Info

7„-4«.<C

r1(—2SCC Nitrogen 7r~0 Of .. .U ... —i-±-

0 1 2 0 <i< i I) 2 0 0 5 1I1D2ÍI 0 y<kW) — If)

0 1 2 0 <i< i I) 2 0 0 5 1I1D2ÍI 0 y<kW) — If)

Fto. S.J. Maximum possible efficiency versus power oulpul ior a Philips Type 1-98 rhombic-drive, single-cylinder engine with diflerent heater and cooler temperatures and three different working fluids. Result were calculated using the Philips simulation program for maximum possible efficiency for an engine with 98 cm1 (5.98 in') piston displacement. Speed and cycle pressure were not disclosed. Speed increases Irom left to rigid. It is thought that pressures also increase from left to right (after Michels 1976).

carefully noted lliat a logarithmic scale was used for the horizontal power axis, so that very significant differences in power existed for the relatively short distances between such points as, for example, A and B in Fig. 8.4(a).

Comparison of the characteristics given in Figs. 8.1 and 8.4 show that the shapes of the curves are different. In F ig. 8.1 the efficiency declined as the power per unit displacement increased, whereas in Fig. 8.4 the reverse is true, for as the specific power increased, the efficiency first increased to a maximum and then fell away. This difference is thought to have arisen because in Fig. 8.1 the maximum pressure was maintained constant whereas no limitation was imposed on the pressure to calculate the results in Fig. 8.4. Now increase in power output can be obtained by increase in cither or both the engine speed and engine pressure level. On Fig. S.4 it is likely, therefore, that the pressure increased from left to right along the curves just as the speed did in both Figs. 8.1 and 8.4.

An interesting feature of Michels' results was that, for a given tempera-

htrp rflnimon f«r i/inoUr i U.. ----- :—1 - — 1 1 1

example, the peak efficiency at a heater temperature or 850 C (1562aF) and a cooler temperature of 0°C (32°F). For hydrogen, point C. Fig. 8.4(b) the maximum indicated efficiency is 56 per cent at 12 kW (16.3 hp) power output. For helium, point D, Fig. 8.4(d) the peak efficiency is 56 per cent at 9 kW (12.2 hp) power output. For nitrogen, point E, Fig. 8.4(f) the peak efficiency is 55 per cent at 2.5 kW (3.4 hp) power output. This correspondence of the same maximum indicated efficiency bui at significantly different power levels was found at other heater and cooier temperatures. It is presumed that the pressures and speeds for helium would be somewhat less than for hydrogen and very much less for nitrogen. What a pity it is Michels did not see fit to include this essential additional data in his paper.

Parenthetically, it will be recalled that air is composed of 79 per cent nitrogen and 21 per cent oxygen so that Michels' results for nitrogen can generally be interpreted as applicable to 'air' without serious error.

Experiment I comparisons

Very little experimental data about the effects of different working fluids have been published. Dros (1965a) published the comparison reproduced in Fig. S.5. of the performance of a large Stirling cycle cooling

using hydrogen .mil helium as the working llutd The machine was supplied with walet cooling at a temperature of 15 "C (59 °F) and flow rate 20 m Vh (706 ft'.'hi Measured value« of refrigerating capacity Pv and shaft power Pm ami also the relative efficiency n/ij, calculated from these two quantities arc shown as functions of the cold-side temperature. At temperatures above 110 K (198°R) the maximum cycle pressure was limited to ot) MPu (870 lb per sq ini. A: tower temperatures the pressure was reduced to maintain the shaft power constant at 334 kW (182 hp), the maximum permissible value (after Dros 1965a).

using hydrogen .mil helium as the working llutd The machine was supplied with walet cooling at a temperature of 15 "C (59 °F) and flow rate 20 m Vh (706 ft'.'hi Measured value« of refrigerating capacity Pv and shaft power Pm ami also the relative efficiency n/ij, calculated from these two quantities arc shown as functions of the cold-side temperature. At temperatures above 110 K (198°R) the maximum cycle pressure was limited to ot) MPu (870 lb per sq ini. A: tower temperatures the pressure was reduced to maintain the shaft power constant at 334 kW (182 hp), the maximum permissible value (after Dros 1965a).

engine with hydrogen and helium as the working fluid. This shows Ihe cooling effect generated and the power input required as a function of refrigerating temperature with the same engine speed and maximum cycle pressure. Comparison of the data, presented separately side by side, for hydrogen and helium, shows that with hydrogen as the working fluid the engine produced a higher refrigerating effect and consumed less power than with helium. In this paper Dros points out that at low cryogenic temperatures helium deviated less from ideal gas behaviour than hydrogen and so the advantage of hydrogen becomes less marked than in power engines.

Loftus (1964) evaluated the performance characteristics of a large four-cylinder 265 kW (360 hp) Stirling engine with hydrogen and helium as the working fluid. Hie engine was made by Philips in 1963 at the

iHydroften

100(1 ISOQ

Pressure (lb/sq in)

•Helium 1500 rpm

Helium

Hydrogen

I lydrogen 1200 rpm

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