|Home | Articles | Forum | Glossary | Books|
1 Storage Systems: Flywheel Storage • Compressed Air Energy Storage • Superconducting Magnetic Energy Storage • Battery Storage
2. Fuel Cells: Basic Principles • Types of Fuel Cells • Fuel Cell Operation
1 Storage Systems
Energy storage technologies are of great interest to electric utilities, energy service companies, and auto mobile manufacturers (for electric vehicle application). The ability to store large amounts of energy would allow electric utilities to have greater flexibility in their operations because with this option the supply and demand don’t have to be matched instantaneously. This is especially true when there are intermittent generating sources like wind and solar present in the grid. The availability of the proper battery at the right price will make the electric vehicle a reality, a goal that has eluded the automotive industry thus far. Four types of storage technologies (listed as follows) are discussed in this section, but most emphasis is placed on storage batteries because it’s now closest to being commercially viable.
The other storage technology widely used by the electric power industry, pumped-storage power plants, is not discussed as this has been in commercial operation for more than 60 years in various countries around the world.
• Flywheel storage
• Compressed air energy storage
• Superconducting magnetic energy storage
• Battery storage
1.1 Flywheel Storage
Flywheels store their energy in their rotating mass, which rotates at very high speeds (approaching 100,000 rotations per minute), and are made of composite materials instead of steel because of the composite's ability to withstand the rotating forces exerted on the flywheel. In order to store energy, the flywheel is placed in a sealed container which is then placed in a vacuum to reduce air resistance.
Magnets embedded in the flywheel pass near pickup coils. The magnet induces a current in the coil changing the rotational energy into electrical energy. Flywheel storage technologies are finding their way into UPS systems and for data center applications where a bridging power supply is necessary after a power failure and before the back-up diesel engine can be started.
1.2 Compressed Air Energy Storage
As the name implies, the compressed air energy storage (CAES) plant uses electricity to compress air, which is stored in underground reservoirs. When electricity is needed, this compressed air is withdrawn, heated with gas or oil, and run through an expansion turbine to drive a generator. The compressed air can be stored in several types of underground structures, including caverns in salt or rock formations, aquifers, and depleted natural gas fields. CAES technology has been in use for over 30 years. A 290 MW CAES plant started operation in Huntorf, Germany in 1978. This plant has demonstrated a 90% availability and 99% starting reliability. A 110 MW CAES plant went into commercial operation in McIntosh, Alabama in the United States in 1991. This plant stores compressed air in a 19-million-cubic-foot cavern mined from a salt dome and has a storage capacity for 26 h operation. For the last 20 years since the Alabama plant went into operation, there has not been any further commercial deployment of this technology. In the last few years there seems to have been a revival of interest in this technology, but no commercial power plant yet.
A typical CAES plant consists of a compressor, an air reservoir, and a combustion turbine genera tor. During the charging mode, the compressor (usually run by an electric motor) uses electricity to compress air which is stored in the air reservoir. While discharging, the compressed air is released from the air reservoir and fed as input to a gas turbine. This compressed gas is combusted in the turbine to produce electricity. In a CAES system the same synchronous machine can perform the functions of compressing the air and then operate as a generator (single compression-generation train). It may also be designed with two separate machines where one is used as a motor to compress the air and the other is used as a generator. In a combustion turbine, the air that is used to drive the turbine is compressed just prior to combustion and expansion and, as a result, the compressor and the expander must operate at the same time and must have the same air mass flow rate. In the case of a CAES plant, the compressor and the expander can be sized independently to provide the utility-selected "optimal" MW charge and discharge rates which determine the ratio of hours of compression required for each hour of turbine generator operation. The MW ratings and time ratio are influenced by the utility's load curve, and the price of off-peak power. For example, the CAES plant in Germany requires 4 h of compression per hour of generation. On the other hand, the Alabama plant requires 1.7 h of compression for each hour of generation. At 110 MW net output, the power ratio is 0.818 kW output for each kilowatt input. The heat rate (LHV) is 4122 BTU/kWh with natural gas fuel and 4089 BTU/kWh with fuel oil. Due to the storage option, a partial-load operation of the CAES plant is also very flexible. For example, the heat rate of the expander increases only by 5%, and the airflow decreases nearly linearly when the plant output is turned down to 45% of full load.
