Abbreviation for power output, given in watts.

Power output, or work done, (P, measured in W), voltage (U, measured in V) and current (I, measured in A) are the most important values of a photovoltaic system. If two of the values are known, then the third value concerning the performance can easily be figured:
current = power output / voltage (A = W / V)
power output = voltage x current (W = V x A)
voltage = power output x current (V = W x A)

For the calculation of a photovoltaic system’s power output , one may not multiply the short-circuit current (I short) by the open-circuit voltage (U open), but rather only the actual current at the momentary voltage of the device—which is unfortunately always lower. The exact values can be taken from the current-voltage graph.

Solar panel Picture: Wagner & Co., Cölbe

Solar modules which are connected together make a panel, or solar panel. As a whole, the modules constitute the Generator.

Parabolic mirrors (also called parabolic troughs) are not used in Central Europe. They concentrate the direct Solar Radiation on the Absorber, which is placed at the centre of the mirror, and thus can reach very high temperatures. However, most diffuse radiation cannot be concentrated in this way, and for this reason a Flat-Plate Collector or an Evacuated-Tube Collector is more useful in Central Europe.

"Plataforma Solar de Almeria"

Concave mirrors and lenses have been used for ages to concentrate the sun’s rays on a single point and therefore multiply its strength. Mirrors with a parabolic cross-section are especially suited to this purpose because they can also focus the outer rays towards the middle. If a mirror is designed in the form of a trough, the solar radiation, concentrated about forty times, can be focused on an absorber tube with a heat-conducting fluid inside.

The best use for these tube collector thermal solar power systems is for domestic water heating and heating support. A well-known high-tech system is the parabolic trough power plant in the Californian Mojave Desert. It has a total of 2.3 million square meters of mirror surface area and produces 354 megawatts of electricity. To improve their performance they can be rotated about their roll axis. The heat-conducting fluid is heated up to 400 ºC and by means of a turbine and generator then produces electric current. Similar large plants are also planned at Crete, Egypt and India and should be able to deliver electricity at a price of about $ 0.07 (0.08 €) per kilowatt-hour.

The Center for Solar Energy and Hydrogen Research (Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSF)) in Stuttgart, Germany operates an experimental plant in Almeria, Spain with oil as the heat-conductor and heat-storage fluid.

Passive building “Rombach” in Engstlatt, Germany. Picture: Suntech; Tübingen, Germany

From the energy-saving point of view, passive buildings are most advanced, and when considering the involved technology they can be constructed almost anywhere.

Heating needs of buildings in kilowatt-hours per square meter per year:

Passive Energy Building max. 15 KWh/m²*a

Low-Energy Building 40 – 79 KWh/m²*a

Three-Liter-Building 16 -- 39 KWh/m²*a

Zero-Energy Building/Energy-Producing Building 0 KWh/m²*a or energy surplus

Existing Buildings Depending on Insulation 80 – 300 KWh/m²*a

In this case, a quantity of heat corresponding to 10 KWh =: 1 liter of heating oil, 1 m³ of natural gas or 2 kg of woodpellets. Therefore, in a passive building only about 1.5 liters of heating oil would be used per square meter of living area each year. Heat gained from internal sources such as body heat or heat radiation from electrical appliances plays a large role in passive buildings.

This classification doesn’t have anything to do with the construction of low-energy buildings. For example, a one-bedroom apartment within a large complex and with an un-shaded south-facing window could, without special construction, reach the status of a passive building. On the other hand, for a single family house with a large south-facing outside surface area (i.e. a roof-top balcony, or bay windows) that is shaded by another building or trees and is also exposed to wind, substantial insulation would be needed in order to meet the energy requirements of a low-energy building.

Passive buildings usually require high quality insulation of the building’s outer layers as wells as a ventilation system capable of preventing heat loss. Detailed and current information is offered by the passive building institute (Passivhaus-Institut) in Darmstadt, Germany www.passiv.de.

