Off-Design behavior and control strategies of small scale cycles with organic fluids.

Energy saving is one of the priority of our society, involved in worldwide challenges such as climate change, energy resource depletion, conflicts between nations and economic crisis. This objective must be achieved both by energy end-users through an increased awareness about wasted energy and the use of high efficiency devices, and by energy producers. In the electric energy production field, the possibility of decentralizing the production allows to reduce the plant size, gives the possibility to end users to become independent energy producers, to exploit renewable energies and to increase the efficiency of the systems by recovering low-temperature heat. Because of the stochastic nature of some of those energy sources, it is expected that future energy systems will be forced to become increasingly flexible, in order to deal with this challenge. In this scenario, Organic Rankine Cycles (ORC) are one of the fastest growing technologies, due to their ability of exploiting low temperature heat sources, typical of renewable energies and of waste heat streams, to their simplicity and low costs. The aim of this doctoral thesis is to contribute to the knowledge of the modeling and optimization of small scale organic cycles systems for low temperature applications, such as low concentration solar application, waste heat recovery and micro-geothermal systems. Particularly various control strategies and control parameters have been defined to improve efficiency, flexibility and system management, both for organic Rankine cycles and for organic flash cycles (OFC), which can be considered an advanced architecture of the basic Rankine cycle. The use of a positive displacement rotary engine, which is particularly suitable for small scale cycles under variable working conditions, due to its high simplicity and flexibility, has allowed to simulate the operation of a small scale solar ORC, according to a sliding-velocity control strategy and without any thermal storage, reducing in this way the number of required solar collectors and simplifying the system layout. The dynamic analysis of the plant highlighted the effect of various transient phenomena which caused a variation in the prediction of annual plant production with respect to steady-state approach. The comparison of sliding-velocity and sliding-pressure control strategy of a small scale Waste Heat Recovery (WHR) ORC, has highlighted the ineffectiveness of both of them in the case of highly variable heat sources and has lead to the definition of an optimal combined sliding-pressure and velocity control strategy, controlled by easy measurable variables: the optimized function which allowed operation according to this control strategy could be extrapolated, in steady-state conditions, both from system data and even from expander data through a simple approximation of the heat exchangers off-design behavior. Dynamic simulations have confirmed that both methods lead to better results in terms of system flexibility, efficiency and safety. One of the major problems of ORCs is the constant temperature evaporation phase, which increases entropy production during the isothermal heat transfer and in the case of sensible WHR systems, keeps the exhaust temperature of the heat flux high. Organic flash cycles could be an alternative solution to bypass this problem: however, the architectures proposed in the literature for low temperature WHR systems have the drawback of high specific costs. A new regenerative architecture with the same thermodynamic performance of the original architecture has been defined, with the result of a decrease in systems cost, leading the specific cost of the main component of the cycle to be equal to that of basic ORC systems, in the case of very small scale applications. The off-design analysis comparison of single flash cycle with and without regeneration has highlighted the higher flexibility of the regenerative solution, and the possibility of adopting an optimal combined sliding-pressure and velocity control strategy, acting on the expander speed and on flash pressure, in a similar manner as in ORC systems

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. Keywords: Inverted Brayton cycle, Organic Rankine cycle, Waste heat recovery, High-temperature exhaust, Techno-economic analysi

Experimental and Numerical Analysis of the Valve Timing Effects on the Performances of a Small Volumetric Rotary Expansion Device

Single stage expansion devices are currently studied for small scale size power plant, often in combination with Organic Rankine Cycles for the employment of solar, geothermal, biomass or waste heat energies. A volumetric rotary single-stage expander was chosen in this study as expansion device for such type of plants. Its main characteristics and performances are discussed as a function of both the working conditions (fluid type, inlet temperature) and the working parameters (rotating speed, admission and recompression grades, valves advance). These analyses were carried out with numerical and experimental techniques. The analysis of the effects of the working condi- tions on the expander performances was carried out through a numerical model created with the simulation tool AMESim. At the same time, a prototype was built and experimented with compressed air to validate the model used by means of air mass flow rate, torque and indicated cycle. This way the isentropic and mechanical efficiency are discussed. The validation of the model was carried out by comparison with the experimental data collected at the engine test bench by operating the engine with compressed air. The indicated cycle, the air mass flow rate and the delivered torque were used as parameters of comparison. Moreover an experimental analysis at the fluid dynamic bench was carried out to validate the numerical 3D CFD model of the valves. The part load performances of this expansion device were studied by hypothesizing different control strategies and comparing them in terms of efficiency reduction respect to the design point. The influence of valves advance was also discussed

