1,720,991 research outputs found
Experimentally-validated models for the off-design simulation of a medium-size solar organic Rankine cycle unit
Get PDFOrganic Rankine Cycle is an efficient and reliable technology for the thermal-to-electricity conversion of low-grade heat sources but the variability in boundary conditions often forces these systems to operate at off-design conditions. The development of reliable models for the performance prediction of organic Rankine cycle power systems under off-design conditions is therefore crucial for system-level integration and control implementation. In this paper, a mathematical model for the evaluation of the expected performance of organic Rankine cycle power units in a large range of operating conditions based on experimental data collected in a medium-size solar organic Rankine cycle power plant is presented. Two different empirical approaches for the performance prediction of heat exchangers and machines, namely, constant-efficiency and correlated-based approaches, are proposed and compared. In addition, empirical correlations based on experimental data are proposed for the preliminary assessment of the energy demanded during the start-up phase and the corresponding duration. Results demonstrate that a good achievement in terms of accuracy of the model and reliability of the simulation performance can be obtained by using a constant-efficiency approach, with average errors lower than 5% and 2.5 K for the expected net power and outlet oil temperature respectively. The use of polynomial correlations leads to a more accurate estimation of the performance parameters used for evaporator and the turbine (in particular the evaporator heat effectiveness and the isentropic and electromechanical efficiency for the turbine), which strongly affect the main output variables of the model and, at the same time, are remarkably influenced by the operating conditions. A reduction in the average error in the prediction of the net power and outlet temperature of the heat transfer fluid to about 4% and 1.5 K respectively is therefore achieved by this approach. Average errors of 18.5% and 12.5% are achieved for the start-up time and the corresponding energy absorbed, respectively. Although the results obtained in terms of accuracy could be improved, these correlations can give an initial indication about the duration and energy required during this phase
Optimal Management of Reversible Heat Pump/Organic Rankine Cycle Carnot Batteries
Get PDFIn the view of reducing the global greenhouse gas emissions, it becomes fundamental to exploit the renewable energy sources at their maximum potential by developing effective strategies for their flexible use. Among the available solutions to realize these strategies are the electric energy storage including the innovative pumped thermal energy storage technology (included in the Carnot battery concept). This can become very interesting in these applications where different energy flows must be handled (both electric and thermal), thanks to the possibility of adding the contribution of a waste heat source, in a thermally integrated energy storage. However, despite the several advantages, the stateof-the-art still lacks experiments and investigation of efficient control strategy for the Carnot battery when inserted into the process. As an original contribution to the current literature, this paper presents the off-design model of a reversible organic Rankine cycle (ORC)/heat pump (HP) Carnot battery configuration with the aim of employing it to simulate the performance of such system and discuss its optimal management when inserted into a generic process. An existing reversible HP/ORC kW-size prototype is considered as reference and its optimal control in both HP and ORC mode under different boundary conditions is assessed
Experimental validation of a multiple model predictive control for waste heat recovery organic Rankine cycle systems
No full textWaste heat recovery systems are today considered as a valuable solution to increase energy efficiency of industrial applications and heavy-duty vehicles, as it uses a thermodynamic organic Rankine cycle system to recover the heat losses to produce electrical or mechanical power. Optimal performance of such machines is often achieved at conditions where complex time-varying nonlinear dynamics are encountered, making the automatic control strategy a fundamental element to maximise the energy efficiency. In this paper the development of a multiple model predictive controller suitable for industrial implementation is presented, and its effectiveness is experimentally validated for the task of maximising output power of a 11kWel small-scale ORC power unit used in a waste heat recovery application. The main advantage of the proposed controller is the possibility to use different model structures to describe local dynamics without increasing complexity of the optimisation problem. Additionally, experimental results illustrate that the entire operating range of the system might be classified in two regions, a quasi-linear and a highly nonlinear region for ‘high’ and ‘low’ superheating degrees respectively. Closed-loops tests lead to the conclusion that a single linear model predictive controller might only be used under suboptimal operation of low power production (on the quasi-linear region for ‘high’ superheating), otherwise leading to poor performance or even instability. Alternatively, the proposed strategy keeps the cycle stable over the entire range of conditions and allows to increase the net electrical energy produced by at least 6%, even under drastic waste heat source variations, when operating closer to the minimum allowed superheating degree
