The Solar PV- Thermal system

On the roof of the UGA demonstration home in Tifton is a photovoltaic/ solar thermal system similar to the Dawn & Englert Solar PV/thermal collection system.  This system uses PEX-AL-PEX tubing under a standing seam metal roof as the collecting surface.  In our application we are using ½” Pex-Al-Pex tubing under standing seam roof to wick heat from roof.  The Pex-AL-Pex is held in place by 24 GA galvanized steel purlins designed to hold the tubing against the standing seam roof; a reflective radiant barrier is under the Pex-Al-Pex and purlin that holds the Pex.

From top, this image (from the Englert Solar Sandwich website – system) shows the PV film going on top of the standing seam roof which lays across galvanized purlins that has ½” PEX-AL-PEX snapped into its grooves. A radient barrier is under the purlins, which, on the UGA home, is layered over Huber’s OSB with an integrated house wrap (Zip system).

On the UGA home we used Unisolar amorphous PV peel & stick film, laying it between the standing seams before mounting the metal roof panels on the roof..  The total roof area theses circuits cover is 864 sq ft. (48’ wide and 18 ft long – 6/12 pitch facing south in Tifton, GA – 50 miles north of FL line) and the maximum rise of the solar thermal system above the solar boilers is 30 feet (the solar thermal is a closed loop system that need only a small pump to operate it).

We installed the Pex-Al-Pex solar thermal system in 4 circuits – each 280 linear feet of Pex arranged as 22 rows 9’ long on 10” centers with 20 ft supply and return “tails” going into attic space to manifold (approx  2.6 gal/circuit).   The collection roof area for each circuit is 12’ by 18’  (216 sq ft).  In the attic space we have two manifolds.  Each manifold handles the supply and return lines for 2 circuits moving the heated transfer solution (60% distilled water/40% propylene glycol mixture) to each of 2 solar boilers (both 80 gallon tanks are solar water heaters with internal heat exchange loops 80 gal -one is a Huch SGL-1 , the other is a Superstore Ultra) in 1” Pex tubing to the connected carriage house (100’ in each direction).

We are bringing circuits 1 & 3 together and 2 & 4 together to give us the capability of testing the effect of differentially cooling the PV on the roof and measuring the effect it on electrical production – thus, our need for two circulating pumps and associated thermometers, valves etc.

To summarize – each pair of circuits has 560’ of ½’ Pex-Al-Pex moving through 44 – 180 degree turns and flowing through a manifold to and from a solar boiler in a total of 200’ of 1” Pex.  Total volume of heat transfer solution in each pair of circuits – including the boiler is approximately 12 gallons, and the total collection surface area for 2 circuits is 432 sq ft.  The lift from the bottom of the storage tank to the highest point on the solar thermal system is 30’ – in a closed loop.

The two solar thermal pumping stations are Taco SPS-PC-3s retrofitted with 009 variable speed Delta-T cast iron circulator pumps.  We went with the pumping stations to make it easier for our plumber to install (rather than building the system out of components).

We swapped out the regular PC-3 pump for the larger 009 pump to give us added flexibility.  As mentioned earlier, we designed it to use two pumping stations to help us determine the change in electrical output of the solar PV panels as we “cool” the roof with the solar thermal system.  One of these Taco pumping stations would probably have been sufficient to handle the entire system.

Given that the whole house is an experiment, we plan on gathering data on how the system functions over time – monitoring flow, electrical use and temperature.  This information will feed into our whole house system that is able to monitor and control most of house functions.

The domestic hot water line moves through the Superstore into the Huch and then out to the hot water circulation loop. We have added a resistance coil and controlling thermostat to the Huch when the solar thermal system is unable to meet demand (set at 100F).

We will divert some of the hot water to regenerate the triethylene glycol (TEG) liquid desiccant humidity control system that is projected to need about 10gal/day of 155F water, and thus may require a tiny electric assist to insure the 155F water.