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A NOVEL APPROACH TO DISTRICT HEATING: USING A TWOPHASE EXPANDER IN REVERSIBLE HEAT PUMP-ORGANIC RANKINE CYCLE SYSTEM
Daniarta, Sindu; R. Imre, Attila; Kolasiński, Piotr
A district heating system involves a central generation and distribution of heat to residential buildings through a network of insulated pipes. Heat can be generated, for example, from the combustion of fossil fuels and biomass. Since there are many environmental restrictions and the growth of heat and energy demand, several industrial waste heat and renewable energy sources, such as geothermal or solar thermal energy may be promising to be utilized for district heating purposes. However, in some districts, various low-temperature heat sources might be available; as a result, a heat pump is installed to increase the temperature to accommodate demand peak loads. The heat pump is operated only for a certain period and sometimes not used during summer. To optimize the utilization of the heat source, the district heating system may be transformed into a power generation system that can generate, for example, electricity instead of heat during the summer. This goal may be achieved by applying a reversible system. Therefore, this article discusses the novel reversible heat pump-organic Rankine cycle (RHPORC) system using a two-phase expansion system. Some working fluids were selected based on thermal properties, as well as safety and environmental issues. Some selected two-phase volumetric expanders were introduced in the novel system. In the end, the performances of the system based on the selected working fluid were compared. According to the results of a study, using RHPORC in district heating systems might save annual energy consumption by up to 50%.
A novel combined cooling and power cycle integrated ejector refrigeration and composition adjustment for stationary engine waste heat recovery
Sun, Xiaocun; Shi, Lingfeng; Tian, Hua; Shu, Gequn
The combined cooling and power cycle has wide applications ascribed to the feasibility of simultaneously providing cooling and electricity. The layout comprised of vapor compression refrigeration cycle and Organic Rankine Cycle by sharing condenser is one of the most common structures. However, for this kind of structure, the high temperatures of working fluid in compressor outlet and expander outlet aggravate the condensation load and lead to the decline of system performance. This study proposes a novel combined cooling and power cycle integrated of ejector refrigeration and composition adjustment running with CO2 mixture. Liquid separation condenser is introduced to realize composition adjustment. Working fluid with higher CO2 mass fraction flows to power sub-cycle and evaporation process of refrigeration sub-cycle. On the other hand, working fluid with smaller CO2 mass fraction is pumped to the inlet of ejector and heated by expansion exhaust. A stationary engine is chosen as the research objective. The co-generation system recovers waste heat of engine exhaust and coolant to provide cooling to refrigerating chamber. The results show the priority of the novel proposed co-generation system. Under the requirement of 0 ℃ chilled fluid, the basic combined cooling and power cycle could export 8 kW cooling capacity and 6.63 kW electricity, while the novel proposed system can export 4.5% more net power output with 8 kW cooling capacity output or export 18.6% higher cooling capacity with 6.63 kW electricity.
ACHIEVING 45% MICRO GAS TURBINE EFFICIENCY THROUGH HYBRIDIZATION WITH ORGANIC RANKINE CYCLES
Escamilla, Antonio; Sánchez Martínez, David; García-Rodríguez, Lourdes
The demand for affordable, secure, and sustainable energy storage solutions has grown significantly with the increasing focus on decarbonization and the adoption of renewable energy sources (RES). Power-to-Power (P2P) energy storage systems (ESS) have emerged as a promising solution, utilizing excess electricity from RES to produce hydrogen for future power generation. This document presents a study on increasing the round-trip efficiency of P2P ESS by improving the electric efficiency of micro gas turbines (mGT) and integrating waste heat to power (WHP) technology. The research investigates the potential of mGTs as prime movers in P2P ESS, aiming to break the 45% electric efficiency barrier that would make them competitive with other alternatives like internal combustion engines (ICE) and fuel cells (FC). Increasing the nominal electric efficiency of mGTs would lead to significant reductions in hydrogen consumption, system footprint, and overall capital expenditure. Thus, this research focuses on increasing the electrical efficiency of the mGT by proposing a hybridization between the recuperative Brayton cycle and bottoming organic Rankine cycles, reaching higher than 45% electrical efficiencies in a hybrid configuration. An exhaustive comparison of the main ORC systems hybridized with the recuperative Brayton cycles is presented. The results reveal that hybridizing an intercoolingrecuperative Brayton cycle with a simple recuperated ORC has the potential to increase electrical efficiency to 46%. The work also presents a sensitivity analysis to assess how the design parameters influence the performance of the hybrid thermodynamic cycle.
