ORC Optimal Design through Clusterization for Waste Heat Recovery in Anaerobic Digestion Plants

Waste heat recovery (WHR) systems through organic rankine cycles (ORCs) in anaerobic digestion plants may improve cogeneration efficiency. Cogeneration unit power output, flue gas temperature, and mass flow rate are not constant during the day, and the thermal load requested by digesters shows seasonal variations. For this reason, a proper design of the ORC is required. In this study, a design methodology is proposed, based on the clustering of the boundary conditions expected during one year of operation and the anaerobic digestion plant operation. The design has to be a compromise between part-load operation and nominal power rating. In this study, the ORC design boundary conditions were partitioned into four representative clusters with a different population, and the centroid of each cluster was assumed as a potential representative boundary condition for the cycle design. Four different ORC designs, one for each cluster, were defined through an optimization problem that maximized the cycle net power output. ORC designs were compared to those resulting from the seasonal average boundary conditions. The comparison was made based on the ORC off-design performance. Part-load behavior was estimated by implementing a sliding-pressure control strategy and the annual production was therefore calculated. ORC off-design was studied through a detailed Aspen HYSYS simulation. Simulations showed that the power output of each design was directly connected to the cluster population. The design obtained from the most populated cluster generated 10% more energy than that from a system designed by taking into account only the year average conditions.

Energy Conversion Using the Supercritical Steam Cycle

A supercritical steam (or Rankine) cycle is used today for more most of the new coal-fired power plants. More recently, it has been proposed as well for future water-cooled nuclear reactors to enhance their efficiency and to reduce their costs. This chapter provides the technical background explaining this technology. Some criteria for boiler design and operation, like drum or once-through boiler design, fixed or sliding pressure operation, and coolant mixing, are discussed in general to explain the particular challenges of supercritical steam cycles. Examples of technical solutions are given for two large-scale applications: a coal-fired power plant and a supercritical water-cooled reactor, both producing around 1000 MW electric power.

Efficiency of operation of 300 MW condensing thermal power blocks with supercritical steam parameters in sliding pressure mode

The previous research of the application of sliding pressure has shown certain advantages in the operation of high-power condensing blocks with supercritical steam parameters in sliding pressure mode in comparison to the one with constant pressure. The maintenance of stable temperature regime and thermal expansion of turbine elements, prolongation of service life of materials of steam pipes and heating surfaces of the boiler due to the decrease in pressure of the working medium are only some of those advantages. On the other hand, the operation mode of a condensing block with sliding pressure is characterized by the change in cost-effectiveness. The result of this change is mainly due to the de-crease of steam throttling in the turbine's balancing valves and the increase of its internal action in a high pressure turbine, then also due to reduced steam consumption of the feed turbo pump just like a drop in the feed water pressure at the steam boiler inlet. A model has been developed within the framework of this study that follows such changes and their graphical interpretation is provided. The analysis results show that switching 300 MW blocks from the constant to the sliding pressure regime in the 30-60% load range increases the block efficiency respectively by 6.70-1.05%.

Optimum Control Strategy for a Low-Temperature Solar Kalina Cycle Power Generation Under Off-Design Conditions

Low-temperature Kalina cycle power generation system shows great potential in the region of solar energy utilization. The variation in solar radiation affects the heat source temperature of Kalina cycle, and additionally the cooling water temperature also varies with the seasons. In this paper, the mathematical model of a Kalina cycle used for low-temperature solar power generation is established to investigate the off-design performance of the Kalina cycle under off-design heat source and cooling water temperature using three control strategies including constant pressure regulation method, sliding pressure regulation method and modified sliding pressure regulation method. The results show that when the heat source temperature varies, the modified sliding pressure regulation method could keep the cycle performance and the efficiencies of turbine and pump at a relatively high value, and it could be applied in a wider range of heat source temperature. When the cooling water temperature varies, different control strategies have similar influence on the variations of the cycle performance and the efficiencies of turbine and pump. Based on performance comparison, the modified sliding pressure regulation method is determined to be the optimum control strategy for the Kalina cycle under off-design conditions.