According to the department of energy, water heating is the third largest energy consuming activity in the average American household, accounting for between 14-25% of the average residential energy consumption. Currently, most systems run on natural gas or electricity, but other options are available, including solar power. With so much attention and effort spent on developing solar electricity technology, there seems to be relatively little discussion about the use of solar thermal energy. I recently spent much of the last two years living in Oaxaca, Mexico and came across some pretty simple and effective home-made solar water heaters (SWH). In this system, a copper pipe was set up to weave back and forth inside of an insulated wooden box topped with a piece of plexi-glass. Water from a tank then flowed through the copper pipe, snaking upwards by thermosiphon. By the time the water reached the top of the box it was steaming hot and flowed out to a separate insulated tank for storage and use. The entire passive solar system required no extraneous energy input and had no mechanical parts. KISS at its best.
While some of the specific conditions are different in Mexico and the U.S. (they generally have water tanks on their roofs to begin with), the basic idea of using solar thermal energy to heat water appears to at least merit some additional focus. Although the use of simple passive systems may not always be feasible, active systems using small electric pumps are also available and can be integrated into any American home. While these systems are more complex and entail a higher initial cost, they are often more energy efficient overall. When assessing the overall technology of solar water heating, some important questions immediately come to mind. I will address a few of them to give a brief overview of this technology and related policies.
Are they cost effective when compared with current systems?
The ultimate determination of whether a SWH system is an economically sound investment will depend on a wide range of factors including average hot water use, geographical location, access to tax breaks or other incentives, and the amount of time expected to recoup the initial investment. However, some of the general analyses appear to be favorable. According to Austin Energy, solar water heaters should already be cost effective compared to electric heaters and competitive in the long run when compared to gas heaters. They have computed that most systems should pay for themselves in about four to eight years, and that they should last anywhere from 15-40 years. The Department of Energy states that maintenance may only be required every three to five years. Additionally, federal renewable energy credits in place through 2016 will now cover 30% of the entire system, with no upper limit. While some states offer additional credits or incentives, Texas currently does not.
Where and how are these systems currently used?
Currently, the main application for solar water heaters in the U.S. is for swimming pools. However, several states are actively pursuing policies to encourage their use. Leading the charge is Hawaii, who recently became the first state to require solar water heating systems in all new home construction. In other parts of the world, solar water heaters are already an integral part of the national energy equation. In Israel, for example, about 85% of the residences rely on solar water heaters.
How much energy do they save?
Water heating in the U.S. accounts for about 8% of all end use natural gas consumption, 8% of residential electricity use, and about 3.5% (3.5 quadrillion BTUs) of total energy consumption. SWH will not eliminate energy used for water heating, however individual systems have the potential to reduce energy expended by anywhere from 40-90%, depending on geographic location. A National Renewable Energy Laboratories report indicates that SWH have the potential to replace about 1 quad of primary energy use. This would translate into 50-75 million metric tons of CO2 emissions, and about $8 billion in retail energy costs.
Would they work in northern climates? Would I ever have to take a cold shower?
While the effectiveness of any system will depend on the amount of radiant sunshine in the specific area of use, systems are available that work in both warm and cold climates. The cold climate models generally heat the water to a certain point, and then utilize another form of heat to get the water really hot. All of the commercial systems I have read about are designed with a backup heater (electric or gas) for use during period of unusually high demand or no sun. So, with a properly designed and integrated system, there should be no shortage of hot water availability.
Overall, the technology of solar water heaters appears to be a simple and relatively viable option to complement the national energy mix. Policies are already in place to promote their use, and it should be interesting to see if they become widely accepted or not.