Feasibility Study of Solar Heating and Cooling Systems in Kuwait

Transcription

1 Feasibility Study of Solar Heating and Cooling Systems in Kuwait Abstract A. A. Ghoneim and A. H. Abdullah Applied Sciences Department, College of Technological Studies, Shuwaikh 70654, KUWAIT This paper presents an investigation of the performance and economic feasibility of solar heating and cooling systems at Kuwait. The water heating load, space heating load, and the cooling load of a typical Kuwaiti house were considered. The performance of hot water and the space heating systems are simulated using the transient simulation model (TRNSYS). A single effect lithium bromide water absorption chiller is implemented to provide air conditioning to the house. The absorption chiller model is based on a commercially available lithium bromide-water absorption chiller, Arkla model WF-36. The weather data generator subroutine included in TRNSYS package is used to generate hourly data from the available average monthly data for Kuwait. The performance of solar collectors is determined using the collector test facility at the College of Technological Studies, Kuwait. The study is carried out first for a conventional flat plate collector and then for the same collector but equipped with transparent insulation material. The results showed that the modified flat plate collector significantly enhances the performance of the solar heating and cooling systems by reducing the optimum collector area needed. Also, a great portions of the heating and cooling loads are satisfied by solar energy. In addition, the cost of energy unit for solar heating and cooling systems using the modified flat plate collector is found to be approximately 64% of the corresponding cost of the conventional fuel at the current prices. INTRODUCTION In hot climate areas like Kuwait, it is necessary to have air conditioning systems. One does not feel comfortable if the temperature and humidity level is too high. A large amount of the operating costs of a building is determined by the energy requirements and the fuel consumption of its heating, ventilation and air conditioning system (HVAC -system). Active solar heating systems are now widely used in many applications, while, active solar cooling systems are not. Numerous publications studied the performance of the solar heating systems (Lunde 1978, Garg 1987, Perers et al 1990, Ghoneim et al 1993). On the other hand, there are fewer publications discussing the performance of solar cooling and air conditioning systems. Lof et al (1974) made an extensive study of solar heating and cooling systems for eight cities in the U.S.A. The system has been optimized to minimize the annual energy cost. Nakahara et al (1977) studied the performance of a solar heating and cooling system. Their results showed that the system was able to provide all the required heating energy in the winter and 70% of the energy needed to drive an absorption chiller on a typical summer day. In Saudi Arabia, a 3.5 ton lithium bromide-water absorption chiller (LiBr-H2O) system with flat plate collectors was installed and tested (Sayigh & Khoshaim, 1981). An experimental study of a solar powered cooling system in Ispra, Italy had been conducted ( Van Hattem & Dato, 1981). The overall cooling efficiency (the ratio of cooling load to total solar energy) was found to be around 11%. The feasibility of utilizing solar energy for comfort cooling in Hong Kong has been studied by Yeung et al (1992). They constructed a solar energy absorption air conditioning system on the campus of the University of Hong Kong. The system has an overall efficiency of approximately 8% and an average solar fraction of 55%. Absorption chiller sys tems are considered as the most suitable option for solar cooling; because they are compact, reliable and can be easily integrated into different building energy systems. An absorption air conditioner or refrigerator does not use an electric compressor to mechanically pressurize the refrigerant. Instead, the absorption device uses a heat source, such as natural gas or a solar collector, to evaporate the already-pressurized refrigerant from an absorbent/refrigerant mixture. 239

