Smart Energy Design Notes is a series of tutorials on some aspects of energy design. It will serve as a reference guide for the general analysis of smart energy design concepts. In this first part, we analyze photovoltaic (PV) systems...
The installation of a photovoltaic system reduces electricity consumption and contributes to CO2 emission reductions into the environment. The latest generation of solar panels guarantees a decent quality, even after more than 20 years of existence.
The solar radiation that arrives on earth is a source of unlimited energy emitted by the fusion processes of the hydrogen contained in the sun. Such power does not reach the earth’s surface often. The quantity varies during the day, from season to season and depends on the cloudiness, the angle of incidence, the location of the site, relief and obstacles, climatic conditions, as well as surface reflectance. The radiation that one square meter of a horizontal surface receives is called global radiation and is the result of the sum of direct and diffuse radiation.
The sum of maximum radiation is obtained on a south-facing surface with an angle of inclination of about 30°. An overlay at a slant of 45° with south-east or south-west orientation records a decrease of less than five (5) percent of the annual average global radiation.
The basic component of a solar system is the photovoltaic cell in which the conversion of solar radiation into electrical current takes place.
When a light ray hits a thin layer of semiconductor material (PN junction) that makes up the photovoltaic cell, the photons, the energy particles that make up the solar ray, transfer their energy to the electrons. The material immediately begins to move in a particular direction creating a DC; other currents of other modules or cells can be added until they reach the power required for the specific desired use.
The photovoltaic cells are in thin, flat layers connected, or they can be obtained by creating a uniform thin film obtained by distributing the pulverized material directly on a support thanks to vacuum technology.
The solar cell is the main component of a photovoltaic system. Each cell can produce power from three (3) to six (6) Watts. More solar cells form a module, and more modules form a photovoltaic panel.
In the modules, the photovoltaic cells are not all identical due to inevitable manufacturing differences, so two blocks of cells connected in parallel may not have the same voltage. As a result, a circulating current is created from the higher voltage cell block to the lower voltage block. A part of the power produced by the module vanishes within it (mismatch losses).
The power of a solar panel is expressed in peak-watts (Wp). This is the power provided by a solar panel at 25°C, with a light similar to that of the sun and a power of 1000 W per square meter.
This standard size allows you to compare several panels to each other: those with a higher peak power will obviously produce more electricity. Finally, it makes it possible to evaluate the annual production of a photovoltaic system according to the place of installation.
The energy dimensioning of the photovoltaic system, connected to the distributor’s grid, is carried out taking into account, in addition to the economic availability, the following aspects: availability of space on which to install the photovoltaic system, availability of the solar source, morphological and environmental factors (shading).
The design principle used for a photovoltaic system is to maximize the capture of the available annual solar radiation. The first step in designing a photovoltaic system is the choice of the photovoltaic panel. It is dependent on the specifications provided by the respective manufacturers, such as performance, size, power, and cost, which must be compared for the different models on the market before choosing which ones to use. The other step is to define the number of such panels that must be connected in parallel to produce the desired current, and those to be connected in series to produce the desired voltage.
Besides, the orientation is another aspect to be considered: the photovoltaic panels must be mounted at a fixed angle (tilt angle) concerning horizontal (a tilt angle +/- 15° for the latitude of the place increases energy production in winter or summer), or on a sun tracking system to maximize production.
In most cases, the photovoltaic generator must be optimally exposed to sunlight, prioritizing south-facing orientation, and avoiding shading phenomena. Depending on the possible architectural constraints of the structure that encapsulates the generator itself, different angles are adopted to obtain the best efficiency.
Besides panels, it is necessary to choose the inverter, a component that has a high impact on the performance and cost of the system. It also transforms the direct voltage of the panels into alternating current. Inverters are chosen based on numerous parameters such as conversion efficiency (the ratio between output and input power), input voltage, nominal and peak power, voltage regulation and protection, current frequency, modularity, power factor, etc.
If the photovoltaic system is a stand-alone system, i.e. isolated, not connected to the grid, it will also be necessary to choose suitable batteries or energy storage and charge controllers to protect them from excessive charging or discharging. The accumulators most commonly used in PV are lead-acid batteries, which are inexpensive and require little maintenance.
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Usually, the systems used are grid-connected, i.e., they share the excess energy by feeding it into a smart grid. When the PV system is unable to produce energy, the electrical grid system helps. In these cases, no batteries are required.
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The generator is modified to accommodate the load produced by the system for energy optimization. Thereby, the operating point always corresponds to the maximum power point. For this purpose, a controlled chopper called Maximum Power Point Tracking (MPPT) is used in the inverter, which instant by instant identifies the torque of the generator voltage and current values for which the power supplied is maximum.