What is a geothermal heat pump?
Geothermal heat pumps (also known as ground source heat pumps) are a renewable alternative to furnaces or boilers. It is an important part of the geothermal system.
The geothermal system consists of two main parts:
Geothermal heat pump located in your home (usually where the furnace is placed)
Underground pipes, called ground loops, are installed in your yard below the frost line
The main difference between a furnace and a geothermal heat pump is the heat source used to heat the home. A typical furnace generates heat by burning oil or natural gas in its combustion chamber, while a geothermal heat pump simply transfers heat from the existing underground.
In addition, although furnaces and boilers can only heat, many geothermal heat pumps (such as dandelion geothermal) can heat and cool.
How does the geothermal system work?
In short, a geothermal system absorbs heat from the ground to heat your house in winter, and discharges the heat from your house to the ground for cooling in summer. This explanation may sound a bit sci-fi, but the way the geothermal system works is very similar to the refrigerator in the kitchen.
Just a few feet below the frost line, the ground remains at about 50 degrees Fahrenheit all year round. A water-based solution circulates through underground pipes, where it absorbs heat from the ground and is carried into a geothermal heat pump.
The solution exchanges heat with the liquid refrigerant in the heat pump. The refrigerant then evaporates and passes through the compressor, where the temperature and pressure increase. Finally, the hot steam enters the heat exchanger, where it transfers the heat to the air. This hot air is distributed through the plumbing system of the home and heated to any temperature set on the thermostat.
How efficient is the geothermal heat pump?
For every unit of energy used to power the geothermal system, 4 units of heat energy will be provided. This is about 400% efficiency! Geothermal heat pumps can achieve this efficiency because they do not generate heat-they just transfer heat. Only about one-third to one-quarter of the energy provided by the geothermal system for heating comes from electricity consumption. The rest is extracted from the ground.
Types of geothermal heat pump systems
There are four basic types of ground loop systems. Three of them-horizontal, vertical and pond/lake-are closed loop systems. The fourth type of system is an open loop option. Various factors such as climate, soil conditions, available land and local installation costs determine which method is most suitable for the site. All of these methods can be used in residential and commercial building applications.
1. Closed loop system
Most closed-loop geothermal heat pumps pass a closed-loop cycle of antifreeze solution-usually made of high-density plastic pipes-buried in the ground or submerged in water. The heat exchanger transfers heat between the refrigerant in the heat pump and the antifreeze in the closed loop.
A closed-loop system called direct exchange does not use a heat exchanger, but instead pumps refrigerant through COPper pipes buried in the ground in a horizontal or vertical configuration. The direct exchange system requires a larger compressor and works best in moist soil (sometimes additional irrigation is needed to keep the soil moist), but you should avoid installing it in soil that is corrosive to COPper pipes. Since these systems circulate refrigerant on the ground, local environmental regulations may prohibit their use in certain places.
This type of installation is usually the most cost-effective for residential installations, especially for new buildings where enough land is available. It needs trenches at least four feet deep. The most common layouts either use two pipes, one buried 6 feet deep and the other 4 feet deep, or two pipes are placed side by side in a 2-foot wide trench 5 feet deep underground. The Slinky™ ring pipe method allows more pipes to be installed in a shorter groove, thereby reducing installation costs and making horizontal installation possible, which cannot be done in traditional horizontal applications.
Large commercial buildings and schools often use vertical systems because the land area required for a horizontal loop can be prohibitive. Vertical loops are also used where the soil is too shallow to dig trenches, and they can minimize disturbance to the existing landscape. For vertical systems, the boreholes (approximately 4 inches in diameter) are about 20 feet apart and 100 to 400 feet deep. The two pipes are connected with a U-bend at the bottom to form a ring, which is inserted into the hole and grouted to improve performance. The vertical loop is connected to the horizontal pipe (ie, the manifold), placed in the trench, and connected to the heat pump in the building.
If the site has sufficient water, this may be the lowest cost option. A water supply line extends from the building’s underground to the water surface and is coiled in a circle at least eight feet below the surface to prevent freezing. The coil can only be placed in a water source that meets the minimum volume, depth and quality requirements.
5. Open loop system
This type of system uses well water or surface water as the heat exchange fluid that circulates directly through the GHP system. Once it circulates through the system, the water returns to the surface through wells, replenishment wells, or surface discharge. This choice is obviously only feasible if a relatively clean water supply is sufficient and all local laws and regulations regarding groundwater discharge are met.
6. Hybrid system
A hybrid system using several different geothermal resources, or a combination of geothermal resources and outdoor air (ie, cooling towers), is another technical option. The mixing method is particularly effective in situations where the cooling demand is significantly greater than the heating demand. Where local geological conditions permit, "pillar wells" are another option. In this variant of the open loop system, one or more deep vertical wells are drilled. Water is drawn from the bottom of the column and returned to the top. During peak heating and cooling periods, the system can drain a portion of the return water instead of reinjecting it all, causing water to flow into the column from the surrounding aquifer. The vent cycle cools the column during heat removal, heats the column during heat removal, and reduces the required drilling depth.