Announcement

Collapse
No announcement yet.

What happens to excess heat in a solar hot water system?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • What happens to excess heat in a solar hot water system?

    I've posted this question on the "Welcome New Members" section too, but I thought maybe here it would be seen more.

    I am new to this forum but I am fairly sure someone will be able to point me to previous discussions of my questions.

    I live in a sunny bit of France, at about 45degrees north, and I have a good unobstructed south-facing aspect in my garden. I have a pool (60 cubic meters - about 15,000 gallons) that is currently unheated, and a fairly large house which has a wood-fired boiler/furnace (wood-chips automatically loaded via an archimedes screw from a big silo).

    I'm right now building a shed in the garden to take solar water heating panels on its roof. The shed will be about 15 feet long, 5 feet wide, and will have a 45 degree roof facing due south. The roof area will be about 10 square meters (about 90 square feet).

    I am hoping that it will be possible to put 10 m2 of solar panels on the roof, to enable me to heat the pool a little in spring and autumn, and provide domestic hot water in summer and maybe even a bit in winter. In mid-summer the pool doesn't really need heating - last year it got to 30 degrees centigrade (86 fahrenheit).

    What I don't know is what will happen if I find I have installed too much heating area - what happens with the excess heat? Is it a minor problem (the hot taps get very hot indeed?) or a major one (everything melts?).

    I should add that the shed and the house are not very close together - about 150 feet apart. I plan to bury the pipe run and make sure the pipes are insulated.

    And this isn't entirely about economics either. The wood-chip boiler is efficient but noisy - from time to time it groans like a cow in pain - and using it less, particularly in summer - would be a benefit in itself

  • #2
    Originally posted by Araucaria View Post

    I live in a sunny bit of France, at about 45degrees north, and I have a good unobstructed south-facing aspect in my garden. I have a pool (60 cubic meters - about 15,000 gallons) that is currently unheated, and a fairly large house which has a wood-fired boiler/furnace (wood-chips automatically loaded via an archimedes screw from a big silo).

    I'm right now building a shed in the garden to take solar water heating panels on its roof. The shed will be about 15 feet long, 5 feet wide, and will have a 45 degree roof facing due south. The roof area will be about 10 square meters (about 90 square feet).

    I am hoping that it will be possible to put 10 m2 of solar panels on the roof, to enable me to heat the pool a little in spring and autumn, and provide domestic hot water in summer and maybe even a bit in winter. In mid-summer the pool doesn't really need heating - last year it got to 30 degrees centigrade (86 fahrenheit).

    What I don't know is what will happen if I find I have installed too much heating area - what happens with the excess heat? Is it a minor problem (the hot taps get very hot indeed?) or a major one (everything melts?).

    I should add that the shed and the house are not very close together - about 150 feet apart. I plan to bury the pipe run and make sure the pipes are insulated.
    Well you are trying to do two different things: Pool heating and hot water heating. These can be accomplished with one system but are typically done with two separate systems.

    Pools require low temperature heating, but lots of heat (watthours). Typically, inexpensive single glazed collectors will be used to heat a pool. these only work in warm weather, don't work in cold weather, but nobody uses the pool in cold weather either. They extend the pool season. Quite a large number of inexpensive panels can be used, because the pool is a large heat sink. Maybe 10 or 20 square meters of panels, depending on the size of your pool.

    Hot water heating happens year round, and requires higher temperatures. Typically people will use more expensive, but efficient panels to heat hot water. They will use just a few panels, maybe 5 square meters (50 square feet)

    Excess heat is always a problem in a properly sized system. There are two major types of solar water systems, and two corresponding ways of dealing with excess heat. The first system is called a drainback. The collectors are dry, untill a pump turns on and circulates water through a heat exchanger to your hot water system. An electronic controlled tells the pump when to turn on - if there is enough sun, the pump turns on, and if the heat is too high, it turns the pump off. Once the pump is off, the collectors drain dry again, and no more heat is collected.

    The second type of system always has water int he collectors. A small pump runs on a solar panel, and runs whenever the sun shines. This system can overheat because there is no limit controlling the temperature of the water. It must circulate water anytime there is sun, because if it does not, the water in the collector will boil. In these systems, several ways of dumping excess heat can be used. One way is to have a solenoid valve that is controlled by a thermostat, and sends hot water through a loop made to waste heat, either passing through a finned tube on the outside of the house or a loop of pipe through the soil. The other way is to have a solenoid valve that dumps hot water directly into a drain, cooling off the storage tank.

    There are several other systems, and several other ways of dealing with excess heat, but these are the ones I believe are most common. Typically a pool heater might use the drainback system and controls would keep it from pumping when the pool temperature was warm enough.

    150 feet is a long way to send hot water. Even with a lot of insulation, it will be a serious drain on the efficiency of your system. I'd recommend putting the domestic hot water system on the house, and a separate system on the shed for the pool. Two different systems with different collectors for the two purposes. You can probably find someone else who says the opposite, there is no right and wrong way to approach this.
    Lawrence Lile, PE

    Comment


    • #3
      Lawrence

      thanks very much for this very helpful reply.

      Unfortunately we can't put the solar collectors on the roof of our house: apart from the fact that the ridge runs not far off north-south, our house is an old stone-built barn with a roof made of "lauzes" - thick stone "slates" that are individually cut and shaped for the roof. We had it fully repaired two years ago and the cost of doing that would have bought us a second house! It looks lovely, but we cannot bear the idea of covering the stones with solar collectors. The shed, on the other hand, is pretty close to the pool.

      We're hoping to minimise heat-losses in the 150 foot pipe run by having it as well-insulated as possible: the half-inch copper pipes will run in the centre of 4-inch plastic pipes, with closed cell foam insulation between the copper and the plastic. Obviously there will be losses, but we get good sunshine in the summer, and I'm more worried about the possibility of too much heat rather than too little.

      So thanks for explaining the alternatives: I'll talk to our plumber about which is best. We might end up with some extra water tanks in the house as a heat-sink and heat store: luckily we have some space where the boiler is to add something of this kind. I think we have to avoid using the pool itself as a heat sink as we don't want to overheat it.

      Comment


      • #4
        Keep in mind that the pool collectors can be used to dissipate heat at night cooling the pool.

        Comment


        • #5
          Thanks Ionized, I hadn't thought of that but it would obviously work if the pool got too hot - I may try to get a manual override put in so we can cool the pool at night if it's ever needed.

          So far as dealing with excess heat, I do already have a drain in the room with the hot water tank and the furnace. It's needed as this room is partly below ground level, and it's impossible to keep rainwater out in the winter - so we just drain it away after it comes in. So maybe the best idea is a dump valve from the tank that opens if it gets too hot. The furnace already has something very similar to deal with overheating, so our plumber will be familiar with the concept.

          Comment

          Working...
          X