Monday, January 24, 2011
This post is in response to comments made regarding the last one. In order to illustrate my points, I have used a modified version of the following picture, which I had originally intended to use in a different manner, so even though it is a bit off-topic, let me start with an explanation this illustration first.
This picture is the result of a design exercise, the object of which was to create the highest capacity station possible with the smallest footprint. In order to do this, I used elevators with curved doors, so that they can retract without needing much space in the walls. There are two of them, one for entering and one for exiting the station. This station is not “off-line,” but would rather be bypassed by a track that is not shown. Four cars can be loading while four are unloading, and (guessing a time of thirty seconds to get seated and on your way) the capacity of the station would be one car per 7.5 seconds, which works out to 480 vehicles per hour. It was designed to be ADA compliant, yet has a footprint of only about 50 square ft. One thing to think about is that if 8 of these stations where operating at capacity, the track they would be feeding would need sub-second headways to handle the passenger load. A station like this would be factory-built and delivered to the site in several pieces. Obviously the design is a bit misplaced in this setting, which isn’t exactly downtown, (so a footprint small enough for a crowded sidewalk isn’t really needed) but I had no other jpeg to “shop” the model into.
This leads me to the next picture, which shows a two-way variation of the same station. In this one, each elevator handles both arriving and departing passengers, with one elevator being for each direction. I drew this in response to alert reader Lars Endre, who suggested the possibility of using sloping track to capture the energy lost in deceleration. While this would be impractical for most PRT designs, it’s a concept that is well suited to self-leveling, hanging systems. The idea rests on the recognition that it takes a great deal of energy to get a vehicle up to speed and that it wastes a lot of kinetic energy to get it to stop. Parking atop a hill, so to speak, addresses both issues. This picture shows such an arrangement.
An alternative (frequently mentioned) approach to the problem is regenerative braking. As the vehicle slows, the momentum of the vehicle turns the wheels, which rotates the motor faster than it wants to go. This turns the motor into a generator and a brake at the same time, and the power is fed back into the track to be reused elsewhere. Sounds good when you say fast. I am not, personally, completely sure that this is an efficient process that is practical to exploit, what with electrical transmission losses, etc., especially with minor voltage supplementation in a DC system.
Regenerative braking raises another fundamental question. How much braking do you want to do? After all, if the system is smart enough, it ought to have vehicles coasting more and braking less, right? The problem boils down to the need for speed. To some, it is assumed that assumed that PRT has a natural speed limit. Studies have shown that as speed increases, the safe spacing between vehicles must increase as well. Thus a system with a densely populated track going slower could move more people than one with faster, more widely spaced vehicles. The problem with those studies is a glaring fault in logic. It assumes that nothing can improve braking ability or crashworthiness of the vehicles. Fix that and you can both pack them tighter and go faster. But then we need brakes, and must deal with those mechanical inefficiencies. Consider the off ramp leading to a station. Making the split-off ultra gradual and giving a very long lead-up track is not very practical.
Here is a different angle to show more track. While my first instinct was to think that raising the boarding area was a waste of materials, I soon realized that this cost could be offset by allowing shorter on/off ramps. Obviously this is more of an attractive option for fast, densely populated systems than for slower ones with few vehicles.
Another consideration is the “Umbrella Effect,” where overhead structures block the sky, a concern for landowners along the route. While this is less of a concern for minimalist track systems like I advocate, in a bi-directional station like the one shown there is still a lot of track up there, as can be seen. (Imagine the ULTra track four lanes wide!) Long acceleration lanes represent additional visual obstruction. If raising the station can shorten these ramps, that would seem to be a plus for public acceptance as well as cost. Even the station itself would appear somewhat less imposing by being higher, as more light would get in beneath it, and individual areas would remain shaded for less time.
Astute reader Andrew F further pointed out that in tight turns, a sloping track could also be used to “bleed off” speed. (and give it back again after the turn) There are plenty of tight turns in a city environment, so this is something to consider. The negatives here are about ride quality, the way the system looks, the extra engineering, etc. Clearly, going very fast downtown would require a system that would be designed like a roller coaster, and I doubt we really want to go that far. On the other hand, in a system fast enough for commuting from the suburbs, there will always be the interface into the slower urban environment, just like freeway exits feeding downtown streets. Such an approach should certainly be in the toolbox. The case for using slopes to slow or speed a vehicle naturally arises, I believe, from the fact that it is so easy to do, considering that the track is raised anyway and the vehicles are designed to handle slopes and turns with minimal discomfort to the rider.
Posted by Dan at 9:38 PM