STUDY ON HYBRID SYSTEM OF SOLAR POWERED WATER HEATER AND ADSORPTION ICE MAKER

Transcription

1 , Volume 6, Number 4, p , 2005 STUDY ON HYBRID SYSTEM OF SOLAR POWERED WATER HEATER AND ADSORPTION ICE MAKER Zhaohui Qi School of Energy Science and Engineering, Centralsouth University, Changsha, China, (Received 23 October 2003; Accepted 14 February 2006) ABSTRACT Based on the vacuum tube type solar collector, a system of hybrid solar powered water heater and adsorption ice maker was designed and a prototype was built. Energy analysis of the hybrid system was presented, and the prototype was experimentally tested in different season. The results of experiment show that, compared with traditional solar powered water heater and cooling machine, the hybrid system is more suitable for household usage, and the utilization of solar energy is more effective. 1. INTRODUCTION The utilization of solar energy is a major feature of modern green building, it is concerned by scientists around the world and some progress is made. There exist two types of solar collector [1]: photovoltaic cells and purely thermal collector, the latter is more common in the application of solar energy. Based on the thermal collector, solar powered heating or cooling systems of building have been developed, some products are used for residential applications. The technology of solar powered heating system has been well developed and successfully used in solar powered water heater. Solar powered water heaters are popularly used in China, more than 15,000,000 water heaters in China are powered or partly powered by solar energy, and the number is still increasing quickly. Solar powered water heater is advantageous for environmental friendliness and energy saving, but it has some disadvantages. The main disadvantage is that the supply of solar energy and the need for heated water does not match with each other in the same season. In summer, the supply of solar energy reaches maximum level, but the need for heated water is minimum; in winter, it is in the opposite situation. So the utilization of solar energy in water heater is not sufficient. The technology of solar powered cooling system has been developed also. It is an excellent application of solar energy, because the supply of solar energy and the need for refrigeration both reach maximum levels in the same season (summer). Liquid absorption system has been proposed and tested at first [2], solid adsorption system has been discussed and tested later. One characteristic of solid adsorption system is that there is no need of rectifying column, so it has advantages of simple construction, no moving parts and no-noise. On the other hand, non-freon refrigerant is used in adsorption refrigeration, so it is environmental friendly. Worsoe-Schmidt has developed adsorption solar refrigerator which has been used in areas of the Third World [3]. The main drawback of adsorption system is that its operation is limited by season. So the solar powered cooling machine could only be used seasonal, that is to say, it is expensive compared with the cooling machine which can be used for the whole year. Requirement for cooling and heating of household is dissimilar in different season. Single solar powered heating or cooling machine could not meet the need of household in a year. The aim of the work described below is to combine the solar powered water heater and adsorption refrigeration machine, to meet different requirement of household in different season, and to improve the efficiency of utilization of solar energy. 2. HYBRID SOLAR POWERED WATER HEATER AND ADSORPTION ICE MAKER Glass vacuum tube has been widely used as collector in solar powered water heater. The technology of vacuum tube type collector is mature, absorption rate of glass vacuum tubes is more than 93%, and thermal radiation rate at 100 o C is less than 6%, water temperature in it can reach 98 o C easily by adjusting the quantity of water. Water at that temperature can be used as heat source of adsorption refrigeration system. 168

2 Fig. 1: Schematic of hybrid solar powered water heater and ice maker According to the above discussion, a hybrid solar powered water heater and ice maker was designed. The schematic of system is shown in Fig. 1. The working principle of the system is just a combination of a solar water heater and adsorption ice maker. Heating of the vacuum tube type solar collector is started in the morning, water in the solar collector is heated slowly. When temperature of water in the collector is 5~7 o C higher than storage tank s water, the water circulation pump works to pump up tank s cold water into the collector, warm water in the collector flows through the adsorber, goes back into storage tank. When the warm water flows through, the adsorber is heated, the temperature of adsorbent in it rises. In a ideal process, the adsorbent temperature could be very close to the maximum temperature of water in the tank at last. When the temperature in the adsorbent rises up to the reaction temperature of desorption, the refrigerant is desorbed from the adsorbent, and the vapor pressure in the adsorbent rises too. When the pressure reaches the condensing pressure, the desorbed refrigerant vapor is condensed in the condenser and collected in the receiver. At night, with the reduction of ambient temperature, the adsorber is cooled by nature convection, the adsorbent in it is cooled too, and the pressure in the adsorber drops to value below evaporation pressure, evaporation begins, ice is made slowly in the ice box. Refrigeration will continue for the whole night until the next morning. The main difference between the hybrid system designed in this paper and other hybrid system of solar powered water heater and adsorption ice maker is that the adsorber of hybrid system designed in this paper is separated from the hot water storage tank, and adsorber of other system is placed in the hot water storage tank, the desorption and evaporation of refrigerant depends on the consumption of hot water. So it is difficult for them to meet different requirements of hot water and cooling. The hybrid system designed in this paper can meet different requirement of hot water and cooling without any difficulty. 3. ENERGY ANALYSIS Solar energy absorbed by the collector will be used in three way [4]: (1) Q u, energy to heat the water in the storage tank and adsorber. (2) Q s, energy storage in the collector. (3) Q l, energy lost due to various losses. The energy conservation equation is: A e G(τα) = Q u + Q s + Q l (1) where G is the solar flux density to the collector, τ is the transmittance coefficient of solar radiation through the cover of the collector, α is the collector efficiency, and A e (m 2 ) is the area of the collector. 169

