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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. A test rig with 10 identical heat storage units with water and 35 kg sodium acetate trihydrate composite was set up. Multiple heating and cooling cycles were carried out with charge temperatures between 84 °C and 94 °C for various durations and with discharge temperatures from 10 °C to 35 °C. The investigations showed that charging the storage units with an inlet temperature of 89 or 92 °C resulted in the highest occurrences of stable supercooling. Discharging with inlet temperatures of 10 °C resulted in higher occurrence of spontaneous solidification during discharge compared to higher inlet temperatures. There was a large deviation in how each tank performed. The best performing tank achieved stable supercooling in 87 % of the test cycles and the worst performing tank achieved stable supercooling in 30 % of the test cycles.

Water pit thermal energy storage (PTES) systems have proven a cheap and efficient storage solution for solar district heating systems. This is partly due to their low cost, deriving from low material usage as the ground is used as the weight-bearing structure. Further savings are obtained by the absence of insulation toward the ground, although this makes the ground-storage thermal interaction more pronounced than other storage technologies. However, it remains unclear how the different operation strategies affect the ground temperature and heat losses, especially in cases where groundwater is present. A simulation model was created using ANSYS and validated using operation data from the PTES in Marstal to investigate this. Using the validated model, it was found that the presence of groundwater could increase heat losses up to 60%, while the heat losses were unaffected when the groundwater table was more than 13 m below the storage bottom. The groundwater temperature could be maintained below 20 °C for a seasonal PTES if the groundwater table was at a depth of 25 m. However, maintaining this temperature for a short-term PTES operation was not feasible. Generally, the soil temperature was unaffected outside of a 100 m radius around the storage regardless of the operation type or the soil's thermal properties.

Pit thermal energy storage (PTES) is an efficient renewable energy storage technology widely used in large-scale solar district heating systems. Accurate modeling of mixing in a PTES due to inlet flow is key in calculating heat storage performance. However, the commonly used one-dimensional PTES models fail to consider inlet mixing due to the three-dimensional nature of the mixing flow. This research adopts a three-dimensional model to analyze the dynamic behavior of inlet mixing inside the PTES. The model is validated against measurements of the Dronninglund PTES. To quantify the inlet mixing impact, two performance indicators (i.e., the penetration height (Z) and the energy distribution ratio (nj)) are proposed. The parametric analysis revealed that is more dependent on the Reynold (Re) number than the Froude (Fr) number, while both the Re and Fr numbers influence nj. According to the dimensional theory, the penetration height shows a power-law relation with time. For the energy distribution ratio , a power-law relation with time is seen, although an asymptotic formula is needed in the region of a negative buoyancy jet. Finally, the inflow mixing inside the PTES is characterized under various operating conditions by empirical correlations. The results of this study could be used to improve the current one-dimensional heat storage models in terms of inlet mixing.

The thermal performance of a 115 L latent heat storage prototype for cooling data centers was investigated. Experimentally, the heat transfer power and heat absorbed by the heat exchanger during the charging and discharging processes were measured at two flow rates (5 and 10 L/min). Numerically, two phase-change models were developed using the enthalpy and effective heat capacity methods, respectively. The results showed that the enthalpy method provides an overall better prediction of the absorbed heat, whereas the other method only agrees well with the measured results during the melting process. Thus, it is suggested that further modification of the effective heat capacity with temperature improves the agreement between the results. For a volume flow rate of 5 L/min, the average heat transfer power predicted by the enthalpy model was 2290 W during the melting process and > 920 W during the solidification process due to the smaller temperature difference for heat transfer caused by supercooling. The prototype achieved the highest average heat exchange capacity rate when melted to a 50% of its total capacity. This study provides a baseline for predicting and improving the thermal performance of latent heat storage.

