Solar thermal

[Nitrams]

Are these evacuated tube arrangements any good as a source of heat. Can you sucessfully generate 250-500kw heat with a large arrangement? Is there any FIT still available. It would need a large buffer tank to store the heat. Mostly for summer use so potential good levels of radiation.

 

[Wisconsonian]

Yes they work. I've always heard that a flat plate collector is better for summer conditions. Evacuated tubes are better for higher temperature differences, and cloudy/overcast conditions.

 

[steveR]

Are these evacuated tube arrangements any good as a source of heat. Can you sucessfully generate 250-500kw heat with a large arrangement? Is there any FIT still available. It would need a large buffer tank to store the heat. Mostly for summer use so potential good levels of radiation.

My understanding from someone who was fitting these, is that PV is now cheaper, and it is easier to manage, simply dump the output into immersion heaters.

I have been tempted to get an array to top up the thermal store, but more PV might be the way I go.

 

[Wisconsonian]

PV is MUCH easier to install and manage. If you're talking 250KW, then flat panel will be cheaper as long as you're looking at reasonable temperatures.

Then again, I don't know if the things are even available these days...

 

[Exfarmer]

Solar thermal is far more efficient though at harvesting energy, the industry suffered through the FIT subsidy as it never qualified.
It would make sense to have panels on all house roofs mose so than PV

 

[Nitrams]

Any ideas on purchase cost/kw

 

[steveR]

Yes they work. I've always heard that a flat plate collector is better for summer conditions. Evacuated tubes are better for higher temperature differences, and cloudy/overcast conditions.

I remember seeing a demo of the tube collectors at the CAT in Wales, 25-30 years ago and being amazed what they would produce on an overcast day, but OMG they were so expensive.

I have a mate with a small array of them on his roof who loved the performance initially, but not so enamoured at their reliability, as the individual tubes broke down. Not seen him for a while, but I think that he was moving to PV

 

[renewablejohn]

Its a shame the technology is far superior than PV but as others have said Fits killed solar thermal. With the right solar thermal technology all uk homes could become self sufficient for heating electric and cooking but the big six saw the danger and snuffed out the competition. Just looking at the maths it was a no brainer in efficiency terms PV 20% Thermal 80% PV for 12hrs a day if your lucky Thermal 24/7. PV only on sunny days Thermal also worked on cloudy days. Biggest problem with thermal was the wrong technology was promoted using water instead of the higher temperature thermal oil.

 

[steveR]

Its a shame the technology is far superior than PV but as others have said Fits killed solar thermal. With the right solar thermal technology all uk homes could become self sufficient for heating electric and cooking but the big six saw the danger and snuffed out the competition. Just looking at the maths it was a no brainer in efficiency terms PV 20% Thermal 80% PV for 12hrs a day if your lucky Thermal 24/7. PV only on sunny days Thermal also worked on cloudy days. Biggest problem with thermal was the wrong technology was promoted using water instead of the higher temperature thermal oil.

I have got to ask, how you get thermal heat production 24hrs/day, unless it is pulling heat from the air in teh summer??

 

[Exfarmer]

Its a shame the technology is far superior than PV but as others have said Fits killed solar thermal. With the right solar thermal technology all uk homes could become self sufficient for heating electric and cooking but the big six saw the danger and snuffed out the competition. Just looking at the maths it was a no brainer in efficiency terms PV 20% Thermal 80% PV for 12hrs a day if your lucky Thermal 24/7. PV only on sunny days Thermal also worked on cloudy days. Biggest problem with thermal was the wrong technology was promoted using water instead of the higher temperature thermal oil.

heating by irradiation is subject to exactly the same constraints as PV you have only so much energy which can be extracted from the solar energy emitted by the sun. Virtually none of this comes at night although we have recorded meters ticking over a tenth of a unit in the night on a 150Kw array. However this may be just one click out of one thousandth of a unit.

however I suspect that you mean that this solar derived heat can be stored and used by the means of superheated oil, or possibly in the future molten salt. however although you can raise oil to around 400 C before it start to degrade, it only has half the specific heat of water and a relatively low heat transfer speed which limits its use in a domestic situation. It is however a lot safer than using water as it does not boil, but we should not underrate the dangers remembering it was a favourite weapon in olden times.

 

[renewablejohn]

I have got to ask, how you get thermal heat production 24hrs/day, unless it is pulling heat from the air in teh summer??

