Review on thermal energy storage with phase change materials and applications

The use of a latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal nature of the storage process. PCMs have been widely used in latent heat thermal-storage systems for heat pumps, solar engineering, and spacecraft thermal control applications. The uses of PCMs for heating and cooling applications for buildings have been investigated within the past decade. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. This paper also summarizes the investigation and analysis of the available thermal energy storage systems incorporating PCMs for use in different applications.     

太阳能生活热水系统相变储能研究进展

该文阐述了太阳能生活热水（SDHW）系统用相变材料的选择与封装情况,综述了相变材料在SDHW系统水箱、太阳能集热器和SDHW系统循环中的储能及其改进与强化换热研究进展情况,并对今后SDHW系统相变材料应用发展方向提出建议和展望。研究表明,石蜡与三水醋酸钠应用于SDHW系统水箱和集热器的研究较为广泛,其封装形式主要是的塑料、铝、不锈钢的宏封装,形状为管、柱体、球等,且相变材料加入到传统的SDHW系统中均能提高太阳能生活热水器的储热性能,其储热性能还有较大的提升与改进空间。     

A review of research and development work on solar dryers with heat storage

Drying of agricultural food products is one of the most attractive and cost-effective applications of solar energy. The solar dryer is less reliable due to the intermittent nature of solar energy. This shortcoming can be overcome to some extent by storing solar energy. Information on sensible and latent heat storage materials and systems is spread widely in the literature. In this paper, we try to gather information about the previous and current research works in the field of thermal energy storage technology for solar air heater and dryer. The relative studies are classified on the basis of the type of storage material used in solar dryers, i.e. phase change material (PCM), rock, water, etc. Several designs of solar dryers with different heat storage materials were proposed by researchers. Recent studies focused on PCMs such as Paraffin and salt hydrate, due to their high heat storage capacity per unit volume.     

Effect of wall conductivity on thermal stratification

The growing desire to find alternate energy sources to meet the ever-increasing energy demands of mankind has led to extensive research in the field of solar energy. Energy devices working on solar energy need energy storage subsystems because of the intermittent nature of solar radiation. Thermal energy storage systems which keep the warm and cold water separated by means of gravitational stratification have been found to be attractive in low and medium temperature thermal storage applications due to their simplicity and low cost. Experimental investigations have been carried out on scaled model storage tanks of various materials and wall thickness to study their effect on the stratification. It has been found that degradation of thermoclines is slower for tanks whose walls have a higher thermal resistance.     

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.     

Alternative storage options for solar thermal systems

The efficient implementation of solar systems in buildings depends on the storage of energy yielded, as it can both increase the solar system's autonomy and make it a feasible solution for zero energy buildings, and make storage vessels more compact, reducing precious space requirements. This is of particular important in places with reduced time of sunshine, where solar systems are less effective, because of the deviation between solar radiation and the demand. The traditional storage options use water, which is practical, safe and low‐cost, especially when the storage requirements are small. However, when larger storage is needed, limits concerning the use of water exist, mainly due to the need for larger installation space and the increased thermal losses. The use of phase change materials (PCM) for thermal energy storage seems an upcoming technology. The main idea is the substitution of water with PCM, which feature larger specific energy storage capacity compared to other conventional materials. In the context of the specific paper, a combined solar thermal system used for the preparation of domestic hot water (DHW) and space heating (Solar Combi System) with two different types of storage is studied, for two Greek cities. The aim is to find out which is the most efficient way of storing energy with respect to the autonomy of the system, for a solar combi system. This is being achieved by determining the comparative autonomy of PCM and water storage system for various climates. It was proven that using PCM is advantageous, as it can extend the autonomy duration of the solar system for 2 to 8 hours, depending on the season and the climatic conditions. However, it was also seen that in solar combi systems used throughout the whole year, PCM are inefficient during summer period.     

Analysis and performance of a low-cost solar heater

One of the alternatives to reduce the consumption of electricity for heating water is by popularizing the use of solar energy. This work contributes with studies on a Low-Cost Solar Heater (LCSH), a new concept of solar water heater made entirely of polymeric materials, which requires a relatively low investment and is user-assembled. The solar collector, which absorbs solar energy and transfers it to water in the form of heat, is composed of uncovered flat panels of rigid PVC. The storage tank that holds the water heated during the day is made of polyethylene coated with polystyrene. The results of the LCSH were compared with those of a conventional solar heater composed of a glass-covered copper collector and a stainless steel storage tank. The efficiency of the systems was evaluated by measuring the incident solar radiation and water temperature with the systems operating naturally (thermosiphon). The heat loss in the hot water storage tanks was measured to estimate the thermal performance of the solar heaters. Considering that the target temperature for the heated water is slightly above the ambient temperature, the results indicated that the LCSH showed a satisfactory global heat transfer coefficient for storage tanks and that it attained an excellent thermal performance, although it is not as efficient as the conventional heater.     

