ProTech Energy

A geothermal heat pump system is a heating and/or an air conditioning system that uses the Earth’s ability to store heat in the ground and water thermal masses. These systems operate based on the stability of underground temperatures; the ground a few feet below surface has a very stable temperature throughout the year, depending upon location’s annual climate. A geothermal heat pump uses that available heat in the winter and puts heat back into the ground in the summer. A geothermal system differs from a conventional furnace or boiler by its ability to transfer heat versus the standard method of producing heat. As energy costs continue to rise and pollution concerns continue to be a hot topic, geothermal systems may hold a solution to both of these concerns.

Geothermal heat pumps are also known as “geo-exchange” systems (a term created by an industry association) and “ground-source heat pumps.” The latter term is useful as it clearly distinguishes the technology from air-source heat pumps. Geothermal heat pumps, which can be used in almost any region, should also be distinguished from geothermal heating. Geothermal heating is used in areas where exceptionally high underground temperatures, such as those at hot springs and steam vents, are used to heat indoor spaces without the use of a heat pump.

This article focuses on geothermal heat pumps that use water to exchange heat with the ground, often referred to as “water-source geothermal heat pumps” or “water loop geothermal heat pumps.” Another technology, the direct exchange geothermal heat pump, is also available and is briefly discussed.

Introduction

A geothermal heat pump is a heat pump that uses the Earth as either a heat source, when operating in heating mode, or a heat sink, when operating in cooling mode.

Geothermal heat pumps can be characterized as having one or two loops. The heat pump itself, explained more fully in the article on heat pumps, consists of a loop containing refrigerant. The refrigerant is pumped through a vapor-compression refrigeration cycle that moves heat from a cooler area to a warmer one.

In a single loop system, the copper tubing refrigerant loop actually leaves the heat pump appliance cabinet and goes out of the house and under the ground and directly exchanges heat with the ground before returning to the appliance. Hence the name “direct exchange” or DX. In a double loop system, the refrigerant loop exchanges heat with a secondary loop made of plastic pipe containing water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the heat exchanger, the plastic pipe then leaves the appliance cabinet, and goes out of the house and under the ground before returning, so the water is exchanging heat with the ground. This is known as a water-source system. Secondary loops are popular for ground use because they are not pressurized, so cheap plastic tubing can be used, and because they reduce the amount of expensive refrigerant required. Copper loop DX systems are gaining acceptance due to their increased efficiency and lower installation costs.

Components

Geothermal systems require three primary components; a length of buried tubing on the property, a liquid pump pack and a water-source heat pump. The tubing can be installed horizontally as a loop field or vertically as a series of long U-shapes (see below). The purpose of the tubing is to transfer heat to and from the ground. The size of the loop field depends on the size of the building being conditioned. Typically, one loop (400 to 600 feet) has the capacity of one ton or 12,000 British thermal units per hour (BTU/h) or 3.5 kilowatts. An average house will range from 3 to 5 tons (10 to 18 kW) of capacity. The second component is a liquid pump pack, which sends the water through the tubing and the water-source heat pump. An example of an installed liquid pump pack can be seen in the image to the right. Lastly, the water-source heat pump is the unit that replaces the existing furnace or boiler. This is where the heat from the tubing is transferred for heating the structure. Heat pumps have the ability to capture heat at one temperature reservoir and transfer it to another temperature reservoir. Another example of a heat pump is a refrigerator; heat is removed from the refrigerator’s compartments and transferred to the outside.

Common Systems

Closed loop fields

A closed loop system, the most common, circulates the fluid through the loop fields