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A chemical fuel is required
Applied Energy 47 (1994) 169-182
A Chemical Fuel is Required 'The outputs from renewable and nuclear-energy sources do not match well in space or time with the needs of the users. This leads to the requirement for an energy storage medium that can be easily implemented.'
Laurence O. Williams Energy in transition. App. En. 23 (1986) 171-87.
ABSTRACT It was shown that fusion energy sources have excellent characteristics for future base load energy needs. However, fusion is limited to the production of heat, and from heat, electricity. This is also a limitation of most of the other alternatives to fossil fuels examined earlier. Solar thermal and geothermal energy sources produce only heat and, from heat, electricity. Power plants using solar photovoltaic, wind, tidal and ocean thermal gradients only produce electricity. Solar energy collected as biomass can yield carbon-based chemical fuels that can be handled in the same manner as fossil fuels, but like fossil fuels they are more valuable for other uses and can cause significant pollution. No matter what future combination of these energy sources is adopted there are many important uses for energy that cannot be satisfied with heat, electricity or biomass fuels. A manufactured chemical fuel is needed. The handling of energy, including transport and storage is examined, and the case is made that a portable, storable energy carrier is required for all users of energy.
CHEMICAL ENERGY TRANSPORTATION Chemical fuels have very desirable characteristics for the transportation of energy. Liquid and gaseous fuels can be p u m p e d continental distances through pipelines without significant loss. All chemical fuels can be carried on trucks, trains, planes and boats. Solid fuels, suspended in water, have been transported through pipelines over modest distances as if they were liquids. The other m e t h o d of energy transport, electricity, is essentially limited to transmission over wires. F r o m a comparison of the 169 Applied Energy 0306-2619/94/$07.00 © 1994 Elsevier Science Ltd, England. Printed in Great Britain
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properties of transport of energy as chemical fuel and as electricity it will be shown that a chemical fuel offers advantages of simplicity, efficiency and reliability when compared with electricity. Pipeline transport is used throughout the world to transport natural gas and oil. At the source, the gas or oil is pumped to high pressure and introduced into a pipe. At this point the pipe must be quite strong to resist the high pressure of its contents. At appropriate points the pipeline is divided into smaller lines for distribution to specific areas or regions. The smaller lines can be operated at the same pressure as the main line or at a lower pressure depending on the length of the smaller line and its desired flow rate. The flow and pressure of the fluid in the line can be controlled by simple valves and orifices. The process of branching into smaller and smaller lines with pressure reduction is continued until the pipe line arrives at its final destination. For reasonable costs the pipeline systems can serve users as large as several states or as small as an individual home. For industrial customers, with large usages, the pipelines are large and the pressure relatively high. For small users, such as individual homes, the pipeline can be slender and the pressure low. The delivery of the energy through this type of system is very reliable. It has been in use for more than 120 years and has provided a high-quality service. Pipelines lose little of their contents by direct leakage, but energy is consumed by friction as the fluid flows through the pipe. To make up for this friction, and to keep the contents moving at the desired rate, pumping stations are established along the lines. Some pumping stations consume fuel from the pipeline to provide the pumping energy, others use electric energy obtained from the local grid. Whichever source of pumping energy is used, the effect on the total system is to deliver less energy to the customer than was initially available. The amount of energy used in pumping is strongly dependent on the design and length of the pipeline system. As a rule of thumb, less than 5% of the energy put into the pipeline is consumed in pumping and, is thus, unavailable to the customer 1.
ELECTRICAL ENERGY TRANSPORTATION A large amount of energy is transmitted by pipelines, but a similar amount is transmitted by electric power lines. To compare these two types of transmission techniques, it will be useful to review some of the characteristics of the electric power transmission system. Electrical systems share some of the characteristics of a pipeline distri-
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bution system. Elcctricity can be produced and fed into the distribution system at a central location, and transported to users over a distribution network. The electric distribution network is made of copper and aluminum wires rather than iron, steel or plastic pipes, but in many respects the gas and electric distribution networks are very similar. The wires fan out from central and substation distribution centers to serve individual customers. In virtually all parts of the world electric power lines can be seen distributing energy. Despite its wide use, electricity is not ideal as a method of transmitting energy. A major problem is power loss during transmission. Resistance in the transmission wires turns a portion of the electrical energy into heat, which is lost to the surroundings and performs no useful task. The amount of heat produced is a function of the voltage used on the transmission line, its length and the diameter of the wire. Very little energy is lost from short, thick wires operating at very high voltage. Short wires do not go anywhere, and thick wires are expensive, hard to string and difficult to support. These factors lead to electrical transmission systems made up of thin wires operating at high voltages 2. The higher the voltage, the more difficult it is to insulate the voltage from the surroundings and the more difficult it is to switch the power off and on. High voltage can form electric arcs. The length of these arcs depend on the shape of the electrodes, temperature, humidity, air circulation, and the presence of ionizing radiation. At r o o m temperature, normal pressure and 50% humidity, a 2.5 cm spark will form between two sharp points at 12 000 V. At 50 000 V the spark is 13 cm long and at 100 000 V it can span nearly 40 cm. To achieve low loss, cross country lines are operated at more than 300 000 V and local distribution lines are operated at more than 10 000 V. Insulators used to handle these high voltages must be large and of high quality to prevent arcing to the support structure. The high voltage wires also must be prevented from coming close to any grounded conductor or, at a minimum, power will be lost. In the worst case, arcs will form resulting in fire and destruction. In the early days of electric power, Thomas A. Edison wanted to use direct current (DC). He found it required very thick wires to transmit energy with low loss. Raising the voltage to decrease these losses made the design of the generator difficult and compelled the user to handle very high voltages; voltages so high the potential for arcs, fires and hazards were extreme. It is difficult to convert direct current from low to high voltage for transmission and back to low voltages for safe use. Because of these shortcomings direct current transmission lost out to alternating current (AC). With AC it is possible to use a transformer to step the voltage up or
