Energy of Solar Radiation

The Sun is a celestial body — a colossal plasma sphere — with an average density slightly higher than that of water. Within its core, ongoing nuclear fusion causes four protons to gradually combine into a single helium nucleus, releasing an enormous amount of energy.

A comparison of the masses of protons and helium shows that roughly 0.7% of the original proton mass is converted into energy. This energy is initially carried away by high-energy gamma-ray photons produced in the reaction. Through repeated interactions with other particles, these photons heat the inner layers of the Sun. The energy is transported outward through the radiative zone and then by convection through the outer convection zone until it reaches the photosphere, the Sun’s visible surface, from which it is emitted as solar radiation. The photosphere is observed from Earth as the familiar solar disc.

At an age of about 4.6 billion years, the Sun is roughly halfway through its main-sequence lifetime, during which hydrogen in its core is gradually converted into helium. The fusion reactions convert approximately 700 million tonnes of hydrogen every second, of which about 5 million tonnes are transformed directly into energy.

Only a small fraction of the Sun’s radiated energy reaches Earth in the form of solar radiation. About 29% of the incoming solar radiation is reflected back into space, around 23% is absorbed by the atmosphere, and approximately 48% is absorbed by the Earth’s land and oceans.

The Earth’s atmosphere filters part of the solar radiation, allowing primarily visible light and portions of ultraviolet (UV) and infrared (IR) radiation to reach the surface. The amount of solar energy that falls on a unit area (1 m²) perpendicular to the Sun’s rays per unit time (1 s) is referred to as solar irradiance.

If this surface is placed outside the Earth’s atmosphere, at a distance of one astronomical unit (the average Earth-Sun distance), the resulting solar irradiance is known as the solar constant. Its value is approximately 1,361 W/m², varying by a few tenths of a percent depending on the Sun’s activity within its 11-year solar cycle.

 

Methods of using Solar Heat

The utilisation of solar thermal energy has been known since ancient times. Proper orientation of buildings and the application of solar architectural elements laid the experimental foundations for passive solar systems. The invention of flat glass further expanded the use of greenhouses and botanical gardens.

It was not until the second half of the 19th century that experiments with the technical use of solar energy began. The French teacher Augustin Mouchot constructed the first steam engine powered exclusively by solar energy, and a few years later, the first parabolic trough collector appeared.

The first solar thermal power plant using parabolic trough collectors was built in 1913 in Meadi, Egypt, by the American engineer Frank Shuman. The facility powered a water pump drawing water from the Nile for irrigation purposes. The same principle — a parabolic trough focusing sunlight onto a receiver tube containing a heat transfer medium — was later used in the SEGS (Solar Energy Generating Systems) plants built in California’s Mojave Desert.

Passive Solar Systems

The simplest way to harness solar radiation is through passive solar systems integrated into building design. Apart from sunlight itself, they require no additional energy or mechanical components for their operation.

The inclusion of suitable passive architectural elements can significantly reduce a building’s energy demand, especially for heating. Most commonly, designs incorporate large glazed façades that utilise the greenhouse effect. Various passive solar solutions are frequently implemented in low-energy building designs.

Active Solar Systems

In contrast to passive systems, active solar systems employ various electromechanical components and processes to improve the absorption of solar radiation or the transfer of thermal energy. From a physical standpoint, active systems can be divided into two main categories:

• Solar Thermal Systems;
• Photovoltaic Systems.

The first category includes all technologies that use solar radiation to heat a heat transfer medium, which is then used to transport the thermal energy to the point of consumption. This group includes thermal collectors for space heating and domestic hot water production, where the absorber area equals the collector’s surface, as well as concentrating solar power (CSP) collectors, where the absorber area is much smaller than the total collector surface.

Typical active solar thermal systems consist of solar collectors that absorb solar energy, thermal storage units, heat exchangers, pumps, valves, piping, and intelligent electronic control systems. These systems can make a substantial contribution to meeting a household’s thermal energy demand.

