# Tag Info

## Hot answers tagged photovoltaics

16

where does that electricity go? The photons from the sun have energy and momentum, but not "electricity". Essentially, a photon (solar or otherwise) striking the solar panel can create an electron-hole pair (EHP) and, if the EHP is within or near the depletion zone, the pair will be separated by the built-in electric field. This results in a separation ...

11

Any high slew rate (fast rate of change of power) stresses the grid. Lots of things cause high slew rates. People getting ready for work in the morning, having showers, turning lights and appliances on. Factories starting up at the same time. Faults on major international HVDC transmission lines. Safety shutdowns at nuclear reactors. Lots of people turning ...

10

Rather than considering quantum efficiencies or such details it's instructive to step back and take a broader view. One of the main fuel crops grown in the UK is miscanthus. There are various figures around for the yield produced by miscanthus, but these people estimate it as about 14 tonnes per hectare per year. The energy content is 19GJ/tonne, so that's ...

7

There are several ways to design the circuit. If it's a Smart system, then when there's surplus power, additional devices will get turned on, to use it: dishwashers, washing machines, or immersion heaters in hot-water storage tanks. If there's still surplus after that, then it's as below. If it's grid-connected, with an inverter, then it's usually designed ...

7

Photosynthesis is less efficient than solar panels. According to the Wikipedia page on photosynthetic efficiency, typical plants have a radiant energy to chemical energy conversion efficiency between 0.1% and 2%. Most commercially available solar panels have more than 10 times this efficiency.

6

In the field of quantum optics "parametric down-conversion" is a process where an intense pump laser shines an optical non-linear crystal and strongly correlated or entangled photon pairs are generated. In an optical non-linear crystal the polarization response of the material to the optical field is non-linear, which typically occurs at high intensity. ...

5

Developments as of summer 2014 The hottest kid on the organic-PV block is perovskites: in February 2012, Hardin, Snaith & McGehee published an article in Nature Photonics announcing "The renaissance of dye-sensitized solar cells". The inventors of one implementation, Oxford Photovoltaics Ltd (a spinoff of the University of Oxford) described their new ...

5

It will vary a lot by site, and on the particular PV technology being proposed. So, you need to know: is it monocrystalline silicon, is it CdTe thin-film, or something else? what's the efficiency of the inverter? what's the guarantee on the kit (5 years, 10 years, 20 years)? Using @Martin Beckett's figure of 2000kWh/y, that means, roughly, that your ...

5

I complete Gerard's answer: up-conversion is the reverse process, where two low frequency photons are converted to a single high-frequency photon. So basically down- and up-conversion correspond to a frequency conversion of the photons through a nonlinear interaction. The up/down term correspond to the "direction" of the frequency change. I have to add ...

5

The E=mc^2 formula only applies to an object at rest, and light is never at rest. You want to use the more general formula: $E^2={m_0}^2c^4+p^2c^2$ Then you can set the mass to zero. $E=pc$ What this says is that light has momentum, which is related to its energy.

4

The Juno mission to Jupiter, if successful, will be the furthest that we've managed to get PV work to date. It was launched on 5 August 2011; it is already beyond Mars orbit, and will, all being well, reach Jupiter in July 2016. There isn't a physics limit to the distance at which photovoltaics work. If a photon of suitable wavelength reaches the PV (and we ...

4

Hmm, I think it would depend on the panel circuit. If the panel is not connected, for example, the charge potential would still be created at the leads, but since it's not being drained into a storage device (or otherwise used), the solar medium would saturate at some measurable voltage boundary. Whether it turns to heat at that point depends on whether ...

4

The website is clearly supported by lots of money which doesn't guarantee that it reflects the most accurate scientific information. The very page you quoted says under the picture: The material is graphene, also known as graphite... Well, no. Graphene is not the same thing as graphite. Graphite is a 3-dimensional material used to produce pencils - and ...

4

Boden & Bagnall (pdf) looked at this question of reflection, in their paper Bio-Mimetic Subwavelength Surfaces for Near-Zero Reflection Sunrise to Sunset. Their estimate of the proportion of photons reflected, from sunrise to sunset, in a fixed PV system is 20%, rather than your estimate of one-third. The proposed etalon is not an efficient or ...

