Mike Moseley

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Welcome to my world of disconnected environmental, community and family projects.

I have been working in the renewable energy field for a number of years and this web site has been set up to give you a flavour of some of the projects that I have been involved in both professionally and as a keen, but slightly mad, DIY fanatic.
Mike

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May 2014 Specifying Ground Source Heating (this article is from my work site REHAU Ltd)

Some five or six years ago, ground source heating had a very high profile as a renewable and sustainable heating source, but the extraordinary rise in solar PV and biomass heating, fuelled in part by government policy, has taken focus away from some of ground source’s undoubtable benefits as a sustainable heating system.  With recently announced increases in the Government’s RHI rate for ground source heating and the imminent introduction of a domestic/homeowner RHI, perhaps now is a good time to look at what we need to consider when specifying a ground source heating system?

 

suffolk one

Mindful of not teaching Grandma to suck eggs, it is a good idea to look first at some of the basic principles of ground source energy and how it works.  Ground source energy is sometimes called geothermal energy, but the source of this energy is definitely not the earth.  Every square metre of earth in northern Europe receives about 800 W/m2 of solar radiation, about 13W/m2 of energy from rain and from the earth's core (below 300m) only 0.06W/m2. True geothermal energy comes from deeper than 400m, often kilometres down and beyond the discussion of this article.  From depths of 15m below ground (in the UK) is a constant temperature of 10 degrees C, corresponding to the average annual air temperature above.  From ground level to 15m the temperature is influenced by the seasonal air temperature above. In the winter, close to the surface, the ground temperature is colder and in the summer, the ground temperature is warmer.  Strangely, the temperature from 5-15m is slightly reversed, being marginally cooler in the summer and warmer in the winter.  This is due to the variations in the surface temperature, which can take up to sixth months to move through the earth and demonstrates that the earth can be a good thermal store in many conditions.  Further on, I will expand on how this can be used to our advantage.

Heating your house to 10 degrees C is not going to make you very comfortable though, so we need to boost this temperature to a level that is useful for keeping us warm.  We do this with a heat pump; a heat pump is just like your fridge, which is cold on the outside because it is dumping heat out the back.  If you put your hand around the back of your fridge you will feel it is quite warm.  The fridge has three separate, but very much interconnected, circuits.  The ground loop circuit has a water glycol mix coming out of the heat pump at freezing, or just below.  The ground loop heats this mixture to a temperature of about 5 degrees C and this heat is transferred, via a plate heat exchanger, to a refrigerant, which will turn into a gas at the corresponding temperature.  The gas can then be compressed with a pump and the increase in pressure results in an increase in temperature.  A second plate heat exchanger transfers the energy into a heating circuit and an expansion valve takes the refrigerant back to the original pressure and low temperature. 

These three circuits are inextricably linked and the higher the temperature of the heating circuit, the harder the heat pump has to work.  Ground source heating is not 'no energy’, but a ‘low energy’ system, and the amount of energy required to run the system is measured in the 'COP' (coefficient of performance) of the system.  For a system returning a ground circuit temperature of 5 degrees C and a heating circuit flow temperature of 55 degrees C, the COP will be just over 3, which means that for every unit of electrical energy used by the system, it will provide 3 units of heating energy.  55 degrees C is a typical flow temperature for a low flow temperature radiator system and this is the minimum you must achieve to run a heat pump heating system and get a reasonable COP.  Running high temperature radiators is not a viable option for this kind of heating.  If the heating is delivered by standard solid floor underfloor heating with a flow temperature of 45 degrees C, then the COP will be closer to 4 and, if you design the underfloor heating to run at a very low temperature of 35 degrees C, the COP will be nearer to 5. The room will still be at a standard temperature of 21, meaning that the flow temperature can be lower.

Another way of improving the performance of a ground source system is to increase the temperature at which the ground circuit returns to the heat pump.  I talked earlier about storing energy in the ground and, if you can recharge your ground source circuits with waste heat, either from summer cooling or waste solar heating, it is possible to increase the overall performance of the system, sometimes as much a doubling its COP.  If you are dumping energy from a high temperature source, such as solar, you must ensure that the pipework you use is suitable for the temperature, many are not.  Reassuringly, REHAU’s PE-Xa can be used from -40°C to 95°C. 

Standards for design of ground source systems can be found in a number of different sources, including VDI Guidelines 4640:

Thermal Use of Underground: Parts 1-4

GSHPA Vertical Borehole Standard

GSHPA Thermal Pile Standard

CIBSE Guide TM51 ‘Ground Source Heat Pumps’, released in Feb 2013

The UK Government’s RHI scheme includes the use of ground source heating and is payable in two tiers. Tier one is 8.6p/kWhrTh for the first 1314 hours of heating and 2.6p/kWhrTh for tier two beyond that.  Payment is over 20 years with increases linked to inflation, but all installations must be installed by a MCS accredited installer and it is currently only available for commercial projects.  A domestic system is likely to be available in spring 2014, but has not yet been finalised.  Interestingly though, a commercial project can be as small as two properties working on one heating system.  Most projects will require the use of a class 2 meter to ensure that you can claim your RHI payments from OFGEM.

Energy can be collected from the ground using a variety of different systems.  Boreholes can be drilled typically from 80 to 150m deep and ground source probes (two or four loops of pipe) inserted.  Field base systems (where a large area of ground is dug to a depth of 800mm and an array of pipes, such as those used for underfloor heating, are installed), look simple but the amount of soil that needs to be moved can be enormous.  Be very careful too if your installer wants to use the so called ‘slinky systems’, where a trench is dug and the trench filled with coils of pipe overlapping each other.  This is an ideal environment to encourage ice radii forming at the overlapping of the pipes, potentially freezing the ground.   A better compromise would be ground source spiral probes and these are available from REHAU.  Here, a 450mm diameter hole is drilled to 5m deep and the probes inserted in the bottom 3m of the hole, maximising output and minimising costs. 

Many commercial projects that are using large diameter structural piles use pipe work fitted inside the pile.  Deciding how much energy is available, and how you can extract it from the ground, is a complex issue.  Most commercial projects, where a number of boreholes are going to be drilled, start by installing a single borehole early in the job.  A thermal response test is then carried out, whereby hot water is pumped down the probe and there is a correlation between the heat loss from this hot water and the subsequent amount of energy that can be extracted from the probe.  This will require some investment before the job can be designed, but it is only when you know the thermal response that you can work out how many boreholes are required on any scheme; and this can have a massive impact on the overall cost.    For most domestic jobs, a simple 100m probe is nearly always sufficient.  Crude estimates of the local ground’s thermal capacity can be found on ThermoMap Project.  This covers most European projects, but detailed calculations still require a local thermal response test.

Having installed your boreholes or field based collectors, the system needs to be connected to the property and it is here that condensation can be a problem, with the water often circulating below freezing.  REHAU has a number of specialist products to manage your system, with manifold chambers (complete with manifolds), insulated header pipe (to prevent the loss of collected energy) and a full catalogue of accessories.

One final word of warning: when letting contracts for ground source heating systems, try to let the contract as a single package for the complete heating system, incorporating both the ground source, heat pump and internal heating, so that nobody can then blame anyone else for failures.  REHAU has a range of partners who can offer this service, from small domestic projects, right through to complex solar recharged ground sources systems.

Mike Moseley is a Commercial Manager for REHAU, based in their offices in the Building Centre.  He is a professional engineer with years of experience in the construction industry.  As well as providing REHAU’s external CPD training, he supervises REHAU's special projects, such as solar storage systems and extracting energy from tunnels.

 

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