Daniel Boone High School, located in Johnson City, Tennessee, USA, installed a water loop heat pump (WLHP) with a closed-loop ground-source heat pump (GSHP) system during the 1995-1996 heating season. Five configurations were examined prior to selecting the GSHP system. These were:

• WLHP with electric boiler;
• WLHP with gas boiler;
• WLHP with electric thermal storage;
• 4-pipe system using a natural gas engine-driven chiller and boiler; and
• WLHP with closed-loop ground-source system.

Detailed simulations were performed using an hourly analysis model. The base case system selected for comparison was the WLHP with natural gas boiler and cooling tower. The model was calibrated with actual energy and weather data gathered prior to the feasibility study. The GSHP system was selected on the basis of the simulation results and the anticipated construction cost. An annual energy saving of US$29,000 was predicted.

The preliminary feasibility study also estimated that maintenance costs for the GSHP system would be reduced on annual basis by approximately US$0.55/m2 of floor space compared to the base case system. These savings were attributed to 1) lower maintenance and operating (labor) costs of the GSHP system compared to the boiler, cooling tower, and heat exchanger of the conventional system, and 2) chemicals for the cooling tower and makeup water usage.

Initial results for the first year of operation showed a net annual saving of US$33,000. Based on these savings, a 6-year simple payback period is achieved. Further monitoring of the building energy-use appears to indicate that the installed capacity and ground loop may be oversized by about one third.

System description

The geothermal heat exchanger consists of 320 boreholes, each 45.7 m in depth. Each borehole contains 91.4 m of 1.9 cm diameter polyethylene pipe. The boreholes are grouped in clusters of 20 holes with 4.6 m centre-to-centre spacing. A 20.3 cm diameter supply and return line enters the school through the existing mechanical equipment room. The total installed cooling capacity is 1,410 kW. The system uses a pair of two-speed circulating pumps of 30 kW each.

The ground-source heat pump system cost US$451,000. This cost exceeded the initial estimate by US$100,000 due to unexpected casing costs. The local utility, Tennessee Valley Authority (TVA), agreed to co-fund the project in order to demonstrate and evaluate the GSHP system, particularly the variable flow pumping and the loop sizing. TVA provided US$104,000 in direct funding plus the system monitoring costs. The cost of a conventional boiler, cooling tower, plate-frame heat exchanger, and associated pumping and controls was estimated at US$150,000. The incremental cost of the GSHP system was US$197,000.

Lessons learned

Parasitic pumping in WLHP and ground-source heat pump systems is an area with considerable potential for energy savings. Traditional designs incorporate constant operation of circulation pumps. This can substantially increase energy use, resulting in lower overall system efficiency.

Significant unexpected costs can jeopardize the economic viability of GSHP systems. A thorough estimate with appropriate provisions is needed. Ground conditions should be evaluated by local experts.

Oversizing of ground-source exchangers is often attributable to an overestimation of the cooling demand.

In new construction applications, a significant credit should be given to the substantial reduction in mechanical equipment room requirements. This further reduces the system payback period.

The big picture

Institutional applications are a perfect platform for demonstrating GSHP system benefits. This market segment will usually tolerate longer payback periods and is more willing to use innovative systems. It is common that institutional applications will be the first GSHP systems in a region. These serve to demonstrate the benefits of GSHP systems and give contractors design and construction practice. It is critical that contractors and well drillers be experienced and capable. Unsuccessful or trouble-prone systems can rapidly tarnish the technology's image. In particular, accurate sizing of the GSHP system is key, since oversizing is more costly with GSHP systems than with conventional systems.

© Minister of Natural Resources Canada 1997-2009
www.retscreen.net