Question on covering solar collectors?

[spareparts]

Hi, I'm some what new to the solar experience. Bought a home with solar collectors on the roof for heating fluid to warm the house in the cold months.

Now that the weather is warm we don't need the panels running. Our first year the glycol toasted in the sun during the summer months.

So I'm looking for ideas on how to cover the solar panels during the summer months. A local solar guy suggested pond liner.

These are 3 each 4x8 foot panels so I'm not quite sure if pond liner is the most economical.

Thanks, appreciate it.

[scrambler]

You could make a cover from a regular tarp, adding some straps to wrap around the corners

[J.P.M.]

I've got what is a workable solution for my situation: Corrugated fiberglass roof panels.

My water heating solar collectors (2 ea., 4' X 8') are mounted in portrait and one above the other on the roof with 6" free space between bottom of collector and top of roof deck. I have 2 ea., 2' (26" actual) x 8' corrugated roof panels covering the lower collector. The corrugated panels are affixed to the collectors with bungy cords that wrap around the collector/corrugated panel at 3 places, bottom, top and middle. The bungy cords stay at their location by attaching to roof supports at those bottom, middle and top locations.

While the method is not exact, rocket science detail and precision is not necessary. usually cover the lower collector on my system to help avoid overheating during the non winter/warmer/sunnier portions of the year. Been doing it since I redesigned the solar DHW system in 2005. The covering scheme has worked fine during that time. I got thye panels from big box

If you use the method, a comment/two:
- From the standpoint of thermally induced mechanical stresses, it's probably best to cover an entire panel. A partial cover over one collector doesn't hurt much, and I do it occasionally, but if you do partially cover a collector, be careful to partially cover it in a way that's perpendicular to the direction of the flow, that is, cover the bottom of a collector rather than the left or right side of a collector.
- Also, cover the coolest portion of any collector or array of collectors first. That'll cost a bit of collector/system efficiency, not doing so may well defeat some of the purpose of partially covering an array in the first place. Reason: Controller sensors are usually and probably best located at points where the coolant temps. are highest. Those are usually at the high points in the system. Covering the highest collector(s) may well "fake" the sensor that tells the collector to turn on/operate when the coolant temp. is high enough into thinking the coolant/system is not hot. Meanwhile, the lower (and now uncovered) collector(s) are really cooking under what are called stagnation (no coolant flow) conditions. Now, thermosiphon action will mitigate this condition some, but the pump will run less with the result that the lower collectors will run hot, maybe hotter than if nothing was covered. Cover the coolest parts of a system first.
- Make sure you have a secure place to store the panels from wind or other things.
- FWIW, I cut a 3" radius into the corner of each fiberglass panel. I lacerate my hands and arms less now than before I cut the radii.
- The bungie cords seem to last about 4 seasons before they lose their elasticity and/or fail.
- I started with white fiberglass panels. They were ugly. I switched to dark gray. I've got the system temps. dialed in pretty good to maybe ~ 0.1 deg. F. or so. The effect on collector outlet temps. from the color change is undetectable by any method I use.

For anyone reading this who is considering but does not yet have a solar thermal domestic water heating system, consider a collector design that has a mix/consideration of a tilt that's closer to the winter optimum, and a preliminary size that's close to the economic optimum for that tilt. Reason: Winter collection will be optimized and summer overheating will be minimized if not eliminated.

That's probably not the most cost effective tilt or design because of the < optimum single tilt for year round consideration, but it is one way to address the summer oversizing and resulting overheating situation if/when limited to a single tilt,

The problem with doing that is most folks are unaware of other things that come into play by doing so that will most likely complicate the design in other ways.

[spareparts]

Thanks, I appreciate the tips. Probably going to look in to using a tarp somehow. I like the idea of building rigid panels, but I don't have a place to store them during the cold months.

[J.P.M.]

Thanks, I appreciate the tips. Probably going to look in to using a tarp somehow. I like the idea of building rigid panels, but I don't have a place to store them during the cold months.

You're Welcome.

[sroof]

I've used a poly tarp to cover my panels when I go on summer vacation, but they wear out fast from the UV. I used a cloth painters drop cloth for a few days and it worked well. I like the idea of corrugated panels, or I was thinking a sheet of foil-covered rigid foam insulation might also work well. Just have to secure it well so the wind cannot get under any cover.

[peakbagger]

When I first installed my SHW panels long ago I mounted then conventionally lined up with a somewhat shallow roof pitch, the result was not enough heat in the spring and fall and way too much in the summer. I ended up tipping them up to a winter orientation. It extended my SHW season by a couple of months and reduced overheating in the summer. The only trade off is aesthetics. One plus is when snow builds up on the roof the snow slides down them and if its sunny day they will start putting some amount of limited warm water from the part that is not covered with snow.

