100 J

Fio. 3.-t. Characteristic temperature regime in a fossil-fuelled, water-cooled Stirling engine. A—Temperature of combustion pioducis, H—Temperature of heater walls, C—Mean temperature in expansion space, D Mean temperature in compression space, li—

'I'^tnttoniinri« «if /•ru»llim miil.T -im.I »•/,«lor might be. considered representative of the temperature range established in a fossil-fuelled water-cooled regenerative engine. The temperatures of the combustion products and cooling water are 28011 K (5040°R) and 280 K (504 °R). respectively. The metallurgical limit oi the materials used for the expansion cylinder and heater is (say), 1000 K (1800 °R). This provides for a steep temperature gradient of 2800 to 1000 K (5040 to 1800 °R) between the combustion products and cylinder wall, with the potential for high rates of heat transfer. Further temperature gradients of (say), 100 K (180 R) between the working fluid and the expansion space, and 50 K, between the working fluid and the compression space, might exist, so that the cyclic temperature excursion of the working fluid varies from (280 + 50) = 330 K <5<M°R) to (1000- 100) = 900 K (1620°R). Whereas the Carnot- (or Stilling-) cycle efficiency foi the system m/g/if be calculated as

77c = (2800 280)/2800 - 2520/2800 - 90 per cent, to give a more realistic picture it should be calculated as yc ~ (900 330)/900 = 570/900 - 63 per cent.

This example demonstrates one of the major diflicultics in the commercial application of Stirling engines—one shared by the gas turbine the question of materials. Some parts of the machine (the heater and expansion space), are exposed, continuously, to a high temperature, and are subject, therefore, to the metallurgical limit of the heater and expansion cylinder materials.

The allowable temperature excursion of the working fluid in a Stirling engine is limited to a fraction of that permissible in an internal-combustion engine using an Otto or Diesel cycle, where the maximum cycle temperatures are attained only momentarily. Thus, although regenerative cycles between given temperature limits are thermodynamically more efficient than Otto or Diesel cycles, in practice regenerative engines are compared with gas (or oil) engines operating with radically different temperature limits.

Not all the heat available from combustion of the I'uel and air can be transferred to the working fluid, since this would require a very large heater. The heat passing to exhaust in the combustion products of a Stirling engine represents a direct loss, because it must be paid for in terms of gallons of oil (or cubic feet of gas burned), but has served no useful purpose in the engine. An important engine accessory, therefore, is another heat exchanger (the exhaust/air prcheatcr). used to warm the incoming air by heat transferred from the exhaust gas. This heat exchanger can be of the recuperative type or the regenerative type. In the r^nnnortif i *>.-» lli/» linn fluwlr- ,.«.l.«..-t "... :-----

separated by walls into separate duets. In the regenerative type, the fluids flow alternately, and usually in contraflow, through the same porous msitrix. Il is important to distinguish carefully between the regenerative heat/exchanger, incorporated as an integral part of the engine, and the recuperative (or regenerative) heat exchanger, used as an accessory of the engine for exhaust/ail preheating.

The continuous motion of the reciprocating elements, the non-isothermal compression and expansion processes, the limited heal transfer in cooling and heating devices, the exhaust-stack loss, the increased dead space, and aerodynamic-flow loss together constitute the principal reasons for the failure of most practical Stirling engines to fulfil their designer's hopes and ambitions. Other causes of disappointment include deficiencies in regenerator operation, high mechanical-friction losses, temperature equalization as a result of relatively massive conduction paths, and fluid leakage owing to imperfectly designed (or imperfectly operating) seals.

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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