DRAFT

Design Considerations and Resources

for

Mobile, Bike-powered Energy Systems

Tailored for

Outdoor Events and Live Performance

Understand the design requirements

Total System Power

Total system power requirements will largely be determined by the audio power amplifiers and, to a lesser extent (?), any tube amplifiers used in signal processing, such as pre-amps or sound effects.

For audio-only power supply systems, the primary power load will be the audio amplifiers, whose consumption may be estimated by applying an efficiency factor (based on the type or ‘class’ of amplifier) to the Total System Power rating of the amplifier(s). Wikipedia has a helpful article on amplifier power with a subsection on Total System Power:

Total system power is a term often used in audio electronics to rate the power of an audio system. Total system power refers to the totalpower consumption of the unit, rather than the power handling of the speakers or the power output of the amplifier. This can be viewed as a somewhat deceptive marketing ploy, as the total power consumption of the unit will of course be greater than any of its other power ratings, except for, perhaps, the peak power of the amplifier, which is essentially an exaggerated value anyway^{[citation needed]}. Shelf stereos andsurround sound receivers are often rated using total system power.

One way to use total system power to get a more accurate estimate of power is to consider the amplifier class which would give an educated guess of the power output by considering the efficiency of the class. For example, class AB amplifiers are around 25 or 50% efficiency while Class D amps are much higher; around 80% or more efficiency. A very exceptional efficiency for a specific Class D amp, the ROHMBD5421efs, operates at 90% efficiency.^[3]

In some cases, an audio device may be measured by the total system power of all its loudspeakers by adding all their peak power ratings. Many home theater in a box systems are rated this way. Often low-end home theater systems’ power ratings are taken at a high level ofharmonic distortion as well; as high as 10%, which would be noticeable.^[4]

SOURCE: Wikipedia: Audio Power

Audio power amplifiers operating with AC power supplies

The specifications for an Altec Lansing model 1593C power amplifier may serve as a very rough guide to estimating power consumption for high quality audio power amplifiers. This 50W amplifier draws power as follows:

Idle (no signal): 13W = 0.25 x of rated power capability

17W music power output: 85W = 1.7 x rated power capability

50W music power output: 145W = 2.9 x rated power capability

I’m no audio nor electronics engineer, but my guess is that an amplifier operating close to capacity for peak output levels will probably be consuming average continuous power closer to 1/2 of peak. So if your equipment shows a red peaking power indicator during normal use, you should probably estimate its continuous average power needs at about 1/2 its power output rating. If you have a 1200 W amplifier you typically operate at or near its limits, then use 600W as your music power, and figure somewhere between 2 and 3 times that as the power load on your AC power supply. So the 1200 W amp operating close to limits for peaks, and around 600 W continuous average, might consume around 1200 – 1800 Watts continuous power.

Audio power amplifiers operating with DC-direct power supplies

DC-direct powered audio amplifiers may consume 65% less energy than AC-powered configurations for the same amplifier. Note that production of DC-direct powered audio amplifiers is in revival (Rock the Bike offers DC-input modified JBL PRX amplified speaker systems). Power losses in your system will be reduced by cutting out the AC/DC conversion needed inside the amplifier, before it can use its own consistent internal DC power supply to amplify a varying-frequency AC audio signal. Perhaps more importantly, the DC output of bicycle generators or other renewable power sources (solar, wind, hydro…) may be substantially more efficient when you do not convert it to AC just to have the audio power amplifier (or signal processing equipment) convert it back to DC again to drive the amplifer gain stage, and then use the DC power to make AC at the amplifier output stage.

The Altec Lansing model 1593C is an old model capable of operating on either AC or 24/28V DC power supplies. Compared to its AC power consumption, in DC mode its power consumption is significantly reduced:

Idle (no signal): 0.2 A * 24V = 4.8W = 0.1 x rated power capability

17W music power: 2.5 A * 24V = 60W = 1.2 x rated power capability

50W music power: 4.0 A * 24V = 96W = 1.9 x rated power capability

So, that 1200W rated amp cited above in AC power input mode might use 1.5 – 2.0 times its rated power in DC-direct mode, to provide continuous music power of 600W with occasional 1200W peaks. This translates to more like 900W – 1200W (vs. 1200W to 1800W for the same amplifier converting AC power supply to DC internally). Considering power losses of up to 15% (verify?) between the DC output of the bicycle generators and the AC output of the inverter, the total power factor of a DC-direct powered audio system compared to an equivalent AC-modulated system may be as much as 1.45 (33% for DC-direct vs. AC in the amplifiers’ power supply, plus 15% for losses converting bike DC output to AC inverter output).

Calculating Total System Power

AC-powered systems

The best approach to calculating your AC power needs may be to set up your stage plugged into house line power through a Kil-a-Watts power meter (or several), for about $20 each.

