SUSTAINABLE ENERGY AND NATURAL SCIENCES

HOW DOES A HEAT PUMP WORK?

Heat pumps are effective solutions to heating and cooling applications for all types of buildings, domestic, commercial and retail premises including hotels and residential complexes.

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 heat – 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. 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 – 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.

How Does a Heat Pump Work?

At the heart of a modern heat pump 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 ultimate release of CO2.

The mechanical refrigeration cycle consists of an arrangement of heat exchangers; one that absorbs heat, the other that rejects 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 rejected 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 some 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.