Solar water heaters
Solar water heaters are devices designed to use solar energy to heat water, providing an eco-friendly alternative to traditional heating methods that rely on electricity or fossil fuels. They are low-impact systems that can significantly reduce energy costs and carbon emissions. Despite their environmental benefits, a few challenges exist, such as their reliance on sunlight, which limits effectiveness during cloudy weather or in high-latitude regions with short winter days. The most common types of solar water heaters include integral collector systems (ICS), flat-plate collectors, and evacuated-tube collectors. While ICS systems are less efficient, flat-plate collectors are prevalent for residential use due to their effectiveness and affordability. Installation costs can be higher than conventional systems due to the complexity of plumbing and the need for specialized labor. However, operational costs are lower, and with potential subsidies and increasing interest in sustainable energy, solar water heaters may gain popularity. Overall, these systems can lead to significant savings on utility bills while promoting a reduction in greenhouse gas emissions.
On this Page
Subject Terms
Solar water heaters
DEFINITION: Devices that heat water by using absorption of solar energy either exclusively or primarily
Heating water by absorption of solar energy does not require any electricity or the burning of fossil fuels, thus solar water heaters have extremely low environmental impacts. Even solar water heaters that use electric pumps to circulate water use far less electricity than water heaters that heat water using electricity.
Heating with sunlight is the easiest and most cost-effective method of utilizing solar energy, and humans have heated water using the sun since ancient times. The biggest drawback to solar water heating, however, has been that the sun heats most effectively during the middle of the day, and hot water is often needed in the evenings. A further difficulty is that if the heated water is not properly insulated from the environment, it will cool off on cold days. These problems, however, are fairly easily overcome by well-designed solar water heaters. Well-insulated systems can keep water warmed during the day and hot through the night and can permit water to be heated even when the outdoor temperature is quite cold.
A more difficult problem for solar water heaters to overcome is that they do not work well on cloudy days. They also do not work as effectively at very high latitudes in the winter, when there are few hours of sunlight during the day. Even in summer at high latitudes, the sun is at a low angle in the sky, reducing the effectiveness of solar water heaters. Many solar water-heating systems thus include gas- or electricity-powered backups.
Types
The simplest form of solar water heater is the integral collector system (ICS), which is basically a tank, often painted black, placed where the sun can shine on it during the day. This is the least effective system and is often used as a preheater for a conventional gas or electric water heater.
The most commonly installed type of solar water heater is the flat-plate collector, which consists of a series of pipes with fluid flowing through a large flat box, often insulated and with a transparent window through which the sun can heat the interior of the box and the pipes. The biggest drawback to simple ICS systems is that the warming tanks have a large volume compared with the surface area being heated. Flat-plate collectors overcome this difficulty by creating surface areas to be heated that are far larger than those of the storage tanks. In warm climates where temperatures almost never reach freezing, water can be circulated directly through the collector to be heated and then collected in an insulated storage tank that operates much like a conventional water heater. In locations where there is a of a freeze, a fluid resistant to freezing (such as water mixed with some sort of antifreeze) circulates in the collector and then through a set of pipes either inside the storage tank or in another tank near the storage tank. The second set of pipes in this indirect system acts as a heat exchanger to warm the water. In many systems, the storage tank has heating elements and serves as a backup heater if necessary.
In very cold climates, an evacuated-tube solar collector may be used. The evacuated-tube collector is very similar to the flat-plate collector, except that the pipes in the collector are replaced by double-walled glass tubes. The space between the tubes is a vacuum, thus the loss of heat through conduction in the tubes is limited, much as heat loss is limited in a vacuum Thermos bottle. Evacuated-tube systems can be either direct or indirect.
Both flat-plate and evacuated-tube collectors can be constructed as either active or passive systems. An active system uses a pump to circulate the fluid through the collectors; it is thus not a zero-energy-use system, as it requires a small amount of electricity to run the circulation pump. A passive system, in contrast, does not require any electricity. Instead, the heated fluid rises to a heat exchanger in a storage tank located above the collector. In most solar water heater installations, however, the collector is placed on a building’s roof, so placement of the storage tank at a greater height than the collector is impractical. Active systems are thus more common than passive systems.
Costs of Installation and Operation
Because solar water heaters involve more plumbing than conventional systems, they are more expensive to install than conventional water heaters. The biggest cost of a conventional water heater is the tank, and a solar water heater also requires a tank in addition to other expensive components. Solar water heaters thus cost considerably more than conventional systems in terms of both parts and installation. A further reason for the high cost of solar water heaters is their rarity compared with conventional water heaters. Fewer plumbers and builders have had much experience with solar water heaters, so specialists who charge more for their services are often needed to install the systems.
ICS heaters are the least expensive solar water heaters, but they are also the least efficient. Evacuated-tube heaters are often so expensive that they are not practical except in commercial installations. Thus, the flat-plate collector is the type of solar water heater most commonly used in residential systems.
Solar water heaters are much less expensive to operate than conventional water heaters, but it can still take many years for a system to save a homeowner enough to cover the cost of the initial investment in the equipment. The high upfront cost is a significant deterrent to many people considering installing such systems.
However, the situation may change in the mid-2020s. With subsidies provided by the Inflation Reduction Act and a renewed desire to combat global warming and climate change, solar water heaters may become popular. Nearly 20 percent of the energy in a home is used to heat water. Homes that use solar water heaters can keep 2 tons of carbon dioxide out of the atmosphere. According to the US Environmental Protection Agency (EPA), this is equivalent to a person not driving their car for four months. Solar water heaters can also cut a household's utility bill by about $500 per year.
Bibliography
Coren, Michael J. "Meet the Other Solar Panel." The Washington Post, 13 Feb. 2024, www.washingtonpost.com/climate-environment/2024/02/13/solar-thermal-water-heater/. Accessed 23 July 2024.
Galloway, Terry R. Solar House: A Guide for the Solar Designer. Burlington, Mass.: Architectural Press, 2004.
Laughton, Chris. Solar Domestic Water Heating. Sterling, Va.: Earthscan, 2010.
National Renewable Energy Laboratory. Heat Your Water with the Sun. Washington, D.C.: Government Printing Office, 2003.
Ramlow, Bob. Solar Water Heating. Gabriola Island, B.C.: New Society, 2006.