Model-based analysis of large-scale renewable energy production
By: J. Rosen, D. Möst, W. Fichtner, O. Rentz Institute for Industrial Production (IIP), University of Karlsruhe, Germany

Introduction

The already ongoing and even more the politically intended future growth of renewable and decentralised power generation in Europe are increasingly challenging for the existing energy system infrastructure and require the development of suitable strategies for energy production.

Among all renewable energy carriers, wind energy is of critical relevance. As wind energy production cannot be arranged, problems can arise for the grid, the operation, and the structure of the conventional plant portfolio, especially in regions with high wind power capacities installed (e.g. northern Germany). Once offshore wind parks with large capacities are realised, this problem will gain even greater importance.

Figure 1: Fluctuating feed-in of wind energy in 2010
(Sensfu� et. al 2003)

The fluctuations of renewable power production will have to be compensated, which can be accomplished by a sufficient provision of stand-by capacities and control power (Krämer 2003). This could partly be realised by hydropower reserves (e.g. in the Alps), while the rest would have to be provided by conventional capacities with good load following characteristics. The grid connection of offshore wind parks, the capacity structure, the role of hydropower, and the scheduling of stand-by capacities, as well as the extension of the grid are interesting aspects for the future of electricity supply.

Two interrelated energy model applications of PERSEUS will be presented in this article, which both arise from an increasing renewable energy exploitation. In the following, a short introduction to the beneficial use of energy models will be given and the PERSEUS model family will be presented.

Energy models have been used for energy system analysis purposes for many years, be it as decision support tools for policy makers or as strategic planning tools for industry players. While the different models have become steadily more sophisticated, the competitive environment of the energy supply industry has experienced significant changes. Market structures and thus competition rules have been altered by the intense re-regulation and restructuring efforts taken by the different European governments in order to put into place the plan of competitively structured, liberalised energy markets. At the same time, the present discussion on climate change and clean air strategies implies new challenges for this sector, given that a significant share of the total emission loads of greenhouse gases, especially CO2, as well as other harmful gaseous emissions like SO2 and NOX are related to power generation and fuel supplies. Furthermore, political decisions and attempts to minimise the environmental and climatic impacts of power production are likely to lead to a large-scale integration of renewables into electricity production.

These new framework conditions require continuously more advanced tools to address the arising research questions: How can existing energy supply systems best be adapted to the new market structures as well as to the existing or expected environmental protection targets? What is the impact of policy measures on existing supply systems regarding costs, technology choice, prices and market shares of different players? How can a large scale integration of renewable energies with a fluctuating energy production characteristic into the existing power system be accomplished? To what degree can the use of hydropower contribute to a compensation of the varying and less predictable energy production of large shares of renewable energies?

The PERSEUS model family

The energy and material flow model family PERSEUS (Programme Package for Emission Reduction Strategies in Energy Use and Supply) has been developed at the Institute for Industrial Production (IIP) of the University of Karlsruhe (TH) in order to be able to address questions related to energy systems not only on national, but also on regional and utility level.

The modelling approach is based on a detailed representation of energy conversion technologies and the interconnecting flows of energy (i. e. electricity and heat) and material (i. e. primary energy carriers, emissions of pollutants and greenhouse gases). The models follow linear and mixed integer programming approaches. Their target functions consist of a minimisation of all decision-relevant expenditure within the entire system. Technical, economic and ecological restrictions are integrated into the models in a suitable way to consider relevant system characteristics of the real energy supply system.

Emissions resulting from electricity and heat generation as well as from the distribution of energy carriers (e. g. natural gas) are calculated. Restrictions can be imposed on individual or cumulated emission levels. Hence, the model cannot only be used as a decision support tool for strategic planning under environmental constraints, but also as an environmental information system which generates data on current and future emission levels of all relevant pollutants and greenhouse gases. A data management system has been designed in order to provide a user-friendly interface.

Using the different models of the PERSEUS model family, a large number of studies, e. g. on regional, national and international emission reduction strategies, international cooperation concepts under the UN Framework Convention on Climate Change, as well as on contracting strategies, have been carried out. Versions of the model have also been developed for electricity suppliers to determine cost optimal strategies for capacity and production planning. The latest version of the model is a multi-regional electricity sector model including 42 European regions (Enzensberger 2003). It has been developed to determine the effects of an international emission trading scheme on the structure of the European electricity sector and the corresponding prices of emission certificates. This comprehensive European model acts as a starting point for two new model applications currently under development.

