Photovoltaic features

The term "photovoltaic" refers to a technology, which uses a device to produce free electrons when exposed to light and thus create an electric current. The word photovoltaic derives from the Greek word "photo" meaning light and the modern word "Volt" or "Voltage" meaning a unit of electrical potential (named in honour of the Italian physicist Alessandro Volta (1745–1827), who also is attributed with inventing the first chemical battery. This is the subject of debate, though, as working batteries may have been used in antiquity.

Photovoltaic technology converts sunlight into electrical energy in a direct way as opposed to the circuitous approach of solar thermal technologies that capture sunlight to heat a gas or fluid and subsequently use heat engines to generate electricity.

The most common PV technology uses solar cells made of semiconductor materials (such as silicon or germanium) doped with small amounts of impurities (typically metals or metalloids).

In simple terms, when sunlight strikes a cell, a certain portion of its energy is absorbed within the semiconductor material. The absorbed energy knocks electrons loose, allowing them to flow freely under the influence of electric fields in a closed electric circuit.

Solar cells have inbuilt electric fields that force the released electrons to flow in a certain direction. Metal contacts on the top and bottom of the PV cell enable the cell to generate a current in an external circuit. This current, together with the cell's voltage (which is a result of its in-built electric fields), defines the power (or wattage) that a solar cell can produce.

This direct current (DC) can be used to recharge batteries and run direct current devices, or can be converted via power electronic inverters into alternating current (AC), the form of electricity most commonly used in homes, offices and industry.

The word photovoltaic derives from the Greek word "photo" meaning light and the modern word "Volt" or "Voltage" meaning a unit of electrical potential (named in honor of the Italian physicist Alessandro Volta (1745–1827), who is attributed with inventing the first chemical battery. This is the subject of debate, though, as working batteries may have been used in antiquity).

Introduction

Individual solar cells create relatively low voltage, typically of around 0.5V. Several cells are combined within a a laminate with the cells effectively wired in series.

The laminate is covered in a weatherproof housing and installed in a frame to form a photovoltaic module or panel. The panel will typically produce around 15 volts or more when under a load (e.g. while charging a 12-volt battery). Open-circuit voltage could be higher, perhaps 20 volts or more.

If panels are connected (electrically) in series, it is possible to obtain very high output voltages. In fact, a number of panels can be connected to form a PV string. Moreover, two or more strings can be fed to an inverter to create a PV array.

In its simplest form, a PV system has its cells or panels directly connected to DC electrical equipment. The obvious shortcoming of this approach is the lack of an energy supply when there is no sunlight. In practice, such direct use is limited to powering small batteries, cell phone chargers, pocket calculators and similar small portable equipment.

PV systems can be designed to operate as a stand-alone installation or to be connected to an electrical grid. In the latter case, their output may be used on site or exported depending on the local power demand. PV systems can also be integrated with other generation plants (e.g. diesel generators) to form hybrid systems. In all these systems, electrical energy can also be stored for later local use or transmission to distant users by using batteries and other devices.

Grid-connected systems

The solar arrays at the University of Queensland’s St Lucia Campus are grid-connected in that they feed power back into the university’s low voltage (415 volts) network. If a particular array generates more power than its host building can consume at a given time, then the surplus can be exported to other buildings.

Special grid-tied inverters are used in a grid-connected system as their AC output has to be synchronised with the frequency and voltage of the grid. Grid-connected systems often have one or more meters that are able to measure the amount of electricity exported to the grid.

In Queensland, a feed-in tariff (FiT) provides households with a credit for electricity exported to the grid when the home's PV panels generate a surplus of solar energy over household needs.

The University of Queensland St Lucia Campus is quite extensive and its electricity demand is large. Thus, it is highly unlikely that enough energy would be generated to enable export to the surrounding off-campus utility grid.

Nonetheless, the output from the solar arrays is metered for energy management and reporting reasons and this metering forms the basis of the PV Data Display.

During hot summer afternoons, the PV arrays at St. Lucia, for example, will reduce the campus peak demand by a considerable 6%. This reduces the pressure on the neighbouring utility network during a period of high demand, thus extending the benefit of UQ's PV arrays to the wider community.

Although this is not yet the norm, grid-connected PV systems can also incorporate batteries or other forms of energy storage as pressure grows to reduce peak demand and shift loads to off-peak periods.

