How Solar Panels Work
Solar Panel Basics for VMSVC Competition
As we are talking about the VMSVC competition, specific solar panel and testing data in this document refers to the Scorpio No. 26 solar panel.
This panel has been singled out as it is used by nearly all boat and Sheridan kit car competitors and is the panel supplied by the committee for the student designed car to race with.
This panel is made specifically for the boat competition, as such is lightweight to enable good boat performance. Being lightweight, it is fragile. Solar cells on their own are about as fragile as egg shells, when encapsulated with a layer of fibre glass each side like the No. 26 panel, some extra strength is imparted to them. These panels will withstand gentle bends without damage, but sharp bending will fracture cells, degrading power.
Motor performance when connected to these panels is given later in this document, the motors tested are those in common use in the competitions, they are:
· Faulhaber 2232 6 volt motor which is in almost universal use in the advanced boats and student design car competition.
· Scorpio SM 403 motor this motor is a special made exclusively for Scorpio to suit the No. 26 panel. This motor is supplied with the Sheridan kit car and is in almost universal use in the junior boat competition.
SOME SOLAR CELL BASICS:
· Connecting cells in series adds the voltages of the cells. Silicon cells produce about 0.5 Volts per cell, so 3 cells in series will give about 1.5 Volts. (Series is when the + terminal of one cell is connected to the – terminal of the next cell)
· Connecting cells in parallel adds the current. If each cell can deliver say 1 Amp at a particular Sun level 2 cells in parallel will deliver 2 Amps at the same Sun level. (More surface area = more current) (Parallel connection is when the + terminal of one cell is connected to the + terminal of the next cell and similar for the – terminal)
· The Open Circuit (or no load) Voltage of each cell varies only slightly with Sun level but the maximum current available (Amps) varies directly with Sun level. The brighter the Sun the more current is available.
· The maximum power will be obtained when the Sun strikes the cells at 90 degrees. (See test results later for an indication to effect of tilting panel.)
· Panel power drops as its temperature increases due mainly to voltage falling. Typical power drop is around 0.4% per Deg C. On a day with ambient temperature of 25 Deg C panel temperature can easily reach over 60 Deg C giving a power drop of about 15%.
· Shading only one cell in a series array will reduce the power output significantly.
· Power = Volts x Amps (in watts)
· Solar cells can only deliver current up to the limit imposed by the prevailing light level. If the electrical load (your motor) has a resistance so low that it wants more current than is available (ohms law applies V=R x I) the voltage of the cells will drop rapidly to near zero. Consequently the power delivered to the load (your motor) will be near zero. (see graph 1 below) Solar panel behaviour is totally different to a battery.
WIRING DETAILS & HINTS for SOLAR CELLS & MODULES
To help explain series and parallel connection of solar cells look at the following depiction. Remember the top surface of the cell is negative and the bottom surface is positive.
Top illustration cells connected in series
Bottom illustration cells connected in parallel
When cells are connected in series that is the +ve of one cell connected to the –ve of the next forming a “string” of cells, the voltages add. So two cells connected in series would have an output voltage twice the voltage of each cell alone. The current (amps) available remains the same as that available from individual cells.
When cells are connected in parallel that is all cell positives connected together and all cell negatives connected together the current available adds. So three identical cells connected in parallel would be able to deliver three times the current that each cell alone could deliver. The voltage however will remain the same at the voltage available from each cell.
CIRCUIT TO CONNECT SCORPIO TECHNOLOGY BOAT PANEL IN PARALLEL:
The Scorpio Technology Boat Panel is in almost universal use in the Advanced Division and very common in the Junior Division of the competition.
How to wire the panel in parallel is shown below.
Typical output in full Sun: 3.5 Volts 1.60 Amps
CIRCUIT TO CONNECT SCORPIO TECHNOLOGY BOAT PANEL IN SERIES:
The Scorpio Technology Boat Panel is in almost universal use in the Advanced Division and very common in the Junior Division of the competition.
How to wire the panel in series is shown below.
Typical output in full Sun: 7.0 Volts 0.80 Amps
CIRCUIT TO CONNECT SCORPIO TECHNOLOGY BOAT PANEL SWITCHED SERIES OR PARALLEL:
The Scorpio Technology Boat Panel is in almost universal use in the Advanced Division and very common in the Junior Division of the competition.
If you wish the Scorpio panel can be wired in series or parallel selected with a switch. If a double throw double pole centre off switch is used it both meets the regulations requirements for an on off switch, as well as providing series or parallel connection of the two panel sections. How to wire the switch is shown below. (Scorpio switch catalogue SSW2W)
Typical output in full Sun: Series 7.0 Volts 0.80 Amps
Parallel 3.5 Volts 1.60 Amps
NOTE: The panels shown above is a 2013 production panel, however the layout of Scorpio Panels has changed several times over the past few years in both the number and size of cells and position of terminals. They all have two strings of cells connected in series. If your panel is a different layout to the diagram above just identify negative and positive terminals of each string of cells and wire as above. The back face of the cells is positive the front face is negative.
Solar panel characteristics & effect of electrical load:
Graph 1. Scorpio solar panel No. 26 Full Sun output.
Examining graph 1 above of solar panel power vs volts it is obvious that maximum power occurs at 6.75 volts and drops off either side of this value.
Note where a stalled Faulhaber motor is on the graph.
What is happening, with no electrical load on the panel it will be at its open circuit voltage (OCV) point, the extreme right point on the graph (8.6 volts). At this point the panel has no electrical load or we can consider it with a load of infinite resistance.
A motor with no load on its shaft will be just to the left of this point as it will be only taking a small quantity of power to overcome its internal losses. (Marked as free running motor)
As the load on the motor shaft increases the motor will take power from the panel following the panel power curve up till at a particular shaft load it will be at the maximum power point of the panel. At this point the motor is taking the maximum power available from the solar panel and producing maximum power available on its shaft.
Increasing the load on the motor shaft will cause it to follow the panel power curve down to the stalled motor position. At this point there is no power available on the motor shaft.
The load on the panel is 0.8 ohms (resistance of a stalled Faulhaber 2232 6 volt motor) with only about 0.7 watts being delivered from the solar panel to the motor. All this power is being dissipated in the motor windings as heat.
To obtain the maximum power possible on the motor shaft, which is what actually drives the vehicle, it is essential to control the motor load so it operates at this maximum power point of the solar panel.
Use caution when configuring the solar panel in parallel. Due to internal losses in the panel increasing with increasing current and motor characteristics varying greatly at lower voltage the power available from the motor at high sun levels is significantly reduced when the panel is configured in parallel.
To gain insight into the effect of series or parallel panel configuration , consider the following graphs. The first two graphs show the output on a Faulhaber 2232 6 volt motor shaft powered by a Scorpio No. 26 solar panel at different Sun levels. The third graph is for the Scorpio SM 403 motor.
100% Sun
Examination of the graph above reveals.
· With the solar panel configured in series the motor output at 100% Sun is 4.3 watts with panel power of 5.25 watts. This equates to a motor efficiency of 78%.
· With the solar panel configured in parallel the motor output at 100% Sun is 2.5 watts with panel power of 5.13 watts. This equates to a motor efficiency of 48%.
Using parallel connection, halves the motor RPM but significantly reduces the power available on the motor shaft. Not exactly what is needed for maximum vehicle performance.
29% Sun
Examination of the graph above shows that power in series and parallel are about the same.
Scorpio SM 403 motor on Scorpio No. 26 panel.