Abstract
We prepared bentonite-based grouts for use in the construction of vertical ground heat exchangers (GHEs) using various proportions of silica sand as an additive, and measured the thermal conductivity (TC) and specific heat capacity (SHC) of the grouts under saturated conditions. Furthermore, we performed numerical simulations using the measured thermal properties to investigate the effects of grout-SHCs, the length of the high-density polyethylene (HDPE) pipe, the velocity of the working fluid, and the operation time and off-time during intermittent operation on performance. Experimentally, the grout TCs and SHCs were in the ranges 0.728–1.127 W/(mK) and 2519–3743 J/(kgK), respectively. As the proportions of bentonite and silica sand increased, the TC rose and the SHC fell. Simulation showed that, during intermittent operation, not only a high grout TC but also a high SHC improved GHE performance. Also, during both continuous and intermittent operation, GHE performance improved as the working fluid velocity increased, and there was a critical working fluid velocity that greatly affected the performance of the vertical GHE, regardless of operation mode, high-density polyethylene (HDPE) pipe length, or grout thermal properties; this value was 0.3 m/s. Finally, during intermittent operation, depending on the operation time and off-time, critical periods were evident when the ground temperature had been almost completely restored and any beneficial effect of intermittent operation had almost disappeared.
Highlights
Given the depletion of fossil energy and the need to reduce the carbon footprint, interest in new and renewable energy techniques has increased worldwide
The ground heat exchangers (GHEs), which is the most important component of the ground source heat pump (GSHP) system, can be either vertical or horizontal in form, and exchanges heat with the ground by circulating a working fluid within a vertical or horizontal closed-loop high-density polyethylene (HDPE) pipe to transfer heat to/from the ground
Mraotiroeoovfesri,liwcahesnanthde, tshoeilhwigth%ervtahlueeTsCaraettahesismamilaer, tphoerhoisgithye,rbtehcearuasteiosiolficsailsicaansdanhdas, tahehihgihgehrerTtChethTaCn abteantsoimniitlea.r porosity, because silica sand has a higher thermal conductivity (TC) than bentonite
Summary
Given the depletion of fossil energy and the need to reduce the carbon footprint, interest in new and renewable energy techniques has increased worldwide. Of the various renewable energy systems currently available, ground source heat pump (GSHP) systems have been widely used because of their efficiency, energy conservation, and low-level emissions [1,2,3]. GSHP capacity increased 1.52-fold from 2010 to 2015 at a compound annual rate of 8.69% [4]. GSHP systems are composed of a heat pump coupled with a GHE. The GHE, which is the most important component of the GSHP system, can be either vertical or horizontal in form, and exchanges heat with the ground by circulating a working fluid within a vertical or horizontal closed-loop HDPE pipe to transfer heat to/from the ground. Vertical GHEs, placed in boreholes of diameter 0.1–0.2 m and depth 20–200 m, usually afford better thermal performance than horizontal GHEs, and have been more widely used throughout the world [5,6,7,8,9]
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