Solar hot water collector and storage

Dreaming on solar hot water.  Mostly for household hot water but also for potentially augmenting household heating.  For those interested in this sort of thing I encourage you to poke as many holes and ask as many questions as possible.  Is my thinking correct and is it complete?  Are there simple things to improve it?

Doing a drain back system into a non pressurized tank looks like the best bet because of its simplicity.
Basic systems is a collector with collector and piping on a gentle slope of 1” in 20' or greater, a pump that runs when the collector is warmer than the tank and an insulated storage tank.  When I first encountered discussion on these systems about a decade ago I got stuck on the word “tank”  Several years ago I got to the youtube video on a wood box lined with insulation and a EDPM pond tank liner that completely changed my thinking.

Original video using the word tank.  5000 gallon system for US in Alaska go to 1 hour and 9 minutes and watch next 45 minutes for the why.  This would be the dream system.  But it is not suited  to retrofit

Dream system

Then tank video way more realistic for retro fit.  Needs the stratifiers from the previous video with one for each different temperature loop.  But the key point is it is pieces that can be carried down the stairs and assembled in the basement.

realistic tank

Only one heat exchanger and it will be between pressurized water and the tank water.  There again keep it simple.  Say a 100 to 200 foot spiral of PEX upward in the tank to preheat the incoming water before it goes into the true water heater.  200 feet of PEX is 2 gallons.  Most of the time when you are running water it is in shorter bursts of 2 gallons or less with short rests in between to allow new water time to warm before going to the water heater.  The water in this tubing inside the tank will be the only pressurized water in the system and being surround by water open to the atmosphere it can not possibly reach boiling so no protections needed on this part of the system.

For now lets just look at the collector.  Now the first piece of KISS for the collector is using PEX instead of copper for it.  There are several articles that show PEX is nearly as good as the copper provided it meets several rules.  Thin cross section which it meets.  Surround by the heat and steady applied heat which it can meet as part of design criteria.  Link for article.

Why plastic pipe may be fine data pdf.

Now can we eliminate all the fitting and connections?  The spiral heaters/collector that are all over the internet will do that.  If we properly implement that, it means all splices and connections can be inside the house and PEX itself is supposed to take some freezing, in case something goes wrong in the drain back part.  Problem is they  can't be sloped for successful drain back.  Look at picture A.  If that was stood up perpendicular to sun angle lots of places to trap water for potential freeze damage and potential for air lock and difficulty priming. But what if we laid it down flat?  Poor draining still.  But what if we made the spiral either a dome or a bowl  while keeping the mounting surface flat so it can do drain back?  This will have sun angle problems and poor winter light foot print. But if we add a reflector behind and a bit on both sides to aim the sun down we can mostly solve that.  By eliminating all the fitting and using PEX the cost compared to common collectors can greatly be reduced.  This also elimates the need for metal absorber plates to conduct heat to the tubing because we are simply using the tubing as both tubing and absorber plate.  Reflector is aluminum foil so also cheap and easy.  Better options?  Ways to improve the thinking?

The glass can then optimized for year round sun angle without affecting drain back. Now given my latitude of basically 45 degrees, optimal fixed glass angle is roughly 45 degrees for year round solar.  The piece of double pane glass I got as salvage is 53”x58”  So ideally my collector clear lid will be a bit larger than that.  58” is a non standard size glass so 60” or 62” should be my up slope length to allow for later cheap replacement of glass if needed as they are standard  Will probably design the lid for 56” x 64” to allow for frame material too.  Now the window needs to be rectangular but if the base is a symmetric trapezoid narrower at the back but tapering up to window width and full width at the front this can give me a focus effect concentrating more heat.  Less than 10% concentration over direct best solar foot print but some.



