Choosing the Right Heat Sink for a Geothermal Heat Exchanger

According to the US Department of Energy the most effective type of heat pump is the geothermal heat pump, GHP, more accurately called ground-source heat pumps. These systems have been proven capable of producing large reductions in energy use in buildings. A study by the U.S. Environmental Protection Agency (EPA) comparing the major HVAC options for residential applications determined that GHPs were the most energy efficient and environmentally benign option. The overall performance of these systems will be to a large extent determined by selecting the ideal heat sink for your site and sizing your heat sink and system correctly. Only about 60,000 GHP units are installed each year in the combined US new built and retrofit market. The number is so small because the market is fragmented, the total systems are difficult to understand, the installation costs appear to be about twice the costs of less efficient system, and the distribution and installation network is inefficient. Better understanding of all elements of these systems is the first step in choosing a GHP.

In winter GHPs collects the Earth's natural heat from a heat sink, typically that is through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. Fluid circulates through the loop via an electric pump and carries the heat to the house. There, an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms. In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. The system cools your home in the same way that a refrigerator keeps your food cool by drawing heat from the interior, not by blowing in cold air.

As you may have experienced in a cave, the temperature at least six feet beneath ground surface is cooler in summer and warmer in winter than the ambient temperature. Using this temperature as its source the GHP can operate within its most efficient range for heat transport. The coefficient of performance or COP, of a GHP (or any heating and/or air conditioning unit) is essentially a measurement of the amount of work (either electricity or other source of power) necessary to change the temperature in the desired direction. A theoretical maximum achievable COP would be 7.8 on the usual design parameters. Test results of the best systems are above 5.0. However, when looking at cost savings on an installed unit this savings would be reduced by the energy needed to operate the pumps for the water and/or antifreeze through the piping systems.

The typical GHP has a water loop heat sink installed either horizontally or vertically. Horizontal ground loops are usually the most cost effective when trenches are easy to dig, the size of the yard is adequate and there is adequate soil. Workers use trenchers or backhoes to dig the trenches six feet below the ground in which they lay a series of parallel plastic pipes or loops. They then backfill the trench. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 400 to 600 feet long for each ton of heating and cooling.

The vertical loop is used where there is little yard space, when surface rocks make digging impractical or the heat exchange properties of the rocky soil are unacceptable, or when you want to disrupt the landscape as little as possible. Vertical holes are typically 150 to 450 feet deep and contain a single loop of pipe with a U-bend at the bottom. Each vertical pipe is then connected to a horizontal underground pipe that carries fluid in a closed system to and from the indoor exchange unit. Vertical loops are generally more expensive to install than horizontal loops, but require less piping than horizontal loops because the Earth's temperature is more stable farther below the surface. The performance of these heat sink systems can also be enhanced by the presence of groundwater to improve the thermal transport properties of the water loop.

These heat sink systems whether vertical or horizontal have two loops: the primary refrigerant loop is contained in the GHP cabinet where it exchanges heat with a secondary water loop that is buried underground. They are thus called water loop systems. The secondary loop is generally HD polyethylene pipe and contains a mixture of water and an anti-freeze. After leaving the GHP, the water flows through the secondary loop outside the building to exchange heat with the ground before being returned to the building. In the extremes of weather, they efficiency of the GHP system can be negatively impacted by the water in the secondary loop’s ability to loose or gain enough heat.

The secondary loop is placed deep enough below the surface (and frost line) that the temperature remains relatively stable. Siltstone, balls bluff and other rocky soil are a poor medium for temperature transport and HD polyethylene is not an efficient heat transport medium. Loop systems installed in wet ground or in water are significantly more efficient than drier ground loops since it easier to move heat in water than air, solids, sand or soil because there is better surface contact.

A variation on the ground loop duel system is the direct loop system. The refrigerant leaves the GHP cabinet, and is pumped directly through a loop of copper tubes buried underground, and exchanges heat with the ground before returning to the pump. The name "direct exchange" refers to heat transfer between the refrigerant and the ground without the use of an intermediate fluid. Heat transfer takes place through the piping wall. It is best to use an environmentally friendly anti-freeze like denatured alcohol or methane because all these systems will someday fail and a simple and easy remediation should be planned in advance. Direct exchange heat pumps are not to be confused with “water loop heat pumps" since there is no water in the ground loop. Ground loop heat pumps have a higher efficiency than the water loop systems.

A groundwater or open loop system is the most efficient in terms of heat transport. These systems were originally designed to pump natural water from a well or body of water into a heat exchanger inside the heat pump. Heat would be either extracted or added depending on the season, and the water is returned to a separate injection well, or body of water. The supply and return lines must be placed far enough apart to ensure thermal recharge of the source. These systems were subject to limescale clogging or fouling if the water contains high levels of salt, calcium carbonate, iron and or magnesium, iron bacteria or hydrogen sulfide. In addition, these open-loop systems may drain aquifers or contaminate wells.