feasibility analysis of coupling an orc to a mgt in a biogas plant

Abstract An increasing interest is devoted to biogas plants as they might play a key role in the reduction of current fossil fuel consumption for power production. The main component of the plant is the anaerobic digester where the organic fraction of waste products is converted in a gas with high concentration of methane and carbon dioxide. This biogas is converted in power and heat in a cogeneration unit that may consist in a micro gas turbine or an internal combustion engine. Electric power is used to satisfy the plant internal need and the surplus is sold to the grid. A portion of the heat is used to keep the digester at a constant temperature as requested by the anaerobic digestion, the reaming is generally dissipated. This study focuses on the potential of using an Organic Rankine Cycle as a possible additional thermal user to reduce the amount of dissipated heat and increase the power production. The study is based on an existing biogas plant operating in the town of Viareggio (Italy) which will be equipped with a 600kWe micro gas turbine. The integration of the two systems was studied in detail to have high values of thermal energy recovery. A reference and a modified solution were simulated in AMESim by considering a yearlong period with actual ambient conditions. Off-design behavior of all the components was also included in the simulation. The results of the investigation showed that a thermal energy recovery up to 77% could be achieved. From the economic point of view, the plant modification for introducing the ORC system has a payback period lower than 6 years and an interesting profitability index

Dynamic modelling of a low-concentration solar power plant: A control strategy to improve flexibility

This paper deals with a dynamic analysis on a low concentration solar power plants coupled with Organic Rankine Cycles (ORC), which can be an alternative to PV systems because of their capability of providing a smoother electricity production due to their thermal inertia. At least within certain restraints, moreover they are able to exploit diffused solar radiation. The dynamic model of a plant with static Compound Parabolic Collectors and an ORC system, using a rotary volumetric expander, was developed using the simulation tool AMESim. All the main components of the plant are modelled: solar collectors field, heat transfer fluid circuit, heat exchangers and the ORC system. The plant response to the radiation of different days was analyzed to quantify the daily production and the trend of various plant parameters. Real ambient conditions were employed for the simulations by using data obtained by historical series. The results showed that the employment of a volumetric expansion device with variable rotating speed allows the plant to operate at different radiations and ambient temperatures without the need of any storage system or external heat sources. Results can be extended to other applications, such as low temperature waste heat recovery or geothermal systems

Numerical and experimental analysis of the intake and exhaust valves of a rotary expansion device for micro generation

The use of ORCs is growing in importance the last years, because this type of cycles permits to exploit energy sources which are characterized by low enthalpies (waste heat, low temperature geothermal, low concentration solar plants, etc.) and low installed power sizes (up to 50-100 kW). In this size range, volumetric machines are very attractive devices because they show better efficiencies than turbines. Volumetric expansion devices flow rate is not continuous as in turbines but on the contrary has a pulsating behavior and the effective flow area of the intake and exhaust ports plays an important role in determining the efficiency of such devices. In this paper an analysis regarding the influence of the intake and exhaust valves features on the performances of a rotary expansion device for micro generation derived from a Wankel engine is presented. The analysis which is presented in this paper was carried out by means of both numerical and experimental techniques. CFD simulations of two different types of valves were performed to obtain the values of discharge coefficient and, subsequently, the results were validated at the fluid dynamic test bench. These solutions were also evaluated by testing the prototype using compressed air as working fluid under various operating conditions (pressure ratio, rotating speed). The delivered torque, the air mass flow rate and the indicated cycle were taken into account for the evaluation of the influence of the valves shape and timing on the device performances

Technologies for energy recovery from waste biomasses: A study about Tuscan potentialities

