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Questions About Solar Water Heaters

HomeQuestions About Solar Water Heaters

What about cloudy weather?

Good question. Actually, a solar water heating system will typically collect about half the solar energy of a clear, sunny day on an overcast day. If you have ever had the experience of going to the beach on an overcast day and still getting a sunburn, you will understand this phenomenon. Clouds block many of the visible wavelengths of sunlight, but much of the solar energy still gets through.

Will my water be hot enough?

Yes. Solar heated water is often hotter than the thermostat setting on your water heater. In fact, for safety reasons our systems include mixing valves to make sure the hot water going into your house isn’t too hot. On the other hand, sometimes we may have extended periods of very cloudy and rainy weather. During these periods, a backup electric heating element in your water heater / storage tank will automatically heat water to the water heater’s thermostat setting.

Will I have hot water during cold weather?

Yes. Solar water heating collectors typically deliver excellent performance in Florida during cold weather because the sky is very clear during winter high pressure waves. The glass cover plate and insulation inside the collector prevent collected heat from escaping to the outside air.

Can you use my existing water heater as the solar storage tank?

Usually, no. Solar water heating systems are designed to heat and store 24 hours worth of hot water during the daylight hours, so the tank has to be large enough to store 24 hours’ worth of hot water. Most conventional electric water heaters in Florida homes have a capacity of about 52 gallons. Standard solar storage tank sizes are typically 80, 100 and 120 gallons, with 80 gallons being appropriate for most three- to four-person households. Also, solar storage tanks typically have better insulation than conventional electric water heaters, to minimize overnight heat loss.

I’ve heard that “evacuated tube” solar collectors are much more efficient than flat plate collectors. Are evacuated tubes better for solar water heating?

While it’s true that evacuated tube collectors can attain higher temperatures than medium temperature flat plate solar collectors, as well as higher efficiencies under certain conditions, research studies have demonstrated that flat plate solar collectors produce greater savings for medium temperature (120–160°F) water heating on a year-round basis, even in colder climates.

And we do not recommend evacuated tube solar collector systems for high wind zones such as Florida. An evacuated tube—or vacuum tube—collector system resembles a bank of flourescant light bulbs on your roof. Naturally, an evacuated tube cylinder will have minimal wind resistance, which is good. But it will have very poor impact resistance if struck by a flying object, which is bad. Modern flat plate collectors have tempered glass cover plates that can sustain direct strikes by such wind-driven objects as, for example, a piece of 2×4 lumber.

Evacuated tube collectors are very popular in Europe, where their high efficiency offsets more frequent overcast weather conditions and lower average air temperatures than Florida. On the other hand, Europe does not experience hurricanes.

Can my home solar water heating collectors also heat my swimming pool?

No. Home water heating systems operate at 125–140°F water temperatures. These systems typically have a relatively small solar collector surface area (10 to 16 square feet per occupant) but the solar collectors have special heat absorbing materials, glass cover plates and insulation to maximize heat gain and reduce losses.

Pool heating systems only reach 75–95°F water temperatures. This allows the use of relatively inexpensive polypropelene solar collectors. However, because even a small residential swimming pool may hold 10,000 gallons of water, a typical residential pool heating system must deliver as much as 10 times more total daily energy than a typical home water heater. So while a less expensive type of solar collector can be used for pool heating, a solar pool heating system needs a much larger solar collector surface area.

References and Notes

  • Trinkl, Christoph et al. “Performance of Vacuum Tube and Flat Plate Collectors Concerning Domestic Hot Water Preparation and Room Heating,” 2nd European Solar Thermal Energy Conference 2005 (estec2005), Freiburg, June 21–22, 2005.
  • Heating 100 gallons of water from an inlet water temperature of 72°F to 140°F requires 56,712 BTU: 100 gallons x 8.34 pounds per gallon x 68 degree temperature rise = 56,712 BTU. Natural gas has an energy content of 100,000 BTU per therm. A typical natural gas tankless water heater is about 82 percent efficient, so it needs 56,712 BTU / 0.82 = 69,161 BTU of energy input, and 69.161 BTU / 100,000 BTU/therm = 0.69 therms worth of natural gas to raise the temperature of 100 gallons of water by 68 degrees. At a price of $2.30 per therm, the cost is $1.59 per day.
  • One kilowatt-hour (kWh) of electricity converts into 3,412 BTU of heat energy. An electric water heater delivers almost all of its electrical energy to the water, so it needs 56,712 BTU / 3,412 BTU per kWh = 16.62 kWh to do the same job. At 14 cents per kWh, the electric water heating cost is $2.33 per day. So the natural gas cost is ($2.33 – $1.59) / $2.33 = 32 percent less expensive. Of course, the daily cost of solar energy is zero.

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