Review of the maximum power point tracking algorithms for stand-alone photovoltaic systems

Abstract

A survey of the algorithms for seeking the maximum power point (MPP) is proposed. As has been shown, there are many ways of distinguishing and grouping methods that seek the MPP from a photovoltaic (PV) generator. However, in this article they are grouped as either direct or nondirect methods. The indirect methods (“quasi seeks”) have the particular feature that the MPP is estimated from the measures of the PV generator's voltage and current PV, the irradiance, or using empiric data, by mathematical expressions of numerical approximations. Therefore, the estimation is carried out for a specific PV generator installed in the system. Thus, they do not obtain the maximum power for any irradiance or temperature and none of them are able to obtain the MPP exactly. Subsequently, they are known as “quasi seeks”.

Nevertheless, the direct methods (“true seeking methods”) can also be distinguished. They offer the advantage that they obtain the actual maximum power from the measures of the PV generator's voltage and current PV. In that case, they are suitable for any irradiance and temperature. All algorithms, direct and indirect, can be included in some of the DC/DC converters, Maximum power point trackings (MPPTs), for the stand-alone systems.

Introduction

In the current century, the world is increasingly experiencing a great need for additional energy resources so as to reduce dependency on conventional sources, and photovoltaic (PV) energy could be an answer to that need. PV cells are being used in space and terrestrial applications where they are economically competitive with alternative sources. Furthermore, the PV industry has demonstrated high growth rates over recent years, 30% per year in the late 1990s. By the year 2010, it is assumed that modules will cost 1.50 €/Wp and systems 3.00 €/Wp.

Generally, PV systems can be divided into three categories: stand-alone, grid-connection and hybrid systems. For places that are far from a conventional power generation system, stand-alone PV power supply systems have been considered a good alternative. These systems can be seen as a well-established and reliable economic source of electricity in rural remote areas, especially where the grid power supply is not fully extended. Such systems are presented in two scales: applications at a smaller scale, from 1 to 10 kW, are used to supply electric power in developing countries, the so-called solar home systems, SHS; and stand-alone PV systems of several hundred thousand watts in size, from 10 to 100 kW, on the roofs of dwellings. In general, they have the advantages of using a simple system configuration and simple control scheme. For our study, only DC loads will be considered. Typical DC voltage levels are 12, 24, 48 and 60 V.

In those systems, the performance of a PV system relies on the operating conditions. Then, the maximum power extracted from the PV generator depends strongly on three factors: insolation, load profile (load impedance) and cell temperature (ambient temperature), assuming a fixed cell efficiency.

The variation of the output I–V characteristic of a commercial PV module as a function of temperature and irradiation is shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4. It can be observed that the temperature changes mainly affect the PV output voltage, while the irradiation changes mainly affect the PV output current.

Nevertheless, PV systems should be designed to operate at their maximum output power levels for any temperature and solar irradiation level at all times. The last significant factor, which determines the PV throughput power, is the impedance of the load. In our case, this consists of a DC load and batteries. However, it should be noted, that such impedance is not constant. When a PV generator is directly connected to the load, the system will operate at the intersection of the I–V curve and load line, which can be far from the maximum power point (MPP).

The maximum power production is based on the load-line adjustment under varying atmospheric conditions.

Another important element of a stand-alone PV system is the battery because of the fluctuating nature of the output delivered by the PV array [1]. Moreover, the load, in many cases, requires a power level that is maintained constant. Commonly, lead-acid batteries are used, because of their wide availability in many sizes, their low cost and their well-understood performance characteristics.

Thus, in most cases, a charge controller is an essential element [2], except in small systems with well-defined loads, using low voltage “self-regulating modules” or using a large battery or small array. If finally, a battery charge is used, it will control the voltage and charge current that is applied to the battery in order to protect it from being over charged, over discharged and from load control functions.

Depending on the level of sophistication in the charger technology, different charge algorithms are presented, i.e., a collection of controls over electrical parameters and a timing, associated with multiple voltage and current levels, applied sequentially to the charging system hardware for the express purpose of recharging the battery appropriately. Generally, battery manufacture refer to four distinct charging modes, or stages, within a battery charging cycle, Fig. 5: bulk, absorption, equalization and float. However, not all battery chargers have the four stages.

The previous charging modes can be implemented in several possible ways. The most common controller topologies for battery charge regulation are shunt and series type configuration. The point is that in the two topologies, time after time, PV panels are usually forced to operate at the battery voltage. This is almost always below the peak power point, so some of the power generating capability is lost.

In order to overcome the undesired effects on the output PV power and draw its maximum power, it is possible to insert a DC/DC converter between the PV generator and the batteries, which can control the seeking of the MPP, besides including the typical functions assigned to the controllers. These converters are normally named as maximum power point trackers (MPPTs). They consist of a topology and control circuit where there will be a MPP seeking algorithm. As Fig. 6 shows, the input DC–DC converter part is formed by the PV array and the output section by the batteries and load. The role of the MPPT is to ensure the operation of the PV generator at its MPP, extracting the maximum available power.

However, their losses have to be small enough to improve the efficiency of the overall system. This could increase the gap between the PV peak power voltage and the voltage at which they are forced to operate in a system without a peak power tracking. Theoretically, it is foreseeable that a MPPT would yield the most impressive gains in cold weather because that would raise the PV array’ peak power voltage point well above the usual battery operating voltage.

Fig. 6 shows the general block diagram for a charge batteries system, with MPPT, using a generic DC/DC converter. This one is connected to the PV generator, a battery and a load profile (such as a resistance, DC/DC motor…).

The main objective is to obtain the maximum power from the PV generator for the first-load batteries stage. That is to say, when the batteries are fully or partially discharged, in some state of charge less than 100%, then it is necessary to control one/or some of the input/output following variables; Table 1.