1.3 Superconducting Magnetic Energy Storage
A third type of advanced energy storage technology is superconducting magnetic energy storage (SMES), which may someday allow electric utilities to store electricity with unparalleled efficiency (90% or more). A simple description of SMES operation follows.
The electricity storage medium is a doughnut-shaped electromagnetic coil of superconducting wire.
This coil could be about 1000 m in diameter, installed in a trench, and kept at superconducting tempera ture by a refrigeration system. Off-peak electricity, converted to direct current (DC), would be fed into this coil and stored for retrieval at any moment. The coil would be kept at a low-temperature superconducting state using liquid helium. The time between charging and discharging could be as little as 20 ms with a round-trip AC-AC efficiency of over 90%.
Developing a commercial-scale SMES plant presents both economic and technical challenges. Due to the high cost of liquid helium, only plants with 1000 MW, 5 h capacity are economically attractive. Even then the plant capital cost can exceed several thousand dollars per kilowatt. As ceramic superconductors, which become superconducting at higher temperatures (maintained by less expensive liquid nitrogen), become more widely available, it may be possible to develop smaller scale SMES plants at a lower price.
1.4 Battery Storage
Even though battery storage is the oldest and most familiar energy storage device, significant advances have been made in this technology in recent years to deserve more attention. There has been renewed interest in this technology due to its potential application in nonpolluting electric vehicles. Battery systems are quiet and nonpolluting, and can be installed near load centers and existing suburban substations. These have round-trip AC-AC efficiencies in the range of 85%, and can respond to rapid changes in electrical load (e.g., within 20 ms). Several U.S., European, and Japanese utilities have demonstrated the application of lead-acid batteries for load-following applications. Some of them have been as large as 10 MW with 4 h of storage. The other player in battery development is the automotive industry for electric vehicle applications.
The commercial launch of several electric vehicles in 2011 has brought a lot of attention to storage batteries.
1.4.1 Battery Types
Chemical batteries are individual cells filled with a conducting medium-electrolyte that, when connected together, form a battery. Multiple batteries connected together form a battery bank. At present, there are two main types of batteries: primary batteries (non-rechargeable) and secondary batteries (rechargeable). Secondary batteries are further divided into two categories based on the operating temperature of the electrolyte. Ambient operating temperature batteries have either aqueous (flooded) or non-aqueous electrolytes.
High operating temperature batteries (molten electrodes) have either solid or molten electrolytes. Batteries in EVs are the secondary rechargeable type and are in either of the two sub-categories. A battery for an EV must meet certain performance goals. These goals include quick discharge and recharge capability, long cycle life (the number of discharges before becoming unserviceable), low cost, recyclability, high specific energy (amount of usable energy, measured in watt-hours per pound [lb] or kilogram [kg]), high energy density (amount of energy stored per unit volume), specific power (determines the potential for acceleration), and the ability to work in extreme heat or cold. No battery currently available meets all these criteria.
1.4.2 Lead-Acid Batteries
Lead-acid starting batteries (shallow-cycle lead-acid secondary batteries) are the most common battery used in vehicles today. This battery is an ambient temperature, aqueous electrolyte battery. A cousin to this battery is the deep-cycle lead-acid battery, now widely used in golf carts and forklifts. The first electric cars built also used this technology. Although the lead-acid battery is relatively inexpensive, it’s very heavy, with a limited usable energy by weight (specific energy). The battery's low specific energy and poor energy density make for a very large and heavy battery pack, which cannot power a vehicle as far as an equivalent gas-powered vehicle. Lead-acid batteries should not be discharged by more than 80% of their rated capacity or depth of discharge (DOD). Exceeding the 80% DOD shortens the life of the battery. Lead-acid batteries are inexpensive, readily available, and are highly recyclable, using the elaborate recycling system already in place. Research continues to try to improve these batteries.
A lead-acid nonaqueous (gelled lead acid) battery uses an electrolyte paste instead of a liquid. These batteries don’t have to be mounted in an upright position. There is no electrolyte to spill in an accident.
Nonaqueous lead-acid batteries typically don’t have as high a life cycle and are more expensive than flooded deep-cycle lead-acid batteries.
