Attic Cooling and Solar Performance

[DavidT]

Thinking about experimenting with connecting an external air intake to my duct work then running the whole house fan instead of the AC unit. The air would pull through the ducts, come out the vents then exit through the attic. Moving so much air through the attic should keep it cooler. I'm wondering if a byproduct of keeping the attic cooler would be increased output for my solar panels. Has anyone noticed a difference in solar output when cooling their attic vs. letting the summer heat build up in it?

[Shockah]

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My very first experiment with PV Solar, I mounted these 2 panels flush on the roof-top of a plastic storage shed.
They were wired to Enphase Micro-Inverters, which I had grid-tied.
The output was monitored by an EKM-25IDS.

On sunny mornings, the PV production was satisfactory but no where near the 250W per panel ratings.
As Solar Noon approached, the production numbers got lower... WTH?

I did a bit of research and found PV panels like to be cooler... so I opened the doors on the storage shed during the day and the numbers shot-up drastically. They shot further up when I mounted both panels facing south, instead of south and north as shown in the pic.

I'm sure if I raised the panels off the roof-top, there would have been air flow to cool the panels.

However, my open-door experience sort of answers your "byproduct of keeping the attic cooler would be increased output for my solar panels" question.

[J.P.M.]

Thinking about experimenting with connecting an external air intake to my duct work then running the whole house fan instead of the AC unit. The air would pull through the ducts, come out the vents then exit through the attic. Moving so much air through the attic should keep it cooler. I'm wondering if a byproduct of keeping the attic cooler would be increased output for my solar panels. Has anyone noticed a difference in solar output when cooling their attic vs. letting the summer heat build up in it?

Many (most ?) attics have dedicated vent vans. I'd start there. If there is a space between the roof deck and the panels of a few inches or so, I doubt lowering the attic temp 10 or 20 deg. F. will make a measureable difference in the array output.

[inetdog]

But blowing the exhaust air past the underside of the panels, possibly in a way that would also pull more outside air into the stream (entrainment) could cool the panels more than free air ventilation.
You would have to experiment and monitor air and panel temperatures carefully to be sure whether it would be better or worse than free flow cooling alone.

[russ]

The air would pull through the ducts, come out the vents then exit through the attic. Moving so much air through the attic should keep it cooler.

Not hot where you are? Don't need the AC?

[J.P.M.]

But blowing the exhaust air past the underside of the panels, possibly in a way that would also pull more outside air into the stream (entrainment) could cool the panels more than free air ventilation.
You would have to experiment and monitor air and panel temperatures carefully to be sure whether it would be better or worse than free flow cooling alone.

If you want to keep the attic cooler, ventilate it when it's cooler outside than inside the attic. Most folks find using a dedicated fan less hassle, more reliable and less expensive. Question: If one goal is to keep the panels as cool as possible, why and how would blowing air across them that is likely as hot or hotter than the roof ambient air help accomplish this ? Might very well have the opposite effect.

[inetdog]

If you want to keep the attic cooler, ventilate it when it's cooler outside than inside the attic. Most folks find using a dedicated fan less hassle, more reliable and less expensive. Question: If one goal is to keep the panels as cool as possible, why and how would blowing air across them that is likely as hot or hotter than the roof ambient air help accomplish this ? Might very well have the opposite effect.

Absolutely right, which is why I suggested measurement and experiment. But if you are ventilating the attic during the day you might not be pushing hotter air.
The fact that the roof under the panels will not be getting direct solar heating except by the heat off the back of the panels while it will be getting heating from below based on the sun hitting all the rest of the roof could be the deciding factor if the array covers only part of the roof area.

In the case of the shed, it looks like the panels cover the whole roof.

[DavidT]

Thanks for all the replies. Good stuff.

I live and Maryland and it does get hot in the Summer. But we set our AC to 80 degrees, so it’s not like we need the unit spitting ice cubes at us. There are plenty of days where we could get away with just ventilation, but opening and closing windows is not practical. We’ll certainly need to use the AC at times though. There lies the first dilemma.. the “C” in “AC” stands for conditioning, not cooling. A couple folks I’ve run this by have mentioned concerns about humidity. Not sure if that’s a comfort concern or health concern (potential for mold) or both.

My 33 panel system is rail mounted, so it sits approx 4 inches off the roof. Another good point was made here that the panels will block a lot of the heat from hitting my roof now. I’m sure the attic is still going to heat up some though. My attic does have both soffit and ridge vents. I already have a whole house fan installed, but no attic venting fan. In prior Summers the heat would deform candles. I wonder…. could the attic heat actually help the panels?.. Could the radiant heat cause a convection current between the roof and the panels which could help dissipate the heat the panels are getting directly from the sun? Would be a bummer if a byproduct turned out to be a detriment.

[inetdog]

. Could the radiant heat cause a convection current between the roof and the panels which could help dissipate the heat the panels are getting directly from the sun? Would be a bummer if a byproduct turned out to be a detriment.

