Open Loop Vs. Closed Loop Geothermal

Open Loop Vs. Closed Loop Geothermal
Geothermal systems are some of the most efficient residential and commercial heating and cooling systems available. They use the consistent temperature of the earth’s surface as a heat source in the winter and a heat sink in the summer. The consistent ground temperature provides a dependable constant which is high enough to provide heat for heating applications and low enough to absorb heat for cooling applications. There are many different ways to utilize geothermal energy for heating and cooling applications which fall into two different categories: open loop geothermal systems and closed loop geothermal systems.

Open Loop Geothermal

Open loop geothermal systems use groundwater, which acts as a refrigerant to transfer thermodynamic energy. Because water is an excellent thermal conductor and groundwater is naturally insulated and much closer in temperature to the surrounding ground, open loop geothermal systems are an excellent choice for efficiency. Open loop geothermal systems can be designed to use water from a well as a source and sink, a pond or lake as a source and sink, or a well as the source and a pond or lake as the sink. Every installation is different and various factors of each system determine cost, space needed, and efficiency of the system.

Closed Loop Geothermal

Closed loop geothermal systems are different from open loop geothermal systems in that they use a mix of antifreeze and water which cycles through pipes buried in the ground instead of using fresh groundwater to transfer heat. There are also different types of closed loop geothermal systems.
  • Horizontal - Horizontal closed loop geothermal systems are systems which use the piping laid horizontally in the ground. These systems can take up quite a bit of space as the loops are laid to run length-wise.
  • Slinky - A variation of the horizontal system is the slinky closed loop geothermal system. In a slinky loop system, the piping is laid horizontally, but first it is designed to look like a flattened and spread out slinky. This reduces the length of the trenches in which the pipes will be laid.
  • Vertical - Another type of closed loop geothermal system is the vertical closed loop geothermal system. Instead of running the pipes out horizontally, the pipes are run vertically between 100 and 400 feet deep in several wells and connected at the bottom by a U-bend. The boreholes are then filled with a grout so that the vertical pipes will get good thermal conductivity.

Which to Choose?

In most situations, the open loop geothermal systems are less costly and more efficient than closed loop geothermal systems due to the constant temperature of the ground water and the amazing conductivity of that water in comparison to the antifreeze in a closed loop geothermal system, which absorbs and releases heat through a polyethylene pipe. However, in many situations, open loop applications are not feasible either because there is no groundwater source available, the groundwater contains too much iron, or local code may prohibit open loop systems.
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River Bergquist
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Thank you! Very informative and helpful.
Porsche
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For a closed loop vertical system, aren't the feed and return pipes usually right next to each other? Doesn't this have a significant reduction in efficiency, where the return pipe cools the incoming water of the feed pipe? Also, for an open loop system, depending on the particular mineral content, can the water be filtered to protect the equipment?
Rebekah Muller
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The outgoing and incoming lines are only next to each other for the short distance required to enter the structure so there will be very little effect on heat transfer for the ground loop[ as a whole. Yes depending on water quality it may be possible to filter or treat the water to make it acceptable.
John Stoicu
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in a open system what kind of filters are used and how often do they need to changed them do you filter it at the pickup or in the house does the water have to be tested can the water change life of the heat pump and how deep do the feed lines need to be in Michigan my pond is 12 feet deep
Rebekah Muller
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With water filters for an open loop configuration and air filters, the recommended change will be what the manufacturer recommends. We recommend cleaning/exchanging an air filter once every 90 days if it is pleated, and once a year maximum for an open loop water filter. Is the water required to be tested? It is possible, but it is not required. It just has to be drinkable. Can the water affect the heat pump's lifespan? The only risk is using a corrosive copper heat exchange with an open loop configuration. Cupronickel, on the other hand, is a super slow corrosive alloy that takes more than fifteen years to degrade. It is 99.9%. In most of the country, including the north, a pond of at least 10 feet in depth is required to maintain a 50-60 degree year round temperature. The goal is to stay below the frost line. A pond with at least 10 feet in the middle is the rule of thumb. This is where the polyethylene piping will be rolled up and sunk. There are numerous YouTube videos that may be useful.
Keith
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Rebekah, not entirely true. In a vertical closed loop system, the incoming and outgoing pipes are beside each other all the way down the borehole... It is not a huge efficiency hit though, since the difference in temperature between the two pipes is less than 10 degrees F.
Matthew Orio
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If there are many loops/ bore holes and long offsets a thermal break between the supply and return is recommended. A very good option is pink or blue board insulation cut in strips. This is a commonly applied to open and closed loop systems. Matthew Orio Certified Geo Exchange Designer Cert #116 Water Energy Distributors , Inc 2 Starwood Drive Hampstead NH 03841
Matthew Orio
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Keith , Open verses closed loop . Open systems if following IGSHPA standards typically operate at 50 DegF Entering water temp in heating and 59 Deg F in cooling. A thermal break is recommended on this type of system here in New England. . On closed loop it is not a common . Where as the closed loops Systems are generally 20% less output using IGSHPA standards of 32 DegF loop solution ,at peak load demand in heating ,and 77 in cooling at peak load demand. A thermal break on the return lines is not going to have a huge impact on output. Matthew Orio GeoExchange Designer cert #116 Water Energy Distributors , Inc 2 Starwood Drive Hampstead NH 03841
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