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ModeratorPermalink Reply by Chris Anderson on August 20, 2008 at 7:34am

These guys just set a US endurance record for solar powered flight. Check out this thread for more.

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Permalink Reply by HugePanic on September 17, 2008 at 9:53pm

Did you know that some guys from switzerland just recently got the "unofficial" record for a long-time-uav flight??

search for "ZEPHYR".
it flew for over 83h.... !!!

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Permalink Reply by Scott James on November 14, 2008 at 9:33pm

Hi Brandon,

This project is on hold as we are starting to build a UAV for the next Outback UAV challenge!

I have considered mounting the cells inside the wing (close to the covering) and then covering the wing with a clear covering over the areas where the cells are. Hopefully I won’t loose too much light from the covering. The other consideration will be somehow letting some airflow through so the cells don’t get too hot and loose efficiency.

I found some good ideas here.
Where are you getting your cells from as I am having a hard time sourcing them?

Scott

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Permalink Reply by bGatti on November 24, 2008 at 8:37pm

After some thought.

For systems with batt and solar,
One could install two ESC's - one to connect the battery to the Motor, and the second to connect the solar cells to the same motor.
This overcomes the need for an MPPT and a DC to DC converter. The solar circuit includes a capacitor.

When the capacitor is charged above the target voltage (peak mppt for current sun conditions), turn on the Solar ESC until the capacitor drops below the target voltage. If you need battery power, you can use batteries during the time the capacitor is charger. You can vary the solar to battery ratio by changing the cycle ratio, But generally, you'll run on one or the other and not both. The total power is controlled by the throttle level to the ESCs. (don't throttle them up together).

To charge the batteries, include a charging transistor and a cheap lion charging circuit. ($5 model) power this circuit when the capacitor is above target voltage AND you're above target altitude.

(probably a very bad idea to power them both up together as even the power up chirp could cause a short.) don't connect the common ground until after power up.

The objective here is to avoid a complex, inefficient and heavy DC chain between your power sources and the motor.
The parallel ESC's offer a single transistor connection between the solar cells and the motor, and alternatively between the batteries and the motor. MPPT tracking can be managed by adjusting the motor power/climb rate and/or charging rate. (I can't think of any benefits of adding extra transistors in a discrete MPPT between the solar cells and motor.)


2 ESC $20
1 Motor $20
1 Lion Charger $5
1 ArduPilot Pro ($?)
1 Power transistor for Charger
1 Capacitor for Solar
1 Current Sensor for Solar Cells
x Solar Cells

MPPT Algorithm includes measuring current and voltage of solar circuit and adjusting the target voltage up and down to find the voltage at which the solar cells are producing peak wattage. In a separate loop, if the solar voltage is above the target, then use the power, if below, then stop using the power. The cell voltage must be higher than the battery to be useful. The second ESC solves the problem of voltage mismatch between the solar circuit and the battery potential.

Ben

P.S The nasty problem with parallel ESC is the potential for shorts - to address this place a fuse, and perhaps a inductor between the battery ground and the solar ground as this will minimize the effect of an unpleasant H-bridge short. Better would be a self-resetting fuse.

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Permalink Reply by Scott James on November 25, 2008 at 2:18am

There sure is allot to consider!

I think I will attempt a solar only UAV first with maybe a low voltage cutout to switch to a very small battery (just for the electronics) so I can still land it if there happened to be a sudden eclipse! :)

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Permalink Reply by Kevin McHale on May 14, 2009 at 11:08am

A properly designed MPPT is a DC/DC converter with a clever feedback loop Basically, an MPPT uses the ripple from the DC/DC conversion to find the peak power point. There are some off the shelf MPPTs that have adjustable battery (output) voltage. If you use one of these, it takes the guesswork out of the solar cells.

An important thing to remember when using bare cells (not a pre-made panel) is to either make sure they have intrinsic bypass diodes or to put your own in. Basically, it will prevent every cell from being reverse-biased and ruined (in case of physical damage).

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Permalink Reply by bGatti on May 15, 2009 at 11:15am

Kevin,
Since a DC/DC converter introduces some inefficiency, I proposed using a separate esc to drive the motor (one at a time) one from batt and one from solar. The MPPT algorithm is then applied by the flight logic such that the plane climbs steeper or less steep in order to optimally discharge the solar cells.

In this configuration, the esc /is/ the mppt.
for battery flight, shut down the throttle on the solar, and run up the batt esc. The batt esc can be smaller.
There is no real option for using both simultaneously. (unless you custom design such).

There is also a charging switch between the solar and battery - but this is low current as the charging current is only what is left after the main propulsion current is satisfied.

For inferior lighting conditions, the charging circuit can be used to combine battery and low solar power.
Ben

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Permalink Reply by Mike on May 15, 2009 at 9:48am

Check out University of Michigan SolarBubbles team goal to design, build and test a solar-powered unmanned aerial vehicle capable of 36+ hours of flight with less than a 15 foot wingspan.

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Permalink Reply by Mike on May 15, 2009 at 2:13pm

Solar UAV QinetiQ Zephyr breaks world record - 54 hours.

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