Most solar parks are
ground mounted PV systems, also known as free-field solar power plants. They can either be fixed tilt or use a single axis or dual axis
solar tracker. While tracking improves the overall performance, it also increases the system's installation and maintenance cost. A
solar inverter converts the array's power output from
DC to
AC, and connection to the
utility grid is made through a high voltage, three phase step up
transformer of typically 10
kV and above.
Solar array arrangements The
solar arrays are the subsystems which convert incoming light into electrical energy. They comprise a multitude of
solar panels, mounted on support structures and interconnected to deliver a power output to electronic power conditioning subsystems. The majority are free-field systems using ground-mounted structures, In some cases, depending on local climatic, topographical or electricity pricing regimes, different tilt angles can be used, or the arrays might be offset from the normal east–west axis to favour morning or evening output. A variant on this design is the use of arrays, whose tilt angle can be adjusted twice or four times annually to optimise seasonal output. To achieve this, arrays can be designed using
two-axis trackers, capable of tracking the sun in its daily movement across the sky, and as its elevation changes throughout the year. These arrays need to be spaced out to reduce inter-shading as the sun moves and the array orientations change, so need more land area. They also require more complex mechanisms to maintain the array surface at the required angle. The increased output can be of the order of 30% in locations with high levels of
direct radiation, but the increase is lower in temperate climates or those with more significant
diffuse radiation, due to overcast conditions. So dual axis trackers are most commonly used in subtropical regions, The angle of the axis is normally horizontal, though some, such as the solar park at Nellis Air Force Base, which has a 20° tilt, incline the axis towards the equator in a north–south orientation – effectively a hybrid between tracking and fixed tilt.
Single axis tracking systems are aligned along axes roughly north–south. Some use linkages between rows so that the same actuator can adjust the angle of several rows at once. There are two primary alternatives for configuring this conversion equipment; centralized and string inverters, although in some cases individual, or
micro-inverters are used. Single inverters allows optimizing the output of each panel, and multiple inverters increases the reliability by limiting the loss of output when an inverter fails. is divided into blocks, each with a centralised inverter These units have relatively high capacity, typically of the order between 1 MW up to 7 MW for newer units (2020), so they condition the output of a substantial block of solar arrays, up to perhaps in area. Solar parks using centralized inverters are often configured in discrete rectangular blocks, with the related inverter in one corner, or the centre of the block.
String inverters String inverters are substantially lower in capacity than central inverters, of the order of 10 kW up to 250 KW for newer models (2020), and condition the output of a single array string. This is normally a whole, or part of, a row of solar arrays within the overall plant. String inverters can enhance the efficiency of solar parks, where different parts of the array are experiencing different levels of insolation, for example where arranged at different orientations, or closely packed to minimise site area.
Transformers The system inverters typically provide power output at voltages of the order of 480 VAC up to 800 VAC.
Electricity grids operate at much higher voltages of the order of tens or hundreds of thousands of volts, so transformers are incorporated to deliver the required output to the grid. Transformers typically have a life of 25 to 75 years, and normally do not require replacement during the life of a photovoltaic power station.
System performance The performance of a solar park depends on the climatic conditions, the equipment used and the system configuration. The primary energy input is the global light irradiance in the plane of the solar arrays, and this in turn is a combination of the direct and the diffuse radiation. A key determinant of the output of the system is the conversion efficiency of the solar panels, which depends in particular on the type of
solar cell used. There will be losses between the DC output of the solar panels and the AC power delivered to the grid, due to a wide range of factors such as light absorption losses, mismatch, cable voltage drop, conversion efficiencies, and other
parasitic losses. A parameter called the 'performance ratio' has been developed to evaluate the total value of these losses. The performance ratio gives a measure of the output AC power delivered as a proportion of the total DC power which the solar panels should be able to deliver under the ambient climatic conditions. In modern solar parks the performance ratio should typically be in excess of 80%.
System degradation Early photovoltaic systems output decreased as much as 10%/year, Many panel makers offer a performance guarantee, typically 90% in ten years and 80% over 25 years. The output of all panels is typically warranted at plus or minus 3% during the first year of operation. == The business of developing solar parks ==