Post-Carbon Living - Heat Pumps


		living: 30 years from now

Our pick of the best reads:

Jeff Rubin "Why your world is about to get a whole lot smaller"

Greg Craven "What's the worst that could happen?"

Lester Brown "Plan B 3.0"

Shaun Chamberlin "Transition Timeline"

Andrew Simms & David Boyle "The New Economics"

Anthony Giddens "The Politics of Climate Change"

Tamzin Pinkerton & Rob Hopkins "Local Food"

Clive Hamilton "Growth Fetish"

Richard Heinberg "Peak Everything"

Richard Heinberg "Oil Depletion Protocol"

"The Green Building Bible" vol 1

Mark Lynas "Six Degrees"

Donella Meadows, Jorgen Randers Dennis Meadow "Limits to Growth"

Aubrey Meyer "Contract & Converge"

Alexis Rowell "Communities, Councils & A Low-Carbon Future"

From the Library Shelf:

Proud Co-Founders of Transition Town High Wycombe

Free Heat anyone?

Think about it. We can use a Solar Thermal Panel to Heat your water. However, the sun is outputting lots of heat and energy which we mostly do not harvest. Rays of sunlight hitting the earth gently warms it up. Day after day after day this heat radiation is soaked up like a sponge. The ground beneath us retains this warmth all year long. Even in winter the temperature of the ground just a few feet down can be a steady 6 to 12 degreesC. Imagine what you could do with all that energy?

Well, you can pump it out. A heat pump is a device to do just that. Don't confuse this with geothermal. There is no need to go THAT deep into the ground, just a few feet will do. This is low grade solar energy that we can 'refine' into high temperature heat. How? Read on.....

Ground Source Heat Pumps

Heat pumps are quite a well established technology - basically every home (in the rich north) has one even if we don't recognise it. The humble refrigerator is an enclosed insulated box attached to a machine that takes the heat from inside the box and pumps it to the outside - where it is wasted. Hence outside hot, inside cold. So it wasn't long before someone thought to invert the priorities and make the heat the primary objective of the heat pump. In this case the fridge interior is the outside of your house or garden whilst the heat extracted is dumped inside your home as useful warmth. The energy is extracted from your nearby environment.

Most people don't understand Heat Pumps. They seem to do the miraculous. Is the energy free? Well, not quite. You still need electricity to run the pump. All the heat pump does is to concentrate the diffuse heat outside into high temperature heat inside. This seems counter intuitive. How come when it is 0 degreesC outside can the heat pump make my radiators 40 degreesC inside? Well, think again about your fridge. It is cold inside but on the outside the cooling fins at the back are hot. Same thing.

Although this process absorbs some energy this is far less than it can deliver as usable heat. The efficiency of this process is described through the CoP (the coefficient of performance). A CoP of 4 means that for every one KW put in you get 4 KW out. This miracle happens because the 1KW is not turned into 4KW. That is impossible. However, the 4KW is just outside waiting to be pumped in by the 1KW of electricity. In the UK alone the heat pump could deliver 160 TWh (terra-watt-hours) annually in terms of space heating for homes. A well insulated home could easily have all of its heating requirement delivered by a heat pump throughout the year.

Even better than that - the system is reversible so, in theory, when you have excess heat in your home in summer you can pump it out back into your garden again. Hence this cools your home as a form of air-conditioning. This application is rare in northern Europe. So we will not dwell upon it.

Keep in mind that a Heat Pump requires electricity that, unless you have your own renewable supply, was generated from fossil fuels. The Power Station conversion efficiency is only 30 to 40%. So take the advice of the Centre for Alternative Technology ("CAT") and ensure that your Heat Pump operates at a high CoP if CO2 emissions overall is your concern. Of course, if you are powering largely through your own Photovoltaics and/or are subscribing to a renewable energy tariff then this may be less of a concern to you. Overall CAT recommend that anyone seriously considering a GSHP should consider a wood fired boiler as a better alternative if you want to go really "Carbon Neutral". However there is something very attractive about extracting free heat from the ground. To make the best of your Heat Pump your house should be well insulated but that is true all the time anyway.

At 2007 prices the GSHP can be cheaper than Oil, LPG or electric storage heaters if the CoP is 4 or higher. The whole GSHP should cost around £1000 GBP per kilowatt. This is made up of £400 to £600 GBP for the Heat Pump and £300 GBP per kW or boreholes £500 GBP per kW. So an 8kW system can cost £7000 to £8000 although a £1200 grant is available in the UK.

