Performance of a horizontal ground-to-air heat exchanger located around the foundations of a residential building

An earth-to-air, or geothermal, heat exchanger, hereafter EAHE, reduces the heating/cooling loads associated with building ventilation. Despite its low-tech nature and status as a nearly passive sustainable system, it remains relatively uncommon in residential settings. It consists of underground tubes through which air circulates before being introduced into the building. As excavation costs are often prohibitive, a configuration installed around the foundations of a house is investigated. TRNSYS is used to predict the thermal performance and economic viability of a residential EAHE coupled with a heat recovery ventilator (HRV), required by the Quebec building code since 2012 in each new building. The study focuses on a typical single-family dwelling in Montréal, but the results can be extrapolated to other types of buildings and locations. A sizing study was first conducted on the system for two primary purposes: 1) to identify the configuration that yields optimal results for this specific building, which is subsequently utilized for the economic analysis; 2) to obtain several design guidelines from the results, although it’s acknowledged that this single study may not necessarily be enough to generalize these guidelines to different buildings. In brief, for this particular house, the optimal configuration around the foundations entails a 31 m long EAHE located 2.5 m deep, installed at 0.65 m from the basement wall, and composed of HDPE tubes with a 0.25 m diameter. The optimal configuration reduces the building’s heating and cooling loads by only 701 kWh/year (4.3%). This is due, in part, to significant losses caused by short-circuiting of the HRV (mandatory in new houses). To evaluate the system’s economic performance, the Levelized Cost of Energy (LCOE) has been used. For the investment cost evaluation, inflation-adjusted values are utilized. The resulting LCOE is 0.182 US$.kWh-1. This is prohibitively high compared to the average domestic electricity rate of 0.059 US$.kWh-1 in Quebec, Canada. This may seem very high from a short-term economic perspective, but it should be noted that this equipment is intended to be used for the entire life cycle of the home. An investment of a few thousand dollars, with little or no asset maintenance costs, represents less than half a percent of the total cost of any residential project. Hence, the use of EAHE in Quebec would require legislation to force constructors to install it.