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INNOVATIVE ENERGY STORAGE SYSTEMS FOR PV GRID-CONNECTED APPLICATIONS H. Colin1, J. Merten1, A.Graillot2, X.Vallvé2, G. Sarre3, A. fedzin4, P. Gaillard5, JP. Smaha6 (1) CEA/INES-RDI, 50 avenue du Lac Léman 73377, BP332, Le Bourget du Lac, France – Tel. +33 4 79 44 45 48- Fax +33 4 79 68 80 49 – [email protected] , [email protected] (2) Trama TecnoAmbiental S.L. (TTA), c/Ripollès, 46, 08026 Barcelona, Spain - Tel. +34 93 446 32 34 - Fax +34 93 456 69 48 - [email protected] (3) SAFT, 111 bd Alfred Daney, 33074 Bordeaux, France - Tel. +33 5 57 10 64 09- Fax +33 5 57 10 64 12 79 44 45 48 – [email protected] (4) ENERSYS, ul. Leszczynska 73, 43-301 Bielsko-Biala, Poland - Tel. +48 33 822 52 23- Fax +48 33 822 52 24 79 44 45 48 – [email protected] (5) MAXWELL Technologies sa, route de Montena 65, 1728 Rossens, Switzerland - Tel. +41 26 411 8539- Fax +41 26 411 8505 – [email protected] (6) HAWKER sarl, rue A. Fleming, ZI Est, BP965, 62033 Arras, France - Tel. +33 3 21 60 24 43- Fax +33 3 21 60 25 74– [email protected] ABSTRACT: The number of decentralised electricity supply systems based on renewable energies has grown exponentially during the last years. These systems improve the energy efficiency of the electric system as they are installed closer to the location of the consumption. However, they are sometimes connected at the end of grid lines where the grid may be weak. Therefore, these systems may have a significant impact on the electric system or be victim of malfunctions. INES-CEA, TTA, SAFT, ENERSYS, MAXWELL and HAWKER have launched a project cofinanced by the European Commission in order to develop an inverter, dedicated to the injection of photovoltaic energy into low voltage grids, with special features so that: The impact on the grid of the PV system is minimised and even more, the system provides grid support on demand, The performance of this PV system is increased, The end user is protected against poor power quality and outages of the grid. The article describes the benefits of the inverter, the sizing of the components, the use of innovative technologies for the storage system and the field validation of the concept. Keywords: Small grid-connected PV systems, Battery storage and control, lithium, supercapacitor 1 INTRODUCTION In order to make PV electricity generation more attractive from a technical point of view and increase its acceptability, it is necessary to demonstrate its ability to supply high-quality service, reliability and profitability. Power electronic devices that decrease the impact of PV generation on the grid, that provide additional services such as power quality to the end-user and the support of the grid for the utility are enabling technologies for this increased penetration. The present paper presents an analysis of the quality of the grid and the interaction between grid and PV systems in developed countries. These considerations have led to the implementation of a project aiming at designing and developing an innovative inverter fulfilling the functionalities previously mentioned. Data about the sizing of the different components, including the inverter itself and innovative storage systems are presented in this paper. 2 CONSIDERATIONS ABOUT THE GRID 2.1 Energy supply instability Disturbances in the voltage supply can cause tripping or even damage to sensitive equipments. These disturbances include voltage sags, dips, transients, swells, harmonics as well as short interruptions. They are generally caused by weather, accidents or utility equipment failures. The interruptions can range from only minor events lasting few seconds to blackouts such as the ones experienced in the USA or in Europe on 5th of November. The frequency of occurrence of these events depends on the strength of the electrical system. The situation is quite heterogeneous in Europe: they are very scarce in centralised and oversized grids, such as in Germany or France, whereas they happen relatively often in weak grids. For instance: A study made in 2004 indicates that costumers in Eastern Europe are facing up to nine interruptions per year with a total duration over five hours [1], A publication of the French Utility EDF reported an average cumulated duration of interruptions in 2005 of less than 1 hour, A case study in Spain has evaluated the interruption duration according to the density of connections to the grid. In urban zones, with high density, the repartition is illustrated by Figure 1, which shows that a device able to supply electricity for at least 3 hours would cover about 90% of the interruptions. Repartition of interruption by duration in urban zone 40 35 30 25 (% ) 20 15 10 5 0 0-3 3-60 60-120 120-180 180-240 >240 Interruption duration (minutes) Figure 1 – Repartition of interruption by duration in urban zone 2.2 Interaction between grid and PV systems The cumulative installed PV capacity has been expanding this last decade by about 30% per year. This increase is related to the share of grid-connected systems, whose market is driven by feed-in tariffs in industrialized countries. Some studies related to PV systems connected to the grid already show the mutual interaction of PV systems and the grid. a) The impact of PV systems on the grid From a quality point of view, utility specialists’ opinion is that PV private installations do not have any impact on the grid due to the limited level of power produced. But in the case of concentrated PV systems, the situation may