Or, if your performance ensemble has jammed a lot, you can just count 1800 Watts for every 120V, 15A circuit (2400 W for each 20A circuit) you need to reliably prevent tripping the breaker (assuming your ‘jams’ are running full power for 30min straight or more — otherwise, the breaker could allow use of tens of thousands of watts for short periods, without tripping). For most bands, this is probably one or two circuits. For a full production with modest lighting and medium- to large-venue or outdoor PA, this could be three or more circuits, or as much or more than 7,200 W (aka: 7.2 kW).

DC-powered systems

Select / Assemble DC-powered audio amplification / PA equipment (with a minimum of auxiliary AC equipment, as absolutely necessary)

DC-powered audio equipment links:

Pyle 3-channel 1/4″ mic inputs to AC/DC powered 50W single-channel (mono) audio/PA amplifier
Altec Lansing model 1593C AC/DC 50W audio power amplifier
A/D/S P-series multi-channel automotive DC 12V powered amplifiers with integrated crossovers can be a flexible PA power system (and will drain a 12V battery in 1/2hour, unless there’s a continuous power supply riding on top of the batteries).

Develop and analyze the power demands for the DC- and AC-powered equipment that will be connected to the main PA system by performers

Necessary instrument amps (that absolutely refuse to be replaced by direct-injection interfaces to the PA). Note that digital signal processors in the main PA system tool-kit may be able to substitute for expensive, heavy, and power-hungry analog ‘processors’ like vintage crate amplifiers used by guitarists and basists.
Signal processors (effects pedals, etc.) and electronic instruments (keyboards, powered mics…) may be configured for AC power for ‘normal’ operations, but adaptable to a DC power supply.

Generating Power to Meet Total System Power Load

Generating Power for AC-powered systems

For AC systems, your inverter output rating should be 1.25 x the calculated load it will have to meet.

Your generating system should probably be 1.5 x the calculated load it will have to meet. For a set of bike generators, you’ll probably want a generous over-production capability, so that empty bike seats will attract pedalers without crippling your PA system.

A typical bike generator can produce 100 watts. If you pedal for an hour a day, 30 days a month, that’s (30 x 100=) 3000 watt-hours, or 3 kWh.

SOURCE: Generating Electricity with a Bicycle on MichaelBlueJay.com

Thus at 0.4 hp [~ 300 W] the “healthy human” becomes exhausted within 10 minutes! Try to decide where you fit in this curve.

SOURCE: http://www.ohio.edu/mechanical/programming/hpv/hpv.html / See also graph, below.

So a modest 3 kW portable off-grid power system will need 10 bike generators driving power through a very small power buffer, or fewer generators driving a pre-charged buffer / storage system that may have been charged by plug-in to the grid or by daytime solar power (such as an array of four 250 W solar panels collecting at full power for an average of 3-4 hours’ full-sun equivalent, per day. This solar array would produce about 6 kWh of energy and store about 1/2 that (due to losses in charging the battery), so your storage buffer would be about 3 kWh — enough to run the 3 kW PA for one hour with no generator assistance.

Generating Power for DC-powered systems

A pure DC-only setup will probably be the cheapest, lightest, and least adaptable. Almost every piece of equipment used will likely require some attention to render it functional on a DC-only power supply. But the generating system will be simple. Bikes driving power through some sort of energy buffer to a few voltage converters to accommodate different equipment needs (probably 3V, 5V, 9V, 12V, 16V, 19V and 24V). Since the biggest losses in voltage conversion will be in the biggest power consumers, the system should be designed to operate at the native voltage of the most powerful equipment components: the power amplifiers.

(Can DC voltage be up-converted efficiently? Maybe the DC generating/buffer system should operate at the maximum required voltage of any significant load anticipated for the system.)

Generating Power for Hybrid AC/DC-powered systems

Although there may be important efficiency advantages to operating an all-DC audio/PA setup, the tragic but practical reality is that many musical performers will have signal processing and even some power amplification equipment that cannot easily adapt to the DC power ultimately used internally in almost all audio and music equipment. A guitarist may have a tube crate power amplifier that forms an integral part of their particular ‘sound’. Their typical setup will be guitar plugged into amp, miked into main mixer board, amped again by the main PA (and stage monitors system). More than likely, such an amp will have an AC cord and no way to bypass the AC power supply. This is all terribly inefficient from a power supply perspective, but it allows the guitarist to get the sound they want on almost any stage.

An adaptable mobile off-grid PA system will focus on using as much DC-direct amplification equipment as possible in the main PA, while allowing for enough AC capacity to handle last-minute equipment changes, and the various difficult-to-adapt AC components of the performers’ equipment. Unless the production has an unlimited budget and virtually unlimited, dependable people-power to move equipment and operate generators, performer participants should be warned well ahead-of-time, that the off-grid mobile stage will be operating within tight design tolerances, and that surprise AC power needs may not always be accommodated.

