Introduction
A heat pump can be an efficient alternative to the standard home heating system. Moreover, during the cooling season, its function can be reversed as it acts as an air conditioner. Heating and cooling buildings using ground source heat pumps (also known as geothermal heat pumps) is becoming more popular.
The Basics
To understand how a heat pump works, think for a moment about a refrigerator. This appliance extracts heat from the interior, cools it down, and dumps the extracted heat to the area around the refrigerator. The mechanical device that performs this function is a heat pump, although refrigerators do not have the ‘reverse’ function that is available for home heating and cooling. Heat pumps heat or cool a building basically by moving heat from one location to another. In the winter, they move heat from the air, ground, or a water source into a building where it is concentrated to provide heating. In the summer, the process can be reversed and the heat from a building is removed, cooling the building. For purposes of this article the information will concentrate mainly on the heating aspects of ground source heat pumps.
There are two types of heat pumps: air source and ground source. An air source heat pump typically extracts heat from the air, concentrates it (even when that air is cool) and transfers it to the indoors or outdoors depending on the season. These units are better suited to mild climates for heating, since their capacity is limited. A ground source heat pump extracts heat from the earth or a body of water, and the home is heated from this source. These units are often equipped with a desuperheater, which utilizes excess heat to produce domestic hot water.
In all heat pumps, a water antifreeze mixture is used as the transfer medium between the heat source (the ground) and the heat pump. The heat pump then concentrates the heat and disperses it into the home. Household air is never in direct contact with the heat source (air, soil, or water).
Ground source heat pumps generally require three main components: the heat exchanger(ground loop), a heat pump, and a distribution system such as air ducts or in-floor tubing. The heat exchanger, or loop, is simply a length of tubing placed underground and used to transfer the heat from one location to another. The heat pump moves water through the loop and passes it through a condensing unit which utilizes refrigerant to concentrate the heat. In the winter, that heat is then released through the buildings air ducting systems or in-floor hot water (hydronic) heating system. The process can be reversed for cooling in a home.
Other names
Geothermal energy and ground source heat pumps are often confusing terms. Geothermal simply means “earth heat” (geo- earth, thermal– heat), so geothermal energy is energy produced from an Earth-based heat source. Ground source heat pumps do not create energy, but simply move heat energy from one location to another. Even though many in the scientific community consider it technically incorrect to use the terms geothermal and ground source heat pump interchangeably when referring to heat pumps, both are often used. Ground source heat pumps are also known by a variety of other names, each meaning basically the same thing. Some of the other names you might see include ground coupled heat pumps, GeoExchange, and Earth energy systems.
Installation
Ground source heating systems can be installed in a variety of ways. The majority of systems installed are closed loop, where piping is installed that continuously circulates a liquid solution through a pipe. Some open loop systems circulate water from a lake, pond, or well. Open loop systems are limited due to availability of water, and because of problems with clogging of the pipe. In either application, the piping is used to transfer the heat. For closed loop systems, the pipes can be installed either horizontally or vertically in the ground, a pond, or a lake.
If the building is on a large lot, trenches are dug, and pipe of the correct diameter is installed. The trench depth varies depending on the possible frost line in your area, but depths of 6-10 feet are standard. The compaction of the soil around the pipe is critical, and extreme care must be taken to ensure the soil is properly packed around the pipe.
For installations where land area is not available, vertical bore holes are used, and the pipe is installed in the vertical bore holes or wells. These wells generally range between 100-400 feet deep, and are placed approximately 15 feet apart. The vertical pipes are hooked in parallel to make the flow loops essentially equal. The buried pipe is hooked to a manifold of larger diameter pipe and runs into heating equipment in the building. Grout is used in the bore holes to ensure good heat transfer between the pipe and the ground.
Ground source heat exchange installations: horizontal and vertical
Advantages
There are numerous advantages of ground source heating systems over other types. The efficiency of the heating systems is the main advantage. Ground source heat pumps simply pump heat from one area to another, and as a result use considerably less energy than other systems. Since they are using less energy, they can be considerably cheaper to run.
Reliability is another important advantage. The systems have few moving parts that can wear out, so the mechanical parts typically have warranties of up to 10 years, and the ground loop piping can be warrantied for up to 50 years. Another advantage is that there are no mechanical parts outside the home, so they are not subject to damage from weather, vandalism, lawnmowers.
Geothermal heat pumps are able to provide heat as well as cooling for a building simply by reversing the direction of flow within the heat pump. The ability of the units to provide warm or cool air also allows for zone temperature control in larger buildings. It is possible to heat one portion of a building, while providing cool air in another.
Excess heat from the geothermal system can also provide domestic hot water to a building. Generally this is most efficient in the summer when buildings have excess heat.
Disadvantages
Depending on the installation, ground source heat pumps may have a higher initial installation cost than other types of heating systems. If designed and installed properly, they should make up for the extra initial investment through operational savings over time.
The air released from a heat pump is generally a lower temperature than gas forced air or electric resistance requiring a higher amount of air to be circulated through a building. If the building was not initially constructed efficiently, the system may not be able to supply enough heat to the structure. Proper home insulation, as well as air sealing in the ductwork and exterior framing of the home, is essential to ensure a return on investment for ground source heat pumps as with any other heat source.
Sizing
Sizing of the systems is done by determining the heating or cooling loads that are needed for the building. There are a variety of ways this can be done. It is highly recommended that heating and cooling load calculations be done by a qualified professional. Simply sizing the system on the square footage of the building is not satisfactory, and could result in comfort and efficiency issues. For far northern climates, units are often sized off the heating needs, while in southern, warmer climates the units are sized for air conditioning and humidity removal.
Depending on the climate of an area and the design of the building, backup heating systems can be installed to provide supplemental heat when the temperature drops below certain levels, which can add additional cost.
Cost
The total cost for the outdoor ground loop and indoor heat pump and distribution system can range from about $15,000-$30,000 for residential installations. Federal and state tax incentives, in addition to local utility rebate programs, can shave as much as 1/3rd of this cost. How much can you save with a ground source heat pump? Individual results will vary, but savings of 25% to as high as 75% off your present heating bill, comparing a heat pump to a conventional electrical resistance furnace or electric baseboard heat, are not unusual. Air conditioning savings will also be noticed. For climates with a moderate heating load, choose ENERGY STAR® rated systems. In colder climates, heaters exceeding ENERGY STAR performance criteria will be economically justified.
Typically, the systems cost twice as much as a conventional heating and cooling systems as a result of the cost of the outside piping to install, but cost three to four times less to operate.
Efficiency
A ground source heating system’s efficiency is reported in a unit called the coefficient of performance (COP). As an example, if a unit has a reported COP of 3.0, this means that the heat pump moves three times the amount of energy into the building as the electrical energy required to operate to the system. The heat pump would effectively have an efficiency of 300% compared to electric resistance heat.
Resources
For a list of qualified contractors consult the International Ground Source Heat Pump Association accredited installers and designers directory.
The U. S. Department of Energy has created a Database for State Incentives for Renewables and Efficiency to help find assistance in your state.
ENERGY STAR has also provided information for federal tax incentives for efficient heating systems.