Heatsink on back of panels?

[PNjunction]

I wonder if adding say finned aluminum heatsink material attached to the tedlar backing would do anything?

Other than picking up the ambient temperature, and assuming there was enough thermal conductivity between the tedlar and the heatsink, maybe it would drop the panel temp a few degrees since the heatsink would not be in direct sunlight? I haven't tried it.

[Sunking]

I think once you price out extruded heat sink you will give up that idea in a hurry.

[Naptown]

there is a company called sundrum that makes a hot water panel that fits on the back of some pv modules. He was here for a short while until I ran him off with concerns about aluminum and mixing of metals in the system. Pricey and not that effective for heating water.

[russ]

A number of companies have tried cooling the PV panels but to date no one has come up with a practical solution.

[inetdog]

A number of companies have tried cooling the PV panels but to date no one has come up with a practical solution.

The best generally effective step is to make sure that air can circulate freely under the panels. That is, do not mount them directly on the roof or enclose the backs to make them look nice. Most people take that for granted, and professional installers always will ensure that, but occasionally a DIY system will be done wrong.

I have speculated whether some sort of controlled ventilation (solar chimney, etc.) would be better than free air flow, but it is very unlikely.

[Sunking]

there is a company called sundrum that makes a hot water panel that fits on the back of some pv modules.

Me thinks your local electrical code and inspector would have a real problem with that. Solar PV and DHW have to be separated.

[PNjunction]

..The best generally effective step is to make sure that air can circulate freely under the panels. That is, do not mount them directly on the roof or enclose the backs to make them look nice.....

I'm toying with the idea of just drilling some 1 inch holes or so in the top and bottom channel of the aluminum frame of my tilted Sharp panel just to see if it might improve convection cooling a little bit by allowing air flow closer to the backing. On windless days, I always wondered if the somewhat deep frame might be impeding airflow, or creating hot spots near the top of the panel. I even go so far as to put my panel's junction box at the bottom where my bypass diodes live without any really good heatsinking..

Also thinking about using U-channel aluminum strips instead of a custom fabricated (and costly) finned heatsink. Taken even further, perhaps square aluminum tubing instead of U-channel to get the chimney effect possibly.

The fact that I don't see anything on commercial panels makes me think they have already been down this road without any major tangible benefit though.

[MikeSolar]

We are about to test with a 10kw PVT system, which is heating DHW and a pool. The added cost to the panels is around $1.20/w but we are trying to get it down. It is a glycol system with 250w panels. So far we have about a 5% increase in electrical output in a much smaller system but this really depends on hot water usage.

[russ]

We are about to test with a 10kw PVT system, which is heating DHW and a pool. The added cost to the panels is around $1.20/w but we are trying to get it down. It is a glycol system with 250w panels. So far we have about a 5% increase in electrical output in a much smaller system but this really depends on hot water usage.

You can't tie that together with hot water usage - makes no sense at all. All you can do is dump the heat. Glycol is not as good as water for heat transfer is it - more complexity

[MikeSolar]

You can't tie that together with hot water usage - makes no sense at all. All you can do is dump the heat. Glycol is not as good as water for heat transfer is it - more complexity

Wrong there Russ. We are getting 60c out of the panels on a hot day with a low flow system, defo good enough to heat DHW most of the way and the pool is just an added load.

The TRNSYS (very detailed computer modeling) modeling was done and our system acts like a traditional plastic pool heater in terms of absorption and emissivity so it is just a mater of m2 of panel area as to heat transfer.

If we can bring the panel down from 70C to 40C we have increased the efficiency by 12% under those conditions (based on a temp degradation of .4%/degC).

[russ]

Not wrong - your calculations are only good if you get to take credit for all the hot water - Have fun but at the end of the day you will find it is a losing cause. You have to find someone that wants and can afford both a solar system and a questionable pool heater - not a large market.

[inetdog]

Wrong there Russ. We are getting 60c out of the panels on a hot day with a low flow system, defo good enough to heat DHW most of the way and the pool is just an added load.

The TRNSYS (very detailed computer modeling) modeling was done and our system acts like a traditional plastic pool heater in terms of absorption and emissivity so it is just a mater of m2 of panel area as to heat transfer.

If we can bring the panel down from 70C to 40C we have increased the efficiency by 12% under those conditions (based on a temp degradation of .4%/degC).

I may be missing something, but I have trouble reconciling the two goals:

If you have a low flow system, you get useful heat out of the panels, but you do not reduce the panel temperature much.
If you have a high flow system, you reduce the panel temperature, but do not get very not water.

Some part of the panel has to be at or above the temperature of the output water.

[MikeSolar]

I may be missing something, but I have trouble reconciling the two goals:

If you have a low flow system, you get useful heat out of the panels, but you do not reduce the panel temperature much.
If you have a high flow system, you reduce the panel temperature, but do not get very not water.

Some part of the panel has to be at or above the temperature of the output water.

Actually, to answers Russ issue, any simulation has to use the same load profile that is used to determine the SRCC/CSA etc data. This testing is being done to a lab standard so it will have to follow the same protocol. you will note that the 12% improvement is an instantaneous one based on the 70-40 dT and the 5% is real world and based DHW load changes which includes times when the system is not capable of removing heat.

As i said before the panels acts somewhat similar to a pool heater and there are a couple of those being turned into SDHW systems for mainly southern use. This means that I we can use either a high flow or low flow pumping depending on what we want to achieve, and what piping size we use. If it is for pool heating, I would use a higher flow because heat transfer is only a function of fluid flow rate and dT, high flow- low dT or low flow-high dT. Pumping power is the only major thing that affects it.

There are a lot of people, especially in places like California, where there is only so much roof area and the owner wants both pool heating and DHW and PV with no room for all. We will see just how well the system responds here in Toronto but it looks good so far.

[PNjunction]

... It is a glycol system with 250w panels. So far we have about a 5% increase in electrical output in a much smaller system but this really depends on hot water usage.

Ok, not so interested in hot water. But, if glycol cooling only nets a 5% increase in current, I will definitely shelve my idea of trying to add DIY heatsinking to my panel. You just saved my panel from developing a big hairy crack due to drilling holes in a perfectly fine frame.

[MikeSolar]

Ok, not so interested in hot water. But, if glycol cooling only nets a 5% increase in current, I will definitely shelve my idea of trying to add DIY heatsinking to my panel. You just saved my panel from developing a big hairy crack due to drilling holes in a perfectly fine frame.

I wouldn't give up on it that quickly. Our first goal was to see how much PV area was needed to approximate a typical SDHW system in performance. The average solar water heater in Toronto gives 50-60% of the heat needed through the year (2-flat panels and a 300L (80usg) tank, which is pretty standard). The computer modeling said that 6 PV panels (1.6m x 1m) would give approx 45% of the DHW based on standard test methods. Increasing the panel area had little impact on heat production IN TORONTO where the we can get some colder temps in the winter.

In California, Florida, etc, it would be a different story. You might hit 60-70%. Under the Toronto conditions, 5-8% improvement makes sense but we are only trying to take the heat out when needed for DHW, not constantly. If you can find a load that is more constant and can keep the panels down to near near STC, obviously you can wipe out the temp based performance reduction. The added bonus of this is that the reduction in output that we see in panels over the years is largely due to the constant overheating of the cells. This can reduce that reduction in power.

A quick financial analysis for Ontario shows that if a 1.5kw system where the utility buys all the power at $.80/kwh (which is the case up to now), has an increase from 1900kwh to 2000kwh (5%)annually, adds another $80 in the pocket, + the reduction in gas or electricity for DHW production. It is up to you if that makes sense.