International International Journal on Architectural Architectural Science, Science, Volume 4, Number Number 1, p.14-23, p.14-23, 2003
DESIGN ANALYSIS OF A REFRIGERATED WAREHOUSE USING LNG COLD ENERGY K.H. Yang and S.C. Wu Mechanical Engineering Department, National Sun Yat-Sen University, Kaohsiung, Taiwan 80424 (Received 27 September 2002; Accepted 9 January 2003)
ABSTRACT An innovative design approach has been performed using latent heat of the Liquefied Natural Gas (LNG) during evaporation to provide cooling cooling for 500 USRT USRT cold storage warehouse. warehouse. In compared with with the conventional, mechanical cooling designs, tremendous energy savings can be experienced and is discussed in detail in the paper.
1.
INTRODUCTION
During the energy crisis in the 1970s, oil prices hiked to a record high of 42 USD per barrel, which imposed a lot of pressure economically on oil importing countries such as Taiwan. To diversify energy resources as a counter-measure, it was decided to import LNG from ASEAN countries such as Indonesia and Malaysia, so that most of the fossil-fueled power plants, which were burning coal and oil, would switch into natural gas (NG). Natural gas is a more environmentally friendly fuel, which contains over 92% of methane, depending on the source of product. When LNG is vaporized into NG, tremendous cooling capacity will be generated during this phase-changing process. Conventionally, this was done by pumping seawater through the open rack vaporizer (ORV), such that heat was transferred from seawater to LNG, and vaporized it into a gaseous state, for industrial and household usage. The chilled water was normally dumped dumped into sea again. Fig. 1 shows the conventional process. process. LNG was was pumped from from the tanker A to the storage tank B. In order to maintain a steady flow condition, the boil-off gas (BOG) from the tank at point C was extracted and cooled by the cooler D and recharged into the tanker at point E to form a circulation loop. Part of the BOG at point F was re-compressed at the BOG compressor or point G, condensed and re-charged into the LNG supply loop at point J. While, part of the boil-off gas shown at point H, was incinerated at the burning tower. On the other hand, pump pump 1 located inside the storage tank transports LNG from the tank, through point I and J, boosted by the pump 2 to a higher pressure at point K, and vaporized at the ORV by the sea water loop shown by point M and N and supply to the customers at the gaseous state, denoted by the point L. During this process, only a few part of the LNG cold energy was recovered by re-condensing BOG.
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This open heat exchanger loop is not only a waste of huge “cold energy” or cooling capacity, but will cause thermal pollution to the coastal fishery area also when cold water was re-injected into the sea. In this project, an innovative design has been o performed to reclaim the cold energy for a -35 C cold warehouse. warehouse. Conventionally, this was was done by installing mechanical refrigeration systems, necessitating tremendous electrical power to drive the refrigerant compressor working in such a low temperature. In this study, a closed closed loop LNG heat heat exchanger system will be designed to replace the mechanical, or vapor-compression refrigeration (VCR) cycle, with a technical and economical feasibility study to justify its application.
2.
EVALUATION OF LNG COLD ENERGY RECLAIM POTENTIAL
LNG imported to Taiwan has a chemical composition of 90.5% CH4, 6.2% C2H4, and 2.33% C3H8 in molar fractions. fractions. Its thermal thermal properties can be calculated by the Redlich-Kwang-Soave Redlich-Kwang-Soave (RKS) State equation [1]. However, to meet the demand side higher pressure at the local power utilities, it is o necessary to pressurize LNG from -162 C at 100 kpa atmospheric pressure, to around 8 Mpa, or 80 atm. Therefore, a calculation model with much broader range of temperature and pressure should be adapted, such as the Peng-Robinson (PR) state equations [2]. Both calculation models have similar accuracy on light hydrocarbons, but the RKS tends to over-estimate on density calculations, so the PR model is used in this study. PR state equations:
P = RT
(V − b)
−a
(V(V + b) + b(v − b))
(1)
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where P is the pressure of LNG, kPa; T is the temperature of LNG, K; V is the volume of LNG, M3; and R is the gas constant, kPa.m3kg-1.K -1. n
n
i
i
a = ∑∑ x i x j (a i a j ) 0.5
E = H Pin −162 C − H Pout t °
n i
a i = a ci α i
a c = 0.45724
0.5
R 2 Tc2 Pc 2
= 1 + (0.37646 + 1.54226ωi + 0.26992ωi )(1 − Tri )
b i = 0.07748
(2)
where the subscripts stand for the enthalpy H at different temperature and pressure Pin, Pout. The temperature of LNG at liquid state is around -162 o C, while the temperature of NG at gaseous state is o about 20 C in maximum. And the reclaimed energy potential is expressed in E.
b = ∑ x i b i
αi
The pressurization process will change the LNG boiling point, which in turn, will change the cold energy amount that can be reclaimed. The cold energy potential can be calculated from the enthalpy change of LNG across the ORV, or:
R Tci Pci
where x is the mole fraction of each substance, ω is the acentric factor, Tri = Ti/Tci. The subscript c means critical state.
