80

30 40 50 Specific power

Fto. 8.3. Comparison of the effects of maximum cycle pressure and temperature on the maximum possible efficiency versus power-density characteristic for engines using hydrogen as the working tluid with a brake power output of 165 kVV (22-1 brake hp) per cylinder and a cooler temperature of 25 "C (77 'Fi Results were calculated using the Philips simulation program No allowance made in computation ot efficiency for losses in the preheater (after

Meijer 1970a).

volume. The engines were optimized for maximum efficiency with an output of 170 kW (225 brake hp) per cylinder.

A total of four curves was included, two at maximum pressures of 110 MN/m* (15 954 lbs per sq in) and two at 220 MN/m2 (31 908 lbs per sqin). For each pressure, two heater temperatures of 700 and S00°C (1292 and I472°F) were represented. The curves indicate that substantial gains in engine efficiency would result from the application of heat-resistant steels permitting an increase in either or both the pressure and temperature levels. The gains are more pronounced at the higher power-density levels.

Later, Michels (1976) reported a study of the effect on efficiency of operating temperatures for the heater and cooler with different working fluids. Michels used the Philips Stirling cycle simulation computer program to calculate the optimum efficiencies attainable with:

(a) three different heater tube temperatures, 850, 400, and 250 °C (1562, 752 and 4S2°F)

(b) two different cooler temperatures, 100 and 0°C (212 and 32 T)

(c) three different working fluids, hydrogen, helium, and nitrogen.

Michels based his calculations for reference purposes on the Philips

Type 1-98 single-cylinder Stirling engine. This has a swept volume of 98 env' (5.98 in*) and is capable of delivering aboui 15kW (20.4 hp) at 3000 revolutions per minute with a maximum cycle pressure of 220 MN/m2 (31 908 lbs per sq in) using hydrogen. For his study Michels maintained the basic engine configuration but allowed changes in ihe dimensions of the heater, cooler, and regenerator within the overall limitation that the revised designs were capable of fitting onto the existing engine geometry. The heat exchangers were optimized for maximum indicated efficiency, this being defined as the ratio of 'the engine power assuming no mechanical friction____to the heal delivered to the heater'.

The results given by Michels are reproduced in Fig. 8.4. They show the optimized indicated efficiency of the Type I-9K engine as ;t function of power output for the different temperatures and working fluids specified above.

Unfortunately, Michels failed to include on his curves the engine speed although he noted 'the speed increased from left to right along the curves'. Similarly, he failed to provide any information on the pressure levels of the working lluid except to note lhat '... the pressures and dimensions were determined such that maximum efficiency was obtained*. The curves he presented were therefore much less useful than would appear at first sight.

Estimates of mechanical losses (principally friction) in the engine were included in the ealeuiationft to obtain thi> hrnfcf».n lin*»« oiu^n in Pin 8 A rif

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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