GLOSSARY

Azimuth angle: The azimuth angle indicates the degree to which the surface of thermal panels or photovoltaic panels deviates from exact south-facing orientation.

Tilt angle: The angle between the inclined receiving surface and the horizontal plane. Depending on the latitude of the installation site, different optimal tilt angles apply to a solar system.

Absorption (Degree of): Indicates the proportion of radiation falling on a given surface that is converted into heat.

Absorber: A device that receives solar radiation, blackened or coated with a selective layer and equipped with an integrated piping system. Solar radiation is converted into heat at the surface and transferred to a heat-transfer fluid, usually a mixture of water and antifreeze.

Biogas: Combustible gas obtained from the fermentation of organic matter in the absence of oxygen. From “bio” = life and “gas”: gas from life.

Biomass (from the Italian Official Gazette, Prime Ministerial Decree of 8 March 2002, Annex III): Plant material originating from dedicated crops; from exclusively mechanical treatment of non-dedicated agricultural crops; from forestry operations, forest maintenance and pruning; from exclusively mechanical processing of virgin wood, consisting of bark, sawdust, wood chips, offcuts and small pieces of virgin wood, granules and residues of virgin wood, granules and residues of virgin cork, and uncontaminated wood pieces meeting the requirements for marketing and use; and from plant material produced through exclusively mechanical processing of agricultural products and meeting the requirements for marketing and use.

BOD: Biochemical Oxygen Demand. It is a measure of the amount of biodegradable organic matter present in a water sample. It may be used to estimate the general quality of water and its level of pollution, and it is a parameter commonly used in water-quality management and wastewater treatment. It is often used as a measurement parameter to assess the efficiency of wastewater treatment plants. Numerous easy-to-use analysis kits are commercially available.

Fuel cell: See Fuel Cell.

Panel circuit (solar circuit): The circuit between the panel and the storage tank in a solar system, consisting of the panel, piping, heat exchanger, pump and safety devices.

Primary circuit (panel circuit, solar circuit): The first piping circuit when viewed from the heat source (panel). It is the circuit in which energy is generated.

Secondary circuit: The second piping circuit when viewed from the heat source (panel). It is the circuit that transfers energy to the end user.

Wood chips: Various types of wood material chopped into pieces approximately 5/8 × 5/8 cm in size, suitable for automatic handling in a wood-fired boiler feeding system.

COD: Chemical Oxygen Demand. Its value, expressed in milligrams of oxygen per litre, represents the amount of oxygen required for the complete oxidation of organic and inorganic compounds present in a water sample. It therefore provides an indicator of the degree of water pollution caused by oxidisable substances, mainly organic ones.

Italian law allows water to be discharged into sewer systems only if its COD does not exceed 700 mg/L. Water with higher values must first be treated to remove pollutants.

Temperature coefficient: Indicates how much the open-circuit voltage or power, and therefore the efficiency of a solar cell or panel, decreases per degree Celsius as the cell temperature rises. Since crystalline solar cells generally have relatively high negative temperature coefficients, panels made from crystalline silicon cells should be adequately ventilated.

Cogeneration: Combined production of electricity and heat.

Collector: The basic component of a solar thermal system designed to convert solar radiation into heat as efficiently as possible. The heat is captured and carried by a heat-transfer fluid and then transferred to a storage tank through a heat exchanger. A flat-plate collector consists of a well-insulated casing containing an absorber. The casing is covered by a solar-glass panel.

Standard Test Conditions (STC): Standardised conditions used to determine the rated output power, through measurement of the current-voltage characteristic, of photovoltaic panels: irradiance of 1,000 W/m² with perpendicular light incidence; electromagnetic spectrum corresponding to AM 1.5; cell temperature of 25°C.

Grid connection: Also referred to as parallel operation with the grid. Connection of a photovoltaic system to the electricity distribution grid through an inverter in order to feed all or part of the electricity generated by the system into the grid. Grid-connected photovoltaic systems do not require energy storage, since this function is effectively provided by the public electricity grid.

COP: Annual coefficient of performance.

Current: The flow rate of electric charge through a conductor, for example electrons travelling through a copper wire, is called electric current. The unit of measurement is the ampere (A).

Alternating current (AC): Electric current whose polarity continuously changes. In the German public grid, alternating current has a frequency of 50 Hz, meaning that it completes 50 cycles per second with positive and negative values of an ideal sinusoidal waveform. Alternating current or voltage is generated by rotating generators or inverters.

Direct current (DC): Electric current flowing in one direction only, such as that produced by batteries or photovoltaic panels.

Short-circuit current: Abbreviated as Isc. The current produced by a solar cell or panel when both terminals are connected directly without any additional resistance, creating a short circuit.

