Experimental and theoretical study on a solar assisted CO2 heat pump for space heating

In this paper, a solar combi-system which consists of solar collector and a CO₂ heat pump is investigated experimentally and theoretically. Two experiments are primarily conducted to show the performance of this solar combi-system under different operation conditions. A system model is developed and validated in TRNSYS to analyze the influence of main components parameters. Subsequently, a multi-parameter optimization is carried out in GENOPT to obtain a final optimal result. The simulated results show that the optimized system can save 14.2% electricity and improve the solar fraction by 8%. The solar fraction of the optimized system can reach 71.1%. Finally, the optimized system performance is studied with the weather and load characteristics in Shanghai. Compared with the CO₂ HP heating system alone, the solar assisted system can save 1790.8 kWh electricity on the basis of year round operation.     

太阳能辅助二氧化碳热泵热水系统实验研究

利用实验的方法,研究了太阳辐照度、外界气温和风速、初始水温、蒸发器出口温度和压力等对太阳能辅助二氧化碳热泵热水系统运行状况和COP的影响。实验结果表明,系统COP随初始水温的升高而增大;太阳辐照度、外界温度和风速对热泵系统性能的影响主要体现在对系统循环水温的影响;在一定范围内,蒸发压力和蒸发温度越高,热泵系统的COP越大。     

Optimization of a community solar heating system with a heat pump and seasonal storage

The optimization of a district solar heating system with an electric-driven heat pump and seasonal heat storage is discussed. The optimization process comprises thermal, economic and system control analyses. Thermal and economic optima have been derived for collector area and storage volume simultaneously. The effects of different collector types and building loads are also investigated. Summertime charging of the storage by off-peak electricity has been applied to avoid severe peaking of auxiliary in the winter and to reduce the yearly energy cost. The thermal co-storage of electric energy is emphasized with systems which fail to supply heat for the heat pump during the winter heating season.‡ It has been found that system cost-effectiveness is only slightly affected as storage volume is increased beyond the optimum size. Large variations in the optima for different system configurations were found. The minimum cost of heat supplied in an optimal 500-unit community with 90% solar fraction was estimated at 8.9 ¢ kWh⁻¹.     

Experimental investigation of thermal performance of a solar assisted heat pump system with an energy storage

An experimental solar assisted heat pump space heating system with a daily energy storage tank is designed and constructed, and its thermal performance is investigated. The heating system basically consists of flat plate solar collectors, a heat pump, a cylindrical storage tank, measuring units, and a heating room located in Gaziantep, Turkey (37.1°N). All measurements are automatically collected as a function of time by means of a measurement chain feeding to a data logger in combination with a PC. Hourly and daily variations of solar radiation, collector performance, coefficient of performance of the heat pump (COP_HP), and that of the overall system (COP_S) are calculated to evaluate the system performance. The effects of climatic conditions and certain operating parameters on the system performance parameters are investigated. COP_HP is about 2.5 for a lower storage temperature at the end of a cloudy day and it is about 3.5 for a higher storage temperature at the end of a sunny day, and it fluctuates between these values in other times. Also, COP_S turns out to be about 15–20% lower than COP_HP. Copyright © 2004 John Wiley & Sons, Ltd.     

The performance of a solar assisted heat pump water heating system

Analytical and experimental studies were performed on a solar assisted heat pump water heating system, where unglazed, flat plate solar collectors acted as an evaporator for the refrigerant R-134a. The system was designed and fabricated locally, and operated under meteorological conditions of Singapore. The results obtained from simulation are used for the optimum design of the system and enable determination of compressor work, solar fraction and auxiliary energy required for a particular application. To ensure proper matching between the collector/evaporator load and compressor capacity, a variable speed compressor was used. Due to high ambient temperature in Singapore, evaporator can be operated at a higher temperature, without exceeding the desired design pressure limit of the compressor, resulting in an improved thermal performance of the system. Results show that, when water temperature in the condenser tank increases with time, the condensing temperature, also, increases, and the corresponding COP and collector efficiency values decline. Average values of COP ranged from about 4 to 9 and solar collector efficiency was found to vary between 40% and 75% for water temperatures in the condenser tank varying between 30°C and 50°C. A simulation model has been developed to analyse the thermal performance of the system. A series of numerical experiments have been performed to identify important variables. These results are compared with experimental values and a good agreement between predicted and experimental results has been found. Results indicate that the performance of the system is influenced significantly by collector area, speed of the compressor, and solar irradiation. An economic analysis indicates a minimum payback period of about two years for the system.     