ADDITIVE MANUFACTURING FOR FAST PROTOTYPING OF A VELOCITY COMPOUNDED RADIAL RE-ENTRY TURBINE
Stümpfl, Dominik; Ráž, Karel; Streit, Philipp; Weiß, Andreas P.
In order to achieve a drastic reduction in CO2 emissions to limit global warming, it must be possible to globally cover the electricity demand from renewable sources. However, the fluctuating availability of solar and wind energy will nevertheless make it necessary to complement the renewables by thermal power plants. Additionally using e.g., the potentials of waste, biomass or waste heat from industrial processes locally in Organic Rankine Cycle (ORC) power plants is a promising approach to finally achieve a stable and sustainable electricity supply. In the present work, therefore, different variants of a radial, velocity compounded re-entry cantilever turbine (Elektra turbine) are investigated regarding their potential for such applications. The paper concentrates on the fast prototyping of an existing 5 kW air turbine demonstrator, recently developed by the authors, by using the possibilities of additive manufacturing with plastic materials to substitute essential parts of the flow geometry. In this context the approach to implement the plastic parts in the turbine is explained. Experimentally determined efficiency parameters of various 3D-printed deflection channel modifications and a printed plastic wheel made of PA12GB are presented, discussed, and compared to the fully milled metal turbine. This results in a quantification of the additional losses due to the higher inaccuracies and roughness of the printed parts, which can be taken into account in further investigations. Furthermore, the various problems and hurdles that must be observed when using 3D-printed plastic parts in high-speed turbomachines are highlighted. With regard to the rotor wheel, the authors conclude that the use of additively manufactured plastic wheels is only feasible with increased preliminary testing. The time required for this is usually not in proportion to the manufacturing time and costs saved. For the stator parts however, 3D-printing turned out to be a reasonable approach.
Advanced control of compressor inlet temperature in supercritical CO2 Brayton cycle
Wang, Rui; Wang, Xuan; Tian, Hua; Shu, Gequn
The supercritical CO2 (sCO2) Brayton cycle has gained much interest because of its flexibility, compactness and high efficiency. The sCO2 at the inlet of compressor should work near the critical point to obtain high system efficiency, while the physical properties of sCO2 near the critical point change dramatically, which brings great challenges to the control of the compressor inlet temperature. At present, cooling far from the critical point and adjusting the cooling water flow rate with Proportion- Integration-Differentiation (PID) controller is the commonly used method, but it loses system efficiency and may be out of control sometimes. Therefore, in this study Linear Model Predictive Control (LMPC) and Deep reinforcement learning (DRL) are used to control the compressor inlet temperature and compared with PID. A dynamic model of a recompression sCO2 Brayton cycle is established, and the cooler model is carefully validated against experiment data. The results indicate that both the LMPC and DRL can control the sCO2 temperature near the critical temperature much better than the PID. LMPC works the best because the cold end parameters fluctuate slightly and the cooler model can be regarded as approximately linear, thus LMPC can find almost the global optimal solution. Nevertheless, DRL control exhibits the fastest real-time computation ability and proves good extrapolation ability.
AERODYNAMIC DESIGN AND NUMERICAL ANALYSIS OF A COUNTER-ROTATING CENTRIFUGAL COMPRESSOR
Brahim ABED, Cheikh; LEROUX, Arthur
Counter-rotating turbomachines offer more design and control options than conventional machines. While axial counter-rotating turbomachines have been widely studied, few studies have focused on centrifugal ones. This paper presents a detailed numerical study on the benefits of counter-rotating impellers for centrifugal compressors, specifically the impact of speed ratio on performance gain. The study used 0D/1D models and optimization methods to meet design criteria. The aerodynamic performance of the new designs was compared to a baseline configuration consisting of a centrifugal impeller and a vaned wedge diffuser. The new design features a first impeller with perfect axial inlet and mixed flow or a purely radial blade at the impeller exit, and a parametrically designed counterrotating bladed diffuser with radial or mixed flow inlet and radial outlet. Optimized solutions were determined using validated 0D/1D loss models and analyzed using a 3D CFD solver. The results show that this new configuration can increase efficiency on a wide range compared to the baseline using speed ratio modulation.
