Passive Design

It is a widely agreed-up fact that the sun provides enough energy every hour to provide all of humanity's energy needs for one year. Thus, it stands to reason that there is likely enough sunlight to provide the average household's energy needs throughout the year. We will save that calculation for a later time, but some related research from the National Renewable Energy Laboratory (NREL) has shown that much of the electricity in the U.S. could be provided by solely installing PV arrays on suitable rooftops in the U.S. The images below show 1) the percent of buildings in each state that have suitable rooftop space and 2) the percent of the total electricity sales that could be provided by suitable rooftop space in each state. You can read the full report here if interested.

Figure 5.3: Top image: Percent of small buildings suitable for PV in each ZIP code. This is based on orientation and shading. Bottom image: The percent of total 2013 electricity sales in each state that could be provided by rooftop PV on suitable sites. Note that modules are generally more efficient now than in 2013 and inverter technology has improved. The potential is likely to be higher now, barring significant increase in electricity sales.

Credit: National Renewable Energy Laboratory

Active and Passive Design

Passive design - including passive solar design - seeks to minimize heating, cooling, and lighting energy use in a building using passive strategies. Before we begin, please note the difference between passive and active solar design, as described by the California Energy Commission:

"Homes constructed with passive solar design use the natural movement of heat and air to maintain comfortable temperatures, operating with little or no mechanical assistance. It's called passive solar because the design of the home maximizes the benefits it receives from the sun with standard construction features. Passive solar takes advantage of local breezes and landscape features such as shade trees and windbreaks, and uses a simple system to collect and store solar energy with no switches or controls.

On the other hand, active solar systems use mechanical devices such as pumps and fans to move heat from collectors to storage or from storage to use. Photovoltaic panels that collect solar energy, turning it into electricity, are also considered an active solar system."

It is important to distinguish between passive and active strategies. While active strategies such as solar PV and high-efficiency heating and cooling equipment are essential components of minimizing energy use and reliance upon unsustainable energy sources, passive design requires a different mentality and in some ways, design intent. For example, many homes in the U.S. have enough roof (and/or yard) space to accommodate enough solar PV modules to provide all of their electricity needs even if the houses were not originally designed with that intent. This is especially possible if the home upgrades to highly efficient heating and cooling, as well as improved insulation and air sealing. However, it would be difficult for most houses in the U.S. to become truly passive, because passive design principles are more specific than simply having enough properly-oriented roof or yard space. 

Though you may see some minor disagreements on these depending on which source you use, the following are generally accepted to be essential passive solar design principles. It is important to note that you can minimize energy use by selectively implementing these, but in order for a building to be truly passive, all of them must be used. As you will see, they often complement each other, and some simply do not "work" if not combined with others. These principles are orientation/azimuth, overhangs/shading, windows, insulation/air sealing, and thermal mass. Please read through the following descriptions of passive solar design and some passive solar design principles. Note that I provide a link as well as a .pdf since the links are archived and don't always work. All of the readings are from the California Energy Commission.

The foolwing provides a brief summary of these principles:

• Orientation/azimuth: As you should know at this point, orientation and azimuth refer to the direction the building is faced. In the northern hemisphere, the long side of buildings should be oriented east-west, which means the long side of the building will be faced south.
• Overhangs and shading: If your building is faced south it should be able to capture winter sunlight and reduce heating costs, but conversely it may also capture summer sun, which would cause you to increase cooling costs. Properly deployed overhangs and/or seasonal shading (such as deciduous trees) can allow winter sun in and block the most intense summer sun.
• Windows: Windows must be used with both of the above, otherwise sunlight will not be able to penetrate the builiding envelope and warm up the house in the winter. Windows should mostly be on the south side. The north side of the builiding will rarely if ever get direct sunlight, and so too many windows will increase energy use because they are such poor insulators.
• Insulation and air sealing: This is pretty straightforward. Insulation and air sealing minimizes heat loss in the winter and heat gain in the summer.
• Thermal mass: This is what really separates a passively designed house from just an efficient one. Thermal masses are materials that have a high specific heat, which means that they absorb a lot of heat before they get warm and they release a lot of heat before they cool down. Essentially, they are good at storing heat. This means that they take a long time to heat up even if they are exposed to a lot of radiant energy, and after the heat/radiant source is removed, they can have a lot of heat stored up and release it very slowly. 

Generally speaking, most of these principles are easy to understand. It's all but common sense that you need good insulation and air sealing for an efficient home, and understanding that good orientation is important is easy to understand if you know that the sun is usually in the south in the northern hemisphere. We all know that windows allow sunlight in. There is some subtlety and basic trigonometry (uh oh) to overhangs, since you must consider sun angle at your latitude for proper design, so that can be a little tricky. But the fact that deciduous trees lose their leaves once a year is common knowledge. Thermal mass is the least commonly understood, but the basics are not difficult to understand (things that heat up slowly and release heat slowly). It bears repeating that for a fully passive solar design, all of these elements must be present and work in conjunction. As noted above, this almost certainly must be considered in the design process, since the building's orientation cannot be changed after it is built and some overhangs are difficult to retrofit. Windows, air sealing, insulation, and possibly thremal mass can generally be added after it is built, though.

Passive Design

Passive solar design refers to taking advantage of ambient daily and seasonal sunlight (or lack thereof) to heat and cool a building. Passive design in a more general sense includes any measure that reduces energy use by taking advantage of local conditions. Natural ventilation is one way to do this, as the video below details.

Ancient Uses of Passive Design

One final note about passive design before we move on: In case you think that these principles represent some modern breakthrough in knowledge, well, you are off by a few thousand years! Many ancient and historic civilizations all over the world not only understood these principles, but deployed them on a wide scale.