Experimental performance analysis on a direct-expansion solar-assisted heat pump water heater

A direct expansion solar assisted heat pump water heater (DX-SAHPWH) experimental set-up is introduced and analyzed. This DX-SAHPWH system mainly consists of 4.20 m² direct expansion type collector/evaporator, R-22 rotary-type hermetic compressor with rated input power 0.75 kW, 150 L water tank with immersed 60 m serpentine copper coil and external balance type thermostatic expansion valve. The experimental research under typical spring climate in Shanghai showed that the COP of the DX-SAHPWH system can reach 6.61 when the average temperature of 150 L water is heated from 13.4 °C to 50.5 °C in 94 min with average ambient temperature 20.6 °C and average solar radiation intensity 955 W/m². And the COP of the DX-SAHPWH system is 3.11 even if at a rainy night with average ambient temperature 17.1 °C. The seasonal average value of the COP and the collector efficiency was measured as 5.25 and 1.08, respectively. Through exergy analysis for each component of the DX-SAHPWH system, it can be calculated that the highest exergy loss occurs in the compressor, followed by collector/evaporator, condenser and expansion valve, respectively. Further more, some methods are suggested to improve the thermal performance of each component and the whole DX-SAHPWH system.     

Thermal performance analysis of a direct-expansion solar-assisted heat pump water heater

A direct-expansion solar-assisted heat pump water heater (DX-SAHPWH) is described, which can supply hot water for domestic use during the whole year. The system mainly employs a bare flat-plate collector/evaporator with a surface area of 4.2 m², an electrical rotary-type hermetic compressor, a hot water tank with the volume of 150 L and a thermostatic expansion valve. R-22 is used as working fluid in the system. A simulation model based on lumped and distributed parameter approach is developed to predict the thermal performance of the system. Given the structure parameters, meteorological parameters, time step and final water temperature, the numerical model can output operational parameters, such as heat capacity, system COP and collector efficiency. Comparisons between the simulation results and the experimental measurements show that the model is able to give satisfactory predictions. The effect of various parameters, including solar radiation, ambient temperature, wind speed and compressor speed, has been analyzed on the thermal performance of the system.     

Exergetic analysis of transcritical CO2 residential air-conditioning system based on experimental data

The experimental system for the transcritical CO₂ residential air-conditioning with an internal heat exchanger was built. The effects of working conditions on system performance were experimentally studied. Based on the experimental dada, the second law analysis on the transcritical CO₂ system was performed. The effects of working conditions on the total exergetic efficiency of the system were investigated. The results show that in the studied parameter ranges, the exergetic efficiency of the system increases with the increases of gas cooler side air inlet temperature, gas cooler side air inlet velocity and evaporating temperature. And it will decrease with the increases of evaporator side air inlet temperature and velocity. Then, a complete exergetic analysis was performed for the entire CO₂ transcritical cycle including compressor, gas cooler, expansion valve, evaporator and internal heat exchanger under different working conditions. The average exergy loss in gas cooler is the highest one under all working conditions which is about 30.7% of the total exergy loss in the system. The second is the average exergy loss in expansion valve which is about 24.9% of the total exergy loss, followed by the exergy losses in evaporator and compressor, which account for 21.9% and 19.5%, respectively. The exergy loss in internal heat exchanger is the lowest one which is only about 3.0%. So in the optimization design of the transcritical CO₂ residential air-conditioning system more attentions should be paid to the gas cooler and expansion valve.     

Performance characteristics of integral type solar-assisted heat pump

The characteristic of an integral type solar-assisted heat pump water heater (ISAHP) is investigated in the present study. The ISAHP consists of a Rankine refrigeration cycle and a thermosyphon loop that are integrated together to form a package heater. Both solar and ambient air energies are absorbed at the collector/evaporator and pumped to the storage tank via a Rankine refrigeration cycle and a thermosyphon heat exchanger. The condenser releases condensing heat of the refrigerant to the water side of the thermosyphon heat exchanger for producing a natural-circulation flow in the thermosyphon loop. A 105-liter ISAHP using a bare collector and a small R134a reciprocating-type compressor with rated input power 250 W was built and tested in the present study. The ISAHP was designed to operate at an evaporating temperature lower than the ambient temperature and a matched condition (near saturated vapor compression cycle and compressor exhaust temperature <100°C). A performance model is derived and found to be able to fit the experimental data very well for the ISAHP. The COP for the ISAHP built in the present study lies in the range 2.5–3.7 at water temperature between 61 and 25°C.     

