Virtual power plant models and electricity markets - A review

In recent years, the integration of distributed generation in power systems has been accompanied by new facility operations strategies. Thus, it has become increasingly important to enhance management capabilities regarding the aggregation of distributed electricity production and demand through different types of virtual power plants (VPPs). It is also important to exploit their ability to participate in electricity markets to maximize operating profits. This review article focuses on the classification and in-depth analysis of recent studies that propose VPP models including interactions with different types of energy markets. This classification is formulated according to the most important aspects to be considered for these VPPs. These include the formulation of the model, techniques for solving mathematical problems, participation in different types of markets, and the applicability of the proposed models to real case studies. From the analysis of the studies, it is concluded that the most recent models tend to be more complete and realistic in addition to featuring greater diversity in the types of electricity markets in which VPPs participate. The aim of this review is to identify the most profitable VPP scheme to be applied in each regulatory environment. It also highlights the challenges remaining in this field of study

Optimal short-term water-energy dispatch for pumping stations with grid-connected photovoltaic self-generation

Increases in the energy costs of irrigation water pumping facilities puts the economic sustainability of recent investments in the modernization of farms at risk. To address this problem, it is essential to apply renewable technologies for the production of electricity, and photovoltaic energy is particularly attractive due to its lower cost and recent technological advances. The aim of this research is to develop a mathematical techno-economic dispatch model that optimizes the hourly schedule of pumping equipment subject to electrical and hydraulic constraints to minimize the weekly operating costs of a real pumping station. The resulting model is formulated as a mixed-integer nonlinear programming problem that determines the optimal hourly combination of pumping equipment and available resources to meet water and energy needs. The proposed model comprises fixed and variable speed pumps, a grid-connected photovoltaic plant, and two water ponds for internal regulation and storage. The results verify that the combination of self-consumption photovoltaic facilities and variable speed drives make it possible to maximize the percentage of self-consumed energy up to 99.41% during the month with the highest demand for water. In this case, the pumping station reduces its energy costs by 21.56%, in addition to improving water management

Water-energy management for demand charges and energy cost optimization of a pumping stations system under a renewable virtual power plant model

The effects of climate change seriously affect agriculture at different latitudes of the planet because periods of drought are intensifying and the availability of water for agricultural irrigation is reducing. In addition, the energy cost associated with pumping water has increased notably in recent years due to, among other reasons, the maximum demand charges that are applied annually according to the contracted demand in each facility. Therefore, very efficient management of both water resources and energy resources is required. This article proposes the integration of water-energy management in a virtual power plant (VPP) model for the optimization of energy costs and maximum demand charges. For the development of the model, a problem related to the optimal operation of electricity generation and demand resources arises, which is formulated as a nonlinear mixed-integer programming model (MINLP). The objective is to maximize the annual operating profit of the VPP. It is worth mentioning that the model is applied to a large irrigation system using real data on consumption and power generation, exclusively renewable. In addition, different scenarios are analyzed to evaluate the variability of the operating profit of the VPP with and without intraday demand management as well as the influence of the wholesale electricity market price on the model. In view of the results obtained, the model that integrates the management of the water-energy binomial increases the self-consumption of renewable energy and saves electricity supply costs

Comparative assessment of different solar tracking systems in the optimal management of PV-operated pumping stations

The integration of photovoltaic energy in pumping systems is complex, and the technical constraints of hydraulic and pumping systems must be considered. Exploitation models that link energy management with water management are necessary to ensure the profitability of these investments. This research proposes the design and application of a mathematical model for optimal hourly operation of pumping equipment at the minimum cost for a pumping station with different configurations of self-consumption photovoltaic generation for one week, subsequently extended to an entire year. The proposed optimization problem is formulated as a mixed-integer nonlinear model. Findings of this paper indicate that a self-consumption photovoltaic plant with single-axis solar tracking can increase production by 33.4% and reduce operating costs by 28.9% compared to a fixed system. Therefore, more energy is self-consumed (81.6%), and a more efficient pumping operation is achieved. The use of a two-axis tracker improves photovoltaic production by 3.2% with economic savings of 4.8% compared to a single-axis tracker, but this difference is small considering its higher investment costs and technical complexity. As a result, the single-axis solar tracker is generally used in pumping stations to achieve efficient management and reduced operating costs

A virtual power plant optimal dispatch model with large and small-scale distributed renewable generation

