Tanks and Collectors for Solar Thermal Systems

You are here

When you install a solar thermal system, you always need a collector (the bit that does the heating) and often need a tank (unless you're able to use a simple "batch system"). The tank is pretty simple but the collector format and setup is more complex.

Tanks

A standard 80-gallon water tank measures about 5 feet high and is about 2 feet in diameter. It's generally enough for an average family home, though it's unlikely to cover many of your needs in winter without a conventional heating system as backup.

If you're planning on using your solar thermal system all year round for most of your hot water needs, you're going to need a much bigger tank. To cover you for three or four days without much sunlight, you'll need something like a 350-gallon tank, which measures about 7 feet in height and is about 3 feet around (diameter). It'll also need to be well-insulated or your water will be much colder after a couple of days.

For large tanks like this – or even bigger ones if you're building a communal system for several houses or apartments - it's worth considering a higher initial outlay to have the tank installed underground, where it'll be out of the way and will benefit from being surrounded by earth. Maintenance is potentially more difficult but the benefits in saved space and energy can counterbalance the trouble and cost.

Collectors

The collector is the part of the system which sits in sunlight and collects heat. In a simple batch system, the tank itself is often the collector. If you're installing a more robust system, the collector is separate.

Collectors are very logical: sunnier, warmer locations need fewer square feet of collector than cloudier, cooler locations. Let's put that into perspective with some very rough guideline figures:

 

Most collectors come in two sizes and two types. The sizes are generally 32 sq.ft. (4ftx8ft) and 40 sq.ft. (4ftx10ft); the types are flat plate and evacuated tube.

Flat-plate collectors consist of a glass-covered box containing copper pipes. The pipes are heated by a flat absorber and transfer that heat to fluid running through them. Water or antifreeze is used to carry the heat. Flat-plate systems work best when you don't need very hot water (under 140°F).

Evacuated-tube systems consist of a series of glass or plastic boxes with a pipe in each, in similar fashion to flat-plate systems. The pipes contain air or antifreeze. The difference is that each box has all the air pumped out from around the tube to create a vacuum, which resists heat transfer. These systems retain captured solar energy very efficiently but, as you might expect, are more expensive. They can provide water to systems that require hotter temperatures (over 140°F) and work much better than flat-plate systems in cloudier, colder locations.