Heat pumps

Heat pumps are effective solutions for heating and cooling applications for all types of buildings. Installations in domestic, commercial and retail premises including hotels and residential complexes are all possible. This well-proven technology has been in use for decades and Heat Pumps are at work all over the world providing safe, reliable heating and cooling at affordable prices.

Reserves of conventional fossil fuels are finite and emissions of Carbon Dioxide and other greenhouse gases add increasingly to the effects of climate change. As a low carbon technology, heat pumps can significantly reduce the UK's Carbon Dioxide emissions. Where Heat Pumps are used for heating, they are capable of highly cost-efficient energy applications because they tap into a limitless supply of clean, pollution-free energy from either the surrounding air or heat captured in the ground, all you pay for is the energy to transport that heat, and in some applications, most of this energy can be reclaimed too.

The Basic Principle

As with many technologies that we use in every-day life, the basic principles of how a heat pump works are simple. All our surroundings, even a block of ice, has heat energy. The purpose of a heat pump is to absorb heat in one place where it is plentiful, then to transport and release it in another location where it can be used for space or water heating. Useful heat can be found in the air outdoors, in the ground, and is present in water, rivers, lakes and the sea. Even on the coldest winter days, sufficient heat is present to warm our homes and offices and what's more, it is free. All we have to pay for is the machine to recover it and the cost of the energy to run the machine. Even then the savings continue. Modern heat pumps allow a significant quantity of the electrical energy that drives the heat pump to be returned to the building as useful heat.

At the heart of a modern heat pump system is a refrigeration system. Paradoxically, the refrigeration cycle is an efficient provider of heat as well as cooling and the basics of its operation are quite easily understood.

There are two principle locations in the transfer of heat; the place where heat is absorbed, (the source), and where it is rejected, (the destination). The compressor in the refrigeration system also produces waste heat, and a significant proportion of this can be recovered, thereby reducing running costs and the ultimately release of CO2.

The mechanical refrigeration cycle consists of an arrangement of heat exchangers; one that absorbs heat, the other that ejects it. All but the largest industrial systems are hermetically sealed and pressurised, thereby reducing noise, space and heat losses. This means that the compressor and the motor that drives it are encased in a welded shell. This heat absorbed is transported through a sealed system of pipes by a fluid, the refrigerant, circulated by a compressor. The refrigerant is a fluid that has a low boiling point. A metering device to control the flow of refrigerant completes the arrangement and it is all connected by pipes. As the refrigerant works under pressure, the whole system is sealed for life.

In order to absorb and release the heat into and from the refrigerant, we exploit the ability of the refrigerant fluid to boil from a liquid to a vapour and then to condense back into a liquid. This is a continual process while the compressor is running and circulating the refrigerant. For all volatile substances, there is a known relationship between its pressure and its boiling point; by controlling these in the refrigerant we can achieve cooling and heating in the same machine at the same time.

High pressure liquid refrigerant is fed through the metering device into the evaporator heat exchanger where it evaporates into a vapour by absorption of heat from the heat source (air, water, ground, other) passing through the heat exchanger. The relatively cool return vapour is drawn back to the compressor. The compressor and the electric motor that drive it are constructed in a fully sealed hermetic shell. The cooled return vapour from the evaporator is passed over the compressor motor windings within the heat pump, thus cooling the windings of the motor. Much of the energy absorbed by the electric motor driving the compressor is absorbed into the refrigerant.

The combined heat from the source, plus much of the waste energy from the electric motor is then compressed to a high temperature vapour and enters the condenser heat exchanger where it is cooled and condensed into a high pressure liquid ready to begin the cycle again.

The heat released during the process of condensing the refrigerant to a liquid is ejected via the heat exchanger directly into air or transferred to water to heat the building. The air or water temperature at this point could be 43ºC to 60ºC, depending on the design of the system.

