Photovoltaic System

A photovoltaic system is a system which uses solar cells to convert light into electricity. A photovoltaic system consists of several components, including cells, mechanical and electrical connections and mountings and means of regulating and/or modifying the electrical output.

Due to the low voltage of an individual solar cell (typically ca. 0.5V), several cells are combined into photovoltaic modules, in turn connected together into an array. The electricity generated can be either stored or used directly (island/standalone plant), fed into a large electricity grid powered by central generation plants (grid-connected/grid-tied plant) or combined with one or more other electricity generators feed into a small grid (hybrid plant) ^[1]. Depending on the type of application, the rest of the system ("balance of system" or "BOS") consists of different components. The BOS depends on the load profile and the system type. Systems are generally designed in order to ensure the highest energy yield for a given investment.

Standalone systems

A standalone system does not have a connection to the electricity mains. Standalone systems vary in size from watches or calculators to remote buildings or spacecraft. If the load is to be supplied independently of insolation, the generated power needs to be buffered with a battery. Where weight is not an issue (e.g. buildings) lead acid batteries are used. A charge controller may be incorporated in the system to a) avoid battery damage by excessive charging or discharging and b) optimizing the production of the cells or modules by maximum power point (MPP) tracking. In small devices (e.g. calculators, parking meters) only DC is consumed. In larger systems (e.g. buildings, remote water pumps) AC is usually required. To convert the DC from the modules or batteries into AC an inverter is used.

Hybrid system

A hybrid system combines PV with other forms of generation, usually a diesel generator. Biogas is also used. The other form of generation may be a type able to modulate power output as a function of demand. However more than one renewable form of energy may be used e.g. wind. The photovoltaic power generation serves to reduce the consumption of non renewable fuel. Hybrid systems are most often found on islands. Pellworm island in Germany and Kynthos island are notable examples (both are combined with wind) ^[2] ^[3]. The Kynthos plant has reduced diesel consumption by 11.2% ^[4]

Grid-connected/Grid-tied System

A grid connected system is connected to a large independent grid (typically the public electricity grid) and feeds power into the grid. Grid connected systems vary in size from residential (2-10kWp) to solar power stations (up to 10s of GWp). This is a form of decentralized electricity generation. In the case of residential or building mounted grid connected PV systems, the electricity demand of the building is met by the PV system. Only the excess is fed into the grid when there is an excess. The feeding of electricity into the grid requires not only the transformation of DC into AC by a special, grid-controlled inverter.

In kW sized installations the DC side system voltage is as high as permitted (typically 1000V except US residential 600V) to limit ohmic losses. Most modules (72 crystalline silicon cells) generate about 160W at 36 volts. It is sometimes necessary or desirable to connect the modules partially in parallel rather than all in series. One set of modules connected in series is known as a 'string'.

Grid connected inverters

On the AC side, these inverters must supply electricity in sinusoidal form, synchronized to the grid frequency, limit feed in voltage to no higher than the grid voltage including disconnecting from the grid if the grid voltage is turned off.

On the DC side, the power output of a module varies as a function of the voltage in a way that power generation can be optimized by varying the system voltage to find the 'maximum power point'. Most inverters therefore incorporate 'maximum power point tracking'.

For safety reasons a circuit breaker is provided both on the AC and DC side to enable maintenance. The AC output usually goes through across an electricity meter into the public grid.

In some countries, for installations over 30kWp a frequency and a voltage monitor with disconnection of all phases is required.

Connection to a DC grid

DC grids are only to be found in electric powered transport: railways trams and trolleybuses. A few pilot plants for such applications have been built, such as the tram depot in Hannover Leinhausen ^[5]

Small-scale PV solar systems

Small scale DIY solar systems

With a growing DIY-community and an increasing interest in environmentally friendly "green energy", some hobbyists have endeavored to build their own PV solar systems from kits ^[6] or partly diy ^[7]. Usually, the DIY-community uses inexpensive ^[8] and/or high efficiency systems ^[9] ^[10](such as those with solar tracking) to generate their own power. As a result, the DIY-systems often end up cheaper than their commercial counterparts ^[11]. Often, the system is also hooked up unto the regular power grid to repay part of the investment via net metering. These systems usually generate power amount of ~2kW or less. Through the internet, the community is now able to obtain plans to plans to construct the system (at least partly DIY) and there is a growing trend toward building them for domestic requirements. The DIY-PV solar systems are now also being used both in developed countries and in developing countries, to power residences and small businesses.

Mounting systems

Modules are assembled into arrays on some kind of mounting system. For solar parks a large rack is mounted on the ground, and the modules mounted on the rack.

For buildings, many different racks have been devised for pitched roofs. For flat roofs, racks, bins and building integrated solutions are used.

