Maximum Power Point Tracker

What is Maximum Power Point Tracking (MPPT) and How Does it Work? Maximum Power Point Tracking, frequently referred to as MPPT, is an electronic system that operates the Photovoltaic (PV) modules in a manner that allows the modules to produce all the power they are capable of. MPPT is not a mechanical tracking system that “physically moves” the modules to make them point more directly at the sun. MPPT is a fully electronic system that varies the electrical operating point of the modules so that the modules are able to deliver maximum available power. Additional power harvested from the modules is then made available as increased battery charge current. MPPT can be used in conjunction with a mechanical tracking system, but the two systems are completely different. To understand how MPPT works, let’s first consider the operation of a conventional (nonMPPT) charge controller. When a conventional controller is charging a discharged battery, it simply connects the modules directly to the battery. This forces the modules to operate at battery voltage, typically not the ideal operating voltage at which the modules are able to produce their maximum available power. The PV Module Power/Voltage/Current graph shows the traditional Current/Voltage curve for a typical 75W module at standard test conditions of 25°C cell temperature and 1000W/m2 of insolation. This graph also shows PV module power delivered vs module Typical 75W PV Module Power/Voltage/Current voltage. For the example shown, the At Standard Test Conditions conventional controller simply connects the module to the battery and therefore forces the module to operate at 12V. By forcing the 75W module to operate at 12V the conventional controller artificially limits power production to ≈53W. Rather than simply connecting the module to the battery, the patented MPPT system in a Solar Boost™ charge controller calculates the voltage at which the module is able to produce maximum power. In this example the maximum power voltage of the module (VMP) is 17V. The MPPT system then operates the modules at 17V to extract the full 75W, regardless of present battery voltage. A high efficiency DC-to-DC power converter converts the 17V module voltage at the controller input to battery voltage at the output. If the whole system wiring and all was 100% efficient, battery charge current in this example would be VMODULE ÷ VBATTERY x IMODULE, or 17V ÷ 12V x 4.45A = 6.30A. A charge current increase of 1.85A or 42% would be achieved by harvesting module power that would have been left behind by a conventional controller and turning it into useable charge current. But, nothing is 100% efficient and actual charge current increase will be

2598 Fortune Way, Suite K • Vista, CA 92081 • Phone 760-597-1642 • Fax 760-597-1731 • www.blueskyenergyinc.com

somewhat lower as some power is lost in wiring, fuses, circuit breakers, and in the Solar Boost charge controller.

Actual charge current increase varies with operating conditions. As shown above, the greater the difference between PV module maximum power voltage VMP and battery voltage, the greater the charge current increase will be. Cooler PV module cell temperatures tend to produce higher VMP and therefore greater charge current increase. This is because VMP and available power increase as module cell temperature decreases as shown in the PV Module Temperature Performance graph. Modules with a 25°C VMP rating higher than 17V will also tend to produce more charge current increase because the difference between actual VMP and battery voltage will be greater. A highly discharged battery will also increase charge current since battery voltage is lower, and output to the battery during MPPT could be thought of as being “constant power”. Typical PV Module Temperature Performance What most people see in cool comfortable temperatures with typical battery conditions is a charge current increase of between 10 – 25%. Cooler temperatures and highly discharged batteries can produce increases in excess of 30%. Customers in cold climates have reported charge current increases in excess of 40%. What this means is that current increase tends to be greatest when it is needed most; in cooler conditions when days are short, sun is low on the horizon, and batteries may be more highly discharged. In conditions where extra power is not available (highly charged battery and hot PV modules) a Solar Boost charge controller will perform as a conventional PWM type controller. Home Power Magazine has presented RV Power Products (now Blue Sky Energy, Inc.) with two Things-That-Work articles; Solar Boost 2000 in HP#73 Oct./Nov. 1999, and Solar Boost 50 in HP#77 June/July 2000, Links to these articles can be found on the Blue Sky Energy, Inc. web site at www.blueskyenergyinc.com.

Richard A. Cullen President Blue Sky Energy, Inc.

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Maximum power point tracker A maximum power point tracker (or MPPT) is a high efficiency DC to DC converter which functions as an optimal electrical load for a photovoltaic (PV) cell, most commonly for a solar panel or array, and converts the power to a voltage or current level which is more suitable to whatever load the system is designed to drive. PV cells have a single operating point where the values of the current (I) and Voltage (V) of the cell result in a maximum power output. These values correspond to a particular resistance, which is equal to V/I as specified by Ohm's Law. A PV cell has an exponential relationship between current and voltage, and the maximum power point (MPP) occurs at the knee of the curve, where the resistance is equal to the negative of the differential resistance (V/I = -dV/dI). Maximum power point trackers utilize some type of control circuit or logic to search for this point and thus to allow the converter circuit to extract the maximum power available from a cell. Battery-less grid-tied PV inverters utilize MPPTs to extract the maximum power from a PV array, convert this to alternating current (AC) and sell excess energy back to the operators of the power grid. Off-grid power systems also use MPPT charge controllers to extract the maximum power from a PV array. When the immediate power requirements for other devices plugged into the power system are less than the power currently available, the MPPT stores the "extra" energy -- energy that is not immediately consumed during the day -- in batteries. When other devices plugged into the power system require more power than is currently available from the PV array, the MPPT drains energy from those batteries in order to make up the lack. MPPT charge controllers are quickly becoming more affordable and are more common in use now than ever before. The benefits of MPPT regulators are greatest during cold weather, on cloudy or hazy days or when the battery is deeply discharged. Solar MPPTs can also be used to drive motors directly from solar panels. The benefits seen are huge, especially if the motor load is continuously changing. This is due to the fact that the AC impedance across the motor is related to the motor's speed. The MPPT will switch the power to match the varying resistance. Solar cell I-V curves where a line intersects the knee of the curves where the maximum power point is located

