Three things to consider before running a heat pumps with solar or wind power
Posted by John Barker-Brown on 30 June 2011 at 10:46 am
There is currently a lot of interest in becoming completely self-sufficient in terms of energy and heat. Security of supply, being green, low running costs, etc are all motives for doing this and more and more people are asking is it possible to run their ground source heat pump from their solar pv panels or small wind turbine.
To do this, there are a number of considerations that need to be taken into account.
1) Start up current:
Heat pumps, due to the compressor; generally have a high starting current. This increases with size. While this can be mitigated by using soft starts on the units the starting current can still be too high for standard PV/Hydro/Wind systems to cope with.
As you cannot guarantee that when you require heat is when the wind or sun is generating power, a battery bank is generally needed to store energy when there is a surplus. These batteries tend to be expensive and have a surprisingly short life. Batteries store DC current, mains power is AC. To convert DC into AC, an inverter is required. This inverter needs to produce an output which is of a similar quality to mains supply. A number of inverters provide a sine wave output based on a stepped output, these should not be used as the generated voltage is too noisy and can cause problems with connected equipment.
3) Amount of power required:
For a typical sized 10kW heat pump you would require at best 2.5kW of energy to run the heat pump. Depending on the heat distribution method or if domestic hot water is being produced it could be even higher (4kW+). To obtain this type of power you would require at least 25m2 of south facing roof covered in PV, (without the other household electricity consumption). If wind is being considered then, for example, a 5.5m diameter wind turbine at a height of 12m produces on average 11kWh per day. Enough to run the heat pump only for just under 3 hours.
Connect the heat pump to the mains supply. Generate on-site electricity using PV or wind, but sell what you generate back to the grid and claim the Feed in Tariffs. This way you overcome the problems above and significantly reduce (if not eliminate) the nett amount of electricity you use.
About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.
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