How to Size a Solar Thermal Space Heating Collector Array

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Space heating with solar thermal installations revolves around Btus. If you've sized the heat generation part of your system already, you'll be familiar with this. The next step is to work out how big the collector array needs to be.

You can use a rough estimate to work out how big your collector array must be or you can investigate the actual ratings of different models to work out your options. As always, the manufacturer and the SRCC are excellent resources for the data you'll need. Remember that a collector's output will depend on your location and its mounting angle, so take those into account when you're working out the amount of solar radiation available to generate heat.

The first thing you'll notice when you calculate collector output is that locations which need the most heating generally produce the fewest Btus per square foot of collector. A standard flat-plate collector can provide about 700-1000 Btu per square foot per day, but colder locations will tend to the 700-800 end of that range on a cold, clear day in the heating season. Unfortunately, this goes with the territory: colder locations need bigger collector arrays to generate the same percentage of the overall heating load.

Using your space heating load figure and the daily generation figures for the collector of choice, you can calculate the size of the array. Simply divide your daily requirement by the amount generated by an individual collector and multiply by the collector size:

(Daily requirement in Btu/ft²/day) ÷ (Btu/ft²/day per collector) = # of collectors needed

(# of collectors needed) x (Size of each collector in ft²) = Surface area of collector array

The big question is "Where is the point of best return?" It's obvious that a smaller collector array is cheaper, both at installation and for ongoing maintenance, while a bigger array offers bigger savings on your heating bill. The difficulty is finding the balance, where the installation and maintenance costs won't cripple you and the savings give a reasonably fast return on investment.

In general, assuming you are only installing a space heating (or combined hot water and space heating) system and do not need the collector space for PV panels, it is best to size as large as possible. Remember that the lifespan of the system is measured in decades. It will cost you more initially but the annual savings are compounded by rising grid-tied fuel costs. This means that the return on your investment accelerates over time and, in the long term, your savings on fuel bills are potentially astronomical.