Bekele, Mulugeta (PhD)Adugna, Tolasa2018-07-092023-11-092018-07-092023-11-092017-06http://10.90.10.223:4000/handle/123456789/7129Three different model heat engines, two of which operate between two reservoirs at inverse positive absolute temperatures and a third one that operates between two in- verse negative absolute temperatures are investigated. As a working substance of the engines, a system of two-level spin-half particles, in the thermodynamic limit, subjected to a time-dependent external magnetic field, is used. We investigate the heat engines under two schemes: the quasistatic and finite-time thermodynamic processes. In the quasistatic process the system and the reservoirs essentially remain in ther- mal equilibrium and exchange energy in the form of heat and work. As they link the isothermal processes, adiabatic changes are also basic components of the quasistatic processes. After setting the models, the expressions for net work done, net heat ab- sorbed and efficiency of each model are analytically derive. For parameter values of energy level spacing, occupation probability in the excited state and inverse tempera- ture, the efficiencies coincide with the Carnot efficiency of each model. In the finite-time process, the expressions for net work done, power and efficiency of the heat engines are derived. In all the three models, power versus period (τ ) initially (τ ≤ τmp -period at the maximum power) shows a rapid increase with period; then it shows a maximum value at mp before it decrease as period becomes longer and longer. In the very long period limit, finite-time quantities including power, approach to their corresponding quasistatic values. Employing a unified criterion for energy converters, the model engines are effectively optimized and found to yield optimum finite-time quantities. Efficiency-wise optimized efficiencies are found to be better than efficiencies at maximum power; however, power- viii wise, the optimized power is smaller than its maximum power. The figure of merit of model I, ψI , plotted against the quaistaitic efficiency, increases from its 1.12 to about 1.3, as its quasistatic efficiency increases from zero to the maximum possible value. So, in the entire range of ηC , optimum working condition is an advantage for the model. However, the figure of merit of model III, ψIII , generally decreases from its peak value of 1.89 with an increase in ηC . Only in the small ηC 0.2 values, the optimum working condition is preferred to the maximum power working condition. Else where, the maximum working condition is better than the optimum working condition for the model. In model II, the figure of merits, ψII , slightly decreases from its value of about 1.15 to 1.1 as at ηC increases from zero to 0.57enThermodynamic QuantitiesThesis