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Renewable Energy

Diagram showing underground pipework in a ground source heat pump system

Ground source heat pump

Geothermal heating case study: ground source heat pumps

What are they?

Just 1m below ground in the UK, soil remains at a steady 8-12°C all year round - a source of free heat that can be used to heat houses. Ground source heat pumps do this by working like a fridge in reverse. A fridge moves heat from inside a sealed box to the outside making the inside colder. A heat pump moves heat from outside a house to the inside making it warmer. Here’s how it works

The big picture. Is a heat pump it worth it,
financially or environmentally?

Ground source heat pumps claim to have an efficiency of between 300% and 400%, i.e. for every 1 unit of electrical energy used to run it, 3 - 4 units of heat energy are delivered to the house.
But where does that electricity come from?

Question: What’s the efficiency of the overall system?

  • Assume for this question that, in the real world, an installed heat pump system delivers an efficiency of 300%. For every unit of electrical energy used in running the pump, three units of heat are transferred into the house.
  • Assume that the electricity required to run the heat pump comes from the UK National Grid – Big Hint

Answer

Answer

Answer: 1:1 - 100%

If the electricity used to run the heat pump comes from fossil fuels, for every 3 units of chemical energy entering the power station as coal or oil, only 1 unit of electrical energy arrives at your wall socket (explanation). 

When that 1 unit of electrical energy is used to run the heat pump it transfers 3 units of heat into your house.

Thus same amount of heat energy is transferred into your house as was originally used at the power station in generating the electricity. This is little different to using a modern gas boiler, where up to 95% of the chemical energy is turned to heat in your home.

So in financial terms, you pay less money for the heat gained. But in environmental terms, the system produces the same amount of CO2 as an ordinary gas boiler.
However, a heat pump can be pollution free IF you source the electricity used to run it from a renewable energy supply. Such sources could be your own solar panels, or from an energy company that generates electricity from wind, water or solar power.

Source: The Green Building Bible, Volume 1, Ed: Keith Hall 3rd Ed 2006

 

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