Making an Off-Grid Solar PV System

Hi everyone, I decided to equip my house with an off-grid photovoltaic system. My main goal is to be as independent from the grid as possible. Of course, due to the lack of sunlight during the short winter (November-February) in my area, full independence is not possible at the moment.

Nevertheless, my calculations predicted that my setup should be able to meet 80% of my home's needs throughout the year. Furthermore, if the excess energy generated during the summer months (June, July and August) could be stored for later use in the winter, complete independence could be achieved. These encouraging figures helped me make the decision to undertake this project.

As a final note, this project would not be complete without a comprehensive remote control and monitoring system. Below you will find a description of the electrical setup (energy production) and the monitoring system.

Supplies

1× Solar Field Irradiance and Temperature Module (SFIT)

Based on the Embedded Artists Android Open Accessory Kit board. Implementing the NXP LPC11C24 Cortex M0. processor

1× Photovoltaic Field Monitoring Panel (PVFM)

Based on LPCxpresso LPC1769 Cortex M3 processor. Application written in C. Hardawer and Software are open source

1× Database and Web Server

Based on Raspberry Pi Model B. Uses open source RRDtool database + Python, Javascript and Json scripts.

1× Photovoltaic Battery Monitoring Board (PVBM)

Based on LPCxpresso LPC1769 Cortex M3 processor. Application written in C. Hardawer and Software are open source

1×Online Source Selector (OLSS)

Related Notes

My photovoltaic (PV) system consists of the following main parts:

The solar field, consisting of 16 solar modules, provides 4300 Wp (Watt-peak) of electricity

Solar battery charger that converts 120 V panel output voltage to 48 V battery voltage

Lead-acid batteries provide energy at night

The inverter converts the 48 VDC voltage provided by the battery into 230 VAC - 50 Hz for household appliances

Homemade online power selector that switches between solar and grid power within 20 milliseconds

Working principle:

During the day, the solar field's 960 cells convert sunlight (photons) into electrical energy. The efficiency of this conversion is 17%. This may seem like a bad number, but since solar energy can reach 1000 W/m², under these conditions the field produces 4300 Watts. This is equivalent to the power of 7 horses. Not that bad! The solar panels are Yingli Panda 270 Wc modules.

The battery charger converts the high voltage (120 V DC) coming out of the solar field to 48 V DC, suitable for charging the batteries. Since the solar power is constantly changing, the charger constantly tracks the maximum power point to optimize the system's output. The battery charger also manages the charging process of the battery. This device is the most complex part of the system. The charger is a Victron Blue Solar Charger 150/70 MPPT.

The battery is loaded during the day and discharged at night. It can store up to 10 kWh of electricity. That's enough for one day's consumption. It's a short time, but it still weighs 500 kg (1/2 ton). I want my battery to have a lifespan of at least 7 years. The batteries are 8 Vipiemme 12V/230 AH semi-stationary cells. Increasing the capacity of the battery pack without increasing its weight and size is an important challenge for the future.

The inverter converts the 48 VDC battery voltage to 230 Volts AC. The inverter is rated at 5KW permanently and 10 KW for 5 minutes. Thanks to such a large rating, we can use all the classic "energy-consuming" household appliances, such as kitchen ovens, dishwashers, washing machines and even dryers. When the inverter delivers 10 kW, the battery current may climb to 200 Amps. The inverter is a VICTRON Phoenix 48V / 230V 5KVA inverter.

The grid is the public electricity grid. In France, electricity is mainly provided and distributed by EDF (Electricité de France). Nuclear power plants currently produce 85% of France's electricity. In my opinion, electricity is (very) cheap: about 0.12 €/kWh. You can bet that this price will go up a lot once it's time to dismantle many of France's dilapidated nuclear power plants. Our children may have to pay the bills.

The source selector is able to switch between solar and grid energy almost instantaneously (less than 20 milliseconds). This ensures that the change in energy is usually imperceptible to the inhabitants of the house. This also prevents PC and electronic equipment from crashing. The power switch occurs when the battery is too discharged to continue using solar energy, or when the battery needs to be charged again after a dull day. The source selector is designed to prevent the solar system from being connected in parallel with the grid at any time. The source selection is a completely personal design.

Effect diagram:

Full sun field

The small panel at the bottom center is the measuring panel. This panel measures the available solar energy at any time.

At night, lights indicate whether the house is powered by the PV system (cyan - blue) or the grid (yellow - red)

Inside the house: battery bank, inverter and solar charger (left to right)

The large grey box contains fuses, circuit breakers and lightning protection. The smaller one contains the PV Field Monitor (PVFM), the Raspberry Pi hosting the database and web user interface, and the CAN bus backbone.

Monitoring system (= system design document)

My goal was to accurately acquire every voltage and current present at any stage of the system. Since this was difficult to achieve using a single acquisition board, I decided to design a distributed architecture using dedicated acquisition boards for each major part of the system. The communication between the boards was performed by a CAN bus operating at 500 Kbits/s.

The monitoring system includes the following modules and boards:

Solar Field Irradiance and Temperature Module (SFIT): Connects to a dedicated small measurement PV panel. Measures available solar energy (irradiance) and site temperature.

PV Field Monitor Board: Gets the current and voltage provided by 4 strings of solar modules consisting of 4 panels. Acts as a gateway between the CAN bus and the serial port.

Raspberry Pi: Gets the data available on the CAN bus and stores it in a database (RRDtool). Hosts the MyPV web server. The Rpi is connected to my home Ethernet.

PV Battery Monitor board (PVBM): Gets battery voltage, current and temperature. Manages automatic switching between MyPV and grid power according to battery charge status. The module is equipped with a Wifi module that acts as a gateway between CAN bus and Ethernet.

Online Source Selector (OLSS): switches between MyPV power and grid power in less than 20 milliseconds. The device is controlled via CAN bus messages.

The following diagram shows how this is organized around the CAN bus:

Human Machine Interface (HMI)

Neither the board nor the modules that monitor the subsystem provide any interface to the user (but there are some activity LEDs).

This is intentional: since the system is supposed to be remotely controlled and supervised, the entire HMI must be available via some “state-of-the-art” remote functionality, such as an Android app and/or a Web User Interface (WUI). From this point of view, any efforts made on a local HMI are futile.

In fact, I have developed two types of interfaces in the last year:

PV System Manager (PVSM) Android Application: This application provides the user with a "real-time" interaction with the system. This application can be used locally and anywhere, provided there is Internet access. It is a true "peer-to-peer" application. It can only run on one device at a time. Since it allows control of the system, this application is private and will never be available on Google Play :-). Nevertheless, the Java project is open source.

The MyPV website: This dynamic site is hosted by a dedicated Raspberry Pi. The built-in RRDtool database captures the data available on the CAN bus on a minute-by-minute basis. Python scripts create valuable graphs that allow the user to understand the status of the system "at a glance". The "Select" tab gives access to "custom brew" graphs. The user can choose between 40 different data displays.

Android PVSM application executed on a 10-inch tablet