Investigation of thermal performance of a ground coupled heat pump system with a cylindrical energy storage tank

An analytical and computational model for a solar assisted heat pump heating system with an underground seasonal cylindrical storage tank is developed. The heating system consists of flat plate solar collectors, an underground cylindrical storage tank, a heat pump and a house to be heated during winter season. Analytical solution of transient field problem outside the storage tank is obtained by the application of complex finite Fourier transform and finite integral transform techniques. Three expressions for the heat pump, space heat requirement during the winter season and available solar energy are coupled with the solution of the transient temperature field problem. The analytical solution presented can be utilized to determine the annual variation of water temperature in the cylindrical store, transient earth temperature field surrounding the store and annual periodic performance of the heating system. A computer simulation program is developed to evaluate the annual periodic water and earth temperatures and system performance parameters based on the analytical solution. Copyright © 2003 John Wiley & Sons, Ltd.     

Analysis of solar aided heat pump systems with seasonal thermal energy storage in surface tanks

Annual periodic performance of a solar assisted ground-coupled heat pump space heating system with seasonal energy storage in a hemispherical surface tank is investigated using analytical and computational methods. The system investigated employs solar energy collection and dumping into a seasonal surface tank throughout the whole year with extraction of thermal energy from the tank for space heating during the winter season. A computational model is presented in this study for the prediction of the annual periodic transient behaviour of the system under investigation. The present computational model is based on a hybrid analytical–numerical procedure which facilitates determination of the annual variation of water temperature in the surface tank, the amounts of solar thermal energy collected during each month and the annual periodic performance of the solar aided space heating system.     

Thermal performance of a solar-assisted heat pump drying system with thermal energy storage tank and heat recovery unit

Thermal performance parameters for a solar-assisted heat pump (SAHP) drying system with underground thermal energy storage (TES) tank and heat recovery unit (HRU) are investigated in this study. The SAHP drying system is made up of a drying unit, a heat pump, flat plate solar collectors, an underground TES tank, and HRU. An analytical model is developed to obtain the performance parameters of the drying system by using the solution of heat transfer problem around the TES tank and energy expressions for other components of the drying system. These parameters are coefficient of performances for the heat pump (COP) and system (COPs), specific moisture evaporation rate (SMER), temperature of water in the TES tank, and energy fractions for energy charging and extraction from the system. A MATLAB program has been prepared using the expressions for the drying system. The obtained results for COP, COPs, and SMER are 5.55, 5.28, and 9.25, respectively, by using wheat mass flow rate of 100 kg h⁻¹, Carnot efficiency of 40%, collector area of 100 m², and TES tank volume of 300 m³ when the system attains periodic operation duration in fifth year onwards for 10 years of operation. Annual energy saving is 21.4% in comparison with the same system without using HRU for the same input data.     

Negative buoyant plume model for solar domestic hot water tank systems incorporating a vertical inlet

Thermal stratification in solar energy storage tanks plays an important role in enhancing the performance of solar domestic hot water systems. The mixing that occurs when hot fluid from the solar collector enters the top of the tank is detrimental to the stratification. Mathematical models that are used for system analysis must therefore be able to capture the effects of this inlet jet mixing in order to accurately predict system performance. This paper presents a computational study of the heat transfer and fluid flow in a thermal storage tank of a solar domestic hot water system with a vertical inlet under negative buoyant plume conditions. The effects of parameters such as the fluid inlet velocity and temperature as well as inlet pipe diameter on the thermal mixing were considered. The work culminated in the development of a one-dimensional empirical model capable of predicting the transient axial temperature distribution inside the thermal storage tank. Predictions from the new model were in good agreement with both experimental data and detailed computational fluid dynamics predictions.     

Optimal flow control of a forced circulation solar water heating system with energy storage units and connecting pipes

This paper focuses on pump flow rate optimization for forced circulation solar water heating systems with pipes. The system consists of: an array of flat plate solar collectors, two storage tanks for the circulation fluid and water, a heat exchanger, two pumps, and connecting pipes. The storage tanks operate in the fully mixed regime to avoid thermal stratification. The pipes are considered as separated components in the system so as to account for their thermal effects. The objective is to determine optimal flow rates in the primary and secondary loops in order to maximize energy transfer to the circulation fluid storage tank, while reaching user defined temperatures in the water storage tank to increase thermal comfort. A model is developed using mainly the first and second laws of thermodynamics. The model is used to maximize the difference between the energy extracted from the solar collector and the combined sum of the energy extracted by the heat exchanger and corresponding energies used by the pumps in the primary and secondary loops. The objective function maximizes the overall system energy gain whilst minimizing the sum of the energy extracted by the heat exchanger and corresponding pump energy in the secondary loop to conserve stored energy and meet the user requirement of water tank temperatures. A case study is shown to illustrate the effects of the model. When compared to other flow control techniques, in particular the most suitable energy efficient control strategy, the results of this study show a 7.82% increase in the amount of energy extracted. The results also show system thermal losses ranging between 5.54% and 7.34% for the different control strategies due to connecting pipe losses.     