Experiments on a solar-assisted heat pump and an exergy analysis of the system

In this work a experimental study of a solar assisted heat pump with direct expansion of the refrigerant within the solar collector, is presented. The maximum exergy efficiency, defined as the ratio of the outlet to the inlet exergy flow in every component of the heat pump cycle, is determined taking into account the typical parameters and performance coefficients. The results of this exergy analysis point out that the main source of irreversibility can be found in the evaporator of the heat pump (that is, the solar collector) emphasizing that incoming solar radiation is not used to full advantage in this piece of equipment.     

Cooling characteristics of ground source heat pump with heat exchange methods

The objective of this study is to investigate the influence of the cooling performance for a water-to-water ground source heat pump (GSHP) by using the counter flow and parallel flow methods. The GSHP uses R-410A as a refrigerant, and its main components are a scroll compressor, plate heat exchangers as a condenser, an evaporator, a thermostatic expansion valve, a receiver, and an inverter. Based on our modeling results, the heat transfer rate of the counter flow evaporator is higher than that of the parallel flow evaporator for a heat exchanger length greater than 0.42 m. The evaporator length of the GSHP used in this study was set to over 0.5 m. The performance of the water-to-water GSHP was measured by varying the compressor speed and source-side entering water temperature (EWT). The cooling capacity of the GSHP increased with increased compressor RPMs and source side EWT. Also, using the counter flow method, compared to the parallel flow method, improves the COP by approximately 5.9% for an ISO 13256-2 rated condition.     

PERFORMANCE OF A HEAT PUMP USING DIRECT EXPANSION SOLAR COLLECTORS

Theoretical and experimental studies were made on the thermal performance of a heat pump that used a bare flat-plate collector as the evaporator. The analysis used empirical equations to express the electric power consumption of the compressor and coefficient of performance (COP), as functions of temperature of evaporation at the evaporator and that of the heat transfer medium (water) at the inlet of the condenser. The experimental heat pump had a compressor with a rated capacity of 350 W and collectors with the total area of 3.24 m². Around noon in winter the evaporator temperature was found to be about 17°C higher than the ambient air temperature of 8°C, and a COP of about 5.3 was obtained when the water temperature at the condenser inlet was 40°C. These measured evaporation temperatures and COPs were in good agreement with those predicted by the analysis. According to the analysis, the total area of the collectors in the experiment was appropriate for the heat pump system. Also, the 1-mm thickness of the collector's copper plate used in the experiment could be 0.5 mm with little reduction of COP. The pitch of the tube soldered to the copper plate for the refrigerant flow was 100 mm in the experiment, but the COP would only be reduced by about 4% if the pitch were changed to 190 mm.     

Transcritical CO2 heat pump systems: exergy analysis including heat transfer and fluid flow effects

This paper presents the exergetic analysis and optimization of a transcritical carbon dioxide based heat pump cycle for simultaneous heating and cooling applications. A computer model has been developed first to simulate the system at steady state for different operating conditions and then to evaluate the system performance based on COP as well as exergetic efficiency, including component wise irreversibility. The chosen system includes the secondary fluids to supply the heating and cooling services, and the analyses also comprise heat transfer and fluid flow effects in detail. The optimal COP and the exergetic efficiency were found to be functions of compressor speed, ambient temperature and secondary fluid temperature at the inlets to the evaporator and gas cooler and the compressor discharge pressure. An optimization study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on heat transfer area has been conducted. The exergy flow diagram (Grassmann diagram) shows that all the components except the internal heat exchanger contribute significantly to the irreversibilities of the system. Unlike a conventional system, the expansion device contributes significantly to system irreversibility. Finally, suggestions for various improvement measures with resulting gains have been presented to attain superior system performance through reduced component irreversibilities. This study is expected to offer useful guidelines for system design and its optimisation and help toward energy conservation in heat pump systems based on transcritical CO₂ cycles.     

