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Thermodynamic bounds and general properties of optimal efficiency and power in linear responses

We study the optimal exergy efficiency and power for thermodynamic systems with Onsager-type "current-force" relationship describing the linear-response to external influences. We derive, in simple analytic forms, the maximum efficiency and optimal efficiency for maximum power for a thermodynamic machine described by a $N\times N$ symmetric Onsager matrix with arbitrary $N$. The figure of merit is expressed in terms of the largest eigenvalue of the "coupling matrix" which is solely determined by the Onsager matrix. Some simple but general relationships between the power and efficiency at the conditions for (i) maximum efficiency and (ii) optimal efficiency for maximum power are obtained. We show how the second law of thermodynamics bounds the optimal efficiency and the Onsager matrix, and relate those bounds together. The maximum power theorem (Jacobi's Law) is generalized to all thermodynamic machines in the linear-response regime. We also discuss systems with asymmetric Onsager matrix (such as systems under magnetic field) where the reversible limit of efficiency can be reached at finite output power. Cooperative effects are found to improve the figure of merit significantly in systems with multiply cross-correlated responses. Application to example systems demonstrates that the theory is helpful in guiding the search for high performance materials and structures in energy researches.