Programmable thermostat: An electronic device, usually battery-operated, that replaces a conventional room thermostat. In addition to switching the boiler on and off according to temperature, it also operates according to a time schedule. During periods when the building is not in use, the programmable thermostat reduces the temperature by a few degrees, increasing energy savings. It does not switch the boiler off for long periods, thereby avoiding excessive boiler operation when rapidly restoring the operating temperature.

DC: Abbreviation for Direct Current.

Denitrification: Denitrification is a process within the soil nitrogen cycle involving the reduction of nitrate nitrogen and the formation of gases released into the atmosphere, such as N₂ and N₂O.

From an agronomic perspective, denitrification in the strict sense is considered a negative process because it causes a permanent loss of nitrogen from the soil, removing it from the biogeochemical cycle and therefore making it unavailable to plants. These losses can only be replaced through nitrogen fixation and fertilisation.

Bypass diode: If a solar cell becomes partially or completely shaded, less current flows through it. Because cells are connected in series within the panel, the shaded cell can overheat and may suffer permanent damage. Bypass diodes protect shaded cells. A solar panel may have up to three bypass diodes, depending on the number of cells.

Renewable energy: Energy sources that are not “exhaustible” within a human timescale, or that are perceived as effectively inexhaustible by people or society. These forms of energy can be renewed, regenerated or replenished through resources such as sunlight, wind, tides and others, without exploiting fossil fuels that increase CO₂ emissions and other pollutants.

Primary energy: Energy obtainable directly from a natural source such as oil, coal, natural gas, water or solar radiation. Primary energy sources can in some cases be used directly by the end user. Most primary energy, however, is converted into secondary energy.

Secondary energy: Energy derived from the conversion of primary energy. For example, coal is converted into coke or briquettes, while petroleum is converted into petrol, diesel fuel or fuel oil.

Solar energy: Strictly speaking, solar energy is the energy that reaches the Earth from the Sun in the form of photons.

Heat-transfer fluid: Fluid in the panel circuit of a solar system that carries heat from the panel to the storage tank. To provide frost protection, the heat-transfer fluid is normally a mixture of water and glycol.

Photovoltaic: Generating electrical energy through the absorption of light. Abbreviated as PV. Photovoltaic technology converts solar energy, in the form of photons, into electrical energy through solar cells.

Fuel cell: A device capable of generating electrical energy through an electrochemical reaction powered by gases such as hydrogen.

Generator: The term generator, or solar generator, refers to the entire set of interconnected photovoltaic panels.

Photovoltaic generator: Individual panels are first connected in series to form strings. These strings are then connected in parallel within the photovoltaic generator in order to achieve sufficiently high voltage and current values, for example to feed electricity into the public grid through an inverter.

Engineering corps: According to the Devoto-Oli dictionary, a technical organisation or military corps responsible for the study and execution of works of public or military interest.

Efficiency: Efficiency indicates the ratio between two power values in a system, namely output power and input power. Efficiency is an instantaneous value and depends on the operating conditions of the system during the period considered. The efficiency of a solar cell or panel is defined as the ratio between the electrical power produced and the power of the incident radiation. Since efficiency depends on surface area, it is necessary to specify which surface area is considered in the calculation, for example the total panel surface or only the active surface of the cells within the panel.

The efficiency of a solar thermal panel is defined as the ratio between the thermal output of the panel and the irradiance at panel level. Panel efficiency is only one of the factors determining the overall efficiency of a solar system.

Biogas plant: A system designed to obtain the maximum amount of natural gas from the controlled decomposition of biodegradable matter of animal, plant or industrial-waste origin.

Roof pitch: The roof pitch is the angle of a roof relative to the horizontal plane. For harmonious integration of a photovoltaic system, roof inclinations between 20° and 40° are recommended. Suitable mounting systems can position panels at the ideal angle even when the roof pitch itself is not optimal.

Solar tracking: With a solar tracking system, the surface of photovoltaic panels rotates during the day to follow the position of the Sun. This can increase the system’s energy yield by approximately 30%.

Inverter: Converts the direct current supplied by the panels into alternating current compatible with the public grid. Using MPP control, the inverter draws power from the photovoltaic generator at the Maximum Power Point of the current-voltage characteristic.

Diffuse radiation: Solar radiation reaching the Earth’s surface is divided into direct and diffuse radiation. Diffuse radiation does not reach the observation point along a direct geometric path from the Sun, but is instead reflected or scattered, for example by particles in the atmosphere.

Direct radiation: Solar radiation that reaches the Earth’s surface directly. Direct radiation combines with diffuse radiation.

