NAHB Research Center performed data collection and analysis of the performance of five residential geothermal systems in northeastern Ohio. The objective of the project was to characterize performance of the systems in occupied homes and to compare the measured output at design conditions to standard sizing methods. Data gathered included ground loop temperatures and flow rates, domestic water heating loop temperatures, hot water load flows and temperatures, air temperatures, electrical inputs, environmental temperatures, and operating status of compressors and auxiliary heating. The equipment of three major geothermal manufacturers was represented.
Each of the systems studied was able to meet the heating loads imposed with very little use of auxiliary heat. Two of the five systems used no auxiliary heat during the period of study, and the highest value for any system was about 4 percent of total heating energy consumption. Low use of auxiliary heat is explained in part by compressor capacities that exceed manufacturers' ratings. In one system, the auxiliary heat was normally left disabled. Measured seasonal Coefficient of Performance (COP) values in heating ranged from 2.9 to 3.3, and compared well with manufacturers' ratings. The maximum measured system output, not including auxiliary heating, was larger than manufacturers’ ratings in four of five cases. Loop temperature trends in heating showed some signs of undersized loops in two of the five systems, emphasizing the importance of proper loop sizing and installation. COP was shown to correlate with loop operating temperature.
The cooling load on each system was much smaller than the heating load, and cooling loads in the homes studied was driven by occupant demand in addition to indoor-outdoor temperature difference. Measured seasonal COP values ranged from 3.5 to 4.5, and the maximum measured system output in cooling ranged from 81 to 112 percent of rated capacity.
Heating and cooling loads at design conditions were estimated from the measured data for each home and compared to Air Conditioning Contractors of America (ACCA) Manual J calculated design loads. In heating, Manual J predicted design loads higher than measured values by a median of 17 percent, providing confidence in the use of Manual J for sizing in this climate. In cooling, Manual J predicted loads that varied widely compared to measured values. Geothermal systems sized for heating loads in cold climates will generally provide adequate cooling capacity.
Compressor capacity ratings in heating ranged from 71 to 109 percent of the estimated design heating load, with four of the five systems sized under 100 percent of the load. Sizing of compressor output to less than 100 percent of heating design appears to be an appropriate way to limit system cost, and, as shown here, does not necessarily lead to high use of auxiliary heat. In cooling, rated capacities ranged from 1.2 to 3.2 times projected design load, an expected result in a heating dominated climate.
The systems equipped with desuperheaters for domestic water heating provided 9 to 47 percent of the water heating load, with higher performance explained by long system operating hours and reduced hot water tank temperature. The failure of one desuperheater system due to pipe blockage was identified from data collected during the study, suggesting the need for system design improvements and for a means of periodically verifying operation. In the absence of optimizing the desuperheater design, the value of desuperheaters is questionable. The two systems equipped with dedicated water heating provided 100 percent of the load from geothermal system operation. However, the thermal losses in these two systems was larger by a factor of over 2.5 compared to the average thermal losses of the desuperheater systems, due to the use of long piping loops and continuous or intermittent pump operation to allow water temperature sampling.
Prepared by:
NAHB Research Center
Prepared for:
National Renewable Energy Laboratory and
Geothermal Heat Pump Consortium.
60 pages