3 In a solar collector, the heat quantity Q u (kj) is used to heat the water and adsorber, which is mainly determined by the efficiency of collector; the heat quantity Q s (kj) is determined by the sensible heat of collector and depends on the collector material; Q l (kj)is the heat losses composed of the face loss, the bottom loss, the four sides loss of collector and the loss of heat exchange in the adsorber when warm water flows through the adsorber. Q s and Q l of vacuum tube collector are less than 10% of the total solar energy absorbed by the collector. The useful heat from the collector Q u contributes both to the heating of the water in the tank and to the heating of the adsorber which will cause the desorption of refrigerant from the adsorbent bed, etc. The energy equation is: Q u = Q water + Q adsorber + Q refrigerant + Q reaction (2) In which Q water (kj) represents the heat added to the water in the storage tank. Q water = M water C water ( T h - T l ) (3) where M water is the mass of water in the tank, kg; C water is the specific heat of water, kjkg -1 K -1 ; T h is the last temperature of water after heating, K; and T l is the temperature of city water, K. Q adsorber (kj) represents the sensible heat of material and adsorbent in the adsorber. Q adsorber = (M m C m + M a C a )(T h -T 0 ) (4) M m is the mass of adsorber material, kg; C m is the specific heat of material, kjkg -1 K -1 ; M a is the mass of adsorbent, kg; C a is the specific heat of adsorbent, kjkg -1 K -1 ; T h is the last temperature of adsorber after heating, K; and T 0 is the ambient temperature, K. Q refrigerant (kj) represents the sensible heat of refrigerant in adsorber. Q refrigerant = M R C R (T e -T 0 ) (5) M R is the mass of refrigerant in adsorber, kg; C R is the specific heat of refrigerant, kjkg -1 K -1 ; and T e is the critical temperature of reaction, K. Q reaction (kj) is the heat of desorption. Q reaction = H R M R (6) where H R is the heat of desorption per kilogram, kjkg -1. The desorbed refrigerant is condensed in the condenser and flows into the evaporator. When the adsorbent bed pressure is lower than the evaporation pressure, the refrigerant liquid in the evaporator evaporates which causes the refrigeration effect in the ice box. The refrigeration quantity is : Q ref = M ice L e (7) In which L e is the latent heat of water vaporization (kjkg -1 ), and M ice is the weight of removed ice (kg). The hybrid system has two useful output, one is refrigeration, its system COP (Coefficient of Performance) is: Q ref COP system = (8) AeG(τα) Another is heating the water in the storage tank, its system efficiency is: Q η water system = (9) AeG(τα) Chloride strontium (SrCl 2 ) ammonia (NH 3 ) is used as working pair of adsorption refrigeration, SrCl 2 is used as the adsorbent, and NH 3 is used as refrigerant, it is desorbed and adsorbed according to the following equation [5]: SrCl 2 8NH 3 <= > SrCl 2 NH 3 +7 NH 3 (10) Ammonia is not a suitable refrigerant for residential applications if it is used in air conditioning, but in the Third World, however, air conditioning is a luxury most people cannot afford. In those countries, refrigeration is primarily needed at the village level, for preservation of foodstuffs and medical supplies only. Ice maker with box can meet this need easily, ammonia is acceptable if the refrigeration system is separated from other systems [3,5]. 4. EXPERIMENTAL RESULTS A prototype hybrid system for water heating and ice-making was developed. The solar energy collector consisted of 30 φ glass vacuum tubes whose collect area of solar energy was 4.2 m 2, structure of adsorber adopted by this paper is shown in Fig 2. It was made up of unit generating tubes. Configuration of the unit generating tube is shown in Fig 3. Unit generating tube was made up of aluminum fin in which SrCl 2 was filled, ammonia-conducted tube and stainless steel tube. Ammonia-conducted tube was the passage of ammonia vapor flowing into/out the generating tube, and stainless steel tube was an heat exchanger in effect where heat exchange was conducted when warm water of collector flowed. 170

