Category Archives: electric vehicles

what are capacitors?

(this is reblogged from the new ussr illustrated, first published August 29 2017)

the shapes and sizes of capacitors – a screenshot taken from the youtube vid – What are Capacitors? – Electronics Basics 11

Jacinta: We’re embarking on the clearly impossible task of learning about every aspect of clean (and sometimes dirty because nothing’s 100% clean or efficient) technology – batteries, photovoltaics, turbines, kilo/megawatt-hours, glass electrolytes, powerwalls, inverters, regen, generators, airfoils, planetary gear sets, step-up transformers, nacelles AND capacitors.

Canto; Enough to last us a lifetime at our slow pace. So what, in the name of green fundamentalism, is a capacitor?

Jacinta: Well I’ve checked this out with Madam Youtube…

Canto: Professor Google’s co-dependent…

Jacinta: And in one sense it’s simple, or at least it sounds simple. Capacitors store electric charge, and the capacitance of a capacitor relates to how much charge it can hold.

Canto: So how does it do that, and what’s the purpose of storing electric charge?

Jacinta: Okay now you’re complicating matters, but basic to all capacitors are two separated pieces of conducting material, usually metal. Connected to a battery, they store charge…

Canto: Which is a kind of potential energy, right?

Jacinta: Umm, I think so. So take your battery with its positive and negative terminals. Attach one of the bits of conducting material (metal) to the positive terminal and you’ll get a flow of negatively-charged electrons to that terminal, because of ye olde law of attraction. This somehow means that electrons are repelled from the negative terminal  (which we’ve hooked up to the other bit of metal in the capacitor). So because the first strip of metal has lost electrons it has become positively charged, and the other bit of metal, having gained electrons, has an equal and opposite charge. So each piece of metal has the same magnitude of charge, measured in coulombs. This is regardless of the size and shape of the different metal bits.

Canto: But this process reaches a limit, though, yes? A kind of saturation point…

Jacinta: Well there comes a point where, yes, the accumulated charge just sits there. This is because there comes a kind of point of equilibrium between the positive battery terminal and the now positively charged strip of metal. The electrons are now caught between the attractive positive terminal and the positive strip.

Canto: Torn between two lovers, I know that foolish feeling.

Jacinta: So now if you remove the battery, so breaking the circuit, that accumulated charge will continue to sit there, because there’s nowhere to go.

Canto: And of course that accumulated or stored charge, or capacitance, is different for different capacitors.

Jacinta: And here’s where it gets really complicated, like you know, maths and formulae and equations. C = Q/V, capacitance equals the charge stored by the capacitor over the voltage across the capacitor. That charge (Q), in coulombs, is measured on one side of the capacitor, because the charges actually cancel each other out if you measure both sides, making a net charge of zero. So far, so uncomplicated, but try and get the following. When a capacitor stores charge it will create a voltage, which is essentially a difference in electric potential between the two metal strips. Now apparently (and you’ll have to take my word for this) electric potential is high near positive charges and low near negative charges. So if you bring these two differently charged strips into close proximity, that’s when you get a difference in electric potential – a voltage. If you allow a battery to fully charge up a capacitor, then the voltage across it (between the two strips) will be the same as the voltage in the battery. The capacitance, Q/V, coulombs per volt, is measured in farads, after Micky Faraday, the 19th century electrical wizz. I’m quoting this more or less verbatim from the Khan Academy video on capacitors, and I’m almost finished, but here comes the toughest bit, maths! Say you have a capacitor with a capacitance of 3 farads, and it’s connected to a nine volt battery, the charge stored will be 27 coulombs (3 = 27/9). 3 farads equals 27 coulombs of charge divided by nine volts, or 27 coulombs of charge is 3 farads times 9 volts. Or, if a 2 farad capacitor stores a charge of 6 coulombs, then the voltage across the capacitor will be 3 volts.

Canto: Actually, that’s not so difficult to follow, the maths is the easiest part for me… it’s more the concepts that get me, the very fact that matter has these electrical properties…

Jacinta: Okay here’s the last point made, more or less verbatim, on the Khan Academy video, something worth pondering:

You might think that as more charge gets stored on a capacitor, the capacitance must go up, but the value of the capacitance stays the same because as the charge increases, the voltage across that capacitor increases, which causes the ratio to stay the same. The only way to change the capacitance of a capacitor is to alter the physical characteristics of that capacitor (like making the pieces of metal bigger, or changing the distance between them).

