Feb 13

Feb 13, 2006

The Perfect Prone Energy Accumulator Human Powered Vehicle Concept


(click on all the images to enlarge)

I've had this idea in my head for a long, long time and like most of my ideas, it just really needed to get explored. I spent a couple hours drafting up this preliminary concept. If you have some expertise in this area, I would really appreciated your input. I'm not sure at this point if it's a valid concept or not, but I think it may have some merit.

Basically, the idea is to take advantage of a super low drag body shape called a tear drop. The tear drop fairing would TIGHTLY cover a rider laying in the prone position. Because the fairing would be so tight against the riders body, there would be no way for the rider to power the vehicle by peddling or any other means. Energy is accumulated while the vehicle is at a stand still by having the rider sit up and charge a battery using an electric hub motor as a generator.

After an hour or so of charging the battery, the rider is enclosed in the tear drop fairing and he uses the electric motor to power his journey around a track until the battery is drained.

Here is how it could all work for an HPV 24 hour distance record:


1. The rider spends 1 hour in the optimum biomechanical position peddling the rear wheel which is lifted off the ground allowing the wheel to spin freely and turn the electric hub motor connected to the rear wheel. The electric motor becomes an electric generator and charges a lithium polymer battery.

250 watts for one hour should result in a total of 209 watts of power storage in the battery according to the following efficiencies:

1. Mechanical drivetrain is 98% efficient
2.Electric motor is 95% efficient
3. Lithium Polymer batteries are 90% efficient (in and out)

Since HPV rules require that nothing is ejected or added to the vehicle during a record attempt, the seat stay would rotate around and couple with a seat tube extension that would allow the rider to get into the power input position. A retractable landing gear would keep the vehicle supported, and raise the rear wheel off the ground. The fairing shells would detach for this stage.


2. After the battery is charged, the rider gets into the prone position on the frame, has the fairing shells assembled and uses the 209 watts of stored electric energy from the battery to power the electric hub motor and accelerate to around 90 kph for one hour. Since the electric motor is 95% efficient, the 209 watts of power stored would result in 198.5 watts of power to the rear wheel.

I have some data that estimates a tear drop fairing enclosing an average sized body has a CdA of .07 (sq ft). If we add faired wheels, that should double the drag bringing the total drag of the streamliner to .14 (sq ft)

Using the following values for the variables in the PDG spreadsheet:

1. .0045 as coefficient of rolling resistance (Crr) from published values for Schwalbe Stelvio tires for 16 x 1 1/8 front tire and 20 x 1 1/8 rear tire.

2. .14 CdA (Aerodynamic drag)

3. 72 degrees F

4. 30 inches of mercury barometric pressure

5. 198.5 watts of input power

We calculate an average speed of 83 kph, or 83 km in one hour.


3. Steps 1 and 2 are repeated every hour for 24 hours which would result in a total of 12 hours of 83 kph resulting in a total distance travelled of 996 km which is only 26 km off of the current 1022 km 24 hour distance record.


Here is a table that compares the energy accumulator approach with the conventional approach using the CriticalPower HPV 24 hour distance record attempt.

HPV CdA Crr # hours power input Average watts of input power mechanical efficiency Average watts of output power Average speed (including losses due to drivetrain, batteries, electrical, etc) distance travelled in 24 hours Total energy invested by the rider in Kilojoules
CriticalPower .32 .0045 22 100 95% 95 46.6 kph 997 km 7920 Kj
Perfect Prone energy accumulator .14 .0045 12 250 80% 200 83 kph 995 km 10,800 Kj

It appears that the energy accumulator approach is about 36% LESS efficient that a conventional direct drive streamliner as measured by the total energy expenditure in both cases in Kilojoules. HOWEVER, according to hundreds of hours of watts, time and hear rate data I have accumulated over the years on BOTH an upright road bike as well as a recumbent lowracer, the average efficiency loss due to the different biomechanical positions as measured by heart rate from a road bike position to a recumbent position is 10%. Here is a summary of the biomechanical differences between the recumbent lowracer and the road bike geometries based on SRM data from a random selection of 35 training sessions:

Geometry Average watts Average heart rate Watts per heart beat % difference
Road bike 141.05 watts 117.76 bpm 1.19 watts per beat 10%
Recumbent lowracer 121.17 watts 111.37 bpm 1.08 watts per beat

Lets assume that using state of the art solar car components, we could get our electrical drive efficiency up to 90% all-in. And including the 10% biomechanical advantage that the road bike geometry offers, here is how the comparison would play out (I increased the wattage input for the CriticalPower so both resulting distances travelled would be approximately the same):

HPV CdA Crr # hours power input Average watts of input power mechanical efficiency

Average watts of output power

Average speed (including losses due to drivetrain, batteries, electrical, etc) distance travelled in 24 hours Total energy invested by the rider in Kilojoules Adjustment for biomechanical advantage of 10%
CriticalPower .32 .0045 22 108 95% 102 48.2 kph 1068 km 7920 Kj 8553 Kj
Perfect Prone energy accumulator .14 .0045 12 250 90% 225 88.5 kph 1069 km 10,800 Kj 9720 Kj

The resulting energy investment into both systems is now a bit closer, but it would appear that the conventional direct drive approach is still more efficient. Other considerations in favour of the energy accumulator approach would be that the rider could benefit from exercising outside of a fairing body by receiving fresh, cooling air and plenty of hydration. Nutrition could be consumed and digested more easily while the rider is at rest piloting the streamliner. I am not certain if there are any physiological advantages to a burst approach compared with a steady state approach. From experience spending 20 hours peddling CriticalPower around the track in Alabama, I would probably prefer alternating hours at a higher intensity, but more comfortable and powerful geometry position than the monotonous 22 straight hours of 100 watt output trapped inside the fairing shell.

I'll ask my coach Jason for any input on the physiological aspects of this burst approach.

One additional area of potential improvement could be the estimated CdA of the super compact tear drop fairing. It is possible that I have overestimated the additional drag the wheels would add to the .07 CdA tear drop shape. Perhaps considering laminar flow and properly faired wheels, the CdA could be improved to as low as .1 sq ft. At 250 watts of power input, and 90% mechanical/electrical efficiency, that would result in an average speed of 96.5 kph, or 1166 km in 12 hours.


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