Solar Energy: Which is Better, Active or Passive?
Solar energy is a hot topic in the news these days. But if you’re not already familiar with the terms, it can be confusing. What's the difference between active and passive solar energy systems? And which one is better?
Active solar technology uses electrical or fuel-driven mechanical equipment to perform some of its functions. For example, some solar swimming pool heating systems use electric pumps to move the heated water from the coils in the solar panels into the pool. Large off grid solar electrical systems that provide 100% of the electricity to buildings usually use diesel or gas-powered generators as part of their backup systems.
Some solar electric systems use motors to allow their photovoltaic arrays to track the sun's movement throughout the day in order to maximize the amount of sunlight they capture. Grid-tied solar electrical systems are also considered to be active systems because they rely on the electric company as a backup source.
Passive solar technology does not use additional mechanical or electrical devices to deliver heat or electrical energy. There are no additional fuel or utility costs, and no added environmental impacts. For this reason, many people think that passive solar technology is superior to active solar technology.
The most common passive solar application is solar water heating. A truly passive solar hot water system can provide enough hot water for washing and bathing. It usually consists of a collector tank located on a south or west-facing wall or on the roof, and a plumbing system that delivers the water using gravity. These types of systems are most successful in moderate climates where temperatures don't get cold enough to cool the water in the storage tank.
It's also possible to combine passive and solar energy systems to get the benefits of both. Any of the active systems that use electric motors can also use solar cells to create the electricity. For example the motors that move photovoltaic arrays can easily be powered by solar cells. And solar cells can also power the electric pumps that move the water in solar swimming pool heaters, and in water heating systems used in buildings.
Some active solar electric systems, like off grid systems, need external generators. They are part of the system to provide backup energy during long periods of cloudy days when there is no direct sun. Solar cells won't work under those conditions. But it's still possible to stay green by using bio-diesel fuel to power the generator.