Questions About Solar Water Heaters

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.[1]

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.

Does a tankless water heater make more sense than a solar water heater?

We don’t think so. Every gallon of hot water you use still has to be heated. And the amount of energy required to heat a gallon of water to a certain temperature does not change just because it is being heated at a faster rate.

The tankless water heater only eliminates energy costs for maintaining the temperature of water that has been heated and is sitting in the storage tank awaiting use. So a tankless water heater saves about 15 percent of the heating energy needed by a conventionally insulated electric or gas water heater. Solar storage tanks have extra insulation, to keep water heated during the day hot overnight, so you get the same temperature maintenance savings with solar.

And tankless water heaters do have drawbacks. According to the U.S. Department of Energy:

“Sometimes … even the largest gas-fired model cannot supply enough hot water for simultaneous, multiple uses … taking a shower and running the dishwasher at the same time can stretch a (tankless) water heater to its limit.”

Most tankless water heaters use natural gas, which is currently about 32 percent cheaper[2] than electricity for water heating. But natural gas is a fossil fuel, so it is subject to the same climate change concerns and cost inflation pressures as powerplant-supplied electricity.

Solar energy is clean, inexhaustible and free, so it’s “greener” than gas and beats the economics of a gas tankless water heater hands down.

And one final point: Tankless water heaters can fall short on performance during periods of high demand; for example, if you are showering while the dishwasher or clothes washer are filling, or if more than one shower is in use at the same time.

Of course, you could overcome this problem by installing multiple tankless water heaters. On the other hand, tankless water heaters cost much more than conventional water heaters to install, so the total installation cost for more than one tankless water heater can quickly surpass the cost of a single solar water heating system.

Why not just install a solar water heater? Solar water heaters provide all the benefits of tankless water heaters but eliminate up to 90 percent of your water heating costs.

Why not use solar electric power for water heating?

This is an excellent question. Understanding the answer provides some insight into why, 50 to 100 years from now, solar water heaters will still be the best way to heat water.

And why you should go ahead and install a solar water heater today.

The issue is conversion efficiency. A typical flat plate solar water heating collector transfers about 63 percent of the solar energy that strikes it directly into the water (or heat transfer fluid, in indirect circulation systems).

A typical photovoltaic (PV) solar electric cell converts only 15 percent or so of the energy striking it into electrical energy under ideal conditions: PV cells lose efficiency as operating temperatures rise. Additional inefficiency occurs when an inverter changes direct solar electric current (DC) into alternating current (AC). The end result is that a PV cell with a 15 percent rated peak efficiency only delivers about 10 percent of the energy striking its surface to a demand load.

So here is the problem: The solar electric PV panels would require more than six times the roof area of a flat plate solar thermal collector to meet the same (water heating) demand load.

Meeting the hot water needs of an average Florida family of four requires about 40 square feet of flat plate solar water heating collector surface area, so to do the same job with PV panels would take about 40 x 6 = 240 square feet of PV panel surface area.

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. For more information on the specific amounts of sun in your area visit, “The Renewable Resource Data Center”.

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.