Variation in Riser Flow For Wide Solar Collectors

Solar space heating systems often use wide solar collectors in which a large diameter bottom manifold distributes a heat transfer fluid (often water) to a large number of vertical risers.  The risers are equiped with fins to absorb the sun and transfer the heat into the water flowing in the risers.  The heated fluid is collected by an upper manifold and returned to the storage tank.  The collectors are often arranged so the inlet is on one lower corner and outlet is on the opposite upper corner in order to improve the evenness of water distribution to the risers.

This test attempts to assess how even a distribution of fluid to the risers this design achieves.  If the distribution is uneven, then the efficiency of the collector will suffer.  Even though this design is very common, there is surprisingly little data that I could find on the evenness of the distribution.

The picture shows the my 11 ft wide collector used for the test.

Same collector with glazing removed to show manifolds and risers.

The test was done by removing the glazing from the collector, and with full sun on the collector, and normal fluid flow through the collector, the temperature of each riser was measured with an IR gun style thermometer.
The idea being that if a good fluid distribution is being achieved, the risers should all have a similar temperature profile with the lower part being near the tank temperature, and the temperature gradually increasing as you go up the riser.

Riser No	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
top	124	123	123	124	126	129	124	128	126	132	127	125	129		125	128	126	123	123	122
mid	123	122	121	123	124	124	120	125	123	125	125	125	125		124	123	123	123	123	120
bottom	122	117	119	117	119	117	115	119	118	119	120	121	119		117	117	115	116	120	115
Time	12:47 PM																			12:56 PM
Temp rise	2	6	4	7	7	12	9	9	8	13	7	4	10	0	8	11	11	7	3	7

The truncated table above shows the results -- see the full report link below for the full table -- there are 20 risers in the full table.

Happily, the distribution is pretty even, with most risers showing similar temperature patterns.  There are variations, but probably not enough to seriously effect efficiency.

It does appear that the evenness of the distribution can be effected by relatively small changes in the details of how the plumbing is done.  Have a look at the full report for more details on this.

All the gory details on riser flow distribution test...

If you have additional data on riser flow pattern tests or analysis, please let me know.

Gary