Pssst! Wanna see my solar panel pictures?

[Yellow Fang]

Have a garden party. That'd make a lot of tea!

Globalti, could the excess heat power a Stirling engine? (and hence generate power)

http://en.wikipedia....Stirling_engine

Hmm, maximum Carnot efficiency is 1- (T[sub]c [/sub]/ T[sub]h[/sub])
T[sub]h[/sub] = 60C+273K = 333K
T[sub]c[/sub] = 20C+273 = 293K
Max efficiency = 1 - (293/333) = 12%

Heat input, Q[sub]1[/sub] = Heat output, Q[sub]2[/sub] + Work, W
There are 5 litres in a gallon.
The specific heat of water is 4.2kJ kg[sup]-1[/sup]K[sup]-1[/sup]
1 litre of water weighs 1kg
Q[sub]1[/sub] = 4.2kJ kg[sup]-1[/sup]K[sup]-1[/sup] x 25000kg x 333K = 3.50x10[sup]7[/sup] kJ
Q[sub]2[/sub] = 4.2kJ kg[sup]-1[/sup]K[sup]-1[/sup] x 25000kg x 293K = 3.08x10[sup]7[/sup] kJ
W = Q[sub]1[/sub]-Q[sub]2[/sub] = 4.2x10[sup]6[/sup] kJ
Also, W = 0.12 x 3.50x10[sup]7[/sup] kJ = 4.2x10[sup]6[/sup] kJ
4.2x10[sup]6[/sup] kJ / 3600sh[sup]-1[/sup] = 1167kWh

That's not bad actually.

 

[Arch]

Hmm, maximum Carnot efficiency is 1- (T[sub]c [/sub]/ T[sub]h[/sub])
T[sub]h[/sub] = 60C+273K = 333K
T[sub]c[/sub] = 20C+273 = 293K
Max efficiency = 1 - (293/333) = 12%

Heat input, Q[sub]1[/sub] = Heat output, Q[sub]2[/sub] + Work, W
There are 5 litres in a gallon.
The specific heat of water is 4.2kJ kg[sup]-1[/sup]K[sup]-1[/sup]
1 litre of water weighs 1kg
Q[sub]1[/sub] = 4.2kJ kg[sup]-1[/sup]K[sup]-1[/sup] x 25000kg x 333K = 3.50x10[sup]7[/sup] kJ
Q[sub]2[/sub] = 4.2kJ kg[sup]-1[/sup]K[sup]-1[/sup] x 25000kg x 293K = 3.08x10[sup]7[/sup] kJ
W = Q[sub]1[/sub]-Q[sub]2[/sub] = 4.2x10[sup]6[/sup] kJ
Also, W = 0.12 x 3.50x10[sup]7[/sup] kJ = 4.2x10[sup]6[/sup] kJ
4.2x10[sup]6[/sup] kJ / 3600sh[sup]-1[/sup] = 1167kWh

That's not bad actually.

And again in English for the Hard of Understanding?

I only know about Stirling engines in the vaguest sense, but that Wiki page seemed to suggest they can generate without the need for the high pressure of a steam engine. I've seen the model ones that can turn a very light crank, from the heat over a cup of tea or just from sitting in your hand....

 

[Globalti]

I guess it's all down to the size of your collector. Ours is only 20 tubes so we expect it to struggle to raise 160 litres of water from 8 degrees to 60 degrees. The bloke who sold me the kit says that if we added a second array we would have excess heat and boiling problems, not a major issue but not really desirable, hence my plans for a heat dump eventually.

My neighbour has two home-made flat collectors and they give him almost all his hot water in summer, he also has an array of PV panels the area of the side of a bus on his barn roof and he feeds back into the grid most of the time, he is actually a registered power station! His total energy bill last year was £75.00.

His car, by the way, is almost entirely filled with three milk float traction batteries, massive things, which occupy the boot and back seat. There are also three normal car batteries, which provide power for ancillaries like lights, wipers etc. shoehorned in between them with a rat's nest of wiring, relays and so on. The curious thing is that he took out the petrol engine and kept the clutch and gearbox so he drives it like a normal car. When you see the amount of space give over to the batteries you realise the limitations of lead-acid batteries for road vehicles.

 

[Yellow Fang]

And again in English for the Hard of Understanding?

