Abstract

Oxy-combustion power plants, where high-purity oxygen instead of air is used for the combustion of fossil fuels, could become more common in the future because they are well-suited for carbon capture and storage (CCS). The supply method of high-purity oxygen is critical for the profitability of those power plants because it is highly energy-intensive and requires expensive equipment. Currently, the most mature method, is cryogenic air separation, but an efficient alternative is membranes that separate oxygen by electronic and ionic conduction, The integration of these membranes in the Graz Cycle, a promising oxy-combustion power cycle, has already led to a remarkable improvement in efficiency. Therefore, it is studied if membranes can also improve the NET power cycle, another interesting oxy-combustion power cycle with supercritical as working fluid, although its high efficiency is based on the utilization of heat from the cryogenic air separation. For this, thermodynamic simulations of the integration of two different membrane technologies were performed using the cloud based process simulation platform IPSE GO.

Case 1 has a 3-end membrane, on whose permeate side pure oxygen is under vacuum pressure while hot pressurized air is fed to the other side. In Case 2, cycle flue gas with low oxygen content is fed to the permeate side of a 4-end membrane. Both membrane cycles use a cascade of compressors, turbines and heat exchangers to achieve optimum efficiency. In this study the membrane area, the temperature differences of the heat exchangers in the oxygen production cycle as well as the membrane operating conditions were varied in search for an optimum. The net cycle efficiency of the optimized Case 1 is still only 50.73%, which is remarkably lower than the 52.72% of the base case with cryogenic air separation. But the efficiency of the optimized Case 2 is with 52.66% almost as high as that in the base case, which can make membranes an interesting alternative to cryogenic air separation.

To explore the potential of oxy-fuel technology, a detailed thermodynamic analysis is performed for four different oxy-fuel combustion cycles: the SCOC-CC, Matiant cycle, S-Graz cycle, and Allam cycle. Based on the literature data, these thermal cycles are modeled and simulated at full load using commercial software Aspen Plus with the same input and boundary conditions. The results show that the Allam cycle and S-Graz reach the highest net energy efficiency with 56.04% and 53.19%, respectively. The Matiant cycle and SCOC-CC have lower efficiencies at 50.02% and 45.23%, respectively. The exergy efficiency is also higher for the Allam cycle (52.87%) and the S-Graz cycle (50.18%). Moreover, the CCS efficiency penalty is comparatively higher for S-Graz, SCOC-CC, and Matiant, with 3.88%, 1.78%, and 1.70%, respectively. This is primarily because of the lower condenser pressure than the Allam cycle, which has a penalty of 0.42%. The ASU efficiency penalty is consistent at 10.31% for all cycles due to the same delivery pressure, flow rate, and oxygen purity. However, in the Allam cycle, it is marginally higher by 0.71 ppt due to an extra oxygen compressor. The results also indicate that the turbine main parameters, such as inlet temperature, inlet pressure, outlet pressure, and condenser pressure, strongly influence the system overall performance. The Allam cycle generally stands out for its high thermal efficiency and simple configuration.

This work focuses on the definition of the operating envelope and the boundary conditions for the start-up of the transcritical CO₂ blended power plant developed in the framework of the DESOLINATION project. The goal of the project is to build and operate a 1.8 MW_el demonstrational concentrated solar power plant, which is currently in the design phase. The demonstrator aims to test a series of innovative systems in a 2000h campaign to prove the system's reliability and efficiency in real operational conditions. The novel technology is mainly represented by CO₂/SO₂ (82% -18% mole) mixture working fluid, implementation of concentrated solar power as a stream of hot molten salt, and unconventional high-pressure mixture condenser for heat rejection. Due to the complex nature of the project, a continuous interaction between component manufacturers and the partners in charge of plant simulation has been established. This paper aims first to present the consolidated plant configuration including the most recent specification of the different components, such as performance maps for pump and turbine and off-design performance of heat exchangers. Then, the operative envelope is derived varying both ambient temperature and mass flow rate of hot molten salt. The system’s inventory is fixed, and an air-cooled condenser unit with an embedded subcooler is implemented. Limit conditions in the operability of the plant at both extreme temperatures and low thermal input were identified and solved by varying air cooled condenser fan speed. Finally, equilibrium pressure in case of plant shutdown (Settling Out Pressure) is calculated in case of both hot and cold conditions highlighting the expected conditions at the beginning of the start-up process. The results provide the expected amount of fluid to be removed from the system to meet the specifications of condenser and turbine manufacturers.

