## Or pniI1rKla l452ij

(19) Heat lifted per unit mass of working fluid tt( 1 + 5 cos 0)8 sin 0

Oraim

(20) Coefficient of performance (COP)

rE i

(21) I leat transferred from the compression space (to cooling medium)

(23) Heat transferred from the hot (compression) space per cycle, n =, v > 777 Z'-^Vj gsin Q

(24) Heat transferred from the hot (compression) space, per unit mass of working fluid,

(25) Coeflicicnl of performance (COP) = TCJ(TK- Tc) = t/( I -t).

(26) Heat transferred from the expansion space (heat source),

(27) Power required to drive the heat pump,

Optimization of design parameters

It is obvious from the Schmidt-cycle equations summarized above that the net cycle power and the thermal loads on the heat exchangers arc-direct linear functions of engine speed (N), pressure of the working fluid (Pmns)t »nd size of the engine, expressed in terms of the total swept volume Vr. The effect which the four principal design parameters (t, k, a and X) have on performance is less obvious. In particular, it is not clear what combination of these should be used to achieve optimum performance. This is an important consideration, since these parameters must be determined at the design stage and, except for the temperature ratio r. cannot readily be varied afterwards, except by structural changes in the machine.

Pigs. 5.9 to 5.12 show the effect on the cycle-power parameter P/(pra„,V|) of independent variation of one of the four parameters r, k. a and X, with the other three maintained constant. In Pig. 5.9 the effect of the temperature ratio on cycle power is explored for expansion-space temperatures (T,;) both above and below the assumed compression-space temperature of 300 K, thus embracing both refrigerating machines and prime movers. With TE> P(> the power parameter is positive, and progressively increases as the expansion-space temperature increases. When 7'k< /'c, the machine is acting as a refrigerator and. as the expansion-space temperature decreases, there is a progressive increase in

Temperature of expansion space (K>

I-10. 5.9. Effect of letnperature on cycle power. The figure shows the effect on the non-dimensional power parameter P/fp,,,,, V'T) of vnrintion in die expansion -.pnee tempera turc 'J',„ with T<: constant ai 300 K. phase angle <» 90°. swept volume ratio ic O.K and dead-volume ratio X 1.0. At expansion-space temperatures below 300 K, r>1.0 and the powei parameter is negative, because the cycle is acting as a cooling cycle, requiring a net input of work.

Temperature of expansion space (K>

I-10. 5.9. Effect of letnperature on cycle power. The figure shows the effect on the non-dimensional power parameter P/fp,,,,, V'T) of vnrintion in die expansion -.pnee tempera turc 'J',„ with T<: constant ai 300 K. phase angle <» 90°. swept volume ratio ic O.K and dead-volume ratio X 1.0. At expansion-space temperatures below 300 K, r>1.0 and the powei parameter is negative, because the cycle is acting as a cooling cycle, requiring a net input of work.

available power may be increased by the rise of high-temperature materials for the expansion-space cylinder and heat exchanger, and for refrigerators the temperature of refrigeration should be as high as possible.

Fig. 5.10 shows the ellcct of the svvept-volume ratio k on the power parameter. The curves show clearly that, for given values of r, a, and A, there is a definite optimum value of k at which the power parameter is a maximum. Comparison of the 'wo curves for 7 = 0.25 and 0.5 shows, however, that the optimum value of k is not constant, but changes from about 0.75, when r = 0.25. to about 1.0, when t 0.5. Changes in « and X also produce an adjustment of the optimum value of k. Thus, there is no single 'best' value of k.

Fig. 5.1 1 shows the effect of the dead-volume ratio X - Vp/Vn on the power parameter. The message of this figure is very clear: increase in the dead space above the absolute minimum required reduces the power

## 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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