Economic Evaluation of the Coaxial Borehole Heat Exchanger

Abstract

Geothermal energy plays a critical role in energy structure improvement and carbon emissions reduction. A newly-developed geothermal energy extraction technology, the coaxial borehole heat exchanger (CBHE) can produce in most parts of the world. Its application requires neither water containing formation nor multiple wells since the working fluid is circulated and heated inside the coaxial pipe. However, the feasibility of CBHE remains unproved. This paper proposes an improved unsteady-state heat transfer model to evaluate the heat production capacity and the economic benefit of CBHE. The heat transfer model is solved by the finite difference method. Then, this paper presents the economic feasibility analysis of a base case, an insulated case, a high-thermal gradient case, and a case in dry hot rock. The temperature profile and output performances during certain production time are calculated. The result shows that the outlet fluid temperature and thermal power will be relatively stable in one month after a sharp decrease at the beginning. The paper's base case has a low production efficiency since the CBHE system must be insulated for the effective prevention of fluid loss in the central pipe. The insulated case can double the thermal power and halve the payoff period under the same conditions compared with the base case. The geothermal gradient and well depth play important roles in the CBHE output performance. An insulated CBHE can supply more than 20000m2 area and need only 5.6 years payoff period once the geothermal gradient and well depth are higher than 0.07K/m and 5000m, respectively. The CBHE is also an economic and environmental method to develop the hot dry rock reservoir. 9.7 years payoff period is needed for a simulated CBHE in a simulated 3500m deep well at Qiabuqia HDR in Gonghe Basin.