Session: 30-10 Systems 2

Paper Number: 127526

127526 - Optimal Part-Load Performance of Supercritical Carbon Dioxide Brayton Cycles During Inventory Control 

Closed loop supercritical carbon dioxide (sCO2) cycles are actively being looked at for efficient and low carbon footprint power plants. Recently, the interest has expanded over waste heat recovery and compressed gas storage applications. All these applications involve off-design operation under the variation of source or sink conditions. Further, power plants must operate at part-load conditions as per the load requirements.  Several control strategies have been proposed for ensuring stable cycle operation. These include heat source bypass, turbine bypass, turbine inlet parameter control and inventory control. Among them, several studies recommend inventory control for maximum part-load efficiencies. The control strategy involves mass transfer between the cycle and inventory tanks to regulate the produced power. It affects the pressures in the cycle affecting the turbine and compressor operation in the cycle. The power can be reduced by discharging mass from the system and vice versa.

This paper deals with the optimisation of the cycle performance during Inventory control. At part-load conditions, the turbomachines do not necessarily operate at the optimal point. Therefore, variable speed operation of the turbomachines is explored to investigate the maximum part-load efficiency. In this way, optimisation is studied for the entire operating range and not just at the design point. Variable speed operation in turbomachine systems with both coupled and decoupled speeds are analysed. In this regard, part-load operation in the case of constant turbine and compressor speeds is chosen as the baseline case. The implication of varying inventory and shaft speeds on the performance of the turbomachines are also investigated.

A 5 MW simple recuperated sCO2 Brayton loop is considered for the purpose of analysis. The cycle is designed using an analytical formulation based on the carbon dioxide equation of state. The heat exchangers are modelled using one-dimensional conservation equations. The turbomachines are based on performance maps. The component models are validated against experimental studies in the literature. The loop volume is calculated by properly sizing all the components and the piping. Based on the loop volume, a single tank inventory management system is designed. The part-load calculations are performed thereafter.

It is observed that the part-load efficiency deteriorates with drop in the power. The results for the baseline case reveals why sCO2 cycle efficiency drops at part-load conditions compared to closed ideal gas cycles which are able to operate at constant efficiency. The results from the variable shaft speed case underscore that cycle efficiency can be improved by adjusting the speed of the turbomachines. The efficiency improvement increases at lower loads. The decoupled shaft systems achieve higher efficiency compared to coupled or single shaft systems.

In conclusion, this work contributes to the understanding of sCO2 cycle operation, particularly under off-design conditions, and highlights the potential for optimizing performance across a wide range of operating points. The insights gained will aid in the control and inventory management design of sCO2 cycles, making them more adaptable and efficient for diverse applications.

Presenting Author: Shrey Gupta Indian Institute of Science, Bangalore

Presenting Author Biography: Hi, I am Shrey Sahai Gupta, a Ph.D. candidate at the Indian Institute of Science, Bengaluru, working on Thermal Controls of Supercritical Carbon Dioxide Power Systems. I have a knack for Interdisciplinary research and am skilled in System Simulations and reduced-order modeling.

After completing my bachelor's in mechanical engineering from the Indian Institute of Technology, Ropar, India, in 2020, I briefly worked at Hindustan Petroleum Corporation Limited, India, for around a year. I joined my Doctoral Degree in August 2021. Currently, I am investigating the potential for sCO2 cycles for modern power needs and cost-competitive heat-to-power systems.

Outside work, I've embraced a dynamic blend of interests, from hitting the gym to framing moments behind the lens. I love to discuss ideas. Occasionally, I find myself on the basketball court shooting hoops or exploring South India's captivating landscapes. Above all, appreciating art is my number one hobby.

Authors:

Shrey Gupta Indian Institute of Science, Bangalore
Pramod Kumar Indian Institute of Science Bangalore