Monitor Your Solar System with a Raspberry Pi

Most photovoltaic systems contain parts such as the solar modules (panels) to provide the electrical power, a battery charger for converting the panel output to the battery voltage, a battery pack to store energy during the day and provide it during the night time, an inverter to transform the battery voltage to the proper line voltage for operating home appliances and an line source selector to switch between the solar and grid power.

When the sun is shining during the daytime, the solar photovoltaic cells convert the sunlight falling on them into electricity. Although the efficiency of the conversion may be only about 17%, solar power can easily reach 1KW/m2 and suitable panels can produce 5000 Watts in these conditions.

Solar panels typically produce a high voltage, 120V DC being a common figure. The battery charger has to convert this to match the battery voltage, generally 48V DC. Solar light power charges the batteries continuously during the daytime; therefore, the charger has to keep tracking the maximum power point to optimize the yield of the system. As the charger has to charge the battery also, this device forms the most elaborate part of the system.

With the above arrangement, the solar panels charge the battery during the daytime and the battery discharges during the night. The size of the battery depends on one day of consumption plus some extra to tide over an overcast day. That also decides the size of the solar panel. Batteries are essentially heavy and the lead-acid types generally have a lifespan of about 7 years.

The batteries feed the inverter, which converts the 48V DC into the line voltage – usually 230V AC or 110V AC. With a 5KW continuous rating, inverters can essentially run almost all household appliances such as the clothes dryer, the washing machine, the dishwasher and the electric kitchen oven. When the inverter is supplying a large load, the battery current may climb up to 200A.

Multiple sensors measure the solar field power from and temperature of the solar modules divided into arrays. The information comes to a PV panel via a CAN bus, which unites all the sensors. The PV panel also acts like a gateway between the CAN bus and a single board computer.

The tiny, versatile single board computer, the Raspberry Pi or RBPi is suitable for gathering data from the PV panel and storing them in a database. On the RBPi is a web server connected to the home Ethernet network.

Another set of sensors monitor the battery voltage, current and temperature. These are also on CAN bus and the information collects on a PV battery monitor board. A Wi-Fi module on the board acts as a gateway between the CAN bus and the Ethernet.

The boards and modules of the monitoring subsystem do not provide any interface with the user, except for a few activity modules. The system is meant for being supervised and controlled remotely. This is possible with a Web User Interface or an Android application.