Geothermal is based on a simple premise: Below the frost line - usually about six feet deep – the earth is a constant temperature of about 50 degrees Fahrenheit all year long.
During the winter, a heat pump absorbs heat from the ground and uses it to warm the air in your home. In the warmer summer months, the processed is reversed, taking heat from your home and transferring it back into the ground.
The basic elements of a geothermal system include:
- Underground loops of plastic piping;
- A liquid antifreeze solution;
- A heat pump; and
- An air distribution system.
The loops of piping are buried in the ground near your home or business, either vertically or horizontally. That ground loop is connected to a pumping module inside your home.
The pump circulates a mixture of water and the antifreeze through the ground loop, where it absorbs heat from the earth.
When the heated liquid reaches the heat pump inside your home, the heat is multiplied and used to warm the air inside the air-handling system. A blower sends the warmed air throughout the building through ductwork.
- How it works: Anatomy of a Heat Pump
- How it works: Open Loop System
- How it works: Closed Loop System
How It Works: Anatomy of a Heat Pump
You already have a heat pump in your home - your refrigerator. If you put your hand behind it, you'll feel the heat that's being pumped from the inside.
It's the same principle that a geothermal heat pump uses to warm your home. Instead of producing heat like a conventional furnace, a geothermal heat pump moves heat from one place to another - from the ground into your home.
The diagram above illustrates the summer cooling process:
- The cool, liquid refrigerant enters the indoor coil during cooling. As it enters the coil, the temperature of the refrigerant is between 40 and 50 degrees.
- As warm, moist room air passes over the cool coil, the refrigerant inside absorbs the heat.
- The new cooler, drier air is circulated back into the room with a blower fan.
- The refrigerant moves into the compressor, which is a pump that raises the pressure so it will move through the system. The increased pressure from the compressor causes the refrigerant to heat to roughly 120 to 140 degrees.
- The hot vapor now moves into contact with the condenser (the underground loops), where the refrigerant gives up its heat to the cooler ground loop, then condenses back into a liquid.
- As the refrigerant leaves the compressor, it's still under high pressure. It reaches the expansion valve, where the pressure is reduced.
- The cycle is complete as the cool, liquid refrigerant re-enters the evaporator to pick up room heat.
During the winter, the reversing valve switches the indoor coil to function as the condenser, and the underground piping to act as the evaporator.
In short, the indoor coil and underground piping cause the refrigerant to change state, absorbing and releasing heat through boiling and condensing. The compressor and expansion valve move the refrigerant through the system by changing the pressure.
How It Works: Open Loop System
While the majority of geothermal installations use a closed loop system, another option is an open loop system.
Instead of using an antifreeze solution sealed inside the buried piping, an open loop system uses water from a surface or underground source - such as a pond, lake or well.
The water is pumped into the heat pump unit where the heat is extracted; the water is then discharged back into the original source.
Well water designs are the most common and most cost-effective. The well supplies both household water and water for the heat pump. Approximately three gallons per minute of well water are needed per ton of cooling capacity. A 3,000-square-foot, well-insulated home would typically require 10 to 15 gallons per minute.
Water quality is an important issue with open-loop systems. Mineral deposits can build up inside the heat exchanger, iron and other impurities can clog a return well, and organic matter from ponds and lakes can quickly damage a geothermal system. Water should be tested for acidity, mineral content and corrosiveness.
How It Works: Closed Loop System
The most common type of geothermal installation is a closed loop system. The heat exchanger - a loop of piping filled with fluid - is buried underground. The fluid circulates continuously inside the buried pipe, absorbing heat from the earth during the winter for use inside your home or business. In warmer months, the fluid takes heat from indoors and transfers it back into the earth.
Types of closed loop systems
A central closed loop system has all heat pumps in a central room; air or water is ducted and circulated to the heated or cooled rooms. Commercial applications include chiller or rooftop unit retrofits.
Distributed systems use a central water pump and heat pumps serving individual rooms and areas. Types of buildings served included offices and schools, both new construction and retrofits.
Since multiple units are heating and cooling simultaneously, the distributed system can provide heat recovery from core zones that have excess heat to perimeter zones that require heat.
A distributed system also permits location of relatively small individual units in restricted areas, such as historic districts.
Modular systems have dedicated heat pumps, water pumps and loops. This type of system allows for independent individual control, operation and maintenance.
Types of buildings suitable for a modular approach include schools, with modules serving individual classrooms, and other buildings where usage and environment are clearly separated.
A hybrid system uses a cooling tower or other means to reject excess heat not needed for winter heating. The cooling tower reduces the size of the ground heat exchanger and the cost of installation.
If adequate land area without hard rock is available, a horizontal loop installation is usually the most economic. Horizontal loops are often used for newly constructed homes and commercial buildings.
A horizontal system uses a number of trenches. The piping can be configured in the trenches in several ways:
- A single pipe;
- Multiple pipes in a narrow trench; or
- Multiple pipes in a wider trench.
The trenches are normally four feet deep or more, and vary in length depending on the number of pipes to be buried. One of the advantages of a horizontal loop system is being able to lay the trenches according to the size of the lot.
If the land area available is limited, a vertical loop may be installed for the geothermal piping.
Vertical installations might also be used where the land is too rocky for trenching, for existing buildings, and for large commercial or educational facilities.
To install a vertical loop, a contractor will bore holes into the ground. Long, hairpin-shaped loops of pipe are then inserted. The hole is backfilled, plugged or grouted, and the pipes are connected to headers in a trench leading back to the building.
The drilling depth is determined by the lowest total cost based on the conditions at the job site. A typical borehole depth is 150 to 250 feet.
The objective of a vertical borehole is to install a specific amount of pipe, not to reach a certain depth. If 1,200 feet of pipe are required, three 200-foot boreholes are acceptable and may be more cost-effective.
Drilling boreholes for geothermal loops is much simpler than drilling to find well water. The borehole is generally smaller, which reduces drilling time, and no casing is required because the hairpin-shaped loop is the casing.
An increasingly popular approach, especially in residential systems, is a "slinky" coil.
A slinky is a coil of plastic tubing spread out and overlapped in a trench and buried. Slinky coils are installed horizontally at the bottom of a three-foot-wide trench.
This method concentrates the heat transfer surface into small volume, requiring less land area and shorter trenching - a big plus for homeowners.
A compact slinky will reduce trench length by about two-thirds; an extended slinky will reduce trench length by about one-third. Specific design lengths will vary with the climate, soil and the heat pump's run fraction.