Storage Of Electrical Energy

Because of the transient nature of electricity, none of the techniques used for the storage of chemical fuels are of use in storing electricity. Capacitors can store tiny bits of energy for a few moments. Most magnetic devices store less energy than capacitors. The only methods used to store significant quantities of electric energy do so by converting it to some other form and then storing the other form.

The common storage container for electrical energy is the battery. There are many types of batteries in use. In single use batteries, as used in flashlights, energy is used to drive a chemical reaction that produces a material used to fabricate the battery. In the manufacture of common batteries, energy is used to produce zinc metal. In this manner, energy is stored as active zinc metal when the battery is assembled. An electrochemical reaction consumes the zinc producing electricity when it is needed. Lithium batteries have recently come into use because they provide more power than zinc, mercury or alkaline batteries. Magnesium batteries can be stored for long periods. They use seawater as the electrolyte. Once filled with electrolyte they must be used. They cannot be recharged. Single use batteries are not rechargeable so are of no value for storage of energy from the power line. Single use batteries only serve as a one-time energy supply. 166

In multiple use storage batteries, input electrical energy is converted into an active chemical within the battery. The chemicals are stored within the battery for later regeneration of the electricity. The chemical reactions used to store energy in both types of batteries are similar. The difference lies in the fact that the storage battery can be recharged, but the single use battery must be replaced.

Storing electrical energy in batteries has many limitations. Batteries store a small amount of energy useful in short cycle applications such as starting an automobile, powering a flashlight, or providing emergency lighting. The only common uses for batteries that involve the storage of a meaningful amount of energy are golf carts and similar small vehicles. Other examples of these uses are the courtesy vehicles used to transport people inside airports and small forklifts used inside warehouses. These vehicles share similar requirements. They need only go short distances at low speeds. They can be taken out of service for long periods to recharge the batteries. Their quiet operation and lack of gaseous emissions are valuable in these special applications; users can accept the high cost, poor performance and lack of flexibility. In a tiny number of locations, batteries are used to store solar or wind energy for routine operation of a home or small business. In these applications, one of the large problems is the tendency for batteries to accept less total charge each time they are recharged. This performance characteristic results in frequent costly battery pack replacement.

166 http://www.eurobatteries.com/sitepages/Battery%20Glossary.htm

A good automobile type lead-acid battery can store about 100 ampere hours of electric energy, about enough to operate a 100 watt light bulb for one hour or a 1200 watt stove burner for 7 or 8 minutes. To operate one average house for 24 hours, without clothes washing and drying or air conditioning, 20 to 30 fully charged automobile batteries would be required. We all know that these batteries must be replaced after 2 to 3 years in cold climates and 3 to 5 years in warm climates. In general, when discarded, the lead is recycled. However, lead is quite toxic. It is dangerous to simply throw a discarded lead-acid battery in the trash. Great care must be taken in disposal of lead-acid batteries.

There are rechargeable batteries with better energy storage capability than the lead acid battery used in automobiles. Nickel-cadmium batteries used in portable, rechargeable power tools store 2 or 3 times more energy than lead acid batteries on a weight basis, but are far more expensive. Like lead-acid batteries nickel-cadmium batteries present a disposal problem. Like lead, cadmium is toxic and must be recycled. Nickel-hydrogen batteries are used on some spacecraft. Recently Toyota introduced a hybrid automobile that uses a nickel-hydrogen battery. Nickel-hydrogen and a type of rechargeable lithium battery are finding use in powering laptop computers and cell phones.

A large amount of money was spent in the late 1970s in an effort to discover new chemical combinations for batteries. The efforts were directed at improving the power to weight ratio, and the ability to go through many cycles of full charge to full discharge without loss of storage capacity. One new type of battery, the sodium-sulfur cell, has resulted from this work.

The sodium-sulfur battery has a greatly improved power to weight ratio and will accept a large number of charge, recharge cycles without significant loss of capacity. Their major shortcoming is they must be operated at a temperature of 625 Kelvin (350 degrees Celsius). When they are cold, they can neither be charged nor discharged. Other problems have been encountered with the physical stability of some of the internal components. Further work will be required before these batteries can be used for bulk power storage in stationary applications.

Sodium-sulfur batteries have been suggested as a possible method of storing energy for over-the-road transportation use. In addition to the problems with heat and component stability, they have shortcomings that are particularly troubling in transportation use. The high operating temperature will be difficult to maintain when vehicles are not in use. If they are allowed to cool, it will take a long time and much energy to heat them to operating temperature. Safety is a major concern with these cells. If they are physically damaged, the hot sulfur burns producing toxic fumes. The molten sodium burns and reacts explosively with water. These characteristics make the use of sodium sulfur cell in transportation unlikely.

Metal-air batteries are advantageous in some situations. Zinc-air batteries are available for hearing aids. Experimental aluminum and magnesium air batteries have been tested. These batteries use oxygen from the air as the second reactant. The metal is consumed to produce the electricity and the metallic oxide builds up in the battery. Metal air batteries produce more energy per initial unit weight than do batteries that carry both reactants. As they are used, they gain weight. When depleted they weigh the same as any other battery using the same chemical system. Metal-air batteries cannot be recharged. The metal oxide produced can be recycled, but the cost is high.