The second category is characterised by the direct conversion of solar energy into electricity, without the involvement of moving mechanical parts or heat-transfer media. These are modern photovoltaic (PV) systems, valued for their simplicity, installation flexibility, and ability to generate electricity directly whenever sufficient solar radiation is available.

At the interface of both categories are thermoelectric generators. These devices use concentrated solar radiation to heat an absorber, but instead of a heat-transfer fluid, they contain numerous interconnected thermoelectric elements whose hot sides are heated by solar radiation. These elements generate electricity through the Seebeck effect, making this a form of indirect solar-to-electricity conversion.

 

Advantages of Solar Energy

One of the principal advantages of solar energy is its inexhaustibility. The Sun is the primary source of energy in our solar system, and the amount of solar radiation reaching the Earth is both substantial and relatively stable. Even large-scale utilisation of this renewable resource would have virtually no impact on the biological or energy balance of the natural environment.

Another significant benefit is its environmental friendliness. Solar energy generation does not depend on fuel supplies and produces no direct combustion-related emissions during operation, although environmental impacts occur during the manufacture, construction and end-of-life stages of solar energy systems.

A further advantage of solar power is its low operating cost and minimal maintenance requirements. Although the initial installation cost of a solar system can be relatively high, the return on investment is typically ensured by the low running costs over the system’s operational lifetime. This is especially true for photovoltaic systems and solar thermal collectors. It is worth noting, however, that concentrating solar power (CSP) systems — which use large numbers of tracking mirrors and steam turbines — do require a higher level of maintenance.

 

Solar Collectors

The primary purpose of solar collectors is to heat a heat transfer medium, which then transports the collected heat to thermal storage units or points of direct consumption.

The simplest collector design is the air-based solar collector, in which air is heated directly. These are used mainly in ventilation and air-conditioning systems, and less frequently in space-heating systems. Due to the lower heat transfer efficiency between the absorber and air — and the lower specific heat capacity of air compared with liquid-based systems — achieving the desired energy output requires larger absorber surfaces and a higher airflow rate through the collector.

The absorbers in air collectors can either be directly exposed to solar radiation or enclosed within a collector frame and covered with a transparent layer, which reduces heat losses and improves the collector’s overall efficiency.

Another type of solar thermal collector is the flat-plate collector. As the name suggests, it consists of a flat copper or aluminium plate coated with a selective surface that absorbs solar radiation efficiently while reducing thermal radiation losses. The plate heats up through absorption and transfers the thermal energy by conduction to pipes containing the circulating heat-transfer medium. To minimise heat losses, the flat absorber is insulated from the collector frame using standard insulating materials and covered with glass or a transparent plastic sheet.

Maximum performance of flat-plate collectors is achieved when they are optimally oriented and tilted. Ideally, the collector surface should always remain perpendicular to incoming sunlight, although a continuous tracking system would be too complex and costly for most applications. In practice, fixed installations are most common, with collectors facing south or south-west and tilted at about 50°, which is optimal for central European latitudes, especially during transitional seasons.

Flat-plate collectors are fundamental components of solar water-heating systems and, with efficiencies of 70—80%, can reduce water-heating costs by up to two-thirds.

Evacuated tube collectors achieve even higher efficiency than flat-plate collectors by using a vacuum as insulation.

From a design perspective, evacuated tube collectors may consist of a single vacuum-sealed tube or two concentric glass tubes with a vacuum between them. Heat removal from the absorber occurs either through the flow of the heat transfer medium via U-tubes or through heat pipes that transfer heat by repeated evaporation and condensation of a working fluid.

A heat pipe is a hermetically sealed tube containing a working fluid with a low boiling point. When heated, the fluid evaporates and the vapour rises to the condensation zone in the upper part of the tube. There, it condenses, releasing its latent heat to the heat transfer medium. The condensed liquid then flows back down to the lower part of the tube by gravity, completing the cycle.

