Searching For Passive Solar Energy? You're Getting Warmer!
As the name implies, passive solar energy is usually described as the process by which solar energy is converted to other energy forms without the intervention of mechanical, electronic or other methodologies.
We say “usually described” because that definition is not, strictly speaking, correct. In fact, pumps and fans are often used to aid the circulation of hot or cool air. However, this intervention is minimal when compared with what is necessary with active solar energy.
You only have to move outside on a sunny day to experience the heat in the sunlight. All that energy is simply waiting to be harnessed into energy that is useful in living and working situations.
How Does Passive Solar Energy Work?
The main use of passive solar energy lies in the design of new buildings, both residential and commercial, as well as the heating and cooling of existing buildings.
Heat transfer between materials and areas in buildings always occurs on the basis of transfer from the higher temperature to the lower temperature until equilibrium is achieved.
This transfer occurs in a number of different ways, as outlined below.
- Conduction is where heat is transferred from one material to another. The heat in one substance causes the molecules of that substance to vibrate and transfer the energy to molecules in the nearby substance. For example, if you hold a metal spoon and place it in a cup of hot water, then the heat will travel from the water to the spoon to your hand.
Some materials are better conductors than others but generally speaking, all metals are good conductors whereas substances such as glass, plastics and wood are poor conductors.
- Convection is where heat is transferred by actual movement of the molecules of a substance and is usually observed in liquids and gases. A simple example is applying a heat source in a room and the resulting hot air rises to the top.
- Radiation is where electromagnetic waves transfer energy through space. The best example is the radiant energy from the sun which travels millions of mile through space where there are no solids, liquids or gases and brings energy to our planet.
Examples of electromagnetic waves are light, microwaves, radio and TV – all with differing wave lengths.
Convection, conduction and radiation all play key roles in passive solar energy systems.
Practical Uses Of Passive Solar Energy
As indicated earlier, the main use of passive solar energy systems is in heating and cooling of residential and commercial buildings.
So let's look at the two major processes of heating and cooling buildings using this great resource...
The most common way to draw heat into a building is through the windows. Depending on which hemisphere you live in, you utilize the south facing windows (northern hemisphere) and the north facing windows (southern hemisphere) so as to ensure maximum exposure to the sun.
Obviously, in the northern hemisphere your main accent will be on heating, whereas in the southern hemisphere it will be on cooling.
However, in both cases you need to remember the balance and provide for both.
In a new building it is possible to build in the best design for getting maximum heat in through the windows. In an older building it is still possible to make changes if you are busy doing a renovation.
In addition to the windows you can use a 'sun-space' such as a room specifically built on as a sun-room. Such rooms are used for dining, relaxing and having an afternoon snooze.
Once you have your maximum exposure to the sun sorted out, it is necessary to to use convection and conduction to transfer the heat throughout the home.
This is done by using appropriate materials that absorb and store the heat during the day and release the heat during the cool evening.
One popular method is to use what is known as a Trombe Wall, where the wall is thick and constructed from absorbent materials such as masonry and protected by glazing on the outside to help retain the heat. In the evening the wall releases its stored heat into the interior of the dwelling.
The following diagram, courtesy of US Dept of Energy – Energy Savers illustrates the five basic elements of passive solar design – collector, control, absorber, thermal mass & distribution.
There are a variety of techniques and provisions that are used in passive solar cooling. Some of the key ones are listed below:
- Appropriate building design
- Window shading
- Radiant Banners
- Reflective Roofs
- Earth cool tubes
- Evaporative cooling
For more information also see our page on Solar Air Conditioning
It should be noted that some of the techniques and provisions used in passive solar heating are also used in passive solar cooling. For example, walls that store heat in the day and release it at night also serve to cool the building during the day.
Passive solar energy is a developing field and more and more people are taking it into account when building new homes or renovating older ones.
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