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.



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 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.



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 (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 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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Wind Science, Energy, and Growing Prevalence

Wind is the kinetic energy of molecules in the air. Wind has powered ships and mills for centuries or longer.

Modern windmills convert the wind into rotational energy by allowing the air molecules to bombard the blades, turning them. The blades are connected to turbines, which generate electricity from that rotational energy.

Wind energy is one of the cleanest forms of energy available because it doesn’t require a fuel or produce greenhouse gas or other bi-products, outside of those from production and maintenance of equipment and transmission.

Wind turbines themselves take up only a small area compared to their generating potential, making it possible to install them on agricultural, forest, or grazing lands.


In just ten years, wind power in the United States grew more than ten-fold, from just over 2,000 megawatts in 1999 to more than 34,000 megawatts in 2009, when wind accounted for 9 percent of renewable energy produced in the country and more than geothermal and solar combined.

Here’s an animated map of wind development from 2000 to 2010.

Texas, Iowa, and Minnesota had the greatest wind capacity in 2010. Additionally, at least 27 other states used wind to generate electricity that year.


Wind is an intermittent resource, meaning that the windmills can’t continuously and predictably produce energy. They only work when the wind blows, and they can only work as hard as the wind is blowing at that time.

Research is ongoing into predicting what regions of the country have significant wind resources suitable for wind development, a process that requires computer programming and meteorological knowledge.

Furthermore, public and private researchers are working to produce better models of wind on an hourly, daily, and seasonal basis to make it easier for wind energy producers to forecast their output and sell it ahead of time.

Another major hurdle to wind power is that it is expensive compared with fossil fuel-based electricity. Modern windmills cost a lot to design and build, especially as they have to be strong enough to endure extreme weather, even though they will mostly operate in moderate weather. That makes competing with other energy sources difficult without government intervention.

Some people don’t like the way windmills look, and windmills can also kill bats and birds, though newer designs have slower and less deadly blades. A 2010 study published in the Journal of Ornithology estimated that windmills kill around 440,000 birds every year. However, the same study showed that house cats kill more than 1,000 times that number, as many as 500 billion per year.

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What Is A Nuclear Reaction?


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Photovoltaic Cells, Solar Power, and LEDs

Most of the world’s energy can go back to our sun. Every day we are heated by its electromagnetic rays, and plants use the sun’s energy to make sugars and ultimately proteins and other good things to eat. Fossil fuels were also once made from these plant and other organisms that relied on the sun’s energy millions of years ago. Today, humans can convert the sun’s energy directly into electricity, through solar thermal and solar photovoltaic systems.


Solar panels, also called solar thermal, convert sunlight to heat and then heat to electricity. Photovoltaic cells, or solar cells, convert sunlight directly into electric current by way of carefully-engineered semiconductor materials.

Though solar photovoltaics are more efficient converters of sunlight, they are also more expensive.

As of May 2011, the world’s largest solar power plant is a concentrating solar thermal power plant in the Mohave desert in California. Solar Energy Generating Systems has a capacity of 310 megawatts and uses parabola-shaped reflective troughs to concentrate electromagnetic radiation.

The world’s largest solar photovoltaic plant is probably the Sarnia Solar Project in Ontario, Canada. It has a capacity of roughly 80 megawatts.


Sunlight heats a design element (water, air, chemical fluids), and that thermal energy is transmitted for other applications, such as heating water, heating space, or generating electricity. In solar thermal power plants, sunlight heats a specialized fluid, which in turn heats water into steam, which can run turbines and produce electricity.

Solar thermal power plants use concentrators that bounce the sunlight off elliptical mirrors to a central tube, in which the specialized fluid lies.


Photovoltaic cells are made of specialized diodes. Electrons (natural components of atoms) in the photovoltaic cells absorb light, which excites them to a state where they can be conducted as electrical current. This difference in energy, between the valence band (the state of a normal electron staying around its home atom) to the conduction band (electron free to move between atoms) is called the band gap.

Solar photovoltaic farm in Indonesia. Photo by Chandra Marsono.

Well-engineered photovoltaics have a band gap that coincides with the energies of as broad a spectrum of light as possible, to convert the maximum amount of the sunlight into electricity.

As sunlight energy pops electrons into the conduction band and away from their home atoms, an electric field is produced. The negatively-charged electrons separate from the positively-charged “holes” they leave behind, so that when electrons are freed into the conduction band, they move as electric current in the electric field, electricity.


An ever-expanding variety of semiconductor materials can be used to make solar cells; universities and companies worldwide are researching these options, from special bio-plastics to semiconductor nanocrystals. Nonetheless, the photovoltaic cells available today require precise manufacturing conditions and are therefore far more expensive to produce than solar panels.

