On the Design of Recuperator for Transcritical Cycle Adopting CO2‐Based Mixture as Working Fluid: A Focus on Transport Properties Prediction

Transcritical cycles working with CO2-based mixtures gain considerable attention due to thermodynamic efficiency gain compared to pure sCO(2) in hot environments. Previous literature works prove that the adoption of CO2 mixtures provides a reduction of the levelized cost of electricity in concentrated solar power applications and medium-high temperature heat recovery. However, for techno-economic analysis and heat exchanger design, proper evaluation of transport properties of the CO2-based mixtures in power cycle conditions is necessary. Herein, it deals with the analysis of the proper transport properties models for CO2 mixtures to assess their actual thermal behavior. A literature review on transport properties models, and their validation with available experimental data, proves that the friction theory is suitable for CO2 blended with dopants having high molecular complexity. The impact of the different model selection on the recuperator sizing, considering optimized power cycle conditions, is assessed on the CO2 mixtures with hexafluorobenzene and decane: The TRAPP and Chung-Lee-Starling models are imported from Aspen Plus, while the friction theory model is implemented and calibrated in an in-house MATLAB code. The optimal design of the recuperator for the CO2 + C6F6 mixture in a 100 MWel power block coupled with a solar power plant located in Sevilla is carried out

Solar hydrogen production with cerium oxides thermochemical cycle

This paper discusses the hydrogen production using a solar driven thermochemical cycle. The thermochemical cycle is based on nonstoichiometric cerium oxides redox and the solar concentration system is a solar dish. Detailed optical and redox models were developed to optimize the hydrogen production performance as function of several design parameters (i.e. concentration ratio, reactor pressures and temperatures) The efficiency of the considered technology is compared against two commercially available technologies namely PV + electrolyzer and Dish Stirling + electrolyzer. Results show that solar-to-fuel efficiency of 21.2% can be achieved at design condition assuming a concentration ratio around 5000, reduction and oxidation temperatures of 1500°C and 1275 °C. When moving to annual performance, the annual yield of the considered approach can be as high as 16.7% which is about 43% higher than the best competitive technology. The higher performance implies that higher installation costs around 40% can be accepted for the innovative concept to achieve the same cost of hydrogen

Simulation of Oxygen Transport Membranes for CPO Reactors in Small-scale Hydrogen or Syngas Production Applications

The proposed work aims at presenting a 1D finite volume steady state simulation model of an Oxygen Transport Membrane for Catalytic Partial Oxidation (OTM-CPO) reactor developed at the Group of Energy COnversion Systems (GECOS) at Politecnico di Milano. The model is able to simulate supported and unsupported perovskite-based reactive membranes by means of a lumped mass and energy transport method; the active ceramic layer is modelled throughout a generalised O2permeation equation, which depends on the micro-structure characteristics and mixed-ion conduction properties of the material. The supporting porous structure is represented by a mass diffusion model dominated by gas-gas, porous and surface exchange transport processes. The model also includes a global chemical reaction kinetic mechanism of CPO on Rh-based catalysts. The model is applied to simulate the behaviour of a membrane reactor operated upstream the Hydrogen Transport Membrane for Methane Steam Reforming (HTM-MSR) installed at the Laboratory of Micro-Cogeneration (LMC) at Politecnico di Milano. The test bench is focused on testing fuel pre-processing systems for low temperature fuel cells (PEM) applications. The simulation object of this work would allow obtaining a feasibility assessment of the system and a preliminary design of the OTM-CPO reactor

Membrane reactors for green hydrogen production from biogas and biomethane:A techno-economic assessment

This work investigates the performance of a fluidized-bed membrane reactor for pure hydrogen production. A techno-economic assessment of a plant with the production capacity of 100 kgH2/day was carried out, evaluating the optimum design of the system in terms of reactor size (diameter and number of membranes) and operating pressures. Starting from a biomass source, hydrogen production through autothermal reforming of two different feedstock, biogas and biomethane, is compared. Results in terms of efficiency indicates that biomethane outperforms biogas as feedstock for the system, both from the reactor (97.4% vs 97.0%) and the overall system efficiency (63.7% vs 62.7%) point of views. Nevertheless, looking at the final LCOH, the additional cost of biomethane leads to a higher cost of the hydrogen produced (4.62 €/kgH2@20 bar vs 4.39 €/kgH2@20 bar), indicating that at the current price biogas is the more convenient choice.</p