PERSEUS applications for large-scale renewable energy production

A comprehensive analysis of all aspects of the forthcoming large-scale integration of renewable and/or decentralised energy conversion technologies into the European electricity system are the main focus of a recently started research project. Starting off with a thorough analysis of the state and market share of renewable energy technologies (wind, solar, hydro, geothermal, biomass, biogas) to date, their future market potential as well as possible difficulties for the existing generation and distribution infrastructure are analysed. Due to the fact that about half of the installed conventional capacities installed today will have to be renewed in the course of the next 20-25 years, the analysis will be able to give valuable indications for an economically and ecologically optimised restructuring of the generation system with simultaneously increasing shares of renewable energy production. The central calculation tool used is a modified version of a comprehensive interregional PERSEUS model of the European electricity sector (Enzensberger 2003). This cost-optimising model is used for the analysis of long-term structural developments. The challenges for energy production in the fields of production, feed-in, and distribution (occurring on a much smaller time scale) are analysed with a coupled short-term model based on a system dynamics approach using the commercial Matlab/Simulink® software package applied to both the existing and the future energy system (forecasted by PERSEUS). The effects and problems identified in the short-term Simulink® model when compared to the results of the long-term PERSEUS model are then used as feed-back for an improved and more realistic representation of the energy system in the long-term model. This will be accomplished by a more detailed representation of wind and control power technologies, including a generalised fluctuation pattern that accounts for the short-term effects.

Especially significant regions (e.g. off-shore regions for the exploration of wind energy) are modelled in greater detail, while other areas with a less critical potential of renewables are modelled on a more aggregated level.

Fig. 2: Interaction of the long- and short-term models

Tasks within the project include the preparation and analysis of a comprehensive data-set, its integration into the optimising long-term energy-system model PERSEUS, plus subsequent model scenario analyses, which simultaneously provide input to the short-term model. Scenario runs conducted with both models will be analysed for each relevant region and examined with a focus on the following short- and long-term aspects:

• Renewable and conventional technology-/fuel-mix
• Effects on conventional capacities
• Influence of a large-scale integration of renewables on the scheduling of existing and planned conventional power production
• Marginal costs of power production, capacity and cost-effects with a special focus on stand-by-capacities and the amount of control power needed
• Infrastructural requirements (grid, fuel-supply)
• Net CO2-reduction

An example of first results generated with the Simulink representation of the German power generation system is depicted in Figure 3. It shows the conventional power production in the course of the first 3 weeks of the year 2010. The top green line represents the total remaining demand after the deduction of the produced wind energy. Wind power production is calculated in a model by Fraunhofer ISI (Sensfuß et al. 2003) from hourly measured wind speeds at different characteristic wind park locations and the projected development of installed wind power capacities. The figure shows quite well the impact of increased wind power feed-in on the production characteristics of the conventional plant portfolio. The lines below the green line of total demand represent peak-load generation, the space down to the blue line, which is base-load generation, represents intermediate load generation. In a system with a limited feed-in of fluctuating renewable energy, where the load profile and also the coverage by the different plant types would be quite similar from one week to another. Here it can be noted, that in contrast to this regular behaviour even intermediate and base-load power plants have to be adjusted in their schedule in order to cope with the fluctuating amount of wind energy produced. This necessity induces more frequent load changes and switching plants on/off will be necessary more often as well, which leads to a less efficient production in the conventional plants. The net emission reductions are consequently not as high as could be expected when counting wind power feed-in alone. These effects can presumably be minimised by finding an optimised production strategy and by restructuring the plant portfolio to cope with these effects in the best possible way economically and environmentally (e.g. by installing low-emission gas-fired combined cycle power plants that can be used for load-following with smaller losses in efficiency), something that will also be analysed in the further course of the project.

Fig. 3: Load curve and scheduling of base load and peak load

PERSEUS application for analysis of hydropower plant operation and competitiveness

Due to the fact, that the large scale integration of renewable energy poses problems for stand-by capacities, it will further be examined, which new possibilities and tasks result from the changing capacity structure for traditional hydropower plants in the Alps. A detailed energy system model for the alpine region will be coupled with a European energy system model and the effects on the capacity structure will be examined. To analyse the competitive position of hydropower in Switzerland, a new modelling approach of the PERSEUS model will be developed. With this approach the following questions are to be answered:

• What influence do the upcoming structural changes in the European energy system have, especially the integration of large shares of fluctuating wind energy, on the economical evaluation of peak load and stand by capacities, with respect to the reservoir power stations in the Alps?
• Which consequences result for the international energy exchange and the grid extension?