Top left drawing A. is the spiral as everyone is doing them for solar collectors.  Mostly they are making the coils touch but that means sun on only one side.    Top right drawing B is a messed up version of the thinking as it is too close to square but the rough ideal of how the base is shaped vs top glass shows even so.  The triangular ends will fold out at the top making the angle top rectangular in spite of the trapezoidal base. The actual base shape is shown in picture H.(sorry out of order)  The blue box not labeled shows the approximate coverage of the glass over the coils.  The angled in back wall covers the rest.   Next 3 pictures  C, D, E show noon sun pattern in the thinking.  Showing reflected light vs direct light.  Notice on the shortest day(aka winter) the reflecting surface extends beyond and above the glass to concentrate even more of the light on the collector from outside the glass.  Since nearly all of the light hitting the coils is reflected light there will be way bigger losses so the extension will likely only come close to breaking even vs summer and mostly will only work for the couple of hours near solar noon.  Also notice the coils are arranged in a dome not flat so the pipes can do the drain back.  Dome is chosen because it fits better under the sloping glass  Pipe slope needs to be roughly 1” in 20 feet so the farther out from the middle the steeper the dome will need to be.  Under it rather than a reflecting domen thinking a simple reflecting pyramid.  A bit of complexity not figured out is the middle to maximize gain. Plain pyramid or octagonal pyramid?  Guessing will want thicker material to screw into so probably ¾” plywood for the pyramid.

Now how do we adjust the height to maintain a nearly constant slope to the PEX?  Thinking is building little tubing clips to hold the pipe with a screw in the middle to hold the pipe up giving easily adjustable height while the clip just keeps the pipe roughly on top.  If we went with the simplest math for the 4'x4' square with a  10 inch circle out of the middle(minimum bend radius for PEX), 1/2” pex which is 5/8” outside would yield (24”-5”)/(5/8”) gives me potentially 30 wraps.  Now the support clips need a bit of space but the trapezoidal shape will give some extra room, so guessing 30 to maybe 35 wraps is pretty close to right.  Near the edge the tubing will fall enough it can actually be stacked a bit too as there will be rough 16 to 20 feet of fall in the outer wrap meaning a full inch lower for a single wrap.  So between shape and stacking guessing I can have 1/8” to 1/4” minimum separation between coils in the tight areas, letting sun down below to reflect up heating some from the bottom side too. (of course at the corners the gaps will be bigger) If it is 1/8” gap that means that  13% of light will potential leak back up having been missed and the rest should make heat on the bottom side either heating the reflector or heating the bottom of the pipes.  Small lose but gives expansion / contraction room and better heat distribution and trapping.  Now if I needed 8 screws per wrap that would be 240 screws.  Will need fewer in the middle and more on the outer perimeter.  So thinking on the order of 250 screws and clips.  Neat thing is if I guess wrong  and need more simply add screws and more clips  Cut the pipe diagonal  to make the clips so it has pieces roughly ¾” long on the bottom and nearly zero on the top making a C shaped clip that snaps on to the pipe.  Drill the bottom part of the clip with a counter sink drill so the screw head can go in and not pull off.  The pipe inside the clip will trap it on the bottom.  Will this need some sort of metal shim inside to protect the pipe from the screw? (Movement of the pipe cause the screw head to wear into the PEX?)  Build the coils from the outside in and simply put screws in to maintain slope.  

Most of the designs are using ¾ black poly pipe and what I got was ½” oxygen barrier PEX.  Choose the PEX for freeze resistance(hopefully) and for oxygen barrier for UV resistance hopefully.  Black paint over the top should help with UV resistance also.  The fact that the collector is 5 or 10 psi worst case and that is purely water drag and PEX at 0 psi is rated 200 degrees F.  PEX derates pressure wise as temperature goes up.  Depending on brand at 100 psi it may be derated to 180 degrees.  Looking at that and the iteration in between if it is any where near linear I should be able to safely go to 190 to 195 degrees on the collector with some margin for error.  This collector it should not matter with but if I am using PEX in the tank as the heat exchanger with city water it is under pressure.  So the storage tank should top out about 180 to maybe 185 degrees to deal with derated coil holding the city water under pressure.  This collector from the other information on double pane collectors, should top out at 140 to 150 degrees worst case so the planning is for when it is dry with full sun or for when  a different type collector heats the tank hotter.  Eventually hope to add a second collector also doing drain back as an evacuated tube system which can exceed boiling potentially.  So the pump limit switch would need to turn off and drain back at that temperature.  Minimum bend radius on 1/2” PEX is 5 inches.  So a 10 diameter hole in the middle and corners I can't quite reach.