The calculation result of the LNG cold energy potential under various temperature and pressure was shown in Fig. 2. It is noticed that the cold energy potential decreased while LNG was pressurized. For example, at 8 Mpa, LNG would -1 have an enthalpy of 786 kjkg , only amounts to 88% of its value at atmospheric pressure. Based on statistics from September 1, 1998 to August 31, 1999, the annual LNG consumption rate in Taiwan is around 396.52 million metric tons, equivalent to a daily average of 10864.31 metric tons, or 452.67 tons.h-1.
Fig. 1: A schematic diagram showing the conventional LNG transporting process using ORV for evaporation
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1000
LNG pressure=1atm LNG pressure=30atm LNG pressure=50atm LNG pressure=80atm
800
) g k / j k (
600
y 400 g r e n e
200
0
-200
-150
-100
-50
0
50
LNG temperature(C)
Fig. 2: Calculation result of the LNG cold energy potential under various temperature and pressure
When LNG was imported from ASEAN countries, it was pumped from the tanker and stored in the 60 m-diameter storage tank at -162 oC liquid state. Pumped out at -148 oC, it was pressurized in two stages, from atmospheric pressure to 3 atm, and then to 83 atm while entering the ORV. It o leaves ORV at 15 C with a pressure drop of around 20 atm, and stays at 63 atm. Therefore, the calculation yields a total heat exchange capacity of 743Mj per ton of LNG across the ORV. Combining with the daily LNG utilization rate in our study, which is around 10,864 ton/day, then it accounts for 1932 Gcal cold energy wasted per day, or equivalent to 26,662 Refrigeration Tons (USRT) cooling capacity, enough to provide residential cooling for around 8800 families during the summer. This has been validated by an experimental investigation through measuring the sea water loop with flow rate and temperature differences entering and leaving the ORV. As shown in Fig. 3, T1 and T2 denote the measuring points of the sea water inlet and outlet temperatures at the ORV, resulting at 27.3oC and 23.5oC respectively. The point F denotes the sea water flow rate measuring location, resulting at 2123.8 ton.h-1. The accumulated heat exchange capacity is 26682 USRT. Compared with our estimation, the deviation is within 3%. Furthermore, Fig. 4 shows a statistics and prediction of the LNG import to Taiwan. Following the completion of the second phase of LNG storage extension project in 1996, the annual import rises from 150 metric tons to 450 metric tons, and expected to reach 900 metric tons at the year of 2020. The huge amount of cold energy to
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be reclaimed is beyond doubt and waiting to be explored.
3.
SYSTEM DESIGN FOR COLD WAREHOUSE USING LNG RECLAIMED COLD ENERGY
In order to reclaim cold energy during LNG evaporation process, it is necessary to construct a closed heat exchange (HX) loop using secondary refrigerant, as shown in Fig. 5. In addition, the ORVs are still needed as a means for adjusting the appropriate portion of heat to be reclaimed without interrupting the normal NG supply to the power utilities. In other words, either the closed-loop heat exchanger or the ORV, each will provide a certain percentage of evaporation heat needed to vaporizer LNG into NG, and can be adjusted subject to the cooling load of the warehouse. On the ORV side, residual LNG was evaporated accordingly so that NG at its outlet still maintains the normal temperature and pressure needed for the power utilities. In this project, LNG cold energy will be utilized to supply cooling to a 110 RT cold storage warehouse o operating at -35 C, another 390 RT warehouse at o -20 C, and to provide 146 RT cooling capacity for air-conditioning in preparative rooms at around 15 o C. Fig. 6 shows a flow chart of the complete warehouse system using cold energy reclaimed from LNG, named the LNGCW system, with major design parameters indicated.
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Computer simulation has been performed following the first law of thermodynamics along the flow process. Heat and mass balance of the system was calculated as shown in Fig. 7 where each component, including heat exchangers, pumps, and
storage tanks were connected according to the system flow chart. After an iterative design and simulation process, the system schematics has been finalized. Fig. 8 gives an outlook of the completed LNG cold warehouse architectural design.
• denotes the measuring point, T for temperature, F for mass flow rate of sea water
Fig. 3: The schematic diagram of the full-scale experiment in measuring the LNG cold energy released by ORV through sea water
The Import of LNG
11 10 9 8
n o 7 t 6
0 1 6 x 1 5 4 3 1995
2000
2005
2010
2015
2020
2025
year Fig. 4: Prediction of LNG import quantity from ASEAN countries to Taiwan in next decades
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Fig. 5: A schematic diagram indicating cold energy reclaimed from LNG evaporation process for cold warehouse application
Fig. 6: A schematic diagram showing the LNGCW system flow
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Fig. 7: An energy and mass balance simulation result of the LNGCW system
Fig. 8: An outlook of the completed LNGCW architectural design
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4.
