Andreas Efstratiadis

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• Associate Professor
• Professor (Associate) at National Technical University of Athens

As the share of renewable energy resources rapidly increases in the electricity mix, the recognition, representation, quantification, and eventually interpretation of their uncertainties become important. In this vein, we propose a generic stochastic simulation-optimization framework tailored to renewable energy systems (RES), able to address multi...

The European Green Deal has set climate and energy targets for 2030 and the goal of achieving net zero greenhouse gas emissions by 2050, while supporting energy independence and economic growth. Following these goals, and as expected, the transition to "green" renewable energy is growing and will be intensified, in the near future. One of the main...

Storm Daniel initiated on 3 September 2023, over the Northeastern Aegean Sea, causing extreme rainfall levels for the following four days, reaching an average of about 360 mm over the Peneus basin, in Thessaly, Central Greece. This event led to extensive floods, with 17 human lives lost and devastating environmental and economic impacts. The automa...

Decentralized planning of renewable energy systems aims to address the substantial spa-tiotemporal variability, and thus uncertainty, associated with their underlying hydrometeorological processes. For instance, solar photovoltaic (PV) energy is driven by two processes, namely solar radiation , which is the main input, and ambient temperature, with...

The ongoing debate about hydropower and its role in the transition towards renewable energy development is strongly associated with its sustainable character, social and environmental footprint, and potential benefits. Multipurpose hydropower reservoirs play a significant dual role as they serve both water and energy uses. However, their operation...

Triggered by the need for developing a comprehensive flood protection strategy (referred to as the Master Plan) for Western Thessaly in Central Greece, we demonstrate a strategic approach for large-scale hydrosystems, where the need for design accuracy is contrasted to extreme computational burden. The area of interest occupies approximately 6,400...

In recent years, both availability and interest in Earth Observations (EO) have increased due to their ability to provide information with extensive spatial coverage, which is valuable for data-scarce regions. This study provides a roadmap to exploit EO/satellite data (EO-based model) for continuous hydrological simulation using the Hydrologic Engi...

Ο Αλιάκμονας είναι ο μεγαλύτερος σε μήκος ποταμός της χώρας με σημαντική υδροηλεκτρική αξιοποίηση, η λεκάνη απορροής του οποίου έχει υποστεί έντονες τροποποιήσεις που έχουν αλλάξει δραστικά την πλημμυρική δίαιτα του ποταμού στα κατάντη. Η υδροηλεκτρική παραγωγή και η λειτουργία των εκχειλιστών του συγκροτήματος αποτελούν κρίσιμο παράγοντα για τη δι...

Οι επιπτώσεις των έργων υποδομής στο τοπίο απασχολούν όλο και περισσότερο την κοινή γνώμη. Την τελευταία δεκαετία, ιδιαίτερη έμφαση έχει δοθεί στα αιολικά και τα φωτοβολταϊκα έργα και τις επιδράσεις τους στον φυσικό, πολιτισμικό και αισθητικό χαρακτήρα των τοπίων. Παράλληλά όμως, η προβληματική αυτή αποτέλεσε αφορμή για να έρθει στο προσκήνιο ένα σ...

The electricity market across Europe, which is key driver of energy systems, has been subject to structural changes in the last years, in order to favor the penetration of renewables and foster decarbonization. A substantial guiding principle was the establishment of the Target Model, configurating a new era of the energy as a trading product. The...

As urban populations swell and infrastructure demands escalate, managing resources sustainably becomes increasingly challenging. This paper focuses on the energy challenges inherent in distributed water management systems, using sewer mining as an example. Sewer mining is a distributed water management solution involving mobile wastewater treatment...

The hydrometeorological processes associated with renewables are characterized by substantial spatiotemporal variability, and thus uncertainty, which is addressed through decentralized planning, thus taking advantage of scaling effects. The objective of this work is to provide a comprehensive investigation of the role of scale regarding solar photo...

