Underground storage of heat in solar heating systems

The thermal interaction between underground heat storage and the surrounding ground is studied. The storage medium may be water in a tank, rocks, the ground itself, etc. For the case of hemispherical geometry, analytic steady-state solutions and numerical solutions to select time-dependent situations are presented and discussed.The quasi-steady-state average daily net heat loss into the surrounding ground from uninsulated underground heat-storage facilities is estimated to be only a few per cent of the heat storage capacity for typical home-size units in low-conductivity ground in the absence of ground-water flow. The economics of added insulation under these circumstances appears to be critically site and system specific.The time required for the average heat-loss rate of a new system to approach the steady-state value is on the order of a year.The ground surrounding a typical heat-storage facility contributes very little to the system's storage capacity either on a daily or a seasonal time scale.     

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.     

A model for latent heat energy storage systems

In this study, a theoretical approach is proposed for the prediction of time and temperature during the heat charge and discharge in the latent heat storage of phase changed materials (PCM). By the use of the average values of the mean specific heat capacities for the phase‐changed materials, analytical solutions are obtained and compared with the available experimental data in the literature. It is shown that decreasing the entry temperature of the working fluid from ?4 to ?15°C has a very dominant and strong effect on the PCM solidification time. The effect of the working fluid flow rate and the material of PCM capsules on the time for complete solidification and total charging is also investigated. The agreement between the present theoretical model results and the experimental data related to the cooling using small spheres and the heat storage using rectangle containers is very good. The largest difference between the present results and the experimental data becomes about 10% when the fluid temperature approaches the phase change temperature at high temperatures. Copyright © 2006 John Wiley & Sons, Ltd.     

一种与坐卧用具结合的太阳能相变蓄热采暖系统

介绍了一种将太阳能利用、相变蓄热与局部采暖技术相结合的新型太阳能相变蓄热坐卧用具采暖系统。阐述了系统的工作原理,分析了其优点,并给出其设计计算实例。     

Investigation of nitrate salts for solar latent heat storage

Solar thermal applications require some means of thermal energy storage. Amongst several storage concepts, latent heat storage is quite suitable because of its high storage density and almost constant temperature during charging and discharging. The temperature range between 200 and 300°C is considered to be important for solar total energy systems. In this temperature range, sodium nitrate and its mixed salts with other nitrates including eutectic and off-eutectic salts are candidates.

The present paper deals with heat transfer in a latent heat storage unit utilizing these salts. A method of rough estimation of the thermal conductivity of the storage materials is described, and the temperature history of the storage material experimentally obtained is compared with numerical solutions and found to be in reasonably good agreement.

It is seen that the temperature of the heat transfer surface quickly drops soon after the appearance of a solid phase due to low thermal conductivity of these salts. Ways to avoid this temperature drop are discussed.     

Solar cooker with latent heat storage systems: A review

Cooking with the sun has become a potentially viable substitute for fuel-wood in food preparation in much of the developing world. Energy requirements for cooking account for 36% of total primary energy consumption in India. The rural and urban population, depend mainly, on non-commercial fuels to meet their energy needs. Solar cooking is one possible solution but its acceptance has been limited partially due to some barriers. Solar cooker cannot cook the food in late evening. That drawback can be solved by the storage unit associated with in a solar cooker. So that food can be cook at late evening. Therefore, in this paper, an attempt has been taken to summarize the investigation of the solar cooking system incorporating with phase change materials (PCMs).     

A solar water heater with a built-in latent heat storage

In this paper, we have investigated the performance of a novel built-in storage type water heater containing a layer of PCM-filled capsules at the bottom. The PCM layer is introduced with a view of getting hot water during off-sunshine hours. The moving solid-liquid boundary layer problem for the PCM material is simplified to a stationary boundary layer problem, and the effect of latent heat is included in the specific heat by replacing the semi-melted PCM by a fictitious solid. The performance of the water heater is then predicted analytically for two depths of PCM and for different flow rates, both constant and intermittent. The case of sudden withdrawal of water over very short periods is also studied.     

