Geothermal Heat Pumps

William Thomson, the first Lord Kelvin, described the theoretical basis for the heat pump in 1852. As a noted freethinker, he would probably be amused that it is now a potentially significant tool in the fight to lower CO₂ emissions. His key theoretical advance was to overturn the notion that heat could only flow ‘downhill' — from the hot to the cold. A heat pump collects low-grade heat and can deliver it at a higher temperature, but needs some imported energy to do so.

Lord Kelvin certainly did foresee its first application in buildings for cooling, and millions of air conditioners, chillers and refrigerators (i.e. heat pumps) are manufactured and installed every year. Indeed, the increased demand for the comfort that they can bring, particularly in very hot climates, is one of the main drivers of the rapid increase in energy use in buildings of all types over the past 50 years.

However, heat pumps can also do exactly what the name says — pump heat! To apply them as heating devices (rather than the now almost ubiquitous cooling devices) low-grade heat is collected from the atmosphere (air), bodies of water (such as lakes or rivers) or from the ground. Using a refrigerant circuit, this heat is upgraded by an electrically-driven compressor and can then be delivered at a useful temperature for heating. For cooling, this process is simply reversed; low-temperature heat is collected from inside a building, upgraded and rejected to the atmosphere, water or ground.

It is difficult to assemble authoritative statistics for the number of heat pumps used for domestic heating. At the five-yearly World Geothermal Congress in 2005, a review concluded that there are in excess of 1.3 million. Writing in Renewable Energy World magazine earlier this year, Eric Martinot of the Worldwatch Institute quoted a figure of more than two million ground source heat pumps used for heating or cooling in over thirty countries.

The key to ensuring that a heat pump is a worthwhile installation — in terms of overall efficiency and related carbon emissions — is the Coefficient of Performance (COP). This is the ratio of the units of heat (kWh th) the equipment delivers for every unit of electricity (kWh e) consumed. A well-designed, properly-sized system using modern components should deliver a COP of 2.5-4.5. Clearly, the efficiency of the generation to provide the electricity is important to the overall worth of the system. If the electricity comes from a conventional power station operating at about 35% efficiency and the heat pump has a COP of 3.5, then the heat pump will be 1.4 times more energy-efficient overall than a gas-fired boiler. If the electricity comes from a more efficient combined-cycle power station, say operating at 45% efficiency and the heat pump has a COP of 4, then the system will be more than twice as efficient as the boiler. Of course, if the electricity were to come directly from a wind or solar renewable energy source, then the heat pump represents an excellent way to generate heat without any associated carbon emissions.

There are a variety of different types of heat pump. They can be categorized by function (heating, cooling, domestic hot water, ventilation, drying, heat recovery etc); by heat source (ground, water, air, exhaust air etc); by working fluids (both for the heat collection and distribution — brine/water, water/water, air/water, air/air etc) and by unit, construction, location, drive power and more.

For buildings, and heating and hot water applications, the main heat input is likely to be air, water or ground. In some parts of the world, particularly North America, the heat is most likely to be delivered as air through ductwork, either warmer in the heating season, or cooler when this is required. Reversible units are thus quite common for areas with both heating and cooling needs at different times of the year. In Europe, by contrast, the output is most likely to be delivered using water either through a radiator or under-floor system.

Almost all of the heat pumps used in buildings will use the wider environment as their energy ‘source'. While air is the most widely available source, a significant difficulty arises in that heat pumps work best when there are lower temperature differences; on cold days when demand for heating is highest a heat pump will then either be working inefficiently, or not delivering sufficient heat to meet the demand. In moderate climates with well insulated buildings they may be able to meet the heating needs, otherwise a secondary heat source will have to be used as well.

Water heat pumps have a big advantage in that water has a much higher heat-carrying capacity than air, better heat-transfer characteristics and can be moved around easily and efficiently. However, relatively few buildings have a conveniently located, appropriate source of water to use.

As a result, interest in geothermal heat pumps (or ground source heat pumps) has burgeoned. Typically, these are closed loop systems, and usually use the ground surrounding or underneath a building. By installing a suitably sized loop of pipework in the ground, water can be circulated to collect the renewable energy stored in the earth and deliver it to a water-source heat pump.

While very simple in concept, these systems can be rather more complex in their design. The loop temperatures are engineered to ensure the movement of heat by conduction through the ground. The systems need to be thought of as coupled, comprising the building, the heat pump and the ground loops, which need to be carefully matched to ensure a long-lasting, reliable system.