Same way as you can have a shower first thing in the morning because you have stored the thermal heat in the tank overnight whereas the PV has to wait while the sun comes up for the electric heater to heat the water. Technically Solar tubes filled with thermal oil take approx 3 hours to cool down after the sun goes down so all that time the thermal store will still be being heated. During the summer there is not long between sunrise and sunset and solar tubes actually start working before the sun is over the horizon.

 

[Exfarmer]

Same way as you can have a shower first thing in the morning because you have stored the thermal heat in the tank overnight whereas the PV has to wait while the sun comes up for the electric heater to heat the water. Technically Solar tubes filled with thermal oil take approx 3 hours to cool down after the sun goes down so all that time the thermal store will still be being heated. During the summer there is not long between sunrise and sunset and solar tubes actually start working before the sun is over the horizon.

what sort of temperature can standard domestic set ups heat the thermal oil to?.
I guess that these figures of 400 degrees would be in a commercial set up, effectively a solar furnace.
the oil in the tubes will only be able to heat thew store if the heat is being drawn offence no new energy will be coming in and heating before sunrise will be extremely limited and thanks to refraction. you have to also remember the huge issue with PV is the same for Thermal and that is 75% of the suns energy falls on us between April and October when we are not gross consumers of heat

 

[renewablejohn]

The normal operating range your looking for is 200C to 150C. 200C is important for the use of thermal oil cookers as used in bakeries and 150C is the key temperature for steam production for use in conventional steam engines. All of this can be achieved with the normal bulb and fin solar tubes but with thermal oil added which makes them approx 25% more efficient with the oil than without. We are still consumers of heat even in summer as we still cook which can be done using thermal oil and you can also get thermal oil heated washing machines.

 

[Wisconsonian]

Really need to know what temperature you're aiming for with this 250-500kw. Flat plate will be more efficient and cost effective at lower temps and clear sky, evacuated tubes at higher temps and clouds.

Here, there was a solar push in the Carter administration, hot water solar was not ready, or most installers got it very wrong at least. Reagan famously "ripped the solar panels off the white house roof" and solar was set back twenty years by poor govt policy, promoting a tech that was not ready. Mostly, solar hot water has been left in the dark ages, Solar means PV now.

 

[Nitrams]

Really need to know what temperature you're aiming for with this 250-500kw. Flat plate will be more efficient and cost effective at lower temps and clear sky, evacuated tubes at higher temps and clouds.

Here, there was a solar push in the Carter administration, hot water solar was not ready, or most installers got it very wrong at least. Reagan famously "ripped the solar panels off the white house roof" and solar was set back twenty years by poor govt policy, promoting a tech that was not ready. Mostly, solar hot water has been left in the dark ages, Solar means PV now.

Min 60c max 100-120c would be primarily a heat source for grain drying so heat delivered through large heat exchanger

 

[Wisconsonian]

unglazed hot air collectors would be the way to go for summer grain drying, I would think. If you need that big, 250KW, then hot water might be needed to move the heat reasonably from such a large collection area, but it would be a huge cost and complexity over hot air.

 

[renewablejohn]

Min 60c max 100-120c would be primarily a heat source for grain drying so heat delivered through large heat exchanger

Very simple with solar thermal tubes but at that temperature I would avoid water as heat carrier and use thermal oil instead. That way you will not need a pressurised system which will be much safer. Only downside is thermal oil pipelines need to be similar quality to gas pipelines. The last thing you want is being fried with hot oil. Have a look at the 20ft shipping containers designed for Asphalt storage as a cheap ready made thermal store.

 

[Nitrams]

The tin cladding sheets on the side of the shed got me thinking about this. They get v hot in summer, usually the drier is running. I thought about running a network of underfloor heating pipe up and down inside of the sheets, maybe spray foam them in. Then just pump it around in a loop through a heatexchanger in front the burner.

 

[Wisconsonian]

Yes, you're on the right track. The simplified version of that is a perforated cladding sheet with ductwork behind it. Warm air is drawn through the perforations, through the ductwork, and into the drier. No need for glazing, or heat exchangers. With a small product like grain with lots of surface area, I'd want the air once through and exhausted. Wood kilns will recirculate the air, as the wood releases water slower, being thicker. I'd bet the Alpine areas of Europe have some solar hay drying ideas, even if it's auxiliary to a heat pump/dehumidifier setup.

 

[flowerpot]

I heard of a plumber who got up on his roof and installed some old radiators, painted black. The water boiled. I don't know the end of the story!