Energy Storage
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Definitions
Energy storage may be defined as the process of storing energy in any form for later use. An energy storage system may be defined as any medium which stores energy in any form such as chemical (batteries), thermal, mechanical (flywheel), electrical (capacitor), or another type of energy (in the form of compressed air, for instance) for use at another time.
Energy Density/Specific Energy/Energy-To-Weight Ratio: This is defined as the amount of energy that an energy storage medium can store per kilogram (per unit mass). It is usually measured in Wh/kg (Watt-hours per kilogram). Another term for energy density is energy-to weight-ratio. It is calculated by dividing the energy storage capacity by the weight of the medium. Energy Density = Energy / Weight.
I will use the Tesla Roadster battery bank as an example:
Weight: 453 kilograms.
Capacity: 53,0000 Watt-hours.
Energy Density: 53000 / 453 = 117 Watt-hours per kilogram.
Power Density/Specific Power/Power-To-Weight Ratio: This is defined as power output per kg of mass. It can be calculated by dividing power output by weight.
A battery bank/battery pack may be defined as a set of batteries which are connected to each other in either series or parallel to achieve a certain voltage and/or capacity.
Energy storage systems also need to be as efficient as possible, because if you want a certain amount of power from a generator setup, such as 1000 watts for example, then the lower the efficiency of the energy storage system, the less power will be available, and the more powerful the generator would have to be to compensate for energy losses. If the energy storage system in this case is 50% efficient (which is the lowest for lead acid batteries), then only 500 watts would be available from this example setup:
1000 watt 12 volt DC natural gas powered generator
50% efficient lead acid battery.
If the energy storage system was a 90% efficient lithium ion battery, then the power available would be 900 watts, because 0.9 * 1000 = 900.
Batteries
Batteries are a frequently chosen energy storage medium for a variety of applications, including electric and hybrid electric vehicles, portable electronics, solar and wind powered generators, and power grid load balancing. They store chemical energy which can be converted into electrical energy via chemical reactions.
Batteries are usually charged by using a power supply which basically uses a step down transformer to lower the voltage of mains electricity from 110-130, 240, or more volts down to a much lower voltage such as 3, 4, 6, 12, or 24 volts (especially 12), other devices may be used to condition the current and prepare it for battery charging purposes, and rectifiers are used to convert that lower voltage alternating current (AC) to direct current (DC) which is then supplied to the battery.
Advantages and disadvantages of using batteries in electric vehicles:
Heavy weight, high cost, undesirable lifespan (this isn't the case with all battery technologies, thin film lithium batteries can be cycled 40,000 times) and slow charge/discharge time are common battery issues. It can take hours to charge and discharge batteries, and they are costly. One of their qualities is that they can be recharged with a variety of electricity sources. Battery technologies are improving, and new ones are being born. Some new batteries can be charged in five minutes, such as the Toshiba SuperCharge battery.
Lithium ion batteries have the lowest environmental impact, the best energy-to-weight ratio, and the longest lifespan, as well as the largest amount of cycles (which is 1000-2000 cycles) of the three most common types of batteries used for electric and hybrid electric vehicles. The main drawback of lithium ion batteries is their high price tag.
Lead acid batteries can be cycled a few hundred times, it varies significantly, and Nickel Metal Hydride batteries can be cycled up to 1000 times. Nickel Metal Hydride and Lead acid batteries have a significantly higher environmental impact than lithium ion batteries, which have a low environmental impact. Lithium ion batteries are the batteries of choice for many electric vehicles now, except for the Toyota Prius which uses Nickel Metal Hydride batteries. There are other batteries such as lithium sulfur and thin film lithium batteries which are currently not being mass produced, but, they might be one day. One advantage that lead acid batteries have over lithium ion batteries is a significantly lower price tag, but, their lifespan is significantly shorter. Deep-cycle lead acid batteries are commonly used to store electricity generated by residential solar panel setups because of their low price tag.
Lead acid batteries are the heaviest of the three main types of batteries used for electric vehicles due to their low energy density. Nickel metal hydride batteries are sometimes much lighter than lead acid batteries, but still heavier than lithium ion batteries.
A few examples of li-ion battery powered vehicles are:
- Upcoming Toyota Prius plug in hybrid.
- Upcoming Chevrolet Volt.
- Tesla Roadster.
- Tesla Model S.
- Fisker Karma.
Anticipated Battery Technology
There was a lithium ion technology breakthrough and the new batteries can be charged in less than 20 seconds, they are also smaller, cheaper, lighter, and more efficient. This technology is expected to take a few years to become mainstream, but it is a significant breakthrough. The fast discharge time means that electric vehicle manufacturers won't have to use the excessively large, heavy, and costly batteries to compensate for their slow discharge rate anymore, permitting the manufacture of faster, lighter, more cost effective, and more efficient electric vehicles. Environmental impact and lifespan depend on the type of battery used, and how it is treated.