Maximum-power-point-tracking solar battery charger Sustainable electrical sources like solar photovoltaic arrays are becoming increasingly important as environmentally friendly alternatives to fossil fuels. But, while they’re nice for the environment, sustainable sources aren’t always easy to apply. These sources are characterized by both stringent peakpower limitations and “use it or lose it” availability. Successful application of sustainable energy sources therefore depends on strict attention to efficiency in both power conversion and energy storage. For small systems, workable energy-management schemes usually include a rechargeable battery and battery charger. A shortcoming of this solution is that ordinary battery chargers, even efficient ones, do an imperfect job of squeezing the last milli-watt from sustainable sources over realistic combinations of ambient and battery conditions.

The circuit shown addresses this problem in small solar power systems (Fig. 1).

It works by continuously optimizing the interface between the solar array and battery. The principle in play, sometimes called Maximum Power Point Tracking, is illustrated in the I/V and P/V curves for a typical photovoltaic array (Fig. 2) exposed to “standard” sunlight intensity (insolation) of 1 kW/m2. To accommodate a useful range of insolation and battery voltage variation, designers of solar panels make the number of cells large enough so that a useful level of charging current is provided even when the light level is low and the battery voltage is high. Consequently, when lighting conditions happen to be more favorable, these panels can produce up to 50% more voltage and 30% more power than the battery wants. Simple direct connection of panel to battery will therefore cause inefficient operation at point “A,” with the excess power lost as heat in the solar panel. Figure 1 does better than that by combining a high-efficiency (≈95%) SMPS circuit (LTC1149) with an analog power-conversion optimization loop. To understand how it works, assume battery B1 is in a state of discharge. In this condition, E1 will accept all of the current the SMPS can supply (subject to the ≈2.5-A current limit set by RSENSE) at a voltage around 12 V. If U1 drives Q1 to a 100% duty factor, inefficient operation at the direct-connect point “A” will result. However, the optimization circuit doesn’t let that happen. Instead, 50-Hz multivibrator S1/S2 causes A2 to continuously dither Q1’s duty factor by about ±10%. The result is a dither of approximately ±1 V in V IN. There’s also a corresponding 50-Hz modulation of the average power extracted from the solar panel as reflected in the return current through RSENSE. The 50-Hz ac waveform across RSENSE is filtered by R1C1 and synchronously demodulated by S3. This dc error signal, whose polarity indicates the slope of the solar panel I/V curve wherever VIN happens to be sitting, is integrated by A1 to close a feedback loop around A2. For example, if the SMPS happens to be operating at a VIN below the maximum power point (VIN < VMPPT), then there will be a positive correlation between VIN and ISENSE, and A1 will ramp toward lower average duty factors and higher VIN. By contrast, operation at VIN > VMPPT reverses the dither phase relationship and A1 ramps toward higher duty factors and lower VIN. Either way we get convergence toward VMPPT and maximum charging current for B1. This mode of operation continues as B1 charges and its voltage rises to the ≈14.1-V terminal-voltage setpoint determined by the R6-R7-R8-RT network. Once reached, A2 saturates with zero output and normal LTC1149 constant-voltage regulation takes over. RT provides temperature compensation appropriate for typical lead-acid battery chemistry. R2 allows for A1 offset nulling, which is particularly important at low panel output levels. The circuit makes no provision for preventing reverse current from being drawn from the battery under no-light conditions, but since the drain—even in total darkness—is less than 3 mA (comparable to typical battery self-discharge rates), adding a blocking diode would actually reduce overall efficiency.

The MPPT technique has much wider application than just photovoltaics alone. That’s because conceptually similar functionality of power output versus loading can be seen in the I/V curves of other sustainable energy sources. Such sources are small water turbines (e.g. the “Peltonwheel” impulse turbine of Figure 3) and fixed-pitch-rotor wind-power turbines, when either is combined with constant field alternators. The voltage, current, and power produced by any of these sources is highly variable in response to ambient conditions (insolation, hydrostatic head, or windspeed) and dramatically dependent on the electrical impedance of the imposed load (V vs. I). Under any combination of ambient conditions, each of these sources is characterized by exactly one ideal load impedance, which will result in operation at VMPPT and maximum power transfer. Also of benefit is the simplifying absence of confusing local maxima in the power versus voltage curves. Of course, the actual physics behind the I/V curves for the various sources are very different. In the case of photovoltaics, the primary energy-producing process is recombination of photoelectric charge carriers and how the rate of such recombination varies with output voltage, temperature, and insolation. For windpower generators, the dominant parameter is the interaction of “Tip Speed Ratio” (defined as turbine peripheral velocity divided by wind speed) with the aerodynamic design of the turbine. For small hydroelectric generators, it’s the fluid dynamics of the turbine or “runner” as they relate to the pressure and volume of the available water source. But the MPPT charger really doesn’t care about these details. It just blindly climbs the I/V curve to the VMPPT summit. Figure 1’s circuit can therefore be easily adapted to any of these systems. The only modification necessary is a bigger C2 (0.1 µF to 1µF) to slow the dither rate to 5-Hz to 0.5-Hz frequencies compatible with the inertial time constant of mechanical power sources. In addition, wind-power applications will benefit from an overspeed preventer. This VIN-limiting circuit is basically just a big Zener diode connected across the input terminals that dumps excess power in conditions of high wind speeds and low battery demand. Consequently, it prevents overrevving of the turbine and alternator. For higher power applications (25 W and up) or other output voltage ranges, consult Linear Technology LTC1149 application literature.