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Author Topic: High temperature thermal store/power generation concept  (Read 1599 times)
Ajarworks
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« on: June 29, 2017, 08:59:42 AM »

Here's an idea I've been thinking about; evacuated solar tubes can have a tip temperature in excess of 200 degrees centigrade (one figure I saw quoted was 274C) so would there be any way of storing heat at this extreme? Then I came across a heat transfer fluid which can work at temperatures up to 345C

So, deep breath, here's the concept;

Set up say four 30 evacuated solar tube arrays producing, on a sunny day, roughly 8KW each and circulate the heat transfer fluid through their manifolds to a highly insulated (surrounded with vermiculite possibly) 5000 litre steel tank filled with a high thermal mass material (100mm diameter flint pebbles spring to mind)

Now you have shed loads of stored energy you can pump it to a Sterling engine generator of which there are many becoming available thanks to the research into CHP boilers. A 3KW unit would provide almost enough power for an average household.

Other uses would be the rapid recovery of a hot water cylinder via the secondary heating coil or pumped to an underfloor heating manifold.

In the winter the system could be maintained by sending the heat transfer fluid to the heat exchanger of a wood or pellet burner.

I did originally consider vaporizing water to drive a steam turbine to generate electricity but the Sterling engines are so much neater.  

Any thoughts or anyone able to put some meaningful numbers to the idea gratefully received  fingers crossed!

    
« Last Edit: June 29, 2017, 02:38:27 PM by biff » Logged
Westie
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« Reply #1 on: June 29, 2017, 11:07:41 AM »

An interesting idea.  I can confirm the tip temp, out of manifold, reaches 240C in sunlight (yes, I burnt myself installing my system;) . I don't see the point of using the heat to generate electricity, PV and battery would be my preference. But a high density high temp heat store sounds interesting. The manifold is brazed so it should be okay for that temp. Heat paste would need to be high temp. What pressure would the system have to hold to stop it boiling?



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4kwp south facing array  SMA 4000TL grid connected.  2x30tube Navitron solar thermal panels (east/west). Arada 5kw S/C WBS. 25000Ltr underground rain water tank. KTM E-Bike  Cool
Ajarworks
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« Reply #2 on: June 29, 2017, 01:17:40 PM »

Atmospheric pressure would be fine. Therminol boils at 395 degrees C. 
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stannn
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« Reply #3 on: June 29, 2017, 04:07:31 PM »

Pedant at work.
Please forgive my intrusion Ajarworks but I believe that the Reverend Stirling did sterling work inventing something quite remarkable.
Now carry on with an interesting line of thought. Wink
Stan
.....and whilst I'm at it Mart, the plural of roof is roofs. fingers crossed!
« Last Edit: June 29, 2017, 04:19:22 PM by stannn » Logged

2.45 kWp PV (Navitron supply), 40 evacuated tubes (Navitron supply), Clearview 650 log burner with back-boiler heating cottage and water, 2 off 50W border collies, 1 off 35W cat, 1 off 25W cat.
dhaslam
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« Reply #4 on: June 29, 2017, 05:00:46 PM »

I have three sets of 30 tube panels and on  very sunny day the output could reach about 11kW. However only about one day in six is sunny.  If you take blue a  maximum of 50% efficiency less storage losses you could be down to an average of a few kWh output per day.




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DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
RIT
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« Reply #5 on: June 29, 2017, 11:31:17 PM »

Any reason why you would not just install a PV system with a large battery store? While the collection efficiency of PV is not as high as evacuated solar tubes, the thermal store is going to suffer from a lot of inefficiencies when trying to maintain a temperature 200C+ above ambient. Such a store would also suffer from a very long reheat time if it was too cool down. The Stirling engine will also suffer from very low overall efficiency if the aim is to produce electricity from the heat store. Firstly due to the limits of efficiency of the engine itself and then because its output will be used to drive a generator which would then feed an inverter.

The winter requirment maybe better meet by directly driving a Stirling engine/generator/inverter from the burner and just charge up the batteries. *

Also as I understand it while a product like Thermionl has a nice high boiling point, it also has a much lower vapour flash point (below 200C). The result is that an installation will have to be done with a lot of care.



* While the UK/EU seems to have spent time focussed on Stirling engines for CHP solutions, another option comes from the USA with products such as this

         http://www.tegmart.com/thermoelectric-generator-products/devil-watt-teg-power-100-watt-water-cooled-wood-burning-stove-thermoelectric-generator/

At the moment the tech seems to all be work in progress (unless you wish to pay a lot), but solutions do seem to be getting larger and cheaper over time. In the future such TEG based devices could provide a way to generate electricity directly from evacuated solar tubes, but they are just to costly at the moment and they also need a place to dump all the extra heat (maybe a great idea if you own a pool).
« Last Edit: June 29, 2017, 11:57:07 PM by RIT » Logged

2.4kW PV system, output can be seen at  - https://pvoutput.org/list.jsp?userid=49083

Why bother? - well, there is no planet B
renewablejohn
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« Reply #6 on: June 30, 2017, 12:05:54 AM »

Ajarworks

Thermal oil and Thermal oil steam evaporators have been discussed many times on here heated by solar or biomass going back to 2007.
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pdf27
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« Reply #7 on: June 30, 2017, 02:03:38 AM »

One comment: 250C or so is the stagnation temperature - i.e. the temperature they reach when you aren't extracting any heat, which is when the losses in the system balance the gains. Take any useful amount of heat out, and the temperature drops significantly - for the 8kW number you're probably down to domestic hot water temperatures or so (if you look up the DIN CERTO certificate you can work out the power versus temperature curve for a given solar input). That will hammer the efficiency of any Stirling engine solution, which will probably leave you vastly less efficient than a PV solution. It's one of those things that can be made to work, but is only worth doing as a hobby or if you can get all the bits and your time for free.
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