Lithium sulfur batteries are another interesting type which have a high energy density of 400-600 watt hours per kg, but cannot be cycled many times.
Silicon Nanowire Li-ion (lithium ion) Battery: This is a battery which has a silicon nanowire anode which has ten times the surface area of the conventional type of anode which is made of carbon, and that silicon nanowire anode increases the energy density by ten times, which is over 1000 Wh/kg. This makes an electric vehicle with an extremely long range possible, a range so long that basically everyone could complete all of their daily transportation after charging overnight at home. Owners could also use a low wattage solar panel to charge the batteries to a fraction of their capacity every morning and still have adequate range. Since the average person does not drive more than 40 miles daily, they would only need the solar panels to charge the batteries every 2 to 6 weeks, and perhaps on a weekend, since the car will be at home on the weekends for most of morning and afternoon, when it is sunny.
Lithium-air Batteries: This type of battery can have an energy density of up to 5000 Wh/kg, which is much greater than all other energy storage systems available today. The main advantage of such a high energy density is that it provides vehicles with very long range. IBM, MIT and other organizations are currently working on this technology.
Lithium Reserves and Sources
Lithium makes up less than 3% of the mass of lithium-ion batteries. Lithium can be extracted from multiple sources, including lithium brine (lithium chloride), seawater (although in low concentrations), it is also found in practically all igneous rocks, as well as minerals such as: lepidolite, petalite, amblygonite, and spodumene (this is the most common mineral that lithium is extracted from).
Lithium-Ion Batteries: Possible Material Demand Issues
Los Alamos National Laboratory - Lithium
Texas A&M University: Concentrations of various dissolved species in interstitial waters
Saga University: Lithium Occurrence
If you are interested in learning about lithium reserves, you can read about that here: USGS Mineral Information.
Sodium-ion Batteries
MIT: Sodium-ion Cells For Cheap Energy Storage
GE: Sodium Metal Halide Battery
Cheaper, Stronger Li-ion Batteries for Electric Vehicles
Argonne National Laboratory: Lithium Air Battery
Navy.mil: PDF File: Extending The Life of Lead-acid Batteries
Fuel Cells
Hydrogen fuel cells are another way to store energy for future use. They use hydrogen and oxygen to generate DC (direct current) electricity via an electrochemical process which involves the oxidization of the fuel (hydrogen) using the oxygen which is fed to the cathode, and the hydrogen is fed to the anode. Fuel cells have had a problem with the high cost of the platinum that they contain, but, significantly cheaper electrode replacements such as carbon nanotubes have been found. Read more
Hydrogen fuel cells only need hydrogen and oxygen to generate electricity (and in some cases, they may use other fuels), and the fuel can be extracted from water via electrolysis. During electrolysis, two electrodes are placed into water and electricity is passed through them, then hydrogen and oxygen bubble up at the cathode and anode respectively. You can even extract your own hydrogen and oxygen, the process is simple, although energy intensive. There are many other types of fuel cells, including phosphoric acid, alkaline, molten carbonate, direct-methanol, solid oxide, and polymer exchange membrane fuel cells. The average fuel cell stack is 50% efficient, and can be up to 70% efficient. An example of a fuel cell powered vehicle is the Honda FCX Clarity
Hydrogen can come from fossil fuels, the electrolysis of water, and other sources as well. It is possible to use wind turbines or solar panels to lower the energy cost of hydrogen production via electrolysis. So even though electrolysis is energy intensive, it has the potential to be the cheapest in the long run. It is a very voluminous and lightweight fuel which can be stored under high pressure inside of a storage tank.
Additional Information
Cornell: Oxidation Reduction Reactions
Supercapacitors/Ultracapacitors
A supercapacitor or an EDLC (Electric Double Layer Capacitor) is an energy storage medium which is capable of being charged and discharged very quickly. A capacitor is an energy storage medium which stores an electric charge between two conductors which are separated by a dielectric (a dielectric is a non-conducting substance, such as glass, air, ceramic material)
They are also more efficient than batteries. Supercapacitors have millions of charge and discharge cycles, unlike conventional batteries which have 100 to over 2000 charge cycles. They have high charge and discharge efficiencies so they don't waste much energy, but they do self-discharge fairly quickly. They are also very safe and environmentally sound.
