Solar PV Technology - How it works
What is solar PV
We have been using solar photovoltaic (PV) cells to harness energy from the sun for a long time. In recent years however, they have become considerably more efficient and affordable.
Individual PV cells usually comprise layers of doped crystalline silicon, sandwiched beneath glass to create a photodiode. These cells are then assembled into panels, which in turn can be linked together to form larger arrays.
For the majority of households in the UK a solar PV system tends to be mounted onto the structure of a building; commonly, though not exclusively, a roof.
How does it work?
Photons from sunlight, hit the PV cell creating a flow of electrons in one direction across the cell – a direct current (DC). For households that are connected to the grid, the DC power generated by their PV system is fed through an inverter, which then generates AC power at 230V, synchronising the PV system with the grid frequency.
This PV produced electricity is either used by the household or, if there is more being generated than being used, is fed automatically back into the grid. Conversely, if the household needs more electricity than their PV system is generating, the extra needed will automatically be imported into the home from the grid in the normal way. Special meters are installed to record generated or exported units of electricty.
How much electricity does it generate?
Myth-buster – it doesn’t have to be a day of uninterrupted sunshine to generate electricity – PV panels in the UK can even work on cloudy days, though output is reduced.
The amount of electricity your PV system will generate depends upon its size, siting and the efficiency of the panels. In the south-west of England, a well-positioned roof-mounted PV system will generate 800-900kWh per year for each kilowatt of rated capacity (kWp) installed. A typical 3kW domestic system will thus produce around 2,500kWh - just over half the annual electricity consumption of a typical household, although a portion of this will be exported.
How much does it cost?
Costs have fallen dramatically over the last few years. Typically (as of May 2013) prices range from £1,650 to £2,300 for each kilowatt of installed capacity, depending upon the type of system installed.
Box-statement – research suggests that with the right level of investment, PV could be competitive with fossil-generated electricity by the end of this decade – the so-called ‘grid parity’ point.
Financial benefits: Feed-in Tariff
You may have heard about the Feed-in Tariff, a government scheme where energy suppliers pay those who have installed electricity-generating technologies a fixed sum of money per kWh they produce. The following fixed unit rates are payable as of 1 October 2014:
- Solar PV (<4kW) 14.38p/kWh
- Solar PV (4-10 kW) 13.03p/kWh
- Solar PV (>10-50kW) 12.13 p/kWh
- Solar PV (>50-100KW) 10.34p/kWh
- Export tariff 4.77p
These rates are paid for all units generated, RPI-linked, guaranteed for 20 years, and tax-free for householders.
There are however, a couple of elibigility rules associated with receiving the feed-in-tariff – firstly, your installation must be approved and registered with the Microgeneration Certification Scheme (www.microgenerationcertification.org) and secondly, your home must have an Energy Performance Certificate rating of ‘D’ or above to receive the above rates; otherwise a lower rate applies.
Looking after your PV system
A PV system requires a surprisingly minimal amount of maintenance; they are designed for a 25-year working life, with performance even after 20 years being typically 80% of that when new. Although the panels need to be reasonably clear of dust and debris, the rain is generally sufficient to clean them on a standard pitched roof.
Siting your PV system
Aim for a SE to SW facing location – as close to due south as possible.
Any shade greatly reduces the productivity of your PV panels, look for a site with as little shade as possible.
The angle of your installation is important. Whilst the majority of systems are placed on pitched roofs, they can be installed on vertical walls or even horizontal surfaces with a frame that raises them to the optimal elevation.