Water Cooled Solar Cell Questions 1

[Kuhuh]

I am looking for the temperature of the effluent water after it cools the solar cells. Anyone have an equation or experience with this? Also, how much does maintenance on one of these systems cost. I have seen many on roof tops out here in California but a lot of them have the frosted over glazing and corroded lines what is the real life span of these systems. I asked these questions to a sales rep for an installer and I got unrealistic answers.

 

[IRstuff]

"temperature of the effluent water "

That depends on what the design requirements are and the operating conditions; it's at least higher that the input.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Kuhuh]

Well I would hope its higher then the inlet conditions because that's the whole purpose of it. Assume a one panel system (96"x24") pump flow is lets say 4 [ft/s] with lets throw a 2 [in] PVC pipe at it. so .34 [ft^3/s]. Surrounding air temp is a bright sunny day at 78-80 [F]. I'm hoping to find at lease someone who has a ratio of temperature rise vs # of panels.

 

[itsmoked]

Sounds like a gimmick to me. Yes they will run a little cooler but now you are adding water that will need to be treated, replaced, filtered, pumped, cooled, returned, kept from freezing, kept from over-pressuring, etc,etc.

Here you have a dry system with decades of life expectancy and now you add a water systems that cannot possibly have the same life expectancy, costs more money to purchase, money to maintain, money to run, and brings corrosion to the electrical party.

If water made sense everyone would be using it. The government would be recommending it.

Run-a-way. Scream if it helps.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Kuhuh]

The Government does recommend it actually in hot areas. The water may cause the corrosion for the pipes but one it increases the efficiency of the panels and the cooler the panels stay the longer the life of the cells to produce higher efficiency. The by product of the system is heated water which is an added bonus free energy. European countries use this system ALOT to heat floors but no one can tell me the stupid temperature after it cools the cells down.

 

[IRstuff]

Again, it depends on the design requirements. If you want the cells to stay close to room temperature, then the water will be barely warm, but there'll be lots of it. If you're willing to let them rise to say, 150°F, then you can get some pretty warm water, but at a much lower flow.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Kuhuh]

Typically when it comes to this system you want the cells to be as cool as possible since their efficiency is directly dependent on their temperature. So basically if I want it cooler I obviously need more water passing by and it will be at a less temperature. But if I ran it at a slower flow I could produce higher temp water while still getting the cells to a cool temperature?

 

[IRstuff]

Is this for school? You're lack of understanding of Newton's Law of Cooling is surprising for an engineer.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Kuhuh]

No I'm actually designing this for my self. My forte is not Thermo I would not be surprised if you had your own Achilles heel on some subjects as well. Hence why we have this wonderful forum to get help and answers not criticism for asking it. I was just confirming that the lower the temp the panels are the longer contact time it would take to get the water warm as I can with that temp.

 

[IRstuff]

The bottom line is that if you want heat transferred, it MUST be from a higher temperature to a lower temperature. If you want 150°F water, then the cells MUST be higher than 150°F.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Skogsgurra]

An aside: It is not so very well known that Si solar panels follow the diode equation. As you know, the forward voltage drop has a temperature coefficient of around -2.6 mV/C. That means that you get 130 mV lower voltage when temperature rises from say 20 C to 70 C. That's quite a decrease in efficiency. So, cooling may be good.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

A large installation might be able to justify the addition of a heat pump. Then you could keep the cells at a lower temperature while still being able to scavenge the heat.

Here's the catch. Locations and times that are good for solar energy typically don't need heat in that place and time. When there's a ~kilowatt per square meter falling from the sky, then air conditioning is what's often required. There are exceptions of course, but the economics often become foolish.

More generally, the problem with overly-complicated, overly-expensive renewable energy systems is that the payback period often stretches out to multiple infinities. This is not only bad economics, it's also a sign that it's a waste of resources. Considering that the manufacturing sector is one of the largest sources of CO2, realize that the capital cost of the installation is roughly proportional to the environmental impact of the project. If you're trying to save money, while saving the planet, spending huge gobs of money at the outset is a bad first step.

Calculating the approximate energy captured by water cooling is trivial. You look up the data for the local solar incident (something one needs for any solar project), subtract the energy taken away as electricity (~20%?), guesstimate how much of what remains escapes through other loss mechanisms (assume half, plus or minus half, to start), and then use the well-known thermal mass of water to calculate the approximate temperature rise.

Another approach would be to find data for a comparable sized water heater solar panel. The missing ~20% of the energy for the electrical output is within the error bounds anyway.

 

[Skogsgurra]

Twenty percent would be a very very fine solar plant.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[wongs9982]

the temperature co efficiency of the solar panel is very small.
ie increase in temperature the out put power drop very little,
about 85% of the sun light energy transfer to heat only 15% transfer to energy. normally the solar cell will be heat up to 50 degree C,
the environment temperature is 30 degree C,
the loss of power due to this temperature different is small.
therefore no need to spend money to cool down the solar by water.
sam wong

 

[IRstuff]

Cooling isn't just about efficiency. Reliability decreases as a function of temperature. You can substantially decrease the failure rate by dropping the temperature 20°C

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[itsmoked]

I can't believe that since the water system is so prone to problems.

How's the reliability when all that water freezes solid?

This is like adding a mechanical piston operated oscillator to a microprocessor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

I was only referring to the failure rate of the solar panels and inverters. The system failure rate is a function of how one makes that tradeoff.

In many cases, that trade will not compute, but in some systems, it might be worth it, particularly if your cooling system is reasonably robust, and you cannot accept high failure rates on the electronics, which is often the case in military systems.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[wongs9982]

solar panel fail because of partial shading.
large current will damage the shaded cell.
that why there are 3 bypass diod, one diod protect two role of cell.

all the information print in a pc of spec sheet.
read it then come to the point of reliability.
use the army term as shell.

 

[TurboXS]

There was a discussion regarding this on;

http://forums.whirlpool.net.au/archive/1609024

 

[Kuhuh]

Yes ambient temperature can cause a cooling effect in PV Cells. but there is a .5% efficient decrease with 1 kelvin. Obviously this is not much but why not collect the rejected energy. The system is a combination of thermal collectors and PV cells. Both are highly used practices. The heated water is use to preheat domestic water which reduces energy consumption for heating and maintain the hot water tanks. For freezing there are two facts 1) we use a glycol mixture 2) if it is cold enough to freeze your PV panels are producing next to nothing or its cold enough to keep your PV panels cold anyways. The systems seem to be only practical when looking at preheating domestic water. If heated water is tried to be used for cooling with heat pump it is still not practical or economic as of yet. IF your interested, preliminary results showed a decrease of 2 years on the rate of return