Potentiality of a biogas membrane reformer for decentralized hydrogen production

This paper investigates the potentiality of membrane reactor for green hydrogen production from raw biogas. The assessment is carried out both from thermodynamic and economic point of view to outline the advantages of the innovative technology with respect to the conventional one based on reforming, water gas shift and pressure swing adsorption unit. Both biogas produced by landfill and anaerobic digestion are considered to evaluate the impact of biogas composition on system design. BIONICO system model is implemented in Aspen Plus and Aspen Custom Modeler to perform respectively the balance of plant with thermal integration and a detailed fluidized bed membrane reactor design. Two permeate side configurations, sweep gas and vacuum pump, were modelled and compared. The adoption of membrane reactor increases the system efficiency by more than 20% points with respect to reference cases. Focusing on the economic results, hydrogen production cost show lower value respect to the reference cases (4 €/kgH2vs 4.2 €/kgH2) at the same hydrogen delivery pressure of 20 bar. Between the landfill and anaerobic digestion cases, the latter has the lower costs as consequence of the higher methane content

Experimental and Numerical Study of a Micro-cogeneration Stirling Engine for Residential Applications☆

Abstract Micro-cogeneration Stirling engines are considered promising for residential applications. The present work covers the experimental and numerical analysis of a commercial Stirling unit capable of 8 kW of hot water and 1 kW of electricity. A previously concluded experimental campaign that focused on external measurements is extended here to include internal measurements. The scope is collecting useful data to validate a detailed numerical model. Three test cases are considered by fixing the temperature of the cogeneration water at the unit inlet at alternatively: 30, 50 and 70 °C. Mass flow rate of the water is kept at the nominal value of 0.194 kg/s. This numerical model is an extension of the well-known work by Urieli and Berchowitz. The model is calibrated on the 50 °C case and compared in the other two cases. Maximum deviations with respect to experiments are about 4% on net power output, whereas they remain below 1% on heat input and rejection. The Stirling unit has shown an electrical efficiency exceeding slightly 9% and a thermal efficiency of 90% (both based on the Higher Heating Value) if the cogeneration water inlet temperature is 30 °C, which decreases down to about 84% with water inlet at 70 °C. The Primary Energy Index is remarkably positive for all cases, ranging from 17% to 22% as the temperature of the water inlet goes from 70 °C to 30 °C

Achievements of European projects on membrane reactor for hydrogen production

Membrane reactors for hydrogen production can increase both the hydrogen production efficiency at small scale and the electric efficiency in micro-cogeneration systems when coupled with Polymeric Electrolyte Membrane fuel cells. This paper discusses the achievements of three European projects (FERRET, FluidCELL, BIONICO) which investigate the application of the membrane reactor concept to hydrogen production and micro-cogeneration systems using both natural gas and biofuels (biogas and bio-ethanol) as feedstock. The membranes, used to selectively separate hydrogen from the other reaction products (CH4, CO2, H2O, etc.), are of asymmetric type with a thin layer of Pd alloy (<5 μm), and supported on a ceramic porous material to increase their mechanical stability. In FERRET, the flexibility of the membrane reactor under diverse natural gas quality is validated. The reactor is integrated in a micro-CHP system and achieves a net electric efficiency of about 42% (8% points higher than the reference case). In FluidCELL, the use of bio-ethanol as feedstock for micro-cogeneration Polymeric Electrolyte Membrane based system is investigated in off-grid applications and a net electric efficiency around 40% is obtained (6% higher than the reference case). Finally, BIONICO investigates the hydrogen production from biogas. While BIONICO has just started, FERRET and FluidCELL are in their third year and the two prototypes are close to be tested confirming the potentiality of membrane reactor technology at small scale.The research leading to these results has received funding from the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreements No 621181 (FERRET), No 621196 (FluidCELL). BIONICO has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 671459. This Joint Undertaking receives support from the European Union's Horizon 2020 research and innovation programme, Hydrogen Europe and N.ERGHY

Off-design of a CO₂-based mixture transcritical cycle for CSP applications: Analysis at part load and variable ambient temperature

This work focuses on the off-design analysis of a simple recuperative transcritical power cycle working with the CO₂ + C₆F₆ mixture as working fluid. The cycle is air-cooled and proposed for a state-of-the-art concentrated solar plant with solar salts as heat transfer fluid in a hot region, with a cycle minimum and maximum temperature of 51 °C and 550 °C at design conditions. The design of each cycle heat exchanger (primary, recuperator and condenser) is carried out in MATLAB with referenced models and the turbine designed in CFD, providing performance maps adopted by the cycle operating in sliding pressure. The off-design of the cycle is developed with a routine simulating the thermodynamic conditions of the cycle at variable ambient temperature and thermal inputs down to 40 % of the nominal value. The results show that the cycle can efficiently run in a wide range of part load conditions and ambient temperatures, from around 0 °C to over 40 °C, with net electric cycle efficiencies from 45 % to 36 %: according to the control philosophy proposed, the condenser fans are fixed at design speed, while the cycle operates in sliding pressure, when is possible. The results evidence the flexibility and good performances of the proposed system in various operating conditions