PERFORMANCE STUDY OF A SOLAR POND USING SELECTIVE COATING
PREPARED BY Muhammad Ashraful Kabir Roll No. 0405023
DEPARTMENT OF MECHANICAL ENGINEERING KHULNA UNIVERSITY OF ENGINEERING & TECHNOLOGY March, 2009
PERFORMANCE STUDY OF A SOLAR POND USING SELECTIVE COATING
A project report submitted to the Department of Mechanical Engineering of Khulna University of Engineering & Technology, in partial fulfillment of the requirements for the degree of “Bachelor of Science in Mechanical Engineering”.
Supervisor:
Submitted By:
Dr. A.N.M. Mizanur Rahman
Muhammad Ashraful Kabir
Professor
Roll No. 0405023
Department of Mechanical Engineering KUET.
DEPARTMENT OF MECHANICAL ENGINEERING KHULNA UNIVERSITY OF ENGINEERING & TECHNOLOGY March, 2009
ACKNOWLEDGEMENT
First of all, thanks to Almighty Allah for his immense blessing and mercy and also for enabling me to complete this project work. I am grateful to my supervisor Professor Dr. A.N.M. Mizanur Rahman, Department of Mechanical Engineering, Khulna University of Engineering & Technology, for his close guidance, valuable suggestions and kind cooperation towards completing this project work successfully and preparing this report. I am also grateful to Professor Dr. Tarapada Bhowmick, Head, Department of Mechanical Engineering, Khulna University of Engineering & Technology, for his kind permission to use the workshop and laboratory facilities of Mechanical Engineering Department. Thanks are extended to the Vice-Chancellor, Khulna University of Engineering & Technology for providing the financial assistance to my project. Special thanks to the staffs of Chemistry Lab, for their co-operation for testing the density of saline water. Gratitude is extended to the Engineering Section for their support during the project work. Finally, I would to like to offer heartiest thanks to the staffs of Heat Engine Lab and Wood Shop for their co-operation towards the completion of this work.
Muhammad Ashraful Kabir
ABSTRACT
Solar pond is an artificially constructed pond in which significant temperature rise occurs in the lower region by preventing convection. To prevent convection, salt water is used in the pond. Those ponds are called ‘‘salt gradient solar pond’’. In the last 15 years, many salt gradient solar ponds varying in size from a few hundred to a few thousand square meters of surface area have been built in a number of countries. Nowa-days, mini solar ponds are also being constructed for various thermal applications. In this project work, a solar pond system was constructed with better insulation, transparent cover on the upper surface and improved absorber coating. The temperatures within the pond was measured at various levels and compared with other works.
In this work, performance of the solar pond was observed with varying salinity. It is seen that maximum temperature developed in the storage zone increases with increasing salinity. The pond also works as storage. Because with varying solar intensity temperature developed in the storage zone reaches to maximum at the end of the day. Thus, the solar pond also works with diffuse radiation.
The present system shows better output than the previous work. Maximum temperature developed in the storage zone is higher than that developed in the previous work. This shows better heat transfer characteristics of the system.
LIST OF FIGURES
Figure No.
Caption
2.1
Schematic Diagram of a typical Salt Gradient
Page No.
Solar Pond
5
2.2
Schematic Diagram of the UCZ Layer
6
2.3
Schematic Diagram of the NCZ Layer
6
2.4
Schematic Diagram of the LCZ Layer
6
2.5
Effect of Shading Area
14
3.1
Experimental Setup of the Solar Pond System
17
5.1
Effect of Varying Salinity in the Pond on Lower Convective Zone Temperature
27
5.2
Variation of Solar Radiation Intensity with Time of Day
28
5.3
Variation of Temperature with Time at Lower Convective Zone
5.4
28
Variation of Temperature with Time at Upper Convective Zone
29
LIST OF TABLES
Table No.
Title
4.1
Temperature Distribution at Various Levels after Mixing 70 kg Salt
4.2
4.4
21
Temperature Distribution at Various Levels after Mixing 80 kg Salt
4.3
Page No.
23
Temperature Distribution at Various Levels after Mixing 90 kg Salt
25
Density of Water after Mixing Salt
26
CONTENTS Page No.
