(Transcript of the lesson commentary.)
Turning Sunlight into Electricity
The energy from the Sun reaching the Earth’s surface is many times greater than the energy consumed by the entire human civilization. There are various ways to harness this energy and convert it into electricity. Solar thermal power plants exploit the sun’s heat. In order to use it to generate electricity, for example in a steam cycle, temperatures of hundreds of degrees must be reached.
Concentrating solar power plants achieve this by bringing the sun’s rays together in one spot. A magnifying glass works on the same principle. Mirrors are used in solar power plants to reflect the sun’s rays falling on a large area to a single point.
Tower power plants have a central receiver located on a tall column surrounded by mirrors called heliostats. Linear power plants have a long tube of heat transfer medium at the focal point. The mirror that reflects the rays onto it may be in the shape of a parabola or a set of long flat mirrors. This is called a Fresnel arrangement. Parabolic dishes are another way of concentrating the sun’s rays into a single point.
The updraft tower has a slightly different approach to harnessing the sun’s heat. It uses a combination of the greenhouse and chimney effects. Under a large circular greenhouse, air is heated, then drawn upwards through a central chimney to spin a turbine.
Photovoltaic power plants use the photoelectric effect in semiconductors to create an electric current. Their basic unit is most often crystalline silicon, either in the form of a polycrystal or a monocrystal. Thin films represent a revolution in photovoltaics as they allow the deposition of a thin layer of photovoltaic material on virtually any surface, including flexible materials. There are several different types of thin films.
A hybrid between concentrating and photovoltaic power plants is concentrating photovoltaics. It focuses the sun’s rays onto a special small photovoltaic panel, which then achieves high efficiency in converting the rays into electricity.
Central Tower Solar Power Plant
A Central Tower Solar Power Plant is a point concentrating power plant. It concentrates light into a single spot using mirrors. Here, high temperatures of around 500 °C are achieved. This is because the light falling on a large area covered with reflective mirrors is concentrated here. This area typically occupies between 1.5 and 3.2 square kilometres. The mirrors are called heliostats and can number up to hundreds of thousands. One heliostat is usually made up of several smaller flat mirrors. These are slightly tilted on the support structure to best concentrate the light on the central receiver. The mirrors actually form a parabola with a focal length corresponding to the distance of the mirror from the receiver.
The heliostat rotates every few seconds to follow the movement of the sun across the sky and reflect its rays continuously onto the central tower. Its movement is controlled by a computer. The heliostats are positioned around the central tower so that they do not shadow each other and reflect the maximum amount of light possible.
A central receiver is located on the tower in the middle of the heliostat field. It’s where the heat transfer medium is heated. There are two types of receivers, external and cavity. The external receiver takes the form of a cylinder whose outer walls are made up of narrow panels of thin tubes. Their surface is painted with a highly absorptive black paint. A cold medium is drawn into the tubes from the bottom and the heated medium is drawn out from the top. It then travels down the centre of the tower where it transfers its energy for further use.
In order to reduce heat loss, the cavity receiver has tubes with a heat transfer medium placed inside an insulated cavity. The cavity only receives light reflected from the heliostats located directly in front of it. Therefore, a multitude of cavities placed side by side, each facing a slightly different direction, or heliostats placed only in front of the single cavity are used.
There are five basic types of heat transfer media used. Oil, water, nitrate salt mixture, liquid sodium, and helium. Oil, either hydrocarbon or synthetic-based, is suitable as a medium for thermal storage. Its maximum operating temperature is 425 °C. It solidifies at temperatures below 10 °C. It’s flammable. Water is cheap and when heated produces steam that can be used directly to run a steam turbine. Its maximum operating temperature is 450 °C. It is unsuitable for use in thermal storage and must be protected from freezing. The nitrate salt mixtures are ideal for thermal storage and allows operating temperatures up to 565 °C. When the plant is not in operation, molten salts must be kept liquid by heating. Liquid sodium is more expensive than nitrate salts, but can be used up to 600 °C. It is suitable for thermal storage but is reactive. It must be heated during plant shutdowns because it solidifies at 98 °C.
If a high-temperature cycle, such as the Brayton cycle, is planned, air or helium can be used as the heating medium. Temperatures up to 850 °C can be achieved with helium, but it cannot be used for thermal storage.
