Numerical investigations of long-term thermal performance of a large water pit heat storage

Water pit heat storage is an effective long-term heat storage. Experimental and theoretical investigations of a 60,000 m3 water pit heat storage in Dronninglund, Denmark was carried out with an aim to study its long-term thermal performance. Experimentally, detailed measurements were analyzed to monitor thermal behavior of the store, including flow rates, temperatures of the inlet and outlet flow, thermal stratification in the store and temperatures of the surrounding soil at four different depths.

Theoretically, a simulation model of the water pit heat storage was developed based on the Type 343 model (ICEPIT) in TRNSYS. The calculated temperatures were compared to the measurements. The developed model was used to investigate the influence of soil properties and inlet arrangement on the thermal performance of the store.

The result show that the simulation model predicts well the thermal behavior of the store. Over the whole year, the calculated average temperature of the store differed within +/- 4.7 K from the measured values. The measurements show that the annual charged and discharged heat are 12,787 MWh and 11,957 MWh respectively.

The deviation of the charged and discharged energy between the simulation and the experiment did not exceed +/- 4%. The annual storage efficiency reached 90% in the second year of operation. The calculated ground temperatures were consistent with the measured ones at different depths. Measured heat loss of the store is 1392 MWh.

As elucidated by the model, the heat loss from the cover contributed to 60% of the annual heat loss, while the heat loss from the sidewall and the bottom contributed 38% and 2% respectively. The influence of soil types on thermal performance of the store is significant. For a PTES surrounded by dry soil, the storage efficiency can reach 90%, while for a PTES surrounded by wet sandy gravel, the storage efficiency decreases by 7%.

Change of the height of the middle inlet/outlet diffuser could slightly improve the storage efficiency by 1%. The restrains of the model were analyzed and the focus areas of further investigations were pointed out. The findings of the paper give good reference for researchers, designers and consultants in planning of a solar heating plant with a large water pit heat storage.