Volatility and sharp increases in the price of electricity are serious economic problems in the primary sector because they affect modernization investments for irrigation systems in Spain. This paper presents a new virtual power plant (VPP) model that integrates all available full-scale distributed renewable generation technologies. The proposed VPP operates as a single plant in the wholesale electricity market and aims to maximize profit from its operation to meet demand. Two levels of renewable energy integration in the VPP were considered: first, a wind farm and six hydroelectric power plants that inject the generated electricity directly to the distribution network, and second, on-site photovoltaic plants associated with each of the electricity supply points in the system that are designed to prioritize self-consumption. The proposed technical-economic dispatch model was developed as a mixed-integer optimization problem that determines the hourly operation of distributed large-scale renewable generation plants and on-site generation plants. The model was applied to real data from an irrigation system comprising a number of water pumping stations in Aragon (Spain). The results of the VPP model demonstrate the importance of the technical and economic management of all production facilities to significantly reduce grid dependence and final electricity costs

Techno-economic model and feasibility assessment of green hydrogen projects based on electrolysis supplied by photovoltaic PPAs

The use of hydrogen produced from renewable energy enables the reduction of greenhouse gas (GHG) emissions pursued in different international strategies. The use of power-purchase agreements (PPAs) to supply renewable electricity to hydrogen production plants is an approach that can improve the feasibility of projects. This paper presents a model applicable to hydrogen projects regarding the technical and economic perspective and applies it to the Spanish case, where pioneering projects are taking place via photovoltaic PPAs. The results show that PPAs are an enabling mechanism for sustaining green hydrogen projects

Optimal scheduling and management of pumped hydro storage integrated with grid-connected renewable power plants

Pumped hydro-energy storage will become a fundamental element of power systems in the coming years by adding value to each link in electricity production and the supply chain. The growth of these systems is essential for improving the integration of renewables and avoiding dependence on fossil fuel sources, such as gas or oil. This paper presents the modeling and application of an optimal hourly management model of grid-connected photovoltaic and wind power plants integrated with reversible pump-turbine units to maximize the monthly operating profits of the energy system and meet electricity demand. The techno-economic dispatch model is formulated as a mixed-integer optimization problem. To assess the proposed model, it is applied to a Spanish case study system, and the results are obtained for an entire year. The combination of renewable energy and pumped hydro energy storage reduces energy dependence by decreasing energy costs by 27 % compared with a system without storage to satisfy the required electricity demand. The findings confirm that storage plays a key role in energy transition to ensure the security and stability of power systems with a higher share of renewable generation

Plantas virtuales de energía para la integración de fuentes renovables de generación distribuida en sistemas de demanda de agua y energía.

