Make Battery Last Longer

[cagiva]

We're looking to install flex solar cells on a Flying Wing UAV for the sole purpose of extending its flight time another 15-20 minutes. The vehicle is currently able to fly for 50 minutes on just one battery, and we're wondering if it would be possible to extend that time by connecting solar cells in parallel with the battery (see diagram below).

Please notice that we're not trying to fly on pure solar, or charge the battery during the flight, or break an endurance flight time record. Just extend the current flight time by helping the battery to last a little bit longer with the small power added from the solar cells. Thanks in advance!

Flex Solar Cells

7.4v Battery

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[SunEagle]

I approved this post and deleted the other one since it was a duplicate. Your posts were unapproved probably due to you being new and having multiple website links.

[jflorey2]

We're looking to install flex solar cells on a Flying Wing UAV for the sole purpose of extending its flight time another 15-20 minutes. The vehicle is currently able to fly for 50 minutes on just one battery, and we're wondering if it would be possible to extend that time by connecting solar cells in parallel with the battery (see diagram below).

Please notice that we're not trying to fly on pure solar, or charge the battery during the flight, or break an endurance flight time record. Just extend the current flight time by helping the battery to last a little bit longer with the small power added from the solar cells.

Sure. Alta has some 30% efficient superlight cells intended for just this purpose. They're also somewhat flexible so they will be able to follow airfoil contours. You'll want to design the array to give you about 30% more voltage than you need, then drop it down with the MPPT's. Be careful if you use film to hold the cells onto foam - that combination (sun exposure+good insulation) can lead to _very_ hot cells.

[Sunking]

Well being a RC Pilot and an engineer you got one little problem called physics and the Laws of Flight you cannot dismiss. Those Laws are:

Lift, Drag, Thrust, and Weight.

They must balance out and cancel each other out. Adding panels adds weight and drag. To cancel that out you must add thrust (power) to generate more lift to carry a heavier plane. To do that requires more power from the power plant.

Now here is where physics can get in your way. Unless you have some very specialized expensive panels in terms of efficiency, watts, and weight per square area, it will take more power than the panels to generate to over come added weight and power required to lift the added weight of the panels. End result is shorter run time.

[cagiva]

SunEagle Thank you for the approval and sorry for the double posting. After my first post, I realized that the "Batteries / energy storage" group was a better place. I'll use this one for now on.

jflorey2 We have a very small budget for our research project; which it might be the reason why Alta Devices has not responded to our inquires. On the other hand, PowerFilm has a alternative product that seems to meet our requirements (flexible to follow airfoil contours and light) and comes with a Pressure Sensitive Adhesive (PSA) backing.

Good suggestion about supplying the MPPT with ~30% more voltage since our MPPT allows that (up to 18v input -> nominal 12V output). The cells getting hot is also concern, but PowerFilm claims that their PSA backing provides some level of insulation. I'll let you know the results.

Sunking In our previews flights (with just the battery), we collected data under the two main flight conditions: at takeoff and at level flight.

At hand launch, the propulsion gives us a thrust higher than 4N; which ensure a good start. During level flight at a speed of 8.3m/s, the propulsion provides a thrust of 1.1 N at an overall efficiency higher than 70 %. In the two cases, we can validate that we have room to add the flex panels; which will add about another 40 grams to the vehicle (including the wires and the mini MPPT board).

Just for an optical reference, our vehicle looks similar to the one posted in the Alta website, but definitely smaller (wingspan 1m).

Thanks again for the prompt comments and feedback.

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[jflorey2]

We have a very small budget for our research project; which it might be the reason why Alta Devices has not responded to our inquires. On the other hand, PowerFilm has a alternative product[/URL] that seems to meet our requirements (flexible to follow airfoil contours and light) and comes with a Pressure Sensitive Adhesive (PSA) backing.

I would recommend against using them. They are amorphous cells that are about the lowest efficiency out there (3%). They may literally be worse than nothing; the weight they add may well reduce your range more than the generation capacity improves it.

If you can't get Alta cells, Sunpower cells are about the best silicon cells out there, and are generally 22% efficient - about 7x more efficient than Powerfilm cells. You can get 3W cells for $5 on Ebay and the like - so they are cheaper than Powerfilm as well. They are super light, slightly flexible and will conform to gentle curves. You'll have to do the interconnects yourself, of course.

The cells getting hot is also concern, but PowerFilm claims that their PSA backing provides some level of insulation. I'll let you know the results.

Insulation will make things WORSE not better. You want some path for the heat to exit the cell.

[Garrett]

Cagiva- It would be interesting to know what the efficiency affects would be with your aircraft on a low density altitude day compared to a high density altitude day; both being sunny of course. It would be tricky to compare the solar characteristics as they would be vastly different from season to season as the angle of the sun would hit the wings differently.

I'm curious to know your results on wind velocity over the wing and how it effects the panels delta-T.

What's the project for? Pleasure? School?

[cagiva]

@Garrett, It's mainly a research project to test a new GNSS PPK (Post Processing Kinematic) system. However, we need to extend the flight time a little bit longer to collect more data; which is why we're trying to add the solar cells to our sUAS. At the moment, our vehicle is setup to fly continuously at low altitude so we've characterized it for the summer conditions.

@jflorey2, Sunpower cells were our first choice, but our current vehicle wingspan is not big enough to accommodate the number of tiles we need to match our required voltage. I agree that the PowerFilm cells have the lowest efficiency, but we will give them a try and post data results back here.

[SunEagle]

My concern is the weight of the MPPT CC's that work with Li batteries. They may reduce the flight time due to their extra weight.

