Stationary Power Generation

Stationary power generation is a term embracing the widest possibilities. Engines that may be stationary power plants in one application may be adapted as the auxiliary equipment or even the main propulsion machinery in space, aeronautic, marine, railway, automotive, or recreational vehicle applications.

Stirling engines have attractive characteristics for stationary power applications. They have a wide multifucl capability, operate without noise, have excellent part-load performance, and respond fast to sudden changes in load. They have the potential to operate for very long periods with minimal maintenance and low lubricant-oil consumption.

In the closing years of the twentieth century, the multifuel capability of the Stirling engine will most likely become important. The engine can operate on any source of heat, and so as oil and gas become increasingly valuable, more and more use will be made in power generation of solid fuel like coal, industrial wastes such as wood bark, forestry trimmings, agricultural wastes, and municipal wastes. Anything that is combustible can be consumed in high-efliciency tluidi/.ed beds, or other advanced combustors to producc the hot gases for heating Stirling-engine systems.

Similarly the combination of a free-piston Stirling-engine/linear alternator with an absorber and solar concentrator may be used as illustrated in lng. 19.6 to produce electricity from a solar input (Pilar 1978).

A major solar energy conversion demonstration is in progress for the U.S. Department of Energy in a program managed by the California Institute of Technology, Jet Propulsion Laboratory. This program includes the construction and evaluation in California of twenty-three 50 kW Stirling engines used in con junction with a similar number of solar collectors each 16 m (52 ft) in diameter. The engines to be used for this program are United Stirling P75, Siemens-type, four-cylinder machines derated to 50 kW. 'ITie program contractor is Ford Aeroneutronics. No doubt technical papers on this program will be appearing in the technical press in due course. The use of 23 engines provides a good probability for 20 systems to be operating continuously at any one time to provide a 20x50= 1 mW capacity. It is said that electrical storage is to be included for power supply during the dark hours.

In solar systems with a tracking concentrator it is possible to locate the engine at the focus of the concentrator but this is not desirable as it requires the concentrator and tracking mechanism to be capable of supporting the relatively heavy engine and generator system. The preferred alternative therefore is to locate the engine remote from the focus of the concentrator in a fixed, secure, location and to indirectly heal the engine with a liquid metal heat pipe or transfer loop. This has some important advantageous characteristics for the engine (see Chapter 16 on Stirling engines for automotive use) principally to eliminate hot spots, increase the upper cycle temperature and hence the efficiency, and, finally, decrease the internal engine dead space with consequent gain in power output and efficiency.

In addition to all these advantages the introduction of indirect heating

Stationary Engines Malta Scrap

Solar Collector

STIRLING ENGINES FOR HEAT PUMPS. Free piston Stirling engine/linear alternator

Absorber

Fig. 19.6. Stirling engine with solar energy input.

with solar power systems offers a number of possibilities for power supply during the dark hours. A thermal storage system (thermal battery) of lithium fluoride may be incorporated in the circuit to be recharged by the thermal energy of concentrated sunlight during the sunlit hours. This would permit 24 hour (or as required) operation of the engine/generator system. Another possibility is that with indirect heating (and possibly thermal storage) a supplementary heat source may be added at very low cost to keep the engine/generator system going whether the sun is shining or not. Thus a lluidized bed combustor providing energy to a liquid sodium heat pipe may be the only additional equipment necessary to keep a large solar-concenrrator/Stirling-gcncrator active during the dark hours. The combustor may consume garbage, used tires, coal, or wood waste. As an accessory to an existing or forthcoming solar plant the cost would be minimal.

Fluidized bed combustion systems offer the possibility of intense thermal flux, in relatively moderate and well-controlled temperature combustion systems able to operate with a variety of solid fuels over a wide range of load. Their use in conjunction with Stirling engines has been studied and reported by Asselman (1976) and by Dunn and Rice (1975). They may be used to heat the working fluid in the heater tubes directly but the preferred use is with indirect heating using a sodium heat pipe or sodium/potassium eutectic transfer loop.

