Geothermal Explained

Geo Energy, geo exchange, geothermal heat pump systems; no need to be confused, they all mean the same thing. They all describe a sustainable resource that lies in the ground at your location. Here's how it works.

A typical air conditioning system draws heat from the inside of a residence or commercial building and discharges it to the air outside using a compressor and fan. As you might expect, on hot days when the A/C is used the most, it is difficult to shed heat into the already hot outside air. The system is very inefficient and must run a long time to remove heat.Conversely in the winter, the air source heat pump tries to draw heat out of already cold air. Likewise, it struggles to draw heat from the frigid air, and must also run a long time.

Geothermal is different. At a depth of 4 to 6 feet below the frost line the ground temperature of the earth remains relatively constant throughout the year. Ground source heat pumps (GSHP) are designed to capitalize on this near constant temperature. Using a properly constructed ground heat exchanger (the ground loop or loops), they harness the thermal energy stored in the ground beneath your site.

Geothermal installations are integrated systems consisting of three major sub-systems.

Air distribution within the structure - air ducts and blowers circulate the air throughout to control the climate within the structure.

Ground source heat pump(s) - mechanical device to drive or "pump" the heat between the two other subsystems. GSHP work most efficiently when the entering liquid from the ground exchanger remains within a range of 40 to 80 degrees.

Ground heat exchangers - in-ground piping (ground loop or loops) arranged in a grid that makes contact with the earth to facilitate heat exchange. Typical ground heat exchangers are constructed of plastic tubing encased in grout creating improved thermal conductivity. These are referred to as closes loop systems. An open loop system uses groundwater from an ordinary well as a heat source. The groundwater is pumped into the GSHP unit where heat is extracted and the water is disposed of back into the ground. These open loop systems are commonly called "pump and dump" systems. Poor water quality can cause serious problems in open loop systems. Mineral deposits can build up inside the GSHP.Impurities, particularly iron, can eventually clog a return well. Because of these issues we will concentrate this discussion on the closed loop systems.

Vertical Loops Diagram

Vertical Loops

Vertical loops are used where space is limited or where soil conditions make horizontal loops impractical. Installing vertical loops require the use of a drilling rig. Multiple holes are bored at a minimum 10 feet apart. A double high density polyethylene (HDPE) pipe connected with a U-bend and filled with a mixture of water and food grade anti-freeze is inserted into each hole. The hole is filled with grout to provide good contact around the pipe and to seal the hole. The vertical pipes are then connected to a header system horizontally a few feet below the surface. The depth of the holes is dependent upon soil/rock conditions and size of the system. Although most holes are bored about 100 to 250 feet deep, there's no "magic depth" that needs to be reached. Capacity is not based on depth: rather how much pipe is in the ground and the overall thermal conductivity of the hole.

Horizontal Loops Diagram

Horizontal Loops

If adequate land is available, horizontal loops can be installed. Trenches are dug using a backhoe or trencher. HDPE pipes are inserted and the trenches are back filled. There are various designs (slinky or race track) of horizontal loops using one, two or three circuits per trench. The more pipes in each trench, the shorter the trenches can be. Trenches normally range from 100 to 300 feet depending on the design. A typical home requires 1/4 to 3/4 of an acre for the trenches.

A variation on the horizontal loop is the directional bore loop. This type of loop is most often used in a retrofit situation to minimize disruption to the landscape. It requires special equipment to bore holes under the surface. The operator can "steer" the drill head to go deeper or shallower, or turn right or left. This machine drills at a slight angle down to a typical depth of 15 to 20 feet, then back to the surface, typically 200 ft. away. At that point, two ends of the pipe are attached to the drill bit and pulled back through the hole until the pipe is buried. This technique allows the loop to be placed underneath homes, basements, wooded lots or even swimming pools.

Pond Loops Diagram

Pond Loops

If an adequately sized body of water is close to your home, a pond loop can be installed. A series of sealed HDPE pipes containing a mixture of water and anti-freeze can be coiled and sunk to the bottom. A 1/2 acre, 8-foot-deep pond is usually sufficient for the average home. Ideally, the pond should be close to the home (less than 200 ft.). If the pond is farther from the home, the benefit of using a pond loop is reduced due to added trenching, materials and pumping costs.

Pond loop coils are connected together on dry land, and then floated into location. Once filled with fluid, they will sink to the bottom and remain there. Generally, a 300 ft. coil is used for each ton of capacity. This is less pipe than is used in an earth loop because water is a better conductor of heat energy. Pond loops are a cost effective way to install a geothermal system, because trenching is limited to only the supply and return piping from the pond to the house.

Some frequently asked questions about geothermal.

How about lower energy bills?

Heat from the ground is free, and the only electricity needed is for moving that heat between your home and the ground. According to the Geothermal Heat Pump Consortium, a geothermal system can lower your heating bill up to 50 percent and lower your cooling bills up to 30 percent. This could mean a payback in as little as two to seven years! Ground-source heat is a naturally renewable energy source and friendly to the environment.

Geothermal systems work very well in below zero temperatures; and when the equipment and loop field are sized correctly, the electrical resistance back up is minimal. The geothermal system in your home is specifically designed to account for the climate and your home's heating and cooling load. Geothermal systems are very efficient in northern climates and work just as efficiently as those in southern zones. As with any home heating and cooling, adequate insulation and overall weatherization are key factors in lowering energy consumption.

What makes a geothermal system different from conventional heating and cooling systems?

