Open Loop Geothermal Heating and Cooling Systems
Open loop geothermal heating and cooling systems use either a surface or underground water source as a heat source in the winter and a heat sink in the summer.
Conventional air source heat pumps and air conditioners must use air as the heat source or sink. The outside air temperature is high in the summer and low in the winter, which is counter-productive to cooling and heating respectively.
This is why geothermal systems operate so much more efficiently than air source systems. Depending on location the temperature of ground water is typically 45 to 55 degrees Fahrenheit and stays stable throughout the year making it a great heat sink in the summer and a good heat source in the winter. Additionally, water has a heat capacity 4 times that of air, so not as much volume is needed to provide the heating or cooling needed as with an air source system.
Open loop systems tend to be slightly more economical than closed loop geothermal systems because they typically require less drilling, excavating, and pipe work, especially if an existing well is used for the open loop. Open loop systems also operate a little more efficiently than closed loop systems. Open loop systems do require periodic cleaning and maintenance that costs more than the maintenance on a closed loop system, but usually open loops are more economical overall even with the maintenance factored in.
Sources of Water for Open Loop Systems
Typical water sources include wells, ponds, lakes, rivers, etc. In many areas use of surface water is prohibited or requires an extensive and costly permitting process, so wells tend to be the most common water source.
As a rule of thumb most systems require around 2.0 gpm per ton (12,000 Btu/hr) of heating or cooling required, whichever is higher. This requirement can vary from 1.5 to 3.0 gpm per ton depending on water temperature, heat pump efficiency, and design conditions. It is important that there is enough water available annually at the site to achieve the required flow rates and to take into account any anticipated changes to either the system size or the water supply.
Some jurisdictions have a limit on ground water usage that may be prohibitive to installing an open loop geothermal system, but in cases where the limit is only marginally exceeded a pressure tank can be installed to boost flow rates during times of peak flow. Many heat pumps have multiple stages or are modulating, so the peak flow rate is only required when the system is operating at 100%, which only happens occasionally in a staged system.
Water quality is also an important factor when evaluating a source for an open loop system. Water that is very alkaline or full of minerals can cause heavy scaling on the heat exchanger, leading to increasingly poor performance over time. Some scaling is expected for most systems and can be removed every 1 to 3 years using an acid wash. This periodic maintenance has to be performed by a qualified technician because specialized pumps and strong acid are involved. If the water PH is less than 7.5 and contains less than 100 ppm calcium descaling can be performed less often or not at all.
In addition to a water source there also needs to be a place to discharge the water. Typically this is accomplished by drilling an additional well for discharge only. In some instances the water can be discharged into a lake, pond, or river or even used as irrigation water. The possibilities will depend on the water regulations for the area.
When dealing with local or regional water regulatory agencies it can be helpful to remind them that open loop geothermal systems are non-consumptive, meaning they don’t deplete the water supply used if the supply and discharge come from the same source. They do, however, change the temperature of the water which could be of concern for fragile ecosystems that may inhabit the water source. This is not a concern for underground water sources, but may come into play for surface water sources.
Geothermal heating and cooling systems tend to be more reliable than conventional heat pumps and air conditioners. This is because the heat pump itself is located indoors and is exposed only to water of a known and consistent quality. Traditional systems are always at least partially exposed to the outside elements, so compressors go out more regularly and the heat exchanger coils are subject to harmful corrosion. Most equipment carries at least a 10 year warranty. In an open loop system the water loop is just as reliable as your domestic water well. The only things that can go wrong are the pump breaking down (easily replaceable) or the well drying up.
Equipment Manufacturers and Performance Rating
Open loop geothermal systems use the same heat pumps as closed loop geothermal systems and can be adapted to both residential and commercial applications. A selection of heat pump manufacturers offering both residential and commercial products is shown below:
• Florida Heat Pump
• Water Furnace
Geothermal heat pumps are subject to standardized testing to determine heating and cooling performance. For cooling heat pumps receive an EER (energy efficiency ratio) in the units BTUs Out/Watts In. For heating the COP (coefficient of performance) is specified and is a unitless value (Watts Out/Watts In). To receive an EnergyStar rating (required for federal incentives) a heat pump must meet requirements set by the EPA and DOE. To view the requirements currently in place and new requirements for 2011 click here.
Costs and Financial Incentives
Although a geothermal heating system costs more initially than a traditional system there are many incentives available to subsidize the initial cost of installing a geothermal heating and cooling system. The federal government offers a 30% tax credit with no maximum for residential heat pumps installed after 2008. Just make sure the heat pump you decide on meets the minimum qualifications for the EnergyStar program.
There are also state and/or local incentive programs that may be available in your area. To find out which rebates or incentives you qualify for visit: http://www.dsireusa.org/ and click on your state.
With the combination of incentives and lower monthly heating and cooling costs most systems pay for themselves in 3-7 years.
Taking the Next Step
Geothermal systems can be retrofitted to existing buildings or integrated into new construction. Each system is unique in many ways and requires attention to design details that can only be provided by an experienced designer.
A geothermal system can use an open well water loop, an open surface water loop, a closed vertical, horizontal or slinky loop, or a closed loop submerged in a pond. Inside the house the system can heat and cool using forced air, radiant heating, or a combination of the two. It can also provide some or all of the domestic water heating required for a building. Each building and site have their own requirements and considerations that must be taken into account when designing a geothermal system.
If you’re ready to see what a geothermal heating system could do for your home or business visit Energy-1 for experienced advice from initial concept development and design through installation and post-installation sevices.