Where the heat comes from.
There are various types of heat sources which
broadly fall into the following categories:-
Air source
Ground source
Water source
(How we utilise the heat from the above source
options is covered further down the page)
Air source.
Many of the wall-mounted air-conditioners that
you see outside offices and shops are reversible,
and can operate in heating-mode as well as cooling.
These are usually optimised for cooling mode,
so their energy efficiency in heating-mode is
often inferior. Systems specifically designed
for heating have been with us for some time.
These are usually much more energy-efficient,
but the build-quality and efficiency will vary
greatly from model to model; In many ways -
you only get what you pay for.
An air source unit usually sits outside you
building, and contains an electrically powered
refrigeration mechanism with a finned heat exchanger
and a large fan. The heat (extracted from the
outside air) is usually transferred to water
that is piped to inside the house.
The air is not the ideal source of heat since
when the heating demand is highest, the air
is at its coldest. However, the majority of
days over the year are somewhere between ‘mild’
and ‘chilly’, where air-source efficiency
is reasonably good. Air source systems have
been improved over recent years, so the efficiency
difference between ground source and air source
may have closed. Recent real-life studies in
Germany suggest that Air Source systems are
on average 20% worse than Ground source, but
it should be noted that there are many factors
that affect the overall energy efficiency of
the system.
At outside air temperatures below around 6 or
7°C, ice will tend to form on the heat exchanger.
This blocks the air passages and reduces the
efficiency. A mechanism is deployed that reverses
the system to melt the ice. This process is
not as wasteful as maybe first thought, but
it still contributes to a reduction in energy
efficiency (in the region of 10%). The key to
energy efficient defrosting is a well-engineered
sensing mechanism. In general terms the most
sophisticated and energy efficient methods are
usually fitted to more expensive heat pumps.
A back-up heater in the form of a conventional
electric heating element is sometimes included
within the heat pump package. This is far more
necessary for air source, than ground source
systems, and usually controlled automatically.
It is important to ensure that this expensive-to-run
heater is not used too much. Boiler-fired back-up
systems are possible, but far less common in
the UK than they are in Germany. An air source
system operating along side an existing oil
boiler, or together with wood burning stoves
may be a viable option. This avoids excess use
of electricity during the coldest spells.
Exhaust air heat-recovery heat pumps take their
heat from ventilation air extracted from a building,
and can help in buildings with damp problems.
However, the electricity consumption of such
heat pumps can be considerable. In countries
like Sweden, where electricity is more commonly
used for heating, exhaust-air heat pumps are
more common. This may be due to their 'cleaner'
electricity-generating network which has a large
hydropower input.
In new-build houses, such systems can work well
if designed well, but there is always a danger
that badly installed ventilation systems increase
the demand on the houses heating system. i.e.
ventilation is in excess of requirements, and
the boiler load goes up. A correctly set-up
system requires an extremely airtight building
Passive heat recovery ventilation (with no heat
pump) is a far simpler energy saving method.
Again, an airtight building is required, but
such units can be very cheap to run.
Ground source promises to deliver
the best year-round energy-efficiency, and can
offer considerable long-term environmental advantages.
The ground source heat pump (GSHP) extracts
heat stored in the ground. This is sometimes
referred to as ‘geothermal’. However,
the heat is mostly from the surface (solar gain),
and very little is from the earth’s core.
(True geothermal is found in several parts of
the world, including Iceland, Japan and even
Southampton).
At depths of 2m and more, the ground temperature
does not deviate very much from the average
summer/winter surface temperatures (around 9°
to 12°C in the UK depending on location).
At this depth, there is an enormous store of
heat that can be usefully tapped for heating
in the winter.
The most practical way of extracting this energy
is to bury a large amount of pipe in the ground.
This is usually laid in horizontal trenches
at depths between 1 and 2m depth. If insufficient
land area is available, or if excavating a garden
is impractical, then vertical boreholes are
an alternative method which gives similar results.
However, the borehole method is usually considerably
more expensive to install.
Click here for
further information on ground source heat pumps.
Water source
A river or small stream can be utilised, and,
in the past systems using copper coils in the
water have been used. However, rules and regulations
and the lack of available manufactured equipment
have made this type of system less attractive.