New battery systems may be useful for specific limited applications, but it is impossible for a battery system to compete with a chemical fuel burned with air. 167 In a battery system there are two chemical

167 http://www.ev.hawaii.edu

electrodes; one gives up electrons to the electrical circuit and the other takes them up to complete the circuit. A rechargeable battery must store all the chemicals involved in the reaction within the confines of the battery case. This requirement, to carry all the reactants at all times, is the primary reason that batteries cannot store as much energy per unit mass as can chemical fuels generating energy by reacting with air. In a chemical - air energy systems, all that is carried is the fuel and the conversion equipment. The second reactant, air, is obtained locally and the waste products are dumped into the air.

The metal air battery is kind of hybrid of the two. Only the fuel and conversion equipment are initially within the battery. Thus the unused metal air battery is lighter per unit power capability than a battery that contains all the reactants. The air supplies the other reactant and the reactions products are confined within the battery.

The most energetic chemical battery possible is a cell using beryllium as the source of electrons and oxygen from the air as the sink. Today it does not appear possible to produce such a beryllium-air cell, but if it were possible, it would store the maximum energy possible for a chemical battery. This best of all possible chemical battery cells would store energy at a rate of 24.5 megajoules per kilogram of reactants. Gasoline stores 45 megajoules per kilogram and hydrogen stores about 120 megajoules per kilogram. The battery case, electrode support structure and electrolyte weight are not included in calculating the storage energy of the beryllium-oxygen battery. In calculating the fuel energies, the weight of the tank was not included.

Sandia Laboratories has examined a concept involving storing electrical energy in magnetic fields. Their system employs interacting magnetic fields generated by super-conducting coils. The potential of this technology is low storage loss and high rates of discharge. It should be re-usable at full capacity for a very large number of charge-discharge cycles. Thus far, only modest research has been performed. It is not clear if the formidable task of constructing these devices, which consist of massive super conducting coils carrying heavy currents at liquid helium temperatures and supported against powerful magnetic fields, can be performed. Nor is it clear if they will provide an economic method of storing electric energy. Much more research is required to determine if these devices will ever be able to store energy on a commercial scale. If successful, the requirement that the super conducting coils be cooled to liquid helium temperatures (4.2 Kelvin) will limit the use of these energy storage devices to large installations.

Electric energy is effectively stored by use of pumped hydro storage. This technology utilizes a reservoir located at a good height over a conventional hydroelectric power generating station. During off peak load times, when other non-hydroelectric portions of the system have excess nuclear or coal fired generating capacity, water is pumped into the reservoir. At peak need times the water is allowed to return through the hydroelectric plant to generate power to satisfy the peak load requirement. This system has reasonable efficiency, but it requires an unusual combination of terrain and non-hydroelectric generation capacity availability to be of value. This energy storage technique is used at a small plant in the mountains west of Denver, Colorado and near Ludington, Michigan. There is little prospect for the wide scale use of pumped hydro storage. Its use is limited to special circumstances.

Unlike electricity, chemical fuels can be stored in a number of safe convenient forms. In the transportation network, the pipelines serve as a huge energy storage reservoir. The large volume of a continent-spanning pipeline filled with gas at a pressure of 40 to 60 atmospheres pressure contains several days' supply of gas for the users. If a section of the line is put out of operation, it can be isolated for a period by valves and the residual gas pressure in the line can for a time, serve the customers. From the customer's view point this adds reliability of supply to the already high reliability of pipelines, even when portions of the pipeline have been damaged.

Petroleum fuels store energy at the rate of 45 megajoules per kilogram. Hydrogen fuel stores energy at a rate of 120 megajoules per kilogram. The much higher storage capacity of the chemical fuels is a result of the lack of the requirement to include all the chemical reactants in the weight of the system. Combustion fuels react with the oxygen of the atmosphere. The weight of the necessary oxygen does not need to be included in the weight of the system, as it is a battery. With chemical fuels the reaction products are placed in the atmosphere and do not need to be saved for the later recharge of the system. Neither the oxygen nor the reaction products need be stored. As mentioned above, in most batteries the fuel, the second reactant (equivalent to oxygen) and the reaction products must be carried about at all times.

For users with special needs it is possible to place storage containers for chemical fuels at the point of use and store an emergency supply of chemical fuel on site. This type of system increases the capital cost of the user's system, but it adds an increment of reliability. Such a high level of reliability is useful for public facilities such as hospitals, police stations, fire stations and air traffic control centers. The common technique is a system with a diesel or gasoline engine wired to start if the main power is interrupted. The engine operates a generator supplying emergency power to the critical equipment in the facility until the main power is repaired. In these facilities, the added cost for on site storage and emergency power generation is acceptable because of the need for extreme reliability in operating the facilities. It has been shown that handling of energy stored as a chemical fuel offers the greatest possible flexibility, utility and efficiency. A recapitulation of the availability of various handling techniques is shown in Table 4.1.

Chemical

Electrical

Transport of energy: Energy to the Stationary Consumer

By networks

Yes by Pipeline

Yes by Wires

Discrete Bulk Shipment.

Yes in Containers

Not Possible

Storage of energy: Load Management, Daily, and Yearly

Bulk Storage

Yes 1 Yes by Batteries & Pumped Hydro

Energy for Transportation: Energy to Moving Vehicles

Rail

Yes by Stored Fuel

Wires or third rails

Highway

Yes by Stored Fuel

Not Possible

Air

Yes by Stored Fuel

Not Possible

Water

Yes by Stored Fuel

Not Possible

Table 4.1 Energy Handling

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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