Dynamic mismatch losses of grid-connected PV-battery systems in residential buildings

Abstract

Battery energy storage devices coupled with photovoltaic (PV) systems have to react to the fluctuating nature of the PV output and electrical demand in residential buildings. However, the charging and discharging power cannot be adjusted to the power fluctuations without any time delay so that an inherent temporal mismatch between the battery power and the residual power (PV output minus load) occurs under dynamic conditions. This paper presents a simulation study analyzing the response time-induced mismatch losses of grid-connected PV-battery systems. Firstly, the response time issue is analyzed theoretically to reveal how the energy flows between the household and the grid are changed due to the delayed battery response. Then, measurements are presented to show the dynamic behavior of two different PV-battery systems. The experimental results reveal settling times of up to 7 s to reach the steady state after an abrupt step change in the load demand. Thirdly, simulations of an AC-coupled PV-battery system are carried out with a time step size of 1 s. A sensitivity analysis is conducted to identify the impact of the dynamic response on the annual amount of energy exchanged with the grid. It is found that longer response times are accomplished by an increase in the grid feed-in and grid supply as well. Consequently, a slow-reacting battery system diminishes the economic benefit for the owner of the PV-battery system. In order to improve the transparency from the end-customeŕs point of view, response time-related specifications should be stated in the data sheets of the products in the future.