Silicon has to be processed under clean room conditions — carefully regulated atmospheres — to remove impurities and prevent introducing contaminants, both of which can change the band gap. Thin film-based photovoltaics require special production methods, like chemical vapor deposition. Semiconductor processing also uses strong acids and often dangerous chemicals for etching.

Today, commercially-sold cells are made from purified silicon or other crystalline semiconductors like cadmium telluride or copper indium gallium selenide.


Silicon is plentiful in the Earth’s crust. Cadmium is a readily available but highly toxic heavy metal, as is arsenic, another chemical used in some cells. As tellurium demand is only recently rising in response to solar demand, it’s unknown what the global supply is for this unusual element but it may be quite abundant. Photovoltaics are a lively area of research, and the future production and environmental costs of starter materials, production, and pollution are difficult to predict.

California, Massachusetts, Ohio, and Michigan produced the most photovoltaics in 2009. However, that year, 58 percent of photovoltaics were imports, primarily from Asian countries like China, Japan, and the Philippines.


Photovoltaic cells work in the opposite direction of light-emitting diodes, or LEDs. LEDs are used interchangeably with other lighting, like light bulbs. However, LED’s work in a completely different manner, far closer to the way photovoltaics work.

Click here see a bar chart comparing how much energy is used by various light sources.

LEDs absorb energy in the form of electricity, exciting electrons into the conduction band. When the electrons in the semiconductor material drop back into the valence band from the conduction band, they emit energy in the form of photons, or electromagnetic radiation.

It’s a highly efficient process because energy isn’t wasted on producing heat, which happens with standard tungsten filament bulbs. LEDs also last a much longer time as they do not have filaments to burn out, and because they are very small and several units are used to replace one large traditional lamp, they do not all burn out at once. That makes LEDs a good choice for stoplights or other safety critical applications.


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Thermodynamics and Thermal Energy

Thermodynamics is the study of how energy moves and changes form, usually by way of heat, as suggested by the components of its name: thermo-dynamics. Its laws and equations help to predict what could happen in various situations, based on the temperature, pressure, materials, and shape of a system.

Thermodynamics tells us how to calculate the ultimate temperature of a refrigerator or how much energy we can get out of a steam engine. Thermodynamics can also be applied to chemistry and the world on an atomic level, predicting which compounds are stable at specific temperatures and pressures. Thermodynamics explains why diamonds form naturally and spontaneously from carbon-based compounds deep inside the Earth, but they cannot form spontaneously here on the surface.

Thermodynamics relies on the idea that energy is conserved, even if it is transferred from or to a system to its surroundings through heat, changes in momentum, or other forms of energy.



Heat and thermal energy are directly related to temperature. We can’t see individual atoms vibrating in solids, liquids, and gases, but we can feel their kinetic energies as temperature. Atoms in solids, liquids, and gases do vibrate. If they didn’t, they would be at absolute zero, a theoretical state of zero thermal energy at ­-459.67 Fahrenheit.

When there’s a difference between the temperature of the environment and a system within it, thermal energy is transferred between them as heat. Something doesn’t have heat. Instead, as an object or system gains or loses heat, it increases or decreases its thermal energy.

Adjacent objects that exhibit different temperatures will spontaneously transfer heat to try to reach the same temperature as each other, or equilibrium. However, how much energy it takes to change the temperature of an object is based on what its made of, a property called heat capacity or thermal capacity.

Water has a higher heat capacity than steel, for example. An empty pot on the stove takes almost no time to get to 212 degrees Fahrenheit, or the boiling temperature of water. A pot with some water in it will take far much longer to reach the same temperature, because water needs to absorb more energy — per weight, per degree — to gain the same number of degrees as metal. (Even though the vaporization temperature of metal is far, far higher than the water’s).



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 and into the ice.

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.

Solar panels use thermal energy storage. The panels absorb the heat of sunlight and store that energy so it can be transformed into electricity with turbines. There are several kinds of solar panels, but all rely on heat for energy, unlike photovoltaic cells.

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Major sources of energy/their advantages and disadvantages

There is no easy answer to what is the best source of energy or electricity. Is the priority reliability, affordability, the economy, international human rights, limiting greenhouse gas emissions, preserving environmental resources, or human health?


It’s undeniable that today — whether we like it or not — humans worldwide are overwhelmingly dependent on fossil fuels: coal, oil, and natural gas. Everything eaten, worn, lived in, and bought is tied to availability of fossil fuels. Even if 100 percent of politicians were determined to stop using them today, society has neither the electricity grid nor the vehicular and industrial technology to sustain the current American lifestyle on non-fossil sources of energy. Yet.

When comparing sources of energy, it’s easy to forget how universal fossil fuels are. These sources continue to dominate for reasons that are difficult to measure, like political influence, advertising clout, and control over energy infrastructure. Other sources have disadvantages purely because they don’t fit in as well.

