Photovoltaic systems design: design guidelines with all the necessary information and technical requirements (part 1)
A photovoltaic system is composed of a set of photovoltaic modules (panels), from an electrical component (cables) and an electronic device (inverter). So the modules are capable of converting the incident solar energy to produce electricity by the photovoltaic effect.
Consequently it’s quite obvious that the main objective for a photovoltaic system is to achieve significant energy savings for a given building.
So the use of this kind technology comes from the need to combine:
• compatibility with architectural requirements and environmental safeguarding;
• no noise pollution;
• saving of fossil fuel;
• production of electricity without emission of pollutants.
In this article, we’ll be seeing the requirements and characteristics necessary for the correct sizing and in conclusion, the final design of a photovoltaic system.
The sun’s apparent position
If we consider the fact that the Earth rotates around itself from west to east, and since we don’t perceive this movement, we have the impression that the celestial sphere rotates in the opposite direction, from East to West, dragging both the stars and the sun with it.
Due to the inclination of the Earth’s axis, in relation to its orbit plane, the Sun appears from season to season, at different heights on the horizon.
This perceptive error, negatively affecting man’s vision of the world and the sky for thousands of years, is however, so comfortable that even today it is used as a simple and intuitive descriptive reference.
Solar radiation means the flow of energy emitted from the sun and absorbed by the earth’s crust.
Solar radiation on the ground
When solar radiation passes through the various atmosphere layers, different phenomena take place. A part is reflected towards space, a part is diffused in all directions, a part is absorbed and a part, called direct solar radiation, reaches the earth’s surface directly.
Energy sizing of the photovoltaic system connected to the distributor’s grid is performed, in addition financial feasability aspects, taking into account the following requirements too:
• availability of spaces on which to install the photovoltaic system
• availability of solar energy
• morphological and environmental factors (shading and albedo)
Identification of spaces for photovoltaic system installation
First of all we need to identify appropriate areas for installing the photovoltaic modules.
Generally speaking, in the case of civilian installations, PV panels are normally placed on roof tops. When possible integrated installation systems are also preferred for their low visual impact. In addition optimum exposure is coincident with the south direction as this orientation offers the highest productivity for an azimuth of 0 degrees.
Then we need to identify the best inclination angle for the panels with respect to the horizontal plane. So this value is defined tilt.
Generally the maximum productivity values are obtained with a tilt value = latitude -10°.
Generically exposed surfaces
To properly assess the energy gains provided by solar radiation on a surface, it becomes necessary to calculate the direct and diffuse solar radiation incident on each of the building’s surfaces, or in any case inclined.
Availability of solar energy
The availability of solar energy for the installation site should be verified using the monthly average daily values of solar radiation on the horizontal plane.
Morphological and environmental factors (shading and albedo)
The shielding effect by the horizon volume, due to natural (hills, trees) or artificial elements (buildings), determine the reduction of solar gains and conseguently the system’s “investment payback time”.
Photovoltaic systems classification
Here are some of the basic elements involved in photovoltaic systems design:
• photovoltaic modules:
devices capable of transforming solar energy into electricity
• charge controller:
circuit protection device for accumulators (regulates their charging and discharging)
devices for storing the energy produced by the photovoltaic system in order to provide it to the user when the modules are not producing due to the lack of solar radiation (night time functioning)
a machine capable of transforming a direct current from the modules output and/or from the batteries into an alternating current at a conventional voltage of 220v.
Note: In the case of loads that require direct current power supplies, inverters can be excluded from the system.
• electrical switchpanel:
used for energy distribution. In case of high consumption or in the absence of an adequate supply of electrical energy produced by the PV system, the extra need of electricity is acquired from the public electricity grid. On the contrary, an excess of electricity produced by the system is again fed into the grid. The electrical panel also measures the amount of energy provided by the PV through a metering system.
• photovoltaic cell:
A single photovoltaic cell, of size around 10x10cm, constitutes the elementary device at the base of every photovoltaic system.
We can have different types of photovoltaic systems: grid connected systems and isolated systems also known as “stand-alone”.