrigerant were stuck on the condensers' wall and didn't flow to the evaporator because the adhesive bond between the methanol and water with the glass is high so we found difficult to turn the refrigerant back in to the evaporator. The second thing is that the condenser was very long this was the greatest obstacle we found when operating the system. So we found that the
generator and the evaporator must be close to each other and this distance is also proportional to the system size and the cooling capacity and if we recommend to be approximately 20 cm, Also we recommend to solve the lubrication problem the condenser must be put under the generator directly so the refrigerant by gravity will flow to the evaporator. y
The charcoal used in the experiment was of an unknown origin and we didn't know its' adsorptivity and we recommend to use activated carbon which has a better adsorptivity or charcoal fibers.
References:
abal, Center for Energy [1] Active Solar Collectors and Their Applications, Ari R abal, and Environmental Studies, Princeton University (New York Oxford University 1985). enewable Energy note book, Dr, Afif Hasan ,(Birzeit University 2009). [2] R enewable [3] Solar angles, http://www.docudesk.com, 12/9/2009.
elationships-www.vistech.net/users/rsturge/dateline.html [4] Sun Earth R elationships-www.vistech.net lwl/classes/astro100/fall03/ [5] The Season http://eeyore.astro.uiuc.edu/~ lwl [6] Solar Electric Systems University of Delaware, ECE Spring 2008 C. Honsberg [7]- Analysis of a Flat-plate Solar Collector, Fabio Struckmann , Dept. of Energy Sciences, Faculty of Engineering, Lund University, Box 118, 22100 Lund, Sweden.
blood collection tube, tube, www.bd.com , 20/9/2209. [8] Evacuated blood [9] Evacuated tube collector www.toodoc.com, 20/9/2209. [10] Flat vs evacuated tube, www.energymatters.com.au, 20/9/2209. [11] Conversion energy, renewable energy sources: Sorenson, [12] Evacuated tube solar hot water collector www.solarwaterwise.com.au , 20/9/2209. [13] Optical Performance Analysis for Concentration Solar collector Applying Parabolic and Cylindrical Stationary R eflector eflector , Jun Dog , Zhifeng Wang Institute of Electrical Engineering , Chinese Academy of Sciences, Beijing 100080,P.R . China. [14]- Enhancement in Thermal Performance of Cylindrical Parabolic Concentrating Solar Collector, K.D.P.SINGH and S.P. SHAR MA MA , Department of Mechanical Engineering,NIT,Jamshedpur,INDIA{
[email protected];
[email protected] o.in},R eceived eceived 13 January 2009, Accepted 26 January 2009. [15]- Types of Solar collector, http://en.wikipedia.org , and 10/9/2009. [16] Absorption Chillers for Buildings :www.eren.doe.gov/power/ , 23/9/2209. [17] Dorgan, C.B., Leight, S.E .and Dorgan, C.E., 1995, Application guide for Absorption cooling/refrigeration using recovered heat, Am .Soc .Heat .R ef ef AirAE(, Atlanta, GA Cond .Engrs )ASHR AE(,
[18] Cooling cycle2006 www.escenter.org , 20/9/2209. [19] (PDF [20] Air
SHC )solar heating and cooling international energy agency
conditioning and refrigeration book, Jerold W.jones , 2nd edition.
[21] Absorption
chiller for building www.energy.wsu.edu/cfdocs/tg/12.htm.
Absorption chiller Niebergall, W., 1959, Sorptions-Kältemaschinen, Vol .7 of Handbuch der Kältetechnik, Ed .R .Plank, Springer-Verlag, Berlin [22]
Direct-Fired Absorption Chillers :Student :Anya WIndira Supervisor :Prof . Dr .Ing .B .Stanzel. [23]
[24] - An energy efficient solar ice maker, make r, K.Sumathy Department of Mechanical Engineering, University of Hong Kong, Hong Kong [25]- Solar energy refrigeration by liquid-solid adsorption technique, Watheq
Hussein, Al najah-university (2008). [26] Investigation of an Adsorption System for the Seasonal Sto rage of Heat Applied to R esidential esidential Buildings, Maria Mottillo, Mottillo, and January 2006. [27] Technology development in the solar adsorption refrigeration systems, K. Sumathy and K.H. Yeung, Li Yong, May 2002. [28] Solar energy refrigeration by liquid-solid adsorption technique, Watheq Hussain, January 2008.
Appendix A.3 Table 1 Dimensions of the doors in the building in millimeters [arch. Plane].
Table 2 Dimensions of the windows in the building in millimeters [arch. Plane].
