Easy Parabolic Mirror from a Trash Can Lid

Parabolic reflectors for solar applications are nice stuff, and making your own is a great project in itself. One of the easiest ways we have seen is that of [GREENPOWERSCIENCE], who uses nothing more than a trash can lid, mylar film, and tape. You need a way to make a partial vacuum though.

The idea is so simple that it´s almost like cheating. Cut a circle of mylar slightly larger than the lid, and tape it all around, taking care of stretching the mylar in the process. After you´re done with this, you end up with a nice flat mirror. Here´s where the vacuum is needed to force the film into parabolic shape. Extract the air from a little hole in the lid that was previously drilled, and tape it to prevent the loss of the vacuum. The atmospheric pressure on the mylar film will take care of the job, and magically you get a nearly-parabolic reflector ready for work.

In this other video, you can see the reflector in action burning stuff. One obvious problem with this technique is the loss of the vacuum after some time, about an hour according to the author. Here´s another way to make a more durable mirror also with mylar as the reflecting element, however the quality of the resulting mirror is not as good.

 

Homebrew Powerwall Sitting at 20kWh

Every now and then a hacker gets started on a project and forgets to stop. That’s the impression we get from [HBPowerwall]’s channel anyway. He’s working on adding a huge number of 18650 Lithium cells to his home’s power grid and posting about his adventures along the way. This week he gave us a look at the balancing process he uses to get all of these cells to work well together. Last month he gave a great overview of the installed system.

His channel starts off innocently enough. It’s all riding small motor bikes around and having a regular good time.  Then he experiments a bit with the light stuff, like a few solar panels on the roof.  However, it seems like one day he was watching a news brief about the Powerwall (Tesla’s whole-home battery storage system) and was like, “hey, I can do that.”

After some initial work with the new substance it wasn’t long before he was begging, borrowing, and haggling for every used 18650 lithium battery cell the local universe in Brisbane, Australia could sell him. There are a ton of videos documenting his madness, but he’s all the way up to a partly off-grid house with a 20kWh battery bank, for which he has expansion plans.

There’s a lot of marketing flim flam and general technical pitfalls in the process of generating your own non-grid electricity. But for hackers in sunny areas who want to dump those rays into local storage this is an interesting blueprint to start with.

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Portable Battery Bank Only Looks Like a Bomb

If one of the design goals of [wsw4jr]’s portable solar battery bank build was to make something that the local bomb squad would not hesitate to detonate with a water cannon if he leaves it unattended, then mission accomplished.

We kid, but really, the whole thing has a sort of “Spy vs. Spy” vibe that belies its simple purpose. A battery bank is just an array of batteries, some kind of charge controller, and an inverter. The batteries are charged by any means possible – in this case by a small array of solar panels. The mains output of the inverter is used to power whatever doodads you have.

[wsw4jr] didn’t mention of the inverter specs, but from the size of the batteries and the wiring – both of which he admits are not yet up to snuff in his prototype – it’s a safe guess that the intended loads are pretty small.  Tipping the scale at 60 pounds, the unit tends toward the luggable end of the portability scale. Still, this could be a great tool for working out in the field, or maybe even tailgating.

We’ve seen expedient battery banks and emergency power from cordless drill batteries before, but this build is quite a bit more sophisticated. We’ll be watching for updates on this one.

Characterizing a Death Ray… er, Solar Oven

Many of you will probably at some point have looked at a satellite dish antenna and idly wondered whether it would collect useful amounts of heat if you silvered it and pointed it at the sun. Perhaps you imagine a handy source of  solar-cooked hotdogs, or maybe you’re a bit of a pyromaniac.

[Charlie Soeder] didn’t just think about it, he did it. Finding a discarded offset-focus DirecTV dish, he glued a grid of 230 inch-square mirror tiles to it and set to investigating  the concentrated solar energy at its focus. 

Cotton waste, newspaper, and scraps of fabric char and burn with ease. A cigarette is lit almost from end to end, and it burns a hole right through a piece of bamboo. Most of the energy is in the form of light, so transparent or reflective items need a little help to absorb it from something dark. He demonstrated this by caramelizing some sugar through adding a few bits of charcoal to it, once the charcoal becomes hot enough to caramelize the sugar around it the spreading dark colour causes the rest of the sugar to caramelize without further help.

Solar furnace calculations
Solar furnace calculations

To gain some idea of the power of his solar furnace, he recorded a time series of temperature readings as it heated up some water darkened with a bit of charcoal to absorb heat. The resulting graph had a flat spot as a cloud had passed over the sun, but from it he was able to calculate instantaneous power figures from just below 30W to just below 50W depending on the sun.

He records his progress in the video you’ll find below the break. Will we be the only ones casting around for a surplus dish after watching it?

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Working in Peace With an Off-Grid Office Shed

Finding a good work space at home isn’t a trivial task, especially when you’ve got a wife and kid. A lot of us use a spare bedroom, basement, or garage as a space to work on our hobbies (or jobs). But, the lack of true separation from the home can make getting real work done difficult. For many of us, we need to have the mental distance between our living space and our working space in order to actually get stuff done.

This is the problem [Syonyk] had — he needed a quiet place to work that was separated from the rest of his house. To accomplish this, he used a Tuff Shed and set it up to run off-grid. The reason for going off-grid wasn’t purely environmental, it was actually more practical than trying to run power lines from the house. Because of the geology where he lives, burying power lines wasn’t financially feasible.

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Dual Axis Solar Tracker with Online Energy Monitor

[Bruce Helsen] built this dual axis solar tracker as one of his final projects for school.

As can be experimentally verified in a very short timeframe, the sun moves across the sky. This is a particularly troublesome behavior for solar panels, which work best when the sun shines directly on them. Engineers soon realized that abstracting the sun away only works in physics class, and moved to the second best idea of tracking sun by moving the panel. Surprisingly, for larger installations the cost of adding tracking (and its maintenance) isn’t worth the gains, but for smaller, and especially urban, installations like [Bruce]’s it can still help.

[Bruce]’s build can be entirely sourced from eBay. The light direction is sensed via a very clever homemade directional light sensor. A 3D printer extruded cross profile sits inside an industrial lamp housing. The assembly divides the sky into four quadrants with a light-dependent resistor for each. By measuring the differences, the panel can point in the optimal direction.

The panel’s two axis are controlled with two cheap linear actuators. The brains are an Arduino glued to a large amount of solar support electronics and the online energy monitor component is covered by an ESP8266.

The construction works quite well. If you’d like to build one yourself the entire BOM, drawings, and code are provided on the instructables page.

 

Perovskite Solar: Coming Soon?

Making solar cells out of silicon is difficult. There’s plenty of manufacturing steps, many of them at very high temperatures, and you need a high vacuum and a clean room. However, perovskite solar cells–cells made with hybrid organic-inorganic materials in a perovskite crystal structure–are relatively easy to make using wet chemistry involving solvents or vapor deposition.

In theory, silicon solar cells could be 30% efficient, but in reality, 25% seems to be a practical limit with commercial cells typically topping out at 20%. Perovskite cells are nearly that high now, and could be higher by stacking thin layers, each sensitive to different wavelengths of light.

A recent development at the Lawrence Berkeley National Laboratory may lead to even more efficient perovskite cells. Researchers found that certain crystal structures had a much higher efficiency than other structures. The problem now is figuring out how to produce the crystals to increase the prevalence of that structure.

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