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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.

2) Batteries:

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.

The solution:

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.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

3 comments - read them below or add one

David Hunt

David Hunt from Comment left on: 23 August 2011 at 9:15 pm

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.

Sorry but the advice here is written by someone with absolutely no understanding of feed in tariffs, solar pv, wind or domestic electricity supply.  It is absolute nonsense.  The better advice comes in the comments. You cannot and would not directly supply your heat pump via Solar or wind. You certainly wouldn't want batteries unless you were off grid. You would always want to be grid connected. The heat pump draws electricity as it requires. You generate your solar/wind energy as the weather dictates. One will never match the other at any given time. We have customers that install heat pumps and solar pv and/or wind to offset some of the extra electrical use of the heat pump. It is never a like for like equation.

And yes, the best advice is to use the renewable (solar/wind) energy as you generate it as you get your Feed in tariff (43.3p by the way) and you save the 12-14p you would be buying from the grid. Surplus to the grid gets just 3.1p. So the best benefit comes from using what you generate.

We have MCS for Solar PV, Solar Thermal, Wind Turbines and Heat Pumps. We design, install, commission and integrate them for many customers. 

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John Barker-Brown

John Barker-Brown from Kensa Engineering LtdComment left on: 23 August 2011 at 10:01 am

Hi Andy,

I'm not an expert in Feed in Tariffs, but the export tariff (3p) is additional to the generation tariff (43p).

What you ideally want to do is run what you can from the electricity you generate (as buying electricity is around 12p), but a mains supply will need to be used for the heat pump to overcome the problems above, particular at start up. 


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andypippa

andypippaComment left on: 21 August 2011 at 4:20 pm

Your last point on solutions confuses me. From what i've read any elec sold back only qualifies for 3p, so surely you are better running the heat pump along with other elec appliances and getting 43p for generate and use?

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