Heat Pumps, Ground Source, Geothermal, John Cantor, Wales, UK  
Types of Heat Pump

Where the heat comes from.

There are various types of heat sources which broadly fall into the following categories:-

Air source
Ground source
Water source

(How we utilise the heat from the above source options is covered further down the page)

Air source.

Many of the wall-mounted air-conditioners that you see outside offices and shops are reversible, and can operate in heating-mode as well as cooling. These are usually optimised for cooling mode, so their energy efficiency in heating-mode is often inferior. Systems specifically designed for heating have been with us for some time. These are usually much more energy-efficient, but the build-quality and efficiency will vary greatly from model to model; In many ways - you only get what you pay for.

An air source unit usually sits outside you building, and contains an electrically powered refrigeration mechanism with a finned heat exchanger and a large fan. The heat (extracted from the outside air) is usually transferred to water that is piped to inside the house.

The air is not the ideal source of heat since when the heating demand is highest, the air is at its coldest. However, the majority of days over the year are somewhere between ‘mild’ and ‘chilly’, where air-source efficiency is reasonably good. Air source systems have been improved over recent years, so the efficiency difference between ground source and air source may have closed. Recent real-life studies in Germany suggest that Air Source systems are on average 20% worse than Ground source, but it should be noted that there are many factors that affect the overall energy efficiency of the system.

At outside air temperatures below around 6 or 7°C, ice will tend to form on the heat exchanger. This blocks the air passages and reduces the efficiency. A mechanism is deployed that reverses the system to melt the ice. This process is not as wasteful as maybe first thought, but it still contributes to a reduction in energy efficiency (in the region of 10%). The key to energy efficient defrosting is a well-engineered sensing mechanism. In general terms the most sophisticated and energy efficient methods are usually fitted to more expensive heat pumps.

A back-up heater in the form of a conventional electric heating element is sometimes included within the heat pump package. This is far more necessary for air source, than ground source systems, and usually controlled automatically. It is important to ensure that this expensive-to-run heater is not used too much. Boiler-fired back-up systems are possible, but far less common in the UK than they are in Germany. An air source system operating along side an existing oil boiler, or together with wood burning stoves may be a viable option. This avoids excess use of electricity during the coldest spells.

Exhaust air heat-recovery heat pumps take their heat from ventilation air extracted from a building, and can help in buildings with damp problems. However, the electricity consumption of such heat pumps can be considerable. In countries like Sweden, where electricity is more commonly used for heating, exhaust-air heat pumps are more common. This may be due to their 'cleaner' electricity-generating network which has a large hydropower input.

In new-build houses, such systems can work well if designed well, but there is always a danger that badly installed ventilation systems increase the demand on the houses heating system. i.e. ventilation is in excess of requirements, and the boiler load goes up. A correctly set-up system requires an extremely airtight building

Passive heat recovery ventilation (with no heat pump) is a far simpler energy saving method. Again, an airtight building is required, but such units can be very cheap to run.

Ground source promises to deliver the best year-round energy-efficiency, and can offer considerable long-term environmental advantages. The ground source heat pump (GSHP) extracts heat stored in the ground. This is sometimes referred to as ‘geothermal’. However, the heat is mostly from the surface (solar gain), and very little is from the earth’s core. (True geothermal is found in several parts of the world, including Iceland, Japan and even Southampton).

At depths of 2m and more, the ground temperature does not deviate very much from the average summer/winter surface temperatures (around 9° to 12°C in the UK depending on location). At this depth, there is an enormous store of heat that can be usefully tapped for heating in the winter.
The most practical way of extracting this energy is to bury a large amount of pipe in the ground. This is usually laid in horizontal trenches at depths between 1 and 2m depth. If insufficient land area is available, or if excavating a garden is impractical, then vertical boreholes are an alternative method which gives similar results. However, the borehole method is usually considerably more expensive to install.
Click here for further information on ground source heat pumps.

Water source
A river or small stream can be utilised, and, in the past systems using copper coils in the water have been used. However, rules and regulations and the lack of available manufactured equipment have made this type of system less attractive.