A standing column well system is a specialized type of open loop system. According to Dr. Zheng Dong O’Neill who has done extensive research on the topic, though there were only about 1,000 standing column systems installed in the United States in 2005 these systems utilize a shorter borehole depth and have more stable water temperature so they are cheaper to install than a vertical loop system and are more efficient than the water loop systems. In standing columns water is drawn from the bottom of a deep well, passed through a heat pump, and returned to the top of the well. Heat is lost or gained from the water as it travels back down the well. This system because it utilizes direct water contact for the heat sink (which is really the bedrock) is more efficient. In addition, because the water is returned to the well there is limited impact on the groundwater system. A high density of these groundwater systems could potentially impact a groundwater basin.

The standing column system is ideal where the bedrock is near the surface and the soils are rocky so that the well casing required is less than 60 feet. The standing column well method is popular in residential and small commercial applications in the New England states and should be ideal for the geology in my backyard. I have a strong shallow aquifer that runs from Bull Run Mountain to the river behind my land. There are acres of woods between the house and the yard (that I own) to allow the aquifer to regain its natural temperature profile. My land is down gradient of all my neighbors with the river serving as a hydraulic barrier to the basin. Any additional wells I install down gradient of my own drinking water well will not impact any other homes. Access to the well location for the well drilling equipment is an important factor. Well drillers tend to want to locate the well where it is easiest to drill. Make sure that the location will not impact drinking water wells, and that the trenching for the connection to the unit in the house is at least six feet deep. If that trench is too long and too shallow you defeat any benefit of the geothermal system. This is also true for the vertical loop wells.

If flow and sizing are properly done, this type of groundwater system can potentially have some heat storage benefits. The idea is that heat is rejected from the building will raise the temperature of the well by a few degrees during the Summer cooling months can then be harvested for heating in the Winter months, thereby increasing the efficiency of the heat pump system. Of course if the temperature of the well is raised too much in the summer the efficiency of the cooling system just falls. As with closed loop systems, sizing of the standing column system is critical. The heat exchange in this system is actually with the bedrock, using groundwater as the transfer medium. A large volume of water flow is not required for a standing column system to work and the water is returned to the well so the net effect on the groundwater table should be negligible. However, if there is adequate water production and the groundwater is well recharged, then the thermal performance of the well system can be increased by discharging a small percentage of the system water flow during the peak summer and winter months.

Since this is essentially a water pumping system, standing column well design is the most difficult to obtain peak operating efficiency. The well designer and driller needs to be fully trained and certified in GHP systems to ensure that the system is properly designed and includes all essential design elements including system shut-off valves to install a system that operates at maximum efficiency. Since the number of national installation has been so limited, experienced well drillers are difficult to find. The American Society of Heating, Refrigerating and Air Conditioning Engineers, ASHRAE, has funded Dr. O’Neill’s research to model standing column wells in GHP systems. Finding a well driller who has been certified in GHP by ASHRAE is a place to begin.

Reducing My Energy Consumption

I have been systematically making small changes to my home to reduce my energy consumption. I started with the easiest steps; lowering the thermostat in the winter and raising the temperature in summer, purchasing energy star eligible appliances and choosing an LCD TV over a plasma (an LED TV is even more energy efficient, but was not available at the time). The next simple step was to change all the incandescent light bulbs for florescent bulbs and when I installed additional lighting it was florescent fixtures. (Though, I warn that the clothes in my closet look oddly colored in florescent light.) The next project was to install solar films on the windows and patio door and drapes and curtains on all the windows. These were small steps, but I learned over the years that small steps do add up.

The following year, after servicing the heat exchanger and furnace to ensure they were working properly, and appropriately sized for the house, and inspecting the attic and accessible areas of the basement and crawl spaces for adequate insulation, I turned to the Building Envelop Research of the Oak Ridge National Laboratory for guidance. The Oak Ridge National Laboratory performs their Building Envelop Research for the US Department of Energy, DOE. The DOE publishes their guidance in their “Insulation Fact Sheet,” which is available on the blog home page. Following the recommendations by the Oak Ridge National Laboratory the attic, crawl spaces, eves, ductwork, underside of a large portion of the main level floor were insulated with cellulose. The pipes, wall end caps, knee walls, sump pumps and all identified areas were sealed, the garage ceiling was insulated and an insulated garage door installed. I was actually surprised at the winter energy savings and pleased with the improved comfort in the master bedroom and bath.

