Fresnel lenses help overcome the problems in focusing solar energy to perform useful work. The sun is the most popular energy source since its light and rays fall on almost every square inch of the earth's surface. Many locations have plenty of ambient light when the sun is shining, but often it is too diffuse to be of much reliable help. One of the most difficult problems is how to focus the available solar energy onto a location where it can do useful work.
Enter Fresnel lenses...
What is needed is to take a larger area of diffuse sunlight and focus all the power into a single point where the total energy is concentrated and delivered.
The Fresnel lens is an accessory to do exactly that. It has great potential in the solar energy realm. This is evidenced by some of its current uses:
It is commonly seen as a part of a rear window of an RV. In this case it is designed to magnify the rear view to give a clearer picture of what is behind the unit. The lens provides much greater safety to anyone driving such a large unit.
Many traffic lights use a Fresnel lens to make controlling traffic flow more efficient. The flat lens will selectively display the control lighting only to a specific viewpoint, such as a specific lane on a busy corner. The light is often not visible to all drivers. These lenses can be designed to direct light in quite a variety of angles.
One of the most common uses is when these lenses are used as a replacement for a magnifying glass during reading or examining maps. Glass is easily broken, and a flat plastic Fresnel lens is a great replacement. They easily slide along the paper to magnify a book's letters for people who need larger print. A standard magnifying glass can be used, but it is much bulkier and difficult to comfortably use.
Fresnel lenses magnify images, or they can be used in the opposite direction and concentrate light. The power contained in concentrated sunlight is often a topic taught in school science classes. Informal experiments focusing light are also common with adolescent boys and their magnifying lenses. There is a huge fascination in burning holes in paper or starting fires with just the sun.
Nowadays, a Fresnel lens, more often than not, is the focusing agent. They are so much cheaper and easier to manufacture than a more conventional and bulky magnifying glass. Besides, they are much easier to handle and can bend light in very different ways.
Even more surprising, its original design had nothing to do with creating heat. The first ones were made of glass but were created to concentrate the light produced in a lighthouse and create a single piercing beam to be easily seen by ships at sea.
Wikipedia gives a good background description of the use of such lenses:
A Fresnel lens (/fre?'n?l/ fray-NEL or /'fr?zn?l/ FREZ-nel) is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.
The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. Compared to conventional bulky lenses, the Fresnel lens is much thinner, larger, and flatter, and captures more oblique light from a light source, thus allowing lighthouses to be visible over much greater distances."”
Once again, technology created well over a century ago is being employed in our contemporary search for ways to lessen our impact on the earth. In this case, the interesting characteristic of the Fresnel lens are the ways they can be manufactured to concentrate a wide area of ambient light into the pinpoint of an extremely hot focal point. It is really a glorified magnifying lens with some very important added properties.
The reason the Fresnel is so important is the material savings during the construction of the unit. Instead of a heavy glass unit, modern Fresnels are lightweight plastic that is often very flat and easy to move. The other advantage of these lenses is that they can be made quite large at a reasonable price. The more area that is concentrated, the hotter and more useful the resulting focused heat can become. Lenses over a meter square can be made quite economically.
These lenses are being used in both active and passive solar capacities.
Active Implementation Of Fresnel Lenses
One of the ways the Fresnel is becoming more popular in the active solar area is to focus a hot point on the heat section of Stirling Engines where the Stirling engine uses differential areas of hot and cold to drive the piston in the cylinder.
By expanding the area where sunlight is being processed as much as possible, the short focal point where the heat is concentrated results in a very hot spot under the lens. When this is focused on the hot area of the Stirling engine, the unit runs without any other energy being used.
The problem is that there are not many small to medium Stirling engines available. Either they are too small to be of any real use, usually models of industrial versions or made to display the concept and how it works, or the engine is a huge machine designed for high end commercial purposes. Both of these are not suitable for the home production of power.
Many of the large commercial installations that focus the sun's heat use some form of Fresnel construction to direct the light to a particular area of the installation. These are often employed even if they use dish arrays to concentrate and help focus the heat on a particular area of the machine.
Passive Implementation Of Fresnel Lenses
Passive solar energy is where the Fresnel can be used more advantageously in the search for home energy production. On a separate page, we discuss a way to build and use a solar oven.
The oven suggested used a sealed clear glass top to allow the sun's rays to enter and generate heat. A compact lens can be substituted for the cover, or even be integrated as the cover if it is large enough. The ability to focus an extremely high temperature pinpoint of heat on a stable area, such as the side of the pot or, better yet, a base heat sink on the bottom can use more of the sunlight striking the immediate area and create high operating temperatures for the oven and speed the cooking process.
The following video gives a further illustration of the lens use in solar cooking.
Fresnel lenses can also be very useful when dealing with solar water heating. They have the advantage of being able to deflect the light to particular areas. The standard water heaters using a grid of pipes that are passively heated by the sun can be made more active by taking light that otherwise might fall between the pipes and directing it to a location directly on the pipe holding the heating water. This construction might be a long rectangular lens with a long focal line directed along the length of the tubes. This way, no heat is lost - especially if the lens is built to be inserted under the thermal installation's cover.
To summarize - Fresnel lenses have quite a number of advantages
They are very flat but perform the same task as their more bulky cousins, the magnifying glass.
Unlike the magnifying glass, the entire surface area of the lens can be used, and it does not have to be round.
Many Fresnel lenses are rectangular and much larger than most magnifying glasses. They can be used to focus a wide area, many inches squared, into a very short focal length that packs a surprising amount of heat.
This heat can be used actively to drive an engine like a Striling Engine, or passively, heating an enclosed area such as a solar water heater assembly or solar oven to a much warmer temperature than passive solar rays can offer if they are left unfocused on the enclosed area.