Residential - Geothermal AC and Heat Pump
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Geothermal heat pump, ground source heat pump (GSHP), or ground heat pump is a central heating and/or cooling system that pump heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems, and may be combined with solar heating to form a geo-solar system with even greater efficiency. Geothermal heat pumps are also known by a variety of other names, including geoexchange, earth-coupled; earth energy or water-source heat pumps. The engineering and scientific communities prefer the terms "geo-exchange" or "ground source heat pumps" to avoid confusion with traditional geothermal power, which uses a high temperature heat source to generate electricity. Ground source heat pumps harvest a combination of geothermal energy (from the earth's core) and solar energy (heat absorbed at the earth's surface) when heating, but work against these heat sources when used for air conditioning.
Depending on latitude, the upper 3 meters (9.8 ft) of Earth's surface maintains a nearly constant temperature between 10 and 16 °C (50 and 60 °F). Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat from there. Heat pumps can transfer heat from a cool space to a warm space, against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cool one. The core of the heat pump is a loop of refrigerant pumped through a vapor-compression refrigeration cycle that moves heat. Heat pumps are always more efficient at heating than pure electric heaters, even when extracting heat from cold winter air. But unlike an air-source heat pump, which transfers heat to or from the outside air, a ground source heat pump exchanges heat with the ground. This is much more energy-efficient because underground temperatures are more stable than air temperatures through the year. Seasonal variations drop off with depth and disappear below seven meters due to thermal inertia. Like a cave; the shallow ground temperature is warmer than the air above during the winter and cooler than the air in the summer. A ground source heat pump extracts ground heat in the winter (for heating) and transfers heat back into the ground in the summer (for cooling). Some systems are designed to operate in one mode only, heating or cooling, depending on climate.
The geothermal pump systems reach fairly high Coefficient of performance (CoP), 3-6, on the coldest of winter nights, compared to 1.75-2.5 for air-source heat pumps on cool days. Ground source heat pumps (GSHPs) are among the most energy efficient technologies for providing HVAC and water heating. Actual CoP of a geothermal system which includes the power required to circulate the fluid through the underground tubes can be lower than 2.5. The setup costs are higher than for conventional systems, but the difference is usually returned in energy savings in 3 to 10 years. System life is estimated at 25 years for inside components and 50+ years for the ground loop. As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity, with an annual growth rate of 10%
How does a Geothermal Heat Pump get the heat from the ground? The heat pump utilizes a loop of refrigerant sucked through a vapor compressed refrigeration cycle moving heat either in or out. The source of heat is actually much more stable and continuing then air based heat which tends to be less controlled. The other benefit is that Geothermal Heat pumps can also be used to pump the heat out of your house for instant cooling during hot weather. Geothermal heat pumps are also more economically friendly and can reduce the effects of pollution, electricity, and global warming.
What are the different kinds of Geothermal Heat Pumps? So far some of the different types of geothermal heat pumps in existence are; Ground Exchange, Direct Exchange, closed loop, vertical, horizontal, pond, open loop, and finally standing column well.
Ground Exchange - This type of heat pump uses the grounds current heat and exchanges it with the temperatures in your home. If you plan to have a ground sourced heat pump then you will need a heat exchanger which sits against either ground or ground water to make the pump function properly.
Direct Exchange - Instead of using a circulating refrigerant the direct exchange geothermal heat pump works by using a SINGLE loop refrigerant in direct thermal contact with the ground.
Closed loop - The typical system uses two loops on the ground side and one loop located in the appliance cabinet which exchanges temperatures with the loop which is pulling the temperatures from the ground. Closed loop systems have much longer and larger pipes implemented into the ground for better accuracy and also have an extra loop located in between the refrigerant loop and the water loop, pumping in both loops.
Closed loops come in two different types themselves one being the vertical closed loop and the other is the horizontal closed loop. The difference between the two being the vertical closed pump has pipes which run vertical through the ground while the horizontal closed loops have pipes running horizontally through the ground.
Pond - Pond based heat pumps are not commonly used due to the need to be close to an external source of water. Pond based heat pumps are more preferred and recommended for people who want to use a pump but have bad quality water, or a low heat source currently containable by a standard heat pump. The Pond based heat pump gets its name from the fact that the loop dragging in the heat is located underneath a large body of water, for example a pond.
Open Loop - An open loop is a heat pump which draws in water from an external source stores it in the main refrigerant where heat is extracted from the water source and the water is then returned to the external source.
Standing column well - A standing column well heat pump is much like an open loop heat pump except it is designed to pull in water from deep within a well and exchange it for heat in the main refrigerant where it is then returned back into the well while traveling down it exchanges heat with the bedrock.
How does a geothermal heat pump work in large buildings?
A geothermal heat pump works in all different ways each make varying depending on its best function for what it is replacing. For example a heat pump moves anywhere in between three and five times faster than the heat or electricity in which it consumes it is actually outputting more energy than it is inputting this causes the efficiency of thermals to exceed anywhere between 100 and 200 percent. While your average electric device will typically never exceed 100%. This proving that a geothermal heat pump will always exceed the performance of the electric device it is equivalently designed against for a specific building or structures size.
What is the environmental impact of geothermal heat pumps?
The impact is to be frank very positive, producing less greenhouse gases, and built to be biodegradable and non toxic in the external sources. But the negative side is that the refrigerant system uses a type of refrigerant which is actually a cause of depleting the ozone layer. This refrigerant is called chloro diflouromethane, it is harmless when used properly but leaks can cause the ozone layer to deplete. But don't worry this product has been favored out and is being replaced with a more environmentally safe product.
Without carrying out a full building heat loss analysis, and calculating its related energy consumption profile and the hot water requirements of the buildings occupants. You cannot accurately design a ground source heat pump system.
The main problem with many renewable energy installations is, that compared to normal energy systems such as gas/oil or electric, the installed costs are generally much higher, thus meaning the economies of scale are more limited. An oversized Heat pump will spend most of its time running under part load conditions, which can result in a shortening of the equipments lifespan and ultimately affect performance.
Under-sizing can result in a system that requires another heating system to be used, instead of the GSHP during periods of cold weather. This is known as an alternative bivalent system and is not very efficient. A top up system would be required to help the system meet its requirements. Whilst it is in fact fairly normal to have what is known as a parallel bivalent system, where two systems work together during periods of peak loads, the Heat pump will work at maximum output providing the base load of the heating, whilst the other system tops up the temperature levels. It is vitally important to know the buildings and its occupant's energy requirements so that the most energy efficient and therefore cost effective system is designed, as generally the use of non renewable supplementary heating should not exceed 5% of the annual energy requirement.
It's not only the sizing of the heat pump that needs to be considered when designing the system. Different ground conditions will have an effect on the performance of the ground coil or borehole system used to collect heat from the ground. The ground collects solar energy and almost all of the ground heating effect comes from the sun, even up to hundreds of meters below ground level. The type of ground ie, sand/gravel, rock, clay have different levels of heat extraction. Even the moisture levels of the ground will have an effect on the performance and therefore the design of the system.
Without a ground condition survey being carried out one could not even say whether or not a coil system is appropriate. If the ground conditions are such that there is let us say, 1m of topsoil over rock then the cost of the installation will vastly increase.