Over the years, the use and storage of solar energy have become incredibly popular for financial and environmental reasons. One of these reasons is using it in an active or passive solar heating system.
We‘ve all experienced the great power of the sun in one form or another. By simply stepping outside, we can feel the warmth and see the impact of the sun's energy on plants.
Simple and complex strategies are available to capture and use solar energy for heating. From complex off-grid solar systems and batteries to simple setups like a black length of hose filled with water to replace costly hot-water boilers, solar energy is both cheap and versatile.
Passive solar heating systems are a simple means of trapping and storing solar energy as heat.
These solar heating systems have become increasingly popular in recent times, and we’re going to explain exactly why this is as well as what you can do to implement passive solar heating in your home.
How Passive Solar Heating Works
“Passive heating” means, in short, that your heating system doesn’t require another mechanism to create or store heat.
Passive solar heating systems take advantage of the existing heat, generated by the sun, to heat living spaces.
Most smart passive solar heating systems rely on windows: cleverly designed and placed windows with unique energy-trapping material that collect the energy from the sun and keep it from escaping your house.
The Four-stage Process
The process of passive solar heating can be broken down into four basic stages.
If you’re living in the Northern hemisphere, the windows which capture the most solar energy are those facing South. In the Southern hemisphere, it’s the inverse.
These windows allow the light from the sun to pass through and heat your living space.
This stage is also known as the “aperture stage.”
After the solar energy has been collected through the windows, it’s stored by an absorber.
An absorber can be anything with a high thermal mass, such as specially-designed bricks or ceramic tiles.
“Thermal mass” refers to a material’s ability to absorb, store, and release heat.
A good example of how thermal mass works occurs during snowfall.
Snow that falls on the road melts faster than snow on grass and trees.
This is because tarred roads have a higher thermal mass than grass and trees.
In this regard, dark surfaces generally make great absorbers, whereas objects with a lighted color are better at reflecting heat. You can use this knowledge by using dark colors in areas you want to keep warm and light ones in places you want to cool.
After solar energy is absorbed, it needs to be released in the form of heat.
There are three ways that heat can be released using passive solar heating systems.
These heat-releasing processes are radiation, convection, and conduction.
Radiation is when heat wicks away from the absorber. For example, if a brick sits in the sun gathering energy during the day, at night, once the sun has set, it stays warm and slowly releases its heat to the surrounding space.
Convection heat involves transferring energy from a fluid medium (a liquid or gas) to another. Think of running a pipe of hot water under your floor to heat it up. If the water is heated passively (e.g., a black pipe left in the sun), you can then use passive convection heating to heat some spaces.
Conduction is when heat moves from one end of an object to another (from hot to cold). This is because energy tends towards equilibrium. If the walls of your house get hot during the day, the energy they store will disperse into your house where it’s colder.
Depending on the material used in the conductive medium, this can take a long time. Metals, for example, transfer their heat relatively quickly, whereas materials like water and ceramic transfer slowly. By carefully selecting the right medium, you can make sure that heat is gathered during the day and transferred slowly so your home is passively heated at night.
After the solar energy has been collected, stored, and released, it has to stay inside the target space.
Control consists of various processes working in tandem to ensure that the heat collected doesn’t escape.
These processes include things like smart window design, limiting external airflow, and maintaining the way the energy is dispersed throughout the system.
For example, if you’ve gathered 1kJ (kilojoule) of energy through passive systems throughout the day, the time over which it’s emitted into your home affects how efficient your system is. If you release 1 kJ into your house over a minute, it’s going to get very hot very quickly, and then cool down just as rapidly.
If it’s released over an 8-hour period, your house won’t get as warm, but it will maintain a comfortable heat overnight.
Because of conduction, once you heat your house, if it’s warmer inside than it is outside, that heat is going to ‘tend’ to disperse outside. The colder it is outside, the faster this transfer happens. If you use the right materials, you can keep the heat inside your house for as long as possible so the space doesn’t reach equilibrium.
Practical Example of Passive Solar Heating
We have all been out on a hot summer's day, perhaps hiking in the woods, or relaxing at the beach.
There's nothing like that feeling you get the minute you step back into your car, which has been sitting in the sun all day, and you are hit with a massive heatwave.
The heat stored in the car is a prime example of how passive solar heating works.
Heat is trapped by the windows after which it’s absorbed by the dark panels, seats, dashboard, and other objects, heating the car. Although it might be a comfortable 70 degrees outside, your car can easily get well over 100 because of the greenhouse effects of the collection and absorption properties of the car.
Advantages of Passive Solar Heating
The increase in awareness of global warming has made the use of solar energy popular.
Most of the electricity we use entails burning fossil fuels such as coal, which release carbon dioxide, various heavy metals, and other toxins into the atmosphere. This promotes the greenhouse effect, much like the passive solar healing used in our homes, only on a global scale.
Solar energy provides an environmentally-friendly means of energy production. We know we can’t simply wind the clock back 300 years and start living like people were back then, so we need to find a way to maintain our current standard of living without placing our planet at risk.
By using a passive solar heating system, you can lower your carbon footprint by reducing your reliance on energy produced by burning fossil fuels.
Your electricity bill is significantly higher during winter. The average home uses the majority of its energy for heating, be it your stove, water boiler, or AC. As much as two-thirds of your energy bill go towards this, so if you can generate even a few degrees of heat through clever home design without relying on your municipal supplier, that directly translates into money in your pocket at the end of the month.
Passive vs. Active Solar Heating Systems
There are many similarities between the two, however, there are a few significant differences between passive and active solar heating systems.
