Solar Energy Availability for Photovoltaic Electricity Generation

The energy output from a photovoltaic (PV) system depends on both device characteristics such as efficiency, and surrounding factors where the device is being operated. The surrounding factors determine the solar energy potential at certain location. The main surrounding factors are solar radiation received by the device, ambient temperature, wind speed, and surrounding coefficient reflection. Prior to installing a PV system, it is necessary to evaluate the potential of solar energy at the location where the application system planned to be installed.

This article discusses the aspects related to evaluation and assessment of solar energy potential in a certain location. The aspects are including solar geometry and position, component of solar radiation, and calculation of amount of solar insulation that can be expected. The assessment in this paper is emphasized for solar electricity PV application.

 The sun is hot sphere of gas heated by nuclear fusions reaction at its centre [1]. The spectrum of the radiation from the sun is similar to that of a 5800 K blackbody with fine structure due to absorption in the cool peripheral solar gas. The existence of nearly all life on Earth is fueled by light from the sun. Outside the atmosphere, at the mean solar distance, the beam irradiance, also known as the solar constant (I₀), is 1367 W.m^-2 Irradiance is the rate of energy received per unit area, thus the units of irradiance is Watts per square metre (W/m²), where 1 Watt (W) is equal to 1 Joule (J) per second.

Solar radiation reaches the earth’s surface is highly variable caused by absorption and scattering in the earth atmosphere.  Solar energy received at the Earth's surface can be separated into two basic components: direct solar energy and diffuse solar energy. Direct solar energy is the energy arriving at the Earth's surface with the Sun's beam. Diffuse solar energy is the result of the atmosphere attenuating, or reducing the magnitude of the Sun's beam. Some of the energy removed from the beam is redirected or scattered towards the ground - the rate at which this energy falls on a unit horizontal surface per second is called the diffuse solar irradiance. The remaining energy from the beam is either scattered back into space, or absorbed by the atmosphere. Global solar irradiance is a measure of the rate of total incoming solar irradiance, both direct and diffuse irradiance, on a horizontal plane at the Earth's surface.

When designing a PV system, it is need to estimate of the insolation expected to solar module panels system. Ussualy, mothly average daily data radiation values, as presented in Table 1, are sufficient and characteristic days near middle of each month are often used to define average monthly values. Besides, portion of direct and diffuse radiation usually need for estimation of the effects of tilted panel, however these need to be estimated from global values if measurement data is not available. The efficiency of a solar cell depends on many factors. It is therefore possible that a single solar cells performance varies widely depending on its location. Solar panels (a solar panel is an array of solar cells) were until recently almost exclusively used in spacebourne applications. Nowadays, rapid technological advances and a sharply risen public environmental awareness have propelled solar panels to the centre stage in humanity’s search for clean energy. The solar industry has exploded, with the photovoltaic industry reporting a combined global revenue of $37 billion over 2008.

Commonly, solar panels are marked by listing their capacity in watt-peak. A solar panels power in watt-peak represents its peak output under standardized test conditions: 25^oC 1.5 airmass, 1 kW/m² intensity. However, such conditions might never be reached, as many factors influence the performance of a solar panel Temperature, shading, tilt angle, power conversion loss, soiling and azimuth plane angle.

With increasing temperature, the efficiency of a solar cell decreases. This is because a higher temperature increases the conductivity of the semiconductor. This balances out the charge within the material, reducing the magnitude of the electric field at the junction. This in turn inhibits charge separation, which lowers the voltage across the cell. The flow of current to increase slightly as result of increases the mobility of electrons by higher temperature however minor and insignificant compared to the decrease in voltage. The listed power of a solar cell is the power measured under ideal laboratory conditions, which prescribe a temperature of 25 °C. However, on a typical hot sunny day, it is not uncommon for a solar cell to reach a temperature of 70 °C . A general rule of thumb is that the efficiency of a solar cell decreases with 0.5% for every 1 °C above 25 °C . This means that on a hot sunny day, the efficiency of a solar cell could drop as much as 25%. It is therefore extremely important to keep a solar panels well ventilated.

Resistance among of solar cells and their wiring connections within the solar panel directly influences both voltage and current of the panel. Increasing resistance of the panel will cause the voltage-current curve of the solar cell to move away from the so-called maximum power point (MPP). At this point, a solar cell produces maximum output ,that is multiplication of current and voltage. Therefore it will give advantageous to maintain this point.

It is important to make sure a solar panel system is as little possible affected by shadows cast by trees, other buildings or other element of the solar panel system. This is because batches of solar cells are connected in series, the entire batch will operate at the current level of the weakest cell. By (partly) shading a single cell, one can thus adversely influence the output of all other cells.When installing the system, it must be consider that the angle of the sun changes throughout the year.