6 Responses to “Q&A: PV Controls for a Solar Thermal System?”


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  1. Alireza

    Why do you consider 18F temperature difference between collector and bottom of tank for turning on the pump? is it the same for all collectors? or it is based on calculations?

    • Dr. Ben

      The 18F turn-on delta T comes from experience running systems. There is a range of probably 16-20F which is satisfactory.

      One can watch the collector temperature sensor when the system is turned on. As the tank water enters the collectors, the delta T drops significantly as the collectors are cooled. If it is a marginal day (low solar radiation), the delta T might drop below 5F and turn the system off. This is called short cycling and produces very poor performance. The system “ain’t busted”, but “don’t work”.

      The 18F number is not magic, just practical.

      On the other hand, if the turn-on delta T is set at 25F, then the system waits longer to turn on, and is well beyond the short cycling problem. It simply wastes some available energy by waiting longer to turn on.

      Another very important parameter that relates to the control modes is the flow rate through the collectors. If the flow is too high, the result is identical to having the turn-on delta T too low, and short cycling will result. A high flow rate also increases the pump power needed. If the flow rate is too low, it is equivalent to having the delta T too high.

      So, the flow rate and the delta T settings have to match. I have found that 0.025 gpm/ft2 is a good value that matches the control modes above.

  2. Alireza

    In which condition a tank may be hotter than the collector?

      • Dr. Ben

        And in the morning when the sun first comes up. There may be enough solar radiation to trigger the PV panel, but the collectors are not warmer than the tank. The radiation level to trigger the PV panel is not related to the radiation level needed to make the collectors 18F hotter than the tank.

        Also in the late evening when the tank is at maximum temperature and the sun begins to go down. The PV panel will continue to generate power with lower solar radiation than what works for the collector circuit.

        There are systems where the PV panel supplies the electricity to run the delta T controller and power the collector pump. In this case, the PV panel replaces the utility electricity and the system can work fine.

  3. BlakeJ

    @Alirez, 18F difference is so once the pump turns on and fills the collectors it will cool down the collectors. Solar loop pipe run length can have an effect on temp differential # between tank and collectors. If you did not have enough of a temp difference, the system would “short cycle” (if 5F difference, pump turns on pumping 8F cooler water, then once hit collectors would shut off). System will not turn on unless collectors are 18F warmer. At night the collectors would be cooler than the tank, so would not turn on.

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