Question on covering solar collectors?

[Captron]

OK, Back to the original OP’s quest: Overheating.

There are so many crazy claims and counter claims about ST that I find it hard to talk to people that already have their minds made up. Here is reality for 2020.

To be clear I am in the business of Solar Thermal and have been for almost 2 decades. We have a new system preferably for new buildings that uses interseasonal storage. We can store MWt in this area and its huge, so this is the best way to prevent overheating, hands down, and it extends the solar heating into dark times and winter.

Retrofitting in low caloric heating systems (ie radian heat) is possible. If you have an existing hydronic heating system, and enough room adjacent, the storage system can be installed there and integrated.

No moving on to solar collectors (not panels) for hot water that overheats. Yes you Can cover the collectors, but it inhibits their performance on poor days of insolation. So how do you keep them cool?
The easiest solution is to divert fluid when it reaches sub 100C let’s say 93C (200F) and run it through a cooler like a baseboard pipe with fins behind the header box on EvT collectors or anywhere behind flat plate, or run to a pool or some other heat dissipating location. Then you can walk away and not worry about overheating. There are several ways to do this, so I won’t go into them here.

Efficiency: An EvT collector typically exceeds 80c efficiency. A PV panel rarely exceeds 20% efficiency.

EvT vs Flat Plate: In cold areas EvT collectors are the option of choice, they even run fairly well even at -30c, so for heating and even hot water they rule. In locations where winters are rarely colder than say -6c (20f), there a new flat plate collectors that will do, if less costly. In really warm winter locations it doesn’t matter.

EvT issues: If your solar EvT tubes are older than 5 years old there can be design issues for some EVT units. There are 3 main types of EvT’s:
Basic Heat Pipe collectors, the typical heat pipe installation: Typically double wall glass with vacuum between and no vacuum in the center. Older collectors had issues with cold weather. The interior of the heat pipe is a narrow heat absorbing pipe typically copper, where a moderate high vacuum is pulled and a water fluid injected. The new fluids have a copper content to stop them freezing up and warping the pipe. They work even in very cold weather. Heat pipe EvT’s are great for Drainback systems as the header allows the Drainback. If you have old Heat Pipes, the good news is you can USUALLY replace them, fairly inexpensively.
High temp heat pipes: These operate as above but are sealed through the glass, usually single glass with vacuum inside. Not recommended for residential use.
U tube EvT’s: Less common but These are the most efficient, often as high as 90%, but lack any control of heating except by increasing heat transfer fluid flow. They don’t drainback due to the network of headers and pipes. Only the glass double envelope can slide off during service.

So there is a primer on these collectors.

And those chatting about Absorption or adsorption cooling/AC, we are deep into this at the moment, to be released next year. One common misconception is PV is a better deal. Only rarely will this happen. A. The area for the PV panels is 4X greater, B. A PV system requires batteries typically to run at night, C. 4X the PV system greatly reduces the cost benefit, and if we are talking about a AC with a compressor, most people forget about the huge draw when starting up. This means the PV system must be over sized by over 150%/r as a rule. ST however can easily store heat or cooling, and store cooling. It not as efficient as a compressor but it doesn’t have to be.

[LucMan]

All that is probably 110 % true for most applications, at least in most of the developed world.

But the OP seems to be looking for thoughts on ways to avoid overheating with respect to an existing and apparently serviceable solar thermal (space heating ? or perhaps seasonal ?) system.

The best answer I can give to that is to convert to a drain back system, remove the glycol fill with plain water, end of problem.

[J.P.M.]

Solar thermal has become obsolete with the current price of PV panels, heat pump water heaters, and Hyper Heat heat pumps and mini splits. PV is maintenance free, no danger of overheating or freezing, no pump failures, no possibility of leaks and the best part of all is the availability of net metering.
There is also the option of using air to water heat pumps if radiant heating, and cooling is desired. Solar thermal is no longer a viable option.

All that is probably 110 % true for most applications, at least in most of the developed world.

But the OP seems to be looking for thoughts on ways to avoid overheating with respect to an existing and apparently serviceable solar thermal (space heating ? or perhaps seasonal ?) system.

