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Stable supercooling of sodium acetate trihydrate (SAT) composites can be used to store heat over long periods. When the melted SAT is supercooled and in temperature equilibrium with the ambient, the latent heat of fusion is essentially stored without further heat loss. The main limitation of this technology is that the supercooled SAT may crystalize spontaneously and release the stored heat before desired if the storage design does not allow for the principle to work. These investigations elucidate the effects of charge and discharge temperatures and durations on the stability of the supercooling in storage units designed for domestic hot water application.

Pit thermal energy storage (PTES) is one of the most promising and affordable thermal storage, which is considered essential for large-scale applications of renewable energies. However, as PTES volume increases to satisfy the seasonal storage objectives, PTES design and application are challenged. These difficulties trigged an interest in PTES investigations. This paper aims to identify the success factors and research gaps of PTES by an up-to-date evaluation of 160 recent publications. Existing technical elements that affect PTES thermal properties inclusive geometry design, inlet/outlet design, cover design, and materials, are outlined in depth. Numerical studies are categorized in terms of their mathematical theory and research purposes for a systematic discussion. The current application status of PTES systems worldwide is summarized from four aspects: storage material, geological design, operation strategy, and storage duration. For projects in operation, special attention is given to gathering and comparing operational data on solar fraction, storage efficiency, storage cycle, and PTES temperature. This review outlines the progress and potential directions for PTES design and numerical studies by identifying the research gaps that require further effort.

A generic numerical model of PCM water energy storage is developed and validated by experiments. The numerical model consists of a water region and a PCM region. Models of the two regions are derived by the energy differential equations and solved by the implicit method. The solid-liquid PCM behavior is modeled based on its enthalpy-temperature relation, in which the melting/solidification phase is linearized. Special treatment is developed for the time steps with a phase change, which significantly improves the prediction accuracies of PCM temperatures and the melting fractions. An iteration method is applied to the two regions for coupling calculation. The energy balance of the model is examined in each time step. Extended functionalities of the numerical model are further developed, including separate heat loss coefficients, auxiliary heaters, flexible inlet and outlet layout, the mixing effect in the water tank, and three PCM supercooling-activation modes. The experimental verification for a PCM water energy storage was carried out at Technical University of Denmark. The PCM and water layer temperatures are calculated. The simulated and measured outlet temperatures and heat content are compared. The results show that the simulated outlet temperatures are maximum of 4.6 K deviation from the measured, with a relative error of 10.4 %. The relative error of the heat content is within 1 %, and the energy balance errors are within 5 %. The extended application and limitation of the numerical model are discussed, and the possible error sources are analyzed.

An accurate and less time demanding model is required when integrating pit thermal energy storage (PTES) into solar heating systems. Multi-node (1D) models are commonly used, but these models face challenges when calculating PTES thermal stratification and heat loss. Therefore, a full-scale computational fluid dynamics (CFD) model of PTES inclusive water and soil regions is developed using FLUENT to improve the accuracy of heat transfer calculation of a multi-node model. The CFD model is validated against the Dronninglund PTES measurements regarding PTES thermal stratification, inlet/outlet energy flow, and soil temperature distribution. The model corresponds well to the measurements in three aspects: (i) a maximum temperature difference of 1 K in the water region; (ii) a maximum temperature difference of 2 K in the soil region; (iii) a maximum outlet temperature difference of 3 K. An indicator RΔT/δdefined as the ratio between the thermocline temperature difference and the thermocline thickness is proposed to assess suitable grid size for PTES models, and the quantitative relationship between RΔT/δand grid size is recommended. Investigations with a range of grid sizes show that by using the recommended grid size, the prediction accuracy of the multi-node model TRNSYS Type 343 is significantly improved. The root mean square deviations of the predicted MIX number are decreased by 11–43 % for different years, and the relative differences of the monthly charge/discharge energy from the measurement are within 5 %. The findings of this study provide guidance for selecting appropriate grid sizes to achieve better calculation accuracy for large-scale PTES.

Water pit thermal energy storage systems have been demonstrated in Denmark and have proven effective in increasing the solar thermal fractions of district heating systems and in covering the mismatch between heat demand and production. This study analyzed five years of measurement data for two PTES systems in Denmark, namely Marstal and Dronninglund. Their efficiency was assessed using energy, exergy, and a seasonal efficiency indicator. The degree of stratification was investigated using the MIX number, the stratification coefficient, and a newly-introduced indicator, exergy destruction. Exergy destruction was shown to be a promising indicator for assessing stratification since it can be used to compare PTES systems with different heat losses, providing a quantitative evaluation of the amount of mixing. In addition, the seasonal efficiency was found to be suitable for estimating the long-term efficiency of combined seasonal and short-term storage systems. The storage in Dronninglund had 92% energy and 73% exergy efficiency, while Marstal had 63% energy and 48% exergy efficiency. All stratification and efficiency indicators showed that the storage in Dronninglund performed better overall than the one in Marstal.