The aim of this study is to evaluate the long term performance of the water pit heat storage of the Dronninglund solar district heating plant in Denmark, located in the peninsula of Jutland. Experimental storage data from 2017 is evaluated, including the charged and discharged energy, its efficiency, the diffusers outlet temperatures, envelope heat losses and many more. In addition, simulation of different models are carried out based on Type 1535 and 1301 elements of TRNSYS, developed by TESS. The objective of these simulations is to analyze not only the long term performance of the storage, but the accuracy that the results have compared with the actual data. The differences between models are the number of layers in which the storage is divided, changes in geometry or modifications in soil properties to study how these changes affect to the performance of the storage. The results show that the models simulate with a pretty high accuracy the outlet temperatures of the storage, especially the ones from the top and bottom diffusers. The actual charged and discharged energy of the year in question is 11868 MWh and 11250 MWh, respectively. The simulations show a high accuracy in terms of charged energy, with only a -3% deviation in the worst case. Although the discharged energy is not as accurate as the charged energy, the results are quite acceptable. Total heat losses of the real storage have a value of 1157 MWh. The results of all models are deviate significantly from the actual value, both overestimated and underestimated, but the possible reasons are explained throughout this report. However, the distribution of the different types of heat losses among all models results in about 65% of top losses, 33% of edge losses and 2% of bottom losses, except in the model where the soil properties are modified, with a different percentage distribution. The same applies for the efficiency of the storage, whose actual value is 90%. The simulated efficiency values are higher f, Objectius de Desenvolupament Sostenible::11 - Ciutats i Comunitats Sostenibles, Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.2 - Per a 2030, augmentar substancialment el percentatge d’energia renovable en el con­junt de fonts d’energia

Thermal stratification in water-based storages can be destroyed by mixing, heat diffusion, and thermal conduction. For this reason, the evaluation of stratification in water-based thermal energy storages is important for assessing their performance. The most promising indicators were identified and assessed based on their suitability for use in practical applications. The selected stratification indicators were calculated for four simulated storage scenarios comprising a fully stratified, a fully mixed, and two realistic storages. It was found that most indicators had severe limitations in their application. For this reason, a new indicator called internal exergy destruction was proposed, which can be used in combination with the overall exergy efficiency for assessing the performance and stratification of thermal energy storages. The main benefit of internal exergy destruction is that it can be used to compare storages with different heat loss coefficients. In addition, it separates the effects of mixing from the heat losses and is easily applied to real-life storages.

Fresnel lenses are used in a wide range of solar energy applications, primarily due to their reduced material usage, low cost, and high optical efficiency. This study presents an investigation of the world’s first full-scale Fresnel lens solar collector field. The collector field consists of 144 two-axis tracking solar collectors manufactured by the Danish company Heliac and supplies heat to the local district heating network in Lendemarke, Denmark. The thermal performance of the solar collector field was determined using the quasi-dynamic test method. It was found that the peak efficiency was 11% lower compared to a brand-new collector and that heat losses from the collectors made up half of the total heat losses of the solar field. The reduction in the peak efficiency was primarily caused by soiling as the collectors were exposed to outdoor conditions for one year without cleaning. Furthermore, the system’s annual performance was determined using a simulation model developed in TRNSYS and validated by comparison to measurement data. For 2020, the heat generation was 373 kWh/m2 (relative to aperture area) when operating with an outlet temperature of 90 °C and inlet temperature of 50 °C. Additionally, a sensitivity analysis of the annual heat generation was performed, varying the ground cover ratio, mean collector temperature, and soiling level. The sensitivity analysis showed that the heat generation was relatively insensitive to changes in the mean collector temperature, demonstrating that the collectors are suitable for generating heat above 100 °C, unlike flat-plate collectors.

A model of PTES from TRNSYS containing 1D water pit and 2D soil is used as a research object. The measured data of 60,000 m3 of PTES in Dronninglund, Denmark, in 2017 are utilized as input parameters and validation data. The stratification coefficient and Mix number are taken as an indicator to evaluate the stratification effect during the energy charging and discharging period. The coupling effects of layer number and time step on the computational accuracy are investigated.

