Selection maps for ORC and CO2 systems for low-medium temperature heat sources
Marco Astolfi, Silvia Lasala, Ennio Macchi
Session: Session 1A: CO2 Power System
Session starts: Wednesday 13 September, 10:30
Presentation starts: 11:50
Room: Building 27 - Lecture room 01
Marco Astolfi (Politecnico di Milano)
Silvia Lasala ()
Ennio Macchi (Politecnico di Milano)
Nowadays, ORC is the most reliable option available on the market for the exploitation of low-medium temperature heat sources in a large range of power outputs: ORC field spans from renewable energy sources like geothermal, biomass and solar applications to waste heat recovery form industrial processes or engines flue gases. Such energy sources are characterized by either a nearly constant or a variable temperature profile. The first case includes direct solar fields and direct biomass boilers where the working fluid flows in the solar collector or in the boiler without the use of an intermediate heat transfer fluid. The maximum temperature of these cycles is defined by the thermal stability of the fluid and the heat-resistance of the material used for the hot sections of the plant. In the second case, the ORC working fluid is heated by a variable-temperature hot stream that is typically cooled down to a certain temperature limit. This is the case of, for example, geothermal brines, flue gases discharged by either a gas turbine or an internal combustion engine or waste heat recovery from a generic plant. This class also includes applications based on a HTF loop.
In recent years, the use of supercritical CO2 for power production has gained a large interest from both the Industry and the Scientific Community. SCO2 cycles are typically envisaged for large and high-temperature power plants coupled with solar tower technology or fossil fuel combustion. In these fields of application, in fact, sCO2 plants can compete with conventional steam cycles thanks to their smaller investment cost, more compact turbomachines, simpler plant arrangement, higher flexibility. Besides the already attested application of high-temperature sCO2 power cycles, this technology may be also considered as a viable solution for the exploitation of low-medium temperature heat sources, competing with ORC.
This work aims at presenting performance maps to enable the straightforward thermodynamic comparison and easier selection between ORC and sCO2 cycles, in a wide range of applications where they may compete. The analysis is thus carried out considering both constant and variable-temperature heat sources, with a maximum heat source temperature ranging from 200 to 500°C. Each point of the map provides the optimal efficiency of both ORC and sCO2 power cycles, considering their most suitable configuration currently available on the market. As regards ORC, they are modelled as saturated Rankine cycles with a limited vacuum in the condenser to limit air leakage. Several different fluids are investigated and cycles are optimized by varying evaporation and condensation temperatures and the use of the recuperator. For sCO2 cycles, different plant layouts are investigated (regenerative, recompressed, intercooled, re-heated). In all cases, the expander efficiency is evaluated with a correlation which accounts for the effect of volume ratio and last stage size parameter. The analysis is performed considering both high and low-temperature heat sinks, representative of ambient air and water. In the first case, heat is rejected to the ambient with an air-cooled condenser, limiting the sCO2 cycle to a non-condensing Brayton configuration while, in the second case, the availability of water enables the reliable condensation of CO2 thus allowing the less-power-consuming compression of highly-dense cool CO2.