Biomass is a form of renewable energy that can be used to provide high energy outputs, to support and in same case replace conventional fossil fuel energy sources. There are many kinds of energy conversion processes in relation both with biomass chemical - physical characteristics and with the form in which the energy is required. In this work the potentialities for energy recovery of the waste biomasses derived from the agro industrial activities of the Tuscan region (Italy) are analyzed: in particular waste derived from food crops (cereals, beet, sunflower, olive tree, citruses, vineyard, ...), zoo technical activities, and wood. The data obtained are examined to make a comparison between the various energetic and economic results employing different kind of energy systems commonly used in biomass energy conversion. The technologies analyzed are the thermochemical processes combustion, gasification, Fischer Tropsch (FT) diesel fuel production and the biochemical process of anaerobic digestion. Each process requires a proper energy conversion plant. For the above mentioned processes, the conversion plants hypothesized are: for the direct combustion the steam turbine plant, for the gasification the Integrated Gasification Combined Cycle (IGCC), for FT biodiesel the compression ignition engine and for anaerobic digestion the gas engine. These conversion technologies were analyzed also from an economical point of view. This analysis was carried out by taking into account costs (harvest and collection, eventually pre-treatment, transportation cost) and incomes (energy saving and sale. Finally the environmental impact is considered studying the avoided emission of CO2 in relation with the avoided use of fossil energy for the power production

Small scale ORC plant modeling with the AMESim simulation tool: Analysis of working fluid and thermodynamic cycle parameters influence

ORC plant transient modeling is an actual issue for the correct assessment of the size of the various components of the system especially when unpredictable fluctuations of the inlet thermal flux are to be considered. This work shows the modeling procedure of a small scale (10-50 kW) Waste Heat Recovery ORC plant which uses an innovative expansion device derived from a Wankel engine. The numerical model here presented was developed with the simulation tools AMESim and simulates the transient behavior of such a small scale system in all its main components: preheater, evaporator, expansion device and condenser. The aims of this work were to evaluate the suitability of the Wankel-derived mechanism to ORC systems and to establish its optimal working conditions for the employment in a low-grade heat recovery system. The application of several working fluids as well as of various operating conditions are presented in this paper. The analysis of the transient response of the plant is also presented with a particular attention to start up operations

Analysis of a Low Concentration Solar Plant with Compound Parabolic Collectors and a Rotary Expander for Electricity Generation

In the last decade many studies have been carried out on low temperature solar compound parabolic collectors (CPC) that are able to collect solar direct and diffuse radiation without the need of a tracking system, especially if coupled with small scale Organic Rankine Cycles for electricity and heat combined production. This paper presents a study on a thermal power plant that uses an expansion device driven with pressurized vapor generated with the heat collected by a CPC solar field. The numerical model of the expansion device was developed with the simulation tool AMESim v.12 and allowed the simulation of the indicated cycle of this machine for the evaluation of delivered power, isentropic efficiency and specific working fluid consumption. At the same time in this paper an analytical model of the evacuated solar CPC is presented for the evaluation of the collected heat as a function of sun incidence angle, external temperature, inlet carrier fluid temperature and mass flow rate. These two models were used in conjunction for the analysis of the electricity that may be generated as a function of the ambient and working conditions. This study was performed in steady state conditions and with several working fluids to evaluate the power that would be delivered by a given displacement expander rotating at a fixed speed. The results are given in terms of delivered power, thermal efficiency and amount of collecting surface needed for a given displacement expansion machine, calculated for several working fluids and different operating conditions

Feasibility analysis of bio-methane production in a biogas plant: A case study

A feasibility analysis, to assess the suitability of converting the biogas produced in an existing anaerobic digestion plant to bio-methane, was carried out. The case study plant was equipped with a micro-gas turbine co-generator. Several upgrading systems of different sizes were considered, to determine the most suitable configuration from a thermodynamic and economic point of view. For this purpose, a model of the whole plant that included digesters, a micro-gas turbine, a sludge line, heat transfer loops, and heat exchangers was developed. A steady-state simulation was performed by using the daily average conditions for the one-year long operation of the plant. The results highlighted that the feasibility depended on the amount of bio-methane produced, as this affected the performance of the cogeneration system and the balance between the costs and revenues. When large amounts of biogas are upgraded to bio-methane, the heat provided by the micro-gas turbine during the winter season is not sufficient to keep the digesters at the desired temperature and, therefore, natural gas integration is necessary. In addition, by increasing the upgrading unit size, the amount of electric energy purchased by the grid increases accordingly. An economic analysis showed that the optimal upgrading system size was strongly dependent on the bio-methane selling price