• Energy Research

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/δ defined 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/δ and 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.

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/δ defined 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/δ and 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.

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 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.

Heat storage is the key factor in future energy systems with a large share of renewable energies. A shell and tube heat storage tank capable of both long and short term heat storage has been developed by utilizing stable supercooling of sodium acetate trihydrate. Theoretical and experimental investigations were carried out to determine power, heat exchange capacity rate (HXCR), and stored energy of the heat storage tank during the charge and discharge. Theoretically, a multiphase computational fluid dynamics (CFD) model of the storage was developed. The CFD model was validated against the measurement. The heat transfer mechanisms of the heat storage were investigated. The results show that the multiphase model can satisfactorily predict thermal behaviour of the heat storage under different operation conditions. The CFD model shows that 21.15 kWh of heat was charged into the heat storage unit within 7.5 h, compared to 21.16 ± 0.85 kWh in the measurement. During discharge, 14.05 kWh of sensible heat was discharged as short term heat storage, and 7.65 kWh of latent heat can be released on-demand as long term heat storage. The measured sensible heat and latent heat during discharge are 13.57 ± 0.54 kWh and 7.56 ± 0.30 kWh, respectively, corresponding to a relative difference of 1.2–3.7 % compared to the CFD model. There is a strong natural convection flow in some of the tubes, which significantly increases the heat transfer rate. The energy-weighted heat exchange capacity rates are 850 W/K and 795 W/K during charge and discharge of the heat storage, respectively. The findings of the paper give a good reference for designers and manufacturers of latent heat/cold storage.

Heat storage is the key factor in future energy systems with a large share of renewable energies. A shell and tube heat storage tank capable of both long and short term heat storage has been developed by utilizing stable supercooling of sodium acetate trihydrate. Theoretical and experimental investigations were carried out to determine power, heat exchange capacity rate (HXCR), and stored energy of the heat storage tank during the charge and discharge. Theoretically, a multiphase computational fluid dynamics (CFD) model of the storage was developed. The CFD model was validated against the measurement. The heat transfer mechanisms of the heat storage were investigated. The results show that the multiphase model can satisfactorily predict thermal behaviour of the heat storage under different operation conditions. The CFD model shows that 21.15 kWh of heat was charged into the heat storage unit within 7.5 h, compared to 21.16 ± 0.85 kWh in the measurement. During discharge, 14.05 kWh of sensible heat was discharged as short term heat storage, and 7.65 kWh of latent heat can be released on-demand as long term heat storage. The measured sensible heat and latent heat during discharge are 13.57 ± 0.54 kWh and 7.56 ± 0.30 kWh, respectively, corresponding to a relative difference of 1.2–3.7 % compared to the CFD model. There is a strong natural convection flow in some of the tubes, which significantly increases the heat transfer rate. The energy-weighted heat exchange capacity rates are 850 W/K and 795 W/K during charge and discharge of the heat storage, respectively. The findings of the paper give a good reference for designers and manufacturers of latent heat/cold storage.

A novel combined solar heating plant with flat plate collectors (FPC) and parabolic trough collectors (PTC) was constructed and put into operation in Taars, 30 km north of Aalborg, Denmark in August 2015. To assess the thermal performance of the solar heating plant, global radiation, direct normal irradiance (DNI) and total radiation on the tilted collector plane of the flat plate collector field were measured. To determine the accuracy of the measurements, the calculated solar radiations, including horizontal diffuse radiation, DNI and total tilted solar radiation with seven empirical models, were compared each month based on an hourly time step. In addition, the split of measured global radiation into diffuse and beam radiation based on a model developed by DTU (Technical University of Denmark) and the Reduced Reindl correlation model was investigated. A new method of combining empirical models, only based on measured global radiation, was proposed for estimating hourly total radiation on tilted surfaces. The results showed that the DTU model could be used to calculate diffuse radiation on the horizontal surface, and that the anisotropic models (Perez I and Perez II) were the most accurate for calculation of total radiation on tilted collector surfaces based only on global radiation under Danish climate conditions. The proposed method was used to determine reliable horizontal diffuse radiation, DNI and total tilted radiation with only the measurement of global radiation. Only a small difference compared to measured data, was found. The proposed method was cost-effective and needed fewer measurements to obtain reliable DNI and total radiation on the tilted plane. This method may be extended to other Nordic areas that have similar weather.

A novel combined solar heating plant with flat plate collectors (FPC) and parabolic trough collectors (PTC) was constructed and put into operation in Taars, 30 km north of Aalborg, Denmark in August 2015. To assess the thermal performance of the solar heating plant, global radiation, direct normal irradiance (DNI) and total radiation on the tilted collector plane of the flat plate collector field were measured. To determine the accuracy of the measurements, the calculated solar radiations, including horizontal diffuse radiation, DNI and total tilted solar radiation with seven empirical models, were compared each month based on an hourly time step. In addition, the split of measured global radiation into diffuse and beam radiation based on a model developed by DTU (Technical University of Denmark) and the Reduced Reindl correlation model was investigated. A new method of combining empirical models, only based on measured global radiation, was proposed for estimating hourly total radiation on tilted surfaces. The results showed that the DTU model could be used to calculate diffuse radiation on the horizontal surface, and that the anisotropic models (Perez I and Perez II) were the most accurate for calculation of total radiation on tilted collector surfaces based only on global radiation under Danish climate conditions. The proposed method was used to determine reliable horizontal diffuse radiation, DNI and total tilted radiation with only the measurement of global radiation. Only a small difference compared to measured data, was found. The proposed method was cost-effective and needed fewer measurements to obtain reliable DNI and total radiation on the tilted plane. This method may be extended to other Nordic areas that have similar weather.