Analytical estimation of the presence of non-condensable gases in the condensate tank of an Organic Rankine Cycle
Dhanasegaran, Radheesh; Uusitalo, Antti; Honkatukia, Juha; Turunen-Saaresti, Teemu
The presence of non-condensable gases (NCGs) can have a negative impact on the ORC performance due to the reduced expansion ratio over the expander. The presence of NCGs can reduce the turbine expansion ratio significantly, especially in such systems in where siloxanes or high molecular weight hydrocarbons are used, requiring condensing pressures well below the atmospheric pressure. The ORC facility that uses siloxane MDM as the working fluid at LUT University has been considered in the present study. Experimental and simulation data of the facility from the previously published studies suggest that defining the working fluid state in the condenser and condensate tank suffers from significant uncertainty due to the existence of NCGs. Therefore, an attempt has been made to analytically estimate the quantities of NCGs present in the condensate tank of the ORC system using 1-D calculations based on ideal gas assumptions. It was assumed, that air is the NCG that is present inside the condensate tank for simplifications. Since the compressibility factor of MDM vapor is close to 1 at such low condensing temperatures, its behavior is close to ideal gas and hence, the assumption of ideal conditions seems viable. Data from three different experimental measurements with varying temperatures have been tested using the proposed analytical method and the corresponding quantities of the MDMair mixture during the system operation have been estimated. The results showed that the presence of the NCG was dominant, especially in the case of measurement data with lower temperatures, and was reduced for higher condensate tank temperatures.
ASSESSMENT OF HIGH TEMPERATURE HEAT PUMP LAYOUTS EQUIPPED WITH A BLADELESS TURBOEXPANDER
Passalacqua, Matteo; Maccarini, Simone; Barberis, Stefano; Traverso, Alberto
pursuing the ambitious targets of electrification of industrial processes and thermal generation via renewable power. This work evaluates the optimal thermodynamic performance features of heat pump cycles comparing conventional layouts with those adopting a Tesla turboexpander, for recovering pressure drop in high temperature applications. The paper focuses on n-pentane (R601), a natural refrigerant, being a favorable substitute of synthetic refrigerants thanks to its low global warming potential (GWP) and adequate thermophysical properties that could allow to reach high temperature thermal outputs (>150°C), thus, being suitable for some specific industrial processes. Tesla or bladeless turboexpanders are a promising technology for small volumetric flows and two-phase fluids, featuring low sensitivity to downscaling effects, while retaining high rotor efficiency, which is being investigated for energyharvesting solutions. The benefit of introducing such expansion device in place of a conventional lamination valve is assessed, in different layout configurations. Simulations were conducted using an improved version of WTEMP-EVO modeling tool, proprietary to University of Genoa. Results show that the use of Tesla expander improves significantly the overall thermal efficiency in terms of coefficient of performance (COP), thanks to power recovered, depending on the cycle layout.