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down. This permits the generator to operate at its optimum voltage. A transformer is used to step the voltage up to a high value for relatively low loss transmission on thin lines. Near the user the voltage is reduced by a second transformer to a value that is safe and easy to handle. On the low voltage user side of the second transformer the wire size is increased to reduce losses. This is acceptable because of the relatively short run from the transformer to the user. This combination of optimum generator operation, step-up conversion to high voltage for low loss transmission and voltage step-down conversion for safe use by the customer has provided the basis for the electric system world-wide. Transformers are remarkably efficient, ranging from 88-96%. Few things have an efficiency this high. Despite their high efficiency, every time the voltage is converted, 2-12% of the energy is lost in the transformers as heat. Depending directly on the distance between the generator and the customers, more power is converted to heat in the transmission lines. The power converted to heat is lost and reduces the efficiency of the energy distribution system. 3 AC is subject to a loss, other than resistance, when transmitted over long distances. Alternating current used in the USA cycles at a rate of 60 Hz. The 60 cycle current can generate 60 Hz radio waves. This can be observed as the buzz heard on a car radio when driving near high voltage power lines. Like electrical resistance, this radiation results in a loss of power. The amount of power radiated is affected by the condition of the lines and their length. The wave length of a 60 Hz radio wave is about 5000 km. As the length of a transmission line approaches a quarter-wave length (1250 km), more and more power is lost by radiation. When the line length reaches 1250 km, more than half of the power input is radiated as essentially useless 60 Hz radio waves. The radiation effect places limits on the distance 60 Hz AC can be transmitted without severe loss. 2 Commercial systems for conversion of high-voltage AC to high-voltage DC are in use today. These systems allow the transport of electric power distances greater than 1250 km, but the equipment for conversion has internal resistance and conversion losses and it is very expensive. Highvoltage direct current systems are used only in locations where the input power cost is quite low and the source far from the customers. This combination is most often found with hydroelectric power generation facilities located at sites remote from large cities. For example, a direct current line carries power from the water power projects on the Columbia River, in Washington State, to San Francisco. These special circumstances demonstrate high-voltage DC transmission can be used successfully. However, ultra voltage-high DC transmission has little use in most power distribution systems.
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In a large system, power lost in transmission is dependent on factors such as the n u m b e r of transformers, the length of the transmission lines, the size of the wires, the type and number of customers and the peak voltage used. The very best systems deliver less than 85% of their generator output to the customer and many deliver much less. 4 When the efficiency of the transport of energy as a chemical fuel is compared to the efficiency of electric energy transport, one can wonder why electricity is used at all. It survives and grows because of the low cost and simplicity of the end-use electrical conversion devices. Electric lights may use more energy than gas lights, but they are far more convenient, can be controlled with great ease and present little fire hazard. Small electric motors incorporated in home appliances and manufacturing equipment are extremely convenient and simple to use. In applications where large amounts of energy are used, such as home heating and cooking, gas is more cost effective and is often selected for this purpose in preference to electricity when both are available. Small motor-driven appliances could be operated with little gasburning internal combustion engines at higher energy efficiency, but enormously reduced convenience, flexibility, and reliability. The electronic gadgets in the home--TVs, video recorders, radios, stereo sound systems and microwave ovens, all require electricity for operation. To supply these devices with energy as gas or oil would not only require engines, but generators and local wires as well. As a result of the convenience of electric devices the relatively less efficient electric power distribution systems will, in the near term, remain a useful method of transporting energy. 5 For all its convenience electricity has several major shortcomings as an energy handling medium. Electricity cannot be stored. At all times the power plant operators are adjusting the o u t p u t of the generators to match the needs of the users. This can create difficulties if the user changes his needs at a rapid rate. To account for these changes most systems have a hierarchy of generating capability. First are the base load generators. These Consist of large coal fired or nuclear generators operated at a constant output power level. These generators produce electric power at the lowest cost, but require hours to start and to shut down. The next level of generators consists of intermediate size units capable of more rapid turn-on and-off. The final level is made up of peaking units that can start from cold to full power in a matter of minutes. The peaking units often burn relatively expensive gas or oil, thus the power generated by these units costs up to 10 times more than power produced by the base-load generators. 6 In normal operations the base load generators operate at all times.