be more critical as it has been shown in Oota City, Japan [2]: under certain circumstances like high level of irradiance or low level of consumption -for instance during week ends- the voltage at end of line, far from the transformer, increases above the security threshold and leads to disconnections and energy loss of the conventional PV system. From a network planning point of view, a large coverage of the energy demand by PV systems can have an influence on the network operation as the production of green electricity enables to smooth the load curve: the daytime peak of consumption may disappear, in a case of pure grid-connection, as it has been illustrated (see Figure 2) by the study [3]: no PV penetration 10% PV penetration 20% PV penetration 30% PV penetration 900 800 700 600 phenomena is of great importance for electric systems managers and producers. The origin of these disconnections is not yet well known as very few studies have been conducted on this topic. Nevertheless some possible criteria are: Voltage (increase of voltage as mentioned in clause a, voltage sags), Frequency, Impedance. These disturbances or fluctuations on the grid are detected by the inverters, which disconnect the PV array very rapidly. In given configurations [2], the loss of production can reach some days more than 50 %. c) Solution These two types of interaction between grid and PV systems are strong arguments in favor of a grid-connected PV system including a storage function. The interest is threefold: Firstly a battery can store the energy in excess In case of inverter disconnection, the PV electricity that cannot be fed into the grid is stored; this avoids losses of energy, and thus leads to an improved performance ratio of the system, Or when the level is consumption is low, storing energy instead of injecting on the grid can limit the increase of voltage, Secondly the stored energy can be supplied to the loads during the evening peak of consumption so as to reduce the amount of energy requested from the grid, and thus smooth the load profile. Finally in case of grid shortage, which can last from few seconds up to few hours, the stored energy can be supplied to the end user during this interruption. We saw that autonomy of 3 hours would cover 90% of the interruptions in urban configurations in developed countries. 3 SOS-PV project These considerations have led to the project of developing an multi-functional grid-connected PV inverter, including a storage function, dedicated to the injection of photovoltaic energy into low voltage grids, with special features so that: The PV system provides grid support on demand, The end user is protected against poor power quality and outages of the grid. W 500 400 300 200 100 0 00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 Day hours 16:00 18:00 20:00 22:00 00:00 Figure 2- Urban load curve in Spain assuming different rates of PV coverage of the energy demands. b) The impact of the grid on PV systems The grid disturbances that PV systems suffer are mainly their disconnections leading to a loss of productivity. The understanding of these disconnection 3.1 Market study To achieve this overall objective, the first step in the project consisted in an analysis of the potential market for such a system. This market can be split between short term and long term applications. The short term application market is deduced from the present quality of the networks, the existing market for PV systems and for uninterrupted power supply systems, The long term application market implies a change in markets and regulations so that the owner of a SoS-PV system receives a monetary advantage for supporting the network by providing energy or by shedding from the grid on demand. 3.2 Choice of scenario In the short term scenario, a single feed-in tariff is applied and the storage is used for UPS purposes and for storing the PV energy in case of disconnection from the inverter. In this case, the size of the array is determined so as to cover the annual consumption and the inverter should be able to feed the totality of the PV power to the grid. In the long term scenario, there are two possibilities: Either the system is designed for grid support purpose and for securing the house. In this case, the inverter does not need to be of the size of the PV array since injection will happen only when the grids requires support. The storage system should have a larger size in order to store the PV energy for later feeding, Or the system is operating with real time pricing and it is of interest either to feed as much energy as possible during the peak times or to shed from the network. For the study it was decided to focus on the long term scenario. 