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Exhaust

Figure 3.5 In the open-cycle Rankine, exhaust vapor is released directly into the environment. A large water tank is required.

Figure 3.5 In the open-cycle Rankine, exhaust vapor is released directly into the environment. A large water tank is required.

Add Reject

Compress heat Expand heat

Add Reject

Compress heat Expand heat

Fuel

Figure 3.6 The Brayton cycle used in jet planes is an internal combustion, open-cycle engine.

Fuel

Figure 3.6 The Brayton cycle used in jet planes is an internal combustion, open-cycle engine.

Table 3.4 Nature of the Four Processes in Different Engines

Cycle

Compression

Heat addition

Expansion

Heat rejection

Carnot

adiabatic

isothermal

adiabatic

isothermal

Otto

adiabatic

isometric

adiabatic

isometric

Diesel

adiabatic

isobaric

adiabatic

isometric

Brayton

adiabatic

isobaric

adiabatic

isobaric

Stirling

isothermal

isometric

isothermal

isometric

Ericsson

isothermal

isobaric

isothermal

isobaric

25 C, as is the case in ocean thermal energy converters), the most appropriate fluid for the Rankine cycle is ammonia. For low operating temperatures such as those in some direct solar engines, one of several available freons may work best. Most fossil-fueled Rankine engines use water vapor, although mercury may be effective at higher temperatures.

Steam engines are ill suited for automobiles—they either require a large condenser or a large water tank. In part, because no heat rejection device is required, the open-cycle Brayton turbine is universally used in jet planes. Various engines that use only gases as working fluids differ in the nature of the processes used, as illustrated in Table 3.4. For good efficiency, mechanical heat engines must operate with high compression ratios.

In gas turbines, two different types of compressors can be used: radial or axial. Radial compressors can be built with compression ratios of about 3:1 per stage. Thus, for a 9:1 compression, two stages are sufficient. The disadvantage is that it is difficult to obtain high efficiencies with this type of compressor, especially when stages are ganged. In radial (centrifugal) compressors, the air is accelerated toward the rim and must be redirected to the intake at the hub of the next stage. However, channeling air around involves losses. Furthermore, part of the energy imparted to the air is rotational and therefore has to be converted to translational energy by means of (lossy) diffusers.

Axial compressors yield only a small compression ratio per stage, say, 1.2:1. Thus, a large number of stages must be employed. The Rolls-Royce "Tyne" aircraft turbine achieved an overall 13.5:1 compression using 15 stages, with a mean compression ratio of 1.189:1 per stage. The large number of stages is practical because of the simplicity with which the output of one stage can be fed to the input of the next. The trouble with axial compressors is that their efficiency falls rapidly with decreasing size owing mostly to blade tip leakage, something that is difficult to avoid in small machines. For this reason, Brayton turbines of relatively low power are still of the radial type.

Gas turbines may be at the verge of opening a new significant market as a bottoming cycle for solid oxide fuel cells whose exhaust consists of high-temperature gases well suited to drive this class of turbines.

In both Rankine and Brayton engines, each of the processes is carried out in a different part of the equipment. Compression and expansion are

Table 3.5 Some Characteristics of the More Common Combustion Engines

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Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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