What is Passive Solar?

Solar Dictionary
What is Passive Solar?

As building techniques improve and we learn more about our climate, the energy efficiency of our buildings continues to improve. This goes for both homes and commercial-use buildings.

By adopting new, environmentally-friendly building techniques, we can cut down both our carbon footprints and our power bills. The first step to understanding passive solar is to cover some of the basic definitions.

Afterward, we can delve into the specifics of each facet of passive solar. This will help us understand the coherent whole, alongside each part. Finally, we can cover how to implement passive solar alongside solar power.

Passive Solar by Definition

Let’s start with a basic definition.

Passive solar is the process of designing buildings and homes to collect, store, reflect, and distribute solar power depending on the season and other infrastructural factors not creditable to the solar array itself; e.g., harnessing windows, walls, and floors to control the building’s environment. 

More specifically, Passive solar refers to combining building techniques and solar technology to produce energy-efficient, solar-powered homes. These homes do not rely on fossil fuels or other forms of dirty energy. Instead, they position themselves such that the south-facing side of the house lets in sunlight, which is then trapped in a “thermal mass.”

This mass then radiates the heat which, guided by various fans, vents, and so on, heats your house. A reverse process, called passive solar cooling, also exists. Passive solar cooling also usually involves a number of non-electric factors, such as shade trees and better insulation. Window shades can also be a good, energy-free way to cool your home. Let’s go over each piece one at a time, then we can put them all together at the end for a clear picture of what passive solar is and how it works.

Passive Solar Defined by Its 5-Step Process

Sometimes terms are best explained in the context of their process, and what these respective processes entail. When it comes to understanding passive solar in the context of its process, there are generally 5 steps needed to explain the passive solar process, end-to-end.

While passive solar is a step in the right direction, it isn’t enough by itself to leverage true energy efficiency. Use our solar savings calculators to see what you could save with passive solar techniques and a residential solar array.

Step 1: Establishing a Designated Solar Room

The solar room is a dedicated room in your house that will be exposed to the sun. Solar energy will be passively generated in this room, which will then be distributed throughout the rest of the house. A solar room is a necessity for any passive solar project. 

Step 2: South-facing Windows

The next step to building a passive solar home is to face your windows south. South facing objects get the most sun, which is why the best roofs for installing solar are those which face south. In a similar vein, a passive solar house wants to have large south-facing windows.

These allow the most sun to shine through them, heating the house. This is particularly useful for winter when the sun doesn’t shine as strongly and every bit of sunlight counts. Additionally, these windows should avoid shading. The more shading you have on these windows, the more energy needs to be diverted for artificial heating. Too much energy is devoted to heating defeats the purpose of passive solar.

Step 3: Installing Thermal Mass

Now that you have your south-facing windows installed, it’s time to install your thermal mass. It can be made of many materials, such as tile, stone, or even water tanks. The thermal mass is a heat sink — think of an asphalt road on a hot summer day. That road takes in a lot of heat and then emits that heat in various ways.

One way that the heat can be emitted is through conduction, direct contact, such as when you step on the asphalt road and burn the soles of your feet. Another way that the heat can be emitted is through the air, called convection, which can be seen through the murky heat waves that you see above the asphalt road.

The thermal mass absorbs heat and stores it. While the sun is shining on it, it’s not going to emit much heat and it’s going to keep absorbing the sun’s rays. Once the sun isn’t directly shining on it, the thermal mass begins to emit the energy that it stored throughout your house. Thus, during the day you don’t need heating because of the sun, and at night you don’t need heating because of the thermal mass.

Step 4: Installing a Distribution Mechanism

Having a thermal mass is all well and good, but without a way to distribute that heat throughout the house, you’re out of luck. Therefore, every passive solar home needs to be constructed with a distribution system that takes that heat to other parts of the house. 

As mentioned above, convection and conduction are the ways in which heat is usually spread throughout the house. In a truly passive solar home, no other distribution system is necessary. The home will naturally heat itself. In other homes, a more active system is needed. Generally, an active heat distribution system for a passive solar home will use fans or ducts to transmit the heat

Step 5: Installing a Control Mechanism

A passive control mechanism will complete the system. You might have been thinking to yourself, “Won’t south-facing windows get really hot during the summer? Maybe too hot?” And you would have been right! One of the big hurdles to overcome with passive solar is keeping the home cool during those months when the sun seems to beat down from the sky. The main solution to overheating in these periods is constructing what’s called a Trombe wall.

A Trombe wall is made of absorbent materials like concrete or stone and painted a dark color to maximize the amount of heat it soaks in. This wall is constructed on the south side of the house to ensure maximum exposure to the sun, particularly during winter. A roof overhang might be constructed to limit the amount of sun that the wall gets during summer. Finally, a sheet of glass is mounted about an inch away from the wall, which covers the entire thing. This glass absorbs the sun’s rays and then emits them into the wall, which stores the energy.

The energy passes through the wall at around an inch per hour, so, depending on how thick your Trombe wall is, the heat from the sun will radiate into your house starting at anywhere from 8-12 hours after it begins absorbing energy. This type of construct will regulate the amount of heat that your home gets and makes sure that you don’t get overheated during the summer.

How Do Solar Panels Factor Into Passive Solar?

Now that we’ve learned about passively harnessing the power of the sun for climate control, what about for your home’s energy? If you’re going to such lengths to make your home environmentally-friendly, when it comes to heating and cooling, it would make sense to finish the job and make it environmentally friendly in all aspects of energy. Solar installations can both make your home environmentally friendly and reduce your power bill to boot.

A home with a large, south-facing wall is likely going to have a south-facing roof as well. This is the perfect placement for a solar installation. Additionally, a passive solar home is an excellent first step towards going off-grid with solar. Reducing your energy usage through a passive solar home will significantly reduce the amount of power you need to generate — thus making it easier to go off-grid.

When you combine passive solar with battery backup, such as the Tesla Powerwall, you can construct a completely energy-independent home. Want to learn more about solar terminology? Visit our Solar Glossary today, which covers and clarifies frequently misunderstood solar terms and basic definitions alike.


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