543 Heat Recovery

Presently, it appears that the most promising way to raise engine efficiency is to recover heat by using a regenerator or recuperator as part of the engine cycle. Therefore, a recuperated cycle has been adopted by nearly all MT manufacturers, although the degree of heat recovery varies considerably. Figure 5.16 shows how recuperation improves the performance of a typical simple-cycle gas turbine engine. Note the dramatic effect in the range of low pressure ratios employed by microturbines (3:1 and higher).

The performance impact on the balance among TIT, pressure ratio, and recuperator effectiveness is shown in Figures 5.17 and 5.18. Note the effect of differing levels of recuperator effectiveness. An engine design with an effectiveness of only 85% would require a TIT of 1800°F and a pressure ratio of 4:1 to reach an engine efficiency of 33% lower heating value (LHV). However, a 1600°F TIT at a pressure ratio of about 3:1 will reach this level only if a 91% effective recuperator is used. An engine running under the latter conditions will enjoy a significantly longer life and reduced maintenance costs.

More effectiveness, however means more surface area and, thus, more weight and volume. Fortunately, the cogeneration, chiller, refrigeration, air compression, and other market opportunities for today's MTs are stationary applications. Within reasonable limits, it does not matter how heavy a system is. Therefore, an MT designed specifically for stationary applications is free to incorporate a very effective recuperator.

Figure 5.19 shows an example of two-shaft MT design that incorporates a high-effectiveness recuperator. The recuperator is contained in the bright metal enclosure in the upper portion of the machine that also provides inlet air and engine exhaust ducting service. The waste heat exchanger for cogen-eration applications is also located within this enclosure.

Like other MT components, recuperator life and cost are critical. Recuperators experience large thermal gradients and load swings in the course of normal operation. Inlet temperatures are also fairly high, but are considerably lower than TITs. Increased TIT and pressure ratios do not cause recuperator inlet temperature to increase by much because the expansion ratio of the turbine increases as the ratio of turbine inlet and outlet temperatures increase. Therefore, metals will still be applicable for recuperators even if future turbine components are switched to ceramics to withstand greater operating temperatures.

Recuperator Performance: 90% effective, 5% dP/P

Recuperator Performance: 90% effective, 5% dP/P

Pressure Ratio

Pressure Ratio

FIGURE 5.16

Recuperator performance improvement.

Recuperator Effectiveness = 0.91

E iij

Recuperator Effectiveness = 0.91

E iij

\ V

4

y*

X4

\5

\

y

2

)

Vv-

1

V

V

>

7

8

¿10

5

170

CO

9 30

Turbi

ie Inlet Te

mperatur

s (Deg F)

60 80 100 120 140 160 180 200 220

Specific Power KJ/Kq

60 80 100 120 140 160 180 200 220

Specific Power KJ/Kq

FIGURE 5.17

Efficiency map (91% effective recuperator).

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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