An Introduction to Concentrating Solar Power and its Sub-systems

By DR. DF Duvenhage

1. Renewable energy technologies, an overview.

Renewable energy technologies are those that can be used to produce energy from direct and indirect effects on the Earth from the sun’s energy. These technologies include solar, wind, hydro, geothermal, ocean, and biomass^[1]. According to the International Energy Agency (IEA), renewable energy supply from solar, wind, hydro, geothermal, and ocean rose by close to 8% in 2022, representing a substantial increase in the share of these technologies in total global energy supply^[2]. Renewable energy is now a commercially attractive investment opportunity in many countries, with new solar photovoltaic (PV) and wind energy plants being more profitable than fossil and nuclear plants in a fast-growing number of regions around the world^[3].

Renewable energy sources have been used to generate heat and power for much of human history, and more recently, electricity. Renewable energy makes up 12% of primary energy use in the United States and 11% worldwide^[4]. In 2020, renewable energy sources generated a record 834 billion kilowatt-hours (kWh) of electricity or about 21% of all the electricity generated in the United States. Renewables surpassed both nuclear and coal for the first time on record^[5].

The major types of renewable energy sources are biomass, hydropower, geothermal, wind, and solar. Biomass is organic material that comes from plants and animals, and it can be used to produce electricity, transportation fuels, and chemicals. Hydropower uses the energy of falling water to generate electricity. Geothermal energy is heat from the Earth’s core that can be used to generate electricity or heat buildings. Wind energy uses the power of the wind to generate electricity. Solar energy uses the sun’s energy to generate electricity or heat buildings^[6].

Renewable energy technologies are at different stages of commercialization. The IEA has defined three generations of renewable energy technologies, reaching back over 100 years. First-generation technologies include hydropower, biomass combustion, geothermal power, and heat. Second-generation technologies include solar heating and cooling, wind power, modern forms of bioenergy, and solar photovoltaics. Third-generation technologies are still under development and include advanced biomass gasification, biorefinery technologies, hot-dry-rock geothermal power, and ocean energy^[7].

Renewable energy can help fill the gap in energy needs, even if we had an unlimited supply of fossil fuels, using renewable energy is better for the environment. Renewable energy technologies produce few if any greenhouse gas emissions and have numerous benefits, including job creation throughout renewable energy industries, reduced emissions and air pollution from energy production, increased energy independence, and expanded clean energy access for non-grid-connected or remote communities^[8].

2. CSP, and how it fills a significant gap.

Renewable energy sources such as solar and wind are variable in nature, meaning that their energy production fluctuates depending on weather conditions. This variability can pose challenges for grid stability and energy storage. One technology that can address these limitations is concentrating solar power (CSP).

CSP systems use mirrors to concentrate sunlight onto a receiver, which creates heat that is used to generate electricity. The benefit of CSP’s exploitation of the thermal component of solar irradiation is that the energy can be stored as heat, or thermal energy storage (TES), which provides a workable solution to the challenge of reduced or curtailed energy production when the sun sets or during periods of weather-related intermittency^[9]. CSP systems can store thermal energy for many hours, which enables them to be flexible and dispatchable options for providing clean, renewable energy^[10]. CSP can provide power grid stability and flexibility, making it a valuable asset as grid demands evolve^[11]^,[12].

Additionally, CSP can provide inertia, which is the kinetic energy stored in spinning generators and is useful in handling transient conditions such as temporary short circuits and momentary disruptions. This inertia is especially useful for low-frequency control^[13]. CSP has the potential to reduce the cost of renewable energy from various sources and provide a reliable source of clean energy^[14].

3. An overview of CSP technologies.

CSP utilizes three alternative technological approaches: Parabolic Trough (PT), power or solar tower (ST), and dish/engine^[15]. CSP systems use mirrors or lenses to concentrate sunlight onto a receiver, which creates heat that is used to generate electricity.

The ideal conditions required for the economic feasibility of CSP include areas of high solar radiation, low humidity, and minimal dust and clouds. These factors affect the amount and quality of sunlight that reaches the receiver, and thus the thermal energy output. The availability of land, water, grid connection, and environmental impact should also be considered^[16].

Countries with CSP in operation and under development include Algeria, Australia, Canada, Chile, China, Denmark, Egypt, France, India, Italy, Mexico, Morocco, South Africa, Spain, Ukraine, the United Arab Emirates, the United Kingdom, and the United States^[17].

Parabolic Trough Systems:

Figure 1: Fundamental PT power plant layout, Adapted from http://energy.gov/eere/energybasics

Parabolic trough linear concentrating systems are used in one of the longest-operating solar thermal power facilities in the world, the Solar Energy Generating System (SEGS) located in the Mojave Desert in California. The facility has had nine separate plants over time, with the first plant in the system, SEGS I, operating from 1984 to 2015, and the second, SEGS II, operating from 1985 to 2015^[18]. Parabolic trough systems are commercially mature and have been in operation for over 30 years. They have a lower cost per unit of capacity than power tower systems^[19].

