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Back up for Solar Hot Water System An Over Looked Area

by Larry Schlussler PhD on May 6th, 2010

Often over looked is the significant effect a back up system can have on the overall performance of a solar hot water system. In a recent letter sent to Solar Industry Magazine I point out some design considerations.

Several years ago I designed a zero energy home, which I now live in. The considerations made when choosing a back up system are described in a paper presented to ASES.

An area often overlooked when the performance of a solar thermal system is evaluated is how the back up is integrated in the hot water system. Assume that showers are taken early in the morning on a sunny day and the storage tank is depleted of hot water. If the back up system then fills the storage tank with hot water that morning there will be no room for solar produced hot water. If more hot water is not needed until the afternoon, the energy used to heat the water that morning will have been wasted.

This type of management problem can be eliminated if an on demand heater is used as the back up. Unfortunately, neither electric or gas back up are the perfect back up. The disadvantage of a gas back up is that even in the modulating heaters the heat cannot be turned down sufficiently. If, for example, you are taking a shower and your solar tank is at 100ºF you may only need a 10ºF boost in temperature to be comfortable but the on demand gas heater may unnecessarily boost the temperature 25ºF. To remain comfortable the cold water must then be turned up; water and energy are then wasted.

With an on demand electric heater the output can be modulated to a lower level so this problem is eliminated. However, electricity is a more valuable form of energy; three units of gas-produced heat energy must be expended to produce one unit of electrical energy so finding the ideal solution is not clear-cut. In my home I oversized my collectors so they would provide a large percentage of my hot water needs. In this situation, where I often only need a small boost in temperature, I use an on demand electric heater as a back up. To determine the best solution in a particular situation requires some whole system thinking: considering the demand size of the array, storage, and usage patterns etc.

  1. Wow! This is amazing!

  2. Curt Moulton permalink

    I recently installed a 400 watt solar system (with the ability to expand) at our seasonal cabin near Lassen National Park in California. We are off the grid and have been reliant on propane for lighting, refrigeration and hotwater. So far we are using solar electricity for lighting and refrigeration. Since we cannot dump surplus power onto the grid, perhaps it makes sense to heat water with it. My thinking is that it could reduce propane use and increase the life of our solar panels. Since I’m not an engineer, it will take some investigation to figure out if this is feasible.

  3. Solarguy permalink

    Nice blog Larry! I just bought a house and have been pondering what to do in this area as well. I would also like to produce close to 100% of my hot water needs.
    One idea for the waste heat in the summer (or winter) that I had is to pipe it around a storage container for composting toilets to keep it warm and happily composting. I still have normal toilets, but the electricity required for many composting toilets is not attractive for me and this could be a DIY alternative.

  4. gas heaters are great but due to the rising prices of petroleum, perhaps an electric heater might be a cheaper option-~~

  5. i prefer to use gas heaters in my home cooked meals because they tend to cook faster *`:

  6. Matt Hahr permalink

    Nice blog Larry. It has helped us with our HSU project immensely. Thank you for making yourself and your business available to us.

    Also, your last comment is correct…this IS something…haha.

  7. Garry Painter permalink

    Wow. Just, wow.

    I think Da Vinci would be impressed with your work.

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