The Water-Energy-Food-Ecosystem nexus is characterized by synergies, complementarities and conflicts, and thus its management is a demanding task. This becomes more challenging when socioeconomic influences are embedded. Key components of this nexus are multipurpose water reservoirs that provide drinking water, electricity, agricultural water for f...

This paper presents a holistic approach to address the challenges associated with deploying laboratory-derived hydraulic models in real operational conditions within Urban Water Systems (UWS). The underlying methods and tools are tested and validated using real-world data from the conveyance system serving the city of Athens, Greece. Initially, a n...

For a long time, the classical problem of identifying the optimal modeling structure and/or parameters followed the calibration-validation norm, originating from the iconic split-sample scheme by Vit Klemeš. A common feature of such approaches is their dependence on the length and representativeness of the available data. This introduces several qu...

Hydropower reservoirs inherently serving as major flood protection infrastructures, are commonly occupied with gated spillways, to increase both their storage capacity and head. From an operational viewpoint, during severe flood events, this feature raises challenging conflicts with respect to combined management of turbines and gates. From the per...

Hybrid renewable energy systems, complemented by pumped hydropower storage, have become increasingly popular amidst the increase in renewable energy penetration. Such configurations are even more prosperous in remote regions that are typically not connected to the mainland power grid, where the energy independence challenge intensifies. This resear...

Hybrid renewable energy systems (HRES), complemented by pumped hydropower storage (PHS), have become increasingly popular amidst the increase of renewable energy penetration. This con-figuration is even more prosperous in remote regions that are typically not connected to the mainland power grid, where the energy independence challenge intensifies....

We investigate the design of small hydropower plants under multiple sources of uncertainty and contrast it with the conventional deterministic practice that leads to a unique solution. In particular, we emphasize three sources of uncertainty, referring to: (a) the rainfall process, (b) the rainfall-runoff transformation, and (c) the flow-energy con...

Complex environmental optimization problems often require computationally expensive simulation models to assess candidate solutions. However, the complexity of response surfaces necessitates multiple such assessments and thus renders the search procedure too laborious. Surrogate-based optimization is a powerful approach for accelerating convergence...

The optimal management of sociotechnical systems across the water–energy nexus is a critical issue for the overall goal of sustainable development. However, the new challenges induced by global crises and sudden changes require a paradigm shift in order to ensure tolerance against such kinds of disturbance that are beyond their “normal” operational...

Managing water resources for growing demands of energy and food while sustaining the environment is the greatest challenge of our era, especially when we are dealing with complex adaptive natural-human systems (Machell et al., 2015). The performance of water-energy systems is strongly depending on hydroclimatic processes and human behaviors, while...

In this work, we investigate whether the parameters of physics-based hydraulic models, omnipresent in every relevant engineering handbook, can be used in blind trust in a real-world complex system. Here, we focus on the discharge coefficient for flows through a sluice gate and the Manning's coefficient for steady flows, and we compare their typical...

The operation of a small hydroelectric power plant (HPS) with two hydro turbines of different types and power is usually done following a hierarchical rule, which is not necessarily the most efficient. Alternatively, other synergetic rules have been proposed that improve the delivered energy. In this paper, the operation of the two turbines is syst...

The optimal management of sociotechnical systems across the water-energy nexus is critical issue towards the overall goal of sustainable development. However, the new challenges induced by global crises and sudden changes require a paradigm shift, in order to ensure tolerance against such kinds of disturbance that are beyond their "normal" operatio...

The energy autonomy of small non-interconnected islands in the Mediterranean, taking advantage of their high renewable energy potential, has been a long-standing objective of local communities and stakeholders. This is also in line with the recently implemented European Green Deal, which has set the goal of increasing the renewable energy penetrati...

Impacts to landscapes have been identified as major drivers of social opposition against renewable energy projects. We investigate how the process of mitigating landscape impacts can be improved and accelerated, through a re-conceptualization of visibility analysis. In their conventional format, visibility analyses cannot be implemented in early pl...

We propose a novel probabilistic approach to flood hazard assessment, aiming to address the major shortcomings of everyday deterministic engineering practices in a computationally efficient manner. In this context, the principal sources of uncertainty are defined across the overall modeling procedure, namely, the statistical uncertainty of inferrin...

