Photovoltaic vs. thermal radiator Solar Panel comparison (using a lens)

[camerart]

Hi,
Has anyone experience with photovoltaic solar panels and thermal radiator type solar panels?

1/ How do they compare when both have the same collection area?

If the thermal radiator was much smaller than the collection area, and a lens the size of the collection area was used, focused on the radiator. This would produce the same amount of heat, but it would be hotter. Is there any advantage in this?
Cheers, Camerart.

 

[anorlunda]

1/ How do they compare when both have the same collection area?

Compare in which way? The purpose of one is to make electricity, and the other to heat water. That's pretty apples and oranges.

There are designs of both kinds of panels that use some kind of focus or concentration mechanism. Since none of them became popular commercially, I suspect that the answer was that simpler is better. For most people, "better" regarding these panels puts low weight on the area used.

 

[camerart]

Compare in which way? The purpose of one is to make electricity, and the other to heat water. That's pretty apples and oranges.

There are designs of both kinds of panels that use some kind of focus or concentration mechanism. Since none of them became popular commercially, I suspect that the answer was that simpler is better. For most people, "better" regarding these panels puts low weight on the area used.

Hi A,
The idea is to extract electricity from the thermal panels, by heat exchange, then a generator. I can see where inefficiencies may be, but there are also inefficiencies in PV.

Re 'better': I doubt they would go on roofs, so wight wouldn't be much of an issue, and they would need to be aimed. Also the focus distance is a bit inconvenient too.

I didn't know there are previous designs for focused panels, although I've seen mild focusing on roofs, but what I'm talking about is a much bigger ratio, thatn those.
C

 

[Rive]

1/ How do they compare when both have the same collection area?

In theory, on the 'thermal way' you might be able to reach around 30% efficiency, compared to the 15-18% efficiency of PV.
In reality, on the 'thermal way' concentrators (and with those: directioning of collectors) is required to have some ~ stable output and high efficiency, and that has plenty of drawbacks.
Also, size does matters. Small size means loss in efficiency (insulation is a difficult topic in itself, and you will need to work with high pressure steam, and find a small turbine too. And then the control - it's a nightmare).

PV in its simplicity is just a lot smoother game.

Random example:


These concentrators can cause really serious burns, by the way. Second or even third grade burns are just matter of momentary carelessness and done in seconds.

 

[camerart]

Hi R,
Thanks.

30% efficiency compared to 18% (at present?) is encouraging.

I'm not too worried about directioning, but can see many obstacles.

Is steamand turbines, the only way to convert the energy into electricity? I doubt there would be much steam generated in a UK winter :)

As for the dangers of focusing, I remember the ants of my childhood.
C.

 

[Rive]

30% efficiency compared to 18% (at present?) is encouraging.

At small size, not really. The truth is, that with all the trouble and in small size you'll be far (!) better of with PV.
This thermal stuff is good only as a hobby.

I doubt there would be much steam generated in a UK winter :)

Also, at any time when there is any cloud or any obstacles to direct sunlight. PV works (albeit with less output) even in a cloudy day. At the end that'll matter more than the twice as big theoretical efficiency.

If you want a calm, comfortable life then just stay with PV.

If you need a DIY hobby for some hopeless but heroic struggle which burns away money like nothing then you can go on with the thermal thing.

 

[camerart]

At small size, not really. The truth is, that with all the trouble and in small size you'll be far (!) better of with PV.
This thermal stuff is good only as a hobby.

Also, at any time when there is any cloud or any obstacles to direct sunlight. PV works (albeit with less output) even in a cloudy day. At the end that'll matter more than the twice as big theoretical efficiency.

If you want a calm, comfortable life then just stay with PV.

If you need a DIY hobby for some hopeless but heroic struggle which burns away money like nothing then you can go on with the thermal thing.

Hi R,
The reason I asked the question was to save wasted time with frutless experimenting, so thanks, I'll forget it.

Before I do can I assume that you know what you're talking about?
C

 

[Rive]

Before I do can I assume that you know what you're talking about?

I've seen many of these kind of DIY projects (when I tried to design my own).
In general they gives a good struggle, with really moderate reward.

