An Experimental PEX Solar Water Heating Collector

These pages cover an experimental solar collector for heating water that uses PEX pipe instead of the usual copper pipe. 

This construction has the potential to dramatically reduce cost, and also to make the collector much easier to build with common DIY skills.

 

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) copper has become very expensive, 2) soldering the copper pipes to the fins is time consuming and not a common DIY skill.

 

 


The prototype test collector using PEX-AL-PEX

 

 

This design uses aluminum fins instead of copper fins, and uses PEX (actually PEX-AL-PEX) instead of copper pipe.  Much effort is put into providing a good thermal bond between the PEX tubes and the aluminum fins.

 

 


PEX pipe with narrow alum strip under,
and wider, grooved strip over.
Silicone fills any tiny air pockets.
 
Aluminum fins must fit very snugly over
the PEX tubing.


The potential advantages of this construction:

  1. The collector is easy to build using ordinary tools and skills -- its a fairly easy DIY project.
     
  2. The collector is about 1/5 th the cost of commercial collectors ($140 vs $700 to $900 for a 4X8 collector)
     
  3. 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 more than the total cost of materials for the PEX collector!

There are also potential disadvantages -- the main ones being:

  1. PEX-AL-PEX does not withstand temperatures as high as copper does.  For practical purposes, it is probably limited to temperatures around 230F.  This means that the stagnation temperature of the collector must be controlled -- see below for more on this.
     

  2. The PEX collector is not as efficient as a copper commercial collector.  My testing indicates that the PEX collector will have about 84% of the heat output of a conventional, all copper collector.
     

  3. PEX is subject  to deterioration when subjected to sunlight (UV light). 
    So, it must be protected from UV exposure -- this is pretty easy -- see below.

You might also want to look at a similar collector that uses copper tubes here...

And, the PEX collector that I used for our solar water heating system here...

 

 

Design Overview

The collector uses a PEX-AL-PEX to convey the heat transfer fluid and to pick up heat from the collector.  The PEX is laid out in a serpentine pattern from the inlet end of the collector to the outlet. 

Aluminum fins that are grooved to snugly fit the PEX tubing are applied over the tubing with silicone used to fill any very small gaps and improve the thermal connection. 

A narrower strip of flat aluminum is used under the PEX tubing to provide some additional heat transfer area into the bottom of the tube.  In this way, the tube essentially surrounded with aluminum.

A lot of care is taken to maximize the quality of the thermal connection between the PEX and the aluminum.

 

A part of the cost saving for this design comes from using plastic instead of metal parts.  Plastics have lower temperature capabilities, so, an important part of the design has to do with protecting the plastics from being damaged by high temperatures.  If you are going to build one of these, it is important to understand the steps taken in the design to protect the plastics from being destroyed by high temperature conditions.
 

Corrugated polycarbonate glazing provides a high transmittance, high temperature, long life glazing at a low cost.  It is important to use polycarbonate glazing, as it has the high temperature capability that is required.

 

The back of the collector is insulated with high temperature, high R value polyisocyanurate rigid foam insulation.


All of the materials are likely to be available at a good lumber yard or big box hardware.

 

Prototype Construction Details

The construction is very simple. 

Much much more on the construction here ...  <----------

 

For the prototype, I used PEX - Aluminum-PEX  -- why PEX-AL-PEX? ...

 

Prototype Performance

Two types of performance testing have been done on the collector:

  1. Some tests that were carried out with small (2 sqft) collector panels that include a baseline collector using traditional commercial collector construction, this PEX collector, and a hybrid copper tube collector using aluminum fins.
    I believe that these tests give the best indication of the kind of performance that can be expected from this PEX collector compared to other collectors.
    Test data here ...
     

  2. Some day long tests using using a full sized prototype of the PEX collector to heat an insulated heat storage vessel.
    These tests may give you a better idea how much collector area might be needed for your application. 
    Performance of PEX/Aluminum and Copper/Aluminum full size prototype collectors ...
    More performance on PEX collector prototype...
     

  3. Monthly performance plots and solar fractions for my solar water heating system (which uses this PEX tube collector)...