Exergy analysis of a passive solar still

This paper presents a steady-state and transient theoretical exergy analysis of a solar still, focused on the exergy destruction in the components of the still: collector plate, brine and glass cover. The analytical approach states an energy balance for each component resulting in three coupled equations where three parameters—solar irradiance, ambient temperature and insulation thickness—are studied. The energy balances are solved to find temperatures of each component; these temperatures are used to compute energy and exergy flows. Results in the steady-state regime show that the irreversibilities produced in the collector account for the largest exergy destruction, up to 615 W/m² for a 935 W/m² solar exergy input, whereas irreversibility rates in the brine and in the glass cover can be neglected. For the same exergy input a collector, brine and solar still exergy efficiency of 12.9%, 6% and 5% are obtained, respectively. The most influential parameter is solar irradiance. During the transient regime, irreversibility rates and still temperatures find a maximum 6 h after dawn when solar irradiance has a maximum value. However, maximum exergy brine efficiency, close to 93%, is found once T_col<T_w (dusk) and the heat capacity of the brine plays an important role in the modeling of collector–brine interaction. Nocturnal distillation is characterized by very low irreversibility rates due to reduced temperature difference between collector and an increase in exergy efficiency towards dawn due to ambient temperature decrease.     

Utilizing the multi-objective particle swarm optimization for designing a renewable multiple energy system on the basis of the parabolic trough solar collector

Today, to preserve fossil resources, mankind has to search for new ways to respond to its ever-increasing energy needs. In this study, a hybrid system of energy and the use of a parabolic trough solar collector to attract solar radiation was investigated to produce clean electricity, cooling, and hydrogen from thermodynamic and economic aspects. The designed system consisted of a parabolic trough solar collector, organic Rankine cycle, lithium-bromide absorption refrigeration cycle, and proton exchange membrane electrolysis system. The evaporator input temperature, turbine inlet temperature, solar radiation intensity, mass flow rate of collector and parabolic trough collector surface area were set as decision variables and the effect of these parameters on system performance and system exergy loss were investigated. The objective functions of this research were exergy efficiency and cost rate. In order to optimize this system, multi-objective particle swarm optimization algorithm was employed. Optimization results with particle swarm optimization indicated that the best rate of exergy efficiency is 3.12% and the system cost rate is 16.367 US$ per hour, at the same time. The system is capable of producing 15.385 kW power, 0.189 kg/day hydrogen and 56.145 kW cooling in its optimum condition. The results of sensitivity analysis showed that increasing mass flow rate at the collector, temperature at the evaporator inlet, and temperature at the turbine inlet have positive effect on the performance of the proposed system.     

Transient simulation of a capacity-modulated, direct-expansion, solar-assisted heat pump

The long-term thermal performance of a direct-expansion, solar-assisted heat pump is determined from the transient simulation of the system. The system employs a bare collector that also acts as the heat pump evaporator. Of particular interest in this study is the configuration in which the compressor and the collector area are properly matched from the long-term thermal performance point of view. This matching is achieved through multistep as well as two-step compressor capacity modulation. In addition to examining the effects of compressor capacity modulation, the effects of various system parameters such as collector area, storage volume, load temperature, wind speed, collector slope, and refrigerant properties are also studied in detail.Monthly averaged thermal performance parameters such as the heat pump system coefficient of performance are determined by executing a computer simulation program that uses the typical meterological year (TMY) solar data for Norfolk, Virginia. Results indicate that the system performance is governed strongly by collector area, compressor RPM, load temperature, and refrigerant properties. The remaining parameters have only weak influence on the long-term system performance of direct expansion solar-assisted heat pump (SAHP) system considered in this study.     