Of course, if you keep the energy as heat (hot water for local usage) then that's a different kind of business.
That may work (depending on your location).

 

[camerart]

I've seen many of these kind of DIY projects (when I tried to design my own).
In general they gives a good struggle, with really moderate reward.

Of course, if you keep the energy as heat (hot water for local usage) then that's a different kind of business.
That may work (depending on your location).

Hi R,
I suppose if the energy is for heating, then the standard panels would take the sting out of the heat difference whan heating a house in winter.

I've had lots of inventions over the years, all went the same way as this. (I was advised to stop)

Later, I'll post my engine design, which would have been ok, but electric is superceding it.
C.

 

[russ_watters]

Is steamand turbines, the only way to convert the energy into electricity?

It's not the only, but it is by far the most common and efficient way to turn heat into electricity.

 

[bob012345]

Hi R,
I suppose if the energy is for heating, then the standard panels would take the sting out of the heat difference whan heating a house in winter.

I've had lots of inventions over the years, all went the same way as this. (I was advised to stop)

Later, I'll post my engine design, which would have been ok, but electric is superceding it.
C.

Solar thermal panels are very efficient in gathering heat to use as heat for your house or hot water but not very efficient to drive a heat engine to generate electricity because they are just not very hot and the maximum possible efficiency of a heat engine depends of the ratio of temperature of the hot side and cold side $\eta = 1 - \frac{T_{cold}}{T_{hot}}$. Then it gets worse because of other inefficiencies in the system.

 

[camerart]

Solar thermal panels are very efficient in gathering heat to use as heat for your house or hot water but not very efficient to drive a heat engine to generate electricity because they are just not very hot and the maximum possible efficiency of a heat engine depends of the ratio of temperature of the hot side and cold side $\eta = 1 - \frac{T_{cold}}{T_{hot}}$. Then it gets worse because of other inefficiencies in the system.

Hi B,
This is why I am suggesting focusing. This is to make the difference between the hot and cold sides a bigger ratio.
C

 

[anorlunda]

Here is an example of focused solar hot water panels.

 

[camerart]

Hi A,
This is better than my lens idea, it looks like it doesn't need to move to stay in focus?
I wonder why the collector tube isn't black?
C

 

[anorlunda]

Hi A,
This is better than my lens idea, it looks like it doesn't need to move to stay in focus?
I wonder why the collector tube isn't black?
C

Focused solar thermal panels have been well researched, well engineered, built and operated. Yet they are rarely seen today. What message does that suggest?

 

[bob012345]

Hi B,
This is why I am suggesting focusing. This is to make the difference between the hot and cold sides a bigger ratio.
C

Of course but that is a whole different beast and also very dangerous if you are not careful. Also it can be expensive but still the efficiencies are likely to top out in the low 40's.

 

[Rive]

I suppose if the energy is for heating

Erm... no. It's for having hot water, and not at winter, but from spring to autumn.

About solar heating: usually (depending on location!) the time when heating is needed and solar heating is actually available is just too short. This - again - is just a good DIY struggle without much return
Regarding heating, some passive solutions may be considered, but that's already architecture.

Consider some of these things instead. Relative easy to install, relative cheap (was cheap when I checked the prices last time), and can work from spring to autumn.
Some reserve system is still needed, though: and return depends on the location/climate.

 

[camerart]

Focused solar thermal panels have been well researched, well engineered, built and operated. Yet they are rarely seen today. What message does that suggest?

Hi A,
What message does that suggest?
It could mean that it isn't practical, or that they haven't figured it out yet.
C

 

[camerart]

Of course but that is a whole different beast and also very dangerous if you are not careful. Also it can be expensive but still the efficiencies are likely to top out in the low 40's.

Hi B,
Over the years, I've seen all sorts of things develop, in surprising ways. I remember milk floats, and look at electric cars now.
Generally power is dangerous.
Costs generally come down, apart from heating bills.
C

 

[camerart]

Erm... no. It's for having hot water, and not at winter, but from spring to autumn.

About solar heating: usually (depending on location!) the time when heating is needed and solar heating is actually available is just too short. This - again - is just a good DIY struggle without much return
Regarding heating, some passive solutions may be considered, but that's already architecture.