Motivated by the challenges induced by the so-called Target Model and the associated changes to the current structure of the energy market, we revisit the problem of day-ahead prediction of power production from Small Hydropower Plants (SHPPs) without storage capacity. Using as an example a typical run-of-river SHPP in Western Greece, we test alter...

As the share of renewable energy resources rapidly increases in the electricity mix, the recognition, representation, quantification, and eventually interpretation of their uncertainties become important. Consequently, we propose a stochastic simulation-optimization framework tailored to renewable energy systems (RES), that is able to address multi...

The challenging task of generating a synthetic time series at finer temporal scales than the observed data, embeds the reconstruction of a number of essential statistical quantities at the desirable (i.e., lower) scale of interest. This paper introduces a parsimonious and general framework for the downscaling of statistical quantities based solely...

The limited availability of hydrometric data makes the design, management, and real-time operation of water systems a difficult task. Here, we propose a generic stochastic framework for the so-called inverse problem of hydroelectricity, using energy production data from small hydropower plants (SHPPs) to retrieve the upstream inflows. In this conte...

The large-scale surface-water monitoring infrastructure for Greece Open Hydrosystem Information Network (Openhi.net) is presented in this paper. Openhi.net provides free access to water data, incorporating existing networks that manage their own databases. In its pilot phase, Openhi.net operates three telemetric networks for monitoring the quantity...

Water, energy, land, and food are vital elements with multiple interactions. In this context, the concept of a water-energy-food (WEF) nexus was manifested as a natural resource management approach, aiming at promoting sustainable development at the international, national, or local level and eliminating the negative effects that result from the us...

Fluvial floods are one of the primary natural hazards to our society, and the associated flood risk should always be evaluated for present and future conditions. The European Union’s (EU) Floods Directive highlights the importance of flood mapping as a key stage for detecting vulnerable areas, assessing floods’ impacts, and identifying damages and...

Hydrological calibrations with historical data are often deemed insufficient for deducing safe estimations about a model structure that imitates, as closely as possible, the anticipated catchment behaviour. Ιn order to address this issue, we investigate a promising strategy, using as drivers synthetic time series, which preserve the probabilistic p...

Small hydropower plants (SHPPs) are subject to multiple uncertainties and complexities, despite their limited scale. These uncertainties are often ignored in the typical engineering practice, which results in risky design. As this type of renewable energy rapidly penetrates the electricity mix, the impacts of their uncertainties, exogenous and endo...

The highly-competitive electricity market over EU and the challenges induced by the so-called “Target Model”, introduce significant uncertainties to day-ahead trades involving renewable energy, since most of these sources are driven by non-controllable weather processes (wind, solar, hydro). Here, we explore the case of small hydropower plants that...

The design of hydropower works typically follows a top-down approach, starting from a macroscopic screening of the broader region of interest, to select promising clusters for hydroelectric exploitation, based on easily retrievable information. Manual approaches are very laborious and may fail to detect sites of significant hydropower potential. In...

The storage-reliability-yield (SRY) relationship is a well-established tool for preliminary design of reservoirs fulfilling consumptive water uses, yet rarely employed within hydropower planning studies. Here, we discuss the theoretical basis for representing the trade-offs between reservoir size and expected revenues from hydropower production, un...

The fundamental concepts in the field of water-energy systems and their historical evolution with emphasis on recent developments are reviewed. Initially, a brief history of the relation of water and energy is presented and the concept of the water-energy nexus in the 21th century is introduced. The investigation of the relationship between water a...

Although storage-reliability-yield (SRY) relationships have been widely used in the design and planning of water supply reservoirs, their application in hydroelectricity is practically missing. Here we revisit the SRY analysis and seek for its generic configuration for hydroelectric reservoirs, following a stochastic simulation approach. After defi...

