Ground-source heat pumps systems and applications
Introduction
Climate change is a real threat to our future, and a major cause is the use of fossil fuels to power homes and businesses. Renewable energy, combined with energy efficiency, offers a viable and potent solution to countering the effects of global warming. By installing any one of the renewable energy technologies, one will be making a major personal contribution to the well being of future generations and could also benefit from lower fuel bills.
Our natural sense of heat is based rather more on instinct than on science. Humans are warm-blooded and judge “heat” by comparing it to touch. Since our body temperatures need to be maintained within a few degrees centigrade, our natural senses have evolved to make extremes of temperature uncomfortable. To us, a hot summer's day feels many times “hotter” than the freezing mid-winter. But in reality the Earth's surface does not vary in “heat energy” as much as we might imagine. Scientifically speaking, there is only 11% less energy in cold river water at 5 °C (40 °F) compared to hot bath water at 40 °C (105 °F) [1].
Ground-source heat pumps (GSHPs) provide a new and clean way of heating buildings in the world. They make use of renewable energy stored in the ground, providing one of the most energy-efficient ways of heating buildings. They are suitable for a wide variety of building types and are particularly appropriate for low environmental impact projects. They do not require hot rocks (geothermal energy) and can be installed in most of the world, using a borehole or shallow trenches or, less commonly, by extracting heat from a pond or lake. Heat collecting pipes in a closed loop, containing water (with a little antifreeze) are used to extract this stored energy, which can then be used to provide space heating and domestic hot water. In some applications, the pump can be reversed in summer to provide an element of cooling.
The only energy used by GSHP systems is electricity to power the pumps. Typically, a GSHP will deliver three or four times as much thermal energy (heat) as is used in electrical energy to drive the system. For a particularly environmental solution, green electricity can be purchased. GSHP systems have been widely used in other parts of the world, including North America and Europe, for many years. Typically they cost more to install than conventional systems; however, they have very low maintenance costs and can be expected to provide reliable and environmentally friendly heating for in excess of 20 years. GSHPs work best with heating systems, which are optimised to run at a lower water temperature than is commonly used in the UK boiler and radiator systems. As such, they make an ideal partner for underfloor heating systems.
Heat pumps offer the most energy-efficient way to provide heating and cooling in many applications, as they can use renewable heat sources in our surroundings. Even at temperatures we consider to be cold, air, ground and water contain useful heat that is continuously replenished by the sun. By applying a little more energy, a heat pump can raise the temperature of this heat energy to the level needed. Similarly, heat pumps can also use waste heat sources such as from industrial processes, cooling equipment or ventilation air extracted from buildings. A typical electrical heat pump will just need 100 kWh of power to turn 200 kWh of freely available environmental or waste heat into 300 kWh of useful heat. Because heat pumps consume less primary energy than conventional heating systems, they are an important technology for reducing emissions of gases that harm the environment, such as carbon dioxide (, sulphur dioxide ( and nitrogen oxides (. However, the overall environmental impact of electric heat pumps depends very much on how the electricity is produced. Heat pumps driven by electricity from, for instance, hydropower or renewable energy reduce emissions more significantly than if the electricity is generated by coal, oil or gas-fired power plants.
Geothermal heating can be more efficient than electric resistance heating. These systems are also typically more efficient than gas or oil-fired heating systems. They are more energy efficient than air-source heat pumps because they draw heat from, or release heat to, the earth, which has moderate temperatures year round, rather than to the air (which is generally colder in winter and warmer in summer than the earth, resulting in less effective heat transfer). It is argued that heat pumps are highly energy efficient, and therefore environmentally benign.