This study presents the design and performance of the Brønderslev hybrid plant – the world's first concentrated solar power (CSP)-biomass plant to utilize waste heat. The combined heat and power plant consist of a 16.6 MW parabolic trough collector field, two 10 MW biomass boilers, and an organic Rankine cycle (ORC) system with an electrical output of 4 MW. The solar collector field and biomass boilers are hydraulically connected and supply heat in parallel to the ORC system. A multi-stage heat recovery process ensures that the waste heat is supplied to the local district heating grid. The solar field can also be operated independently of the biomass plant, supplying heat directly to the district heating grid. The plant operation was investigated by analyzing measurements for 2020, and the monthly heat and electricity generation was calculated for each component. Additionally, the performance of the solar field and ORC was elucidated, and the ORC was found to have a maximum electrical efficiency of 20.6 %. Detailed results are shown for one day with joint heat supply to the ORC from the solar field and biomass boilers, and the corresponding energy flows are visualized using a Sankey diagram. The total efficiency of the biomass plant for the investigated year was 93.6 % based on the higher heating value of the fuel. Overall, it was found that the hybrid plant performed reliably and successfully demonstrated the potential of CSP-biomass hybridization.

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.

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.

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.

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 techeconomic and feasibility investigations. This paper experimentally and theoretically investigated the longterm thermal performance of a 60,000 m3 PTES 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.

The solar preheating wall provides an effective solution to the conflict between the requirement for fresh air and the need for heat conservation in winter by preheating fresh ventilation air. In this paper, a study of the glazed solar preheating collector wall is carried out by numerical simulation with the aim of improving heat collection efficiency through structural optimization. The influence of the environment as well as its range of applications are investigated. The characteristics of the internal flow and temperature distribution are obtained to facilitate the structural analysis. The influence of air volume and structural parameters on thermal efficiency is investigated, providing a good reference for structural design optimization. In addition, it is observed that the exergy efficiency of the optimized design is 1.3 times higher than that of the conventional solar wall, while the thermal efficiency is 15% higher than traditional ones. The heat loss rate is reduced by 40% in high wind and extremely cold regions compared to that of the unglazed one. The maximum supply percentage can reach 69.4% and the temperature rise is approximately 20 °C. Finally, a parametric investigation is carried out on different functional buildings, and an optimal structure in Xi'an is given as a reference. The energy collection and transfer efficiency increase with the increase of air volume, but the energy saving rate decreases. The results indicate that this solar wall is ideal for buildings with low fresh air demand, such as single residences and small offices. It also has a considerable preheating effect on high fresh air demand ones.

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.

In this paper, an open source method for calculating self-shading in fields of two-axis tracking solar collectors of arbitrary geometry was developed and validated. The method was used to investigate the impact of latitude and collector aperture shape on annual shading loss. Simulations were carried out for the entire design space of field layouts by uniformly discretizing the layout parameters, i.e., aspect ratio, offset, rotation, and ground cover ratio. Results showed shading losses generally increase with latitude, and the optimum aspect ratio decreases with distance from the equator. Aperture shape was shown to significantly impact power output; the annual shading loss was lowest for the rectangular collector and highest for the square collector. Also, the impact of sub-optimal rotation of rectangular arrays was presented.

A flexible thermal energy storage with 137.8 kg PCM and 75 L water was introduced. The heat storage combined short-term and long-term heat storage functions by utilizing sodium acetate trihydrate as heat storage material. The thermal performance and flow characteristics of the heat storage were investigated by experiments and by CFD simulations. The result show that after a full charge to 92 °C, the heat storage can flexibly release heat in two steps: In the first step, 13.7 kWh sensible heat can be released for short-term use; in the second step, 7.8 kWh latent heat can be released for long-term use. When releasing heat, there is laminar flow of water and a high degree of thermal stratification in the tank. During discharge of the sensible heat, the heat storage can provide 294 L hot water with an average temperature of 68.2 °C. During discharge of the latent heat, 334 L hot water with an average temperature of 46.7 °C were drawn from the heat storage. In the charging process, a fluid short circuit existed inside the water region. Part of hot water flows out of the tank before sufficiently heated. There was no obvious thermocline inside the tank during charging. The flow rate has a big influence on the thermal characteristics of the heat storage during the charge and discharge periods. By increasing the flow rate from 4 L/min to 14 L/min, the charging time was shortened by 67% from 276 min to 92 min while the discharging time was shortened by 65% and 83% respectively in releasing sensible heat and latent heat.