We show that for systems with broken time-reversal symmetry the maximum efficiency and the efficiency at maximum power are both determined by two parameters: a "figure of merit" and an asymmetry parameter. In contrast to the time-symmetric case, the figure of merit is bounded from above; nevertheless the Carnot efficiency can be reached at lower and lower values of the figure of merit and far from the so-called strong coupling condition as the asymmetry parameter increases. Moreover, the Curzon-Ahlborn limit for efficiency at maximum power can be overcome within linear response. Finally, always within linear response, it is allowed to have simultaneously Carnot efficiency and non-zero power. The understanding of the fundamental limits that thermodynamics imposes on the efficiency of thermal machines is a central issue in physics and is becoming more and more practically relevant in the future society. In particular due to the need of providing a sustainable supply of energy and to strong concerns about the environmental impact of the combustion of fossil fuels, there is an increasing pressure to find best thermoelectric materials [1][2][3][4].A cornerstone result goes back to Carnot [5]. In a cycle between two reservoirs at temperatures T 1 and T 2 (T 1 > T 2 ), the efficiency η, defined as the ratio of the performed work W over the heat Q 1 extracted from the high temperature reservoir, is bounded by the so-called Carnot efficiency η C :The Carnot efficiency is obtained for a quasi static transformation which requires infinite time and therefore the extracted power, in this limit, reduces to zero. For this reason the notion of efficiency at maximum power has been introduced. An upper bound for the efficiency at maximum power has been proposed long ago by several authors [6][7][8][9] and is commonly referred to as Curzon-Ahlborn upper bound:The range of validity of this bound has been widely discussed in several interesting papers [10][11][12][13][14][15]. For the thermoelectric power generation and refrigeration, within linear response and for systems with time-reversal symmetry, both the maximum efficiency and the efficiency at maximum power, are governed by a single parameter, the dimensionless figure of meritwhere σ is the electric conductivity, S is the thermoelectric power (Seebeck coefficient), κ is the thermal conductivity, and T is the temperature. The maximum efficiency is given bywhere η C is the Carnot efficiency; the efficiency η(ω max ) at maximum power ω max reads [10]The only restriction imposed by thermodynamics is ZT ≥ 0, so that η max ≤ η C and η(ω max ) ≤ η (l) CA , where η (l) CA = η C /2 is the Curzon-Alhborn efficiency in the linear response regime. The upper bounds η C and η (l) CA are reached when the figure of merit ZT → ∞. This limit corresponds to the so-called strong coupling condition, for which the Onsager matrix L becomes singular (that is, det L = 0) and therefore the ratio J q /J ρ , with J q heat currrent and J ρ electric (particle) current, is independent of the applied temp...