Photovoltaic solar farms

Nowadays it is normal to hear about renewable energy. Solar energy is a renewable energy becoming very popular. Moreover, some people even call it “The edge of sunshine”. In this post we will provide a short introduction about solar power.

Figure 1. Solar farm
Source: PV magazine

Solar energy can be captured by thermal systems and photo-voltaic systems. In this post we will introduce photo-voltaic systems. Solar power is based on photo-voltaic, the conversion of light into electricity at the atomic level. When a given material absorbs photons of light, it releases electrons; thus creating an electric current that can be used as electricity. This property of capturing photons and releasing electrons is known as the photoelectric effect.

Figure 2. Photovoltaic system
Source: DitrolicSolar

Sunlight is composed by ultraviolet rays, the visible spectrum rays and infrared rays. All of them are ray lights but with different wavelengths. The so called Solar Constant is the total energy flux outside the Earth’s atmosphere and it is equal to 1367 Watt per sq m. Considering a cross sectional area of the Earth’s surface of 127.4 million sq km, the solar energy flux received by the earth is about 1.7 *10^17  Watt. Thus, in one day the Earth receives more solar energy than the energy consumed by world’s population in 1 year. Unfortunately, not all this energy is converted to electricity. However, in order to estimate the potential solar electricity we have to consider some limitations such as available sunlight hours, angle of incidence, atmospheric absorption and efficiency. 

Astronomical movements
Because of astronomical movements from the Earth, the solar insolation varies constantly. For instance, because of the earth’s rotation no energy can be stored during the night; only daylight hours can be used to store energy. This sunlight hour’s variation is more critical at northern and southern latitudes because of latitude and the Earth’s tilt. During winter season it is normal to have Septentrional and Austral latitudes with several months of only a few sunlight hours (or none).  Besides, it is important to consider that the solar intensity changes within the day. At noon the sun is at its highest point in the sky and the energy reaches the ground perpendicularly. Thus, we have the highest insolation, also known as full sun. This is a very important point for designing solar farms; the inclination given to the panel will define the energy capacity.

Angle of incidence
The optimum angle of incidence is the one that allows a solar panel to be perpendicular to the Sun’s rays. Thus, the whole solar panel will receive energy. Otherwise, when the panels are not perpendicular the effective collecting area decreases according to the non-perpendicular angle. 

Figure 3. Incidence correction]
Source: Solar power

Atmospheric absorption
It is important to consider that the atmosphere is a mass that absorbs energy. Thus, when the light passes throughout the air, some energy is absorbed by the air. Meteorological conditions such as clouds increase the absorption rate. Locations at high altitude have more solar potential because less energy is absorbed by the atmosphere. Thus, it is important to consider meteorological conditions and the elevation of the proposed location for a solar farm.

Efficiency.
Unfortunately, any solar panel and any transformation device has an efficiency far from 100 %. Most of the solar panels have an efficiency between 15% and 22%, depending on the material and manufacturer. Material Science Engineering is performing important research for creating new improved solar panels. Thus, every year the solar panels have higher efficiency and their price tends to reduce.

How many Watts can be produced by solar farms?
The capacity of a solar farm depends on many factors such the geographical location, the size, the number of solar panels and the type of solar panel. Thus, it is not possible to give an average capacity. In this post we provide the capacity of the 5 largest photo-voltaic farms in the world.

Table 1. Top 5 largest photo-voltaic solar farms (Source:Imeche
Solar farm Location Capacity Area
Solar Star California USA 579 MW 13.0 sq km
Desert Sunlight California USA 550 MW 15.4 sq km
Topaz California USA 550 MW 24.6 sq km
Longyangxia Dam Solar Park Qinghai China 320 MW 9.16 sq km
Golmud Solar park Qinghai China 200 MW 5.64 sq km

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