Dr. Ambedkar Institute of Technology
Question
Asked 29th Jan, 2016
We are using magnetic Generators, Why can't we use electro static generators to produce Electric power?
coil wound alternators are used everywhere to Generate electric power in all kind generating stations.Though it is demonstrated by vande graf (static generator) it is is not popular in power generation. Let me know the limitations and constraints if any
Most recent answer
Dear David Gibson
That is an excellent in depth answer with mathematical calculation.Your answer definitely gives insight in to the practical limitations of the process.Thank you very much for sharing your knowledge here.
with regards
vasanth
Popular answers (1)
University of Exeter
Some of the answers posted here so far have, I think, rather missed the point; but then, so did the question. The salient point is that the 'opposite' of 'magnetic' is not 'electrostatic' but simply 'electric'. Forget the term 'electrostatic', as that is just misleading. Conventional so-called electrostatic generators are designed to generate high voltages (for 'static electricity') rather than power. If we want to generate power then we simply look to the electric analogue of a magnetic power generator. As I will demonstrate, electric machines are theoretically much better at generating power than magnetic machines; but there are good reasons why they have not caught on.
Consider, first, how a generator works using magnetic induction. A moving magnet (relatively speaking) causes a time-varying magnetic field which generates a voltage in a loop of wire. If you complete the circuit to allow a current to flow, you dissipate energy and this arises because of the energy required to move the magnet against the field induced in the loop. A magnet is manufactured by allowing a material to 'solidify' in an area of high magnetic flux, so that the magnetic domains are 'frozen' in place.
A magnet is, more formally, a ferromagnetic material. Now consider the electric equivalent of a magnet, which is a ferroelectric material, also known as an electret.
A moving electret causes a time-varying electric field which generates a current in a wire/plate. If you complete the circuit to allow a potential to build up, you dissipate energy and this arises because of the energy required to move the electret against the field induced in the wire/plate. An electret is manufactured by allowing a material to 'solidify' in an area of high electric flux, so that the electric domains are 'frozen' in place.
The analogy is interesting, yes?!
Although 'magnetic induction' is the more familiar phenomenon, you will probably have done physics experiments at school with 'electric induction' using an electrophorus, which is a device for transferring electric charge from a reservoir. One of the key points in those physics lessons was to get the pupil to explain why it apparently allows you to transfer charge indefinitely, i.e. where does all the charge come from? That’s a very clever question, and it baffles most students, but it is only like asking why a magnet lasts 'for ever'.
Now, the energy storage density of a magnet is simply ½ B.H which, for a NdFeB magnet with a relative permeability of about 1 and a saturation flux density of (say) 1.3 T [Tesla, equal to As/m^2], gives about 0.8 MJ/m^3. The energy storage density of an electret is, similarly, ½ D.E which, for barium titanate, with a relative permittivity of about 200 and a saturation flux density of 0.16 Vs/m^2 [that unit has no common name, unlike the magnetic analogy], gives about 7 MJ/m^3. The energy storage is not, in itself, directly relevant but it is useful as a figure of merit to describe the material and its potential (excuse the pun) as a generator of power.
So you can see that an electric generator could be ten times better than a magnetic generator for the same volume of active material. So why do we not use electric generators?
Well, of course, they are used. For very small devices (not necessarily "nano" but just "micro" or "milli" devices) they can work better than a magnetic generator because they do not require a bulky copper coil to be added to the assembly.
However, for larger devices there are a number of engineering reasons why they are not used. To achieve the theoretical maximum flux density in an electret requires a high electric field strength and, due to the breakdown voltage of the material, this may limit it to a thickness of perhaps 1mm - so large devices are not practical. There are several other reasons, including those of cost, and of 'historical investment' by which I mean that there is a large body of knowledge on magnetic generators and no real incentive to find an alternative – except for specialised and very small devices. But, if there ever were a need for such generators, one can assume that suitable ferroelectric materials will swiftly find their way into the marketplace.
Hope this helps!
5 Recommendations
All Answers (8)
University of Nottingham
Static electricity is usually high voltage and low current, the product being power.
Current static generators, Wimshurst machine Van de Graf etc. Only produce very low power levels, even in big machines, although voltages can me in the Mega-volt region.
Even lighting strikes, (i.e. a machine comparable to the size of the earth) where current can be in the 100,000 amp region at mega-volts, hence produce large amounts of power are not useful as the strikes are very short, in the nanosecond region, so the energy content is low. (power x time = energy)
Static generating machines are also very inefficient in terms of work in, to power out.
Magnetic generators are only limited by their size, and efficiency can be close to 100% if designed correctly.
Basically, magnetic devices are more power dense than electrostatic ones. Certainly at larger sizes. (i.e. things humans can see)
At nano size, electro-static forces tend to dominate, so static force and generation is sometimes more appropriate.