CHAPTER I
CHAPTER II
INTRODUCTION
1-2
1.1 Overview
1
1.2 Objectives
2
LITERATURE REVIEW
3-15
2.1 Solar Energy
3
2.2 Solar Pond
4
2.3 Working Principle of Solar Pond
5
2.4 Types of Solar Ponds
7
2.4.1 Nonconvecting Pond
7
2.4.2 Convecting Pond
8
2.5 Application of Solar Pond
8
2.5.1 Greenhouse Heating
9
2.5.2 Process Heat in Dairy Industries
9
2.5.3 Desalination
9
2.5.4 Power Production
10
2.5.5 Hot Water Applications in Agriculture
10
2.5.6 Economics of Solar Ponds for Heating
10
2.5.7 Industries with Potential Applications for Solar Ponds 11
CHAPTER III
CHAPTER IV
2.6 Advantages and Disadvantages of Solar Pond
12
2.7 Theoretical Analysis of the LCZ Temperature
12
2.8 Factors Affecting Pond Performance
13
2.8.1. Water Turbidity and Bottom Reflectivity
14
2.8.2 Wall Shading Effect
14
2.8.3 Effect of Energy Extraction
15
DESIGN CONSIDERATION AND ANALYSIS
16-19
3.1 Transparent Cover Design
16
3.1.1 Size of Cover Frame
16
3.2 Selection of Materials
17
3.2.1 Glazing Materials
17
3.2.2 Absorber Surface Coating
18
3.2.3 Insulating Material
18
CONSTRUCTION AND EXPERIMENTAL PROCEDURE
20-26
4.1 Construction of Solar Pond
20
4.2 Method of Making Salinity Gradient
20
4.3 Maintenance and Working Procedure
20
4.4 Experimental Data
21
CHAPTER V
REFERENCES
RESULT AND DISCUSSION
27-30
5.1 Result of the Work
27
5.2 Discussion `
29
5.3 Conclusion
30 31
CHAPTER I INTRODUCTION 1.1 Overview: As technology develops, the energy needs of communities increases. This energy need is provided from different energy sources known as traditional energy sources, such as coal, fuel oils, geothermal energy, hydraulic energy, and nuclear energy. These energy sources have some disadvantages. The first three of these energy sources have limited life time. Hydraulic energy is an insufficient energy source and nuclear energy has some unsolved environmental and safety problems. Therefore, the researchers have condensed their studies on new alternative energy sources known as renewable energy sources [1]. Solar energy is a form of renewable energy sources. Solar Energy is the radiation produced by nuclear fusion reactions deep in the Sun’s core. The Sun provides almost all the heat and light which earth receives and therefore sustains every living being. People can make indirect use of solar energy that has been naturally collected. Earth's atmosphere, oceans, and plant life, for example, collect solar energy that people later extract to power technology. Now-a-days, salinity-gradient solar technology is a useful form of utilizing solar energy. It is a generic name given to the application of a salinity gradient in a body of water for the purpose of collecting and storing solar energy. One type of salinity-gradient technology is called the salinity-gradient solar pond. A solar pond is a shallow body of saline water several meters deep, set up in such a way that there is increasing salinity with depth. Solar radiation entering the pond is stored as heat in the lower layer. This heat (up to 80°C) is then available on a 24 hour basis [2]. Solar pond is used for various thermal applications like green house heating, process heat in dairy industries, desalination and power production. The solar pond provides a unique opportunity to do research in such areas as double diffusive convection, wind/wave interaction, flow in stratified fluids, and computer modeling. In addition, the state of the art equipment on site provides an excellent opportunity for energy efficiency studies, cost analysis, system studies, heat exchanger [3].
Some years back an undergraduate project work was done on solar pond system which was constructed by using ferrocement and tested. But it had some limitations such as inadequate insulation of the side walls, no attempt to prevent convection and radiation heat loss to air from upper surface, absorber surface coating having limited life etc. The main objective of the present work is to improve the solar pond performance by removing above limitations and comparing it with the previous one. For this some steps were taken considering the various factors affecting solar pond performance such as water turbidity, bottom reflectivity, heat insulation etc. In order to increase the thermal capacity of absorber surface special type coating was applied which was supposed to have more durability than the coating used in previous work. Two types of coatings are available in market: Epoxy coating and Synthetic Enamel paint (matt finish). The first one is better but costs too high. The later is a type of cement paint with superior adhesion qualities. It is available in two types of finishes glossy and matt and can stick to all primers [4]. To prevent water turbidity and convection and radiation heat loss to air from upper surface, transparent cover over the pond surface was used to improve pond efficiency.
1.2 Objectives: 1. To identify the limitations and problems of the solar pond constructed earlier. 2. To modify the solar pond system thus avoiding previous limitations. 3. To monitor the performance of the solar pond.
CHAPTER II LITERATURE REVIEW 2.1 Solar Energy: Solar energy is the utilization of the radiant energy from the Sun. Solar radiation along with secondary solar resources such as wind and wave power, hydroelectricity and biomass account for over 99.9% of the available flow of renewable energy on the Earth. The flows and stores of solar energy in the environment are vast in comparison to current human energy needs. The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850 Zettajoules (1021J) per year, while global wind energy within 80 m height, the minimum height of modern large wind turbines, is estimated as 2.25 ZJ per year. Photosynthesis captures approximately 3 ZJ per year in biomass. In contrast, worldwide electricity consumption was approximately 0.0567 ZJ in 2005, and total worldwide primary energy consumption was 0.487 ZJ in the same year [5]. Solar energy has been used since prehistoric times, but in a most primitive manner. Before 1970, some research and development was carried out in a few countries to exploit solar energy more efficiently, but most of this work remained mainly for academic purposes [5]. Solar power is often used interchangeably with solar energy but refers more specifically to the conversion of sunlight into electricity, either by photovoltaics and concentrating solar thermal devices, or by one of several experimental technologies such as thermoelectric converters, solar chimneys and solar ponds. Solar energy and shading are important considerations in building design. Thermal mass is used to conserve the heat that sunshine delivers to all buildings. Day lighting techniques optimize the use of light in buildings. Solar water heaters heat swimming pools and provide domestic hot water. In agriculture, greenhouses expand growing seasons and pumps powered by solar cells (also known as photovoltaics) provide water for grazing animals. Evaporation ponds are used to harvest salt and clean waste streams of contaminants. Solar energy is the fastest growing form of energy production. Solar distillation and disinfection techniques produce potable water for millions of people worldwide. Family-scale solar cookers and larger solar kitchens concentrate sunlight for cooking, drying and pasteurization. Clothes lines are a common
application of solar energy. More sophisticated concentrating technologies magnify the rays of the Sun for high-temperature material testing, metal smelting and industrial chemical production. A range of prototype solar vehicles provide ground, air and sea transportation. Solar energy technologies use solar radiation for practical ends. Solar technologies such as photovoltaic and water or air heaters increase the supply of energy and may be characterized as supply side technologies. Technologies such as passive design and shading devices reduce the need for alternate resources and may be characterized as demand side. Optimizing the performance of solar technologies is often a matter of controlling the resource rather than simply maximizing its collection. A solar pond is large-scale solar energy collector with integral heat storage for supplying thermal energy. It is simply a pool of water which collects and stores solar energy. It contains layers of salt solutions with increasing concentration (and therefore density) to a certain depth, below which the solution has a uniform high salt concentration. 2.2 Solar Pond: A salinity gradient solar pond is an integral collection and storage device of solar energy. By virtue of having built-in thermal energy storage, it can be used irrespective of time and season. In an ordinary pond or lake, when the sun's rays heat up the water this heated water, being lighter, rises to the surface and loses its heat to the atmosphere. The net result is that the pond water remains at nearly atmospheric temperature. The solar pond technology inhibits these phenomena by dissolving salt into the bottom layer of this pond, making it too heavy to rise to the surface, even when hot. The salt concentration increases with depth, thereby forming a salinity gradient. The sunlight which reaches the bottom of the pond remains entrapped there. The useful thermal energy is then withdrawn from the solar pond in the form of hot brine. The heat trapped in the salty bottom layer can be used for many different purposes, such as heating of buildings or industrial hot water or to drive a turbine by using special working substance for generating electricity. The pre-requisites for establishing solar ponds are: a large tract of land (it could be barren), a lot of sun shine, and cheaply available salt (such as Sodium Chloride).
