Organic Brayton Cycles with solid sorption thermal compression for low grade heat utilization

• The concept of Organic Brayton Cycle (OBC) for power generation is introduced.
• Thermal compression in a solid sorbent bed is employed.
• R134a, CO2, R507a, propane, R32 and R410a are considered with activated carbon as adsorbent.
• Among the fluids investigated R32 is preferable with Maxsorb III as adsorbent.

While organic Rankine cycles have been widely used for power generation using low grade thermal energy, Brayton cycles have not been considered feasible because the work required to compress the gas nearly compensates the turbine work output. However, if the low grade energy can be used for thermal compression of the working fluid, it may be possible to gainfully operate the Brayton cycle. With this in mind, a solid sorption based Brayton cycle is proposed in this paper. R134a, CO2, R507a, propane, R32 and R410a with activated carbon as sorbent, were considered in this proof-of-concept study due to the ready availability of adsorption data. Even though the thermal efficiency is low (<8%), the proposed scheme could add an option for distributed power generation using solar or waste heat. It is found that if irreversibilities in turbine and thermal compression are considered R32 gives a better performance than CO2 and R410a.