My next project was to spend the winter saving money eating and entertaining at home, watching DVDs for “nights out” on my LCD, eliminating trips to the mall and saving up money for my next energy saving project. Back in October 2008 President Bush had signed the Emergency Economic Stabilization Act of 2008 (P.L. 110-343). The Act extends the 30% investment tax credit for residential solar Photovoltaic or geothermal heat pump installation for eight years through December 31, 2016 and removed the cap on qualified solar photovoltaic projects and geothermal projects (from the previous $2,000). This allows taxpayers to use the credit to offset dollar for dollar their federal tax liability, and to carry unused credits forward to the next succeeding taxable year. Essentially Uncle Sam was now willing to pay 30% of the cost of my next energy savings project. I couldn’t believe it.
According to the DOE heating and cooling account for 56% of the energy use in a typical U.S. home, making it the largest energy expense for most homes. So that is where I looked for my next project. A wide variety of technologies are available for heating and cooling your home, and they achieve a wide range of efficiencies in converting their energy sources into useful heat or cool air for your home. Heat pump systems provide both heating and cooling and offer the benefit of delivering more useful energy than they consume. Unfortunately, on very hot days or very cold days they do not do as effective a job as an air conditioner and a furnace. For climates with moderate heating and cooling needs, heat pumps offer an energy-efficient alternative to furnaces and air conditioners.

Higher energy efficiencies are achieved with geothermal (ground-source or water-source) heat pumps, which transfer heat between your house and the ground or a nearby water source. Although they cost more to install, geothermal heat pumps have low operating costs because they take advantage of relatively constant ground or water temperatures. However, the installation is expensive because of the need to bury coils to deliver constant temperature fluid or install a groundwater pump and injection well to supply constant temperature water to the system. Ground-source or water-source heat pumps can be used in more extreme climatic conditions than air-source heat pumps, and are more effective at cooling and heating at the extremes.
According to the heating and cooling experts and the manufacturers of the various equipment that I have, my heating and cooling system, which is a split system with a gas furnace and air conditioner for the lower level and an air heat exchanger for the upper level, should last another 7-12 years. The most sustainable approach would be to use the current system for its entire expected life despite the fact that I could probably reduce my energy consumption somewhat by changing from my current equipment to two geothermal (ground source) heat exchangers. Though geothermal heat exchangers are more expensive to purchase and install than a traditional furnace and air conditioner, they are far more efficient, reportedly consuming 25-30% less energy to operate. The most reasonable thing to do was to wait and continue using my current system even with availability of the tax credit. Thought for the next several years I will continue to keep an eye on my equipment condition.

In October 2009 Virginia announced that a portion of the stimulus dollars for the state would be allotted to its Residential and Commercial Solar and Wind Incentive Program to provide rebates to partially reimburse the costs of renewable energy systems. For residential users on the first 10 kilowatts, the rebates will be $2.00 per watt for Photovoltaic Solar systems, $1.50 per watt for small wind turbines and $1.00 per watt for solar thermal units (solar hot water heaters). The rebate is less than you might think because system capacity is defined as the installed system’s predicted peak alternating current (AC) output which is around 75%-80% of the DC rating. Combining this incentive with the federal tax credit of 30% and the sale of the renewable energy credits, REC’s, which can be sold to utilities needing RECs and suddenly, there is a positive return on the investment. It was still a big decision because even with rebates and tax credits we have to come up with the cash to pay for the system and while current prices quoted for RECs are $220-$300 per kilowatt/year and are sold in 4 or 5 year contacts there is no guarantee that the REC’s will have any value in the future.

One of the selection criteria for my home was the large southern roof span, perfect for solar panels. I was able to reserve funds from the Virginia Renewable Energy Rebate Program for a 6 kilowatt solar photovoltaic system before all the money was gone and we put the deposit down for an American made solar photovoltaic system installed by a local company. We will be installing a 6 kilowatt system that we estimate will save us approximately $1,300 per year on our electric bill. That is about twice the savings we achieved by insulating the house; however, the cost (before rebates and incentives) is more than ten times the cost of the insulation project. Even after all the rebates and incentives (assuming I successfully navigate these) this energy savings was many more times more expensive than the insulation project.

Geothermal Heat Pumps

The most effective type of heat pump is the geothermal heat pump, GHP. It doesn't create heat by burning fuel, like a furnace does. Instead, in winter it collects the Earth's natural heat through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. As you may have experienced in a cave, the temperature six feet beneath ground surface is cooler in summer and warmer in winter than the ambient temperature. Using this temperature as its source the GHP can operate within its most efficient range. In winter, fluid circulates through the loop and carries the heat to the house. There, an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms. In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. The system cools your home in the same way that a refrigerator keeps your food cool by drawing heat from the interior, not by blowing in cold air.