Las instalaciones de bombeo de agua para riego son grandes consumidores de energía eléctrica además de ser un complejo sistema de gestión de agua. España es el país de la Unión Europea con mayor extensión para el regadío con casi 4 millones de hectáreas, destacando Aragón con una superficie de 415.998 hectáreas. No obstante, la agricultura se está enfrentando a serios problemas. Por un lado, como consecuencia del cambio climático, los periodos de sequía son cada vez más frecuentes y la disponibilidad de agua para el regadío agrícola se está reduciendo. Además, España se sitúa como uno de los países de la Unión Europea con el mayor coste de electricidad, debido entre otras razones por los costes regulados que afectan especialmente a los consumos estacionales, como es el caso de los sistemas de bombeo de agua para riego que concentra su consumo principalmente entre los meses de mayo a septiembre. La obligación de contratación de la potencia eléctrica para todo el año sin la posibilidad de modificarla durante el mismo perjudica seriamente a este sector. Para reducir el coste de suministro eléctrico, las comunidades de regantes están invirtiendo en instalaciones de generación renovable.La rápida penetración de estas fuentes de generación en el marco de un mercado eléctrico cada vez más competitivo requiere de nuevas tecnologías y sistemas de operación para hacer frente a los nuevos retos técnicos y económicos derivados de la integración óptima de los recursos disponibles. Las plantas virtuales de energía aparecen, así como ejes claves para hacer posible esta integración. Dada la tendencia actual de generar energía de forma distribuida es primordial el control conjunto de las unidades de producción para conseguir el mayor rendimiento del sistema.Esta tesis doctoral realizada por compendio de publicaciones pretende ofrecer soluciones a los problemas planteados anteriormente. El objetivo principal de esta investigación es el estudio, desarrollo y aplicación de nuevos modelos de operación óptima integrada de la generación y el consumo de energía eléctrica junto con las infraestructuras de agua de los sistemas generales de regadío, integrando los recursos de producción eléctrica, la demanda horaria de electricidad y la gestión del agua mediante el modelado matemático de una planta virtual de energía que participa en el mercado eléctrico mayorista para maximizar el beneficio de operación.En primer lugar, se realiza una clasificación y evaluación de los trabajos publicados en los últimos años sobre el modelado de plantas virtuales de energía con participación en distintos tipos de mercados eléctricos. La clasificación se basa en los criterios más relevantes para el modelado, tales como el objetivo del problema, el tipo de problema matemático y método de resolución, los tipos de mercados eléctricos y la aplicabilidad del modelo a casos reales de estudio. Además, se identifican los retos todavía pendientes en este campo de estudio, entre los que destaca la aplicación simultánea de varias estrategias de compra-venta de energía de la planta virtual de energía en los distintos mercados energéticos, además de la utilización de técnicas de inteligencia artificial con el fin de proporcionar al modelo de planta virtual de energía un método de aprendizaje capaz de garantizar un margen de anticipación en sus decisiones.Posteriormente, para analizar el alcance real de la gestión de energía de acuerdo al diseño y operación de una planta virtual de energía, se desarrolla y aplica un nuevo modelo matemático de despacho horario técnico-económico a un gran sistema energético real de bombeo de agua para riego agrícola con recursos de producción renovable que evacuan directamente su producción a la red de distribución (centrales hidroeléctricas y un parque eólico), plantas de autoconsumo fotovoltaico asociadas a cada estación de bombeo y demanda eléctrica para maximizar el beneficio de operación conjunto de la planta virtual de energía. El comportamiento del modelo se ilustra para un año entero.Como ampliación del modelo anterior y afrontar el complejo reto de gestión eficiente del binomio agua-energía en las instalaciones de bombeo de agua, se desarrolla y aplica un nuevo modelo de despacho horario con la integración de los recursos energéticos e hídricos para la optimización de los costes de energía y de los cargos por demanda máxima en un gran sistema de regadío para un año entero, convirtiéndose en un modelo de tipo no lineal mixto-entero. A partir de los resultados obtenidos, el modelo con integración de la gestión del binomio agua-energía consigue aumentar el autoconsumo de energía renovable y ahorrar costes de suministro eléctrico al reducir la potencia contratada anualmente en los periodos horarios con mayor coste energético.Por otra parte, la aplicación de fuentes de energía renovable debe considerar el acoplamiento de la producción de electricidad con la demanda de energía eléctrica de las estaciones de bombeo y contemplar las limitaciones técnicas de las instalaciones hidráulicas de bombeo, almacenamiento y distribución del agua. Por tanto, por último, se propone el desarrollo de un modelo matemático de despacho a corto plazo con gestión técnica y económica para obtener la programación horaria óptima de los equipos de bombeo, minimizando los costes de operación de una estación real de bombeo de agua con autoconsumo fotovoltaico, sujeto a las restricciones eléctricas e hidráulicas de los sistemas de bombeo, y garantizando la demanda de riego. A partir de los resultados obtenidos, se puede comprobar que gracias a la combinación de instalaciones fotovoltaicas de autoconsumo y variadores de velocidad se consigue maximizar el porcentaje de energía autoconsumida y así, reducir los costes energéticos de la estación de bombeo además de mejorar la gestión del agua.En definitiva, esta tesis doctoral pone de manifiesto la importancia de desarrollar estrategias de gestión óptima de fuentes de generación eléctrica renovable e infraestructuras de agua para minimizar los costes energéticos y mejorar la eficiencia energética.<br /

Competencias profesionales y su desarrollo mediante aprendizaje-servicio en la universidad: perspectiva de los empleadores

