Additional Solar Thermal System Sizing Considerations

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Evacuated Tube and Concentrating Collectors

Each individual model of evacuated tube and concentrating collector is different. If you intend to install anything other than a flat-plate collector array, you will need to refer to the manufacturer's manual for sizing guidelines.

If no guidelines are available, you can work out the basics for yourself, using the collector's efficiency (though you should probably also ask why there are no sizing figures for something so important). The SRCC records the number of Btu per square foot each tested collector will produce, which you can use to work out the output for your location. This is typically 600-700 Btu per square foot per day for evacuated tube collectors, though concentrating models vary widely.

Conversion table for calculating the thermal value of different fuelsNext, you need to convert your daily hot water use into Btus. You can use the table of fuel thermal values to get an estimate of how much you use, then divide that by the collector output to get the required array size:

(Thermal value in Btus x Daily fuel usage) ÷ Btu output of collector model

This method also works with any kind of collector setup, including flat plate models. It's just a bit more involved and there are simpler guideline calculations for flat-plate installations.

Too Big or Too Small?

Sizing a collector array can be tricky because collectors are only available in standard sizes and building an array that is precisely a given size may not be possible. In these cases, it is often better to under-size the array to avoid any potential overheating issues, though over-sizing by a little bit is rarely catastrophic.

If you are building an array that will be partially shaded, you'll want to over-size to compensate. The same goes for mountings that point more than 30° away from direct south – they will need to be oversized by 1%-2% per degree over 30°.

Remember that all the figures on this site are averages and guidelines, so you may be able to upgrade to the next size bracket without any issues. As always, the best option is to consult an experienced professional with local knowledge.

Plan Ahead

The final consideration when you're sizing a solar thermal hot water system is the future. Remember that the equipment you install is going to operate for as long as 40 years (or more). What will change in that time? Are you likely to have a bigger family? Is the area in a prolonged period of urban growth, which will potentially mean obstructions to your solar window? Are you two people living in a four-person home?

There are many reasons that you may want to install a larger system than you need right now. However, be very careful: installing a bigger tank (an extra 20-30 gallons) is usually fine but larger systems are built for larger loads. If you don't use all that solar radiation, you risk overheating your solar loop and collectors. Balance your current requirements against future use and build the best system for both.