Green Living
ISDN2001/2002: Second Year Design Project
Independent Study
An Investigation into the Type of Light Collection Device and Their Effects
01
Prologue
As our system utilizes sunlight from the outside world, the collector is of utmost importance to its functioning. The design of the sunlight collector will not only affect the light collection capabilities, but also have an impact on how it tracks the sun, the weather resistance, ease of installation and many other factors.
02
Choices
In optics, we primarily manipulate the direction of light via reflection(as in mirrors) and refraction(as in lenses).
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02.1 Reflectors
Cassegrain Telescope
Cassegrain reflectors utilise revolved conic surfaces, usually a paraboloid primary mirror and a hyperboloid secondary, which increases the focal length of the whole system and places the focal point in a more convenient location for the optic fiber to access.
02.2 Refractors
Fresnel Lens
For refractors, we usually use spherical lenses, which can also converge parallel light beams into a spot. However, as the aperture size increases, to maintain curvature over the entire lens, it will become very thick and heavy.
Fresnel lenses solve this problem by slicing the lens into concentric rings, which do not require the internal thickness while retaining the local surface angle. Thus the lens can be made thinner and lighter while maintaining its focal length.
03
Investigation
An investigation into the light-transmission performance deviations between Fresnel lens-based collectors and Cassegrain reflectors was performed.
Modelling and Simulation
To perform our tests, we produced digital and physical models of both the Cassegrain reflector and the Fresnel lens .
05.1
Results for the Cassegrain
We performed the test using a 1:10 scale model, which has a reflective surface made from mirrored aluminium panels, and another using electroplated chromium. After accounting for differences brought by variations in geometry, we reached the above results.
05.2
Results for the Fresnel lens
We utilised an acrylic lens for testing.
While it is less stiff than glass, testing and simulation show that a 5mm thickness should be sufficient for supporting its own weight and retaining its geometry for our target size of 1m diameter.
For this thickness, acrylic can transmit approximately 90% of the incident light. We can assume this single lens will focus the light onto the optic fiber.
06
Conclusion
As we can see, the optical transmissivity of the two systems is very similar. However, the Cassegrain system folds the optical path to a more compact size so a smaller frame is needed, whereas the Fresnel lens requires the optical fiber to be fixed at the focal point and requires more space and a structure to maintain the alignment.