PEX Collector -- Performance

This section provides some preliminary performance measurements and estimates for the PEX collector:



Prototype Collector Heating 35 Gallon Water Tank

I've recorded several days with the prototype 4X8 ft collector hooked up to an 35 gallon insulated tank --see the picture.  This gives some idea how the real life performance might work out for solar water or space heating.


Even though its April, the weather has been part sunny with low ambient temperatures -- probably more typical of winter in most places -- the joys of Montana Springs -- 5 inches of new snow today!


On days with descent sun, the prototype has managed to heat up the 35 gallons to a useable hot water temperature on part cloudy days.  See plots below.


The prototype would do even better with: 1) no gaps between the aluminum fins -- I think this might be good for an about 7% improvement, and 2) a water storage tank that was inside or better insulated. 


At any rate, the current prototype performance seems about right for 2 peoples typical hot water needs.  I think this is pretty close to what one would expect from a similar size commercial unit.


Test Setup:

Prototype with 35 gallon tank in
insulated box below.

Prototype from other side.
Small black gadget attached to right
side measures sun intensity.

This is the 35 gallon tank before insulating.  The 12 volt submersible March pump can be seen at the right.
All the scum on the bottom comes from a previous use.

Click on pictures to enlarge

Pictures show the test setup for the prototype.


The instrumentation consisted of a temperature logger to measure 1) collect water inlet temperature, 2) collector water outlet temperature, 3) collector air temperature at the top, 4) ambient air temperature, and 5) solar intensity using an Apogee pyranometer.  The flow rate was measured by recording the time to fill a half gallon container.


Performance Plots:





This is a good sun day with only some clouds in the morning.


On the plot:

Blue solid line is the collector inlet temperature (that is, the pump outlet temperature)
Red short dash line is the collector outlet temperature

Red long das line is the temperature of the collector air near the top.
Black dash-dot line is the ambient temperature
Green line is the solar intensity in w/m^2


The flow rate for this test was XX gpm.

So, the collector raised the tank water temperature (35 gallons) from about 65F to 116F on this mostly sunny, but cold day.  Ambient temperatures started at 20F and got only up to about 40F by afternoon.


As noted above, performance would have been even better with a well insulated or indoor tank and with more care in not leaving gaps between the fins in the collector. 



Another Day Plot:


April 3, 2008 performance.


This was a part sunny day with some clouds in the afternoon.


I was late to turn the pump on, so there is no collection until about 12:00 on the plot.

The tank started at about 80F, and reached about 125F by the end of the collection period.


It is interesting to note what happens to the collector air temperature in the morning.

Without water flow to cool the collector, it climbs steadily up to nearly 210F at 12:00.  At this point, the pump is turned on, and the air temperature drops quickly to about 160F.  So, even with the cool ambient temperatures (50F), the need to consider high stagnation temperatures is apparent.


Note that the peak in ambient temperature in the morning just before noon is bad data -- the ambient temperature sensor was in the sun during this half hour or so.  Its OK before 11:00 and OK after 12:15.



PEX to Aluminum Fin Tests

I'm just getting started on some testing to try to optimize the fin to PEX heat transfer.   The little test board shown below allows the thermal performance of two or three fin to PEX connection arrangements to be directly compared to each other.  Each different arrangement consists of about 4 ft of PEX and fin in a U shaped arrangement.  Water from a bucket is pumped through the 4 ft using a small submersible pump in each bucket.  The higher the rate of temperature rise in the bucket, the more efficient the fin to PEX arrangement is.


PEX and fins being installed on the
test board.

This arrangement tests two different
fin/PEX couping arrangements.
Each of the two U shaped tubes
is supplied by a separate bucket and
pump.  The one that heats its bucket
of water fastest is better.


I'm planning to test at least the following:

- Different materials between the aluminum fin and the PEX pipe (silicone, filled silicone, ...)

- Different fin arrangements (the prototype arrangement, two fins with PEX sandwiched between, ...)

- A copper pipe with aluminum fin with silicone between (just to verify the effect of the thermal resistance of the PEX tube wall)


Its going to be a couple weeks before I can get back to this.
I am not expecting anything revolutionary out of this.  I think the test is likely to confirm that the major resistance to heat transfer from the fin to the water inside the PEX tube is the wall of the PEX tube itself.   But, I think some modest improvements might be possible, and, if not, the performance of the current prototype arrangement seems good enough.


It seems to me that the things that are likely to make the performance improve are:

  1. Maximum contact area between the aluminum fin and the PEX -- wrapping it 360 degrees would be ideal.

  2. Tight fit between the aluminum and the PEX -- a paper thin gap

  3. A good filler between the aluminum and the PEX to eliminate any poorly conducting air film

  4. Using PEX-AL-PEX instead of PEX because it has better thermal conductivity.

  5. Narrower fins because the narrower the fin, 1) the more efficient the fin is, and 2) the less heat has to be transferred into a unit length of PEX

  6. Larger diameter PEX, as this increases the heat transfer surface area.

  7. What else?



Calculated Performance Compared to Copper Absorber

The calculation below attempts to estimate how much hotter the aluminum fin must run compared to an all copper collector in order to transfer the same amount of heat into the water.   The slope of a typical collector efficiency curve is then used to estimate how much of an efficiency decrease the PEX results in compared to an all copper absorber.



< show the calculated temperature drop across the PEX wall>


<what would 5/8 PEX do?




Gary April 5, 2008