Geothermal Heat Pumps


A geothermal heat pump is the most efficient type of heating and cooling system available today. Unlike other types of heating systems that convert fuel to heat, a heat pump is designed to move heat from one place to another. Even the more common air source heat pumps are able to deliver much more heat per unit of energy than systems that convert electricity to heat. But because they draw their heat from the outdoor air, their efficiency varies with the outdoor temperature. As outdoor temperatures get colder, there is less heat available, so the capacity (amount of heat delivered) and efficiency of the system decline.

But even in locations with extreme high or low temperatures, the temperature of the ground below the surface is relatively constant year-round. Geothermal heat pumps take advantage of this by absorbing heat from below ground or from ground water and delivering it inside the home. In summer, they remove heat from inside the home and transfer it to the ground where temperatures are cooler.

Because geothermal heat pumps have fewer mechanical components than other systems, and most of those components are either indoors or underground, protected from the weather, they are very durable and require little maintenance. The underground piping used in the system is typically guaranteed to last 25 to 50 years.

How It Works


The heat pump operates on the principle that heat can be transferred by a cycle of alternating vaporization and condensation - the same cycle used by refrigerators, freezers and air conditioners. When a liquid vaporizes, heat is absorbed, and when a vapor condenses, heat is released. By alternately pressurizing and depressurizing a substance with a very low boiling point (a refrigerant), the heat pump can absorb heat from a relatively cool medium and transfer it to a warmer one.

Geothermal heat pumps use an indirect or two-stage heat transfer process. They circulate a fluid (water or a water/antifreeze mixture) through piping underground, where it is able to absorb more heat than it could from outside air. The fluid is then circulated through the heat pump's heat exchanger where the heat is transferred to the refrigerant, which then heats air that is distributed via the home's duct system.

During the summer, the cycle is reversed. Instead of absorbing heat from the water and transferring it to the indoor air, the refrigerant now absorbs heat from the indoor air and transfers it to the water, where it is given off to the ground or ground water. Because of the constant relatively cool temperature of the ground or water, the geothermal system is more efficient than a standard air conditioner, which must reject heat to hot outdoor air.

Efficiency


Heating efficiency of geothermal heat pumps is expressed with a Coefficient of Performance or COP. A COP of 1.0 is equivalent to 100% efficiency. Because heat pumps move heat from one environment to another, rather than converting electricity directly into heat, they can achieve efficiencies much higher than 100%. Geothermal heat pumps typically have COP ratings of 2.5 to 4 (250-400% efficiency), and some have COPs over 5.

For cooling, geothermal heat pumps are rated with an Energy Efficiency Ratio or EER (also used for some air conditioners). Typical EERs for geothermal systems range from 13 to 20, and some very high efficiency systems are rated as high as 30.

Although geothermal heat pumps are considerably more expensive than air source heat pumps, their higher efficiency can make them a cost-effective option, particularly if the cost for the ground loop is not excessive. Some developers have addressed the problem of the cost of the ground loop by installing "community loops" in subdivisions, allowing the cost to be shared between multiple homes.

Types/Configurations


There are two basic types of geothermal systems - open loop and closed loop. An open loop system typically uses a conventional well as its heat source. Water is pumped from the well through the heat pump's heat exchanger and then returned to a pond, stream, or second well. Local water use and disposal regulations may limit the use of open loop systems.

Closed loop systems circulate a heat transfer fluid (usually a water/antifreeze solution) through a system of buried plastic piping, arranged either horizontally or vertically.

Horizontal loop systems draw their heat from loops of piping buried 6-8 feet deep in trenches or ponds. This configuration is usually most cost-effective, as long as sufficient land is available. A typical horizontal loop will be 400 to 600 feet long for each ton of cooling capacity.

Vertical loop systems use holes bored 150 to 400 feet deep with U-shaped loops of piping. They work the same as horizontal loop systems, but can be installed in locations where space is limited. This type of loop can be used where there is little yard space, when surface rocks make digging impractical, or when you want to disrupt the landscape as little as possible.

Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the temperature is more stable as you go further below the surface.

Other Options


Another type of geothermal heat pump is called a "Direct Exchange" or "DX" system. This type of system uses a much shorter loop of copper piping buried underground, through which the refrigerant itself is circulated, replacing the heat transfer fluid used in other geothermal systems. Because the heat is transferred directly between the ground and the refrigerant, the amount of piping can be drastically reduced. This type of system is ideal for situations where the amount of space for the piping loop is very limited.

There are also "dual-source" heat pump systems that combine an air-source heat pump with a geothermal heat pump. Dual-source heat pumps have higher efficiency ratings than air-source units, but are not as efficient as geothermal units. The main advantage of dual-source systems is that they cost much less to install than a strictly geothermal system, and perform almost as well.

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