Due to their negligible storage capacity, small hydroelectric plants cannot offer regulation of flows, thus making the prediction of energy production a very difficult task, even for small time horizons. Further uncertainties arise due to the limited hydrological information, in terms of upstream inflow data, since usually the sole available measur...

As made for most of renewable energy sources, wind energy is driven by highly uncertain and thus unpredictable meteorological processes. In the context of wind power scheduling and control, reliable wind predictions across scales is a challenging problem. However, since the generation of wind energy is, in fact, a nonlinear transformation of wind v...

Wind turbines are large-scale engineering infrastructures that may cause significant social reactions, due to the anticipated aesthetic nuisance. On the other hand, aesthetics is a highly subjective issue, thus any attempt towards its quantification requires accounting for the uncertainty induced from subjectivity. In this work, taking as example t...

Over the last 30 years, numerous water resources planning and management studies in Greece have been conducted by using state-of-the-art methodologies and associated computational tools that have been developed by the Itia research team at the National Technical University of Athens. The spearhead of Itia’s research toolkit has been the Hydronomeas...

The Open Hydrosystem Information Network (OpenHi.net) is a state-of-the-art information infrastructure for the collection, management and free dissemination of hydrological and environmental information related to Greece’s surface water resources. It was launched two years ago as part of a the national research infrastructure “Hellenic Integrated M...

We present a distributed hydrological model with minimal calibration requirements, which represents the rainfall-runoff transformation and the flow routing processes. The generation of surface runoff is based on a modified NRCS-CN scheme. Key novelty is the use of representative CN values, which are initially assigned to model cells on the basis of...

In November 2017, a storm event of substantial but unknown local intensity caused a flash flood in Western Attica, Greece, which was responsible for 24 human fatalities and large-scale economical losses. Our focus is to the neighbouring catchment of Sarantapotamos, which has been equipped with an automatic stage recorder that was destroyed during t...

The generation of hydrometeorological time series that exhibit a given probabilistic and stochastic behavior across multiple temporal levels, traditionally expressed in terms of specific statistical characteristics of the observed data, is a crucial task for risk-based water resources studies, and simultaneously a puzzle for the community of stocha...

The construction and operation of large dams has been questioned in recent years as, despite their positive effect on the economy, they are regarded as negative to the environment. The size of a dam, in particular, is an important aspect of this debate as it is thought to increase its economic benefit but also its environmental impacts. We investig...

The protection and management of water and environmental resources require the availability of reliable data, collected by properly designed, equipped and functioning monitoring networks. However, for many years in Greece, the status of data collection and archiving has been far from adequate, thus preventing the country from managing its water res...

For more than two decades, the use of artificial neural networks (ANNs) in hydrology has become an effective and efficient alternative against traditional modeling approaches, i.e. physically-based or conceptual. These can take ad-vantage of any type of available information to predict the hydrological response of complex systems, with missing data...

Integrated modeling of hybrid water-energy systems, comprising conventional and renewable energy sources, pumped-storage facilities and other infrastructures, which aim to serve combined water and energy uses, is a highly challenging problem. On the one hand, such systems are subject to significant uncertainties that span over all associated input...

Design and management of water resource systems are arguably challenging tasks, as they are mainly driven by hydrological processes that are dominated by “structured” randomness. In this vein, the stochastic simulation of the input processes is regarded an essential component for such studies. Typically, the objective of stochastic models is the ge...

Lecture Notes for the course "Renewable Energy and Hydroelectric Projects" (8th semester, School of Civil Engineering, National Technical University of Athens) on the subject "Simulation and optimization for the design and management of hydroelectric works"

The objective of this article is to provide a simple and effective tool for low flow forecasting up to six months ahead, with minimal data requirements, i.e. flow observations retrieved at the end of wet period (first half of April, for the Mediterranean region). The core of the methodological framework is the exponential decay function, while the...

Over the last decade, Environmental Flow Assessment (EFA) has focused scientific attention around heavily-modified hydrosystems, such as flow regulated releases downstream of dams. In this light, numerous approaches of varying complexity have been developed, the most holistic of which incorporate hydrological, hydraulic, biological and water qualit...

