Geothermics 57 (2015) 56–72 Contents lists available at ScienceDirect Geothermics jo ur nal homep age: www.elsevier.com/locate/geothermics Impact of coupled heat transfer and water flow on soil borehole thermal energy storage (SBTES) systems: Experimental and modeling investigation Ali Moradi a,∗ , Kathleen M. Smits a , Jacob Massey a , Abdullah Cihan b , John McCartney c a Center for Experimental Study of Subsurface Environmental Processes (CESEP), Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO, USA b Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA c Department of Structural Engineering, University of California, San Diego, CA, USA a r t i c l e i n f o Article history: Received 6 October 2014 Accepted 25 May 2015 Keywords: SBTES systems Vadose zone Convective heat transfer Phase change Numerical model Experimental investigation a b s t r a c t A promising energy storage option is to inject and store heat generated from renewable energy sources in geothermal borehole arrays to form soil-borehole thermal energy storage (SBTES) systems. Although it is widely recognized that the movement of water in liquid and vapor forms through unsaturated soils is closely coupled to heat transfer, these coupled processes have not been considered in modeling of SBTES systems located in the vadose zone. Instead, previous analyses have assumed that the soil is a purely conductive medium with constant hydraulic and thermal properties. Numerical modeling tools that are available to consider these coupled processes have not been applied to SBTES systems partly due to the scarcity of field or laboratory data needed for validation. The goal of this work is to test different conceptual and mathematical formulations that are used in heat and mass transfer theories and determine their importance in modeling SBTES systems. First, a non-isothermal numerical model that simulates coupled heat, water vapor and liquid water flux through soil and considers non-equilibrium liquid/gas phase change was adopted to simulate SBTES systems. Next, this model was used to investigate different coupled heat transfer and water flow using nonisothermal hydraulic and thermal constitutive models. Data collected from laboratory-scale tank tests involving heating of an unsaturated sand layer were used to validate the numerical simulations. Results demonstrate the need to include thermally induced water flow in modeling efforts as well as convective heat transfer, especially when modeling unsaturated flow systems. For the boundary conditions and soil types considered, convective heat flux arising from thermally induced water flow was greater than heat transfer due to conductive heat flux alone. Although this analysis needs to be applied to the geometry and site conditions for SBTES systems in the vadose zone, this observation indicates that thermally induced water flow can have significant effects on the efficiency of heat injection and extraction. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The rapidly growing gap between consumption and produc- tion of energy can be addressed by introducing new cost-effective and renewable energy sources such as wind or solar energy. An important issue limiting the implementation of renewable energy sources is energy storage, as it is not possible to control the tim- ing of the supply of solar or wind energy. For example, unlocking solar energy’s full potential becomes relatively complex because its ∗ Corresponding author. Tel.: +1 3032733767. E-mail address: amoradig@mines.edu (A. Moradi). rate of generation is highest mid-day and during summer months, which is offset from the timing of the highest rates of heat consump- tion in winter (Pinel et al., 2011). Although a significant amount of research is being devoted to storage of electricity, the storage of heat can be more cost-effective and can be done on different scales. Soil-borehole thermal energy storage (SBTES) systems are one such technology that has been shown to be effective at storing heat collected from solar thermal panels in the summer so that it can be later extracted during the winter (Sibbitt et al., 2007, 2012). SBTES systems involve direct circulation of heated fluid through closed-loop geothermal heat exchangers in closely spaced vertical borehole arrays (Claesson and Hellstrom, 1981; Pinel et al., 2011; Bas ¸ er and McCartney, 2015). The geothermal heat exchangers are http://dx.doi.org/10.1016/j.geothermics.2015.05.007 0375-6505/© 2015 Elsevier Ltd. All rights reserved.