Heat could be collected in roof cavities and stored in batteries.

Lithium batteries are freely available now

Has anyone worked out the energy created by the sunlight heat in a typical roof cavity per sq m?

How best to collect that heat with a lightweight system that could be be 2 separate systems working together to generate electricity

One system could consist of roof tiles with pipes built in, the heat heats up water in the pipes to turn a generator, a size ratio would have to be worked out to determined the best size for the pipes which would just be hollow tubes in the ceramic tile.

Another system could be placed in the roof cavity to collect heat like a heat bank, and it too drives a generator.

There must be lots of ways to collect heat in a suburban house roof.

I suspect a very slow way to charge a very expensive battery.

I expect you could get a few tens of watts out of a simple rooftop rotary ventilator, by attaching a generator.

But would it be worth the cost and trouble? I dunno.

dv said:

I expect you could get a few tens of watts out of a simple rooftop rotary ventilator, by attaching a generator.

But would it be worth the cost and trouble? I dunno.

That could be a third system operating in unison with the others.

Tau.Neutrino said:

dv said:

I expect you could get a few tens of watts out of a simple rooftop rotary ventilator, by attaching a generator.

But would it be worth the cost and trouble? I dunno.

That could be a third system operating in unison with the others.

The ceramic tiles could allow have airflow channels built in around the water tubes.

Using the principle of heat goes up, create solar assisted fans that suck the heat up the ceramic airflow channels for more heat generation

Running a BOE. Raising a cubic metre of air one degree takes around 1200 Joules. A roof space contains (SWAG) 300 cubic metres. A hot day raises the temperature by (SWAG) 20 degrees.

So the energy ‘stored’ is approximately 7.25MJ or 2 kWhr. Getting the energy out and stored into a battery is the hard part. A peltier (seebeck) device just sucks for efficiency, but it may be the best you can do. Call it 10%. So you’ll end up with 200 Whr getting put into the battery.

sibeen said:

Running a BOE. Raising a cubic metre of air one degree takes around 1200 Joules. A roof space contains (SWAG) 300 cubic metres. A hot day raises the temperature by (SWAG) 20 degrees.

So the energy ‘stored’ is approximately 7.25MJ or 2 kWhr. Getting the energy out and stored into a battery is the hard part. A peltier (seebeck) device just sucks for efficiency, but it may be the best you can do. Call it 10%. So you’ll end up with 200 Whr getting put into the battery.

what sort of numbers could you get by setting up your whirlybird like a horizontal wind turbine?

It’s fine to have hot roofs in winter but wouldn’t the corresponding heat transfer in Summer negate any gains over the long term?

Witty Rejoinder said:

It’s fine to have hot roofs in winter but wouldn’t the corresponding heat transfer in Summer negate any gains over the long term?

The idea is to used any heat stored in the roof (really delta T) to charge a battery. So you would be cooling the roof space in summer which is a good thing. You’d also be doing the same in winter which will probably be a negative :)

it’s a very impractical way to (try to) collect energy, of a typical roof space, for as you say

transition said:

it’s a very impractical way to (try to) collect energy, of a typical roof space, for as you say

Yeah, a solar panel on the roof may just (sic) be a better way of doing it :)

sibeen said:

Running a BOE. Raising a cubic metre of air one degree takes around 1200 Joules. A roof space contains (SWAG) 300 cubic metres. A hot day raises the temperature by (SWAG) 20 degrees.

So the energy ‘stored’ is approximately 7.25MJ or 2 kWhr. Getting the energy out and stored into a battery is the hard part. A peltier (seebeck) device just sucks for efficiency, but it may be the best you can do. Call it 10%. So you’ll end up with 200 Whr getting put into the battery.

I’m pretty sure you could get more than 300 m3/day raised through 20 degrees with some air circulation.

Whether enough to justify the effort is another matter.

If only we had some way of converting light directly into electricity.

sibeen said:

transition said:

it’s a very impractical way to (try to) collect energy, of a typical roof space, for as you say

Yeah, a solar panel on the roof may just (sic) be a better way of doing it :)

yeah, those things.

>If only we had some way of converting light directly into electricity.

what are typical efficiencies these days of photovoltiacs, ones with a multiple decade service life, that don’t cost too much

..?

If solar heated hot water is possible then why not build one directly into the whole roof.

I wonder what those numbers would be?

Place transparent solar panels over them as well.

If solar heated hot water is possible then why not build one directly into the whole roof.

I wonder what those numbers would be?

Place transparent solar panels over them as well.

Tau.Neutrino said:

If solar heated hot water is possible then why not build one directly into the whole roof.

I wonder what those numbers would be?

Place transparent solar panels over them as well.

Better to have part hot water & part electricity.

Tamb said:

Tau.Neutrino said:

If solar heated hot water is possible then why not build one directly into the whole roof.

I wonder what those numbers would be?

Place transparent solar panels over them as well.

Better to have part hot water & part electricity.

Yes. Converting light into electricity, then using some of that electricity to raise the temperature of water by about 30C using a resistive heater makes no sense at all.

If you applied the solar hot water technique to the whole roof what energy production could you obtain from it?

Tau.Neutrino said:

Heat could be collected in roof cavities and stored in batteries.

Lithium batteries are freely available now

Has anyone worked out the energy created by the sunlight heat in a typical roof cavity per sq m?

How best to collect that heat with a lightweight system that could be be 2 separate systems working together to generate electricity

One system could consist of roof tiles with pipes built in, the heat heats up water in the pipes to turn a generator, a size ratio would have to be worked out to determined the best size for the pipes which would just be hollow tubes in the ceramic tile.

Another system could be placed in the roof cavity to collect heat like a heat bank, and it too drives a generator.

There must be lots of ways to collect heat in a suburban house roof.

If solar heated hot water is possible then why not build one directly into the whole roof.

I wonder what those numbers would be?

Place transparent solar panels over them as well.

Another mollwollfumble fantastic claim coming up. I have calculated the heat collected in roof cavities as part of my work for CSIRO. The roofing material chose for the task was corrugated iron, which is infamous for heating up rapidly in the hot sun.

1) The heat in the corrugated iron sheeting is way more than that in the whole roof cavity, because roof cavities are naturally cooled by free convection.

2) You want roof spaces to be cool in the afternoons when the roof is hottest, in order to keep the house underneath cool.

3) Next door neighbour has black pipes on the roof through which water is circulated in order to heat his swimming pool. This is an older technique than solar panels and until recent improvements in solar panels it was much more efficient.

4) You can’t have solar cells either over or under a solar heat collector because solar cells need to be kept as cool as possible and a solar heat collector has to be kept as hot as possible.

5) I may be able to dig up some numbers for the corrugated iron sheeting from my CSIRO work, but you could probably get more reliable figures from a solar heat collector manufacturer.

6) I’ve “recently” been working with CSIRO on the opposite, evaporative cooling of roofs and walls. Designing walls as wicks to bring water up and down in such a way that it isn’t lost by dripping off.