THE SOL-ION SYSTEM: REALIZING SAFETY AND EFFICIENCY FOR A PV STORAGE SYSTEM

ZSW, Stuttgart, Germany ABSTRACT: PV Storage systems are a first step to move PV home installations towards home energy systems. Their ability to shift the “sun to the loads” qualifies them for optimizing the self consumption and to support grid stability. The Sol-ion system is one of the first PV storage system, using lithium-ion batteries, which is used to optimize self consumption. First installations in laboratory and in private house holds have already been realized. An important aspect of the development was the achievement of system safety and efficiency. Keywords: PV-battery system, lithium-ion battery, self-consumption, grid integration 1 INTRODUCTION Over the past years the perception of PV home installations starts to shift from being an object of investment towards being a component of a home energy system. This change is supported by the decreasing feed-in tariffs, the funding of self consumption and the approach of grid parity in several countries [1,2]. Additionally, grid quality has become a more and more important aspect since most PV plants are installed on the low voltage grid, where only few grid control measures are feasible [3]. Hence, the requirements for PV home installation start to change. The production of PV power needs to be in accordance to the need of the household and should not destabilize the grid. In case of grid malfunction, the PV home installation should also be able to support the grid actively or at least not destabilize the system any further. PV Storage systems are able to meet these requirements. Their ability to shift the “sun to the loads” qualifies them for optimizing the self consumption and to support grid stability. There are several ongoing research projects which have realized PV storage systems. While for larger PV installations the dominant storage technologies are Redox-flow batteries and Natrium-Sulfur batteries [7,8], lead acid and lithium-ion batteries are mainly used for small PV installations [4,9]. The French-German Research project Sol-ion falls into the latter category. The goal of this project is the realization and testing of a PV storage system using Lithium-Ion battery for use in private households [10,11]. Besides the functional requirements for these systems, system safety and efficiency considerations have to be addressed, since the systems are installed in private households. 2 THE SOL-ION SYSTEM The development of the Sol-ion system has been completed in winter 2010, while first prototype systems have already been installed in Summer 2010 at different partner sites. Additional systems are currently being installed at private households in French oversea department and southern Germany. Figure 1 shows the architecture of the Sol-ion system. A string inverter is responsible to provide a mixture of PV power and battery power to the grid. This inverter is also able to provide a stable island grid in order to allow off-grid or backup applications. The battery converter is responsible for charging und discharging the battery and has also the capability to stabilize the inverter operation. The overall operational strategies are managed by the energy management system. This unit is also responsible to decide on the most efficient mode of operation.