Convert my flat panel thermo siphon system to a pumped one

[Bladegrabber]

to update from my last post, I cleaned the panels internally which helped restore their efficiency and the system was performing much better.
the backup electric immersion element didn’t seem to be doing much so I opened up the tank and removed it to find it had 50mm of solid lime scale attached and the tank walls and bottom were covered with the same . After a good jet wash and manual removal of as much scale as I could get out it’s back online and working better.
My next plan is to install a thermal water store in the garage which is directly below the panels and convert the system to a pumped one
I can get a 500 Lt tank with integrated serpent coil for around £800 but shocked to see that pump units for the glycol mix are around £300 which seems very expensive for what they are ?

can anyone point me to best buys / manufacturers for the thermal store / heat exchanger ?
can I use a standard heating system pump for the glycol as I have a spare one ?
which differential temp controllers are good buys .....I don’t need anything fancy like remote access/ control systems .

thanks again for any advice offered which is all appreciated .

roy

[LucMan]

The pump units for glycol are set up so that purging the system of air is simplified, they also include check valves, pressure relief valves, and the "proper expansion" tank for the system. You can build your own and maybe save some money.
The components that I use are available in the US so that may not help you, Caleffi products should be available in Europe and they are high quality. Resol makes several controllers pick the simplest one or try to source a Tekmar 155 or 156.
I f your storage tank is undersized be aware that you may need a dump zone if the panels near stagnation temps.
I suggest looking into converting to a drain back system. This would require remounting the panels so that they can drain properly, 1/4" per ft slope. No glycol, expansion tank, pump package, or dump zone required just a 5 gallon SS drain back tank and pump.

[Bladegrabber]

Hi Lucman, thanks again for the good advice .
i need to look at a drain back design and see how it works but always happy to learn hence asking the questions .
will keep working it as there is no rush now that it’s actually working for us .
seasons greetings .

roy

[J.P.M.]

Thank you for the update. Do you plan on getting rid of/not using the old tank
Unless you increase the solar collector array surface area, changing the storage mass size by adding all that storage mass will lower the highest daily storage temp increrase by ~ 1-(200/(200+500)), or ~ 70% or so.

If it was me, once I got the system cleaned and working as you seem to report, and I'd live with for a yr. and keep an eye on performance, read up on solar DHW system design logic before I started what can be a big modification.

How often go you see temps. that would cause water to freeze in the panels or the lines ? Reason for the question: I'd guess southern Spain has a mediterranean type climate. I also have a mediterranean type climate. Because of my lower, but not zero probability of temps. and conditions such as low radiant sky temps. that will freeze collector water and burst a collector riser, I also have a direct type SDHW system that circulates potable H2O through the collectors w/out a secondary glycol loop. The controller fires up the pump and circulates water to avoid collector freezeups if the collector temperature sensors gets to ~ +5 C or so, circulating warm(er) tank H2O through the collectors for a couple of minutes. I estimate I lose maybe ~ 2% or so of the energy the system collects over a year's time with the method. While I'm intimately aware of freezing potential, and that it only takes one such incident to ruin a collector, mine is a common method of freeze protection that's been around for 50 + years that's now frowned upon but has not failed me (yet). I believe such controllers are still available. If interested, check Google and also check your local ordnances. Such controllers are not allowed in some areas.

Just offered as something to think about: If you're determined to change the system, maybe getting rid of the glycol loop would result in a better system. Just know the consequences and tradeoffs involved. Again, I'd respectfully suggest reading up on solar DHW systems if you change stuff. And, almost as an aside, if you do get any new tank, insulate the crap out of it.

On material compatibility, glycol/H2O systems aren't that much different from potable H2O systems for material requirements. Sometimes the seals and gaskets need more surveillance/maint/changeouts, but both need some maintenance. If your current system hasn't been maintained (?), it sounds like your water is about typical in terms of scaling. If it's been maintained, 5 cm. of scale would tell me you're water is pretty hard. Difficult to say w/out some history. Glycol loops need PH checks and probably fluid changeout every couple of years or so, maybe longer. Perhaps interestingly, if you went the other way, that is, from a potable water circulation system to a glycol/H2O system, you'd need to address/get rid of any zinc and aluminum contact with the glycol, but otherwise going from glycol/H2O to straight water shouldn't be a problem except as noted above.

Point of all this: If the water is not too hard and you're determined to get a bigger/different storage system, changing to a direct circulation system may have advantages - provided the water isn't so hard it'll foul/scale the inside of the collector headers and risers.

