ReRev, Human Dynamo,
and the Green Revolution

Generating Electricity from Exercise Machines

Psomas - President & CEO
Hudson Harr started with an elliptical machine
and parlayed his invention into ReRev.
By Tom Gibson
Just four short years ago, Hudson Harr was a 21-year-old freshly minted college graduate armed with electrical and mechanical engineering degrees from the University of Florida. He had a vision: to create an exercise machine for health clubs that would generate net electrical power. Operating out of his mother’s house, he amassed a collection of parts and cobbled prototypes together. “I was on a bootstrap budget, and I spent everything I had on ellipticals and inverters,” he recalls.
Where did this vision come from? One day, Harr says, “I thought, we’re trying to harness all these kinetics in the world, and here we have an indoor environment, tons of infrastructure, stable kinetics day in and day out. There are so many things that make for the right kind of application, and it just suddenly hit me that nobody had done this before.”
Harr’s efforts paid off, as he went on to found a multi-million-dollar company ReRev in Clearwater, Florida to produce fitness machines that generate electricity, and he now serves as its president. And he has expanded his reach by applying the same renewable energy technology to commercial solar and wind power applications, creating a second company called Sunquest.
Actually, Harr is one of a trio of entrepreneurial engineers that have started companies in the last four years to develop and market these fitness machines, each taking a unique approach. They find themselves creating a new industry, one that combines physical fitness with renewable energy. Such a concept has been viewed as a novelty for a long time, but now it’s going mainstream and becoming a serious entity. Most aerobic equipment, including stationary bicycles, Stairmasters, rowing machines, and treadmills, can be configured to generate power to the electrical grid as people sweat.
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The Green Revolution retrofits onto existing machines such as these exercise bicycles in a spinning class.
People argue that an individual machine doesn’t generate enough power to make a difference. But Mike Taggett, founder, R&D director, and president of Henry Works Research and Development in Hurricane, Utah and inventor of the Human Dynamo machine, points out that “a lot of gyms have large spinning group exercises with 30 or 40 or 50 people exercising at the same time, and that’s a perfect environment to use a machine like this.” With dozens of machines running for 10 or more hours a day, they harvest enough energy to pay back their owners within 8 to15 years, roughly the same payback as solar electric.

Use Existing Technology
Many of us may remember using a generator mated to the rear tire of a bicycle to power front and rear lights as we pedaled along. Generating power in a gym is based on the same principle. New technology enters the picture in the form of inverters and controllers developed for renewable energy applications such as solar and wind power systems. In most cases, existing machines are retrofitted, but some companies have designed new machines from the ground up.
It turns out most exercise equipment already comes with a generator on it to power its display. Increasingly, such machines are free standing in that the generator supplies all their electrical needs. Harr reports, “Gym equipment over the last few years has transitioned from alternators to generator-based drive trains. We make use of the algorithms and control board interface used to load these.” These generators use permanent magnets rather than electromagnetic coils to avoid having to supply a current to the rotor to magnetize it; renewable energy equipment such as wind generators have increasingly converted to this. By their rotating nature, alternators and generators generate AC power, and components within them convert it to DC.
Psomas - President & CEO
Jay Whelan at The Green Revolution has an
industrial engineering background.
“We tap into the power and create another control sequence to feed it back to our equipment,” Harr continues. “So essentially we remove the internal resistance the machine has and give it an external resistance load, which is our equipment and processing units.” Taking place at the individual machine, this increases the resistance inside the generator, increasing the power needed to drive it and consequently the power coming out of it. With this, someone using the exercise machine can vary the difficulty of their workout and the power they produce.
It varies by company, but typically, the DC power coming off each machine is wired to a central processing unit containing an inverter, and this box in turn is connected to the building’s electrical system and ultimately the grid. The power coming off is not clean enough, U.L. approved, or in the right frequency. Harr states, “We actually invert the DC power through the inverter, which cleans it up and exports it back to the utility as a cosine wave with low harmonic interference.”
To give an idea of how much power humans can crank out, a person in decent shape can generate 60 to 120 watts during an hour of strenuous exercise, and an elite cyclist can average 300-400. If a person averages 75 watts (about one-tenth of a horsepower) for an hour, they will produce 75 watt-hours of energy. As a reference, ReRev says 50 watt-hours of electricity can power a CFL (compact fluorescent light) bulb for 2.5 hours, a cell phone charger for six full charges, a laptop for one hour, or a desktop computer for 30 minutes.