The first solar power plants used water as the heat transfer medium. Later on, they switched to molten sodium and current solar tower power plants mostly work with molten nitrate salts. The accumulated heat is converted by the steam cycle. The heated water turns into steam and spins a turbine that drives a generator. This cycle requires cooling. Because water cooling is difficult in a desert power plant, air-cooled cycles are preferred.
The hot heat transfer medium is ideal to be used for thermal energy storage. Solar energy can be stored in a storage tank and used later, for example after sunset. Thus, the electricity production from solar power plants does not necessarily depend on how the sun shines. In principle, the plant can choose one of three modes in which it wants to operate. The plant will follow the daily cycle unless it is equipped with a thermal storage tank. It will produce maximum power at midday and will not produce electricity at night or under cloudy skies.
A plant with a medium-sized storage tank will produce a steady amount of electricity for most of the day. It will store surplus power from the midday hours in a storage tank. In the evening, it will then use the reservoir to heat the operating medium and produce electricity for several hours after sunset. With a large enough storage tank, the solar thermal plant can even achieve baseload mode, i.e. the ability to supply a steady amount of electricity twenty-four hours a day. Very often a gas-fired power plant is built along with a solar power plant. It operates at night, when it heats the heat transfer medium and generates electricity.
The largest Central Tower Solar Power Plants include Invanpah in California, USA, Ashalim in Israel and Quarzazate in Morocco, also called Noor III.
Linear Concentrating Power Plants
Linear concentrating power plants concentrate sunlight into a focal point running along the axis of the device. Among their most typical representatives are trough power plants. They are considered the most mature and of all solar thermal installations, troughs account for 85 percent.
The history of trough solar power plants dates back to 1913, when Frank Shuman constructed five trough reflectors, each 61 m long and 4 m in width, in the small farming community of Al Meadi in Egypt. Steam from the system powered irrigation pumps.
Trough concentrating power plants have the form of a long parabolic mirror. At the focal point of this parabola is an absorber tube. It consists of a pipe through which the heating medium flows. Most often it is mineral or synthetic oil. Water or molten nitrate salts are also used. The tube is painted with absorptive black paint. To reduce heat loss, a glass cylinder is placed around the tube and the space between it and the tube is evacuated. The vacuum is indeed an excellent insulator. In principle, it is a Dewar flask.
The troughs can be connected in long rows and the rows can be interconnected by pipes. The heat transfer medium flows through the piping to a steam generator where it transfers heat to the water. This turns into superheated steam and drives the turbine. Trough concentrators reach temperatures of 400—500 °C. Like tower plants, trough plants often use thermal storage tanks to store excess heat. This allows them to supply electricity even after the sun has set.
To keep them facing the sun, the trough concentrators need only be rotated by a single-axis tracking mechanism. This is technically simpler and less energy-intensive than rotating the system in two axes. Most troughs are oriented north-south and track the sun during the day. This way the most efficient use of solar irradiance is achieved.
Less common is the east-west orientation, which does not require tilting during the day, only minor seasonal adjustments. This system has fewer demands on daytime operation, but in turn operates with less efficiency.
Due to the modularity of the troughs, impressive power plant outputs can be achieved. Among the largest trough power plants in the world are the Mojave Solar Project, Solana Generating Station and Genesis Solar Energy Project in the USA, and Solaben in Spain.
A cheaper but less efficient variant of parabolic troughs are Linear Fresnel reflectors. The first power plant based on this principle was built in 1961 by Giorgio Francia in Italy.
The concave mirror is replaced in linear Fresnel reflectors by a series of narrow long flat mirrors. Because the mirrors are placed low to the ground, they are less likely to be damaged in a strong wind, and the cheap and quick production of flat mirrors makes them easier to replace. Each row of mirrors has a slightly different inclination so that all of them cast the reflected sunlight on the bottom part of an absorber suspended above the mirrors. Oil is used as a heat transfer fluid, but often also water, which directly generates superheated steam.
Because these are flat mirrors, they do not concentrate the light on the absorber perfectly. Often the absorber is enclosed by a secondary parabolic mirror to help focus the light. Alternatively, the absorber takes the form of a cavity in which tubes of heat transfer medium are placed. Due to a lower concentration factor than that of a parabolic trough collector, the lower operating temperatures, around 300—400 °C, are normally achieved. Thus, such plants operate at a lower efficiency.
This type of plant also allows units to be connected in long rows and rows to be interconnected to form large solar farms. Large power plants operating on the linear Fresnel reflector principle are Dhursar in India, Dacheng Dunhuang in China and Puerto Errado 2 in Spain.
