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Solar panel overheat issues

hr Member Posts: 6,106
possible for system temperatures to easily exceed a T&P setting of 210°. A system without any load or DHW draw can reach 240 degrees or more. Suppose a homeowner leaves town for a week!

Solar friends tell me even in a Montana winter climate this is possible with the high performance evacuated tube type collectors.

This fellow has a system that uses a 16 psi radiator cap, which allows a 247° boiling point (about where glycol cooks) and a piece of baseboard as a dump radiator.

Then a small overflow tank, similar to a car radiator overflow tank.

This assures if you do flash to steam you don't airlock your system.

These systems use the new Laing D-34 12volt pumps, which can be run off a 20W PV with a linear current amp.

I ordered one of his patened HX that slips into a water heater. Thought it might drive a small radiant zone from an exisiting tank.

This guy has been a solar tinker for over 20 years. Lots of good stories to tell :)


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  • Dale Pickard
    Dale Pickard Member Posts: 231
    Hey Hot Rod

    I'll check out the fellow's web site to see what he's up to.

    Generally, a differential temp controller will provide a tank high limiit that shuts the collector loop down when the tank gets too hot. Very high water temps dramatically shortens the life of storage tanks of all kinds, but especially glass lined tanks. Also, in a dhw system, water can get dangerously hot and which can also destroy tempering valves. So for safety purposes, I don't think that we ever want the water that hot.

    He may be running the DC pumps from the current provided by a small PV panel thinking that this simplifies the system by getting rid of the DT control. This is somewhat of a misconception though as the function of the DT control is prevent pump operation whenever there are greater losses than gains, that is whenever the collectors are cooler than the storage tank. There may be enough sun to drive the pump, but not enough to heat the collectors hotter than the storage tank. The pump can simply dump the heat out of the tank to the collectors. These simple systems are ok for Arizona or Hawaii, but I wouldn't do it in Montana, where we could easily lose everything we gained. Unlike therma collectors, PV panels, being made of semiconductors, are more efficient in the cold.

    Also, the DC pumps available, like the Laing, don't put up much flow which can allow the collector temp to escalate. While we don't want to run pumps that are larger than necessary, because of the power consumption, we do want flows high enough to scrub the heat out of the collectors so as to run a minimal DT across the collectors to miinimize heat losses. There is no situation where low flows are desireable, especially low sun, hot tank. Heat losses increase as the DT across the collector loop increases. A PV powered pump will slow down just when you don't want it to.

    When the pump shuts down, the collector loop will continue to increase in temp. A good flat plate collector can stagnate at temps greater than 400F and evacuated tubes can go as high as 800F!!
    This is a real problem without good solutions. Etube collectors like the Viessmann/ThermoMax collectors use valve driven by a shape memory alloy that shuts down the flow of vapor in the heat pipe in the collector under stagnation conditions, which prevents transfer of heat to the glycol solution and keeps it from over pressurizing the collector loop. Also the heat pipes are tuned somewhat so that under very high input, all of the fluid in the heat pipe vaporizes and stops moving heat up to the condensor element.

    The best, but not perfect, answer to all of this is to run closed loop drainback systems that are pressurized to keep the collector loop from boiling over or vapor locking. Also, generously sizing the storage tank to allow longer operation.
    So that's the little I know about that.

    BTW, HR, thanks for your recent ThermoFin plug. All David needed was a slingshot, but we need all the help we can get.

  • Constantin
    Constantin Member Posts: 3,796
    Had a look...

    ... and liked what I saw. Mr. Butler has a patented system to deal with stagnation, consisting of a radiator, a automotive radiator cap, and a small reservoir. Looks like a pretty good way to ensure you won't have too much boiling coolant coursing through pipes...

    As for the controller running the pumps, the system seems to be organized around pumping only whenever the ΔT is large enough to add heat to the storage tank. I believe Mr. Butler is going with SS316L alloy tanks for the very reason you raise, longevity under high temperature duress.

    Anyway, I am going to contact him about coming up with a diagram, quote for a H30-equivalent. No need to spend 2x just to get the viessmann name on the stuff, particularly if all they do (in this instance) is repackage.
  • hr
    hr Member Posts: 6,106

    His system does include a DT control with high limit. But shutting down the pump, yet still seeing gain has led him to come up with the over heat radiator and dump tank.

    With his PV pump (less DT control) a simple snap disc stat on the tank cuts off the pump before the tank overheats. I agree that the thermostatic mixers should not be overheated on the tank. Nor should the tank reach 210¯ to pop the T&P!

    His concept is to use the exisiting tank with that small HX. So he does not have a lot of "parking space" for BTU above what can be safely, temerature wise, stored in a 40 or 50 gallon tank. I think this is what drove him to the over heat and over pressure system.

    Interesting guy if you get a chance to chat with him.