Volume brings another difficulty in comparing sources of energy. There is so much more fossil energy, and it’s been used for a long time, so we know a lot more about its hazards and benefits. More modern technologies are harder to quantify. Some are renewable but still pollute (biofuels), some are very clean except in accidents or waste disposal (nuclear). Most electricity sources (renewable or not) use steam turbines, and all the water to make steam has to come from somewhere, but how important should that factor be?

Clicking the graphic above will give an abbreviated chart comparing sources line by line, but that doesn’t provide anywhere close to the whole story.

Each of the following topics compares the major sources of energy  through a different lens. Though environmental and local issues may seem the most important to those of us who don’t own power plants or utility companies, the cost of energy drives which sources are actually in place today and which sources will see investment tomorrow.












Source: U.S. Energy Information Administration

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Physics and How Machines Work

Machines are so complicated these days it’s difficult to quickly explain how they work. Nonetheless, today’s machines were built using the basic principles of physics that we began harnessing hundreds of years ago.


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Water Depends on Energy, Or Is It The Other Way Around?

The United States took more than 400 billion gallons of water out of the ground, lakes, rivers, and reservoirs daily in 2005.  (more…)

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Ocean Energy

You don’t have to talk about hurricanes and tsunamis to know that the oceans are powerful. People have dreamed about harnessing their energies for centuries, and today there are many projects worldwide experimenting with just how to plug into the oceans.

However, ocean energy projects are expensive because of the nature of their energy source. The salty seas can be corrosive, unpredictable, and destructive.

Several aspects of the ocean’s energy can be exploited to generate power;  we’re not limited to the crashing waves. The three most well-developed ideas are tidal power, wave power, and ocean thermal energy conversion.

There are many different projects in various stages of development in coastal states today. However, as yet, ocean energy isn’t a significant source of energy nationally.

Ocean energy is renewable, and it’s clean because of its lack of emissions. However, using ocean energy along coastlines can cause conflict with other coastal uses – transportation and scenic oceanfront – and ocean energy can as affect marine life and environmental conditions.



Wave energy capitalizes on the power of waves as they roll through the ocean. There are small wave systems generating small amounts of electricity today, though the development costs are high and it is difficult to design equipment that can withstand the salt water, weather and water pressures.

Systems have to be designed for average waves but must also withstand the much stronger waves that occur in seasonal storms and the extreme waves that appear only rarely. Waves shift direction, so systems are designed to move to optimize direction.

Prototype plants currently operating have capacities of fractions of a megawatt, which is the tiniest drop in the bucket compared to average-sized power plants in the hundreds of megawatts.

There are over 100 wave energy technologies in various states of planning and testing or in operation as prototypes. However only one type is operating commercially, the Pelamis Wave Power, according to the World Energy Council.

In the United States there are other projects in planning or testing in Hawaii, New Jersey, Oregon, Texas, and California.



Using the potential energy of rising and falling ocean tides is called tidal energy.

One way of harnessing the tides is to trap the high tide behind dams.When the ocean rises to its highest tide, the dam is closed and high water is held in a reservoir by the dam. After the water recedes in low tide, the trapped water can be released through turbines like in hydroelectric plants.

Tidal energy plants of this type demand a large height difference between high and low tides, a condition that applies to only select global locations. However, research is ongoing to bypass this limitation.

The one major tidal power plant in operation is the 240 megawatt plant in La Rance, France, which has been operating since 1966, according to World Energy Council. There is also an 18 MW experimental plant in Annapolis Royal, Nova Scotia and a 0.4 megawatt plant near Murmansk, Russia.

Tidal energy can have the same drawbacks as hydroelectric power, such that dams may interfere with aquatic life.



Thermal energy conversion harnesses the difference in temperature between the warm, surface waters of the ocean and the colder, deep water. The two temperatures of water are matched to a fluid that has a low boiling point, like ammonia. Using the heat of the warmer water in a heat exchanger, the ammonia is evaporated and, once in gas phase, it rotates a turbine. Then, the colder seawater cools the ammonia back to liquid in a second heat exchanger. The rotating turbine generates electricity.

Open-cycle thermal energy conversion is similar but uses low pressure vessels to boil the warm surface water, instead of employing a fluid like ammonia. Water will boil at lower than its boiling point if the pressure is less than atmosphere. The steam runs a turbine, and then the cold seawater cools the steam back into fluid water.

These projects are expensive and difficult to site, since they must have deep enough water to get a substantial enough difference in temperature, yet the site must also be close enough to shore to transmit electricity.

Thermal plants can change the temperature gradient of the ocean around them, having a potential affect on marine life.

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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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