直膨式太阳能热泵微通道集热/蒸发器制冷剂分布特性

为研究微通道集热/蒸发器内制冷剂分布及对直膨式太阳能热泵系统性能的影响,搭建以丙烷（R290）为制冷剂的系统实验平台。基于实验数据,提出一种利用红外成像技术分析微通道集热/蒸发器内两相态制冷剂分布的方法,获得了电子膨胀阀开度、太阳辐射强度以及环境温度对集热/蒸发器内两相态制冷剂分布情况的影响特性。结果表明：当电子膨胀阀开度由20%增至60%时,集热/蒸发器的制冷剂分布参数（RDP）提高10.6%,系统性能系数（COP）从2.8升至5.5。较高的太阳辐射强度或环境温度可有效避免制冷剂回流现象。     

太阳能热泵多功能复合机系统数学模型

简要介绍了太阳能热泵多功能复合机系统的组成及工作原理,建立了系统各主要部件的数学模型。根据系统各部件的特点,用分布参数法建立了太阳能集热器的动态参数模型,用分相集中参数法建立了冷凝器和蒸发器的数学模型,用集中参数法建立了压缩机和电子膨胀阀的数学模型,并给出了模型的求解方法,为下一步计算机仿真奠定了基础。     

Exergy assessment of an optimized capillary tube‐based transcritical CO2 heat pump system

A capillary tube‐based CO₂ heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. A comparative study for various operating conditions, based on system COP and exergetic efficiency, of a capillary tube and a controllable expansion valve‐based transcritical carbon dioxide heat pump systems for simultaneous heating and cooling at 73 and 4°C, respectively, is presented here. Two optimized capillary tubes having diameter of 1.5 and 1.6 mm are compared with an equivalent controllable throttle valve. Heat transfer and fluid flow effects are included in the gas cooler and evaporator model and capillary tube employs the homogeneous flow model to simulate two‐phase flow. Subcritical and supercritical thermodynamic and transport properties of CO₂ are calculated employing a precision in‐house property code. Optimization of effective distribution of total heat exchanger area ratio between gas cooler and evaporator is investigated. The exergetic efficiency is better in case of the capillary tube than that of a controllable throttle valve‐based system. Capillary tube‐based system is shown to be quite flexible regarding changes in ambient temperature, almost behaving to offer an optimal pressure control just like the controllable expansion valve yielding both, maximum system COP and maximum exergetic efficiency. Relatively at a smaller diameter, the capillary tube exhibits better exergetic efficiency. Capillary tube length is the critical parameter that influences system optimum conditions. The exergy flow diagram exhibits that compressor, gas cooler and capillary tube contribute a larger share, in that order, to system irreversibility. It is fairly established in this study that a capillary tube can be a good engineering option for small capacity systems in lieu of an expansion valve, which has been thought of as the only possible solution to attain the pressure optimization, an important feature of all transcritical CO₂ systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermodynamic study of a simplified model of the solar chimney power plant

A simplified model of solar chimney power plant (SCPP) consists of a heating air collector, turbine and chimney. Thermodynamic interpretation of processes occurring in these SCPP components is based on the derived energy and exergy balances. The examples of the energy and exergy flow diagrams show how the SCPP input of 36.81 MW energy of solar radiation, corresponding to 32.41 MW input of radiation exergy, is distributed between the SCPP components. Responsive trends to the varying input parameters are studied. Additionally, the concept of mechanical exergy (ezergy) of air is applied and it allowed for quantitative determination of the effect attributed to the terrestrial gravity field on the component processes of the SCPP.     

Thermal performance of a direct expansion solar-assisted heat pump

A direct expansion solar assisted heat pump, in which a bare flat plate collector also acts as the evaporator for the refrigerant, Freon-12, is designed and operated. The system components, e.g. the collector and the compressor, are properly matched so as to result in system operating conditions wherein the collector/evaporator temperature ranges from 0 to 10°C above ambient temperature under favorable solar conditions. This operating temperature range is particularly favorable to improved heat pump and solar collector performance. The system thermal performance is determined by measuring refrigerant flow rate, temperature and pressure at various points in the system. The heat pump COP_H and the solar collector efficiency ranged from 2.0 to 3.0 and from 40 to 70 per cent, respectively, for widely ranging ambient and operating conditions. Experimental results indicate that the proposed system offers significant advantage in terms of superior thermal performance when compared with results gotten by replacing the solar evaporator with a standard outdoor fan-coil unit.     

