Photovoltaic Systems

Photovoltaic Systems image


1.  How Solar Electric Technology Works

Image below shows a residential Grid-Connected Photovoltaic System.

1. solar panels 2. inverter 3. breaker box 4. home power and appliances 5. meter 6. utility power grid.

(1) Solar Electric or PV modules convert sunlight to electricity. The PV modules generate DC electricity - or direct current - sending it to the inverter.
(2) The inverter transforms the DC power into AC electricity for ordinary household needs.
(3) Existing electrical panel distributes solar electricity and utility power to
(4) loads (appliances). For systems with a battery backup (optional), the inverter also regulates the charge of batteries. The electricity stored in the batteries can be used at night or during blackouts.

A valuable feature of photovoltaic systems is the ability to connect with the existing power grid which allows owners to sell excessive electricity back to the utility with a plan known as (5) Net Metering. At times when you are not using all of the electricity produced by your system, your meter will spin backwards selling the electricity back to the (6) utility power grid at retail rate.

Types of PV Systems

Systems are generally classified according to their functional and operational requirements, their component configuration, and how the equipment is connected to the other power sources and electrical loads (appliances).

2. Utility Intertie PV Systems (Grid-Connected)

Intertie systems use an inverter that converts the energy collected from the PV panels into conventional AC power and feeds it to your electrical circuit breaker panel. As long as there is enough electricity flowing in from your PV system, no electricity will flow in from the utility company. If your system is generating more power than you are using, the excess will flow back into the grid, turning your meter backwards. If you live in one of the majority States that allow Net Metering, then everything goes in and out through a single residential meter.

Grid-Connected System is the simplest and most cost effective way to connect PV modules to regular utility power. Grid-Connected systems can supply solar power to your home and use utility power as a backup. If utility power is reliable and well maintained in your area, and energy storage is not a priority, you don't necessarily need a battery. But if the utility power goes down, even if there is solar, the PV system will be off for the safety of the utility workers.

Grid-Connected System with Battery Backup can supply power 100% of the time: At night, on cloudy days and when the utility power is down.

Grid-Tie Solar Systems (GTS)

A valuable feature of grid-tie or grid-connected photovoltaic systems is the ability to connect with the existing power grid and sell excessive electricity back to the utility with a plan known as Net Metering. At times when you are not using all of the electricity produced by your system, your meter will spin backwards selling the electricity back to the grid at retail rate. This systems do not include a battery. Power is obtained from the utility grid when the system is not producing electricity.

Grid-Tie Systems with Battery Backup (GTB)

Grid-Tie Solar Electric Systems with Battery Backup have all the features of the Grid-Tie Systems with the addition of a Battery. The battery can store power for use when the system is not producing electricity such as during the night or during blackouts.

Both Stand-Alone and Intertie systems can operate with or without a battery. Batteries add a substantial amount of cost and maintenance to the system but it allows the systems to operate when there is no other energy input (at night, during cloudy days, or during utility power outages). Without a battery if there is no solar energy flowing in and your utility power goes down, so will your system.

3. Stand-Alone Solar Electric Systems

Stand-alone PV systems are designed to operate independent of the electric utility grid, and are generally designed and sized to supply certain DC and/or AC electrical loads. Stand-alone systems may be powered by a PV array only or may use utility power as a backup power source.

Stand-Alone System - Since there is no battery to store electrical energy, energy is used immediately. Common applications are direct power to DC loads, water pumping and telecommunications. With an inverter it can also power AC loads. This system only works when it's sunny.
Stand-Alone System with Battery Backup - Can supply power 100% of the time: At night, on cloudy days and when the utility power is down. Excellent for remote applications where utility grid is inaccessible; cabins, boats, RVs and for emergency backup systems.

The size and type of the PV system that will meet your expectations depends on your individual needs, site location and climate. Please contact us and our engineers and consultants will gladly assist you.

4. Typical PV System Components

PV Power generation systems are made up of interconnected components, each with a specific function. One of the major strengths of PV systems is modularity. As your needs grow, individual components can be replaced or added to provide increased capacity. The selected components will vary depending on the applications, what follows is a brief overview of the components of a typical Solar Electric PV system.

4.1 Solar Array

The solar array consists of PV modules which convert sunlight into electric energy. The modules are connected in series and/or parallel to provide the voltage and current levels needed. The array is usually mounted on a metal structure and tilted to face the sun.

4.1.1 Photovoltaic (PV) power

PV is emerging as a major power resource, steadily becoming more affordable and proving to be more reliable than utilities. Photovoltaic power promises a brighter, cleaner future for our children.

Using the technology we have today we could equal the entire electric production of the United States with photovoltaic power plants using only about 12,000 square miles.

In 1839, Edmund Becquerel discovered the process of using sunlight to produce an electric current in a solid material, but it wasn't until a century later that scientists eventually learned that the photovoltaic effect caused certain materials to convert light energy into electrical energy.

The photovoltaic effect is the basic principal process by which a PV cell converts sunlight into electricity. When light shines on a PV cell, it may be reflected, absorbed, or pass right through. The absorbed light generates electricity.

In the early 1950s, photovoltaic (PV) cells were developed as a spin-off of transistor technology. Very thin layers of pure silicon are impregnated with tiny amounts of other elements. When exposed to sunlight, small amounts of electricity are produced. Originally this technology was a costly source of power
For satellites but it has steadily come down in price making it affordable to power homes and businesses.

Cells: Semiconductor device that converts sunlight into direct current (DC) electricity
Modules: PV modules consist of PV cell circuits sealed in an environmentally protective laminate and are the fundamental building block of PV systems
Panels: PV panels include one or more PV modules assembled as a pre-wired, field-installable unit
Array: A PV array is the complete power-generating unit, consisting of any number of PV modules and panels

Photovoltaic Cell

A single PV cell is a thin semiconductor wafer made of two layers generally made of highly purified silicon (PV cells can be made of many different semiconductors but crystalline silicon is the most widely used). The layers have been doped with boron on one side and phosphorous on the other side, producing surplus of electrons on one side and a deficit of electrons on the other side.

When the wafer is bombarded by sunlight, photons in the sunlight knock off some of excess electrons, this makes a voltage difference between the two sides as the excess electrons try to move to the deficit side. In silicon this voltage is .5 volt

Metallic contacts are made to both sides of the semiconductor. With an external circuit attached to the contacts, the electrons can get back to where they came from and a current flows through the circuit. This PV cell has no storage capacity, it simply acts as an electron pump.

The amount of current is determined by the number of electrons that the solar photons knock off. Bigger cells, more efficient cells, or cells exposed to more intense sunlight will deliver more electrons.

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