1,721,048 research outputs found
Multi-effect distillation plants for small-scale seawater desalination: thermodynamic and economic improvement
No full textThe growing global demand for fresh water coupled to the increasing interest in renewable energies and waste heat recovery has resulted in flourishing attention to the multi-effect distillation process for seawater desalination. The low operating temperature makes this technology attractive in the case of low temperature heat sources such as geothermal, solar or waste heat recovery. The low energy density of these heat sources requires small-scale desalination systems whose layout and operation may differ from large-scale plant. In this work, new plant configurations for a small-scale multi-effect distillation system are proposed and analyzed from a thermodynamic and economic point of view. Each configuration tends to better exploit the energy content of the various streams by improving heat recovery, according to an increasing layout complexity. These configurations were studied in two layouts, differing in the way seawater and brine fed the various effect. The feed mass flow in each effect was varied to maximize the recovery ratio by imposing the maximum salt concentration in the brine related to calcium sulphate precipitation. Numerical simulations were conducted in Aspen Plus environment by varying the top brine temperature with a fixed bottom brine temperature of 40 °C. The electrolyte non-random two liquid equation of state was adopted to evaluate saltwater properties and an inter-model comparison with a validated algebraic model was carried out. The configurations implementing seawater preheating increased the performance ratio up to 10% due to the better exploitation of the energy content of distillate streams. The proposed solutions with the maximization of the recovery ratio demonstrated to be cost-effective with respect to the base multi-effect distillation configuration when thermal energy cost became relevant. In the case of negligible thermal energy cost (waste heat recovery) the base configuration was the preferable solution in terms of water cost, despite the lower performance ratio
Dynamic modelling of a low-concentration solar power plant: A control strategy to improve flexibility
Get PDFThis 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
Electrical production of a small size Concentrated Solar Power plant with compound parabolic collectors
Get PDFThe use of the solar energy for electricity or useful heat generation has been extensively investigated as an alternative to fossil fired energy conversion. Particularly in the last decade, many studies have been carried out on Concentrated Solar Power (CSP) which was developed worldwide with Spain acting as the leading country in this field. Concentrating solar energy requires complex mirror systems which continuously move to track the sun. In comparison with flat mirrors, Parabolic Through Collectors (PTCs) have allowed to reduce costs, but they still remain quite an expensive solution. Instead, compound parabolic collectors (CPCs) are able to collect a higher fraction of both the direct and the diffuse radiation, although they have a lower efficiency at high temperature. Moreover, at least within certain limits, they do not require a tracking system. Their employment is therefore suited for the collection of medium temperature heat (up to 200°C) and is useful for the reduction of the installation cost of Concentrated Solar Power (CSP) heating/cooling and energy generation systems. Small size plants (10-50kW) were studied in this paper since they are more likely to be realized due to their smaller initial investment cost and to the capability of being installed on the roof of existing buildings. While the Organic Rankine Cycle (ORC) solution is well established to be the optimal for small size, distributed generation plants, the technology of the expansion device is still to be defined for the investigated installed power range. Accordingly to previous studies, an expansion device based on the Wankel mechanism was employed. Based on these considerations and prior to more detailed analyses, a study of the annual energy production of a small scale ORC power plant using CPCs as a heat source and a volumetric machine as an expansion device was carried out. The influence of the thermodynamic cycle parameters, the working fluid, the concentration and the tilt angle of the collectors on the electrical energy production were taken into account. The thermal module power output, the expansion device isentropic efficiency and the overall efficiency were evaluated by means of a numerical model developed within the simulation tool AMESim v.12.0. The aim of this work is to provide a contribution in the assessment of the optimal configuration of such kind of plants in terms of collectors concentration and tilt angle on one hand, and thermodynamic parameter of the thermal module on the other. The annual electricity production was used as a criterion of comparison among the various parameters combinations. The number of operating hours per year was also taken into account for the sake of ensuring a regular production of energy. A selection of commercial solar tubes for the realization of the solar field was carried out and the optimal configuration for both the solar field and the thermal module was found. The results of this study are encouraging and constitute the basis for the development of future analyses
Optimization of supercritical CO2 cycles for biomass cogeneration for industrial applications
No full textThe industrial sector emitted 9.0 gigatonnes of CO2 in 2022, representing 25 % of global emissions, highlighting the urgent need for decarbonisation strategies. Biomass-fuelled Combined Heat and Power (CHP) systems offer a promising pathway to reduce primary energy demand and industrial emissions. This study evaluates three supercritical CO2 (sCO2)-based cogeneration architectures compared to standard biomass-fuelled Rankine cycles to assess their potential for enhanced energy and economic performance. The systems were designed to serve an industrial load of 10 ton/hour of 16-bar steam and 8 MW of electricity, typical of a tissue paper mill. Key parameters, including electrical efficiency, primary energy savings (PES), capital expenditures (CAPEX), and levelized cost of electricity (LCOE), were optimized and analyzed. Results demonstrate that at a turbine inlet pressure of 300 bar, sCO2 cycles achieve a PES of up to 13.8 %, significantly outperforming the Rankine cycle (1.9 %). CAPEX for sCO2 systems ranges from €32 million to €40 million, comparable to or lower than Rankine cycles of similar size. Biomass consumption is reduced by 3,500–4,000 tons annually, as reflected in LCOE values of €0.086–€0.095 per kWhel. These findings suggest that sCO2 cycles are a viable and efficient alternative for biomass-based CHP systems, particularly in biomass-scarce scenarios
Feasibility analysis of coupling an ORC to a mGT in a biogas plant
Get PDFAn 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
Technical and economic analysis of organic flash regenerative cycles (OFRCs) for low temperature waste heat recovery
Get PDFOrganic 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
Dynamic modeling of a solar ORC with compound parabolic collectors: Annual production and comparison with steady-state simulation
Get PDFIn this paper the dynamic behavior of a small low-concentration solar plant with static Compound Parabolic Collectors (CPC) and an ORC power unit with rotary volumetric expander has been analyzed. The plant has been simulated in transient conditions for a year-long operation and for three different sites respectively located in northern, central and southern Italy, in order to evaluate the influence of the latitude on the production. Hourly discretized data for solar radiation and for ambient temperature have been used. The adoption of a sliding-velocity control strategy, has allowed to operate without any storage system with a solar multiple (S.M.) of 1, reducing the amplitude of the solar field and simplifying the control system. Different collectors tilt angles and concentration factors, as well as thermodynamic parameters of the cycle have been tested, to evaluate the optimal working conditions for each locality. Results highlighted that specific production increased with the concentration ratio, and with the decrease of latitude. The comparison with the steady-state analysis showed that this type of control strategy is suited for those configurations having a smaller number of collectors, since the thermal inertia of the solar field is not recovered at all during the plant shut-down phase
Experimental and Numerical Analysis of the Valve Timing Effects on the Performances of a Small Volumetric Rotary Expansion Device
Get PDFSingle 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
Dynamic control strategies for distributed microgeneration and waste heat recovery power plants
Get PDFIn 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
Numerical and experimental analysis of the intake and exhaust valves of a rotary expansion device for micro generation
Get PDFThe 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
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