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From Room to Field: Solar-Battery Coupling Feasibility Study
O. Astakhov, T. Merdzhanova, L.-C. Kin, U. Rau
PV Systems and Storage – Modelling, Design, Operation and Performance
Subtopic: Storage
Event: 37th European Photovoltaic Solar Energy Conference and Exhibition
Session: 5EO.2.4
ISBN: 3-936338-73-6
0,00 EUR
Document(s): presentation


Integration of photovoltaics (PV) with electrical energy storage (battery) at different levels and scales from tiny PV-harvester for an office room application to a PV-system in the field is a straightforward approach to turn intermittent power source into stable power supply. This PV-battery unit has to ensure proper power coupling of a PV device to a battery and finally a load. Main solutions to the power coupling include use of maximum power tracking (MPPT) electronics or “direct coupling” / “direct impedance matching”. For direct coupling parameters of a PV-array and a load are preselected to provide impedance matching. The direct coupling of a PV to a battery reduces system complexity and cost, may improve system reliability and give highest energy efficiency for specific conditions. The system is free of interference signals and fluctuations related to MPPT operation. These advantages come at a cost of system flexibility in terms of the load parameters and limited range of PV-module/array operating conditions. The range of these “specific conditions” is in focus of our study where we address the potential performance of directly coupled PV-battery system in variety of irradiances and load power demands via simulations. The matching in PV-battery system is quantified with a “coupling factor” C - a ratio of the working point power PWP to the maximum power PMPP of a PV-module. The coupling factor has been studied as a function of irradiance Ee, battery voltage VOCB and load resistance RL for two distinct PV-battery utilization cases: “PV-harvester” – predominantly low irradiance, non-demanding load, but full system autonomy; and a “PV-system” – predominantly high irradiance, highly demanding load, and partial system autonomy. Detailed analysis of IVs of PV-module and battery shows that series resistance of a battery may have positive effect on the PV-battery coupling, i.e. the loss related to the battery series resistance can be partially compensated by better matching of PV-battery device. We show that it is possible to find a range of PV-battery combinations with C ≥ 0.9 over the whole target range of irradiances for both PV-harvester and PV-system cases even if specific load resistance or its power demand vary in wide range. We confirm feasibility of intrinsically matched PV-battery combination to provide excellent power coupling under variety of conditions and demonstrate that it is a viable alternative to commonly used systems with MPPT electronics. The study opens a new prospective of the usage of batteries in PV-systems not only as a storage, but in addition as an efficient impedance matching element.