Solar (PV)

Photovoltaic (PV)

Photovoltaic solar system absorbs the available light from the sun to generate Electricity and feed the main electricity grid or load. The PV panel converts the light. 

reaching them into DC power. The Amount of power they produce is roughly proportional to the intensity and the angle of the light reaching them. They are therefore positioned to take maximum advantage of available sunlight within sitting constraints. Maximum power is obtained when the panels are able to ‘track’ the sun’s movements during the day and the various seasons.

The power generating capacity of a photovoltaic system is denoted in Kilowatt peak.

A common rule of thumb is that average power is equal to 25% of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 6 kWh per day (24 hours x 1 kW x 25% = 6 kWh).

In 1954, Photovoltaic technology is discovered when American Bell Labs scientists develop the silicon photovoltaic (PV) cell. The solar cell had just 4% efficiency. At this time, solar panels were not commercially available, and were very expensive. In 1955, Western Electric gets in the market by selling licenses for PV technology. NASA has launched the first spacecraft, powered by a 470-watt PV array. The new technology to power these spacecraft’s were expensive. In the 1980s solar power worldwide production increases from 1  megawatt to over 20 megawatts, and efficiency increases to 20% in 1985.

 

PV plant which is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material which are classified as wafer-based cells.

Three key elements in a solar cell form the basis of their manufacturing technology. The first is the semiconductor, which absorbs light and converts it into electron-hole pairs. The second is the semiconductor junction, which Separates the photo-generated carriers (electrons and holes), and the third are the contacts on the front and back of the cell that allow the current to flow to the external circuit.

A photovoltaic array (also called a solar array) consists of multiple photovoltaic modules, casually referred to as solar panels, contains an array of photovoltaic (PV) modules, one or more DC to alternating current (AC) power converters (also known as inverters), a tracking system that supports the solar modules, electrical wiring and interconnections, and mounting for other components.a photovoltaic system may include any or all of the following: net metering, maximum power point tracker (MPPT), battery system and charger, GPS solar tracker, energy management software, solar concentrators, solar irradiance sensors, anemometer. The number of modules in the system determines the total DC watts capable of being generated by the solar array; however, the inverter ultimately governs the amount of AC watts that can be distributed for consumption. Large grid-connected photovoltaic power systems are capable of providing an energy supply for multiple consumers. PV systems are generally designed in order to ensure the highest energy yield for a given investment. The cells can be arranged in a module to produce a specific voltage and current to meet the particular electrical requirements. Similarly, the PV modules can be arranged to form a solar array to produce a specific voltage and the current.

Solar Photovoltaic System Components:

  • PV Array: An electrical assembly of photovoltaic modules that convert sunlight to DC electricity.
  • Inverter: A device that converts DC power from batteries or PV arrays into utility-grade AC power.
  • Energy Storage: Electrical or other storage devices sometimes used to store energy produced by PV arrays for later consumption.
  • System Charge Control: A device used to protect batteries from overcharge and over discharge, sometimes provide load control functions.
  • Load: Energy consuming electrical appliances served by the system.
  • Balance of System (BOS) Components: Other equipment required to control, conduct, protect and distribute power in the system.

 

Inverter: The output of the inverter must synchronize automatically its AC output to the exact AC voltage and frequency of the grid. Inverter Efficiency considered 94% in the PV system. Inverter shall be provided with isolating protection to isolate it from the grid in case of no supply, under voltage or over voltage conditions so that there is no chance of accident in any case. PV systems shall be provided with adequate rating Fuses, fuses on inverter input side (DC) as well as output side (AC) side for overload and short circuit protection and disconnecting switches to Isolate the DC and AC system for maintenances are needed. Fuses of adequate rating shall also be provided in each solar array module to protect them against short circuit.

DC and AC cables:

One common factor for most of the photovoltaic power systems is outdoor use, characterized by high temperatures and high UV radiation. Single-core cables with a maximum permissible DC voltage of 1.8 kV and a temperature range from -40°C to +90°C are generally used. A three-core AC cable is used for connection to the grid if a single-phase inverter is used, and a five-core cable is used for three-phase feed-in.

Junction boxes:

Used to assemble wire dc and ac to protect the solar system from the (overload and short circuit current).

Earthling equipment / material:

Photovoltaic systems are typically installed on roof of the houses or on large open spaces, where are bigger possibility of lightning strike. Consequences of lightning strike are destroyed photovoltaic modules also other electrical equipment in the house because of connection between the photovoltaic system and electrical installations in the home.

To ensure safe and uninterrupted operation of PV system throughout its life cycle it is necessary to be provided complete protection from lightning and induced high voltages. Protection should be provided on DC side also on AC side from the DC/AC converter.

Solar Cell:

The basic photovoltaic device that generates DC electricity when exposed to light. A typical silicon solar cell produces about 0.5 volts and up to 6 amps and 3 watts for larger area cells.

Module:

A complete, environmentally protected unit consisting of solar cells, optics, and other components, exclusive of tracker, designed to generate DC power when expose to sunlight.

Panel:

A collection of modules mechanically fastened together, wired, and designed to provide a field installable unit.

