Solar panel optimization- More backpacking watts?
Solar panel optimization experiments using a solar focusing mast and lightweight reflectors with backpacking solar panels.
Background
Whether skiing or walking, the full-on activities of the day will usually not leave much quality time of sunshine to charge a battery with a backpacking solar panel. This means that it will be advantageous to optimise and increase solar panel charging when the opportunity arises.
Better solar panel aiming
In a post on running a blower stove on a solar panel, I have already described a simple solar aiming device. It uses the absence of a shadow from a simple mast to set the panel surface square to the sunbeams.
“This ‘sundial’ showed me how bad my eyeball aiming was and also how quickly the shadow length can grow over a relatively short time. My tinkering with various sun reflectors also revealed that the shadow could be masked by the extra light falling from different directions, so there is another device on the drawing board that will be immune from this problem and it is there in plain sight in the photo below.”
Solar panel optimization using reflectors
Steve, the inveterate innovator behind the Ultralighthiker has made a Tyvek solar reflector trough that can enhance the charging performance. It was intended to be able to be used on a backpack while walking. I was less ambitious and thought that with the way I walk and ski that this arrangement would not be good for me. While putting my panel up on top of my pack would still be useful, I thought that a reflector that could be used in a static position during breaks in my activities would be a more practical option.
Diffuse reflection. I accept that Tyvek is highly reflective as Steve indicates, but, if I understand correctly the reflected sunlight from Tyvek is diffuse (going in all directions https://en.wikipedia.org/wiki/Diffuse_reflection).
Consequently, I thought that more directional reflectors might be more effective if they could be made as light, simple components. They could possibly be backpacked safely as flat components that are hinged and laid flat against the stiff solar panel. Then they could be rapidly and accurately deployed for optimum solar collection when required without too much fiddling about. These important practicalities can wait, as I wished to first find how much a reflector could improve solar panel output and what type of reflector materials might get the most extra watts in a busy backpacking context?
My speculation about specular reflection. I thought that I might try a shiny metal reflector (not foldable like Tyvek) that has much less diffuse reflection in favour of specular reflection (single angle reflection). “The rule of thumb is; if you see it easily then it is diffuse reflection. Does it all come back from school days? The incoming incident light angle to the reflector is the same as the reflection angle from it.”
Testing specular sunlight reflection for solar panel optimization in a backpacking context
My winter sun, as for Steves, was low in the sky and the feeble rays are arriving at 30 degrees. So I set my panel at 60 degrees to afford the best direct light interception. This also meant that I could put a horizontal reflector along the bottom of the panel and it would have a 30-degree incident angle to the sunbeams and the same reflected angle up onto the panel.
A trivia note: “Just recently I found out that the photon of sunlight that just arrived on my panel today had its birth from hydrogen fusing to helium in the core of the sun about 1,000,000 years ago. It has taken that time bouncing around, inside the sun, slowly working its way outwards so that it can finish its ~8minute journey to my solar panel. To put that escape time in context, it is about half of the duration of human existence on earth. Probably not a good fit with intelligent designers!”
With the above arrangement of the solar panel and reflector, I estimated that a mirror that was as wide as the panel body would be the most efficient as most of the specularly reflected light would target the panel. Having a wider reflector would simply cause the extra reflection to overshoot the panel.
“I even calculated that ideally, one such reflector would theoretically increase the charging power by about 50%. This is because the geometry of the 30-degree set of the reflector means that the reflector intercepts an area of solar radiation that is half the area that the panel intercepts. Read on to see if my estimation was crazy!
A control test. I started with the panel set on a dirty pale plastic table and I immediately could see that the magic 50% gain was not going to materialise. I thought that perhaps the table surface was acting as a diffuse reflector.
Consequently, I put a rectangle of black silage plastic in front of the panel to reduce the diffuse reflection. However, the shiny nature of the black plastic may make it less than perfect for this purpose. Maybe matt-black paint would be better? Possibly it should be much bigger to more effectively reduce diffuse reflection?
With an inline power meter in the cable, I connected the output of the panel to a USB power bank. After pointing the panel at the sun (no shadow from the mast), I reset the timer and mAh meter to zero and let the power meter do its thing for 11 minutes. I recorded the average output voltage as 4.84V. As 12 minutes clicked over, I disconnected the power bank and recorded the accumulated 67 mAh of current passed through the meter.
The instantaneous amperage measured by the inline device was quite variable so I used a combination of the integrated mAh and run-time as a surrogate. The average current was calculated as 0.335A by dividing the mAh by the test duration in hours.
An aluminium foil reflector test. The next test was similar to the above one, but I substituted the black plastic with a reflector made from aluminium cooking foil (shiny side up for maximum specular reflection). I re-tuned the sun angle and ran the test for 8 minutes. The average potential was 4.85V and an accumulated 65mAh of current passed through the meter. The average current was calculated as 0.4875A by dividing the mAh by the test duration in hours.
Discussion & conclusion
A pointing mast for the solar panel is a great start for practical solar panel optimization especially when the sun is invisible behind clouds. It also helps with objective comparative testing of various reflectors. However, the use of powerful specular reflectors can ‘eat’ the shadow, so I have a tricky alternative on the way to turn this technique and the problem on its head.
Making reflector/s the same size as the panel would make them very practical and able to be carried safely and efficiently with the panel in or on a backpack.
The use of a single aluminium foil specular reflector apparently increased the power of the solar panel by ~46%. This increase approximates the 50% gain that I calculated if the reflector was 100% efficient. I did this in ignorance of the high proportion of diffuse reflection that comes from such foil. I based this estimation on the fact that the effective sunlight interception surface area of the reflector, measured perpendicular to the sun’s rays, was approximately half the area of the solar panel.
