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.     

Energy analysis and modeling of a solar assisted house heating system with a heat pump and an underground energy storage tank

An analytical model is presented and analyzed to predict the long term performance of a solar assisted house heating system with a heat pump and an underground spherical thermal energy storage tank. The system under investigation consists of a house, a heat pump, solar collectors and a storage tank. The present analytical model is based on a proper coupling of the individual energy models for the house, the heat pump, useful solar energy gain, and the transient heat transfer problem for the thermal energy storage tank. The transient heat transfer problem outside the energy storage tank is solved using a similarity transformation and Duhamel’s superposition principle. A computer code based on the present model is used to compute the performance parameters for the system under investigation. Results from the present study indicate that an operational time span of 5–7 years will be necessary before the system under investigation can attain an annually periodic operating condition. Results also indicate a decrease in the annually minimum value of the storage tank temperature with a decrease in the energy storage tank size and/or solar collector area.     

Experimental investigation of a solar energy heating system under the climatic conditions of Edirne

Seasonal storage of solar energy to supply the heat requirement of buildings in Edirne (41°39′54″N) has been examined experimentally. Solar energy has been stored in a cylindrical underground storage unit. Measurement values have been recorded per hour by means of a computerized recorder between July 2005 and May 2006. Monthly average temperature values of the heat storage unit and the surrounding ground have been calculated through the measurement results. The transient heat transfer which takes place between the heat storage unit and the surrounding ground has been calculated by means of the QuickField finite-element analysis program. It has been determined that the most significant deviations between the theoretical and the experimental temperature values turn out to be in question during the heating period. The annual solar fraction of the solar energy heating system has been determined as 53% for space heating and 85% for domestic water heating.     

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.     

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.     

A numerical study on heat storage water tank containing phase change materials for air source heat pump systems

构建空气源热泵-相变蓄热水箱供暖系统,通过相变储能技术的合理应用,优化了太阳能、空气热能等非连续能源的供能方式,有效提高了建筑中可再生能源的利用率。相变蓄热系统采用了6 m³的保温水箱作为蓄热容器,选取46#石蜡为主要相变材料,304#不锈钢管为封装材料。建立蓄热系统的三维数学模型,采用有效热熔法对相变材料的焓值进行处理,运用Fluent数值模拟软件,研究相变蓄热系统的蓄放热性能。模拟结果显示,系统的蓄热时间为9.2 h,理想蓄热量为102.4 kW·h,能够单独提供低能耗建筑连续采暖11.1 h。空气源热泵-相变蓄热水箱供暖系统能实现大跨度的间歇供暖,在利用非连续能源供暖领域具有良好的前景。     

Investigation of a combined solar–heat pump system for residential heating. Part 1: experimental results

In order to investigate the performance of the combined solar–heat pump system with energy storage in encapsulated phase change material (PCM) packings for residential heating in Trabzon, Turkey, an experimental set‐up was constructed. The experimental results were obtained from November to May during the heating season for two heating systems. These systems are a series of heat pump system, and a parallel heat pump system. The experimentally obtained results are used to calculate the heat pump coefficient of performance (COP), seasonal heating performance, the fraction of annual load meet by free energy, storage and collector efficiencies and total energy consumption of the systems during the heating season. Copyright © 1999 John Wiley & Sons, Ltd.     

A computational model of a domestic solar heating system with underground spherical thermal storage

This theoretical study deals with a domestic heating system assisted by solar energy stored in an underground spherical container. The system includes a heat pump. The analytical model employed calculates the water temperature in the storage vessel, as well as the temperature distribution in the surrounding geological structure, by using the monthly-average solar radiation and ambient temperature. Storage temperature, collector efficiency, performance coefficient of the heat pump (COP) and annual solar fraction are computed and presented in various graphs. The importance of seasonal solar energy storing in the ground is demonstrated.     

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.     