Canto: Okay so to give an example, a capacitor might be connected to an 8 volt battery, but its capacitance is, say, 3 farads. It will be fully charged at 24 coulombs over 8 volts. The charge increases with the voltage, which has a maximum of 8. The ratio remains the same. Yet somehow I still don’t get it. So I’m going to have a look at another video to see if it helps. It uses the example of two metal plates. They start out as electrically neutral. You can’t force extra negativity, in the form of electrons, into one of these plates, because like charges repel, and they’ll be forced out again. But, according to the video, if you place another plate near the first, ‘as electrons accumulate in the first metal plate, they will repel the electrons in the second metal plate’, to which I want to respond, ‘but electrons aren’t accumulating, they’re being repelled’. But let’s just go with the electron flow. So the second metal plate becomes depleted of electrons and is positively charged. This means that it will attract the negatively charged first metal plate. According to the video, this makes it possible for the first plate to have more negative than positive particles, which I think has something to do with the fact that the electrons can’t jump from the first plate to the second, to create an equilibrium.

Jacinta: They’re kind of attracted by absence. That’s what they must mean by electric potential. It’s very romantic, really. But what you’ve failed to notice, is that a force is being continually applied, to counteract the repulsion of electrons from the first plate. If the force no longer applies then, yes, you won’t get that net negative charge in the first plate, and the consequent equal and opposite charge in the second. My question, though, is how can the capacitance increase by bringing the plates closer together? I can see how it can be changed by the size of the conducting material – more electrons, more electric potential. I suppose reducing the distance will increase the repulsive force…

Canto: Yes, let’s assume so. Any, a capacitor, which stores far less charge than a similarly-dimensioned battery can be used, I think, to briefly maintain power to, say, a LED bulb when it is disconnected from the battery. The capacitor, connected to the bulb will discharge its energy ‘across’ the bulb until it achieves equilibrium, which happens quite quickly, and the bulb will fade out. If the capacitor is connected to a number of batteries to achieve a higher voltage, the fully charged capacitor will take longer to discharge, keeping the light on for longer. If the metal plates are larger, the capacitor will take longer to charge up, and longer to discharge across the LED bulb. Finally, our second video (from a series of physics videos made by Eugene Khutoryansky) shows that you can place a piece of ‘special material’ between the two plates. This material contains molecules that change their orientation according to the charges on the plates. They exert a force which attracts more electrons to the negative plate, and repel them from the positive plate, which has the same effect as increasing the area of the plates – more charge for the same applied voltage.

Jacinta: An increase in capacitance.

Canto: Yes, and as you’ve surmised, bringing the two plates closer together increases the capacitance by attracting more electrons to the negatively charged plate and repelling them from the positively charged one – again, more charge for the same voltage.

Jacinta: So you can increase capacitance with a combo of the three – increased size, closer proximity, and that ‘special material’. Now, one advantage of capacitors over batteries is that they can charge up and discharge very quickly. Another is that they can endure many charge-discharge cycles. However they’re much less energy dense than batteries, and can only store a fraction of the energy of a same-sized battery. So the two energy sources have different uses.

Canto: Mmmm, and we’ll devote the next post to the uses to which capacitors can be put in electronics, and EVs and such.

 

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electric vehicles in Australia, a sad indictment

(this is reblogged from the new ussr illustrated, first published August 15 2017)

Toyota Prius

I must say, as a lay person with very little previous understanding of how batteries, photovoltaics or even electricity works, I’m finding the ‘Fully Charged’ and other online videos quite addictive, if incomprehensible in parts, though one thing that’s easy enough to comprehend is that transitional, disruptive technologies that dispense with fossil fuels are being taken up worldwide at an accelerating rate, and that Australia is falling way behind in this, especially at a governmental level, with South Australia being something of an exception. Of course the variation everywhere is enormous – for example, currently, 42% of all new cars sold today in Norway are fully electric – not just hybrids. This compares to about 2% in Britain, according to Fully Charged, and I’d suspect that the percentage is even lower in Oz.

There’s so much to find out about and write about in this field it’s hard to know where to start, so I’m going to limit myself in this post to electric cars and the situation in Australia.

First, as very much a lower middle class individual I want to know about cost, both upfront and ongoing. Now as you may be aware, Australia has basically given up on making its own cars, but we do have some imports worth considering, though we don’t get subsidies for buying them as they do in many other countries, nor do we have that much in the way of supportive infrastructure. Cars range in price from the Tesla Model X SUV, starting from $165,000 (forget it, I hate SUVs anyway), down to the Toyota Prius C and the Honda Jazz, both hybrids, starting at around $23,000. There’s also a ludicrously expensive BMW plug-in hybrid available, as well as the Nissan Leaf, the biggest selling electric car worldwide by a massive margin according to Fully Charged, but probably permanently outside of my price range at $51,000 or so.