The geothermal loop that is buried underground is typically made of high-density polyethylene, a tough plastic that is extraordinarily durable but which allows heat to pass through efficiently. The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system. Earliest systems were open loop, but those could impact the groundwater supply and are not used as much today. There are two types of closed loops used to provide constant temperature to the GHP. Horizontal ground loops are usually the most cost effective when trenches are easy to dig and the size of the yard is adequate. Workers use trenchers or backhoes to dig the trenches six feet below the ground in which they lay a series of parallel plastic pipes. They then backfill the trench. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 400 to 600 feet long for each ton of heating and cooling.

The vertical loop is used where there is little yard space, when surface rocks make digging impractical, or when you want to disrupt the landscape as little as possible. Vertical holes are typically 150 to 450 feet deep and contain a single loop of pipe with a U-bend at the bottom. Each vertical pipe is then connected to a horizontal underground pipe that carries fluid in a closed system to and from the indoor exchange unit. Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the Earth's temperature is more stable farther below the surface.

Geothermal heat pumps (GHPs), more accurately called ground-source heat pumps, have been proven capable of producing large reductions in energy use and peak demand in buildings. Although the U.S. was once the world leader in GHP technology and market development, European markets now absorb 2 to 3 times the number of GHP units annually as do the U.S. domestic markets. In 2007 the Intergovernmental Panel on Climate Change identified the building sector as having the highest green house gas emissions, but also the best potential for dramatic emissions reductions. In their report GHPs were specifically identified as a solution that is economically feasible under certain circumstances‖ in continental and cold climates. Their report cited cases where total electricity use decreased by one third and heating energy use by 50 to 60 percent.

Tax credits for home and business owners investing in GHP systems were enacted in October 2008 through 2016 and increased in the stimulus plan of 2009. Hopefully these tax credits will help GHP achieve wider market acceptance despite its large upfront capital costs. The largest hurdle to the widespread adoption of GHP technology seems to be the capital cost for initial installation. The outside portion of the GHP system can be half or more of the overall GHP system cost (and equal to the total cost for a traditional furnace and air conditioner). The technology while economically viable, is little known or understood and has suffered from the high upfront and installation costs. If the costs of the exterior coils were excluded, GHP systems have about the same price as competitive alternatives. In addition, due to the lack of demand, there are few design and installation firms in the market.

Buildings, both residential and commercial, account for about 40 percent of primary U.S. energy consumption, 72 percent of U.S. electricity consumption, 55 percent of U.S. natural gas consumption, and significant heating oil and propane consumption in the Northeast. While industrial use of electricity has been flat for about 15 years, electrical use to power commercial and residential building has grown by more than 50 percent since 1985. U.S. resources and investment have been deployed to build the infrastructure required to generate, transmit, and distribute electricity to serve that growth. Reducing the peak electricity demands for air conditioning and heating could alleviate peak demand on the electrical grid. Buildings present one of the best opportunities to economically reduce energy consumption and limit green house gas emissions. A recent study by McKinsey & Company study performed for the Department of Energy found that reducing the consumption of energy in buildings is the least costly way to achieve large reductions in carbon emissions.

A study by the U.S. Environmental Protection Agency (EPA) comparing the major HVAC options for residential applications determined that GHPs were the most energy efficient and environmentally benign option. Yet only about 60,000 units are installed each year in the combined new built and retrofit market. This languishing of the market is attributed to several federal policy lapses. A program at the DOD ran for several years in the late 1990’s intended to increase use of GHPs in federal buildings. This program’s authority was allowed to lapse. Although DOD took the initiative to restore the program 14 months later by then much of the GHP project pipeline had diffused away. A second policy mistake damaging to federal agency use of GHPs occurred in 2005 when the Energy Policy Act defined renewable energy that counted toward agency renewable goals as power generation only, excluding thermal forms of renewable energy such as GHPs. No lobbyists were paid to identify this oversight. Federal utilization of GHPs might have created the critical mass for the market; instead it was once more forgotten.

The basics of GHP technology have changed very little over the decades but awareness, understanding, and acceptance of the systems is limited. The systems are truly misnamed, GHPs are often confused with geothermal power production, in which the extreme heat of subsurface geological processes is used to produce steam, and ultimately to generate electricity. GHPs are also sometimes confused with the direct use of geothermal heat in which greenhouses, aquaculture ponds, and other agricultural facilities are heated using lower-temperature sources such as hot springs. Ground source heat pumps can be used economically anywhere and utilize the earth stored solar energy to function. There are at least 16 manufacturers of GHPs in the United States that participate in the residential and commercial markets. The GHP market began to develop in the late 1970s, and has had its ups and downs due to the cyclic nature of the buildings industry and volatility in government and utility support and the prices of competing forms of energy. The current tax incentives and awareness of US energy consumption may serve as an opportunity for the GHP market to achieve critical mass.