Drawing F is what would be a better ideal for reflector behind the pipe.  So it is shown for dreaming purposes but with absolutely no idea how to accomplish it.  For commercial use it might be molded but as DYI the pyramid basically wall papered in aluminum foil is the best idea I see.  Any suggestions that are cheap and easy?  It would allow slightly wider pipe spacing and probably better heat transfer.

Drawing H.  Yes I know out of order shows a more realistic look at how the top down on the coils will look.



The glass needs hinge up next to the back wall with arms extending into space cleared behind the back wall so a counter balance weight can be added to help lift that glass as it is heavy and will be worse with the added frame.  The reflector outside the glass maybe should hinge with the glass or be removable as a separate piece.  Might even want to hinge so it is adjustable angle too?  Another part of the planning is the need for a built in set of supports so a work scaffolding can be assembled inside the collectors to wash the glass and work over the tubing without harming the tubing.  Mounts for lid supports while doing this work need to be included too.  Storage for the support scaffolding between the sloped wall of the collector and the house wall maybe?

So that completes the thinking on the collector itself lets look at cabinet design next and over all system assembly. This brings us to drawing G which is a cross section of the collector and cabinet.



Now about the base under the collector.  Since the neighbors cows will be by I don't want an angled glass down where they can step on it.  That means raising the collector by 2 to 3 feet.(and probably adding some sort of bumper as well?) I would like to add more heat gain.  The windows I have to use to are 3 feet.  How do I take advantage of that.  1.  It gives room for snow to slide off the sloping glass and fall. 2.  It gives room for a vertical glass so I have a space for a vertical glass front.    The reason for doing this in glass is to create a preheater for the angled glass above to melt the snow off if the main glass is covered in snow.  Goal is to do the same air based collector type on a smaller scale that I am running on the other end of the house already for household heating.  Glazing, multi layer fiber glass window screen painted with black bbq paint as the absorber and an aluminum foil covered back wall reflecting any sunlight missed in the first pass thru the screens back at the back of the screen.  Draw a the air in from the bottom back of the upper collector box.  Across the roof of the base box, down behind the front wall of the vertical collector and up thru the screen part of the collector.  Then discharge it under front of the angled glass.  It should naturally convect till it reaches a hot stall temperature with a temperature of 100 to 120 degrees.  The heat radiating thru the angled glass should melt it off without any effort.  It also adds more BTU's to the total system in winter.  So what do I do with the rest of that space inside the base?  2 possibles.  1. storage, possibly for the scaffolding to work inside the collector, 2.  insulated cat house.  I am leaning towards doing the cat house in the lower space.  Build it right and it would be the equivalent of an envelope home in miniature.  One other detail to add here is that the bottom edge of the sloped glass needs to be protected but protector should be as sloped and/or smooth as possible so the snow has as little to stick to as possible.

Now about the scaffolding to go over the coils.  This is done to provide some place to be while washing the inside of the glass and someplace to be while repairing or adjusting the PEX pipe.  If it is done as separate boards then one small section at a time can be taken out to work on sections of coil.  Thinking is to store them in the triangular space behind the back reflector wall of the collector between it and the house wall.  Each board will be a different length.  Will likely want notches in the side for each to lock into.  Since the notches will potentially catch concentrated reflected heat(fire hazard?) will a removable cover be needed.  Probably just a piece of aluminum flashing hung on some nails on both sides.

