1、 （一） What is Maximum Power Point Tracking (MPPT) and How Does it Work? Photovoltaic (PV) generation is becoming increasingly important as a renewable source since it offers many advantages such as incurring no fuel costs, not being polluting, requiring little maintenance, and emitting no noise, amon
2、g others. PV modules still have relatively low conversion efficiency； therefore, controlling maximum power point tracking (MPPT) for the solar array is essential in a PV system. The amount of power generated by a PV depends on the operating voltage of the array. A PVs maximum power point (MPP) varie
3、s with solar insulation and temperature. Its V-I and V-P characteristic curves specify a unique operating point at which maximum possible power is delivered. At the MPP, the PV operates at its highest efficiency. Therefore, many methods have been developed to determine MPPT. Maximum Power Point Trac
4、king, 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
5、 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 conjuncti
6、on with a mechanical tracking system, but the two systems are completely different. To understand how MPPT works, lets first consider the operation of a conventional (non-MPPT) charge controller. When a conventional controller is charging a discharged battery, it simply connects the modules directly
7、 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
8、standard test conditions of 25C cell temperature and 1000W/m2 of isolation. This graph also shows PV module power delivered vs module voltage. For the example shown, the conventional controller simply connects the module to the battery and therefore forces the module to operate at 12V. By forcing th
9、e 75W module to operate at 12V the conventional controller artificially limits power production to53W. 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
10、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
11、 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 con
12、ventional controller and turning it into useable charge current. But, nothing is 100% efficient and actual charge current increase will be 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 ope
13、rating 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
14、 VMP and available power increase as module cell temperature decreases as shown in the PV Module Temperature Performance graph. Modules with a 25C 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 g
15、reater. 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”. What most people see in cool comfortable temperatures with typical battery conditions is a charge current increas
16、e 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 conditi
17、ons 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. WHY MPPT? MPPT loads solar
18、 panel that it operates at the maximum power point. MPPT is a DC-DC converter that uses high frequency switching and control algorithm. It is desired to design the circuit at high efficient, light weight, small size, and reliable for the event. Solar cells are dependent on environment conditions lik
19、e temperature, sun light, and load voltage. While this parameters always changing daytime, solar cell or solar panel output characteristics vary also. Some hours photovoltaic system may give lower power than rated power of load. If solar panel is connected to battery by directly, a power loss occurs
20、 in the system since solar panel operating characteristic will change at some period of time up to environment conditions. This problem can be solved by using larger solar panels, but the solution is expensive. Instead of this, maximum power point tracker circuit is located between solar panel and b
21、attery. INCREASE SOLAR CHARGING WITH AN MPPT POWER TRACKING CHARGE CONTROLLER A new feature is showing up in charge controllers. Its called maximum power point tracking (MPPT). It extracts additional power from your PV array, under certain conditions. This article explains the process by a mechanica
22、l analogy, for people who do not understand basic electricity.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. Thats because the engine is running either slower or faster than its ideal speed range.
23、 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 acceeration and terrain. Lets compare a PV module to a car engine. Its voltage is analogous to engine speed. Its ideal voltage is that at which it
24、can put out maximum power. This is called its maximum power point. (Its also called peak power voltage, abbreviated Vpp). Vpp varies with sunlight intensity and with solar cell temperature. The voltage of the battery is analogous to the speed of the cars wheels. It varies with battery state of charg
25、e, and with the loads on the system (any appliances and lights that may be on). For a 12V system, it varies from about 11 to 14.5V. In order to charge a battery (increase its voltage), the PV module must apply a voltage that is higher than that of the battery. If the PV modules Vpp is just slightly
26、below the battery voltage, then the current drops nearly to zero (like an engine turning slower than the wheels). So, to play it safe, typical PV modules are made with a Vpp of around 17V when measured at a cell temperature of 25 C. They do that because it will drop to around 15V on a very hot day.
27、However, on a very cold day, it can rise to 18V! What happens when the Vpp is much higher than the voltage of the battery? The module voltage is dragged down to a lower-than-ideal voltage. Traditional charge controllers transfer the PV current directly to the battery, giving you NO benefit from this
28、 added potential. Now, lets make one more analogy. The cars transmission varies the ratio between speed and torque. At low gear, the speed of the wheels is reduced and the torque is increased, right? Likewise, the MPPT varies the ratio between the voltage and current delivered to the battery, in ord
29、er to deliver maximum power. If there is excess voltage available from the PV, then it converts that to additional current to the battery. Furthermore, it is like an automatic transmission. As the Vpp of the PV array varies with temperature and other conditions, it tracks this variance and adjusts t
30、he ratio accordingly. Thus it is called a Maximum Power Point Tracker. What advantage does MPPT give in the real world? That depends on your array, your climate, and your seasonal load pattern. It gives you an effective current boost only when the Vpp is more than about 1V higher than the battery vo
31、ltage. In hot weather, this may not be the case unless the batteries are low in charge. In cold weather however, the Vpp can rise to 18V. If your energy use is greatest in the winter (typical in most homes) and you have cold winter weather, then you can gain a substantial boost in energy when you need it the most!
