solar tank with back up electric, one tank or two tanks

peacefulsoular

? I have an existing solar hot water system.  I have a separate shed built to house a 300 gallon non-pressureized drain back tank and 280 square feet of collector area.  In the house I have an electric water heater- which provides the DHW and heat (through a flat plate heat exchanger) to cast iron radiators.  I want to increase my efficiency of delivering solar heated and pre-heated water to the the water heater in the house.  I am thinking to use a 120 gallon electric water heater with just the top elements on low as backup.  But I am concerned about overly mixing the water in the tank, and not maintaining stratification of the water (hotest on top).

A friend with a lot of solar experience suggested 2 separate tanks, the electric back up tank being small, 20-30 gallons, and having two 5500 W elements for fast recovery.  The idea is to have more control over mixing,to maintain stratifaication and not to mix electrcally heated water  with the solar heated water, unless the solar heated water is hotter than the back up elecrically heated water.  It would accomplish what a on demand water heater does, minimum storage of hot water, but ability to deliver a lot when the demand is high.

Do you know about systems that use these concepts.  It complicates things to have 2 tanks, and I have read that there is stratification in a solar /electric tank(unless a pump is mixing the tanks.  In my situation, I have a pump moving the heated water from the drain back tank (through heat exchanger) and into the electric water heater.  I have temperature sensors and am loging the temperature at the bottom and top of both my tanks.  By making graphs in excel, I have seen that there is almost no difference between temp at tank bottom or top.

hot_rod

pumping tanks

if you are pumping through the tank(s) it is hard to keep them stratified. Some of the more "engineered" tanks have multiple ports at different levels to load without mixing the entire tank.



Also you can "time out" the electric element in the morning, allow all day for solar harvest, then switch the element back on in the afternoon before you go home. Only enable the backup when it is needed, and after the solar has a chance to cover the load.



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