R 11 1I I1 IJ

Alternator .stalor

Cooler

'Vacuum furnace

Gas bcariocs

;Gas bearings

Displacer gas spring

Power piston

Displacer

Heater head'

Piston gas spring and bearing compressor

•Regenerator

Alternator plunger

Fio. IK.?.. MTI-Hcatc free-piston Stirling engine and linear alternator for I kW isotope space power-system «'after Gohlwuter and Morrow 1977).

can be easily disabled by hostile action so that it is likely that military spacecraft will continue to use isotope sources.

The intensity of solar energy diminishes as the square of the distance from the sun. Therefore spacecraft engaged in exploration of the remote solar system find it increasingly difficult to collect adequate solar energy as their journey proceeds away from the sun. Power levels of a few watts are all that is required to remain in radio contact with the Earth and to provide power levels lor space experiments. Where higher power levels are necessary one possibility being vigorously explored for energizing remote spacecraft is to use laser beam transmission. This would supply power by optical beam that could be collected, absorbed, and converted to heat and hence using a Stirling engine to electrical energy. A laser is a device which can highly concentrate light energy to a coherent beam or ray.

stirling engines for underwater power

The situation with regard to Stirling engines for underwater power is much like that for space power—lots of promise and potential but little actual application experience.

A major United States report (Chapman 1968) on underwater power systems in the I to 100 kW (1.4 to 136 hp) power range recommended that significant effort be invested in Stirling engine development in preference to other dynamic converters and fuel cells. The particular advantages were that the reciprocating Stirling engine was admirably suited for the modular concept. There was little penalty in system weight, volume, or fuel consumption if four 10 kW (13.6 hp) units were used instead of one 40 k\V (54.4 hp) unit. The modular system would allow the use of a small inventory of parts and components to fulfill a broad power range.

They judged the reciprocating Stirling engine to have a higher thermal efficiency potential than any of the rotary engines, and to have the favourable characteristic of operating on any source of heat at different temperature levels without noise and without the need to dump waste products overboard.

The Stirling dynamic converter system was judged to have a weight and volume advantage over fuel cell systems up to 3000 kW'li (4079 hp hour) capacity. It was thought to have the advantage at all levels of energy requirement when fuelled with a halogen-metal reaction. Cost comparisons of the fuel cell and Stirling engine were established in favour of the Stirling engine.

The isotope-powered Stirling engine appeared particularly attractive in the power range from I to l5kW (1.4 to 20.4 hp). It was found that a high proportion of U.S. Navy requirements for future deep-sea activities could be adequately served by three sizes of power generators of about 3, 10, and 25 kW (4, 13.6. and 34 hp) coupled in various combinations with three sizes of energy storage containers of 40. 100, and 1000 kWh (54.4, 1.36, and 1360 hp hours).

Despite these favourable recommendations no substantial activity in Stirling engine development was undertaken by the U.S. Navy. Indeed in a later study (McCartney and Cates 1975) of power sources for remote ocean-Oriented applications Stirling-engine systems were not even mentioned despite the inclusion of other highly esoteric concepts.

During the tenure of their licence agreement with Philips, 1958-1970, General Motors invested considerable effort in studies of Stirling engines for underwater power systems. Percival (1967) reviewed some of this effort with particular reference to thermal energy storage systems and liquid-metal combustion. General Motors' interest in thermal-energy storage systems predates the Philips licence, for in 1957 a proposal was made to the U.S. Navy for a Rankine-cycle system combined with lithium hydroxide thermal storage, l ater in 1959 the proposal was converted to lithium fluoride thermal storage combined with a Stirling engine.

THERMAL STORAGE SYSTl-MS

A thermal-storage system consists essentially of an insulated tank containing some material having a high heat capacity. The material can be heated by combustion, by electric heating, or by nuclear source. When the temperature of the material increases on heating the heat is said to be 'sensible* heat. When during heating the material melts at constant temperature, changing phase from solid to liquid, the heat is said to be 'latent' heat. Energy-storage materials should have a high heat capacity, low vapour pressure, and high density. They should be chemically stable and compatible with the container and heat transfer surface materials.

The heat capacity on a weight and volume basis of possible energy-storage materials is shown in Fig. 18.3. This figure was prepared for the energy transfer per unit mass or volume to etfect a change in temperature from 538°C (HXH)"F) to the maximum temperature shown alongside each material. The maximum temperatures were selected on considerations relating to the heat-storage materials and without regard to containment or insulation characteristics.

The first four materials are molten salts in which a high proportion of the heat transfer is latent heat associated with a phase change occurring at the maximum cycle temperature. Lithium hydride has an exceptionally

100 0 Energy density

20 30

100 0 Energy density

20 30

FiG; 18.3 Energy density of thermal energy-storage materials on a weight and volume basis (minimum temperature 1000 °F. after Mattavi et al. 19fi9).

Starter

■Utility

1 langer

Alternator

Crankshaft-LH Water pump

Compressor-helium /-Governor x Pressure capsule -Crankshafl-RH

Stirling engine (30 blip)

Lithium fluoride

\\_ Stornge tank helium

J^-j Thermal r insulation (in vacuum)

"Heating clement ■Eng. heater tubes

Support-engine assembly

Fig. IS.-t. Cross-section of spherical underwater power-system module including a 30hp Stirling engine and a lithium fluoride thermal reservoir (after Pcrcival 1967).

high heat capacity on a mass basis but this salt readily dissociates at temperatures only slightly above its melting point. A preferred material is lithium fluoride which has a higher melting temperature and a better volumetric heat capacity. Unfortunately the latent heat capacity on a mass basis is less than half that for the hydride. High latent heat capacity is favourable to maintain a constant temperature as heat is withdrawn to operate the engine.

Most of the General Motors work on thermal-energy storage was done with lithium fluoride but other materials were studied, particularly aluminum oxide. Percival (1967) describes one experimental unit using 60 000 hexagonal pellets of aluminum oxide in an insulated tank. The system operated between temperature limits of 1482.2°C (2700°F) and 815.6 °C (1500 °F) with a storage capacity of approximately 73.6 kWh brake power (100 brake hp hours) when used in conjunction with a 22 kW (30 hp) Stirling engine.

Fig. 18.4 is a cross section of a 22 kW (30 hp) Stirling engine underwater power-plant with lithium fluoride thermal energy-storage, (Percival

1967). Approximately 453.6 kg (1000 lb) of lithium fluoride was contained in the lower hemisphere of the spherical capsule, 106.7 cm (42 in) in diameter. The engine heater tubes were immersed directly in the molten salt, healed initially by electric-resistance elements. In this system, the heat losses through the 2.54cm (I in) of multi-layer insulation were said to be 0.1 per cent of the stored heat per hour. A pump was used to circulate engine coolant to an ambient-temperature, sea-water heat sink located outside the pressure hull to eliminate high-pressure sea-water inside the hull. Power output was regulated by varying the pressure of the working fluid, hydrogen, or helium in the engine.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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