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Renewable energy
Energy derived from renewable non-fossil sources – wind energy, solar, aerothermal, geothermal, hydrothermal and ocean energy, hydropower, biomass, landfill gas, gas from wastewater treatment plants, biogases.

Raw materials in PV panels
Silicon is the basic component in most PV cells. It is a chemical element with atomic number 14. Silicon is abundant in the Earth’s crust in the form of quartz sand (SiO2). It is used for production of pure silicon ingots (through melting and subsequent cooling). The ingots can be monocrystalline or polycrystalline. Monocrystalline ingots contain homogenous crystals and have a homogenous coloring, while polycrystalline ingots have multi-crystalline structure.

Silicon “wafers” are the thin discs into which the ingot is sliced. These wafers or substrates comprise the basic crystalline “platform” that photo-voltaic semiconductor cells are grown on via doped silicon epitaxial processes.

PV (solar) cell
The PV cell converts sunlight into electricity through the photovoltaic effect. The produced voltage is about 0.5-0.6 VDC. The amount of current produced depends on the amount of sunlight that falls upon the cell and the cells conversion efficiency.

PV (solar) panel or module
A single cell cannot produce enough power to serve as a larger scale power source. Therefore, several cells are combined into a PV panel through an interconnect and lamination process. PV panels in turn can be mounted on roofs, facades or on open ground spaces. The PV cells are placed on a substrate, encapsulated and covered with a protecting glass and aluminum frame, to form the PV panel.

PV System
The PV system is the comprehensive system for generating electricity from the Sun. Components of the PV system are the PV panels plus:
Inverter – a device which transforms the DC current which is generated by the PV panels, into alternating current, which is used in the electricity grid. The inverter also serves to maximize PV panels efficiency;
Cables – DC cables should be double-insulated and more heat-resistant than AC cables;
Junction box – contains a main disconnect that isolates the panels from the remaining part of the circuit, and surge protection. It serves also for parallel connection of the panels;
Mounting hardware - static racks and fasteners forming a backbone the panels are safely secured to with stanchions and risers interfacing with the roof or ground foundations.
Tracking device – some systems are mounted on tracking devices (trackers) and in this way can follow the Sun on its path in the sky. Two types of trackers exist: one-axis trackers, which follow the Sun on its “east-west” trajectory, and two-axis trackers, which follow it on its “north-south” trajectory (throughout the year);
Charge controller and batteries – they are used in the stand-alone systems. In this sense, one can distinguish two types of PV systems – grid-tied and stand-alone.

Stand-alone PV system

The stand-alone PV systems are used to power consumers that are not connected to the main grid, like bus stops, city lighting, telecommunication antennas, satellites, telecommunications towers, and a variety of remote application, etc.

Grid-tied PV system
The grid-tied PV system are connected to the power supply grid and feed energy into it. They are used in distributed electricity generation and require an inverter.

PV system efficiency
The PV system efficiency limits the amount of electricity that can be produced from a certain amount of sunlight. It is based on the efficiency of the cell. The efficiency of the mass-produced cells is about 18%. The efficiency of the system itself is lower due to inefficiencies in the system such as the inverter. One can distinguish two types of system efficiency – laboratory and open-field (which is achieved under real conditions). The open-field efficiency is lower than laboratory one mainly due to environmental affects such as varied sunlight conditions, angle, etc. The maximal output that a PV system can achieve within its efficiency level, is achieved under the so-called Standard Test Conditions (STC) – solar insolation of 1000 W/m2, air mass 1.5g AM, and cell surface temperature of 25 degrees C ambient. This maximal output is denoted with Wp (watt-peak).

Losses in PV systems
Usually, the losses in a PV system are about 25% from the cell produced energy. Different types of losses exist – losses from suboptimal location and angle of the system and from shading, losses in cables, inverter, diodes, junction box, etc. Also, there are losses from sunlight reflection from the panel surface, losses from panel heating or dirt piling on the panel surface.

Also called bypass diodes, these are installed in the PV junction box. Diodes are used for preserving output and the cells if shade falls upon one cell. If no diodes are present in the circuit, when a cell is shaded the panel efficiency will decrease vastly and abruptly, and the cell may even break.

Open circuit voltage
Voltage which the panel produces in open circuit – it is the maximal possible voltage the panel can produce.

Short circuit current
Short circuit current flows when the panel terminals are connected in short circuit. This is the maximal current that can flow in the circuit.

Net metering
Net metering is performed by sophisticated electricity meters. It allows the excess electricity produced by a grid-tied PV system that serves a local consumer to be fed back into the grid, metered and sold to the utility company. In most countries supportive legislations are in place, accordingly this electricity is sold at preferential rates.

Preferential (feed-in) tariff
The rate at which in many countries the private PV electricity producers sell their electricity to the utility companies. It is regulated by an energy regulator or by the state. The feed-in tariff is introduced because the cost of production of PV electricity is still too high. It is expected that after 5 to 6 years, the cost of PV electricity will drop and reach the peak electricity retail rate. This is the so-called grid parity. Then, households and businesses will be able to use their own PV electricity for fully covering their needs.

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