3.3 Sizing of components The sizing depends basically on the general requirements for the system, the energy demand, the solar radiation and the feed-in tariffs in each country. a) PV array We made the assumption that feed-in tariffs may decrease and disappear in the long term. In this situation there is no interest to have a large PV array: the right size is the one enabling to cover the energy consumption. According to the evaluation of the yearly consumption of southern and northern countries in Europe, and the optimum yearly productivity of 1 kWp of PV modules, it was possible to determine the range of power to cover the needs of a household: between 4 and 6 kWp. b) Inverter In a grid-connected system, the inverter only runs at 100% in few occasions (high irradiance), but has a low efficiency the rest of the time. Having a battery in the system permits to undersize it as it is possible to store the PV energy during the peak of generation and use it afterwards. An analysis of the consumption profiles in Spain of residential users and mixed type (i.e. residential and SME) users in winter and summer times showed that an inverter of 1,5 kW would be sufficient to secure the critical loads (size for grid support and house secure mode). In the real time pricing mode, the inverter should have the same size as the PV array to supply not only critical loads. In this case connecting in parallel two 2,4 kVA inverters can be an optimum (as it falls below the single phase limit of the German regulation) and gives high modularity. c) Battery The battery has to be sized in order to fulfill two ways of operation: Normal daily operation where the battery is used to delay the injection of energy into the grid, Operation in case of grid shortage where the battery should provide energy to critical loads. The previous consumption profiles were also used to evaluate the size of the battery bank in the first mode. The most constraining scenario led to a daily need of 11000 Wh. As said before 90% of interruptions would be covered by a storage system with 3 hours of autonomy; the need for critical loads is therefore 4500 Wh (3 hours at 1,5 kW). This makes a total capacity need of 15,5 kWh. 4 INNOVATIVE STORAGE SYSTEMS The energy for the power quality and UPS functions as well as for the grid support will be provided by 2 types of storage systems that proved to be most adapted to this application after evaluation in the INVESTIRE thematic network. A lithium-ion based system, A hybrid system combining a lead-acid battery and supercapacitors. Both types of storage systems have never been used yet in this application. They have the following advantages: Maintenance free operations, Long life duration, Positive impact on environment. 4.1 Innovative aspects The first innovation in this project is to associate in parallel a valve-regulated lead-acid (VRLA) battery with supercapacitors as a hybrid system in order to provide the following advantages for the lead-acid battery: Decrease the effect of the “coup de fouet”, Limit the depth of discharge, Absorb the peak power pulses in discharge, Allow to use a lower capacity thus decrease the cost, Decrease the weight and volume. The second innovation in terms of storage is to use a lithium battery of large size in an application where it was never used yet. This type of battery provides : High energetic efficiency (> 95 %), Operation whatever the state of charge of the battery, Low weight and volume, Low life cycle cost. 4.2 Lithium-ion based system The battery bank is based on Saft’s Li-ion VL45E elements, which can supply a maximum of energy within a compact and light packaging. It has a low self-discharge and gives an excellent reliability during its whole lifespan. It is highly suited to any charge/discharge cycling application that demands a battery with drastically reduced weight and volume. Elements are gathered by group of 14 cells in a module; each module includes an electronic board for safety management, cells balancing and data acquisition (thermal and electrical state of the storage system). According to the energy needs, 8 modules are assembled in a cabinet according to a rack configuration (see Figure 3). Figure 4 – Lead-acid battery Figure 3 – Li-ion modules in cabinet During normal operation the battery voltage will stay between 380 V and 448 V; thus, there is no need of a DC/DC converter to connect it to the 400 V DC bus linked to the inverter. In case of emergency, during an interruption of supply from the grid, this voltage may drop down to 336 V. The 112 Li-ion cells are able to provide the requested 15,5 kWh especially at the end of life of the battery (20 years). 4.3 Hybrid storage system The hybrid storage system is composed of a lead-acid battery and a supercapacitor, which will cover the power peaks. The goal of this parallel construction is to extend the lifetime of the battery by suppressing the high currents. a) Lead-acid battery The battery bank is based on Enersys’ VRLA batteries with AGM construction. Internal electrochemical design has been adapted to the cycling requirement in order to achieve a long cycle life (7-10 years). To ensure this long life, daily cycles are made between 50 and 90 % of the state of charge, which statement leads to a global energy capacity of near 28 kWh. The reserve autonomy, in case of grid shortage, is higher than 5 hours (between 50 and 20% SOC). A prototype of the VRLA battery is designed to achieve a low cost battery with longer life and better reliability. This prototype is based on a 12 V module in an existing box and lid with revised electrochemistry: adapted grid design, adapted paste formulation, specific AGM separator (see Figure 4 ). The battery bank is made of 32 cells of 2 V put in series, giving a nominal voltage of 64 V. A DC/DC converter steps up the voltage up to the 400 V of the DC bus. The battery management system is included in the inverter. b) Supercapacitor The supercapacitor is based on Maxwell’s BOOSTCAP® cells. It is sized to level off the peak current from the battery and thus ensures the function of a low pass filter. The role of the supercapacitor is to supply current to the loads if the demand exceeds the maximum tolerable for the battery (i.e. maximum battery current allowed). During the charge and discharge