Another detail that needs to be included in the thinking is radically changing temperatures is going to cause the volume of the box to breath in and out.  So needs screens and air filters somewhere to keep dust and bugs out to minimize how often it is opened to be cleaned.   Breather screen in one end of the storage triangle maybe and filter mounted on the back wall of the collector down low.  Likely want a bigger filter with a small breather hole to minimize heat loss.

One final possible wild thought on this.  This is dream thinking and if done will be a later retrofit. What if the mini roof was replaced with an insulated hot box.  If the collector is exceeding 140 degrees could this be turned into a slow cook solar oven.  Insulated box heated by convection from below.  Put some sort of welded steel box in it fill with wax with a melting point just over 140 degrees that surround a oven type space.  The phase change of the wax would store this higher heat to hold the oven hot for longer periods.  Would likely need steps to get high enough to add and remove things.  Say 1/2 inch of shell,  4 inches of insulation, 1 inch air gap, 2 inches of steel and wax.  Times 2 =15 inches.  My good roaster is just under 11 for 26 total.   Will have roughly 22 to work with at the top behind the exterior reflector section so if the collector moved 4 inches out from the wall it might be workable.  Another reason to think about this it it would let the house wall breathe better.

That completes the collector part of this write up. What might I be missing so far?  Now on storage part of the discussion.  

Now that we have the collector thinking in, it is time to look at moving that heat in and storing it.

Now a quick discussion of drain back systems.  By having my storage tank in the basement and the collector 3 to 7 feet above ground level this is basically the ideal location.  Here is a video with a really good discussion of doing drain back systems and the advantages and how and why to use it to simplify the system.  You only need the first 10 minutes as the rest is flat plate collector info that does not apply here.

Really good drain back explanation.

The other collector videos I linked have more complex systems.  I want to borrow the stratifier from the 5000 gallon Alaskan tank system.  In fact in the long term I think I need maybe as many as 6 in my tank for various returns from various systems.  But for now I only need the one.  The piping because of temperature would need to be CPVP and the shell pipe at say 4” is bit pricey especially if I wanted to install 40 feet of it from the beginning. (all 6 downspout pipes).  But what if I added a short piece to get it below the water line and simply hung a nylon fabric sock off it with a weight ring at the bottom.  Lowest max for nylon is 180 degress and many types are well over 200.  That way I could wad it up and drop new ones in later.  So only the one needed to be installed to start.  To punch the outlet breather holes in the sock if a sharp edge hot cutter pipe was use it would heat fuse the edges of the holes so the couldn't unravel and would be fast to make.  The T's and spacer pipes of the inner pipe system could be assembled one piece at a time over the tank maybe using stainless screws instead of glue so they can be removed if they later needed too and can be done with a low ceiling over the work area.  After all minor leaks at the joints would probably be a good thing as part of the diffuser system.  

The pump is the next to last piece.  The pump I have for this is 24V DC so it should readily run off a solar panel eventually but is easy to power off its brick and AC so dual potential power systems  It is high efficiency and only uses 30 watts.  It will only have to run when the sun shines so no batteries for this part.  At best it will run about 6 hours a day summer and winter because that is the only time the collector will have sun.  On the longest day of the year the sun doesn't get around to the front of the house until 9:30 to 10 and goes away proportionally.  Winter that is your whole of the good part of the sunny day.   It is a magnetic drive so no seals to fail and has a nearly 4 meter head rating.  Just over 2 of that will be used in lifting to the high point in the collector leaving 1.5 meters of that head to overcome the drag of the tubing.  Pump is not self priming so a shut off in case of low water needs to be designed in.

