An Experimental Copper/Aluminum Solar Water Heating Collector

These pages cover an experimental solar collector for heating water that uses aluminum heat absorbing fins and copper pipes for conveying the heat transfer fluid. The key to good performance in this design is a good thermal connection between the aluminum fin and the copper pipe -- a lot of attention is given to providing this good thermal bond.

The aims of the collector are to: 1) performance that is very close to commercial collectors, 2) low cost - perhaps a quarter of commercial collectors, 3) long life, and 4) easy construction using readily available materials.

Copper/Aluminum Collector Concept

Solar water heating collectors are used to heat water for domestic use and also to provide hot water for space heating. The most common commercial collectors consist of copper fins that are soldered or welded to copper pipes. In operation, the sun heats the fins, which in turn transfer their heat to the copper pipes and then to the water flowing through the pipes. This is a good design that works efficiently and has a long track record. But, the design, does have some drawbacks from a build-it-yourself viewpoint: 1) the materials have become very expensive, 2) soldering or welding the copper pipes to the fins is time consuming and not a common DIY skill.

Note that the recent drop in copper prices have made the
copper collector more attractive.
The copper collector now costs about $5 per sqft compared to the
$6 per sqft when this was first written.
April 4, 2009

This design uses aluminum fins instead of copper fins over a grid of copper pipes. Since the thermal bonding of the aluminum fin to the copper pipe is a potential thermal bottleneck, every attempt is made to provide a good thermal bond.

The finished, full size, prototype collector using copper tubes and
aluminum fins.

Some overview pictures -- see the Construction section for much more detail.

The copper grid -- using copper pipe.

The aluminum heat transfer fins
that transfer absorbed solar heat
to the copper tubes.

The key to good performance without
soldered or welded fins is lots of
heat transfer area, and very good
thermal contact.

The potential advantages of this construction:

The collector is easy to build using ordinary tools and skills -- its a fairly easy DIY project.
The collector is about 1/4 th the cost of commercial collectors (a saving of about $700 on a 4X8 collector)
Materials to build the collector are locally available -- the cost and hassle of shipping large assembled collectors is avoided.
For one case I looked at, shipping on a commercial collector was nearly as much as the total cost of materials from the collector.
Compared to the PEX collector, this collector offers better performance, and is more resistant to the effects of high stagnation temperatures, but the materials cost more. The build time is about the same.

Prototype Construction Details

The construction is very simple.

The frame is made with 2 by 4's and a 2X6 for the top plate.
The glazing is corrugated SunTuf polycarbonate ($1 per sqft at HD)
The absorber is built on a sheet of plywood. The copper pipe grid secured to the plywood, and then the aluminum fins are applied over the copper pipe grid and stapled tightly in place using silicone to fill any tiny gaps and provide better heat transfer.
A sheet of polyiso insulation is set into the back of the frame to insulate the back.

Much much more on the construction here ...

Prototype Performance

Some day long performance tests of the full size prototype copper/aluminum collector are here ...

These small panel tests of the copper/aluminum collector as well as the PEX/aluminum collector and the traditional all copper commercial collector construction give a better idea of how the performance of the copper/aluminum collector compares to other collector designs.

Cost of Prototype

Note that the reduction in cost of collectors is the result of a number of factors:

- Aluminum is cheaper than copper

- Polycarbonate glazing is cheaper than glass

- The cost of truck shipping is avoided.

- You don't have to pay yourself any wages.

Cost for the 4X8 ft prototype -- using all new materials:

Item	Quantity	Unit Size	Unit Cost	Total Cost
SunTuf Corrugated polycarbonate glazing	2	2X8 ft	$18	$36
Framing 2 by lumber (studs)				$10
Plywood or OSB absorber backing	1	4X8 ft	$10	$10
Atlas R-Board polyisocyanurate insulation	1	4X8 ft	$16	$16
Aluminum absorber fins	32 sf		$1	$32
Copper pipe	60 ft		$1.00/ft	$60
Copper fittings (mainly T's)				$14
Silicone	3 tubes		$3/tube	$9
Screws, paint, wiggle strips, ...				$10
Total				$197

A bit over $6 per sqft.

All of this adds up to a 4X8 ft collector that cost about $200 rather than $600 to $900 plus shipping that may run $150 ish.

Protecting the Collector From Overheating

This collector uses all high temperature materials and does not need any special protection for stagnation conditions. I still think that is a good thing to not leave collectors stagnated for extended periods (as do commercial collector manufacturers). So, try to protect your collector in some way if you can -- shade cloth, high tilt, use the heat for something...

Stagnation Test ... (for PEX collector, but should be very similar)

Drainback Test

On the PEX collector, I did a drain back test with clear tubing just to make sure that its serpentine tubing layout would drain well.

This copper/aluminum collector should have no drain back issues, and with the half inch risers should drain back very reliably. When I did the leak test in the shop, at the end of the test I opened the valve at the bottom to let the fluid in the collector drain into a bucket, and it drained with great gusto.

As with all drain back systems, the collectors and the collector plumbing should slope toward the storage tank.

Tentative Conclusions and Remaining Issues

Compared to the PEX collector, this version that uses copper instead of PEX tubing offers better performance, and more resistance to stagnation temperatures at an additional cost of about $50. For most people I think this is probably the better choice between the two.

Additional Possibilities

While I have talked mostly about using this collector for domestic solar water heating, it would also be fine for things like winter space heating, hot tub heating, and even pool heating into fall and winter in cold climates.

One space heating possibility would be to integrate the collector into your south facing wall. This would reduce the cost and material use for both, and (I think) should save some money and be a good use materials.

Bear in mind that this collector could be built in just about any shape you want to. It could be used to fit odd spaces you may have available. I suppose that even triangular, trapezoids, and circles might be possible -- collector art :)

I suppose you could even go around obstacles with it, just leaving a hole in the collector for the obstacle (e.g. roof vent pipes).

I think that large collectors might be a good way to go (e.g. build a single 64 sqft collector rather than two 32 sqft collectors. But, the larger sizes do get heavy and awkward to handle, so make sure you have a way to move it into position when you are done (or build it in position).

Example Solar Water Heating System using this Collector

For an example of a simple solar water heating system that could use this collector, go here...

This is my solar water heating system, and it uses the PEX versions of the collector, but the copper versions described here would work even better.

While this system uses a collector of about 48 sqft, a copper/aluminum collector would only require about 40 sqft to achieve the same performance.

The system was put in place in September of 2008 -- full details on how its is performing are at the link listed above.

Gary September 23, 2008, September 29, 2008, October 4, 2008