Array:

A mechanical integrated assembly of modules or panels with a support structure and foundation, tracker, and other components, as required, to form a direct-current power producing unit.

Types of PV Systems

Mainstream materials presently used for photovoltaic’s include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide. Wafer-based silicon is divided into two different types:


Mono crystalline and multi crystalline (sometimes called ‘polycrystalline’). The poly crystalline wafer thickness is around 150-200μm. Single-junction wafer-based c-Si cells have been independently verified to have record energy conversion efficiency of more than 18 % for poly crystalline silicon cells under standard test conditions.

The majority of PV modules (85 percent to 90 percent of the global annual market) are based on wafer-based crystalline-Si. The manufacturing of c-Si modules typically involves growing ingots of silicon, slicing the ingots into wafers to make solar cells, electrically interconnecting the cells, and encapsulating the strings of cells to form a module.

Multi-crystalline silicon modules have a more disordered atomic structure, leading to lower efficiencies. But they are less expensive and more resistant to degradation due to irradiation. They have a well proven and reliable technology, long lifetimes, and abundant primary resources.

Advantages  of PV System

PV is an excellent energy efficiency source, particularly in distributed generation, where transmission
and distribution losses are reduced. If there is significant PV capacity on roof tops and close to urban centers or demand loads, PV output be a maximum during mid-day, coinciding with the peak load.

Today’s crystalline PV panels have a very long lifetime, at least 25 years, and possibly much longer. This is because crystalline silicon is very stable (silicon crystals can remain intact on geological time scales). The primary cause of failure is due to degradation of the transparent laminates that protect the cells from the elements, and from problems such as broken contacts.

Solar power plants play an important role in decreasing the environmental pollution; they contribute directly to the CO2 reduction that caused by the conventional fossil fuel power plants.

Main advantages of the PV System are:

  1. Environmentally friendly.
  2. No noise, no moving parts and no emissions.
  3. No use of fuels and water.
  4. Minimal maintenance requirements.
  5. Long life time up to 40 years.

PV General Rules 

  • PV Stores the excess in batteries during the day, then draw off the batteries at night, or when it’s cloudy.
  • The utility grid is a two way electron flow, electricity can be “sent back” to the grid by the customer, this eliminates the need for batteries, reduces cost and maintenance and ensures a constant supply of electricity.
  • Excess electricity generated goes through the net meter and into the grid, that will get credit for “stored” electricity.
  • The direction of the slope is up to South.
  • Crystalline PV efficiencies range from 15-22%. Space required: 90-150 square foot per kW (3 – 5 square meters).
  • Thin-Film PV efficiencies range from 5-10%. Space required: 170-300 square foot per kW(6 – 10 square meters).

How Does PV Work?

The basic idea of photovoltaic effects is simple, electrons will emit from matter as a result of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as “photoelectrons”. However the electron movements have no clear direction; therefore, to create electricity, it is necessary to collect electrons. The semiconductor material is therefore polluted with ‘impure’ atoms. Two different kinds of atom produce an n-type and a p-type region inside the semiconductor and these two neighboring regions generate an electrical field. This field can then collect electrons and draws free electrons released by the photons to the n-type region. And the holes move in the opposite direction, into the p-type region.

However, not all of the energy from the sunlight can generate free electrons. There are several reasons for this. Part of the sunlight is reflected at the surface of the solar cell, or passes through the cell. In some cases, electrons and holes recombine before arriving at the n-type and p-type regions. Furthermore, if the energy of the photon is too low-which is the case with light of long wavelengths, such as infrared-it is not sufficient to release the electron. On the other hand, if the photon energy is too high, only a part of its energy is needed to release the electron and the rest converts to heat.

PV Design Requirements

PV products undergo rigorous internal tests and have obtained external certifications (ISO 9001, CE, IEC and TUV) to ensure optimal performance and safety. For best performance, PV systems aim to maximize the time they face the sun. Solar trackers achieve this by moving PV panels to follow the sun.  The increase can be by as much as 20% in winter and by as much as 50% in summer.

Static mounted systems can be optimized by analysis of the sun path. Panels are often set to latitude tilt, an angle equal to the latitude, but performance can be improved by adjusting the angle for summer or winter. Generally, as with other semiconductor devices, temperatures above room temperature reduce the performance of photovoltaic.

Testing PV modules and PV inverters at PV Laboratory of the National Energy Research Center (NERC):

  • Outdoor Testing of PV Modules and Arrays:
  • Testing up to 100 kWp PV arrays (NERC received recently a new IV curve tracer, the only one in Jordan)
  • Testing according to IEC 60904-1 entitled “Photovoltaic devices-Part 1: Measurements of PV current-voltage characteristics”
  • Correction according to IEC 60891 standard entitled “Procedures for temperature and irradiance corrections to measured I-V characteristics of crystalline silicon photovoltaic (PV) devices”
  • Measurement of PV inverter efficiency:

1– Testing up to 10 KVA.

2– Testing according to IEC 61683 standard entitled “Photovoltaic Systems". 

3– Power Conditioners-Procedure for measuring efficiency.