Calculating Flow Rate and Head in Solar Thermal Systems

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Choosing the right pump for your solar thermal installation is very important and for that you need to work out the flow rate and head for your chosen setup. Solar loops which are always full only have friction head, whereas drainback systems also have static head.

Calculating Flow Rate

Example friction, head and flow ratings for Type L copper pipesThe quickest way to work out flow rate in any solar thermal system is to borrow the SRCC's standard. They use a rate of 0.88 gallons per minute (GPM) for each flat-plate collector in a system. Once you know which model of collector you'll be using, you can consult the manufacturer's documentation – most recommend a flow rate of 1 GPM, though there is some variation.

The flow rate affects system efficiency and operating temperature. Slower flow rates keep the solar fluid in the collectors for longer, so it comes out hotter, but overall efficiency is reduced as the heat is transferred slower. Higher flow rates do the opposite: more fluid is pumped around the system for better efficiency, but the overall temperature is lower.

It is important to understand which your project requires, so that you choose the right pump. Do you need lots of lower-temperature water for the domestic supply or do you need less volume at a higher temperature for solar cooling?

Calculating Friction Head

Head ratings for valves and other componentsFriction head is a consideration for all system types. In pressurized loops, it's the only kind to overcome; in drainback systems, it's part of the overall total. The calculation is simple but long: take the standard friction head per foot for the type of piping you intend to use and multiply it by the total length of all the pipes in the system, then add the friction head for each individual component.

Friction head for pipes is easy because it's standard: any plumbing reference can provide the figures you need – the diagram shows an example for Type L copper pipes, with friction head expressed in feet of head per 100 feet of pipe.

Component head can be a bit trickier. Most manufacturers express component head in an equivalent length of tubing, so you'll have to total all the equivalents and work out the friction head for that much piping. Of course, some components are individually rated, as is often the case for collectors and heat exchangers, so you'll end up totaling those and adding it to the pipes and equivalent pipes. And to make matters even more complex, many are rated in PSI (pounds per square inch), which you'll have to convert – 1 PSI is 2.31 feet of head; 1 foot of head is 0.4335 PSI.

Piping friction head = (Total length of pipes in feet ÷ 100) x Head per 100 feet of pipe
Component friction head = [(Total equivalent pipe length in feet ÷ 100) x Head per 100 feet of pipe] or  [Listed PSI x 2.31 feet]

Total friction head = Piping friction head + Component friction head

Calculating Static Head

This bit's easier, because gravity is the same everywhere in the world! You only need to worry about