Study On Optimum Analysis And Varialbe Capacity Operation Of Direct Expansion Solar Assisted Heat Pump Water Heater

This paper concentrated on direct-expansion solar-assisted heat pump water heater (DX-SAHPWH) for domestic use. The thermal performance of a DX-SAHPWH was investigated and the effects of various parameters on its thermal performance were analyzed. Three experimental prototypes of DX-SAHPWH were developed and researched on subsequently to overcome current obstacles for the commercialization of DX-SAHPWH in China. The main works are summarized as follows:The DX-SAHPWH (A) system was designed and fabricated under meteorological conditions of Shanghai. It consisted of an unglazed aluminum solar collector/evaporator with exposure area 4.20 m2, R-22 rotary-type hermetic compressor with rated input power 750 W, a hot water tank with water volume 150 L and polyurethane insulation thickness 38 mm (with an immersed copper tube coil heat exchanger as condenser), and a external balance type thermostatic expansion valve (TEV). According to experiments done during spring sunny days, the DX-SAHPWH (A) system took 90-98 minutes to heat 150 L water from about 14-20 oC to 50 oC and the total electric consumption for compressor was 0.98-1.06 kWh. The average COP andηcollwas 5.21-6.61 and 88%-105%, respectively. Exergy analysis for each component of the DX-SAHPWH (A) system was calculated and found out that the highest exergy loss occurred in the compressor, followed by solar collector/evaporator, condenser and expansion valve, respectively.In order to maintain a proper matching between the heat pumping capacity of the compressor and the evaporative capacity of the solar collector/evaporator under widely varying ambient conditions, the electronic expansion valve or/and variable frequency compressor are suggested to be utilized in the DX-SAHPWH.To optimize the solar collector/evaporator performance, lower cost and facilitate integration with buildings, DX-SAHPWH (B) system with smaller capacity was developed. DX-SAHPWH (B) system consisted of an unglazed copper solar collector/evaporator with exposure area 2.08 m2, R-22 rotary-type hermetic compressor with rated input power 400 W, a hot water tank with water volume 150 L and polyurethane insulation thickness 38 mm (with an immersed copper tube coil heat exchanger as condenser), and a external balance type thermostatic expansion valve (TEV) parallel connected with electronic expansion valve (EEV). According to experiments done during autumn sunny days, it was found that it took 164-201 minutes to heat 150 L water from about 23-25 oC to 50 oC and the total electric consumption for compressor was 0.90-1.12 kWh. The average COP andηcollwas 4.13-5.12 and 84%-101%, respectively. Under the limitation of the available products in the market, the actual collector/evaporator utilized in the system (B) is made of 8 solar collector slab cores. Its "sheet-under-tube" structure type makes the heat exchange between the copper solar collector/evaporator and the ambient is not good enough. Then the evaporating temperature and COP of the system (B) is lower than that of the system (A). But correspondingly, the compressor suction pressure/temperature can be decreased so as to benefit the operation safety of DX-SAHPWH. After setting controlling parameters under some different typical working condition by means of "response curve method", "open loop Proportional - closed loop Proportional Intergral (PI)" control strategies were applied to effectively control EEV to maintain a proper superheat degree of the solar collector/evaporator so as to realize the "dynamic matching" working conditions between the compressor and the solar collector/evaporator under widely varying ambient conditions. According to comparing experiments, it was found that the superheat degree could be controlled more accurately and stably by EEV than by TEV.To realize the "dynamic matching" working conditions better (especially the matching between compressor and evaporator), DX-SAHPWH (C) prototype with "variable capacity" was developed. DX-SAHPWH (C) system consisted of an unglazed aluminum solar collector/evaporator with exposure area 4.20 m2, R-22 rotary-type hermetic variable frequency compressor with rated frequency 72Hz and rated input power 860 W, a hot water tank with water volume 150 L and polyurethane insulation thickness 38 mm (with an immersed copper tube coil heat exchanger as condenser), a external balance type TEV parallel connected with EEV, and a special microcontroller. According to experiments done in autumn sunny days, it was found out that it took 62-87 minutes for heating 150 L water from about 23-30 oC to 50 oC and the total electric consumption for compressor was 0.54-0.84 kWh. The average COP andηcollwas 5.29-6.93 and 88%-99%, respectively. By comparing system C with systems A and B, it was observed that DX-SAHPWH (C) took less time 6% and 48%, less electric consumption 29% and 20%, had higher COP 25% and 31%, and higher total exergy efficiency 16% and 55% than the system (A) and the system (B) respectively. This can be attributed to the better matching between components.Dynamic mathematical simulation models for solar collector/evaporator, compressor, condenser, water tank, TEV, and the whole DX-SAHPWH (C) system were proposed and solved based on lumped parameters. The effects of structural, operational and environmental parameters of the DX-SAHPWH (C) system on thermal performance were investigated. The simulation results based on the meteorological conditions of Shanghai and with the real operational parameters as the inputs to the model were shown to agree reasonably with the experimental data. The database for predicting the DX-SAHPWH (C) system performance was built and control strategies mainly including start/stop control regulations, variable capacity control regulations, and superheat degree control regulations were set up. Finally, a cheap microcontroller based on 8-bit micro processing unit (Renesas-M37544 chip) special for the DX-SAHPWH (C) system was developed including hardware design and software programming. By cooperating with an enterprise majoring in renewable energy field, small-lot trial manufacture and performance determination of the DX-SAHPWH (with solar collector/evaporator installed on balcony) was done. It is an important symbol of commercialization for DX-SAHPWH in China.