Improvement of heat-led CHPs based upon ORC-technology

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2016
Thesis identifier
  • T14351
Person Identifier (Local)
  • 200871573
Qualification Level
Qualification Name
Department, School or Faculty
  • Drawn by the benefits of decentralised and renewable power supply,over 150 Organic Rankine Cycle systems, in a range from 400kWel to2MWel, have been installed in Central Europe between 1998 to 2014. The majority of modules are biomass fired and heat-led by district heating networks. Combined heat and power (CHP) using the ORC technology has a great potential to provide heat and the electric baseload for households. The impact of such systems in terms of CO₂ and primary energy consumption is very low in comparison to conventional heating systems (gas, oil). With rising fuel prices over the last years, the economic situation has become critical for many of these facilities and improvements in efficiency are essential. The research reported here, concludes five years of practical and theoretical experiences and investigations. An operating power plant with a design power of 1MWel has served as validation. Monitoring data have been gathered over a period of eight years for the entire biomass power plant including the district heating network. In addition, a detailed measuring campaign was conducted for three years on the ORC-unit. Approaches to improve the performance of power plants of this category have been developed based on the long-term monitoring. - The influence of a condenser reservoir controller is very limited, based on the proportions of the thermal inertia. Control implementation of the heat rejection systems is vital in order to obtain smooth load transitions. The control of the heat rejection system has been optimized in order to provide stable sink temperatures. It could be shown that this measure leads to a smoother operation of the unit. - Acceptable electrical efficiencies can only be achieved with high pressure ratios across the turbine. Therefore, the recuperator was focused upon. A significant increase of power output (2% points) could be achieved by improving the heat transfer while reducing the pressure losses across the aggregate. Besides the frictions losses, the potential of the cycle can only be utilized if the fluid contains no contaminants, such as low-boilers or inert gases. If this is the case, the condenser can reach a minimum of pressure. Quality management and, if necessary, fluid recycling are vital. Approaches for these two points are discussed. The implementation of a fluid management system for working fluid contaminated with low-boilers and high-boilers is shown. The effectiveness of these measures are proved by monitoring data. It could be shown, that 54% to 63% of the MDM contained in contaminated working fluid can be recycled with just one distilling procedure. In addition to the practical experiences gained, the obtained data of the ORC-unit has been used to develop, calibrate and validate correlations and computer models. The data inform partial models and cycle models, of physical and empirical nature, to simulate ORC units of that type in modelica. - A turbine model for single-stage, super-sonic axial units has been developed. It is accurate within a range of ±0.5%. - The steady-state validation of the pre-heater model show a fair correlation within boundaries of ±5%. Taking a look a the correlation quality of the dynamic component model of the preheater,even better results can be expected. With a nodal model of only two nodes the entire calculation correlates within a range of ±0.5% - The comparison of the various pool boiling correlation in literature has shown that evaporation of siloxanes in a kettle-type tube evaporator cannot be solved without significant adaptations. While the most correlations delivered a good trend the offsets are enormous. Therefore, a new correlation for this specific case has been developed. Finally a correlation based on was chosen with the basic structure of Goreno's approach using the pressure correlation function of Mostinski. The prediction quality, with a range of ±8%, is fairly good for a boiling correlation. - The proposed model for the recuperator unit correlates within a range of ±5% in the relevant U-value range of 200W=m²K to 325W=m²K. The deviation for lower U-values can exceed 10%, but it is only relevant during start-ups - The simulation results show that an operational mode with a low super-heating rate lead to higher thermal efficiencies. This implies a reduction of fuel in the furnace there from including all consequences. The fuel mass flow is reduced, which leads to a decrease of wear on the transport systems and heat exchangers. Additionally this strategy lowers the risk of hot spots in the ORC, leading to a longer cycle fluid lifespan and a win-win(-win) situation economically speaking. The electric efficiency improves by 1%-point almost across the whole load range. The surplus during an average year amounts up to 28000€ for the case study unit.
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