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To do well in Solar BikeRayce, you need to be able to go both far and fast with a very small amount of energy. In order to do this, you have to focus on energy efficiency and on correctly sizing your motor and other systems. This section will help you get a ballpark understanding of the energy available to your team during the Rayce. We suggest that you make more detailed calculations that are specific to your bike and riders.
In Solar BikeRayce, the A/B/X Class has 4 hours of raycing to complete 100 km (with a recharge break after the first 1.5 hrs). The top team in 2002 finished in just under 2.5 hours of raycing. For the calculations below, we will be using 2.5 hours as the time to complete 100 km.
POWER AND ENERGY
It is important to get a handle on your energy supplies (solar array, battery, rider). Since both the solar array and battery produce electrical energy, we will use electricity units.
- Power - The power available depends mostly on your motor and rider. Power is measured in units called watts (W) or in larger units called kilowatts (1 kW = 1000 W). Power is an instantaneous measurement.
- Energy - The energy available comes from the solar array, battery, and rider (similar to gas in a conventional car--only much less!). Energy is POWER x TIME and its units are watt-hours (Wh) and kilowatt-hours (1 kWh = 1000 Wh).
YOUR POWER
What power is available? This will vary with how your bike is designed and how prepared your riders are.
- Rider - An outstanding bike rider can put out around 225 watts of power for a short period of time. A healthy athletic person can put out about 160 watts (or just over 0.2 horse power) for 2.5 hours. (Remember that you can change riders.)
- Solar Panel - A well-designed solar panel of the maximum size permitted (1.6 m²) can deliver almost 200 W. On a sunny clear day in the middle of May in Topeka (where SBR is often held), the power may range from 120 W to 195 W throughout the raycing day. A minimum sized array (0.15 m²) can deliver almost 20 W. Over the rayce day, the power may range from 11 W to 18 W.
- Battery - The battery will deliver whatever power that you ask of it (within reason), so you'll need to regulate its power delivery in order to make it last for the entire Rayce.
YOUR ENERGY
What energy is available? The amount of energy available will vary with how long it takes you to complete the 100 km (this will also change the amount of energy that you use). The numbers below are for 2.5 hours of raycing time.
- Rider - Assuming the rider(s) are in top shape and can deliver 225 watts for 2.5 hours, about 560 Wh will be available from that source (225 W x 2.5 hrs).
- Solar Panel - Over the 2.5 hours, a maximum sized solar panel can deliver about 420 Wh (on sunny day). For a minimum size bike panel over the same time, the energy from the panel is about 40 Wh.
- Battery - The battery size permitted is about 480 Wh. During the charging break, assuming full sun and a full sized pit charger, the solar panel can deliver another 480 Wh to the battery for a total battery supply of 960 Wh.
The total energy ranges from 1560 Wh to 1940 Wh, depending on the array size. With a maximum sized raycing array, 28% of the energy is from your riders, 22% from your raycing array, and 50% from battery and pit array. With a minimum sized raycing array, 36% of the energy is from your riders, 3% from your raycing array, and 61% from battery and pit array.
To make things even more exciting, these numbers for the amount of energy available are for BEFORE the system losses due to transmissions, matching the array and battery voltages, and battery discharge efficiency. What does this mean? The actual energy available to turn your wheel will be reduced by the inefficencies in your systems.
CONCLUSION
You can't afford to ignore the importance of the riders, the solar panel, or the battery. Incidentally, the energy available under ideal conditions isn't a gallon of gasoline--it's about 1/2 a cup! Over the rayce course, that works out to better than 1500 mpg. Your job is to create a super efficient vehicle and win the rayce with this small amount of energy. Every electron counts! It is a challenge and it will be fun!
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