An Engineer in Wonderland – Arguing about solar heating

w-b-solar-panel-boiler-type-6.jpgThe solar panels are up, and on a not-very-sunny day got the water in the domestic cylinder up from 22 to 40 degrees, an 18°C rise, requiring 20°C of additional heating to get the cylinder to its operating temperature.

The temperature rise in the cylinder would have been greater, as the panel exit temperature was 61°C, but the long feed pipes from the loft to the cylinder are not yet insulated.

But it got me thinking.

At the moment, the two panels are plumbed in parallel. 

If there is not enough sun to heat the panel working fluid above 60°C on a particular day, would it be advantageous to switch the panels to a series configuration. It would only take a couple of three port valves.

I think it would, and that it would almost double the temperature rise in the working fluid. A friend of mine disagrees and reckons it will make little or no difference.

Does anyone out there know the answer?

And can any one explain it using an electrical analogy so that I can understand without having another go at thermodynamics?

If you can settle this argument, please comment at the bottom of this page, or to

It also occurs to me that a variable speed pump would help here, the installation is fixed speed, with a feedback loop to throttle back the pump until the panel exit temperature is regulated to the required temperature.

Do people do this?


Tags: Wonderland

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  1. 'Alice'
    June 11, 2008 11:09

    Thanks for your comments.
    I have still not got to the bottom of the parallel series switching argument, but am now convinced that if there is an advantage, is can also be gained with a variable speed pump.
    I am still also convinced that to calculate heat flow into the working fluid there is something like an ‘equivalent solar temperature’ – my expression – that would have to be calculated from solar irradiation, cloud filtering, latitude, and some factor from the panel structure itself.
    There must also be some form of heat flow resistance figure from the panel to finish off any equation.
    Simon, is there a speed controller in your system as well as variable PV power to set solar pump speed?
    And do you have flat plates or evacuated tubes.
    The good news from the roof is that by 14:30 on Monday, the whole 175 litre storage tank was up to its limit of 80°C using only the panels.
    An automatic mixing valve cuts this to 50°C for domestic use.

  2. Simon
    June 10, 2008 14:49

    Putting the panels in parallel or series will have no significant effect as the water will pass through the panels twice as fast and as such absorb half as much energy from each panel.
    As for variable speed systems mine has the pump run from a small PV panel so that when there is less sun the water stays longer in the panel giving it more time to absorb energy.
    Seems to work fairly well.

  3. Ian
    June 04, 2008 12:51

    I agree with your friend. If you put them in series you will double the volume of water flowing through each solar panel so it won’t have the same exit temperature.

  4. Cushie
    June 03, 2008 09:42

    Yes i think you are correct. What about thinking about when you connect batteries in series or parallel. When you connect 2 in series you get double the voltage and in parallel the same voltage. Not sure if that helps.

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