Trackers

A solar tracker can substantially improve the amount of power produced by a system by enhancing morning and afternoon performance. It is only worth installing trackers for non-concentrating applications in regions with mostly direct sunlight. In diffuse light (ie under cloud or fog), tracking has no value. For concentrated photovoltaic systems a tracker is necessary.

Solar cell I-V curves where a line intersects the knee of the curves where the maximum power point is located

A maximum power point tracker (or MPPT) is a high efficiency DC to DC converter which functions as an optimal electrical load for a photovoltaic (PV) cell, most commonly for a solar panel or array, and converts the power to a voltage or current level which is more suitable to whatever load the system is designed to drive.

Photovoltaic (PV) cells have a single operating point where the values of the current (I) and Voltage (V) of the cell result in a maximum power output. These values correspond to a particular resistance, which is equal to V/I as specified by Ohm's Law. A PV cell has an exponential relationship between current and voltage, and the maximum power point (MPP) occurs at the knee of the curve, where the resistance is equal to the negative of the differential resistance (V/I = -dV/dI). Maximum power point trackers utilize some type of control circuit or logic to search for this point and thus to allow the converter circuit to extract the maximum power available from a cell.

Battery-less grid-tied PV inverters utilize MPPTs to extract the maximum power from a PV array, convert this to alternating current (AC) and sell excess energy back to the operators of the power grid.

MPPT charge controllers are desirable for off-grid power systems to make the best use of all the energy generated by the panels. MPPT charge controllers are quickly becoming more affordable and are more common in use now than ever before.

The benefits of MPPT regulators are greatest during cold weather, on cloudy or hazy days or when the battery is deeply discharged. Solar MPPTs can also be used to drive motors directly from solar panels. The benefits seen are huge, especially if the motor load is continuously changing. This is due to the fact that the AC impedance across the motor is related to the motor's speed. The MPPT will switch the power to match the varying resistance.

System performance

At high noon on a cloudless day at the equator, the power of the sun is about 1 kW/m², on the Earth's surface, to a plane that is perpendicular to the sun's rays. As such, PV arrays can track the sun through each day to greatly enhance energy collection. However, tracking devices add cost, and require maintenance, so it is more common for PV arrays to have fixed mounts that tilt the array and face due South in the Northern Hemisphere (in the Southern Hemisphere, they should point due North). The tilt angle, from horizontal, can be varied for season, but if fixed, should be set to give optimal array output during the peak electrical demand portion of a typical year.

For large systems, the energy gained by using tracking systems outweighs the added complexity (trackers can increase efficiency by 30% or more). PV arrays that approach or exceed one megawatt often use solar trackers. Accounting for clouds, and the fact that most of the world is not on the equator, and that the sun sets in the evening, the correct measure of solar power is insolation – the average number of kilowatt-hours per square meter per day.

For the weather and latitudes of the United States and Europe, typical insolation ranges from 4 kWh/m²/day in northern climes to 6.5 kWh/m²/day in the sunniest regions. Typical solar panels have an average efficiency of 12%, with the best commercially available panels at 20%. Thus, a photovoltaic installation in the southern latitudes of Europe or the United States may expect to produce 1 kWh/m²/day. A typical "150 watt" solar panel is about a square meter in size. Such a panel may be expected to produce 1 kWh every day, on average, after taking into account the weather and the latitude.

In the Sahara desert, with less cloud cover and a better solar angle, one could ideally obtain closer to 8.3 kWh/m²/day provided the nearly ever present wind would not blow sand on the units. The unpopulated area of the Sahara desert is over 9 million km², which if covered with solar panels would provide 630 terawatts total power. The Earth's current energy consumption rate is around 13.5 TW at any given moment (including oil, gas, coal, nuclear, and hydroelectric).

Photovoltaic cells' electrical output is extremely sensitive to shading. When even a small portion of a cell, module, or array is shaded, while the remainder is in sunlight, the output falls dramatically due to internal 'short-circuiting' (the electrons reversing course through the shaded portion of the p-n junction). Therefore it is extremely important that a PV installation is not shaded at all by trees, architectural features, flag poles, or other obstructions. Sunlight can be absorbed by dust, fallout, or other impurities at the surface of the module. This can cut down the amount of light that actually strikes the cells by as much as half. Maintaining a clean module surface will increase output performance over the life of the module.

Module output and life are also degraded by increased temperature. Allowing ambient air to flow over, and if possible behind, PV modules reduces this problem. However, effective module lives are typically 25 years or more ^[12], so replacement costs should be considered as well.

Standardization

Increasing use of photovoltaic systems and integration of photovoltaic power into existing structures and techniques of supply and distribution increases the value of general standards and definitions for photovoltaic components and systems. The standards are compiled at the International Electrotechnical Commission (IEC)and apply to efficiency, durability and safety of cells, modules, simulation programs, plug connectors and cables, mounting systems, overall efficiency of inverters etc.

Legality of photovoltaic systems

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