Intra-day electro-thermal model predictive control for polygeneration systems in microgrids

François Maréchal, Ramanunni Parakkal Menon, Mario Paolone
2016
Article

Résumé

This paper is framed within the context of intra-day optimal control of polygeneration systems and storage connected to microgrids. In particular, the paper proposes an optimal control strategy that accounts for both electrical and thermal processes taking place in multi-building energy networks. To this end, the proposed optimal control strategy, based on the use of a Model Predictive Control, has been developed with the aim of providing optimal resource set-points for the coming 24 h. The proposed approach takes into account: day-ahead and spot prices of electricity, resource prices in terms of fuel cost and shut-down, start-up costs as well as the presence of electrical and thermal storage. The ability of this thermo-electric optimal control strategy is then demonstrated through simulations considering three case studies, namely: (a)fixed electricity price, (b)electricity spot-price and (c)different seasonal cases. The paper also studies the shaving of peaks through the use of optimally sized units in a system with the proposed method.

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https://infoscience.epfl.ch/record/218393?ln=fr

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Model Predictive Control Strategies for Low-Voltage Microgrids

François Maréchal, Ramanunni Parakkal Menon, Mario Paolone

Modern concepts for grids are being developed owing to the increasing stress put on the electrical producers and the burgeoning need for thermal demands due to varying climate conditions. The increasing awareness of the depletion of natural resources has also been a driver in the direction of modern grids which are more efficient and less susceptible to failures. One solution which is being considered as an alternative for the existing centralised electrical grid and thermal needs is the concept of microgrid. Microgrids are decentralised subunits which consist of different cogeneration units and combined heat and power units along with renewable sources and heat and electricity consumers. Multiple microgrids combine together to aid and complement the existing producers thus, reducing the stress on these producers while increasing the efficiency of the entire electricity grid. The presence of different storage devices and electricity production units mean that the microgrids can also maintain very high quality of electricity, thus reducing losses during transmission and distribution. Another advantage of a microgrid is it's ability of being self-sufficient for a short period time which means that microgrids have the ability to island themselves voluntarily or accidentally from the rest of the electricity grid which could be used to prevent entire grid failures during meteorological problems or when the economic factors are not conducive. The presence of decentralised sources, storage devices and consumers mean that there is a strong need for control of all the different components that constitute a microgrid. There are many different approaches to control a microgrid. This paper attempts to demonstrate a model predictive control (MPC) strategy developed to provide strategies for a low-voltage microgrid with both thermal and electrical devices and requirements. The aim of the strategy is to satisfy the demands and comforts of the consumers while minimising the costs associated by performing a multi-layered mixed-integer optimisation which provides strategy that needs to be employed by each unit of the system. The optimisation consists of two layers, the first of which provides the setpoints for both the thermal and electrical components and the second layer solves a problem which is only takes the electrical units into account and is performed over a shorter timestep. This is owing to the different dynamics of the thermal and electrical units. The fast dynamics of the electrical systems requires us to perform the second layer to maintain the efficiency of the microgrid. The paper then shows the importance of optimal sizing for the model predictive control strategy employed for varying periods of the year and the advantages of this strategy for microgrids over the existing strategies. The paper also attempts to show the impact of certain variables employed in the MPC strategy in particular, for LV (Low-Voltage) microgrid.