Any radiant heat or conducted heat from the underside of the panels would, still just IMHO, dominate over any air conduction driven by heat coming up from the roof.
a hot roof surface cannot make cooling of the panels any better.

[Volusiano]

I wonder if the energy drawn from the fan to cool the attic negates any energy savings you stand to get from cooler operating panels, ASSUMING that a cool attic can result in cooler panels, which we don't know for sure. The shed example is not a good one because the panels sit right on top of the shed's roof without any clearance underneath like a normal installation would be.

Another thing to consider is that if the panels are already keeping that area of the attic cooler than the rest thanks to the shading it gives, circulating air up in the attic may simply equalize the attic temperature and cause the cooler area under the panels (thanks to the shading) to become uniformly warmer like the rest of the attic, losing the benefit of the shade from the panels.

I think if you want to cool the attic as a home to improve whole house energy efficiency, that's fine. But doing it strictly for the purpose of helping the panels stay cooler (IF it's any help at all, we don't even know) is overkill.

Another consideration is that if the roof is already designed with convection cooling in mind using softits and ridge vents or what-have-you, blowing air around in the attic now may interfere with the operation of the convection cooling and cause a decrease in attic cooling efficiency instead.

[DavidT]

The primary objective is to bypass having to use the AC as much as possible. That unit is drawing several thousand watts whenever the compressor is running. The volume of air that a whole house fan moves would have to lower the attic temperature some. So IF lowering that temperature increases panel output at all, that would be a bonus. Between the AC and the swimming pool pump, Summertime is by far my highest electric consumption season (Gas heat and hot water).

Not sure what the payback would be for a project like this. I’d probably need to swap in a two speed motor for the fan and some type of thermostatic control. I wish I could experiment with this without having to butcher my duct work when attaching an intake.

[Volusiano]

Between the AC and the swimming pool pump, Summertime is by far my highest electric consumption season (Gas heat and hot water).

As soon as I saw you mention the pool pump, I gotta ask if you have already switched over to a variable pool pump yet? If not yet, I would focus on that first. I used to have a single speed 3HP pump drawing 2550W at 3450rpm. I recently switched to a 3HP variable speed pump and set it to run at 1400rpm and now it only draws 250W (1/10 of the energy). I have to run it twice as long for the same turn-over rate, but that's still an 80% energy saving to me. And the old pool pump sucked up 30% of my total energy bill. So that's a low hanging fruit that can pay for itself in easily just a couple of years.

[DavidT]

As soon as I saw you mention the pool pump, I gotta ask if you have already switched over to a variable pool pump yet? If not yet, I would focus on that first. I used to have a single speed 3HP pump drawing 2550W at 3450rpm. I recently switched to a 3HP variable speed pump and set it to run at 1400rpm and now it only draws 250W (1/10 of the energy). I have to run it twice as long for the same turn-over rate, but that's still an 80% energy saving to me. And the old pool pump sucked up 30% of my total energy bill. So that's a low hanging fruit that can pay for itself in easily just a couple of years.

Yes, I have replaced the pool pump with a two speed pump. I only run it about 8-10 hours a day at low speed and the pool stays clear. Several years ago I did have a single speed pump and I pretty much let it run all Summer long, day and night. I cringe when I think of how much electric that motor was sucking up.

Like you said… go for the low lying fruit first. Two speed pump… LED and CFL light bulbs… all in place. I have to admit, it’s kind of fun treating finding ways to save on energy expense as a hobby.

Tinkering with a solar pool pump would be intriguing, but I think that would have to be just for the fun of it. Not sure if that would be cost effective since the main pump would still have to be maintained and used at times.

[Volusiano]

Tinkering with a solar pool pump would be intriguing, but I think that would have to be just for the fun of it. Not sure if that would be cost effective since the main pump would still have to be maintained and used at times.

If you already have a grid-tied solar system, I'm not sure why you'd need to do a solar pool pump anymore. Your existing pump in a way does run on solar energy already via your grid-tied solar system. Neither will a dedicated solar water heater makes sense.

[DavidT]

If you already have a grid-tied solar system, I'm not sure why you'd need to do a solar pool pump anymore. Your existing pump in a way does run on solar energy already via your grid-tied solar system. Neither will a dedicated solar water heater makes sense.

One of the reasons I come here is in case I need a dose of common sense. Good point re. the pool pump. My original question here involved using a whole house fan, and would the effect of cooling the attic help my Summertime solar production. A couple of folks – whose opinions I do respect – question the cost effectiveness of my idea. I do as well. But if a byproduct is increased solar production then it might be worthwhile. My solar system went live in late January, and while I’ve clipped some in Feb and March, I have no history yet to know how the Summer heat will impact my panel performance. If I’m still clipping in the Summer, then cooling the attic is sort of a mute point. But what I gather is that Summertime clipping is highly unlikely.