This post describes the electrical and electronic circuitry installed that controls and monitors the power generated by the two 130 watt solar panels installed above the electronics lab. A previous post describes how I installed the panels.
The Okanagan College facilities management was instrumental in getting the panels actually hooked up and generating power. They gave us permission to put the panels on the roof and even did all of the electrical installation. The electrical installation needed to be done by an electrician and Zdenek did a fantastic job not only with the installation, but also with laying out the wiring in the installation to make it easy to see and trace (this was very important since the electronic circuit is to be a teaching/demonstration platform for solar energy). So a special thanks to Zdenek and facilities management department for helping out with the installation.
The wires from the solar panel came in through a conduit in the roof. In this picture, you can see the conduit at the top and the electronics in the background.
Here is a closer, if unexciting, look at the conduit:
A more exciting and closer look at the layout of the electronics:
The DC voltage from the solar panels comes in to a DC-DC converter at the right. Next to that is a 12 volt, deep cycle, 18Amp-hour battery. To the left of the battery are some shunts for measuring the charging and discharging current and connected to the shunts is the monitoring equipment. Finally on the left hand side is the inverter which converts the 12V DC voltage into a 60 Hz, 120V (RMS) AC voltage.
This DC-DC converter outputs 12 volts regardless of what the solar panel is providing as input. Since the sun intensity falling on the solar panels can vary throughout the day and throughout the year, the DC-DC converter is a very important piece of electronics for ensuring that the voltage level used by the system stays constant.
This particular converter is a Solar Boost 2412 25 amp, 12 volt Maximum Power Point Tracker (MPPT) photovoltaic battery charge controller from Blue Sky Energy. Given the voltage and maximum current outputs, what do you think the maximum power output of the DC-DC converter is?
In the picture above, you can see 6 screw connections along the bottom. The two on the right connect to the PV panels and the next two connect to the battery. The two that are not connected can be used for charging an auxiliary battery.
Take a look at all of the components in the circuit. See how many you can identify. I was interested to see that they use a PIC microcontroller from Microchip which is a family of microcontrollers the students in the electronic engineering technology program become very familiar with.
Next to the DC-DC converter is the 12 volt battery. It is rated for 18Amp-hours which means, in theory, it could output 18 amps of current for 1 hour (or 9 amps of current for 2 hours) etc. I actually undersized the battery a bit and should have chosen a larger one. In the wintertime when there is less sun and therefore less energy to recharge the batteries when the bench is not in use, there might not be enough energy to run the lab bench solely from the solar panels. In fact a couple of times, students had the computer shut off on them as they were working, but fortunately not too much work was lost. We now plug the computers in to the wall outlets.
Four points are monitored in the system are monitored by the PentaMetric PM5000 Input Unit:
- Battery Voltage
- Current output from the DC-DC converter (source)
- Current input to or output from the battery
- Current to the inverter (load)
To monitor the currents, three 100 amp, 50mV shunts are used. These shunts work by inserting them in series with the point in the circuit you wish to measure, then the voltage across the shunt is accurately measured by the PentaMetric unit. Since the resistance of the shunt is known, the PentaMetric unit can then calculate the current at the measurement point using Ohm’s Law.
Here is a closeup of the three shunts:
The top one measures the current to the system, the middle one measures the current to the battery and the bottom one measures the current from the DC-DC converters. Unfortunately the supplier mixed up the shunt for measuring the battery and it is rated for 100 A, 100 mV. Since the PentaMetric unit is expecting 100A, 50mV, the current measurements that it does for the battery current are twice what they should be. See if you can calculate the resistance of each of the two shunts (one drops 100mV at 100A and the other drops 50mV at 100A).
Finally, the inverter converts the 12V DC signal to a 60Hz, 120V AC signal. The inverter is a pure sine wave with less than 4% total harmonic distortion. Inverters involve some pretty cool electronic circuitry which maybe I will get in to in a later post, but basically they take the DC input and using transistors and some control circuitry switch the input between positive and negative which creates a square wave. Then the signal is filtered to eliminate the harmonics and create a sine wave.
The inverter then connects through a fuse to a standard three-prong wall outlet.