Solar Thermal Plans

I have been wanting to install solar thermal panels to help heat the hot tub water since the hot tub is the major draw on our electrical power. We have prime sunshine and solar hot water heaters were very popular here in Colorado in the 1980’s. I already have two solar panels that I started renovating but have not completed. But I happened to see six more available and thought I could use them. So I arranged to pick them up but they are big, 4 x 8 instead of the 3 x 7 size that I already have. Unfortunately one of the glass panels broke in the truck from the weight of the panels on top! I can recycle the copper in the panel or use it instead of one that has a tear in the casing and switch the glass to the broken one.

Buying used panels is a gamble but an enjoyable one. Nothing like a new project when several others are pending. I now have five Novan 4 x 8 panels, another Novan heat exchanger although a larger size than the first and two 125 gallon tanks that are still to be removed from the basement.

I would like to have a drainback system to prevent both overheating and freezing in the panels. And I want to use the tanks on their sides so as to not block our view from the terrace. I’m planning to mount the panels on the terrace roof and run the hot water down to the storage tanks in front of the patio wall.

The system design is a standard drainback type.

Solar Drainback with Heat Exchanger

Solar drainback with heat exchanger schematic

I am planning to build an insulated shed for the equipment. It will need doors on top and in front for access since it won’t be tall enough to walk in. I’m thinking it should be about 4′ tall, 8′ wide and 6′ deep. I have lots of Roxul insulation left to use inside. And I have several 4 x 4’s that could be used, but I’m not sure yet about the shed design.

The plan above requires a drawback tank and I have not found a promising commercial source. The best tanks are stainless steel but many people use a small electric water heater. It has to be large enough for double the total amount of fluid in the panels and pipes.

Here are some other stats I found for planning purposes.

  • Panel array size: determine the optimal percentage of a building’s heating or domestic hot water load based on available unshaded panel area and an economic analysis. Too many panels result in over-capacity during low loads in summer and a longer payback. Typically, solar thermal systems are most cost-e ective when sized to provide approximately 50 percent of peak demand
  • Storage versus panel square footage: Provide from 1.5 to 2 gallons of storage per square foot (3.75 kL/m3 to 5 kL/ m3) of collector and insulate the storage systems
  • Pumping flow rate per collector: Provide 1 gpm to 1.25 gpm (0.06 to 0.08 L/s) per panel – based on a typical array of six to eight panels, resulting in flows of 6 gpm (0.38L/s) to 10 gpm (0.63L/s) for each array
  • Drain-back tank size: Provide approximately 1.35 gallons (5.1 L) per panel – based on calculating water capacity according to roof panel and tubing
  • Heat exchanger size: Calculate to derive the maximum energy available from panel arrays. Typically, the flow rate into and out of the heat exchanger equals the flow into the panel arrays.

There is a lot more engineering to do but I have a record of heat use in the hot tub from the Efergy electricity monitor. The spa is heating approximately 12% of the time. It runs 24/7 or 8760 hours a year, so it is heating about 1051 hours a year. There are two 7.5 watt heaters in the spa. The highest use of electricity recorded is 12.38 KW. On that day it appears that the spa used about 56KW of electricity to heat water for about 10 hours. That is about 190,000 Btu’s.

Each panel is capable of about 12,000 Btu’s/hour in moderate sunshine to raise the heated liquid temperature 90 degrees. 22,000 Btu’s are possible if the temperature only needs to be raised 30 degrees.

Novan Panel Specs

Novan Panel Specs

Five panels on a reasonably sunny day would produce about 60,000 Btu’s of heat per hour or about 17.5 KW. But some would be lost in transfer and storage. The pump for the panels would be capable of about 6 gpm. I am hoping to use a DC pump powered by a solar panel. The pump for the storage water is a Taco 06 on the heat exchanger system.  There would be a total of 240 gallons of storage in two tanks. That water would circulate through the heat exchanger to transfer the panels heat to storage.

The pump in the spa is 5 HP. That is a very strong pump. On high it uses 14 amps at 230 volts or about 3 KW. High is only used for the jets while in the tub. Low uses 1.2 amps or 276 watts. That is the speed that pushes the water through the heaters and filter. It is likely the system uses about 15 psi although it does not have a pressure gauge. My first thought was to connect the storage water to the spa piping in between the return and the heater. It may work best to have a separate pump and a heat exchanger on the spa too though.

So the system is just in the planning stages and there will be many design changes and issues to resolve before it is installed. I still have to get the solar tanks out of the sellers basement and it does not look like an easy job.



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