Cascaded latent heat storage for parabolic trough solar power plants

The current revival of solar thermal electricity generating systems (SEGS) unveils the still existing need of economic thermal energy storages (TES) for the temperature range from 250 °C to 500 °C. The TES-benchmark for parabolic trough power plants is the direct two tank storage, as it was used at the SEGS I plant near Barstow (USA). With the introduction of expensive synthetic heat transfer oil, capable to increase the operating temperature from former 300 °C up to 400 °C, the direct storage technology became uneconomical. Cascaded latent heat storages (CLHS) are one possible TES alternative, which are marked by a minimum of necessary storage material. The use of a cascade of multiple phase change materials (PCM) shall ensure the optimal utilization of the storage material.This paper reports experimental and numerical results from the investigation of cascaded latent heat storages with alkali nitrate salts like NaNO₃, KNO₃ and others more. The experiments were conducted with vertical shell and tube type heat exchanger devices under realistic operation parameters. The experimental results were used for a numerical model to simulate different CLHS configurations. Dymola/Modelica was used to conduct the simulation. The outcome of this work shows on the one hand, that the design of CLHS for this temperature range is more complex than for the temperature range up to 100 °C. And on the other hand, the low heat conductivity of available PCM is an obstacle which must be overcome to make full use of this promising storage technology.     

Review of latent thermal energy storage systems for solar air-conditioning systems

Solar air conditioning is an important approach to satisfy the high demand for cooling given the global energy situation. The application of phase-change materials (PCMs) in a thermal storage system is a way to address temporary power problems of solar air-conditioning systems. This paper reviews the selection, strengthening, and application of PCMs and containers in latent thermal storage system for solar air-conditioning systems. The optimization of PCM container geometry is summarized and analyzed. The hybrid enhancement methods for PCMs and containers and the cost assessment of latent thermal storage system are discussed. The more effective heat transfer enhancement using PCMs was found to mainly involve micro-nano additives. Combinations of fins and nanoadditives, nanoparticles, and metal foam are the main hybrid strengthening method. However, the thermal storage effect of hybrid strengthening is not necessarily better than single strengthening. At the same time, the latent thermal storage unit has less application in the field of solar air-conditioning systems, especially regarding heat recovery, because of its cost and thermal storage time. The integration of latent thermal storage units and solar air-conditioning components, economic analysis of improvement technology, and quantitative studies on hybrid improvement are potential research directions in the future.     

Exergy analysis of latent heat storage systems with sensible heating and subcooling of PCM

The effect of sensible heating of the phase-change material (PCM) before melting and subcooling after solidification, on the performance of the latent heat storage system (LHSS), is studied in terms of first- and second-law efficiencies for the overall charge–discharge cycle. The external heat transfer irreversibilities on account of the interaction between the heat transfer fluid and the storage element are characterized and the optimum phase-change temperature for maximum second-law efficiency is studied. The performance of the LHSS operation is assessed with and without sensible heating before melting and subcooling after solidification. It is observed that the optimum phase-change temperature is higher, and, the overall second-law efficiency is greater for LHSS with sensible heating and subcooling beyond a certain phase-change temperature compared to latent heat storage alone. In addition, the first-law overall efficiency is found to exhibit a minimum and a pre-heated discharge stream is shown to result in a substantial improvement of the second-law efficiency. © 1998 John Wiley & Sons, Ltd.     

Load optimization in solar space heating systems

When load variables, such as window and insulation types, are included in the economic optimization of a solar space heating system, the over-all cost is lower than that resulting from optimization of collection area for a fixed load (as by FCHART [1] and SOLCOST [2]). In this paper an algorithm is derived for choosing insulation levels, as well as solar collection area, so as to minimize the over-all cost of constructing and heating a building. The general algorithm is applicable with any solar performance prediction method, and with any economic criterion where the “cost” is a linear function of collection area and of auxiliary energy consumption. A specific algorithm is also derived for active solar systems using the Relative Areas method of performance prediction [3] and a conventional present worth life cycle cost analysis. The degree-day model is used for the load calculations.     

Single- and multi-tank energy storage for solar heating systems: fundamentals

A multi-tank liquid-water system for storing low-temperature solar-derived heat is investigated experimentally and analytically. The motivation is a perceived economic advantage of the proposed system over single-tank systems in solar heating systems where the required total volume of water is rather large — say 2000 l or more. In the proposed system, the individual tanks in the multi-tank system (each with a volume of about 200 l) are interconnected by means of two strings of immersed-coil heat exchangers: the first string serially connects exchangers that have been immersed at the bottoms of the tanks, the second connects exchangers that have been immersed at the tops of the tanks. The first string will be connected to the solar collector loop and the second in the load loop. The present work experimentally demonstrates the degree of effective stratification that the multi-tank system can achieve, as well as a thermodynamically advantageous ‘thermal diode’ effect. It also describes a model for a multi-tank system as well as experiments that validate the model. Part of the multi-tank model is a model for a single tank with immersed coil heat exchanger, and this model drew upon a ‘reversion-elimination’ algorithm from the recent literature. The validity of the reversion-elimination algorithm is supported by the present experiments.     

