When researching solar energy heating options you will often come across two different concepts; active and passive solar heating. This article looks at what passive solar energy is and how it works.
So, what is passive solar? Well, this term refers to the direct use of solar energy for heating purposes. It is different from active solar energy in that passive solar doesn’t involve the use of any mechanical or moving parts. This can also include electronic controls and other systems commonly found on active solar technologies.
How Passive Solar Energy Works
Passive solar energy works by absorbing solar radiation and retaining heat through the use of thermal mass. This is a term that refers to any material that stores heat.
Examples of passive solar energy use include the following:
- Greenhouses – These make use of large panes of glass that absorb solar radiation. Once heat builds up inside the greenhouse, it can be retained in thermal mass such as water and concrete.
- Passive Solar Homes – These are designed to absorb solar radiation for heating purposes. There are many elements that go into the design of passive solar homes. The most important are correct window placement and the efficient use of thermal mass.
Ideal materials for thermal mass should have both a high density and high specific heat capacity. The following materials are often found in passive solar energy structures.
- Concrete (including concrete foundations, breeze blocks, and insulated concrete panels)
- Clay (including traditional clay house bricks)
- Earth (mud/dirt)
- Rock & Stone (Sandstone specifically)
Passive Solar Building Design Principles
When designing a passive solar building, there are several principles that should be adhered to. These help to enhance the level of solar radiation a building can absorb and retain heat within the structure.
1. Correct Placement of Windows
In passive solar homes, south-facing windows help a building to absorb a larger amount of solar radiation than if they were facing north. South-facing glass is a key principle of passive solar design.
Steps should be taken to minimize obstacles that may inhibit the level of sunlight entering the home. As a result, trees and shrubs may need to be removed. Dirty windows also reduce the level of sunlight entering the home and therefore should be kept clean.
2. Efficient Use of Thermal Mass
To help retain heat inside a building, its structure must include an efficient amount of thermal mass. This mass should be exposed to adequate levels of sunlight where possible.
Exterior materials are generally ok providing there aren’t any large trees or other obstacles nearby. Inside a building, large windows can help solar radiation reach internal walls. These should be kept clear of large obstacles such as sideboards and display cabinets.
The correct materials should be chosen to help retain heat in the winter months and expel heat in the summer months. Materials such as concrete blocks, clay bricks, stone, and tile can help to achieve this. Darker materials are better than light colors as they absorb more heat.
Most modern homes are built with energy efficiency in mind and therefore adhere to this principle.
3. Distribution of Heat
In most cases, not all rooms of a building will face south. To mitigate the effect of this, heat can be distributed throughout a building. Passive solar building designs rely on the natural conduction and convection of heat to achieve this.
Conduction in passive solar homes involves the transfer of heat between connected structures. This may include a south-facing room passing heat to a north-facing room on the other side of a wall.
Convection involves the transfer of heat through the air and will happen naturally within a building. Warmer air has more energy than cooler air and moves to the place where there is less energy (i.e. cooler rooms). This process can be improved through the use of fans and blowers, however, the use of such devices would be considered active solar heating rather than passive.
4. Cooling in Summer
Whilst the main aim of passive solar energy is to heat a building, cooling should also be taken into account. It’s great having a warm passive house in the cold winter months, but you don’t want the same effect on a hot summer’s day.
To mitigate this, passive cooling elements should be included in the design of a building. These can help to reduce the level of sunlight entering a building during the summer months. They can also help to expel heat that does build up inside. Some elements of passive cooling include the following:
- Awnings & Verandas – Installing retractable overhangs to the exterior of a building helps to reduce the level of sunlight that can enter it. In winter, you can retract them to allow the maximum amount of sunlight into your home.
- Blinds & Shutters – These can help shut out intense sunlight during heatwaves. They are however less desirable in living spaces as they will darken the room and shut it off from the outside world.
- Trees – Although trees can inhibit sunlight from entering a room during the winter, they offer an excellent barrier against sunlight during the summer. Choose trees that have lots of greenery in summer and lose their leaves in winter. This will help to mitigate the blocking effect in winter.
- Ventilation – Window openings, air vents, and other features can help to expel heat that does build up inside a building. In most cases, these should be kept closed during warm summer days until the inside air temperature is greater than the outside air temperature.
How Effective is Passive Solar Energy?
Passive solar energy is a highly effective solution in helping to heat your home during the winter months. But of course, this does depend on the climate where you live and the design of your home.
Buildings that use passive solar heating are generally not as effective as those designed to use active solar technologies. This is because active systems can make use of mechanical and electrical control systems to enhance their efficiency. For example, thermostats can trigger fans that quickly move heat through air ducts. Active solar heating systems can also provide hot water for your home.
So, we have learned what passive solar energy is and how it can be used. Passive solar heating works by absorbing solar radiation and retaining heat within the thermal mass of a structure. This can reduce heating bills by making use of free renewable energy.
Passive solar buildings should also feature elements of passive cooling. These help to keep buildings cool in summer, providing more comfortable temperatures all year round.