A simulation study on a solar heat pump heating system with seasonal latent heat storage

Solar heating systems with seasonal energy storage have attracted an increasing attention over the past decades. However, studies of such systems using a phase change material (PCM) as seasonal storage medium have not been found in the open literature. In this paper a solar heat pump heating system with seasonal latent heat thermal storage (SHPH–SLHTS) is firstly described. This is followed by reporting the development of a simplified mathematical model for a SHPH–SLHTS system. Using the model developed, the operational performances of a SHPH–SLHTS system which provided space heating to a villa building have been investigated by simulation, and simulation results are reported in this paper.     

Experimental investigation of a calcium chloride-water heat storage system for off-peak energy supply and solar heating

One of the main difficulties in applying heat storage systems utilising the latent heat of calcium chloride hexahydrate is the incongruent character of the melting of the hexahydrate. The heat storage capacity of the system is considerably diminished by the separation of α-CaCl₂.4H₂O, forming a new phase. In externally non-mixed systems the separation of the tetrahydrate is practically irreversible and this causes the rapid deterioration of the heat storage system. During the experiments described in this paper the effects of the change in composition of the CaCl₂, 6H₂O heat storage material, of the impurities (NaCl, KCl, MgCl₂) and of thermoconvective flow on the formation of α-CaCl₂.4H₂O were investigated in externally non-mixed systems. It has been established that with a heat storage packing of more dilute (CaCl₂.6·3H₂O) total composition than the peritectic composition, favourable storage properties can be attained. To achieve this, the solid impurities have to be removed from the molten phase and the electric cable providing the heat input must be arranged to provide for adequate concentration equalisation by the convective thermal flow.     

太阳能热泵地板辐射供暖系统

太阳能热泵低温地板辐射供暖系统是将太阳能、水源热泵和地板供暖结合在一起的新型采暖方式。介绍了太阳能热泵低温地板辐射供暖系统的工作原理和构成,并以青岛地区为例,设计了太阳集热器、蓄热器、地板换热盘管等设备,探讨了系统的设计方法。     

Simulation of an absorption heat pump solar heating and cooling system

An absorption heat pump (AHP) is a heat driven heat pump utilizing the absorption process. A continuous, liquid absorbent AHP with chemical storage is modeled using mass and energy balances and assuming mass transfer equilibrium. This model is used with the TRNSYS program [5] to simulate the performance of an AHP in a residential solar-driven heating and cooling system. The effects of collector area for an AHP using the NaSCN---NH₃ chemical system are investigated for the Columbia, MO, Madison, WI, and Fort Worth, TX climates. The AHP system is compared to a conventional solar heating and cooling system and the effects of heat exchanger effectiveness, storage mass, additional thermal capacitance and alternative control strategies are studied for the Columbia climate.     

Experimental studies on a ground coupled heat pump with solar thermal collectors for space heating

This paper presents experimental studies on a solar-assisted ground coupled heat pump (SAGCHP) system for space heating. The system was installed at the Hebei Academy of Sciences in Shijiazhuang (lat. N38°03′, long. E114°26′), China. Solar collectors are in series connection with the borehole array through plate heat exchangers. Four operation modes of the system were investigated throughout the coldest period in winter (Dec 5th to Dec 27th). The heat pump performance, borehole temperature distributions and solar colleting characteristics of the SAGCHP system are analyzed and compared when the system worked in continuous or intermittent modes with or without solar-assisted heating. The SAGCHP system is proved to perform space heating with high energy efficiency and satisfactory solar fraction, which is a promising substitute for the conventional heating systems. It is also recommended to use the collected solar thermal energy as an alternative source for the heat pump instead of recharging boreholes for heat storage because of the enormous heat capacity of the earth.     

