The last 20 years has seen massive worldwide investment in heat pump technology. The countries shown in green on the map below contain either manufacturers or suppliers of heat pumps.
In some regions of the world, heat pumps already play an important role in energy systems. This is generally as a result of government incentives and advertising.
The heat pump market in the UK is drastically different to that of the world at large as can be seen in the 2 pie charts below. The UK market is dominated by the non-domestic sector whereas the global picture is the reverse.
About 96% of all heat pumps sold in the UK are for non-domestic buildings - over 60,000 units in 1996 alone. Sports centres, particularly those having swimming pools, are ideal candidates, where the heat pump can provide both heating and dehumidification. Retail outlets and office buildings where there is a need for simultaneous heating in one area and cooling in another can benefit from substantial energy cost savings.
There are relatively few heat pumps currently installed in industry world wide in comparison to those installed in the residential or commercial markets. However, as environmental regulations become stricter, industrial heat pumps can become an important technology to reduce emissions, improve efficiency, and limit the use of ground water for cooling.
Industrial heat pumps are mainly used for:
1) space heating
2) heating and cooling of process streams
3) water heating for washing, sanitation and cleaning
4) steam production
In Japan, Sweden and the Netherlands, multi-MW heat transformers operating on the absorption cycle are used for waste heat recovery in petrochemical and steel works.
Heat pumps are a feature of many homes in, for example, Switzerland, Norway and the Netherlands, but not many systems have been installed in the UK.
The UK market is strongly influenced by first cost for heating systems, and gas heating is fairly cheap, widely available and fairly clean. Apart from some purpose-built demonstration houses, domestic heat pumps in the UK tend to be confined to rural areas without gas supplies. In these areas oil is the main alternative which is currently very economical.
Heat pumps for heating and cooling buildings can be divided into four main categories depending on their operational function:
1) Heating-only heat pumps, providing space heating and/or water heating.
2) Heating and cooling heat pumps, providing both space heating and cooling. The most common type is the reversible air-to-air heat pump, which either operates in heating or cooling mode
3) Integrated heat pump systems, providing space heating, cooling, water heating and sometimes exhaust air heat recovery.
4) Heat pump water heaters, fully dedicated to water heating. Heat and cold distribution systems.
The largest domestic market for heat pumps is in regions with a warmer climate such as Florida, USA. These regions make use of the dual functionality possible with heat pumps, providing both heating and/or air conditioning.
Air is the most common distribution medium in the mature heat pump markets of Japan and the United States. The air is either passed directly into a room by the space-conditioning unit, or distributed through a forced-air ducted system. Water distribution systems (hydronic systems) are predominantly used in Europe, Canada and the north eastern part of the United States. Conventional radiator systems require high distribution temperatures, typically 60-90°C. Today's low temperature radiators and convectors are designed for a maximum operating temperature of 45-55°C, while 30-45°C is typical for floor heating systems Because a heat pump operates most effectively when the temperature difference between the heat source and heat sink (distribution system) is small, the heat distribution temperature for space heating heat pumps should be kept as low as possible during the heating season.
As the term "pump" implies, a heat pump
moves heat from one place to another. It reverses the natural flow of heat from
a warmer to a cooler place. Heat pumps use the refrigeration cycle to
accomplish this. The advantage of pumping heat is that it takes less electrical
energy than it does to convert electrical energy into heat (as in electric
furnaces, baseboards and radiant heaters). In fact-in mild winter temperatures
you can get three times as much heat out of each watt of electricity as you get
from an electric furnace. Energy cost savings differ for each installation.
Properly sized and installed heat pumps can reduce heating costs by 30 to 50
percent compared to electric furnaces. Heat pumps are named for their source of
heat. Air-source heat pumps get heat from the outdoor air. There also are
water-source heat pumps, which get heat from water, usually well water.
Ground-source heat pumps get their heat from ground below the frost line. Most
heat pumps have two main parts; the outdoor unit and the indoor unit. The
outdoor unit includes the outdoor heat exchanger, the compressor and a fan. This
is where the heat from the air outside is picked up during the heating season,
and where the heat from inside the house is rejected during the cooling season.
The indoor unit contains the indoor heat exchanger and the fan that distributes
heated or cooled air to the distribution system of the house. Some systems have a second
indoor cabinet that contains the compressor.