There are poly tarps with silver color to them that seem to hold up far better than the blue ones. They arent that hard to customize, just cut them large and then fold the edges over. 3M sells special duct tape for tarps. Grommets kits are cheap and a couple of grommets on the corners where the tarp wraps behind the panels makes running bungee cords easy.

[spareparts]

Thanks for all the good tips.

Per a suggestion from a local solar tech, I ended up using pond liner and a bunch of cheap Harbor Freight 3" c-clamps to secure it to the framing of the collectors.

If the pond liner holds up, I'll attach it to some wood frames next spring, and rig up an attachment system.

I live at about 7500 feet in New Mexico and the UV kills a tarp pretty quick...many things actually. So hopefully this liner material holds up ok. Otherwise I may look in to that plastic roofing (can't recall what it's called right now) and see if I can get a piece big enough to do the job. My neighbor just got her roof done with it, and it's supposed to be rated for 15 years if I remember right.

[alpcool]

Why not use the hot water for an air conditioning system (absorption cooling) and regulate the room air or simply connect the shower, kitchen appliances and washing machine etc.. Covering the solar cells somehow sounds like a waste of energy

[peakbagger]

Absorption chillers are the "holy grail" for solar AC but I am not aware of anything less than 5 tons. They are inefficient so they need a lot of heat rejection capability.

[J.P.M.]

Why not use the hot water for an air conditioning system (absorption cooling) and regulate the room air or simply connect the shower, kitchen appliances and washing machine etc.. Covering the solar cells somehow sounds like a waste of energy

Solar DHW systems and loads are relatively small to try to gain any economic benefit from what's most likely the equipment costs and hassle involved for the relatively small amount of cooling provided by equipment sized for a DHW load would probably make absorption cooling an inappropriate method of using the excess energy.

It's probably easier and more cost effective to get a 1500 W, 5000 BTU window shaker.

Besides partially covering a DHW system that's oversized for summer duties and conditions, Peakbagger's way is probably the easiest and most practical way to modify an existing solar DHW system that has been sized and built for maximizing annual load without considerations for excess summertime production or summer overheating.

Design, sizing and orientation of DHW solar thermal systems to optimize for winter loads, conditions and irradiance availability instead of average total loads and so giving optimization of summer conditions second priority - beyond making sure overheating is accounted for - but otherwise letting those summer conditions sort of take care of themselves usually allows for less summer oversizing/overheating while still meeting most or all of the summer DHW load.

But, when designing from scratch, because DHW loads are relatively small with respect to most DHW collector size availability (usually in increments either 2m^2 or 4m^2 or so), some creativity in orientations is often necessary, or might be thought of as design options, both for tilt and azimuth and also shading, such as placing a DHW collector in partial summer shade such as under an overhang.

As for tilt, and as an example only, while 4 m^2 of solar thermal collectors orientated at (latitude - 10 deg. tilt) may not meet the winter design duty, it may well overheat in summer, at least partially due to higher summer ambient temps. as well as a higher summer solar zenith angles that result in increased irradiance.

6 m^2 at the same tilt will meet that winter duty but may well result in summer overheating. However, a 6 m^2 collector at a different tilt, say, lat. + 10 deg. or greater tilt may meet both duties (or most of them) and avoid summer over heating by virtue of the lower summer production from the less than optimum (higher) tilt, while still meeting all/most of that summer duty.

Another way to mitigate summer overheating - if, for example, flush mounting to a low slope (say 20 deg.) roof is mandated - is to partially cover the collector array in the summer. It's less cost effective but easier and, if done right, not too obtrusive. I use that method for my solar DHW system using dark grey corrugated fiberglass panels and bungie chords, but that's in an HOA that disallowed panels that are non parallel to the roof they're on when I constructed the system. Otherwise, it would have been at a 55 deg. tilt and about 3.3 m^2 instead of ~ 5.5 m^2 at a 18.75 deg. tilt as it is now. The solar fraction for either system is ~ > 0.95. The higher tilt system would have been less expensive by ~ 15 % or so.

In a prior residence and different location that's tough for winter solar DHW (Buffalo, NY, 43 N. lat., winter clearness index ~ 0.30 or less 4 months of the year), I found the most cost effective, practical orientation for annual collection was ~ 160 deg. azimuth and about 64 deg. tilt. Again, that choice is a bit complicated by the idea that the DHW loads are relatively small compared to the relatively large collector size increment and the fact that Buffalo weather in the winter is simply terrible (but I must say summers are about as good as it gets in the NE U.S., just not good enough to make up for the winters).

Take what you want of the above. Scrap the rest.

[alpcool]

Absorption chillers are the "holy grail" for solar AC but I am not aware of anything less than 5 tons. They are inefficient so they need a lot of heat rejection capability.

