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.
