Power Generation Concepts
Efficiency
Our research has ascertained that bicycles are the most efficient way for people to create mechanical power. Our task is to transform the mechanical power into electrical power suitable for powering the Earth Hour concert as efficiently as possible. To do this we need to choose the best generator and most efficient method of transmitting power from the pedals to the generator.
Choosing the generator
The first requirement for the electricity generator is to be able to connect to the “Grid Connect Inverter” that will then feed the power into the electricity grid. Grid Connect Inverters are primarily designed for solar panel installations and so take DC electricity in the range 54-400V. This requirement also immediately rejects automotive alternators as they maintain a constant 12V (24V for trucks) output. We are using a Conergy WR3300 Grid conect inverter which has an input voltage range of 150-500V and can convert up to 3.6kW of DC electricity to AC.
The next option is the permanent magnet alternator. These are commonly used for low power wind and hydro power generation, similar applications to bike power. There are many permanent magnet alternators (/motors) available, but the ones that caught our eye were Fisher & Paykel Smart Drive washing machine motors. These motors are very configurable, allowing us to tune them to give maximum efficiency at the correct power point and are reported to have reached 97% efficiency when tuned correctly (More Info (external pdf)…).
We’ve tested these motors extensively and have found that the motors have peak efficiency at a particular current. This current can be increased by wiring coils in parallel, but cannot be decreased from the point of the unmodified motor. The voltage the motor generates is proportional to the number of coils and the rotation speed. This allows us to predict the most common power level the cyclists will output and tune the system to be most efficient at that point.
Power transmission
We have reviewed a number of options on transmitting the power from the pedals to the generator. The main requirements are comfort for the rider and efficiency of transmission. To keep the system accessible for all riders, and allow them to choose their own cadence (ie, how fast they cycle), we have decided to keep the bicycle gearing on the bikes.
This gives us a rotation speed of approximately 150 RPM on the rear wheel of the bicycle. To step this up to a more useful speed we are running the rear wheel of the bicycle onto a 260mm friction wheel. This is in turn connected to the generator. There will be one generator per bicycle to avoid the difficulties inherent in mechanically linking the bicycles together. We reviewed other options for linking the bicycles to the generator’s drive shaft – like using pulleys or geared wheels – but our tests showed that the friction wheel was most efficient as well as the simplest.
Inertia
The large friction wheel also has an important advantage over other drive systems – inertia. As a cyclist spins their pedals, the power they create is not constant. Nearly all of the power comes from the down stroke on the pedals and almost no power is generated at the top of the stroke, when the feet are moving forwards and backwards.
On a system with not much inertia, the generator will rapidly consume all of the mechanical power stored in the system and make it difficult to move the feet around before start of the next down stroke. This results in the pedals slowing down at the top of the stroke and speeding up during the power stroke, making for an irregular, uncomfortable and unsustainable cycling experience. If we give the system some inertia, it will greatly assist the cyclist in continuously generating power. This means at the start of the power stroke, they are not starting from scratch again.
Sorry mate inertia and friction are two different things.Friction wastes your energy. Inertia is something quite different.
The rubber wheel drive will eat up about 50% of the available energy - leaving you about 100 to 150 Watt at most from a person.
The big metal frame is energy and materials intensive.
The high centre of gravity is not useful
We agree that a chain drive would be more efficient than the friction drive, but this was the simplest solution to give the required running speeds at the motor. Research shows that rolling resistance for bicycles wastes around 50W when running at 30kph (our wheels will be spinning at around this speed) with an 85kg load. We are only using one wheel and have a much lower contact force (~10kg), and so our losses from rolling resistance will be much smaller. The friction wheels have inertia, and are acting as flywheels to smooth out the power transfer as well as facilitating the power transfer.
The trailer has been energy and materials intensive, but it is designed to be an educational tool, to help people get a better understanding of the costs of power and energy, so that they can make informed decisions about their use. Their efforts will make the trailer a net benefit for the environment.
I'm not sure what you mean by "The high centre of gravity is not useful"
I am exceedingly keen to speak with someone regarding the bicycle driven generators. I was very impressed with the demonstration I witnessed today at Federation Square
-2nd try to submit--
27" wheel = 680mm diam
.68 * 3.1415 = 2.14 meters per rev of tyre.
150 rpm x 2.14 = 320 meters/ min => 19 km/h.
A friction wheel of 260mm Diam would turn at 258 rpm.
The linked article at mtbest.net looks to use a much higher generator speed of 700 to 900 rpm ?
The gearing ratio between the back wheel and the friction wheel is 0.68:0.26 (2.62), so the generator is spinning at 392rpm when the back wheel is at 150rpm.
Spinning the generator faster will get more power out of it, but you still have to provide that power. If we were to run the generators at 700rpm, very few people would be able to sustain the power output needed to keep the generators running near the point of maximum efficiency. Running the generators at 400rpm instead shifts the most efficient power output to within the capacity of a normal person.
The most efficient point is dependent on the torque on the motor rather than the speed, so we are able to slow down and speed up whilst maintaining efficiency, so long as the electrics follow correctly.
Amazing... kinda amazing issue. I'm goin to blog about it too.
what I was looking for, thanks