Improved analytical modeling and system performance evaluation of deep coaxial borehole heat exchanger with segmented finite cylinder-source method

Abstract

The deep borehole heat exchanger is attracting more and more attention due to its higher performance. But there is no suitable analytical model for its fast simulation and system design. Classic analytical models of borehole heat exchangers (BHEs), assume that the initial soil temperature and heat flux of the borehole wall are uniform. Those makes them not applicable to deep borehole systems in which geothermal gradient is dominant. This study introduced and formulated a segmented finite cylinder-source model (SFCS), which can provide an accurate simulation tool. Data of distributed thermal response test and finite difference model are used for model comparison. A comprehensive comparative study is implemented to identify the performance differences between the classical and the new model. It is shown from simulation results that the newly proposed SFCS model can better describe the water as well as soil temperature field, especially for deeper borehole systems. In addition, three new influencing factors, namely pipe sizing ratio, inlet thermal flow rate and initial soil temperature distribution function, are introduced for system performance evaluation. Some valuable findings are identified, which can promote a better system design and achieve a higher system performance.