Wednesday, July 8, 2015

As I Stare at My Smoke-Clouded Sky, a Thought or Two About Tipping Points

Arctic sea ice is vanishing and quite rapidly to boot. The ice cover that once reflected solar radiation, heat, back into space has walked off the job. No brilliant white ice means dark green ocean that is a heat sink. That warming Arctic ocean warms the atmosphere that causes the tundra to dry out and catch fire. As the tundra burns it creates black soot that winds up turning the Greenland Ice Sheet a dirty colour and that accelerates the melting of the ice sheet and sea level rise.

The thawing, burning tundra also exposes the permafrost underneath that, as it thaws, releases massive amounts of once safely sequestered, formerly frozen methane, a very powerful greenhouse gas. As the Arctic ocean warms it also triggers the thawing of ancient, frozen seabed methane clathrates - methane ice if you like - that bubbles to the surface and then onward to the atmosphere.

From rampaging wildfires to tundra fires to ice caps covered in black soot to the release of ancient stores of methane from the permafrost and seabed clathrates these are all the feedback mechanisms your mother those scientists warned you about. They're happening now, not forty years from now, not even twenty years from now.

Have we passed the point of no return. The good news is that's a conversation we're not really having right now. We're still proceeding - although not very quickly and not very well - with talks that assume we're not there yet and can, if we just try hard enough dammit, avoid the worst - maybe.

Today we're at just 0.8 degrees Celsius above pre-industrial levels. We're not at the 1.5C mark yet because that persistent atmospheric greenhouse gas needs time to work its magic. It will and as it does our children and grandchildren will experience the changes in creates.

There are two things that we must understand, and that includes you.

First off. That 1.5C is something we've already bequeathed our kids and theirs. What we need to realize is that emissions are cumulative which means our greenhouse gas emissions from today onward add to that 1.5C. Every tonne of CO2 we emit goes on top of that 1.5C pile. We're experiencing the impacts of barely 0.8C of warming (and it's a real bitch). As today's warming keeps getting hotter, those who follow us will endure a variety of impacts that are even greater, more dangerous, and demanding of new adaptation responses.

Second. These numbers don't include the natural feedback mechanisms we already seem to have triggered. The greenhouse gas emissions they create - CO2 from forest fires, methane released from the permafrost and seabed clathrates - also go atop that 1.5C we have already locked in.

Saturday, July 4, 2015

Evolving Tesla Electric Drivetrains

Electric drivetrains are much simpler, much lighter and much less costly than internal combustion drivetrains of similar power. Tesla's drive units consist of just an inverter, induction motor and single-speed reduction gearbox with differential. The Model S drive unit, for instance, is so compact that it fits entirely within the rear suspension assembly.

In mechanical terms, Tesla's drive unit is what one would get if the starter motor of an ICE drivetrain were mated with the differential, and everything else - the engine, transmission, drive shaft - were thrown away.

The complicated part is the inverter which changes the DC voltage of the battery into AC voltage of varying amplitude and frequency that operates the motor. Much like a mechanical transmission adjusts the ratio between speed and torque, the inverter adjusts the ratio of voltage to current. Low voltage and high current produce high motor torque at low speeds while higher voltage and lower current result in lower motor torque and higher speeds.
The cost, size, performance and efficiency of the inverter depends largely on the power switching transistors, and these are improving at a semiconductor pace. New silicon carbide and gallium nitride power transistors switch faster and operate at higher temperatures than silicon transistors. Faster switching reduces the size of filtering components and supports higher motor speeds, that in turn allow smaller, lighter motors. High temperature operation allows simpler mechanical design for cooling within the inverter. The result is that electric drivetrains are getting smaller, lighter and cheaper, quickly.

An indication of the weight and cost of electric drive systems can be found in this DOE presentation. Note (slide 5, bottom) that a 2010 development program at GM achieved the projected 2015 weight target, suggesting that the technology was even then 5 years ahead of DOE's optimistic expectations. Our cost and weight estimates for Model 3 drivetrains is based on this DOE data, extrapolated forward to 2017. Cost estimates for the high performance drivetrain versions used in the 366PD car include a substantial premium for "high spec" electronic components, high performance magnetic materials in the motor and presumably low production rates.