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Virtual Power Plant model 

Virtual Power Plant model 

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Conference Paper
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The exploitation of distributed generation based on intermittent renewable energy sources (RES) has increased the load and generation profile variability. The resort to distributed energy storage systems (DESSs) is usually proposed to compensate the volatility introduced by RES. In particular, plug-in electric vehicles (EVs) are considered one of t...

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... order to determine the EVs plug-in availability, the data from the survey “Mobility in Germany” (MID 2008) [12] have been considered. These data have been filtered out according to several criteria: number of trips traveled each day, mean of transportation, travel distance and time per day, and purpose of trips. Six different modes of transport have been examined: passenger car driver, car passenger, bike, on foot and public transport. Moreover the travel motives have been classified in seven groups: leisure, work, education, private business, shopping (daily needs) and accompanying. Afterwards the data of [12] have been employed for calculating the hourly percentage of cars en-route. The Fig.1 shows the statistical time distribution of cars on the road in Germany for an average week. In addition for each trip purpose the percentage of trips traveled by car has been evaluated and used as input for the proposed modeling. The data are reported in Table I. A deeper analysis has highlighted the main destinations for leisure and private business journeys. The results are depicted in Fig. 2. It can be observed that the prevalent destinations are places where a charging station could not be available. As a consequence the V2G mobility model has to take into account both the mobility behavior (vehicle on the road or parked) and the statistical time distribution of destinations with the aim of evaluating the available access points. In fact this data is fundamental for the real time estimation of EVs capacity usable for applying V2G services. Basis on these considerations a novel mobility model devoted to calculate hour by hour plug-in EVs connectivity is proposed. In order to carry out the distribution of parked EVs according to motives and destinations of trips, a hourly model for an average week is proposed. The calculations have been performed with the following equations: where t is the time step, V p (t) indicates the EVs parked at time t and V r (t) represents the EVs on the road at the same time. The EVs connected to the grid V c have been determined as follows: where D i (t-1) is the share of EVs en-route in the hour t-1 , differentiated by travel destinations i ; p represents the share of parked EVs that are connected (plug-in ratio) for each destination i ; C i (t) is the share of EVs on the road in the hour t which are coming back home, starting their homeward journey from the position i . In Germany, as stated in [12], the trip distance driven by about 87% of vehicles is less than 25 km and the driven time does not exceed 30 minutes in about 77% of car trips. For these reasons in the simulation it has been assumed that trips starting in hour t last less than one hour. As a consequence in the hour t+1 such EVs will be parked and available for V2G, according to their destinations and charging structures accessibility. The simulation of the EV battery storage system is based on the model proposed in [13]. Several tests were carried out on a LiFePO 4 battery and charging and discharging models were developed. The obtained results confirmed a good matching between the simulations and the experimental tests. The model has been used to develop the charging and discharging process simulations of a LiFePO 4 battery used in a commercial EV, characterized by the technical parameters reported in Table II. The developed battery model allows to simulate the time evolution of energy, voltage and SoC for an EV LiFePO 4 battery. The results obtained have then been used to perform the V2G simulations in the VPP model. More details about the LiFePO 4 battery model and simulations can be found in [13]. III. V IRTUAL P OWER P LANT M ODEL The proposed VPP is schematically reported in Fig. 3. It is supposed to be geographically sited in Germany and characterized by a weakly interconnection to the main grid. The VPP is basically composed of three power plants (a CHP plant, a wind park and a PV farm), an industrial zone and a city district, characterized by the power capacities and demands reported in Table III. A gas turbine, which burns natural gas, is considered as the prime mover of the CHP plant. Additionally, it is supposed that the CHP is heat driven and the heat to power ratio is set to be one. An external boiler is assigned to the CHP plant, in order to avoid excess electrical production during heat demand peak. The city is considered to have 20,000 habitants. The car fleet of the overall energy district is characterized by 11,000 vehicles, equal to about 550 passenger cars per 1,000 inhabitants [14]. The total share of annual wind and photovoltaic energy production is equal to 20% of the annual district electrical energy demand, respecting the EU goal and reducing the energy dependence from fossil fuel plants. To evaluate the wind and PV annual production, generation profiles of German wind and PV farms have been used. The hour profile of the city electricity demand has been modeled referring to yearly measured data of German power networks. Contrariwise, the thermal and electrical behavior of the industrial zone has been evaluated employing data found in the scientific literature. An EMS coordinates the energy production depending on weather conditions, storage capacity and availability, and load demand. The EMS is modeled in Matlab environment as a real time management system, which works on time hour base. Moreover the developed algorithm allows to carry out an annual energetic analysis for the proposed VPP. The EMS task is the achievement of energy balance conditions between the hourly local energy generation and the hourly consumption. To make this possible, the EMS: • controls the CHP and external boiler energy production with an active regulation; • manages the RES production, reducing the power coming from wind plant and PV farm in case of high production; • regulates the industrial load demand with a Demand Side Management (DSM) control which allows to avoid or decrease peak load; • coordinates the charge and discharge cycles of the battery storage systems in order to exploit the RES electrical production and to support the system during peak load. In the developed model an EVs fleet is introduced. In order to be able to charge and discharge according to EMS signals, providing V2G services and exploiting their batteries as DESSs, the vehicles have been supposed to be equipped with bidirectional power connection and adequate meter and communication systems [15]. IV. A NNUAL VPP C OST A NALYSIS This work aims to analyze the impact of accessibility of EVs charging points on the economies of the proposed VPP by the evaluation of the Total Cost (TC) function. The VPP TC calculation has been integrated into the EMS algorithm and determined on an annual basis, according to ...

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