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During the morning hours the intermediate generators are activated to supply the basic power for the operation of routine daily activities. Under some conditions, intermediate generators may carry the total load. On a very hot summer afternoon, when the demand for electricity to drive air conditioning is at its peak, it may be necessary to bring the peaking units into operation to carry the short term peak load. 7 As the day progresses the loading process is reversed. As the demand drops the peaking units are shut down followed by the intermediate units. Finally late in the evening all extra power is shut off and the base load generators supply the low needs through the late night and early morning hours. This system works well as long as events follow the predicted course. When events deviate, a series of problems can result starting with slight brownouts to momentary interruptions progressing on to long interruptions and finally disaster such as happened in the northeast section of the USA in the 1960s when a multi-state area was without power for several days. Power outages are usually a result of weather conditions that rapidly change the demand in unexpected ways or violent weather that damages parts of the system. Most electric systems have a dozen or more power interruptions during the summer when thunderstorms are common. Lightning strikes some part of the system. The surge of voltage causes protective circuits to cut the power off for a short time. If the protective circuits are inadequate, lines and transformers are damaged causing interruptions lasting for hours while the equipment is repaired. Lightning induced power surges can occasionally cause a section of a system to fail resulting in interruptions requiring several days to repair. Ordinary storms, tornadoes and hurricanes have winds capable of knocking down wires and poles. Often, interruptions in electric service can be repaired in a few hours but when many poles and kilometers of wire must be replaced the interruption can last for weeks. In winter, lightning is not a severe problem, but snow and ice can collect on power lines and tear them down. This results in interruptions that last for a number of days because thousands of kilometers of wire must be replaced. Winter interruptions are less common than summer interruptions, but when they occur they often last longer because the damage is greater and the repair crews have a difficult time working in ice and snow. 8 Underground transmission of electric power over long distances is impractical. Long distance transmission requires high-voltage. Most ground is somewhat electrically conductive so very thick, high quality insulation is needed to protect the lines from arcing. The thick insulation is
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expensive and the installation of thick stiff transmission lines is difficult. Consequently, long distance underground transmission of electricity is quite costly and is used only in specialized situations. One of the common uses for underground transmission lines is in the final run from the local transformer to the private home. This is usually a short length of thick wire carrying a relatively low-voltage. The short path keeps the cost of the thick wire to an acceptable level and the low voltage makes the use of simple insulation feasible. The country is crossed and recrossed by above ground high-voltage power lines for implementation of the distribution system. Plant growth must be prevented close to the lines because trees and other vegetation are sufficiently conductive to short circuit the high-voltage. For much the same reason buildings cannot be allowed near the lines. As a consequence, all of the cross country power transmission lines have a swath of clear cut ground about 100 m wide along their path. These are not pleasing to the eye. Because of the danger from arcs, the land under the lines has few uses. The high-voltage AC induces small electric currents in everything near the path of the transmission line. At a distance the effect is weak but it becomes stronger close to the power line. This effect is commonly observed as the buzz in a car radio that gets stronger as you near the transmission line. Fragmentary data indicate the tiny electric currents produced in people living near the power lines can cause health problems. We live with and tolerate the power lines because they are necessary for the use of electric power. 9'1° The power lines stretch across the country delivering electric energy at relatively low efficiency and marginal reliability. In performing this task they provide visual insult and possible environmental harm. A similar amount of energy is transmitted by the nation's pipelines, but most of us are totally unaware of their existence. When compared to electric power lines, pipelines carrying a chemical fuel are more efficient, more reliable, invisible and only harm the environment by accidental leakage. The major justification for the transmission of electricity stems from its ability to directly power electronic devices of the user. These include common every day items such as electric lights, electronic devices (radios, TVs, VCRs), small motors and controls. The foregoing discussion of electric power transmission is presented to demonstrate that pipelines carrying a chemical fuel are far more efficient and reliable at delivering energy than are electric wires carrying electricity. Under most circumstances pipelines are buried a meter or two underground and are protected from the destructive effects of weather. Pipelines of any size can be buried without significant problems. Pipeline
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failures are limited to the occasional break due to materiel failures, digging for construction and earthquakes. To reap the full advantages of the use of pipelines for the transportation of energy it is necessary to transmit most energy in the form of a chemical fuel.
S T O R A G E OF C H E M I C A L E N E R G Y Chemical fuels can be easily stored for long periods. The solid fuels present the least problems of storage. For simple temporary storage, coal can be d u m p e d in a pile on the ground. If storage is for only a short period of time the coal can be left in the open. For longer term storage it is desirable to cover the coal with some type of water p r o o f cover. The cover prevents rain from soaking the coal and interfering with the combustion process. Rain water can also leach toxic sulfur c o m p o u n d s from coal stored in the open thereby creating significant environmental damage. 7 In long term coal storage facilities, great care must be exercised to prevent unacceptable environmental damage. To protect the ground from contamination, the coal can be placed on plastic or rubber sheets or on a concrete pad. In either case it is desirable to provide a method of catching the rain water that filters through the coal. When coal comes in contact with water toxic chemicals, such as sulfur, are leached and contaminate the water. For environmental protection the contaminated water must be purified before allowing it to enter the water table. The storage of oil and oil-derived liquid chemical fuels is somewhat more complicated than the storage of coal. These fuels must be confined in some sort of tank. It is usually best to cover the tank so water and dirt do not contaminate the fuel. In this case the cover serves the purpose of protecting the oil from contamination and evaporation rather than protecting the soil from water that has leached toxins from the fuel. The tanks designed for liquid fuel storage are somewhat more costly than the simple coal pile, but the cost is still low. Tank storage of liquid fuels ranges from large ocean-going tankers, to onshore storage depots, storage at local distribution points, local gas stations, automobile tanks and the small tank that stores the fuel to power small tools such as the lawn-mower and the chainsaw. Gas is the most complicated chemical fuel to store. It must be placed in a container of sufficient strength to resist the gas pressure and prevent all leakage of air and water. It has been found the most cost effective method of storing large quantities of gas is in low pressure storage gas holders that change their volume in response to gas input and with-
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drawal. These tanks are large, but of relatively low cost. When low pressure storage is used there is no need to spend a large amount of energy compressing the gas. In a large scale distribution system, the pipelines serve as a large volume continent-spanning storage reservoir in addition to their primary function as a conduit. In other applications, where modest capacity is required and cost is not a major consideration, gas can be stored in high pressure cylinders as is done with welding gas.1 The storage methods used with chemical fuels are satisfactory regardless of the storage duration required. Chemical fuels can be stored for hours, days, months, or years allowing maximum flexibility for the fuel consumer. If sufficient storage space is available, chemical fuels can be purchased when the price is low for use at any later time. Chemical fuels can be purchased and stored against future disruptions in the fuel distribution system. In the summer when river barge transportation is available at low cost, power plants can buy their winter fuel and place it in storage. In the winter when the river is frozen and fuel cannot be shipped, the power plant can continue operating using its stored fuel. Emergency power systems can be equipped with a supply of fuel adequate for several days operation. With proper design, fuel can be reliably stored for years ensuring its availability if the main power systems fails.