分区蓄热水箱应用于太阳能热泵中的蓄放热分析

针对冬季太阳能辐射弱且不稳定的特点,提出一种应用于太阳能热泵的分区蓄热水箱,以水泵驱动蓄热水箱循环区与蓄热区的热量传递,并运用热力学原理对水箱循环区与蓄热区的运行状况进行模拟,分析了水箱两区在不同水泵体积流量下的逐时温度变化并与整体式水箱进行对比。结果表明,分区蓄热水箱克服了整体式水箱的热惰性,启动灵活,能在较短时间内达到热泵运行的理想温度,显著提高了系统的性能。     

Performance of solar assisted heatpump systems in residential applications

In this experimental study, several solar-assisted heating and cooling configurations have beenconsidered for a basic system comprised of a two-speed heat pump, photovoltaic (PV) arrays, solar thermal collectors, and thermal storage. The objective of the study was to determine the performance of the PV arrays at decreased insolation, the effects of air preheat by solar thermal energy on heat pump operation, and cooling system performance under two different configurations. During the entire operation, the PV arrays converted 4.7 per cent (9.5 MWh) of the incident solar insolation to d.c. power, of which 54.6 per cent was used by the residence. This contributed 23.4 per cent of the total house electrical demand. The remaining 45.4 per cent of the output was fed to the utility, indicating the arrays and the heat pump were not properly sized with each other. Based on results from the winter heating operation, it is shown that for the particular heating system consdered, the best performance is attained when the solar heating is used alone. By using the heat pump as a booster, the remaining available solar energy left in the storage tank can be used with good seasonal performance factor. Summer cooling operation consisted of two sequential cooling configurations. In the first cooling test, the heat pump was operated to either the house or storage when the PV array generation level was greater than the energy demand of the heat pump and associated equipment. When the array output level was less than the cooling system demand, the operating strategy was that of an off-peak cooling operation to chill the water storage. Utilization of chilled water storage was not realized in the first cooling test because of the inherent inefficient design of the Tri-X coil. The capacity at low-speed heat pump operation was too small to effect significant cooling of the water loop; whereas high-speed heat pump operation in attempting to chill water (fan operation absent) caused frosting of the coil. The heat pump was utilized only to maintain chilled water storage in the second cooling test, without heat transfer through the Tri-X coil. Cooling system performance obtained in cooling test 2 using the Ametex exchanger was considerably improved over the test 2 performance with the Tri-X coil.     

Thermal performance of a solar-aided latent heat store used for space heating by heat pump

In this study, the cylindrical phase change storage tank linked to a solar powered heat pump system is investigated experimentally and theoretically. A simulation model defining the transient behaviour of the phase change unit was used. In the tank, the phase change material (PCM) is inside cylindrical tubes and the heat transfer fluid (HTF) flows parallel to it. The heat transfer problem of the model (treated as two-dimensional) was solved numerically by an enthalpy-based finite differences method and validated against experimental data. The experiments were performed from November to May in the heating seasons of 1992–1993 and 1993–1994 to measure both the mean temperature of water within the tank and the inlet and outlet water temperature of the tank. The experimentally obtained inlet water temperatures are also taken as inlet water temperature of the simulated model. Thus, theoretical temperature and stored heat energy distribution within the tank have been determined. Solar radiation and space heating loads for the heating seasons mentioned above are also presented.     

正交各向异性相变材料的无网格法传热模型及应用

利用无网格迦辽金(EFG)法建立正交各向异性相变材料的传热计算模型,基于该模型编程完成各向异性材料太阳能相变蓄热水箱和管壳式相变蓄热单元的相变传热分析,并探讨热导率因子和材料方向角对复合材料相变传热特性的影响.研究表明:在相同节点布置下EFG法的温度场和相界面计算精度均高于有限元法,EFG法在动态相界面追踪方面具有明显...     

Experimental and theoretical investigation of a solar heating system with heat pump

In this study, the performance of a solar heating system with a heat pump was investigated both experimentally and theoretically. The experimental results were obtained from November to April during the heating season. The experimentally obtained results are used to calculate the heat pump coefficient of performance (COP), seasonal heating performance, the fraction of annual load met by free energy, storage and collector efficiencies and total energy consumption of the systems during the heating season. The average seasonal heating performance values are 4.0 and 3.0 for series and parallel heat pump systems, respectively. A mathematical model was also developed for the analysis of the solar heating system. The model consists of dynamic and heat transfer relations concerning the fundamental components in the system such as solar collector, latent heat thermal energy storage tank, compressor, condenser, evaporator and meteorological data. Some model parameters of the system such as COP, theoretical collector numbers (N_c), collector efficiency, heating capacity, compressor power, and temperatures (T₁, T₂, T₃, T_T) in the storage tank were calculated by using the experimental results. It is concluded that the theoretical model agreed well with the experimental results.     

