How Many Tanks for a Simple Solar Hot Water System?

I was just asked this question the other day, which reminded me that this is a frequent question that people ask and don’t always understand. There are many manufacturer’s today that are promoting single tank solar hot water systems. Whether or not they are a good idea depends mostly on your situation. To keep this simple, I am only talking about simple residential solar hot water systems.
A single tank system means that the water heater and the solar storage tank are the same tank. On the surface, this sound like a super idea. Most people already have a water heater, so you don’t have to spend the additional money for a separate solar storage tank. This also means that you don’t have to plumb between the tanks, install a valve tree, etc.
However, you need to look at how the system will perform to understand what you are letting go by not having a separate solar storage tank. First, with only one tank, you don’t have as much storage. Typical water heaters are only 40-50 gallons. Whereas, typical solar storage tanks are 80 gallons or more. That means you could have at least half the storage in a single tank system that you would have in a two tank system. How much energy you can store in the solar storage tank depends on the temperature that the tank is in the morning and the temperature of your high limit for the tank. This is what frequently occurs with many solar hot water systems in the summer time in Colorado and around much of the USA. If the tank is 90F in the morning and your high limit is set at 170F, Then that gives you 80 degrees of an 80 gallon tank to store energy. With water, you can easily calculate how much energy that is, known as a BTU (British Thermal Unit).

The equation is: Q=Mass of water x (finish temperature.-start temperature)

Please be aware that this equation is simplified to make this example easy to understand.

First, you need to find the mass of 80 gallons of water, a quick Google search finds 1 gallon of water weighs 8.34 lbs. (pounds)

So, the equation looks like this for the 80 gallon solar storage tank.
Q= 80 x 8.34 x (170 – 90)
Q= 53,376 BTU’s

For the single tank system, you have 40 gallons, with the minimum temp no less than 120F and the high limit the same as for the two tank system, 170F

So, the equation looks like this for the 40 gallon single tank system.
Q= 40 x 8.34 x (170 – 120)
Q= 16,680

Comparing the two amounts of energy that you can store. The two tank system can store 3.2 times the amount of energy of the single tank system.

Second, if you are using flat plate solar collectors, their efficiency drops dramatically at higher temperatures, which will reduce your energy collected by even more. I will go into more detail on this in a future post.

Finally, I do think that the single tank systems can make sense. Mostly, they make sense for applications where there is no room for a second tank. If the option is ‘no solar system’ or a single tank system. Then, the single tank system will perform much better than no solar system. As with everything solar thermal, always consider the application.

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About Daimon Vilppu, President Simply Efficient Solar and Wind

I can't remember when I wasn't interested in energy, machines and other mechanical equipment. This desire led me to a degree in Mechanical Engineering (Northern Arizona University, graduated BSME1993). By the time I graduated, I was dismayed at our countries myopic energy policy. I couldn't understand why we were continuing to focus on oil and nuclear energy when there were so many possibilities with alternative energy technologies that languished with no support by our Government. I have since devoted myself to learning about alternative energy technologies, with a particular focus on solar thermal technologies. With that Knowledge, and continual education, I want to engage myself with others to refine, improve and educate as many people as possible so that we all can see why alternative energy technologies have to be a vital part of our countries energy future. Without many different technologies, I strongly believe that we will never reduce or eliminate our independence on foreign countries and they will continue to have power over our economy until we do embrace the best technologies available.
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