2.3 Working Principle: Most people know that fluids such as water and air rise up when heated. The salinity gradient stops this process when large quantities of salt are dissolved in the hot bottom layer of the body of water, making it too dense to raise to the surface and cool [6].
Fig. 2.1: Schematic Diagram of a typical Salt Gradient Solar Pond A typical salinity-gradient solar pond has three regions. The top region is called the surface zone, or upper convective zone (UCZ). The middle region is called the main gradient zone (MGZ), or nonconvective zone (NCZ). The lower region is called the storage zone, or lower convective zone (LCZ). The lower zone is a homogeneous, concentrated salt solution that can be either convecting or temperature stratified. Above it the NCZ constitutes a thermal-insulating layer that contains a salinity gradient. This means that the water closer to the surface is always less concentrated than the water below it.
Fig. 2.2: Schematic Diagram of the UCZ Layer
Fig. 2.3: Schematic Diagram of the NCZ Layer
Fig. 2.4: Schematic Diagram of the LCZ Layer The surface zone is a homogeneous layer of low-salinity brine or fresh water. If the salinity gradient is large enough, there is no convection in the gradient zone even when heat is absorbed in the lower zone because the hotter, saltier water at the bottom of the gradient remains denser than the colder, less salty water above it. Because water is
transparent to visible light but opaque to infrared radiation, the energy in the form of sunlight that reaches the lower zone and is absorbed there can escape only via conduction. The thermal conductivity of water is moderately low, and if the main gradient zone (MGZ) has substantial thickness, heat escapes upward from the lower zone very slowly. The insulating properties of the main gradient zone, combined with the high heat capacity of water and large volume of water make the solar pond both a thermal collector and a long-term storage device [7]. 2.4 Types of Solar Ponds: There are two main categories of solar ponds [8]: 1) Nonconvecting solar ponds, which reduce heat loss by preventing convection from occurring within the pond. 2) Convecting solar ponds, which reduce heat loss by hindering evaporation with a cover over the surface of the pond. 2.4.1 Nonconvecting Ponds: The nonconvecting solar ponds again can be divided into two types: the salt gradient ponds and membrane ponds. A salt gradient pond has three distinct layers of brine (a mixture of salt and water) of varying concentrations. Because the density of the brine increases with salt concentration, the most concentrated layer forms at the bottom. The least concentrated layer is at the surface. The salts commonly used are sodium chloride and magnesium chloride. A dark-colored material—usually butyl rubber— lines the pond. The dark lining enhances absorption of the sun’s radiation and prevents the salt from contaminating the surrounding soil and ground water. As sunlight enters the pond, the water and the lining absorb the solar radiation. As a result, the water near the bottom of the pond becomes warm—up to 93.3oC (200oF). Although all of the layers store some heat, the bottom layer stores the most. Even when it becomes warm, the bottom layer remains denser than the upper layers, thus inhibiting convection. Pumping the brine through an external heat exchanger or an evaporator removes the heat from this bottom layer. Other method of heat removal is to extract heat with a heat transfer fluid as it is pumped through a heat exchanger placed on the bottom of the pond. Another type of nonconvecting pond, the membrane pond, inhibits convection by
physically separating the layers with thin transparent membranes. As with salt gradient ponds, heat is removed from the bottom layer [8]. 2.4.2 Convecting Pond: A well-researched example of a convecting pond is the shallow solar pond. This pond consists of pure water enclosed in a large bag that allows convection but hinders evaporation. The bag has a blackened bottom, has foam insulation below, and two types of glazing (sheets of plastic or glass) on top. The sun heats the water in the bag during the day. At night the hot water is pumped into a large heat storage tank to minimize heat loss. Excessive heat loss when pumping the hot water to the storage tank has limited the development of shallow solar ponds. Another type of convecting pond is the deep, saltless pond. This convecting pond differs from shallow solar ponds only in that the water need not be pumped in and out of storage. Usually double-glazing covers are used in deep saltless ponds. At night, or when solar energy is not available, placing insulation on top of the glazing reduces heat loss [8]. A non-convective solar pond constructed in accordance with the foregoing features provides a number of important advantages: First, even though the bottom layer is not completely impermeable, there will be only negligible seepage through it since the pumping means, returning the liquid which permeates into the intermediate layer, prevents the building-up of pressure by the liquid overlying the bottom impermeable layer. In addition, the gases evolved at the bottom of the pond are vented to the atmosphere before the drained liquid is returned from the sump to the pond, and therefore the evolved gases cannot destroy the concentration gradient in the pond or foul its water. Further, the bottom construction of the solar pond can be made with relatively inexpensive materials (e.g., compacted earth for the impermeable layers and course sand and/or crushed stone for the permeable layer), thereby avoiding the need for expensive heat-resistant materials [9]. 2.5 Application of Solar Pond: The solar ponds are widely considered as the low temperature energy storage devices having use in wide range of process applications. The following section deals with the scope of the applications of solar pond heat adopted in various processes.
