Allocation Mechanisms without Reduction

We study a simple variant of the house allocation problem (one-sided matching). We demonstrate that agents with recursive preferences may systematically prefer one allocation mechanism to the other, even among mechanisms that are considered to be the same in standard models, in the sense that they induce the same probability distribution over successful matchings. Using this, we propose a new priority groups mechanism and provide conditions under which it is preferred to two popular mechanisms, random top cycle and random serial dictatorship. (JEL C78, D44, D82)

Thermodynamic Assessment of Using Water as a Refrigerant in Cascade Refrigeration Systems with other Environmentally Friendly Refrigerants

For current and future sustainability, refrigerants with high global warming potential (GWP) are being phased out and replaced with environmentally friendly refrigerants. To this end, research into the current and possible future low-GWP refrigerant alternatives in cascade refrigeration systems caught much attention. In this paper, a mathematical model is developed to assess the optimum energetic, exergetic, and operational parameters of a cascade refrigeration system using water as a refrigerant in the upper cycle with R744, N2O, R41, R717, R290, and R1270 in the lower cycle for a cooling load of 10 TR (35.2 kW). Multiple studies have been conducted for evaporator temperatures between -25 and 5 °C. Results show that R41 and R717 as low- and intermediate-temperature refrigerants, respectively, are recommended for the bottom cycle. Furthermore, R717-water showed improved COP compared to other top cycle refrigerants, with a COP improvement of 2.9% to 8.6%. This study demonstrates the thermal feasibility of using water as a refrigerant in low-temperature cascade systems. Using water as a refrigerant in the top cycle showed good results in low-temperature applications without the risk of solidification. However, the drawbacks are the high volumetric flow rate and compressor discharge temperature, requiring a large capacity water injected compressor.

Power and Efficiency Optimization for Open Combined Regenerative Brayton and Inverse Brayton Cycles with Regeneration before the Inverse Cycle

A theoretical model of an open combined cycle is researched in this paper. In this combined cycle, an inverse Brayton cycle is introduced into regenerative Brayton cycle by resorting to finite-time thermodynamics. The constraints of flow pressure drop and plant size are taken into account. Thirteen kinds of flow resistances in the cycle are calculated. On the one hand, four isentropic efficiencies are used to evaluate the friction losses in the blades and vanes. On the other hand, nine kinds of flow resistances are caused by the cross-section variances of flowing channels, which exist at the entrance of top cycle compressor (TCC), the entrance and exit of regenerator, the entrance and exit of combustion chamber, the exit of top cycle turbine, the exit of bottom cycle turbine, the entrance of heat exchanger, as well as the entrance of bottom cycle compressor (BCC). To analyze the thermodynamic indexes of power output, efficiency along with other coefficients, the analytical formulae of these indexes related to thirteen kinds of pressure drop losses are yielded. The thermodynamic performances are optimized by varying the cycle parameters. The numerical results reveal that the power output presents a maximal value when the air flow rate and entrance pressure of BCC change. In addition, the power output gets its double maximal value when the pressure ratio of TCC further changes. In the premise of constant flow rate of working fuel and invariant power plant size, the thermodynamic indexes can be optimized further when the flow areas of the components change. The effect of regenerator on thermal efficiency is further analyzed in detail. It is reported that better thermal efficiency can be procured by introducing the regenerator into the combined cycle in contrast with the counterpart without the regenerator as the cycle parameters change in the critical ranges.

Refining Tournament Solutions via Margin of Victory

Tournament solutions are frequently used to select winners from a set of alternatives based on pairwise comparisons between alternatives. Prior work has shown that several common tournament solutions tend to select large winner sets and therefore have low discriminative power. In this paper, we propose a general framework for refining tournament solutions. In order to distinguish between winning alternatives, and also between non-winning ones, we introduce the notion of margin of victory (MoV) for tournament solutions. MoV is a robustness measure for individual alternatives: For winners, the MoV captures the distance from dropping out of the winner set, and for non-winners, the distance from entering the set. In each case, distance is measured in terms of which pairwise comparisons would have to be reversed in order to achieve the desired outcome. For common tournament solutions, including the top cycle, the uncovered set, and the Banks set, we determine the complexity of computing the MoV and provide worst-case bounds on the MoV for both winners and non-winners. Our results can also be viewed from the perspective of bribery and manipulation.

