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Extra Components & Design Decisions for Solar Powered Server
The solar panel in our recommended system is a monocrystalline silicon panel. The embedded energy of monocrystalline silicon is among the highest compared to other types of panels, at approximately 6,000 MJ/m^2. The EPBT is also among the highest, at approximately 8 years, and the EROI, at 9.0, is among the lowest. Despite all this, the mono-Si panel has among the highest efficiency at roughly 14%. The high efficiency of mono-Si panels make them highly desirable for our purposes, as this allows us to use a single panel on a roof instead of installing an entire array to power the server.
MPPT charge controllers are some of the most efficient controllers on the market and are specifically designed to harness the full power from the solar panels to charge the batteries most effectively. For example, if it becomes cloudy, a MPPT charge controller will decrease the amount of current drawn in order to maintain a desirable voltage at the output, viceversa when it becomes sunny again, the MPPT controller will allow more current from the solar panel once again. MPPT charge controllers also limit their output to ensure batteries don’t get overcharged as they will monitor and adjust their input accordingly, therefore extending the life of the battery. There are many MPPT charge controllers on the market, some far cheaper than the one we are using, but we wanted to be able to get actual real-time data from our system to keep track of specific data points such as PV Voltage, PV overall Power, Battery Voltage as well as Load Voltage/Current instead of deriving this information from external sources and calculations, therefore we chose a reliable charge controller that speaks Serial communication in order to capture this data in real time directly from the charge controller with minimal external hardware.
We chose to use Sealed Lead-Acid batteries (SLAB) because it’s a very mature technology and these types of batteries are fairly resilient to abuse, overcharging and different environmental conditions. Since we are hoping people across the world can replicate this system, we wanted to use components for our system design that are widely available in most markets around the world, that are affordable and, when not available, that can be shipped easily through post and courier services.
Another strategy we implemented to protect our system components but in particular the battery, was to add a battery protect circuit by Victron in order to allow the battery to get properly charged before the system would allow power to flow to the server, so that the server and battery would not be simultaneously competing for power. In most renewable energy systems, batteries are usually the most expensive components as well the ones with the shortest life span as most batteries usually need to be replaced in ~5 years if properly maintained. Through the use of an MPPT charge controller and the implementation of a battery disconnect circuit we wanted to ensure the healthiest life for our battery.
We employed 4 fuses in different parts of the circuit to protect the most critical elements in our system. In-line fuses allow certain lines to be isolated in the event of a fault so that the rest of the wiring system won't be affected.
- A Renogy 15A waterproof in-line fuse holder with fuse is used on the power line of the PV panel to protect the panel and charge controller.
- A 20Amp circuit breaker in-line fuse is used to protect and isolate the battery.
- A 10Amp circuit breaker in-line fuse is used to protect the charge controller output line, connected to the battery disconnect circuit.
- A 10Amp in-line fuse is included in the USB power outlet socket to protect the USB ports from being overloaded.
The Voltage output of the charge controller is equal to the Voltage of the battery the system is connected to, therefore if using a 12-14V battery, a Voltage regulator circuit is needed to power 5V devices safely. We have chosen to use off-shelf USB Charger Socket Power Outlet (normally used in car, boat and RV mobile installations) rated at 2.1A or 3A and equipped with in-line fuse to safely and convenienty power the RaspberryPi or other USB 5V devices.
Because the Charge Controller we chose to use offers a data port but speaks the RS485 Serial protocol, we had to translate signals using an USB to RS485 Converter Adapter based on the high performance ch340T chip which allows the RaspberryPi (and other microprocessor and microcontroller devices) to read the Charge Controller data. This chip is supported by most Operating Systems and therefore drivers are widely available for this specific hardware.
Our biggest hardware inspiration for this project was LOW←TECH MAGAZINE solar-powered website system. We began our explorations using the same single board computer, an Olimex board, which seemed really promising because of its low power consumption, but the board required a backup power supply to gracefully shut down and restart. RaspberryPi 4s had just been released when we embarked on this research and as part of their launch news circulated that the RaspberryPi Foundation was running their website off of a cluster of RPi4 computers, which do deal more effectively with power outages.