Experimental Analysis of a Solar Energy Storage Heat Pump System

This paper introduces a novel solar-assisted heat pump system with phase change energy storage and describes the methodology used to analyze the performance of the proposed system. A mathematical model was established for the key parts of the system including solar evaporator, condenser, phase change energy storage tank, and compressor. In parallel to the modelling work, an experimental set-up of the proposed solar energy storage heat pump system was developed. The experimental data showed that the designed system is capable of meeting cold day heating demands in rural areas of Yanbian city located in Jilin province of China. In day-time operation, the solar heat pump system stores excess energy in the energy storage tank for heating purposes. A desired indoor temperature was achieved; the average coefficient of performance of solar heat pump was identified as 4.5, and the system showed a stable performance throughout the day. In night-time operation, the energy stored in the storage tank was released through a liquid-solid change of phase in the employed phase-change material. In this way, the provision of continuous heat for ten hours was ensured within the building, and the desired indoor air conditions were achieved.     

Modeling and optimization of a solar assisted heat pump using ice slurry as a latent storage material

This paper presents the modeling and optimization of a solar assisted heat pump using ice slurry. Solar collectors are used as the primary source of thermal energy, with two distinct loops allowing the collectors to operate in series with an ice tank, or a warm water tank. Thermal energy stored in the ice tank is transferred to a warm water distribution tank via a heat pump. First, a new mathematical model of an ice slurry storage tank is presented. Validation of the model with experimental data confirms its ability to predict the ice mass and tank fluid temperatures during the charging and discharging modes of operation. The developed ice tank model is combined with the TRNSYS energy simulation program to formulate a complete model of the proposed heat pump system. This computer model then serves as a base for a mathematical optimization with the objective to minimize the energy use for heating and DHW over a single heating season. Simulated results demonstrate the potential of the optimized system in reducing the heating operating energy use of a high performance home in Montreal, QC.     

Modeling of the CSU heating/cooling system

This paper resents a thermal simulation of the Colorado State University solar house. A computer model of the solar energy system was developed and computer runs were made using one year of meteorological data to determine the important design features. The system consists of a flat plate solar collector, main storage tank, service hot water storage tank, auxiliary heater, absorption air conditioner with cooling tower and heat exchangers between the collector and storage, storage and service hot water tank and storage and residence. This system very closely models the CSU house in operating mode one.The results are in the form of monthly integrated values for the pertinent energy quantities. In addition, results are presented which show the effect on the system performance of the collector tilt, collector area and number of covers.     

Modeling, design and thermal performance of a BIPV/T system thermally coupled with a ventilated concrete slab in a low energy solar house: Part 1, BIPV/T system and house energy concept

This paper is the first of two papers that describe the modeling, design, and performance assessment based on monitored data of a building-integrated photovoltaic-thermal (BIPV/T) system thermally coupled with a ventilated concrete slab (VCS) in a prefabricated, two-storey detached, low energy solar house. This house, with a design goal of near net-zero annual energy consumption, was constructed in 2007 in Eastman, Québec, Canada - a cold climate area. Several novel solar technologies are integrated into the house and with passive solar design to reach this goal. An air-based open-loop BIPV/T system produces electricity and collects heat simultaneously. Building-integrated thermal mass is utilized both in passive and active forms. Distributed thermal mass in the direct gain area and relatively large south facing triple-glazed windows (about 9% of floor area) are employed to collect and store passive solar gains. An active thermal energy storage system (TES) stores part of the collected thermal energy from the BIPV/T system, thus reducing the energy consumption of the house ground source heat pump heating system. This paper focuses on the BIPV/T system and the integrated energy concept of the house. Monitored data indicate that the BIPV/T system has a typical efficiency of about 20% for thermal energy collection, and the annual space heating energy consumption of the house is about 5% of the national average. A thermal model of the BIPV/T system suitable for preliminary design and control of the airflow is developed and verified with monitored data.     

Experimental investigation on a combined sensible and latent heat storage system integrated with constant/varying (solar) heat sources

The objective of the present work is to investigate experimentally the thermal behavior of a packed bed of combined sensible and latent heat thermal energy storage (TES) unit. A TES unit is designed, constructed and integrated with constant temperature bath/solar collector to study the performance of the storage unit. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the constant temperature bath/solar collector to the TES tank also acts as sensible heat storage (SHS) material. Charging experiments are carried out at constant and varying (solar energy) inlet fluid temperatures to examine the effects of inlet fluid temperature and flow rate of HTF on the performance of the storage unit. Discharging experiments are carried out by both continuous and batchwise processes to recover the stored heat. The significance of time wise variation of HTF and PCM temperatures during charging and discharging processes is discussed in detail and the performance parameters such as instantaneous heat stored and cumulative heat stored are also studied. The performance of the present system is compared with that of the conventional SHS system. It is found from the discharging experiments that the combined storage system employing batchwise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.     