Unlike ordinary systems, Geothermal systems do not burn fossil fuel to generate heat; they simply transfer heat to and from the earth to provide a more efficient, affordable and environmentally friendly method of heating and cooling. Typically, electrical power is used only to operate the unit's fan, compressor and pump.

Is the efficiency rating actual or just a manufacturer's average?

All heating and cooling systems have a rated efficiency from a U.S. governmental agency. Fossil fuel furnaces have a percentage efficiency rating. Natural gas, propane and fuel oil furnaces have efficiency ratings based on laboratory conditions. To get an accurate installed efficiency rating, factors such as flue gas heat losses and cycling losses caused by oversizing, blower fan electrical usage, etc., must be included.

Geothermal heat pumps as well as all other types of heat pumps, have efficiencies rated according to their coefficient of performance or COP. It's a scientific way of determining how much energy the system produces versus how much it uses. Most geothermal heat pump systems have COP's of 3 to4.5. That means for every one unit of energy used to power the system, 3-4.5units are supplied as heat. Where a fossil fuel furnace may be 78 to 98 percent efficient, a geothermal heat pump is about 400 percent efficient. Some geothermal heat pump manufacturers and electrical utilities use computers to accurately determine the operating efficiency of a system for your home or building.

Do geothermal systems require much maintenance?

No. Geothermal systems are practically maintenance free.When installed properly, the buried loops will last for generations. And the other half of the operation - the unit's fan, compressor and pump - is housed indoors, protected from the harsh weather conditions. Usually, periodic checks and filter changes are the only required maintenance.

What does geothermal mean for the environment?

Geothermal systems work with nature, not against it. They emit no greenhouse gases, which have been linked to global warming, acid rain and other environmental hazards. Geothermal Eco Options uses an environmentally friendly anti-freeze exclusively in all the ground loops that we install in case of the unlikely event of a leak.

How does a geothermal heat pump work?

Anyone with a refrigerator or air conditioner has witnessed the operation of a heat pump, even though the term heat pump may be unfamiliar.All of these machines, rather than making heat, take existing heat and move it from a lower temperature location to a higher temperature location.Refrigerators and air conditioners are heat pumps that remove heat from colder interior spaces to warmer exterior spaces for cooling purposes. Heat pumps also move heat from a low-temperature source to a high-temper space for heating.

An air-source heat pump, for example, extracts heat from outdoor air and pumps it indoors. A geothermal heat pump works the same way,except that its heat source is the warmth of the earth. The process of elevating low-temperature heat to over 100 degrees Fahrenheit and transferring it indoors involves a cycle of evaporation, compression, condensation and expansion. A refrigerant is used as the heat-transfer medium which circulates within the heat pump. The cycle starts as the cold liquid refrigerant passes through a heat exchanger (evaporator) and absorbs heat from the low-temperature source (fluid from the ground loop). The refrigerant evaporates into a gas as heat is absorbed.

The gaseous refrigerant then passes through a compressor where the refrigerant is pressurized, raising its temperature to more than 180degrees. The hot gas then circulates through a refrigerant-to-air heat exchanger where heat is removed and pumped into the building at about 100degrees. When it loses the heat, the refrigerant changes back to a liquid. The liquid is cooled as it passes through an expansion valve and begins the process again. To work as an air conditioner, the system's flow is reversed.

Does a geothermal system heat and cool?

One thing that makes a geothermal heat pump so versatile is its ability to be a heating and cooling system in one. With a simple flick of a switch on your thermostat, you can change from one mode to another.

Do I need separate ground loops for heating and cooling?

No. The same ground loops work for both. To switch heating to cooling, or vice versa, the flow of heat is simply reversed.

Does the underground pipe (ground loop) system really work?

The buried pipe, or ground loop, was an important technical advancement in heat pump technology. The idea of burying pipe in the ground to gather heat energy originated in the 1940's. New heat pump designs and more durable pipe materials have been combined to make geothermal heat pump systems the most efficient heating and cooling systems available.

What is a closed loop system?

A closed (ground) loop system uses a continuous loop of buried high density polyethylene pipe (HDPE). The HDPE pipe is connected to the indoor heat pump to form a sealed, underground loop through which an environmentally friendly anti-freeze and water solution is circulated. A closed loop system constantly re-circulates its heat-transferring solution in pressurized polyethylene pipe. Many closed loop systems are horizontally bored in areas adjacent to the building. However, where adequate land is not available, loops are vertically bored. Any area near a home or business with appropriate soil conditions and adequate square footage will work.

How long will the ground loop last?

Closed loop systems should be installed using only high density polyethylene pipe. HDPE is expansive, therefore it is able to expand and contract. Properly installed, these pipes will last for 50 years or more.They are inert to chemicals normally found in the soil and have good thermal conducting properties. PVC pipe is not expansive and therefore should never be used in a ground loop.

How are the pipe sections of the loop joined?

Pipe sections are joined by thermal fusion. Thermal fusion involves heating the pipe connections and then fusing them together to form a joint that's stronger than the original pipe. This technique creates a secure connection to protect from leakage and contamination.

Will the ground loop affect my lawn or landscape?

No. Research has proven that ground loops have no adverse effects on grass, trees or shrubs.

I have a nearby pond; can I put a loop in it?

Yes, if it's deep enough and large enough. A minimum of six feet in depth at its lowest level during the year is needed for a pond loop to be considered. The amount of surface area required depends on the heating and cooling load of the structure.

Can I install the ground loop myself?

It's not recommended. Good earth-to-loop contact is very important for successful loop operation. Nonprofessional installations may result in less-than-optimum loop system performance.