Not that Easy
But while the benefits of exercise machines that produce power in a group situation may seem obvious, developing and marketing them has proven an uphill battle. Harr cites expensive hardware as an obstacle. Jay Whelan, founder and CEO of The Green Revolution in Ridgefield, Connecticut, the third company in the trio of those pursuing the market, relates, “It’s a combination of two things. One is that it’s brand new equipment; we’re almost creating a new industry that didn’t exist two years ago. The other thing is, it takes awhile for gym owners to understand how enthusiastic their club members are about sustainability and renewable energy.” Previously a partner at Accenture, Whelan received a B.S. in Industrial Engineering from Rochester Institute of Technology and an MBA from Boston College.
Psomas - President & CEO
Henry Works founder Mike Taggett (left) and his
machinist Carlos Esqueda show a system they’ve
developed for sending power generated by a
machine to the electrical grid through a wall outlet.
“It’s kind of funny how it evolved,” Whelan says in telling how The Green Revolution came about. He recalls, ”While working out one day, I started to calculate the amount of energy expended at the gym. Stationary bikes create resistance and through this friction, heat is produced. The industrial engineer in me said, ‘What a waste! There’s got to be a way to capture and use this energy.’” He continues, “Mark Sternberg, a friend and millwright, had a ten-speed bike, and he put the rear wheel up on a platform like a triangular-type structure. Then we attached a car alternator to the back wheel – very rudimentary.” They used a DC dimmer switch to increase the field coil currents to draw more power, increasing the resistance level. But they found that the ten-speed bike wheel just didn’t have enough mass from an inertia standpoint. So Sternberg found one of those big metal turning wheels from a lock on a river that weighs 80 to 100 pounds.
Their original concept was to build bikes with the generators on them, but when they held focus groups with club owners, the owners made it clear they didn’t want to buy all new equipment. They decided to build an attachment. Whelan reports, “We saw that spinning bikes, or the indoor cycles, have a 55-to-60-pound flywheel on the front, so we didn’t have to change a thing. We attach it right to the front wheel of the indoor cycles.” Green Revolution employs a 24-volt system with two 12-volt batteries in series. When a user starts pedaling, the batteries charge, and when they hit 28 volts, the inverter kicks in and sends power to the grid, converting 24-volt DC to 110-volt AC.
For engineering, The Green Revolution uses Gyre 9, an outside consulting firm in nearby Oxford, Connecticut with which they’ve forged a strategic partnership. The full-service engineering firm knows electronics for building circuit boards and electricity for the generators and grid-tied equipment.

Trade Show Attraction
Mike Taggett at Henry Works grew up in Arizona and went to the University of Arizona. “I bounced around in liberal arts and ended up in Latin American studies,” he recalls. He worked as a river guide and started a business making an eyeglass retainer he invented in the off-season called Chum, a little tubular fabric retainer with elastic in the ends. In the late 1980s, “I had the idea for our trade show booth to have a converted exercise machine that would generate electricity so we could power a blender or lights or something just for fun. We made smoothies and stuff.”
Psomas - President & CEO
Human Dynamos connect by a shaft to a single
generator to become a Team Dynamo.
Taggett later came up with the Human Dynamo design, which has a few unique twists. For starters, they added hand cranks to go with the leg cranks to provide a rigorous upper-body workout and generate additional electricity. They’re chained together in the drive train, so the arms and legs spin at the same speed to turn the bike’s 18-inch flywheel. An average person spins the pedals about 60 times a minute, according to Taggett, and this translates to 300 rpm in the flywheel.  A belt then connects the flywheel to the generator, taking the speed up to 1500 rpm.
As another unique aspect, Taggett is testing a way of connecting the machine’s output directly to the power grid without going through the wiring, conduit, and boxes found on other systems. This system is called the Firewheel Inter Grid Generator (FIGG). When a person first gets on a Human Dynamo, the FIGG acts like a motor, taking a small amount of electricity from the grid to power up the bike. Once the pedals begin spinning, the FIGG turns back into a generator, and the controls convert DC power from the generator to AC power, as usual. The twist: power flows through a wall socket into the grid, like plugging in a normal appliance.
Their inverter scheme incorporates silicon-controlled rectifier (SCR) regenerative control, according to Taggett. “These are off-the-shelf control modules used on things like conveyor belts or elevators where the elevator going up is motoring and using power, but the elevator coming down is braking and producing power, but instead of shunting the power to a resistor load bank, the DC power is chopped up into something close to a sine wave and put back into the grid.”
But Taggett likes to talk about perhaps the most innovative concept with his machines, one that has multiple machines driving a single generator — the team approach. “You really cut down expense and maintenance because you have one big generator and one electronics package for up to 10 machines.” Known as the Team Dynamo, the machines bolt together with a common driveshaft that propels the generator.
Psomas - President & CEO
The generator, inverter, and controls
on the Team Dynamo
Henry Works uses two electronics engineering consultants in designing the Human Dynamo machines, one for circuit board layout and buildup and the other to handle software code (C++). Taggett has worked with industrial designers on the aesthetics of the machines and with another engineer or two on SolidWorks CAD modeling.
In detailing ReRev’s operations, Harr says the firm has a 15,000-square-foot production facility staffed by15 employees, including Keith Beaver, a mechanical engineer, and Dave Desilva, an electrical engineer. They now have 150 machines installed at more than a dozen gyms around the country. They have installed systems at many colleges, with their largest fleet, 30 machines, at Texas State University.
The outlook for electric-power-generating exercise machines looks bright, according to Harr. “It’s been really exciting to see the feedback in the marketplace. I think we’ve got a lot of growth ahead of us.” He actually likes the competition from the other two companies. “Whereas we may be competitors, at the same time, they’re proving a concept out there in the market. It’s just good to have company so we don’t feel alone in the world.”


Clearwater, FL
The Green Revolution
Ridgefield, CT
Henry Works Research and Development
Hurricane, UT

Developers of exercise machines that generate electrical power

Types of engineers they use:
Mechanical and electrical

Outlook for hiring engineers:
ReRev: Hudson Harr, president, reports, “We use a lot of outside consultants, mainly P.E.s that sign and seal drawings or give us an independent review.” Will they hire more engineers? “Oh yeah. That’s a big part of our business, not only new product development and R&D but also making sure our solution fits the space. I can foresee a lot of new hires.”
Henry Works: Mike Taggett, CEO, says he has started a new division called Griffin Materials to make products from recycled rubber and plans to hire engineers more for that than exercise machines. “Our focus now will be more towards chemical engineering and process-related stuff.”

Contact for submitting resumes:
The Green Revolution: Ed Gilchrest, president of Gyre 9,
Henry Works: Mike Taggett, CEO,