Passive Solar Energy

What is Passive Solar Energy and How Does it Work?

Passive solar energy, or passive solar building design, is a simple, yet powerful concept. It can help people save money on both heating and cooling by using the natural energy the sun provides. And the best part is that passive solar energy doesn’t require the use of solar panels or other specialized equipment. Indeed, that is why it is called passive. All that is needed is for a home to be built the right way.

That being said, unlike rooftop solar panels, passive solar energy is not something that can be easily installed. It takes advantage of the home’s construction to harness the sun’s energy. Thus, passive solar energy is something that is better to consider when buying (or building) a new home.

Now, let’s take a look at everything that is needed for passive solar energy to work.

Passive Solar Energy Components

In general, there are five things a passive solar energy system needs to work properly. Let’s go through the details of what is needed to harness the sun’s energy passively.

Aperture or Collector

The first thing the system needs is an aperture, also known as a collector. This is a fancy name, but all it simply refers to a way to capture the sun’s energy. The most common way to do this is by having windows that face the equator. Thus, in the northern hemisphere, you would have south-facing windows.

The equator is the part of the earth that receives the most sunlight; thus, having your windows face that direction is how you can take in the most sunlight. Specifically, the aperture should face within 30 degrees of true south. It also should have exposure to sunlight and not be blocked from 9 a.m. to 3 p.m. local time.

Absorber

The absorber, as its name implies, will absorb the heat that will be used to heat the home. This could be water, a masonry wall, or a floor. It should be in the direct path of sunlight for maximum effectiveness.

One way to make the absorber more effective is to make it a dark surface. You might have noticed that in the summer, wearing a dark shirt is usually a bad idea if you will be outside for a long time. That’s because the dark color of the shirt absorbs sunlight; light colors do the opposite.

Exactly the same concept applies to the absorber for your passive solar energy system. Using a dark color will help it in more sunlight.

Thermal Mass

After the absorber takes in as much light as it can, the thermal mass retains that sunlight. Thus, the thermal mass is directly beneath or behind the absorber. The thermal mass can be materials such as concrete, stone, ceramic tiles, or bricks. These are heavy materials that retain heat well.

Water and phase change products can also be used and are more efficient at storing heat than the other materials mentioned above, but they cannot double as building materials. In more temperate climates, even drywall and in-home furnishings can serve as a thermal mass.

Distribution

Now that we have captured and stored our heat, it’s time to put it to work heating out home. There are three different ways to distribute heat to the home: conduction, convection, and radiation. In some homes, small fans may be used to help distribute heat.

Conduction

Conduction occurs when two objects come into direct contact with each other. Think about a time when you may have touched a hot surface with your finger–that is conduction at a very basic level. When the hot surface comes into direct contact with the not-so-hot surface, heat is transferred from one to the other.

Convection

Convection refers to heat moving through the air or through water. A passive solar energy system may use convection to move warm air throughout the house.

Radiation

Radiation occurs when you can feel the heat radiating off of something. For example, if you put your hand above an electric stove top burner and feel warmth, heat is radiating from the burner.

Control

The last part of a passive solar energy system is control. One example of a control measure is a roof overhang that shades the collector during the summer months. Of course, you won’t want the system to heat the home while it is hot outside.

Another control might be a smart thermostat that turns fans on and off.

Direct Gain

Direct gain works by taking in sunlight through the aperture (remember, south-facing windows). It then hits the masonry floor and walls, absorbing sunlight. As mentioned, there may also be an absorber in place to help the thermal mass absorb as much sunlight as possible.

Passive Solar Energy
Direct gain uses light from south-facing windows to heat the home.

Once the heat is captured in the thermal mass, it is radiated into the room at night to provide heat. These systems are very efficient, using about 60-75% of the sunlight to create heat. To make it even more effective, the thermal mass should be insulated if necessary so that it doesn’t lose that energy as the day progresses.

Indirect Gain

Where indirect gain is the most obvious implementation of passive solar, indirect gain works a little differently. In particular, a Trombe wall is the most common way to achieve indirect gain.

The biggest difference with these systems is that the Trombe wall sits between the south-facing windows and the living space. This wall is eight to 16 inches thick and there is one inch or less between the wall and the south-facing windows. The wall absorbs heat which then radiates into the living space at night.

In this setup, heat moves through the thermal mass at a rate of about one inch per hour. Once the temperature of the living space falls below that of the thermal mass, heat will be radiated into the room.

You can also have a control, such as vents, to help draw in heat only when desired.

Passive Solar Cooling

You may not think it would be possible to cool a home using solar energy given that the sun produces heat, not cold. However, there are a few methods you can use to help cool a home with solar power.

Convective Cooling

Remember that convection occurs when heat moves through the air and that is exactly how convective cooling works. Simply put, it brings cool air in during the nighttime and pushes warm air out during the day. You can have a vent at the top of the home to let hot air out since heat rises. Then, you might have a vent at the bottom of the home on the opposite side to let in cool air.

There might also be a thermal chimney, which is exactly what it sounds like. You would position it at the top of the home to let hot air out, just like a chimney would do for an old-style fireplace. You would still have a vent at the bottom of the home to let in cool air at night.

Shading

Shading is a simple idea. We mentioned this above when talking about having an overhang as a control, and that is essentially solar cooling. In addition, windows should have blinds or some other form of shading to prevent heat from entering the home during the summer. You can also use trees and shrubs to block the sun’s rays during the hot summer months.

Ventilation

Ventilation works by drawing warm air away, which means it won’t help if the indoor temperature is already lower than the outdoor temperature. However, since the temperature tends to drop at night, this can be an effective way to draw out warm air overnight and then bring in cooler air.

If your area has a lot of breezes during the day, you can create cross ventilation. In this setup, you draw in cooler air from one side of the house. Then, on the opposite side, you would let out the hot air as it moves through the house.

Passive Solar Energy: Conclusion

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