Consider some of these things instead. Relative easy to install, relative cheap (was cheap when I checked the prices last time), and can work from spring to autumn.
Some reserve system is still needed, though: and return depends on the location/climate.

Hi R,
An interesting idea, but I won't be pursuing it.
Thanks all for you help in my decision.
Cheers, C.

 

[sophiecentaur]

This thermal stuff is good only as a hobby.

That's a bit of a sweeping statement. Carbon neutral homes make very good use of Solar Thermal - sometimes just directly with south facing windows and effective blinds at night. PV has advantages but not so much when you can't rely on Feed-in Tariff (which is a bit of a snare and delusion)

 

[bob012345]

We often talk about getting energy from the sun either as heat or electricity but It may be far more efficient to reflect heat in hot climates. I mean the effective cooling power of shade trees, cool roofs, radiant barriers as such. Even placing outdoor condenser units in shade rather than sun helps. On top of that good insulation helps to both keep cool and warm.

I saw today a presentation of geothermal systems for home or business and what made it efficient is that it has effectively the same efficiency whether the outside temp is 100 or 0. I was surprised to learn these systems still rely on compressors but they use the constant temperature of ground loop water flow to draw off the compressor heat rather than outside air temperature.

 

[camerart]

That's a bit of a sweeping statement. Carbon neutral homes make very good use of Solar Thermal - sometimes just directly with south facing windows and effective blinds at night. PV has advantages but not so much when you can't rely on Feed-in Tariff (which is a bit of a snare and delusion)

Hi S,
My house is definitely not carbon neutral, and can't be turned round easily. But when the sun shines here, it should be captured and stored somehow.
It's hot here at the moment, so any stored energy, should be converted to electric, for other uses than heating.
C

 

[camerart]

We often talk about getting energy from the sun either as heat or electricity but It may be far more efficient to reflect heat in hot climates. I mean the effective cooling power of shade trees, cool roofs, radiant barriers as such. Even placing outdoor condenser units in shade rather than sun helps. On top of that good insulation helps to both keep cool and warm.

I saw today a presentation of geothermal systems for home or business and what made it efficient is that it has effectively the same efficiency whether the outside temp is 100 or 0. I was surprised to learn these systems still rely on compressors but they use the constant temperature of ground loop water flow to draw off the compressor heat rather than outside air temperature.

Hi B,
Effective cooling, should mean heat extraction, and strorage.
It wouldn't take much effort to use cooling from 'just' undergound, where the temperature is generally cooler.

Ground loop water is a special subject and needs studying and monitoring to see if there is enough cooling for what's needed.

General presentations, can be misleading!
C

 

[Rive]

That's a bit of a sweeping statement.

Nope. It was a very specific reply in a very specific topic.

 

[sophiecentaur]

Nope. It was a very specific reply in a very specific topic.

I'd agree with you, in that particular case but your sentence implied a more general criticism of thermal solar.

 

[Rive]

more general criticism of thermal solar.

Looking at the various company ADs, there is some room for that too.
But here, it was very specific.

 

[sophiecentaur]

But here, it was very specific.

I appreciate that now. Perhaps just a few more words would be appropriate in posts on this sort of multi-faceted topic.

 

[camerart]

I appreciate that now. Perhaps just a few more words would be appropriate in posts on this sort of multi-faceted topic.

Hi S,
I agree with you. A snap reply, or a Google link, can be time consuming, and is impossible to know if the answer is from knowledge or a 2minute look up.
C

 

[sophiecentaur]

or a Google link,

I seldom appreciate post consisting of just a link. The information may well have passed through the poster's computer without touching the sides.

 

[Opera]

I produce domestic hot water with PV. While less efficient in collecting space vs thermal solar, it is far more economical. Supplemental PV water heating on a cost basis can be better than a heat pump. True success in a water heater is it turns off. That PV energy can be used elsewhere like air conditioning. I built my own controller to keep the PV array at optimal voltage for energy transfer to a resistive element making the cost for a system not much more than the panels themselves. This method of water heating is relatively new with the drop in panel prices. This still isn't understood in the solar community, but new products are starting to appear. This is a paper comparing PV against thermal at several locations in the world https://www.hindawi.com/journals/ijp/2017/7540250/
Any questions of my practical experience with PV heating are welcome. I've been experimenting in this technology since 2016.