indicated that the APCL ranged from 91% during the summer and down to 76% in the winter. Assuming 330 operation days a year, the LNG consumption rate of the LNGCW system can be calculated as shown in Fig. 10, where the daily average flow rate is 11.2 ton.h-1 and reached its -1 peak of 12.3 ton.h in July. Steady state energy estimation method is used for the cooling load calculation, energy analysis and the power consumption of refrigeration system in this project. It can be calculated for different values of outdoor temperature and multiplied by the corresponding numbers of hours. This method can be accurate if the indoor temperature and internal gains are relatively constant and if the systems are to operate for a complete season as recommended by the ASHRAE Handbook [3]. The calculated power consumption and energy savings of the LNGCW system was plotted in Fig. 11, indicating a monthly energy savings of 1,050 MWh on the average.
ECONOMIC FEASIBILITY STUDY
Based on the design analysis shown above, major components were sized accordingly enabling an economic feasibility evaluation of the LNGCW systems vs. a conventional mechanical refrigeration (CMR) system. The LNGCW system necessitates 3 specially designed close-loop heat changers and storage tanks etc., introducing higher initial investment at the cost of 12.8 million USD. On the other hand, the CMR system consists of huge refrigerant compressors and condensers, etc. at a cost of around 8.3 million USD. In this way, the operation cost plays a key role in justifying economic feasibility. The CMR system suffers from operating in low evaporator temperature causing reduced refrigerant mass flow rate and smaller cooling capacity. In addition, the high compression ratio between condenser and evaporator demands higher power consumption which worsens the case. The operation power demand is around 2321 kW. On the other hand, the LNGCW system, with an inherent low evaporative temperature of -162 oC, can easily provide cooling capacity at a “higher” temperature of -30 oC by way of a simple heat exchanging process. The operation power demand is 379 kw, or 16% of that of the conventional CMR system.
Economic assessment is performed using the life cycle cost (LCC) method by calculating the present worth during each period of time. Pv = A ×
(1 + i) n − 1 i × (1 + i) n
(3)
where Pv is the present worth, A is the annual money saving by reduce electricity consumption, i is the effective discount rate, and n is the total number of years.
Based on the outdoor design temperature of 34 oC, the average partial cooling load (APCL) of the cold warehouse was calculated as shown in Fig. 9. It
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% 90 d a o l g 85 n i l o o c 80 l a i t r a 75 p e g a r e 70 v a 65 1
2
3
4
5
6
7
8
9
10
11
month
Fig. 9: Annual cooling load estimation of the cold warehouse in this study
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) 12.5 h / n o t ( 12 W C G 11.5 N L r o 11 f e t a r w 10.5 o l f G 10 N L e 9.5 g a r e v a 9 1
2
3
4
5
6
7
8
9
10
11
12
11
12
month
Fig. 10: Annual LNG consumption rate of the LNGCW system
1200
) h W 1150 M ( h t n o 1100 m r e p 1050 g n i v a s 1000 r e w o 950 p W C 900 G N L 850
1
2
3
4
5
6
7
8
9
10
month
Fig. 11: Annual operational power savings estimation of the LNGCW system
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The input parameters include annual inflation rate at 1%, rediscount rate at 4%, assuming an double declining depreciation rate for 15 years of major equipment and no residual value left after that. Local power tariff was inputted at 6.5 cents per kwh. Fig. 12 indicated that the monthly average operation cost savings is about 70,000 USD. Fig. 13 furthers shows the simulation result, indicating that the payback is at around 3.5 years. This represents a promising project with both technical and economical feasibility.
5.
CONCLUSIONS
During its vaporization process, tremendous cooling capacity can be reclaimed. In this study, the feasibility to design and construct a refrigerated warehouse by using LNG cold energy has been demonstrated both technically and economically. The payback of the 500 USRT cold warehouse is expected to be within 3.5 years, which is now funded by the Chinese Petroleum Corporation of Taiwan for construction.
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D 76 S U 74 0 0 72 0 1 t s 70 o c 68 y t i c 66 i r t 64 c e l e 62 g n 60 i v a s 58 56 1
2
3
4
5
6
7
8
9
10
11
12
month
Fig. 12: Annual operational cost savings estimation of the LNGCW system
8000 ) s r a l l o 6000 d S U 3
0 4000 1 ( e u l a V 2000 t n e s e r 0 P t e N
Payback Year
0
2
4
6
8
10
12
14
16
year
-2000
Fig. 13: The life cycle cost analysis of the LNGCW system in this study
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REFERENCES 1.
H.T. Liu and L.X. You, “Characteristics and applications of the cold heat exergy of liquefied natural gas”, Energy Conversion & Management, Vol. 40, pp. 1515-1525 (1999).
2.
W.C. Edmister, Applied hydrocarbon thermodynamics, Vol. 1, Gulf Publish Company (1984).
3.
ASHRAE Handbook - fundamentals, Chapter 28, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, USA (2001).
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