Large-scale water pit thermal energy storage (PTES) promotes solar district heating (SDH) system as one of the most potential renewable applications for carbon neutrality. PTES needs vast investment and operates in a complicated system with numerous components, highlighting the need for a suitable simulation tool for tech-economic and feasibility investigations. This paper experimentally and theoretically investigated the long-term thermal performance of a 60,000 m3PTES in Dronninglund, Denmark. Five years measurements were analyzed to investigate the development of temperatures, heat flows, and thermal stratification in heat storage. A modified 2D model was proposed to calculate the thermal performance of the large-scale PTES based on the XST model in TRNSYS. The results showed that the developed model predicts well the storage temperatures and the heat flows. For one-year validation, the deviations of annual charged/discharged energy, internal energy content, and annual thermal loss between the model and the measurements were 2.0%/1.8%, 2.8% and 1.3%, respectively. The Dronninglund PTES showed 90.1% storage efficiency in the five-year investigation due to its high storage utilization cycle of 2.16. Even without any insulation on the sidewall and the bottom of the PTES, the average thermal loss from the two parts only accounted for 32.3% of total heat loss. Meanwhile, the soil region needed four years to stabilize. Approx. 24.4% of the heat loss in summer from the sidewall and the bottom is regained by the PTES in winter, when there is a low temperature in the heat storage. The findings of the paper serve as a good reference for designers and practitioners of water pit heat storage.

This paper presents numerical investigations on a heat storage utilizing sodium acetate trihydrate (SAT) as phase change material (PCM). The heat storage can be used both in short-term and in long-term by utilizing stable supercooling of SAT. The store contains 137.8 kg PCM and 75 L water. Based on a validated CFD model, the flow conditions of the heat storage was analyzed. Uneven flow distribution inside the heat storage was revealed. Three design optimization methods were investigated to eliminate the uneven flow distribution. The results were analyzed using key performance indicators inclusive charging time, charged heat, degree of thermal stratification and the mixing of the heat storage. The influence of flow direction, inlet size and addition of a porous plate on the thermal performance of the heat storage were elucidated. Concerning flow direction, after moving the inlet from the bottom to the top of the storage, the time needed to completely melt the PCM was shortened by 50%. The best storage design was identified: For charge of the storage, the top inlet should be used, while for discharge, the bottom inlet should be used. Concerning the size of inlet opening, the charging time of the heat storage was reduced from 75 min to 51 min by using 3.0 mm radius instead of a 11.2 mm inlet. The small inlet size (3.0–8.0 mm) was suggested to make a uniform temperature distribution inside the heat storage. Short circuit was completely eliminated by adding a porous plate with 10% porosity. The charging time of the heat storage was shortened 28% by adding the porous plate. Finally, recommendations were proposed for different applications of the heat storage. The findings of the paper serve as a good basis for designers and manufacturers of PCM heat storages.

•A flexible heat storage utilizing stable supercooling of SAT investigated.•Thermal performance determined for the first time by experiments and simulations.•Validity of the CFD model documented.•Short term and long term heat storage capacities determined.•Power and HXCR of the heat storage under different conditions investigated.

Purpose – Low flow bikini solar combisystems and high flow tank-in-tank solar combisystems have been studied theoretically. The aim of this paper is to study which of these two solar combisystem designs is suitable for different houses. The thermal performance of solar combisystems based on the two different heat storage types is compared. Design/methodology/approach – The thermal performance of Low flow bikini solar combisystems and high flow tank-in-tank solar combisystems is calculated with the simulation program TRNSYS. Two different TRNSYS models based on measurements were developed and used. Findings – Based on the calculations it is concluded that low flow solar combisystems based on bikini tanks are promising for low energy buildings, while solar combisystems based on tank-in-tank stores are attractive for the houses with medium heating demand and old houses with high heating demand. Originality/value – Many different Solar Combisystem designs have been commercialized over the years. In the IEA-SHC Task 26, twenty one solar combisystems have been described and analyzed. Maybe the mantle tank approach also for solar combisystems can be used with advantage? This might be possible if the solar heating system is based on a so-called bikini tank. Therefore, the new developed solar combisystems based on bikini tanks is compared to the tank-in-tank solar combisystems to elucidate which one is suitable for three different houses with low energy heating demand, medium and high heating demand. [ABSTRACT FROM AUTHOR]

•Flexible heat storages utilizing stable supercooling of SAT characterized.•Heat exchange capacity rates of short/long term SAT heat storages analysed.•Main issues of application reviewd: Phase separation and spontaneous crystallization.•Success rates of controllable supercooling and solidification analysed.•Benefit and advantages of SAT heat storages reviewed and identified.

•A reliable economic analysis tool developed for household solar heating systems.•An optimal SF of 10%−13% for a household solar heating system with ASHPs.•The lowest LCoH for solar combisystems found with 100% solar fraction.•A low temperature heating network reduces the LCoH of household heating systems.•An increase in heating load intensity results in an increase of LCoH.