As a ref. only, my water's pretty hard, but after keeping an eye on pressure drops and max. system flow rates since the system was installed 04/2008, I'd guess - but not much better than a guess - the scale layering in the collector risers due to scaling from hard water to be less than ~ 1 mm in thickness or so, and that's assuming the scale layer top surface is as smooth as the surface of a new copper tube.

But whether or not you change the system, know that more storage without increasing the collector area of other measures such as side booster mirrors will lower the max. water temp. your system will achieve. That may or may not result in a better system depending on your goals. Too much storage and you'll need more aux. boost for high enough delivery temps. at the points of use. But, lower system operating temps. will improve system thermal efficiency. Too little storage mass relative to the collector area will result in perhaps too high a storage temp. and increased system thermal losses that will do little more than decrease system efficiency and produce water that's too hot to use w/out tempering before use.

One last point, water is a better heat transfer fluid than glycol/H2O. systems with water collect more heat and require less pumping power or (depending on the pump) produce a higher flowrate than the same system running with glycol/H2O mixtures. Most of the time, the improved performance is not a big amount, but if you change the system, consider putting a ball valve at the pump outlet to throttle the flow, as well as one on the inlet to also make pump maint. easier.

I'd be interested to read what Lucman's thoughts might be with respect to your most recent post.

Good luck.

[Bladegrabber]

Hi JPm, all good information and appreciated .
currently I have a 200L storage vessel on top of the collector frame and I could do with maybe 300-500L but that would be a new tank not incorporating the old one. I won’t be rushing into buying anything this year as you suggested I already planned to monitor what we have and see how it goes. The climate is south Mediterranean/ Malaga Spain so freezing is not an issue as nightime temps go down to around 10c at worst so I have already changed the old glycol out for water and it works just fine. As for reading up on systems I am doing that as well as learning from you guys as knowledge is power as they say.
we are seeing around 4 hours of collector support just now and I will increase that by cutting down a large tree that’s blocking the afternoon sun once I have the permit to remove it from the local authorities and that should give us more late afternoon sun hopefully.

the water here is not really hard and I’m guessing the scale problem was more to do with lack of maintenance by the previous owners but I will plan to open the system up again before we head north to the UK for spring / summer . Photo attached of the installed immersion element local scaling issue for info .14247BE5-FD6E-4B5F-8752-02F489AA90F9.jpeg

cheers
roy

[J.P.M.]

I'd only reiterate that increasing the size of the storage will, to a first approximation decrease the temp. gain of the storage over a collection period in approx. inverse proportion to the ratio of the new/old storage sizes.

A fixed size thermal collector system will gain a certain amount of heat over any collection period. So, say a solar DHW system manages to collect and deliver to storage (using customary units) 60,000 BTU net after collector, storage and piping losses over the course of a sunny day.

And, say the storage size is 120 gal ( = ~ 1,000 lbm of H2O or a little less). That means that the heat delivered to storage will increase the temp. of that 120 gal. storage by ~ ( 60,000 BTU/day)/(1,000 BTU/deg. F) = 60 deg. F./day.

Now, say you double the storage size to 240 gal ~ = 2,000 lbm. The collector system will, to a first approx., deliver about the same amount of heat to the larger storage (actually more because of lower system losses form lower system operating temps., but that's another story - holler more if you want details), but the temp. rise of the new, larger storage will be approx. half as much: (60,000 BTU/day)/(2,000 BTU/deg. F) = 30 deg. F/day.

Now, depending on how aux. heat is required to be added to the DHW before it gets to the point(s) of use, that may be just fine, and, because of the lower ave. system temps. that lead to lower thermal losses, in more than a few - but by no means all - circumstances that'll be a more efficient way to do things. But the increased storage size may well lead to more aux. energy being used over the course of a year to boost the (now) lower storage temps. up to a high enough point of use temp.

The moral of the story: Size the storage carefully. Too much is probably better than too little, but neither is as good as a goldilocks size. Very rough rule of thumb might be ~ 2 gal. of storage or so per ft^2 of collector surface (very ~~ 80 l H2O/m^2 of collector surface.

Too small storage sizes decrease system efficiency from higher system operating temps. that only increase system losses with no benefit. Such higher point of use temps. may also require heat dumping to avoid boilovers wasting still more energy.

Larger storage system capacities result in lower system capacity utilization and often cause increased aux. energy use to bring poiny of use temps. to required levels.

If you do a system redesign, one trick to consider for solar DHW systems is to use a higher than normal tilt for the thermal (note, NOT PV) collectors while using a less oversized storage. The winter collection increases over that of a lower collector tilt arrangement, and the tendency for summer overheating is somewhat mitigated by the (now) less than optimal tilt that's made less of a penalty by virtue of the generally higher summer ambient temps. which reduce the thermal losses. For example, the tilt that results in best annual collection for me with less/no summer overheating/heat dumping while attaining and max. winter heat gain is ~ latitude + 15 deg. or ~ 48-50 deg. collector tilt. That tilt may make things a bit harder to achieve, but solar energy utilization in nothing if not a bunch of tradeoffs anyway.