ASSESSMENT OF TRILATERAL ORGANIC RANKINE CYCLE FOR SOLAR APPLICATIONS WITH INNOVATIVE TURBOEXPANDER CONCEPT
Romei, Alessandro; Giostri, Andrea; Spinelli, Andrea
Low-temperature solar collectors coupled with thermal energy storage can enable stable and carbonfree energy production. The key issue is to efficiently convert solar energy into electricity without resorting to complex architecture that may hinder the technical and economic feasibility of the entire system. In this work, we propose a fully integrated organic Rankine cycle (ORC) with the solar field and energy storage, targeting 200 kW. The system consists of a single circuit of the selected organic fluid that passes through the solar collectors, the thermocline thermal energy storage, and the ORC unit. The organic fluid remains liquid inside the solar field and the thermal energy storage, leading to a trilateral thermodynamic cycle. Leveraging the thermodynamic behavior of molecularly complex fluids, the radial-inflow turbine expands from saturated liquid to superheated vapor. Following the idea of White (App. Therm. Eng., 192 (2021), 116852), the nozzle cascade expands the two-phase flow mixtures and delivers superheated vapor to the rotating cascade. In this way, the rotor processes dry organic vapors without incurring mechanical damage or suffering from additional losses due to twophase interactions. Three maximum temperatures are investigated, and each of them entails a different fluid selection to have the liquid-to-vapor expansion through the stator. Preliminary designs of the radial-inflow turbines are carried out by employing a meanline code, validated for single-phase organicfluid flows, revealing that feasible designs can be obtained. Based on these results, the proposed technology appears feasible and promising on the technical ground.
Basic design of an ORC demonstrator system for implementation in an Iron & Steel plant through the DECAGONE project
Windfeldt, Magnus; Deng, Han; Andresen, Trond; Schifflechner, Christopher
Waste heat recovery (WHR) technologies offer great opportunities for improving energy efficiency and reducing CO2 emissions for energy intensive industrial processes. The DECAGONE project is developing an innovative ORC-based WHR system to be demonstrated in an iron & steel plant located in the Czech Republic. The design of the ORC system considers the practical site-specific conditions and limitations, such as variations in heat source conditions, heat sink availability, and size and space limitations for the ORC components. Various cycle configurations are compared with a thermodynamic optimization model for maximizing the net power output subject to the process constraints, including (1) recuperative vs. non-recuperative designs, (2) air vs. water as heat sink and (3) direct vs. indirect evaporation. The recuperative cycle with indirect evaporation and direct air condensation is deemed the most suitable solution for the project site conditions. The results provide recommendations for performance improvement and indications for performance subject to practical plant operating conditions, such as large range of temperature and flow rate of waste heat source. Analyses in this work provide the basis for detailed component design and decision processes towards finalizing the design of the demonstrator.
Calibration of multi-hole probes for measurements in compressible organic vapor flows
Hake, Leander; Sundermeier, Stephan; aus der Wiesche, Stefan; Passmann, Maximilian
Multi-hole probes are necessary devices to measure flow characteristics and losses in turbomachinery. The present contribution reports the first experience with calibrating and using multi-hole probes in high-speed flows of the organic vapor Novec 649 at elevated pressure and temperature levels. It is found that the classical scheme, established for perfect gas applications, must be extended by an additional quantity reflecting the impact of the initial thermodynamic state on the pressure signals and the data reduction. The practical use and operational issues of three- and five-hole probes for ORCturbomachinery research are discussed based on the new results. A promising hybrid experimentalnumerical calibration approach significantly reduces the tremendous calibration effort.
COLD ENERGY RECOVERY IN LNG PLANTS WITH ORGANIC RANKINE CYCLE
Marchiori, Gabriele; Giudici, Marta; Ruffato, Giorgia; Astolfi, Marco
During the production phase, natural gas (NG) is compressed and chilled to liquid state (liquified natural gas, LNG) for sake of volume reduction, hence to ease transportation. Approximately, 500 kWh of electricity or mechanical power per ton of LNG is needed for LNG compression and refrigeration during the production phase, which makes cold energy exploitation interesting from a thermodynamics point of view. An effective way to harness the cold energy contained in LNG is through an Organic Rankine Cycle system. A Cold Energy Plant (CEP) is an efficient system based on ORC technology which regasifies liquified natural gas and converts the heat absorbed from sea water into electricity by recovering the valuable exergy content of LNG. This solution allows to produce electrical power onsite without any additional external fuel consumption and no emissions at stacks. The CEP is a lowmaintenance and cost-effective solution that can be efficiently applied in the LNG industry to increase energy efficiency and sustainability of operations. The CEP system designed by Exergy achieves high efficiency thanks to the use of the proprietary Exergy Radial Outflow Turbine combined with the Multilevel Condensation Cycle, maximizing the revenue from electricity production. This paper will outline the technical features of a CEP power plant and will present a case study based on a Liquified Natural Gas regassification system engineered to produce Nominal 4 MWe (4000 kWe) net electric power while simultaneously fulfilling the regasification of 180 ton/h (~1 MTPA) of LNG.
