You
probably own a heat pump without realising it
- a refrigerator is actually a type of heat pump.
When used for heating, heat pumps save energy
by extracting heat from an outside source, and
delivering it for use within the building. They
can be used for any normal heating need. This
apparently magical technology is not new: in the
1950's several heat pumps were installed in a
bid to save energy and fuel costs. One of the
most famous of these was used to heat the Royal
Festival Hall in London by extracting heat from
the River Thames.
HOW CAN ENERGY BE EXTRACTED FROM COLD
WATER?
Our natural sense of heat is based more on instinct
than on science. Humans are warm-blooded and
judge "heat" by comparing it by touch.
Since our body temperatures need to be maintained
within a few degrees centigrade, our natural
senses have evolved to make extremes of temperature
uncomfortable. To us, a hot summer's day feels
many times "hotter" than the freezing
mid-winter. But in reality the Earth's surface
does not vary in "heat energy" as
much as we might imagine. Scientifically speaking,
there is only 11% less energy in cold river
water at 5°C (40°F) compared to hot
bath water at 40°C (105°F).
HOW DOES A HEAT PUMP WORK?
The most familiar form of heat pump is the domestic
refrigerator. Here, heat is extracted from the
cabinet to keep food fresh and the extracted
heat is expelled through the radiator grill
at the back of the unit. In this case the heat
is merely a waste product. In the heat pump,
we utilise this heat, and put the "cold
part" outside. To make this more understandable,
imagine that the "ice box" of your
refrigerator is immersed in a small garden stream
and the hot grid from the back is placed inside
a house. The "ice box" will attempt
to freeze the stream and, if we stopped the
stream from flowing, freezing of the water would
actually occur. But the passing water will constantly
warm up the very cold "ice box". The
temperature of the flowing stream will actually
be reduced very slightly. So we are extracting
heat from the stream, heat which ends up as
heat in the radiator grill, available to warm
the house. In every case, the useful heat delivered
to the house will be greater than the energy
required to drive the heat pump itself. So we
have extracted "free" heat from the
stream.
Here is an alternative simple
way of looking at the principle;-
Let us consider an example of an opposite scenario:-
If an electric kettle element was immersed in
the river or stream and switched on, then the
heat would be rapidly absorbed by the water.
This would be a continuous loss of heat energy
to the stream. The heat would be dispersed and
the stream would hardly heat up.
Back to our heat pump, if, conversely, the element
was colder than the stream, then the stream
would be warming it up, we are therefore absorbing,
hence gaining energy.
This principle can be hard to
grasp, since it is not an immediately obvious
process. The diagram below may help the penny
drop.