The reflectors can shade each other or reflect light at too sharp an angle. The solution offers a compact linear Fresnel reflector, which has two receivers above the mirror array. The mirrors can be set to reflect alternately onto one or the other absorber, thus making maximum use of the incident light. With this configuration, more mirrors can be placed side by side and the area is used more efficiently.
Parabolic Dish Concentrators
A parabolic dish concentrator is a point concentrating solar power plant. All the sun rays that hit the surface of the parabola-shaped dish are reflected into the focus. The receiver in the focal point is fixed to the dish and the whole system rotates to follow the sun. Two rotation axes ensure that the device is always perfectly oriented directly towards the sun. Because it concentrates light from a relatively large area, the dish concentrator can reach high temperatures in the focal point, on average around 700 °C. This makes it more efficient than other solar concentrating systems.
The utilization of heat is possible in two ways. The first is the heating of the heat-transfer medium, which is led from the parabolic concentrator to the turbine. It requires piping in which heat losses may occur, thereby reducing the efficiency of the plant.
The second option is to convert heat to electricity directly in the focus. This option is used most often. A Stirling engine or a small gas turbine is placed in the focal point. The Stirling engine operates best at temperatures up to 950 °C, at higher temperatures a turbine is used. No heat transfer medium is required, the engine workspace is heated directly.
The equipment in the focus should be as small and light as possible so as to minimally shade the parabolic dish itself and not encumber the support structure. The surface of the reflecting parabola is made of glass, less often plastic, with a silver or aluminium reflective layer. Silvery polymers are being experimented with to make the whole mirror structure significantly lighter. The dish must be able to withstand the weather conditions, especially the wind, which leans on the large parabola like a sail.
Due to weight and dimension constraints, the dishes do not reach extreme size and therefore performance. Their power output ranges typically between 0.01—0.4 Megawatts. They are mostly used as individual units to produce smaller amounts of power and rarely combined into larger farms. When they are equipped with a motor located directly in the focal point, they do not use thermal storage methods.
The history of solar dish concentrators is relatively old. Augustin Mouchot experimented with them in the second half of the 19th century. A reflective funnel directed the sun’s rays onto a glass cylinder, which enclosed a black-painted absorber with water. The machine generated steam capable of powering engines. But the widespread availability of cheap coal put the development of solar power on hold.
An alternative method of using solar dish concentrators is not energy production, but cooking. Even a small mirror can reach temperatures suitable for meal preparation in the focal point. In sunny areas it can be used as an ecological substitute for wood burning. In addition, it is suitable for sterilising drinking water.
Solar Updraft Tower
A solar updraft tower is a still not widely used technology for harnessing the sun’s energy, combining the greenhouse and chimney effect. It has been developed since the 1980s but has not yet found widespread application. A solar updraft tower, also known as a solar chimney, consists of a tower and a large greenhouse at its base. The bigger the greenhouse, the greater the efficiency of the updraft tower, so a greenhouse up to 7 km in diameter is considered. In the greenhouse, the air is heated by the sun’s rays. The greenhouse rises slightly towards its middle. The warm air travels to the centre, where it is drawn in by the tower’s chimney effect. The air above the tower is cooler and colder, so the warm and light air rises upwards. The higher the tower, the higher the air temperature difference, and the more intense the flow. Ideally, the tower should be up to 1 km high. At the entrance to the tower, or low from the base of the tower, are turbines that are spun by the airflow.
No special materials are needed to build a solar updraft tower, just metal, concrete, glass or plastic. It can therefore be easily built from locally available materials. The ability of the greenhouse to accumulate heat can be increased by using suitable thermo-accumulating materials. For example, transparent bags of water or painting the substrate black. The air intake works as long as the temperature difference between the air in the greenhouse and above the top of the tower lasts. Theoretically, the solar chimney is therefore able to operate even after sunset. The high temperature in the greenhouse can be used to grow warm-loving plants.
The structure of the tall tower is heavily stressed by weather conditions. An alternative may be a slope updraft tower. In suitable locations, a sun-facing slope is used on which a greenhouse is built. A turbine and shorter chimney will be located at the upper end of the greenhouse. This will make the whole structure less stressed by wind and remove the complications associated with erecting an extremely tall tower.
The efficiency of converting the accumulated heat into electricity in a solar updraft tower is still very low, around two percent. This is significantly lower than the efficiency of photovoltaic panels or solar concentrating power plants. Although the operation of an updraft tower is almost maintenance-free, the construction costs are high. That’s another reason why the towers have not spread much.