I am still somewhat sceptical about my result where I measured a 46% improvement. It seems that the reflector is just working too well. This gets worse if we consider the splits between specular and diffuse reflectivities described below. Possibly, the diffuse reflection from the aluminium foil is more effectively targeting the solar panel than I am imagining. However, it is hard to imagine how a large portion of spherically distributed radiation from the reflector could intercept the solar panel. Most of it will just be bouncing back out into the universe.
Reflectivity book values. Polished stainless steel as reflectors is reported as having about 60% reflectivity. Lucky for us, simple aluminium cooking foil is much better with 86% total reflectivity (spectral +diffuse) on both the dull and the shiny sides. Another reference (as used in the calculation below) defines bright aluminium as having reflectivities of 88.1% specular and 11.9% diffuse.
A better explanation of the 46% gain. Using the higher specular reflectance value of bright aluminium it results in a theoretical gain of 44%. This makes my experimentally determined value of 46% look quite sensible, given the potential for error in these first tests that entails a lot of learning.
“I would appreciate any constructive comments about mistakes that I may have made.”
“I will do more comparative tests to see if they stand up to the time-honoured test of replication. This may take time as there is not much cloud-free sunlight and skiing is a current priority. More to come…as time permits…….”
If my results stand the test of replication, it appears that two very light, aluminium reflectors might make effective and practical reflectors for backpacking. When combined with solar focusing, they could nearly double or triple the backpacking solar panel power under marginal sunlight conditions.
Staying within reasonable limits. I should make it clear, that the idea of this solar panel optimization is not to make the panel have a greater output than it was designed for and ‘blow its grommet’. Rather, it is to improve power output under adverse conditions and make the most of brief sunshine opportunities out on the trail (more time for skiing and fishing etc). Two reflectors might be better than two panels. “And no, inverting the panel and using a big parabolic reflector is not a backpacking option for me!”
More reflections and speculations on diffuse and specular solar reflections
Spectral plus diffuse reflection. I find that is interesting to think about the nature of diffuse reflection. Pretend that we have our vision through a little hole in the panel. All the things that we see will be a source of diffuse radiation for the solar panel. This is very different to the concentrated specular reflections that can only come from a small area of a foil reflector. They both may simultaneously contribute to solar panel optimization while backpacking. Both coexist in most reflectors and diffuse reflectors abound in many backpacking kits.
The more I know what I know, the more I know what I don’t know. I find any investigation usually comes up with some useful answers that may be simple or small. However, more often than not, the list of new questions raised is far greater than those answered. This is a case in point, so here is my short list of ideas for further investigation (brain dump):
- Replicate the aluminium foil reflector test with casual diffuse reflection eliminated.
- Test a grossly oversized diffuse reflector formed from a Tyvek ground sheet or thinner polymer.
- Test a hybrid reflector composed of a small specular metal reflector that is surrounded by an oversized soft diffuse reflector.
- Test my silver-coated breathing polyester fabric as a reflector.
- Penultimately, what about two giant diffuse reflectors? Could one part of the silver wall of a tent and the other be Steves Tyvek, in the form of my regular Tyvek ground sheet pegged out below? Would this satisfy the rules of the ultralight crowd (nitpickers) if nothing extra was carried?
Backpacking charging & storage capacity
The requirement for backpacking electrical energy charging is very dependent on the storage capacity of the power bank/s. As an example, for many years, I primarily use USB power for blower stove cooking and lighting, but topping up power for other USB power devices would also be a nice to have feature.
Small power banks. When using my traditional small power banks (shown in the photo below) cooking time from each unit was about 2-3 hours at full power, but they have no charge level indicator and have erratic charging properties. As a consequence, I would invariably carry at least three of these little power banks (even for a beach fishing picnic) and they collectively weigh about 186g. Using them is a bit like Russian roulette, waiting for the fan to suddenly stop and hoping there is enough left for the remainder of the journey.
Large power banks. Now I have a Nitecore NB 10000 power bank which provides 27h of cooking time and it only weighs 150g and includes charge level indication. I have evaluated this device in another post and for shorter remote overnight backpacking trips it can provide infinite worry-free cooking and power to spare for other purposes.
Large power bank and solar charging. For extended trips in remote areas, a combination of a large power bank and a solar charger seems to me to be a wonderful combination as the charging from an optimised solar panel can easily be stored at times when the sun is kind to us for when it is not. The certainty of the spare electrical energy reserves will give peace of mind and encourage its judicious use for less important things than cooking. For example, charging phones and GPSs, particularly if some of the charge can be replenished from the sun.
For me, part of the joy of remote area activities is the lack of dependence on ‘the grid’. Providing my own limitless resources from the sun is very satisfying. Even at home, there is a feeling of resilience and comfort that comes from having these sun-provided resources available when the grid fails through excess demand, fire and floods. You can still have a hot meal from road kill and the garden and nice hot drinks and even a hot shower while the grid is restored.
Lastly, maybe I am a bit perverse, but I find considerable pleasure in running my blower stove directly on solar power using solar-made dead sticks for fuel. For example, running a tiny blower stove directly from a solar panel on a beach while fishing (even when I have a charged battery available). I am sure some readers will understand what I mean.
Oh, and this techno reflectivity babble about solar panel optimization seemed like good stuff for an ode before we finish.
A search for a weightless backpacking reflectaar, If you will excuse my bleeding Aussie vernacular, Finding specular surfaces that are minimally diffuse, Using techniques not commercial and likely obtuse, Will one be a success or both be failures specular?
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Tim
