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Solar electricity: Grid-connected photovoltaic systems

Photovoltaic power plants convert sunlight to electric energy. The energy output of such PV plants will therefore reach it's peak at midday, meeting the daily energy consumption peak, when the spot prices on energy are highest.

A PV system with a nominal capacity of 8,96 kilowatt peak, as pictured, covers the electricity needs of two four person households. Picture: Sharp Electronics (Europe) GmbH

Despite this economic benefit photovoltaic power has not yet reached grid parity, the point at which the costs are equal to grid power (except some sunny islands like Hawaii that use diesel fuel to produce electricity).

Net metering (US and Canada) and feed-in tariff systems

Since no local energy storage facilities are needed, the limiting factors sizing grid-connected photovoltaic systems are the available space - often a roof - the investment costs and the regulatory frameworks including subsidy and promotion programs. Such programs can include investment subsidies, net metering or feed-in tariffs. With net metering the inflow of electrical energy is charged up against the electrical consumption at the same estate, using mostly a bi-directional working electricity meter. This system is widely in use in the US and Canada. Since there is in most cases no compensation for an inflow exceeding the yearly consumption, photovoltaic facilities will be mostly sized to provide no more energy than consumed at the same estate during the year; the grid is used only as a storage facility. Within a feed-in tariff system on the other side, providing (like in Germany) fixed and guaranteed payments per kWh, more output means more profit; so facilities will be bigger sized.

Schematic diagram of a photovoltaic system. Illustration: LGABW

Solar Power: Sunlight becomes the source of electricity.

A grid coupled PV system essentially consists of the PV panels (modules), one or several solar inverters, a protections device for automatic shutdown in case of a grid breakdown and a counter for the fed in solar electricity

The components of a grid-connected PV system include the PV modules, a power inverter, a safety device to power down at failures in the grid and an electricity meter. The "mains-commutated" inverter converts the direct current (DC) provided by the modules to alternating current (AC), simultaneously synchronizing the AC output to the AC in the grid.
The power-generating capacity of a photovoltaic system is denoted in kilowatt peak (measured at standard test conditions and a solar irradiation of 1000 W per m²). Today's PV modules will cover an area between 7 and 10 m² per kWp. Assumed that the modules are oriented to south and inclined at an angle between 30° and 35° such a PV system will generate in Middle and West Europe - depending on the exact latitude and other factors - between 800 and 1.000 kWh electrical energy per year and per kWp of nominal capacity. To exemplify: On a roof in Cambridge or Oxford (UK), a 4-kWp-plant with optimized module orientation and module inclination angle will provide about 3.380 kWh per year, at Sevilla (Spain) 5.640 kWh per year. The plant at Sevilla will therefore need an inverter with an higher input voltage than the one at Oxford.

Some tips for planning a grid-connected PV system

- Size of the PV generator

The economically optimal size of a grid-connected PV system depends mostly on different financial incentives and legal parameters, since grid parity - meaning the costs of photovoltaic generated electricity are equal to or cheaper than the price of grid power - is achieved only in a very few regions today.Net metering concepts, as they are widely in use in the US and Canada, provide - like with stand-alone systems - no incentive to build systems that generate more electrical energy than consumed at the same estate during the year; the grid replaces only a local battery storage. Feed-in tariff systems on the other side render big systems with net excess profitable.

A PV system may cover the whole roof; the pictured solar roof (233 square meters) has a nominal power output of 24,2 kilowatt (kWp). Picture: Hieronimi regenerative Energien GmbH

- Required module space:

Within bigger systems mostly crystalline silicon modules are used today. To install a nominal capacity of 1 kWp (Kilowatt Peak) with such modules an area between about 7 m² (using monocrystalline cells) and 10 m² (using polycrystalline cells) is required.Otherwise unused pitched roofs are in many cases the most cost-efficient places to install a PV system, especially if they are oriented to south and inclined to a degree of about 30° to 37°.

- PV Orientation and Output

The efficiency of the photovoltaic process is at its highest if the sun rays hit the panel vertically. Therefore PV modules should be oriented to south (speaking of the northern hemisphere) and somewhat inclined; the optimal inclination angle depends on the location (including latitude, altitude and other factors). As a rule of thumb the inclination angle would be best between 3/4 and 4/5 of the latitude – resulting in angles of 32° to 38° in Middle and Western Europe or 30° to 36° in most of the US. However: Small divergences from the optimal orientation and inclination result only in even smaller reductions of energy output per year.

In order to most effectively use Solar Radiation, a PV Module or Collector of a photovoltaic system and Solar Heating System, respectively, is aligned to absorb or collect as much of the radiation as possible. The radiation's angle of incidence, the tilt angle of the module or collector, and the azimuth angle all play roles in achieving the greatest possible power production.

The azimuth angle (β) in the picture at right) specifies how many degrees the surface of the module or collector diverges from the exact south-facing direction. The tilt angle (α) specifies the divergence from the horizontal.

Experiments show that photovoltaic systems operate most effectively with an azimuth angle of about 0° and a tilt angle of about 30°. Of course small variances in these values are not at all problematic: with the system oriented towards the south-east or south-west, about 95 % of the highest possible amount of light can still be absorbed. Large systems with arrays are fitted with electric motors which track the sun in order to optimise output.

Installation of power inverters of a 123 kWp PV system in Germany.

- Power inverter:

PV systems provide direct current (DC) voltage. To feed to the grid, this DC voltage has to be inverted to the grid alternating current (AC) voltage by a »mains-commutated« or grid-tied inverter, synchronizing automatically its AC output to the exact AC voltage and frequency of the grid.

This MPP fluctuates during operation in an interval depending on the radiation, the cell temperature and the cell type und has so to be tracked by the inverter controlling unit.

The second important job of the solar power inverter is to control the PV system to run near its Maximum Power Point (MPP), the operating point where the combined values of the current and voltage of the solar modules result in a maximum power output. This MPP fluctuates during operation in an interval depending on the radiation, the cell temperature and the cell type und has so to be tracked by the inverter controlling unit.