In 1978 the U.S. Department of Energy initiated the External Combustion Engine project under the management of the University of Chicago Argonne National Laboratory in Illinois. This program will explore the feasibility of Stirling-engine power generators of 500 to 2000 kVV for modular application in situations where appreciable combustible wastes do exist but in quantities which do not justify the installation of a full-blown, base-load. Rankine-cycle steam-turbine plant. There are thousands of local municipalities which fall into this category. Publication of the Program Opportunity Annouccment attracted a response by over sixty U.S. corporations. The Request for Proposal and subsequent concept design and prototype stages are expected to initiate innovative engine arrangements previously unexplored, but which may prove particularly appropriate for large Stirling engines.

Earlier. Hoagland and Percival (1978) have reported on a very comprehensive technology evaluation of Stirling engines for stationary power generation in the 375 to 1500 kW (500 to 2000 hp) range. They concluded that the Stirling engine was well suited, principally on account of the multifuel capability, for commercial development as an alternative to diesel, gas turbine, and steam plants. On a considerably larger scale Stirling engines of megawatt capacity have been envisaged by staff of the Atomic Energy of Canada. Such engines would be associated with advanced nuclear reactors.!

Small stationary power generators, say up to 10 kW capacity are of intense interest on a broad front for a variety of military and civil applications. In many cases quiet operation is the attractive feature, particularly in small power generators for the Array or as propulsion motors for rubber assault boats or clandestine operations. Quiet low-power generators are attractive for recreational vehicles, camp-grounds, and similar situations.

F.F.V., the Swedish defence-related company, half owner with Koc-kums of United Stirling (see Chapter 15). has carried out a decade of r Private discussions with J Bradley. Chalk River Laboratories. Canada.

independent development of a 10 kW generator. This is near the point of introduction to commercial application (Johansson 1978) and may well be the lirst commercial multiple-horsepower Stirling engine outside of existing cryogenic cooling systems. It is understood the engine has a moderate efficiency (20 per cent) to allow for the use of non-exotic materials for the hot parts.

In yet smaller sizes (up to 200 watts) there appears to be a substantial market for small thermally-activated power generators for navigation signal generators, particularly light-ships and buoys.

Present systems on buoys utilize acetylene gas supplied in bottles as the energy source for the periodic emission of light. The timing and flow control device is activated by the acetylene gas pressure and extremely reliable systems have been developed. Unfortunately there is little that can be done to optically magnify or concentrate the emission of an acetylene flame so that visibility is limited.

The size of large marine tankers is now so great and their inertia so high that the limit of visibility of an acetylene flame is in fact less than the distance travelled by the ship between the start of a change of course and a response by the vessel. Radar systems are of course used in the main but these have proved fallible and an alternative supplementary warning system of unfailing reliability is required.

The preferred system would appear to be periodic flashes from an electric Xenon lamp. The flash can be tailored to provide intense energy emission for an extremely short interval and can be further concentrated optically to effect an order-of-magnitude improvement in visibility. Electric batteries arc presently used and a variety of turbo-power generators using wave action are being evaluated. An alternative would be a small Stirling-engine power generator operating on diesel oil, liquid petroleum gas. or radioisotope. Such a unit capable of operating six months (diesel oil) or two years (radioisotope) unattended and with guaranteed reliability would most likely find a substantial market in the international lighthouse and buoy market of the maritime regulatory authorities.

One promising development in this field at the University of Calgary, for a generator operating with cobalt 60 radioisotope, was abandoned when the isotope power program of the Atomic Energy of Canada Ltd. was cancelled in the Canadian recession of 1971.

