Geothermal Exchange: Ground Source Heating And Cooling
Posted on Mar 08, 2008 - 04:55 PM
By: Adam Beazley
Geothermal ground-source heating and cooling, technically known as geothermal exchange, is the process of harnessing the earth’s constant underground temperature to heat and cool a structure. The term geothermal exchange really describes the process of exchanging heat energy with the earth. Throughout the world, while above ground air temperatures vary drastically between sub-zero and over 100°F, the earth’s temperature 6 feet below grade remains relatively constant and only varies between 50°F and 70°F. This is due to the fact that the earth is a thermal mass and has thermal inertia otherwise known as Volumetric heat capacity. This thermal inertia provides a constant source of energy which can be used effectively to heat and cool a building or residence.
All geothermal exchange systems use devices called heat pumps. Heat pumps are extremely efficient at heating and cooling due to the fact that they move heat as opposed to generating heat. A typical HVAC system will use gas or electricity to generate heat in the winter and remove heat and moisture in the summer. This type of setup will never exceed 100% efficiency because you only get out what you put in. On the other hand, a heat pump, because it simply moves heat can effectively provide 4 times the heating and cooling with the same unit of energy used in a typical HVAC system. That means that in certain conditions, heat pumps can see efficiencies in excess of 400%.
The only drawback to typical air exchange heat pumps is that when interior and exterior temperature differentials exceed 30°F, they have a hard time keeping up. For instance, when the outside air temperature is below 0°F, it is nearly impossible for a heat pump to move heat from that condition into the home, and it is for this reason that typical air exchange heat pumps come fitted with supplemental heaters for cold winter days. This drawback can be solved by using a geothermal exchange setup as opposed to an air exchange setup. As stated above, the sub surface temperatures of the earth stay relatively constant throughout the year, so these ground source heat pumps never see 30°F temperature differentials.
Geothermal exchange systems come in many different types, sizes and configurations, but all of them have have two main components, the heat pump and the ground loop. There are basically two types of geothermal exchange ground loop systems, an open loop system and a closed loop system.
Open Loop Geothermal Exchange System:
An open loop system is quite uncommon due to the fact that it relies on a nearby body of water( stream, pond, lake or underground well). In an open loop system, the water from the body of water flows (underground) to the heat exchanger in the heat pump and then back out again. This type of system is slightly more efficient than a closed loop system because the source water comes into direct contact with the heat exchanger. In a closed loop system, the water or antifreeze circulating in the plastic pipe is isolated from the ground temperature, so the heat exchange is slightly less than in an open loop system.
Closed Loop Geothermal Exchange Systems:
Closed loop geothermal exchange systems, unlike open loop systems, use an environmentally friendly antifreeze liquid to exchange the heat between the earth and the heat exchanger in the heat pump. A closed loop system is the most common type of geothermal exchange system simply because it does not require a nearby body of water to pull from. Closed loop systems come in a variety of different configurations all of which have their own benefits and detriments.
Vertical Loops:
Vertical loop configurations are the most expensive of all of the closed loop configurations due to the fact that large well holes must be drilled down to a suitable depth, piped and then filled with a thermal grouting compound. These types of closed loop systems are ideal for areas where yard space is limited and the required area needed to cool the structure with a horizontal closed loop system is exceeds what is available. Also, in situations where the earth is rocky close to the surface or when the geothermal exchange system is retrofitted to an existing structure and limited land disturbance is required, the vertical loop configuration is perfect. The number of vertical drill wells is directly related to the depth of the wells, in other words, less holes for deeper wells. The depths of the drilled holes will generally vary between 150 to 450 feet where they will ultimately depend on the needs of the structure as well as the cost.
Horizontal Loops:
Horizontal closed loop configurations are the most popular and most cost effective of all geothermal exchange systems. Horizontal loops require the use of a trencher or a backhoe to dig to a depth of at least 8 feet. Individuals who are able and willing to do some of this work themselves can see a drastic reduction in the total cost of their geothermal exchange system. As a rule of thumb, a typical horizontal loop system will require 400 to 600 linear feet per ton of cooling capacity.
Another type of horizontal configuration is the slinky loop. While a slinky loop will require less land area, it will generally require more pipe than a typical parallel type horizontal loop configuration. As with a parallel loop, the earth is excavated to a depth of approximately 8 feet and the pipe is coiled like a flattened slinky and then buried.
Pond Loops:
In specific situations, where an open loop system cannot be applied to a nearby body of water, a closed loop system may be the best choice. Just like the other closed loop systems, an environmentally friendly antifreeze is circulated through the closed polyethylene piping except the exchanging medium is water instead of the earth. Bodies of water, like earth are thermal masses and hold the same thermal inertia as the earth does, so at the same depth, the body of water will provide very similar constant temperatures as the earth. Pond loop configurations are generally slinky like in nature and extend a minimum of 6 to 8 feet below the the lowest water level to assure proper heat exchange.
Heat Exchanger Variations:
There are two basic ways of exchanging the heat from the earth to the heat exchanger inside the heat pump, using a fluid to water heat exchanger or a fluid to air heat exchanger. In both cases, the fluid refers to the water or antifreeze in the ground loop and the water or air refers to the medium used within the structure as the delivery method. In a fluid to water heat exchanger, the water in the structure is typically used for radiant heating and cooling where the floor and/or walls act as thermal masses with the water lines embedded inside. In a fluid to air heat exchanger, the air within the structure is typically ducted, using conventional ducting methods to the spaces throughout the structure. The delivery method really comes down to the preference of the end user, however, fluid to air exchangers will typically cost less due to the fact that ducting is less labor intensive than laying radiant flooring.
Another heat exchanger variation is the use of supplemental heat to generate your domestic hot water. In the summer, when the heat pump is moving heat from within your home, that excess heat can be used to generate your domestic hot water for free. During the winter when the heat pump is moving heat from outside to inside, the extra heat generated from the actual heat pump motor can be recovered and used to generate your domestic hot water very efficiently, but not free.
Geothermal exchange systems are extremely efficient, cost effective heating and cooling systems and can be used in almost any environment. These ground source systems can reduce energy cost by up to 60% when compared to typical HVAC system of equal tonnage. Although most geothermal exchange systems will cost more up front, if those cost are added to the mortgage, the monthly energy savings will generally offset the added mortgage cost each month. Not only can these geothermal exchange systems save money, but because of their efficient nature, they will help the environment by reducing your energy consumption.
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Show/Hide Comments (4)
By Jim Bisnett on 05/20/2008
This sounds like a great energy and cost savings to the individual. On average, how many years does it take to make up the initial cost increase of going geothermal as opposed to conventional natural gas heating?
By Adam Beazley on 05/20/2008
When comparing a ground source system to a conventional system a homeowner can expect to save anywhere from 30% to 70% annually on utilities. Even though it has a high initial costs, the system the payback period is relatively short, typically between three and five years.
By Sell on 05/27/2008
Nice read. I didn’t know about Geothermal but it’s seems very viable. I think it’s a matter of time before most houses that use this more. Thanks.
By Adventure Art Travel on 09/04/2008
This is a great energy source. The main reason I don’t think it is catching on in the UK is the big capital cost and the fact that people move house more frequently now days. With rising fossil fuel prices it must gradually take market share. Ideally kick started when adopted by major house builders.