Solar irradiance is concentrated by long, curved reflectors onto collector tubes spanning the length of the parabolic troughs. These reflectors consist of a reflective material (such as mirrors or polished metals) that has been bent and orientated to reflect the sunlight onto a focal line ^[20]. The collectors consist of a glass outer tube, a high-conductance, a coated metal tube filled with a heat transfer fluid (HTF), and a vacuum between the two tubes. The selective coating on the metal tube allows high radiation absorption from the mirrors and limits heat loss due to the emission of infrared radiation from its surface^[21].

The HTF within the metal tubes at current PT power plants that are operational or under development are either synthetic oils, molten salts, or steam (water). In the case of using an HTF other than water, heat is exchanged within a boiler, between the HTF and water, to produce high-temperature, high-pressure steam. In the case of direct steam generation, water is cycled through the metal tubes, producing the critical steam^[22]. Finally, the steam drives a conventional Rankine-cycle turbine and generator and is condensed either by wet-, dry- or hybrid cooling.

The PTs within the solar field are modular and are arranged in long parallel rows. The structures on which the reflectors and collectors are mounted, in the solar field, track the sun on one axis only. They are aligned in a North-South orientation and track the sun from East to West throughout the day^[23]. The HTF must be circulated through the entire solar field, hence they can reach great lengths. This results in the HTF having to be pumped kilometres before reaching the power block where it can exchange heat with water, generate steam, and drive the turbine and generator^[24]. As a result of this fact, PT power plants generally need to be located in areas that are not steep and are near flat. This is another key difference between PT and ST technologies.

The temperature of the heat transfer fluid flowing through the pipe, usually thermal oil, is increased from 293ºC to 393ºC, and the heat energy is then used in the thermal power block to generate electricity in a conventional steam generator.

Power Tower Systems:

Figure 2: Fundamental ST power plant layout, Adapted from http://energy.gov/eere/energybasics

Solar tower power plants also make use of the basic Rankine cycle for energy generation from steam. The key difference between it and PT plants is that the solar irradiance is concentrated by many mirrors in a solar field onto a single point. These mirror structures are called heliostats and track the sun on both the vertical (tilting)- and horizontal (rotating) axis^[25]. They reflect and concentrate the sunlight onto a central receiver mounted on a tall tower. Here, the sunlight can be concentrated 600-1000 times, or more, resulting in temperatures in excess of 1000 degC^[26].

The fact that the source of the heat is at a central location results in much less need for piping to transport the working fluid. The power block, storage, and condensers can all be located at the base of the tower, meaning that the working fluid does not have to be circulated for such long distances, as is the case with PT technology. The dual-axes tracking system required for the vast solar field is however more complicated than the single-axis system for PT power plants and requires more sensitive calibration.

The HTF used in ST power plants can also range between synthetic oils, molten salt or direct steam. In addition to these mediums, air can also be used as the working fluid because of the high temperatures reached^[27]. If any medium other than water is used as the HTF, then heat must be exchanged between it and water to produce the steam required for the Rankine cycle. If direct steam is used, then the same principles as for PT power plants apply. The higher temperature, super-critical steam from ST power plants does however result in higher Rankine efficiencies. Beyond this fact lies the improvement in thermal storage capacity due to the higher temperatures since hotter molten salts can retain heat for longer periods^[28].

Spain has several power tower systems. Planta Solar 10 and Planta Solar 20 are water/steam systems with capacities of 11 and 20 megawatts, respectively. Gemasolar, previously known as Solar Tres, produces nearly 20 megawatts of electricity and utilizes molten-salt thermal storage^[29]. South Africa currently has two power towers; Khi Solar One, near Upington, and Redstone near Postmasburg. Khi, a 50MW direct-steam power tower, has been operating since 2017, while Redstone, a 100MW molten salt tower, is currently under construction. Power tower systems are less mature than parabolic trough systems, but they have the potential to achieve higher operating temperatures and efficiencies^[30].

4. Detailed breakdown of the underlying sub-systems of CSP.

The two CSP technologies that are suggested to be most applicable for large-scale baseload generation are comprised of different technological subsystems. Each subsystem, in turn, can employ different technology variations for the process it is involved in. The selection of subsystems depends on a variety of factors, most notably of which is maturity and the resulting bankability. Each different technology option for each subsystem leads to many possible configurations. There are certain physical and practical limitations to which technology options can be combined^[31]. Furthermore, certain technologies require other specific technologies for the entire system to function successfully. Figure 3 shows the possible configurations based on the commercially available subsystem technologies.

