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Abstract: Thermal energy storage with phase change materials (PCMs) offers a high thermal storage density with a moderate temperature variation, and has attracted growing attention due to its important role in achieving energy conservation in buildings with thermal comfort. Various methods have been investigated by previous researchers to incorporate PCMs into the building structures, and it has been found that with the help of PCMs the indoor temperature fluctuations can be reduced significantly whilst maintaining desirable thermal comfort. This paper summarises previous works on latent thermal energy storage in building applications, covering PCMs, the impregnation methods, current building applications and their thermal performance analyses, as well as numerical simulation of buildings with PCMs. Over 100 references are included in this paper. [Copyright &y& Elsevier]

Latent heat thermal energy storage is an attractive technique as it can provide higher energy storage density than conventional heat energy storage systems and has the capability to store heat of fusion at a constant (or a near constant) temperature corresponding to the phase transition temperature of the phase change material (PCM). This paper provides a state-of-the-art review on phase change materials (PCMs) and their applications for heating, cooling and electricity generation according to their working temperature ranges from (−20 °C to +200 °C). Four working temperature ranges are considered in this review: (1) the low temperature range from (−20 °C to +5 °C) where the PCMs are typically used for domestic and commercial refrigeration; (2) the medium low temperature range from (+5 °C to +40 °C) where the PCMs are typically applied for heating and cooling applications in buildings; (3) the medium temperature range for solar based heating, hot water and electronic applications from (+40 °C to +80 °C); and (4) the high temperature range from (+80 °C to +200 °C) for absorption cooling, waste heat recovery and electricity generation. Different types of phase change materials applied to each temperature range are reviewed and discussed, in terms of the performance, heat transfer enhancement technique, environmental impact and economic analysis. The review shows that, energy saving of up to 12% can be achieved and a reduction of cooling load of up to 80% can be obtained by PCMs in the low to medium–low temperature range. PCM storage for heating applications can improve operation efficiency from 26% to 66%, depending on specific applications. Solar thermal direct steam generation (DSG) is the most common electricity generation application coupled with PCM storage systems in the high temperature range, due to the capability of PCMs to store and deliver energy at a given constant temperature. The recommendations for future research are also presented which provide insights about where the current research is heading and highlights the challenges that remain to be resolved. [ABSTRACT FROM AUTHOR]

Sorption thermal energy storage (STES) is perceived as one prospective way of thermal energy storage (TES) owing to the advantages of high energy density, negligible heat loss, flexible working modes and long-term storage capability. Based on STES, this paper exhibits an innovative resorption thermal energy storage (RTES) system, and the experimental prototype is established for heat and refrigeration cogeneration. Working pair of MnCl 2 CaCl 2 NH 3 is chosen, 4.8 kg MnCl 2 and 3.9 kg CaCl 2 impregnated in expanded natural graphite treated with sulfuric acid are filled in two sorption beds respectively. Experimental results indicate that the largest energy storage density reaches 1706 kJ/kg when charging and discharging temperature are 160 °C and 30 °C, respectively. The maximum average cooling power achieves 1.07 kW during discharging phase and corresponding SCP is 27.33 W/kg within the overall cycle period. When charging temperature increases from 135 °C to 160 °C, the energy efficiency of the resorption system for heat and refrigeration cogeneration augments steadily from 0.72 to 0.87 while the exergy efficiency rises slowly from 0.29 to 0.35. [ABSTRACT FROM AUTHOR]

A small-scale pilot project was built for the pavement-solar energy utilization in this paper. An automatic data acquisition system was designed to measure the effectiveness of the pavement solar energy system based on the operation data of 24 h a day in both summer and winter. Through 69 days (1656 h) of operation in summer, 2821 kW h of heat energy were stored in soil underground. In the transitional season, 4598 kW h of heat energy were taken out from soil during 104 days (2496 h) of operation in winter. The analysis showed that in summer, solar heat collection of asphalt pavement could effectively reduce 7 °C of its temperature. Under conditions of natural radiation, the average heat absorptivity of pavement was 37% and the average thermal storage effectiveness of the system was 17%. The electrical energy consumed by the system is only 11% of stored heat. During the winter, the asphalt pavement absorbs heat from underground soil which effectively increases its temperature, cutting 32% days of surface temperature below the freezing point. This not only save the energy for ice/snow removal but also mitigate associated safety risks. [ABSTRACT FROM AUTHOR]

