The power supply for DIY projects located "remote" from AC/wall sockets, generally rely upon batteries, optimally recharged via solar cells. For example, the Garden Light Project for illuminating a walking path made from six salvaged high current LED's and controlled via Bluetooth/PIC microcontroller.
Solar cell battery charge regulators obviously add to the cost of such a DIY project (and while relatively minor, require mounting and wiring etc). This lead to the investigation of constructing DIY solar cell battery charge controllers. Which in turn, via exploring maximum power point tracking (MPPT) charge controller algorithm/circuitry, resulted in this project for monitoring the voltage-current(IV) characteristics of solar cells I had on hand (partly to determine if a MPPT approach would produce significant power/charge advantage over a simpler design).
Further, concurrently I was learning about utilising the ESP8266 WiFi module to enable WiFi connection of DIY projects to the home intranet/web servers. Thus, the IV-curve tracker was a good example of a project that would benefit from WiFi connection to the home intranet and a WAMP server to store collected voltage, current, temperature and ambient light data to a mySQL database. Thus this project had a two-fold objective of collecting IV-curve data from solar cells under various ambient real world conditions and practice receiving and storing data via WiFi/HTML/php/mySQL (with a web-based GUI enabling IV-curve calculations and statistical analysis).
The actual circuit used to realise the IV-curve tracer is discussed in the Circuit Details and Schematic Diagram sections, with example results detailed in the Testing/Experimental Results Section. These sections also contain details about the ESP8266 WiFi transfer of collected data to the web server/mySQL database. Some further background material on IV-curve tracing for solar cells follows.
Solar Cell Maximum Power Point (MPP)
A solar cell converts light directly into electricity by the photovoltaic effect, and as such has a non-linear relationship between applied light, ambient temperature (and other factors) and the maximum power that can be produced (1). A full discussion of the IV characteristics of a solar cell and maximum power requires consideration of the photovoltaic effect (and associated quantum physics concepts), silicon dopants and the notions of PN junctions with charge carriers such as "holes" through to dark current, PN junction baising and the Shockley diode equation. A relatively "math" free and "intuitive" discussion of such topics in relation to the photovoltaic and operation of solar cells is given in the following reference (2).
However, for the purpose of background to the utility of having a IV-curve tracer (and hence the DIY project to construct such a device), consideration of the IV-curve's (obtained "manually" by using a DIY Dummy Electronic Load under differing illumination conditions) in the following figure is sufficient.
The solid red line in Figure 1 (with data points denoted by the red squares) shows that for a particular illumination level the voltage of the solar cell decreases as the current increases, and that the current plateau's over much of the range. If for each data point in the IV curve, the voltage is multipled by the current (i.e., calculate the power at the particular voltage) the resultant calculated power can be plotted against voltage (the dotted red line in Figure 1 is the PV curve corresponding to the solid red line which is the IV curve). It is readily apparent from the PV curve (e.g. dotted red line in Figure 1) that there is a particular voltage and current for which the power from the solar cell is a maximum (for the particular light illumination level and ambient temperature).
The data for Figure 1 was produced by using a DIY Dummy Electronic Load to increase the load on a solar cell and measure the corresponding voltage. The readings where performed on a cloudless day at noon in order to obtain relatively consistent illumination conditions (with the readings taken as quickly as possible). In order to "reproducibly" vary illumination conditions, various layers of shade cloth were used to cover the solar cell (see the Photographs Section for this "manual" IV curve recording setup). The "green" and "blue" lines in Figure 1 show the effect of illumination level on the IV curve of the solar cell. Hence, the PV curve and resultant maximum power point.
Battery Charging and Solar Cell MPP
While the data from Figure 1 shows that the solar cell MPP changes in response to illumination level (with temperature being another major factor) how does this impact on battery charging?
A battery typically has a much smaller internal resistance than the resistance of the solar panel. Hence, when a solar panel is connected to a battery, the voltage across the solar panel will be approximately equal to the open circuit battery voltage (i.e., the solar cell voltage will be "pulled down" to match the battery voltage). With this in mind, the following example calculation/scenario shows the potential utility of using maximum power point tracking (MPPT) as part of a solar cell charger controller.
From the specifications of the solar cell (used to determine the data in Figure 1) voltage at maximum power (Vmp) is 16.4V with current at maximum power (Imp) 0.54A. Therefore, power at MPP is 16.4V x 0.54A = 9.5W. This solar cell is then used to recharge a battery which has a voltage of 12V. The solar cell voltage will be "pulled down" to 12V hence the power transfered to the battery will be 12V * 0.58A = 6.96W (not the previously calculated 9.5W available). This is a loss of approximately 36.5%. From the data obtained in Figure 1 of the solar cell under actual operating conditions (which takes into account the age etc of the solar cell) Vmp=13.7V and Imp=0.5A therefore Pmax=6.85W. Again, if the solar cell was used to recharge a battery currently at 12V this would equate to approximately 12.4% loss of potential power available.
A charge controller with MPPT determines the MPP of the solar cell (Vmp and Imp) together with the state of charge of the attached battery, and then changes the voltage supplied from the solar cell (by DC-DC conversion either buck and/or boost circuitry) to maximise the power transfered to the battery. "The function of a MPPT is analogous to the transmission in a car. When the transmission is in the wrong gear, the wheels do not receive maximum power. That's because the engine is running either slower or faster than its ideal speed range. The purpose of the transmission is to couple the engine to the wheels, in a way that lets the engine run in a favorable speed range in spite of varying acceleration and terrain" (3).
The most benefit from MPPT comes from when the solar cell is located in a cold climate (increasing temperature of a solar cell decreases the voltage) and if the battery being recharged is deeply discharged. The "literature" about MPPT (particularly from those selling MPPT controllers and competing devices) seems to have a healthy "disagreement" if MPPT is worth it (i.e., the cost of a MPPT charge controller for a potential extra 10% in power, or just spend the equivalent money on extra solar cells which are comparatively cheap).
However, the actual circuity involved in a MMPT charge controller, in addition to that of a PWM based controller for example, is relatively minor. In the case of DIY projects that would benefit from incorporating a solar cell charge controller for trickle charging the battery source, and which have a microcontroller as part of the necessary functionality in any case, a MMPT function potentially does not add significant cost (setting aside the time to develop the MMPT firmware/algorithm). Since I'm not actually spending money on purchasing an off-the-shelf MMPT controller, and I can't make a solar cell, but I can make a MMPT controller (and the potential fabrication/component costs are minor), seems like MMPT for a DIY controller has benefit.
The circuitry/approach used for the IV-curve tracer is discussed further in the Schematics and Circuit Details Sections.
The Testing/Experimental Results Section discusses the data obtained from the IV-curve tracker and the WAMP server software/web interface etc for receiving and storing the data via WiFi.
The Construction Notes/Trouble Shooting Section details some of the difficulties encountered in producing a working prototype, which lead to changes in the circuitry (e.g., signal filtering, high/low side current sensing options etc).
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