1.4.3 Nickel Iron and Nickel Cadmium Batteries
Nickel iron ( Edison cells) and nickel cadmium (NiCad) pocket and sintered plate batteries have been in use for many years. Both of these batteries have a specific energy of around 25 Wh/lb (55 Wh/kg), which is higher than advanced lead-acid batteries. These batteries also have a long cycle life. Both of these batteries are recyclable. Nickel iron batteries are nontoxic, while NiCads are toxic. They can also be discharged to 100% DOD without damage. However, these batteries have memory problem, which reduces useful energy recovery if the battery is not fully discharged before recharging. The biggest drawback to these batteries is their cost.
1.4.4 Nickel Metal Hydride Batteries
Nickel metal hydride batteries are offered as the best of the next generation of batteries. They have a high specific energy: around 40.8 Wh/lb (90 Wh/kg). According to a U.S. DOE report, the batteries are benign to the environment and are recyclable. They also are reported to have a very long cycle life. Nickel metal hydride batteries have a high self-discharge rate: They lose their charge when stored for long periods of time. They are already commercially available as "AA" and "C" cell batteries, for small consumer appliances and toys. In the past these batteries have been used for EV applications, but most automobile companies are now converging to Lithium ion batteries for electric vehicles.
1.4.5 Sodium-Sulfur Batteries
The sodium-sulfur (NaS) battery is a high-temperature battery, with the electrolyte operating at temperatures of 572°F (300°C). The sodium component of this battery explodes on contact with water, which raises certain safety concerns. The materials of the battery must be capable of withstanding the high internal temperatures they create, as well as freezing and thawing cycles. This battery has a very high specific energy: 50 Wh/lb (110 Wh/kg). During the last several years NaS batteries have found applications in power systems where large-scale storage is necessary. There are several examples in the United States and Japan where NaS batteries have been deployed in conjunction with wind farms.
1.4.6 Lithium Ion and Lithium Polymer Batteries
These batteries have a very high specific energy: 68 Wh/lb (150 Wh/kg) and store the same charge over their lifetime. These are widely used in laptop computers. Lithium ion (Li-ion) batteries use an anode of carbon, a cathode made of lithium cobalt oxide and an electrolyte gel. Under the stress of rapid charging or heavy use, a Li-ion battery may overheat very quickly with the possibility of fire. As a result, Li-ion batteries have an active protection circuit that prevents the battery from overheating. These batteries allow a vehicle to travel distances and accelerate at a rate comparable to conventional gasoline-powered vehicles. Lithium polymer batteries have the same basic chemistry as Li-ion batteries but use a porous separator which when exposed to the electrolyte, turns into a gel. Because this gel is not flammable, these batteries have a different architecture and are not subject to overheating and fire. Since there are no liquid electrolytes, these batteries can be manufactured in sizes as small as credit cards.
1.4.7 Zinc and Aluminum Air Batteries
Zinc air batteries are currently being tested in postal trucks in Germany. These batteries use either aluminum or zinc as a sacrificial anode. As the battery produces electricity, the anode dissolves into the electrolyte. When the anode is completely dissolved, a new anode is placed in the vehicle. The aluminum or zinc and the electrolyte are removed and sent to a recycling facility. These batteries have a specific energy of over 97 Wh/lb (200 Wh/kg). These batteries have been used in German postal vans which carry 80 kWh of energy in their battery, giving them about the same range as 13 gal (49.2 L) of gasoline. In their tests, the vans have achieved a range of 615 miles (990 km) at 25 miles per hour (40 km/h).
2 Fuel Cells
In 1839, a British Jurist and an amateur physicist named William Grove first discovered the principle of the fuel cell. Grove utilized four large cells, each containing hydrogen and oxygen, to produce electricity and water which was then used to split water in a different container to produce hydrogen and oxygen.
However, it took another 120 years until NASA demonstrated its use to provide electricity and water for some early space flights. Today the fuel cell is the primary source of electricity on the space shuttle.
As a result of these successes, industry slowly began to appreciate the commercial value of fuel cells. In addition to stationary power generation applications, there is now a strong push to develop fuel cells for automotive use. Even though fuel cells provide high performance characteristics, reliability, durability, and environmental benefits, a very high investment cost is still the major barrier against large-scale deployments.
