Who’d have thought running an electricity grid and running a railway would have so much in common?

I was at a transport industry networking event the other night, chatting to Peter White of the University of Westminster. He was outlining strategies for reducing emissions from rail travel, and I was explaining the potential role of electric vehicles in grid balancing, and we realised that both problems were surprisingly alike.

The problem with running a railway is that you have to invest in loads of rolling stock to meet rush hour demand, and then it sits there idle most of the day. Here’s the total number of passengers in and out of London stations, hour by hour, on average (DfT stats):

That’s a lot of unused capacity outside the rush hour. If we could just spread out that morning rush, by employers allowing (or even encouraging) employees to have staggered start and finish times, the railways would be a whole lot cheaper to run – and more pleasant to travel on.

Turns out, that’s just what happened during the Olympics. As well as encouraging businesses to promote home-working during the Olympics, the ODA and Transport for London asked them to stagger shifts.

One of the most striking successes was with City trading firms. They’d always maintained there was no way they could allow flexible working, because of trading times, but with the Olympic spirit behind them they tried out an ‘early’ and ‘late’ shift. Turns out it worked beautifully, because the early traders were clued up on the Eastern markets, and the late traders got to see how things played in the US. How many other types of business could benefit from this approach?

So how’s all this like running the electricity grid? Let’s say you’re National Grid, and you’re responsible for supplying us all with electricity. Your biggest problem is variable demand – everyone wants power at 5.30pm, and nobody wants it at 4am. Here’s the graph of demand on the UK grid through one day (1st January 2012 as it happens, data from National Grid):

So you have base-load generators, like coal and nuclear, which are cheap but hard to turn on and off, and you have peak-load generators (usually gas), which are only turned on when you need them, but which are therefore more expensive.

Just as with the railways, if we could spread demand more evenly, or store electricity, we could generate our power more efficiently. Grid operators call it ‘filling the bath-tub’. This is one reason why the electricity companies are so interested in electric cars – they all come with an energy storage device (i.e. the battery).

If we all went and bought electric cars tomorrow, and charged them overnight, it would allow for a much more even load on the grid. This would allow the energy companies to run more base load generation, and therefore provide cheaper electricity.  Of course, the opposite is also true – if everyone put their car on charge when they got home from work, we’d need even more peak capacity, making power more expensive.

Incidentally, price and greenhouse gas emissions are pretty closely linked here. If you buy an electric car in the UK right now, and charge it from midnight to 7am, you’ll be saving GHG compared to an equivalent petrol or diesel car. However, if you put your car on charge anytime from around midday to 10pm, you’ll be emitting around twice as much GHG as charging off-peak, and more than a petrol or diesel car.

Adding renewables like wind or solar to the mix is even more of a headache, because you don’t know when you’ll get the power. That’s when electric cars really come in handy – but more of that in a later post…

A bluffer’s guide to the difference between a ‘parallel’ hybrid and a ‘series’ hybrid. (And why the parallel hybrid came first.)

So, we’ve had the Prius around for a while, and that’s a hybrid – right? Then the Ampera/Volt comes along (with billboards everywhere you look), and it’s an electric car – or is it? Because it’s got a petrol engine too.

Well, when they’ve got their petrol engines running, the Prius and the Ampera represent two different ways of making a ‘hybrid’ – parallel and series, respectively. I’ve explained which technology I think is superior to many people at parties, and I still seem to have friends, so hopefully you’ll find this interesting too.

A ‘parallel’ hybrid has two complete drive-trains connected to the wheels, which operate in parallel, hence the name. Basically, you start with a regular petrol car (or a diesel, more of that in a later post) and you bolt on an electric motor and a battery that can also drive the wheels. The electric motor ‘assists’ the petrol engine when needed, allowing a car like a Prius to have a smaller petrol engine, and use less fuel, for a given level of performance.

The electrical energy in most parallel hybrids thus far has come from ‘regenerative braking’. Essentially, when you apply the brake, the wheels drive the electric motor in reverse, generating power which is stored in the battery. The new ‘plug-in’ Prius has a bigger battery, which you can charge up from a wall socket. This means that you can get a little over 10 miles on the electric motor alone, but most of the time that electrical power is still ‘blended’ with power from the petrol engine.

OK, sounds good right? Well yes, until you consider that a petrol engine is incredibly inefficient. At its most efficient, a ‘sweet spot’ of revs and load, it converts about 25-30% of the energy in the fuel into forward motion – the rest is mostly lost as heat out of the exhaust or the engine block. But that’s when you’re cruising at a steady, optimum speed. The rest of the time, when you’re accelerating, or decelerating, or in the wrong gear, the efficiency drops much lower. This means that the engine in a car has to be oversized to cope with all this variation.

Suppose you only needed the petrol engine to deliver a constant power, at constant revs? Like, say, the engine in an electrical generator. Hmmnn… and suppose you only had to have one drive train connected to the wheels, that would save a load of weight, right? Bingo! – you’re thinking of a ‘series’ hybrid. Electric motors drive the wheels, and they’re 85-90% efficient (very little heat loss), and that efficiency doesn’t vary much at different speeds. Then you charge up the battery using a small engine acting as a generator.

In theory, the series hybrid should be more efficient than a parallel hybrid, because the petrol engine only has to run at its optimum speed and load. It only has to deliver the average power requirement to the battery, which buffers this power, supplying more or less energy to the motors driving the wheels as the car accelerates, cruises etc. This is essentially what an Ampera does, although like a plug-in Prius it has a bigger battery, that you can charge up from the wall, so you can run the car on ‘pure electric’ mode for about 30 miles before the generator kicks in.

So why did we get a parallel hybrid, like the Prius, years before a series hybrid, like the Ampera? Well, it’s a simple question of what happens if it goes wrong. A parallel hybrid is in essence just a regular car with the electric motor helping out – if the electric drive train fails, you can still drive the car. But with a series hybrid, if the battery, the motor, or any of the complex systems managing them were to fail, you’re stuck at the side of the road waiting for a tow truck (or an electrician!). It’s only now, with ‘pure’ electric cars like the Leaf on the market, that big manufacturers have got enough confidence in the technology.

In my opinion, the Ampera, or cars like it, will lead the mass adoption of electric cars, for several reasons. But that’s for another post…