Figure 1

There are numerous different possible configurations for a heat pump. A schematic showing a heat pump refrigeration cycle is shown in Figure 1. This example has input air at 7ºC and output water at 50ºC. Below is an overview of the main types of electrically-driven, vapour compression heat pumps typically used in single and multiple family homes, and in industrial, commercial and community buildings.

Water/water and brine/water

These are frequently used for heating operation (usually ground source) as well as cooling, heat recovery and hot water production. Water/water systems are used with an available water source such as a lake or borehole, while brine/water systems will be closed loop, ground-coupled installations. While in early systems ‘brine' may originally have literally been brine (saltwater), the term is now used to refer to any water circuit with antifreeze in it.

They are generally compact units suitable for indoor use. Typically, they will have a hermetic compressor chosen for extremely quiet operation, and modern heat pumps will include some acoustic shielding and vibration isolation as well. A steel, flat-plate heat exchanger is usually used for the evaporator and condenser, though shell and tube or coaxial ones can also be used. A chlorine-free refrigerant should be chosen. A controller will usually be integrated or externally mounted, and displays are usually kept as simple as possible

For simple installation, the heating system circulation pump, and/or the source side circulation pump will be integrated into the heat pump casing, and for extra compactness, the hot water tank is sometimes built in as well.

Direct expansion/water heat pump

These do not have a heat exchanger on the source side. Instead, a loop of pipe puts the refrigerant in direct contact with the ground or water body. The compressor circulates the refrigerant directly, eliminating the heat transfer losses. It is particularly important that the refrigerant loops are totally sealed and corrosion resistant.

Direct expansion/direct condensation

These systems are similar, but the distribution of heat is achieved directly using the refrigerant, usually in a radiant floor heating system. The condenser, like the evaporator, is composed of seamless, plastic-sheathed copper tubing, and the latent condensation energy is released at a constant temperature.

Air/water heat pump — split units

These use air as their heat source and usually operate in bivalent heating systems. They are particularly well suited for retrofit and renovation as they need less structural work. The indoor unit will contain the main components and be protected from frost. Typically a fully hermetic compressor will be used, and a stainless steel flat plate heat exchanger for the condenser. The outdoor unit will then be connected through refrigeration lines; with the elimination of air ducts it is possible to get fans to run very quietly.

Air/water heat pump — compact units, indoors

In this case the outside air is the heat source — units typically operate in bivalent heating systems. They are compact, and frequently used in indoor installations. They use a hermetic compressor and often a copper-tube, aluminium-finned evaporator. The refrigeration cycle should be fully insulated to prevent thermal losses and condensation.

Air/water heat pumps — compact units, outdoors

These compact units can be installed in any open location. All components of the refrigeration cycle are integrated into the unit, so no air ducts are required, though the housing unit needs to provide protection against the weather. The connection to the heating system will consist of two insulated pipes for supply and return, and will be installed underground, along with the pump power supply and control cable.

Domestic hot water/heat pumps — air source, compact units

These are supplied as fully-integrated compact units, normally as exhaust air heat pumps with a hot water storage tank. All components are integrated into the unit. These will often have fairly sophisticated electronic controls to ensure simple and effective operation of all the main controls that may be needed, such as Legionella treatment, the use of off-peak power and so on.

Domestic hot water heat pumps — air source, split units

With these units, external hot water tanks are used, and a built in feed pump circulates water from the heat pump to the hot water tank. This allows tanks of any model or capacity to be used with these systems.

Other types of heat pump include ground or water heat pumps with domestic hot water, split units, air/air heat pumps for ventilation, various designs of exhaust air heat pumps, and some for heating and cooling.

Conclusion

Geothermal heat pumps are an often-overlooked member of the set of renewable energy tools available for reducing CO₂ emissions. To some, the fact that they need an electrical input counts against them significantly; indeed, if poorly sized, with a low COP and using power from an inefficient generation source, they might actually be a poor choice of technology resulting in an increase in CO₂ emissions (compared with burning the fuel directly for heat).

However, systems with good COP, appropriately sized, and particularly those using renewable electricity to provide the electrical power, can make a big contribution to higher efficiency. This is particularly true for many urban buildings; some planning authorities require a percentage of a building's energy to come from renewable sources as a condition of granting planning permission. Since many urban sites, particularly compact ones, may have little wind resource and only a small or shaded area to try to collect solar energy, a geothermal heat pump may be the only option for on-site renewable energy generation.

This article is based on material in the book Geothermal Heat Pumps — a Guide for Planning and Installing by Karl Ochsner with an introduction by Robin Curtis. The book is published by Earthscan. http://www.earthscan.co.uk/?tabid=415