Muscle power drives battery-free electronics
Your sweat and a super-cap replace the battery.
Alexander Bell, Infosoft International Inc, Rego Park, NY; Edited by Brad Thompson and Fran Granville -- EDN, November 21, 2005
Recent developments in electric-double-layer-capacitor technology have made it possible to replace rechargeable batteries in certain secondary-power-storage applications (Reference 1). Capacitors offer significant advantages over rechargeable batteries, including a practically unlimited number of charge/discharge cycles, survival of short circuits, and simple charging circuits that require only overvoltage protection. In addition, storage capacitors recharge quickly and pose no toxic-waste-disposal problems when the product reaches the end of its service life.
This Design Idea extends an earlier one by describing a muscle-power-driven capacitor charger. The combination of a muscle-powered electrical generator and a high-value capacitor provides a highly autonomous and environmentally clean power approach for emergency equipment and survival kits. Applications of such an alternative "renewable" energy source span a range of modern portable electrical and electronic devices, including cellular phones, MP3 players, AM/FM radios, PDAs, handheld PCs, and flashlights.
A muscle-powered capacitor charger contains only a few components: a storage capacitor, a bridge rectifier, and a voltage-limiting zener diode that protects the capacitor from excessive voltages (Figure 1). For practical energy-storage experiments, you can use 1 or 0.47F capacitors with 5.5V maximum ratings, such as those available from NEC-Tokin America (www.nec-tokin.com, Figure 2). For more storage, you can use higher capacitance capacitors, such as Elna's (www.elna.co.jp) 100F, 2.5V Dynacaps (Figure 3).
You can remove the lamp from an inexpensive, hand-powered flashlight and use its generator as a capacitor charger (Figure 4). Also, a variety of manually powered products now appearing on the market offer possibilities for experimentation. For higher outputs, you can use a stationary-bicycle-powered generator. Depending on the individual providing pedaling power, these generators can deliver average powers ranging from 20 to 100W. The hand-cranked flashlight in Figure 4 originally lit a 2.5V, 0.15A, filament-type bulb, which consumes approximately 0.4W at full brightness. However, measurements show that the generator could deliver more power and could charge a 1F capacitor to 5V in approximately 10 sec. Thus, the following equation calculates the energy, E, stored in the capacitor of value C: E=½C×VMAX2=12.5J, and the following equation calculates the average maximum muscle-generated electrical power over time, T: TMAX=E/T=12.5/10=1.25W.
You can use the following equation to calculate the effective energy, EEFF, that the capacitor can deliver during its discharge cycle while its terminal voltage changes from maximum to minimum voltage: EEFF=½C(VMAX2–VMIN2), where VMAX2 and VMIN2 represent the maximum and minimum operating voltages, respectively, applied to the powered devices. You can connect storage capacitors in parallel or in series. In both cases, make sure that the circuit includes proper overvoltage protection for the capacitors. To obtain additional voltages, you can add a dc/dc switched regulator to produce stable output voltages.
Important design considerations relate to the maximum voltage and current ratings of the diode-bridge rectifier and the zener diode, DZ. Experimental measurements on the hand-cranked generator yield the following approximate values for its open-circuit voltage: maximum voltage of 10V rms, peak voltage of 14V, and maximum short-circuit current of 200 mA rms. For this application, an inexpensive bridge rectifier with 20V minimum peak-inverse voltage and 0.5A minimum forward current provide adequate margins. DZ's breakdown-voltage rating should be slightly lower than the storage capacitor's maximum working voltage, and the diode's power rating—2W in this application—should exceed the product of the generator's maximum output current and the zener's conduction voltage.
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More on Ultracaps.
Conducting some Energy Research and Googleing™ Internet on the variety of Energy topics I came to conclusion, that the idea of Capacitor-powered electronic and electrical devices eventually entered its mature stage. Reading about the latest progress made in carbon nano-tubes technology I have almost no doubts that the future portable Electronics will be powered by Supercapacitors (or Ultracapacitors), not by Batteries.
Retrospectively, the key milestones in the history of the evolution of this idea (i.e., Capacitor-based mobile power sources replacing Batteries) could be concisely described as
-3 Decades ago: it sounds like an Engineering Prophecy
-2 Decades ago: Vision
-1 Decade ago: Opportunity
- This Decade: Reality
Most likely till the end of this decade we will witness the dramatic shift from battery powered electronic and electrical devices to Capacitor Powered ones. And this goes further; Capacitors as intermediate Energy storage will not only replace the Batteries in “Mobile Energy” applications (portable devices, transport vehicles, etc.), but will find the way to the “Stationary” power sources as well in order to ease the burden of typical “peak power” issue in Power grid. Well, life is getting interesting! :)
Alexander Bell - 2006-7-12 16:07:00 PST -
FYI: “GEL Initiative”, detailing and extending the idea of muscle-powered generators in conjunction with multi-Farad power Capacitors, replacing the Batteries is now topping Google™ search list; just query on “GEL Initiative” and find the links to the original publications and other online resources. Enjoy the reading.
Alexander Bell - 2006-17-11 14:07:00 PST -
Hi Mario, Thanks for your kind attention and the posting. In this regards my comments follow:
1. This is a design idea plus a bit more: a brand new design concept of alternative mobile power source. Just watch the news for the next couple years…:)
2. As it is stated in the article – big variety (better to say - whole lot!) of existing applications (commercial, industrial, military and emergency) could benefit from this type of energy source. I would like to add just a couple examples of micro-power energy consumption devices: digital clocks and calculators…
3. Flashlight runtime: take a bigger capacitors – up to 100 F are COTS. You could also connect them in series or in parallel (but don’t forget about over-voltage protection, e.g. – by Zeners).
4. Zener’s Power rating: as it was stated in the article, originally this flashlight was intended to drive a filament-type bulb of 2.5 x 0.15 A = roughly 0.4 W. Suggested Zener diode has almost 5-times power rating (2W) over the bulb (0.4W), which should be enough for this particular application. Another estimate: maximum measured short-circuit output current was about 200 mA, which gives us a figure of 0.2 x 2.5 V = 0.5 W power dissipation. Still, it is well below 2W power rating and presumably we have a sufficient safety margin (unless something extraordinary happen, e.g. the device get into the hand of King-Kong or likes; and even though I would expect that the Generator itself will break first :-)
5. You are right: V must be squared in the equation. It is just a typo – obviously subscript/superscript is mixed (please notice that Vmax2 should actually read (Vmax)^2 and Vmin2 should be (Vmin)^2: here I am using the popular VB syntax to make things clear). BTW, the numeric estimates are accurate as they are based on the correct underlying formulas.
That’s mostly it.
Let me take this opportunity to wish you a Merry Christmas and Happy New Year.
Kind Regards,
Alexander Bell
Infosoft International Inc, NY, USA
Alexander Bell - 2005-19-12 17:26:00 PST -
Is this a design idea ? I am very surprised. If your 1F supercapacitor discharges from 5V to 3V, it deliveres an energy of 8 Joules [1/2(25 - 9)= 8]. If you are delivering, say 100 mW to the high-brightness LEDs of a torch, that will last for 80 seconds...
Apart from this, looking at fig.1, I would be very careful about the power rating of zener diode Dz: once the capacitor is charged, all the current will flow into it!
I would also recommend more care when printing equations: the stored energy equation is not 1/2CxVmax2 but 1/2C x Vmax squared. The same goes for Eeff which is 1/2C x (Vmax squared - Vmin squared)....and these mistakes are present both in the "non printer friendly" and in the pdf version of the article.
Mario Pazzini - 2005-30-11 00:29:00 PST
