Announcement

Collapse
No announcement yet.

Do Solar Hot Water Panels overheat?

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

  • Do Solar Hot Water Panels overheat?

    I was hoping someone can answer my question regarding whether hot water solar panels can overheat when the flow is cut off to the panels either manually or by a Thermosyphon Arrestor Valve. I am having trouble finding any information about overheating via Google or forums as all the information I receive after a search is related to the overheating in the storage tank

  • #2
    I was told that they need multiple pressure relief valves. As the liquid has a tendency to flash to steam.
    4400w, Midnite Classic 150 charge-controller.

    Comment


    • #3
      Yes they can stagnate and boil the antifreeze making it acidic. As long as its circulating a flat plate will not exceed about 80 deg F over ambient, but once it stops flowing bad things happen.

      Comment


      • #4
        Thanks for your advice. I do have a standard pressure relief valve on the storage tank but I’ll look into adding another pressure relief valve to the system. I live in a non frost area so do not have antifreeze. So far my system hasn’t expelled water through the pressure relief valve. I was just thinking about when I go on holidays. Should I play it safe and just cover the panels while I’m away?

        Comment


        • #5
          Originally posted by paulshea57 View Post
          I was hoping someone can answer my question regarding whether hot water solar panels can overheat when the flow is cut off to the panels either manually or by a Thermosyphon Arrestor Valve. I am having trouble finding any information about overheating via Google or forums as all the information I receive after a search is related to the overheating in the storage tank
          Short answer: They can overheat, but the question then becomes what is considered overheating ?

          Longer answer:

          An energy balance on a solar thermal collector will show that the energy input (solar energy) will always equal the useful energy output (that is, the energy added to the fluid flowing through the collector) plus the losses to the surroundings, mostly by thermal radiation to the surroundings and by convective losses to the air flowing around the collectors via wind and natural (gravity) convection. There are some thermal losses to the collector supports but those are small, usually of the order of a % or less of the total losses.

          Under operating conditions with coolant flowing, the collector temp. will rise until the sums of the product of the thermal losses (the radiation and convection heat loss coefficients times the appropriate collector to ambient temp. differences) plus the coolant energy gains equals the irradiance input.
          If irradiance is high, wind velocities are low and particularly if coolant flow = = zero, the collector temp. will rise until the combined thermal losses == the irradiance input to the collector.

          Check your collector spec sheet. It should list a thermal loss coefficient in units of W/(m^2- deg.C). To a first approx., the stagnation temp. (no coolant flow) under a bright and clear sky will be something like the P.O.A. irradiance divided by that listed thermal loss coefficient + whatever the ambient temp. is around the collector.

          So, for example, if the loss coefficient is, say, 6W/(m^2/deg.C) and the P.O.A. irradiance is, say, 800 W/m^2, and the ambient air temp. is, say, 20 C, to a rough 1st approx., the stagnation temp. of the collector will be something like (800W/m^2)/(6W/m^2-deg.C) + 20 C.= 133.3 + 20 = ~ 153 C.

          But, if you have a maintained and code compliant solar thermal water heater it will be equipped with ASME pressure /temp. relieving device(s) (as it should). The relieving device(s) is (are) designed to operate if the temp. and/or the vapor pressure of the coolant is high enough. If such devices it do operate, they will reduce the temp. (and pressure) of the collector if the operating/set parameters of pressure/temp. are low enough so that the device(s) will operate (lift). If the system is an open one (potable water cooled), the relieving device(s) will continue to operate, probably intermittently, until the irradiance is reduced (or the collector is covered).

          If the relieving device is for a domestic water tank in the U.S or Canada (and many other countries), it will lift at 150 PSI and/or 212 F. with 212F. usually happening long before the vapor pressure in a pressurized system will rise and actuate the valve.

          Sometimes, DIY stuff will skip the safety system components. Or, relieving devices are not checked and maintained as necessary. Such situations are common and unfortunately dangerous. In such cases the energy balance info described above is likely to apply.

          Comment


          • #6
            Thanks JPM for your advice, that gives me much more information for me to utilise in improving my system. My system hasn’t actually overheated since installation, during a couple of hot summers, but we have been at home using hot water regularly. Looks like I definitely need to improve my system for when we go away.

            Comment


            • #7
              Originally posted by paulshea57 View Post
              Thanks JPM for your advice, that gives me much more information for me to utilise in improving my system. My system hasn’t actually overheated since installation, during a couple of hot summers, but we have been at home using hot water regularly. Looks like I definitely need to improve my system for when we go away.
              You're welcome.

              One trick to avoid overheating when away and hot water use is nil and that's mostly a no brainer is to cover the collector(s) when no one is home.
              I use grey fiberglass corrugated panels from big box. 2 ea. 2' X 8's work well on a 4' X 8' collector. I strap them across the collector with bungie cords and tie off to the collector supports. Easy install (provided there's reasonable roof access). Trouble free since 2005 install. The panels for my installation last ~ about 10 years before they get brittle. I store them under the collectors and weigh them down with spare concrete roof tiles.

              Because mine is a direct system, it's easy to drain the collectors and that's what I do when the house will be vacant for more than a few days. particularly in the winter. My circulating system used for freeze protection has not failed but will be useless if there is a power outage and the controller is told by the collector sensors to circulate if the ambient temp. gets cold enough, particularly at nite a few nites/year and then particularly just before dawn.

              Comment


              • #8
                Hi guys,

                If I drain my thermal solar panels in the Spring, but I keep them in direct path of the sunlight, will the empty panels be susceptible to damage. I'm thinking they could get up to 300 or so degrees deep inside on a hot day.

                Bright Days,

                Paul

                Comment


                • #9
                  Originally posted by Promero View Post
                  Hi guys,

                  If I drain my thermal solar panels in the Spring, but I keep them in direct path of the sunlight, will the empty panels be susceptible to damage. I'm thinking they could get up to 300 or so degrees deep inside on a hot day.

                  Bright Days,

                  Paul
                  Thermal collectors subject to stagnation temperatures (either drained or under no flow conditions such as during power outages or pump failures) will most likely deteriorate faster than thermal collectors that do not undergo stagnation temperatures, and the more cycling up and down at stagnation temperatures occur, the more likely will be the damage and/or thermal performance degradation.
                  The soldered joint of copper tubes to copper sheet for example will probably fare better than 2 dissimilar metals for many reasons, but the rest of the collector may be subject to other problems. For example, gaskets and glazing seals will most likely be affected.

                  Get some 2' X 8' corrugated fiberglass sheet and affix them as I suggested in a prior post.

                  Last edited by J.P.M.; 01-09-2023, 11:58 AM.

                  Comment


                  • #10
                    Thank you JPM!

                    Common sense advice helps a lot. Although common sense did not prevail today. I broke a replacement glass pane for a 4x8. Do you know if there is a “big box” poor cousin replacement?

                    Bright Days,

                    Paul

                    Comment


                    • #11
                      Originally posted by Promero View Post
                      Thank you JPM!

                      Common sense advice helps a lot. Although common sense did not prevail today. I broke a replacement glass pane for a 4x8. Do you know if there is a “big box” poor cousin replacement?

                      Bright Days,

                      Paul
                      You're most welcome.
                      Short answer: I do not.
                      Slightly longer answer: In replacing a 4' X 8" lite of collector glazing in situ there may well be challenges in first off, finding someone willing to do it, and then the cost, which may well be more than it's worth.

                      Comment

                      Working...
                      X