Performance analysis and parametric study of thermal energy storage in an aquifer coupled with a heat pump and solar collectors,for a residential complex in Tehran,Iran

Aquifers are underground porous formations containing water. Confined aquifers are the formations surrounded by two impermeable layers, called cap rocks and bed rocks. These aquifers are suitable for seasonal thermal energy storage.In the present study, a confined aquifer was considered to meet the cooling and heating energy needs of a residential complex located in Tehran, Iran. Three different alternatives were analyzed in this aquifer thermal energy storage (ATES), including: using ATES for cooling alone, for cooling and heating, as a heat pump, and for heating alone, employing flat plate solar energy collectors. A numerical simulation, based on the finite difference method, was carried out for velocity and temperature distributions as well as the heat transfer in the aquifer. The thermal energy recovery factor and the annual coefficient of performance of the system were determined under various schemes of operation, revealing that the combination of the ATES with the heat pump, to meet both cooling and heating needs of the complex, is the best. The study was repeated for different aquifer properties.     

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.     

Performance and economics of annual storage solar heating systems

Annual storage is used with active solar heating systems to permit storage of summer-time solar heat for winter use. This paper presents the results of a comprehensive computer simulation study of the performance of active solar heating systems with long-term hot water storage. A unique feature of this study is the investigation of systems used to supply backup heat to passive solar and energy-conserving buildings, as well as to meet standard heating and hot water loads.

Findings show that system performance increases linearly as storage volume increases, up to the point where the storage tank is large enough to store all heat collected in summer. This point, the point of “unconstrained operation”, is the likely economic optimum. In contrast to diurnal storage systems, systems with annual storage show only slightly diminishing returns as system size increases. Annual storage systems providing nearly 100% solar space heat may cost the same or less per unit heat delivered as a 50 per cent diurnal solar system. Also in contrast to diurnal systems, annual storage systems perform efficiently in meeting the load of a passive or energy-efficient building. A breakeven cost 4¢–10¢/kWh is estimated for optimal 100 per cent solar heating in the U.S.A.     

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.     

Discharge of a thermal storage tank using an immersed heat exchanger with an annular baffle

A number of solar domestic hot water systems and many combined space and water heating systems have heat exchangers placed directly in the storage fluid to charge and/or discharge the tank. Operation of the heat exchanger produces a buoyancy-driven flow within the storage fluid. With a view toward controlling the flow field to increase heat transfer, a cylindrical baffle is inserted in a 350 l cylindrical storage tank. The baffle creates a 40 mm annular gap adjacent to the tank wall. A 10 m-long, 0.3 m² copper coil heat exchanger is placed in the gap. The effects of the baffle on the transient heat transfer, delivered water temperature, heat exchanger effectiveness, and temperature distribution within the storage fluid are presented during discharge of initially thermally stratified and fully mixed storage tanks. The baffle increases the storage side convective heat transfer to the heat exchanger by 20%. This increase is attributed to higher storage fluid velocities across the heat exchanger.     

Thermal and economical analysis of a central solar heating system with underground seasonal storage in Turkey

Thermal performance and economic feasibility of two types of central solar heating system with seasonal storage under four climatically different Turkey locations are investigated. The effects of storage volume and collector area on the thermal performance and cost are studied for three load sizes. The simulation model of the system consisting of flat plate solar collectors, a heat pump, under ground storage tank and heating load based on a finite element analysis and finite element code ANSYS™ is chosen as a convenient tool. In this study, the lowest solar fraction value for Trabzon (41°N) and the highest solar fraction value for Adana (37°N) are obtained. Based on the economic analysis, the payback period of system is found to be about 25–35 years for Turkey.     