In 1982, a tower 194 metres high surrounded by a greenhouse 244 metres in diameter was built in Manzanares, Spain. It operated for almost seven years before it was destroyed by a hurricane. But its conversion efficiency was very low, only 0.8%. Another notable project is Jinshawan in Inner Mongolia, China, launched in 2010. It should provide 200 kilowatts of power.
Photovoltaics
Photovoltaics produce electricity in a completely different way than solar thermal power plants, by direct conversion from incident solar radiation. It exploits a photovoltaic phenomenon discovered as early as 1839 by French physicist Alexandre Edmond Becquerel.
The photovoltaic phenomenon occurs in semiconductors, which are materials that conduct electricity better than an insulator but worse than a conductor. A typical semiconductor is silicon. Electrons are bound in the crystal lattice of silicon in such a way, that when they receive an extra portion of energy, for example from a photon, they are released and start moving freely through the material. The vacant space left behind is called a hole, and it can also move. The oriented movement of charged particles is called an electric current. In an ordinary semiconductor, this electron-hole pair soon recombines, that is, the electron takes up space in a hole. In order for an electric current to be generated, the electrons and holes need to be separated. This is what the P-N junction is for. In the N-layer, the semiconductor is enriched by atoms with a higher number of electrons in the valence layer, e.g. phosphorus or arsenic. There is an excess of electrons. In the P-layer, the semiconductor is doped with atoms with a lower number of electrons in the valence layer, such as boron or indium. There are holes as the main charge carriers. If an electron-hole pair is formed in such a modified material after the photon impact, the electron is attracted to the n-layer and the hole to the P-layer. A voltage is created between the accumulated electrons and holes. When the load is applied, an electric current can be drawn.
Many types of photovoltaic cells have been developed. Among the most widely used are crystalline silicon cells. They are either monocrystalline or polycrystalline. Monocrystalline ones have higher efficiency but are more expensive. They are made from a single large crystal of silicon They are very dark in colour and usually octagonal in shape. Their efficiency varies between 14 and 20%.
In contrast, polycrystalline panels are easier to produce. The material for them is created by solidifying molten silicon, so there are many differently oriented silicon crystals. They are blue in colour and the edges of the individual crystals are clearly visible. Their efficiency is 12 to 16%. These are the most widely used type of panels.
Instead of growing semiconductor crystals and then cutting them into wafers and modifying them, there is the possibility of depositing a thin layer of material with the desired properties on the substrate. Thin film cells are created that can have a number of interesting properties. Among the materials that can be used for their production are amorphous silicon, cadmium telluride, copper indium gallium selenide, gallium arsenide, organic materials, perovskite or quantum dots. Applying several different layers can create a multi-junction cell. Each layer is adjusted to use a slightly different wavelength of light, so together they will utilize the spectrum much more efficiently and achieve efficiencies of up to 45 per cent. The application possibilities of thin film cells are sharply increased by the use of a flexible substrate.
The output of one solar cell is small. It produces a voltage of about 0.5 volts and provides a power output of just over one watt. Therefore, the cells are interconnected and installed in a rigid frame covered with glass — a photovoltaic panel. The panels are then connected into larger units as required. They have an extremely wide range of applications. They can power homes, offices, or electric car charging stations. Small photovoltaic cells, most often thin films, are used to power small electrical devices, calculators, information boards or power banks for charging mobile phones.
A large array of photovoltaic panels can work as a power plant. Its output depends on the number of panels connected and the amount of time the sun shines on their surface. Most photovoltaic power plants use fixed panels facing south and inclined at an angle of about 35 degrees. The panels are connected in a row. Since photovoltaic panels produce direct current, the output of one row is usually fed into a string inverter, which converts it into alternating current compatible with the requirements of the electricity grid. Panels are usually built on agriculturally unusable sites, and in areas with a high number of sunny days.
During cloudy skies, the solar power plant produces minimal power, and none at night. The annual utilisation of photovoltaic panels varies between 15 and 25 per cent depending on the location. For a balanced output, it is necessary to store the surplus, typically from the midday period, in batteries or other storage.
In order to have an installed capacity of 1 megawatt, a photovoltaic power plant must have approximately 30,000 square metres of photovoltaic panels installed. Besides, a considerable area for battery storage must be taken into account. In 2022, the global installed capacity of photovoltaic power plants was 1,185 GW and together they produced 1,300 TWh.