STORAGE OF E L E C T R I C A L E N E R G Y 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 even 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 that. 7 The most common storage container for electrical energy is the battery. 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 process of manufacture of common batteries the energy is used to produce zinc metal. In this manner energy is stored in the form of active zinc metal when the battery is assembled. An electrochemical reaction consumes the zinc producing electricity when it is needed. In multiple use storage batteries, input electrical energy is converted to a chemical form within the battery. The chemicals are stored within the battery for later regeneration of the electricity. The chemical reactions
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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 singleuse battery must be replaced. Storing electrical energy in batteries has many limitations. Batteries store a small amount of power 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 fork lifts 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 so valuable in these special applications, users can accept the high cost, poor performance and lack of flexibility. In these applications one of the large problems is the tendency for lead/acid batteries to accept less total charge each time they are recharged. This performance characteristic results in frequent costly battery-pack replacement. A good automobile type lead/acid battery can store about 100 Ah of electric energy, about enough to operate a 100 W light bulb for 1 hour or a 1200 W stove burner for 7 or 8 min. To operate one average house for 24 h, without clothes washing and drying or air conditioning, 20 to 30 fully charged automobile batteries would be required. There are batteries with better energy storage capabilities 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. Lithium batteries have recently attracted attention because they provide more power than zinc, mercury or alkaline batteries. They are not rechargeable so are of no value for power line energy storage and only serve as a one shot energy supply. 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. This 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 350°C. When they are cold they can neither be charged or discharged. Other problems have been encountered with the physical stability of some of the internal components. Further studies
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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 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. New battery systems may be useful for specific limited applications, but it can easily be shown it is impossible for a battery system to compete with a chemical fuel burned with air. In a battery system there are two chemical 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. The most energetic chemical battery possible is a cell using beryllium as the source of electrons and oxygen as the sink. Today it does not appear possible to produce a beryllium/oxygen 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 o f 6.8 kWh kg -~ of reactants. The battery case, electrode support structure and electrolyte weight are not included in this calculation. Sandia Laboratories has examined a concept involving storing electrical energy by means of interacting magnetic fields generated by superconducting coils. The advantages of this technology are low storage loss and high rates of discharge. It should be re-useable 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 superconducting coils carrying heavy currents at liquid helium temperatures and supported against powerful magnetic fields, can be performed. N o r 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
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superconducting coils be cooled to liquid helium temperatures (~4.2 K) will limit the use of these energy storage devices to large installations. Electric energy can be 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. Its use is limited to special circumstances and there is little hope that it can be placed in wide use for the effective storage of electric power 7. 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 40-60 atm 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 customers can, for a time, be served by the residual gas pressure in the line. From the customer's viewpoint this adds reliability of supply to the already high reliability of pipelines, even when portions of the line have been damaged. Petroleum fuels store energy at the rate of 12.5 kWh kg. Hydrogen fuel stores energy at a rate of 33.5 kWh kg -1. 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. 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. In a battery, the fuel, the second reactant (equivalent to oxygen) and the waste 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 improves 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
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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.
ENERGY CARRIER END USES
It has been shown that handling of energy in the form of a chemical fuel offers the greatest possible flexibility, utility and efficiency. A recapitulation of the availability of various handling techniques is shown in Table 1 . TABLE 1 Energy Handling Chemical energy
Transport of energy By networks Discrete bulk shipment Storage of energy Load management Daily Seasonal Emergency use Transportation energy Rail Highway Air Water
Electric energy
Pipeline Yes
Wires Not possible
Stored fuel Stored fuel Stored fuel
Varying generators Pumped hydro Not possible Not possible
Stored Stored Stored Stored
Wires Not possible Not possible Not possible
fuel fuel fuel fuel
C H E M I C A L FUELS F O R T R A N S P O R T A T I O N The ability to provide a lightweight, high energy chemical fuel to deliver energy for transportation is essential for any future energy system. Whereas stationary users can use fuels in any physical state--solid, liquid, or gas--the transportation sector must use a fuel that can be quickly and easily loaded into the vehicle storage tank. As a result, the selection of the future chemical fuel is strongly driven by the needs of transportation in the energy use economy. The next paper in this issue
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will examine the characteristics of possible manufactured fuels to serve the needs of transportation. It is assumed that any fuel successfully used for transportation can be used for stationary applications. The detailed discussion of application of a manufactured chemical fuel for transportation has been deferred to the paper on 'Hydrogen as a Transportation fuel'.
SUMMARY It has been shown that for transport and storage of energy chemical fuels offer many advantages over the competitive electrical systems. Even though chemical fuels have many advantages, electrical transmission of energy has special characteristics favoring it for specific end uses. The new energy system must be able to supply electricity and chemical fuel in an appropriate balance to best serve the customer. An ideal system should allow the simple conversion in either direction between the chemical fuel and electricity. Later in this issue a system will be described that embodies all the advantages of electrical and chemical energy handling (The Fusion Hydrogen Energy System).
REFERENCE 1. Jensen, E.K. & Ellis, H.S., Pipelines. Sci. Am., 216 (1) (1967) 62. 2. Barthold, L.O. & Pfeiffer, H.G., High voltage transmission. Sci. Am., 210 (5) (1964) 38. 3. Coltman, J. W., The transformer. Sci. Am., 258 (1) (1988) 86. 4. Snowden, D. P., Superconductors for power transmissiom Sci. Am., 226 (4) (1972) 84. 5. Ross, M., Improving the efficiency of electricity use in manufacturing. Science, 244 (4902) (1989) 311. 6. Glavitsch, H., Computer control of electric-power systems Sci. Am., 231 (5) (1974) 34. 7. Kalhammer, F. R., Energy storage systems. Sci. Am., 2,41 (6) (1979) 56. 8. Abelson, P.H., Reliability of electric service. Science, 245 (4919) (1989) 689. 9. Anon, Biological effects of power frequency electric and magnetic fields. Office of Technology Assessment, Report No. OTA-BP-E-53, US Government Printing Office, Washington DC, May 1989 10. Edwards, D. D., ELF: The current controversy. Sci. News, (131) (1987) 107.
