Saturday, August 9, 2008

Energy efficiency of PRT and MRT

Someone asked me, "How can the PRT be more efficient than the MRT?" The MRT has economy of scale and should use less energy.

I think that the PRT can be more efficient because:

1. the number of podcars can vary according to demand.
2. the podcars can go straight to the destination, without having to stop at the intermediate stops

2 comments:

  1. A Transportation EnthusiastAugust 9, 2008 at 10:18 PM

    Consider weight alone as a rough estimate of energy usage. This is not fully accurate, but can give a ballpark number for energy efficiency.

    According to Wikipedia, MRT trains weigh about 214,000 kg unloaded and can carry a maximum of 1920 passengers (492 sitting, 1428 standing).

    ULTra PRT weighs about 700kg unloaded. Assuming the average passenger weighs about 70kg, with 1.2 passengers per PRT vehicle and 30% empty vehicle movement, ULTra PRT moves an average of ((700 + 70*1.2) / 0.7) = 1120 kg per passenger.

    For the MRT train, the weight for x passengers is (214,000 + 70x), and therefore the weight per passenger is (214,000/x + 70). So, for example, if there are 100 passengers, MRT is moving 2140 kg per passenger.

    From this we find that MRT must average at least 203 passengers for MRT weight per passenger to exceed ULTra's. At crush MRT load of 1920 passengers, MRT only moves 181 kgs per passenger (very efficient), but we also have to consider that during rush times the trains must often travel back nearly empty to pick up more passengers, so even then the average capacity is perhaps no larger than 960 per train (293 kg/passenger).

    And then there is the non-rush periods. If the trains are running at 8-minute intervals (at least) for 18 hours a day, the trains will likely be nearly empty for long stretches, and this brings the overall average down. I don't know where you would obtain such statistics for Singapore, but in the US I believe that the average passengers per train across the US is somewhat less than 100.

    Now consider how efficient a multi-mode transit system (MRT + PRT) could be. If a complementary PRT network could handle ALL off-peak traffic, you could perhaps run the MRT trains only during busy times (maybe less than 10 hours a day rather than 18), and run at lower frequencies during moderate daily demand. This would drive the MTA passenger load factor up by eliminating the inefficient off-peak schedules. PRT would efficiently handle all the non-peak rush MTA would handle the peak, therefore both would be operating with maximum efficiency.

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  2. Hi
    I will approach the calculation in this way:

    Train 214,000 kg / 1920 passenger = 111 kg p passenger

    Podcar = 700 kg / 4 = 175 kg p passenger

    In both cases, I assume an average occupancy of 50% during the peak and off-peak hours.

    So, the train appears to be relatively more efficient than the podcar by the ratio of 0.6.

    We have to consider the energy used in starting and stopping the train. Assume that this adds to an additional 20% in energy use. The relative ratio changes to 0.72.

    The final adjustment is for distance travelled. The podcar can take the direct route and cut down the distance. The train follows the same schedule and forces all passengers to travel longer.

    Assume that the difference in distance is 30%. Then the relative ratio changes to 0.94.

    Both train and podcars can probably consume the same energy.

    The podcar beats the train in the following respects:

    1. Shorter travelling time, due to direct route

    2. All passengers in the podcar are seated.

    If the podcar can be filled with 4 passengers (through pod-pooling, to reduce cost per passenger), the pod beats the train in energy consumption as well.

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