# Solar heat gain on a typical solar cell

## Summary:

heat gain of a black solar cell if both thermal and electrical power is needed

## Main Question or Discussion Point

what will be the joules per meter using thermal gain and wattage gain from a solar cell if the cell is in an insulative vacuum not counting the thermal degredation of 25% for wattage loss due to heat gain

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berkeman
Mentor
Summary: heat gain of a black solar cell if both thermal and electrical power is needed

what will be the joules per meter using thermal gain and wattage gain from a solar cell if the cell is in an insulative vacuum not counting the thermal degredation of 25% for wattage loss due to heat gain
Can you say more about your question? Solar cells generally get less efficient as they get hotter. Are you trying to prevent that by extracting heat from the cell assemblies as thermal energy to be used for something like heating hot water?

And what do you mean by "thermal gain" in "joules per meter"? Is this a standard term?

https://sciencing.com/temperature-affect-solar-panels-20687.html

anorlunda
Mentor
what will be the joules per meter using thermal gain and wattage gain from a solar cell if the cell is in an insulative vacuum not counting the thermal degredation of 25% for wattage loss due to heat gain
I am also confused by your question. Solar panels become less efficient when they are hot. That means they need cooling. Usually, that means allowing air to circulate around the panels to cool them. You could heat the air and use that thermal energy perhaps. But then you say vacuum, so air cooling is out. I don't understand.

Maybe you are trying to forge a marriage between solar cells and solar heat pipes?

Given the question, I'm assuming the OP is interested in the utility of putting PV cells inside an evacuated tube, for combined heat/power.

Me too, though my need isn't immediate so I haven't done any in-depth research, yet.

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OmCheeto
Gold Member
Maybe you are trying to forge a marriage between solar cells and solar heat pipes?
It looks as though they exist: R-Volt, by Systovi, technical characteristics

Using their data, I came up with quite a different number than my back of napkin number this morning.

Summary: heat gain of a black solar cell if both thermal and electrical power is needed

what will be the joules per meter using thermal gain and wattage gain from a solar cell if the cell is in an insulative vacuum not counting the thermal degredation of 25% for wattage loss due to heat gain
Guessing you want the "watts per square meter", as "joules per meter" is equal to newtons, and I'm guessing you're not trying to make a light sail, or, whatever.

Anyways, this morning I came up with an answer of: 1170 watts per square meter.
Then I found the the "R-Volt" panel data, and came up with a different answer: 470 watts per square meter.

Scratching my head as I usually do, when things aren't quite right, I decided the second number, 470, was probably more accurate.

ps. I was thinking about trying to be funny this morning, and solve your problem in "joules per meters", but I would have had to have added "per meter second", but that really confused the heck out of me.
But after a bit of googling, I discovered that "meter seconds" actually exist: Absement

a solar panel at midday receives on average here in the great northwest approx 1000watts per metersquared

but if i want to pull as much energy from a standard solar cell w a given solar watts per square meter and dump it into say a heater or pre hot water tanks for minimizing the cost of home heating
how much energy can be pulled out of the solar cells
water pre heat tank is 50 degrees f
using a fast flow sytem to keep cells cool
average solar watts/meter squared 900 at midday if in full sun and proper orientation
i want both watts ,and joules and thermal rise in a 80 gal tank

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OmCheeto
Gold Member
a solar panel at midday receives on average here in the great northwest approx 1000watts per metersquared

but if i want to pull as much energy from a standard solar cell w a given solar watts per square meter and dump it into say a heater or pre hot water tanks for minimizing the cost of home heating
how much energy can be pulled out of the solar cells
water pre heat tank is 50 degrees f
using a fast flow sytem to keep cells cool
average solar watts/meter squared 900 at midday if in full sun and proper orientation
i want both watts ,and joules and thermal rise in a 80 gal tank
I get 5 °C temperature change per square meter for such a system.
November 5th, at 45.5° latitude, with a 2 axis tracking system.
Total electrical energy collected was 1 kwh.
I would provide the input watts/m^2, but it varies continuously during the day, from zero to 790 watts.
I used the efficiencies of the unit I mentioned in my previous post.
Electrical: 16.7%
Thermal: 30%

Fun problem. I'm glad I never bothered to finish building my solar thermal collector. My sister talked me into buying all the parts when I told her my idea. I told her that I hadn't completed the maths, and that it would be silly to buy everything unless I knew it wouldn't be a waste of time. But she insisted.

i want to pull as much energy from a standard solar cell w a given solar watts per square meter and dump it into say a heater or pre hot water tanks for minimizing the cost of home heating
The very problem with using solar thermal for heating is, that the time when you need it the heat available from the Sun is the lowest. I've made a DIY solar air heater (sum cost for 2m2 surface was ~ equivalent to 200USD) and it works four months in a year: mid-Autumn and late Spring. At winter it is rare to start up at all. At summer I have to cover it. It's more than ten years old, but I'm still not sure if it worth it or not. Likely not.

