Solar Water Heating: A Viable and Reliable Water Heating Method, but One That Needs a Backup

Propane fills the bill for an energy-efficient, environmentally friendly back-up energy source.

While scientists might debate issues related to global warming and the depletion of the ozone layer, none would find an argument with this statement: "Solar heating systems are reliable and adaptable . . . because they use renewable energy from the sun."

That statement is straight out of a publication (A Consumer's Guide: Heat Your Water with the Sun) by the Department of Energy's Energy Efficiency and Renewable Energy (EERE) division. There's also little disagreement about the need for alternative sources of energy to heat water, which typically accounts for the second largest amount of total energy used in a typical single-family home.

According to the National Renewable Energy Laboratory (NREL), water heating accounts for a substantial portion of energy use at many residential, commercial, institutional, and federal facilities. Nationwide, approximately 18 percent of energy use in residential buildings and 4 percent in commercial buildings are for heating water. Solar currently provides just 1 percent of the water heating market. Energy Star, as seen in the chart below, shows a lower amount of energy used to heat water (14 percent). But both estimates show that water heating accounts for a significant amount.


Water heating typically accounts for the second-largest amount of total energy used in a single-family home, after heating and cooling. Chart courtesy ENERGY STAR

While solar water heating systems are widely recognized as being reliable, good for the environment, and an excellent way to lower utility bills, one thing is certain: All solar water heating systems need a backup system for cloudy and/or rainy days and times of increased demands.

Because solar alone can't provide hot water at all times, the most energy-efficient and environmentally friendly back-up system is a propane tankless water heater. Generally, a tankless water heater offers lower operating costs for the homeowner and lower CO2 emissions.

Rinnai, a forerunner in tankless technology, has recently developed the RH360, a "boost" kit for tankless technology that's been designed specifically for use with solar water heating systems. It features a rapid system recovery; is safe for use with any indoor or outdoor Rinnai tankless unit; provides consistent hot water, even when the solar tank temperature falls below the set-point; has been engineered to optimize solar gain by minimizing tank-mixing; and is compatible with all types of solar thermal systems. It is an accepted Energy Star appliance and is eligible for a federal tax credit of up to $1,500.

Propane: the best backup energy source for a solar water heater

Solar water systems all need a backup system for cloudy days or during times of increased demands. Like all water heaters, these backups can be powered by electricity, natural gas, or propane. There are pluses and minuses associated with all three sources, from costs (actual and environmental) to convenience to accessibility.

Because there are very few electric tankless water heaters on the market (only eight models of electric heat pump water heaters are Energy Star approved), the fuel choice is realistically between natural gas and propane.

While natural gas has a number of benefits, its distribution system limits its use to urban and suburban locations. While there are no statistics available on the distribution of solar water heaters in the United States, it's safe to assume that many are in rural locations where, typically, natural gas is not available.

With up to 56,000 miles of pipeline and more than 6,000 retail dealers, propane is readily available throughout the whole country. Because it is 270 times more compact as a liquid than as a gas, it is highly economical to store and transport. Propane is a well established, domestically available clean-burning fuel. Using propane in place of less environmentally friendly fossil fuels reduces acid rain, the greenhouse effect, urban smog, and the thinning of the ozone layer.

The most prevalent greenhouse gas – carbon dioxide – is a necessary byproduct of fossil fuel combustion. The mass of carbon dioxide released per Btu of fuel – the carbon content, also called the carbon footprint—is a good first-order indicator of the carbon dioxide emissions comparison between fuels.

Natural gas (methane) generates slightly fewer CO2 emissions per Btu than propane, but natural gas is chemically stable when released into the air and produces a global warming effect 25 times that of carbon dioxide. This means that one pound of methane produces the same effect on climate change as 25 pounds of carbon dioxide. At 52.7 (kg CO2 per million Btu), propane's Greenhouse Gas (GHG) footprint is relatively small compared to other fuels in terms of total emissions and emissions per unit of energy consumed.

Using propane and related applications can earn more than 100 points toward the National Green Building Standard™. High points can be achieved in the categories of indoor environmental quality and resource, energy, and water efficiency. In fact, just building with propane and related applications can propel a project halfway to a Bronze designation level. You can learn more about the Green Building Guidelines from National Association of Home Builders' green website.

Click here to take "Propane-Enhanced Solar Water Heating," a free online course offered at the Propane Training Academy website.


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