Ground, Air and Water Source Heat Pump Systems
Ground Source Heat Pumps either use the heat in the soil at 1 - 3m depth (networks of piping) these operate at soil temperatures varying from 0 - 8°C; or they use the heat from the bedrock by drilling a borehole of 80 - 100m (a loop of piping with backfill in each borehole) these operate at rock temperatures varying from 14 - 16°C depending on depth and rock type. NB Most of the rest of the world, considers these systems under the heading of Geothermal (Shallow).
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While these systems are normally used for heating houses, they could also be used to heat a Swimming Pool or even an Aviary/Pigeon loft etc.
The first step is to assess how many kWh you spend on heating your property, as this will give the supplier an idea of the size of installation that is needed in your soil/rock type.
In order to maximize the available temperature, it is normal for the top of a ground source system, to be buried at a depth of 1m - it needs to be below the frost level for maximum use of the heat in the ground. The Slinky - a favourite with many plumbers - is a coil of piping having a diameter of approx. 1m. To be laid correctly for maximum contact with the soil, and therefore heat abstraction, the Slinky must be laid in a trench that is wide enough to take the 'tunnel' of pipe when it is pulled out square at both ends (see diagram of Slinky Loop). This means that the bottom of the trench needs to be dug down to a depth of just under 2 metres. Unfortunately, many installers leave the coils of pipe overlaping and touching in the ground; therefore, concentrating the heat abstraction from a much smaller volume of soil. They do this by setting it in the ground either horizontally at a depth of 1m or in a vertical slot with the top of the pipe at 1m and the bottom at 2m. This is inefficient, and you will need to ensure that they follow the first method above which does involve more digging, but results in an improved yield of heat as you are abstracting from a much larger volume of soil. A horizontal network of copper piping, in the form of several joined loops (Horizontal Loop shown) may also be used and will also need to be buried at a minimum depth of 1 metre.
At a depth of 1 - 2 metres, there is a constant annual temp of 6 - 8°C. Temperatures in the top 2 feet of the soil fluctuate with the solar input and are liable to freeze. Hence one of the reasons for burying the piping network well below this level; the other being to get it below the depth of any future gardening excavations!
The first question is: which of these three systems could fit into the space you have available?
- If you live in a town you may only have a small patch of garden at the front or a bit of a yard at the back. In which case you will be limited to the Borehole option. This will leave you with an inspection hatch surrounded by approx. 1sq.m of concrete on the surface. Below this will be a hole of up to 100m, in which will be a loop of piping supported by a back fill of sand. For max. heat uptake the best suppliers recommend separation of the inflow and outflow pipes by approx. the diameter of a dinner plate (ca.30cms) Ideally the top of the pipe will also be lagged to avoid heat loss as the fluid used comes up through the cooler/freezing surface soil. From about 10m depth a constant temperature of ca. 10°C (50°F) can be expected; thus this will supply hotter liquid to the heat exchanger.
- If you have a bit of a lawn, but don’t want it all dug up, then a Slinky is the obvious answer. You will need to carefully remove the turf from a generous meter wide strip, and ideally lay a strip of plastic alongside to take the soil from the digging, this makes it much easier to get all back without a mess after the digger has done its job and the slinky is in place. Both the slinky and the borehole piping will be bedded on sand to avoid puncturing with sharp stones, the soil can then be returned to the trench and the turfs relaid. There will be some extra soil to be removed, or used in landscaping for a rockery etc. The ideal season for this work will be when the weather is neither cold nor raining. The wetter it is, the greater the mess and the greater the tendency for the sides of the trench to collapse.
- If you have a bigger garden, and a vegetable patch that you can arrange to avoid planting until late spring, then this will provide the ideal site for either an horizontal ground loop, or ground network, or indeed a slinky. The ideal time to do the work is as above. Since the pipes will be buried under at least a metre of soil, they will be well below the roots of your vegetable crops and will not be endangered by your spade.
The actual Heat Extractors for both: the Air, Soil and Water heat sources will require housing. This could be a lean to shed, or under-stairs cupboard, or cellar, or a small room dedicated to housing the Heat Pump itself, plus accumulator tanks and associated electronic controls for the house's heating system. The volume of space needed will depend on the size of the house and the system installed, but a rough estimate would be for a minimum area of 3 x 2m that is tall enough to stand/operate in.
There are two techniques to avoid an hour of a cold house in the early morning, or hours of cold offices on a Monday morning.
- The Heat Pump can be left running all the time, preferable to avoid condensation forming in offices; and usually recommended by the manufacturers using the most efficient modern systems. However, this does mean that the installation is also using electricity all the time; and most people sleep better at a lower air temperature.
- The Heat Pump can be turned off at night, but an insulated Accumulator/Bulk tank can be plumbed into the system to store extra hot water from the day. This hot water can then be used to prime the system first thing in the morning until the cooled water in the piping can be returned to the pump and heated up from the fluid in the outside piping network/loop. This means that the house will warm up quickly as soon as the Heat Pump is turned on again.