[LucMan]

Solar thermal has become obsolete with the current price of PV panels, heat pump water heaters, and Hyper Heat heat pumps and mini splits. PV is maintenance free, no danger of overheating or freezing, no pump failures, no possibility of leaks and the best part of all is the availability of net metering.
There is also the option of using air to water heat pumps if radiant heating, and cooling is desired. Solar thermal is no longer a viable option.

[bcroe]

Its always interesting how different parts of the world and regions have different approaches to heating and cooling. Evaporative coolers are popular in the western US due to drier climate while they are non existent in the eastern US where relative humidity tends to be high during hot weather.

Yes every area has different needs. Here it seems most advertisements are for
the oldest, noisiest, least efficient technology, but some change is appearing.

Here as we today have quite high temps and high humidity, my mini splits are at peak
efficiency, while being so quiet it is hard to tell when they are even running. Being able
to heat and cool, I have no need to supplement with gas heat some parts of the year.
Not even being connected to the gas line saves me not only the energy charges, but
the very expensive minimum connection fees that keep rising. The electricity comes
from my PV panels, which do not need to be close (and are 200 meters away).

The energy systems here have no effect on my income taxes beyond the installation
year. Bruce Roe

[peakbagger]

Its always interesting how different parts of the world and regions have different approaches to heating and cooling. Evaporative coolers are popular in the western US due to drier climate while they are non existent in the eastern US where relative humidity tends to be high during hot weather. Local incentives also factor in. I looked for some of the manufacturers you mentioned and didnt find any US distributors. Generally manufacturers follow the money and sell products where they can make money, if they dont, distributors move in. In this case of absorption chillers in the US neither seems to have happened. Therefore its an opportunity for someone to make their next fortune or far more likely that the economics do not work out.

I will keep an eye out for these smaller capacity units and if and when they become commercially available in the US then its worth considering them in some areas.where the climate lines ups with their benefits. .

[alpcool]

Of course, these systems are very expensive, but have become much cheaper in recent years + with the ionic liquids, the production costs are reduced even further.
If a sufficient number is ordered then the prices are higher.
Without strong subsidies here (tax relief, for example) or political increases in the price of electricity (which is what we like to have here), such systems are not interesting.if you don't have your own proper manufacturers, then import them from abroad. I also have an import machine from Germany + cooler from italy.





first of all what maintenance costs? I have no maintenance costs. Absorption refrigeration systems are very primitive.

coefficient of performance is somthin of the 90s or 2000`s. We do use since 200er energy efficiency ratio (EER).
EAW in 98630 Römhild for example in Germany "Li-Br." starting 15 KW ( Prior 5KW) and is selling in projects in Switzerland. Lets stay stay in outdated unity "COP" 0,75. Starting at 172,4 F°Normal
"COP" for manufacturers at our market + CE at the moment is between 0,7 - 0,83 (Manufacturer Entropie for his very mig unites for example 0,83) on Li-Br. Most prototypes with ionic liquids are between 0.8 and 0.85 " COP" starting between 68° C and 78° C and will start the next 2-5 years.

I am amazed at the efficiency of "typical" solar collectors in your market of "typical" 65%. Mybe in US you do use another word for "efficiency" ? My collectors from 1999 do have 70% here.
Optical efficiency: The maximum solar thermal efficiency is achieved when the solar power absorber is at the same temperature as the surroundings. However, 100% efficiency is impossible.
The efficiency of solar thermal energy usually drops / warmer the absorber is in comparison to the surroundings. With regard to this phenomenon, one speaks of “optical efficiency”. The average solar efficiency here is between 70 and 85 percent. Manufacturers report the values of the optical efficiency for their products, which enables the efficiency of the technical systems to be compared in order to find the optimal solar thermal product. Chinese do it here too.


In summary, your calculations do not work with our manufacturers and conditions. It looks like you do have completely outdated technology + no suppliers etc.. I recommend an absorption chiller from Germany EWA for example to everyone here in my Canton who owns an normal house + normal payed tax payer job and if he produces enough excess heat energy in the summer ( a lot of sollar collectors).
This has the advantage that you do not need the ugly "usual" air conditioning systems made in china, you save electricity and money, as well as noise and can at least strongly reduce your income tax burden here. You also protect the environment from unnecessary power plant systems + the escape of climate-damaging gases + does not have to cover solar panels in summer.

[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

[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.

[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.

[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

[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.

[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.

[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

[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.