SOLAR ELECTRICITY

Solar Photovoltaic Systems (PV)

What is a Solar Photovoltaic System?
Solar photovoltaic (PV) systems tap into the same abundant and free energy resource as solar thermal technology. PV cells consist of semiconductor materials that convert sunlight into electrical energy through the photovoltaic process. On their own, the voltage and current levels produced by individual cells are not particularly useful, but when many cells are connected together to form modules (panels), more practical energy can be produced.

Various materials and technologies exist for solar PV, each having its own characteristics, advantages and disadvantages. Despite having a tendency to cost more than other renewable energy technologies, they offset the most expensive energy bought and used in the home ? electricity. This, together with recent and significant electricity price rises coupled with falls in the cost of manufacturing make PV an ever more attractive option for domestic renewable energy.

PV produces on-site electricity, bypassing all the losses associated with electrical transmission over the national grid and saves hundreds of kilograms of carbon emissions per year in the process.

Lacking any mechanical components, PV is silent in operation and highly reliable, minimising the need for maintenance. The modular nature of PV allows for system design and/or rating to be amended at any time with ease. Additionally, PV can be integrated into the fabric of the building as well as being mounted on the roof, or onto a specially design frame.

Types of PV Technology
•Crystalline Silicon ? most common for domestic installations, and most widely available. The following crystalline silicon is used in PV modules and roof slates (see figure below)
oMonocrystalline ? the most efficient, but more expensive single silicon crystal technology
oPolycrystalline ? slightly less efficient and lower cost than monocrystalline. Polycrystalline cells each consist of a number of individual crystals, producing a different appearance.
•Amorphous Silicon ? these are thin films of non-crystalline PV material, deposited on a rigid, transparent sheet of glass or plastic, but also available as flexible laminates. Commonly used for roof tiles instead of crystalline silicon. 



Fig 3. (a) PV roof mounted modules.












(b) Crystalline silicon roof slates

How does a Solar PV System Work?
PV cells convert incident sunlight into electricity; light falling onto a PV cell imparts energy to electrons, freeing them from their ground, or normal state, and leaving them free to move around the semiconductor material. The most common semiconductor material used is crystalline or amorphous silicon.
These free electrons are transported by electric fields to an electrical contact (terminal) where they can be conducted away and do work in an electrical circuit, i.e. to power your household appliances.

Individual silicon cells provide in the region of 0.55Volts and 4.7Amps (=2.59 Watts). To make their outputs more useful for domestic installations they are connected together in a series string to form modules of, commonly, 36 cells, providing outputs of 18V and 4.7A (=84.6 Watts). This is just an example however, as specific values depend upon particular characteristics.

Modules are usually connected together in a mixture of series and parallel strings to form an array. The collective power output from the array is quoted as the system rating in kilowatts peak (kWp). Individual module ratings can range from a few Watts to over 300 Watts.

Direct Current (DC) electricity is produced by photovoltaic cells, which needs inverting to Alternating Current (AC) electricity before being used by appliances around the home or exported to the local electricity grid (for grid connected systems). This requires the use of an inverter. Another function of the inverter is to ensure that the voltage output of the system is compatible with that of household appliances.

For stand alone systems (systems not connected to the grid) it is usual to store surplus energy produced during the day in a bank of batteries ready for use at another time. For this a charge controller and converter/inverter are required for proper charging of the batteries, and for stepping up or converting the energy produced to useable levels by appliances. This can be used as a backup supply during power outages.

As with solar thermal water heating systems, PV can be installed into the home in a variety of ways. All that is needed is a site with little or no shading for most of the year and good access to south-eastern to south-western skies.

Other options include the use of PV integrated roof tiles. These are designed to appear more like a conventional roof, although the physics of operation is the same.

What components comprise a Solar PV System?
Off grid systems vary slightly from grid connected systems. Stand alone systems usually comprise:
1.PV array (modules, slates, tiles, shingles)
2.Power conditioning equipment (converter/inverter, battery charge controller)
3.Battery Bank
4.Load

Grid connected systems usually comprise:
1.PV array
2.Inverter (grid connected)
3.Safety switches and diodes
4.Load (including grid)

How much energy can I expect a Solar PV System to produce?
This depends strongly on the technology of cells used, array area, location and array orientation in addition to whether or not direct shading occurs. A well designed system in a suitable location should produce between 700 and 850 kWh of electricity per year per kWp of installed capacity.

A PV system of 1.5 ? 2.5 kWp could generate between a third and fifty percent of the electricity requirements of a two or three bedroom household.

It should be noted that shading of even a small area of the array may have a disproportionate impact on the performance of the system. Modules containing bypass diodes reduce this effect.

What are the benefits of PV?
•PV has a very low maintenance requirement as there are no moving parts to service. As a result operation is also silent
•If replacing the roof of a building, PV integrated roof tiles, slates and shingles form an ideal weatherproof alternative to conventional tiles
•PV reduces the amount of electricity imported to the house; the most expensive and polluting energy when produced from fossil fuel sources
•Connection of PV modules is such that almost any size or shape of roof can be accommodated by the array. This versatility also allows a degree of creativity in the appearance of the system
•Long operational life-time; well in excess of 20 years. The end of life for a PV module is when its actual maximum output has degraded to 80% of its rated output
•PV can be positioned on the roof of a building or near the building on a suitable mounting, e.g. an A-frame

Suitability
Ideally, for best performance any PV array should face between southeast and southwest, and have an angle of tilt of 30-45 degrees to the horizontal for the UK; arrays should not be angled towards the north.

Shading should be kept to a minimum. Even a small amount of shading on a single module can reduce the overall output of the whole system disproportionately; however modules with bypass diodes act to reduce this effect. If some shading cannot be avoided, try to ensure that it does not affect the array between the hours of 10am and 4pm.

Each kWp requires an area of about 8-15 m2 for mono/polycrystalline modules, but this increases for amorphous silicon roof tiles, etc. due to their lower conversion efficiency.

Grid connection and exporting excess energy
Before connecting any electricity generating equipment to the local grid, you should seek advice and approval from your Distribution Network Operator (DNO), and make sure that your equipment satisfies the G83/1 regulations. Your PV installer should be able to help with this.
In the event of grid failure (power cut), your PV system will automatically shut down in order to ensure the safety of any engineers working on the power lines.

Excess electrical generation can be sold to the national grid; contact your energy supplier for more details of the options available to you.

Maintenance requirements
Having no moving parts, PV systems are very low maintenance. It is advised to conduct occasional visual inspections of the array to ensure no significant build up of dust and debris occur. Panels inclined on a roof tend to be ?washed clean? naturally by the weather, reducing the need for any cleaning.

Occasional inspections of the inverter to ensure it has not tripped or become faulty should also be made. These are usually very easily and quickly remedied without the need to call out an engineer.

Installation Costs
A significant proportion of the cost of a PV system is the array and the grid-connect inverter(s), which both can vary substantially. The installed and commissioned cost of a grid connected PV system should be in the region of £4,000 - £9,000 per kilowatt peak of installed capacity, with an average of around £6,500 per kWp.

© 2008 West Wales ECO Centre