The final part of the water circulation is system is suction snorkel.  To reduce the risk of draining the tank in the event of a failure want to come thru the wall at about 4 foot water depth with one snorkel going clear to the bottom of the tank.  That way the pump should always have 2 feet of head above it to prime the pump.  Outside the tank a sight glass tube up from that fitting to see water level.  It will also serve as an air break to stop the tank from siphoning the bottom 2/3 in the event of a leak hopefully reducing spill amounts.  Have wondered if a second snorkel is needed if using part of the capacity of the tank for air conditioning?  I can make arguments both ways.  For now the fact that it increases both risk and cost has me thinking not to do it.  By only having one bulkhead fitting thru the tank wall and having it up 2/3 or more of the water depth I think the risks can be minimized.  All other lines,fittings and connections come in from the top of the tank  The snorkel does need to be at least somewhat insulated to keep it from cooling the upper tank.  May want a second snorkel if needed for air condition use too. Plan is to  do a snorkel inside a snorkel meaning I need to install a bigger bulkhead style fitting thru the tank wall initially.  Lesson learned the hard way in the shop circulation system is rust destroys magnet drive pumps so the suction snorkel need a magnet filter to keep protect the pump.

And that brings us to the “tank”.  When I first got to these collectors I got stuck on the word tank.  Round and metal was the thinking.   No real way to retrofit that into my basement. Then I got to the first of the videos showing a wood box lined with insulation and a EDPM pond liner or other plastic high temperature liner.  2X4 or 2x6 frame work and plywood liner.  Carry it to the basement as panels and assemble there.  And size is easily modified to match my space.  The light bulb goes on.  This I can do!  EDPM liner is rated to 200 F.  Polyiso insulation is over 200  The PEX line to wind around inside the tank is rated to 200 degrees F if it is not under pressure.  Under the high end of  city water pressure as preheat for the main water heater and worst tubing I have found I still has a  180 degrees  temperature/pressure derate with a safety factor.  If I can find a metal replacement for that line then I could go 200.  And boiling point of water at this elevation is just over 203 degrees so the tank automatically levels out at basically an acceptable temperature assuming I could ever get it that hot.  So everything is coming out roughly even with temperature being regulated by physics for worst case potential problems.  Using the PEX hot water line through the tank as the heat exchanger gives roughly 2 gallons preheated to go into the main water heater.  Most of the things I do run under that 2 gallons for a given burst.  Each cycle of the dishwasher is about 2 gallons.  Washing machine is only major one that runs over.

Current thinking is rather than a 2X framework holding the plywood lets go with thicker plywood and use some steel to carry some of the side loads.  It will be more expensive but will be in smaller area.  Saving 7inches to 11 inches of space on the tank.  Floor to ceiling in the basement is just over 7 feet.  Out from the wall is at best 44 inches without major disruptions in traffic patterns and length probably 4 to 6 feet.  Materials available will sort of dictate the size of the box.  One potential cost cutting measure here is if I can get sheet material pallet toppers from local manufacturing.  Size of those sheets would set the more efficient sizes.  So for now the numbers are floating.  This gives a tank ranging from roughly 300 to 500 gallons.  Walls would be ½” for air gap, 1½ “ for plywood and 6” inches of insulation.  Best bet would be 6 inches of polyiso or some of the commercial aircrete giving an R 6 per inch thus an R 36 for the insulation.  Current thinking is I want to change the inner 1” or 2” to rockwool.  The goal it to plan for leak detection of the liner.  Rockwool is not supposed to be hurt by water and it is not supposed to support bacterial growth.  Thus if the liner leaks and it can get in the rockwool it can work its to a drain with moisture detection.  Then a poly liner between the rock wool and the main insulation as a backup leak barrier.  I get one other advantage out of this in the high thermal mass of the rock wool but at the cost of  R2 to R4 on the final insulation level.  Rockwool is rated for a maximum of 7 psi compression with minimal insulation loss.  If I have 6 feet of water at 0.433 psi per foot that is less than 3 psi worst so acceptable that direction.  

Now about top and bottom.  

