solar ponds, etc. The low temperature heat is converted into useful work, that can itself be converted into electricity.
Organic Rankine Cycles seem to be a promising technology in the perspective of a decrease in plant size and
investment costs. They can work at lower temperatures, and the total installed power can be reduced down to the kW
scale. The market for ORC's is growing at a rapid pace. At the present, Organic Rankine Cycle (ORC) raises
considerable interest as it makes it possible to produce electricity from cooler geothermal sources, typically within
the 100–130 °C temperature range, exceptionally down to 90–95 °C, often available from below 1000 m deep
production well increasing the number of geothermal reservoirs in the world that can potentially be used for
generating electricity. Among the literature studies concerning this topic, Franco [1] presented an overview of
current R&D in the field of small-scale ORC for the exploitation of geothermal sources with reduced temperature
below 130 °C. He analyzed the performance of such those new cycles and to consider the potential improvements
that will result in higher cycle performance or lower resource utilization and lower cost of electricity generation. He
showed that the geothermal power plant with a regenerative Organic Rankine Cycle is an interesting and promising
option, in particular the benefit gained by adding a regenerative heat exchanger which provides some of the
preheating heat from the vapor exiting the turbine. Ghasemi et al. [2] provided numerical models for an existing
commercial ORC operating by a regenerative cycle and using isobutane as working fluid. The condensation system
was of air-cooled type. From their simulation results, validated by comparison with experimental data, it appears that
at high ambient temperatures, the net power output of the ORC is limited by the capacity of condenser system. They
also observed that at low ambient temperatures, the inlet of turbine should be in a saturated vapor state and the
maximum feasible pressure as suggested by previous studies. However, as the ambient temperature increases, this
conclusion does not hold anymore and a significant superheat is required to obtain the maximum in net power output
of the ORC. This was considered a consequence of the off-maximum operation of the turbines and consequently
variable isentropic efficiency. It means that at high ambient temperatures, the condenser system should be at full
capacity for the optimal operation, but at low ambient temperatures, the cooling capacity of the condenser system
need to be adjusted to obtain the optimal operation. A theoretical analyses of 12 natural and conventional working
fluids-based transcritical Rankine power cycles driven by low-temperature geothermal sources have been carried out
by Guo et al. [3] with the methodology of pinch point analysis using computer models. Their calculated results
include the optimum turbine inlet pressure and the corresponding thermodynamic mean heating temperature, the net
power output, thermal efficiency, heat transfer capacity as well as the real expansion rate in the turbine. From those
parameters they were able to strike a balance about the more suitable working fluid depending functional conditions.
Similar analyses were carried-out by Saleh et al. [4] and by Hung et al. [5]. In [4] the BACKONE equation of state is
used for screening 31 pure component working fluids for ORC applications. A pinch point analysis for the external
heat exchanger is also performed and results are discussed with relation to the optimization of the heat source. In [5]
the suitability of several working fluids in terms of system efficiency is otherwise analyzed in relation to low-grade
energy sources, such as solar pond and ocean thermal energy conversion systems. Quoilin et al. [6] developed a
thermodynamic model of a waste heat recovery ORC in order to compare both the thermodynamic and the thermoeconomic
performance of several typical working fluids for low to medium temperature-range ORCs. Recently, a
systematic comparison of ORC configurations by means of comprehensive performance indexes was proposed by
Branchini et al. [7]. In the present framework, this paper reports a thermodynamic analysis of ORC applications for
generating energy by exploiting geothermal resources. Results are carried-out for different working fluids and
several operational and environmental conditions.