Intrigued by Geothermal?
February 13, 2012
In my experience, geothermal systems are generally desired but widely misunderstood. Many of us understand that geothermal systems take advantage of the earth’s temperature to heat and cool buildings, that they involve deep drilling and that they are expensive to install, but cheaper to run that conventional heating and cooling equipment. Beyond that, general knowledge gets dicey.
This blog will give you an overview of geothermal systems, especially as they relate to residential applications. My description of geothermal systems is pulled from April’s Architect Magazine with additional input from Melanie Head at EnergySmart Alternatives. If after reading this, you are interested in geothermal for your own home, I would strongly suggest that you consult with a trained and experienced expert to find out more. My go-to local geothermal expert is EnergySmart Alternatives out of Wakefield, MA. Not only are they experienced installers and contractors, they have a team of engineers who make sure that every installation is done right.
Geothermal Systems – What Are They?
Geothermal systems for buildings, also known as geothermal heat pumps or ground-source heat pumps (GHPs), use the thermal energy stored in the upper portion of the Earth’s crust to heat or cool a building, replacing conventional heating and air-conditioning systems. “The temperature of the Earth down 20 or 30 feet is a relatively constant number year-round, somewhere between 50 and 60 degrees , says John Kelly, the COO of the Geothermal Exchange Organization, a nonprofit trade organization in Washington, D.C. “A geothermal heat pump moves heat to and from the Earth by circulating water through a well.”
In other words, in winter, a GHP moves the thermal energy from the earth into a building, and in summer it reverses that process, transferring heat from a building into the earth. These systems incorporate a piping loop buried in the ground through which anti-freeze is circulated, and the heat pump extracts the temperature from the anti-freeze and distributes it through the building, much in the same way that central air conditioning works. Alternatively, groundwater is directly circulated through a series of wells.
Either way, GHPs are significantly cheaper to operate than conventional heating and cooling systems. “The cost savings occur because the ground offers starting temperatures closer to what is desired for heating and cooling than the seasonal temperature extremes upon which many conventional air-source HVAC systems rely,” says John Rhyner, a senior project manager at P.W. Grosser Consulting in Bohemia, N.Y., a civil engineering firm that specializes in geothermal. “It takes less energy to make up that smaller difference in temperature,” Rhyner says.
diagram showing heat transfer to and from the Earth in Cooling and Heating Seasons
The three most common types of GHP systems are closed-loop, open-loop, and standing column well.
Open loop systems circulate anti-freeze through a sealed network of pipes buried underground. The anti-freeze within the pipes transfers heat from the earth to the building during the winter, and vice versa during the summer, by way of a heat exchanger. Since the anti-freeze flows in a closed loop, it does not exchange all of its temperature; it can get as warm as 80 to 90 degrees F in summer and as cold as 40 to 30 degrees F in winter. For this reason, the anti-freeze is usually a food-grade antifreeze with freeze protection between 15F to 20F (for example, ethanol) to keep the fluid from gelling during the winter months.
Closed-loop systems can be laid out either horizontally in fields, buried just beneath the frost line, or vertically in wells, bored typically 200 to 500 feet deep. Horizontal systems are generally used for smaller or residential projects with plenty of space. In geographic locations where there are few rocks and bedrock is not present close to the surface, horizontal loops are cheaper to install. However, horizontal loops are affected by outdoor air temperatures, meaning that they can become less efficient as a season progresses as the soil takes on the characteristics of the air temperature.Horizontal loop systems typically require large amounts of land. “For a closed-loop system, it’s all a function of how much pipe you can get in the ground with the open land area you have available to work with,” Rhyner says.
Vertically drilled closed-loop systems are more efficient than horizontal systems, as more of the pipe is in contact with a more constant earth temperature. They are most efficient if they can be drilled into groundwater rather than dry ground, since water is a good conductor of heat. “You get a certain number of tons per linear footage [a ton of heat is 12,000 British thermal units per hour], and can get more pipe in the ground going vertically than horizontally,” says Rhyner.
Standing column wells are another type of open-loop system that is well suited where bedrock is close to the ground surface. Standing column wells are typically less deep than vertical closed-loop systems with similar heat output capacity. Whereas vertical closed-loop borings are typically 250 to 400 feet deep, standing column wells can be anywhere from several hundred feet to over 2,000 feet deep. Steel casing is installed to hold the borehole open up to the depth of bedrock. The remaining depth is drilled through bedrock and is left as an open rock borehole. In these systems, the groundwater is pumped up from the bottom of the well, passed through the GHP, and then returned to the top of the well, where it filters slowly downward, exchanging heat with the surrounding bedrock.
Choosing which of these systems is right for a specific project requires calculating a building’s heating and cooling demand and conducting a subsurface analysis to determine the thermal capacity of the site, and how many wells or how large of a loop field will be needed. If the calculations are done correctly and the system is properly designed, GHPs can handle all of a building’s heating and cooling loads, no matter what climatic conditions prevail.
High Upfront cost versus Return on Investment
When designed and installed correctly, GHPs drastically reduce the amount of energy needed to heat and cool a building. According to the U.S. Environmental Protection Agency, GHPs are 48 percent more efficient than the best gas furnace and 75 percent more efficient than the best oil furnace. They require 25 to 50 percent less energy than other HVAC systems and bring down operation and maintenance costs by as much as 40 percent.
