As I was walking through a brightly lit grocery store the other day, it occurred to me that there must be a lot of energy spent on lighting, and a huge percentage of that light just hits the floor. What are the economics of recapturing that energy using solar panels in the floor? Is indoor light too low in intensity for solar panels to use it? Are solar panels so expensive that you would never get a return on the investment?
I suspect it would be a combination of low light intensity- artificial light is WAY dimmer than the sun, startup/installation costs, and maintenance issues that would preclude it from being effective.
When solar panels get dirty, they lose efficiency. Putting them in a place where they will be constantly walked over is not a good idea if you want to generate a lot of power.
It would make the floor dark, which would make the store darker and things like under-shelf shadows harsher and more contrasty - the light bouncing off the floor is not wasted - it’s just providing infill illumination.
Better to put solar panels on the roof, where the sun can get at them.
Note I painted a dark colored kitchen white. Suddenly the lights in the kitchen were no longer needed during the day! It became quite bright in that kitchen.
So the opposite would be true. The light colored ceiling, walls, and floor of a retail store help to light it up by reflecting light. If you suddenly turn the floor dark, you might need more lighting!
This is a greatly simplified thought process so electricians feel free to correct my data. But consider that solar panels are at best around 25% percent effective at capturing light and turning it into electricity. So you would only be able to recapture a fourth of the energy of a room. Consider you have two 75 watt bulbs, at best you could get around 20 watts back (and probably much less than that). The solar panel on the roof of my house capture about 5000 watts at their peak. It would be much more cost effective to use more efficient light fixtures.
I strongly suspect it would be much more efficient to put the solar panels on the roof than on the floor. Even though the roof panels would be generating only 12 hours a day (average over the course of a year), the intensity of daylight will generate much more electricity than floor panels lit 24/7.
Incandescent bulbs convert about 2% to 3% of energy in to visible light. Florescent bulbs around 10%, LEDs get up to around 14%. So of that 150W there is max 4.5W of energy that could be recovered by solar panels. The light in a room is not a lot of energy.
Just looking at Wikipedia, the most efficient indoor lamps (LED and fluorescent) max out at around 15% efficiency, including the inefficiency of the power supply.
Indoor light fixtures don’t just illuminate straight down - otherwise you can’t read anything on a vertical surface. But let’s be optimistic and say 50% of the light ends up hitting the floor; the rest hits walls, tables, shelves, etc.
Most commercial solar panels are also around 15% efficiency.
Under those assumptions, the solar panel on the floor only gives you back 0.150.50.15=0.012= 1.2% of the electricity you’ve used to illuminate the room.
My advice would be to forget the solar panels and just turn down the light by 1.2%. It’ll save just as much electricity, it won’t cost you anything, and nobody would ever notice.
(I won’t even get into the difficulty of covering the solar panels with something transparent, durable enough to be used as a floor, and not slippery. And don’t even get me started on solar freakin’ roadways.)
I designed a solar simulator for testing solar cell arrays and can provide some real life figures. This was about 25 years ago, so things may have improved since then.
Total lamp output: 20,000 watts
Visible light output from the lamps (8%): 1600 watts
Power from solar cells: 150 watts, less then 1% of the lamp power.
This used halogen projector bulbs that put out 90% infrared, 2% UV and 8% visible light, which was quite good at that time. They had dichroic reflectors to let the infrared through to a cooling system while only the visible and UV was reflected forward.
The lamps produced an insolation at the surface of the solar cells of about .5 suns, so quite bright.
Even if you could find 25% solar cells (not sure if they really exist for practical uses), that is only 25% of the visible light that hits them. I think the output in the situation you described would be negligible.
And every shadow across them would really complicate things. If the voltage in one section of a solar array drops, then it receives back voltage from the adjoining cells which drops the efficiency even further. Illumination on all cells needs to be within 5% of each other.
Dennis
The solar panels would have to be the same price as regular flooring for this to work.
Better idea: leave the floor as it is - put the solar panels on the roof.
Even then, you’re better off using a light colored floor than a black solar panel. A solar panel can maybe convert 15% of the light into electricity. A tan or light gray floor can easily reflect 30% of light back up into the room.
And they are very expensive and require designing the entire building around them. Let’s cut to the chase with a WAG:
You would spend $100,000 to save $75 a year.
Dennis
Indeed - so even if we assumed the solar panels were free of charge to procure and install, the small amount of electricity they would generate could probably be saved by painting more things in the store lighter colours, then reducing the lighting.
You know if the floor was a solar panel, it would have to be dark… which means that there would have to be more lighting power in the ceiling…
So what they actually do is use a white coloured floor … this minimises the amount of power used in lighting.
But its true that there is not enough light to get the solar panel to make any wattage… you know how a 11 watt LED can do what a 100 Watt incandescent can do ? Well they both only make one watt of light ! The rest is wasted as heat. The LED still sends a large % of energy to heat, but the absolute amount is far less than incandescent… Anyway the point is that even though Flouro and LED save al ot, there still isn’t much light power to recover…
It would be hideous expensive to install and maintain floor photovoltaic panels … so they’d be very expensive up front and produce very little…