2.1 Basic Principles
The fuel cell works by processing a hydrogen-rich fuel-usually natural gas or methanol-into hydrogen, which, when combined with oxygen, produces electricity and water. This is the reverse electrolysis process. Rather than burning the fuel, however, the fuel cell converts the fuel to electricity using a highly efficient electrochemical process. A fuel cell has few moving parts, and produces very little waste heat or gas.
A fuel cell power plant is basically made up of three subsystems or sections. In the fuel-processing section, the natural gas or other hydrocarbon fuel is converted to a hydrogen-rich fuel. This is normally accomplished through what is called a steam catalytic reforming process. The fuel is then fed to the power section, where it reacts with oxygen from the air in a large number of individual fuel cells to produce direct current (DC) electricity, and by-product heat in the form of usable steam or hot water.
For a power plant, the number of fuel cells can vary from several hundred (for a 40 kW plant) to several thousand (for a multi-megawatt plant). In the final, or third stage, the DC electricity is converted in the power conditioning subsystem to electric utility-grade alternating current (AC).
In the power section of the fuel cell, which contains the electrodes and the electrolyte, two separate electrochemical reactions take place: an oxidation half-reaction occurring at the anode and a reduction half-reaction occurring at the cathode. The anode and the cathode are separated from each other by the electrolyte. In the oxidation half-reaction at the anode, gaseous hydrogen produces hydrogen ions, which travel through the ionically conducting membrane to the cathode. At the same time, electrons travel through an external circuit to the cathode. In the reduction half-reaction at the cathode, oxygen supplied from air combines with the hydrogen ions and electrons to form water and excess heat. Thus, the final products of the overall reaction are electricity, water, and excess heat.
2.2 Types of Fuel Cells
The electrolyte defines the key properties, particularly the operating temperature, of the fuel cell.
Consequently, fuel cells are classified based on the types of electrolyte used as described next.
1. Polymer electrolyte membrane (PEM)
2. Alkaline fuel cell (AFC)
3. Phosphoric acid fuel cell (PAFC)
4. Molten carbonate fuel cell (MCFC)
5. Solid oxide fuel cell (SOFC)
These fuel cells operate at different temperatures and each is best suited to particular applications. The main features of the five types of fuel cells are summarized in Table 1.
2.3 Fuel Cell Operation
Basic operational characteristics of the four most common types of fuel cells are discussed in the following sections.
TABLE 1 Comparison of Five Fuel Cell Technologies
Polymer electrolyte membrane (PEM) Alkaline (AFC) Phosphoric acid (PAFC) Molten carbonate (MCFC) Solid oxide (SOFC)
Solid organic polymer poly-perflouro-sulfonic acid Aqueous solution of potassium hydroxide soaked in a matrix Liquid phosphoric acid soaked in a matrix Liquid solution of lithium, sodium, and/or potassium carbonates soaked in a matrix Solid zirconium oxide to which a small amount of yttrium is added.
Operating Temperature (°C): 60-100 90-100 175-200 600-1000 600-1000
Applications: Electric utility, transportation, portable power; Military, space; Electric utility, transportation, and heat; Electric utility; Electric utility
Advantages: Solid electrolyte reduces corrosion, low temperature, quick start-up Cathode reaction faster in alkaline electrolyte; therefore high performance Up to 85% efficiency in co-generation of electricity Higher efficiency, fuel flexibility, inexpensive catalysts Higher efficiency, fuel flexibility, inexpensive catalysts Solid electrolyte advantages like PEM
2.3.1 Polymer Electrolyte Membrane
The polymer electrolyte membrane (PEM) cell is one in a family of fuel cells that are in various stages of development. It’s being considered as an alternative power source for automotive application for electric vehicles. The electrolyte in a PEM cell is a type of polymer and is usually referred to as a membrane, hence the name. PEMs are somewhat unusual electrolytes in that, in the presence of water, which the membrane readily absorbs, the negative ions are rigidly held within their structure. Only the positive (H) ions contained within the membrane are mobile and are free to carry positive charges through the membrane in one direction only, from anode to cathode. At the same time, the organic nature of the PEM structure makes it an electron insulator, forcing it to travel through the outside circuit providing electric power to the load. Each of the two electrodes consists of porous carbon to which very small platinum (Pt) particles are bonded. The electrodes are somewhat porous so that the gases can diffuse through them to reach the catalyst. Moreover, as both platinum and carbon conduct electrons well, they are able to move freely through the electrodes. Chemical reactions that take place inside a PEM fuel cell are presented as follows:
2 4 4 2 H H e ? + + -
O H e H O 2 2 4 4 2 + + ? + -
Net reaction: H O H O 2 2 2 2 2 + =
Hydrogen gas diffuses through the polymer electrolyte until it encounters a Pt particle in the anode.