When working well, with a well-designed pipe layout, a Ground Source Heat Pump can have a CoP (Coefficient of Performance) of 1:4 i.e. 1kW of electricity used to drive the system will result in 4kW of heat energy abstracted. However, the lower the temperature of the incoming fluid, the more electricity will be needed to abstract the same amount of heat; in this case the CoP can fall to 2.5 or below.
The depth or length of piping installed outside will depend on the amount of heating that you require the installation to supply; as well as the type of installation including the efficiency of the compressor and gas used, and the material used for the piping network/loop. Originally copper pipe was used because it is a very good conductor of heat. However, most companies now work with alkathene or pvc piping since it is easier to work with and a lot cheaper.
Ideally the system installed should be reversible; so that in summer, excess heat can be returned to the air, soil, rocks or water, thus cooling the house. This is an especially attractive feature in warmer climates. Though not so useful for the soil beneath a summer vegetable patch or the muds of a pond that are already being warmed by the heat of the summer sun, this can usefully help to increase the restoration of heat to the soils a couple of metres below the lawn, or to the bedrock surrounding a Ground Loop system.
Currently no payments equivalent to FiTs (Feed in Tariffs) are payable for non-electric heating systems. However, from June 2011 there will be RHI (Renewable Heat Incentive) payments for any installations completed after 15th July 2009. This will equate to the FiTs available for electricity generation. They are to be calculated to bridge the gap between the cost of heating by Conventional and by use of Renewables. A component will be towards the establishment of the system and the RHI will also offer a Rate of Return of 12% on the additional cost of Renewables, or 6% for Solar Thermal systems - the input from the sun being free - whereas Ground source has an element of electrical input to extract the heat and Biomass etc. requires growing and payment for the crop.Top of page
- The systems can be run with under-floor distribution or with radiators. In the first instance - suitable for new builds or conversions - a thermostat controls the temperature of the whole room or house. However, if radiators are to heat a whole room to an even temperature, then the radiators would have to be kept very hot. Inevitably there will be a temperature gradient across the room from the site of the radiator, and to deliver the heat to where it is required most will require careful thought. In this scenario the electrical input is often greater, depending on where the thermostat is situated.
- With all types of ground source heating, the distribution system does not have to be fitted throughout, but can be limited to specific rooms in the house. It should be possible to switch the flow to specific rooms off and on as required e.g. for a guest bedroom.
- It should also be possible to arrange the plumbing so that the house heating could be boosted by use of a wood fuel boiler or a solar thermal system; instead of using extra electricity to provide point sources of heat.
Pro's of the Borehole system
- The temperature of the water in the loop is at a much more stable temperature of 10 - 14ºC depending on whether it is into soil or rock - even the type of rock can make a difference.
- The CoP will hold good throughout the coldest winter. This means that
- The electrical energy needed to drive the system can be predicted accurately.
- The area required for the installation is merely 1sq.m. and could easily be disguised as an ordinary part of a patio.
Con's of the Borehole system
- It is more expensive to drill out the borehole than it is to dig a trench, ca. £1,200/ borehole (2009) for all the whole of the outside work - the cost of the inside work has to be added on to this - and as for all systems will depend on the size of the house and type of heating (under-floor/radiator).
- The nature of the spoil means that it will be unsuitable for a garden use and will have to be removed from the site.
Pro's of the Horizontal Loop and the Slinky systems
- They usually cost less to install in the ground than the Borehole system and the ground preparation is usually carried out by the owner, prior to the installation job, and therefore costed separately.
- The overall costs are approx £800 - £1,200 per kWh of installation. The cost of the heat distribution system within the house, be it radiators or under floor, has to be added onto this.
- If the Slinky is laid properly, then the spacing between the coils should not cool the soil unduly. This means that the Trench must be dug to 2m and the Slinky pulled out as it is back-filled with sand, so that the bottom of each loop is at 2m and the top is at a depth of 1.25 - 1.5m.
- In cases of new build or renovation, where the floors have had to be taken up, then the costs of an under-floor heat distribution system in the house should be the same as that for any other system.
Con's of the Horizontal Loop and the Slinky systems
- The owner is expected to do the preparatory ground work. This is therefore an extra cost that will need to be added to the quote from the outside installer when calculating the overall cost. NB some installers will do the whole job.
- If the pipes are close together, or overlapping as in a Slinky when laid flat, then the continual removal of heat and passing of cold fluid into the system will actually cool the soil. This will result in a lower CoP i.e. it will be necessary to use more units of electricity to drive the system for the same units of heat abstracted.
- The closer the pipe loops are to the surface the more they are influenced by solar radiation, or more importantly, the lack of it in winter.
As more households take this new technology on board, it becomes cheaper. The more work they get, the more the installation companies can benefit from the economies of bulk buying. Presently a lot of the gadgetry used is bulk purchased from USA to mainland Europe and is then sold on to the UK in small amounts. Once the usage in UK is high enough to bulk buy on its own behalf, or better still to manufacture in this country, the price of installations should fall towards parity with those in the USA.
