• Energy Research

The development of photovoltaic (PV) technology is now a reality. The inclusion of lithium-ion batteries in grid-connected PV systems is growing, and the sharp drop in prices for these batteries will enable their use in applications such as PV water pumping schemes (PVWPS). A technical solution for the monitoring and tracking of PV systems is shown in this work, and a novel quasi-real-time monitoring system for a PVWPS with a Li-ion battery is proposed in which open-source Internet of Things (IoT) tools are used. The purpose of the monitoring system is to provide a useful tool for the operation, management, and development of these facilities. The experimental facility used to test the monitoring system includes a 2.4 kWpk photovoltaic field, a 3.6 kVA hybrid inverter, a 3.3 kWh/3 kW lithium-ion battery, a 2.2 kVA variable speed driver, and a 1.5 kW submersible pump. To address this study, data acquisition is performed using commercial hardware solutions that communicate using a Modbus-RTU protocol over an RS485 bus and open software. A Raspberry Pi is used in the data gateway stage, including a PM2 free open-source process manager to increase the robustness and reliability of the monitoring system. Data storage is performed in a server using InfluxDB for open-source database storage and Grafana as open-source data visualization software. Data processing is complemented with a configurable data exporter program that enables users to select and copy the data stored in InfluxDB. Excel or .csv files can be created that include the desired variables with a defined time interval and with the desired data granularity. Finally, the initial results of the monitoring system are presented, and the possible uses of the acquired data and potential users of the system are identified and described.

[EN] This article deals with the analysis of energy efficiency optimization in battery-based photovoltaic pumping schemes. The study builds on previous findings derived from the comparison between a direct photovoltaic water pumping system (DPVWPS) and the equivalent system including a lithium-ion battery (LIB). The initial experimental results of the battery-based photovoltaic water pumping system (PVWPS+LIB) were obtained with the motor-pump group operating at its rated condition of 50 Hz. In the present work, an analysis of the efficiency and the performance ratio of the variable speed drive (VSD) and the motor-pump group, showed that both parameters improve when operating in the low frequency range of the VSD and far from the rated frequency where the flow rate is maximized. Several fitting models were performed with the data obtained with the monitoring system and it was concluded that an overall energy efficiency optimization can be achieved with a VSD frequency equal to 37 Hz. A comparison of the experimental results obtained with the VSD working in the direct mode and with the battery-based solution (setting different VSD frequencies - 50 Hz, 37 Hz, and a combination of the two frequencies designed as 37/50/37 Hz) was made to determine the efficiency and performance ratios in each case. The results presented in this study also establish criteria for improving efficiencies in the LIB charging and discharging processes. This work was supported in part by the Universitat Politecnica de Valencia (UPV) (Program ADSIDEO-Cooperation 2017 through the Project Characterization of Sustainable Systems for the Pumping of Water for Human Consumption in Developing Regions and/or Refugee Camps in Kenya Through the Implementation of Isolated Photovoltaic Systems With New Generation Lithium-Ion Batteries); and in part by the International Organization for Migration under a contract entitled `Design, Assembly, Testing and Documenting Parameters of Solar and Ion-Lithium Energy Storage Equipment for Powering of Water Pumps under Laboratory Conditions' with funding from the Bureau for Humanitarian Assistance-United States Agency for International Development (USAID).

[EN] Energy efficiency in photovoltaic systems is especially important when batteries are part of the system. Performance optimization includes the selection of the best efficiency point for each stage of the system, and the inclusion of devices that reduce energy losses when some elements of the system are not operating. This paper proposes improving battery-based photovoltaic pumping systems by using high-voltage lithium batteries, combined with the inclusion of IoT switches and the operation of the pumping system at its most efficient operating point. Experimental results of a system working with different irradiance profiles are included for an analysis of the main working parameters. These parameters demonstrate how the inclusion of IoT switches improves system efficiency and provides extra energy that can be used to pump additional water or for other purposes. A comparison of the results obtained with the estimates made for direct pumping shows that the battery-based solution yields an average increase of 10.85 % on a sunny day (with 5.29 PSH) and 16.37 % on a cloudy day (with 2.88 PSH). Some water is pumped on days with less than 2 PSH, while direct pumping is unable to pump on those very cloudy days. This work was supported in part by the Universitat Polit`ecnica de Val`encia (UPV - Program ADSIDEO-cooperation 2017 - project Characterization of sustainable systems for the pumping of water for human consumption in developing regions and/or refugee camps in Kenya through the implementation of isolated photovoltaic systems with new generation lithium-ion batteries ) and in part by the International Organization for Migration under a contract entitled Greening humanitarian responses through enhanced Solar Energy harvesting with funding from the Bureau for Humanitarian Assistance-USAID and Humanitarian Aid department of the European Commission (ECHO).

Videos that show the evolution of the instantaneous temperature profile measured and determined with the models and in the cases developed in the paper "Minimum number of experimental data for the thermal characterization of a hot water storage tank" by the same authors. Thermocline thickness evolution is also shown. Video 1. Evolution of the instantaneous temperature profile estimated with the 5 MEL models during the charging period. Evolution of thermocline thickness during the same period is displayed in a separate frame also for all MEL models. Video 2. Evolution of the instantaneous temperature profile estimated with the 5 MDH models during the charging period. Evolution of thermocline thickness during the same period is displayed in a separate frame also for all MDH models.