Although most systems are configured for heating only, reverse cycle heat pumps use an electrically operated reversing valve with four pipe connections to change the direction of refrigerant flow within the system so that the system is able to deliver both heating and cooling as desired. Many commercial systems are capable of cooling as well as heating, with fully automatic control enabling the user to receive year-round operational benefits.

Heat pumps work most efficiently with low temperature heating distribution systems such as underfloor heating. The lower the outlet temperature from the heat pump the more efficient the unit is. At a temperature of 35.8C, for every one unit of electricity used in a ground source heat pump 4 units of heat energy will be produced i.e. 400% efficient. As heat pumps use renewable energy they also have low CO2 emissions compared to traditional heating systems and can cut emission rates by up to 50%.

Types of Heat Pump

Air Source

An air source heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its inside. It can extract heat from the air even when the outside temperature is as low as minus 15° C. Heat pumps have some impact on the

environment as they need electricity to run, but the heat they extract from the ground, air, or water is constantly being renewed naturally. There are two main types: * An air-to-water system uses the heat to warm water. Heat pumps heat water to a lower temperature than a standard boiler system would, so they are more suitable for underfloor heating systems than standard radiator systems, however, recent developments have seen the introduction of High Temperature Heat Pumps that can work with existing radiator systems. * An air-to-air system produces warm air

which is circulated by fans to heat your home. Heat from the air is absorbed into a fluid which is pumped through a heat exchanger in the heat pump. Low grade heat is then extracted by the refrigeration system and, after passing through the heat pump compressor.

 

Ground Source

A ground source heat pump circulates a mixture of water and antifreeze around a loop of pipe - called a ground loop - which is buried in the ground. Heat from the ground is absorbed into this fluid and is pumped through a heat exchanger in the heat pump. Low grade heat is then extracted by the refrigeration system and, after passing through the heat pump compressor, is concentrated to a higher temperature.

Heat capable of heating the property and domestic hot water systems. The ground loop fluid, now cooler, passes back into the ground where it absorbs further energy from the ground in a continuous process whilst heating is required. The length of the ground loop depends on the size of your home and the amount of heat you need - longer loops can draw more heat from the is concentrated to a higher temperature, useful heat capable of heating water for the heating and hot water circuits of the house. The efficiency of air source heat pump systems is measured by a coefficient of performance (COP) - the amount of heat they produce compared to the amount of electricity needed to run them. A typical COP for an air source heat pump is around 2.5 to 3.0 when used with under floor heating. The COP means that for every unit of electricity used to power the pump, 2.5 to 3.0 units of heat could be generated.

Normally the loop is laid flat in trenches about one metre deep, but if there is not enough space in your garden you can install a vertical loop or borehole, to a depth of up to 100 metres for a typical domestic home. Heat pumps have some impact on the environment as they need electricity to run, but the heat they extract from the ground, air, or water is constantly being renewed naturally.

The energy efficiency of a ground source heat pump is measured by a coefficient of performance (COP) - the amount of heat it produces compared to the amount of electricity needed to run it. A typical COP for a ground source heat pump is around 3.2 to 4.0 if used with under floor heating. This means for every unit of electricity used to power the pump, you could get 3.2 to 4 units of heat.

Water Source

Water source heat pump systems produce the initial heat in a similar way to ground source systems. Pipes are submerged in a river, stream or lake, where temperatures remain at a relatively constant level of between 7 and 12 degrees. Liquid in the pipes absorbs the heat. This heat is passed to a heat pump located inside the house. For buildings located near a suitable water source,this type of heat pump offers an attractive alternative to ground source systems. Water source heat pumps are virtually silent, maintenance needs and costs are negligible and there are no visible external units. Water source heat pumps can be installed in existing properties, and are particularly recommended for new builds and larger domestic and commercial premises.

Because of the constant level of heat, water source heat pumps are even more efficient than air source heat pumps, though they cost more to install. Once installed and operating, you can expect a cut in fuel bills of between 30% and 70%.