The heat pump is most efficient when the temperature gap between source (ie, your garden) and demand (ie, inside your house) is minimal. For a CoP of 4 you should use underfloor heating with a water temperature of 35^oC. You can run your Heat Pump to get your water up to 50^oC but then the efficiency drops to CoP 3. A water temperature of 75^oC means you CoP drops to only 2.5. CAT recommend some form other form of heating for your hot water. They recommend Solar Panels for this purpose.

To enable a heat pump to work at maximum efficiency all year round it is usually sized to meet most, but not all, heating requirements. Because of this, some form of backup heating is necessary during very cold spells.

In installation the trenches should be at least two meters deep. For an 8kW GSHP you'll need trenches 50 to 80 meters long. However this is only 10 metres of (so called) "slinky" (coiled pipe). Bore holes need 20 to 50 metres of pipe per kW and will be 100 to 150 metres deep (2 to 4 pipes per borehole). Pipe diameter should 20 to 40mm for best performance.

Underfloor Heating vs Radiators

Traditionally the Ground Source Heat Pump has been limited in its appeal because it was normally associated with underfloor heating. This is because a GHP can't get water hot enough for a conventional Central Heating system (70^oC vs only 40^oC). Hence the thermal gradient (difference between room temperature and radiator temperature is so much lower. The lower the 'gradient' the less heat is willing to flow from hot to cold. So you need a big area, that is slightly warmer than its environment, to pass sufficient heat, quickly enough, in order warm up you house. As most homes do not have underfloor heating then this would significantly add to the cost and disruption caused by such a project. Hence it often only applied to new builds.

Companies pushing the GHP idea now have spurned the underfloor heating solution because it limited their appeal and market. They are now pushing for the use of traditional radiators. These wouldn't be 'normal' radiators as these would be too small and inefficient. The type of Radiators we mean are those that would be at least 50% bigger than normal (the GHP Companies say only 30% but that is optimistic). They will also be very thin and made of aluminum. This reduces their thermal mass and makes them very good at transferring heat. There are several types on the market but they are very expensive. Either option remains unappealing.

Air Source Heat Pumps

Less well known than their Ground-Source cousins are the Air-Source Heat Pumps. These work in exactly the same way as other heat pumps. In fact the Air Source Heat Pump works much as an Air Conditioning Unit in reverse. The compressor can sit somewhere 'outside' the living space. This can literally mean 'outside' or it could be in an attic space where wasted house warmth can be recycled back into the home living space. It is claimed that heat can be extracted from air at an ambient temperature of just -15^oC. It is also claimed they have a CoP as high as 4.

The Air Source Heat Pump looks exactly like an air conditioning unit. Hence they normally only provide warmth to a specific location of the home. Alternatively there are now some versions that will heat water for the home, eg, the "BWarm" from Heatking. Others on the market include the IVT Nordic Inverter however, a read of the small print reveals they are designed for unoccupied holiday chalets in the dead of winter where they keep the interior temperature at +10^oC whilst it is -17^oC outside. However the brochure suggests that an interior temperature of 20^oC is possible with an outside temperature of 6^oC. CoP for an outside temperature of +7^oC is 5.1 which is excellent (Modell 12 FR-N) but this falls to only about 2 when the outside temperature falls to -18^oC.

Post-Carbon Girl

Resource

This Ground Source Heat Pump thing is a mystery to me. Daddy explains it to me that we will suck the sunshine energy right our of the ground outside of our home. This sounds great - we use this energy to make our home warm. Like a fridge backwards. Mummy likes a warm house in Winter. So do I! We don't have this heat-pump-thing today, in our current house. However we will move to a new one and fit it there. Can't wait.

Conclusion

Low Carbon Man

You need underfloor heating for it to work and electricity to run the compressor. Hardly practical nor resilient.
If you have a resilient source of carbon-free electricity powering a purpose-built home then this is a winner.

References:	zerocarbonbritain.pdf downloadable from www.zerocarbonbritain.com "How a Heat Pump Works" illustration from the IceEnergy Electronic Newsletter "GroundSourceHeatPumps.pdf" downloadable for free from www.cat.org.uk and here.