I only know about Stirling engines in the vaguest sense, but that Wiki page seemed to suggest they can generate without the need for the high pressure of a steam engine. I've seen the model ones that can turn a very light crank, from the heat over a cup of tea or just from sitting in your hand....

Stirling Engines turn a heat difference into work, which can be turned into electricity. Unlike an internal combustion engine, the heat is applied externally, and it doesn't matter how the heat was produced, i.e. they don't depend on any particular type of fuel. They've been around for a long time, but they didn't take off initially because steam engines and then internal combustion engines were more efficient. Now there's a lot more interest in them.

The maximum possible efficiency of a heat engine in which the hot temperature was 60C and the low temperature is 20C is about 12% by my reckoning. You have to convert the temperatures into absolute temperatures in Kelvin by adding 273. The maximum efficiency (Carnot efficiency) is calculated by subtracting the cold temperature from the hot temperature and dividing the result by the hot temperature again. You get an answer between 0 and 1, which you multiply by 100 to get the percentage efficiency. The actual efficiency of the Stirling engine will be lower than this.

I think the heat content of the water is its mass multiplied by its specific heat and its absolute temperature, but I would have to check. I'm a bit unsure how water being ice under 0C affects the calculation.

 

[Shaun]

Wow ...

I've never developed any practical skills, well, not with D.I.Y./plumbing/decorating/building type stuff anyway, so I'm always well impressed when I see things like this (which doesn't look like a 'small' project by any means).

Well done.

Cheers,
Shaun

 

[beanzontoast]

I guess it's all down to the size of your collector. Ours is only 20 tubes so we expect it to struggle to raise 160 litres of water from 8 degrees to 60 degrees. The bloke who sold me the kit says that if we added a second array we would have excess heat and boiling problems, not a major issue but not really desirable, hence my plans for a heat dump eventually.

My neighbour has two home-made flat collectors and they give him almost all his hot water in summer, he also has an array of PV panels the area of the side of a bus on his barn roof and he feeds back into the grid most of the time, he is actually a registered power station! His total energy bill last year was £75.00.

His car, by the way, is almost entirely filled with three milk float traction batteries, massive things, which occupy the boot and back seat. There are also three normal car batteries, which provide power for ancillaries like lights, wipers etc. shoehorned in between them with a rat's nest of wiring, relays and so on. The curious thing is that he took out the petrol engine and kept the clutch and gearbox so he drives it like a normal car. When you see the amount of space give over to the batteries you realise the limitations of lead-acid batteries for road vehicles.

Indeedy. A cost-effective solution to that one would really change the way road transport evolves.

 

[biggs682]

we keep looking at having it done but cant make the final decision due to cost but at that price its got to worth doing , fancy doing ours

 

[twentysix by twentyfive]

This was also my concern; I simply did not want to be drilling or cutting the tiles. This panel is held on by four metre-long stainless steel straps, which bolt to each corner and are thin enough to slip under the tiles and then fold around battens fixed across the rafters. The 10mm pipes, even with a layer of insulation, fit through grooves that I ground in the bottom edges of the tiles, so again the roof remains completely intact.

Ah Ha - Good job. I may have a go myself. But I don't have a good head for heights.......

 

[twentysix by twentyfive]

Stirling Engines turn a heat difference into work, which can be turned into electricity. Unlike an internal combustion engine, the heat is applied externally, and it doesn't matter how the heat was produced, i.e. they don't depend on any particular type of fuel. They've been around for a long time, but they didn't take off initially because steam engines and then internal combustion engines were more efficient. Now there's a lot more interest in them.

The maximum possible efficiency of a heat engine in which the hot temperature was 60C and the low temperature is 20C is about 12% by my reckoning. You have to convert the temperatures into absolute temperatures in Kelvin by adding 273. The maximum efficiency (Carnot efficiency) is calculated by subtracting the cold temperature from the hot temperature and dividing the result by the hot temperature again. You get an answer between 0 and 1, which you multiply by 100 to get the percentage efficiency. The actual efficiency of the Stirling engine will be lower than this.

I think the heat content of the water is its mass multiplied by its specific heat and its absolute temperature, but I would have to check. I'm a bit unsure how water being ice under 0C affects the calculation.

Ah thanks for reminding me of Thermodynamics.
I think the ice thing would need the Latent Heat bit taken account of (somehow). Possibly as an input when melting and an output when freezing (somehow).
12% Carnot efficiency is pretty good. IIRC Steam Engines are ~30% Max and they run very hot.