Central solar heating plants with seasonal storage in Germany

In the house building sector, central solar heating plants presently offer the most cost-favourable application of all possibilities of solar-thermal systems. By the integration of seasonal heat storage, more than 50% of the annual heating demand for space heating and domestic hot water can be supplied by solar energy. Since 1995, eight central solar heating plants with seasonal heat storage have been built in Germany within the governmental R&D-programme ‘Solarthermie-2000’. This report describes the technology of central solar heating plants and gives advice about planning and costs. The pilot and demonstration plants for seasonal heat storage already built in Germany are described in detail to give an idea about possible system design and applications of central solar heating plants.     

Simulation studies of the expected performance of Kerava solar village

The Kerava solar village is the first regional building complex in Finland with a combined solar heating and heat pump system using seasonal heat storage. The village will be completed at the beginning of 1983. In this study we describe the heating system of Kerava solar village. The heat demand was estimated by computer simulations. Different weather conditions and operational situations were considered. Under normal weather and operating conditions approximately 60 percent of the total heat demand of the Kerava solar village is estimated to be obtained from solar energy.     

German central solar heating plants with seasonal heat storage

Central solar heating plants contribute to the reduction of CO₂-emissions and global warming. The combination of central solar heating plants with seasonal heat storage enables high solar fractions of 50% and more. Several pilot central solar heating plants with seasonal heat storage (CSHPSS) built in Germany since 1996 have proven the appropriate operation of these systems and confirmed the high solar fractions.Four different types of seasonal thermal energy stores have been developed, tested and monitored under realistic operation conditions: Hot-water thermal energy store (e.g. in Friedrichshafen), gravel-water thermal energy store (e.g. in Steinfurt-Borghorst), borehole thermal energy store (in Neckarsulm) and aquifer thermal energy store (in Rostock). In this paper, measured heat balances of several German CSHPSS are presented. The different types of thermal energy stores and the affiliated central solar heating plants and district heating systems are described. Their operational characteristics are compared using measured data gained from an extensive monitoring program. Thus long-term operational experiences such as the influence of net return temperatures are shown.     

太阳能季节性地下水池蓄热供热系统的模拟研究

分析了太阳能季节性地下水池蓄热供热系统的运行原理,建立了蓄热水池的数学模型,以哈尔滨一栋采用该供热模式的别墅建筑为实例,以满足太阳能保证率为目标、地下水池中水温为约束,通过改变集热器面积和地下水池的体积及其他相关参数,模拟计算得到太阳集热器面积和地下水池半径的最佳匹配、系统集热量与热损失量和水温与集热效率的关系.     

Experimental and numerical analysis of seasonal solar‐energy storage in buildings

This paper presents seasonal‐energy storage of solar energy for the heating of buildings. We distinguish several types of seasonal storage, such as latent, sensible, and chemical storage, among which the thermochemical storage is used and analysed in this research. In the first part, a laboratory heat‐storage tank, which was made in the laboratory for heating, sanitary, and solar technology and air conditioning from the Faculty of Mechanical Engineering, University of Ljubljana, Slovenia, was presented. The experimental model was tested for charging and discharging mode. Two types of numerical models for sorption thermal‐energy storage exist, which are microscale and macroscale (integral). For microscale analysis, the analysis system (ANSYS) model can be used to simulate the behaviour in the adsorption reactor. On macroscale or integral scale, TRaNsient SYStem (TRNSYS) model was used to perform the operation of the storages on the yearly basis. In the second part the simulation of the underfloor heating system operation with a built‐in storage tank was carried out for two locations, Ljubljana and Portoro?. Furthermore, the comparison between a thermochemical and sensible‐heat storage was performed with TRNSYS and Excel software. In this comparison, the focus was on the surface parameters of the SCs and volume of the thermal‐storage tank for the coverage of the energy demand for selected building. With this analysis, we would like to show the advantage of the thermochemical storage system, to provide greater coverage of the energy demand for the operation of the building, compared with the seasonal sensible‐heat storage (SSHS). Such a heat‐storage technology could, in the future, be a key contributor to the more environmentally friendly and more sustainable way of delivering energy needs for buildings.     

The ITW solar heating system: an oldtimer fully in action

Public awareness of energy in the early 1970s stimulated a number of projects on alternative ways of heating. The ‘Institut für Thermodynamik und Wärmetechnik’ (ITW) of the University of Stuttgart has been operating a solar heating system since 1985. Ever since, this system has been minutely monitored. In particular, the storage was painstakingly considered as it was intended to serve as a pilot facility for the much discussed problem of seasonal storage. This storage unit should be simple and cheap but heavily instrumented in order to obtain many and accurate data and it should be versatile in order to gain knowledge for operation. The solar heating is provided by collectors that are unglazed, so a heat pump is required for appropriate heating temperatures. However, the heat pump allowed for a combination of heating and cooling in our system and this proved to be very advantageous, as cooling energy is more expensive and more in demand in our building than heating energy.

