control variables and strategies for the optimization of a whr orc system

Abstract In this paper, the dynamic behavior of a WHR (Waste heat Recovery) ORC system with positive displacement rotary expander has been analyzed and an optimal control strategy was defined to increase the system efficiency and flexibility. Input heat flow was varied in time by varying the heat source mass flow and inlet temperature, according to two different load cycles. Three different control strategy were implemented and compared. The first strategy was sliding pressure, where expander speed was kept constant and system power output was controlled by evaporator pressure variations. The second control strategy was sliding velocity, where expander speed was varied to keep the evaporating temperature to a constant set point value. The third control strategy was a combination of sliding-pressure and sliding velocity: the set point of evaporating pressure varied according to a suitable function of easily measurable variables, with the objective of maximizing system efficiency. A function of the heat source admission temperature and of the product of the volume flow rate by the admission pressure was used to define the evaporating temperature set point. This function was evaluated in steady-state conditions from the model of the plant. Results showed that the last control strategy, maximized system efficiency and flexibility, and that the control parameter chosen were suitable to drive the set point variation

Small-scale desalination plant driven by solar energy for isolated communities

In the last years, an increasing number of countries has been affected by water shortage. Seawater desalination driven by solar energy, which is usually available in arid regions, might be a solution to satisfy the freshwater demand. In this study, the feasibility of a stand-alone multi-effect desalination (MED) plant driven by solar energy for an isolated community was studied. The system was made up of a solar field, a MED unit, and a thermal storage that mitigated solar energy fluctuations. Simulations with different top brine temperature and inclination and number of the solar panels were carried out in Matlab and Aspen Plus on an hourly basis by considering one typical meteorological year for ambient temperature and solar radiation. Two different sources of electrical energy were considered: A photovoltaic (PV) field and a diesel generator. The results were compared from an energetic and economic point of view, by considering the adoption of plastic as a material for MED heat exchangers. The maximum water production was obtained with December as the design month. Polytetrafluoroethylene heat exchangers allowed the cost of water to be reduced up to 9.5% compared to conventional exchangers. The lowest cost of water (7.09 $/m3) was obtained with September as the design month and a tilt angle of 45◦ with the PV field as the electrical power source

experimental results of a wankel type expander fuelled by compressed air and saturated steam

Abstract The work presented in this paper deals with the experimental tests which were carried out on a prototype of a rotary volumetric expansion device based on the Wankel mechanism. This expansion device is addressed to small size power plants (in the range 5-50 kW) for distributed micro-generation using various sources of thermal energy, such as sun, biomass and waste heat. The prototype was built using an internal combustion Wankel engine, employing the shaft, the rotor, the bearings, while the statoric case was newly built on the design of the University of Pisa. Firstly, the tests were carried out with the compressed air produced by a compressor, then the prototype was fed with the saturated steam produced by a biomass boiler. In the first case, the exhaust back-pressure was the atmospheric one, in the second case vacuum conditions were employed thanks to a condenser. The inlet pressure was between 4 and 8 bar. The results showed the capability of the prototype to rotate regularly at 3000 rpm, and allowed the validation of numerical models presented in previous papers. Moreover, the expansion device showed the capability of developing the expected power

Dynamic control strategies for distributed microgeneration and waste heat recovery power plants

In this paper the modeling activity on a waste heat recovery microgeneration ORC plant is presented together with the results of the application of two different load diagrams and three different control strategies. The overall energy production and the average efficiency were compared and a proper control strategy was evaluated to optimize the energy recovery process as well as the dynamic response of the plant

poly generation capability of a biogas plant with upgrading system

Abstract Biomass, together with other renewable sources, is increasingly used to provide energy to minigrids and distributed generation systems. Particularly, biogas production seems an interesting solution as it can be used to produce electricity, heat and bio-methane (through an upgrading system). In addition, biogas can be relatively easily stored in gasometers to compensate for small request variations. On the other hand, the amounts of heat, electricity and bio-methane produced are strictly dependent one on the others. A poly-generation scenario was considered starting from an existing case study made up of a digester, a 600kWel micro gas turbine and an upgrading system for bio-methane production. An off-design system simulation was carried out to analyze the energy and mass fluxes between plant components as a function of the fraction of the biogas sent to the upgrader. The constraints and relations between heat, electricity and bio-methane production were extensively analyzed. Results show that this system can be a versatile poly-generation unit