A Chemical Fuel is Required 'The outputs from renewable and nuclear-energy sources do not match well in space or time with the needs of the users. This leads to the requirement for an energy storage medium that can be easily implemented.'
Laurence O. Williams Energy in transition. App. En. 23 (1986) 171-87.
ABSTRACT It was shown that fusion energy sources have excellent characteristics for future base load energy needs. However, fusion is limited to the production of heat, and from heat, electricity. This is also a limitation of most of the other alternatives to fossil fuels examined earlier. Solar thermal and geothermal energy sources produce only heat and, from heat, electricity. Power plants using solar photovoltaic, wind, tidal and ocean thermal gradients only produce electricity. Solar energy collected as biomass can yield carbon-based chemical fuels that can be handled in the same manner as fossil fuels, but like fossil fuels they are more valuable for other uses and can cause significant pollution. No matter what future combination of these energy sources is adopted there are many important uses for energy that cannot be satisfied with heat, electricity or biomass fuels. A manufactured chemical fuel is needed. The handling of energy, including transport and storage is examined, and the case is made that a portable, storable energy carrier is required for all users of energy.
CHEMICAL ENERGY TRANSPORTATION Chemical fuels have very desirable characteristics for the transportation of energy. Liquid and gaseous fuels can be p u m p e d continental distances through pipelines without significant loss. All chemical fuels can be carried on trucks, trains, planes and boats. Solid fuels, suspended in water, have been transported through pipelines over modest distances as if they were liquids. The other m e t h o d of energy transport, electricity, is essentially limited to transmission over wires. F r o m a comparison of the 169 Applied Energy 0306-2619/94/$07.00 © 1994 Elsevier Science Ltd, England. Printed in Great Britain
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properties of transport of energy as chemical fuel and as electricity it will be shown that a chemical fuel offers advantages of simplicity, efficiency and reliability when compared with electricity. Pipeline transport is used throughout the world to transport natural gas and oil. At the source, the gas or oil is pumped to high pressure and introduced into a pipe. At this point the pipe must be quite strong to resist the high pressure of its contents. At appropriate points the pipeline is divided into smaller lines for distribution to specific areas or regions. The smaller lines can be operated at the same pressure as the main line or at a lower pressure depending on the length of the smaller line and its desired flow rate. The flow and pressure of the fluid in the line can be controlled by simple valves and orifices. The process of branching into smaller and smaller lines with pressure reduction is continued until the pipe line arrives at its final destination. For reasonable costs the pipeline systems can serve users as large as several states or as small as an individual home. For industrial customers, with large usages, the pipelines are large and the pressure relatively high. For small users, such as individual homes, the pipeline can be slender and the pressure low. The delivery of the energy through this type of system is very reliable. It has been in use for more than 120 years and has provided a high-quality service. Pipelines lose little of their contents by direct leakage, but energy is consumed by friction as the fluid flows through the pipe. To make up for this friction, and to keep the contents moving at the desired rate, pumping stations are established along the lines. Some pumping stations consume fuel from the pipeline to provide the pumping energy, others use electric energy obtained from the local grid. Whichever source of pumping energy is used, the effect on the total system is to deliver less energy to the customer than was initially available. The amount of energy used in pumping is strongly dependent on the design and length of the pipeline system. As a rule of thumb, less than 5% of the energy put into the pipeline is consumed in pumping and, is thus, unavailable to the customer 1.
ELECTRICAL ENERGY TRANSPORTATION A large amount of energy is transmitted by pipelines, but a similar amount is transmitted by electric power lines. To compare these two types of transmission techniques, it will be useful to review some of the characteristics of the electric power transmission system. Electrical systems share some of the characteristics of a pipeline distri-
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bution system. Elcctricity can be produced and fed into the distribution system at a central location, and transported to users over a distribution network. The electric distribution network is made of copper and aluminum wires rather than iron, steel or plastic pipes, but in many respects the gas and electric distribution networks are very similar. The wires fan out from central and substation distribution centers to serve individual customers. In virtually all parts of the world electric power lines can be seen distributing energy. Despite its wide use, electricity is not ideal as a method of transmitting energy. A major problem is power loss during transmission. Resistance in the transmission wires turns a portion of the electrical energy into heat, which is lost to the surroundings and performs no useful task. The amount of heat produced is a function of the voltage used on the transmission line, its length and the diameter of the wire. Very little energy is lost from short, thick wires operating at very high voltage. Short wires do not go anywhere, and thick wires are expensive, hard to string and difficult to support. These factors lead to electrical transmission systems made up of thin wires operating at high voltages 2. The higher the voltage, the more difficult it is to insulate the voltage from the surroundings and the more difficult it is to switch the power off and on. High voltage can form electric arcs. The length of these arcs depend on the shape of the electrodes, temperature, humidity, air circulation, and the presence of ionizing radiation. At r o o m temperature, normal pressure and 50% humidity, a 2.5 cm spark will form between two sharp points at 12 000 V. At 50 000 V the spark is 13 cm long and at 100 000 V it can span nearly 40 cm. To achieve low loss, cross country lines are operated at more than 300 000 V and local distribution lines are operated at more than 10 000 V. Insulators used to handle these high voltages must be large and of high quality to prevent arcing to the support structure. The high voltage wires also must be prevented from coming close to any grounded conductor or, at a minimum, power will be lost. In the worst case, arcs will form resulting in fire and destruction. In the early days of electric power, Thomas A. Edison wanted to use direct current (DC). He found it required very thick wires to transmit energy with low loss. Raising the voltage to decrease these losses made the design of the generator difficult and compelled the user to handle very high voltages; voltages so high the potential for arcs, fires and hazards were extreme. It is difficult to convert direct current from low to high voltage for transmission and back to low voltages for safe use. Because of these shortcomings direct current transmission lost out to alternating current (AC). With AC it is possible to use a transformer to step the voltage up or
