GROUND-SOURCE HEAT PUMPS: A CASE STUDY OF SUSTAINABLE ENERGY

Ground-source (geothermal) heat pumps utilize the heat capacity of the earth as a heat source or sink for heating and cooling. These systems currently generate approximately 2,700 MW in the US, equivalent to burning 60 million barrels of oil/year. Theoretically, geothermal systems can be up to 600% efficient and can cut energy bills for homeowners by 30-40 percent. Although engineers have studied geothermal systems for years, geologic investigation of ground thermal response is critical for assuring the sustainability of the system.

For this study we investigated a closed-loop geothermal system installed for a household in West Chester, Pennsylvania. Dataloggers equipped with thermistors and current meters were used to measure bore fluid temperature, indoor and outdoor air temperature, and electrical power consumed by system components. The response of the borefield was modeled using semianalytical (Theis solution) and finite difference (MODFLOW) techniques.

We found it necessary to record temperature and electrical current every 2 minutes due to the highly transient nature of the system. This resulted in a very large dataset of 1.8 million measurements over a 6-month period. Although ambient ground temperature for the study area is 54 degrees F, the system lowered ground-loop temperature to 33 degrees F in the winter and raised it to 79 degrees F in the summer. System efficiency ranged from 180 percent in the winter to 270 percent in the summer. This is less than the manufacturer’s predicted efficiency of 300-400 percent, likely a result of accessory electrical demand by circulators and air handlers. Modeling efforts revealed a ground thermal conductivity of 1.37 W/m K and a volumetric heat capacity of 2.1 million J/m^3 K. These parameters were then used in the forward-mode to predict future thermal response, which will allow us to optimize efficiency and maintain system sustainability.