In this paper, an off-design analysis of a lab-scale cycle sCO2 Thermally Integrated - Pumped Thermal Energy Storage (TI-PTES), based on Brayton type cycle, is presented.

Off-design conditions arise due to load-schedule following, or due to variations in storage tank temperatures (e.g. for incomplete charging/discharging of the TES) or due to the temperature of the external heat source integrated in the cycle or the ambient temperature. The goal of this off-design analysis is to identify the potential operating envelope for the off-design temperatures in the cycle. To that end, the off-design envelope for the off-design of ambient temperature and the integrated heat source is evaluated. The off-design envelope is mapped for the rotational speed of the hot machines in the charging and discharging cycles of the TI-PTES. Based on the off-design envelope, mitigation strategies have been proposed with the target to bring the storage temperatures to design point. Inventory control for the heat transfer fluid of TES has been proposed for all the temperature related off-design conditions, as it is a less complex strategy than controlling the mass flow rate of the CO2 loop. The operational range of hot turbomachines, storage temperatures, pressure ratios, powers and performance of the cycles have been evaluated for the selected off-design cases.

Energy storage is crucial for reducing the imbalances between energy demand and generation and thus an important asset to increase the share of renewable energy. This work proposes an efficient, zero-pollution, self-contained integrated energy system for energy conversion and storage. The integrated energy system comprises a supercritical CO₂ gas turbine, an electrolyzer, and a methanol synthesis unit. With this system, excess renewable electrical energy is stored by producing hydrogen in the electrolyzer and converting it into methanol in the methanol synthesis unit. The renewable methanol is used as a fuel in a supercritical CO₂ gas turbine system with direct combustion, to generate electrical energy and heat on demand. This paper investigates the supercritical CO₂ gas turbine systems with the aim of achieving high thermal efficiency. The thermal characteristics of the supercritical CO₂ gas turbine cycle are assessed and the influence of different design and operating parameters is determined.

To assess the thermal characteristics of supercritical CO₂ gas turbines a thermodynamic model is developed in MATLAB. To calculate the properties of CO₂ and CO₂/H₂O mixture CoolProp is used. In an environment of recirculating CO₂, the methanol is oxidized using the stored oxygen from the electrolysis. High pressure and high temperature gases (CO₂/H₂O mixture) expand in the gas turbine, generating mechanical energy. Later, the combustion products, H₂O and CO₂ are separated from the recirculating CO₂ at the cold side of the regenerator and stored to be reused for fuel production. The influence of various operating conditions, such as (i) compressor outlet pressure, (ii) gas turbine inlet temperature, (iii) compressor isentropic efficiency, (iv) gas turbine isentropic efficiency, (v) regenerator temperature difference, and (vi) pressure drop in heat exchangers, is determined.

The results of the investigation show that high thermal efficiency of the supercritical CO₂ gas turbine (above 60 %) can be achieved. Hereby, the low compression work needed to compress the fluid near the critical point to the desired pressure is essential to achieve this high efficiency. The supercritical CO₂ gas turbine is a crucial component of the integrated energy system for energy conversion and storage, which generates electricity with high efficiency and zero pollutants. The mass and energy flow balances of the supercritical CO₂ gas turbine and the renewable fuel generation lead to a self-contained closed energy system with high round-trip efficiency.

This study deals with the off-design operation and controller tuning of a 5 MWth, air-cooled recuperated supercritical carbon dioxide (sCO₂) Brayton Cycle. Previous studies indicated that implemented PID controllers require gain scheduling for efficient operation under fluctuating air temperatures or load levels. However, regarding sCO₂ cycles, limited literature explicitly covers controller tuning, with a majority relying on manual methods. Therefore, this study presents and implements a gain scheduling approach for the air-cooled heat sink of the cycle in a systematic and automized manner, includes the following steps: Calculation of the steady-state boundary conditions, determination of transfer functions from step tests, controller tuning using Matlab and Internal Model Control and, finally, controller testing.

It was found that the Internal Model Control approach, with a closed loop time constant equal to 1/10 of the open loop one, yields a stable and efficient controller performance for all boundary conditions, except cases with a low air mass flow rate. Consequently, it is recommended to avoid such extreme part-load conditions, e.g. by cooler modularization. Finally, important considerations for the following tuning of the inventory control are presented, highlighting the interaction of component design and controllability.