Thanks for the interest on this project!
I will try to answer everyone.
now this obviously shows your lack of basic knowledge for mechanical engineering. more weight = more traction?!?!? i guess it's technically true, but then, what would you be doing with the traction that you get? how are you going to utilize the traction to your advantage? have you thought through everything??
Here is the wikipedia article on
train traction.
Steel on steel is slippery. If there is not enough weight on motorized wheels, 15kW will create too much torque, even close to top speed.
In our current design, only one axle out of four is motorized with about one ton of weight on it. We will probably use sanding or add weight on the motorized wheels to increase traction.
More importantly, you intend to propel your 30m, 3 ton train to 65mph using only 15kW?? seriously? You probably did your engineering weight/aerodynamic calculations and came up with that number (at least I hope you did), but what about acceleration? how long are you gonna take to get to 65mph with 15kW?do you have like 5 minutes of time and track to spare to go there?
As long as we don't take more than 10 minutes to reach top speed, we should enough time to do two attempts under one hour and we will have about 10 kilometers of tracks. Sure, we would have prefer to accelerate faster so we are stil trying to not be stupid on the weight...
Anyway, most motors are about 85-95% efficient, and controllers are about 95-97% efficient, so unless you're running at the absolute peak efficiency all the time (which is not possible in the real-world), you won't be making your 92% efficiency. And this doesn't include the efficiency of the batteries, drivetrain etc. I hope you did your homework on those too.
Only efficiency at to top speed matter (as Duncan pointed out), so yes I hope to be in the 92% efficiency range... If you have some experience on a specific engine, I would be happy to know which efficiency you reach under some specific conditions.
ok, so this probably answers my question about you doing engineering weight/aerodynamic calculations. I think you probably did it, that's why you said no storage and solar energy will be used directly. You need to figure why you need storage, which will then answer the part on why using 15kW to go 65 mph is not the best of ideas.
We have no choice. To qualify for this record, no energy storage is allowed.
The Tesla Model S weighs just under 2.5 US tons and iirc requires ~15kW to cruise at 60MPH. Obviously the Tesla is extremely aerodynamic and even if you make your train as 'slippery' you'll need more than 15kW to break the speed record.
I am interested in knowing where did you find this info (but seems right!).
I chatted with a Telsa aerodynamic engineer about this project and here is what he said: "If you don't have styling design constraints, you could get the Cd of a train down to 0.15 I would guess."
Most cars are not designed to be aerodynamic, aesthetic and handling characteristic are more important. Here is
a car with a 0.15 Cd.
Per
this page, Telsa S has a Cd of 0.24, slightly better than a Prius.
We are shooting for Cd of 0.15. Even if we don't reach it, we should be able to compensate by make the train longer since parallel drag is much lower than front drag and longer will give us more power.
An
a bicycle powered by a man can reach 83mph with less than 1kW.
Can you turn the train around or must it run backwards at full speed?
Either are fine, however aerodynamic is not symmetric, so we are planning to turn around the train. There are railroad loops that would flip around a train in our area like
this one. However, we are not planning on being able to drive on curves since the axles will be fixed and far apart (no bogies, so it can only drive on straight tracks). And it is difficult to secure those specific tracks. We will use cranes to lift and turn the train.
I estimate your solar array could produce ~18kWp at noon in the Bay Area if you're using cells with 20% efficiency (see SF Solar Map
here).
I have a similar number: 16kW
Here is my estimation:
Solar energy @90deg/clear sky: 1025W/m2
Solar panels: 56 x
Sunpower X22-360 22.2% efficiency at 25C (the best commercial panels on the market), 1.63m2 each
Efficiency at 35C: 19.3% (0.29% lost per degree above 25C)
Attenuation due to dust / humidity: 7% (20% humidity, 50km visibility)
Sun altitude 75deg (May / Jun): 3.5% attenuation
Have you considered fitting a Tesla Model S with track wheels and towing the solar array? I'm not sure whether this would qualify as a 'train' but it might meet your aerodynamic requirements
We actually considered that! Car wheel distance and train wheel distance are pretty close, so it is possible to have car running a train track with few tweaks!
However, the Telsa S didn't fit in our budget
The idea of using the rails to get a very low rolling friction is a good one
Yes, rails give advantages. Rolling friction is one (although only 9% of the energy is lost in rolling friction, per my calculation), but also carrying heavy weights (not need to optimize weight with custom solar panels) and no problem for a long structure.
I suggest you aim for a much lower weight - 600 Kg rather than 3000 Kg
A lightweight frame as optimised as possible and as aerodynamic as possible
It is tricky to reduce significantly the weight: the solar panels alone will weight 1000 Kg and we will need steel beams to have a 10 meters spans between the axles... And at the same time, we are very limit in traction...
You wants some rails that are at the best possible orientation to the sun - and then design your vehicle around that
This was one of the ideas, but not to maximize power.
In May / June,
sun altitude is around 75 deg in California, which only reduces the power received on an horizontal surface by 3.5%.
However, solar panels are very sensitive to the temperature: efficiency drops with height. So putting the solar panels at an angle would have allowed us to attempt the record much earlier in the year, when the air is cooler which would have cooled down the solar panels to keep a better efficiency.
We ruled out this idea, since this adds more constraints to the design and does seem to be needed to beat the record...
.
The most difficult aero will be around the wheels and the bottom - as th wheels don't have to steer the body and wheels must be as close as possible
Right, here are some
examples of aerodynamic train designs.
All of these are much much more important than motor efficiency
Sure! But I am the one in charge of finding the right motor, so even a few % of efficiency will reduce top speed by a few mph !
That only makes sense if the locomotive is pulling something... and this is not.
Actually, it is! To qualify as train, this should have at least two vehicles.
Here is the train definition I found online: "a series of railroad cars moved as a unit by a locomotive or by integral motors."
We are planning on a 20 meters locomotive pulling a 10 meter car.
The locomotive by itself can attempt the solar-powered vehicle speed record (one driver is enough to qualify as a vehicle), but two vehicles are required to be a train. There is no solar-powered train speed record so we are sure to get this one regardless our speed!
Why has a specific size already been chosen? Is this the result of some design optimization (balancing solar power and drag), or just an arbitrary guess?
I made a spreadsheet that computes max speed with length as an input.
Above 20 meters, extra length only adds a few extra mph.
We decide to build a 20 meters motorized car and a 10 meter car, attached together to be a train. The 20 meters car by itself should be able to beat the record, but won't qualify as a train. The two cars couple together may have additional drag, so may be slower than only the 20 meters car but will be a train.
Aerodynamics are also interesting. The rail vehicle should have much lower frontal area than a large sedan, and the length-to-area relationship is favourable, but the sheer bulk of this 30-metre-long thing will be an aerodynamic challenge in anything but zero crosswind and straight track.
You are correct, the front drag is much lower than the drag created by skin friction of the air flow parallel to the body of the train.
Fortunately, flat surfaces are much easier to build than a fancy aerodynamic frontal area!