Pumping river water through a heat pump is another
option, and can give very good results, but
heat pump units require water at temperatures
above 5 to 8°C (varying depending on type).
Whilst delivering very high-efficiencies for
much of the time, this system may fail to operate
in the middle of winter during lower temperatures-
just when you need the most heat, so a back-up
heat source is usually advisable.
Oxygen and contaminants in the river water may
also be a concern in some circumstances, causing
pump failure and possibly a system refrigerant
leak. But don't let me put you off; this system
can give excellent results if installed correctly.
For those lucky enough to have a spring, this
is a much more stable and better heat source.
Its an opportunity not to be overlooked, offering
excellent efficiencies. Again, acidity and impurities
in the water can sometimes make its use prohibitive,
but some heat pump units will tolerate ‘corrosive’
water (some Dimplex models etc). A possible
alternative is to use an intermediate heat exchanger
in such circumstances. However, the added temperature
drop and necessary extra pump can reduce the
performance considerably.
The water source should ideally be fairly close
to the property, and should not require pumping
up any significant height or the power for pumping
it may detract from the energy savings. Having
said that, water could be taken over considerable
distances if the pipe diameter is big enough,
especially in a downward direction. It is a
relatively simple exercise to calculate the
pump power required (if any) to get the water
to and through the heat pump unit.
Permission should be sought from the relevant
authorities as an abstraction licence may be
needed.
HYDRO-POWERED HEAT PUMP SYSTEMS
The mechanical power from a water turbine or
wheel is usually used to generate electricity.
A heat pump can extract energy from the same
water to produce a heat-energy output of three
or more times that of a conventional hydro-electric
system. Since space-heating is usually the biggest
single energy load, it is sometimes better to
put all the hydro energy into a heat pump, whilst
remaining grid-connected for general electrical
needs. The heat pump can be driven directly
by a mechanical belt-drive etc. However, this
system requires a lot of maintenance. An electric
drive heat pump driven from a hydro-electric
source is probably the most practical solution,
since electric compressors are now so well developed
and significantly more efficient than any belt-drive
design available.
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How is the heat delivered to the building?
Heat pumps usually deliver heat in
the form of hot water, as do most conventional
central-heating systems. However, to maintain
a high energy-efficiency, the emitter system
should be designed so that the water temperature
is as low as possible. Ideally, a well-designed
underfloor heating system should be used that
is designed specifically for use with a heat
pump. Such systems are energy efficient and
very comfortable. Radiators may be the only
alternative, but should be significantly larger
in area than normal. Several radiators in one
room could be considered. Heat pumps are less
forgiving of the design details compared to
boilers, so designs and installations must be
carried out carefully.
Ducted hot air is an alternative method of distributing
heat into a building; however, this is often
not as comfortable as radiant (underfloor) heating
and should be installed with caution. It is
particularly undesirable in badly insulated
buildings.
COOLING.
Buildings can overheat in summer. The
main cause is often sunlight (solar gain). One
square metre of sunlight through a window can
contribute almost 1kW of heat to the room. This
is the same amount of heat as that from a small
electric room heater. The solar heat often falls
on the floor, and therefore heats the room.
Air-to-air systems (air-conditioners), are used
throughout the world for air cooling. Their
energy consumption is significant, so they are
to be avoided if possible. It is far more energy-responsible
to reduce the solar gain in the first place
by shading the sun. There are many ways to minimise
heat build-up in houses including limiting the
heat sources and good ventilation. Older buildings
that have a high thermal mass tend not to have
an overheating problem. In our climate, it is
perfectly possible to design modern buildings
that keep sufficiently cool without the need
for air-conditioning.
If air-conditioning is deemed to be necessary,
then a good water or ground-coupled heat pump
system will be the most energy efficient. This
type rejects the heat to the ground coil or
borehole.
Passive cooling
If the liquid in the ground source is pumped
directly around the underfloor heating pipes,
then a certain amount of ‘free’
cooling can result. This is known as 'Passive'
cooling. It will have a limited effect, and
will only work with a borehole, or with a very
good ground collector with moving ground water.
The passive cooling option may have a limited
effect, but if coupled with good housekeeping
as outlined above, it can help to curb excessive
internal temperatures with minimal use of energy.
It will also put heat back into the ground for
use in heating mode; however, the advantageous
of this re-generation may be fairly minimal.