Requirements for all PA portable off-grid power systems

Make Bike Generators:

A 12v generator will likely need to be connected in series with one or three others (system voltage is additive in series), to reach the 24v or 48v ideal voltage input for most off-grid type power inverters.

What’s realistic in terms of human power production via pedaling?

Ohio.edu has some good information on human power performance, summarized in the unit of horsepower (746 Watts, not counting losses in the generator):

Time to exhaustion vs. human output power, in horsepower.

SOURCE: http://www.ohio.edu/mechanical/programming/hpv/hpv.html

Healthy adults should be able to reliably produce 200-300 watts from an efficient generator, for around 20 or 30 minutes — a good generous time-frame to allow for down-time during rider changes and to be sure riders are pedaling at a rate that’s sustainable for them.

Bicycle Generator links:

http://forums.makezine.com/comments.php?DiscussionID=5251
Harbor Freight $15 drills as hand-crank generator (adaptable to bike stand power take-off?)

Energy buffer (battery and/or capacitor?)

Human bicyclists may not always deliver power when and as needed. Some systems may draw more power than can be reasonably expected from a bank of bike generators, so you’ll need to incorporate pre-stored energy. How much can one or a few batteries, or a capacitor bank, do to supplement, when people aren’t pedaling, or are not pedaling hard enough? Advantages of one over the other? Combination?

Battery:

Batteries will require less technical know-how to design the system and operate it. The hazards of up to 5 interconnected batteries are easily-comprehensible ones of chemical spill and the need for ventilation for off-gasing during charge/discharge.

The energy density (kWh per pound) of a battery is probably not dissimilar to capacitors. The energy density (kWh per volume) of a battery is going to be MUCH greater than capacitors — capacitors will take up a LOT of space to deliver the same energy as one 12V marine (deep cycle) battery.

Capacitor:

Capacitors could be very light-weight, but will take up more room in supplying even 1/10 of the stored energy of a battery. Also, they will have to be more engineered and better protected from untrained end-users, since the wrong connection could cause a capacitor to explode, causing serious injury. Also, people may be familiar with batteries as a source of islanded energy, while capacitors’ ability to deliver lethal shocks and explosive short-circuits while disconnected from other components may surprise those unfamiliar with safe handling.

Although a battery-sized capacitor case will provide much less energy buffer than a battery, it will weigh about the weight of the case the capacitors live in, times 2, whereas the battery will weigh 50 lbs. and must always be stored upright and in a ventilated area and, ideally, in cooler places. A capacitor may improve amplifier audio performance by providing capacity for high power, transient (micro-second) demand that the slower chemistry of a battery cannot always deliver.

Requirements for portable off-grid power systems incorporating some AC loads:

Combine 12V Bike Generator outputs into regulated 48V DC inverter supply:

Multi-component system using separate charge controller(s), battery system, and inverter

Insert a Charge Controller between generators and battery bank. The EcoPower Model 182 MPPT charge controller features a power level and charge indicator display. Although not as dramatically visible as the visual display from Rock-the-bike, it could provide a bike-power station conductor with the necessary information to direct the pedalers. This model is capable of 300W charge power operating into 12V battery system, or 600W operating into a 24V battery system. Each unit could handle up to 8 or 10 bicycles generating at moderate effort (fewer when pedalers are working as hard as they can).
Battery bank should consist of one or more batteries as needed to deliver the ideal voltage for the off-grid type inverter (probably two or four 12V batteries for 24V in series/parallel or up to 48V with four in series). A typical car-sized deep cycle/marine battery may store about 50 Ah of energy, or 600 Wh, or 0.6 kWh. So a one-hour storage buffer for a 3 kW power system would need about 5 or 6 batteries. Each battery will weigh around 50 lbs. for a total storage system weight of about 300 lbs. This would allow you to operate a minimum of 1 hour and probably for 1.5 hours with 5 or 10 pedalers contributing the whole time.
Inverter could be a simple inverter with no display or frills (or a bank of several). Inverter replacement would be easier (and cheaper) in an emergency. Ten (10) of a Model 237 (about $650 for all) would provide 3,000 W of power on (10) receptacles (possibly none of which would provide enough power for some audio amplifiers).

Secure a robust off-grid inverter from physical damage, dust/smoke, and moisture; keep it cool

The inverter (with battery charge controller?) will be the heart of any mobile system capable of supplying AC loads (cord-and-plug connections like you have in your house). It will also be the single most expensive component.

HomePower Magazine has a beginner-oriented “choosing an inverter” article that provides some fundamental information about inverters, as well as specific information about available products in various categories.

Bike-powered cooling fans in a convertible closed/semi-closed road case? Will the bike fans correspond sufficiently with bike 12V energy input, to cool the inverter at appropriate times?

avancerichs

Tuesday, April 9, 2013

Bike-powered mobile AC/DC stage power system