PowerSwitch

Coal mines to be transformed into multi-million pound renewable energy system:
https://bdaily.co.uk/articles/2020/04/2 ... rgy-system

Abandoned coal mines in South Tyneside are set to become an integral part of a new multi-million pound renewable energy project. Geothermal energy will be drawn from mines in the former Hebburn Colliery to heat council-owned buildings in the town as part of a £7m scheme developed in collaboration with the Coal Authority and Durham University. The project will see water extracted from the flooded mines by drilling vertical boreholes 300-400m underground, before being compressed to a much higher temperature and then distributed to the heat network.

The project, which has secured preliminary approval for £3.5m funding from the European Regional Development Fund, is set to take a step forward next week with the expected appointment of a main scheme designer by South Tyneside Council. Cllr Joan Atkinson, lead member for area management and community safety, commented: “This is a highly innovative scheme, which will be one of the first council minewater district heating systems in the UK. “It is expected to deliver a reduction of 319 tonnes of carbon emissions a year, which will make it a key component in our drive to make the council carbon neutral by 2030. “This is an exciting project which will make a significant contribution to our ambition for carbon neutrality and a greener, more sustainable borough.�

Mark wrote:,...,.........before being compressed to a much higher temperature

The journalist obviously had to have a heatpump explained to him. Good scheme though.

Believe that other former mining areas, such as Oldham, are also thinking along similar lines

I would like to know how much thermal energy can be transferred per mine before a steady state temperature inside can is achieved. I would also be interested in how much energy in the form of carbon dioxide emitting electricity generation is 'saved' by this.

The figure 319 tonnes of carbon emissions seems to give a clue but is also misleading.

It seems to me that this is some form of ground-source heat pump.

BritDownUnder wrote:I would like to know how much thermal energy can be transferred per mine before a steady state temperature inside can is achieved. I would also be interested in how much energy in the form of carbon dioxide emitting electricity generation is 'saved' by this.

The figure 319 tonnes of carbon emissions seems to give a clue but is also misleading.

It seems to me that this is some form of ground-source heat pump.

Yep!! That's just what it is with the ground coil replaced by a bloody great pool of earth warmed water. The total available output would be proportional to the size of the flooded mine and the heat input from the ground times a COP of about 3.

There are other systems of this sort where they drill holes into granite, frack it and then pump water through to extract the heat from the radioactive decomposition of the granite - a sort of nuclear energy powered ground source heat pump although they sometimes don't need the heat pump if the temperatures in the rock are high enough.

Iceland does a similar thing with their volcanic heat source but they definitely don't need the heat pump.

kenneal - lagger wrote:

BritDownUnder wrote:I would like to know how much thermal energy can be transferred per mine before a steady state temperature inside can is achieved. I would also be interested in how much energy in the form of carbon dioxide emitting electricity generation is 'saved' by this.

The figure 319 tonnes of carbon emissions seems to give a clue but is also misleading.

It seems to me that this is some form of ground-source heat pump.

Yep!! That's just what it is with the ground coil replaced by a bloody great pool of earth warmed water. The total available output would be proportional to the size of the flooded mine and the heat input from the ground times a COP of about 3.

There are other systems of this sort where they drill holes into granite, frack it and then pump water through to extract the heat from the radioactive decomposition of the granite - a sort of nuclear energy powered ground source heat pump although they sometimes don't need the heat pump if the temperatures in the rock are high enough.

Iceland does a similar thing with their volcanic heat source but they definitely don't need the heat pump.

Now it is a Sunday I have had time to think about this. It is also a refreshing change not to have to constantly read about Covid-19 or what guns to buy on this forum. With help from this nice document from the British Geological Survey I found a few facts.

In the UK the ground gets hotter as you go down by about 25 C per kilometre and the average heat flux from the centre of the earth outwards is about 100 milliwatts per square metre.

Thinking like the late David MacKay for a moment this will give a water temperature underground at 400 metres of 10 C (average ambient ground temperature of the soil in the UK) plus 10 C as it is 400 metres closer to the centre of the earth. You will also get a flux of 0.1 watt per square metre of mine surface area which is about one thousand times less than the average solar flux that a square metre of the earths surface receives. Given the surface area of all the galleries and longwalls (I think in Australia the technical term is 'panels' where the coal has been mined from) of a coal mine is probably quite large - maybe 1 square kilometre or 1,000,000 square metres you could probably get 100,000 watts of heated water at 20 C to play with. So about 100kW of heating.

A quick scan of the internet for the amount of carbon dioxide produced per kwh of electricity generated by gas (the UK's favourite source of power these days) gives 0.2 kg of CO2 per kWh. So 319 tonnes of CO2 will equate to 1,595,000 kWh per annum or a constant generation of 182 kW. This number squares well with the earlier guess of heat available from the mine.

We could discuss the efficiency of the heat pump used but the heat still has to come from somewhere to 'recharge' the heat from the mine otherwise you will lose efficiency and end up with an iced up mine.

A while back in Australia I looked at geothermal heating via a ground source heat pump taking water from a borehole i would have had to get dug (the garden was too small to dig up and put coils of pipes under the lawn).
The cost would have been about 15000 pounds equivalent in dollars.
I did not go ahead with it but the efficiency would have been quite good for heating and terrible for cooling. I would do about half heating (and hot water) and half cooling. Here in Australia the ground temperature is about 22 C year round so would have made a small heat pump heating to 45 C a COP of about 10 which I thought was great. The cooling COP was terrible though.

In the end I just use solar for hot water and air source heat pumps for heating and cooling. Over here it just makes more sense to use solar and air source heat pumps and grid electricity.

Does it make sense for the UK? Probably, but one coal mine will probably just about provide enough hot water to heat a commercial building and not a city.

There is a company called Star Refrigeration based in Scotland that makes megawatt scale heat pumps that get their 'heat' from sea water for large district heating schemes and freezing works and have a COP of about 3 as well. I think the working fluid is ammonia.