2 This takes place in a device called the vapour generator. Although absorption coolers require electricity for pumping the refrigerant, the amount is small compared to that consumed by a compressor in a conventional electric air conditioner or refrigerator. When used with solar thermal energy systems, absorption coolers must be adapted to operate at the normal working temperatures for solar collectors. Solar space cooling is quite costly to be implemented alone. So, it is better to use a solar system for both heating and cooling purposes. Significant space heating and/or water heating can be accomplished with the same equipment used for the solar cooling system. Bruno et al (1996) discussed the integration of an absorption chiller in a combined heat and power plant. They concluded that the absorption chiller is economically viable with the gas turbine when the chiller is used to cool the inlet air to the turbine to increase generator capacity. Meloche et al (1996) examined running the gas turbine at an input of 100 units of fuel and determined the maximum cooling using a compression chiller and an absorption unit. The calculation is based on maximum chilled water production from all the available energy. The half-effect absorption unit can be also used in applications where the heat source temperature is too low to be used to fire a single-effect unit (Herold et al, 1996). The advantage of the half-effect cycle is that the heat-input temperature can be less than the single-effect for the same evaporator and heat rejection temperature. The disadvantage of the half-effect cycle compared to the single-effect is that the coefficient of performance (COP) of the halfeffect cycle is approximately half of the single-effect, which means that for a given capacity the heat rejection of the half-effect cycle is larger. A small scale experimental single-effect lithium bromide absorption chiller was used for an experiment in the Hot Springs of Sivas, Turkey (Kececiler,et al,1999) The main purpose of the present study is to investigate the thermal performance and the feasibility of solar system designed for space heating, domestic water heating, and cooling requirements of a typical house in Kuwait. The single effect lithium bromide water (LiBr-H2O) absorption chiller model is based on a commercially available LiBr-H2O absorption chiller, Arkla model WF-36. The Arkla chiller has a nominal cooling capacity of three tons (37980 kj/h). Units of different capacity are approximated by scaling the Arkla performance. In the present work, transient simulation program, TRNSYS (Klein et al1993) is adapted to simulate the thermal performance of different solar heating and cooling system components. The weather data file for Kuwait is provided by Kuwait Institute for Scientific Research (KISR). The weather file contains monthly average values of daily radiation on horizontal surface, clearness index, ambient temperature, and wind speed. The weather data generator subroutine included in TRNSYS package is used to generate hourly data from the available average monthly data for Kuwait. High temperature applications like solar cooling neceissate a high performance solar collector like evacuated tube collector. Evacuated tube collectors are expensive and require high technology making their use inappropriate specially for developing countries. An interesting alternative to evacuated tube collector is the use of a conventional flat plate collector but equipped with transparent insulation materials (Rommel & Wagner 1992, Reddy & Kaushika 1999, Nahar 2001). The collector test facility installed at the college of technological studies, Kuwait is used to evaluate the performance of solar collectors. The test is performed first for a conventional flat plate collector (FPC) and then for the same collector equipped with honeycomb transparent insulation material (FPCHC). The economic calculations for this study are based on life cycle savings (LCS) method (Duffie & Beckman, 1991). SYSTEM DESCRIPTION AND CONTROL STRATEGY Figure 1 shows a schematic representation of the system us ed. The main system components are a solar collector, a storage tank, an absorption chiller, heat exchanger, and auxiliary units. There are four different modes for the system operation. When solar energy is available for collection and there is a load demand, heat is supplied directly from the collector to the heating or cooling unit. When solar energy is available for collection and there is no heat or cooling demand, heat is stored in the storage unit. On the other hand, if solar energy is not available for collection and there is a load demand, storage then supplies heat to the heating or cooling unit. However, if storage temperature is not sufficient, the heating or cooling load is supplied by the auxiliary source. 240

3 Figure 1 Schematic diagram of the solar heating and cooling system Figure 2 shows a schematic of a single-effect lithium bromide water absorption chiller. The main components of the absorption unit are; generator, condenser, evaporator, absorber, and low temperature heat exchanger. The lithium bromide solution is pumped from the absorber to the generator where the water is boiled off. The heat source is passed in a counter-flow arrangement through the generator to boil off water vapour from the LiBr-H2O solution. A cooling water loop is needed to condense the water vapour boiled off from the generator and to aid in the absorption of water vapour back into the LiBr-H2O solution. This cooling water is passed first through the absorber and then the condenser. The evaporator takes in low-pressure cold water and produces a cooling effect by evaporating the water and passing it to the absorber. A circulation pump is used to ensure complete wetting of the tubes. Heat Source Return Generator Condensing Water Condenser Heat Exchanger Evaporator Chilled Water Pump Absorber Condensing Water Figure 2 Schematic of single effect absorption chiller 241