A refrigerant is a fluid, which
vaporizes (boils) at a low temperature. The refrigerant circulates through tubes
(refrigerant lines) that travel throughout the heat pump. We'll begin our
description of the refrigeration cycle at point A on the illustration below,
which describes the heat pump when it is heating the house. At point A the
refrigerant is a cold liquid, colder than the outdoor air. The refrigerant
flows to the outdoor coil (point B). This coil is a "heat exchanger" with a
large surface area to absorb heat from the air into the colder refrigerant. The
heat added to the refrigerant causes the fluid to vaporize, so this heat
exchanger is called the "evaporator coil" during the heating cycle. When
materials change state (in this case from liquid to gas), large amounts of
energy transfer take place. At point C the refrigerant is a cool gas, having
been warmed and vaporized by the outdoor air. It is too cool to warm the house,
so that's where the compressor (point D) comes in. The compressor raises the
pressure of the gas. When that happens, the gas temperature rises. One way to
think about it is that the compressor concentrates the heat energy. The
compressor is often thought of as the "heart" of the heat pump, since it does
most of the work of forcing heat "uphill." The compressor also forces the now
hotgas (point E) further into the cycle. The indoor coil (point F) is where the
refrigerant gives up its heat to the indoor air. A fan blows air past the indoor
coil to distribute heat to the house. This cools the refrigerant to the point
where much of it condenses, forming a liquid. In the heating season, the indoor
coil is called the "condenser coil." This change of state results in a large
transfer of heat energy. The warm mixture of liquid and gas (point G) continues
through the cycle to point H, the expansion device (sometimes called a
"metering device"). This device reduces the pressure, causing the refrigerant,
to become cold again - cold enough so that it is once again ready to absorb heat
from the cool outdoor air and repeat the cycle.
Low Energy Use
The biggest benefit of GHPs is that they use 25-50% less electricity than conventional heating or cooling systems. This translates into a GHP using one unit of electricity to remove three units of heat from the earth.
Architects and building owners like the design flexibility offered by GHPs. Historic buildings like the Oklahoma State Capital and some Williamsburg structures use GHPs because they are easy to use in retrofit situations and easy to conceal, as they don't require cooling towers. GHP systems eliminate conventional rooftop equipment, allowing for more aesthetically pleasing architectural designs and roof lines. The lack of roof top penetrations also means less potential for leaks and ongoing maintenance, and better roof warranties. In addition, the aboveground components of a GHP system are inside the building, sheltering the equipment both from weather-related damage and potential vandalism.
Low Environmental Impact
Because a GHP system is so efficient, it uses a lot less energy to maintain comfortable indoor temperatures. This means that less energy, often created from burning fossil fuels, is needed to operate a GHP. According to the EPA, geothermal heat pumps can reduce energy consumption and corresponding emissions-up to 44% compared to air-source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment.
According to a study completed for the Geothermal Heat Pump Consortium (GHPC), buildings with GHP systems had average total maintenance costs ranging from 6 to 11 cents per square foot, or about one-third that of conventional systems. Because the workhorse part of the system, the piping, is underground or underwater, there is little maintenance required. Occasional cleaning of the heat exchanger coils and regularl changing of air filters are about all the work necessary to keep the system in good running order.
Because GHP systems have relatively few moving parts, and because those parts are sheltered inside a building, they are durable and highly reliable. The underground piping often carries warranties of 25 to 50 years, and the GHPs often last 20 years or more.
This type of installation is generally most cost-effective for residential installations, particularly for new construction where sufficient land is available. It requires trenches at least four feet deep. The most common layouts either use two pipes, one buried at six feet, and the other at four feet, or two pipes placed side-by-side at five feet in the ground in a two-foot wide trench. Or, the Slinky method of looping pipe allows more pipes in a shorter trench, which cuts down on installation costs and makes horizontal installation possible in areas it would not be with conventional horizontal applications.
Large commercial buildings and schools often use vertical systems because the land area required for horizontal loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching, also they minimize disturbances to the existing landscaping. For a vertical system, holes (approximately four inches in diameter) are drilled about 20 feet apart and 100 to 400 feet deep. Into these holes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with a horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building.
If the site contains an adequate body of water, this may be the lowest cost option. A supply line pipe is run underground from the building to the water and coiled into circles at least eight feet under the surface to prevent freezing. The coils should only be placed in a water source that meets minimum volume, depth, and quality criteria.
This type of system uses well(s) or surface body water as the heat exchange fluid that circulates directly through the GHP system. Once it has circulated through the system, the water returns to the ground through the well, a recharge well, or surface discharge. This option is obviously practical only where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met.