2.5.1 Greenhouse Heating: As mentioned in [10], Sokolov and Arbel demonstrated the use of fresh water solar pond for greenhouse heating purpose. The pond comprised an excavation in the earth with liner and a thin top cover. Water was used as a heat transferring fluid during periods of solar radiation. Energy was delivered to the greenhouse by pumping hot water from the upper layer of the pond through a heat exchanger. The water returned after heat extraction to the bottom of solar pond. In another study, Arbel and Sokolov studied different collector materials having different material properties and concluded that the use of appropriate material improves the solar pond performance. As mentioned in [10], Riva studied a 20 m2 solar pond for two years before constructing a bigger pond of 140-160 m2 area. The energy efficiency was found to be 10 to 20 percent during preliminary testing. The energy was intended for air heating in a dryer of 40-50 m2 area [10]. 2.5.2 Process Heat in Dairy Industries: Studies have indicated that there is excellent scope for process heat applications, when a large quantity of hot water is required, such as textile processing and dairy industries. The hot water requirements for sterilization and pasteurization in a dairy plant at Bhuj of Kutch district of Gujarat State in India are being met from a solar pond of 6000 m2 area. The hot water temperature was in the range of 84 to 95o C during the pond operation period [10]. 2.5.3 Desalination: Desalination involves the process of obtaining fresh water for drinking and irrigation from either brackish or saline water after suitable treatment. The solar energy has been utilized for distillation of brackish or saline water for a very long time. The fresh water is produced through repetitive cycles of evaporation and condensation, using low temperature heat from the solar ponds. As mentioned in [10], Tabor showed that a pond of 1/3 km2 area could operate a multi-effect distillation unit, with an annual mean output of 4000 m3 /day at a rate of US $ 0.67/ m3. He further remarked that a solar pond desalination plant produces about 5 times the quantity produced from simple tray
type solar still. A 20000 m2 solar pond in Italy was used for desalination of seawater to produce 120 ton of fresh water/day [10]. 2.5.4 Power Production: In solar pond power plants, the solution from the lower convective zone is pumped to a heat exchanger that acts as evaporator for an organic Rankine cycle. As mentioned in [10], Trieb made a comparative analysis of different solar electricity generation options and found that solar pond produces electricity at a cost of 0.254 German Marks (DM)/kWh as against 1.198 German Marks (DM)/ kWh for photovoltaic cells [10]. 2.5.5 Hot Water Applications in Agriculture: Many of the agricultural operations involve hot water application for different purposes. Some of them include paddy soaking in parboiling, sugarcane sett treatment, vegetable blanching, washing of cans in dairy industry and domestic hot water consumption. Traditionally, parboiling process involves soaking of raw rice in water at ambient temperature in masonry tanks for 3 days and steaming of drained paddy. The method was later improved to soak the paddy in hot water at around 70o C for few hours depending upon the type of parboiling method. This method could eliminate unwanted odors’ associated with traditional method and reduce the soaking time from a few days to a few hours. Heat therapy of sugarcane setts before planting is desirable to raise the crop free from seed piece diseases and certain insect pests. Conventionally the setts are treated in hot water at a temperature of 50oC for 2 hours and at 54oC for 4 hours in humid hot air. It is clear that the solar ponds have a great scope in agricultural applications with low temperature requirements [10]. 2.5.6 Economics of Solar Ponds for Heating: It is estimated that solar ponds in climatic regions similar to northern Victoria can produce process heat (40 - 80°C) for a wide range of applications at an average cost between $10 and $15/GJ. The results from the current project will provide actual data on cost of energy delivered, for the demonstration facility and in commercially
available systems. Heat from solar ponds is therefore expected to be competitive with the use of LPG and electricity in rural areas. Currently heat in rural areas costs over $20/GJ for LPG (at 43 c/litre). Heat from electricity (that is, direct heating rather than from heat pumps) costs over $45/GJ at peak rate, and $9/GJ off-peak. Solar pond heating would not be competitive in areas where natural gas is available, since this is priced typically at only $4-5/GJ [2]. 2.5.7 Industries with Potential Applications for Solar Ponds: Solar ponds have the potential to provide low-grade heat in industries such as the following:
salt production (for enhanced evaporation or purification of salt, that is production of vacuum quality salt)
aquaculture, using saline or fresh water (to grow, for example, fish or brine shrimp)
dairy industry (for example, to preheat feed water to boilers)
fruit and vegetable canning industry
grain industry (for grain drying)
water supply (for desalination)
The applications of the technology are certainly not limited to these industries. Basically the generic requirements for a practical solar pond application are these:
no access to natural gas, and hence reliance on more expensive fuels such as LPG, electricity or fuel oil
demand for heat in the 40 to 80 °C temperature range
saline water and salt preferably available locally
availability of relatively flat land on which to construct the solar pond
relatively high annual average solar radiation
Solar ponds may also be considered as a source of heating factory and office space and water heating at suitable rural sites [2]. 2.6 Advantages and Disadvantages: The solar pond system has some advantages and disadvantages. These are mentioned as follows: •
Low investment costs per installed collection area.
