Back in the 1960s, the caretaker at a summer lodge that was in our family rigged a makeshift solar water heater: he laid a black sheet of plastic against the hillside next to the pool, hooked up a small pump, and attached a hose that directed the water down the plastic, where it was warmed by the summer sun before it flowed back to the pool. While not particularly attractive, his invention worked very effectively to provide low-cost hot water in the mountains of Colorado, where nighttime July temperatures often dipped below freezing.
Heating pools is still the most common use of solar thermal, or solar water heating (SWH): systems designed for domestic use have never fully caught on in the U.S., even as more complicated, more expensive, and less efficient photovoltaic (PV) modules have become more common. Why hasn’t solar water heating become more mainstream? When is it worth considering, and what does it take to pay for it? Before we explore these questions, let’s review the current technologies and applications.