Numerical simulation of geothermal-PVT hybrid system with PCM storage tank

As the increase in greenhouse emissions, climate changes, and other irreversible repercussions stems from environmentally destructive energies such as fossil fuels, exploiting solar and geothermal energy as unlimited clean sources of energy in the renewable energy technologies can survive the planet earth, which is facing a catastrophe on a global scale. The main purpose of this research is to study Techno analysis of the combined ground source heat pump (GSHP) and photovoltaic thermal collectors (PVTs) with a “phase-change material” (PCM) storage tank to fulfill the energy demands of a residential building. In the first step of this study, in order to model the heat pump behavior in multi-usage operation modes (heating and cooling), a numerical transient simulation of a water-to-water GSHP, which includes a vertical U-type ground source heat exchanger (GSHX) and a variable speed drive (VSD) compressor, was conducted by developing a numerical code in Engineering Equation Solver software. To study the thermodynamic aspect of the hybrid system in terms of exergy and energy, a transient numerical simulation was accomplished using the TRNSYS program. Also, the impact of effective characteristics of ingredients such as areas of PVT panels and the volume of the storage tank of PCMs on the performance of the hybrid system are investigated. On top of that, the two types of the GSHP-PVT-PCMs and GSHP-PV from the energy and exergy points of view are compared. The obtained results demonstrate that the irreversibility of the solar modules of the GSHP-PVT-PCMs is 6.6% lower than that of the GSHP-PV. Furthermore, the calculation of the annual required load of the building for these two kinds of hybrid systems shows that the use of collectors in this combined system has reduced the total load of the building by 6.5%. The use of collectors in the GSHP-PVT-PCM gives rise to a difference in the value of solar factor (SF) of this system by 1.4% more than the one for the hybrid system without thermal collectors.     

Experimental results of a sensible heat storage system for residential and light commercial application

This paper presents the performance results for a sensible heat storage system. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating, and domestic water heating, this paper emphasizes the space heating performance of the unit. The heat storage system was tested for different indoor and outdoor conditions to determine parameters such as heating charge rate, compressor power, and coefficient of performance (COP). The thermal storage tank was able to store a full charge of heat. The rate of increase of storage tank temperature increased with outdoor temperature. The heating rate during a charge test, best shown by the normalized rate plots, increased with evaporating temperature due to the increasing mass flow rate and refrigerant density. At higher indoor temperature during the discharge tests, the rate of decrease of storage tank temperature was slower. Also, the discharge heating rate decreased with time since the thermal storage tank temperature decreased as less thermal energy became available for use. Copyright © 1999 John Wiley & Sons, Ltd.     

Modelling and analysis of the dynamic behavior of a forced circulation solar water heating system with storage tanks in series

A generalized model for a forced circulation solar water heating system with storage tanks in series is presented in which the loss of heat through an insulation lagging is considered, and the periodic time variation of the intensity of solar radiation, as well as both the ambient air temperature and the temperature of cold water entering the first main tank, is taken into account. Using the Laplace transformation, an exact solution is presented which, under certain conditions, reduces to an approximate solution. The conditions for convergence to the approximate solution are discussed, and figures are presented comparing it with the exact solution for several different sets of conditions. In this communication, the effect of the number of storage tanks on the outlet temperature of the hot water and the effect of various water heating system parameters on its performance have been analytically investigated. Numerical calculations have been made for a typical cold day.     

Mass and energy storage analysis of an absorption heat pump with simulated time dependent generator heat input

This communication presents an assessment of the feasibility of energy storage via refrigerant mass storage within an absorption cycle heat pump with simulated time dependent generator heat input. The system consists of storage volumes with the condenser and absorber of the conventional absorption cycle heat pump to store liquid refrigerant, weak and strong solutions during the generation period, which are required for the heat pump operation during the generation off period. A time dependent mass and energy storage analysis based on mass and energy balance equations for various components of the heat pump system has been carried out to evaluate energy storage concentration and storage efficiency for combined and separate storage schemes for the weak and strong solutions. Two possible performance modes, viz constant pumping ratio or the constant flow of the strong solution from the absorber to the generator have been analysed: the latter is preferable over the former from a practical point of view. Numerical computer simulation has been made for a typical winter day in Melbourne (Australia) with the desired heating load specified. It is found that the concept of refrigerant storage within the absorption cycle heat pump is technically feasible for efficient space heating. The energy storage concentration in the condenser store is slighly higher while that in absorber store is slightly lower for the separate storage mode as compared to the combined storage. However, the combined storage has an advantage of less storage volume and hence is more cost effective than separate storage and the disadvantage of limited system operation due to the decrease of solution concentrations.     

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.