What is Passive Solar Energy?

Passive solar energy is an efficient, cost-effective way to heat your home by leveraging the sun’s natural power without relying on electricity or mechanical systems. By integrating smart building design, strategic window placement, and thermal mass building materials, homes can maximise heat absorption in winter and minimise overheating in summer. This guide explains how this system works, its key components, and why it’s a more sustainable alternative to conventional heating methods.

Let’s get started.

Passive solar energy refers to a system designed to produce heat for the home without using any electrical or mechanical elements. In simple terms, it’s a system that converts sunlight directly into heat. It uses components like efficient home design, building materials, and proper orientation to collect, reflect, store, and distribute thermal (heat) energy throughout the home.

The basic idea behind these systems is to use sun shining through south-facing windows that can be absorbed and stored in ‘thermal mass’ materials. How much thermal energy you can collect and store depends on the surface area of the windows and the amount of building materials you have that can hold thermal mass. To create a system, the following items must be considered in your design.

How much heat you can capture depends entirely on orientation. When constructing your home, you need to position it to optimise the sun's position in the sky at various times of the year. How much potential warmth you can gain depends on the amount of sun in your area and the number of cloudy days you have. If you know this, you can design a building to maximise thermal gain in winter and minimise it in summer using effective design and placement of your windows.

Storing thermal energy requires thermal mass materials that absorb and hold warmth. The most commonly used materials include stone, concrete, brick, tile, and adobe. While water and phase change products are more efficient in storing warmth, stonework serves dual purposes as both a thermal mass and structural finishing material for your home. To work, thermal mass materials must be left bare without any obstacles blocking them from exposure to direct sunlight.

Passive systems are heavily dependent on efficient building design. The home and the window size and positioning must be oriented to optimise heating during the winter by allowing as much direct sunlight to hit the thermal mass materials as possible. Stonework interior floors and walls are typically utilised to maximise thermal mass. The design must also optimise cooling during the summer by blocking direct sunlight.

The general rule of thumb in the UK is to have properties face within 5-30° of true south and not have shade from other buildings or trees between 9 am and 3 pm during the winter.

Optimally-sized double or triple-glazed windows should be used, with more windows placed on the south-facing side than on the others. The total glass area should generally be 7 to 12% of the house’s floor area. The south-facing windows must also be adequately shaded when optimising cooling to prevent internal heating. This can be done by placing windows to receive direct sunlight during winter but with overhangs that prevent direct sunlight from getting in during summer when the sun is higher. Overhangs can be added or removed on older homes to block or allow sunlight to enter. Additionally, window shades or blinds can be used to optimise cooling.

Finally, there’s a misconception that skylights are helpful in passive builds. However, they should not be used because their placement on the roof means they usually maximise heating in the summer while causing heating losses in the winter.

Another factor to consider is how the warmth that enters the home on the south-facing windows can be distributed to the rest of the house. This can be achieved using convection, conduction, or radiation.

Convention is the transfer of warmth through air or water. Here, fans and blowers in the ventilation systems can be used to transfer heat. Conduction is the transfer of heat from direct contact. This happens naturally, where colder areas of the home naturally attract thermal energy from thermal mass materials. Finally, thermal radiation transfers warmth from warmer to cooler surfaces. Using darker colours that absorb more heat and placing them strategically can help provide routes for heat to travel between thermal masses throughout the home.

Different strategies are used to control the heat that enters the home when you don’t want the extra thermal energy. For instance, roof overhangs can be strategically placed to shade south-facing windows during the summer. These can be attached internally or externally. Additionally, insulated shades and shutters can block summer sun, as can exterior awnings and window films. Another option is to use an automatic thermostat to signal the ventilation system to turn on. Additionally, vents and dampers can enhance or restrict heat flow.

Finally, proper insulation in your home is crucial for anyone, especially those using passive systems. The insulation holds the heated air inside and the cold air out, or vice versa, in the summer months if you use air conditioning. Using insulated shades and drapes is also a good idea, and having them close at night and open during the day can also improve your heating efficiency.

Passive systems differ from active solar energy, which captures incoming sunlight using solar panels, also called photovoltaics, and converts it into electricity. They rely on external structures like photovoltaics, fans, and battery backup systems like the EcoFlow Power Ocean (Single-Phase) to store the energy for later use. Solar energy can be used for electricity, heating, or cooling. When heating, active systems generally rely on water, other liquids, or air in tubes to transfer warmth throughout the home.

However, passive systems convert incoming sunlight directly into heat without water or other substances, making them incredibly efficient. They rely on strategically placed windows to allow maximum sunlight to penetrate. They also use thermal mass materials like brick, concrete, and stone installed where they’ll receive direct sunlight to store that heat directly and passively. However, they cannot generate or store any electricity; they only store warmth.

Virtually any home is suitable for solar, both passive for heating and using solar panels to power a house. In fact, using both systems allows you to maximise your solar power benefits. If you already have photovoltaics, you can easily retrofit your EcoFlow Power Ocean (Three-Phase) battery storage system. There’s no need to re-do your system when you use the EcoFlow Power Ocean DC Fit to connect your photovoltaics and your existing inverter, making installation a breeze.

One of the most significant benefits of using passive heating technology is that it’s much cheaper than active forms to install and use since it’s designed into your home rather than needing to purchase additional equipment. Passive heating also cuts down enormously on energy consumption and, therefore, bills, since heating and cooling account for most of our energy use in the home.

These also have tremendous benefits for reducing our environmental impact. Since passive systems capture and utilise the energy from the sun directly, no extra equipment is needed, and no fossil fuels are consumed. So, your carbon footprint will be significantly reduced. Lowering your energy consumption also reduces your reliance on grid power, with further environmental benefits.

Rather than using fossil fuels for heating, passive systems don’t rely on these, creating much better indoor air quality without using combustion and forced air. The natural lighting it provides also benefits us, particularly in the winter, to help combat Seasonal Affective Disorder.

In a well-designed home, it’s incredibly effective. Depending on the house and the location, it can become the primary heating source. However, it’s essential to note that passive solar relies heavily on the weather and maximising exposure to sunlight. In northern areas with very long winters or locations with a lot of shade, a supplemental active system is needed to supply enough thermal energy. Still, even in those instances, these systems can significantly reduce the amount of heating your active system has to produce.

The only drawback with passive systems is their reliance on the weather. Those with longer winters may need to supplement their system with another active heating source. Still, passive solar alone is sufficient for some homes, while it will significantly reduce heating bills for others.

Passive systems use efficient home design to heat a building using the sun's natural heat without any electrical or mechanical devices. They rely on allowing sunlight to enter the home and being absorbed by thermal mass materials like stonework while blocking sunlight in summer.

Passive solar energy is a simple yet highly effective way to harness the sun’s natural warmth, reducing energy costs and environmental impact. While climate and home design influence its effectiveness, passive heating remains a valuable solution for enhancing efficiency. Homeowners can enjoy sustainable heating with minimal reliance on external power sources by optimising home orientation, thermal mass, distribution mechanisms, control strategies like overhangs and insulated drapes, and good insulation. To further boost your home’s solar capabilities, consider integrating it with an active photovoltaic system and battery storage products like the EcoFlow Power Ocean (Three-Phase) for a complete renewable energy solution.