Add: If you are going to redesign the system as a direct system, and so intend to plumb it to have potable water circulating thorough the collectors, scrap any idea of using a collector loop separate from the storage. Pipe the storage directly through the collectors.

Reason: Using a "loop" or "coil" inside the storage tank (or an external heat exchanger) only introduces more losses and a bunch more surfaces to foul up and need replacing. All for no gain.

I worked with something called "Heat Exchanger Factor", and modified it to include multipass heat exchangers. Figure a performance penalty of ~ 8-10 % for a well designed system with a HX, over a direct system.

Also, and contrary to what's known as conventional wisdom (which I question as "wisdom") if a redesign, I'd consider plumbing the collectors in series rather than the usual parallel arrangement. The pumping requirements will increase and maybe (maybe - depending on the current pump) increase the pump size, but, and for a lot of reasons, the system will foul less and collect more heat for the same ambient conditions. My 2 collector system operates about 5-10 % more efficiently by virtue of flow velocities 2X higher than if the collectors were in parallel, but I'm running a bit higher flowrate than most DHW systems to begin with.

[Bladegrabber]

JPM thats great info and fascinating insight into storage optimisation so many thanks.

Will soak it all up and work with it to make future plans .

Roy

[J.P.M.]

JPM thats great info and fascinating insight into storage optimisation so many thanks.

Will soak it all up and work with it to make future plans .

Roy

You're most welcome. Take if for what it's worth. Most of it's no more than opinion based mostly on stuff in the open literature and what I've learned from an engineering career with process and power equipment. But know that if you stop at no more than the info you get on an unvetted source such as this forum, you're headed for trouble.

Good luck.

[LucMan]

Just for your reference 6 sq meters of flat plate collector is good for 300 litres, true south with a tilt angle of 35-40 degrees. You can expect 80 C water in in the summer. You can stretch that to 450 L but your temps will be around 50 C.
The age of your collectors may also decrease the max temperature of your water.

[J.P.M.]

Just for your reference 6 sq meters of flat plate collector is good for 300 litres, true south with a tilt angle of 35-40 degrees. You can expect 80 C water in in the summer. You can stretch that to 450 L but your temps will be around 50 C.
The age of your collectors may also decrease the max temperature of your water.

More anecdotal and site specific ref. info:

W/300 l (80 gal.) of storage, no daytime draw, about 5.5 m^2 of collector surface (2 Sunearth EC32's) @ 18.75 deg tilt and 195 az.., lat. 33.2, I'll lift the P. & T. relief valves (@ ~ 100 C.) most sunny days between 04/01 and 10/01 before 1400 solar time or so unless I cover some collector surface or dump some heat. I can easily get a 55-60 C rise in storage temp. over a sunny spring/summer/fall day. If I ever replace the tank, I'll increase the size to 450 l (120 gal.).

As best as I can estimate/guess, my solar and environmental situation is quite similar to the OP's.

If I was the OP and if, after some experience with the cleaned out system, I choose to redesign the system with all options on the table (including some serious conservation measures before any system changes as lifestyles permit), I might consider a 2 tank design but only add the second tank if things got regularly overheated, and then plumb the second tank so it could be bypassed if/as determined necessary. Still, a higher collector tilt and/or a slightly increased storage size may also be an option, or some mix of both and/or other options.

[Bladegrabber]

Gents, really good information thanks,I have 4M2 of collector surface area and approx 300L of storage and on a decent sunny day it’s warming up to 50C so I’m sure in the summer the system will work fine . For now I am going to distill all your good advice, read the books and try different ideas out on paper before deciding what to build . By the way I have a really good heat dump if the system did overheat and that’s a 56m3 swimming pool system so nothing will go to waste for sure.

roy

[LucMan]

More anecdotal and site specific ref. info:

W/300 l (80 gal.) of storage, no daytime draw, about 5.5 m^2 of collector surface (2 Sunearth EC32's) @ 18.75 deg tilt and 195 az.., lat. 33.2, I'll lift the P. & T. relief valves (@ ~ 100 C.) most sunny days between 04/01 and 10/01 before 1400 solar time or so unless I cover some collector surface or dump some heat. I can easily get a 55-60 C rise in storage temp. over a sunny spring/summer/fall day. If I ever replace the tank, I'll increase the size to 450 l (120 gal.).

As best as I can estimate/guess, my solar and environmental situation is quite similar to the OP's.