Numerical simulation of solar assisted ground-source heat pump heating system with latent heat energy storage in severely cold area

Solar assisted ground-source heat pump (SAGSHP) heating system with latent heat energy storage tank (LHEST) is investigated. The mathematical model of the system is developed, and the transient numerical simulation is carried out in terms of this model. The operation characteristic of the heating system is analyzed during the heating period in Harbin (N45.75°, E126.77°). From the results of the simulation, the average coefficient of performance (COP) of the heating system is 3.28 in heating period. In the initial and latter heating period, the COP of the heating system is higher, and the highest value is 5.95, because the system can be operated without heat pump. During the middle heating period the COP of the heating system and the operation stability of the system are improved due to solar energy and soil alternately or together as the heat source of heat pump. LHEST is a very important role in operation of the system. The system can be operated more flexibly, effectively, and stably by the charge and discharge heat of LHEST, and the effect becomes especially obvious in the initial and latter heating period.     

The evaluation of heat capacity and choice of materials for short-term storage of diurnal solar heat surplus in passive solar heating systems

The results of numerical studies aimed to evaluate the heat capacity of short-term phase change heat accumulators applied in passive solar heating systems (PSHS) with a three-layer energy active translucent enclosure (TE) are presented. An example is given of calculation of specific (per unit of TE surface area) weight of heat accumulator using dimethyl sulfoxide, (СН₃)₂SO, with melting temperature 18.6°C and latent heat of phase transition 173 kJ/kg, as the heat storage agent.     

Latent heat storage for solar energy systems: Transient simulation of refrigerant storage

This paper presents a brief review of the available latent heat storage systems for solar energy utilization. A new concept of latent heat storage of solar energy via the refrigerant-absorbent mass storage in absorption cycle heat pump systems used for solar space heating/cooling has been proposed and assessed thermodynamically. A computer modelling and numerical simulation study shows that the concept of refrigerant storage is fundamentally sound, technically feasible and yields the following advantages over other storage methods: (i) the storage capacity per unit volume is high as the latent heat of vaporization of the refrigerant is high; (ii) the heat loss from the storage to the surroundings is minimum as the storage temperature is near the ambient; (iii) prolonged energy storage is possible with no degradation in system performance and hence suitable for combined solar heating and airconditioning. The effects of operating parameters on the energy storage concentration and storage efficiency have been studied in detail.     

Sizing phase-change energy storage units for air-based solar heating systems

A simple method for sizing phase-change energy storage (PCES) units for air-based solar heating systems is presented. An effective heat capacity for the phase change unit is obtained as a function of its mass, latent heat, specific heat, and melting temperature. The effective heat capacity can then be used, along with any convenient design method for systems with sensible heat stores, such as the f-chart method, to estimate the thermal performance of the system utilizing PCES.     

Collector and storage efficiencies in solar heating systems

A general definition of the effective efficiency of solar collector operating in a solar energy system is presented which gives a fair method of comparison of different collectors operating in that particular application. Based on comparison between the area required for the actual collector and that of a perfect collector-both giving the same fraction solar—the definition permits the definition of the effective average value of the collector input parameter, P = (T_fi − T_a)/S. The concept of the perfect collector also leads to a fair definition for the efficiency of the storage component in a solar heating system. These parameters are evaluated for the special case of residential space heating and service hot water systems of the standardized f-chart type operating in a number of Canadian cities. Simple methods for collector comparisons result from the study and indications are that a simple solar system design method will follow.     

Comparison of heat storage systems employing sensible and latent heat

This study makes an evaluation of the performances of heat storage systems destined for power plants with a discontinuous power source such as nuclear fusion. Two classes of heat storage systems are investigated: heat storage systems based on first sensible heat storage and second latent heat storage. Both classes of heat storage systems are evaluated both from a thermodynamic point of view, inquiring whether the irreversibility of the system stays limited or not, and from an economic point of view, examining whether the system makes proper use of the heat storage capacity present. It is shown that an unambiguous conclusion that one is superior to the other is not possible and that the operating conditions and the configuration of the phase‐changing materials play an important role. Copyright © 1999 John Wiley & Sons, Ltd.     

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.     

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.