Solar Electricity PhotoVoltaic (PV) Systems

Posted by: Solar Trade Association

When electricity rather than heat is required, PV is a robust, flexible, scaleable technology which can provide energy for both houses and other buildings. The typical sized household PV system for example will provide at least 2,000 units of electricity per annum and save at least 1.14 tonnes of carbon dioxide annually. PV has a proven working life of 45 years plus and modules typically come with a 25 year manufacturers warranty. Electricity generation in the UK today is still largely sourced from fossil fuels, which causes the release of harmful gasses into the atmosphere – including CO2. The use of solar electricity generation through Photovoltaic (PV) systems will help to reduce the emission of these gases thereby helping government to meet their binding 2020 emission reduction targets.

Energy from the sun can be used to generate electricity through a PV system. Solar PV systems convert the sun’s rays directly into electricity through the use of solar cells. The technology can be used for domestic as well as larger industrial or commercial applications and has various applications to suit specific needs.

A large number of homes in the UK have recently been installing PV systems, and the number is increasing exponentially since the introduction of the Feed in Tariff (FiT) – the financial incentive scheme introduced by Government on 1st April 2010. The aim of the FiT is to increase the uptake of on-site electricity generating technologies including PV.

Types of PV System

PV systems can be grouped into grid connected and stand-alone/off grid systems.

The grid connected system allows the building in question to use a certain amount of the generated electricity, with the rest feeding directly onto the National Grid. An advantage of the grid connected system is that it allows you to get the balance of your electricity needs from the grid and it also allows you to export excess generated electricity back to the grid. Through the recently introduced Feed in Tariff for every unit (kWh) of electricity generated you will receive between 29.3p/Kwh and 41.3p/Kwh (generation tariff), as well as 3p/kWh for every unit exported back to the grid (export tariff). The grid connected system would have a grid connect inverter as well as a meter. The inverter would transform the DC power from the PV modules into AC power at a voltage and frequency that can be accepted by the grid while the meter would monitor the energy produced and used on site; as well as total energy exported to the grid.

Stand alone systems used for domestic and industrial applications in which a household or company is able to produce its own electricity and its yield can be matched to its demand. The electricity generated can be stored in batteries for use at a later time. This type of system is also very useful for remote locations where it is inconvenient or expensive to connect to the grid, an example of this is in navigation buoys. They are also used for other applications such as garden lights, parking meters, etc

Domestic PV systems are usually roof mounted in an array of panels and are placed at angles best suited to receive the energy coming from the sun. The orientation of these arrays would ideally be southerly, though PV systems will perform at almost any orientation (though output may be lower and the system should be sized accordingly). They can also be pole or ground mounted. An important consideration is that the panels are mounted in areas where there are no tall buildings or trees so as to minimise shading which will adversely affect the performance of the system.

PV System Components

The typical PV system is made up of the following parts:

PV panel:
This is made of cells that help to convert energy from the sun directly into electricity. The cells are made of silicon and are connected to form modules which may be either in series, parallel or both. The modules should be durable and be able to withstand temperature variations and humidity. There are three main types of solar cells as below:

  • Mono-crystalline: These are cells made from pure mono-crystalline silicon with a single thin crystalline structure. They are very efficient; however the cost of production is high as it is energy and labour intensive. This would be useful for small installations or areas with very limited mounting space.
  • Poly-crystalline: These are made from small grains of mono-crystalline silicon. They are cheaper and easier to manufacture but are also found to be less efficient than their mono-crystalline counterpart.
  • Amorphous Silicon: These are cells made from thin films of silicon in a form known as amorphous silicon. Here the silicon structures are in less order than the crystal lattice structure of the mono crystalline and the poly crystalline. Its production method though complex, is not as energy intensive as crystalline cells; however its efficiency is low. This technology is most often used in smaller applications such as calculators, or garden lights. It is also often found in multi-layer technology which increases cost, performance and efficiency typically to crystalline levels.

Inverters:
PV systems generate electricity in direct current (DC), however most appliances found in the home operate on alternating current (AC). Transformation of the DC to AC is done by the inverter, and they come in various sizes depending on the user’s energy needs. Battery powered devices also work on DC whilst the mains is supplied through 230 volts AC. The inverter also has several important safety features which protect electricians and householders from electrical dangers.

Storage batteries:
These are used to store excess energy produced by the system which can be used at night when the system can’t produce energy. Alternatively, excess energy stored here can be exported to the grid. If the grid is available, it is always preferable to use grid-connected PV circuits.

How They Work

Sunlight is made of a stream of photons which are absorbed by the cells of the PV panels. These photons interact with electrons found in the cells to ‘upgrade’ the electrons to a higher energy level which can be exploited as an electric current. These currents are conducted by wires to the electrical system of the building to be used for a range of appliances; directly to the grid or to batteries for storage purposes.

Feed in Tariff (FiT)

The use of PV systems is not only cost effective by saving you energy bills, but it is also a form of investment especially with the introduction of the feed in tariff. The FiT scheme is available for people who have an electrical micro generation technology installed in their homes and this could be in form of on-site wind turbines, anaerobic digestion, hydro electricity and combined heat and power (mCHP) and of course PV.

How does the FiT work?
To qualify for the FiT, the PV system must be installed by an MCS certified installer and supplier anytime from 15th July 2009.

Benefits of the FiT
There are three ways in which the owner of the technology can benefit from the scheme. These are:

  • Generation Tariff: in which your energy supplier gives money for every unit of electricity produced. The tariff varies depending on the capacity of the installed technology; for PV it ranges from 29.3p/Kwh to 41.3p/Kwh
  • Export Tariff: in this case, there is a payment of 3p/Kwh for all excess generated electricity exported to the grid. This payment is currently “deemed” which means an estimated amount of energy is exported and so receives an export payment.
  • Energy bill savings: by using the power from the installed technology, there is less dependence on the conventional electrical power from the grid thereby allowing savings on energy bills.

Please visit the DECC website for further information on Feed-in Tariffs.

Cost and Maintenance

Cost
The cost of a PV system would largely depend on the installed capacity, however on average one would anticipate paying between £4000 & £8000/kWp installed cost. Please note that this is only an estimate as there are many building factors that affect the cost of the quote. We always recommend that if someone is unsure about the first quote they receive that they should then obtain another quote from another member of the Association.

Maintenance
PV has a proven working life of 45 years plus and modules typically come with a 25 year manufacturers warranty. These systems require little maintenance as rain water should keep the panel surface sufficiently clean. Occasional additional cleaning may be required from time to time, particularly if the panels are located near a large tree (sap & birds!)

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Information Updated on: 1 February 2012