Page 256 - Low Temperature Energy Systems with Applications of Renewable Energy

242 Low-Temperature Energy Systems with Applications of Renewable Energy

The ODP is normalized at 1.0 for refrigerants R-11 and R-12 which are the worst in
this regard. The GWP is normalized at 1.0 for carbon dioxide. Owing to their high
ODP and GWP, both R-12 and R-114 have been banned from use by the Copenhagen
Amendment (effective as of 1994) to the Montreal Protocol (signed in 1987, effective
as of 1989). The binary plant at Paratunka in Russia that was installed in 1967 used
R-12 as its working fluid in conjunction with a very low temperature geofluid,

w81 C [8].
All of the hydrocarbon candidate fluids obviously are flammable and necessitate
appropriate fire protection equipment on site, over and above the usual requirements
for any power plant.

6.5 Optimization of geothermal CHP cycles

In Section 6.2 we performed a system optimization for a CHP plant to determine which
separator temperature yielded the highest revenues from combined output of heat and
power when the two outputs had different monetary values. The individual compo-
nents of the plant were not examined. The efficiency of exergy transfer is an important
consideration in the design of each component of the plant. In this section, we examine
this aspect of the optimization process.

6.5.1 Thermal and exergetic efficiencies

Thermal efficiency is a long-established measure of performance for power plants
operating on a cycle in which the working fluid passes through a series of components
undergoing various processes and periodically returns to some “initial” state. During
the cycle of processes, heat and work may be exchanged with the surroundings. The
thermal efficiency is defined as the ratio of the net work output to the heat input:
_
W net
h ¼ (6.30)
th
Q _ in
Exergy efficiency is a more recent measure of plant performance that is defined as
the ratio of the net exergy output to the exergy input associated with the heat input.
Exergy is the maximum theoretical output allowed by the Second Law of thermody-
namics. It applies to heat transfer, work transfer, and mass flows. The exergy associ-
ated with heat transfer is the work that could be delivered by using that heat in an ideal
Carnot cycle operating between the temperature of the heat source and the ambient
heat sink or the atmosphere. Thus the equation is:

_ _ T 0
E Q ¼ Q 1 (6.31)
T
where T 0 is the ambient temperature or dead-state temperature, in absolute degrees.