In single-tube evacuated collectors, a flat absorber with a selective coating is located inside a single vacuum-sealed glass tube and is thermally connected to a metal tube. The seal between the metal and glass must be extremely tight to prevent degradation of the vacuum over time.

A double-tube evacuated collector (one tube nested inside another) operates on a principle similar to that of a thermos flask — the vacuum space between the tubes minimises heat loss from the absorber. Solar radiation passes through the outer glass tube and is absorbed by a selective surface on the inner tube. The captured solar energy is then transferred by conduction to an aluminium fin in contact with a copper tube or a heat pipe in the non-vacuum central section.

When heat pipes are used, their condensing ends are inserted into a copper manifold, where the heat is transferred to the flowing heat transfer medium.

 

Solar Concentrators

A parabolic dish concentrator is a large-scale solar collector that focuses incoming direct solar radiation onto a single point — the focus of the parabola — where the receiver is located. By concentrating radiation from a large surface area, temperatures of up to 1,000 °C can be achieved at the collector’s focal point.

Manufacturing a precise parabolic mirror several metres in diameter is technologically challenging. For this reason, larger concentrators are typically composed of numerous smaller mirrors arranged on a supporting structure to form the overall parabolic surface.

The complexity of the design also stems from the need to support not only the heavy mirrors themselves but also the receiver assembly, since the absorber must track the Sun’s movement along with the parabola.

In many parabolic dish systems, the focal point contains the receiver of a Stirling engine, which uses the concentrated heat as its energy source. The output of individual units is typically in the range of tens of kilowatts, so solar power plants often deploy dozens or even hundreds of parabolic concentrators operating together. Due to the moving components involved, these systems generally require more intensive maintenance.

 

Central Tower Solar Power Plants

Central tower solar power plants are another type of solar concentrator system. Unlike parabolic concentrators — which move the entire mirror assembly as one unit — these systems use a large number of independently controlled flat mirrors, known as heliostats, positioned around a central tower containing the absorber. Each heliostat tracks the Sun individually, continuously reflecting and focusing sunlight onto a small area at the top of the tower.

The heat captured by the absorber is transferred via a heat transfer medium — such as water, oil, or molten salts — to the plant’s power conversion system, where it is converted into electricity using a conventional steam cycle. Because of the high concentration of solar energy, the absorber reaches temperatures typically above 500 °C, which significantly enhances the overall efficiency of the power plant.

The entire power conversion system is usually located at the base of the tower or in an adjacent engine room. In some installations, thermal energy storage units are also included, enabling electricity generation after sunset or during adverse weather conditions.

Among the pioneering examples of central tower solar power plants are the Solar One project in California’s Mojave Desert, with an installed capacity of 10 MW, and the PS20 power plant in Spain, which achieves a capacity of 20 MW. Another pioneering project was Gemasolar in Spain, a 19.9 MW central-tower plant with molten-salt thermal storage.

 

Solar Farms

Solar farms are power plants that generate thermal energy from solar radiation using parabolic trough collectors. They belong to the broader category of solar concentrators, although in this case, the radiation is not focused into a single point as in central tower power plants. Instead, parabolic mirrors reflect incoming sunlight onto their focal line, through which a dark tubular absorber carrying a heat transfer medium is positioned.

The basic unit of a solar farm consists of a series of interconnected parabolic trough collectors. At one end of the series, the heat transfer fluid — usually a type of synthetic oil — is supplied into the absorber located at the focal line. At the opposite end, the heated fluid is directed to the plant’s power conversion system, where the captured thermal energy is either converted into electricity or utilised in industrial processes.

The total output of a solar farm essentially depends on the number of installed collector units.

The outlet temperature from the absorbers of parabolic trough collectors can reach up to 400 °C, and when high-quality mirror surfaces are used, thermal efficiencies of nearly 90% can be achieved. The most common configuration for trough collector arrays is north-south alignment, allowing the collectors to rotate along their longitudinal axis to track the Sun’s movement.

