Solar passivity is much discussed among the avant-guard of renewable energy experts. But what is it, exactly, and how is it used? This article aims to answer a few basic questions about the passive solar concept, and show small ways to incorporate it even in your current home. Those looking into building a new home may discover that they wish to dig deeper into passive solar constructs, and include many more aspects of solar passivity in their new homes.

Passive means that something is receiving an action, rather than doing the action. Passive solar is no different. In a non-passive solar construct, for example, photovoltaic cells on a roof convert solar energy to electricity to run an electric radiator to heat a house. In a passive solar construct, the house has large windows facing the south, with shutters on the outside that can be closed during the hot part of the summer. The sun does all the heating. There's no expensive conversion from light to heat to electricity and back to heat; simply an understanding of the way the sun heats a room when the windows face south, and an ability to use that understanding.

You may hear passive solar enthusiasts speak about "thermal mass." Thermal mass is an important concept in passive solar. There are many scientific principles behind it, things such as heat density and conductivity, but these aren't important if you only want to understand the basic principles behind passive solar. If you want to use passive solar energy to heat and cool your own house, we suggest you find a good book, such as "The Passive Solar Energy Book" by Edward Mazria, that can explain the science behind thermal mass.

For the rest of us, it's enough to know that some materials, like stone and concrete, get hot when the sun shines on them (did you ever walk across a parking lot barefoot when you were little?) and stay hot for a long time. Other materials, such as wood, get hot quickly, then cool down quickly. In passive solar heating and cooling, you want materials that take a long time to get hot and a long time to cool down. Those include concrete, stone, and, oddly enough, water.

In winter, the sun shines through the large glass windows of a home made of concrete and stone (among other materials, of course). The stone and concrete slowly heat up throughout the day, and at night, they continue to radiate that stored heat. In the morning, just as they've exhausted all the heat they stored the day before, the sun comes up and the cycle begins again.

But what about cooling? Can you cool a home using passive solar energy? Certainly. Or, at least, you can cool a home by understanding how the sun heats it, and working against that. For example, our perfect solar home above, with all those south-facing windows, would have shutters on the outside that could be closed during the hot part of the day. Why on the outside? Think about it for a moment. Sunlight streams through glass, hits that stone floor, and becomes heat. (Simplified version!) When you put blinds up inside a window, you can't see the sunlight anymore. But the sunlight can still get through the glass. It hits the blind instead of the floor, and all that heat is still there, inside your house, right behind the blinds, leaching out into the room. If, on the other hand, you put shutters on the outside of your windows, the sun can't get to the glass at all. It becomes heat outside, where it can't bother you.

Also, the perfect solar passive house will have many deciduous trees around it. In the summer, their leaves offer shade from the sun and can drop the temperature several degrees. In the winter, they lose their leaves, allowing sunlight to stream through and warm the house.

So how can you incorporate principles of passive solar energy into your existing house? It's worth looking into; just using the power of the sun to heat and cool can significantly drop your heating and cooling bills.

For the long-term, plant shade trees, if you can. Critical spots that need shade are your house itself, any concrete (such as driveways and patios), and any brick or stone (walkways, walls, etc.). If you have a garden, of course you don't want to shade it out. But shading any other areas will help more than you realize.

Short-term, install outside shutters onto your windows whenever possible. If you have to leave for the day (such as for work, for example), then make closing up the shutters part of your routine, along with locking the doors.

Add thermal mass wherever you can. Maybe that means a small raised bed of stone and concrete up against the side of your house. Maybe it means stone pillars on the side of your door. Maybe it means replacing your entire siding with brick! Adding a small fountain or pond under a deciduous tree is a great way to add thermal mass to your garden. Remember: thermal mass should be shaded in the summer, but in sun in the winter. If you have thermal mass in the sun (such as a driveway), you're adding heat. If you have thermal mass in the shade, you're removing it.

If you are in the fortunate position of building your own house, find a good book that will explain in detail the concepts of passive solar energy, and incorporate them into your own designs. For the rest of you, using a few passive solar techniques can help your current systems function better and reduce your heating and cooling bills. Plus, you'll be able to discuss passive solar concepts with your friends!