I suspect you do write about machines that work on the basis of lithium bromide / water ?. These machines also no longer weigh 5 tons, but can buy the smallest systems with the appropriate cooler together with a delivery weight of around 3 to 4 tons. The volume flow of hot water required for this has also been reduced to less than 3 tons per hour in the meantime.

no matter how, the absorption cooling systems with ionic liquids will massively reduce the delivery weights and the flow requirements. If there is sufficient demand, these systems can be produced at reasonable prices. Many usable ionic liquids are PH neutral, so they are not corrosive and are harmless to the environment. This does not require demanding materials that make a machine so "difficult".Of course, these systems will never be too productive like cold compression systems, but they will not release any nefarious environmental gases and will instead consume much less electricity.


can you make links in the forum? If so, I could make German links to some manufacturers. In the meantime there are some homeowners who use their solar energy in summer to cool the house. Me too since 2017

[alpcool]

Solar DHW systems and loads are relatively small to try to gain any economic benefit from what's most likely the equipment costs and hassle involved for the relatively small amount of cooling provided by equipment sized for a DHW load would probably make absorption cooling an inappropriate method of using the excess energy.

It's probably easier and more cost effective to get a 1500 W, 5000 BTU window shaker.

Besides partially covering a DHW system that's oversized for summer duties and conditions, Peakbagger's way is probably the easiest and most practical way to modify an existing solar DHW system that has been sized and built for maximizing annual load without considerations for excess summertime production or summer overheating.

Design, sizing and orientation of DHW solar thermal systems to optimize for winter loads, conditions and irradiance availability instead of average total loads and so giving optimization of summer conditions second priority - beyond making sure overheating is accounted for - but otherwise letting those summer conditions sort of take care of themselves usually allows for less summer oversizing/overheating while still meeting most or all of the summer DHW load.

But, when designing from scratch, because DHW loads are relatively small with respect to most DHW collector size availability (usually in increments either 2m^2 or 4m^2 or so), some creativity in orientations is often necessary, or might be thought of as design options, both for tilt and azimuth and also shading, such as placing a DHW collector in partial summer shade such as under an overhang.

As for tilt, and as an example only, while 4 m^2 of solar thermal collectors orientated at (latitude - 10 deg. tilt) may not meet the winter design duty, it may well overheat in summer, at least partially due to higher summer ambient temps. as well as a higher summer solar zenith angles that result in increased irradiance.

6 m^2 at the same tilt will meet that winter duty but may well result in summer overheating. However, a 6 m^2 collector at a different tilt, say, lat. + 10 deg. or greater tilt may meet both duties (or most of them) and avoid summer over heating by virtue of the lower summer production from the less than optimum (higher) tilt, while still meeting all/most of that summer duty.

Another way to mitigate summer overheating - if, for example, flush mounting to a low slope (say 20 deg.) roof is mandated - is to partially cover the collector array in the summer. It's less cost effective but easier and, if done right, not too obtrusive. I use that method for my solar DHW system using dark grey corrugated fiberglass panels and bungie chords, but that's in an HOA that disallowed panels that are non parallel to the roof they're on when I constructed the system. Otherwise, it would have been at a 55 deg. tilt and about 3.3 m^2 instead of ~ 5.5 m^2 at a 18.75 deg. tilt as it is now. The solar fraction for either system is ~ > 0.95. The higher tilt system would have been less expensive by ~ 15 % or so.

In a prior residence and different location that's tough for winter solar DHW (Buffalo, NY, 43 N. lat., winter clearness index ~ 0.30 or less 4 months of the year), I found the most cost effective, practical orientation for annual collection was ~ 160 deg. azimuth and about 64 deg. tilt. Again, that choice is a bit complicated by the idea that the DHW loads are relatively small compared to the relatively large collector size increment and the fact that Buffalo weather in the winter is simply terrible (but I must say summers are about as good as it gets in the NE U.S., just not good enough to make up for the winters).

Take what you want of the above. Scrap the rest.

What you wrote is certainly obvious, but also outdated and will not hold the future. I do not know his house, state, Law city etc. I imagine you have much more heat in summer than we do here in Switzerland do have. Don't know how much solar collectors he has and whether he has air conditioning in house. In my house (in Switzerland) I have 32m2 hot water collectors that I installed in 1999. In winter they do heat my house (with, support the gas heating). In summer I use the thermal energy to operate an absorption chiller. This produces up to 8 ° in Celisius cold water and cools my house ( 9 rooms) the same quality with out sound 3 air conditioning systems each with 2 KW cooling energy in past. The cold water goes into the radiators.
Electricity costs the equivalent of USD 0.14 per kilowatt hour here, the cold absorption machine I installed in 2017 did cost the equivalent of USD 39,000. I did deduct 70% of the acquisition costs and installation costs from the cantonal income tax over three years (we have municipal, cantonal and confederative income tax here).
I no longer have to cover my solar collectors in summer and save a lot of electricity. The A(Cs did heavy consume electricity + I also protect the environment from harmful greenhouse gases

In the meantime, there are still smaller machines based on lithium bromide that cost less than USD 30,000, but the first ionic liquid refrigeration machines also come onto the market. They are getting cheaper again.

