PV modules use semiconductor materials to generate dc electricity from sunlight. A large area is needed to collect as much sunlight as possible, so the semiconductor is either made into thin, flat, crystalline cells, or deposited as a very thin continuous layer onto a support material. The cells are wired together and sealed into a weatherproof module, with electrical connectors added. Modern modules for grid connection usually have between 48 and 72 cells and produce dc voltages of typically 25 to 40 volts, with a rated output (see box) of between 150 and 250 Wp.
In order to supply electricity into a mains electricity system, the dc output from the module must be converted to ac at the correct voltage and frequency. An electronic inverter is used to do this. Generally a number of PV modules are connected in series to provide a higher dc voltage to the inverter input, and sometimes several of these ‘series strings’ are connected in parallel, so that a single inverter can be used for 50 or more modules. Modern inverters are very efficient (typically 97%), and use electronic control systems to ensure that the PV array keeps working at its optimum voltage. They also incorporate safety systems as required in the country of use.
Many grid connected PV systems are installed on frames which are mounted on the roof or walls of a building. Used in this way the PV does not take up land that could be used for other purposes. Ideally the PV faces towards the equator (i.e. South in the northern hemisphere) but the exact direction is not critical. However, it is important to make sure that there is minimal shading of the PV. The inverter is housed inside the building and connected to the mains electrical supply, usually with a meter to measure the kWh generated. If the PV electricity production exceeds building demand then the excess can be exported to the grid, and vice versa.
A grid connected system rated at 1 kWp (1000 Wp) has an area of between 5 and 14 m2, depending on the type of semiconductor. The photo shows 1 kWp systems using crystalline PV modules, installed by Ashden Award-winner Kirklees Council.
If the PV system is installed during construction or refurbishment, it can sometimes be used as part of the building fabric, such as a roof or wall-cladding. Ashden Award winner Solarcentury has integrated PV arrays into a wide range of buildings.
Where space and sun are available, large stand-alone PV arrays can be built and connected to the public grid. In 2010, the largest operating system is a 55 MWp ‘photovoltaic park’ in Olmedilla, Spain, but plants of up to 550 MWp capacity are being planned.
Grid-connected systems do not usually include batteries for storage, because the mains grid can accept or provide power as needed. However, if rechargeable batteries are included, a grid-connected PV system can be used as a standalone ac supply in the event of a power cut, to allow essential loads to keep working. Ashden winner Deng solar provided a 9.2 kWp grid backup system for the central courts in Accra, Ghana, which maintains lighting and thus enables court business to keep going during power cuts. The Aryavart Gramin Bank has provided PV grid backup systems for its rural branches, so that their IT systems and cash machines still work during power cuts and voltage fluctuations.
What are the benefits of grid-connected PV systems?
By reducing the need for fossil-fuel generation, grid-connected PV cuts greenhouse gas emissions (and other air pollution), because no emissions are produced during PV operation.
In the past there has been concern about the greenhouse gases emitted (‘embodied’) in the manufacture of PV systems, particularly in the production of ultra-pure semiconductors. With current production techniques, these embodied greenhouse gases are saved within two to four years of use of grid-connected operation, depending on the amount of sunlight.
PV is the easiest renewable electricity source to incorporate into buildings. The electricity is supplied at the point of use, thus avoiding the losses which occur in electricity distribution (these average 7% in the UK). It can be used at any scale – from less than a kWp on an individual home up to MWp scale systems on large public buildings - and is simple and reliable. Because of this, it is a valuable way to raise awareness of electricity supply and use, and helps highlight the potential for renewable energy. Several schools that have won Ashden Awards like Cassop Primary School and Ringmer College have installed PV, to supply part of their electricity and as an education aid.
Cost
The capital cost of grid-connected PV varies between countries. As a guide, for the USA in mid-2010 the typical installed cost (excluding tax) ranges from about US$2.2 million for a 500 kWp utility scale system (US$4.4 per Wp), to about US$9000 for a 1 kWp domestic system (US$9 per Wp). The PV modules account for just over half of this cost, and the inverter, frame, wiring and labour for the rest. Costs have decreased substantially over the past three decades.
Translating these installed costs to the cost of electricity depends on a number of factors. One major factor is the amount of sunshine. A 1 kWp system on a South-facing roof in the UK supplies about 800 kWh/year of ac electricity (20% of average UK household electricity use), but about 1700 kWh/year in California. Another major factor is how the system is financed. For California, assuming 5% interest and 20-year payback, the utility-scale PV system above generates at about 19 US cents per kWh, and the domestic system at about 39 US cents per kWh.
These electricity costs are higher than the consumer price for grid electricity, and substantially higher than the cost of fossil-fuel generation. A number of governments support the installation of PV as part of their drive to reduce greenhouse gas emissions, by providing subsidies towards the capital cost, or schemes to buy PV-generated electricity at preferential rates (feed-in tariffs)
Numbers
The global PV market has experienced rapid growth over the past three decades, averaging 35% per year. Most of the recent growth has been in grid-connected PV which in 2009 accounted for about 95% of total PV sales (7.6 GWp out of 7.9 GWp) and 87% of total installed capacity (21 GWp out of 24 GWp). PV currently supplies about 0.1% of global grid electricity.
The future
The price of PV modules is decreasing rapidly. For crystalline cells, new ways of processing silicon and increased volume manufacture are driving down prices. The market share of thin film PV is growing rapidly as materials which have been proved in the laboratory go into volume production, and these promise even greater price reductions. However, there is less potential for price reduction in the ‘balance of system’, and these costs will soon dominate the overall system cost.
Because of the decreasing prices, the rapid growth in the market for grid-connected PV is expected to continue even if government support is reduced. The market will really take off when electricity from PV becomes cheaper than other grid sources. When PV feeds directly into a building supply, this grid-parity price is the consumer purchase price, currently 10 to 20 US cents per kWh. A recent ‘roadmap’ by the International Energy Agency suggests that this point may be reached in sunny countries by 2020. For systems connecting directly to the national grid, the grid-parity price is less than 5 US cents per kWh. The roadmap suggests that even this point could be reached by 2030, and that PV could then be supplying about 5% of global electricity.
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