4 The chiller model is based on a commercially available LiBr-H2O absorption chiller system, Arkla model WF-36. The Arkla chiller has a nominal cooling capacity of three tons (37980 kj/h). Units of different capacity are approximated by scaling the Arkla performance. Hot water is supplied to the air conditioner at a temperature of 87 C (minimum), 93 C (maximum) and leaves this unit 10.5 C cooler than the supply and returns to the storage ( or to the auxiliary heater if storage is below 77 C). Whenever hot water from storage is cooler than 87 C, the auxiliary heat is supplied to raise its temperature to 87 C. When storage is cooler than 77 C, it is not used, and the auxiliary heater carries the full cooling load. The coefficient of performance (COP), and the ratio of actual cooling capacity to the rated capacity (f), depend on the generator temperature and the condensing water inlet temperature. Both COP and f are found from curve fits to the Arkla WF-36 performance. The cooling tower is modelled using a constant approach to ambient wet bulb temperature. In addition, a constant COP model of a vapour compression air conditioner is included as a secondary cooling auxiliary so that the energy required to meet space cooling is provided even if the absorption machine cannot meet the full load. The control strategy and the multistage room thermostat used is shown in Table 1. Table 1. Control Strategy for Room Thermostat On Temperature ( C) Off Temperature ( C) Solar- AC TR > 25.0 TR < 24.4 Auxiliary I- AC TR > 25.7 TR < 25.1 Auxiliary II-AC TR > 26.8 TR < 26.2 Solar- Heat TR > 20.0 TR < 21.5 Auxiliary Heat TR > 18.5 TR < 19.9 THEORETICAL ANALYSIS The collector efficiency, c, can be expressed using the well known formula : Q u ( Ti Ta ) c = = FR ( τα ) FR UL (1) A cg G where Qu is the useful energy rate gained by the collector, Ac is the collector area, G is the incident radiation on horizontal surface, FR is the heat removal factor, (τα) is the transmittance-absorptance product, UL is the collector overall heat transfer loss coefficient, Ti is the inlet collector temperature, and Ta is the ambient temperature. The storage tank is modelled as a stratified tank. The energy balance of the water in the storage tank accounts for energy gain from the collector, energy removed by the load, and energy lost to the surroundings. The equation describing these quantities is given by : Mscp dts dt = mc cp (To Ts ) + (ml + mf ) c p (TRN Ts ) + ULsA s (Ta Ts ) (2) where Ms is the mass of the water in the storage tank, c p is the specific heat of the water, T S is the temperature of the water in the energy storage tank, t is the time, m c is the mass flow rate of water to the collector, To is the outlet collector temperature, m L is the mass flow rate of water to the service hot water system, mf is the mass flow rate of water to the air conditioner or air heater, TRN is the temperature of the combined stream of water returning from the air conditioner or air heater, and service hot water systems, ULS is the storage tank heat transfer loss coefficient, and As is the storage tank surface area. 242

5 FINANCIAL ANALYSIS The financial viability of a solar system depends on many factors. One of the most important factors is the cost of the conventional fuel energy. The economic calculations for this study are based on life cycle savings (LCS) method (Duffie & Beckman, 1991). There are two variables which characterize the life cycle savings method: the duration of the analysis and the discount rate. The discount rate is defined as the rate of return which can be obtained from the best alternative investment. The life cycle savings of a solar system (LCS) over a conventional system can be defined as the difference between the reduction in fuel costs and the increase in expenses resulting from the additional investment for the solar system and is given by the following equation : LCS = P1 CF LFt P2 (CA Ac + CE ) (3) where P 1 is the factor relating life cycle fuel cost to first year fuel cost savings, P 2 is the factor relating life cycle by additional capital investment to initial investment, C A is the solar energy investment cost which is directly proportional to collector area, C E is the solar energy investment cost which is independent of collector area, C F is the unit cost of delivered conventional energy for the first year of analysis, L is the total load, and Ft is the total solar fraction of the solar system. For a particular locality and set of economic conditions, the economic analysis can be used to evaluate the economic feasibility of the solar system in terms of the life cycle savings. For example, optimization is made with respect to collector area to obtain the maximum life cycle savings for a given locality and a set of collector parameters. However, when two parameters were considered simultaneously, optimization was made in terms of the life cycle cost and not the life cycle savings. SYSTEM PARAMETERS AN D DATA Table 2 shows the system parameters for the different components of the system studied. Table 2. Solar System Parameters House UA house 700 W/ K latent cooling load 0.3 x sensible cooling load house capacitance 6000 W/ K Hot Water Load daily hot water demand 240 kg/day demand temperature 50 C inlet temperature 25 C volume of preheat tank 0.24 m 3 Uloss of preheat tank 0.60 W/m 2 K Storage Tank volume 0.08 m 3 /m 2 height/ diameter 1 ULS 0.60 W/m 2 K Tmax 100 C Solar Source Space Heating mass flow rate of water kg/s.m 2 mass flow rate of air kg/s.m 2 Auxiliary space heating max. rate at which heat can be provided W max. temp. at which air is returned to the load 46 C 243