Try aiming for solar water heating. Hot water is needed all year around and you can buy complete, easy to install solar heat tube based non-pressurized boiler sets for cheap.

I get 5 °C temperature change per square meter for such a system.
November 5th, at 45.5° latitude, with a 2 axis tracking system.
Total electrical energy collected was 1 kwh.
I would provide the input watts/m^2, but it varies continuously during the day, from zero to 790 watts.
I used the efficiencies of the unit I mentioned in my previous post.
Electrical: 16.7%
Thermal: 30%

Fun problem. I'm glad I never bothered to finish building my solar thermal collector. My sister talked me into buying all the parts when I told her my idea. I told her that I hadn't completed the maths, and that it would be silly to buy everything unless I knew it wouldn't be a waste of time. But she insisted.
i have the acreage and the material but no time ,i think as a whole its well worth the time to make a free energy accumalator that has no issues other than producing energy that ,in my case ,can be utilized on demand rather than losing it

i have the acreage and the material but no time ,i think as a whole its well worth the time to make a free energy accumalator that has no issues other than producing energy that ,in my case ,can be utilized on demand rather than losing it
also
my son asked about energy gained by the solar cell reducing the thermal gain gained by photon absorbtion
is there a loss of heat gain equivalent to the gain of electrical wattage

is there a loss of heat gain equivalent to the gain of electrical wattage
Of course ; given that the cell doesn't change during the process, any energy extracted as electricity is not available for other considerations, ie: heat.

OmCheeto
Gold Member
i have the acreage and the material but no time ,i think as a whole its well worth the time to make a free energy accumalator that has no issues other than producing energy that ,in my case ,can be utilized on demand rather than losing it
Are you willing to pay $160,000 to accomplish this? As Rive and I have pointed out, without some type of month+ long thermal storage units, it's a waste of time and money to do any of this. ps. The graph is for professionally installed units. DIY is probably 10% of that. 160,000? What kind of system can I buy for 160,000 E Bay has cheap panels and whole systems for less than 15.000 they could be easily modified or even rebuilt to suit my needs for partial and total house heating requirements Even the water heater demands could be partially met if I wanted to go big 160,000 will buy me a 2000 square ft superinsulated house and I can spend an extra 1000 just for giggles God I love building things 160,000 will buy me a 2000 square ft superinsulated house and I can spend an extra 1000 just for giggles Yes. It is often said that insulation first. Now, you know why. Also, keep in mind that with the reduced amount of heat needed for a well insulated house the return time of any extensive, complicated heating equipment becomes very-very long. Yep Yes. It is often said that insulation first. Now, you know why. Also, keep in mind that with the reduced amount of heat needed for a well insulated house the return time of any extensive, complicated heating equipment becomes very-very long. Yep The problem is I just built my house using standard construction w up to code insulation Currently it's 27 f and the house cools a lot by morning A small setup that is strictly for heat will really help w my outrageous power bills Even a simple thermal mass unit under the house would slow the cold a little bit as long as I block off the vents to allow the heat to rise through the house I wanted solar panels because it has a usable voltage and a large black area w enormous thermal gain It seemed funny that only 10 percent of the energy is used when a hellofa lot more is wasted and slowing the energy production to boot A 160.000 dollar system would have an a I equipped smart sensor w gold plated soldering tabs just for bling A optional drive feature so it will power the electric car wirelessly and have a beeper for bubbles in the jacuzzi room Or so I have heard My last quote for a simple house system in Fresno California on South facing 1000 sq ft roof was 21.000 Installed The thermal gain in the winter use beyond awesome .in the summer I can make Ice cubes from the heat w the ammonia cycle because those babies get smoking hot All those beautiful wasted therm units while I am damp and dreary in the great northwest Sigh Of course ; given that the cell doesn't change during the process, any energy extracted as electricity is not available for other considerations, ie: heat. Is there literature for layman to see how that would work Just because the photon collides w and dislodges an electron .that is considered work by the photon andno thermal units are part of the process or is there 2 different forms energies available instead of just the one OmCheeto Gold Member 160,000? What kind of system can I buy for 160,000 E Bay has cheap panels and whole systems for less than 15.000 they could be easily modified or even rebuilt to suit my needs for partial and total house heating requirements Even the water heater demands could be partially met if I wanted to go big 160,000 will buy me a 2000 square ft superinsulated house and I can spend an extra 1000 just for giggles God I love building things You earlier said i have the acreage and the material but no time I interpreted "I have no time" as "I don't have time to do this myself", which is why I included in my postscript that you can do this yourself for$16,000, which is very close to your "E Bay has cheap panels and whole systems for less than 15.000" comment.

