Collector loop and domestic hot
water preheat heat exchangers.
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
If you have any thoughts or questions on the system,
please add a comment ...
