Environmental Engineering Reference
In-Depth Information
After doing work, the working substance can be either exhausted into the envi-
ronment or sent back to the heat source to start the cycle over. If the working sub-
stance is returned to its initial state (gas or liquid), there has been no change in its
total energy. Consequently, from the first law of thermodynamics, the total energy
of a system can be increased by doing work on it or by adding heat, and the total
work done by the system is just equal to the heat added (i.e., heat in minus heat
out). The heat source may be direct solar radiation, geothermal steam, geothermal
water, ocean water heated by the sun, combustion fuel, or nuclear energy. The two
types of heat engines that are most commonly associated with renewable energy
processes such as solar power and geothermal energy are the Rankine cycle and
Stirling engines.
R ankine C yCle H eat e ngine
Figure 1.2 illustrates the four processes as they occur in a closed-cycle Rankine
cycle heat engine. The expansion can be accomplished through a cylinder with a
piston, as in the locomotives one still sees in the old western movies or through a
turbine. In the Rankine cycle heat engine, as described for turbine use, the working
fluid changes state and is also called a vapor gas (i.e., hot air; not to be confused
with fuel natural gas) cycle. Rankine cycle engines are all external combustion
devices, such as external combustion gas turbine engines widely used in units for
electrical generating stations.
As shown in the figure below, the first phase pumps the working fluid from low
to high pressure; because the fluid is a liquid at this stage, the pump requires little
input energy. In the second phase, the high-pressure liquid enters a boiler, where it
is heated at constant pressure by an external heat source to become a dry saturated
vapor. In the third phase, the dry saturated vapor expands through the turbine, thus
generating power. In the process, both the temperature and pressure of the vapor
are lowered, and it is possible that some condensation may occur. In the final phase,
the wet vapor enters a condenser, where it is condensed at a constant pressure and
temperature to become a saturated liquid. The pressure and temperature of the con-
denser are fixed by the temperature of the cooling coils as the fluid is undergoing a
phase change.
Add Heat
Expand
Remove Heat
Liquid
Vapor
Vapor
Pump
Boiler
Turbine
Condenser
Liquid (for reuse)
FIGURE 1.2
The four processes of the Rankine cycle.
 
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