Second Time Around: Energy Recapture

What Is It, and How Does It Produce Electricity?

America has been called a “throwaway” society. Research has shown that we “throw away” most of our energy. Over 50 percent of all the energy we consume is wasted—thrown away—mostly as waste heat up a smokestack or out a tailpipe instead of performing useful work. But we throw away unused electricity as well. Why not recapture and save that wasted electricity for future use?

Electric utilities have always been a zero-inventory operation. They produce electricity each moment to meet the exact demand of that moment. Each time someone turns on (or off) a light, some power plant has to wiggle its output up or down a tiny bit. Any extra electricity produced is literally thrown away at the plant. It is a very inefficient system.

The Smart Grid is supposed to help improve that efficiency. But what we really need is a way to recapture lost electric energy and recycle it back into the grid. The U. S. Department of Energy estimates that the utility industry could save $100 billion over the next 20 years by increasing and developing energy storage capacity to recycle produced—but not used— electricity. Efficient storage systems will allow utilities to produce electricity when it is most economical to do so and then save it to meet the daily (or seasonal) peaks in demand and to even out the peaks and valleys of daily demand.

Several innovative and developing storage systems were mentioned in part 1 of this book, including compressed air, molten salt heat exchanger, sodium-sulfur batteries, and hydrogen. Any of them could be used to recapture the electricity wasted as excess at power plants.. But several other recapture systems are worthy of mention:

• Pumped Storage. Pumped storage systems are not a new idea. They have been around for over 50 years but are now being studied for greatly expanded use. To use a pumped storage system, a utility keeps its most efficient plants and generators working 24 hours a day—even at night when demand is low. During off-peak hours, the utility uses the excess electricity to power giant electrical pumps that pump water from one reservoir up to a higher elevation reservoir. During peak demand periods, that same water is allowed to surge back down pipes to spin turbines at the lower reservoir, creating extra electricity.

The idea is for a utility to run only its most efficient and cleanest units, to run them 24/7, and to never (or at least rarely) have to fire up the less efficient (and more polluting) units and generators it typically fires up for a few hours each day to meet afternoon peak demand. The pumped storage power would satisfy this peak demand instead.

Pumped storage systems are being used in California (Pacific Gas & Electric’s Helms Pumped Storage, for example) and in other mountainous Western states. Also, the Robert Moses Plant near the base of Niagara Falls is a pumped storage system. The concept could be expanded and used wherever suitable reservoir systems exist.

• Flywheel. Small flywheels were used by ancient potters at least 6,000 years ago, who cranked them up with a foot pedal. Modern flywheels are drawing renewed interest as an energy recapture and storage technology.

Modern industrial flywheels are huge, heavy spinning wheels—picture a giant metal wheel 10 to 20 feet in diameter that weighs many tons and spins with almost no friction. Once it is cranked up to 50,000 to 60,000 rpm, there is a lot of energy stored in that flywheel’s motion. During low demand times, a power plant shunts excess energy (which it would otherwise waste) into making a flywheel spin faster. During peak demand periods, the flywheel connects to a turbine, spinning it to produce electricity.

• Footfalls. Every time you walk up (or down) a flight of stairs, you waste energy. In addition to lifting you up, a part of your energy goes to pushing the stair down. The excess downward pressure of your foot on the stair can be converted into flexing pressure on the stair, which in turn can be converted into mechanical motion. That mechanical motion can be used to generate electricity.

Footfall recapture systems are ideal for schools and other commercial buildings where large numbers of people climb up and down stairs each day. Initial experiments have shown that such systems can produce about 6 watts per person per stair. That is enough energy for a footfall recapture system to power the entire building served by that staircase.

What’s Happening Now?

If flywheels spin fast enough, even solid metal flywheels begin to physically distort. Then, like an automobile tire that is out of balance, they vibrate and spin much less efficiently. Several research laboratories are studying new high-strength carbon-fiber materials and new lower-friction ball bearings so that utility flywheels can spin ever faster and hold even more energy. So far, only minor improvements have been achieved.

An international research team just discovered that crystals of indium selenide (a chemical compound) efficiently convert heat into electricity. No direct sunlight required! Just heat. Such thermoelectric materials could recover waste heat from power plant smokestacks, industrial processes—even the exhaust of cars! The potential is enormous. But it is still too early to tell if this process, which works in a laboratory, will work well when scaled up to mass production.