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down. This permits the generator to operate at its optimum voltage. A transformer is used to step the voltage up to a high value for relatively low loss transmission on thin lines. Near the user the voltage is reduced by a second transformer to a value that is safe and easy to handle. On the low voltage user side of the second transformer the wire size is increased to reduce losses. This is acceptable because of the relatively short run from the transformer to the user. This combination of optimum generator operation, step-up conversion to high voltage for low loss transmission and voltage step-down conversion for safe use by the customer has provided the basis for the electric system world-wide. Transformers are remarkably efficient, ranging from 88-96%. Few things have an efficiency this high. Despite their high efficiency, every time the voltage is converted, 2-12% of the energy is lost in the transformers as heat. Depending directly on the distance between the generator and the customers, more power is converted to heat in the transmission lines. The power converted to heat is lost and reduces the efficiency of the energy distribution system. 3 AC is subject to a loss, other than resistance, when transmitted over long distances. Alternating current used in the USA cycles at a rate of 60 Hz. The 60 cycle current can generate 60 Hz radio waves. This can be observed as the buzz heard on a car radio when driving near high voltage power lines. Like electrical resistance, this radiation results in a loss of power. The amount of power radiated is affected by the condition of the lines and their length. The wave length of a 60 Hz radio wave is about 5000 km. As the length of a transmission line approaches a quarter-wave length (1250 km), more and more power is lost by radiation. When the line length reaches 1250 km, more than half of the power input is radiated as essentially useless 60 Hz radio waves. The radiation effect places limits on the distance 60 Hz AC can be transmitted without severe loss. 2 Commercial systems for conversion of high-voltage AC to high-voltage DC are in use today. These systems allow the transport of electric power distances greater than 1250 km, but the equipment for conversion has internal resistance and conversion losses and it is very expensive. Highvoltage direct current systems are used only in locations where the input power cost is quite low and the source far from the customers. This combination is most often found with hydroelectric power generation facilities located at sites remote from large cities. For example, a direct current line carries power from the water power projects on the Columbia River, in Washington State, to San Francisco. These special circumstances demonstrate high-voltage DC transmission can be used successfully. However, ultra voltage-high DC transmission has little use in most power distribution systems.
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In a large system, power lost in transmission is dependent on factors such as the n u m b e r of transformers, the length of the transmission lines, the size of the wires, the type and number of customers and the peak voltage used. The very best systems deliver less than 85% of their generator output to the customer and many deliver much less. 4 When the efficiency of the transport of energy as a chemical fuel is compared to the efficiency of electric energy transport, one can wonder why electricity is used at all. It survives and grows because of the low cost and simplicity of the end-use electrical conversion devices. Electric lights may use more energy than gas lights, but they are far more convenient, can be controlled with great ease and present little fire hazard. Small electric motors incorporated in home appliances and manufacturing equipment are extremely convenient and simple to use. In applications where large amounts of energy are used, such as home heating and cooking, gas is more cost effective and is often selected for this purpose in preference to electricity when both are available. Small motor-driven appliances could be operated with little gasburning internal combustion engines at higher energy efficiency, but enormously reduced convenience, flexibility, and reliability. The electronic gadgets in the home--TVs, video recorders, radios, stereo sound systems and microwave ovens, all require electricity for operation. To supply these devices with energy as gas or oil would not only require engines, but generators and local wires as well. As a result of the convenience of electric devices the relatively less efficient electric power distribution systems will, in the near term, remain a useful method of transporting energy. 5 For all its convenience electricity has several major shortcomings as an energy handling medium. Electricity cannot be stored. At all times the power plant operators are adjusting the o u t p u t of the generators to match the needs of the users. This can create difficulties if the user changes his needs at a rapid rate. To account for these changes most systems have a hierarchy of generating capability. First are the base load generators. These Consist of large coal fired or nuclear generators operated at a constant output power level. These generators produce electric power at the lowest cost, but require hours to start and to shut down. The next level of generators consists of intermediate size units capable of more rapid turn-on and-off. The final level is made up of peaking units that can start from cold to full power in a matter of minutes. The peaking units often burn relatively expensive gas or oil, thus the power generated by these units costs up to 10 times more than power produced by the base-load generators. 6 In normal operations the base load generators operate at all times.
