Comparison of Pressure Driven Electrolytic Membranes (PDEM) and Solid Electrolyte Oxygen Pumps (SEOP) for Small Scale Oxygen Production

AbstractThis work evaluates the thermodynamic performances of two oxygen separation technologies, Pressure Driven Electrolytic Membranes (PDEM) and Solid Electrolyte Oxygen Pumps (SEOP), focusing on the application to small scale oxygen production. We show that PDEM systems operated with a specific flux of 5 liters of oxygen per minute per square meter of active membrane surface (5 LO2/min-m2) can reach an energy consumption as low as 0.39 kWh/kgO2. In the same conditions with a SEOP, the optimized energy consumptions are 0.52 and 0.49 kWh/kgO2 respectively for atmospheric and pressurized configurations

Thermodynamic assessment of liquid metal–steam USC binary plants to break 50% efficiency in pulverized coal plants

Nowadays the state-of-the-art technology to convert coal energy of combustion into electricity is to adopt a pulverized coal boiler coupled with an Ultra Super Critical (USC) steam cycle. The total installed capacity of this well-proven configuration is of hundreds of GW worldwide with an increasing share respect to both supercritical and subcritical cycles. Typical coal USC cycles have maximum pressures of around 270 bar and maximum temperatures of 600-620°C for the high pressure and the mid pressure steam respectively. Maximum attainable efficiency is close to 45% in favorable locations and is mainly penalized by two irreversible processes: coal combustion (about 30%) and heat introduction (about 10%) that is characterized by large temperature differences between the hot flue gases and the steam. The main strategy to reduce the second loss is focused on the development of new super alloys able to withstand higher temperatures, higher pressures and water corrosion and so bring efficiencies close to 49% in the so called Advanced USC plants (AUSC). However, the increasing of maximum cycle pressure and temperature results in a relatively small increase of cycle efficiency due to the large increase of specific heat around the critical point but, on the other hand, it involves a considerably increase of equipment’s cost. Another option to increase cycle efficiency is represented by the introduction of a high temperature and low pressure power cycle between the flue gases and the steam cycle. In this case, the topping power cycle could be (i) an external combustion gas cycle, (ii) an open gas cycle fueled by syngas produced by coal gasification or (iii) a Rankine cycle that uses a proper working fluid with a very high critical temperature. This study aims to define a number of optimized binary plant configurations with saturated Rankine potassium cycle as top cycle and a conventional USC plant as bottom cycle. Top cycle receives heat from the flue gases within the coal-fired boiler while bottom cycle recovers heat from the top cycle fluid condensation and the flue gases cooling before the Ljunström air preheater. Potassium thermodynamic properties are computed with a proper equation of state calibrated on experimental data from reference [2] and able to predict accurately both the volumetric and the thermodynamic behavior of potassium in liquid, vapor and two-phase conditions. Different liquid metal cycles have been designed and the trends of the main quantities (heat of condensation, turbine isentropic enthalpy drop and plant efficiency) have been correlated to both evaporation and condensation temperatures. This information is implemented in the USC scheme, calculated with an in-house process simulation code GS developed at the Department of Energy at Politecnico di Milano [3], which has been validated and used on hundreds of publications and projects. Analysis is completed by the evaluation of the potassium turbine design in terms of number of stages, need of cross-over and optimal rotational speed. A double condensation level configuration is also considered for the top cycle in order to further reduce the temperature difference between the top cycle condensation and evaporation process in the bottom cycle, which further increases the efficiency. The thermal input of coal to the burner is fixed for all the simulations to 1.66 GW, five plant configurations have been selected as the most promising ones and fairly compared with a conventional USC coal-fired power plant having a calculated efficiency equal to 44.72%. Limiting the maximum potassium temperature at 800°C, which corresponds to an evaporation pressure of 1.5 bar, it is possible to reach electric efficiencies close to 51% with a single condensation level top cycle and value close to 52% with a double condensation level top cycle. Power produced by the metal cycle ranges between 25 and 30% of the net system power output. As general conclusion the adoption of binary cycles with a top Rankine liquid metal cycle is demonstrated to be an attractive option from a thermodynamic point of view leading to an electric efficiency larger than in AUSC plants. However, these binary metal-steam cycles still need to face a number of technical and safety issues mainly related to the use of liquid metals. Technical issues are related to the high temperature of heat exchange surface of the boiler, to the very high vacuum at condenser, the need of limiting air leakages and the design of a turbine expanding a fluid with an increasing liquid fraction. Safety issues are due to working fluid reactivity with water that requires the need of expensive solution to limit fire hazard. [1] World Energy Council, 2016. World Energy Resources: Coal. [2] Reynolds, W.C. Thermodynamic properties in SI - graphs, tables and computational equations for 40 substances. Department of Mechanical Engineering, Stanford Univ., 1979 [3] GECOS, GS software. www.gecos.polimi.it/software/gs.ph

Small scale CO2 based trigeneration plants in heat recovery applications: A case study for residential sector in northern Italy