Organic Flash Cycles: Off-design behavior and control strategies of two different cycle architectures for Waste Heat Recovery applications

Off-design characterization of energy systems has become interesting, especially for waste heat recovery application, where the heat source temperature and mass flow rate can vary over time. Low-grade heat is generally converted into power through ORC modules: the problem of the constant temperature evaporation lead to the definition of alternative architectures, among which organic flash cycles. In this work, the off-design behavior of two different architectures of single-stage Organic Flash Cycles has been analyzed in steady-state condition, for small scale waste heat recovery (WHR) purposes. The main difference between the two architecture is the regeneration: in the first architecture (Single-Stage Organic Flash Cycle SS-OFC), the liquid of the flash evaporator, after lamination is mixed with the vapor from the expander and then sent to the condenser; in the second architecture Single-Stage Organic Flash Regenerative Cycle, SS-OFRC, the liquid from the flash evaporator is mixed with the liquid from the condenser, to regenerate the cycle. The most appropriate fluid for the two cycles was selected from a list of sixteen fluids with the objective of minimizing volume flow rates and maximizing the system efficiency and i-Pentane was chosen. For the off-design behavior, a rotary volumetric expander derived from a Wankel engine was considered, taking into account the performance variation of the device at various rotating speed and pressure ratios. Three different control strategies were considered and compared in off-design analysis for both the cycle architectures: sliding-pressure, in which the expander speed was constant and flash pressure varied with the load; sliding-velocity, in which the load was controlled by the speed variation of the expander and flash pressure was retained constant; combined strategy in which the expander speed was varied to drive the flash pressure according to a function which maximized the system efficiency. Results showed that the efficiency of the two cycles was similar in all the operating field whatever was the control strategy considered: SS-OFRC demonstrated a better behavior at low temperatures of the heat source (<170 °C), while SS-OFC had a better efficiency at higher temperature. The maximum absolute efficiency difference in off-design conditions between the two cycles was lower than 0.3%. SS-OFRC however had a wider field of operation than SS-OFC, due to the better flexibility of this type of cycle. As for the control strategy, with both the architectures, combined strategy maximized the system efficiency and flexibility for every temperature and mass flow rate of the heat source considered

Extensive techno-economic assessment of combined inverted Brayton – Organic Rankine cycle for high-temperature waste heat recovery

This work is a techno-economic study of the combination of inverted Brayton cycle, and organic Rankine cycle (combined IBC-ORC) applied for high-temperature waste heat recovery (WHR) of the engine exhaust energy. In IBC, exhaust gases expand to subatmospheric pressure in the turbine, transmit heat residuals to ORC, and restore pressure to 1 atm in the compressor. The system is analysed in the Aspen Hysys software in design conditions at the case study of 1.4 MW gas-fueled internal combustion engine as a high-temperature waste heat source (470–570 °C). Firstly, the paper shows the performance of the system optimised for different ambient temperatures. The role of water condensation contained in flue gas is emphasised for these bounds. Then, the paper presents a multi-objective optimisation illustrated by Pareto fronts for the objective functions of system electric efficiency and levelized cost of energy (LCOE) in the mentioned range of exhaust temperatures. TOPSIS-based Pareto-front analysis results in recommendations of the best sets of cycle parameters in this trade-off. For exhaust temperatures 470 °C, 520 °C, and 570 °C, optimal configurations identified via TOPSIS methodology demonstrate 10.8%, 12.1%, 13.3% efficiencies with LCOE equal to 185.5 $/MWh, 162.4$/MWh and 146.1 $/MWh correspondingly

Technical and economic analysis of organic flash regenerative cycles (OFRCs) for low temperature waste heat recovery