    He sold a system up your way to keep a Clivus composting toilet warm through the winter to keep the composting "product" warm enough to work.

    hot rod

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  • hr
    hr Member Posts: 6,106

    You really ought to call him up. You would get a ton of good info. He likes to talk, be prepared :)

    hot rod

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  • Dale Pickard
    Dale Pickard Member Posts: 231
    They do more than repackage

    As I understand it Viessmann takes up most of the ThermoMax production capacity. They also use their own controls and storage tank and header. The header is far and away a better design and manufacture than the ThermoMax. All that said, I would opt for flat plates. Viessmann is now bringing over their flat plate collectors which are the nicest flat plates I've ever seen. Much better made than any of the domestic stuff. Unless you have an extraordinary application, a good flat plate will run economic rings around the etubes. E tubes are marginally more efficient than flat plates under some conditions but they aren't magic. Area adds up much faster than efficiency differences.

  • Carl PE
    Carl PE Member Posts: 203
    My hippie neighbors

    did something very similar in the 70's. They chopped the neck out of an old car radiator and brazed it to a 1¼ or 1½ tube. Used a 5-gal bucket as an overflow.
  • magic evacuated tubes

    They seem like magic, you put one in the sun for one minute and it is too hot to touch. Water boils at a much lower temp. in a vacuum. An instructor told me the tubes are much more efficient in the cold weather and better suited for radiant heating. Yesterday it was raining and cloudy all day and my vacuum collectors were 120 degrees, while my box collector was about the same as the outside temp. The first morning I started up the tubes the sun was just barely poking through the trees and the collector manifold was banging and boiling and setting off the relief valve and it melted the insulation near the collector. these tubes put out a lot of heat at a much higher temp than panels and I feel it is better suited to use for domestic and heating. bob

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  • Dale Pickard
    Dale Pickard Member Posts: 231

    I'm not sure what kind of tubes that you have. Or what kind of flat plates. I don't know of any tube designs that expose the heat transfer fluid to the vacuum. In the Thermomax/Viessmann design the HTF is contained in the manifold and the fluid in the heat pipe is contained in the copper heat pipe.

    Because of the low losses, the etubes should not be warm to the touch on the outside. The Ownes Illinois / SunMaster tubes that we used to work with were a tubular envelope - 3mm deep. The vacuum was contained inthe envelope. The tubes were amazing. You could take a tube and stagnate it in the sun and measure temperatures as high as 700F inside the tube, yet you could hold it in your hands. It was warm, but not 700!

    If the system is banging and opening the relief valve and melting the insulation, then the flow must be very low.

    It's important not to confuse temperature with energy. Energy is a function of the mass flow rate and the temp rise across the collector loop. If the tank is cold the collector outlet temp should run no higher than about 10F above the inlet temp.

    Collector performance is well explained in the performance curves which plot efficiency as a function of delta T (across the collector) divided by radiation, aka heat loss. If you compare the curves of etubes to good flat plates you will see that the etubes provide perhaps as much as 10% greater efficiency at high delta Ts. Some of the designs have poor optical performance compared with flatplates, so their peak efficiency, at low dt's, is actually lower.

    The efficiency you get is strongly related to the application and the climate, so while etubes are great at providing high temps in cold weather, flat plates also provide good performance generating moderate temps in moderate weather.

    Solar is all about economics. The main point I was trying to make was that collector area adds up much faster than efficiency differences. The etubes are expensive enough that you can generally afford much more area in flat plates than in tubes. In a domestic water heating system, producing temps in the 140 range, even in a cold climate, one extra flat plate will produce more hot water on an annual basis than the smaller sized etube array.

    If you are south of about 40 deg north latitude.....flat plates will almost always outperform etubes in a heating application. Etubes would be useful in Arizona for absortion air conditioning but not for heating.

    Solar collectors are not boilers.

  • tubes

    I have thermomax tubes and the glass part of the tube stays cool, but the copper heat transfer bulb at the end got very hot, in one minute. My system was banging on the startup because the controls were not set up correctly, but that was first thing in the morn. when the sun was just peeking through the trees. I just attended a class put on by Peter Biondo who uses vacuum tubes in Arizona for heating 7000' up in the mountains. He said people don't even install air conditioning there because it is so cool. With the high temp these tubes get if you had enough tubes why wouldn't it be considered a boiler?

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  • Brad_9
    Brad_9 Member Posts: 29
    burnt antifreeze

    There's been some talk about stopping a solar circulator when the tank gets too hot. In an antifreeze system this will lead to vapoizing the antifreeze and burning it at stagnation temperatures. I do not see how having an overflow radiator would change this, except it might not blow all the fluid out, or might recover it, but you still have boiling fluid in the collector. This will make a coating of very acidic, corrosive, gunk on the inside of your collector, not a good thing to do. In a vacuumm tube collector header it would be even worse. You must keep the circulation going. Sort of like a failed circulator in a boiler without a low water cutoff.

    The only type of system that works well with this type of overheat control is a drainback system, where the fluid is just water, and when the pump turns off, the collector water drains back into the holding tank. Then it's OK because the collector will handle the stagnation temperature and there's nothing to cook inside.
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