|Image courtesy of ClimateMaster|
How does geothermal heating and cooling work?
Geothermal systems transfer heat between the ground and a building. In the cooling mode they absorb heat from the building and transfer it to the ground. Conversely in the heating mode they absorb heat from the ground and transfer it to the building. Typically they do this by circulating water through the ground and then through a heat pump. The heat pump transfers the heat either to or from the water to the buildings via a compressor and refrigerant circuit coupled with a coil and a fan.
Below the frost line the earth temperature stays at a near constant, 55-60° F year round. This provides a reliable source of heat during the winter. Because the ground temperature does not drop like outside air temperature the heat pump is able to provide a higher air temperature which eliminates the cool drafts associated with a typical heat pump. Additionally, geothermal dramatically reduces the amount of hydrocarbon fuels burned to create heat.
There are two types of geothermal systems: an open system and a closed loop system. In an open system a pump is used to draw water from the ground through a well. The water is pumped through a heat pump and then returned back to the ground via a second well. While this type of system is very efficient one major draw back is that the ground water can be heavily saturated with minerals and suspended solids which coat the interior of the pipes reducing heat transfer. These substances can also clog the pumps and piping. This kind of system can also be affected by water table level and droughts.
In a closed loop system the water is circulated through loops of piping buried in the ground and then through a heat pump. These loops can be installed either vertically or horizontally and can even be installed under parking lots. This type of installation requires little additional water after the initial fill. The vertically installed loop system is not affected by a drought.
There are significant savings with a geothermal system. While the initial cost of the system installation is higher than a conventional heating and air conditioning system it is generally paid back in about seven years (sometimes less) via utility bill savings. Energy savings can be increased even more by recovering heat from the building during the cooling mode and generating domestic hot water rather than transferring it into the ground. In a comparison of two 155,000 square foot high schools with the same footprint it was found that the utilities for a geothermal school were approximately $60,000 less per year than a conventionally heated and cooled school.
Geothermal systems are safer than conventional systems since there is no need to run propane or natural gas or fuel oil piping inside the building for heating. The potential for carbon monoxide poisoning is also eliminated because the heat pumps are not burning hydrocarbon fuels such as natural gas or oil. Finally, since the outside air conditioning unit is eliminated the danger of a spinning fan blade is removed.
The piping loops for many geothermal systems are warranted for 50 years. The geothermal heat pumps may carry a 25 year warranty. The underground piping has no moving parts and requires no maintenance. All piping joints are heat welded to prevent the problems that occur with glued joints. The school division fully expects the geothermal systems installed in the schools to last 30 years. Since geothermal heat pumps do not cycle through a defrost mode like conventional heat pumps there is less stress on the compressor which lengthens the life of the compressor. Geothermal heat pumps are typically located inside the building which protects the units against the weather. The outdoor equipment for conventional heat pumps and air conditioning systems begins to deteriorate and rust after only a few years of service. The hail damage and vandalism which occurs to conventional outdoor equipment is also eliminated.
The noisy, rusty outdoor equipment for conventional heat pumps and air conditioning systems is eliminated providing a cleaner appearance for the building.
Due to the elimination of defrost controls and electric resistance heaters, geothermal heat pumps are more dependable and easier to maintain than conventional systems. Geothermal heat pump compressors also do not experience the stresses caused by the defrost mode in conventional heat pumps which enables the compressor last longer. Since geothermal heat pumps are typically located inside buildings where they are protected from the weather they last longer and require less maintenance.