Appendix B.1 Absorption chiller catalogue
Appendix B.2 Collector catalogue
Appendix B.3 Boiler catalogue
Appendix B.4 Pumps catalogue
Appendix B.5 Expansion tank catalogue CAL-PRO CE drawing 20013
Technical table for standard closed expansion tank (ZILMET) product red colour.
Appendix C.2 Cooling load design condition & calculation variable
Table (1): Cooling load calculation ca lculation variables. Wall Direction North
CLTD
North East
18.143
East
19.72
South East
17.977
South
15.404
South West
18.807
West
20.55
North West
18.973
15.155
SHG
CLF
SC
117
0.75
0.83
445
0.2
0.83
691
0.19
0.83
571
0.24
0.83
350
0.32
0.83
571
0.49
0.83
691
0.52
0.83
445
0.51
0.83
Table (2): Temperature and relative humidity for the design conditions. con ditions(summer) Inside design conditions(summer) Temp. 23 inside (C) elative R elative humidity 47% Outside design conditions(summer) conditions(summer) Temp. 35 outside (C) elative R elative humidity 62%
Appendix D.1 Figure of pressure drop in straight, circular, sheet-metal, 20 QC air, absolute roughness 0.00015 m
Appendix D.2 Duct sizing of case study
Table 1 : Ground floor duct sizing. Space
Duct length(m)
Main duct Number Section Number Size of section Friction of length(m) of (mvm) loss section Branches P a
FTP (Pa)
m
Hall 120 seat
#2x12
0.45
4
Entrance lobby
#2x8
0.48
3
3 3 3 3 2.7 7.7 2.7
0 0 0 0 0 0 0
0.53 v 0.26 18.8 0.45 v 0.23 0.39 v 0.2 0.13 v 0.07 0.451 v 0.74 8.5 0.42 v 0.21 0.32 v 0.16
Table 2: First floor duct sizing. Space
Duct length(m)
Main Number Section Number duct of length(m) of Friction section Branches loss
Size of section (mvm)
Size of branches (mvm)
FTP (Pa)
P a
Multipurpose
#2x8
m 0.45
Office(4,5) Office(4,5)
3
0.5
2
Corridor
7
0.6
3
Office(1,2,3)
15
0.65
3
3
2.7 2.7 2.7 3 4 2 2.5 2.5 2 5 8
0 0 0 1 0 0 0 1 1
0.4 v 0.33 0.37 v 0.18 0.28 v 0.14 0.372 v 0.186 0.304 v 0.152 0.38 v 0.16 0.34 v 0.17 0.27 v 0.137 0.405 v 0.2 0.36 v 0.18 0.28 v 0.14
8.5
39.8
0.304 v 0.152 10.4
0.26 v 0.2 0.36 v 0.18
15.75
Appendix D.3 DC Fan coil catalogue
Figure (1): Ceiling Basic Models with Plenum.
DCP fan coil designed for concealed ceiling installation above false ceiling with ducted supply air distribution and free return of air above false ceiling. The plenum encloses the fan section of the basic unit. Unit of this type consist of a coil, fan and flat filter.
Figure (2): fan coil water strainer
Appendix D.4 Pill of quantity
Equipment Absorption chiller
Type/Model
Quantity
Explanations
Sanyo /LCC-E02
1
Cooling capacity 40 RT
Petra/DC fan coils
9
Medium static pressure
Salmson LRL 20313/1.1
1
Flow rate 3.5 m 3 h and head 19 m
Salmson LRL 20310/0.55
1
3.5 m 3 h flow rate and total head 12 m
Salmson LRL 40413/0.25
1
0.43 m 3 h flow rate and total head 5 m
Salmson LRL 20316/1.1
1
22 m 3 h flow rate and total head 27.5 m
(ZILMET) expansion tank(2)
1
35 Liter
(ZILMET) expansion tank(1)
1
25 liter
Storage tank
Double jacket
1
3 2.5 m
Evacuated tube solar collectors
CPC-18 OEM collector
15
3.41 m 2 area for each one
Boiler
Buderus G115E/G115 S
1
heating capacity 34KW
Fan coils
Pumps
Expansion tanks
steel 1/2 inch
60 meter
steel 3/4 inch pipes
steel 1 inch
1
steel 1 2 inch
100 meter quantity In length
44 meter 8 meter
steel 2 inch
16 meter
elbow 90
30
with pipe size
tees
10
with pipe size
water collectors collectors
5
suitable for fan coils chilled water distribution
valves
30
globe and gates
square ducts
9
Total area can can be founded founded in appendix D.2
Fittings
Ducts
Duct insulator
For all the supply duct surface area
Chilled water supply pipe
100 meter pipe
Insulator