One of the largest operating parabolic-trough developments is the 600 MW parabolic-trough section of Noor Energy 1 at the Mohammed bin Rashid Al Maktoum Solar Park in Dubai. Other large solar farms have been built in Europe, North Africa, and the United Arab Emirates.

A more cost-effective alternative to parabolic trough collectors is the use of linear flat mirror arrays based on the principle of a Fresnel lens. Each row of mirrors is mounted at a slightly different angle so that all mirrors reflect sunlight onto the underside of a fixed absorber positioned above them.

Inside the absorber tubes, water typically circulates and is heated to around 300 °C, where it vaporises directly. The resulting steam is then fed into a turbine in a conventional steam cycle.

Since flat mirror arrays are significantly cheaper to produce than parabolic mirrors, solar plants using Fresnel reflectors require lower investment costs compared to similarly rated parabolic systems.

Pioneering projects in Fresnel-based solar power generation include the Puerto Errado 1 plant in Spain, with an installed capacity of 1.4 MW, and the Kimberlina facility in California, which achieved a capacity of 5 MW.

 

The Largest Solar Power Plants

Electricity generation in solar tower and solar farm power plants is based on heating a heat transfer medium to several hundred degrees Celsius and using the captured heat to generate electricity via a conventional Rankine cycle. The core of this cycle is a steam turbine driven by superheated steam and coupled to an electric generator.

One drawback, particularly for installations in desert environments, is the need for effective cooling. In the case of water-based cooling systems, solar thermal power plants require approximately 3 m³ of water per megawatt-hour (MWh) of electricity produced.

The electrical connection of a solar thermal power plant to the grid depends mainly on its generator capacity and grid voltage, while thermal storage influences when and how flexibly the plant can supply electricity.

The world’s largest solar thermal power plants use both parabolic-trough and central-tower technologies. Noor Energy 1 in Dubai combines 600 MW of parabolic-trough capacity with a 100 MW central-tower unit, giving the project a total CSP capacity of 700 MW.

Energy Use of the Photovoltaic Effect

The photovoltaic effect is a physical process in which light generates charge carriers in a material, producing an electric potential difference. In the case of photovoltaic cells, the material is most commonly a semiconductor, typically silicon, and the radiation consists of a stream of photons with sufficient energy that can excite electrons from the valence band into the conduction band, creating electron–hole pairs.

The condition of a specific wavelength means that a minimum photon energy is required for the electron release process to occur. This threshold energy is 1.12 eV, which corresponds to a wavelength of 1,105 nm. Photons with higher energy are absorbed and release electrons, whereas photons with lower energy cannot initiate the process.

The released electrons become free charge carriers within the material. Simultaneously, the absence of an electron at the original site creates a hole, which behaves as a positively charged particle. The existence of these electron-hole pairs makes it possible to generate electromotive voltage and thus produce an electric current.

A photovoltaic cell is essentially a diode, composed of two layers of semiconductor materials forming an electric potential via a so-called P–N junction. The P-type region is commonly produced by doping crystalline silicon with boron, while phosphorus is typically used to produce the N-type region. At the interface between the two regions, an internal electric field is created that helps separate the electrons and holes generated by absorbed light.

The internal electric field separates the charge carriers generated by absorbed light, producing a voltage of around 0.6 V in a typical crystalline-silicon solar cell. When suitable electrodes are connected, this potential can be used to generate electric current.

Types of Solar Cells

At present, the most widely used types of solar cells are monocrystalline, polycrystalline, and thin film.

Monocrystalline silicon solar cells are currently the dominant crystalline-silicon technology and typically achieve efficiencies above 20%, with the most efficient commercial designs approaching 25% and are produced from thin, dark wafers cut from a single-crystal silicon ingot. Although they are more expensive, they are highly efficient and have a long service life.

Polycrystalline, or multicrystalline, solar cells generally have lower efficiency than monocrystalline cells, and their share of the photovoltaic market has declined substantially. They are made from silicon composed of multiple small crystals. Their dark blue, usually square wafers display visible crystal boundaries.

