An idiot's guide to ground source heat pumps
Written by James
Ahh, warmth...we all want it. Here's a way of getting some.
Worldwide, more heating energy is produced this way than electricity is
produced via solar photo-voltaic panels and wind power put together.
In Sweden, 90% of new homes are heated with ground source heating.
In 2002, approximately 41,000 heat pumps were installed across Europe,
of which the UK installed 150. We're very slow at developing this
technology. But things are changing fast.
What is it?
The simplest way of describing a heat pump is to use it's most
common form: the fridge. A fridge takes heat from the air inside your
fridge and discharges that heat out of the back. If you feel the grille
at the back of the fridge you’ll notice it's warm.
The process works because as a material is compressed, the amount of
energy that it can hold is reduced, so energy is pushed out
of the pressurised material. Imagine pumping up a bicycle tyre - you
start with air at ambient temperature, and as you pump it into the
tyre the valve gets really hot.
Likewise, if you let something depressurise, it cools down, like a camping gas cylinder when you take the burner off the top.
Inside your fridge, you have a big white plate. This is full of a
volatile liquid at roughly normal (atmospheric) pressure. At the top of this plate is
a copper tube which passes out of the top of the fridge, down the back,
and into a big black cylinder at the base, this is the “heat pump”. The
pump compresses the liquid. As the liquid compresses, it can’t hold as
much energy, so it gives it off in the form of heat. The heat is
dispersed via the large black grille at the back of the fridge (be
careful when touching it, they’re really hot!!).
The newly compressed liquid is pushed back into the plate on the inside
of your fridge via a very narrow pipe. When it exits this pipe, it
expands. As it expands, it can absorb energy again, so it takes heat
from the air inside the fridge and in so doing it cools the fridge
down. The liquid returns to a normal pressure again, rises to the top of
the plate and exits to be re-pressurised. And so it continues….
Fridges aren’t the most efficient things in your kitchen…in fact if
you’ve got an old fridge like mine, its probably using more electricity
than anything else in your entire house (apart from my old freezer
sitting next to it!). But this inefficiency can be turned on its head
if you utilise the heat, not the cold.
Now lets relate this to ground source heat. Instead of the air inside
you fridge, you're taking energy from the ground, instead of the little
compressor you’ve got a big compressor and instead of grilles at the
back of the fridge you’ve got central heating. That’s it, really.
How much space do I need to get the energy I need?
From the above principles, many variants have evolved.
The two most common sources of energy for this technology are
groundwater or rock. Across the UK, These vary in average temperature
between 9- 11 centigrade, with an annual fluctuation at any location of
only a degree or two. The smaller the fluctuation, the more you can fine-tune
your system to a specific set of working parameters. To access
groundwater or rock, you need at least one borehole
You can drill two boreholes and take water from one, pass it through
you heat pump and re-inject to the other. This is called an OPEN LOOP
system. The advantage with open loops is that the coolled water is put
back into a different place from were the energy is taken out, so in
theory, there’ll never be a ‘short circuit’ (see later)
Or you can drill just one borehole and put a heat exchanger down
this single borehole. This is called a CLOSED LOOP system. Normally,
the borehole is filled in with special material to encourage high
thermal conductivity. The advantage of this system is that its cheaper,
and even though its not as efficient when it comes to large buildings
like hospitals, its great for small buildings like houses.
If boreholes are too expensive (they’ll set you back at least a few
thousand pounds each) and if you’ve got plenty of space (a paddock,
for example) you could run an overlapping coil of PVC water pipe in
rows approximately 1.5 –2 m below ground. At this depth, the annual
fluctuation is minimal, so again the system can be optimised. This
method is called a ‘slinky’ because it looks like a flattened spring.
Water is passed through the pipes, effectively using the paddock as
a massive solar collector, with the PVC pipes acting as a heat exchanger.
A slinky coil laid out close to surface
Other methods include dropping coils of PVC pipes into lakes (the
Queen’s favourite!) using atmospheric air (prone to variations in
temperature), dropping slinkys vertically into deep trenches, using
mine water discharges, building coils into pile foundations, or even
linking the system to solar water heating panels. The list is almost
endless. I’m going to concentrate on “traditional” ground source
heating (GSH) for two reasons- I don’t know much about the other types,
and traditional GSH is now a tried and tested technology that can be
trusted.
How much energy will I get and what are the costs?
Per metre of excavation, the energy you get from a borehole is very
consistent and isnt dependant on rock type, water level or whether its
closed or open loop. The energy you get is between 60 and 100 watts per
drilled metre. The lower value will be for dry, loose rock and the
higher value is for wet dense rock. Given that most domestic properties
have a heating requirement of between 4.5-7 kilowatts, this equates to
a borehole of between 50 and 100m, depending on conditions. If you
choose to have an open loop (very unlikely for a small system), you
would need two such boreholes.
Some indicative scheme costs:
House, Cornwall: Closed loop system, one borehole to 70m, 4kW output £5000
Bungalow, Nottinghamshire, closed loop system, one borehole to 68m, 6 kW output £6,000
Office block (location?), closed loop, 12 boreholes each to 48 metres, 60 kW output £38,000
Church, open loop, two boreholes unknown depth, 350 kW output, £140,000
For a small domestic system, a closed loop system can be installed for around £5-6000.
Is it environmentally beneficial?