Comparison of Different Evaporator Topologies for Industrial Heat Pumps
Vermani, Sanjay; Anand, Nitish; Van Bael, Johan; Touliankine, Evgueni; Serafino, Aldo; De Servi, Carlo
Reducing the energy consumption associated with industrial process heating is one of the crucial steps toward building a green and climate-neutral Europe, as the thermal energy demand of industry accounts for a significant portion of the total primary energy consumption and carbon emissions of this sector. In this regard, waste heat utilization using high-temperature heat pump systems is one of the most promising solutions to increase industrial energy efficiency. To efficiently recover and convert the available waste heat, the performance of the individual heat pump components, in particular the heat exchangers, is key. This paper reports a comparison between two different evaporator concepts considered for high-temperature heat pump systems, namely the shell and tube and the falling film evaporator. Four key characteristics of the two evaporator concepts are analyzed and compared: the overall heat transfer coefficient, the induced pressure losses, the footprint, and the required refrigerant charge. The results indicate that the falling film evaporator features a higher overall heat transfer coefficient and enables approximately a 77% reduction of the required working fluid charge with respect to the shell and tube evaporator.
COMPREHENSIVE ANALYSIS OF ORC-VCC SYSTEM FOR AIR CONDITIONING FROM LOW-TEMPERATURE WASTE HEAT
Witanowski, Łukasz
Due to the need to reduce electricity consumption, methods of increasing the efficiency of energy use are being sought. One of the possibilities is the use of low-temperature waste heat for the production of electricity or cooling. Systems that convert heat into cold seem particularly attractive. This approach allows, above all, the removal of the generator. It eliminates the electrical part of the expander and, as a consequence, increases the efficiency of waste heat utilization, simplifies the construction of the ORC turbogenerator, and removes certain restrictions. The paper presents an analysis of the ORC-VCC cycle using waste heat at a temperature of 95℃ and heat up to 130kW. Various novel working fluids (e.g. R1224yd(Z), R1233zd(E), R1336mzz(Z)) are considered. Selected fluids are characterized by low global warming potentials. An in-house code was developed and used to calculate cycle parameters. Single and multi-objective optimization was performed, aiming at the maximization of the ORC efficiency, Net Present Value (NPV) and the Internal Rate of Return (IRR). Pareto frontiers were obtained, and a decision-making method was used to select an optimum solution. The obtained ORC efficiency was 5.25%, COPVCC 0.47 and the cycle efficiency was above 27.1%.
COST-EFFECTIVE OPTION OF COLD ENERGY UTILIZATION IN PHARMACEUTICAL INDUSTRY
Daniarta, Sindu; Sowa, Dawid; Havas, Ádám; R. Imre, Attila; Kolasiński, Piotr
In pharmaceutical industries, nitrogen is used for several purposes, including inerting storage containers to prevent chemical reactions with oxygen at a temperature range of 2 to 8 °C. To meet these specifications, the regasification of liquefied nitrogen (LIN), transforming the liquefied nitrogen into gas phases at certain temperatures and pressures, is necessary. One potential solution that can be applied to increase efficiency and reduce costs while maintaining strict quality and safety standards in the pharmaceutical industry is a replacement of the conventional LIN regasification process with an organic Rankine cycle (ORC). This process could utilize cold energy from the process while using nitrogen as a cooling source and ambient air as the hot side of the thermodynamic cycle. Using the ORC system for this cold energy utilization is promising as the technology is now more developed, compact, relatively reasonable cost, and reliable. Since there are few investigations in cold energy utilization as power generation, this article discusses the techno-economic feasibility of the ORC system in the case of its application in pharmaceutical industries with a particular focus on cold energy utilization in LIN regasification. In this analysis, propane was selected as the working fluid of the ORC system as it has good criteria such as thermal properties, zero ozone depletion potential, and low global warming potential. The analysis was optimized for different heat source conditions. Several designs (with and without the direct expansion system) were developed. In the end, their techno-economic performances and cost-effectiveness were compared. The obtained results show that replacing the conventional LIN regasification in the pharmaceutical industry with an ORC system may improve the efficiency of the system and reduce power consumption. The results of the study additionally indicate that, in terms of cost-effectiveness, reusing existing components of the prior system – specifically, the nitrogen vaporizers and pump – would result in a 23.81% reduction in investment costs and a 22.00% decrease in levelized cost of energy (LCOE).