This study investigates the potential of trigeneration systems utilizing CO2-based power cycles to harness hightemperature excess heat. Various CO2-based cycles are proposed, comprising pure CO2 and CO2-mixture, emphasizing integration into district heating and cooling networks. Given the non-isothermal heat rejection of CO2-based cycles, performance maps for absorption chillers at different thermal levels and temperature drop of the heat source are generated. These maps are beneficial not only for the current study but also for generic applications. Various cycle layouts are studied, employing strategies to maximize overall electrical efficiency, electrical power output, or thermal production, starting from available high-grade heat above 500 degrees C. Depending on the specific cycle layout and strategy, the optimal cycle-thermal user coupling is evaluated. The economic and environmental viability of the proposed solution is evaluated in comparison to an existing case-study in northern Italy where the exhaust gases of 10 MWel gas turbines are currently exploited for district heating purposes and centralized vapour-compression chillers meet the residential cooling demand. Compared to the case-study, the adoption of a simple recuperative CO2-mixture bottoming cycle, at a minimum cycle temperature of 70 degrees C, allows not only a primary energy saving of 16 % but also an 8 % reduction of levelized cost of electricity

Adoption of CO2 blended with C6F6 as working fluid in CSP plants

The adoption of CO2-based mixtures as power block working fluid for CSP plant can turn supercritical CO2 cycles into efficient transcritical cycles even at high ambient temperature, with significant performance improvement and potential power block cost reduction. In this work, the use of CO2+C6F6 mixture as working fluid for a power cycle coupled with a solar tower is analyzed. Two different cycle maximum temperatures (550°C and 650°C) are considered and for both configurations the overall plant design is performed. The yearly energy yield is computed with hourly data and the LCOE is minimized varying storage and cycle recuperator sizes. Results show comparable results for the innovative working fluid and for the sCO2 cyclesEuropean Union’s Horizon 2020 No 81498

Thermal efficiency gains enabled by using supercritical CO2 mixtures in Concentrated Solar Power applications

Supercritical Carbon Dioxide (sCO2) power cycles have been proposed for Concentrated Solar Power (CSP) applications as a means to increase the performance and reduce the cost of state-of-the-art CSP systems. Nevertheless, the sensitivity of sCO2 systems to the usually hot ambient temperatures found in solar sites compromises the actual thermodynamic and economic gains that were originally anticipated by researchers of this innovative power cycle. In order to exploit the actual potential of sCO2 cycles, the utilization of dopants to shift the (pseudo)critical temperature of the working fluid to higher values is proposed here as a solution towards enabling exactly the same features of supercritical CO2 cycles even when ambient temperatures compromise the feasibility of the latter technology. To this end, this work explores the impact of adopting a CO2-based working mixture on the performance of a CSP power block, considering hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4) as possible dopants. Different cycle options and operating conditions are studied (250-300 bar and 550-700ºC) as well as molar fractions ranging between 10 and 25%. The results in this work confirm that CO2 blends with 15-25%(v) of the cited dopants enable efficiencies that are well in excess of 50% for minimum cycle temperatures as high as 50 or even 55ºC. It is also confirmed that, for these cycles, turbine inlet temperature and pressure hardly have any effect on the characteristics of the cycle that yields the best performance possible. In this regard, the last part of this work also shows that cycle layout should be an additional degree of freedom in the optimisation process inasmuch as the best performing layout changes depending on boundary conditions.Unión Europea SI-1900/10/201

Innovative combined heat and power system based on a double shaft intercooled externally fired gas cycle.

2noThis paper presents a new CHP solution based on a double shaft intercooled gas cycle with external combustion (EFGT cycle). This configuration exploits the turbocharger technology widely diffused in the automotive industry, taking advantage of the capital cost reduction due to the economy of scale typical of the automotive market. Thermodynamic calculations based on actual turbochargers maps available from manufacturers data are carried out in order to evaluate the performances of the system. It is shown that the system results competitive both with natural gas fuelled solutions such as internal combustion engines and microturbines achieving conversion efficiency of 25-30%, as well as with low grade fuels like biomass. For a 50. kW biomass system with a grate combustor boiler and turbine inlet temperature of 750. °C, an electric efficiency of 21% is obtained, which is higher than the available state of the art solutions based on ORC technology for this size. A preliminary design of the system is performed, including the sizing of the boiler, showing its technical feasibility, while complete economic evaluations will be considered in future works.restrictedrestrictedIora P.; Silva P.Iora, Paolo Giulio; Silva, P

Effect of Ambient Temperature on Electric Vehicles’ Energy Consumption and Range: Model Definition and Sensitivity Analysis Based on Nissan Leaf Data

In this paper, a general quasi-steady backward-looking model for energy consumption estimation of electric vehicles is presented. The model is based on a literature review of existing approaches and was set up using publicly available data for Nissan Leaf. The model has been used to assess the effect of ambient temperature on energy consumption and range, considering various reference driving cycles. The results are supported and validated using data available from an experimental campaign where the Nissan Leaf was driven to depletion across a broad range of winter ambient temperatures. The effect of ambient temperature and the consequent accessories consumption due to cabin heating are shown to be remarkable. For instance, in case of Federal Urban Driving Schedule (FUDS), simplified FUDS (SFUDS), and New European Driving Cycle (NEDC) driving cycles, the range exceeds 150 km at 20 °C, while it reduces to about 85 km and 60 km at 0 °C and −15 °C, respectively. Finally, a sensitivity analysis is reported to assess the impact of the hypotheses in the battery model and of making different assumptions on the regenerative braking efficiency