Optimization of Photovoltaic Power Systems

Photovoltaic is the direct conversion of light into electricity. It uses materials which absorb photons of lights and release electrons charges. It can be used for making electric generators. The basic element of these generators is named a PV cell.

In order to obtain a realistic view of the behavior of a photovoltaic system, it is necessary to achieve computer simulations. For that purpose, the most important data is the light irradiance of the photovoltaic array at a small time scale (some minutes) during a significant duration (one year or more). Unfortunately, complete experimental data (irradiance for all module inclination and orientation) are never available. For example, the only available measurement result is often hourly or daily global irradiance on a horizontal plane. Sky modeling is thus necessary in order to deduce from the available partial data a realistic estimation of the module irradiance and some spectral characteristics of that irradiance. Of course, that first result is useful only in conjunction with a model of the photovoltaic modules, in order to deduce from it the electrical power generation for varying irradiance, spectrum and temperature.

Photovoltaic generators are almost always associated with some control and power electronics. Even in the case of a direct connection between a solar array and a battery, a non-return diode is needed. Generally, more complex power electronic converters are needed in order to adapt the electrical frequency and voltage level to the planned use. In addition, the rational use of photovoltaic generators is possible only in association with power electronic converters in order to adjust the voltage of the photovoltaic array to the maximum power point independently of the output voltage of the system. Power electronic converters associated to photovoltaic generators have to verify constraints on quality of energy supplied (low level of harmonics), electromagnetic perturbations (low level of EMC) and security.

The source of photovoltaic electrical energy is the solar cell. Commercial solar cells reach maximum conversion efficiencies of 20–21%, while an efficiency of 25% may be achieved in laboratory [62]. The overall efficiency of a module ranges from 15 to 17% [62]. Under real operating conditions, a lower efficiency than the nominal efficiency could be observed [73]. PV arrays must be installed so that they maximize the amount of direct exposure to the sun. This usually means placement in an area clear of shading, in a southward direction and at an angle equal to the latitude of the location. The power provided by the PV array varies with solar irradiance and temperature, since these parameters influence the I–V characteristics of solar cells. In order to optimize the energy transfer from the PV array to the load, it is necessary to force the working point to be at the maximum power point (MPP) [31, 63].

Battery is generally needed when PV array cannot be functional as at night or on a cloudy day.

Photovoltaic pumping has become one of the most promising fields in photovoltaic applications. To achieve the most reliable and economical operation, more attention is paid to their design and their optimal use. Depending upon the intended application, the pumping system can be selected from surface, submersible or floating pumps type [147]. Submersible pumps remain underwater, surface pumps are mounted at water level at the vicinity of the well or, in the case of a floating pump, on top of the water. Pumps can be classified according to their operating modes. Mainly there are centrifugal and positive displacement pumps [147]. In the centrifugal pump, the rotation of an impeller forces water into the pipe. The water velocity and pressure depend on the available mechanical power at the rotating impeller and the total head. The displacement pump uses a piston or a screw to control the water flow. Compared to the centrifugal pump, the positive displacement pump presents a better efficiency under low power conditions.

Hybrid power systems (HPS) combine two or more sources of renewable energy as one or more conventional energy sources. The renewable energy sources such as photovoltaic and wind do not deliver a constant power, but due to their complementarities their combination provides a more continuous electrical output. Hybrid power systems are generally independent from large interconnected networks and are often used in remote areas. The purpose of a hybrid power system is to produce as much energy from renewable energy sources to ensure the load demand. In addition to sources of energy, a hybrid system may also incorporate a DC or AC distribution system, a storage system, converters, filters and an option to load management or supervision system. All these components can be connected in different architectures. The renewable energy sources can be connected to the DC bus depending on the size of the system. The power delivered by HPS can vary from a few watts for domestic applications up to a few megawatts for systems used in the electrification of small villages. Thus, the hybrid systems used for applications with very low power (under 5 kW) generally feed DC loads. Larger systems, with a power greater than 100 kW, connected to an AC bus, are designed to be connected to large interconnected networks. Hybrid systems are characterized by several different sources, several different loads, several storage elements and several forms of energy (electrical, thermal).