Exergy analysis is an implicit comparison of the performance of real thermal systems with the performance of ideal, reversible thermal systems. Systems must have basic similarities and equivalent boundary conditions to be comparable. Thermal power and refrigeration systems are considered to operate between high-temperature and low-temperature reservoirs. The reservoirs constitute the boundary conditions for thermal systems. Effective reservoir temperature and specific humidity are defined to enable boundary conditions for real and ideal systems to be considered equivalent. It is also shown that equating dead-state temperature and specific humidity to effective reservoir temperature and specific humidity, respectively, yields complete compatibility of real and ideal systems and satisfies the principle that the environment cannot be a source of energy convertible to work. The theory is discussed in relation to systems based on vapour-compression refrigeration and Rankine cycles.

KEYWORDS: exergy, analysis, comparing, performance, heat pumps, chillers, coolers, refrigeration, temperature, vapour compression heat pumps, Rankine cycle heat pumps, humidity, compatibility