[peakbagger]

I guess I should have qualified my comment to a current commercially viable supplier of small absorption chillers in the US. Converting three 2 KW cooling output units to BTUs gives me 20470 btu/hr or just under 2 tons. I can buy two cold climate minisplits of similar capacity for about $5000 installed, if I went with three smaller units I might be in it for $7,500. For the $25,000 difference I could buy a lot of solar panels to run these units and still have money in the bank. I tend to avoid suggesting new technology that does not appear to be on the market yet for current projects.

Sure the promise of small absorption chillers has been around for 30 plus years and at some point someone will start offering commercial designs but as of now I am unfamiliar with any. Yazaki has made smaller units for years but they seem to only get used where someone is willing to pay a big subsidy for them. I think the smallest they offered in the US was 5 tons (60,000 btu/hr) the last time I checked. I have installed a few larger absorption chillers on various CHP projects and in all cases the only reason they get installed is a heavy upfront utility subsidy and ongoing credits for waste heat utilization. The hot water units I am familiar running on 180 F input and 140F output at best have a COP of 0.65. Add in a typical solar hot water collector efficiency of 65% and the collectors get big and expensive and require much larger cooling towers to deal with all the waste heat. COPs on variable speed compressor minisplits are running in the 3.5 or higher range so even with 18% efficient solar electric panels, the economics dont appear to work well for absorption units. In general I find a lot more clients have removed them due to tube leaks and freezeups. I find that on the new installations we just design around the units limitations by baseloading them and running rotary units for varying load. With modern controls they do baseload pretty well but tube longevity is reportedly still an issue.

The other aspect is SHW type collectors have fallen out of style in the US, the initial capital cost is high and not well suited to a heating and cooling climate. As the OP has found out they put out lots of heat in the summer when there is little demand and far less in the winter when its needed.Yes I keep mine running as they are paid for but I would be hard pressed to recommend them to someone considering a new installation.

PV panels have dropped in price substantially so the current approach for hot water is a heat pump hot water heater fed from a PV system generally tied to net metering tariff.

By the way a ton of cooling is 12,000 btus/hr.

[J.P.M.]

I guess I should have qualified my comment to a current commercially viable supplier of small absorption chillers in the US. Converting three 2 KW cooling output units to BTUs gives me 20470 btu/hr or just under 2 tons. I can buy two cold climate minisplits of similar capacity for about $5000 installed, if I went with three smaller units I might be in it for $7,500. For the $25,000 difference I could buy a lot of solar panels to run these units and still have money in the bank. I tend to avoid suggesting new technology that does not appear to be on the market yet for current projects.

Sure the promise of small absorption chillers has been around for 30 plus years and at some point someone will start offering commercial designs but as of now I am unfamiliar with any. Yazaki has made smaller units for years but they seem to only get used where someone is willing to pay a big subsidy for them. I think the smallest they offered in the US was 5 tons (60,000 btu/hr) the last time I checked. I have installed a few larger absorption chillers on various CHP projects and in all cases the only reason they get installed is a heavy upfront utility subsidy and ongoing credits for waste heat utilization. The hot water units I am familiar running on 180 F input and 140F output at best have a COP of 0.65. Add in a typical solar hot water collector efficiency of 65% and the collectors get big and expensive and require much larger cooling towers to deal with all the waste heat. COPs on variable speed compressor minisplits are running in the 3.5 or higher range so even with 18% efficient solar electric panels, the economics dont appear to work well for absorption units. In general I find a lot more clients have removed them due to tube leaks and freezeups. I find that on the new installations we just design around the units limitations by baseloading them and running rotary units for varying load. With modern controls they do baseload pretty well but tube longevity is reportedly still an issue.

The other aspect is SHW type collectors have fallen out of style in the US, the initial capital cost is high and not well suited to a heating and cooling climate. As the OP has found out they put out lots of heat in the summer when there is little demand and far less in the winter when its needed.Yes I keep mine running as they are paid for but I would be hard pressed to recommend them to someone considering a new installation.

PV panels have dropped in price substantially so the current approach for hot water is a heat pump hot water heater fed from a PV system generally tied to net metering tariff.

By the way a ton of cooling is 12,000 btus/hr.

Amen.

As I wrote, for the relatively small system sizes as the OP's, or for most any SDHW system, for so small a cooling benefit to be gained from using/adding absorption cooling systems in domestic /household solar systems, besides the increased maint., without a lot of subsidies, the economics don't work out.

I