From the diagram in Figure 3, it can be seen that there are a total of 36 different technology configurations for steam as a power block working fluid. It is clear that there are very few subsystem technologies that are limited to working in conjunction with other specific ones. Such is the case for molten salts as the working fluid, however, where it can only be used in conjunction with molten salt TES. CSP technology configurations are flexible, with synthetic oils, molten salts, and steam being compatible with HTF and TES combinations. There are however certain subsystem configurations that are preferred in the CSP industry due to their proven track records and maturity.

Figure 3: CSP subsystem technology combinations

The individual subsystems that comprise a CSP plant each have their own purpose in the energy conversion cycle. There are various external factors that are considered before selecting a final total system configuration. The most important of these factors are the financial considerations, specifically, the tariff structure offered by the national utility or energy regulator^[32].

The yield of the CSP plant will be chosen based on whether the tariff structure incentivizes baseload or peaking generation and the maximum limit on net capacity imposed by the energy regulator^[33]. The tariff structure, on the other hand, depends on various complex national aspects, unique to each country’s respective economic, infrastructural, political, and social circumstances.

The most generic CSP power plant layout is given in Figure 4. It gives an indication of which technologies are applicable to which fundamental subsystem and how each subsystem affects certain characteristics of the final power plant. An indication of which resource impacts each subsystem will also be highlighted in order to briefly analyze the dependencies of each, and the total resource dependence of the entire power plant as a whole.

Figure 4: Generic CSP power plant layout – Technology configurations

From Figure 4, it becomes clear that the main resource involved with the operation of a CSP plant is water. Water is mainly used in the collector field, power block working fluid, and cooling subsystems, although it can also be used in the HTF and TES if direct steam generation and storage are selected. Generally, the uses of water in a CSP plant are threefold: collector field cleaning, make-up water for Rankine-cycle and HTF and TES if direct steam generation is used, and cooling of steam from Rankine-cycle.

4.1. Parabolic trough – Solar Field

The solar field is the area where the mirrors or lenses are located that reflect and concentrate sunlight onto a receiver. The solar field is typically made up of hundreds or thousands of mirrors or lenses that track the sun throughout the day to maximize the amount of sunlight that is concentrated onto the receiver. The solar field is a critical component of a CSP plant, as it is responsible for capturing and concentrating the sun’s energy onto a receiver.

In the case of parabolic troughs, the solar field consists of parabolic-shaped mirrors. These track the sun in an east-west direction as the sun moves across the sky from sunrise to sunset. There are various designs for the parabolic trough frame and tracking system by different OEMs. However, the major components include the mirror support structure and frame, mirrors,

Frame and Structure:

The frame and structure provide the overall support and stability for the parabolic trough collector assembly. It is essential for keeping the trough’s shape and orientation intact.
The frame is typically made of sturdy materials like steel or aluminum to withstand environmental conditions.

Mirrors (Reflectors):

The mirrors, also known as reflectors, are the primary components responsible for concentrating sunlight onto the heat collector tube.
They are typically made of highly reflective materials, such as silver-coated glass or specially designed solar reflector material.
The mirrors have a parabolic shape that focuses incoming sunlight onto a specific focal line or point.

Rotating Assembly (Tracking Mechanism):

The rotating assembly is responsible for tracking the sun’s movement throughout the day to ensure that the parabolic trough always faces the sun.
It typically includes a tracking mechanism with motors and sensors that adjust the orientation of the trough to maximize sunlight capture.
The tracking system can be either single-axis (tracking the sun’s movement along one axis, usually east-west) or dual-axis (tracking the sun’s movement along both east-west and north-south axes).

Heat Collector Tube (Receiver Tube):

The heat collector tube, often referred to as the receiver tube, is the component where the concentrated sunlight is collected and used to heat a heat transfer fluid (HTF).
It is positioned along the focal line of the parabolic trough, where sunlight is most concentrated.
The receiver tube is usually a cylindrical or tubular structure, often made of materials with high thermal conductivity and resistance to high temperatures, such as stainless steel.
The HTF inside the collector tube absorbs the concentrated solar energy, heating up in the process.

Heat Transfer Fluid (HTF):

The heat transfer fluid (HTF) circulates through the heat collector tube and carries the captured thermal energy to a heat exchanger.
Common HTFs include synthetic oils or molten salt, chosen for their ability to absorb and transfer heat efficiently while maintaining stability at high temperatures.
The HTF transfers the heat to a secondary fluid or directly to a steam generation system, depending on the specific design of the solar thermal power plant.

Support Structure and Mounting Hardware:

In addition to the primary frame, various support structures and mounting hardware are used to secure mirrors, receiver tubes, and tracking mechanisms in place.
These components are designed to withstand wind loads, including wind loads, and maintain the precise alignment of the parabolic trough.

