Maximum Power Point Tracking techniques for photovoltaic systems: A comprehensive review and comparative analysis

Abstract

Maximum Power Point Tracking (MPPT) is an important concern in Photovoltaic (PV) systems. As PV systems have a high cost of energy it is essential that they are operated to extract the maximum possible power at all times. However, under non-uniform environmental conditions, which frequently arise in the outdoor environment, many MPPT techniques will fail to track the global peak power. This review paper discusses conventional MPPT techniques designed to operate under uniform environmental conditions and highlights why these techniques fail under non-uniform conditions. Following this, techniques designed specifically to operate under non-uniform environmental conditions are analysed and compared. Simulation results which compare the performance of the common Perturb and Observe (P&O) method, the Particle Swarm Optimisation (PSO) and the Simulated Annealing (SA) MPPT approaches under non-uniform environmental conditions are also presented. The research presented in this review indicates that there is no single technique which can achieve reliable global MPPT with low cost and complexity and be easily adapted to different PV systems.

Introduction

Operating Photovoltaic systems efficiently involves ensuring that the maximum possible energy is extracted from the system at all times. However, the maximum power available may vary rapidly due to changing environmental conditions and shadow from objects in the environment. The passage of clouds over a PV system will cause a change in the irradiance on a panel and subsequently, determining the maximum power is a time-varying problem. To enable maximum power production under changing environmental conditions Maximum Power Point Tracking techniques are proposed which will track to the optimal power point. Several limitations exist with these techniques, particularly under non-uniform environmental conditions. Other techniques designed to achieve the same goal include Maximum Power Point Estimation (MPPE) techniques, such as [1], [2], [3], [4] which rely on the use of models of the PV cells. Generally these techniques are defined for uniform conditions so may fail to track the Global MPP (GMPP) during non-uniform environmental conditions.

PV systems are made up of a number of individual PV cells that are connected in series to form strings, and then in parallel to form modules [5], [6]. Multiple modules can be connected in series and/or parallel combinations to further boost the voltage and current and form PV arrays. When the cells are all exposed to the same environmental conditions, in terms of irradiance and cell temperature, they exhibit identical characteristics which are simply scaled to give the overall current–voltage (I–V) and power–voltage (P–V) characteristics for a system. Sample I–V and P–V characteristics under uniform conditions for three series-connected modules are given in Fig. 1. If the cells are exposed to non-uniform environmental conditions, which may arise due to some cells being shaded by objects in the environment, the I–V and P–V characteristics become more complex. Shaded cells have a lower capacity than their series-connected counterparts and, as such, could limit the power producing capability of an entire string. To avoid this effect, and to reduce the risk of hot spot formation and damage to the shaded cell, bypass diodes are often installed across either individual cells or substrings to provide an alternative path for the current [5], [7]. In the presence of bypass diodes, the I–V and P–V characteristics become more complex and will potentially exhibit several local peaks. For three series-connected modules, the corresponding I–V and P–V characteristics under non-uniform conditions are shown in Fig. 2.

The effective utilisation of PV systems is an area that has attracted the attention of the international community. Studies are completed in many countries around the world exploring the potential available solar resource and ways to effectively utilise this. A small subset of these studies is described below. A study into the potential utilisation of wind and solar energy in India highlights that the available solar energy in this region is 5000 trillion kW h per year [8]. This large available energy has the potential to be utilised in the presence of increasing electricity costs. In 2013, India has a PV capacity of over 1430 MW accounting for 6% of the renewable energy generation [8]. The integration of renewable sources such as wind and solar are considered in a study of the European Union (EU) for their capability to provide flexibility in the network [9]. By exploring a region composed up of many different countries, specific factors to specific regions that affect the network flexibility can be explored. Additionally, the solar utilisation and availability within each country can be quite different. Germany and Spain are the two countries in the EU which are leading the way with PV integration and research with an installed capacity making up over 51% of the installed capacity in the EU in 2013 [10]. Further studies in the EU investigate the potential for PV systems in Finland, a country where traditionally, due to the high latitude of the country, PV systems have not been utilised [11]. Despite the high latitude of the country it experiences a level of irradiance similar to Germany which is one of the leaders in PV integration. The benefits of cooperation between regions as described with respect to the EU include reducing the potential ramp rates experienced by each country [9]. A study completed in the UAE, explores how the angle of tilt and orientation of the panels for this particular area can improve the solar yield [12]. Studies have been completed in other regions exploring the same idea which highlights that the same principles apply in selecting the optimal tilt and orientation; however, these values can vary considerably from region to region. Saudi Arabia lies in the ‘sun belt’ and experiences high levels of irradiance between 4.479 kW h/m² and 7.004 kW h/m² depending on the geographical location [13]. This significant available energy could be well utilised in PV systems and is being increasingly integrated in the Saudi Arabian grid to minimise environmental and health issues and with falling prices to generate electricity from PV systems. In developing countries including China, the importance of developing solar power resources as the electricity network is further developed is attracting increased attention [14]. Solar utilisation in China takes the form of PV in addition to direct methods of utilising the energy including using solar energy for heating buildings and water. In 2007 the PV energy available in China was 1200 MW which is a significant amount. Due to the large geographical area of China, different zones can be defined based on their solar potential. Approximately 67% of China lies in a zone with a large amount of solar potential [14]. Recent figures from Australia indicate that nearly 15% of all residential dwellings have a PV system installed [15]. These studies have highlighted that the integration and utilisation of PV systems around the world is becoming increasingly important as each country tries to move away from fossil fuel based energy sources and promote clean energy and sustainability. While each region will have different irradiance conditions, all regions have the capability to produce power from PV systems. All PV systems, regardless of where they are situated in the world are limited by the same P–V characteristics under uniform and non-uniform conditions. For this reason developing an effective MPPT method is of great importance for the further effective utilisation of PV systems around the world.

A variety of MPPT and MPPE techniques have been proposed in the literature [16], [17], [18], [19], [20]. Many MPPT methods are designed to work in uniform irradiance conditions [16], [17]. Recent developments in MPPT approaches focus on designing methods to achieve MPPT under Partial Shading Conditions (PSC) [18]. The impact of varying the system configuration and other methods to mitigate the effects of PSC and improve the performance under PSC have also been described [19]. The effects of measurement noise, bias and the optimisation of parameters is also explored in the literature [21], [22], [23], [24], [25]. Additionally, it is essential that converters are designed appropriately to guarantee tracking feasibility [26].

The review presented in this paper is different as it also provides consideration of the Power Electronics (PE) based approaches to mitigate PV power maximisation issues under PSC, in addition to identifying how conventional MPPT approaches fail and highlighting newer techniques designed to work under PSC. Additionally, this paper presents simulation results comparing the performance of the common Perturb and Observe MPPT method, against two strategies design to achieve Global MPPT (GMPPT). The strategies considered are Particle Swarm Optimisation and Simulated Annealing, which are both developed in the context of achieving GMPPT.

Section 2 will define criteria to assess the global maximum power extraction approaches. Section 3 will discuss conventional MPPT approaches and why these frequently fail under PSC. In Section 4, MPPT approaches to address partial shading, including the enhancement of conventional techniques and methods specifically designed for PSC, are discussed. Section 5 explores the PE based approaches. Simulation results comparing the performance of the P&O, PSO and SA based MPPT methods are described in Section 6. Section 7 provides a discussion and comparison of the techniques based upon the criteria established in Section 2, and Section 8 provides conclusions.