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During the morning hours the intermediate generators are activated to supply the basic power for the operation of routine daily activities. Under some conditions, intermediate generators may carry the total load. On a very hot summer afternoon, when the demand for electricity to drive air conditioning is at its peak, it may be necessary to bring the peaking units into operation to carry the short term peak load. 7 As the day progresses the loading process is reversed. As the demand drops the peaking units are shut down followed by the intermediate units. Finally late in the evening all extra power is shut off and the base load generators supply the low needs through the late night and early morning hours. This system works well as long as events follow the predicted course. When events deviate, a series of problems can result starting with slight brownouts to momentary interruptions progressing on to long interruptions and finally disaster such as happened in the northeast section of the USA in the 1960s when a multi-state area was without power for several days. Power outages are usually a result of weather conditions that rapidly change the demand in unexpected ways or violent weather that damages parts of the system. Most electric systems have a dozen or more power interruptions during the summer when thunderstorms are common. Lightning strikes some part of the system. The surge of voltage causes protective circuits to cut the power off for a short time. If the protective circuits are inadequate, lines and transformers are damaged causing interruptions lasting for hours while the equipment is repaired. Lightning induced power surges can occasionally cause a section of a system to fail resulting in interruptions requiring several days to repair. Ordinary storms, tornadoes and hurricanes have winds capable of knocking down wires and poles. Often, interruptions in electric service can be repaired in a few hours but when many poles and kilometers of wire must be replaced the interruption can last for weeks. In winter, lightning is not a severe problem, but snow and ice can collect on power lines and tear them down. This results in interruptions that last for a number of days because thousands of kilometers of wire must be replaced. Winter interruptions are less common than summer interruptions, but when they occur they often last longer because the damage is greater and the repair crews have a difficult time working in ice and snow. 8 Underground transmission of electric power over long distances is impractical. Long distance transmission requires high-voltage. Most ground is somewhat electrically conductive so very thick, high quality insulation is needed to protect the lines from arcing. The thick insulation is
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expensive and the installation of thick stiff transmission lines is difficult. Consequently, long distance underground transmission of electricity is quite costly and is used only in specialized situations. One of the common uses for underground transmission lines is in the final run from the local transformer to the private home. This is usually a short length of thick wire carrying a relatively low-voltage. The short path keeps the cost of the thick wire to an acceptable level and the low voltage makes the use of simple insulation feasible. The country is crossed and recrossed by above ground high-voltage power lines for implementation of the distribution system. Plant growth must be prevented close to the lines because trees and other vegetation are sufficiently conductive to short circuit the high-voltage. For much the same reason buildings cannot be allowed near the lines. As a consequence, all of the cross country power transmission lines have a swath of clear cut ground about 100 m wide along their path. These are not pleasing to the eye. Because of the danger from arcs, the land under the lines has few uses. The high-voltage AC induces small electric currents in everything near the path of the transmission line. At a distance the effect is weak but it becomes stronger close to the power line. This effect is commonly observed as the buzz in a car radio that gets stronger as you near the transmission line. Fragmentary data indicate the tiny electric currents produced in people living near the power lines can cause health problems. We live with and tolerate the power lines because they are necessary for the use of electric power. 9'1° The power lines stretch across the country delivering electric energy at relatively low efficiency and marginal reliability. In performing this task they provide visual insult and possible environmental harm. A similar amount of energy is transmitted by the nation's pipelines, but most of us are totally unaware of their existence. When compared to electric power lines, pipelines carrying a chemical fuel are more efficient, more reliable, invisible and only harm the environment by accidental leakage. The major justification for the transmission of electricity stems from its ability to directly power electronic devices of the user. These include common every day items such as electric lights, electronic devices (radios, TVs, VCRs), small motors and controls. The foregoing discussion of electric power transmission is presented to demonstrate that pipelines carrying a chemical fuel are far more efficient and reliable at delivering energy than are electric wires carrying electricity. Under most circumstances pipelines are buried a meter or two underground and are protected from the destructive effects of weather. Pipelines of any size can be buried without significant problems. Pipeline
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failures are limited to the occasional break due to materiel failures, digging for construction and earthquakes. To reap the full advantages of the use of pipelines for the transportation of energy it is necessary to transmit most energy in the form of a chemical fuel.
S T O R A G E OF C H E M I C A L E N E R G Y Chemical fuels can be easily stored for long periods. The solid fuels present the least problems of storage. For simple temporary storage, coal can be d u m p e d in a pile on the ground. If storage is for only a short period of time the coal can be left in the open. For longer term storage it is desirable to cover the coal with some type of water p r o o f cover. The cover prevents rain from soaking the coal and interfering with the combustion process. Rain water can also leach toxic sulfur c o m p o u n d s from coal stored in the open thereby creating significant environmental damage. 7 In long term coal storage facilities, great care must be exercised to prevent unacceptable environmental damage. To protect the ground from contamination, the coal can be placed on plastic or rubber sheets or on a concrete pad. In either case it is desirable to provide a method of catching the rain water that filters through the coal. When coal comes in contact with water toxic chemicals, such as sulfur, are leached and contaminate the water. For environmental protection the contaminated water must be purified before allowing it to enter the water table. The storage of oil and oil-derived liquid chemical fuels is somewhat more complicated than the storage of coal. These fuels must be confined in some sort of tank. It is usually best to cover the tank so water and dirt do not contaminate the fuel. In this case the cover serves the purpose of protecting the oil from contamination and evaporation rather than protecting the soil from water that has leached toxins from the fuel. The tanks designed for liquid fuel storage are somewhat more costly than the simple coal pile, but the cost is still low. Tank storage of liquid fuels ranges from large ocean-going tankers, to onshore storage depots, storage at local distribution points, local gas stations, automobile tanks and the small tank that stores the fuel to power small tools such as the lawn-mower and the chainsaw. Gas is the most complicated chemical fuel to store. It must be placed in a container of sufficient strength to resist the gas pressure and prevent all leakage of air and water. It has been found the most cost effective method of storing large quantities of gas is in low pressure storage gas holders that change their volume in response to gas input and with-
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drawal. These tanks are large, but of relatively low cost. When low pressure storage is used there is no need to spend a large amount of energy compressing the gas. In a large scale distribution system, the pipelines serve as a large volume continent-spanning storage reservoir in addition to their primary function as a conduit. In other applications, where modest capacity is required and cost is not a major consideration, gas can be stored in high pressure cylinders as is done with welding gas.1 The storage methods used with chemical fuels are satisfactory regardless of the storage duration required. Chemical fuels can be stored for hours, days, months, or years allowing maximum flexibility for the fuel consumer. If sufficient storage space is available, chemical fuels can be purchased when the price is low for use at any later time. Chemical fuels can be purchased and stored against future disruptions in the fuel distribution system. In the summer when river barge transportation is available at low cost, power plants can buy their winter fuel and place it in storage. In the winter when the river is frozen and fuel cannot be shipped, the power plant can continue operating using its stored fuel. Emergency power systems can be equipped with a supply of fuel adequate for several days operation. With proper design, fuel can be reliably stored for years ensuring its availability if the main power systems fails.
