· Available heat or maximum net useful heat that can be evaluated as mechanical exergy.
As a consequence of the concept of interconvertibility (Gaggioli et al.[5, 6, 7]): 'useful work is not better than useful heat, and the available energy results in maximum net useful heat or, equivalently, maximum net useful work or the combination of both'.
Thus, the definition of mechanical exergy that represents, in this case, the maximum net useful heat obtained from the available energy in infinitesimal terms may be expressed by the following relationship: d E X M = δ Q REV NET = δ Q REV CONVER + δ Q REV TRANSF Open image in new window (16).
As a main advantage, both evaporator and condenser pressures can be optimized simultaneously by optimizing only working fluid mass flow rate to get maximum net work output or heat recovery efficiency for given heating fluid and cooling fluid inlet conditions using selected working fluids.
A multi-objective optimisation by using genetic algorithms was carried out seeking both the maximum net electric power and the minimum heat exchanger capacity.
The maximum net power output is determined by the heat source rather than working fluids with a low inlet temperature of heat source.
According to the maximum net power output, suitable working pressure, total heat transfer capacity and expander SP (size parameter), R114, R245fa, R123, R601a, n-pentane, R141b and R113 are suited as working fluids for subcritical ORC under the given conditions in this paper.
The results show that at small flow inlet velocity, the maximum net power output in TEG with plate-fin heat sink is higher, while the TEG with cross-cut heat sink has higher maximum net power output at high flow inlet velocity.
Furthermore, an optimum geothermal mass flow rate and an optimum geometry of the downhole coaxial heat exchanger were determined for maximum net power output.
The maximum net power output is equal in the TEGs with plate-fin heat sink and cross-cut heat sink.