 

[camerart]

I produce domestic hot water with PV. While less efficient in collecting space vs thermal solar, it is far more economical. Supplemental PV water heating on a cost basis can be better than a heat pump. True success in a water heater is it turns off. That PV energy can be used elsewhere like air conditioning. I built my own controller to keep the PV array at optimal voltage for energy transfer to a resistive element making the cost for a system not much more than the panels themselves. This method of water heating is relatively new with the drop in panel prices. This still isn't understood in the solar community, but new products are starting to appear. This is a paper comparing PV against thermal at several locations in the world https://www.hindawi.com/journals/ijp/2017/7540250/
Any questions of my practical experience with PV heating are welcome. I've been experimenting in this technology since 2016.

Hi O,
After suggestions from here, I've stopped concentrating on this kind of technology, although if I wasn't working on other 'interesting' projects, I would experiemnt, in case something different appeared.

My idea was mainly centred around stroring the energy, in a block, in my case for experiment it would have been water, insulated and underground. I assume it would have been compartmetalised, with high heat at the centre etc.

I think the weak link is converting very hot water, into electric, so if you think of something let me know.

I am a fan of PV, and hope to have an array sometime.

Thanks for the offer of questions.
C

 

[bob012345]

Hi O,
After suggestions from here, I've stopped concentrating on this kind of technology, although if I wasn't working on other 'interesting' projects, I would experiemnt, in case something different appeared.

My idea was mainly centred around stroring the energy, in a block, in my case for experiment it would have been water, insulated and underground. I assume it would have been compartmetalised, with high heat at the centre etc.

I think the weak link is converting very hot water, into electric, so if you think of something let me know.

I am a fan of PV, and hope to have an array sometime.

Thanks for the offer of questions.
C

Just a note FYI that if you do store energy underground in an insulated container, hot sand can be stored at much higher temperatures;

https://www.renewableenergymagazine.com/storage/first-commercial-sandbased-thermal-energy-storage-is-20220707#:~:text=As a material, sand is,8 MWh of energy capacity.

 

[camerart]

Hi B,
I looked into different heat storage materials, and decided on water as the most suitable for experimenting with. e,g, between each sized test, water can be pumped, and doesn't need shovelling, stroring or much cost.

When I'm experimenting with anything, I don't go for the best in the first instance, that would come later if the earlier tests were sucessful
Thanks, c.

 

[solvejskovlund]

Old thread, but still.... I've played with the idea of adding copper pipes on the back of PV panels, and add insulation on the back to keep the heat in. Then circulate water in the pipes. The PV panels do get hot in sunlight. PV panels are more efficient when cold. After all, they have 17-22% efficiency. The reminding energy, that is not reflected, becomes heat. That heat I'd like to collect by circulating the water, and at the same time it provides cooling to the PV panels.

10 years ago solar heat collection was a big thing. In the past years PV panels became cheap and I've seen calculations concluding that PV powering a hot water tank is more economical than solar heat collection.

 

[sophiecentaur]

I've played with the idea of adding copper pipes on the back of PV panels, and add insulation on the back to keep the heat in.

These ideas are always worth playing with but, when you get down to it, it's the numbers that count. It would be necessary to compare the heat that gets to the cooling pipes with what you'd get from black pipes (plus other fancy additions) with what you can expect from your 'in behind' proposal. Then you'd need to look at the economics of installing a solar heat collection system (plumbing isn't cheap). Solar heat collectors work mainly on radiant energy from the Sun and they would be in the shade if behind PV panels. Afaik, thermal solar collectors are not usually a system of choice and PV seems is now the runaway favourite so I'd suggest that keeping them in the shade (albeit in contact with the PV panel backs) would disadvantage them. So I'd guess that any benefit would be marginal.