Experimental investigation of a vapor compression heat pump used for cooling and heating applications

The present work aims at evaluating the performance characteristics of a vapor compression heat pump (VCHP) for simultaneous space cooling (summer air conditioning) and hot water supply. In order to achieve this objective, a test facility was developed and experiments were performed over a wide range of evaporator water inlet temperature (14:26 °C) and condenser water inlet temperature (22:34 °C). R134a was used as a primary working fluid whereas water was adopted as a secondary heat transfer fluid at both heat source (evaporator) and heat sink (condenser) of the heat pump. Performance characteristics of the considered heat pump were characterized by outlet water temperatures, water side capacities and coefficient of performance (COP) for various operating modes namely: cooling, heating and simultaneous cooling and heating. Results showed that COP increases with the evaporator water inlet temperature while decreases as the condenser water inlet temperature increases. However, the evaporator water inlet temperature has more effect on the performance characteristics of the heat pump than that of condenser water inlet temperature. Actual COP of cooling mode between 1.9 to 3.1 and that of heating mode from 2.9 to 3.3 were obtained. Actual simultaneous COP between 3.7 and 4.9 was achieved.     

Experimental results of a solar assisted heat pump rice drying system

In the last rice harvest season, experimental results have been obtained on the efficiency and drying quality of a solar assisted heat pump drying prototype system. The system has been operated as a solar and heat pump system and drying curves for the different options have been obtained. The advantage of the low temperature and better control in the drier shows that the heat pump assisted solar drying system is an excellent alternative to traditional drying systems.     

First and second law analysis of a solar assisted heat pump based heating system

We propose a model for the heating system of an ecological building whose main energy source is solar radiation. The most important component of the heating system is a vapour compression heat pump. Both the first law and the second law were used to analyse the heat pump operation. The state parameters and the process quantities were evaluated by using, as input, the building thermal load. The second law analysis emphasised that most of the exergy losses occur during compression and condensation. Preliminary results show that the photovoltaic array can provide all the energy required to drive the heat pump compressor, if an appropriate electrical energy storage system is provided.     

太阳热水器与空气源热泵辅助加热集中供热水系统的工程设计

结合实例论述宾馆用太阳热水器与空气源热泵辅助集中供洗浴热水系统的工程设计方法,并着重介绍了空气源热泵的配置、布局、安装和控制.     

Experimental study of photovoltaic solar assisted heat pump system

A novel photovoltaic solar assisted heat pump (PV-SAHP) system has been proposed in this study. Performance tests with a range of condenser supply water temperature were conducted on an experimental rig. The dynamic performance of this PV-SAHP system in a 4-day period with very similar weather conditions was analyzed and the influencing factors were identified. The results indicate that this PV-SAHP system has a superior coefficient of performance (COP) than the conventional heat pump system and at the same time, the photovoltaic efficiency is also higher. The COP of the heat pump was able to reach 10.4 and the average value was about 5.4. The average photovoltaic efficiency was around 13.4%. The highest overall coefficient of performance (COP_p/t), bringing into consideration both the photovoltaic and thermal efficiency, was about 16.1.     