Section snippets
Earth-energy systems
Renewable forms of energy such as solar, wind, biomass, hydro, and earth energy produce low or no greenhouse gas (GHG) emissions. Geothermal heating and cooling systems (also called earth-energy systems (EESs), GSHPs or GeoExchange systems) are heat pumps that collect and transfer heat from the earth through a series of fluid-filled, buried pipes running to a building, where the heat is then concentrated for inside use. GSHPs do not create heat through combustion—they simply move heat from one
Heat pump efficiency
A heat pump can save as much as 30%–40% of the electricity used for heating. If you use electricity to heat homes, consider installing an energy-efficient heat pump system. Heat pumps are the most efficient form of electric heating in mild and moderate climates, providing two to three times more heating than the equivalent amount of energy they consume in electricity. Air source heat pumps are recommended for mild and moderate climate regions, where the winter temperatures usually remain above
Heat pumps
One of the most energy-efficient methods of domestic heating is to use heat pumps. Heat pumps use electrical energy to reverse the natural flow of environmental heat from cold to hot. A typical heat pump requires only 100 kWh of electrical power to turn 200 kWh of freely available environmental heat into 300 kWh of useful heat. In every case, the useful heat output will be greater than the energy required to operate the pump itself. Heat pumps also have a relatively low carbon dioxide output, less
Description of ground-source types for heat pump
The ground system links the heat pump to the underground and allows for extraction of heat from the ground or injection of heat into the ground. These systems can be classified generally as open or closed systems, with a third category for those not truly belonging to one or the other.
Open systems: Groundwater is used as a heat carrier, and is brought directly to the heat pump. Between rock/soil, ground water, and the heat pump evaporator is no barrier, hence this type is called “open”.
Closed
Heat pump capabilities
Factors that can effect the life-cycle efficiency of a heat pump:
Local method of electricity generation.
Climate.
Type of heat pump (ground vs. air source).
Refrigerant used.
Thermostat controls.
Size of the heat pump.
Quality of work during installation.
Energy efficiency of home's layout, insulation and ducts.
Safeguards against damage to coils from construction, gardening equipment, etc.
Heat pump types and arrangements
The two major types of heat pumps are the water-to-air heat pump and the water-to-water heat pump. Water-to-air units deliver either hot air or cold air to the space using water or glycol solution as the transfer medium and the ground as the heat sink or heat source. The major components include casing, compressor, expansion valve, reversing valve, refrigerant-to-water heat exchanger, supply fan, and connections for the source water “in” source water “out” condensate drain, controls, and other
Heat pump efficiency
Three types of heat pumps are typically available for residences: (1) air-to-air, (2) water source, and (3) ground source. Heat pumps collect heat from the air, water, or ground outside homes and concentrate it for use inside. Heat pumps operate in reverse to cool homes by collecting the heat inside the house and effectively pumping it outside.
Heat pumps have both heating and cooling ratings—both in terms of capacity and efficiency. Capacity ratings are generally in Btu per hour or tons (one
Environmental benefits
Geothermal/GSHPs work with the environment to provide clean, efficient, and energy saving heating and cooling year round. GSHPs use less energy than alternative heating and cooling systems, helping to conserve our natural resources. GSHPs are housed entirely within the building and underground. They are quiet, pollution free and do not detract from the surrounding landscape (Fig. 14).
Governments and energy planners prefer EE technology because it is an environmentally benign technology, with no
Ground temperatures
At depths below four feet, ground temperature stays a constant 50 to 55 °F year-round. During the winter, a geothermal system absorbs this extra heat from the earth and transfers it into homes. During the summer, the system takes heat from indoors and moves it back underground. Annual air temperature, moisture content, soil type and vegetative cover (i.e., trees and plants) all have an effect on underground soil temperature (Fig. 15, Fig. 16, Fig. 17. As you might expect, the earth's temperature
Energy efficiency
In general, energy efficiency is calculated as the “useful work” or “energy delivered” divided by the amount of energy supplied to do that work. With heat pumps, energy efficiency is measured in two different ways.
Heating efficiency is expressed as a COP. The higher the COP, the more efficient the system. For example, a residential-sized geothermal system might have a COP of 3.4 or higher, meaning for every one unit of energy used to power the system, more than three units are put back into the
The future
Energy prices have increased significantly since the second half of 1999. Plans already drafted at the end of the 1990s, but partly delayed, by Indonesia, Philippines and Mexico aim at an additional 2000 Mwe before 2010. In the direct use sector, China has the most ambitious target: substitution of 13 million tons of polluting coal by geothermal energy.
The short to medium term future of geothermal energy is encouraging, providing some hurdles that have recently slowed its growth are overcome.
Conclusions
A ground-source heat pump (GSHP) system utilises the earth, ground water, or surface water as a heat source/sink for providing heating and cooling. The GSHP is generally recognised to be one of the most outstanding technologies of heating and cooling in both residential and commercial buildings, because it provides high coefficient of performance (COP), up to 3–4 for an indirect heating system and 3.5–5 for a direct heating system. The main benefit of using GSHPs is that the temperature of the
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