 

[solvejskovlund]

The main benefit is area saving. If you have available area to install PV or thermal, go for PV. If you've run out of installation area, pipes behind may be worth an experiment

 

[camerart]

The main benefit is area saving. If you have available area to install PV or thermal, go for PV. If you've run out of installation area, pipes behind may be worth an experiment

Hi S and S,
I agree that pipes behind the PV panel would not be good, as they would be in the shade.
I would try to seal the top of the PV panels, so water could flow over them (To keep them cool) and extract any heat, though heat exchanger/heatpump, if economical.
Use plastic or glass for the cover and plastic central heating pipe for transfer.
C

 

[Rive]

I've seen calculations

Entirely depends on the amount of machinery involved.

There were those low emission dream houses a decade (or so...) ago with 1-2 m³ boilers with multiple heat exchangers within, fed parallelly by wood stove or solar heat (gas options and heat pumps are optional) and hooked on a high tech floor and wall heating system... Yeah, no return point is expected within the millennia.

But those simple, straightforward vacuum pipe&boiler on roof kind of systems are still kickin'.

I've played with the idea of adding copper pipes on the back of PV panels, and add insulation on the back to keep the heat in.

The temperature is just too low for any practical usage/return calculation.
Might be OK for a pool heating, but if you have some free area on the roof then utilizing that'll still beat it by far.

 

[camerart]

Entirely depends on the amount of machinery involved.

There were those low emission dream houses a decade (or so...) ago with 1-2 m³ boilers with multiple heat exchangers within, fed parallelly by wood stove or solar heat (gas options and heat pumps are optional) and hooked on a high tech floor and wall heating system... Yeah, no return point is expected within the millennia.

But those simple, straightforward vacuum pipe&boiler on roof kind of systems are still kickin'.The temperature is just too low for any practical usage/return calculation.
Might be OK for a pool heating, but if you have some free area on the roof then utilizing that'll still beat it by far.

Hi R,
This is quite complicated, and what do we know what developments have happened since the decades old house?

I'm always interested in subtleties, and what gets forgotten with long thread. The OP mentioned cooling the PV panels, this must have an added affect along with some heat capture.

I did have a thought about low level heat, as in a warm water tank. If it wasn't circulating and was well insulated, would heat rise to the top for reclaim?
C

 

[sophiecentaur]

pipes behind may be worth an experiment

Entirely depends on the amount of machinery involved.

The temperature is just too low for any practical usage/return calculation.

There is one huge problem in experimenting with this. You can't just go up on the roof with some pipes and even that is inconvenient for most people. You would need the pipes to be installed just before the PV panels and the PV panel fixing brackets would need to be non standard. I can't imagine any installer being interested in doing the job; they like to be up, down and off. This would involve probably double the on-roof time.

The only realistic experiment you could do would be to buy a single panel, mount it in an accessible spot with a home brewed system and play with various piping systems. Actually, you'd really need two PV panels with one as a control.

There must be lots of data about PV performance at different temperatures. That wouldn't require any of one's own experiments. Pipes with various covers would be easy to try . . . .

This is getting out of hand for anyone but the real enthusiast.

 

[Rive]

This is quite complicated, and what do we know what developments have happened since the decades old house?

They pretty much got retired as dream houses by now (except, maybe, for absolute enthusiasts of the 'money does not matter' type).

There is a nice paradox around insulation. The more you spend on insulating a house (bringing down the energy required to run it) the less room you'll have for additional machinery to reach a return point.

In a really well insulated house only (if any...) the cheapest, most direct heating system would worth it.

 

[sophiecentaur]

the less room you'll have for additional machinery to reach a return point.

Predicted return point is only accurate in a lifetime situation of stable prices. Also, the proportion of a person's income that's spent on Energy will be relevant. (Is it actually always worth it?) There's a good parallel with replacing primary cells with re-chargeable cells for small devices. Very often it's pretty marginal when you take into account the drag of managing the system. You have to be a suitable sort of person [this isn't a quality judgement :-) ] to organise it successfully

Imagine how 'smug' could turn into 'sick' when Fusion Power drastically reduces the cost of energy, half way towards your predicted return point. But then, I can't imagine the Energy Companies letting us get away with free Energy!!