Solar Centipede – Part I

drone with car

 

*caveat* I don’t have any mechanical engineering, automotive etc experience.
Feedback on anything I’ve missed or gotten wrong is welcome!

Scenario

There are two locations roughly on the equator, with a 1,000 km road between them. Both locations need to send 5,000 kg of goods between them every second day. Each location has no resources. The vehicle that travels between them must be completely self-sufficient in energy.

  • Distance to cover in 24 hours is 1,000 km (41.76 km/h)
  • Cargo load to carry 5,000 kg, with an area of 25 m^3
  • No traffic, so no collision avoidance required
  • No restriction on vehicle size
  • Vehicle must not require external sources of energy
  • The route is completely flat with no elevation change
  • There are no clouds or other impediments to sunlight
  • There are no cross winds
  • The road does curve, but a tight turning circle is not required
  • It will move 1,800,000 kg of goods per year, over 540,000 km
  • The scenario starts at dawn, with no residual energy stored (batteries etc are empty)
  • It’s assumed components don’t suffer degraded performance due to wear
  • It’s assumed a self driving system can run on a low speed vehicle at 50w

Design Considerations

  • Solar is the only source of energy. We need to optimise for its collection
  • Aerodynamic drag will be a major consumer of this scare source of power. Optimising to minimise it will be crucial
  • There is a time limit to when the destination must be reached. This means there will be a trade off in the most efficient speed to move over a 24 hour period balancing aerodynamic losses and losses of storing energy
  • Keeping the vehicle lightweight will be important
  • To keep costs down, the design could be modular to maximise the repeat ability of construction
  • A long vehicle, with a low coefficient of drag and a minimal frontal area will be the key to effiency

The Solar Centipede

whole 2

  • Proposed solution to this problem is a modular carriage system consisting of 24 identical carriages, joined end to end. 1 m wide, 55 m long and 0.5 m high.
  • It has a single front specialised aerodynamic carriage that is heavier and has more powerful motors
  • Unlike a train, where an engine carriage is pulling the carriages behind it. Each module is overcoming its own frictional and aerodynamic losses
  • The front vehicle would be responsible for overcoming the frontal aerodynamic force and cutting the vehicle through the air.
  • Each module is running an energy surplus which is transmitted to front carriage not by force, but by electricity. With this approach, it doesn’t require structural strength on the part of the carriages to transmit the energy.
  • Spreadsheet of Energy Input / Energy Output

Front Carriage (1 x)

front carriage 1

  • 2.1 m long, 1m wide and 0.5 m wide
  • 15 kw/h of battery capacity
  • 500 w motor, at 41.76 km/h is it outputting 300 w
  • It has a coefficient of drag of 0.18 and a frontal area of 0.6 m^2
    • At 41.76 km/h it requires 220 w to offset aerodynamic forces, plus another 70 w for rolling friction

Cargo Carriage (24 x)

 

 

  • 2.2 m long body, 1 m wide, 0.5 m high
  • At an average of 41.76 km/h, the cargo carriage runs a surplus of 50 w over 24 hours. 100% concentrated in the 10 am to 2 pm time window.
  • Carriage weight 125 kg
  • 2.5 kw/h energy storage
  • Cargo carried 200 kg
  • Rolling Resistance 45 w
  • Aerodynamic resistance at 41.76 km/h is 25 w as it is largely protected from drag by the carriage in front of it

Energy Collection & Storage

  • Solar input per carriage, average of 200 w per 12 hours of daylight. Peak power 340 w
    • This gives a solar budget of 2.4 Kw/h per carriage, for a total power budget of 57.6 kw/h per day.
    • Assuming 35% of the energy is used directly, with no loss. The total budget is reduced to 48.96 kw/h

Energy Breakdown

  • Total Energy budget of 55 kw/h
  • Energy Expenditure on aerodynamic losses
    • Front Carriage  (7 kw/h)
    • Cargo Carriage (1.6 kw/h, times 24 carriages 38.52 kwh)
  • Energy Expenditure on Rolling Friction
    • Front Carriage (1.68 kw/h)
    • Cargo Carriage (1.08 kw/h, times 24 carriages 25.92 kw/h)

Spreadsheet of Energy Input / Energy Output

Guesses/Assumptions

  • The coefficient of drag and rolling resistance are estimates taken from other similar objects
  • The solar radiation captured per day is crudely modelled

Optimisations not used

  • It would make sense move slightly quicker the stronger the sun was shining as energy is wasted during the storage process. (move slightly quicker at midday, slower at night)
  • The gaps between the carriages could be more effectively covered reducing drag further, 50% of the energy lost is the aerodynamic losses from the carriages

Losses not estimated

  • The last carriage will experience extra drag from the turbulence left behind it
  • Loss of efficiency within the electric motor
  • Connection loss transmitting power between 24 carriages.
  • Energy used to turn the wheels, or to correct path
  • Headwinds, cross winds

References

Spreadsheet of Energy Input / Energy Output

Rolling Resistance
Large Battery

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