Aidic Servizi Srl2014

Chargement

The electricity sector in Switzerland is undergoing important changes following the liberalisation process and the facilitation of renewable energy technologies. Furthermore, the phasing out of nuclear power will increase the demand for new domestic electricity generation. According to the Federal Energy Strategy 2050, additional generation will have to come from hydropower (currently 57% of Swiss electricity production), including small hydropower (currently 6%, i.e. 3.8 TWh). The small hydropower technology, with an installed capacity between 100 kW and 10 MW (whereby 100 kW till 1 MW is considered to be mini hydropower), is a renewable energy technology (RET) which is well developed. However, the technology still requires further innovation to improve its environmental integration and reduce costs. Small hydropower (SHP) provides electricity with a high energy payback ratio and, generally, with lower production costs than other RETs, aside from large hydropower. SHP can be combined within multipurpose infrastructures such as drinking water and irrigation networks. The institutional framework of SHP is conditioned by multi-level (i.e., Federal, Cantonal and Communal) and cross-sectorial institutions (e.g., within the electricity and water sectors, spatial planning). SHP still has significant potential in Switzerland with the possibility of increasing the production of 2010 by 40-50% by 2050. However, SHP requires appropriate policy instruments for its development within a liberalised electricity market as it is, on average, not yet cost-competitive. In 2009, for example, a Federal feed-in remuneration scheme was introduced. To further develop the SHP potential, the institutional framework has still to evolve. This research was aimed at identifying changes in the institutional framework in Switzerland which can contribute towards developing SHP and increasing the alignment between the technology and its institutions. To this end, the literature on co-evolution and the framework of coherence between institutions and technologies in the case of network industries, such as electricity, were used for the analysis. This thesis contributes towards further development of the coherence framework. Policy instruments are identified that can support the development of SHP. Measures to simplify and harmonise administrative procedures are evaluated, even though their implementation remains difficult. A promising endeavour, however, is to reduce opposition, thus duration of procedures, by developing regional master plans and/or multi-criteria evaluations of projects at the very early project development phase. This enables the pursuit of projects for which all stakeholders are favourable to their realisation. Another required measure is guaranteeing the technical quality of plants which receive the feed-in remuneration by introducing a global efficiency criterion. Finally, other policy instruments are analysed such as green certificates, the feed-in remuneration scheme, and CO2 credits, which will become necessary following the opening of new gas-fired plants. The current institutional facilitation of RETs generating electricity focuses solely on quantity, i.e. kWh. It does not consider the need for flexible production and energy storage to deal with the intermittent generation of some RETs and to align the electricity demand and supply. This is not coherent. The institutional facilitation should take into account flexible production and energy storage, and thus specifically support technologies such as storage and pumped-storage SHP. This research investigated, using an explorative and bottom-up approach, the technical potential of small storage and pumped-storage plants by focusing on existing and planned reservoirs in order to reduce investment costs and environmental opposition. Eleven projects were identified in the Canton of Valais. The potential in Switzerland is evaluated at roughly 200-300 MW for storage SHP plants (today 106 MW is used) and 70-150 MW for pumped-storage SHP plants (today 15 MW is used). In order to further develop this potential, which is complementary to the large storage and pumped-storage hydropower potential, some remuneration instruments are identified. The instruments include adapting the feed-in remuneration scheme to facilitate not only run-of-river plants, introducing requirements for ancillary services from RETs (in addition to large hydropower), and CO2 credits and green certificates depending on the production profile (e.g., peak and off-peak). The identified instruments lead to policy recommendations which would further facilitate the development of SHP in Switzerland, including storage and pumped-storage plants. In summary, the main findings of this research are four-fold: The institutional framework has to further evolve to be aligned with the small hydropower technology. The institutional facilitation of renewable energy technologies must not only focus on the quantity of energy, i.e. kWh, but also on the "quality", such as flexible production and energy storage. Storage and pumped-storage small hydropower could play an important role in producing distributed peak and balancing electricity and in contributing to distributed energy storage. Its technical potential in Switzerland is significant enough to shape the institutional framework adequately. The coherence framework offers a very useful lens to analyse technological and institutional changes in the network industries. However, it still needs to be improved to become more robust and less conceptual.

EPFL2012

Chargement

Large scale energy storage may play an increasingly important role in the power generation and distribution sector, especially when large shares of renewable energies will have to be integrated into the electrical grid. Up to now pumped-hydro is the only technology that has been widely used for that purpose. However the spread of this technology is limited by geographic constraints. In the present work, a particular implementation of the storage concept based on thermodynamic cycles, as introduced by ABB Switzerland ltd. Corporate Research, has been analysed thermoeconomically. This concept can be scaled up to a GWh range of stored energy, and is site independent. The underlying idea is to use electricity to power a heat pump transferring heat from a cold source to a hot source (charging mode), while storing the heat and the cold at both sources. Subsequently the energy that has been stored/removed to/from the hot/cold sources can be used to run a heat engine, hence re-delivering the electricity when it is needed (discharging mode). In the implementation presented here, using the ambient temperature as the hot source for both the heat pump and the heat engine allows taking benefit from daily variations of the atmospheric temperature. Indeed, the heat-pump can operate with a higher Coefficient of Performance (CoP) during the night and the heat-engine at higher efficiency during the day. This results in an effective increase of the ratio between the redelivered electricity (discharge) and the stored electricity (charging), compared to a situation where the hot source of both cycles are at the same temperature. The use of solar collectors is a further way to enhance this ratio by increasing the hot source temperature above the ambient temperature during the day. In charging mode, the system consists in an ammonia based Rankine heat pump interfaced at the evaporator with a latent heat cold storage, and at the condenser to the ambient through aircoolers. In discharging mode an ammonia based Rankine heat-engine is interfaced at the condenser with the latent heat cold storage, and the evaporator(boiler) of the thermal engine is interfaced with hot water produced by a field of flat plate solar collectors. A steady state multiobjective optimization of a 50 MW power output plant was carried out, with the objectives of minimizing the investment costs, and maximizing the ratio between electricity output and input. Several types of cold storage substances have been implemented in the formulation and two different types of solar collector were investigated. A storage efficiency of 57% at a cost of 1200 USD/kW was calculated for an optimized plant using solar energy. Finally, a computation of the behaviour of the plant along the year was carried out, reaching a yearly availability of 84.4%.

Pergamon-Elsevier Science Ltd2011

Chargement

Model Predictive Control Strategies for Low-Voltage Microgrids

François Maréchal, Ramanunni Parakkal Menon, Mario Paolone

Aidic Servizi Srl2014

EPFL2012

Pergamon-Elsevier Science Ltd2011