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Solar power is the world’s fastest-growing renewable source of electricity generation and a crucial component of the transition toward a decarbonized future.
But how does it actually work?
And what’s the difference between passive and active solar energy?
Both active and passive solar power are produced by harnessing the sun’s rays and using them to generate either heat or electricity.
Passive solar energy refers to a specific type of home design that utilizes sunlight to store heat.
Active solar energy can also refer to systems that use the sun’s heat. But the definition also includes the form of solar power you’re most familiar with. Using solar panels to convert the sun’s energy into household electricity.
Let’s focus on electricity generation first, then move on to passive and active solar heating.
(Source: Energy Education)
What Is Solar Energy?
Solar energy consists of light and heat generated by nuclear fusion reactions inside the sun. The most frequent modern use of solar energy relies on the photovoltaic effect. Solar power systems capture photons the sun irradiates and convert them into DC electricity.
Photovoltaic (PV) modules — most commonly solar panels —utilize numerous solar cells under a transparent protective surface like tempered glass to capture the sun’s energy. Solar cells are typically monocrystalline or polycrystalline silicon and conductive metal contacts.
Each solar cell contains two semiconductors — p-type and n-type. Together, they form a p-n junction to create an electric field and convert solar energy into DC electricity.
The solar cells inside a PV panel capture solar energy. But a portable power station or other balance of system is required to convert the Direct Current produced by solar panels into usable Alternating Current (AC) or “household” electricity.
The above process is what most people think of when they think of “solar power.”
However, there’s more than one way to harness solar energy. It doesn’t require solar panels — and it’s all about heat.
(Source: 2030Palette)
What Is Passive Solar Energy?
Passive solar energy doesn’t require solar panels, a portable power station, or other balance of system.
Instead, passive solar power relies on the peak sunlight your house receives and uses materials like brick, stone, and water (thermal mass) to store heat.
Passive solar energy can heat your home in the winter and help keep it cool in the summer.
Here’s what you need to make it work.
- South-Facing Windows (Aperture): To capture sufficient energy to make passive solar heating effective for your home, it must have south-facing windows unobstructed by shade during daylight hours: roughly between 9 am and 3 pm. Your windows should be shaded during the cooling season — typically summer and parts of spring and fall. Ideally, your windows should be positioned within 30 degrees of true south.
- Thermal Mass is required to capture the sun’s energy during winter and other times when you need to heat your home. During the hotter months, thermal mass can retain heat from warm air in the house, helping to reduce the ambient temperature overall. Thermal mass can consist of common building materials like bricks, concrete, and tiles. Other options for thermal mass include water and phase change materials (PCM). Solid thermal mass should be painted dark to help the available surface area (absorber) capture more heat.
- Distribution: Once your thermal mass retains passive solar energy from your south-facing windows, it must be distributed throughout the rest of your home to keep it warm. Truly passive solar heating relies on three natural means of distribution (sometimes just one, often all three): radiation, convection, and conduction. Many designs also incorporate fans or other blowers to move hot air around.
- Sunlight Control: Depending on various factors — including the time of day and the time of year — you will require one of several mechanisms to control the amount of sunlight coming in through your south-facing windows. Control is particularly crucial during summer when you don’t want your thermal mass to collect passive solar energy. If unshaded, it will continue to heat — not help cool — your home. Typical passive solar control methods include awnings, blinds, and shudders. For a more permanent solution, an intentionally designed overhang can allow sunlight in during the winter and block out the summer sun.
Passive solar energy alone will rarely be sufficient to fully heat or cool a home year round — especially in climates with extremes of heat and cold. Additional HVAC systems will almost certainly be required. But, correctly designed, passive solar systems can be a cost-effective way to at least partially heat or cool your home.
There are multiple other factors to consider, including whether your passive solar system will utilize direct, indirect, or isolated gain.
(Source: OSU)
Isolated gain is probably the most well-known passive solar system — especially in new-built homes and commercial buildings. It relies on a sunspace (or solarium) to collect sunlight which is stored in the thermal mass and distributed to the rest of the house.
Sunspaces can be built as additions to existing homes. But most properties are unlikely to have a suitable area that meets all the preconditions for a passive solar system to work effectively.