•
Thermal storage is incorporated into the collector and is of very low cost.
•
Diffuse radiation (cloudy days) is fully used.
•
Very large surfaces can be built thus large scale energy generation is possible.
•
Expensive cleaning of large collector surfaces in dusty areas is avoided [11].
•
Solar ponds can only be economically constructed if there is an abundance of inexpensive salt, flat land, and easy access to water. Environmental factors are also important. An example is preventing soil contamination from the brine in a solar pond. For these reasons, and because of the current availability of cheap fossil fuels, solar pond development has been limited [12].
2.7 Theoretical Analysis of the LCZ Temperature: A salt gradient solar pond collects and stores solar energy. The stability of the solar pond is maintained by the salt. Both UCZ and LCZ have uniform and constant temperature and salt concentrations, whereas the temperature and the salt concentration increase with depth in the NCZ. The energy balance for the solar pond can be written as follows [13]. In steady state: Rate of heat input = rate of heat stored in the lower convective zone + rate of heat losses to side and bottom of the pond. In UCZ approximately 45% of the incoming solar radiation is absorbed and the remaining is lost by evaporation, convection and re-radiation. In NCZ, due to the
increasing density, convection currents are suppressed with the effect that warmer water cannot rise to the surface and cool down as in an ordinary pond. Therefore heat losses are only due to heat conduction. Hence, this layer is acting as a good transparent insulation. Depending upon the thickness of the NCZ, around 15–25% of the incoming radiation is absorbed under clear water conditions. By a blackened bottom in the pond, in LCZ, up to 40% of the total received solar energy can be absorbed. The temperature of this zone varies between 80oC and 900C. At the bottom of the pond, proper insulation is provided to minimize heat losses. If sand layer is used as insulation, it will also act as a storage device. The temperature of the storage zone (LCZ) at the end of the period, Tt+∆t is written as follows [13]:
where T is the temperature (oC), t the time (s), ∆t the time intervals (s), As the surface area (m2), h(z) the fraction of solar radiation penetrating to the depth z in the pond, I the hourly insolation incident upon a horizontal surface (W/m2), kw the stored water’s thermal conductivity (W/mK), Ta the ambient temperature (oC), dncz the nonconvective zone vertical extent (m), m the mass of water in the store (kg), cp the specific heat of stored water (J/kg K). 2.8 Factors Affecting Performance: The thermal efficiency of the pond depends on the thickness of the various zones in the solar pond. An increase in thickness of the UCZ reduces the amount of solar energy reaching the storage zone. Therefore the thickness of the UCZ should be very less. The various factors, affecting thermal performance of the solar pond are discussed below: As mentioned in [13], Xiang did the thermal calculation by changing the incident rays from a Xe-lamp into natural ray and halogen lamp. As a result, it was found that the temperature distributions in the solar pond were notably different due to spectral characteristics of the incident ray. Therefore, the spectroscopic consideration for thermal performance of any solar pond is necessary to obtain a correct solution under the spectral incidence with special wavelength distribution.
2.8.1. Water Turbidity and Bottom Reflectivity: The suspended matter in solar pond salt water, which prevents the penetration of light inside water, is called turbidity. Jackson turbid meter is used to measure turbidity. The unit for turbidity is helometric turbidity units. As mentioned in [13], Wang and Yagoobi studied the effect of turbidity on the thermal performance of a salt gradient solar pond. They found that high turbidity levels could prevent ponds from storing energy in the LCZ. As mentioned in [13], Husain proved that reflective bottom and turbidity with certain limits improve the efficiency of pond. As mentioned in [13], Giestas studied the gravitational stability of a salty layer of a fluid subject to an adverse temperature gradient as a result of heat absorption. 2.8.2 Wall Shading Effect: The thermal performance of a solar pond is a function of solar irradiation, heat losses from the sides to the surroundings and from the LCZ towards the upper layers, ultimate storage capacity, and the effectiveness of the heat exchanger system. In small vertical wall solar ponds, the shading of walls plays an important role on reducing the sunny area of the pond and its thermal performance. As mentioned in [13], Jaefarzadeh analyzed the effect of wall shading on the LCZ temperature. A numerical investigation was conducted by Jubran. They developed a model to predict the generation of convective layers on the solar pond walls.
Fig. 2.5: Effect of Shading Area
2.8.3 Effect of Energy Extraction: The temperature of the UCZ and LCZ are almost uniform. The temperature of the NCZ increases, when the depth increases. When the thickness of the NCZ increases, the temperature of the UCZ also increases. As mentioned in [13], Al-Jamal and Khashan proved that the thickness of the NCZ was dependent on the amount of heat extracted from solar pond.