Abstract: Transport and storage of low temperature sensitive products is an issue worldwide due to changes of the lifestyle and population increase. In the recent years, thermal energy storage (TES) using phase change materials (PCMs) is being highly studied and developed for cold storage applications. Furthermore, the PCM are normally encapsulated in containers and added in the available systems, usually in food processes. Therefore safety constraints as the compatibility of the PCM with other materials have to take into account. Hence the main goal of the paper is to study the corrosion effect of different metals and polymer materials in contact with some PCM used in low temperature applications. Results show that copper and carbon steel must be avoided as PCM containers, and aluminium is not recommended; stainless steel 316 is recommended when in contact with the tested PCM. Moreover, PP, PS, PET, and HDPE are not affected by a process of degradation and are also compatible with the PCM studied. [Copyright &y& Elsevier]

Abstract: Within this paper, the characteristics of aquifer thermal energy storage (ATES) for building cooling are discussed for the example of an existing ATES, which has been used for the cooling of the German Parliament Buildings for almost 10years. Based on the analysis of measurement data, it will be shown that the studied system has reached a COP between 3.6 and 7.8 over the time period considered. Based on the results of numerical simulations it will be shown that the COP can be even increased. Choosing proper operating conditions and design parameters, such as the temperature level of the cooling network or the regeneration temperature of the ATES, the efficiency of both, the studied system and future ATES projects, can be increased remarkably. [Copyright &y& Elsevier]

In the past decade, the interest in phase change materials (PCMs) has grown significantly due to their ability to store large amounts of thermal energy in relatively small temperature intervals. Accurate knowledge of thermo-physical properties is a prerequisite for any reliable utilization of these materials. The T-history method is widely used for the investigation of PCM. This paper presents an improved measurement technique for the characterization of PCM using the T-history method. The suggested improvements include the arrangements made in three different prospects: the experimental setup, data processing and data representation. T-history measurements of organic RT21 and inorganic SP22 A17 (RUBITHERM® GmbH) PCM were performed. The applied arrangements resulted in the temperature accuracy of ±0.3°C and the reduction of uncertainty associated with heat stored/released between the cooling and heating measurements. The obtained results showed some important aspects of the T-history PCM investigation and could provide more effective design and development process of the thermal energy storage systems based on the investigated materials. [ABSTRACT FROM AUTHOR]

This paper demonstrates the potential of a post-industrial ceramic commercially called Cofalit® as a promising, sustainable, and cheap filler material in a molten salt direct thermocline storage system. This ceramic, which comes from industrial treatment of asbestos containing waste, demonstrates relevant properties to store thermal energy by sensible heat up to 1100°C and is very inexpensive. In the present study, the compatibility of this ceramic with two different molten salts—the conventional binary Solar salt and a promising ternary nitrate salt also called HITEC XL—is tested at medium temperature (500°C) under static state. The objective is to develop a molten salt thermocline direct storage system using low-cost shaped ceramic as filler material. It should significantly decrease the cost of parabolic trough storage systems and simultaneously increase the efficiency of the plants by producing superheated steam at higher temperature. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the results of an experimental study conducted on a multi-tank thermal storage for solar hot water heating applications. The purpose of the study was to investigate the thermal behaviour of the stratified tank system when subjected to constant temperature charging and constant volume hourly draws. The experimental setup consisted of a charge loop to simulate a solar collector input, three commercially available 270L domestic hot water tanks and three side-arm, natural convection heat exchangers. Tests were performed over 8h periods for three different plumbing configurations and two different hourly draw volumes. Simulations were conducted using the TRNSYS simulation environment, and the results showed that the TRNSYS model was in good agreement with the experimental results, where the discrepancies between data were found mainly in the regions of high temperature gradients within the storage tanks. Preliminary simulations were also conducted using realistic charge and draw profiles to illustrate the thermal behaviour of the tanks under non-ideal operating conditions. [Copyright &y& Elsevier]