Within a first step, a PERSEUS model will be built up and applied to the Swiss energy sector. Particularly hydropower plants � pump storage, reservoir and run-of-the river plants � will be modelled in detail to analyse their ability to compete in the changing European energy sector. Thus it is necessary to modify and adapt the existing PERSEUS model approaches for national energy systems to achieve a more detailed and realistic model representation of hydropower operation. The methodology of the existing PERSEUS modelling approaches will be improved by the following measures:

• the seasonal aggregation of the model will be more exact, so that e.g. two months with typical days will be summarised (instead of half years). The seasonal aggregation has to be as detailed as possible within the limits of the modelling system and the calculation time.
• cross linking and networking of water power plants, respectively stages of weirs, will be considered within the detailed modelling approach.
• control energy supply and demand plays an important role and will be considered within the new approach.

The developed model applied to water power operation in Switzerland will be linked to the interregional European PERSEUS model (Enzensberger 2003). In order to realise the integration of the models, results of both models will be exchanged in an iterative process. The results of the respective optimisation will be handed over to the other corresponding model, which will then be optimised in turn producing new results. Transfer and preparation of the exchanged data automatically take place on the basis of defined rules. The iteration process ends when the model results of the system optimisation only change marginally. The integration of the detailed Swiss power system into the European model will be realised using a decomposition algorithm. Within the overall system optimisation and the supposed framework conditions a cost optimised electricity system for Europe will be determined, while a special focus is set on the impact on Swiss water power operation.

Within this linked modelling approach special consideration will be given to the impact of a large-scale integration of fluctuating wind energy into the European energy system (especially in Germany, Denmark, Spain, and prospectively also in France) on water power production, mainly on pump storage and reservoir power plants for peak load.

Consequently, the competitiveness of Swiss hydropower will be examined within the European energy system in order to find a basis for decisions in investment strategies in water power and for the prolongation of water concessions. Furthermore, the daily energy exchanges and the interconnections between the European countries, especially Germany � Switzerland, will be studied and evaluated in detail. Questions arising in the field of energy transmission and grid extension can consequently be answered.

Fig. 4: design of a selected water power cascade and grid

Implementation of the models

The PERSEUS models are implemented as a PC version that can be run on commercial PCs. They require state-of-the-art hardware components due to their high complexity and the resulting large problem size.

An MS Access based data management system has been designed, which permits easy data handling and a fully automated link to the mathematical module. Programming is realised in GAMS (Brooke & al. 1988). The formatted and structured results are made available in MS Excel spreadsheets for further processing. Commercial solvers like CPLEX can be applied to find the solutions to the respective optimization problems. The system dynamics model is realised using the commercially available Matlab/Simulink� package (The Mathworks 2001).

Remarks

The article above is based on an article published in the proceedings of the EnviroInfo 2003 conference in Cottbus (Rosen et al. 2003).

Bibliography

Brooke, A & al (1988). GAMS A User�s Guide. Washington: Scientific Press

Enzensberger, N. (2002). Expert tool for energy system analyses under emission trading schemes, Proceedings of ECOS 2002, 15th International Conference on Efficiency, Costs, Optimization, Simulation and Environmental Impact of Energy Systems, July 3-5, 2002, Berlin

Kr�mer, M. (2003). Modellanalyse zur Optimierung der Stromerzeugung bei hoher Einspeisung von Windenergie, Fortschritt-Berichte VDI, Reihe 6 Energietechnik, Nr. 492

Rosen, J., M�st, D., Fichter, W., Rentz, O. (2003). Use of the PERSEUS models to analyse the effects of large-scale renewable energy production, Proceedings of the 17th International Conference Informatics for Environmental Protection (EnviroInfo) 2003, Cottbus

Sensfu�, F., Ragwitz, M., Wietschel, M. (2003). Fluktuationen der Windenergie und deren Vorhersagbarkeit bei einem verst�rkten Ausbau des Offshore Anteils in Deutschland, Proceedings und CD-ROM 3. Internationale Energiewirtschaftstagung IEWT03- die Zukunft der Energiewirtschaft im liberalisieren Markt, 12. bis 14.Februar, Technische Universit�t Wien, Wien, 2003

The Mathworks, Inc. (2001). Using Simulink, The Mathworks, Inc., Natick, MA, USA, 2001