Only figuring 6 inches on the bottom.  1/2” plastic blocks so the wood doesn't touch the concrete and to provide air flow between the floor and the plywood with 1½” of plywood + wood spacers built on a taper so the leak warning drain point can come out one corner that is lower than the rest.  Then 2 inches polyiso, secondary poly liner and 2 inches rockwool.  Lower insulation value at the bottom of a tank designed to stratify because the lowest water should insulate too plus it should likely never be as hot or as cold.  Concern here is if I ended up doing air conditioning too in this system or a 2nd tank and doing a folded path tank.  

Originally my thought was the top of the tank needed to be complicated.  Concern was it breathing out humidity daily into the basement and if the tank wasn't mostly staying at 140 degrees or more potentially Legionella.  But I came to realize the amount of air that needed to move was smaller in one way and larger in another.  If the air gap for stratifier making this open loop is inside the sealed system the water will replace the air in the collector and vice versa so that volume won't change directly.  Now when the water is back in the tank for the night and the collector potentially cools to say 40 below that air outside in the tubing will shrink greatly drawing more air in is the bigger one.  Other one is as the air temperature change as water temperature changes inside the tank.  But I think I can nearly completely deal with all of those with an air bag hung up in the floor joists.  Hanging vertically its sides can expand in and out with almost no change in pressure and it can extend a fair ways if needed or even to multiple joist cavities.  Primay criteria is the top be held sort of open (stick held by a couple of magnets inside the bag so no breaks) and the bottom be sloped so any condensation goes back inside the tank.  If a breather is still needed add a copper riser plate type breather from alcohol distallation.  Water seal and because that water is in a region of high copper content it will kill off any disease organisms while removing most of the moisture.  Don't want to seal system completely in case the collector reaches boiling and needs to vent.(no risk of it with just the collector but adding other systems might get it there.

Otherwise an insulated top using probably side wall level insulation continuous but with bags of insulation set up between the floor joists to boost this.  There will be some thermal bridging from this but the floor joist should be a lower conductor if surround by insulation on 3 sides

Short term probably just need the mounts for the stratifier air gaps at one end but dreaming for add ons So lets divide it into 2 sections with a curtain down into the water in between say 3 inches below normal water line.  Small section space for stratifiers.  Large section floated insulation.  What if we had a diaphragm floated on the water it will breathe out dry hot air instead with the air coming from wet side.  Beginning thinking was to build a raft of vacuum filled glass tubes (ideally mirrored) to act as an insulator.  Then I got to the vaccuum insulated glass windows.  The fully mirrored version there was R18 in less than 1/2”.  Would probably want the glass just off the water with a tiny bit of air gap.  Has the potential to greatly reduce upward heat loss but at serious cost.  But if included in the original thinking retrofit should be readily doable at some future date

Another major gain possible is the extreme insulation vacuum panels. If the outer layer of insulation was done with them would have great potential but also great cost.  R30 to R50 per inch.  There again high cost to implement to begin with.  Ideally sandwiched between the plywood and the first layer of other insulation so retrofit would mean draining the tank and disassembly.

So what else is involved in the tank.  Lines into tank.  Will need 1 or maybe 2 suctions.  Because all the best pumps are not self priming need this to be the bulk head fitting.  But to eliminate risk all the rest of the lines should go over the top edge of the tank in some form.
1. stratifier in for main collector
2. city water in and out as heat transfer from this preheat tank to the water heater.  At the very least it should reduce how hard the water heater has to work and at best it should get the water heat over its temperature so it never runs

From here on optional add ons to plan for the possibility.

3.  stratifier in for evacuated tube collector to increase water temperature
4.Brine line in and out loop for desiccant air conditioning.(almost no chance will need this one)
5. 3 ground loop stratifiers for summer AC and winter base load limiting how cold the house can get
6. stratifier for PV panels cooling circuit.

