The Oil Drum: Net Energy | Wave/Geothermal - Energy Return on Investment (EROI) (Part 6 of 6)

98 comments on Wave/Geothermal - Energy Return on Investment (EROI) (Part 6 of 6)

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BobE on May 15, 2008 - 4:53am

I agree with Nick Rouse above.

Ground source heat pumps are only sold as 'geothermal' because it sounds good. Basically the heat comes from the air by making the ground a bit colder than it would have been otherwise.

Air source heat pumps have a problem of the heat exchanger becoming coated with ice in damp climates like the UK. This means that their midwinter performance deteriorates rather seriously and at worst they may have to revert to plain resistance heating. There was interest in them in the UK in the 1970s but, as I understand it, the electricity industry dumped the air source technology when they realised that it would not reduce the peak winter load on the grid.

Ground source heat pumps get round this by using the thermal mass of the ground to coast through midwinter cold periods without performance dropping too much.

You have to be careful in assigning an EROI to heat pumps. Most are electrically operated, so they are really part of the electricity distribution network rather than an actual energy source. A true EROI calculation needs to start with the EROI of the fuel used to generate the electricity. Their advantage is that they can compensate for the efficiency loss at the power station. If 3 units of coal go in to make 1 unit of electricity, then the heat pump with a COP of 3 can convert this back to 3 units of heat.

The same applies to engine-driven heat pumps. The Festival Hall in London was originally designed to use diesel driven heat pumps sucking heat from the Thames. I'd guess at a diesel to heat COP of around 1.5 for this, so the overall EROI would be the EROI of the diesel fuel multiplied by 1.5 less a bit for the energy used to make the diesel heat pump.

Finally can I ask again -

Where is the EROI of building insulation as a potential generator of negawatts (i.e energy not used)? Surely this is just as valid as positive energy flows in the bubble diagram?

I reckon it has potential EROIs in the range 10-100 and potential equivalent negawatt flows in the US of the order of 5 quads or more.

BobE

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DaveMart on May 15, 2008 - 6:31am

Bob, I referred to ground source heat pumps as geothermal because that is the commonly accepted term, not out of ignorance as to their source of heat.

On the issue of icing etc, that may well have been a problem in the 70's, as might inadequate performance, but it seems clear that the latest Japanese designs have overcome both, and have very good performance.
They are extensively used in the island of Hokkaido, where the winters are harsh and humidity is often high.

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