Abstract: Heat load variations in district heating systems are both seasonal and daily. Seasonal variations have mainly its origin from variations in outdoor temperature over the year. The origin of daily variations is mainly induced by social patterns due to customer social behaviours. Heat load variations cause increased costs because of increased peak heat load capacity and expensive peak fuels. Seasonal heat load variations are well-documented and analysed, but analyses of daily heat load variations are scarce. Published analyses are either case studies or models that try to predict daily heat load variations. There is a dearth of suitable assessment methods for more general analyses of existing daily load variations. In this paper, a novel assessment method for describing daily variations is presented. It is applied on district heating systems, but the method is generic and can be applied on every kind of activity where daily variations occur. The method was developed from two basic conditions: independent of system size and no use of external parameters other than of the time series analysed. The method consists of three parameters: the annual relative daily variation that is a benchmarking parameter between systems, the relative daily variation that describes the expected heat storage size to eliminate daily variations, and the relative hourly variation that describes the loading and unloading capacity to and from the heat storage. The assessment method could be used either for design purposes or for evaluation of existing storage. The method has been applied on 20 Swedish district heating systems ranging from small to large systems. The three parameters have been estimated for time series of hourly average heat loads for calendar years. The results show that the hourly heat load additions beyond the daily averages, vary between 3% and 6% of the annual volume of heat supplied to the network. Hereby, the daily variations are smaller than the seasonal variations, since the daily heat load additions, beyond the annual average heat load, are between 17% and 28% of the annual volume of heat supplied to the network. The size of short term heat storage to eliminate the daily heat load variations has been estimated to a heat volume corresponding to about 17% of the average daily heat supplied into the network. This conclusion can also be expressed as an average demand of 2.5m3 of heat storage volume per TJ of heat supplied by assuming a water temperature difference of 40°C. The capacity for loading and unloading the storage should be equal to about half of the annual average heat load for heat supplied into the network. [Copyright &y& Elsevier]

Abstract: Thermal energy storage (TES) has shown potential in improving the overall performance in energy systems, through shifting of thermal load demand, and through matching of uneven energy availability in time and in space. Latent heat TESs demonstrate advantages over sensible heat TESs for their high storage density and small temperature swing; however, lack of accurate knowledge in novel material properties and lack in a holistic design protocol often lead to difficulties in reaching technically viable storage design. With the aim of proposing a sound latent heat based TES design-to-validation protocol, this paper covers material property characterization through Temperature-history (T-history) method, heat exchanger design through heat transfer modeling, and model validation through experimental verification. A model for submerged cylindrically finned heat exchanger latent heat storage unit with phase change material was built. The results show that performance of gelled salt-hydrate based TES can be assessed with a pure conduction based model. This material property characterization-to-model verification approach may serve as a standard in providing accurate storage design for performance evaluation. [Copyright &y& Elsevier]

Abstract: The paper presents an analysis of thermodynamic losses in thermal reservoirs due to irreversible heat transfer and frictional effects. The focus is upon applications to large-scale electricity storage for which it is the loss in availability (or exergy) that is most relevant. Accordingly, results are presented as loss coefficients which are defined as the fractional loss of the entering availability. Only losses stemming from irreversibility are considered – heat losses to the surroundings are not included in the analysis. A number of simplifying assumptions have been adopted, but the results nonetheless clearly demonstrate the dependence of the losses on operating temperatures, reservoir geometry and mode of operation, and point the way towards methods of optimisation. Estimates for a typical installation suggest that the losses are not insignificant, particularly for one-off charge and discharge (i.e., for long-term storage), but remain acceptable for cyclic operation, so as to make the use of thermal reservoirs attractive for electricity storage schemes. [Copyright &y& Elsevier]

Abstract: In the paper, two systems for solar thermal power plants (STPPs) are devised for improving the overall performance of the plant. Each one attempts to reduce losses coming from two respective sources. The systems are simulated and compared to a reference STPP. They consists on: (a) a double thermal energy storage (DTS) with different functionalities for each storage and (b) the subdivision of the solar collector field (SSF) into specialised sectors, so that each sector is designed to meet a thermal requirement, usually through an intermediate heat exchanger. This subdivision reduces the losses in the solar field by means of a decrease of the temperature of the heat transfer fluid (HTF). Double thermal energy storage is intended for keeping the plant working at nominal level for many hours a day, including post-sunset hours. One of the storages gathers a fluid which is heated up to temperatures above the nominal one. In order to make it work, the solar field must be able to overheat the fluid at peak hours. The second storage is the classical one. The combination of both allows the manager of the plant to keep the nominal of the plant for longer periods than in the case of classical thermal energy storage. To the authors’ knowledge, it is the first time that both configurations are presented and simulated for the case of parabolic through STPP with HTF technology. The results show that, if compared to the reference STPP, both configurations may raise the annual electricity generation (up to 1.7% for the DTS case and 3.9% for the SSF case). [Copyright &y& Elsevier]