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Heat pumps

What is a heat pump?

A heat pump is a machine or device that moves heat from one location to another via work. Most often heat pump technology is applied to moving heat from a low temperature heat source to a higher temperature heat sink.

How does a heat pump work?

The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and highly pressurized gas is cooled in a heat exchanger called a condenser until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device like an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. This device then passes the low pressure, barely liquid (saturated vapor) refrigerant to another heat exchanger, the evaporator where the refrigerant evaporates into a gas via heat absorption. The refrigerant then returns to the compressor and the cycle is repeated.

Why is so good a heat pump?

Heat pumps use a little electrical energy to move available energy as heat from the heat source to the heat sink. For every unit of energy purchased as electricity, several units of heat are delivered. So, relating the energy purchased to the energy delivered, heat pumps can be 300% or 400% efficient, which ratio is represented by the Coefficient of Performance - COP number.

How could I use a heat pump?

A heat pump can be used where there is a low temperature source of heat. For example, heat can be transferred from a source at 5C and delivered as heated water at 45 to 50C.

Applications include space heating and cooling, pre-heating domestic hot water, heat recovery and dehumidification in both domestic and industrial sectors.

What is necessary for the operation of a heat pump?

There are three main parts of any heat pump system:

1. A heat source (air, water, ground)
2. The circuit of working fluid within the heat pump itself and a power source
3. A distribution system to deliver the energy in the required form

The heat source can be the ambient air, water, soil or rock. The outside heat exchanger (the collector) transfers energy as heat to the circuit of working fluid within the heat pump itself. It is preferable, in terms of maximising efficiency, to have constant temperature differences between (a) the source and the working fluid and (b) the working fluid and the sink, but this often not possible in some heat pump models.

The distribution system takes the heat from the heat pump (often as hot water) and delivers it to the end-use. Heat can be distributed within a building using underfloor pipes, fan coil units, an air handling system, or wall-mounted radiators.

It is necessary to include a power supply to the compressor to drive the heat pump. It is usually an electric motor which drives the compressor, but a gas engine is used in some designs.

Does a heat pump use renewable energy?

Yes, the source is usually renewable energy from an ambient heat source or waste energy. For example, with a 3:1 performance ratio, for every three units of heat delivered, two units can be from the renewable heat source and one from the electrical power supply. A heat pump, operating on a green electricity supply from an accredited renewable source, offers emissions-free heating and then three units of renewable energy would be delivered for every one unit of energy purchased as green electricity.

What are the different heat sources?

Ambient heat from water, air, or the ground; or waste heat from industrial processes or combined heat and power units are often used.

In air-source heat pumps, external air at ambient temperature is cooled by passing over a finned heat exchanger, thus heat is extracted into the evaporator of the heat pump. In a water source heat pump, river, lake or ground water is cooled as heat is extracted.

In the case of ground source heat pumps, the collector pipe is installed in one or more trenches or boreholes. A water/anti-freeze solution is pumped around this loop of plastic pipe, extracting heat from the ground.

The heat extracted from the ground or lake or river water is replaced by heat from the atmosphere.

What are the advantages of a heat pump system?

Energy Efficient: Heat pumps are very energy efficient as they consume a little energy (usually as electricity) in order to deliver three or four times as much energy as heat. The ratio of energy-out to energy-in depends on the operating conditions and typically varies between 2 to 1 and 5 to 1. The overall average is called the seasonal performance factor.

Environmental: Heat pumps access renewable or waste energy and so displace consumption of conventional fossil fuels (gas, oil, coal). As electricity generation technologies improve, the emissions performance from the combustion of fossil fuels and renewable electricity generating capacity increases, so the greenhouse gas emissions associated with electricity consumption are reducing - making heat pumps even more environmentally beneficial.

Physical: Different types of heat pumps have different benefits. Ground source heat pumps have

1. No visible external equipment
2. No external fans
3. Quiet running
4. Very little risk of accidental damage
5. Adaptable to many locations

Economics of heating houses: The running costs of a heat pump can be less than those of a traditional gas boiler heating system and definitely less than some other forms of heating (LPG, oil, electricity). The initial capital cost is usually higher than other conventional heating systems. The whole-life cost, combining the capital and running costs, can be favourable for heat pumps compared to fossil fuelled systems and, especially, compared with other forms of electric heating.

In addition to providing domestic heating, heat pump systems are used in commercial premises (offices, hotels, supermarkets) often providing heating and cooling and also in some industrial processes and applications.

Is there any restriction on the use a Heat Pump?

Ground-source heat pumps can be used with collector pipes in horizontal trenches or in boreholes. The area of available open ground adjacent to the property may limit the use of trenching. Collector pipes in boreholes require much less open ground than trenching for the same size of heat pump; however, drilling is more expensive than trenching. In either installation, there is nothing visible above ground. Air-source heat pumps can be located in the roof space.

If the heat pump is to be part of an industrial process, e.g. for heat recovery, then any restrictions might be site specific and would require consideration in the system design. Industrial heat pumps tend to be of bespoke design for specific applications.

If the difference in temperature between the source and the sink is too great, then this will limit the operational performance of the heat pump. The smaller the temperature difference, the better is the performance.

Where do I get detailed information about heat pumps?

Yes; look elsewhere on this website for links, case study materials and references or contact us.

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