Control strategy and performance of a small-size thermally integrated Carnot battery based on a Rankine cycle and combined with district heating
Get PDFTo encourage decarbonization and promote a widespread penetration of renewable energy sources in all energy sectors, the development of efficient energy storage systems is essential. Interesting grid-scale electricity storage technologies are the Carnot batteries, whose working principle is based on storing electricity in the form of thermal energy. The charging phase is performed through a heat pump cycle, and the discharging phase is conducted through a heat engine. Since both thermal and electric energy flows are involved, Carnot batteries can be adopted to provide more flexibility in heat and power energy systems. To this aim, efficient scheduling strategies are necessary to manage different energy flows. In this context, this work presents a detailed rule-based control strategy to schedule the synergetic work of a 10-kWe reversible heat pump/organic Rankine cycle Carnot battery integrated to a district heating substation and a photovoltaic power plant, to satisfy a local user's thermal and electric demand. The coupling of a Carnot battery with a district heating substation allows for shaving the thermal demand peaks through the thermal energy stored in the Carnot battery storage, allowing for a downsizing of the district heating substation, with a considerable reduction of the investment costs. Due to the multiplicity of the involved energy flows and the numerous modes of operation, a scheduling logic for the Carnot battery has been developed, to minimize the system operating costs, depending on the boundary conditions. To investigate the influence of the main system design parameters, a detailed and accurate model of the Carnot battery is adopted. Two variants of the reference system, with different heat pump cold source arrangements, are investigated. In the first case, the heat pump absorbs thermal energy from free waste heat. In the second case, the heat pump cold source is the return branch of the district heating substation. The simulation results show that, in the first case, the Carnot battery allows the downsizing of the district heating substation by 47 %, resulting in an annual gain of more than 5000 €. About 70 % of the economic benefit is due to the possibility of reducing the power size of the district heating substation, which can be from 300 to more than 500 kW. The payback period is estimated to be lower than 9 years, while in the second case, the Carnot battery is not able to provide a gain. Eventually, the influence of some parameters, such as the photovoltaic power plant surface, the storage volume, the electricity price profile and the reversible heat pump/organic Rankine cycle specific investment cost, on the techno-economic performance of the system, is investigated through a wide sensitivity analysis. According to the results, the photovoltaic panels surface does not significantly affect the economic gain, while the storage capacity strongly affects the system scheduling and the operating costs. Indeed, it is possible to identify that 13 m3 is the size of the storage volume that minimizes the payback period to 8.22 years, for the considered application. An increase in the electricity price without an increase in the thermal energy price leads to a decrease in economic gain because the benefit brought by the downsizing of district heating is less significant on the economic balance. The specific investment cost of the reversible heat pump/organic Rankine cycle does not influence the operating cost; thus, it does not change the Carnot battery management, nor the economic gain. The specific investment cost affects the payback period, which increases from 8.6 years for a specific cost of 2000 €/kWe to 15.7 years for a specific cost of 5000 €/kWe
Nonlinear identification and control of Organic Rankine Cycle systems using sparse polynomial models
No full textDevelopment of a first principles model of a system is not only a time- and cost-consuming task, but often leads to model structures which are not directly usable to design a controller using current available methodologies. In this paper we use a sparse identification procedure to obtain a nonlinear polynomial model. Since this is a NP-hard problem, a relaxed algorithm is employed to accelerate its convergence speed. The obtained model is further used inside the nonlinear Extended Prediction Self-Adaptive control (NEPSAC) approach to Nonlinear Model Predictive Control (NMPC), which replaces the complex nonlinear optimization problem by a simpler iterative quadratic programming procedure. An organic Rankine cycle system, characterized for presenting nonlinear time-varying dynamics, is used as benchmark to illustrate the effectiveness of the proposed combined strategies
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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