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What is Passive Solar?

Passive solar design is the utilization of the sun's energy, the geographical climate, and the properties of different materials to heat and cool buildings. It includes a variety of methods that use no human-made energy to operate and can reduce the amount of energy needed for heating and cooling by considerable amounts.

Although passive solar design might seem new to you, the basic principles have been around for centuries. In years past, indigenous people who lived in harsh desert locations built partially-underground homes that kept them cool during the day and warm at night. They also built adobe homes in cliff-side caves that were chosen because the winter sun warmed them and the summer sun couldn't reach them.

Passive solar should not be confused with active solar design or photovoltaic solar cells. While active solar design is similar to passive, it uses small amounts of energy to help transport the heat created. For example, if a solar wall heats up air that then naturally rises, it is called passive solar; if a fan was used to help move the air, then it would be considered "active." Active solar design still uses considerably less energy than standard methods of heating and cooling; its small amount of energy used greatly increases its ability to move hot and cold air around. Photovoltaic solar cells are solar panels used to generate electricity; they could be coupled with active solar designs so that the small amount of power needed comes entirely from "clean" energy.

Why Passive Solar?

Passive solar design is an important aspect of building design because people and businesses are looking to save money on energy costs and be environmentally responsible. The appliances that create hot or cold temperatures in our buildings require large amounts of energy and although they work rather well, they have two main drawbacks. The first drawback is the cost associated with running these devices. The average US family spends approximately $1,900 a year on energy of all kinds, including electricity and gas, with about half of that cost due to just heating and cooling the house. That means a family spends about $950 a year to control the temperature inside its home — and that cost goes up considerably for larger homes.

The second issue has to do with the greenhouse gases emitted due to the production of energy needed for these appliances. Air conditioning units are usually powered by electricity while central heating units are often powered by gas.  Most electricity and gas are created by methods that pollute the atmosphere with greenhouse gases such as carbon dioxide. So, the more energy we use, the more greenhouse gases are emitted. Civil engineers who study and design heating, ventilating, and air conditioning (HVAC) systems look for ways to reduce these negative effects of heating and cooling.

One way is to make the appliances more energy efficient so that they use less energy to do the same job. Even when made very efficient, most of these modern appliances still require large amounts of energy to do the job. If the standard appliances used could be replaced, or at least complimented, by methods of heating and cooling that require no human-made energy, this would help with the problem! Unfortunately, passive solar heating is not an instant replacement to conventional heating methods because homes would need to be re-designed around the various methods to maximize the amount of heating produced. And, even then it sometimes does not produce enough heat as is desired. For now, engineers combine passive solar heating with conventional methods to reduce the need for energy-guzzling heating appliances.

Passive Solar in Practice

To see a building with active solar features, please view the Sustainable Library video below.

• Sustainable Library

If you have ever spent some time in the sun on a hot day, you know that the sun has an incredible ability to heat things up. Think of how hot the inside of a car gets after it has been in the sun for awhile. Tapping the sun's power is useful in working towards becoming more energy efficient because its energy is free and in near endless supply. That's why we consider solar energy a "renewable" source of energy.

The simplest method of passive solar heating is sunlight shining through windows. Since we know that the sun rises higher in the sky during the summer than in the winter, engineers and architects design buildings that allow sunlight through the windows during the winter months when the building needs heating, but block the sunlight during the summer to help keep the building cool.

After sunset, have you ever felt the warmth from a big rock or a concrete bench that has been in the sun all day? The rock and the bench absorbed and stored the heat, and released it slowly. Working in the same way, a key passive solar technique is for the radiant heat of sunlight that enters a building to be absorbed by a thermal mass inside the structure. A thermal massmight be a big wall or area of floor that is composed of a construction material that is able to absorb large amounts of heat, such as concrete, brick, tiles or even water. As the sun sets and the air temperature lowers, the thermal mass slowly releases the heat it gathered all day to help maintain a comfortable indoor temperature through the night. In the summer, the same thermal mass can draw warmth from the surrounding air to cool a space. In all seasons, the ability of thermal mass to store heat helps to maintain a uniform temperature.

The two main ideas behind solar cooling are ventilation and the prevention of heat from entering the building. Proper ventilation is achieved by strategically placing windows so that as much air as possible can be circulated when open. One way to prevent heat from entering a building is to design the building with thermal mass on the outside so that it absorbs the heat before entering the building. Another method is to plant large shade trees so they shade the building from direct sunlight during the summer months.