Pumping river water through a heat pump is another option, and can give very good results, but heat pump units require water at temperatures above 5 to 8°C (varying depending on type).
Whilst delivering very high-efficiencies for much of the time, this system may fail to operate in the middle of winter during lower temperatures- just when you need the most heat, so a back-up heat source is usually advisable.

Oxygen and contaminants in the river water may also be a concern in some circumstances, causing pump failure and possibly a system refrigerant leak. But don't let me put you off; this system can give excellent results if installed correctly.
For those lucky enough to have a spring, this is a much more stable and better heat source. Its an opportunity not to be overlooked, offering excellent efficiencies. Again, acidity and impurities in the water can sometimes make its use prohibitive, but some heat pump units will tolerate ‘corrosive’ water (some Dimplex models etc). A possible alternative is to use an intermediate heat exchanger in such circumstances. However, the added temperature drop and necessary extra pump can reduce the performance considerably.

The water source should ideally be fairly close to the property, and should not require pumping up any significant height or the power for pumping it may detract from the energy savings. Having said that, water could be taken over considerable distances if the pipe diameter is big enough, especially in a downward direction. It is a relatively simple exercise to calculate the pump power required (if any) to get the water to and through the heat pump unit.
Permission should be sought from the relevant authorities as an abstraction licence may be needed.


HYDRO-POWERED HEAT PUMP SYSTEMS
The mechanical power from a water turbine or wheel is usually used to generate electricity. A heat pump can extract energy from the same water to produce a heat-energy output of three or more times that of a conventional hydro-electric system. Since space-heating is usually the biggest single energy load, it is sometimes better to put all the hydro energy into a heat pump, whilst remaining grid-connected for general electrical needs. The heat pump can be driven directly by a mechanical belt-drive etc. However, this system requires a lot of maintenance. An electric drive heat pump driven from a hydro-electric source is probably the most practical solution, since electric compressors are now so well developed and significantly more efficient than any belt-drive design available.

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How is the heat delivered to the building?

Heat pumps usually deliver heat in the form of hot water, as do most conventional central-heating systems. However, to maintain a high energy-efficiency, the emitter system should be designed so that the water temperature is as low as possible. Ideally, a well-designed underfloor heating system should be used that is designed specifically for use with a heat pump. Such systems are energy efficient and very comfortable. Radiators may be the only alternative, but should be significantly larger in area than normal. Several radiators in one room could be considered. Heat pumps are less forgiving of the design details compared to boilers, so designs and installations must be carried out carefully.
Ducted hot air is an alternative method of distributing heat into a building; however, this is often not as comfortable as radiant (underfloor) heating and should be installed with caution. It is particularly undesirable in badly insulated buildings.

COOLING.

Buildings can overheat in summer. The main cause is often sunlight (solar gain). One square metre of sunlight through a window can contribute almost 1kW of heat to the room. This is the same amount of heat as that from a small electric room heater. The solar heat often falls on the floor, and therefore heats the room. Air-to-air systems (air-conditioners), are used throughout the world for air cooling. Their energy consumption is significant, so they are to be avoided if possible. It is far more energy-responsible to reduce the solar gain in the first place by shading the sun. There are many ways to minimise heat build-up in houses including limiting the heat sources and good ventilation. Older buildings that have a high thermal mass tend not to have an overheating problem. In our climate, it is perfectly possible to design modern buildings that keep sufficiently cool without the need for air-conditioning.

If air-conditioning is deemed to be necessary, then a good water or ground-coupled heat pump system will be the most energy efficient. This type rejects the heat to the ground coil or borehole.

Passive cooling

If the liquid in the ground source is pumped directly around the underfloor heating pipes, then a certain amount of ‘free’ cooling can result. This is known as 'Passive' cooling. It will have a limited effect, and will only work with a borehole, or with a very good ground collector with moving ground water. The passive cooling option may have a limited effect, but if coupled with good housekeeping as outlined above, it can help to curb excessive internal temperatures with minimal use of energy. It will also put heat back into the ground for use in heating mode; however, the advantageous of this re-generation may be fairly minimal.


             

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