Tue May 1 13:25:01 EDT 2012

come on up, get your extremely impractical ideas here

So I've got a dumb idea, and in the hopes of getting it out of my head, I'm going to write a blog post about it.

In 2006, Daniel Rutter wrote about the "genset" option for the t/zero vaporware electric car. The basic idea was this: A plug-in hybrid, but with the gasoline engine on a trailer. If you're taking a short trip, then you don't take the genset with you, because you don't need the weight. If you're going on a long trip, then you hook up the trailer, and enjoy the long range and fast refueling that a gasoline engine gives you. (Note that this scheme means you'll have a fairly large genset sitting around at home, which spends most of its time not being used.) This is part one.

Part two: Heat engines have a surprisingly low maximum theoretical efficiency. The endoreversible heat engine efficiency equation is thus: (units are absolute degrees, Kelvin or Rankine)

Endoreversible heat engine effciency equation

The problem is that the Earth just isn't cold enough. Room temperature is 300 Kelvin! You're not going to want Th to go much above 830K, or else you'll be producing nitrogen oxides in the exhaust. This gives a final endoreversible efficiency of 0.40. That's the maximum thermodynamic efficiency, and there ain't no way to beat thermodynamics.

You can bend the rules a little, though. It's impossible to extract any more mechanical work from the waste heat, but it's still heat, you can use it for things that don't need low-entropy heat sources. Like, say keeping a house warm, or heating water.

This is called Cogeneration, and depending on how you do it, you can get up to 90% total thermal efficiency. A popular home cogeneration system is the MicroCHP furnace, a natural gas turbine that provides hot water and forced-air heating from its waste heat outflow. It can also run as a backup generator during power outages, in which case it vents waste heat outside.

The synthesis of these ideas is obvious: Make a plug-in hybrid whose engine is a cogenerating furnace built around a multi-fuel gas turbine.

A brilliant idea! There are some problems.

1.) MicroCHP turbines are designed to run on natural gas only. The "multi-fuel" version would be a second generation product. The first gen would have problems: the natural gas fueling infrastructure in the United States is a lot more sparse than the liquid-fuels infrastructure.

2.) The turbine is going to want a lot of cooling. Does a trailer get enough airflow?

3.) How well does the turbine tolerate road vibration?

4.) There's a user design problem: for a cogenerating furnace to work well, you want it at the center of your house, where all the waste heat goes to keeping you warm. This essentially means that every user of this car has to have an insulated, attached garage, or else they'll be losing some efficiency.

5.) There's a user interface problem: You can drive out of the garage with the trailer attached easily enough, but you'll then have to back into the garage, trailer first, and then reattach all the connections. Manually. Every time you want to use it.

6.) There's a use-case problem: Every time you drive away with the turbine, you drive away with the house's sole source of heat and hot water. Fine with a single-occupancy home, might be a problem for a family.

7.) It is certain that in the near future, cripplingly heavy taxes will be applied to fossil-fueled vehicles. Will this count as one? Maybe?

8.) This idea isn't terribly original. There's a lot of prior art here. Getting a patent will be an uphill battle.

So, there. Eight reasons why I'm talking about this idea in a blog post, rather than a room of serious men in suits.

Posted by Samuel Bierwagen | Permanent link | File under: important, Engineering