The bottom line is that the PEX collector appears to offer about 84% of the performance of a commercial collector at 1/5th the cost -- but you have to build it :)

 

 

Cost of Prototype

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

- PEX is cheaper than copper

- Aluminum is cheaper than copper

- Polycarbonate glazing is cheaper than glass

- The labor cost of building the collector is avoided by making it easy to build yourself

- The cost of truck shipping is avoided.

 

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 X 4's and one 2X6       $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
PEX-AL-PEX pipe 55 ft   $0.43/ft $24
Silicone 3 tubes   $3/tube $9
Screws, paint, wiggle strips, ...       $10
Total       $137

A bit over $4 per sqft!

All of this adds up to a 4X8 ft collector that cost about $140 rather than $700 to $900 plus expensive shipping.  While the home built collector may not have quite the same efficiency or life as a high quality commercial collector it will (I think) offer good efficiency and good life at a much lower cost (plus some of your sweat).

 

Protecting the Collector From Overheating

If you aim this collector south, tilt it at 45 degrees and leave it that way through the summer, along with periods of stagnation, the collector will likely destroy itself from the heat.   This kind of treatment is not recommend for commercial all metal collectors, and will certainly dramatically shorten the life of a PEX collector. 
 

Stagnation is the condition in which the collector is exposed to sun, but no fluid is being pumped through it to remove the heat.  It will heat up until it gets hot enough to lose all the heat out its glazed surface -- this can be pretty hot.  Stagnation occurs when there is a failure in the system (e.g. power failure or pump failure), or when the controller shuts down the pump because the maximum allowed storage temperature has been reached.   While its not good to subject collectors to routine stagnation, they must be designed to handle it.

 

Some means is needed to keep the air temperature inside the collector to not a lot more than about 230F.

Some potential strategies to manage the stagnation temperatures:

  1. Mount the collectors vertically or at a steep tilt angle.
    Vertical south facing collectors get automatic protection during the summer because the summer sun is much higher in the sky, and provides less effective radiation on the collectors. 
    For space heating applications, vertical collectors perform very well, and for year long domestic water heating applications a steeply tilted collector can provide good year round performance if sized correctly.
    This may be the preferred method, since it does not require any human interaction.
     

  2.  Cover the collector with something like shade cloth during the times of year when high stagnation temperatures are likely.  This method probably will work well, but it does depend on the collectors parents be very diligent about applying the cover.
     

  3. There may be other possibilities such as automatically opening air vents.  These might be opened with a simple bimetallic spring.  A nice project for someone to tackle.

If this stagnation issue is a show stopper for you, then have a look at the copper/aluminum collector...
 

Stagnation Test ...

 

 

Drainback Test

One potential problem with this collector is how well the serpentine design will drainback for freeze protection.

 

This simple test looks into this issue and indicates that drainback will not be a problem as long as the serpentine pattern is laid out correctly, and the collector is reasonably level.

Drainback test ...

 

 

Tentative Conclusions and Remaining Issues

I think that the work to date demonstrates that it is feasible to make a collector that uses PEX tubing to convey the heat transfer fluid.  And, that if care is taken to obtain a good thermal connection between the PEX tube and the aluminum fin that the performance of the PEX collector is surprisingly good and quite acceptable.

 

The collector is relatively easy to build, and given the materials used, should have a long life.

 

The collector must be protected from high stagnation temperatures, and this deserves careful attention.

 

The collector is very low in cost, and has a very high heat output per dollar spent.

 

My solar water heating system uses this PEX collector and has been performing quite well since it was put in place in September of 2008.  Full details on this system, including monthly performance plots and monthly solar fractions here...

 

Be sure to also consider the variation on this collector that uses the same aluminum fins and attaches them the same way, but uses copper tubing instead of PEX tubing.   This design turns out to be about 12% more efficient, and eliminates the need to be so careful about protecting the collector from stagnation temperatures.  The cost is higher at $6 per sqft, but still very low priced compared to commercial collectors.

Update: March 2009 -- copper prices have fallen by about 50%, and make the copper collector even more desirable  -- about $5 per sqft at current copper prices.

 

Gary March 23, 2008, September 23, 2008, October 4, 2008, April 8, 2009