Analysis and optimization of underground thermal energy storage using depleted oil wells

Underground thermal energy storage (UTES) is an important technology to utilize the industrial waste heat and the fluctuating renewable energy. This paper proposed a new deep UTES system by using single depleted oil well (DOW), and the coaxial borehole heat exchanger with insulation is introduced to retrofit the DOW for seasonal TES. At first, a comprehensive model combing wellbore and formation heat transfer was built, evaluating the performance of heat storage in summer and space heating in winter. Monte Carlo method was used for parameters sensitivity analysis, to obtain the correlation between storage efficiency and five influence parameters. The results indicated that inlet temperature showed the strongest influence on the storage efficiency. By the optimal design, it was found that for an existing heat source, there was an optimal well depth to obtain maximum storage efficiency. During the annual operation, total heat storage for the DOW with depth of 2000 m was about 4.7 × 106 MJ and total heat extraction was about 2.9 × 106 MJ more than 8 × 105 MJ from that without previous heat storage progress. Consequently, the DOW-UTES system could be used to implement seasonal heat storage or waste heat recovery in summer to provide space heating in winter.