4 Fig. 2: Schematic of adsorbor (cross-section) Fig. 3: Schematic of unit generating tube Values of the parameters mentioned in equations (3) to (7) are compiled in Table 1. The prototype was built in Changsha, a city in South China, A typical experiment was demonstrated at the end of August, Overall insolation was measured by a pyranometer parallel to the collector. Temperatures of water in tank, inlet and outlet of adsorber and temperature of evaporator were measured by thermocouple, ambient temperature, temperatures of city water and adsorber were measured also. The measurements and their accuracy are compiled in Table 2. Weight of the removed ice can be determined as the last quantity of refrigeration. The sun began shining normally at 7:00 am, the initial temperature of water was 18 o C, initial temperature of adsorber was 32 o C (the same temperature as environment). After 6 hours, the 180 kg water tank temperature reached 85 o C, temperature of the adsorber reached 80 o C, water temperature inlet adsorber was 91 o C, the desorption process began obviously. After 5 hours of continuous heating, the temperature of water tank reached a maximum value, 93 o C, water temperature inlet adsorber was 98 o C, temperature of the adsorber was nearly 90 o C. Heating and desorption stopped at that time (6:00 pm ). Total solar heat density was 20 MJm -2, that is to say, about 84 MJ solar heat was collected. At night, the temperature of adsorber reduced slowly. At about 11:00 pm, the temperature reduced to 29 o C, adsorption took place. One hour later, the temperature of evaporator reduced to -15 o C, ice was made obviously in the ice box. Refrigeration continued in the whole night until the next morning kg ice was made at last. Fig. 4 shows the temperature variation of different parts of the hybrid system in the experiment. Table 1: Relevant parameters in energy analysis Parameter M water M m M a M R M ice C water C m C a C R H A e α value Table 2: Measurements and their accuracy in experiment Measured quantity Measurement Accuracy Temperature of water, evaporator Thermocouple (NiCr-Ni) 0.1 K Temperature of city water, environment Alcohol thermometer 0.5 K Solar flux density Pyranometer 1.0 Jm -2 Temperature ( o C) Time 171 Fig. 4: Temperature histories of water, adsorber and environment

5 Table 3: Experimental results of the hybrid system Date Aug , 2000 Oct , 2000 Energy collected / MJ Hot water output / o C kg Ice out / kg COP system η system η system without ice maker Here, COP system and η system defined by equations (8) and (9) were and 0.67, η system of the same solar powered water heater system without ice maker was COP system of other kinds of solar power adsorption ice maker varied from [1]. Table 3 shows the experimental result discussed above and another experimental result of hot water and ice output in a typical season (autumn). Obviously, with the reduction of collected solar energy, system efficiency of heating and cooling became smaller. The value of refrigeration efficiency (COP) reduced more relatively. On the other hand, system efficiency without adsorption ice maker was almost not changed. It was due to poor heat transfer in the adsorber. Technology of heat transfer enhancement should be adapted in the adsorber in order to improve the efficiency of refrigeration. But generally, the results of refrigeration are acceptable. The refrigeration quantity produced after one day operation can be used to keep a 200 liter cold box for more than 24 hours with temperature less than 5 o C. REFERENCES 1. M. Pons and J.J. Guilleminot, Design of an experimental solar-powered, solid-adsorption ice maker, Journal of Solar Energy Engineering, Vol. 108, pp (1986). 2. J.C.V. Chinnappa, Performance of an intermittent refrigerator operated by a flat Plate collector, Solar Energy, Vol. 6, pp (1962). 3. P. Worsoe-Schmidt, Solar refrigeration for developing countries using a solid-absorption cycle, International Journal of Ambient Energy, Vol. 4, pp (1983). 4. R.Z. Wang, M. Li, Y.X. Xu and J.Y. Wu, A new hybrid system of solar powered heater and adsorption ice maker, Solar Energy, Vol. 68, pp (2000). 5. A. Erhard, K. Spindler and E. Hahne, Test and simulation of a solar powered solid sorption cooling machine, International Journal of Refrigeration, Vol. 21, No. 2, pp (1998). 5. CONCLUSION The hybrid system of solar powered water heater and ice maker uses one solar collector as heating and refrigeration source, it is suitable for household application. Adsorber is separated from collector, thus high efficiency glass vacuum tube can be used for solar energy collection. Adsorber is also separated from the water tank [4], thus the refrigeration quantity is not influenced by the consumption of hot water. It can meet different household requirement for heating and cooling in different season. Compared with single solar powered water heater and ice maker, system efficiency of heating and cooling is lower [1], but the hybrid system has two useful outputs, the utilization of solar energy is more sufficient and comprehensive. 172