Active Solar Heating Systems
Active solar heating mechanisms serve the same purpose as passive ones but work a little differently.
Active solar heating systems (and solar systems in general) use things like solar panels, generators, and batteries to gather energy from the sun.
While these systems can be a bit more expensive, they also have their advantages over passive systems.
For one, active solar heating systems are significantly more powerful. In a passive system, you have little control once everything has been set up. If you’re experiencing an exceptionally cold winter, for example, the measure you put in place to keep your house warm might not be enough.
In an active system, though, you have direct control over how the energy is used. Solar panels convert sunlight into electricity which is then transferred to a generator or battery system. You can then plug the generator into whatever devices you want to power (provided you have a large enough inverter).
If it’s a warm night and you find you don’t need to turn any electrical heating on, you can use it to run something else, like a fridge or television.
Best Conditions for Passive Solar Heating Systems
A long-debated question in the solar energy world has been that of weather conditions.
Solar energy is produced by the sun every day, however, the amount your solar heating system receives can vary depending on obstructions or weather phenomena.
When it comes to location, the ideal spot for solar panels or passive solar heating collectors is in direct contact with the sun, unobstructed by buildings or trees.
In the Northern hemisphere, South-facing windows or solar panels receive the most light, thus they gather more solar energy.
Passive solar heating systems often struggle in areas with high amounts of snowfall.
These systems thrive in moderate climates which keep a consistent temperature, depending on the season.
Passive Solar Heating and Its Uses
We have already discussed how passive solar systems are used to heat living spaces, but did you know you can cook using them?
Due to the high price and high demand for supplementary fuel such as gas, solar cooking has gained considerable popularity.
One of the most common methods of solar cooking is the use of solar ovens.
Traditionally, these ovens contain wide, reflective surfaces which collect light and concentrate it.
The light collected is concentrated on an object with a thermal mass, usually a black surface, where it is converted into heat to cook.
An important aspect of solar ovens is that they must be regularly adjusted in order to obtain a maximum amount of the sun's energy.
We all jumped into a crystal blue swimming pool and have been met with that blood-curdling stab of cold water.
There are passive solar heating systems available to help avoid these icy situations.
Most swimming pools are heated using conventional electric or gas heaters, or by using active solar heating systems.
However, the most common passive solar heating system for pools involves an easy layout of piping on the roof.
Water is captured and pulled through a system of black, insulated pipes, which heat up during the day.
This water is then heated as it passes through the pipes and is in turn released back into the pool, making it significantly warmer.
This system isn’t as effective as an active solar heater, but it’s quite a lot cheaper.
The world's resources are rapidly decreasing and the need for optimizing energy consumption has grown.
In 2022 alone, the world experienced shocking increases in fuel prices, increasing the need for solar energy.
It begs the question though, how energy efficient are passive solar heating systems?
Passive Solar Heating Energy Efficiency
The best and cheapest form of energy is solar energy.
There are a few key components to a passive solar heating system that are crucial to maintaining energy efficiency.
The first component is well-sealed doors and windows, which should be double or triple-paned.
The second main aspect regarding efficiency is insulation.
A well-insulated house on its own can maintain a certain temperature.
Geographical Limitations and Solutions
As we have learned thus far, all solar systems, active or passive, require sunlight to function.
The best location for any solar panel is an unobstructed one, within a fairly moderate climate.
When solar panels are kept in extremely high or cold temperatures, the panel will begin to degrade. This means that its lifespan is shorter and it has to be replaced more frequently.
Areas with high snowfall or very low temperatures, such as Alaska, aren’t ideal for passive solar heating systems as the heat collected from the system will not be sufficient.
In these areas, active solar systems are preferred for their adaptability, and these systems are usually off-grid solar systems.
Refining the Usual Design
In the early years of solar energy, all designs were simple. However, in modern times we have refined the design with the aid of computers, 3D printers, and the benefit of hindsight.
The design is affected by factors such as budget, aesthetic preferences, performance expectations and so on.
There are a few elements the designer will consider when refining his design:
- The window location, the type of glaze, and window shading.
- The location and type of thermal mass used.
- Insulation and sealing of air around the area.
- Other existing auxiliary cooling or heating systems.
There are three main passive solar design techniques, namely, direct gain, indirect gain, and isolated gain.
Within this design, light enters the house and hit floors and walls as it passes through the south-facing windows.
The masonry walls absorb and store the solar energy until night time, when the thermal mass releases the heat into the house, warming it.
Some owners use water containers as absorbers as water can hold twice as much heat as masonry walls.
This design differs from direct gain as the thermal storage is located between the south-facing windows and the living space, therefore concentrating the heat.
The most popular indirect gain system is the Trombe Wall.
It consists of a masonry wall, usually 10 to 16 inches thick, positioned on the south side of the house.
A double or single-paned glass window is placed 1 to 2 inches in front of the dark wall to let light pass and heat the wall.
The heat then spreads throughout the wall and infiltrates the home, heating it up.
This is the most common passive heating design, also known as a sunspace or sunroom.
It is a separately sealed room of a house, usually south-facing, made up of glass windows all around to let in as much light as possible.
These work very effectively in sunny areas and provide the home with a source of natural light and heat.
The need for alternative energy will forever increase as the world depletes its resources.
Solar energy is a free, renewable source of energy and there are many ways to collect and use it.
Passive solar heating systems offer an easy, affordable alternative to the often expensive active solar systems.
They can be used to heat homes and pools, or even cook and pose no health risk to their owners.
Passive solar heating systems are growing in popularity and it's easy to see why.
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