A similar program carried out at the British Atomic Energy Authority Harwell Research Centre resulted in the successful development of prototype generators using strontium 90 radioisotope or liquid petroleum gas as the energy source (Cooke-Yarborough and Yeats 1975b). This unit has passed the laboratory prototype stage and is presently being evaluated in a navigation buoy by the Irish Lighthouse Authority. It is projected for production by the A.G.A. Ltd.. a British affiliate of the celebrated Swedish industrial and maritime manufacturer.

Radiator

Gas supply

A.C. electrical output

Metal diaphragm

Spring

Gas displacci

Burner

FlG. 19.7. Murwell lice-piston Stirling engine power generator

Radiator

Gas supply

Gas displacci

Burner

A.C. electrical output

Metal diaphragm

Spring

FlG. 19.7. Murwell lice-piston Stirling engine power generator

The Harwell engine, shown in Fig. 19.7. is a large, heavy, free-piston Stirling engine utilizing a flexing diaphragm coupled to the generator unit. It has demonstrated long life with excellent reliability.

Another interesting development at Harwell is the Fluidyne engine (West 1971). a free-piston Stirling engine with liquid pistons used for water pumping. A water column acting as the piston resonates at natural frequency to operate a Stirling engine system contained in the cylinder above the liquid piston. Arrangements can be made for the resonant water column acting as the piston to pump water. The engine can be heated by combustion, by electric heating or by solar heat. Elrod (1974) has given a good description of the Fluidyne engine and instructions for construction of a prototype. Rallis et al (1977) have described their experience with experimental Fluidyne engines at the University of Witwatersrand.

synchronous operation

Many stationary power engines are required (a) to operate at constant speed and (bj to be relatively unaffected by sudden changes in the load. Utility power companies are required by the regulatory agencies to provide electric power at a near constant frequency with close limitations in the permissible frequency variation.

The control devices for Stirling engine stationary power systems will include (a) a fuel/air regulator to increase or decrease the fuel supply so as to maintain a constant maximum temperature in the combustion region, (b) an engine torque control so as to vary the engine power and hence maintain constant speed during variation in the load.

The various torque-control systems available are described in Chapter 10 and include:(a) mean-pressure control, (b) pressure-amplitude control, (c) phase-angle control, and (d) stroke control.

Martini (I978)t has described another type of control system well-suited to the constant-speed regulation required for stationary power systems with close frequency stability. The Martini control is illustrated in Fig. 19.8. The engine is a piston/displacer system driving an electric alternator. The displacer is driven by a synchronous motor consuming a fraction of the output of the alternator and operating at a speed corresponding exactly to the speed of the alternator. Motion of the displacer induces flow of the working fluid between the hot space and cold space so the pressure varies cyclically at the same frequency as the motion ol the piston-displacer. The engine pressure variations act upon the piston causing it to reciprocate in the cylinder and so driving the alternator to produce power, part of which is consumed to drive the displacer. Thus the frequency of reciprocation of the piston in the cylinder is determined by the frequency of reciprocation of the displacer. This in turn is determined by the speed of the synchronous drive motor which itself is determined by the speed of the alternator driven by the piston. The system is therefore completely self-stabilizing with regard to speed control. The phasing of the piston and displacer motion will automatically adjust to assume the phasing required to produce the exact amount of torque required to generate the load demanded from the engine.

Starting would, of course, present a special problem. One way would be to start the displacer motor using battery power passing through an inverter set to produce alternating current at the frequency eventually required. Once up to speed, the displacer drive would automatically be transferred to the alternator output.

t Private communication

Cryogenic Stirling Engine

Pig. 19.8, Constant-spued control system for piston/displaccr Stirling engine (after Martini, private communication 1978).

Pig. 19.8, Constant-spued control system for piston/displaccr Stirling engine (after Martini, private communication 1978).

In most instances the displacer drive would consume a small fraction (maybe one per cent) of the total electric power developed. In other cases, the alternator would be simply sufficient in size to provide power for the displacer drive and for battery charging. The principal output of the engine would be used as mechanical work for driving a gas compressor, fluid pump or some other mechanical system.

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