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.     

Experimental study of a multi-tube ICS solar water heating system in mild climates

Since a majority of residential and industrial building hot water needs are around 50°C, an integrated solar water heater could provide a bulk source that blends collection and storage into one unit. These collectors incorporate thermal storage within the collector itself. The storage pipe surface serves as the absorber surface. Most ICS systems use only one tank, but some use a number of tanks in series. While the simplicity of ICS systems is attractive, they are generally suitable only for applications in mild climates with small thermal storage requirements. A multi-tube ICS solar hot water system with eight cylindrical water storage tanks (pipes) in horizontal (East-West) orientation was designed and outdoor tests of experimental model were performed in mild climate of north (Caspian Sea) region of Iran. Experimental results such as water temperature profiles, mean daily efficiency, water temperature stratification and thermal losses during night are presented and discussed for this model. Experimental results showed that the daily mean efficiency is comparable to other ICS systems and also acceptable thermal performance of this type of ICS system has been observed.     

Five Solar-Energy Desalination Systems

This paper gives an overview of the Solar Energy Storage Program at the Solar Energy Research Institute. The program provides research, systems analyses, and economic assessments of thermal and thermochemical energy storage and transport. Current activities include experimental research into very high temperature (above 800^°C) thermal energy storage and assessment of novel thermochemical energy storage and transport systems.

The applications for such high-temperature storage are thermochemical processes, solar thermal-electric power generation, regeneration of heat and electricity, industrial process heat, and thermally regenerative electrochemical systems.

The research results for five high-temperatuTe thermal energy storage technologies and two thermochemical systems are described.     

Review of thermal energy storage for air conditioning systems

Thermal energy storage is very important to eradicate the discrepancy between energy supply and energy demand and to improve the energy efficiency of solar energy systems. Latent heat thermal energy storage (LHTES) is more useful than sensible energy storage due to the high storage capacity per unit volume/mass at nearly constant temperatures. This review presents the previous works on thermal energy storage used for air conditioning systems and the application of phase change materials (PCMs) in different parts of the air conditioning networks, air distribution network, chilled water network, microencapsulated slurries, thermal power and heat rejection of the absorption cooling. Recently, researchers studied the heat transfer enhancement of the thermal energy storage with PCMs because most phase change materials have low thermal conductivity, which causes a long time for charging and discharging process. It is expected that the design of latent heat thermal energy storage will reduce the cost and the volume of air conditioning systems and networks.     

Review of the phase change material (PCM) usage for solar domestic water heating systems (SDWHS)

The shortage in energy resources combined with the climb in greenhouse emissions is the main incentive beyond the deployment of solar energy resource in various applications. One of the most successful applications is the utilization of solar energy in the domestic water heating systems (DWHS) because 70% of the consumed energy in the residential segment is utilized for space heating and appliances in cold climates 1 . However, the full deployment of solar energy in domestic water heating is only possible when an energy storage system with acceptable price is available. Recently a new tendency for deploying phase change materials (PCMs) as an energy storage system is introduced in several solar DWHS. These systems are known as integrated PCM in solar DWHS and offer several advantages including high storage capacity, low storage volume, and isothermal operation during the charging and discharging phases. The present study reviews various techniques utilized for integrating the PCM in solar water heating systems and the utilized methods for enhancing the heat transfer characteristics of the PCM through the usage of extended surfaces and high conductive additives. Copyright © 2017 John Wiley & Sons, Ltd.     

Optimization study on a solar-assisted air source heat pump system with energy storage based on the economics method

Currently, hybrid renewable energy systems with thermal energy storage have various advantages and are widely used. This paper investigated the performance of a solar-assisted air source heat pump system with energy storage (SASHPS-ES) in Beijing, China, and proposed an optimal operation mode based on economic evaluation. The results indicate that with the optimal heat storage ratio of 50%, the rated capacity of the air source heat pump (ASHP) of the SASHPS-ES system can be reduced by 16.7%, decreasing its annual total cost by 26.5% under a peak-valley electricity price policy. The price of 620¥/m² is critical for the solar collectors. The economics of SASHPS-ES is better than that of an air source heat pump system with energy storage (ASHPS-ES) when the price of the selected solar collector units is less than this critical price (without subsidies from the government). In the current local market, the promotion of SASHPS-ES systems and other solar energy applications requires government subsidies for a period of time. The results can guide the utilization and popularity of SASHPS-ES in China.     