Diagram of a house with a sun room. Picture: LGABW. Animation: Heindl Internet AG

Buildings themselves, or parts of them, are used as collectors. A typical example is a paned sun room. The glass construction prevents heat loss from the building, hence contributing to a reduction of energy consumption. The air which is heated by the sun can be vented from the sun room and can then be used for space Heating.

Current/Voltage curve of a solar cell (current in A; voltage in V). Illustration: DGS/ISES

The electric characteristics of Solar Cells, and therefore of the entire Generator, vary with respect to various general conditions, especially the radiation intensity. In Photovoltaics, the maximum possible output of a solar generator operating under standard conditions is defined as its peak output, which is measured in watts or kilowatts and stated as either Wp (watt, peak) or kWp, respectively. An optimal Solar Radiation of 1000 W / m² is defined as the standard condition, and it can be reached early afternoon on a sunny summer day (however, the mean output over the period of a year is only about one thenth of the peak output due to night-time and less than optimal day-time sun conditions). The peak output is so based on measurements under optimal conditions, and, specifically, the peak output (some Manufacturers also designate this as the nominal or rated value: rated power, rated output, nomial power, nominal output) results from the product of the nominal voltage and the nominal current. More enlightening information over the properties of solar cells or generators can be found on the current/voltage curve at the right. When planning a photovoltaic system, its Performance Ratio (conversion Efficiency) is important as it describes which part of the radiation energy is converted into useable electric current.

Within the realm of Photovoltaics, the term “performance ratio” refers to the relationship between actual yield and target yield. The performance ratio of a photovoltaic system is the quotient of alternating current (AC) yield and the nominal yield of the generator’s direct current (DC). It indicates which portion of the generated current can actually be used. A photovoltaic system with a high Efficiency can achieve a performance ratio over 70 %. The performance ratio is also often called the Quality Factor (Q). A Solar Module based on crystalline cells can even reach a quality factor of 0.85 to 0.95 (performance ratio = 85 - 95 %).

Schematic diagram of a photovoltaic system, all of which rely on the photoelectric effect. Picture: MBW.NRW. Animation: Heindl Internet AG

When light strikes certain solids, positive and negative charge carriers are freed and electricity then flows. This property is known as the photoelectric effect and makes the production of Solar Current by Solar Cells, and hence Photovoltaics, possible. The effect was first analyzed by Albert Einstein who later recieved the Nobel Prize in physics for his related work.

Quantum of electromagnetic radiation, i.e. Solar Radiation. Every light particle, or quantum of light, contains a small amount of energy which is usually measured in electron volts (eV). Blue light (3 eV) has more energy than red light (1.5 eV).

Production of electric current from Solar Radiation. In Solar Cells, usually made of silicon, positive and negative charge carriers are freed when light strikes the cell (Photoelectric Effect). In this manner direct current (DC) is produced which can then be used to directly power a motor or can be stored in a Battery. If Solar Energy is to be used by consumers with 110 V alternating current (AC), or fed (“sold”) directly into the grid (Grid Coupling), then an Inverter is needed. Advantages of photovoltaics include its clean, “ecological” electricity production and the possibility to provide consumers with off-grid, or grid-independent, power (Off-Grid System), i.e. at a weekend home, in a garden or park, or to light small covered waiting areas. The relatively high costs are an important disadvantage when compared to a Solar Heating System--the price/output relationship must constantly be monitored (today it is about $ 4 / Wp.)

Photovoltaic modules mounted on a horizontal roof. Picture: Wagner & Co., Cölbe

A photovoltaic, PV, or solar module consists of many interconnected Solar Cells which are embedded between two glass or plastic plates to protect them from weather. Photovoltaic modules are normally mounted on top of a roof (Roof-Mounted Solar Power System), or a holding rack of some sort, within a frame structure. Modules are delivered in standard voltages, i.e. 12 V.