Organic Flash Cycles (OFCs) can improve the overall efficiency of waste heat recovery or geothermal systems due to a better match of the hot and cold heat transfer curves. However, the lower mean temperature difference between the heat transfer curves implies larger exchanger areas and therefore higher heat exchanger costs. In order to reduce the exchanger size, a new cycle configuration is introduced in this paper, consisting in a new type of organic flash regenerative cycle (OFRC) for heat source temperatures in the range 80–170 °C. The regeneration allows to recover part of the enthalpy of the liquid phase from the flash evaporator increasing the temperature of the liquid at the exchanger inlet, thus reducing the exchanger size. The thermodynamic performance of OFRCs are practically the same as of the OFC, but the unit cost of the system per kW installed power can be 20% lower. A variety of working fluids was tested and results have shown that long molecular chain alkanes provide the best thermodynamic efficiency, but those fluids have the main drawback of a low vapor density, resulting in very large expansion devices and condensers. R601a is the working fluid featuring the best tradeoff between thermodynamic efficiency and components size in the heat source temperature range between 80 °C and 170 °C. The comparison of the OFRC with conventional ORCs has shown the thermodynamic superiority of the OFRC with every tested fluid. Finally the cost analysis has highlighted that OFRCs specific cost has the same magnitude as ORCs for mini and micro scale plants

Techno-economic analysis of combined inverted Brayton – Organic Rankine cycle for high-temperature waste heat recovery

Many practical cases with waste heat recovery potential such as exhaust gases of reciprocating engines, cement kilns or heat-treating furnaces, are nowadays often integrated with organic Rankine cycle to convert waste heat to the mechanical power. However, when dealing with high-temperature waste heat, organic Rankine cycle faces efficiency limit due to the physical properties of the working and thermal fluids. That gives room for further enhancement of the waste heat recovery technologies via the investigation of different non-conventional schemes as one of the possible ways. In the present work, a system introducing the combined inverted Brayton plus organic Rankine cycle is under investigation. Aspen Hysys models of both conventional organic Rankine cycle and combined cycle were designed, orienting on waste heat recovery from the heavy-load gas-fueled reciprocating engine exhaust. In this way, the performance of the combined scheme was benchmarked versus the conventional organic Rankine cycle. An assessment of the organic Rankine cycle working fluids was provided, and pentane has shown the best thermodynamic performance. The study on inverted Brayton cycle defined the remarkable effect of the water condensation in the gas duct on the inverted Brayton cycle performance. Finally, both thermodynamic and economic optimizations of the models were conducted, setting the stage for the comparison of solutions. Results have shown the 10% advantage of the combined scheme over organic Rankine cycle in generated power and system efficiency. The levelized-cost-of-energy-based optimization for variable capacity factors has highlighted above 6% advantage of the investigated solution. The analysis of the sensitivity from machines’ efficiencies and heat exchangers’ pinches has shown that with some sets of parameters, the studied scheme may concede to the organic Rankine cycle

Techno-economic analysis of combined inverted Brayton – Organic Rankine cycle for high-temperature waste heat recovery

Many practical cases with waste heat recovery potential such as exhaust gases of reciprocating engines, cement kilns or heat-treating furnaces, are nowadays often integrated with organic Rankine cycle to convert waste heat to the mechanical power. However, when dealing with high-temperature waste heat, organic Rankine cycle faces efficiency limit due to the physical properties of the working and thermal fluids. That gives room for further enhancement of the waste heat recovery technologies via the investigation of different non-conventional schemes as one of the possible ways. In the present work, a system introducing the combined inverted Brayton plus organic Rankine cycle is under investigation. Aspen Hysys models of both conventional organic Rankine cycle and combined cycle were designed, orienting on waste heat recovery from the heavy-load gas-fueled reciprocating engine exhaust. In this way, the performance of the combined scheme was benchmarked versus the conventional organic Rankine cycle. An assessment of the organic Rankine cycle working fluids was provided, and pentane has shown the best thermodynamic performance. The study on inverted Brayton cycle defined the remarkable effect of the water condensation in the gas duct on the inverted Brayton cycle performance. Finally, both thermodynamic and economic optimizations of the models were conducted, setting the stage for the comparison of solutions. Results have shown the 10% advantage of the combined scheme over organic Rankine cycle in generated power and system efficiency. The levelized-cost-of-energy-based optimization for variable capacity factors has highlighted above 6% advantage of the investigated solution. The analysis of the sensitivity from machines’ efficiencies and heat exchangers’ pinches has shown that with some sets of parameters, the studied scheme may concede to the organic Rankine cycle