225 blip/cylinder 11 eater leittp.700°C Cooling Water temp. 25%: p- llOkgf/cm3

30 40 Specific power

70 80 (blip/litre piston swept volume)

Fit;. 8.1 Comparison of calculated performance for Stirling engines with different working fluids. Engines have the same output of 165 kW (225 brake hp) per cylinder unci have been optimized for I lit* minimum itn««t1tU HlTincnnu <ultl. I -«-• --------------— -'

engine, tjlt>„ was shown as a function of the power output per unit displacement, in horsepower per liire. The size of the engine decreased as the curve moved from left to right of the figure.

Three curves were given, for air, for helium, and for hydrogen. At points on each curve the engine speed was quoted corresponding with the maximum efficiency and engine output. The engine speed increased as the curve moved to the right of the figure.

Near the extreme left of the figure there was little difference between the three curves. At a speed of 250 revolutions per minute the air engine had a somewhat lower efficiency, 38 per cent compared with 47 for helium and perhaps 49 with hydrogen. However, the power density was not markedly different, at about 8.9 W/cm3 (12 hp per litre). This suggested that in low-power, slow-running engines there was little or no advantage in thermodynamic operation to be gained using hydrogen or helium compared with air. However, with air as the working fluid the requirements for fluid sealing are greatly relaxed and less reservoir storage of surplus fluid is necessary. The air can be simply replaced as required by a small compressor. Thus, small, low-power, stationary electric generators with extended life requirements and unsophisticated design are as likely to be using air ior the working fluid as hydrogen or helium.

Moving to the right of Fig. 8.1 it becomes clear that air cannot be used for high speed engines of high specific output. Further, at the highest speeds and power levels, hydrogen becomes significantly superior to helium. Therefore, in automotive applications where power density is vitally important, it is likely that hydrogen will be the preferred working fluid. A contributory and increasingly significant benefit is that the thermal efficiency with hydrogen can be appreciably higher.

I lelium would likely be selected on considerations of safety fot use in confined situations: ships, underwater power systems, total energy plants, heat pumps, or stationary generators in buildings. Hydrogen is highly reactive with oxygen with extremely wide flammability limits. Helium is inert.

Outline designs for a large 660 kW (900 hp) 4-cylinder engine were presented by Meijer (1970a) using the results given in Fig. 8.1, The resultant engine design envelopes are shown in Fig. 8.2 and are presumed to be rhombic drive engines. The subsequent development of double-acting machines would allow further reduction to about half the sizes given in Fig. 8.2.

Curves to show the effects of heater temperature and maximum pressure with hydrogen as the working fluid were also given by Meijer (1970al and are reproduced in Fig. 8.3. As before, the efficiency was shown as a function of the brake power output per litre of piston swept

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.

Get My Free Ebook

Post a comment