4 Steps to Basic Solar Thermal Design

I’m sure you’ve noticed that as fuel and electricity prices continue to rise, the interest in solar hot water (SHW) systems is also continuing to increase, and your customers may be asking about it. Why? It’s a simple, basic, and proven technology that has been around forever.

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If you’re interested in selling and installing solar how water systems, you’ll need to understand the basics of design so that you can perform proper site visits and understand what drives the costs of the systems.

There are four basic steps in designing a pressurized, anti-freeze based solar hot water system. The details of each step can get more involved than what I’ve written here, but these are the basics.

1) Make sure the roof has solar access and enough room for collectors. Solar access is simply exposure to the sun from 9 am to 3 pm all year round and within 25 degrees, east or west, of true south. It’s best not to have any shading at all, but solar thermal collectors are much less susceptible to shading than photovoltaic systems. If you do have some shading, you can often compensate by installing larger collectors.

 How much room do solar collectors take up? Flat plate collectors are typically larger, with an average size of four feet by eight feet.

2) Next, you need to measure hot water usage. Typically, one person uses 20 gallons of hot water per day. This is a good rule of thumb. This number could be less — if there are five people living in a home, they will probably only use 80 to 90 gallons instead of the expected 100. However, some people could use far more water daily, and some could use far less. It is key to collect this information during the site visit.

3) Then, measure the size of the solar storage tank. Sizing solar storage tanks is very simple–count one gallon of storage per gallon of usage. This makes sense, right? Every day the goal will be to heat up the water that the household will use, and then use that water. The tricky part is this: since tanks only come in standard sizes, it is recommend that you round up. If you use 50 gallons of hot water and can’t get a 50 gallon tank, purchase a sixty gallon tank.

4) Finally, you must size the collectors. In order to size collectors, assess how many square feet of collector is needed per gallon of storage. This ratio will change based on your climate. In the northeastern quadrant of the U.S., the ratio is 1.5 to 2 square feet of collector per gallon of storage. This metric internalizes climate data. Thus, in southern climates (with more sun and heat) you need less square feet of collector per gallon of storage.  To use the previous example, if you have a 60 gallon storage tank, you need between 90 (60*1.5) and 120 (60*2) square feet of collector. There are two more things to remember, though. First, this ratio only works with flat plate collectors. Second, collectors only come in standard sizes. A common size is four feet by eight feet–equally 32 square feet. In that case, you would use three collectors, or 96 square feet of collector in total, which is between your range of 90 and 120. If you had 4 by 10 foot collectors (40 square feet), you could use 3 collectors as well to get 120 square feet — still within the range.

Solar Hot Water Cost?

One last question is how much does it cost? As a rule of thumb, two flat plate collectors (4 foot by 8 foot) and a 60 storage tank would be priced around $10,000. Obviously, the price can differ from this depending on the site.

As with all renewable technologies, it is important to learn more than just the basics. When you start dealing with commercial applications or evacuated tubes, there are many calculation and design considerations to take into account.

Want to Get Deeper Into SHW Design?

If you’re interested in getting deeper in the subject, check out our Solar Thermal courses.

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