Utilizing sunlight to illuminate underground spaces through fiber optics

Question

I want to design an underground farm that would be protected from extreme weather conditions to avoid harvest failures.

I'm thinking about using fiber optics to light the underground farm. Is this concept practical/feasible?

Answer 1

Sunlight has on average a nominal 1000 Watts per meter squared. Ignoring the need to track the sun, this would require a bundle of fibers 1 meter x 1 meter x however meters deep. Making the cost to raise one tomato plant economically prohibitive. Light pipes do exist to fill this roll more economically. They are essentially just a tube of polished aluminum with windows on either end. They would probably have similar thermal loss to glass fibers.

A way to improve the performance of fibers or a light pipe would be to concentrate the solar radiation with mirrors before feeding it down the pipe. This would minimize the thermal losses in the growing area, but would require increased complexity to track the sun and safely concentrate, and re-distribute it below.

Another easier and cheaper method would be to use a commercially ready solar photovoltaic array to generate electricity. Then directly consume that electricity with high efficiency blue and red LEDs designed for maximum absorption by chlorophyll in the plants leaves. While there are losses associated with the conversion to electricity and back; there are no thermal losses from the growing area to the outside which can be substantial in cold and hot climates.