Leicester Hackspace again had the opportunity presented by Instructables to acquire some Brown Dog Ultra Compact 5W Folding Solar Panels and hold a workshop in March 2015.
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Step 1: Purpose of Tests
I was interested in how the efficiency of such a panel could be enhanced by tracking the sun with a solar tracker. But rather than build a tracker I thought it would be a good idea to measure the output of the panel at various sun angles. This led me to find some interesting facts about Maximum Power Point tracking. The output of the Brown Dog Solar Panel is a USB socket, and for that reason the panel contains some sort of voltage regulation/limiting, since it may have a device plugged in which could be damaged by overvoltage. In order to measure the panel power output it was necessary to load it with a resistive load sufficient to pull the voltage to less than the regulation voltage. I have been able to plot both power out against sun angle and power out against voltage for a given sun angle
Step 2: Parts Required
Wooden panel with screws to support the panel flattened out
USB to wire lead
2 X Digital multimeters
Resistive load - 100 ohm wirewound potentiometer (variable resistor) with knob and scale
500W floodlamp on stand
Telescope equatorial mount on tripod
Step 3: Initial Tests Indoors
Initial tests were performed in the workshop using the 500W flood lamp. The normal solar irradiance at the earth's surface is around 900W per square metre, so I thought that mounting the lamp 50 cm from the solar panel would be representative, and tests were done at two different angles - 90 deg and 45 deg. The load resistance was changed and measurements made of current and voltage. We can see from the graph that adjusting the potentiometer to load the panel to 4 to 4.5V produced the maximum output power, but that was only 1W total power output when the lamp was normal to the surface of the panel and 0.7W when at 45 degrees. These figures are a lot lower than the designated 5W nominal o/p from the panel, so it was decided to perform a more representative test outdoors on a sunny day.
Step 4: Outdoor Tests
The wooden panel holding the solar panel was mounted on an equatorial telescope mount so that the azimuth drive could be adjusted to point the panel towards the sun at a certain time of day (in this case 2:30pm and 4pm in late March). The elevation drive could be adjusted so that the panel pointed directly towards the sun or at angles up from the sun towards the zenith. The season here in the UK is Spring, so rather than wait (possibly forever) for a sunny day with no clouds, the tests were performed when the sun was shining through gaps in the cloud, but the clouds in the sky also produce light, and probably have the affect of 'flattening' the response against elevation angle sensitivity.
Step 5: Outdoor Test Results
Voltage and current was measured at two values of load resistance - 5 and 10 ohms. There must also have been some clouds come over when I was taking the 5 ohm load 2:50pm data because above 20 degrees the power dropped off faster than expected. Some additional data taken at noon a couple of days later also has one point (10 deg) affected by thin cloud - showing the difficulty of taking this sort of data in a British climate.
Some extra data was taken when the panel was at 70 degree to the sun with higher values of load resistance to determine the load required for peak power output. The graph shows a peak power of 1.55W when the load resistance was 20 ohm and o/p volts 4.7V. The data taken on the main graph with 5 and 10 ohm loads showed only 0.36W and 0.86W showing what happens if the panel is loaded too much for the power it can produce.
There is no step 6 and I dont know how to delete it ;-)
Step 7: Conclusions
I was surprised how flat were some of the curves of the power versus angle. Certainly within the 0 to 40 deg range the panel produces little variation in power output - ie it has a good acceptance angle. The effort required to produce a solar tracker is probably not very worthwhile, especially as for this low power panel, there could be quite a power drain in the tracker itself. What was more evident was that the power that can be taken from the panel needs to be carefully controlled else the panel is operating at a low efficiency. There are circuits around for Maximum Power tracking which may need to contain voltage upconverters if a 5V output is to be maintained.
Future work that can be done is some experiments with the solar tracker - using the telescope mount itself. The mount has drive motors, so the RA drive (right ascension) can automatically compensate for the earth's rotation, and the Declination (elevation) drive can be operated by a photocell circuit to find the maximum power from the panel, or from two photo diodes into a differential amplifier circuit.