How passive solar home design saves you money on energy
Updated
Passive solar home design is a great way to reduce energy use and save money.
Passive solar design is a way of using surfaces in and around your home to collect energy from the sun to provide warmth during cold seasons, as well as block energy from the sun to avoid unnecessary warming during hot seasons.
By carefully choosing and placing windows, walls, and floors based on their heating properties, you can reduce your home’s energy consumption by 25% or more.
The concepts of passive solar design can be combined with an electricity-generating solar photovoltaic system to increase energy efficiency and even eliminate your reliance on fossil fuels.
Pros and cons of passive solar
| Pros | Cons |
|---|---|
| Easily implemented in building design | Professional installation required |
| Low additional cost of new construction | More expensive to retrofit older homes |
| Reduces energy usage by 25% | Trees and/or hills may block sunlight |
| Saves you money on power bills |
Benefits
- Passive solar heating does not add much to the cost of a newly-designed home
- Can reduce energy usage by 25% or more
- Can save you money on electricity even if your home isn’t suitable for solar panels
- Designing apartment buildings to use passive solar heating can dramatically reduce energy usage for many people at once
Drawbacks
- The engineering necessary to design a passive solar home takes professional work and installation, but this isn’t necessarily a drawback, since any major update to your home should involve professionals anyway
- Older homes might not have the proper insulation needed to store heat energy captured during the day, and retrofitting is generally not very cost-effective; we strongly recommend getting a home energy audit before upgrading to passive solar or installing solar panels
- If your home is surrounded by large trees and/or steep hills, it might not receive the amount of sunlight to make a huge difference, so removing trees that already provide other benefits is not recommended
Five key concepts of passive solar design
These five elements can be combined to reduce your heating and cooling needs.
There are five important components or mechanisms of a passive solar heating system: apertures, absorbers, thermal mass, distribution, and control.
Apertures
Apertures are essentially windows or open spaces that have complete, or close to complete, access to the sun, and are ideally south-facing.
They can also be referred to as “glazing,” and the amount of glazing necessary on a sun-facing surface is determined by the climate the building exists in. Windows specially designed for solar heating typically have a special coating that reflects ultraviolet (UV) rays away from the home.
Absorbers
Absorbers are hard surfaces that are directly in the path of the sun, designed to capture (not reflect) solar energy in the form of heat.
Thermal mass
Thermal mass is the material behind or under an absorber that stores heat and releases it slowly. Thermal mass is often made of materials like brick, stone, and tile, but can also be water, stored inside an absorber like a dark-colored tank.
Distribution
Distribution of solar heat happens in three primary ways, sometimes aided with fans and blowers:
- Conduction happens when heat is transferred between two objects in direct contact, like your bare feet on a hot floor
- Convection happens when heat is transferred via air or water. Warm air will naturally flow to cooler areas; this is why your food is more thoroughly cooked in a convection oven than in a standard microwave
- Radiation occurs when you feel the heat from sources around you, like your skin on a hot day
Control
Control mechanisms like awnings, roof overhangs, and blinds provide custom control over the amount of heat entering your home, while insulation and ventilation methods provide control over heat loss.
This is important in the summer months, when keeping things cool requires blocking the sun from getting to absorbers.
The four methods of passive solar heat gain
The above components can be used in four different methods of heat collection: direct gain, indirect gain, sun-tempering, and isolated gain.
The four methods can be achieved based on how the building is designed, and can be used separately or together in whatever combinations make sense for your needs.
Image source: Building Green
1. Direct gain
Direct gain is the most straightforward method of heat collection.
Sunlight enters south-facing windows (apertures), and shines on a dark, hard surface. The energy from the sun is transferred to the thermal mass described above by conduction, and slowly radiates through the living space at night after the sun goes down.
2. Sun-tempering
Sun-tempering is a more modest approach to direct heating.
A sun-tempered building is usually built with a long surface containing most of the building’s windows facing south. Sun-tempering requires no thermal mass, but the building’s walls and roof are typically very well-insulated to keep heat from escaping.
3. Indirect gain
Indirect gain places the thermal mass directly behind south-facing windows, blocking sunlight from directly entering the home. At night, the thermal mass releases stored energy in the form of radiant heat.
In some cases, indirect gain systems use a Trombe wall, which is a south-facing wall made of materials like concrete or heavy masonry (brick).
This masonry wall is separated from the south-facing glass by an air gap, in which air becomes very hot while the sun is shining. The air in this space is then vented to other rooms in the home to provide convection heat where there are no suitable windows.
4. Isolated gain
Isolated gain is heat from sunlight that is confined to isolated spaces—often called sunrooms or sunspaces—that can be closed off from the rest of the home.
The warm air can be kept in this space to function like a greenhouse, and fans can move the warm air to other rooms as needed to prevent overheating in the summer or distribute warmth in the winter.
Passive solar cooling
One of the most exciting aspects of passive solar building design is that homes can be designed to both retain heat in the winter and reject and eliminate heat in the summer.
Passive solar cooling systems rely on methods of shading, ventilation, and convection.
Shading
Shading considers the natural changes in the position of the sun throughout the year.
Passive solar homes are built with overhangs that block the sun from shining through south-facing windows during the summer while allowing it to shine in at its lower winter angle. Not only that, shade from well-designed landscaping and vegetation can prevent late afternoon sun from continuing to heat the home.
Overhangs keep your home cool in the summer.
Ventilation
Smart home design requires adequate natural ventilation of living areas and places that trap heat like attics and crawl spaces.
Buildings should be designed with windows placed perpendicular to prevailing winds, with corresponding windows on opposite sides to allow for air movement and cross-ventilation.
Convection
Convection refers to the tendency of heat to rise within a fluid or air mass.
Passive solar design accounts for convection by placing vents high on a building, such as on the roof of a sunroom or solarium, with intake vents lower on the structure. In the summer, these vents can be open to allow for convective air flow that quickly replaces warm indoor air with cooler outdoor air as the sun sets.
Using the principles of passive solar design and simple methods of using vents to control heat flow, homes across much of the United States can be cooled in this way with less of a need for air conditioning systems.
How passive solar can work with photovoltaic solar panels
Passive solar reduces energy use, but the only way to eliminate your electricity bill and reduce emissions is to choose a photovoltaic solar system that can turn the sun’s energy into electricity. The good news is if you’re designing a passive solar home, you’ll likely need fewer solar panels on the south side of your roof.
Homeowners in America typically use 900 kWh of electricity per month and need a 6 kW solar system to make that much electricity. That 6 kW system costs an average of about $18,000 before incentives.
Using passive solar building techniques reduces your usage by 25%. Therefore, the number of solar panels you need can also be reduced by 25%. That 6 kW system could be reduced to 4.5 kW, saving you $4,500 in upfront costs. In addition, a roof-mounted photovoltaic system itself acts like a barrier between the sun’s rays and your roof, reducing the need for cooling on sunny summer days.
Combining passive solar techniques and renewable energy from a solar panel installation can truly transform your house into a solar home.
Key takeaways
- Passive solar design relies on controlling and distributing heat from the sun within a building.
- Special building materials can be used to store solar energy that can then be used for space heat after the sun sets.
- Using passive solar techniques can make your home much more energy efficient, and adding a photovoltaic solar system can complement the passive design by making the energy you need more cheaply than it would cost from the electric utility company.
Ben Zientara
Solar Policy Analyst and Researcher
Ben is a writer, researcher, and data analysis expert who has worked for clients in the sustainability, public administration, and clean energy sectors.