Hull University in London installed a test footfall recovery system in two heavily used stairways in the University Commons building. The generator is coupled with a flywheel to even out the release of energy back to the building. The test has shown that footfall recovery systems could power the entire building. A similar system has been installed in a high school in Norwich, Connecticut. After the first summer of testing, the system has been pronounced a definite success.

How Does It Measure Up?

(The Good, the Bad, and the Ugly)

On the plus side for energy recapture systems:

• They use free energy that is otherwise being wasted.

• They help slow the demand growth on the grid and give other supply technologies more time to develop.

• They reduce total pollution from the electrical industry.

On the negative side for energy recapture systems:

• These systems are still untested. No other negatives have yet been detected.

What’s the Bottom Line? (How Much Can It Help?)

• Potential: Large but still undetermined. Recapture technologies are off-grid systems that reduce demand on the electrical grid.

• Key Factors: Energy recapture systems are expensive to retrofit into existing buildings. They are better as part of the design of new buildings.

• Timeline: Look for more pumped storage systems to develop in mountainous states and for other energy recapture technologies to begin to be built into building and plant designs by 2020.

Classroom Activities

The idea of recapturing the excess energy in a step—that energy we use with each step not to lift us up, but to depress the surface we step on down—is intriguing. Think of how many steps you take each day. What if you could recapture a little bit of energy from each one of them?

1. Let’s do some tests to see how much potential exists at your school for footfall energy recapture. Are there any stairs at your school? That seems to be the best place for this type of energy recapture—anywhere where people have to exert pressure to lift themselves up a step or to slow themselves as gravity pulls them down the stairs.

Can you calculate how much total energy your school could save if it had energy recapture systems installed on all the stairs? To do that, you will need to calculate how many times each day a person walks up or down each of the school’s stairs. How many times each day do you climb up or down? How many students are there at your school who regularly use the stairs? How often (on average) do they climb either up or down in a day? How many stairs do you step on each time you climb either up or down? (That is, how many stairs are there in each flight of stairs, and how many flights do you have to climb each time you use the stairs?) Discuss the problems you might face in making such a measurement. Use the Internet to research how government agencies measure how many people or cars pass a given spot—something they regularly have to do.

As a class, plan how to measure the total number of stair footfalls in a typical day at your school. Run your count each day for a week. That will give you five totals to average to decide on the number of stair footfalls in a typical day. Use the average figure of 6 watts per person per stair step. Multiply that number by the typical number of total stair steps you have calculated for your school. How does that potential energy recapture compare to your school’s daily electricity use? Could footfall recovery technology supply your school with all of the electricity it needs?

2. Research the few facilities that have installed footfall recovery systems in the United States and England. How much energy did they recover and turn into useful electricity? How do their savings compare to what you calculated for your school?

3. Have each student pick one of the energy recapture schemes mentioned in this section or in part 1 and research that technology. Is it being used anywhere? How successfully? What research and development is ongoing to prove or improve that technology? Where could it be used in your community?

For Further Reading

Orme, Helen. Energy for the Future. New York: Bearport Publishing, 2008.

Rau, Dana. Alternative Energy: Beyond Fossil Fuels. Mankato, MN: Coughlan Publishing, 2009.

Reilly, Kathleen. Energy : 25 Projects Investigate Why We Need Power & How We Get It. White River Junction, VT: Nomad Press, 2009.

Royston, Angela. Sustainable Energy. London: Arcturus Publishers, 2010.

Sechrist, Darren. Powerful Planet: Can Earth’s Renewable Energy Save Our Future? New York: Gareth Stevens Publishing, 2009.

Web Sites

www. ieeexplore. ieee. org/iel5/8455/28364/01267762.pdf? arnumber=1267762

Explores several energy recapture concepts.

www. greentechmedia. com/…/Grid-Scale-Energy-Storage-CAES-Black-


Describes several pumped storage systems.

www. er. doe. gov/bes/reports/abstracts. html

Department of Energy site on energy storage and recovery.

www. renewableenergyworld. com/…/pumped-hydro-and-new-energy

Another site describing pumped storage systems.

www. bioedonline. org/news/news. cfm? art=3171

Description of innovative schemes for recapturing footfall energy and flywheel energy recapture systems.

www. elp. com/index/display/article…2/…/energy-storage_solving. html

Overview of flywheel systems.

Updated: October 23, 2015 — 12:39 pm