Storing Energy: Fuel Cells and Beyond

Fuel input for hydrogen-fuel-cell-powered vehicle. Photo from Siemens PLM in Cypress California.

Storing energy is important for both long-term and short-term uses: to meet changes in energy supply and demand and to iron out irregularities in energy output, whether that’s in a car engine or on the power grid.

Unfortunately, we can only store a tiny fraction  of the electricity we produce in a single day. Instead, power plants have to send their thousands of megawatts of electricity to the right place, at the right time. For more details about electricity transmission, see Power Grid Technology.

Our current electric grid has various quick storage solutions to help make energy delivey smooth, and we use energy storage in cars, phones, and anything else that needs to be moved around. However, batteries and other storing options leave much to be desired.

Devices like capacitors and flywheels can store energy for extremely short periods.  Few technologies exist to store large amounts of energy over time periods ranging to several days: only pumped (water) storage is widely used to store energy on the scale of a power plant.

As energy sources are expanding to include more renewable and intermittent resources like wind and solar onto the electricity grid as we try to both meet growing energy demand and control greenhouse gas emissions. Likewise, there is increased interest in having reliable energy storage for vehicles, instead of gasoline and diesel fuels.

 

BATTERIES

Depending on the type of battery, these devices can store energy on location, like at home or in the car, in a laptop or cell phone. Note that though batteries and fuel cells can help integrate renewable energy sources, most electricity is still generated from fossil fuels. Both charging batteries and making hydrogen for fuel cells  thus produce greenhouse gas through reliance on the prevailing sources of electricity, and using these devices is less efficient than plugging into the wall because there’s always energy loss to byproducts like heat.

Batteries store chemical energy. Chemical reactions in the battery release that energy as needed. Eventually all the starting materials of the reaction are consumed and the battery is dead, or at least unusable until it’s recharged. There are many kinds of batteries, made of a wide array of chemicals. Polysulfide Bromide (PSB), Vanadium Redox (VRB), Zinc Bromine (ZnBr), Hydrogen Bromine (H-Br), and sodium sulfide batteries are some that the electricity industry has interest in.

Electric utilities use lead-acid batteries, which can be recharged, but there is research into other materials for utility and transportation use. Some of the best batteries used today for cars are nickel metal hydride and lithium ion.

 

HYDROGEN FUEL CELLS

Hydrogen fuel cells aren’t the same as batteries, but they can serve a similar purpose. Fuel cells are lumped with batteries because they both function through stored chemical energy. However, in practice, fuel cells are more like engines. They run off hydrogen “fuel” and produce energy and waste products, mostly  water vapor. As long as hydrogen keeps being added, the cell can run, just like a gasoline engine can keep running as long as more gasoline is added. A battery has a finite amount of energy unless it’s recharged with electricity.

For more about how to make hydrogen for fuel cells, see The hydrogen economy, hydrogen sources, and the science behind these.

For a description of different hydrogen fuel cells in development right now, see here.

 

FLYWHEELS

One way to smooth bumps in electricity delivery is through flywheels, which store energy in the form of rotational kinetic energy. A spinning potter’s wheel stores the energy of a good kick to be used moments later to mold a clay pot, and flywheels operate on a similar principle. In automobile engines, flywheels ease the transition between bumpy firing pistons and the drive shaft.

Flywheels can store energy for limited periods of time, from seconds to a few minutes.

 

PUMPED STORAGE

Pumped storage (of water) is the only widely-used method for storing huge amounts of energy for long periods of time. The United States has a capacity of more than 20,000 megawatts of pumped storage, according to the National Hydropower Association.

During times of excess electricity production, that excess energy is used to pump water to a higher altitude, increasing its gravitational potential energy. When extra energy is needed, the water is allowed to flow back down by way of turbines, turning that potential energy back into electricity.

For a figure of pumped storage see the National Hydropower Association

 

OTHER WAYS TO STORE ENERGY

Other technologies are constantly being investigated for energy storage. Compressed air storage is when air is forced into spaces like mines or caves and held at high pressure, using up energy in the process. When the compressed air is let out again, it can turn turbines to generate electricity.

Thermal energy storage exploits the difference in temperature between a system and the environment. In the late 1800s, Americans used thermal energy storage by cutting blocks of lake ice during the winter and storing them underground packed in insulating wood shavings. When the summer rolled around, they retrieved that stored ice to make food cold, exploiting the difference in temperature to force thermal energy out of the food.

