A solar hot water system can be sized to meet all of your hot water needs, year round. But what can you do with the excess heat that will inevitably be generated during the hottest, brightest days of the year? Put it to work, heating a hot tub or pool.
The first step in sizing a solar water heating system is to identify the load—how much hot water the household uses per day. Then we can calculate how large of a solar water heating system will be needed to produce that much hot water. The variables include the consistency of the load and the consistency of the solar resource. Hot water use varies some from day to day but is usually fairly consistent over the whole year. The solar resource changes seasonally, producing more hot water per day during summer and less in winter.
It would be great to have a solar water heater that provides all of your domestic hot water all year long. Unfortunately, that is not economical or practical for most—if we design the solar water heater to provide 100% of the hot water load on the worst solar day of the year, then every other day of the year we would have more hot water than we need. Another limitation is that a sunny day will provide most or all of that day’s hot water—but if it’s cloudy the next day we will run out of solar-heated water. If we size our system so that one sunny day provides two days’ worth of hot water supply, then two sunny days in a row would result in the solar water heater generating excess heat on the second day.
To many, the economics are not as important as avoiding using conventional, backup water heating. But then, what about the excess heat that system will produce? Another scenario that can get us into the same predicament is when a home with solar water heating is vacant for periods of time, especially during the summer.
In most locations in the United States, a solar hot water system requires some kind of freeze protection. There are two types of freeze-protected systems that are used—drainback and pressurized—and both use a differential temperature controller to turn the system on and off at appropriate times. Whenever it is warmer in the collector than it is in the water storage tank, the controller turns the system on; the system stays on until the collectors are no longer warmer than the water storage tank.
Drainback systems offer both freeze protection and overheating protection by simply shutting down, allowing all of the fluid to drain out of the collector and pipes. Controllers appropriate for drainback systems turn off the system when a set high temperature limit is reached in the storage tank. In some homes, it is not possible to install the piping in such a way that guarantees complete drainage, so sometimes a drainback system is not an option.
In that case, a pressurized system is called for. They don’t drain and instead use a nontoxic heat-transfer fluid—which is also an antifreeze, and remains in the collectors and piping at all times. Pressurized systems allow more piping and installation options than drainback systems.
The potential overheating problem in pressurized systems is due to the heat-transfer/antifreeze solution used—almost exclusively it’s propylene glycol mixed with water. Glycols are rated for various operating temperature ranges. But temperatures produced by flat-plate and evacuated-tube collectors can exceed glycol’s highest operating range, resulting in thermal breakdown of the fluid, which causes collector and pipe corrosion.
When drainback systems reach their high-temperature limit, they turn off the pump, the system is drained, and no more heat is collected. If we employ the same strategy with pressurized systems and turn off the circulating pump when the storage tank reaches a high limit, the heat-transfer fluid will stop circulating but it will stay in the collector. If the sun is still shining on the collector when the circulation stops, the collector temperature and heat-transfer fluid temperature will rapidly rise. The temperatures in flat-plate collectors can reach more than 300°F—and the temperature of evacuated-tube collectors can be even higher.