Technology
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Technology
Ground source heat pump systems draw energy from the relatively moderate and consistent temperatures found just below the Earth’s surface. Nearly 47 percent of the sun’s energy that reaches the Earth is absorbed into the ground. This allows the Earth to maintain warmer ground temperatures than the air temperatures in the winter, and cooler ground temperatures in the summer, offering a perfect environment from which to draw energy.
There are other heat pumps that draw their energy from the air (air to air heat pumps). The performance of an air source heat pump declines as the temperature of the air decreases; below –7°C, the heat pump performance is equal to the input energy and a backup system must be used for heating. This limitation doesn’t exist for systems extracting heat from the ground, with the groundwater or water body at least 8 feet in depth.
Although the heat transfer process may seem complicated, the heat pump system is similar to a refrigerator. It’s simply a process of moving heat energy from one point to another using heat exchangers. During heating, the heat pump extracts heat from the earth loop (or well water) and, using a refrigeration process, the heat is intensified and transferred to the air, which is then delivered into the home. During cooling, the process is reversed. Hot air is removed from the home by the cool refrigerant, and then transferred to the ground.
Transferring the low temperature heat from the earth is made easy with the use of refrigerants. As technology improves, newer refrigerants are being designed that provide wider temperature ranges and are more environmentally friendly. Because the ground loop is designed to operate below the freezing point, the closed ground loop must have an anti-freeze solution. A few of them are methanol, ethanol and propylene glycol, each mixed with water to achieve a freeze point of –10°C.
Heat Pump Unit
The heat pump unit
The heat pump unit can heat or cool by forced air. There are also units that heat water for hydronic floor heat or fan coils. The forced air sizing can range from 7,600 BTU/HR to 220,000 BTU/HR (GLHP) and hydronic units range from 5,000 BTU/HR to 462,000 BTU/HR (GLHP). The efficiency of 500 percent or 5 - C.O.P (Coefficient of Performance) can be reached in some installations.
Loop Types
The ground loop is a type of heat exchanger that's purpose is to extract heat from the ground, the groundwater or water body at least 8 feet in depth, is referred to as a loop and can be installed in various ways:
- Vertical loop
- Horizontal loop
- Pond loop
- Open loop
Vertical Loop
Vertical loops are used when space is limited and, or soil disturbance is a problem. Holes are bored using a drill rig. A pair of pipes with special u-bend fittings are inserted into the holes. The holes are sealed with a grout. About 200 feet of bore/per 10,000 BTU/HR (heating), (heavy saturated soil conditions).
Horizontal loop
Used where adequate land is available and soil conditions are favorable. Horizontal loops involve trenches that are dug using a backhoe, track hoe or chain trencher. Horizontal directional boring is also an option. Polyethylene pipes are inserted and the trenches are backfilled. Some types of horizontal boring have to be grouted to ensure pipe contact with the surrounding ground. About 250 ft of trench/per 10,000 BTU/HR (heating), (7 ft depth, 2 pipe configuration with 2 foot spacing and heavy damp soil).
Pond Loop
A pond loop consists of a series of closed loops coiled or slinky style, to be sunk to the bottom of an adequately sized body of water. The pond should be at least 8 feet in depth. About 300 ft of pipe/10,000 BTU/HR (slinky formation).
Open Loop
An open loop is used where there is an abundant supply of quality well water. Two wells are required. One for the supply of the water and one for the return of the water. The spacing between wells should be approximately 100 feet. 2 US/GPM @ 44 deg/F per/10,000 BTU/HR. The well supply should be more than required in case the production of the well drops over time.
In ground water (open loop) situations where scaling could be heavy or where biological growth such as iron bacteria will be present, a closed loop system is recommended, The heat exchanger coils in groundwater systems may, over a period of time, lose heat exchange capabilities due to a buildup of mineral deposits inside. The ground water temperature should also be a minimum temperature of 6 degrees Celsius. With well water there is a tendency for regular maintinance not only with the heat pump but also with the well pump. It is also important to size the well pump for the system; too much water can be as problematic as not enough water. With the development of variable speed pump, many of the pumping problems like water volume and power consumption have been minimized.
Soil & Flow Characterisitcs
Soil Characteristics
It is important to note that the ground loop (source) must be matched for the equipment selected. The soil conditions determine the system's heat transfer capability. Heavy soil performs better than lighter soils. Moisture content is also a factor. Saturated soils perform better than dry or moist soils. During the cooling season, heat rejected to the damp earth will dry the soil and reduce its heat transfer ability (residential cooling loads in Canada have not been a concern). In the winter, freezing soil-moisture around the piping releases a large number of BTU's due to the release of latent heat from the moisture in the soil changing from 0° C water to 0° C ice. Freezing allows the extraction of energy from the soil without the normal drop in soil temperature in the pipe's vicinity. These extra BTU's keeps the loop at or around 0 degree Celsius. Which is very good.
Soils can be classified as the follows:
· Dry, light soil.
· Damp, light soil.
· Dry, heavy soil.
· Damp, heavy soil.
· Wet soil (rock has similar thermal properties).
Flow characteristics
Also important in the design of the loop is flow characteristics. The flow of liquid within a pipe can be considered either Laminar, with the fluid molecules traveling in a straight line, or turbulent, where mixing occurs. The best heat transfer occurs when the flow is turbulent.
Air in the pipes
Because the ground loop is connected under ground or water, the design must also include the ability to flush the air out of the pipes while filling.
The heating/cooling sustainability
The heating/cooling sustainability, of a properly sized ground loop can last indefinably due to the fact that 47% of the suns energy that reaches the Earth is absorbed. This combined with air conditioning (cooling) that injects heat back into the loop will be close to the same earth temperature as first installed or can even be higher if a large air conditioning (cooling) load is present.
BOS
Monitoring of geothermal systems is being done throughout the country and databases are being developed at this time.
The data that is being logged is ground temperatures, loop temperatures; power used (KW input) and average COP.
Life Expectancy & Challenges
Life Expectancy
Life expectancy for various systems according to ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers http://www.ashrae.org/) is as follows;
Average life expectancy by equipment type:
- Water Source Heat Pump 19 Years
- Air-Air HP/Air Conditioner 15 Years
- Furnace (gas or oil) 18 Years
One thing to note is that these numbers are dated and as the need for technology grows the Manufacturing practices are becoming more advanced and we are seeing longer equipment life. The market is also demanding the higher end equipment.
The design and manufacturing technology is in the mature stage.
The challenges with the geothermal technology is in converting the practicing of old technologies (oil, gas) and adopting the new technologies as the standard. Also being that the geothermal industry is relatively new (25 years) there is a shortage of qualified engineers and trades people to support mass deployment at this time.
Modified: 02-19-2008
