What is a solar tube? A solar tube is a way of channeling natural sunlight into a house to provide natural illumination without needing power or a window.
Solar tubes are also called light tubes, solar pipes, or sun tunnels. These names pretty much tell you exactly what they are designed to do. A lot of houses have a skylight, which is a window in the roof. A skylight addresses an unavoidable design problem with a house. If you have windows on every wall of your house, each window will only be exposed to the sun for a certain period of time because the sun moves through the sky. A skylight, if it is flat on the roof and pointing directly up, will naturally be exposed to the sun for a longer amount of time, bringing in more light into the house. Now, again, there is a problem with a skylight because it can only bring the sunlight into the room that it is connected to. A solar tube aims to fix that.
A solar tube works by reflecting light down a tube, where it can be used to light a room. There are two things that are necessary for this. The first is a way to get light into the tunnel and the second is a way to get light all the way down to the bottom of the tunnel. The top of a solar tube is basically a funnel for light, called a rooftop dome, but it needs to have a few properties that a regular funnel doesn’t have. The funnel needs to direct light downwards, but all of the angles need to be carefully considered. If you have a funnel for water, the water will hit the funnel and then travel around the edge of the bowl until it goes down the funnel. Some of the water may bounce out, but most will stay in the funnel. Light doesn’t work like that. With a regular funnel, most of the light would just reflect back out and not go down the spout. To prevent that, the rooftop dome uses angled mirrors that can redirect sunlight coming from any direction down the funnel. They also use specialized lenses to magnify the incoming light and to filter out the unwanted parts, such as the UV and the Infrared light. The funnel needs to filter out the UV because that causes fading and the IR because that would heat everything up and cause problems. One of these lenses is called a Fresnel lens and it works by collecting the incoming light, redirecting it, and concentrating it into a narrow beam that can travel down the tube. A Fresnel lens has a number of flat, concentric rings that bend light one after the other.
The next step, once you have managed to magnify and redirect the sunlight into the tube, is to get the light to the bottom of the tunnel. The problem here is that the longer the tube is, and the wider the beam of incoming sunlight is, the more light you will lose. The inside of the tube is coated with a highly reflective material to reduce this light loss. Some of the solar tubes on the market claim that the materials they use reflect 99.7% of the light coming in through the dome. There are different ways of making a material that reflective. One of them is to alternate high and low reflective materials in very thin layers. The layers reflect different amounts of light, but the thickness of each layer is controlled so that the light waves undergo constructive interference, which increases their size and means that almost all of the light is reflected. Another way is to have nanomaterials with nano-level bumps that force the light to reflect. This is similar to the structure of a moths wing cells. Whatever material is used, the idea is to reflect almost all of the light down the solar tube.
Once the light reaches the bottom of the tube, it is spread out by a diffuser and can bring a lot of light to rooms far away from windows. Solar tubes are environmentally friendly and economically friendly because they don’t require any energy. However, the amount of light you end up getting is of course dictated by how much sunlight you have. The lenses can magnify the available sunlight, but if it is extremely cloudy, or dark, you are going to need to use regular lights. Still, as a way to bring extra light to a building at no energy cost, solar tubes are incredible. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Fresnel_lens
https://thesolarguys.com/solar-lighting-solutions/solatube-daylighting-system/how-solatube-works
https://en.wikipedia.org/wiki/Light_tube
https://www.solarreviews.com/blog/how-solar-tubes-work
https://waaree.com/blog/solar-tubes-everything-you-need-to-know
Image By Delusion23 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=20545826