The size of the PV panels
Since May 2014 I have used the solar bike more than 1000km without any problems. But initially, most people find the solar panels to large and awkward. This is because they have never seen the solar bike before. But is my solar bike unmanageable? Here are examples of other large bikes, which often have a length of 260cm.
Solar panel roof
Another option is a roof panel. It has some advantages:
- The panel does not suffer from shadow from the rider
- The bike is more manageable
- Larger solar surface area is possible
The disadvantages are:
- High center of gravity
- More wind load
- More weight
PV panel air drag
In the ideal situation of the absence of side wind and where the panels are horizontal to the wind, the air drag of the two PV panels is quite low. The skin friction drag is negligible with air. The shape should be aerodynamic like this:
With a PV panel width of 100cm and a thickness of 2cm, the air drag per panel is:
P = 1/2 * ρ * A * v3 * Cw = 1W
- Air density ρ = 1.23
- A = 1 * 0.02 = 0.02
- v = 20km/h = 5.55m/s
- The drag coefficient (Cw) of the PV panel is not precisely known but estimated at 0.5.
Wind gradient graph
The higher you go the higher the wind speed. On the ground the wind speed is zero. The wind profile is logarithmic, but close to the earth surface it is linear, see the wind gradient graph:
PV panel tilt angle mismatch
The PV panels are not pointed into the sun. But in summer, the loss due to the tilt angle mismatch is less than 20%. See this graph:
Bifacial solar panels
The back side of a bifacial PV panel generates electricity from ambient light reflected by surrounding surfaces; this results in up to 30% higher energy generation in comparison with a unilateral module.
The sun's power
Global CO2 emissions have to be drastically reduced within the next few years in order to prevent a disastrous climate change. This is where DESERTEC offers a solution which can be implemented worldwide: Sufficient clean power can be generated in the world's deserts to supply mankind with enough electricity on a sustainable basis.
The total solar collectors surface (for concentrating solar thermal power) needed to provide the electricity for the humankind is 300x300 km², see here:
Yearly average horizontal solar radiation
The annual energy in kWh/m2 from a solar panel depends on the location:
Interactive map of solar radiation
On SolarGIS iMaps you can get the solar energy values at every location on earth.
How to pack solar cells
Solar cells are very fragile, as baubles. Not well packed solar cells will be damaged easily during transport. Note that a stack of 125 solar cells weigh a kilogram. This weight forms a high stress to the solar cells during shipping; it demands a special attention to packaging. Just packaging solar cells in Styrofoam is inadequate.
- The solar cells should be properly stacked, outstanding solar cells will break.
- Pack the cells between solid plywood, not cardboard.
- Seal the package with shrink foil.
- Put the whole package into an exact fitting Styrofoam box.
Solar cell model SPICE simulation
The solar panel with bypass diodes is simulated in Multisim. The models of the solar cell and bypass diode have to be created by ourselves. Here I describe how to make a solar cell Spice model for Multisim. The solar cell is a hierarchical block which contains a current source and a diode:
Download here the Multisim solar cell model.
This is the solar cell test circuit:
Download here the Multisim solar cell dc sweep simulation circuit.
Do a DC sweep analysis with these settings:
- Source = ii1
- Start value = 0A
- Stop value = 5A
- Increment = 0.1A
- Selected variables for analysis = V(1)
This is the Multisim simulation output:
In the Multisim Grapher View window do: Tools > Export to Excel and make a graph in Excel.
The diode parameters can be changed such a way that the model graph equals the solar cell graph. These are the figures of a particular solar cell:
- Open Circuit Voltage: 0.670 V
- Short Circuit Current: 5.9 A
- Maximum Power Voltage: 0.560 V
- Maximum Power Current: 5.54 A
From the solar cell data, the values of the current source and the diode can be determined:
- Current source 5.9A
- Diode 0.67V - 5.9A
- Diode 0.56V - 0.36A (because 5.9A - 5.54A = 0.36A)
This diode formula is used by Multisim and Spice:
Use this Multisim simulation for the solar cell diode and try successively different values for Is (saturation current) and for N (emission coefficient) to get the proper currents of 5.9A and 0.36A:
These values give a good result:
- Emission coefficient N = 1.52
- Saturation current Is = 0.234 μA
Finally, we get a solar cell model graph which equals the solar cell graph. Here is the graph for a current of 5A:
Single solar cell powered step-up converters
By using special low-voltage step-up converters, a single solar cell is capable to deliver power to electronic circuits.
Vertical solar panels for indirect sunlight
Although not applied for the solar bike it is interesting to tell something about a different PV panel approach. Usually, solar panels take advantage from direct sunlight and are therefore as much as possible facing the sun. The power is up to 1000W per m2 earth's surface.
Vertical solar panels receives direct sunlight, this is just 200W/m2 or less. But, you can imagine that the higher the vertical solar panel, the greater the power, without increasing the used earth's surface. So, calculated per m2 of earth's surface, a tower of solar panels can generate more power than a usual solar panel. Per watt solar power, a vertically PV panel is a lot more expensive than a usual solar panel because many more solar cells are required. See more at the article "Solar energy generation in three dimensions".
Lux to Watt conversion
For the specific spectral composition of sunlight: 1 lux ~ 0.0079 W/m2
- Direct sunlight: max 130000 lux = 1030W/m2
- Indirect sunlight: max 25000 lux = 200W/m2