CHAPTER III DESIGN CONSIDERATION AND ANALYSIS
Some years back, a solar pond was constructed in the Department of Mechanical Engineering as an undergraduate project. The previous work had some limitations. It had no provision for preventing convection and radiation heat loss from upper surface. In the present work, some steps were taken to improve the solar pond performance and comparing them with the previous work. A transparent cover was used over the pond surface to minimize heat loss from upper surface, better insulating material was used for side walls and special type of coating was applied over the absorber surface of the pond system. 3.1 Transparent Cover Design: Increase of heat performance is possible with the insulation of the zones surrounding the pond. UCZ on top of the pond is one of the zones where the most heat loss takes place due to affects of wind, ambience, dumbness, evaporation etc. The increase of the heat performance is possible also with the insulation of the walls. For that reason, the top of the pond was designed to be covered with the transparent material to reduce the loss of heat energy from water in to the air [14]. The suspended matter in solar pond salt water, which prevents the penetration of light inside water, is called turbidity. Transparent cover eliminates the effect of turbidity. 3.1.1 Size of Cover Frame: The frame for transparent cover was made from wood. The frame was made inclined so that water droplets formed inside the cover due to evaporation from pond surface can’t be stagnant there. Dimension for the frame is given as follows: Length of the frame=183 cm. Width of the frame=183 cm. Inclination of the frame=9.5o
Fig. 3.1: Experimental Setup of the Solar Pond System. 3.2 Selection of Materials: The principal criteria for selection of materials and developing the design of solar pond system are based upon local availability of materials and cost involvement, efficiency and easy to construction and maintenance. 3.2.1 Glazing Materials: Transparent covers or glazing materials can be made of glass, plastic or fiber glass. Commonly used glazing materials are — 1. Glass cover 2. Plastic cover
One of the common cover materials is glass for good reason. Glass is readily available, quite transparent to visible light and opaque to infrared wave lengths beyond about three micrometer, meaning that it block heat loss from the absorber due to radiation. Transparent glasses have an effective outdoor lifetime of cover
twenty years. But cost involvement for glass cover is too high. For this reason plastic cover was used in order to minimize cost. 3.2.2 Absorber Surface Coating: In order to increase the thermal capacity of absorber surface special type coating was applied which was supposed to have more durability than the coating used in previous work. As a result, life of solar pond system increases. Two types of coatings are available in local market: (1) Epoxy coating (2) Synthetic Enamel paint (matt finish) The first one is better but costs too high. The later is a type of cement paint with superior adhesion qualities. It is available in two types of finishes - glossy and matt and can stick to all primers. 3.2.3 Insulating Material: The purpose of insulation is to reduce heat loss, which is given to the back side of the base plate and outside of side wall. There are many types of insulation available in the market, such as glass wool, vegetable cork, cotton, asbestos rope, straw etc. Out of these glass wool and asbestos rope is best but it is costly and difficult to set. Vegetable cork is more suitable as insulation material, because it is available in the local market, cheap and easy to set. So, vegetable cork was selected as the insulating material in the present work. Different properties of vegetable cork are given below: (1) Flash point temperature: 575 K (2) Density kg/m 3 : 9.61*10 5 -1.28*10 6 (3) Specific heat J/(kg.K) : 1.8*103 (4) Thermal diffusivity m 2 /s : 1.8*10 -7 at 343 K (5) Vibration resistance : Good (6) Water absorption, % by volume : 5% (surface only) (7) Specific gravity (apparent) : 0.96 to 0.128
(8) Temperature limits, service for
Short time: 366 K Continuous: 355.22 K (9) Strength compressive flexure: 3.45*104 N/m2 at 5% deformation
CHAPTER IV CONSTRUCTION AND EXPERIMENTAL PROCEDURE
4.1 Construction of Solar Pond: The present structure of the solar pond was constructed few years back. It was a trapezoidal reservoir constructed by using ferrocement technology. The reservoir was dipped in the earth by digging a hole in the earth. But one side of the wall had a crack which needed repairing. Repairing was done with the help of cement mortar and then it was kept dried. The absorber surface was scrubbed and brushed up to make it prepared for new coating. Then it was painted with synthetic enamel paint (matt finish). After the surface being dried, another coating was provided. The hole for the solar pond was enlarged slightly and then polythene sheet was spread out on the soil. Then rice husk and cork sheet was put on the polythene and the pond structure was placed over these materials. The cork sheet was attached to the side walls with the help of adhesive. After that rice husk was provided through the wall side to fill the void space. A wooden frame was made for transparent cover having few degrees of inclination. Then transparent polythene was attached to frame and the frame was placed over the solar pond to avoid falling of foreign materials to the pond. 4.2 Method of Making Salinity Gradient: The height of the solar pond is 80cm. Water level of the solar pond was maintained at a height of 70cm. First the pond was filled with water up to the height of 40cm. Then 70 kg natural salt was mixed with water. After that fresh water was fed from top up to 70 cm height. Then salinity gradient was formed automatically in between these two layers. 4.3 Maintenance and Working Procedure: Water becomes vaporized from the upper surface when subjected to solar radiation. As a result, density of the upper surface increases and height of water level decreases. In a solar pond the most important factor is the density gradient of different layers. To maintain correct density fresh water was fed from top up to required height at certain