Combined Heat and Power Plants Based on Mirror Heat Exchange Brayton Cycles

As gas turbine exhaust gases leave the turbine at high temperature, heat recovery is often carried out in a combined heat-and-power system or in the steam section of a combined-cycle plant. An interesting alternative is a mirror cycle, which involves coupling together a direct Brayton top cycle and an inverted Brayton bottom cycle; this results in significantly higher power output and efficiency, though at the expense of added complexity. The research illustrated in the present paper was based on two in-house codes and aimed to analyze different plant configurations, one of which was a heat recovery (regenerative) top cycle with the heat exchanger hot side located between the top and bottom cycle turbo-expanders. The authors call this configuration a distorting mirror, as the hot side may not be at atmospheric pressure. A parametric analysis was carried out in order to optimize plant performance vs. pressure levels. Simulation showed that, at the design point, very good performance is obtained: efficiency close to 0.50 with plant cost (per megawatt) about half vs. combined-cycle plants. An off-design analysis showed that the mirror plant is a little more sensitive to changes in load than a simple Brayton, single-shaft GT.

Power and efficiency optimization for combined Brayton and two parallel inverse Brayton cycles. Part 2: Performance optimization

The power and efficiency of the open combined Brayton and two parallel inverse Brayton cycles are analysed and optimized based on the model established using finite-time thermodynamics in Part 1 of the current paper by adjusting the compressor inlet pressure of the two parallel inverse Brayton cycles, the mass flowrate and the distribution of pressure losses along the flow path. It is shown that the power output has a maximum with respect to the compressor inlet pressures of the two parallel inverse Brayton cycles, the air mass flowrate or any of the overall pressure drops, and the maximized power output has an additional maximum with respect to the compressor pressure ratio of the top cycle. The power output and the thermal conversion efficiency have the maximum values when the mass flowrates of the first and the second inverse Brayton cycles are the same. When the optimization is performed with the constraints of a fixed fuel flowrate and the power plant size, the power output and thermal conversion efficiency can be maximized again by properly allocating the fixed overall flow area among the compressor inlet of the top cycle and the turbine outlets of the two parallel inverse Brayton cycles. The numerical examples show the effects of design parameters on the power output and heat conversion efficiency.

Power and efficiency optimization for combined Brayton and two parallel inverse Brayton cycles. Part 1: Description and modelling

A thermodynamic model for open combined Brayton and two parallel inverse Brayton cycles is established using finite-time thermodynamics in part A of the current paper. The flow processes of the working fluid with the pressure drops of the working fluid and the size constraints of the real power plant are modelled. There are 17 flow resistances encountered by the gas stream for the combined Brayton and two parallel inverse Brayton cycles. Six of these, the friction through the blades and vanes of the compressors and the turbines, are related to the isentropic efficiencies. The remaining flow resistances are always present because of the changes in flow cross-section at the compressor inlet of the top cycle, combustion inlet and outlet, turbine outlet of the top cycle, turbine outlets of the bottom cycle, heat exchanger inlets, and compressor inlets of the bottom cycle. These resistances control the air flowrate and the net power output. The relative pressure drops associated with the flow through various cross-sectional areas are derived as functions of the compressor inlet relative pressure drop of the top cycle. The analytical formulae about the relations between power output, thermal conversion efficiency, and the compressor pressure ratio of the top cycle are derived with the 17 pressure drop losses in the intake, compression, combustion, expansion, and flow process in the piping, the heat transfer loss to ambient, the irreversible compression and expansion losses in the compressors and the turbines, and the irreversible combustion loss in the combustion chamber. The performance of the model cycle is optimized by adjusting the compressor inlet pressure of the bottom cycles, the mass flowrate and the distribution of pressure losses along the flow path in part B of the current paper.