Carnot cycle | EPFL Graph Search

A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynamic engine during the conversion of heat into work, or conversely, the efficiency of a refrigeration system in creating a temperature difference through the application of work to the system. In a Carnot cycle, a system or engine transfers energy in the form of heat between two thermal reservoirs at temperatures T_H and T_C (referred to as the hot and cold reservoirs, respectively), and a part of this transferred energy is converted to the work done by the system. The cycle is reversible, and there is no generation of entropy. (In other words, entropy is conserved; entropy is only transferred between the thermal reservoirs and the system without gain or loss of it.) When work is applied to the system, heat moves from the

Investigation of 2R-Cell Degradation under Thermal Cycling and RedOx Cycling Conditions by Electrochemical Impedance Spectroscopy

Stefan Diethelm, Vaibhav Singh, Jan Van Herle

A 2R-cell with a 10 cm2 active area LSCF cathode on a CGO buffer layer was tested at 780°C under thermal and RedOx cycling conditions; 20 thermal cycles were performed between 780°C and 100°C, using 200°C/h heating ramps and natural cooling, followed by 20 RedOx cycles, during which air was fed to the anode for 1 hour at 780°C, causing the cell potential to drop to zero. V-i polarisation curves and electrochemical impedance spectroscopy (EIS) measurements were performed before and after each consecutive cycle in order to monitor the performance losses. The OCV remained stable during the whole test whereas the area specific resistance (ASR) measured at 0.5 A/cm2 increased by +1.3 mΩcm2 per thermal cycle and by +1.8 mΩcm2 per RedOx cycle. The cell was examined after the test by scanning electron microscopy (SEM).

2015

Dynamic behaviour of SOFC short stacks

Nordahl Autissier, Nicolas Badel, Raphaël Ihringer, Diego Larrain, Joseph Sfeir, Jan Van Herle

Electrical output behaviour obtained on solid oxide fuel cell stacks, based on planar anode supported cells (50 or 100 cm2 active area) and metallic interconnects, is reported. Stacks (1–12 cells) have been operated with cathode air and anode hydrogen flows between 750 and 800 ◦C operating temperature. At first polarisation, an activation phase (increase in power density) is typically observed, ascribed to the cathode but not clarified. Activation may extend over days or weeks. The materials are fairly resistant to thermal cycling. A 1-cell stack cycled five times in 4 days at heating/cooling rates of 100–300Kh−1, showed no accelerated degradation. In a 5-cell stack, open circuit voltage (OCV) of all cells remained constant after three full cycles (800–25 ◦C). Power output is little affected by air flow but markedly influenced by small fuel flow variation. Fuel utilisation reached 88% in one 5-cell stack test. Performance homogeneity between cells lay at ±4–8% for three different 5- or 6-cell stacks, but was poor for a 12-cell stack with respect to the border cells. Degradation of a 1-cell stack operated for 5500 h showed clear dependence on operating conditions (cell voltage, fuel conversion), believed to be related to anode reoxidation (Ni). A 6-cell stack (50 cm2 cells) delivering 100Wel at 790 ◦C (1kWel L−1 or 0.34Wcm−2) went through a fuel supply interruption and a thermal cycle, with one out of the six cells slightly underperforming after these events. This cell was eventually responsible (hot spot) for stack failure.

2006

Simulation and Optimization of Heliostat Fields using Radiance and MOO fot the Design of a 300 MWth Central Receiver

Germain Augsburger

This work discusses solar power towers and especially the simulation and optimization of heliostat fields. The technological stateof the art is given through typical characteristic values and dimensions of some existing central tower power plants such as th so-called Solar Two in California and the PS10 in Spain. Leaving cycle losses aside, the field losses involved are described and roughly estimated thanks to commonly measured and computed values, typically coming to an 80% overall field efficiency. On the other side, the coats that must be taken into consideration when designing a heliostat field are listed and approached through correlations, giving for instance a total investment cost of $15 million for relatively large fields ( approximately 700 [m]). The raytracing software Radiance is then chosen for modeling an entire heliostat field with a central receiver, and thus for running simulations over time for different sun positions. Some given field layouts such as those obtained in published cases can be visualized and heat flux at the receiver‘s surface is computed thanks to Radiance tools before being displayed through a MATLAB post-processing. For validating Radiance calculations, the results are first compared with published cases: the receiver‘s total thermal energy produced over a year is generally between 5 and 10 [%] lower than these published results. Secondly, the Radiance heat flux profile has to be compared with some published experimental results: based on measurements led at the PSA Solar de Almeria). the heat flux profile (gradient) is close to measurements, but computed and measured total thermal powers do not match when computed and measured peak values do match. Therefore the Radiance heat flux still requires further validation probably involving some model adjustments. Nevertheless, this does not prevent from implementing some very large fields (more than 3000 heliostats) optimized with the MOO multi-objective optimizer after having modified routines for generating surrounding fields. and thus resulting in a first overview of what the heat flux profile on a cylindrical receiver may look like for a total thermal power of 300 [M Wth].

2008

Simulation and Optimization of Heliostat Fields using Radiance and MOO fot the Design of a 300 MWth Central Receiver

Germain Augsburger

2008