For the present, the best way of getting a good deal is to decide on the things you would like to do, know how many kWh that you use currently each year, and then approach the companies advertised to assess the best deal for your situation.Top of page
For a detailed explanation see en.wikipedia.org/wiki/Heat_pump
The 35% of the infrared radiation that reaches the earth causes the behaviour of our winds, tides, water cycle (rain and evaporation) through the action of heating and cooling; as well as powering all of the flora and Fauna on the planet. A 1sq.m area of plant leaf intercepting 0.8kw is able to make use of ca. 0.1% of this to produce the chemical energy needed to power photosynthesis. A lot of the infrared radiation is reflected and much of the ‘stray’ energy causes the soil to heat up from the surface downwards. The top 3m are also heated from the earth’s crust, but the temperature in the top 1m is influenced more by the solar radiation reaching the surface and the season. The deeper one goes, the warmer the temperature becomes and the less it is affected by events on the surface; after all the molten core of the earth (centre at 6,500km below the surface) has a temperature of ca. 5,500°C. As cavers will know: in summer, caves always seem cool and refreshing when one enters from the heat outside; but on a frosty winters day, the same cave seems warm and cosy by comparison. In fact cave entrances are often found in winter by looking for patches of vegetation that, warmed by the air from below ground, are not white with frost/snow.
In most maritime temperate climates, at a depth of approximately 0.6096m (2 feet), the soil does not freeze. All Ground Source systems make use of the fact that the temperature from this depth downwards gradually increases. The temperature of the top 2 feet (0.6096m) varies widely with the ambient air temperature at the surface. But below this the minimum temperature will be >0°C (-32°F) and the max. can be as high as 10°C at 3m depth. This provides a varying level of heat throughout even the winter months. Obviously if the 2 feet above is frozen solid, then the temperature below will be nearer to 0°C than 10°C. In order to maximize the available temperature without going into mining, it is normal for the top of a ground source, piping network system, to be buried at a depth of 1 - 1.5m. Therefore, this is the depth of the whole of a horizontal network, whilst the Slinky - a favourite with many plumbers - having a diameter of approx. 1m can be set in the ground either horizontally at 1.5m or in a vertical slot with the top of the pipe at 1m and the bottom at 2m; or pulled out to form a spiral. The latter arrangement obviously gives the best heat extraction.
A simple heat exchanger is just like your refrigerator, in that the fridge takes heat out of the food put in to it and pumps it out into the kitchen. The heat exchanger takes heat out of the air or fluid that is pumped into it from outside, bulks it up using a compressor, and passes the final amount of heat on to the fluid circulating in your heating pipes. Electricity is used to power the compressor, as in a fridge, and to pump the circulating liquids around the pipework; though systems can be set up - and the early ones always were - that use the difference in density between hot and cold water to move hot water around the system. However, with the electric pump, the energy used works out at approximately 1 unit in for 2.5 - 4 units of heat out; so over all, it saves energy. The electricity needed to needed to run the heat pump can be supplied from the Grid, or it it could be generated by Wind or Solar installations, and stored in modern, highly efficient accumulators (batteries).
The Heat Pump itself consists of a closed loop of piping containing a condensable gas, or a gas such as carbon dioxide that remains in its gaseous state at normal atmospheric pressure until cooled to -78°C. When it is allowed to expand, so that its molecules are well spaced out, the gas becomes cold - or liquid - at which point it can absorb heat from the incoming air. This heating makes its molecules move faster and if a liquid it will be converted back to the gaseous state in the 'evaporator' chamber. The warmed gas is now passed through a compressor causing the molecules to knock into each other in the smaller space available. This procedure increases the heat of the gas, which is then transfered to the fluid in the central heating pipes. The molecules of the cooling gas in the 'condenser' chamber move more slowly until they either change back to the liquid state, or are so spaced out that they no-longer interact, this stage ends by passing the gas/liquid from the 'condenser' through an expansion valve into the 'evaporator' where the cycle begins again.
Basic chemistry explains this phenomenon by noting that in a substance in its gaseous state the individual atoms/molecules are far apart and rarely come into contact. However, as they are compressed into a smaller volume, increasing numbers of the molecules collide; the individual atoms are also jostled and the result is that the electrons surrounding the atoms lose energy with each collision and move from an higher to a lower energy level releasing heat in the process. A more pictorial way of thinking of it, is to consider the electrons as toddlers and the nucleus as their teacher. Whilst their teacher stops to talk they are are told to stay at her side. Stationary, they cool down, but their metabolism continues to store energy rich compounds in their muscles, where it remains as a potential source of energy. However, as soon as the teacher moves on they are free to run around again and the stored energy is released becoming kinetic energy. This energy is expended in mechanical activity and the heat produced.Top of page
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