Thermal energy storage can also happen in the other direction. Electricity or other forms of energy can be used to heat various materials, which are stored in insulated containers. Later, when the energy is needed, the hot materials can heat water into steam, and that steam can push turbines, which in turn produce electricity.

Thermal energy storage can also be used through ocean energy.

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The Hydrogen Economy, Hydrogen Sources, and the Science Behind These

The hydrogen-filled Hindenburg in 1936 or 1937. Photo from DeGolyer Library at Southern Methodist University.

THE HYDROGEN ECONOMY

The hydrogen economy is a hypothetical future in which energy can be bought, sold, stored, and transported in a currency of hydrogen, much like today’s energy is often exchanged in electricity. Because hydrogen doesn’t need to be attached to the electricity grid, it can be used in forms of transportation like buses and cars.

The end-user of the hydrogen, for example an automobile driver, doesn’t experience significant pollution beyond the formation of water from burning the hydrogen.

For more details about the hydrogen economy see here.

Hydrogen, a gas, isn’t a fuel like gasoline or coal; hydrogen is a way to store and transport energy made from other fuels, like a battery or electricity. Unlike fossil fuels, pure hydrogen isn’t stable, so forming hydrogen in the first place requires energy and produces carbon dioxide, and storing hydrogen involves special considerations because this light gas is very flammable and also quickens rust and oxidation in pipelines and storage containers.

HOW HYDROGEN IS DIFFERENT FROM FOSSIL FUELS

Allowing hydrogen (a gas) to burn in the presence of oxygen releases that stored energy in the form of heat. Hydrogen can also be reacted in a fuel cell to produce electricity. In either case, electricity or heat can then be used to power cars or any number of other devices. Gasoline, biofuels, wood, and other carbon-based fuels all produce carbon dioxide when they are burned, and rising carbon dioxide levels are widely implicated in climate change. Burning hydrogen produces energy, water and a few trace compounds, but it doesn’t produce carbon dioxide.

2 H2 (hydrogen gas) + O2 (oxygen gas) = 2 H2O (water vapor) + energy

It’s unclear what widespread emission of water vapor could do. According to recent published estimates, atmospheric water vapor is responsible for 75 percent of the greenhouse effect. However, water vapor can condense, and it’s naturally-occurring in the atmosphere. It is much easier to trap and transform to liquid than the carbon dioxide normally emitted by burning gasoline. Carbon dioxide won’t form a liquid at atmospheric temperatures and will solidify into dry ice only below -108.4 Fahrenheit, so proponents say it can be easier to trap the vapor in hydrogen-powered machines.

If the energy used to generate and purify and store and ship hydrogen doesn’t require emitting greenhouse gases or toxics, proponents argue that hydrogen is a clean alternative.

SOURCES OF HYDROGEN: THE UNFORTUNATE REALITY TODAY

Hydrogen, not carbon, is the most prevalent atom in the human body. There are two hydrogen atoms in every water molecule, and as many as hundreds of hydrogen atoms on the basic building blocks of life, from DNA to plant fibers. Nonetheless, harvesting the hydrogen atoms out of any of these structures to make hydrogen fuel isn’t easy because hydrogen likes to be bonded to carbon or oxygen; it doesn’t like to be elemental gas.

To produce pure hydrogen today, industries use primary fuel source like petroleum, natural gas, coal, or biomass. Through chemical processing, the hydrogen atoms are stripped from the fuel by way of an input of energy from electricity (more than 80 percent of which comes from fossil fuels in the United States). Furthermore, the leftover material from the stripping is carbon dioxide, the same carbon dioxide that would have been produced if the fuel was burned in an engine.

The reactions for various fuel to hydrogen conversions can be found on the U.S. Department of Energy website here.

Hydrogen can also be produced, at great energy loss, through the electrolysis of water: using electricity, water is divided into its constituents, hydrogen and oxygen. However, water electrolysis is the least carbon-neutral hydrogen production method, and it is very expensive ($3 to $6 per kilogram instead of a little more than $1 in the case of using coal for hydrogen), according to the U.S. Energy Information Administration. All hydrogen production methods result in a net energy loss.

 

 

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Petroleum, Natural Gas, and Coal

The world depends on fossil fuels for its energy, and the United States is no exception. The vast majority of U.S. energy — more than 80 percent in 2009 — comes from burning fossil fuels. (more…)

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