Will this heater idea work

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 22/09/2023 16:09:47:

Years ago, in a TV series … Dick Strawbridge showed a thermal energy store using a pit full of crushed glass

But I think it was only supplying night-time warmth for his greenhouse

MichaelG.

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Haven’t found a video, but here’s a reasonably detailed write-up explaining the principle:

**LINK**

http://www.reuk.co.uk/print.php?article=Solar-Greenhouse-Heat-Sink.htm

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bazyle on 22/09/2023 20:15:53:

Most of the adverts for '200w solar panels' on ebay won't even produce 10W if you work out the area of them. […]

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Which reminds me … there’s a handy but of ‘schoolboy physics’ here:

**LINK**

Measuring the Power of a Solar Panel

MichaelG.

[Russell EberhardtParticipant @russelleberhardt48058]

Just been doing a bit of thinking about solar panels. My weather station records incident solar radiation. Here, in the south of France during the winter solstice the maximum I recorded was 335 W/square metre at 12:49 pm on a sunny day. The average over 24 hours was about 50 W/m2. Now the conversion efficiency of solar panels is about 15 to 20 % so I would be getting, at best 10Wh of electricity. Depending on your lattitude in the UK you would be getting about half that (on a sunny day).

How big is your 300W panel?

Russell

[not done it yetParticipant @notdoneityet]

I’ll keep out of this thread, except to say that one solar panel will be about as useful as a chocolate teapot in the winter period. Better posted on s renewable energy forum.

Too much twaddle about how good chinese diesel heaters are and kerosene purchases. Also some misunderstandings re solar panels? Natural gas is as cheap, or cheaper than oil at the present time, btw.

[Nigel Graham 2Participant @nigelgraham2]

Sand or crushed glass?

Chemically they are the same so theoretically each kg of either would store the same heat for the same input.

However the problem may be that of heating enough of the substance to be effective. It is a poor conductor of heat and the coarser the particles I would expect the less conduction through the mass due to the intervening air, also a poor conductor.

Might a more effective approach be a solar water-heater that transfers its heat to the sand or glass via a matrix of tubes, rather like a boiler "inside out"? Or use storage-heater bricks?

Certainly no harm experimenting as you don't run into problems with dodgy fuels, fire safety, badly-made oil-heaters, etc.

My brother has installed a very effective solar-panel water heater on his home's flat roof, using two commercially-made panels plus one he built from an ordinary central-heating radiator painted matt black, inside a suitable glass-fronted enclosure. I can vouch for it having used the shower heated indirectly by the installation. (They also have an instant-heat shower for when there is not quite enough of Old Sol, in Southern Scotland.)

For my workshop (concrete-block single-skin walls with internal insulation, wriggly asbestos-cement roof with an insulated ceiling, crude "double-glazing&quot, I use a small electric radiant heater when really necessary, but warm clothes and an hour of machining seems enough for bearable conditions.

[duncan webster 1Participant @duncanwebster1]

Water has a far higher heat capacity than glass, by a factor of ~5 so just use a big tank of water. The only advantage of adding glass to the equation is it's heavier, so takes up less volume, but only by about factor 2.5 even for solid, less for crushed, so I think you'd be better of with just water. The potential advantage of electrical solar panel is you can get a higher temperature, so need less mass, like a storage heater.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Nigel Graham 2 on 24/09/2023 22:15:50:

Sand or crushed glass?

Chemically they are the same so theoretically each kg of either would store the same heat for the same input.

However […]

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I think the ‘Strawbridge’ design used crushed glass for two reasons:

1. He was making a clever TV series, to get people inspired
2. The glass, being crushed bottles normally destined for re-processing was much coarser than your average sand

The design used a small fan to draw air through the ‘heat-reservoir’, thus making better use of the mass.

Surface area to volume ratio would be enormous compared with a pit full of sand … and therefore it should work [at least sufficiently well for demonstration purposes] in a greenhouse.

It’s all fairly well documented in the link I provided 22/09/2023 21:54:50 but I don’t think enough emphasis was ever put on the fact that the ‘packing density’ of the glass product is very significant.

MichaelG.

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This is worth a look:

https://gaskellswaste.co.uk/recycling/glass-reprocessor/#:~:text=Glass%20recycling%2C%20or%20glass%20reprocessing,quality%20to%20the%20end%20product.

Edited By Michael Gilligan on 25/09/2023 06:55:36

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 25/09/2023 06:48:09:

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[…]

Surface area to volume ratio would be enormous compared with a pit full of sand

[…]

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Sorry … I expressed that rather badly

What’s important is that the useable surface area to volume ratio is enormous

Drawing air through the mass, using a small fan requires the porosity to be optimised.

[ The sand has the higher surface area, but would require much more energy to draw air through it. ]

MichaelG.

Edited By Michael Gilligan on 25/09/2023 08:10:07

[Nigel Graham 2Participant @nigelgraham2]

Thank you for that explanation, Michael.

In which case I wonder if moderately fine gravel would work just as well and may be easier to obtain. Flint gravel is the same material as sand and glass – silica- but the type of stone might not matter very much.

[noel shelleyParticipant @noelshelley55608]

Having seen the difference bringing a solar panel off the sun by even a few degrees makes, any figures as to output would need taking with a generous portion of salt  Since the OP has welding gear why not make a small wood burner ? The fuel will be free and if built to be almost air tight will be both economical and controlable. OR plan B make a larger one and only have to fuel it once every 24Hrs. All shut down mine was known to go 36Hrs on a fill. Pallet wood is free on most industrial estates ( and other places ) and a small circular saw will convert it into fuel. A walk in the country will keep you warm and a few bits of wood picked up will keep you warm when you get home ! Noel.

