The application of geothermal (ground-source) HVAC systems is growing rapidly in both the residential and commercial markets. These systems offer premium efficiency because they use the relatively constant temperature of the earth to heat and cool. Energy removed from a building in the summer cooling season is sent to the earth and stored for retrieval during the winter heating season.
Because the earth's temperature is generally more moderate than the outdoor air temperature, the annual energy efficiency is improved. In addition, the stored energy is effectively and efficiently recovered when heating is required.
A geothermal system consists of a ground-source loop coupled to the building's HVAC system to reject or add heat to the building as required. The ground-source loop can be in contact with:
• a large body of water (surface water heat pump),
• water pumped from the ground (ground water heat pump), or
• a closed-loop borefield of pipe buried in the ground (groundcoupled heat pump).
Traditionally, geothermal systems have been distributed systems. As the name suggests, a distributed geothermal system is one in which the equipment is distributed throughout the building. An example is a heat-pump system that uses small, unitary water-source heat pumps installed in or near each building space.
The efficiency of some geothermal heat-pump systems can be further enhanced by the recovery of heat within the building. If properly configured, the energy removed by units cooling one part of the building is routed to units providing heat to other spaces. As HVAC designers know, the cheapest Btu to use for heating is the one already in the building. Many building owners and HVAC designers hesitate when considering this multiple-unit, distributed system configuration because the maintenance required has to be performed in or near the occupied space. There are also the acoustic challenges of creating a quiet space with fans and compressors relatively near the occupied spaces. With the advent of high-efficiency, higher-capacity chiller/heaters, a new class of geothermal systems is gaining momentum—the central geothermal system.