If I was the OP and if, after some experience with the cleaned out system, I choose to redesign the system with all options on the table (including some serious conservation measures before any system changes as lifestyles permit), I might consider a 2 tank design but only add the second tank if things got regularly overheated, and then plumb the second tank so it could be bypassed if/as determined necessary. Still, a higher collector tilt and/or a slightly increased storage size may also be an option, or some mix of both and/or other options.

Just 1 advantage of sunny southern California!
I have 2 80 gallon tanks, #2 is turned off at this time as I'm having trouble getting to 80F even on sunny days of which we have had 2 in the last 2 wks. Just me and the wife now so in the summer 80 gallons would suffice, but i harvest all the energy that I can anyway. Costs me an extra $.02 a day for running the pump a little longer. May, June, July, August I can go 3 days with out sunshine before I need to turn on the boiler for HW.

[J.P.M.]

Just 1 advantage of sunny southern California!
I have 2 80 gallon tanks, #2 is turned off at this time as I'm having trouble getting to 80F even on sunny days of which we have had 2 in the last 2 wks. Just me and the wife now so in the summer 80 gallons would suffice, but i harvest all the energy that I can anyway. Costs me an extra $.02 a day for running the pump a little longer. May, June, July, August I can go 3 days with out sunshine before I need to turn on the boiler for HW.

Thank you. Understood.

FWIW, today (12/13/2019) was about as clear as it gets around here this time of year with a clearness index of ~ 0.654. (== extraterrestrial horizontal insolation/global horizontal insolation at the array) during the period 0645 - 1530 hrs. when the water heater was unshaded.

The 300 l store temp. @ 0645 hrs. was ~ 52 C. With no draw during the day (SWMBO was Christmas shopping ), the store was ~ 76 - 77 C @ ~ 1530 hrs. when the collector shading began. Average ambient air temp. at the array over the same period was 21.8 C with an ave. wind velocity at the array of 1.4 m/sec (~ 3 MPH) over the period. The amb. air temp. at ground level on the (shaded) north side of the house for the same period was 16.5 C. The roof amb. air temp. commonly runs ~ 5-8 C warmer than the ground level amb. air temp., mostly as f(irradiance, wind velocity).

My average use of electricity to augment the solar water heating in Dec. is ~ 40-50 kWh/month, including standby losses. Total resistance heating for DHW is ~ 130 - 150 kWh/yr. The solar water heater provides the rest.

A far cry from the average winter days I remember from Buffalo.

[Bladegrabber]

Wow you guys record every possible paramator i am impressed. Our winter weather has been mixed this year with 70% clear sunny days upto 22c but the rest have been cloudy. Being a recreational pilot i have a different set of weather indices i monitor normally ie wind/ cloudbase and associated forecasts but now i will be adding much more to it. On a sunny day i am making 50c in the storage tank by 3pm when the sun drops too low to help and with an average backup supply of 8 KWH from a 1500W resistance element during the night when power is cheaper.

all the best
roy

[J.P.M.]

Wow you guys record every possible paramator i am impressed. Our winter weather has been mixed this year with 70% clear sunny days upto 22c but the rest have been cloudy. Being a recreational pilot i have a different set of weather indices i monitor normally ie wind/ cloudbase and associated forecasts but now i will be adding much more to it. On a sunny day i am making 50c in the storage tank by 3pm when the sun drops too low to help and with an average backup supply of 8 KWH from a 1500W resistance element during the night when power is cheaper.

all the best
roy

Not every possible parameter but a few more than skimmed over here.

As best as I can guess/est., your climate is similar to mine, including typically yearly insolation.

Suggestion: For solar thermal stuff, to measure the effect of performance improvements, or performance in general, measure storage temp. changes over a time period before and after the improvement while taking into account the effects that (collector temp. - amb. temp.) has on performance, not necessarily end storage temps. or increase over ambient temp. only.

Also, depending on what you want to measure, for ~ $1K U.S., you can get a Davis Instruments Pro II Plus Weather station. Semi professional and fit for the task. Weather including temps., barometer, wind vector, moisture, and global horizontal irradiance and more if you want it.

If you change/update the system, and want to get more system monitoring info, get something called a rotometer (a flow meter), a couple of pressure gauges - one on each side of the pump, a few valves for flow control, a few thermometers/thermocouples and a stopwatch.

If you're the curious type, Duffie & Beckman: Solar Engineering of Thermal Processes is still the solar thermal bible. It will explain most all there is that's useful with respect to solar thermal and more and help you to learn what to do w/all the instrumentation. You'll need some tech. background, but that tome will answer all your questions and more with respect to solar thermal. You want good information, go to the source.

Good luck.