Edited By noel shelley on 25/09/2023 10:00:35

[SillyOldDufferModerator @sillyoldduffer]

Posted by duncan webster on 24/09/2023 22:42:03:

Water has a far higher heat capacity than glass … so I think you'd be better of with just water…

A sound engineering explanation for why heating and cooling systems often contain water!

Not the whole story though. Water being liquid adds an extra problem, which is the need for leak-proof containment. That bumps up the initial cost and adds a maintenance problem. In contrast glass doesn't require a well-made container, the system is low maintenance, and sand on the carpet is trivial compared with hot dirty water. However, for a little more dosh, cast-iron or fire-brick store more heat than glass, which might be important.

Solid heat storage systems create a new problem: how is the heat to be transferred to where it's needed? Hot water can be piped through radiators. Solid usually means fan-blown air, requiring trunking etc. Despite needing a fan, the system is much more reliable than gas central heating, which might be more important.

Which brings us to the cost of fuel : gas is cheap (at the moment), electricity expensive (at the moment). Coal is messy hard work and has to be delivered into a bunker. Oil is cleaner, fairly cheap (at the moment), and has to be pumped into a large storage tank. Wood is hard work, needs dry cover, and the cost depends on where you live.

My view is all heating systems have pros and cons, making it unwise to generalise. Engineers have to understand the full requirement and come up with a matching answer. Considering the full requirement often changes the answer, for example, Vehicle Diesel Heaters come with a long list of disadvantages making them inappropriate for heating homes. In the same way, the clean convenience of electricity often outweighs the advantage of cheap fuel, and costly but reliable local fuel is a better bet than unreliable cheap imports. There are many possibilities, and few shortcuts!

Dave

[John HaineParticipant @johnhaine32865]

Phase change heat storage is the way to go though probably beyond the scope of home construction.

Plentigrade

**LINK**

[duncan webster 1Participant @duncanwebster1]

My response was in answer to Nigel's suggestion which used solar water heaters. If you're starting with electricity, I wouldn't mess with water

[Martin KyteParticipant @martinkyte99762]

Posted by John Haine on 25/09/2023 11:36:41:

Phase change heat storage is the way to go though probably beyond the scope of home construction.

Plentigrade

**LINK**

Not sure about the beyond scope comment, there are various companies experimenting with encapsulating waxes for mixing with plaster etc to construct rooms that ‘hang’ at a certain temperature. The idea is that during the day when the solar gain from windows is high the room temperature rises to the point where the encapsulated wax changes phase from solid to liquid. So the room absorbs energy but doesn’t get any hotter. In the evening when the outside temperature drops the reverse happens and the beads give out heat as they solidify and hold the room at a comfortable temperature. The result is less burden on the air con or the heating system and it’s totally passive to boot.

regards Martin

[MacolmParticipant @macolm]

The disadvantage of water as a thermal store is that the maximum temperature must be less than 100C (unless the store is constructed like a steam boiler with all that entails). If it were a good choice, electrical storage heaters would use it. Instead, the material they use stores much more heat by exploiting a large temperature range, even though the specific heat capacity is less than water.

The Sunamp idea is good as a hot water store because water taken from storage is delivered at a constant temperature until the capacity is exhaused. The inverse, "wax coolers" using latent heat have been used for decades for rocket and missile electronics, since there is no air for cooling once out of the atmosphere.

[Michael GilliganParticipant @michaelgilligan61133]

Not straying too far off-topic, I hope

Does anyone have experience of these: **LINK** https://sahp.info

Solar Assisted Heat Pumps for domestic hot water

… I am tempted by the SAHP 130

MichaelG.

[SteviegtrParticipant @steviegtr]

In Spain there were lots of these on rooftops. Free hot water.

Not sure how good in uk they would be

Steve.

[File HandleParticipant @filehandle]

One suggested way of heating a greenhouse is to use 45gal oil drums painted black and full of water.

[Michael GilliganParticipant @michaelgilligan61133]

SAHP … They tell a good story, but seem very coy about the pricing

**LINK**

Solar Assisted Heat Pumps

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Reality check : https://energysavingtrust.org.uk/advice/solar-assisted-heat-pumps/

Solar assisted heat pumps

MichaelG.

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Correct me if I’m wrong, but that looks like something close to 100years to amortise my cost 

Edited By Michael Gilligan on 26/09/2023 19:43:27

[DMBParticipant @dmb]

Greenhouse heating? Only grow stuff off floor, on staging. Keep a rabbit on the floor, its body heat will do the job for the price of some bunny food!

[Michael GilliganParticipant @michaelgilligan61133]

Posted by DMB on 26/09/2023 21:04:03:

Greenhouse heating? Only grow stuff off floor, on staging. Keep a rabbit on the floor, its body heat will do the job for the price of some bunny food!

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… which you can grow in the greenhouse !
Is this an example of perpetual motion[s]

?

MichaelG.

[DMBParticipant @dmb]

Noel, that's all very well, burning free pickup wood, like we do at the club for a BBQ, but Brighton and Hove council are not alone in snotty attitude to woodburning. I can smell why, cold evenings the air is very wood smokey here.

John

[DMBParticipant @dmb]

MichaelG,

I consider that 'coy' pricing = hiding something! Beware!!

John

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