phases the voltage across the BOOSTCAP® terminals changes from a maximum value to a minimum value. In order to use the available stored energy at high efficiency a voltage variation of 50% should be set. The unit is coupled to the VRLA battery DC-bus through a bidirectional DC/DC converter. One aim concerning this component was the increase of the cell voltage of the capacitor, which results in an increased power as well as energy density via appropriate selection of the electrode/electrolyte combination and improved electrode design, and the increase of the electrochemical stability regarding temperature domain and cycle life via the selection of electrode materials with tailored pore structure, novel electrode design, and a matching electrolyte. The supercapacitor is made of 2 modules of 18 cells of 2,7 V put in series, giving a operating voltage of 97,2 V. The capacity of each module is 165 F (see Figure 5). Figure 5 – Supercapacitor The module includes cell voltage management electronics where each cell is monitored; this leads to improved efficiency and operation. c) Operation The principle is that the battery delivers the energy needs and the supercapacitor supplies the power needs. In the case the storage system has to provide the loads with energy, if the demanded peak current exceeds the maximum current allowed for the lead-acid battery, the supercapacitor is requested to supply this current; otherwise the battery is discharged accordingly. The supercapacitor is recharged by the battery itself or the PV generator according to the load demand. The battery is recharged by the PV generator (normal daily charge and equalization charge regularly or when the low state of charge threshold is reached). 5 VALIDATION The individual components are tested individually to check their characteristics and performance before integration, and then the whole system is tested in the field. 5.1 Battery testing Tests are performed to check the initial characteristics of the storage systems (capacity, efficiency, internal resistance, self-discharge) and their cycling ability according to the load profiles defined previously. The test of the storage system (whole unit that is connected to the 400V DC bus, i.e. storage device + converter + BMS) are done independently of the technology and all procedures are applied to one system of each technology. 5.2 Inverter testing Tests are performed to check the characteristics and performance of the inverter: static power efficiency, total harmonic distortion, power factor, start-up sequence, losses, reaction to disconnections, and behavior of MPPT. Some tests related to the inverter with the storage function will also be performed according to the standard concerning UPS installations IEC 62040. Electromagnetic compatibility tests are also foreseen. 5.3 System testing Four PV systems (two with the Lithium based storage and two with the hybrid storage) are going to be tested in the field to validate the new functions: 3 sites (2 in Spain and 1 in France) have been selected which exhibit different classes of grid weaknesses and different ratios between PV generation, storage, and consumption. Typical single household system with a PV power of 2 to 3 kWp will be considered. The installations and site management will be executed including monitoring equipments. The systems will be tested (reaction to grid interruptions and voltage perturbations) and monitored during at least 5 months of operation for what concerns the energy flows and the grid stability parameters. The data resulting from the field tests and the field operation will then be analysed for quantifying grid stabilisation services provided and the efficiency of the systems components as well as PV production comparing to conventional PV inverters. 6 CONCLUSION The present paper deals with the integration of innovative storage systems in the PV grid-connected installations in order to address the different problems encountered in the low voltage distribution grids. The interest of a storage function in the PV gridconnected systems is to improve the mutual impact between grid and PV systems, and increase the performance ratio of the PV installations. The modular architecture of the system enables to select two different storage technologies using the same system components: either on lithium-ion or a combination of lead-acid batteries and supercapacitors. Based on a long term scenario, in which the system is designed for grid support purposes and the security of supply for the house needs, sizing has led to a large storage system in order to store PV energy for a deferred feeding (11 kWh for daily cycling and 4,5 kWh as reserve for emergency use) and an inverter power (2,4 kVA) much lower than the PV generator power (4 to 6 kWp). Test procedures have been defined on component and system level in order to validate the concept in the field and assess the different storage solutions selected. 7 REFERENCES [1] ERRA, EU accession Countries Working Group, Quality of Electricity Supply – Comparative Survey, April 2004 [2] Y. Ueda, K. Kurosawa, Performance Analyses of battery integrated grid-connected residential PV systems, 21st European Photovoltaic Solar Energy Conference, Dresden, 2006 [3] A. Graillot, X. Vallvé, M. Perrin, E. Bosch, Interest of a storage system in PV grid-connected installations, 21st European Photovoltaic Solar Energy Conference, Dresden, 2006 Acknowledgments: This program has been conducting with the support of the European Commission.