Feasibility study of a digital measurement control algorithm for maximum power point tracking of photovoltaic arrays.

摘要

Photovoltaic (PV) energy is a very important resource of energy because it is essentially maintenance-free, pollution-free, and will last almost forever. However, installation cost is still quite high, and an energy conversion mechanism is required in order to interface with a load. It becomes quite important to operate these energy conversion systems at maximum power. In order to extract the maximum possible power from the PV module, a maximum power point tracking (MPPT) system is often used. Many MPPT techniques are in use today. In this thesis we propose a digital measurement control algorithm for MPPT. The solar cell produces a nonlinear current-voltage (I-V) characteristic as output. As environmental conditions change (such as irradiance or temperature) the IN characteristic also changes. Each individual IN curve (corresponding to a specific set of environmental conditions) has a "maximum-power" point at the knee of the I-V curve. This is the point where maximum power can be drawn from the solar cell. Our MPPT technique combines two concepts - windowing and hill-climbing - to perform the algorithm. First, a window is formed around the instantaneous value of solar-cell output power. When environmental conditions change, the power vacates the window, and the system is notified of this. The circuit then begins tracking to maximum power by using a hill-climbing technique. In this manner, the maximum power point can once again be obtained. The system was analyzed in detail and then constructed. The feasibility of several different state machines (the digital "brains" behind the algorithm) were investigated. While maximum power point tracking was achieved, the algorithm did not work perfectly. One of the challenges revealed through the course of this study is that it is very important that the algorithm allow for the two concepts - windowing and hill-climbing - to work successfully together. Additionally, it was concluded that it would be necessary to develop a comprehensive, systematic testing procedure for the system, where "test points" are embedded within the state machine itself.