4

You raise 4 very good points. Without exploiting temperature gradients or fission/fusion of photogenerated charge carriers, spectral splitting and spectral modification are the only routes to very high efficiency solar cells. Tailoring the cell to the spectrum Spectral splitting can be achieved in two ways: Selective reflectors (which split light like ...

4

The colour you're seeing is from the very small fraction of light that the panels are reflecting. The vast majority of light is being absorbed to generate electricity. Why some of the panels appear slightly blue while others don't I don't know. Presumably there must be small differences in the manufacturing process. The absorptance of solar panels does fall ...

4

The sun is an extended source. This means that it occupies a definite solid angle in the sky $\omega = 6.8\times 10^{-5} Sr$. To visualise this (not to scale), let say that the black area in the following diagram is the angular extend of the sun as seen from the surface of the Earth (ignore the other labels), What happens when we concentrate sunlight ...

3

The Wikpedia article on solar cell efficiency gives a number of reasons that solar cells are less than 100% efficient. One of the large ones is the thermodynamic limit-a photon of less energy (longer wavelength) than the silicon band gap cannot produce an electron and one with higher energy can only produce as much voltage as the band gap. Even if you ...

3

For the middle east, typically around 2000 KWh/m^2/year A good place to start is wiki page for insolation (technical term for sunlight arriving)

3

It sounds like what you're really getting at is a way to make the oft-quoted performance numbers for various electricity generation options (PV included, of course) more understandable to non-experts. Having spent a few years teaching this material, I find that the right approach is to do more than simply quote the fact that X familiar object uses Y power or ...

3

The fundamental mechanism is the same. They are both photovoltaics. The cells on satellites are far more expensive and efficient. Because satellite launch costs dominate any other cost, you might as well pay for a high-efficiency cell. Terrestrial cells are 10-20% efficient, usually made from silicon. The ones in satellites can be double that efficiency, and ...

3

Have a look at the Wikipedia article on Solar cell efficiency. The efficiency is the electrical power output divided by the total energy received across all wavelengths. As you say, this would be unlikely ever to reach 100% as some of the energy is at long wavelengths.

3

It would certainly require a material that allows electron release from energies lower than those of the visible spectrum. The energy of a wave is given by E=hf where h is the planck constant (6.63 x 10^-34) and f is the frequency. The wavelengths of IR light range from 0.001 m to 750 x 10^-9 m. (Hyperphysics.com, infrared) Using this knowledge you can get ...

2

Essentially yes they work in the same way. There are slightly different designs, traditionally the space ones had highest performance because money was no object but because it takes years to space qualify a piece of hardware the gap with the latest cheap tech has shrunk. The challenge on most spacecraft is mounting and deploying the panels, which usually ...

2

Yes you can make transparent solar cells. Obviously their efficiency won't be nearly as high as their opaque cousins. http://web.mit.edu/newsoffice/2011/transparent-solar-windows-0415.html There is nothing impossible about a single material that absorbs UV and IR but not visible. Organic semiconductors can do that. Inorganic materials like silicon almost ...

2

Yes, you can have photovoltaic tuned to different areas of the EM Spectrum. No, you can't cover the whole EM spectrum with one detector.[1] In fact, limiting yourself to photovoltaics is throwing out most of the spectrum- you are only looking at materials where EM radiation will cause an electron to jump bands. Is there a particular bandwidth you are ...

2

The short answer is that when an electron in the valance band absorbs energy from a photon to become a conduction band (mobile) electron, both that electron as well as the hole "left behind" in the valence band can participate in an electric current. Thus, it is said, an electron-hole pair is created. Where there was no mobile charge carrier, there are now ...

2

Yes nuclear batteries are reasonably common, it's generally easier to just use the heat given off to drive either a stirling cycle motor or a thermo-electric generator rather than use the energy of the emitted particle directly. They are very useful when you need power for many years without being able to recharge or replace the batteries eg. in space ...

2

The trouble with an etalon is getting the light into it in the first place. Bearing in mind we want normal inidence to maximise the light intensity our solar panel looks something like: But solar panels aren't transparent (obviously since they reflect two thirds of the incident light) so if you wanted to use a second panel you'd need something like: It'...

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