Evaluation and selection of energy storage systems for solar thermal applications

Energy storage is one of the key technologies for energy conservation and therefore is of great practical importance. One of its main advantages is that it is best suited for solar thermal applications. This study deals with a comprehensive discussion of the evaluation and the selection of sensible and latent heat storage technologies, systems and applications in the field of solar energy. Several issues relating to energy storage are examined from the current perspective. In addition, some criteria, techniques, recommendations, checklists on the selection, implementation and operation of energy storage systems are provided for the use of energy engineers, scientists and policy makers. Copyright © 1999 John Wiley & Sons, Ltd.     

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.     

Solar water heaters with phase change material thermal energy storage medium: A review

Latent heat thermal energy storage is one of the most efficient ways to store thermal energy for heating water by energy received from sun. This paper summarizes the investigation and analysis of thermal energy storage incorporating with and without PCM for use in solar water heaters. The relative studies are classified on the basis of type of collector and the type of storage used i.e. sensible or latent. A thorough literature investigation into the use of phase change material (PCM) in solar water heating has been considered. It has been demonstrated that for a better thermal performance of solar water heater a phase change material with high latent heat and with large surface area for heat transfer is required.     

Thermal stratification within the water tank

To sufficiently store and use high-quality heat energy, thermal stratification is gradually applied in many kinds of energy storage fields such as solar thermal utilization system. Because of the unsteady characteristics of solar radiation, thermal storage becomes very essential in long-term operation of heating load. The wide application of thermal stratification lies in the minimization of the mixing effect by use of the thermal stratification, which is caused by the thermal buoyancy because of the difference of temperature between cold and hot water. According to the review, the conception of thermal stratification allows a wide variety of different design embodiments, which essentially extends the fields of practical application of these devices. In this paper a survey of the various types of thermal stratification tanks and research methods is presented, and reasons of energy storage with efficiency problems related to the applications are introduced and benefits offered by thermal stratification are outlined. The structure designs based on theoretical prediction of thermal-stratified water tank performed at many organizations are introduced and are compared with their experimental results. Finally, the development of the tank with thermal stratification in the future application is predicted.     

Optimization of the heat exchanger in a flat plate indirect heating integrated collector storage solar water heating system

Due to the environmental impact of energy usage, consumers need to be encouraged to use renewable energy sources such as solar energy. The indirect heating flat plate integrated collector storage solar water heating system is one of the compact systems for domestic water heating. It incorporates the collection of a solar energy component and a hot water storage component in one unit. The objectives of this study were to investigate the effect of different parameters on the thermal performance of this system with the aim of reducing both the initial and the running costs. The outlet service water temperature was used as a measure of performance, because it is an indicator of the energy acquired from the solar radiation. The continuity, momentum and energy equations of the fluids involved in the system were numerically solved in a steady state condition, using FLUENT software. Three-D CFD models were developed and validated using previous experimental results. A standard k–ω turbulent model was used in the optimization of the heat exchanger because it produced good agreement with the experimental results. The surface-to-surface radiation model was included. The effect of single and double row heat exchangers with different lengths was investigated. Circular and elliptic cross-section pipes were also examined. Mass flow rates of 500 and 650 L/h were chosen. The results showed that the single row HX of 10.8 m length for both the elliptical and type B tube gave high service water outlet temperature (acceptable for heat exchanger design) and with low pumping power. This resulted in an increase in the thermal efficiency and a significant reduction in both the initial and the operating costs of the system.     

Industrial application of PV/T solar energy systems

Hybrid photovoltaic/thermal (PV/T) systems consist of PV modules and heat extraction units mounted together. These systems can simultaneously provide electrical and thermal energy, thus achieving a higher energy conversion rate of the absorbed solar radiation than plain photovoltaics. Industries show high demand of energy for both heat and electricity and the hybrid PV/T systems could be used in order to meet this requirement. In this paper the application aspects in the industry of PV/T systems with water heat extraction is presented. The systems are analyzed with TRNSYS program for three locations Nicosia, Athens and Madison that are located at different latitudes. The system comprises 300 m² of hybrid PV/T collectors producing both electricity and thermal energy and a 10 m³ water storage tank. The work includes the study of an industrial process heat system operated at two load supply temperatures of 60 °C and 80 °C. The results show that the electrical production of the system, employing polycrystalline solar cells, is more than the amorphous ones but the solar thermal contribution is slightly lower. A non-hybrid PV system produces about 25% more electrical energy but the present system covers also, depending on the location, a large percentage of the thermal energy requirement of the industry considered. The economic viability of the systems is proven, as positive life cycle savings are obtained in the case of hybrid systems and the savings are increased for higher load temperature applications. Additionally, although amorphous silicon panels are much less efficient than the polycrystalline ones, better economic figures are obtained due to their lower initial cost, i.e., they have better cost/benefit ratio.