Solar power comes in a number of flavors. We get a lot of inquiries about photovoltaic (PV) panels -- solar energy systems that convert sunlight into electricity. But we recently got a question about non-PV technologies:
I'm interested in using solar for heating my home's water. What's the difference between active solar energy systems and passive solar energy systems?
Beyond the link above -- which provides some detail on active solar and passive solar power systems -- I thought I'd add a quick overview here. To start, "active solar" and "passive solar" are the two main categories of solar thermal systems. Unlike PV panel systems, solar thermal systems don't generate electricity. Rather, they capture the sun's energy in the form of heat, and use it for heating water, or for space heating (or cooling). The main difference between the two technologies is that active systems use electric pumps and valves to "actively" transport heat, while passive systems instead rely on natural convection to "passively" transport heat. Confused? I'll do my best to explain.**
Active solar thermal systems are commonly used to heat water for use in the home or business. In closed-loop systems, an antifreeze-type liquid flows through a flat-plate solar collector or evacuated tubes. Sunlight heats the liquid, which is then pumped into either a storage tank or a heat exchanger for immediate use. Put simply, the heat energy that's been absorbed by the antifreeze is transferred the water you use for showers and washing dishes. Open-loop systems are similar, but they forgo the antifreeze and the heat exchanger. In this type of setup, potable water flows through the solar collector, is heated, and is then pumped directly into a storage tank for household use. Open-loop systems are simpler and more affordable, but close-loop systems tend to be more efficient.
The U.S. Department of Energy's office of Energy Efficiency and Renewable Energy maintains a great resource on active solar heating.
As noted above, passive solar water heating systems rely on something called natural convection -- in simple terms, the tendency of hot water to rise above cold. One of the most popular passive hot water systems is the Thermosiphon. It works like this: A solar collector is placed at an angle to the sun and is filled with water. The heated water migrates up the collector and flows into a tank at the top. Inside the tank, the hottest water -- which is more buoyant and thus stays at the top -- may be skimmed off by opening the spigot and used. The coldest water gets recycled, joining fresh water supply before cycling back through the collector. Passive solar thermal systems are affordable and don't require any electricity to operate.
Depending on your interest and budget, solar thermal systems -- either active or passive -- may be a lower-cost solution than a PV panel system.
**Note: I focus here on solar thermal technologies for heating water. There are, however, a number of solar thermal technologies for space heating and cooling.
Check out this helpful diagram of a Thermosiphon from the Wikimedia Commons:
1. Water tap
2. Isolated container
3. Warm water inlet
4. Solar thermal collector
5. Fresh water supply