What other connections?  Will likely want temperature sensors at 3 to 5 levels in the tank.  Install long bend conduit sweeps to thread PEX lines in capped on the bottom end and put standard cheap sensors on the ends of wires down into them.  Will also want tank leak detection.  Simple float switch on the drain corner of the tank probably.  Will also want a low water sensor.  Thinking is to combine operations here.  Have the T line going up from the suction bulkhead fitting outside the tank.  Want it as a clear tube so I can see water level at a glance.  Probably want that as a float for better visibility so glass bottle with a magnet inside and a hall effect sensor near the bottom of the tube?  Now one other thought here is I would like the float to really show.  So uranium glass and a UV source to cause it to glow?  The reason to need low water sensing is to recognize it and keep the pumps from running since they are not self priming and low water could burn them up.

So what additional plumbing is needed to connect?  
1.  The cold water inlet to the hot water heater cut and 3 valves added.  A bypass valve so it can operate just as it does now or be blocked off to force city water thru the preheat cool in the tank and 2 valves to disconnect the preheat loop out into the tank in case of problems there.
2.  Will also want tempering valve added to the water heater outlet to keep the outlet below 120
when the preheat tank is over.
3. If a pump and check valve is added then it would be possible to use the water heater to directly heat the solar heat tank.(reverse of normal operation.)  Thinking this gives a potential back if using this tank for household heating too.  Should be extremely rarely used as it would be wasteful of energy.  During low use runs would also give a way to warm the water heater back up since it's tank is lower insulation than the storage tank and should cool off far faster.  Might want to extend the plumbing here to prime the heat out to the faucets at the same time.  That way 3 separate uses might be manage out of a single pump.

4.  If seriously hotter is managed.(would probably need the evacuated tube collector to reach) another tempering valve and another line to the dishwasher so it could run at preheat temps without preheating thus saving power.  Worth it??

Current pump thinking and what I have on hand for this is a Topsflo TD5.  Stainless steel head, 24V DC so easily solar powered or AC powered thru its power brick, magnetic drive so no seals to fail.

Primary control system is dead simple.  Check collector temperature and temperature at bottom of tank.  If collector is hotter than tank run pump.  If tank gets too hot also turn pump off.

One final thought on the main system.  Original plan was to build stands to set in the bottom to hold the coils of the city water PEX in the tank.  Later thinking was that this would be nearly impossible to service without emptying the tank.  So better thinking is to create a removable support structure up top and support the coils on stainless cables from the top instead.  This way take the top off the tankand basically wind the coils up and out in the narrow gap between the top of the tank and the floor joist.  If brine system added it would have to be added to that as a second hanging system.

Another wild and crazy thought.   I would like to be able to move the whole tank without draining it.  But a 500 gallon tank plus its material would be over 2 tons in weight.  Casters to move it would be big and expensive and take a lot of space.  But what if I could put a hover craft skirt around it?  Just over 3100 inches of base area meaning I would need less than 1½ psi to float the full tank of water on air cushion.  Thinking is say a perimeter built of say 3/4” heater hose with holes drilled so it vented to the inside.  Build the base so it mildly crushes when install.  Then it would pull into place and could be supported by an angle iron perimeter.  Do slightly less holes than needed so the hose inflates slightly.  As long as the concrete is smooth with no major holes the tank should float free on light air pressure.  The hose could then be pulled out so the base had good air flow under it to prevent mold grown and rotting wood in the base.  Alternate answer would be the flat lay woven poly hose in say 1 ½” as the skirt material.  Would be harder to anchor but easier to take in and out.

Most of this design is borrowing other peoples thinking.  There are only a few maybe innovations.  Mounting the spiral flat and reflecting light down into it I haven't seen anywhere.  Dome or dish for drain back system not seen anywhere.  Using a vertical collector to melt snow off the sloped collector not seen in this form and using a bag bellows to stop air interchange not mentioned anywhere.

So Thoughts?