I have seen other RC planes that included solar panels into their design but they were mostly large wing powered gliders that used the solar cells to extend the receiver & servos power requirements ( think it was 3S). The main thruster motor was on a different battery system (4S) which allowed them to have two different voltage systems.

[J.P.M.]

[QUOTE=Garrett;n381898]
I'm curious to know your results on wind velocity over the wing and how it effects the panels delta-T. /QUOTE]

FWIW, from convective heat transfer considerations, and doing an energy/heat balance on the wing/panel assembly, and using a stated air velocity over the wing of ~ 8.3 m/sec, that the wing/panel assembly temp. will probably be fairly close to the ambient air temp., particularly given the likely thin wing section and light weight.

While convective heat transfer theory is quite well developed, empirical wind convective coefficients over (mostly) flat plates, either one or 2 sided, are all over the board, and quite dependent on the application. For a couple of examples only, for one sided heat transfer, McAdams suggests a value of h ~ = 5.7 + 3.8 V for one sided surfaces (with the other side insulated), where h is the convective heat transfer coeff. in W/(m^2*deg. C) and V is the wind vector in m/sec. But, as Duffie & Beckman note, that correlation may have radiation effects buried in it. A guy by the name of Watmuff (some names you can't make up !) suggests using h = 2.8 + 3.0 V, with the radiation heat transfer effects removed but without stating the mechanism of removal. Stuff I've done with my array suggest a gross two sided heat transfer coeff. of 21.7 + 6.32 V W/(m^2*deg. C) per m^2 of panel area. My number has thermal radiation effects in it, but preliminary stuff I've done looking at that suggests that lowering the result by about a third seems to produce a number for convective heat transfer alone that makes some sense with respect to what the radiation heat transfer might be after accounting for panel, effective sky and the roof temp. under the array.

I too would be interested in what investigations by the OP may turn up. There are about as many empirical heat transfer correlations in the literature as there are investigators to look for them. Fertile ground for the white collar welfare of graduate theses in mechanical or chemical engineering. Theory is a good guide, but for convective heat transfer, the number of variables that can and do influence the actual heat transfer means that theory will never be as accurate as actual and careful experiment and measurement for a particular application, probably more so than for some other disciplines.

[cagiva]

SunEagle We're using a lightweight MPPT (picture below). An additional function of this MPPT is to monitor the current and the voltage of each solar module and make that information available for the control and navigation system through an I2C protocol. This information is communicated to the interface on the ground control station so that the operator is aware of the energy received from the sun in real-time.

J.P.M. Our Matlab model estimates that 3 % to 5 % percent of the solar energy will be lost and converted into heat, especially in the diodes, transistors, and inductances. Considering the very small surface of the MPPT, this can make its temperature increase up to 110◦C at noon. In order to monitor this effect, two temperature sensors are placed on the printed circuit board and connected to the microcontroller. It is thus possible to react from the ground if necessary by stopping the MPPT or limit the current to a certain threshold.

I'll post results once we have completed new test flights with the panels.

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[J.P.M.]

SunEagle We're using a lightweight MPPT (picture below). An additional function of this MPPT is to monitor the current and the voltage of each solar module and make that information available for the control and navigation system through an I2C protocol. This information is communicated to the interface on the ground control station so that the operator is aware of the energy received from the sun in real-time.

J.P.M. Our Matlab model estimates that 3 % to 5 % percent of the solar energy will be lost and converted into heat, especially in the diodes, transistors, and inductances. Considering the very small surface of the MPPT, this can make its temperature increase up to 110◦C at noon. In order to monitor this effect, two temperature sensors are placed on the printed circuit board and connected to the microcontroller. It is thus possible to react from the ground if necessary by stopping the MPPT or limit the current to a certain threshold.

I'll post results once we have completed new test flights with the panels.

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We're talking about two different animals. That would be 3% to 5% of the electricity that's generated. Actually, depending on the type of PV device you'll wind up with something like ~ 85 - 90+ % or more of all the incident solar radiation that will hit the wing/PV assembly will be rejected as heat via convective and radiation heat transfer mechanisms with the remainder of the incident solar energy being turned into electricity. You may well lose about 3 - 5% of that generated electricity through other parts/components of the system, but compared to what's incoming, that's a relatively small amount. Like ~ 0.05 * 0.15 = ~ 0.008 of the incident solar radiation.

Given all the likely heat transfer going on via convection and the probably thin cross sections and lack of thermal insulation, I seriously doubt that's enough thermal energy to raise the temperatures by anything that can be measured much less of any impact.

[jflorey2]

jflorey2, Sunpower cells were our first choice, but our current vehicle wingspan is not big enough to accommodate the number of tiles we need to match our required voltage.

So use a boost MPPT instead of a buck MPPT.

Anyway the MPPT you have posted above looks very big and heavy; those ferrite inductors are not light. There's no way you need those size components to deal with the power you are anticipating (<1 watt per array.) Even if you switch to Sunpower you wouldn't need something even a fraction of that size.

[Mike90250]

i'd be comfortable saying ditch the charge controller and use a zener diode. something like a 5w zener, at the right voltage for the batteries, would be the ticket, won't need much of a heat sink either.

[bcroe]

i'd be comfortable saying ditch the charge controller and use a zener diode. something
like a 5w zener, at the right voltage for the batteries, would be the ticket, won't need much of a heat sink either.

I will second that, just try to get a good match between the cells and the battery operating voltage. If
that is done you have little to gain from any kind of charge controller. All that other stuff is going to
add weight. With a motor load greater than the cells can deliver, there is no danger of over charging
the battery. I would even skip the zener, if the cells are never connected when the motor is off. Bruce Roe