By Bud Coburn
Solar Thermal Worldwide
While solar thermal systems have only recently become popular in the United States, they have been in use since as early as the 1890s. Israel began pioneering work in the 1950s in response to fuel shortages and, today, solar water heating is exploited by 85% of that country’s population. In this way, Israel saves an astonishing 2 million barrels of oil per year – 3% of their total energy consumption – making it the highest user, per capita, of solar energy of any form worldwide.
Spain was the second country (after Israel) to require the installation of solar thermal systems. In China, systems are much cheaper than similar models sold in Western nations, and an estimated 30 million Chinese households make use of them. Solar thermal technologies have seen tremendous growth in Australia, Japan, and many other countries that receive abundant sunshine.
Components and Operation
Unlike photovoltaic solar panels, solar water heaters generate no electricity; rather, they directly heat water through sunlight. These systems are generally composed of solar thermal collectors, a water storage tank, interconnecting pipes, and a fluid to move the heat from the collector to the tank.
Solar thermal collectors are fastened to a roof or a wall that faces the sun, heating fluid that can be pumped (in an active system), or driven by convection (in a passive system). Collectors are made from a glass-topped insulated box with a flat solar absorber made of sheet metal attached to copper pipes, and then painted black, or a set of metal tubes surrounded by an evacuated (near-vacuum) glass cylinder. Solar water heating systems are usually supplemented by conventional backup systems for cloudy days and times of increased demand.
- flat-plate collector. These are weatherproofed boxes that contain a dark absorber plate beneath one (or more) glass or plastic cover. Solar pool heating systems use unglazed flat-plate collectors, which lack a cover or enclosure.
- integral collector-storage or batch systems. These feature black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which warms the water before it is sent to a conventional backup water heater, which then fully heats the water. Batch systems should be installed only in milder climates because the exterior pipes can freeze in cold weather; and
- evacuated-tube solar collectors. These systems feature rows of parallel, transparent glass tubes, each containing a glass outer tube and metal absorber tube attached to a fin. The fin’s coating absorbs solar energy but prevents radiative heat loss. While occasionally used in residences, this design is more common in commercial applications.
Inspection and Maintenance
Solar water heaters require periodic inspections and routine maintenance to ensure efficient operation. Inspectors can recommend that the homeowner or a qualified solar technician perform certain tasks. The owner’s manual should also be consulted for maintenance and inspection tips. The following components should be inspected:
- dampers. If equipped, ensure that the dampers open and close properly;
- mineral buildup. If circulated in the system, mineral-rich water known as hard water can lead to mineral buildup in the pipes. This can be removed by adding an acidic de-scaling solution to the water every few years;
- seals and glazing. Make sure the seals are in good condition, and check for cracks in the glazing. Plastic glazing that has become excessively yellowed may need to be replaced;
- shading. Both new construction and vegetation can reduce the performance of the collector, so check to make sure that neither of these things interferes with the path of sunlight. While inspectors can check for shade once, they can recommend to their clients to check the collector three times (annually) – morning, noon and afternoon – to be on the safe side;
- piping, duct and wiring insulation. Look for degradation or other damage to these components;
- plumbing, ductwork and wiring connections. Check duct connections and seals to make sure there are no leaks where pipes connect. Ducts should be sealed with a mastic (plant resin) compound. Wiring connections should not be loose;
- pressure-relief valve. If equipped, ensure the valve is not stuck open or closed;
- pumps or blowers. The distribution pumps or blowers should activate when the sun is shining on the collectors after mid-morning. Listen to verify that they have turned on. If you cannot hear anything, then it’s likely that either the pumps/blowers or the controller has malfunctioned;
- roof penetrations. Roof penetrations require flashing and sealant, which should be in good condition;
- soiling. Soiled or dirty collectors will perform weakly. Homeowners in dry, dusty environments may have to clean their collectors periodically;
- storage tanks. Check storage tanks for leaks, cracks, rust and other signs of damage; and
- support structures. If the collector has a support structure, make sure all nuts, bolts and other connections are tight.