Tuesday, January 19, 2010

Practicalities Concerning The Copenhagen Wheel

Good morning!

For lack of a super-stimulating post today, I thought I'd immerse myself a bit in the hype surrounding the Copenhagen Wheel before the subject becomes rusty and unwrap this pretty looking present.

So I'm taking it for granted that you folks will have some prior knowledge of this new hybrid e-bike introduced by MIT's Senseable City Lab.

Well, the folks are not actually marketing a bike technically.

They actually demoed a hub-motored wheel during the Climate Summit in December that you can shift around and adapt to different bikes for "city riding", the idea being that it stores energy through regenerative rear wheel braking action and gives it back to the rider when they need a 'boost'. It is sort of a leaner, maybe meaner version of the gas powered Revopower idea that never took shape. Remember that?

An attractive proposition is that this wheel can be monitored by a smart phone via blue tooth and all the electro-mechanical aspects are packaged into one assembly - the wheel itself. And then it measures environmental variables like noise, pollution and so on and throws all that data into your phone for your city riding pleasure.

It is by all means an academic project, funded by the city of Copenhagen, Ducati Energia and the Italian Ministry for the Environment.

Now when I hear of projects like these, I switch off the ads and start looking into specifics. I thought for an institution like MIT, you would expect to see some numbers posted on their website at the least, or a whitepaper indicating weight, wattage requirements, cost, efficiency etc. I certainly haven't found much other than a Java based applet you can sort of play around with.

So here are my questions & concerns for the boys and girls at MIT, which I think I sounded off a month back to Bicycle Design.

What is the energy content of the battery (Wh/kg) and energy use per distance (Wh/km) of this product? Multiplying both will give us an estimate of distance/kg of battery weight and total energy capacity per charge. Knowing cost of electricity, someone can easily calculate the true cost of riding the bike or a bike equipped with the wheel and then compare it with with a bike not equipped with it.

Then comes the question of braking energy recovery. How many stops would a cyclist have to make to recover at least 10% of the total battery capacity back and by how much will their range be extended?

Let me give you a perspective. A 180 lb man with a 20 lb bike traveling at 15 mph has 2026 joules or 0.56 Wh of kinetic energy to shed, in order to come to a stop. Note : These are back of the envelope calculations I just roughly did.

So, with what efficiency will the recovery system be able to capture some of that? 70%? 80%? Let's assume 75% which is a reasonable number. With that, I can get back 0.42 Wh or 1516 joules back, but I can only put this energy back into the battery at a certain charge rate which really depends on the battery's specs.

So for illustration, if battery's max charge rate is 100 joules/sec, 1516 joules will take 15 seconds or more to put back in.

Now realistically, no one takes that long to stop, even though the US CPSC recommends that brakes should stop a cyclist in 15 seconds from 15 to 0 mph.

So realistically, let's say people take about 3-4 seconds every time to stop. Using the 100 J/s charge rate, cyclist can only put in 400 Joules or 0.11 Wh back every stop. So the question again becomes, how many times does a cyclist have to stop to put back 10% of kinetic energy of braking, at 75% efficiency to get the extra amount of miles, given the limitations of the battery?

If its a reasonably good amount, it may make some sense in a crowded urban area like Copenhagen, otherwise not really considering buying costs and life cycle costs. Again, only the average numbers over long riding in the city will give you any solid perspective on the practicality.

In short, without specifics like what I'm calling for, no one should start rooting for this product as a game changer just yet, for that is just silly in my book - a case of inflated yet unwarranted attention that a lot of blog fodder command these days.

Sometime back, I noted that the city of Copenhagen is en route (forgive the pun) to "biking superhighways". With that kind of a future for this city, I expect some of the stopping usually encountered in tight city riding conditions to reduce. So then the question is, will the Copenhagen Wheel even be deemed necessary then?

As usual, I ask too many bothersome but nevertheless important questions. The answers are reluctant to arrive. Or it just might. Right MIT? :)

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