6 Absorption Chiller with Auxiliary I capacity chiller transients start up time constant cool down time constant 6944 W s 3780 s Cooling tower approach 10.5 C max. useful solar source temp 77 C Auxiliary energy to generator temp. of firing water delivered 93 C series auxiliary if solar source temp. > 87 C parallel auxiliary if solar source temp. < 87 C Air Conditioner Auxiliary II capacity COP kj/h Economic Parameters cost per unit area (FPC) 250 $/m 2 cost of transparent insulation material 120 $/m 2 area independent cost 3000 $ cost of conventional fuel in 1st year annual increase in fuel cost 5% % extra insurance and maintenance in year 1 2% term of mortgage period of economic analysis $/kw.hr 10 years 10 years RESULTS AND DISCUSSION The collector test facility installed at the college of technological studies, Kuwait is used to test the performance of the flat plate solar collector without honeycomb material (FPC) and the flat plate collector equipped with honeycomb material (FPCHC). The experiments are carried out on a 30 -tilted collector surface for global solar radiation between 600 and 1000 W/m 2. The water flow rate of all the experiments ranges from 0.01 to kg/m 2 s and the inlet water temperature is changed from around the ambient temperature up to 85 C in 5 C steps. The collected data are examined to ensure that it presents quasi steady state conditions according to the recommendations outlined in (ASHRAE, 1986). The experimental data are fitted with linear equations to provide the characteristic parameters of the different collectors. The parameters obtained for FPC are F R (τα) = 0.68 and F R U L = 7.78, while those for FPCHC are F R (τα) = 0.60 and F R U L = 3.8 (Abdullah & Ghoneim, 2003). These results indicate the significant improvement in solar collector thermal performance which can be attained when the solar collector is equipped with honeycomb material. The thermal performance of a solar system is usually measured by the solar fraction (F). Solar fraction is defined as the fraction of load met by solar energy. Figure 3 shows the variation of the solar fraction of space heating (F s ), domestic water heating (F D ), and cooling load (F Ac ) as well as the total solar fraction (F t ) with collector area. As seen from the figure, a significant portion of the solar fraction for space heating and domestic water heating is satisfied at areas around 30 m 2. Conversely, the space cooling requires much greater areas. 244

7 F D F t Solar Fraction (-) F s F AC Area (m 2 ) Figure 3. Solar fraction variation with collector area The variation of total solar fraction, life cycle savings, and overall system efficiency (ratio of solar energy provided to the total incident radiation) with collector area is presented in Figure 4. This typical figure shows the choice of optimum collector area which is approximately equal to 35 m 2 in this case. It is clear from this figure that this optimum area neither corresponds to maximum system efficiency nor to the maximum solar fraction LCS 2500 F t Solar Fraction (-) LCS ($) Area ( m 2 ) 0 Figure 4. Variation of system parameters with collector area 245