The system was used in various seasonal cycles with changing conditions. It has now been operating for almost 15 years. During this period, neither storage nor collectors caused any trouble. Some difficulties were experienced with the heat pump. The first one had to be replaced; we made suggestions for improvement of the second one. This provided good COPs but there has been an occasional defect. The experience with our solar heating system was so satisfactory that, based on the knowledge gained from it, large housing projects in Friedrichshafen and Hamburg and an office building project in Chemnitz were conceived and built and are now being monitored.     

Feasibility study of a district energy system with seasonal water thermal storage

In this paper, performance of three types of district heating/cooling and hot water supply system with natural and unused energy utilization were examined by using system simulation. An area zoned for both commercial and residential buildings was chosen for this study. The first system is the conventional system in which an electric driven turbo chiller and a gas-fired boiler are installed as the heat source. This is considered as the reference system. Two alternative systems utilize waste heat from space cooling and heating. One is designed based on short-term heat recovery and the other employs the concept of an annual cycle energy system (i.e. seasonal heat recovery). All of the three systems use solar thermal energy for hot water supply to the residential zone. The index for evaluation is the coefficient of performance of the overall system, based on primary energy. As a result, it was found that the seasonal storage system could decrease the energy consumption by about 26% and the short-term heat recovery system could decrease it by about 16% compared with the reference system. In designing the heat recovery system, a balance of cooling/heating demand is an important factor. Therefore a sensitivity analysis of performance of the overall system and the seasonal thermal storage for several load patterns was performed. From these results, it was found that if the amount of heating/cooling demand were well balanced, an improvement of energy performance could be achieved and the utilization factor of the seasonal tank would become higher. Furthermore, the volume of the seasonal storage tank could be reduced.     

High temperature solar heated seasonal storage system for low temperature heating of buildings

A preliminary study of a solar-heated low-temperature space-heating system with seasonal storage in the ground has been performed. The system performance has been evaluated using the simulation models TRNSYS and MINSUN together with the ground storage module DST. The study implies an economically feasible design for a total annual heat demand of about 2500 MWh. The main objective was to perform a study on Anneberg, a planned residential area of 90 single-family houses with 1080 MWh total heat demand. The suggested heating system with a solar fraction of 60% includes 3000 m² of solar collectors but electrical heaters to produce peak heating. The floor heating system was designed for 30°C supply temperature. The temperature of the seasonal storage unit, a borehole array in crystalline rock of 60,000 m³, varies between 30 and 45°C over the year. The total annual heating costs, which include all costs (including capital, energy, maintenance etc.) associated with the heating system, were investigated for three different systems: solar heating (1000 SEK MWh⁻¹), small-scale district heating (1100 SEK MWh⁻¹) and individual ground-coupled heat pumps (920 SEK MWh⁻¹). The heat loss from the Anneberg storage system was 42% of the collected solar energy. This heat loss would be reduced in a larger storage system, so a case where the size of the proposed solar heating system was enlarged by a factor of three was also investigated. The total annual cost of the solar heating system was reduced by about 20% to about 800 SEK MWh⁻¹, which is lower than the best conventional alternative.     

太阳能-土壤源耦合蓄热特性及潜力分析

在分析太阳能与土壤蓄热特点的基础上,提出了利用非采暖季蓄热的节能系统。由于太阳能照射强度具有随季节性变化的特点,非采暖季太阳能辐射强度一般较高,可借助地埋管系统向土壤中蓄热。通过对不同时段太阳辐射强度的监测,在进行数据分析的基础上,建立了太阳集热器数学模型。分析多孔介质蓄热过程的传热传质特性及土壤蓄热特性、非均质性以及地下水径流对热传播半径的影响等特点,提出了在非采暖季进行土壤蓄热的理论和可行性。     

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.     

Energy management in horticultural applications through the closed greenhouse concept,state of the art