DECISION-MAKING MATRIX FOR THE SELECTION OF MIXTURE IN ORC APPLICATION
Combaluzier, William; Tauveron, Nicolas; Beaughon, Michel; Serafino, Aldo
ORC is an established and affordable technology to convert efficiently low/medium grade thermal energy to power. The choice of the working fluid is critical to the performance of the ORC. The use of zeotropic mixtures as a cycle working fluid could lead to an efficiency enhancement, thanks to the nonisothermal phase change occurring at both the condenser and the evaporator. Therefore, there has been a growing interest for mixtures, which are often studied by the means of optimization algorithms or thermodynamic calculations that lead to interesting results (Bederna et al., 2021, Lecompte et al., 2014). This work proposes an alternative approach. A selection methodology is used to conduct a preliminary screening to optimize the mixture choice in accordance with the targeted objectives. This methodology is applied to study cyclopentane-based mixtures. For this purpose, a large list of criteria is considered to achieve improved safety, maximized performances, ORC using more environmentally friendly fluids and respecting heat exchangers sizing while following specific technical constraints. Among these criteria, the temperature glide should be carefully taken into account: indeed, it is directly linked to the fractionating risk, which must be prevented for the good functioning of the cycle. Performing this study prior to the classical energetic analysis reduces the risk of exploring mixtures of incompatible or irrelevant fluids. This whole analysis yields a decision-making matrix, gathering selection criteria, relevant properties, and cycle performances. By adapting the different threshold of these criteria, this methodology is adaptable to large-scale ORC applications.
DESIGN AND CONSTRUCTION OF A REVERSIBLE ORC TEST RIG FOR GEOTHERMAL CHP APPLICATIONS
Kaufmann, Florian; Schifflechner, Christopher; Wieland, Christoph; Spliethoff, Hartmut
for their ability to supply heat or electricity flexibly. While many publications focus on their application in Carnot batteries or domestic systems, geothermal applications got less attention. In geothermal combined heat and power plants, reversible ORCs offer the possibility to generate electricity in times of low heat demand and supply additional heat to a district heating network (DHN) in peak load times. In previous work, the authors showed that this increases plant utilisation and reduces the load of commonly used peak-load gas boilers. This work presents the design and construction of a fully reversible ORC test rig capable of flexible operation as ORC or high-temperature heat pump (HTHP). It extends the state of the art by design and construction of a test rig for experimental validation of a previously untested cycle layout for geothermal applications. The test rig is supplied by a 200 kW hot water heating circuit as a heat source and uses a fully reversible 20 kW twin-screw machine. A closed loop intermediate cooling circuit allows simulating a DHN with varying temperature levels and mass flows during HTHP operation. This paper provides insights into the design methodology and the test rig’s intended operating range and performance. Thus, the paper provides valuable insights for the research community regarding conceptual and experimental activities on reversible ORC systems. Moreover, an in-depth description of the constructed test rig, its components and instrumentation and finally the preliminary control concepts are given. Commissioning and first experimental results are expected in the year 2023.
Design and Modeling of a Demonstration-scale ORC Cycle for the Liquid Air Combined Cycle
Pryor, Owen M.; Conlon, William M.; Rimpel, Aaron M.; Venetos, Milton J.
