use of capacitor to store sunlight energy

Hi Hari,

Yes the photo voltaic energy produced by sunlight on a photo cell (solar cell) can be stored in a capacitor at least to the limit of the capacitor. The amount of energy that a capacitor can store has been improving but it still is less than can be stored in the comparable cost of batteries. Batteries also provide an easier source for recovery of the energy for a practical use. When the energy is being recovered from a capacitor for use the voltage drops steadily. This means that special circuits must be used to maintain a steady voltage level. A battery on the other had has a tendency to maintain a much more level voltage output and thus requires less circuitry to make it useful. Sometimes Batteries and capacitors are used together in special cases as the capacitors can deliver a higher instantaneous power while the batteries a good for the long haul. In reference to concentrating sunlight with a lens this is practical in only a limited sense. As light intensity is increased on a Solar Cell the output does not track in a linear fashion. There will come a point where more sunlight will not produce more current. The cost of large lenses is also likely to be more expensive than just purchasing a greater surface area of solar cells. This is why we see roof tops covered with solar cells and not just one small solar cell and a big lens.

It is important to the advancement of our knowledge that experimenters like yourself conduct experiments and make new innovations. Please continue to do this.

One other consideration is efficiency. Efficiency is a measure of how well we have taken a particular quantity of energy and converted it to a different form. The only 100% conversion is to heat. A good example is your example of the solar cell. I will make up some numbers to illustrate this. Let's say that 100 joules of solar energy fall on your solar cells. Of this 100 Joules perhaps 20 Joules are converted to electricity and the other 80 joules is dissipated as heat into the environment. Now we have 20 joules of energy that we are going to store in a capacitor or a battery. In the case of the capacitor this energy is going to be stored as potential energy in an electric field. In the case of a battery the energy will be stored as potential energy in a chemical system. Both of these are not 100 % efficient so between the loss to heat or other parasitic going in and the loss to heat coming out we may eventually have 10 joules of energy to deliver to the final application where the available energy will be turned into light, sound, motion, an other form of energy, and ultimately to heat. The most efficient thing to do is always eliminate as many conversions as possible in the chain from our energy source to our application.

John

Hi Han,

The problem with capacitors is charge storage capability. It takes a lot of capacitors to store the same amount of charge that a rechargeable battery contains.

You just cannot beat batteries for condensed charge storage for later use.

You also have an issue with charge leakage in capacitors. The design and insulating material is not intended for the long term storage of charge.

Even with the supercapacitors, you just cannot beat the economics of batteries.

DAB

As others have mentioned, capacitors are not great for storing large amounts of charge (they're not energy dense), and they're not good for storing charge over long periods (they leak charge). More than that, they also have a voltage characteristic which is less stable than batteries, making it more difficult to work with, but on the up-side, they are capable of a higher rate of charge/discharge.

But the other issue is that you want to concentrate sunlight onto a solar panel to increase the output. This is not a great idea for a number of reasons:

• Commercial solar panels are optimised for a lifetime under a maximum standard irradiance of 1kW/m^2 or thereabouts. Operating under concentrated conditions will void the warranty on most panels.
• Concentrating higher amounts of light onto a single panel will increase the rate of degradation of the encapsulating material (ethylene-vinyl acetate) which will cause it to go brown and start absorbing light, reducing efficiency.
• Higher amounts of light also translate into higher temperatures at the panel. If the EVA goes brown, that will naturally increase heat production further. Panel power output falls slightly as the temperature increases - but as most cells are silicon, they will get damaged once they go above about 150C.
• Higher amounts of light will produce more current, but internal cell resistive effects will work against you. The screen printed grid on the front of the cell is designed to only collect a regular amount of current - increasing the current means the cell's bulk resistance and the grid (and rear contact) will start acting as resistors, wasting the energy you collected as heat. Thus twice the amount of light will produce less than twice the output.
• Increasing metalisation to overcome this results in shadowing effects, which reduces the cell's net efficiency. Some cells designed for concentrator use are designed differently, and are often exotic multi-junction cells to maximise output by using the full spectrum more efficiently.
• Optics to focus the light is not cheap, require mechanics to track the sun which often break, require energy to drive the mechanics, and also go "opaque" over time. Plastics can't handle UV, and glass doesn't stay transparent forever, also absorbing some parts of the spectrum depending on the sort of glass. The glass never has 100% transmission efficiency either.
• Physics tells you that the amount of energy falling on a surface on a sunny day is fixed at around 1kW/m^2. If you want 1kW output, your harvesting area needs to be at least that much, taking into account optical and optical-to-electrical efficiency. In the end, concentrator systems using PV are possible but not common because they offer little-to-no-benefit especially when you aren't space or weight constrained. They're normally only seen in military/satellite usage. Even tracking (single and dual-axis) are rarely found due to reliability problems. Solar-thermal concentrators are more common for their secondary storage benefits - using molten salts as a heat storage medium.

If it was a good idea, I'm sure several people would have done it already.

- Gough