Best Regards
Paul
2 Recommendations
Dr. Ambedkar Institute of Technology
You are absolutely correct sir. But in exponentially growing demand for electricity , can we adopt small scale static generators to illuminate our houses fitted with LED bulbs?This is my concern sir.
with Regards
vasanth
University of Nottingham
There are inventions that do what you ask for example:
Also there is much interest in energy harvesting for low power devices:
Where mains or battery energy sources are not available.
A question you ask yourself is "why use a large and inefficient static generator, when a magnetically based one is smaller, cheaper and requires less input energy?"
Please mark up answers you find useful.
Best Regards
Paul
2 Recommendations
Dr. Ambedkar Institute of Technology
well, i have gone through the links you suggested,and they are really useful to a greater extent.Thank you sir
with best regards
vasanth
1 Recommendation
University of Tasmania
Dear G.s. Vasantha Kumar,
According to your application, LEDs like small power consuming applications could minimize the large amount of power it still keep arising the question about applicability and public acceptances. Power industries are still getting stuck on supporting the bulk power consumer. Thats why they are concentrating on SG or large power generation sources. In your case, It must has huge prospects to society but huge a big branch of further study to meet the demand and production of this.
Université de Technologie de Belfort-Montbéliard
In nanotechnology, there are electrostatic generator !
It is limited as a generator in the power stations seen their performance
University of Exeter
Some of the answers posted here so far have, I think, rather missed the point; but then, so did the question. The salient point is that the 'opposite' of 'magnetic' is not 'electrostatic' but simply 'electric'. Forget the term 'electrostatic', as that is just misleading. Conventional so-called electrostatic generators are designed to generate high voltages (for 'static electricity') rather than power. If we want to generate power then we simply look to the electric analogue of a magnetic power generator. As I will demonstrate, electric machines are theoretically much better at generating power than magnetic machines; but there are good reasons why they have not caught on.
Consider, first, how a generator works using magnetic induction. A moving magnet (relatively speaking) causes a time-varying magnetic field which generates a voltage in a loop of wire. If you complete the circuit to allow a current to flow, you dissipate energy and this arises because of the energy required to move the magnet against the field induced in the loop. A magnet is manufactured by allowing a material to 'solidify' in an area of high magnetic flux, so that the magnetic domains are 'frozen' in place.
A magnet is, more formally, a ferromagnetic material. Now consider the electric equivalent of a magnet, which is a ferroelectric material, also known as an electret.
A moving electret causes a time-varying electric field which generates a current in a wire/plate. If you complete the circuit to allow a potential to build up, you dissipate energy and this arises because of the energy required to move the electret against the field induced in the wire/plate. An electret is manufactured by allowing a material to 'solidify' in an area of high electric flux, so that the electric domains are 'frozen' in place.
The analogy is interesting, yes?!
Although 'magnetic induction' is the more familiar phenomenon, you will probably have done physics experiments at school with 'electric induction' using an electrophorus, which is a device for transferring electric charge from a reservoir. One of the key points in those physics lessons was to get the pupil to explain why it apparently allows you to transfer charge indefinitely, i.e. where does all the charge come from? That’s a very clever question, and it baffles most students, but it is only like asking why a magnet lasts 'for ever'.
Now, the energy storage density of a magnet is simply ½ B.H which, for a NdFeB magnet with a relative permeability of about 1 and a saturation flux density of (say) 1.3 T [Tesla, equal to As/m^2], gives about 0.8 MJ/m^3. The energy storage density of an electret is, similarly, ½ D.E which, for barium titanate, with a relative permittivity of about 200 and a saturation flux density of 0.16 Vs/m^2 [that unit has no common name, unlike the magnetic analogy], gives about 7 MJ/m^3. The energy storage is not, in itself, directly relevant but it is useful as a figure of merit to describe the material and its potential (excuse the pun) as a generator of power.
So you can see that an electric generator could be ten times better than a magnetic generator for the same volume of active material. So why do we not use electric generators?
Well, of course, they are used. For very small devices (not necessarily "nano" but just "micro" or "milli" devices) they can work better than a magnetic generator because they do not require a bulky copper coil to be added to the assembly.
However, for larger devices there are a number of engineering reasons why they are not used. To achieve the theoretical maximum flux density in an electret requires a high electric field strength and, due to the breakdown voltage of the material, this may limit it to a thickness of perhaps 1mm - so large devices are not practical. There are several other reasons, including those of cost, and of 'historical investment' by which I mean that there is a large body of knowledge on magnetic generators and no real incentive to find an alternative – except for specialised and very small devices. But, if there ever were a need for such generators, one can assume that suitable ferroelectric materials will swiftly find their way into the marketplace.
Hope this helps!
5 Recommendations
Dr. Ambedkar Institute of Technology
Dear David Gibson
That is an excellent in depth answer with mathematical calculation.Your answer definitely gives insight in to the practical limitations of the process.Thank you very much for sharing your knowledge here.
with regards
vasanth
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