8 Figure 5 shows the variation of space heating efficiency (s), domestic water heating efficiency (D), space cooling efficiency (AC), and the total system efficiency (t), through the year for the optimum area. As expected, the efficiency of the space heating and the space cooling diminishes during the summer and winter seasons respectively, while the domestic hot water efficiency remains approximately constant. 0.5 System Efficiency (-) s D AC t Month Figure 5 Variation of system efficiency through the year It should be noted that the previous results are obtained for flat plate collectors without honeycomb material. The calculations are repeated again for the flat plate solar collector equipped with honeycomb material (transparent insulation material). Table 3 presents a comparison between the results obtained for FPC and those obtained for FPCHC. As seen from the table, the optimum area required for FPCHC is less than half that of flat plate solar collector without honeycomb material. Also, the total solar fraction (F t ) for flat plate solar collector without honeycomb and the one equipped with honeycomb are nearly equal and satisfies a great portions of the load. On the other hand, the effect of the solar collector equipped with honeycomb on (t) is very pronounced. The collector efficiency ( c ) behaves the same as the system efficiency with slightly higher numerical values. Finally, the coefficient of performance (COP) of the absorption chiller is within the accepted practical values of the conventional lithium bromide system. Table 3 Thermal Performance Results for Kuwait A op (m 2 ) F t t c COP FPC FPCHC FPC FPCHC FPC FPCHC FPC FPCHC FPC FPCHC

9 Table 4 presents the values of life cycle cost (LCC), life cycle savings (LCS), and the cost of unit energy provided by solar system for both FPC and FPCHC. The results prove the feasibility of the solar heating and cooling systems in Kuwait climate as indicated by the life cycle savings value. The life cycle cost of the conventional and the solar system varies significantly due to the corresponding variation of the total load. However, for the optimum conditions in the present study, the cost of 1 kw.hr provided by the proposed solar system is $/kw.hr which represents 64% of the value provided by the conventional fuel system ( $/kw.hr). Table 4 Economic Results for Solar Heating and Cooling System Load ( x10-7 kj ) LCC (x10-4 $ ) LCS (x10-4 $ ) Solar Cost ($/kw.hr) FPC FPCHC FPC FPCHC FPC FPCHC These results should encourage wide installation of solar heating and cooling systems in Kuwait which will reduce energy consumption of conventional fuel as proved by the results of the analysis. Also, solar systems can greatly help in economical development programs. In addition, wide utilization of solar energy systems, will help in reducing environmental pollution. CONCLUSIONS 1. Using flat plate collector equipped with honeycomb material significantly reduces the optimum area required for solar heating and cooling systems. 2. Great portions of the total heating and cooling loads are satisfied by solar energy at the optimum conditions and the overall system efficiency is within the previously published results. 3. The cost of unit energy for solar heating and cooling systems approximately equals to 64 % of the corresponding cost of the conventional fuel at the current prices. 4. The results of the present study should encourage wide utilization of solar energy systems which will help in reducing environmental pollution. ACKNOWLEDGEMENTS The authors wish to thank Kuwait Foundation for the Advancement of Sciences (KFAS) for its financial support of the research through the project no. ( ). REFERENCES ASHRAE, ASHRAE Standard (1986). Methods of Testing to Determine the Thermal Performance of Solar Collectors. Atlanta, Georgia, USA, Abdullah A.H., and Ghoneim A.A. (2003). Thermal performance of Flat Plate Solar Collector Equipped with Rectangular Cell Honeycomb. Paper presented to Destination Renewables Conference, Melbourne, Australia. Bruno J.C., Fernandez F., and Castells F. (1996). Absorption Chillers Integration in a Combined Heat and Power. Ab-Sorption, 2, Duffie J.A, and Beckman W.A. (1991). Solar Engineering of Thermal Process. Wiley Interscience, New York. Garg H.P. (1987). Advances in Solar Energy Technology. D. Reidel Publishing Company, Holland. Ghoneim A.A., Fisch N., Ammar A.S., and Hahne E. (1993). Design of A Solar Heating System for Alexandria, Egypt. Renewable Energy. 3 (6/7), Herold K.E., Radermacher R., and Klein S.A. (1996). Absorption Chillers and Heat Pumps. CRC Press. 247

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