Introducirse en el mundo laboral es costoso y uno de los primeros obstáculos es el déficit competencial que, en ocasiones, presentan los universitarios. Para subsanar el exceso de una teoría descontextualizada, el EEES (Espacio Europeo de Educación Superior) ha auspiciado un cambio metodológico que fomenta una docencia más práctica, ya que el aprendizaje más efectivo es activo y está conectado con la experiencia. De este modo, la enseñanza universitaria se enfoca, entre otros aspectos, en la consecución de resultados de aprendizaje en forma de competencias asociadas al mercado laboral. En este contexto se están ensayando en la universidad [española] metodologías más prácticas y aplicadas, entre ellas, el Aprendizaje-Servicio. El objetivo de este trabajo es conocer si para los empleadores participantes en nuestro estudio, los universitarios que participan en actividades de Aprendizaje-Servicio desarrollan el perfil competencial que buscan, haciéndoles candidatos más atractivos en los procesos de selección, mejorando en consecuencia su empleabilidad. Para ello, se desarrolló una investigación cualitativa genérica. Se realizaron tres grupos focales en los que participaron por muestreo intencional trece empleadores de egresados de distintas facultades de la universidad, pertenecientes a distintos sectores. La guía temática incluía preguntas sobre las competencias más demandadas por los empleadores, el Aprendizaje-Servicio y su influencia en el perfil profesional. La transcripción textual fue analizada mediante análisis de contenido inductivo por dos investigadores. En el apartado de resultados y discusión se constató que, para los empleadores consultados, los alumnos que participan en actividades de Aprendizaje- Servicio en la universidad, desarrollan algunas de las competencias profesionales más demandadas en el mundo laboral, incidiendo positivamente en su empleabilidad.Entering the labour force is onerous and one of the first obstacles is, on occasion, the competence shortcomings of university students. To correct the excess of a decontextualized theory, the EHEA (European Higher Education Area) has sponsored a methodological change that encourages more practical teaching, since the most effective learning is active and connected with experience. In this way, university teaching focuses, among other aspects, on achieving learning results in the form of competences associated with the labour market. In this context, more practical and applied methodologies are being tested at the university, including Service-Learning. The objective of this study is to find out whether, for the employers participating in our study, the university students who participate in these Service-Learning activities develop the competency profile they are looking for, making them more attractive candidates in the selection processes, consequently improving their employability. Generic quantitative research has been carried out. A purposive sampling of the employers of graduates from different university faculties was adopted. Three focal groups were formed using a topic guide, which addressed: the professional competences most demanded by employers; the repercussion of the teaching focus on employability; the social involvement capacity of their employees; Service-Learning and its influence on professional profile. 13 employers from different fields participated. Audio clips of the focal groups were recorded and their transcription was analysed by two researchers using content analysis. In the view of the employers consulted, students who participate in Service-Learning activities at the university develop some of the most in-demand professional competences, which positively affect their employability

Adjunctive rifampicin for Staphylococcus aureus bacteraemia (ARREST): a multicentre, randomised, double-blind, placebo-controlled trial.

BACKGROUND: Staphylococcus aureus bacteraemia is a common cause of severe community-acquired and hospital-acquired infection worldwide. We tested the hypothesis that adjunctive rifampicin would reduce bacteriologically confirmed treatment failure or disease recurrence, or death, by enhancing early S aureus killing, sterilising infected foci and blood faster, and reducing risks of dissemination and metastatic infection. METHODS: In this multicentre, randomised, double-blind, placebo-controlled trial, adults (≥18 years) with S aureus bacteraemia who had received ≤96 h of active antibiotic therapy were recruited from 29 UK hospitals. Patients were randomly assigned (1:1) via a computer-generated sequential randomisation list to receive 2 weeks of adjunctive rifampicin (600 mg or 900 mg per day according to weight, oral or intravenous) versus identical placebo, together with standard antibiotic therapy. Randomisation was stratified by centre. Patients, investigators, and those caring for the patients were masked to group allocation. The primary outcome was time to bacteriologically confirmed treatment failure or disease recurrence, or death (all-cause), from randomisation to 12 weeks, adjudicated by an independent review committee masked to the treatment. Analysis was intention to treat. This trial was registered, number ISRCTN37666216, and is closed to new participants. FINDINGS: Between Dec 10, 2012, and Oct 25, 2016, 758 eligible participants were randomly assigned: 370 to rifampicin and 388 to placebo. 485 (64%) participants had community-acquired S aureus infections, and 132 (17%) had nosocomial S aureus infections. 47 (6%) had meticillin-resistant infections. 301 (40%) participants had an initial deep infection focus. Standard antibiotics were given for 29 (IQR 18-45) days; 619 (82%) participants received flucloxacillin. By week 12, 62 (17%) of participants who received rifampicin versus 71 (18%) who received placebo experienced treatment failure or disease recurrence, or died (absolute risk difference -1·4%, 95% CI -7·0 to 4·3; hazard ratio 0·96, 0·68-1·35, p=0·81). From randomisation to 12 weeks, no evidence of differences in serious (p=0·17) or grade 3-4 (p=0·36) adverse events were observed; however, 63 (17%) participants in the rifampicin group versus 39 (10%) in the placebo group had antibiotic or trial drug-modifying adverse events (p=0·004), and 24 (6%) versus six (2%) had drug interactions (p=0·0005). INTERPRETATION: Adjunctive rifampicin provided no overall benefit over standard antibiotic therapy in adults with S aureus bacteraemia. FUNDING: UK National Institute for Health Research Health Technology Assessment