Energy and exergy analyses of an ice-on-coil thermal energy storage system

In this study, energy and exergy analyses are carried out for the charging period of an ice-on-coil thermal energy storage system. The present model is developed using a thermal resistance network technique. First, the time-dependent variations of the predicted total stored energy, mass of ice, and outlet temperature of the heat transfer fluid from a storage tank are compared with the experimental data. Afterward, performance of an ice-on-coil type latent heat thermal energy storage system is investigated for several working and design parameters. The results of a comparative study are presented in terms of the variations of the heat transfer rate, total stored energy, dimensionless energetic/exergetic effectiveness and energy/exergy efficiency. The results indicate that working and design parameters of the ice-on-coil thermal storage tank should be determined by considering both energetic and exergetic behavior of the system. For the current parameters, storage capacity and energy efficiency of the system increases with decreasing the inlet temperature of the heat transfer fluid and increasing the length of the tube. Besides, the exergy efficiency increases with increasing the inlet temperature of the heat transfer fluid and increasing the length of the tube.     

Optimal sizing of a heat pump/thermal storage system based on the linear programming method

An optimal planning method is proposed for a heat pump/thermal storage system that utilizes time-of-use pricing of the electrical utility. Equipment capacities are determined so as to minimize the annual total cost in consideration of system's operational strategies for energy demand requirements. This optimal planning problem is solved by the linear programming method. Through a numerical study on a heat pump/thermal storage system for a commercial building, the effect of thermal storage tank is investigated on the long-term economics of the system. A parametric study is also performed with respect to the initial capital unit cost of thermal storage tank. The relation is clarified among the optimal capacities of thermal storage tank and other pieces of equipment. It is ascertained that this optimal planning method is a useful tool for evaluating the economic properties of heat pump/thermal storage systems.     

Solar multi-mode heating system based on latent heat thermal energy storage and its application

为克服太阳能间断性和不稳定性的缺点进而实现太阳能集热与采暖的能量供需调节和全天候连续供热,提出了基于相变储热的太阳能多模式采暖方法（太阳能集热直接采暖、太阳能集热采暖+相变储热、太阳能相变储热采暖）,并在西藏林芝市某建筑搭建了太阳能与相变储热相结合的采暖系统,该系统可根据太阳能集热温度和外界供热需求实现太阳能多模式采暖的自动控制和自动运行。实验研究表明：在西藏地区采用真空管太阳能集热器可以和中低温相变储热器很好地结合,白天储热器在储热过程中平均储热功率为10.63 kW,储热量达到92.67 kW·h,相变平台明显;晚上储热器在放热过程中供热量达85.23 kW·h,放热功率和放热温度平稳,储放热效率达92%,其储热密度是传统水箱的3.6倍,可连续供热时间长达10 h,从而实现了基于相变储热的太阳能全天候连续供热,相关研究结果对我国西藏地区实施太阳能采暖具有一定的指导作用。     

Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector

Analysis of energy and exergy has been performed for a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. CaCl₂·6H₂O was used as PCM in thermal energy storage (TES) system. The designed collector combines in single unit solar energy collection and storage. PCMs are stored in a storage tank, which is located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM. Exergy analysis, which is based on the second law of thermodynamics, and energy analysis, which is based on the first law, were applied for evaluation of the system efficiency for charging period. The analyses were performed on 3 days in October. It was observed that the average net energy and exergy efficiencies are 45% and 2.2%, respectively.     

Modeling and optimization of a solar driven membrane distillation desalination system

The desalination technology using membrane distillation driven by solar energy is a feasible solution for reducing the energy cost. A dynamic simulation model for a solar driven membrane distillation desalination system (SMDDS) is developed on the Aspen Custom Modeler^® (ACM) platform for the system performance and optimization study. The rigorous model for the spiral-wound air gap membrane distillation (SP-AGMD) module takes into account the heat and mass transfer resistances associated with each composing layer. The effects of adopting different objective functions, solar radiation conditions, thermal storage tank configurations, as well as the flowrates of the membrane distillation module and the thermal storage tank on the optimized performance are reported. Simple thermal storage tank and lower flowrate of the membrane distillation module are advantageous to higher water production rate. A control system using conventional PI (Proportional/Integral) controllers is proposed and the water production rate can reach about 87% of the optimal result for clear sky operation.