不同调节方式对循环加热式CO2热泵性能影响的实验研究

为研究膨胀阀开度和压缩机频率对循环加热式CO₂热泵制热性能的影响,采用实验的方法研究了阀开度35%,37%,40%和频率75,80,85 Hz工况下制热量和制热系数COP瞬时值和平均值以及加热时间的变化。结果表明：随着压缩机频率增加,平均制热量和最大瞬时制热量增加,平均COP和最大瞬时COP减小,加热时间缩短;随着阀开度增大,平均制热量和平均COP减小,最大瞬时制热量和COP增大,加热时间延长;当压缩机频率一定时,在不同的水箱温度范围采用合适的膨胀阀开度可以有效提高热泵的平均COP;以本文75 Hz工况为例,当水温小于25.9℃时采用40%开度,当水温在25.9～35.4℃之间时采用37%开度,当水温大于35.4℃时采用35%开度;合理匹配开度和频率可以最大程度提高平均制热量,同时抑制平均COP的减小。     

Experimental investigation and a dynamic simulation of the solar-assisted energy-storaged heat pump system

In order to improve the performance of the solar-assisted and energy-storaged heat pump system, an experimental setup was constructed. In this study, the solar-assisted energy-storaged series heat pump system and other conventional heat pump systems with no energy storage (series and parallel heat pump systems) are experimentally investigated and compared. The experiments were made in July, August, September, October, November, and December in 1990, under the clear-sky conditions for three heat pump systems. The experimentally obtained results are used to calculate the collector efficiency n_k, heat pump COP, and system COP_sys (coefficient of performance). On the other hand, a dynamic simulation program has been developed for a solar-assisted and energy-storaged heat pump system. The experimental results were compared with the dynamic simulation results.     

A new version of a solar water heating system coupled with a solar water pump

This research target was to improve the thermal efficiency of a solar water heating system (SWHS) coupled with a built-in solar water pump. The designed system consists of 1.58-m² flat plate solar collectors, an overhead tank placed at the top level, the larger water storage tank without a heat exchanger at the lower level, and a one-way valve for water circulation control. The discharge heads of 1 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 10–18 kPa. It was found that water circulation within the SWHS ranged between 15 and 65 l/d depending upon solar intensity and discharge head. Moreover, the max water temperature in the storage tank is around 59 °C. The max daily pump efficiency is about 0.0017%. The SWHS could have max daily thermal efficiency of about 21%. It is concluded that the thermal efficiency was successfully improved, except for the pump one. The new SWHS with 1 m discharge head or lower is suitable for residential use. It adds less weight to a building roof and saves electrical energy for a circulation pump. It has lower cost compared to a domestic SWHS.     

Experimental investigation of temperature and flow distribution in a thermosyphon solar water heating system

Natural circulation solar water heating systems are available in varying collector geometries (and materials), storage tank capacities and specifications of individual components. Though theoretical and experimental studies including the test procedures are available to estimate the performances of these systems, detailed experimental studies showing the temperature profiles of the absorber plate, water temperature in the riser and water flow in the riser are few. This paper presents details of experimental observations of temperature and flow distribution in a natural circulation solar water heating system and its comparison with the theoretical models. The measured profile of the absorber temperature near the riser tubes (near the bottom and top headers) conforms well with the theoretical models. The values at the riser tubes near the collector inlet are found to be generally much higher than those at the other risers on a clear day, while on cloudy days, these temperatures are uniform. The mean absorber plate and mean fluid temperature during a day has been estimated and compared with theoretical models. The temperature of water near the riser outlets was found to be fairly uniform especially in cloudy and partly cloudy days at a given plane during a day. The temperature of water in the riser depends on its flow rate. Measurements of glass temperature were also carried out.     

Experimental comparison of a solar-assisted heat pump vs. a conventional thermosyphon solar system

A solar-assisted heat pump system (SAHPS) for hot-water production has been developed and compared for its experimental performance, under similar ambient conditions with a conventional thermosyphon solar system (CTSS) with a single direct tank. Both systems were monitored from 1993 to 1997 during summer and winter time periods. The performance of CTSS was seriously affected by weather conditions, whereas SAHPS could always operate with no significant variation and with a COP above 3·0. A comparison between the two systems proved the performance of the SAHPS to be better than that of CTSS under all climatic conditions. © 1998 John Wiley & Sons, Ltd.