time intervals. Temperature of water at different height was measured using thermocouple with the help of supporting scale considering the upper surface as reference point. Solar energy was measured using solarimeter which was placed at height 1 m. above the roof of Mechanical Engineering building. Salt was mixed with water in three steps. First, 70 kg salt was mixed with water. Then after collecting data for few days, 10 kg salt was added to the pond and the performance was noticed. After few days, another 10 kg salt was added to the pond. In the later two steps, salt was added to water by mixing with water externally collected from upper zone. Then the water was directly fed to lower zone. As a result, water at lower zone remains denser. 4.4 Experimental Data: The experimental data were taken for 10 days from 02.01.09 to 02.03.09 and are shown in Table 4.1 to 4.3. The experiment was conducted after mixing 70 kg salt, 80 kg salt and 90 kg salt in the solar pond. Densities of water at different layers of the solar pond are shown in Table 4.4 after mixing salt in three steps. 4.1 Temperature Distribution at Various Levels after Mixing 70 kg salt: Height from the upper surface of the pond in cm. Time
0
10
20
30
40
50
60
Total 70
Solar Radiation
Temperature in oC
(whm-2)
02.01.09 9.00 am
22
23
24
26
27
28
29
29
10.00am
23
24
25
26
28
28
29
30
11.00am
23
24
25
26
28
29
30
30
12.00pm
24
25
26
27
28
29
30
31
1.00 pm
24
25
26
27
28
29
30
31
2.00 pm
25
26
27
28
29
30
31
32
3.00 pm
25
26
27
28
29
31
32
33
4.00 pm
25
27
28
29
30
31
32
33
5.00 pm
26
27
28
29
31
32
33
34
6.00 pm
26
27
29
29
31
32
33
34
2557
06.01.09 9.00 am
22
23
24
26
27
28
29
29
10.00am
24
25
26
27
28
29
30
31
11.00am
25
26
26
27
28
29
30
32
12.00pm
26
27
28
30
31
31
32
33
1.00 pm
26
28
28
30
31
32
32
33
2.00 pm
27
28
29
30
32
33
33
34
3.00 pm
27
29
29
30
32
33
33
34
4.00 pm
28
28
29
31
32
33
33
34
5.00 pm
28
28
29
31
32
33
34
35
6.00 pm
28
28
29
31
32
33
34
35
2829
08.01.09 9.00 am
22
23
24
24
25
26
27
28
10.00am
23
24
24
25
26
28
29
29
11.00am
24
24
25
25
26
28
29
30
12.00pm
25
24
25
25
26
28
29
31
1.00 pm
25
24
25
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26
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29
32
2.00 pm
25
26
28
30
31
32
33
34
3.00 pm
26
27
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30
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32
33
34
4.00 pm
27
28
29
30
32
34
34
35
5.00 pm
27
28
29
30
32
34
34
35
6.00 pm
27
28
29
30
32
34
34
35
2890
4.2 Temperature Distribution at Various Levels after Mixing 80 kg salt: Height from the upper surface of the pond in cm. Time
0
10
20
30
40
50
60
Total 70
Solar Radiation
Temperature in oC
(whm-2)
18.02.09 9.00 am
24
25
26
26
27
28
29
31
10.00am
25
26
27
28
28
29
30
31
11.00am
26
27
28
29
29
30
31
32
12.00pm
27
28
29
29
30
31
33
33
1.00 pm
27
28
29
29
31
32
34
35
2.00 pm
29
29
30
30
31
33
35
36
3.00 pm
29
30
31
31
32
34
35
36
4.00 pm
30
30
32
32
33
35
36
37
5.00 pm
30
31
33
33
34
35
37
38
6.00 pm
30
31
33
33
34
35
37
38
2557
19.02.09 9.00 am
23
25
26
26
27
28
29
32
10.00am
24
26
27
28
28
29
30
33
11.00am
25
27
28
29
29
30
31
34
12.00pm
26
28
29
29
30
31
33
35
1.00 pm
27
28
29
29
31
32
34
35
2.00 pm
27
29
30
30
31
33
35
36
3.00 pm
28
29
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32
34
35
36
4.00 pm
28
30
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32
33
35
36
37
5.00 pm
29
31
33
33
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35
37
38
6.00 pm
29
31
33
33
34
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37
38
4554
22.02.09 9.00 am
23
24
27
28
30
31
32
33
10.00am
23
25
27
28
31
32
32
33
11.00am
24
26
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31
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34
12.00pm
25
27
29
29
32
33
33
35
1.00 pm
26
28
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32
34
34
35
2.00 pm
26
29
30
30
33
34
35
36
3.00 pm
26
30
31
31
33
35
35
37
4.00 pm
27
30
32
32
34
35
36
37
5.00 pm
27
31
33
33
34
36
37
38
6.00 pm
27
31
33
33
34
36
37
38
4580
24.02.09 9.00 am
24
25
26
28
30
31
32
34
10.00am
25
26
27
28
31
32
33
34
11.00am
26
27
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31
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33
35
12.00pm
27
28
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33
36
1.00 pm
27
29
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34
34
36
2.00 pm
27
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37
3.00 pm
28
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37
4.00 pm
28
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5.00 pm
28
30
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34
36
37
38
6.00 pm
28
30
33
33
34
36
37
38
3306
