FAYETTEVILLE, Ark. — While heat pumps provide significant savings in operating costs for heating and cooling, the high installation costs have been a limiting factor, particularly in commercial buildings. University of Arkansas researcher Darin Nutter has developed a model that can substantially reduce the installation costs in commercial applications.

"Models are used to determine the depth and spacing of the vertical bores in the geothermal heat pump systems," explained Nutter, associate professor of mechanical engineering. "Previous models have assumed that the underground structure is a solid, homogeneous material, but we know that is an oversimplification. We developed a model that can better incorporate the actual site characteristics, which will allow engineers to design a cheaper, more effective system."

Nutter worked with Ralph Davis, associate professor of geosciences; graduate student Matt Sutton and associate professor Rick Couvillon from mechanical engineering in developing the model. Details of their work appear in the current issue of Transaction of the American Society of Heating, Refrigerating, and Air Conditioning Engineers.

Subsurface soils are not uniform and may include layers of sand, clay and rock, in addition to aquifers. Each of these materials affects heat transfer differently. For instance, in some designs the convection heat transfer from groundwater flow is significant and should be added to the conduction through solid materials.

Geothermal or ground source heat pumps take advantage of the earth's relatively constant temperature to provide heating, cooling and hot water for homes and commercial buildings. They start with a closed loop of buried pipes containing a fluid that can carry heat.

The pipes may be arranged in a long, shallow horizontal trench, or they may run vertically, deep into the ground. While residential systems usually have horizontal underground pipes, the increased needs of commercial buildings usually require use of the vertical borehole heat exchanger (VBHE) system.

A VBHE system includes a field of vertical and horizontal piping. The number and depth of the vertical pipes depends on the size of the building to be heated and cooled. For example, an elementary school in Nebraska has a pipe field of 100 boreholes, each 300 feet deep.

Recently Nutter discussed the importance of incorporating groundwater flow into VBHE designs at the international Ground Source Heat Pup Association Annual Technical Conference. He showed research results for a two-layer geology, with soil above an aquifer, and a three-layer geology, where the aquifer is sandwiched between two layers of soil. While their initial work has focused on a single bore, they are currently expanding the model to account for multiple bores.

"The primary cost of installing a VBHE system is drilling the boreholes, which can cost $4 to $10 per foot," explained Nutter. "By taking into account the groundwater and soil types, engineers can design borehole fields that aren’t as deep, saving both time and money without reducing efficiency."