Well, yes and no. You still need to put electricity into the system to
get it to work. Normally, the amount of electricity you put in is a
quarter of the energy you get out in terms of heat. This is called the
coefficient of performance (COP), a heat pump often has a COP of 4.
Under the former clear skies grant government grant scheme, heat pumps
were required to have a COP of greater than 3.7 to be eligible for
government grant.
In 2005, the Building Regulations published a standard assessment
procedure for different forms of domestic central heating.
Please note that although recent, these figures pre-date the recent
rapid increases in domestic oil and gas prices. Some figures:
Mains gas (in an modern efficient combi boiler) cost per kWh 1.63p, produces 0.194 kg CO2 per kWh
Bottled LPG cost per kWh 4.32, produces 0.234 kg CO2 per kWh
Heating oil cost per kWh 2.17p, produces 0.265 kg CO2 per kWh
Coal cost per kWh 1.91p, produces 0.291 kg CO2 per kWh
Electricity cost per kWh 7.12p, produces 0.422 kg CO2 per kWh
Ground source heat pumps cost per kWh 1.7p, produces 0.106 kg CO2 per kWh
So before the recent rapid increase in gas prices, GSH came in around
the same price as gas per unit of heating, but much less carbon is used
in producing that energy. Electricity is extremely costly and wasteful
due to the loss of charge within the national grid, most electricity
that’s produced disappears before it ever gets close to your domestic
system.
Often, people ask if a wind turbine could be used to provide the
electricity. In theory, yes it could, but in reality, the start up
power needed is a surge of around 4.5-6 kilowatts for a household unit.
That’s one heck of a large charge for a small domestic turbine, often
rated at high hundreds or low thousands of watts. So you’d either need
lots of batteries or still be attached to the mains. For this reason,
connected wind turbines are the perfect compromise, because after
initial power up of the heat pump, the energy requirement greatly
reduces.
A domestic ground source heat pump installation-the stuff above ground looks very dull.
What are the problems and drawbacks?
The technology is best installed during construction. You can retro-fit
to an existing building, but it’s more expensive and your choices are
limited.
This is primarily because of the level of heat this system can produce.
Its not actually a very great increase in temperature. So whereas your
existing radiators may run at 60 centigrade, this system will probably
only run at 40 centigrade. This means you can’t power your existing
central heating radiators on it. GSH works well by warming a large area
by a few degrees, unlike traditional radiators that warm a small area
by many degrees. So the most efficient methods heating with GSH are
under floor heating or large panel radiators, both of which are cheaper
to install during construction.
In North America, they like having warm air central heating instead of
radiators. I suppose it's so they can use the same system to pass cold
air through during summer months. This system is easier to retro fit to
an existing building, but it also runs slightly less efficiently.
A word of warning of the Coefficient of Performance (CoP) numbers- many
companies are springing up at present and the market is increasing
rapidly. The supplier who claims the highest CoP is more likely to make
the sale. Some American companies are now advertising CoPs of around 6.
However, it is very hard to corroborate CoP figures for GSH. Although
the Clear Skies scheme required a CoP of 3.7, to the best of my
knowledge the figures supplied by the manufacturers have never been
checked or tested. Scandanavian companies have a long track record and
are likely to have an accurate feel for the true CoP of their
equipment. They are less likely to be interested in over-blowing their
claims, because their markets are so large in their home countries.
The systems need to be designed well. At present, there are a number of
companies who can do it well, but there are many new companies jumping
on the band wagon. The consequences could be very expensive.
One such consequence is short circuiting of the system. This is
were cold water being discharged back to the aquifer starts being taken
up by the abstraction instead of ambient temperature water. The result
is a reduction in efficiency, and a downward spiral in temperatures
eventually resulting in freezing of the ground, and damage to your
equipment. This is overcome in open loop systems by ensuring the
discharge borehole is down gradient of the abstraction borehole. In
closed loop systems, its overcome by ensuring that pumping is
undertaken at low rates to allow cooling to disperse and the ground
temperature to equilibriate (this is why closed loops are more suited
to small heating schemes)
Although you will not need a water abstraction licence, you (or your
designer/ agent) are strongly advised to contact the Environment Agency
at an early point. You can use the general contact number from the Environment
Agency’s web site, tell them which river catchment
you live in, and ask to speak to the groundwater officer for that area.
They will be pleased to help, and are a good starting point for ground
conditions. They are also likely to give you some recommendations
regarding drilling a borehole, whether you're drilling into a major
aquifer or a zone protected for a public water supply, for example.
Remember, it's your taxes that pay for them so don’t be afraid to use
them…Unfortunately, they can't recommend companies.
The Environment Agency are at present drawing up guidance regarding the
implementation of ground source heat pump schemes. At the time of
writing, (late summer 2006), there is no national policy on the
management of these projects.
A last word, throughout this note, I’ve assumed that most people in the
UK will be interested in this technology for heating. The same system
can be also used for cooling but this is very, very much less efficient
than heating.
The images in this article are from:
Running on Empty News Group
http://groups.yahoo.com/group/RunningOnEmpty2/
and Navitron
http://www.navitron.org.uk/heatpumps.htm
Link to Environment Agency home page :
http://www.environment-agency.gov.uk/
Association Of Environmentally Conscious Builders is a useful site:
http://www.aecb.net |
