But I do believe in the idea of the self-driving electric taxi as the best answer for our future urban transportation. So how do you make it happen?
There’s a big problem with this vision. Electric cars today have perhaps 100 to 150 miles of range, which means 3 to 6 hours of operation, depending on the speeds you go. You can make more range (like a Telsa S) but only by adding a lot of weight and cost. But an effective taxi is on shift all day, or at least all the waking hours, and could easily operate 8 to 10 hours per day. While any taxi will have downtime during the day (particularly at off-peak hours) the recharge of the battery takes so long it’s hard to do during the day. Ideally you want to charge at night, when power is cheap. So let’s consider the options.
Large battery pack
You could make a vehicle with enough battery for a full day’s work, and charge it at night. This is very expensive today, and also takes a lot of room and weight in the vehicle, reducing its efficiency. You also need taxis at night so either way you have to have some taxis that work at night and recharge in the day, but not as many.
While battery swap did not pan out for Better Place, it actually makes much more sense for a robotic taxi fleet. You just need a few swap stations in the city. It doesn’t bother the robots if they take a while for a swap (mainly it bothers you because you need more stations.) And while humans would get very angry if they came to the swap station and saw they were 4th in line at 5 minutes/swap, robots would just schedule their swaps and get in and get out.
That’s all good, and it solves a few other problems. Taxis will be putting on lots of miles every day, and they will probably wear out their battery quickly. If the rest of the vehicle has not worn out, swap becomes ideal — replace the vehicle’s components when you need to, including the most expensive part, the battery. It also makes it easier to charge all batteries only at night, on cheap baseload power.
Swap also allows the batteries to be only used in the “easy” part of their duty cycle (from 80% to 20%) without much hassle. You only get 60% of the range, but you don’t care a lot, other than in having to buy more battery packs. You just do the math on what’s cheaper.
A working supercharger that can recharge a vehicle in an hour solves the problem as well. Robotic taxis can always find a spare hour without much loss of efficiency. (Indeed with none as they will age by active mile or hour.) The big problem is that supercharging generally is felt to stress the batteries and reduce the lifetime of the packs. Certainly running a car on full cycle every day and supercharging it is not going to produce a happy battery.
A robocar supercharging station could do a few extra things, though. For example, you could hook the car up to a special cooling system to pump externally chilled coolant through the batteries, as heat is the big killer in the supercharge. You might even find a way to put some pressure in to keep the cases from expanding that much, as this is a big stressor when charging.
Supercharging probably has to be done in the day, with more expensive power. One charge for the morning peak and another for the evening. Some speculate it’s worth using your inventory of old battery packs to store power during the night to release in the day. Solar can also help during the day — on sunny days, at least.
While automated connection is good, you really would not have many supercharging centers, due to their high power needs, so they could have human staff to do the work.
Both the supercharging and battery swap stations do not need to be located all that conveniently for humans. Instead, you can put them near power substations where megawatts can be purchased.
More vehicles and ordinary L2 charging
If the batteries are more expensive than the vehicles, then perhaps just having more vehicles to house all your battery packs is the answer. Then you have vehicles spending their time idle, and charging at ordinary level 2 (6kw) rates. Full level 2 can add about 20 miles of range to a car like a Leaf in an hour. Depending on the usage patterns that might not be too bad. Of course it’s daytime power again, which is expensive. Urban taxis won’t go more than about 25mph on average if they are lucky, often less, particularly at rush hour. Suburban will go a bit faster. You need stations that allow a robot to recharge, which could mean inductive, or human-staffed, or eventual robotic plug-in systems. Don’t laugh at the idea of human-staffed. The robot will not be in a super rush, so stations near retail shops or existing gas stations would work fine as long as somebody can come out and tend to the robot on connect and disconnect within 5 minutes.
It may seem like more vehicles is more expensive, but that’s not necessarily true. It depends on how and why the vehicles wear out. Ideally you design the vehicle so battery and most major vehicle components all reach end-of-life at a similar time or that they can be replaced easily. That may mean a battery that can be swapped — but in the shop, not at an automatic swap station.
Plug in hybrids?
Plug in hybrids of course solve the problem, and they can charge when they can to be mostly electric and only use that gas engine more rarely. This actually creates a downside — it’s expensive to have a fossil fuel power train around to barely use it. And it adds a fair bit to the maintenance cost. This does allow for highway travel. Otherwise, you send a liquid fuel car to anybody wanting to do a long highway trip - save the electrics for the urban travel.
Very light vehicles
Today’s electrics use about 250 to 300 watt-hours/mile. OK, but not great. Efficient designs can go below 100 watt/hours per mile. That means doing 300 miles, which is enough for a full day in a city cab, needs only 30kwh (probably a 45kwh battery.) That’s a $22K battery today, but it will be a $9K battery by the end of this decade according to predictions. This might be quite reasonable.