The Pt catalyzes the dissociation of the hydrogen molecule into two hydrogen atoms (H) bonded to two neighboring Pt atoms. Only then can each H atom release an electron to form a hydrogen ion (H+) which travels to the cathode through the electrolyte. At the same time, the free electron travels from the anode to the cathode through the outer circuit. At the cathode the oxygen molecule interacts with the hydrogen ion and the electron from the outside circuit to form water. The performance of the PEM fuel cell is limited primarily by the slow rate of the oxygen reduction half-reaction at the cathode, which is 100 times slower than the hydrogen oxidation half-reaction at the anode.
2.3.2 Phosphoric Acid Fuel Cell
Phosphoric acid technology has moved from the laboratory research and development to the first stages of commercial application. Turnkey 200 kW plants are now available and have been installed at more than 70 sites in the United States, Japan, and Europe. Operating at about 200°C, the phosphoric acid fuel cell (PAFC) plant also produces heat for domestic hot water and space heating, and its electrical efficiency approaches 40%. The principal obstacle against widespread commercial acceptance is cost.
Capital costs of about $2500 to $4000/kW must be reduced to $1000 to $1500/kW if the technology is to be accepted in the electric power markets.
The chemical reactions occurring at two electrodes are written as follows:
At anode H H e At cathode O H e H O
2.3.3 Molten Carbonate Fuel Cell
Molten carbonate technology is attractive because it offers several potential advantages over PAFC. Carbon monoxide, which poisons the PAFC, is indirectly used as a fuel in the molten carbonate fuel cell (MCFC). The higher operating temperature of approximately 650°C makes the MCFC a better candidate for combined cycle applications whereby the fuel cell exhaust can be used as input to the intake of a gas turbine or the boiler of a steam turbine. The total thermal efficiency can approach 85%. This technology is at the stage of prototype commercial demonstrations and is estimated to enter the commercial market by 2003 using natural gas, and by 2010 with gas made from coal. Capital costs are expected to be lower than PAFC. MCFCs are now being tested in full-scale demonstration plants. The following equations illustrate the chemical reactions that take place inside the cell:
At anode: H CO H O CO e
And: CO CO CO e
2.3.4 Solid Oxide Fuel Cell
A solid oxide fuel cell (SOFC) is currently being demonstrated at a 100 kW plant. Solid oxide technology requires very significant changes in the structure of the cell. As the name implies, the SOFC uses a solid electrolyte, a ceramic material, so the electrolyte does not need to be replenished during the operational life of the cell. This simplifies design, operation, and maintenance, as well as having the potential to reduce costs. This offers the stability and reliability of all solid-state construction and allows higher temperature operation. The ceramic make-up of the cell lends itself to cost-effective fabrication techniques.
The tolerance to impure fuel streams make SOFC systems especially attractive for utilizing H2 and CO from natural gas steam-reforming and coal gasification plants. The chemical reactions inside the cell may be written as follows:
At anode H O H O e
And: CO O CO e
At cathode O
Fuel cells can convert a remarkably high proportion of the chemical energy in a fuel to electricity. With the efficiencies approaching 60%, even without co-generation, fuel cell power plants are nearly twice as efficient as conventional power plants. Unlike large steam plants, the efficiency is not a function of the plant size for fuel cell power plants. Small-scale fuel cell plants are just as efficient as the large ones, whether they operate at full load or not. Fuel cells contribute significantly to the cleaner environment; they produce dramatically fewer emissions, and their by-products are primarily hot water and carbon dioxide in small amounts. Because of their modular nature, fuel cells can be placed at or near load centers, resulting in savings of transmission network expansion.