Global radiation: The sum of direct and diffuse radiation on a horizontal plane. The Earth’s atmosphere reduces the power of extraterrestrial solar radiation, known as the solar constant, through absorption, reflection and scattering. As a result, radiation at the Earth’s surface at our latitudes is reduced to approximately 1,000 W/m² under clear-sky summer conditions at midday. The availability of solar energy varies according to weather conditions and astronomical laws, which also determine seasonal variations. For example, the average annual global radiation on a horizontal surface in the Hanover region is approximately 1,000 kWh/(m²·year). This corresponds to the energy content of approximately 100 litres of heating oil or 100 cubic metres of natural gas.

Stand-alone system: An autonomous photovoltaic system used to supply energy to isolated users located far from the electricity grid, for example in mountainous or rural areas.

Kilowatt-peak: Abbreviated as kWp. Unit used to express the theoretical maximum output of a photovoltaic system. 1 kWp = 1,000 Wp.

Kilowatt-hour: Abbreviated as kWh. Unit of energy. 1 kWh = 1,000 Wh.

LED: An electronic component capable of emitting light in various colours with relatively low power consumption. LEDs are currently used in technological devices as indicator lights. Research is also being carried out into their use for lighting. They are already widely used in the automotive industry to replace position and brake lights because of their longer lifespan compared with conventional bulbs.

Maximum Power Point: Abbreviated as MPP. The operating point on the current-voltage characteristic of a solar cell or panel at which maximum power can be obtained. MPP tracking makes it possible to locate and maintain this point under different operating conditions.

Ground-mounted installation: The term refers to a photovoltaic system installed on an independent surface, such as a field.

Roof-integrated installation: In a roof-integrated installation, the panels are incorporated into the roof covering. The existing roof covering is removed, or in the case of a new building, the area intended for the panels is left uncovered.

Façade-mounted installation: A photovoltaic system permanently mounted on or integrated into the façade of a building.

Roof-mounted installation: In a roof-mounted installation, solar panels are installed parallel to the roof covering using dedicated mounting systems.

Flat-roof installation: For optimum energy performance, photovoltaic panels installed on a flat roof should be positioned at an inclination of approximately 30° using a supporting structure.

Nitrification: According to Wikipedia, nitrification is a complex biological process. It is the transformation of ammonia (NH₃) and ammonium ions (NH₄⁺) into nitrite ions (NO₂⁻).

Nitrifying bacteria belong to genera such as Nitrosomonas, Nitrocystis and Nitrospira. They generally reach optimum activity at neutral or slightly alkaline pH and require the presence of calcium and potassium carbonates. Their highest efficiency is therefore found in grassland soils.

Shading: Shade cast onto a roof by trees, buildings or antennas is one of the main problems affecting photovoltaic systems. Since solar cells are connected in series, any shaded cell interferes with the regular flow of energy and therefore reduces the system’s performance.

Orientation of a photovoltaic system: For optimal orientation, the surfaces of photovoltaic panels should face south and have an inclination of between 20° and 40°.

Panel: An electrical assembly consisting of several encapsulated solar cells, protected from weather and environmental influences and electrically insulated. A panel is the basic unit of a photovoltaic system.

Pellet: Fuel consisting of dried and compressed virgin wood formed into small cylinders without the addition of additives. The bulk density of pellets is approximately 600–700 kg/m³, significantly higher than that of other uncompressed wood fuels such as wood chips or shavings. Their calorific value can reach approximately 4,200 kcal/kg, with an energy density of 3,000–3,400 kWh/m³.

Thin-film technology: A manufacturing technique in which thin-film solar cells are applied to an inexpensive supporting material such as glass, metal or plastic. The advantages of thin-film technology include lower material and energy consumption during industrial production, ease of application and the possibility of producing large-area solar cells with integrated series connections.

Reflection losses: Radiation reflected by the surface of a collector, panel or solar cell and therefore no longer available for the production of heat or electricity.

Power: The amount of energy consumed or supplied per unit of time. The unit of measurement is the watt (W) or kilowatt (kW). 1 kW = 1,000 W = 1,000 J/s.

Rated power: The maximum possible power output of a solar cell or panel. Rated power is defined as the maximum power at the Maximum Power Point under Standard Test Conditions and is measured in watt-peak (Wp).

Photovoltaic principle: Describes the creation of an electrical voltage in a semiconductor when charge carriers are excited by light radiation, or photons, through the internal photoelectric effect. By extracting the charge carriers, electrical energy can be obtained in the form of current.

Project Financing: Project finance is a bond, debt security or financing arrangement structured so that the borrowing entity can repay the principal at maturity and pay the related interest through the revenues generated by the investment financed with the borrowed capital. Project financing is widely used, for example, for major public infrastructure projects.

Lightning protection: A photovoltaic system does not normally increase the risk associated with lightning. Photovoltaic systems are nevertheless installed in accordance with lightning-protection standards for safety reasons and to prevent damage.

