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, Mon, 10 Sep 2012 15:53:43
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Many years ago I installed a solar hot water system. It was single glazed and had selective surface copper fins and tubes. The afternoon that they were installed was cloudy. The clouds cleared off and when I was connecting the tubing I burned my finger on the panel's outlet. I put some spit on my finger, not the burnt one, and wiped that on the connection fitting. It sizzled and boil dry in a few seconds. WOW I was happy, never had a burn feel that good.
There was controller with a thermal couple at the water tank and one in the collector. It would run the pump when it was 20F. The coolant loop was polypropylene glycol, food safe and sometimes used in ice cream, also for antifreeze for pluming in boats, RVs and cottages.
The tank has a regular over pressure and over temperature relief valve that is used for regular hot water tanks. One day when I was at work, it blew off and sprayed across a caret on the basement floor, which had a permanent witness mark. I obtained a replacement valve that never did that again. In the later spring and summer, when we turned on the kitchen sink hot water, the water seemed to be boiling as it came out of the faucet; but I never did measure the temperature of the water when that happened.
The controller worked well, but what it also needed was a lower over-temp setting to prevent such extremes. When the pump shut off and the sun was shining, the coolant in the collector would boil and then steam would flow through the deliver tube to the tank and that would further heat the tank and heat the top of the tank, which may be why the water temp could get so extreme. The coolant was 50:50 and the boiling point of that would be higher than 212F creating a greater over-heat potential.
The coolant loop was "micro flow", using a pump used for beverage delivery. It experienced the heat of the bottom of the tank. The delivery tubes were 1/4" OD tubing and not insulated. Because of the small size of the tubing, the heat losses might have been less than insulated large tubes. Something that was explained in Engineering lectures.
So all of that was very interesting given my background. I had two degrees in ME and did research on solar thermal panel heat loss; coupled convective and radiation heat transfer, through different aspect ratio horizontal honeycombs that suppress laminar losses and the change over from laminar to turbulent convective flow. The variables where the emissivity of the collector plates, honeycomb geometry, air gap thickness, angle of the collector, heat losses [flux] from the collector plate. The pressure in the test cell was varied from a deep vacuum to high pressure. All of those variables were accommodated in a theory driven formula that allowed the research to be used to design collectors. I also was the first to describe a mode of heat loss driven by the linear temperate gradient through the air interacting with the non-linear gradient in the honey combs driven by radiation heat transfer. The work was done at the University of Waterloo, Ontario. The research was funded from the US Department of Energy. Money for such things rarely flows outside of the USA. What happened was that the leaders in heat transfer in the USA had been working on heat transfer in space capsules, Shuttle and Space Stations, where natural convection does not exist. Natural convection research had been neglected in the USA. The professor who obtained the grants had done a lot of work modelling natural convection in the large concrete cooling towers that are emblematic of nuke plants.
posted by 65.69.235...
, Mon, 10 Sep 2012 15:53:43
, Wed, 12 Sep 2012 09:29:24, Tue, 11 Sep 2012 10:58:36, Fri, 14 Sep 2012 15:13:49, Tue, 11 Sep 2012 08:31:04
, Tue, 11 Sep 2012 00:48:38, Tue, 11 Sep 2012 20:54:01
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