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Integrating Hot Water Systems (solar + backup)

posted 1:36 PM Wednesday, October 01, 2008 by pgandhi2

I have been looking at combining the solar thermal hot water system with various types of electric backups this week. A backup system is generally required for cloudy days and during times of increased demand. One idea is to combine the solar thermal system with one or multiple tankless hot water heaters. Or we could go with a conventional tanked water heater, since we will need a tank for the solar thermal system anyway. It was very difficult to find good information about combining hot water systems. Most of what I did find were stories of people who had built their own integrated system, but with very little technical information or specifics.

Although a combined solar-thermal system will probably end up being too expensive, I would still like to consider trying to make it work. In class we discussed putting the evacuated tube collectors on the side of the house or integrating them into the architecture somehow since putting them on the roof would take away PV panel real estate. Before we can proceed too much farther with the solar thermal idea, we need to know what the house exterior looks like and what the room layout is (for tankless location).

Since my searching was not very fruitful this week, I went back into the rules and extracted the hour-by-hour hot water demand by day and contest. For dishwashing and washing clothes, we are given about a three hour window each. For hot water draws, we have just 30 minutes. Usually these tasks overlap in time, but if we space them out properly, we probably won't have to draw hot water for multiple tasks at the exact same time very often. To size any hot water system, we should figure out when we will use the appliance, how long it will take and what flow rate (gal/min) is required.

--Priya Gandhi

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