My WAY OVER optimistic calculation in post #8 was for a 100% sunny day.
A quick check at wikipedia for average (Seattle+Spokane)/2 actual sunny hours/month yielded the following curve:

It's the fact that for the months of November, December, and January, you get less than 3 hours of sunlight per day, on average, is why this idea doesn't quite work in our great northwest.
Unless you're made of money, of course.

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It's the fact that for the months of November, December, and January, you get less than 3 hours of sunlight per day, on average, is why this idea doesn't quite work in our great northwest.
On the other hand you can save some on the cost of hot water from March to October and buy more firewood.
That works.

OmCheeto
Gold Member
On the other hand you can save some on the cost of hot water from March to October and buy more firewood.
That works.
It's still hardly worth the effort. If I took 5 gallon showers every day of the year, it would cost me $30. Like you, I paid$200 for the components of my solar water heater. So that would pay for itself in about {$200/($15/year) } 13 years.
My annual heating bills(Oct thru May) run about $1100. (I actually used my heating use in analyzing the OP's situation, as I live only 10 km south of Washington.) The heating bill is why I'm contemplating borehole thermal energy storage(BTES). I'm fairly confident I can install a system for about$10,000. Unfortunately, one source I researched says my design won't work. And the maths involved is a bit horrific, so I'm not sure why. Until I know it's going to work, I'm not going to invest that kind of money.

If I took 5 gallon showers every day of the year, it would cost me $30. Like you, I paid$200 for the components of my solar water heater.
Everything depends on those numbers. For us, it is a bit more water for a bit higher price and I'm expecting further price rise: so I'll go for it, sooner or later - unless we decide on replacing the house instead of replacing the roof only

You earlier said

I interpreted "I have no time" as "I don't have time to do this myself", which is why I included in my postscript that you can do this yourself for $16,000, which is very close to your "E Bay has cheap panels and whole systems for less than 15.000" comment. My WAY OVER optimistic calculation in post #8 was for a 100% sunny day. A quick check at wikipedia for average (Seattle+Spokane)/2 actual sunny hours/month yielded the following curve: View attachment 252091 It's the fact that for the months of November, December, and January, you get less than 3 hours of sunlight per day, on average, is why this idea doesn't quite work in our great northwest. Unless you're made of money, of course. I am a contractor I am made of money It's going to somebody else but I am dripping w money Lol But During the coldest days here when the burn ban is on I have blue Sky's ,warm sunshine and 0 degrees On those days it would be nice to have supplement heat for my heat pump When it rains during the rest of the year The heat pump can handle the 40s and 50s The 3 he Average is just that An average Very cold days are Max sunny days and temp gain is maximum And we have over a month of cloud free days in the winter It's still hardly worth the effort. If I took 5 gallon showers every day of the year, it would cost me$30. Like you, I paid $200 for the components of my solar water heater. So that would pay for itself in about {$200/($15/year) } 13 years. My annual heating bills(Oct thru May) run about$1100. (I actually used my heating use in analyzing the OP's situation, as I live only 10 km south of Washington.)

The heating bill is why I'm contemplating borehole thermal energy storage(BTES). I'm fairly confident I can install a system for about $10,000. Unfortunately, one source I researched says my design won't work. And the maths involved is a bit horrific, so I'm not sure why. Until I know it's going to work, I'm not going to invest that kind of money. I worked w a drilling company who made a water well for heating Turns out that if you take 50 degree water and run it thru a heat pump It is a cheap easy way to heat large spaces if there is a large deep body of water to draw on I considered running a 1 inch pipe down my well to do the same thing but until I get certified on heat pumps I had to wait Last edited by a moderator: russ_watters Mentor Are you willing to pay$160,000 to accomplish this?

View attachment 252083

As Rive and I have pointed out, without some type of month+ long thermal storage units, it's a waste of time and money to do any of this.

ps. The graph is for professionally installed units. DIY is probably 10% of that.
It isn't clear to me what that graph is actually trying to say...

russ_watters
Mentor
Is there literature for layman to see how that would work
Just because the photon collides w and dislodges an electron .that is considered work by the photon andno thermal units are part of the process or is there 2 different forms energies available instead of just the one
All you really need here is conservation of energy: 1000 Watts in = 1000 Watts out. If your solar cells are 20% efficient (200W) and you lose another 200 to radiated/convected heat, that leaves 600W of recoverable heat. You don't need to know the science of photovoltaics for that (I don't).

A dedicated and optimized heat capture system could capture virtually all of that 1000W. But as others have said, the benefit of PV over thermal is PV is better aligned with demand and has a ready sink for selling the excess energy, so you don't need to store it.