Re.Future: The name of the non-profit association, based on an English acronym: “re” = renewable energy and “future” = future. In addition to expressing a wish for the future, the name summarises the association’s vision: a Europe capable of producing all of its energy from renewable sources, thereby protecting the health of future generations and the planet.

Charge controller: Used in stand-alone systems as the connecting element between the photovoltaic generator and the battery storage system. It regulates and controls the charging process. Additional functions include protection against deep discharge of the battery.

Differential temperature control: Controls heat transfer from the panel to the storage tank by switching the relevant pump on and off. The control signal is based on the temperature difference between the panel outlet and the lower section of the storage tank.

Feed-in remuneration: Payment associated with feeding renewable electricity generated by a system into the public electricity distribution grid.

Panel circuit yield: The amount of heat transferred from the panel circuit, or solar circuit, to the solar storage tank. A specific panel-circuit yield is normally stated. For a flat-plate collector system, a good annual yield is more than 400 kWh/(m²·year), while the highest yields of large systems exceed 500 kWh/(m²·year).

Solar yield: The usable solar heat, meaning the heat that can actually be used from the storage tank after subtracting the thermal losses of the collector system.

Anti-reflective coating: A transparent coating applied to the surface of solar cells to reduce reflection losses. Thanks to this special layer, more light penetrates the cell, since an untreated silicon surface would otherwise reflect approximately 30% of sunlight.

Selective coating: A special coating applied to the absorber of a solar panel that absorbs almost all incident short-wave solar radiation while simultaneously reducing the increasing radiation losses caused by higher temperatures through the emission of long-wave thermal radiation.

Net metering: Defines the rules governing an energy-exchange agreement between the grid operator and a renewable-energy producer. In practical terms, electricity produced from renewable sources and fed into the grid is offset against the producer’s own electricity consumption.

Silicon: A chemical element capable of forming four bonds with other atoms and creating hard, rough crystals with a stable structure similar to that of diamond. After oxygen, silicon is the second most abundant element in the Earth’s crust, although it occurs naturally only in the form of silicon dioxide, SiO₂, such as quartz and sand. Silicon is the semiconductor that has so far played the most important role in the electronics and photovoltaic industries. Silicon dioxide can be processed to obtain monocrystalline, polycrystalline or amorphous silicon.

Amorphous silicon (a-Si): In amorphous material, atoms are arranged irregularly. Because a-Si has a high absorption capacity, a solar cell made from this material requires a coating only a few micrometres thick, making it suitable for thin-film technology.

Microcrystalline silicon: Polycrystalline silicon made up of multiple crystals.

Monocrystalline silicon: Silicon present in the form of individual single crystals.

Polycrystalline silicon: Silicon consisting of small interconnected crystals ranging in size from a few millimetres to several centimetres. A common manufacturing method for polycrystalline silicon is zone melting.

String: A series connection of multiple panels.

Aperture area: The glazed surface of a collector through which solar radiation is captured. The aperture area is the reference surface used for determining efficiency according to DIN 4757 and EN 12975 standards.

Sustainable development: Sustainable development is a form of development, including economic, urban and community development, that does not compromise the ability of future generations to continue developing while preserving the quality and quantity of natural resources, which are finite. Its goal is to maintain economic development that is compatible with social equity and ecosystems, thereby operating within a condition of environmental balance.

Voltage: The difference in electrical potential between two points, for example between the two terminals of a battery. Voltage (U) is the driving force behind electric current (I). The two quantities are related through the resistance (R) of a conductor according to Ohm’s law: U = R × I. The unit of electrical voltage is the volt (V).

Open-circuit voltage: The electrical voltage of a solar cell or panel when its two terminals are not connected and therefore no current flows between them.

Thermography: A type of imaging carried out using a special device capable of displaying different temperatures within the observed area using different colours.

Solar thermal: The use of solar energy for the direct production of heat. The corresponding energy-conversion device is called a solar collector or solar thermal panel.

Trigeneration: Cogeneration in which the heat produced is used both for heating and cooling.

U-value: The U-value, formerly known as the k-value, is the thermal transmittance coefficient indicating how much heat passes through one square metre of a building element for a temperature difference of one degree between the inside and outside. Its unit is W/m²K, or watts per square metre kelvin. A low U-value indicates lower heat transfer and therefore a high level of thermal insulation.

Volt (V): Unit of measurement of electrical voltage.

Wafer: A thin slice of semiconductor material, such as silicon. Wafers are used as a base material in the production of computer chips and crystalline solar cells. Crystalline wafers are generally cut from semiconductor ingots and typically have a thickness of between 0.2 and 0.3 millimetres.

Watt-peak (Wp): Unit of measurement of the power capacity, or rated power, of solar cells and panels. Panel prices are commonly expressed in euros per Wp to allow comparison between different products.