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3.3.5. Discussion of Fiber Optics
Fiber optics are thin strands of either glass or plastic that are used to transmit light or signals. In
this application the light in the visible wavelength range that is reflected by the cold mirror will
be transmitted to the PBR using fiber optic lines. The fiber optics are grouped into a bundle and
located along the inside of the tower columns with the upper end exposed to accept the
concentrated light reflected from the cold mirror. The rest of the fiber optics are coated with a
cladding material which keeps the light contained in the individual fiber optic line until it reaches
the light plates inside the PBR. At this point the ends of fiber optics are exposed to allow the
light to escape from the lines and be transmitted throughout the glass plates. Homogeneous
distribution of the ends of the fiber optics within each light plate is necessary to ensure a near
homogenous spread of the light across the surface of each light plate.
The fiber optics used in this design are made of borosilicate glass, which has an attenuation of
0.002% per inch (78.7% per km) in the visible light range [46]. The length of the transmission
lines is estimated to be at most 150 m. Fresnel losses, or back reflection due to the difference
between the refractive index of glass and air, result in an additional 4% loss per side exposed to
the air (8% total). The combined losses estimated for the fiber optic transmission system are
assumed to be 20%.
3.3.6. Design Summary using Photovoltaic Cells
The photovoltaic system was designed to capture the excess energy from the sunlight to supply
light to the PBR so that operation could proceed 24 hours per day. The calculated input from the
sunlight, in the form of visible light, was calculated to be 5.01x10 7 kWh/day. This value
accounted for the losses occurred from the reflection of the sunlight to the central towers and the
losses from the attenuation of the light during transmission through the fiber-optic lines. The
assumed efficiency of reflection was taken as 95 percent of light collected by the heliostats
would be reflected to the towers, and 80 percent of the visible portion of the light received at the
towers would arrive at the PBR, assuming 20 percent loss in transmission through the fiberoptics.
The excess light required in the PBR would then be supplied using the PV system, which is
composed of the PV cells mounted on the inside of the light collection towers, a DC/AC inverter,
and an electricity transmission system to supply the generated electricity to the grid for “Storage”.
The decision was made to send the generated electricity to the grid as a storage mechanism
because storage of the power in a battery would likely lead to an additional efficiency loss.
There is also a potential to gain a ”Green Energy” tax deduction depending on location, as the
electricity supplied to the grid would be from renewable resources. This will require proper
coordination with the utility company and acceptance by the governing commission. There is
also be the potential benefit of selling the electricity during peak hours when the cost of
electricity is greater, and then buy it back at night when the cost per kilowatt-hour could be less,
which could help to offset the initial capital cost for the system.
The sun’s energy that is reflected to the central towers and not reflected by the spectrally
selective cold mirrors will be absorbed by the PV system in the combined towers. The design
also includes six towers that are present only for the generation of electricity (see Solar
Collection section). The wavelengths of the light that will be absorbed by the PV cells in the
combined towers are in the 700 -10,000 nm range. The “electricity generation only” towers will
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