The ever-increasing energy demand has led to overexploitation of fossil fuels deposits, while renewables offer a viable alternative. Since renewable energy resources derive from phenomena related to either atmospheric or geophysical processes, unpredictability is inherent to renewable energy systems. An innovative and simple stochastic tool, the cl...

Since the prime days of stochastic hydrology back in 1960s, autoregressive (AR) and moving average (MA) models (as well as their extensions) have been widely used to simulate hydrometeorological processes. Initially, AR(1) or Markovian models with Gaussian noise prevailed due to their conceptual and mathematical simplicity. However, the ubiquitous...

An operational framework for flood risk assessment in ungauged urban areas is developed within the implementation of the EU Floods Directive in Greece, and demonstrated for Volos metropolitan area, central Greece, which is frequently affected by intense storms causing fluvial flash floods. A scenario-based approach is applied, accounting for uncert...

A recent storm event, of substantial yet unknown local intensity, in Western Attica (west of Athens, Greece) has caused a flash flood with many fatalities in the city of Mandra as well as material damages. After this incident a debate started on whether the devastating results were due to the extreme nature of the rainfall event or to the poor floo...

In contrast to great advances on stochastic simulation techniques in hydrology and their importance on water management and uncertainty assessment studies, operational software packages for generating synthetic data are limited and hardly accessible. This limits their adoption to a narrow audience, excluding the vast majority of researchers and pra...

From the origins of hydrology, the time of concentration, tc, has been conventionally tackled as constant quantity. However, theoretical proof and empirical evidence imply that tc exhibits significant variability against rainfall, making its definition and estimation a hydrological paradox. Adopting the assumptions of the Rational method and the ki...

In this paper, a new methodology for delineating Hydrological Response Units (HRUs), based on the Curve Number (CN) concept, is presented. Initially, a semi-automatic procedure in a GIS environment is used to produce basin maps of distributed CN values as the product of the three classified layers, soil permeability, land use/land cover characteris...

Stochastic models in hydrology traditionally aim at reproducing the empirically-derived statistical characteristics of the observed data rather than any specific distribution model that attempts to describe the usually non-normal, statistical behavior of the associated processes. SPARTA (Stochastic Periodic AutoRegressive To Anything) offers an alt...

Precision irrigation constitutes a breakthrough for agricultural water management since it provides means to optimal water use. In recent years several applications of precision irrigation are implemented based on spatial data from different origins, i.e. meteorological stations networks, remote sensing data and in situ measurements. One of the fac...

Lecture given at Moscow State University, in the context of School for Young Scientists “Modelling and forecasting of river flows and managing hydrological risks: towards a new generation of methods” (20-23 November, 2017)

Lecture given at the Moscow State University, in the context of the School for Young Scientists “Modelling and forecasting of river flows and managing hydrological risks: towards a new generation of methods” (20-23 November, 2017)

We present and validate a global parametric model of potential evapotranspiration (PET) with two parameters that are estimated through calibration, using as explanatory variables temperature and extraterrestrial radiation. The model is tested over the globe, taking advantage of the Food and Agriculture Organization (FAO CLIMWAT) database that provi...

Περιγράφεται μια ολιστική προσέγγιση για τη διαχείριση σύνθετων συστημάτων ταμιευτήρων, οι οποίοι εξυπηρετούν πολλαπλούς και, συχνά, αντικρουόμενους στόχους, όπως είναι η παραγωγή ενέργειας και η κάλυψη αρδευτικών/ υδρευτικών ή άλλων αναγκών. Η εν λόγω διαδικασία είναι κατά κανόνα πολύπλοκη λόγω του στοχαστικού χαρακτήρα των υδρομετεωρολογικών διερ...

We present a stochastic approach accounting for input uncertainties within water-energy simulations. The stochastic paradigm, which allows for quantifying the inherent uncertainty of hydrometeorological processes, becomes even more crucial in case of missing or inadequate information. Our scheme uses simplified conceptual models which are subject t...

