4 Responses to “Analyzing European Influence on American Solar Hot Water Systems”


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  1. JamieLeef

    Seems like there are three other significant European ideas now informing thermal design:
    – Low-flow rather than high-flow closed loop
    – Active storage tank stratification
    – Steam-back overheat protection

    The first two can apply to drainback systems as well. Thoughts?


    • Dr. Ben


      Low flow means low efficiency. The higher the temperature inside a collector, the more energy is lost to the outside world through convection, conduction, and radiation. The argument is that lower flow requires a lower HP pump, which is true.

      However, glycol system have a 15% lower heat transfer capacity to start with, so it is recommended that the pump be upsized by one model anyway.

      The bottom line is glycol starts out as a bad choice and only compounds the problem when you get to efficiency of the collectors, heat transfer reductions, maintenance, etc.

      The active storage tank stratification question has me slightly puzzled. The idea is to have the hot fluid in the solar tank float (thermosyphon) to the top so the hot water is on the top and the cold water is at the bottom. This occurs naturally anyway because hot water is less dense than cold water and naturally rises to the top. If you take a bath in a tub, the water on top will be warmer than the water at the bottom.

      We want the coldest tank water at the bottom going to the collectors for maximum efficiency as described above.

      The only way to mess this up is to stir the tank up with a high volume of water flowing through. A single tank solar system has this problem. The single tank does double duty as the regular water heater with electric elements in the top, and the solar storage volume at the bottom. The last thing you want is the electric heat going into the solar storage and up to the collectors. Therefore, single tank manufacturers have to spend a significant amount of effort keeping the solar energy separate from the electric energy.

      Manufacturers have devised various kinds of baffles inside the tanks to minimize mixing the water volumes, and clever tubes to introduce the cold water at the bottom without mixing the tank too much. These specially made tanks cost considerably more than conventional water heaters.

      In a two tank solar system, these problems are largely moot. Only solar energy goes into the solar tank and only electric (or gas energy) goes into the conventional water heater. A heat exchanger in the hot part of the solar tank supplies energy to the cold water flowing through on the way to the cold water inlet of the conventional water heater. Think of it as having a hot well as the water source to the water heater. No special water heaters, no high price.

      The solar storage tank stratifies naturally as described above.

      As for steam back, don’t get me started. One of the blog posts discusses that methodology.

  2. Dear Dr Ben,

    Thank you very much for your assistance. In your previous letter you have written:

    {I usually use .04 times the gallons inside the solar tank to determine the exchanger area}

    We are to design our first drain back system for a hotel and we are to use an external plate heat exchanger which is to be cleaned twice a year. It remains to estimate the heat exchanger area. From the above sentence I understand that the heat exchanger area will be 0.04 times gallons expressed in square feet. That means if I have 200 gallons the area will be 8 feet? Am I right ?

    Thank a lot,
    Best regards,
    C. Oikonomidis,
    Civil Engineer

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