A Closed Loop, Roof Mounted Version of the $1K Solar Water Heater

This is a very interesting $1K style system with quite a few interesting design variations.  Among the unique features:

  • Closed loop rather than drain back for freeze protection
  • Glazed with dual wall polycarbonate glazing.
  • A unique collector frame that uses EPDM rubber roofing for weather protection.
  • Using a custom made PVC tank liner instead of the usual folded EPDM liner.
  • Lots of detail on roof mounting the collector, including the site made collector hoist shown just below.
  • A unique design approach to handing differential expansion issues in the absorber design.

hoist for roof mounted solar collector
Getting ready to hoist collector to roof

Thanks very much to Bill for sending in good detailed information on the system!!

All the Construction Details from Bill

This is a 13 page pdf manual from Bill with lots of detail on building the system... 

A few highlight pictures from the above pdf.

solar water heating system roof collector
Collector on roof.
solar collector EPDM cover
The EPDM weather protection wrap.
Bill's two piece fin design
Bill's unique fin design.
PVC liner for solar heat storage tank
Custom made PVC tank liner
solar tank heat exchanger
Collector loop and domestic hot
water preheat heat exchangers.
diy solar collector on roof
First collector on roof.

Some Thoughts on the Unique Features in Bill's Design

Bill provides full detail on the system in the pdf at the link above -- these are just a few small comments on some of the unique features of the system.  If you have any further suggestions, ideas, or comments, please use the Comments Section below to share them.

Roof mounted

Bill's collector is roof mounted, and he provides quite a bit of helpful data on the ins and outs of mounting a collector on the roof.  I really like the hoist that he worked out to get the collector up on the roof.  Its easy to underestimate how much of a job it is to get a heavy collector up to a roof, and having the hoist makes the whole operation much easier and safer.

Twinwall Polycarbonate Glazing

Bill used twinwall glazing for the collector.  I think this is a good choice for colder climates and will improve performance.  It also is very nice to work with and looks very good.

Custom  PVC Tank Liner

One interesting feature of Bill's tank is that it uses a custom made PVC tank liner from www.cover-tech.com  -- this proved to be a reasonable price alternate to using the single sheet EPDM liner that most of the $1K systems use.  It worked out particularly well for Bill in that his tank is relatively small and width and depth for its height, so it gets harder to fold a single piece EPDM liner in place.

I think the custom made PVC liner is definitely worth looking into if you are building a tank.

Collector Frame with EPDM Rubber Weather Protection

Bill's collector frame is very unique.  The outside is wrapped in EPDM rubber roofing membrane for weather protection.  It will be interesting to see how this does over time -- I'm inclined to think it will do well, as rubber roofs have a good reputation.

Another idea along this line that has been used by some is to wrap the outer surface of the collector frame with pre-painted aluminum flashing.

Another approach that I have been thinking about trying is to build the perimeter frame from MDO (Medium Density Overlay) plywood.  This is a premium plywood that has resin impregnated face sheets on both sides.  It is quite durable and is used for applications like concrete forms and chemical tanks.  It costs about twice as much as regular plywood, but may make a very durable collector box.  Protection of exposed cut edges might be the biggest issue, but they do seem to hold up pretty well even when not protected.

Closed Loop Plumbing

Bill decided to use a closed loop system with antifreeze for freeze protection rather than the usual $1K drain back system.  Bill did this because of the large vertical distance between the tank and the collector and because some sections of the plumbing route from collector to tank were hard to provide a consistent down slope on. 

If you are up against a similar situation, you can look at Bill's details and see an example of what converting to a closed loop system involves.  Basically an extra heat exchanger between the collector loop and the storage tank, and an expansion tank -- certainly manageable changes.

Alternatives to going to the closed loop system for high vertical lifts are to either use a single high lift pump (e.g. a Taco 009), or to use two smaller pumps connected in series with one of the two pumps only running during the startup period.  The two pump arrangement reduces the power required for pumping compared to the one larger pump arrangement. 

Another option is to locate a small drain back tank up closer to the collector in a non-freezing area.  Many commercial drain back systems use a small drain back tank and pump. 

Whatever you do, don't take a chance on having sections of the plumbing that do not have a good slope toward the tank -- this is a sure way to end up with freeze damage to the system.

Fin Design and Differential Expansion

Bill is concerned about the long term effects of differential expansion between the copper tubes and the aluminum fins, and has gone to a new variation on the fin to tube attachment to address this -- he describes this in detail in the pdf. 

 I understand where Bill is coming from on this, but my own sense is that in practice the differential expansion does not prove to be a problem.   I've just not seen or heard from anyone who has seen evidence of a problem, and some of these kinds of copper/alum collectors go back a very long time. 

I still have the original prototype for the copper/aluminum collector (built in 2008).  It has been used and abused quite a bit -- at first it was used in a small prototype domestic hot water system, then for a series of tests, and it just finished a full summer as an unglazed pool heating collector for a small swimming pool.  The prototype has spent quite a bit of time stagnated in full sun, so it has seen a lot of extreme temperature cycles.   I took a careful look at it yesterday, and tried to find any evidence of the aluminum separating from the copper.  I looked at the ends of many of the fins, and tried to pull the fin metal away from the copper.  In all cases, the alum fins were still very tightly bonded to the copper by the silicone.   In many cases, the staples that were used to hold the aluminum fins to the plywood backing had pulled out of the plywood, but the purpose of the staples is just to hold the aluminum in place until the silicone sets -- the silicone is the long term bond between the copper and the aluminum.

A number of different fin grooving and attachment designs have been tried -- most are described here...   My favorite to date is the one I used on the $2K system.   It is a little more work, but I believe it results in a little tighter a fit of aluminum to copper, and it eliminates the need for a plywood backing to staple into, which reduces the thermal mass of the collector.

Anyway, there are lots of fin designs to choose from now, including this new one from Bill.

Your Comments

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