Thin-film photovoltaic technologies include amorphous silicon, cadmium telluride (CdTe) and CIGS. Commercial thin-film modules typically achieve efficiencies of roughly 10—20%, and some designs can be made flexible. They are produced by depositing a thin layer of photosensitive material (such as amorphous silicon) onto a glass, plastic, or stainless-steel substrate. These cells are less sensitive to orientation and temperature and perform well even under low-light conditions.

Photovoltaic Panels

Individual solar cells are too small for practical use, so they are combined into larger assemblies known as solar panels. Cells are interconnected within a photovoltaic module, usually predominantly in series, to provide a voltage and current suitable for practical applications. While the voltage is defined by the circuit configuration, the current output is directly proportional to the amount of light falling on the panel.

A typical crystalline-silicon photovoltaic module consists of interconnected solar cells encapsulated between protective layers, with tempered glass on the front and a polymer backsheet or a second sheet of glass on the rear. The entire assembly is enclosed within an aluminium frame.

Solar panels are most commonly installed on the roofs of residential and industrial buildings, but they can also be mounted on simple ground-based support structures suitable for installation in virtually any location.

 

Photovoltaic Farms

A photovoltaic farm is created by installing a large number of solar panels in a single location. Because solar radiation has a relatively low power density and the panels must be spaced to avoid excessive shading, large photovoltaic farms can cover extensive areas.

For grid compatibility, photovoltaic farms must be equipped with inverters and power transformers. Solar panels are mounted either on fixed supporting structures or on tracking systems that follow the Sun to increase energy yield.

Photovoltaic systems are generally classified according to their installed capacity:

• Small-scale photovoltaic systems typically have capacities ranging from a few kilowatts to several tens of kilowatts (kW);
• Large-scale photovoltaic farms can have installed capacities ranging from several megawatts to hundreds of megawatts or even several gigawatts.

The electricity generated is usually fed directly into the power grid, with the exception of standalone (off grid) photovoltaic systems, which usually use batteries to store electricity for periods when solar power is unavailable. These batteries then supply power to devices and infrastructure, for example, in remote areas without access to the central grid.

Photovoltaic systems are now being developed worldwide, with China by far the largest market, followed by other major markets including the European Union, India and the United States.

 

Solar Energy and the Environment

Solar power plants produce no direct combustion-related air pollutants or greenhouse gas emissions during normal operation. Their environmental impacts occur mainly during the manufacture of components, construction of the plant, land use and the management of equipment at the end of its service life. Photovoltaic systems require no fuel during operation, but their overall environmental impact must therefore be assessed over their entire life cycle.

Similarly, photovoltaic power plants require no fuel or working fluids for their operation and generate electricity without emissions or waste. The only notable environmental drawback is the energy-intensive manufacturing of solar panels — a process that still relies partly on fossil fuels.

The environmental compatibility of solar energy has already been well demonstrated, and combined with its virtually inexhaustible primary source, it is considered one of the most promising technologies for future energy generation.

Development of Solar Energy

Given the enormous potential of solar energy and the growing global trend of installing solar systems for both thermal energy and, above all, electricity generation, the future development of solar power is both inevitable and highly anticipated. This development will follow two main directions:

• The construction of large-scale solar power plants covering many square kilometres;
• The widespread integration of solar panels into various forms of infrastructure.

Current photovoltaic development focuses on higher-efficiency silicon cells, tandem technologies, energy storage and applications such as building-integrated, floating and agrivoltaic systems. In addition to established tower- and trough-based solar plants, the region could in principle also host solar updraft tower systems — simple concepts that use solar-heated rising air to drive turbines and generate electricity.

The ambitions of photovoltaic technology reach even further. A major space-based solar power project is being considered, which would exploit the continuous availability of solar radiation at the level of the solar constant — 24 hours a day. The main challenge lies in finding an economically viable method for transporting solar panels into orbit and transmitting the generated electricity back to Earth.