STORAGE OF E L E C T R I C A L E N E R G Y 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 even 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 that. 7 The most common storage container for electrical energy is the battery. 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 process of manufacture of common batteries the energy is used to produce zinc metal. In this manner energy is stored in the form of active zinc metal when the battery is assembled. An electrochemical reaction consumes the zinc producing electricity when it is needed. In multiple use storage batteries, input electrical energy is converted to a chemical form within the battery. The chemicals are stored within the battery for later regeneration of the electricity. The chemical reactions
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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 singleuse battery must be replaced. Storing electrical energy in batteries has many limitations. Batteries store a small amount of power 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 fork lifts 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 so valuable in these special applications, users can accept the high cost, poor performance and lack of flexibility. In these applications one of the large problems is the tendency for lead/acid batteries to accept less total charge each time they are recharged. This performance characteristic results in frequent costly battery-pack replacement. A good automobile type lead/acid battery can store about 100 Ah of electric energy, about enough to operate a 100 W light bulb for 1 hour or a 1200 W stove burner for 7 or 8 min. To operate one average house for 24 h, without clothes washing and drying or air conditioning, 20 to 30 fully charged automobile batteries would be required. There are batteries with better energy storage capabilities 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. Lithium batteries have recently attracted attention because they provide more power than zinc, mercury or alkaline batteries. They are not rechargeable so are of no value for power line energy storage and only serve as a one shot energy supply. 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. This 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 350°C. When they are cold they can neither be charged or discharged. Other problems have been encountered with the physical stability of some of the internal components. Further studies
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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 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. New battery systems may be useful for specific limited applications, but it can easily be shown it is impossible for a battery system to compete with a chemical fuel burned with air. In a battery system there are two chemical 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. The most energetic chemical battery possible is a cell using beryllium as the source of electrons and oxygen as the sink. Today it does not appear possible to produce a beryllium/oxygen 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 o f 6.8 kWh kg -~ of reactants. The battery case, electrode support structure and electrolyte weight are not included in this calculation. Sandia Laboratories has examined a concept involving storing electrical energy by means of interacting magnetic fields generated by superconducting coils. The advantages of this technology are low storage loss and high rates of discharge. It should be re-useable 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 superconducting coils carrying heavy currents at liquid helium temperatures and supported against powerful magnetic fields, can be performed. N o r 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
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superconducting coils be cooled to liquid helium temperatures (~4.2 K) will limit the use of these energy storage devices to large installations. Electric energy can be 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. Its use is limited to special circumstances and there is little hope that it can be placed in wide use for the effective storage of electric power 7. 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 40-60 atm 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 customers can, for a time, be served by the residual gas pressure in the line. From the customer's viewpoint this adds reliability of supply to the already high reliability of pipelines, even when portions of the line have been damaged. Petroleum fuels store energy at the rate of 12.5 kWh kg. Hydrogen fuel stores energy at a rate of 33.5 kWh kg -1. 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. 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. In a battery, the fuel, the second reactant (equivalent to oxygen) and the waste 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 improves 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
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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.
ENERGY CARRIER END USES
It has been shown that handling of energy in the form of a chemical fuel offers the greatest possible flexibility, utility and efficiency. A recapitulation of the availability of various handling techniques is shown in Table 1 . TABLE 1 Energy Handling Chemical energy
Transport of energy By networks Discrete bulk shipment Storage of energy Load management Daily Seasonal Emergency use Transportation energy Rail Highway Air Water
Electric energy
Pipeline Yes
Wires Not possible
Stored fuel Stored fuel Stored fuel
Varying generators Pumped hydro Not possible Not possible
Stored Stored Stored Stored
Wires Not possible Not possible Not possible
fuel fuel fuel fuel
C H E M I C A L FUELS F O R T R A N S P O R T A T I O N The ability to provide a lightweight, high energy chemical fuel to deliver energy for transportation is essential for any future energy system. Whereas stationary users can use fuels in any physical state--solid, liquid, or gas--the transportation sector must use a fuel that can be quickly and easily loaded into the vehicle storage tank. As a result, the selection of the future chemical fuel is strongly driven by the needs of transportation in the energy use economy. The next paper in this issue
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will examine the characteristics of possible manufactured fuels to serve the needs of transportation. It is assumed that any fuel successfully used for transportation can be used for stationary applications. The detailed discussion of application of a manufactured chemical fuel for transportation has been deferred to the paper on 'Hydrogen as a Transportation fuel'.
SUMMARY It has been shown that for transport and storage of energy chemical fuels offer many advantages over the competitive electrical systems. Even though chemical fuels have many advantages, electrical transmission of energy has special characteristics favoring it for specific end uses. The new energy system must be able to supply electricity and chemical fuel in an appropriate balance to best serve the customer. An ideal system should allow the simple conversion in either direction between the chemical fuel and electricity. Later in this issue a system will be described that embodies all the advantages of electrical and chemical energy handling (The Fusion Hydrogen Energy System).
REFERENCE 1. Jensen, E.K. & Ellis, H.S., Pipelines. Sci. Am., 216 (1) (1967) 62. 2. Barthold, L.O. & Pfeiffer, H.G., High voltage transmission. Sci. Am., 210 (5) (1964) 38. 3. Coltman, J. W., The transformer. Sci. Am., 258 (1) (1988) 86. 4. Snowden, D. P., Superconductors for power transmissiom Sci. Am., 226 (4) (1972) 84. 5. Ross, M., Improving the efficiency of electricity use in manufacturing. Science, 244 (4902) (1989) 311. 6. Glavitsch, H., Computer control of electric-power systems Sci. Am., 231 (5) (1974) 34. 7. Kalhammer, F. R., Energy storage systems. Sci. Am., 2,41 (6) (1979) 56. 8. Abelson, P.H., Reliability of electric service. Science, 245 (4919) (1989) 689. 9. Anon, Biological effects of power frequency electric and magnetic fields. Office of Technology Assessment, Report No. OTA-BP-E-53, US Government Printing Office, Washington DC, May 1989 10. Edwards, D. D., ELF: The current controversy. Sci. News, (131) (1987) 107.