The main inhibitor to the wide-scale adoption of GHPs today is the relatively high up-front cost of installation. The main difference in cost between GHPs and conventional systems is the drilling cost. The mechanical equipment itself—the heat pumps and heat exchangers—is no more expensive than high-efficiency conventional heating and cooling systems. Annual savings on energy bills, however, offset the up-front cost. When taking advantage of the available incentives, payback periods for commercial GHP systems can be as little as 5 to 7 years when replacing an aging, inefficient HVAC system. GHP systems are especially cost-competitive against many conventional systems in new construction. In the past, GHPs were primarily popular with municipal and institutional clients, building owners who planned to inhabit and operate their facilities over the long term, and those who were simply more interested in environmental stewardship than the bottom line. With the currently available incentives and the high price of fossil fuels, payback periods have been significantly reduced making GHPs an attainable investment for more building owners.
The cost of installing a geothermal system can vary depending on site specifics. In existing buildings, challenges like duct routing, construction type, and space restrictions can affect the cost significantly. Such challenges are more easily overcome in new construction where these issues can be discussed with the architect or builder early in the design process. Your chosen geothermal company will be able to assist you with a cost analysis for the system that is best suited for your home. Their analysis will factor in the cost of installing a traditional heating and cooling system, the cost of fossil fuel and the available local and federal incentives. The following links have information on these incentives from both federal and local programs.
Federal Residential Renewable Energy Tax Credit (30% of the price of the system)
Local energy company incentives may also be available.
a geothermal drilling rig
Common Myths About Geothermal
It’s surprising how often the same questions and comments arise regarding geothermal systems. The following, part of Energy Smart Alternatives’ ‘Geothermal Demystified’ series, sheds some light on some of these common misunderstandings regarding geothermal installations.
Myth #1: Backup Heating
There is a common misconception that GHPs are not able to provide 100 percent of heating requirements. This simply isn’t true. A properly designed GHP system will provide all of the heating and cooling requirements of the building. There is no need whatsoever to install a gas or oil boiler to provide a backup heat source.
Myth #2: Winter Installation
Transitioning from a fossil fuel heating system to a GHP in the winter can be a challenge. In most cases, the home will be without heat for one or two days while the new geothermal system is being installed. Although a temporary heat source can be used while the transition is being made, some homeowners choose to just add a few layers of clothing.
The drill rig used for vertical installations can drill through bedrock and certainly has the capacity to drill through frozen soil and ice. Trenching in winter can be difficult, though; the degree of difficulty depends on your geographic location and ground cover conditions. When trenching in a small area, a few straw bails can keep the ground from freezing long enough to complete the installation. In some cases, excavators may not be willing to dig in the winter because of wear-and-tear on equipment.
Myth #3 Concerns about bedrock or ledge
Installing a vertical geothermal boring through bedrock is not a problem. Geothermal boreholes are created by cutting and grinding a 6-inch core through bedrock; there is no blasting, hammering, or pile driving. An experienced driller can drill between 200 and 300 feet through solid bedrock in one day. In New England, bedrock will usually be encountered within 50 feet of the ground surface and is encountered on almost every single geothermal installation.
Some homeowners have expressed concerns about drilling through bedrock in close proximity to their own, or their neighbors’, basement foundation wall. To my knowledge, no foundation damage has ever occurred – even when the borings were advanced within 10 to 15 feet of a foundation wall. The drilling will not cause an earthquake. It will not rattle the entire neighborhood.
Shallow bedrock can be an obstacle to horizontal closed-loop installations where hundreds of linear feet of trench are required. It can also be a problem when trenching between the location of vertical borings and the basement foundation wall. A careful evaluation of the site prior to digging will dictate the location of drilling or excavation so as to minimize encounters with ledge during excavation activities.
Myth #4 Concerns about wasting money on drilling.
EnergySmart’s team has installed over 200 tons of geothermal heating systems throughout New England and there has never been a situation where drilling has occurred and the installation has not been completed. First, it starts with an understanding of how the underground components of a geothermal systems actually work.
For both horizontal and vertical closed-loop systems, the heat transfer occurs between the soil or bedrock and the geothermal piping to the antifreeze circulating through the pipe. While groundwater improves the heat transfer properties of the underground portions of a closed-loop system, the presence of copious amounts of groundwater is not absolutely critical to the operation of the system. The presence or absence of groundwater should be accounted for in the design process but does not preclude the installation and effective operation of a GHP system.
Open-loop systems circulate groundwater through the GHP system. It is imperative that the well has enough capacity to support the geothermal system. Low well capacity can be overcome by fracking the well or deepening the well to increase its capacity and yield (this is a chemical-free fracking technique that is completely different from that used by the natural gas industry). In extreme cases, systems that were originally intended to be open-loop can be converted to closed-loop when the well doesn’t produce sufficient good quality water. Similarly, if salt water or hard water is encountered, systems originally intended to be open-loop can be converted to closed-loop where water quality will have no impact.
Thank you to Melanie Head at EnergySmart Alternatives for her valuable information. Feel free to contact EnergySmart Alternatives for more information.
Juli MacDonald, GreenBridge Architects