The commercial greenhouse has the highest demand for energy as compared to all other agricultural industry sectors. Here, energy management is important from a broad sustainability perspective. This paper presents the state-of-the-art regarding one energy management concept; the closed greenhouse integrated with thermal energy storage (TES) technology. This concept is an innovation for sustainable energy management since it is designed to maximize the utilization of solar energy through seasonal storage. In a fully closed greenhouse, there is no ventilation which means that excess sensible and latent heat must be removed. Then, this heat can be stored using seasonal and/or daily TES technology, and used later in order to satisfy the heating demand of the greenhouse. This assessment shows that closed greenhouse can, in addition to satisfying its own heating demand, also supply the demand for neighboring buildings. Several energy potential studies show that summer excess heat of almost three times the annual heating demand of the greenhouse. However, many studies propose the use of some auxiliary system for peak load. Also, the assessment clearly point out that a combination of seasonal and short-term TES must be further explored to make use of the full potential. Although higher amount of solar energy can be harvested in a fully closed greenhouse, in reality a semi-closed greenhouse concept may be more applicable. There, a large part of the available excess heat will be stored, but the benefits of an integrated forced-ventilation system are introduced in order to use fresh air as a rapid response for primarily humidity control. The main conclusion from this review is that aspects like energy efficiency, environmental benefits and economics must be further examined since this is seldom presented in the literature. Also, a variety of energy management scenarios may be employed depending on the most prioritized aspect.     

The self-sufficient solar house in Freiburg—Results of 3 years of operation

Energy efficiency is the key to buildings with minimized environmental impact. The combination of this approach with active and passive solar energy utilization can lead to purchased energy demand close to zero. Nevertheless, energy autonomy under Central European climatic conditions requires seasonal energy storage, especially energy with a high exergy content. The Fraunhofer Institute for Solar Energy Systems built an energy autonomous house in Freiburg, Germany, in 1992. Highly efficient solar systems are combined with a hydrogen-based seasonal-storage system. Long-term storage of low-temperature heat was avoided by extensive passive solar energy utilization. After describing the project, this paper focuses on the experience of nearly 3 years of operation.     

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.     

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.     

The use of ground heat storages and evacuated tube solar collectors for meeting the annual heating demand of family-sized houses

The use of storages for sensible heat is limited because parts of the input thermal energy end up as unavoidable heat losses. In order to minimize this loss, it is necessary to keep the surface area to volume ratio (S/V) as low as possible. This occurs when the volume of a body with a certain shape increases. In addition to a large volume it is important to use materials with a high volumetric thermal capacity, as long as sensible heat is being used for storage. This condition is best met by water or a combination of substances with water. In the field of interseasonal storages, for solar heat to cover the heating demands of small residential buildings, the general belief is that the relative small volume needed, results in too much heat loss and therefore individual seasonal storages seem to be of no useful solution.However, the theoretical considerations and simulations in this paper show that this is a prejudice. It is possible to supply a great deal of the thermal energy needed for small residential homes with interseasonal ground storage for solar heat. The loss of heat is acceptable if the storage is designed in the correct way.The ground heat storage should be of cuboidal shape, using the local soil as storage material, if possible. The storage containment must be heat-insulated and damp-proof. The placement of the storage could be within the heated building, adjacent to it or nearby. As such systems may be useful as retrofit for existing houses this study assumes that the storage system has no contact with the heated house. The heat is supplied by evacuated tube solar collectors and their feature to produce effective heat with high temperature (above 100 °C) is used.     

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.     

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.     

A review on thermal energy storage systems in solar air heaters

The drying needs of agricultural, industrial process heat requirements and for space heating, solar energy is one of the prime sources which is renewable and pollution free. As the solar energy is inconsistent and nature dependent, more often there is a mismatch between the solar thermal energy availability and requirement. This drawback could be addressed to an extent with the help of thermal energy storage systems combined with solar air heaters. This review article focuses on solar air heaters with integrated and separate thermal energy storage systems as well as greenhouses with thermal storage units. A comprehensive study was carried out in solar thermal storage units consisting of sensible heat storage materials and latent heat storage materials. As the phase change heat storage materials offer many advantages over the sensible heat storage materials, the researchers are more interested in this system. The charging and discharging characteristics of thermal storage materials with various operational parameters have been reported. All the possible solar air heater applications with storage units have also been discussed.     

Isothermal storage of solar energy in building construction

The role of advanced isothermal heat storage systems in buildings is discussed. A storage system encapsulated with phase change materials in which energy is absorbed in the hot period and released in the cold period is analyzed. The thermal behaviour of isothermal heat storage composites is examined using numerical techniques.Two methods of heat transfer with latent heat storage are described in the first part. Based on the initial results, the “effective heat capacity” method was selected and implemented into ESP-r. Numerical studies on the effect of isothermal storage of solar energy in specific building material components are discussed in the second part. Numerical simulations were conducted for two cases of multi-zone, highly glazed and naturally ventilated passive solar buildings. PCM-impregnated gypsum plasterboard was used as an internal room lining in the first case study and transparent insulation material combined with PCM was applied for the external south-oriented wall in the second case study. The behaviour of a TIM–PCM wall and its influence on the internal surface temperature are estimated. Air, surface and resultant temperatures are compared with a “no-PCM” case for both case studies and the diurnal and the seasonal latent heat storage effect is analyzed.     

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。