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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.

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.