Advantages of Passive Solar Systems
- Can be constructed with conventional building materials like glass, brick, concrete, or tiles
- No solar panels or balance of system required
- Affordable option for new builds
- Zero carbon emissions
- Noise-free
Disadvantages of Passive Solar Systems
- Typically not a viable option for existing homes
- Unlikely to heat or cool your house completely. Auxiliary HVAC systems are likely to be a necessity
- Thermal mass doesn’t store passive solar energy (heat) for long, leaving you highly reliant on available sunlight
(Source: Energy.gov)
What is Active Solar Energy?
Generating electricity using active solar energy captured by solar panels and converted into AC power by a balance of system is covered above.
But electricity generation isn’t the only active solar energy application.
The other is gathering heat.
(Source: Penn State)
Active solar heating differs from passive in that it uses additional technology and equipment to capture, store, and circulate heat.
Liquid-based active solar heating systems are the most common and typically used to provide central heat. Solar water heaters are another widespread application of active solar power systems, and the principles behind both are similar.
- Collecting Heat: Flat-plate collectors may look deceptively like solar panels but don’t use the photovoltaic effect to generate DC current. Instead, they collect heat. No solar cells are used, and the construction of flat-plate collectors is quite simple. Often it’s essentially a large sheet of copper or aluminum painted or chemically treated to be black — enabling it to capture more solar energy as heat.
- Heat Transfer: The solar energy captured by flat-plate (or other) collectors is used to heat a liquid — typically water or antifreeze. Once the fluid absorbs enough heat, a circulating pump transfers the hot liquid for either immediate use or storage in the home.
(Source: Rethinking the Future)
- Central Heating and Storage: If the active solar energy collected is used to heat your home immediately, the hot liquid will be directed to a heat exchanger that converts it into hot air that is then circulated throughout your home using conventional methods like heating ducts and radiators or less traditional means like a radiant floor system.
Active solar energy that’s not immediately required can be stored in hot water tanks or radiant slab systems and accessed when additional heat is needed.
Other methods of active solar heating include room air heaters, transpired air collectors, and ventilation preheating.
Advantages of Active Solar Heating
- Potential long-term savings on utility bills in cold climates with adequate peak sun hours
- Possible eligibility for government incentives
- Clean, renewable energy source
- Decades-long lifespan
- Active solar heating systems can be configured to heat water as well, providing value in summer months or when central heat is not required
- More easily retrofitted to existing homes than passive solar systems
Disadvantages of Active Solar Heating
- Systems can be costly
- Only provides heat — not cooling
- May still require a supplementary/backup heating source
- Significant building code and permitting restrictions
- Requires professional installation
- Risk of damage from freezing in extreme cold
Frequently Asked Questions
It depends on what you’re trying to achieve. If you want to generate electricity using solar energy, photovoltaic systems — typically using solar panels — are your only option. If your goal is to heat and cool your home, passive solar can be effective and inexpensive — especially for new builds. Active solar heating is also an option that doesn’t use solar panels but different technology for collecting the sun’s heat. But if you want to generate electricity, there is no passive option.
There is no difference. Both simply mean light and heat irradiated by the sun. The difference lies in how you capture and convert solar energy. When most people think about solar power, they think about solar panels and systems that help generate electricity. Passive solar energy involves storing the sun’s heat to heat and cool your home.
The best-known example of active solar energy is using photovoltaic modules — typically solar panels — to capture the sun’s rays and convert them into AC (household) electricity. Active solar heating is another application that doesn’t require PV modules but relies on other technology to capture the sun’s heat. Passive solar heating doesn’t need much technology — it’s all about the design and construction of your home.
Final Thoughts
Many people believe photovoltaic (PV) systems using solar panels are the only type of solar energy applications.
But passive and active solar heating systems can be viable, eco-friendly alternatives to electrical and fossil-fuel-powered air and water heating.
In certain circumstances, they may save you significant amounts of money on heat supplied by utility companies — especially if you’re building a new home and designing it with solar in mind from the outset.
If you’re interested in an active solar energy system that produces enough electricity to power your entire house off-grid, check out EcoFlow’s whole home generator solutions.