Energy storage is becoming an increasing focus for the future energy markets. One potential hybrid system for ling duration energy storage is the Liquid Air Combined Cycle (LACC). The LACC utilizes excess renewable energy to liquefy and store air during the charge cycle. During its discharge cycle, the system uses the exhaust heat from a conventional combustion turbine and an ORC bottoming cycle to vaporize and superheat the stored air that has been pressurized, which is subsequently expanded to atmosphere through a turbine. During the development of the cycle, it has been identified that the main technologies to advance the cycle are the ORC bottoming cycle machinery and the coupled operation between the liquified air subsystem and the ORC subsystem. This paper presents modeling and simulation of the LACC that involves ORC conditions that fall outside the operating regime of more common applications. Due to the low temperatures of liquified air, the ORC system operates on the order of -70°C for the pump, and the turbine has a pressure ratio around 40. The conceptual design of a demonstration system has been developed that focuses on these challenges in order to advance the overall system.
DEVELOPMENT OF A GENERALISED LOW-ORDER MODEL FOR TWIN-SCREW COMPRESSORS
Kaufmann, Florian; Irrgang, Ludwig; Schifflechner, Christopher; Spliethoff, Hartmut
Twin-screw compressors (TSC) are commonly used in heat pump processes due to their robustness and flexibility. They exhibit two core properties, i.e. the swept volume and the built-in volume ratio (BVR), which heavily influence their capacity limits and off-design efficiency. Several models of vastly different computational costs have been proposed in literature to calculate the performance of TSCs. For applications that rely on large amounts of simulation runs, the computational cost of the compressor model becomes an essential factor. This work presents a new low-order model, which can accurately predict a TSC’s behaviour. First, a semi-empirical model from literature is slightly adapted and used to generate performance data for a large operational field. Then a polynomial model with a linearisation for high pressure ratios is fitted to this data. The model uses the external pressure ratio and volumetric compressor inlet flow to calculate isentropic efficiency and compressor speed. Both input parameters are normalised with a reference flowrate (calculated from the swept volume) and the BVR, respectively. This results in a generalised model of low numerical cost, which can be used for explorative studies independent of the specific machine size and BVR. A gain in computational speed by a factor of roughly 375 is achieved compared to a semi-empirical reference model. The model displays good predictive accuracy when used to predict the performance of machines with similar BVRs, but different sizes. When there is a difference in size and BVR, the prediction accuracy is still reasonable but significantly declines for small pressure ratios. Nevertheless, the proposed new approach extends the state-of-the-art by introducing a low-order model, which combines the advantages of low computational cost, good accuracy, physically correct predictions over a wide operational range and scalability to different machine capacities and BVRs.
DOUBLE-STAGE ORC SYSTEM BASED ON VARIOUS TEMPERATURE WASTE HEAT SOURCES OF THE NEGATIVE CO2 POWER PLANT
Stasiak, Kamil; Ziółkowski, Paweł; Mikielewicz, Dariusz
Analysed is the modification of the thermodynamic cycle with the negative CO2 power plant concept by its combination with the organic Rankine cycle. The analysed power plant operates on a gas produced from the gasification of sewage sludge. The negative emission term comes from the aggregated CO2 balance resulting from the capture of the CO2, while the sewage sludge is one of the inevitable environmental sources of CO2 to be avoided. In short, the principle of this power plant is to produce electrical power by converting sewage sludge fuel as the substrate to CO2 as a product, which is an intricate process in-between, with many opportunities for waste heat recovery. There are four main sources of waste heat in such a system. One is the drying process of the producer gas, which must be properly cooled from the high temperature after gasification to the temperature at which no moisture is present in the gas. In the wet combustion chamber, the syngas is oxy-combusted under high temperatures with water injection to control the combustion temperature. This mixture is then expanded in the gas turbine. The mixture leaving the turbine is a major source of heat supply for the ORC. The second heat source is a mixture of steam and gas – a major supply of heat source for ORC. Next, the mixture is undergoing separation process in a cyclone separator and then the CO2 (with a small content of moisture) is directed to carbon capture unit. The CO2 is then compressed in a system that requires intercooling. Due to the wide range of temperatures of the listed waste heat sources, the double ORC combination is investigated. The combined ORC cycle is connected by a cascade heat exchanger. The ORC fluid parameters are selected computationally to match the temperature distribution lines. The power plant processes are simulated in the steady-state process simulator using the most accurate equations of state from the literature. Optimum operating conditions of the ORC integrated power plant are obtained through optimization techniques.

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