Presently, residential rooftop PV systems usually supply energy to the utility grid during the day (when few people are home) and then draw power from the utility grid at night when the PV system cannot generates electricity.

In a sense, the utility grid can be thought of as a "battery".

PV cell technology

There are many different types of solar cells. However, the most common and commercially available types are amorphous, polycrystalline, and monocrystalline cells; which derive their names from the nature of the silicon used to create their substrates.

The conversion efficiency of a PV panel (see the table below) and its cost will depend on the nature of the silicon used to manufacture the panel's solar cells.

Silicon Composition	Efficiency of Commercial Panels*	Known Max Efficiency*
Amorphous Silicon	6	12.5
Polycrystalline Silicon	9.5-15.3	20.4
Monocrystalline Silicon	13.3-15.9	25

* Efficiency (%) of Silicon Cells as measured at Standard Test Condition (STC)

Solar cells are tested at Standard Test Conditions (STC). Incident sunlight of 1000 W/m² and a cell temperature of 25^oC are two of the standard conditions. You can read more about the effect of environmental factors and weather on PV output, at the Weather & Local Environment page.

Monocrystalline solar cells are manufactured from crystals of very pure silicon. A crystal is grown in a complex process to produce a long rod (also called "ingot"). The rod is sliced into 0.2 to 0.4 mm thick discs or wafers, which are then processed into individual cells. These are wired together to create a solar panel. Under standard conditions, their conversion efficiency is much the same as those of polycrystalline cells. Monocrystalline panels are noted, however, for their quality and are often used where high reliability is needed.

Monocrystalline solar cells (and polycrystalline cells) experience a significant reduction in output at elevated cell temperatures. In Queensland, panels can often operate at a temperature of 50^oC which is well above the 25^oC used for the standard test conditions. As a result a reduction of between 12% and 15% in output can be expected on a bright sunny day and this needs to be factored into a project.

Solar panels made with polycrystalline cells (also called multicrystalline cells) are a bit cheaper and generally, slightly less efficient than those made up of monocrystalline cells. The silicon is not grown as a single cell but rather as a block of crystals. These blocks are then sliced into wafers to produce individual solar cells. If you look closely at a polycrystalline panel, you will notice a shattered glass-like look to the cells - this is an indication of the many crystals making up each cell. Polycrystalline cells have an even higher temperature-degrading factor than monocrystalline cells.

A thin layer of silicon is deposited on a base material such as metal or glass to create an amorphous solar panel. The silicon has no regular crystalline stucture. The manufacturing process is reasonably straightforward which results in relatively cheap panels. Amorphous solar cells are, however, only about half as efficient as polycrystalline and monocrystalline cells. Thus, in order to get the same electrical output as the crystalline cells, it is necessary to have twice the area of amorphus panels. This could be a problem when space availability is limited. Amorphous cells do not suffer reduced output with increased cell temperature (i.e. temperature de-rating).

Several different types of solar cell types are used at the various University of Queenland PV sites.

The table below provides a summary for the main installations at each site:

Manufacturer	Model	Site	Type	Wp	Efficiency at STC
Schott	POLY 235	MBRS	Polycrystalline	235W	14.04%
Coenergy	P170M	Gatton	Monocrystalline	170W	13.30%
Trina	TSM-240PC05	St Lucia	Polycrystalline	240W	14.70%
Kyocera	KD210GH-2PU	Heron Island	Polycrystalline	210W	14.14%

Stand alone systems

A standalone system is not connected to an electricity grid. It is typically used at remote sites, such as outback farms, telecommunication repeater stations, etc.

The simplest form of a standalone system has one or more PV panels, a set of batteries and ideally, a charge controller.

PV panels are selected so that their output voltage is sufficient to charge the batteries. Typically, this means voltages a bit higher than the 6, 12 or the 24 volts of standard batteries. A charge controller ensures that the batteries are not damaged through overcharging by the panels.

The batteries can be used to directly power low-voltage equipment such as lights.

Inverters can be wired to both the batteries and the PV panels to provide AC to power 240 volt household appliances, power tools and other AC equipment.

There are two main types of inverters: modified sine wave and pure sine wave inverters. The latter produce AC with a near perfect sine wave that is very close to that delivered by the electricity grid. This type of inverter can therefore be used to power virtually the same wide range of appliances as grid electricity. The cheaper modified sine wave inverter creates a modified square wave output that is suitable for most common electrical equipment but not all sensitive electronic devices.