Stochastic simulation of hydrological processes has a key role in water resources planning and management due to its ability to incorporate hydrological uncertainty within decision-making. Due to seasonality, the statistical characteristics of such processes are considered periodic functions, thus implying the use of cyclo-stationary stochastic mod...

Precision irrigation constitutes a breakthrough for agricultural water management since it provides means to optimal water use. In recent years several applications of precision irrigation are implemented based on spatial data from different origins, i.e. meteorological stations networks, remote sensing data and in situ measurements. One of the fac...

Precision irrigation constitutes a breakthrough for agricultural water management since it provides means to optimal water use. In recent years several applications of precision irrigation are implemented based on spatial data from different origins, i.e. meteorological stations networks, remote sensing data and in situ measurements. One of the fac...

Non-connected islands to the electric gird are often depending on oil-fueled power plants with high unit cost. A hybrid energy system with renewable resources such as wind and solar plants could reduce this cost and also offer more environmental friendly solutions. However, atmospheric processes are characterized by high uncertainty that does not p...

Small islands are regarded as promising areas for developing hybrid water-energy systems that combine multiple sources of renewable energy with pumped-storage facilities. Essential element of such systems is the water storage component (reservoir), which implements both flow and energy regulations. Apparently, the representation of the overall wate...

Low flows commonly occur in rivers during dry seasons within each year. They often concur with increased water demand which creates numerous water resources management problems. This paper seeks for simple yet efficient tools for low-flow forecasting, which are easy to implement, based on the adoption of an exponential decay model for the flow rece...

We propose a systematic framework for delineating Hydrological Response Units (HRUs), based on a modified Curve Number (CN) approach. The CN-value accounts for three major physiographic characteristics of a river basin, by means of classes of soil permeability, land use/land cover characteristics, and drainage capacity. A semi-automatic procedure i...

Following the legislative EU targets and taking advantage of its high renewable energy potential, Greece can obtain significant benefits from developing its water, solar and wind energy resources. In this context we present a GIS-based methodology for the optimal sizing and siting of solar and wind energy systems at the regional scale, which is tes...

One-dimensional and quasi-two-dimensional hydraulic freeware models (HEC-RAS, LISFLOOD-FP and FLO-2d) are widely used for flood inundation mapping. These models are tested on a benchmark test with a mixed rectangular-triangular channel cross section. Using a Monte-Carlo approach, we employ extended sensitivity analysis by simultaneously varying the...

Within the ongoing research project “Combined Renewable Systems for Sustainable Energy Development” (CRESSENDO), we have developed a novel stochastic simulation framework for optimal planning and management of large-scale hybrid renewable energy systems, in which hydropower plays the dominant role. The methodology and associated computer tools are...

Modelling of large-scale hybrid renewable energy systems (HRES) is a challenging task, for which several open computational issues exist. HRES comprise typical components of hydrosystems (reservoirs, boreholes, conveyance networks, hydropower stations, pumps, water demand nodes, etc.), which are dynamically linked with renewables (e.g., wind turbin...

Many hydrological applications require the use of long rainfall data across a wide range of fine time scales. To meet this necessity, stochastic approaches are usually employed for the generation of large number of rainfall events, following a Monte Carlo approach. In this framework, Bartlett-Lewis model (BL) is a key representative from the family...

We present a multi-criteria simulation-optimization framework for the optimal design and setting of flood protection structures along river banks. The methodology is tested in the lower course of the Arachthos River (Epirus, Greece), downstream of the hydroelectric dam of Pournari. The entire study area is very sensitive, particularly because the r...

The combined use of weather forecasting models and hydrological models in flood risk estimations is an established technique, with several successful applications worldwide. However, most known hydrometeorological forecasting systems have been established in large rivers with perpetual flow. Experience from small- and medium-scale basins, which are...

In water resources optimization problems, the calculation of the objective function usually presumes to first run a simulation model and then evaluate its outputs. In several cases, however, long simulation times may pose significant barriers to the optimization procedure. Often, to obtain a solution within a reasonable time, the user has to substa...