While solarwaterheaters can provide nearly all the domestic hot water for a home, there are, in most climates, just too many cloudy days to do 100%. That being said, because these systems do provide most of the domestic water heating, they are very cost effective investments, typically providing positive cash flow from day one.
So you will still need to have a back-up water heater to provide hot water during extended cloudy periods.
I know people who are off the grid and use their stand alone solar hot water for "all" their needs. Its not that the water isn't cold after a few days of cloudy skys, its more that they have adapted to only taking showers after a sunny day. They wait for the sun.
I also wanted to share this link. Its from a good site for perusing other people's solar and alternative energy projects. This page shows lots of home made solar hot water projects. (all thermosyphon systems that require no power).
fits right in with my using energy in the form it is delivered to you as much as you can
Location: Amherst, Wisconsin
posted 5 years ago
karl is certainly correct, you could only use a solar water heating system to provide your domestic hot water. For a while back when I was young we had no running water and no water heater. As time went on we got running water and eventually a solar water heater. We only had solar hot water after sunny days. Summer was rarely a problem here in the North Woods, but during the winter there were often weeks when we only had one or two days of sun. If this is the lifestyle you are after, then indeed solar can provide 100%. The reason I posted what I did is because after 30+ years installing solar water heating systems I have seen one client who relied 100% on solar for their domestic hot water and eventually I was asked to install an on-demand back-up. They had a regular job and had to bathe regularly regardless of the weather.
Important considerations to make in trying to decide whether solar hot water can work as a standalone system is as follows:
1) Where is your hot water tank located (indoors or out?). Below freezing temps at night will sap the heat out of all but the most expensive/well insulated tanks. A minimum of 55mm of high density polyurethane insulation is required to reduce overnight heat loss to less than 20 degrees @ 32 degrees outside temp.
2) Water storage tank size. My rule of thumb is 10 gallons of hot water per person using it. This will provide 2 days of showers per person. We limit showers to 2 gallons per person, but once that 2 gallons of hot water comes out - 2 gallons of cold goes into the tank to replace it and maintain pressure - tank temps will drop dramatically after half of the hot water is used and replaced with cold. Using a mixing valve and low flow shower features will help you sip hot water thus making it last longer during periods of poor sunlight. A 40 gallon tank managed properly works very well for my family of 4. If we had six trying to use it - it would really be tough.
3) Secondary heating element - Does your storage tank also have a means for boosting tank temps on cloudy days? Either an electric element of a gas flame will work miracles in getting a 65 degree tank back up to the 150 range. In my opinion a secondary heating element on the storage tank is preferable over a secondary tank. Having to circulate water between two tanks not only looses heat - it takes more energy and mechanical equipment A good aquarium heater is an example of a secondary heating element that would fit the bill on a 50 gallon tank system. The more passive the design - the better. I have an electric element on mine, but I am also operating on a stand alone PV system, so boosting the tank on cloudy days is not an option, but on real sunny days when my batteries are fully charged and tank temps are pretty low (below 105) I will redirect my surplus power production to heat the water tank to max temps. Thus a secondary storage of energy is accomplished. If we get a week of clouds and rain - I may not have hot water. We have had to take 60 degree showers before - it is exhilarating to say the least, but we lived. I can count on one hand the times we have had to do it, but it is something anyone living stand alone solar hot water should expect to do. IMO it is not worth the cost to add an on-demand LP heater to my system to avoid 3-4 cold showers a year. The cost of both the heater and the additional gas storage capacity and usage is just not worth it to me.
4) Can you limit hot water use only to navy showers (2 minutes/2 gallons of water)? I use hot water from a pot on my wood stove to warm my dish washing water and to warm up my laundry water for really soiled loads. The solar hot water is strictly for bathing. In the summer when sun is plentiful - I might splurge and use some hot water for dishes and dog baths - but that is about it. I treat it like gold.
Why just one stand-alone system? The solar heater is a passive system. A rocket stove heater is an active system, meaning you have to physically operate it, unlike the solar heater that automatically works when the sun is out regardless of your actions or presence.
Passive systems are usually at the mercy of the weather; solar & wind I'm thinking of here. Active systems are not at the mercy of the weather, but they're not automatic, either. You have to do something to make it work.
Why not marry the two and get the best advantages of each? Put a rocket stove near the solar batch heater, install a line that will circulate the water through a heat exchanger in the rocket stove, then back into the batch heater. This way, you have the option of heating the water on cloudy days if you need to and you still get the passive heating on sunny days.
rocket stoves are economical to build and operate, so you might as well back up the solar heater with one.
Best of all, you can still keep the standard electric or gas hot water heater for times when it's cloudy and you don't feel like firing up the rocket stove.
Years ago, when I lived down in the Piedmont area near sea level in Georgia, had a solar hot water heater. It had a back up system, but not once in 4 years or so I lived in that house did it ever kick in. I did a lot of laundry so used a lot of hot water, and never ran out. I think it was a 80 gallon holding tank and the solar panels had something like antifreeze in them and the water circulated over those pipes as they came down into the house (so it didn't freeze in winter). Even on cloudy days, it held water for 3 days and then got a little tepid, so I just didn't do laundry then and did quicker showers due to the slightly cool water. That unit was a professionally installed one and back then, about 30 something years ago it cost about $2500, but they are not hard to build. Even at that price, it was amazing and so cost effective. I want to do one again, some day.
That said, I realize we do have more cloudy days in the mountains and colder, but somehow cold does not prevent it from making hot water. When it is cold more solar gain comes through. Something to do with molecules and how fast they are moving I think. (learned that in Greenhouse school).
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