organic Rankine cycle (ORC) applies the principle of the steam Rankine
cycle, but uses organic working fluids with low boiling points, instead of
steam, to recover heat from a lower temperature heat source. Fig. 1 below
shows a schematic of an ORC and its process plotted in a T-s diagram in
Fig.2. The cycle consists of an expansion turbine, a condenser, a pump, a
boiler, and a superheater (provided that superheat is needed).
Fig.1 A schematic of
an organic Rankine cycle
Fig.2 The process of a
organic Rankine using R11 as the working fluid
The working fluid of an organic Rankine cycle is very importmant. Pure
working fluids such as HCFC123 (CHCl2CF3), PF5050 (CF3(CF2)3CF3), HFC-245fa
(CH3CH2CHF2), HFC-245ca (CF3CHFCH2F), isobutene ((CH3)2C=CH2), n-pentane and
aromatic hydrocarbons, have been studied for organic Rankine cycles. Fluid
mixtures were also proposed for organic Rankine cycles [1-8]. The organic
working fluids have many different characteristics than water . The slope
of the saturation curve of a working fluid in a T-S diagram can be positive
(e.g. isopentane), negative (e.g. R22) or vertical (e.g. R11), and the
fluids are accordingly called “wet”, “dry” or “isentropic”, respectively.
Wet fluids, like water, usually need to be superheated, while many organic
fluids, which may be dry or isentropic, don’t need superheating. Another
advantage of organic working fluids is that the turbine built for ORCs
typically requires only a single-stage expander, resulting in a simpler,
more economical system in terms of capital costs and maintenance .
Examples--Organic Rankine cycle power plant
Among all these thermodynamic cycles for low-grade heat-to-power conversion,
organic Rankine cycle is so far the most commercially developed one. Both
large scales and small scales power plants and units can be found in
Arizona Public Service Company (APS) completed construction of a solar
trough organic Rankine cycle power plant in the United Stats in 2007, which
is the first new organic Rankine cycle power plant built in the past two
decades, and the first power plant that combines solar though technology
with an organic Rankine cycle power block (See Fig.3).
Fig. 3 Organic Rankine
cycle power plant in Saguaro,
Turbine is the most important part in a organic Rankine cycle system. Ormat
and Infinity are among the leading companies that specialize in turbine
design and manufacture for organic Rankine cycles. The turbine used in the
above mentioned organic Rankine cycle power plant in Saguaro, Arizona is
from Ormat International. Beside the large scale systems, portable
system for decentralized users are also available. Below is a10 kilowatt
organic Rankine cycle power generation unit. A unit like this could be very
useful for remote areas.
Fig. 4 A portable
organic Rankine cycle power generation system
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