Part-load analysis and preliminary annual simulation of a constant inventory supercritical CO2 power plant for waste heat recovery in cement industry

The present work investigates the part-load performance of a MW-scale sCO2 2 power plant designed as waste heat recovery unit for an existing cement plant located in Czech Republic, in the framework of the H2020 funded project CO2OLHEAT. The study first presents the selected power plant configuration and then focuses on the evaluation of its part-load operation due to variation of flue gas mass flow rate and temperature. The range of flue gas conditions at the outlet of the upstream process is retrieved from a preliminary statistical analysis of historical trends obtained through the cement plant monitoring. The numerical model developed for this study aims at providing realistic results thanks to the adoption of turbomachinery performance maps provided by the turbomachinery manufacturer of the project. Moreover, heat exchangers have been modelled through a discretized approach which has been validated against manufacturer data, while piping inventory and pressure losses have been assessed through a preliminary sizing that considers the actual distances to be covered in the cement plant. Performance decay is estimated for the whole range of flue gas conditions, reporting the most significant power cycle parameters, and identifying the main causes of efficiency loss. The part-load analysis is carried out considering a constant CO2 2 inventory, in order to reduce the system complexity and capital cost and simplify plant operation. Results show that the operation entails minor variation of the compressors operative points in the whole range of operating conditions of the cement plant, avoiding the risk of anti- surge bypass activation. Moreover, the plant is able to work close to the nominal thermodynamic cycle efficiency (20.5 %-23.0 - 23.0 %) for most of the year and benefits from part-load operation in terms of overall performance. In the last part of the work, a preliminary techno-economic analysis of the plant is also presented to highlight the potential advantages of sCO2 2 technology for waste heat recovery applications. The results of the part-load performance of the plant are combined with the flue gases data obtained from the preliminary statistical analysis and the cement plant historical monitoring. An annual electricity production equal to 13 ' 909.7 ' 909.7 MWh is obtained, corresponding to 6560 equivalent hours and a system capacity factor of 74.9 %. The investment cost of each CO2OLHEAT plant component is estimated by means of cost correlations obtained from literature and the non-discounted payback time is computed as a function of the electricity selling price. The results show that, even considering electricity prices before 2022, the payback time of the CO2OLHEAT plant is estimated to be lower than 8 years, justifying the industrial interest in the proposed technology

Off-design study of a waste heat recovery ORC module in gas pipelines recompression station

This study investigates the use of an ORC as heat recovery unit in a natural gas pipeline compression station powered by a gas turbine with the aim of increasing the process energy efficiency. A flexible Matlab® suite, able to investigate both subcritical and supercritical cycle, has been developed for the plant sizing and for the part-load simulation. The methodology to compute the system energetic performance is discussed. The ORC configuration that guarantees the maximum power output (7.22 MWe) is identified. The yearly electricity yield (42615.9 MWh) reveals good perspectives of implementing ORC with the aim of reducing the environmental impact of gas compression stations

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

This is an open access article under the CC BY-NC-ND licenseThis 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 aircooled 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

GeoProp: A thermophysical property modelling framework for single and two-phase geothermal geofluids

The techno-economic evaluation of geothermal resources requires knowledge of the geofluid's thermophysical properties. While the properties of pure water and some specific brines have been studied extensively, no universally applicable model currently exists. This can result in a considerable degree of uncertainty as to how different geothermal resources will perform in practice. Geofluid modelling has historically been focused on two research fields: 1) partitioning the geofluid into separate phases, and 2) the estimation of the phases’ thermophysical properties. Models for the two fields have commonly been developed separately. Recognising their potential synergy, we introduce GeoProp, a novel geofluid modelling framework, which addresses this application gap by coupling existing state-of-the-art fluid partitioning simulators, such as Reaktoro, with high-accuracy thermophysical fluid property computation engines, like CoolProp and ThermoFun. GeoProp has been validated against field experimental data as well as existing models for some incompressible binary fluids. We corroborate GeoProp's efficacy at modelling the thermophysical properties of geothermal geofluids via a case study on the heat content of different geofluids. Our results highlight the importance of accurately characterising the thermophysical properties of geofluids in order to quantify the resource potential and optimise the design of geothermal power plant

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

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

Part-Load Strategy Definition and Preliminary Annual Simulation for Small Size sCO2-Based Pulverized Coal Power Plant

In the near future, due to the growing share of variable renewable energy in the electricity mix and the lack of large-scale electricity storage, coal plants will have to shift their role from base-load operation to providing fluctuating back-up power. However, current coal power plants, based on steam Rankine cycle, are not optimized for flexible part-load operation, resulting in an intrinsic inadequacy for fast load variations. The founding idea of the H2020 sCO(2)-Flex project is to improve the flexibility of pulverized coal power plants by adopting supercritical CO2 Brayton power cycles. Despite the extensive literature about the design of sCO(2) plants, there is still limited discussion about the strategies to be implemented to maximize system efficiency during part-load operation. This paper aims to provide deeper insight about the potential of sCO(2) power plants based on recompressed cycle with high-temperature recuperator (HTR) bypass configuration for small modular coal power plants (25 MWel). Analysis focuses on both design and part-load operation providing a preliminary sizing of each component and comparing different operating strategies. Results demonstrate that sCO(2) coal power plants can achieve competitive efficiency in both nominal and part-load operation thanks to the progressive increase of heat exchangers effectiveness. Moreover, they can be operated down to 20% electric load increasing power range of coal plants. Finally, the possibility to optimize the cycle minimum pressure ensures a safe operation of the compressor far from the surge line and to increase the performance at low load