I could only afford a bottom of the range hybrid vehicle, so how do hybrids work, and can you run your hybrid mostly on electricity? It seems that for this I would want a (more expensive) plug-in hybrid, as this passage from the Union of Concerned Scientists (USA) points out:

The most advanced hybrids have larger batteries and can recharge their batteries from an outlet, allowing them to drive extended distances on electricity before switching to [petrol] or diesel. Known as “plug-in hybrids,” these cars can offer much-improved environmental performance and increased fuel savings by substituting grid electricity for [petrol].

I could go on about the plug-ins but there’s not much point because there aren’t any available here within my price range. Really, only the Prius, the Honda Jazz and a Toyota Camry Hybrid (just discovered) are possibilities for me. Looking at reviews of the Prius, I find a number of people think it’s ugly but I don’t see it, and I’ve always considered myself a person of taste and discernment, like everyone else. They do tend to agree that it’s very fuel efficient, though lacking in oomph. Fuck oomph, I say. I’m the sort who drives cars reluctantly, and prefers a nice gentle cycle around the suburbs. Extremely fuel efficient, breezy and cheap. I’m indifferent to racing cars and all that shite.

Nissan Leaf

I note that the Prius  has regenerative braking – what the Fully Charged folks call ‘regen’. In fact this is a feature of all EVs and hybrids. I have no idea wtf it is, so I’ll explore it here. The Union of Concerned Scientists again:

Regenerative braking converts some of the energy lost during braking into usable electricity, stored in the batteries.

Regenerative braking” is another fuel-saving feature. Conventional cars rely entirely on friction brakes to slow down, dissipating the vehicle’s kinetic energy as heat. Regenerative braking allows some of that energy to be captured, turned into electricity, and stored in the batteries. This stored electricity can later be used to run the motor and accelerate the vehicle.

Of course, this doesn’t tell us how the energy is captured and stored, but more of that later. Regenerative braking doesn’t bring the car to a stop by itself, or lock the wheels, so it must be used in conjunction with frictional braking.  This requires drivers to be aware of both braking systems and how they’re combined – sometimes problematic in certain scenarios.

The V useful site How Stuff Works has a full-on post on regen, which I’ll inadequately summarise here. Regen (in cars) is actually celebrating its fiftieth birthday this year, having been first introduced in the Amitron, a car produced by American Motors in 1967. It never went into full-scale production. In conventional braking, the brake pads apply pressure to the brake rotors to the slow the vehicle down. That expends a lot of energy (imagine a large vehicle moving at high speed), not only between the pads and the rotor, but between the wheels and the road. However, regen is a different system altogether. When you hit the brake pedal of an EV (with hand or foot), this system puts the electric motor into reverse, slowing the wheels. By running backwards the motor acts somehow as a generator of electricity, which is then fed into the EV batteries. Here’s how HSW puts it:

One of the more interesting properties of an electric motor is that, when it’s run in one direction, it converts electrical energy into mechanical energy that can be used to perform work (such as turning the wheels of a car), but when the motor is run in the opposite direction, a properly designed motor becomes an electric generator, converting mechanical energy into electrical energy.

I still don’t get it. Anyway, apparently this type of braking system works best in city conditions where you’re stopping and going all the time. The whole system requires complex electronic circuitry which decides when to switch to reverse, and which of the two braking systems to use at any particular time. The best system does this automatically. In a review of a Smart Electric Drive car (I don’t know what that means – is ‘Smart’ a brand name? – is an electric drive different from an electric car??) on Fully Charged, the test driver described its radar-based regen, which connects with the GPS to anticipate, say, a long downhill part of the journey, and in consequence to adjust the regen for maximum efficiency. Ultimately, all this will be handled effectively in fully autonomous vehicles. Can’t wait to borrow one!

Smart Electric Drive, a cute two-seater

I’m still learning all this geeky stuff – never thought I’d be spending an arvo watching cars being test driven and  reviewed.  But these are EVs – don’t I sound the expert – and so the new technologies and their implications for the environment and our future make them much more interesting than the noise and gas-guzzling stink and the macho idiocy I’ve always associated with the infernal combustion engine.

What I have learned, apart from the importance of battery size (in kwh), people’s obsession with range and charge speed, and a little about charging devices, is that there’s real movement in Europe and Britain towards EVs, not to mention storage technology and microgrids and other clean energy developments, which makes me all the more frustrated to live in a country, so naturally endowed to take advantage of clean energy, whose federal government is asleep at the wheel on these matters, when it’s not being defensively scornful about all things renewable. Hopefully I’ll be able to report on positive local initiatives in this area in future, in spite of government inertia.