Low cost and energy dense ultracapacitors could also help with one of the major energy storage problems that off-grid solar and wind powered homes have, which is the cost to replace the batteries due to their short lifespan. They could also be good for electric vehicles because of their long lifespan and their ability to discharge energy very quickly (usually in seconds) to provide short bursts of power, provided that they are affordable, but they are currently cost prohibitive. Their energy densities are low (and their power densities are high), but they are improving.
JM Energy: Lithium Ion Capacitor
MIT: Carbon Nanotube Ultracapacitor May Replace The Li-ion Battery
Thermal Energy Storage
Energy may be stored in the form of heat and utilized in a variety of ways, some of which include:
- Powering solid-state thermoelectric modules by taking advantage of the Seebeck effect. When one side of a thermoelectric module is heated (with heat from the sun, generators, engines, or heat from the earth), and the other side is kept cool, a potential difference (voltage) occurs and that causes electrical current to flow.
- Heating a liquid to produce steam which would be used to turn turbines.
- Heating a Stirling engine. A stirling engine is a mechanical device which converts heat energy into mechanical energy when one cylinder is heated, and the other is kept cool.
Thermal energy is often stored in insulated tanks filled with a liquid. The insulation inhibits heat transfer, therefore, it helps to trap heat inside those tanks.
Flywheel Energy Storage
Energy may be stored as rotational energy by using a motor (such as an electric one which would also act as an alternator) to accelerate a flywheel up to a high speed (for example, 41,000 RPM), and maintaining that as rotational energy. Flywheel energy storage systems do not suffer from the memory effect that some (especially older) battery technologies do, and they can help to stabilize the supply of power. Flywheels have many applications due to their heavy weight, which causes inertia (which is the resistance of a body to change its velocity). Some devices, such as video cassette recorders, take advantage of their inertia by using them to keep the speed of the VCR's gears and wheels constant. They also have a higher power density than batteries and can be charged within seconds and discharged within seconds, unlike most batteries.
Reliability and Waste: Flywheels tend to be much more reliable than batteries and are more able to withstand frequent charging than batteries and are not constructed using toxic materials like certain (older) battery technologies, such as lead acid and nickel cadmium, resulting in a much greener life cycle.
Wikipedia: Flywheel Energy Storage
PDF File: Energy.gov: Flywheel Energy Storage
Pumped Hydroelectric Storage
Water may be pumped into a large tank which has a small drain which releases the water through it at a high and controlled pressure to turn turbines which generate electricity.
FERC: Pumped Hydroelectric Storage Projects
Compressed Air Energy Storage
An air compressor may be used to force air into storage tanks so that it can be released from the tanks at a controlled rate to turn a turbine which would generate electricity. According to uoregon.edu, compressed air energy storage systems can have an energy density of 2,000 watt-hours per kilogram.
Load Balancing and How Power Plants Benefit From Economical and Efficient Energy Storage
Energy storage systems are sometimes used for load balancing. Load balancing is the process of storing excess electricity generated during off peak hours especially (in the night) and using it to help power plants to meet electricity demand during the day by jointly supplying electricity with the existing generators (example: steam or wind turbines).
Without load balancing, power plants such as coal, nuclear, natural gas (steam), and geothermal generate too much electricity at night when electricity demand is lower, and just about enough, or too little during the day, so more electricity is wasted at night. Power plants of that type are not load-following power plants, their power output cannot be adjusted much.
Gasoline powered generators' efficiency varies with speed, and they can benefit from economical and efficient energy storage by operating at their most efficient speed at all times and charging cheap, efficient batteries, which would then supply just enough electricity to meet demand, and at all times.
Wind turbines generate electricity dependent on the speed of the wind. Wind speed patterns do not consistently follow electricity demand patterns, and the wind blows intermittently, therefore, energy storage helps wind power plants by enabling them to supply exactly the right amount of electricity demanded and at all times.
Intermittency of Electricity Sources
Some electricity sources, such as solar and wind powered generators produce electricity intermittently or of varying intensity. There are a variety of ways to counter this, some of which include the use of one or more of the following:
- Batteries
- Supercapacitors
- Mirrors
- Thermal Energy Storage
- Compressed Air Energy Storage
- Using separate generators as backup for when the sun stops shining or when the wind stops blowing.
Backup Generators
Generators such as solar panels and wind turbines can be used with backup generators which would be switched on before evening time, to enable the power plant to provide a constant supply of power all the time.
Wind turbines can be operated with backup generators which would supply the electricity needed when wind speeds are too low.
Links
- University of Oregon - Energy Storage
- Department of Energy: Alternative Fuel Stations
- MIT - Open Courseware: Capacitors
- Lessons In Electric Circuits: Batteries and Power Systems
- Platinum-Free Fuel Cell With Nickel Catalyst
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