基于平板热管的太阳能-空气能双源集热蒸发器性能实验研究

设计一种基于平板热管的太阳能-空气能双源集热蒸发器及由其组成的新型直膨式热泵系统,并对其进行实验研究与分析。实验测试平板热管在制冷剂低温取热条件下的均温性与导热性能,热泵运行工况下集热蒸发器表面温度分布、光电光热性能,以及在不同天气条件不同运行模式下热泵系统性能。结果表明,平板热管在低温取热条件下当量导热系数可达6.8×10⁵W/(m·℃),集热蒸发器运行时纵向最大温差为3.9℃;在夏季晴朗天气条件下运行太阳能模式制热水时热泵平均COP为3.62;在低辐照阴天下运行太阳能-空气能双源模式与太阳能模式相比,单位面积集热功率提高18.8%,系统平均COP提高5.7%;在无辐照的夜晚,运行空气源模式系统COP为2.54。     

Exergetic performance assessment of a variable‐speed R404a refrigeration system

The goal of this study is to carry out exergy analyses for an experimental variable‐speed refrigeration system working with R404a in order to determine irreversibility rates and exergetic efficiencies of system components and the overall system. For this aim, an experimental refrigeration system was designed with a frequency inverter mounted on compressor electric motor. Controlling the rotational speed of the compressor with a frequency inverter is one of the best methods to vary the capacity of the refrigeration system. The experiments were made for different compressor electric motor frequencies. The results showed that at low‐frequency values, irreversibility rates of the system decreased and exergetic efficiencies were increased. In addition, the major irreversibility occurs in the compressor by 61.47–61.83% followed by condenser by 17.00–16.52%, evaporator by 12.39–13.73% and expansion valve by 6.24–6.76% for different compressor frequencies. Copyright © 2009 John Wiley & Sons, Ltd.     

A thermoeconomic model of a photovoltaic heat pump

In this paper the model of a heat pump whose evaporator operates as a photovoltaic collector, is studied. The energy balance equations have been used for some heat pump components, and for each layer of the photovoltaic evaporator: covering glaze, photovoltaic modules, thermal absorber plate, refrigerant tube and insulator. The model has been solved by means of a program using proper simplifications. The system input is represented by the solar radiation intensity and the environment temperature, that influence the output electric power of the photovoltaic modules and the evaporation power. The model results have been obtained referring to the photovoltaic evaporator and the plant operating as heat pump, in terms of the photovoltaic evaporator layers temperatures, the refrigerant fluid properties values in the cycle fundamental points, the thermal and mechanical powers, the efficiencies that characterize the plant performances from the energy, exergy and economic point of view. This study allows to realize a thermoeconomic comparison between a photovoltaic heat pump and a traditional heat pump under the same working conditions.     

太阳能-空气复合热源热泵热水器的性能模拟与分析

介绍了一种新型太阳能—空气复合热源热泵热水装置(SAS-HPWH)。该装置通过使用独特设计的螺旋翅片蒸发管的平板型集热/蒸发器,可以在不同的天气情况下切换运行太阳能热源热泵模式、太阳能与空气双热源热泵模式和空气源热泵模式,制取生活热水。论文主要针对自行设计的一台150L的SAS-HPWH,建立系统的数学模型,并以太阳能输入比例为准则研究系统的运行模式与特性。模拟结果显示该热水器在不同天气特征情况下可高效率地制造55℃热水。论文还分析了太阳辐射、环境温度以及压缩机的容量对系统特性的影响,提出使用变频压缩机,根据不同的天气情况调节制冷剂流量,进一步提高系统的整体性能。