4.3 Temperature Distribution at Various Levels after Mixing 90 kg salt: Height from the upper surface of the pond in cm. Time
0
10
20
30
40
50
60
Total 70
Solar Radiation
Temperature in oC
(whm-2)
28.02.09 9.00 am
27
28
29
30
32
34
35
36
10.00am
28
29
30
30
32
34
36
37
11.00am
29
29
30
31
33
35
36
37
12.00pm
29
30
31
32
33
34
36
38
1.00 pm
29
30
31
32
34
35
37
38
2.00 pm
30
31
32
33
34
35
37
39
3.00 pm
31
32
32
33
34
36
38
39
4.00 pm
32
32
33
34
35
37
38
40
5.00 pm
32
33
34
35
35
37
39
40
6.00 pm
32
33
34
35
35
37
39
40
4652
01.03.09 9.00 am
27
28
29
30
32
34
35
36
10.00am
28
29
30
30
32
34
36
37
11.00am
29
29
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35
36
38
12.00pm
30
30
31
32
33
34
36
39
1.00 pm
30
30
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34
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37
39
2.00 pm
30
31
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33
34
35
37
39
3.00 pm
31
32
32
33
34
36
38
39
4.00 pm
32
32
33
34
35
37
38
40
5.00 pm
32
33
34
35
35
37
39
40
6.00 pm
32
33
34
35
35
37
39
40
4693
02.03.09 9.00 am
26
27
29
30
32
34
35
36
10.00am
27
28
29
30
32
34
36
36
11.00am
27
29
30
31
33
35
36
37
12.00pm
28
30
31
32
33
34
36
38
1.00 pm
29
30
31
32
34
35
37
38
2.00 pm
29
31
32
33
34
35
37
39
3.00 pm
30
31
32
33
34
36
38
39
4.00 pm
30
32
33
34
35
37
38
40
5.00 pm
30
32
34
35
35
37
39
40
6.00 pm
30
32
34
35
35
37
39
40
4405
4.4 Density of Water at Different Layers after Mixing Salt to the Pond: Density of Water, kglit-1
Amount of Salt
Upper Layer
Middle Layer
Lower Layer
70
1.028
1.037
1.082
80
1.032
1.041
1.085
90
1.06
1.078
1.1214
in kg
CHAPTER V RESULT AND DISCUSSION
5.1 Result of the Work: The experimental data was taken from 02.01.09 to 02.03.09. During this period daily solar radiation data was taken. The relations between salinity, solar intensity and temperature are shown below. Fig. 5.1 shows the effect of varying salinity in the pond on lower convective zone temperature. Fig. 5.2 shows variation of solar intensity with time. Fig. 5.3 and Fig. 5.4 show variation of temperature with time at lower convective zone and upper convective zone.
Fig. 5.1:Effect of Varying Salinity in the Pond on Lower Convective Zone Temperature
Fig. 5.2: Variation of Solar Radiation Intensity with Time of Day.
Fig. 5.3: Variation of Temperature with Time at Lower Convective Zone.
Fig. 5.4: Variation of Temperature with Time at Upper Convective Zone.
5.2 Discussion: Fig. 5.1 shows, effect of varying salinity in the pond on LCZ temperature. In the figure it is seen that for density 1.082 kg/lt maximum temperature obtained is 35oC. For density 1.085 kg/lt and 1.1214 kg/lt temperature at LCZ rises to 38oC and 40oC respectively. It is seen that heat capacity in the LCZ increases with salinity Fig. 5.2 shows, variation of solar intensity with time of day. It is seen that as maximum solar intensity obtained at 12.00 pm. After that intensity decreases to almost zero at end of the day. Fig. 5.3 and Fig. 5.4 shows, variation of temperature developed with respect to time of day at lower convective zone and upper convective zone. In the Fig. 5.2, it was seen solar intensity increases to maximum value then decreases to lowest value. But the significant thing is maximum temperature in the LCZ was developed at the end of the day. So, it can be said that solar pond works not only as a collector but also as heat storage. In the present work, cork sheet was used as insulation. This insulation system minimizes heat loss from side walls and from bottom. Thus temperature was developed in the LCZ at the end of the day and heat was stored there.
5.3 Conclusion: Solar pond was constructed in which three modifications were made. Transparent cover was placed over the system, cork sheet was used as insulator for the side walls and special type of absorber surface coating was used. The present work shows better output than the previous work. In the previous work, maximum temperature obtained at the LCZ was 32oC where as the maximum temperature obtained in the present work is 40oC. Temperature difference between UCZ and LCZ was 6oC in the previous work where 10oC temperature difference between UCZ and LCZ was obtained in the present work. Increased lifetime of the solar pond is expected because of insulation provided at the bottom and absorber coating used are of better quality.
REFERENCES [1] ] http://www.ece.utep.edu/research/Energy/Pond/pond.html [2] http://www.rmit.edu.au/browse;ID=905wa9169827 [3] http://www.solarpond.utep.edu [4] http://www.gharexpert.com [5] http://www.wikipedia.org [6] http://teriin.org/division/eetdiv/docs/ca02_1.htm [7] Huanmin Lu, John C. Walton, Andrew H.P. Swift. Desalination coupled with salinity-gradient solar ponds, September 11–13, 2000. Desalination 136 (2001) 13–23. [8] www.eren.doe.gov/consumerinfo/refbriefs/aa8.html [9] http://www.freepatentsonline.com/4345581.html [10] G.R. RAMAKRISHNA MURTHY and K.P.PANDEY, Solar Ponds- A Perspective from Indian Agriculture, Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur-721302, India [11] http://www.teriin.org/case/solar.htm [12] http://www.eere.energy.gov/consumerinfo/factsheets/aa8.html [13] Velmurugan V, Srithar K. Prospects and scopes of solar pond: A detailed review. Renew Sustain Energy Rev (2007), doi:10.1016/j.rser.2007.03.011 [14] Nuri Ozek, Mehmet Karakilcik, Nalan Cicek Bezir. A SOLAR POND MODEL WITH INSULATED AND GLASS COVERED SURFACE (IGCSP). Bulgarian Journal of Physics 27. No. 4 (2000) 67-70.
