PV Gap

Philip Said:

Alexandre-Edmon Becquerel - what exactly did he discover?

We Answered:

For one thing , don't call him an old man!! He was only 20 when he showed that light can initiate chemical reactions that produce an electric current. His discovery opened up the idea that light energy might be transformed into electrical energy and led to the investigations that uncovered the photoelectric effect.
Don't be too bothered about the different names used for the effect. If light energy is being transformed into electric energy then folk can call it what they like ( and obviously do ) but its all the same thing.

Helen Said:

What is the photovoltaic effect? What is doing? What is N-type and P-type material? Why aren't solar cells?

We Answered:

Sci-Tech Encyclopedia: Photovoltaic effect

The conversion of electromagnetic radiation into electric power through absorption by a semiconducting material. Devices based on this effect serve as power sources in remote terrestrial locations and for satellites and other space applications. Photovoltaic powered calculators and other consumer electronic products are widely available, and solar photovoltaic automobiles and aircraft have been demonstrated.

The basic requirements for the photovoltaic effect are (1) the absorption of photons through the creation of electron-hole pairs in a semiconductor; (2) the separation of the electron and hole so that their recombination is inhibited and the electric field within the semiconductor is altered; and (3) the collection of the electrons and holes, separately, by each of two current-collecting electrodes so that current can be induced to flow in a circuit external to the semiconductor itself.
http://www.answers.com/topic/photovoltai…


A bipolar (junction) transistor (BJT) is a type of transistor. It is a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes, as opposed to unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow.

Although a small part of the transistor current is due to the flow of majority carriers, most of the transistor current is due to the flow of minority carriers and so BJTs are classified as 'minority-carrier' devices.

An NPN transistor can be considered as two diodes with a shared anode region. In typical operation, the emitter–base junction is forward biased and the base–collector junction is reverse biased. In an NPN transistor, for example, when a positive voltage is applied to the base–emitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or "diffuse") through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base.

The base region of the transistor must be made thin, so that carriers can diffuse across it in much less time than the semiconductor's minority carrier lifetime, to minimize the percentage of carriers that recombine before reaching the collector–base junction. To ensure this, the thickness of the base is much less than the diffusion length of the electrons. The collector–base junction is reverse-biased, so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base junction.

http://en.wikipedia.org/wiki/PNP_transis…

Victor Said:

Why can't we convert heat to electricity?

We Answered:

We can.

Thermoelectric devices work backwards. It uses the Seebeck Effect.
That's for a direct conversion. Heat Energy to Electrical Energy. BAMN! It works... you can buy a lantern that powers a small radio.

You can also go from Heat (Thermal Energy) to heat up water and make stream, which turns a turbine or steam engine (Mechanical Energy) to turn a generator to make electricity (Electrical Energy).

In fact, you can go from any energy to any energy.
Vibrations to Electrical - YES (Piezoelectric devices)
Sound to Electrical - YES (Acoustic generators)
Ah... it's late - but the list can go on and on....

Stella Said:

do colored light filters give you a single wavelength range?

We Answered:

There are two ways for a color filter to make the light look green. It can either just pass green light, which will means it restricts the light to a single range (the green range). Or it can pass blue and yellow light, which means it will restrict it to two ranges. Because of the way your eye works, there is no way to tell based solely on looking at the color. However, there are ways to tell. What you should do is get a prism (those little glass wedges... some cut glass crystal like from a chandelier will also work, but not as well) and shine the light into that. The prism will split light by its wavelength or color, so that white light comes out looking like a rainbow. If you put true green light in, then you will only get green light out. If you put blue and yellow light that looks green in, then you will get a blue and a yellow band of light out. Chances are your filters will be a mix of these two colors.

Now if you have such a prism, an easier way to get monochromatic (one color) light is to shine white light into the prism. That will separate the colors into a rainbow. Then if you take a piece of cardboard and cut a slot in it, you can let the rainbow of light hit that slot. If you orient the slot in the right way, you can only let a single color through while blocking the other colors. That color that gets through will be very close to monochromatic. But to do this and get a lot of light, you will need a very bright white light to start out with.

Ok, now that I have said that, let me clue you in on an issue that will plague both my idea and your idea. These different colors of light, be they from using my prism or your filter, will have different powers. With the prism, the power, or intensity will depend on how much power there is in each band of light in the white light source. With the filters, it will depend both on that and on how dark the filter is. And you will get more power from a solar cell when you have more light. So you might think that one color has more energy per photon because you get more power, but it might just be that there is more of that light in your source. And this is the type of thing that can not be judged just using your eye, because your eye is also more sensitive to some colors (green) than others (red). However, if you connect correctly to your solar cell, you may notice a difference in the voltage you get from each color and that voltage should not be intensity dependent, just color dependent. My point is you might have to think carefully about what you are measuring and whether or not it is being influenced by things you don't intend to measure.

Good luck with your experiment.

Jacqueline Said:

Why is the photovoltaic effect only predominantly useful for small appliances?

We Answered:

Solar doesn't have a high power density compared to the needs of most devices we use, so it's impractical to put solar panels on, say, a laptop. There wouldn't be enough power to run the laptop, although theoretically, you might charge the battery over a very long time.

Since solar usually costs more (at least initially) than making a device battery powered, a manufacturer won't sell a solar version unless consumers are willing to pay more for it. In the case of a camping or emergency radio, this might be so. For a DVD player, unlikely.