4.2. Parabolic trough – Heat collectors/Receivers

The absorber tube is a critical component of a parabolic trough mirror-collector assembly in a CSP plant. It is responsible for collecting the concentrated sunlight and transferring the heat to the heat transfer fluid (HTF). The absorber tube is typically made of steel or glass and is coated with a selective coating to maximize the absorption of solar radiation^[34]. The absorber tube has several sub-components, including the selective coating, expansion bellow, and HTF flow path.

The coating is designed to absorb as much solar radiation as possible while minimizing the amount of heat that is lost through radiation. The coating is typically made of a material with high absorptivity in the visible and near-infrared regions of the solar spectrum, such as black chrome or black nickel. The coating is also designed to have a low thermal emissivity, meaning it does not radiate heat away from the absorber tube as effectively as other materials. This helps to maximize the amount of heat that is transferred to the heat transfer fluid (HTF). The selective coating is a critical component of a parabolic trough CSP system, as it is responsible for maximizing the amount of solar energy that is collected and transferred to the HTF. The coating must be able to withstand high temperatures and vacuum conditions, as well as resist degradation from UV radiation and moisture.

4.3. Parabolic trough – HTF system

The heat transfer fluid (HTF) system is responsible for transferring the heat from the absorber tubes to the power block.

The type of fluid used in the HTF system can vary, but it is typically a synthetic oil or molten salt that is heated in the receiver and then circulated through a series of pipes to transfer the heat to the power block^[35]. The selection of the HTF is a crucial decision in designing a CSP plant. Common HTFs include synthetic oils (such as Therminol and Dowtherm) and molten salt (typically a mixture of sodium nitrate and potassium nitrate). The choice depends on various factors, including operating temperature, efficiency, cost, and system compatibility. Synthetic oils are often used in lower-temperature systems, which are currently the industry norm, while molten salt is favored for higher-temperature applications.

For molten salts, the eutectic mixture of sodium nitrate and potassium nitrate used in CSP plants typically freezes (solidifies) at approximately 220°C (428°F) and melts at approximately 288°C (550°F). These values represent the lowest freezing point and highest melting point for a mixture of these two salts in varying proportions. These properties make Solar Salt a suitable heat transfer fluid in CSP plants because it remains in a liquid state over a wide temperature range, allowing it to efficiently carry and transfer heat in the solar thermal system. The high melting point also ensures that it can withstand the high operating temperatures encountered in CSP applications without solidifying, which would disrupt the system’s operation.

However, the use of molten salt as the HTF results in an increased risk of freezing of the molten salt within the receiver tubes in the solar field, since this happens at around 200°C. Synthetic oils, however, such as Therminol and Dowtherm, are designed to operate within a temperature range typically spanning from approximately -20°C (-4°F) to 400°C (752°F) or even higher. This means that much less heat tracing is needed to ensure the HTF remains well above the freezing temperature, hence them being the more widely adopted HTF for parabolic trough CSP plants.

To minimize heat loss, piping is typically insulated with high-temperature insulation materials like mineral wool or aerogel. Proper insulation helps maintain the temperature of the HTF during transport, reducing energy losses.

Properly sizing the pipes is essential to maintain the required flow rates and minimize pressure drop. Larger pipes can reduce flow resistance and pressure losses, but they also increase material costs. Computational fluid dynamics (CFD) simulations are often used to optimize pipe sizing. Pressure drop in the piping network should be minimized to ensure efficient fluid circulation. This involves careful selection of pipe diameters, flow velocities, and the use of well-designed fittings and valves.

Given the temperature variations in CSP systems, the piping must accommodate thermal expansion and contraction. Expansion joints or flexible connections are used to prevent stress on the pipes and associated equipment. Expansion joints are typically made of materials that can withstand the temperature range and chemical compatibility with the HTF. Common materials include stainless steel, Inconel, or other high-temperature alloys. The bellows, the flexible component of the expansion joint, must be designed to accommodate the expected thermal expansion and contraction. The number of convolutions (the corrugated sections) and their geometry are critical factors.

Proper anchoring and support of expansion joints are essential to ensure they move as intended without imposing excessive stress on adjacent components. The design should allow for axial, lateral, and angular movement. Expansion joints must be designed to withstand the pressure and vacuum conditions of the HTF system. This is especially important if the CSP plant includes a thermal storage system, which can introduce varying pressure conditions.

Finally, an expansion tank is used to accommodate the expansion and contraction of the HTF as it heats up and cools down^[36]. As the HTF heats up and expands, it would lead to a significant increase in pressure within the closed piping system if there were no means to accommodate this expansion. This overpressurization could potentially damage the piping, valves, and other components. Expansion tanks help protect the integrity of the HTF piping system by preventing stress and fatigue on the pipes, fittings, and other equipment. This reduces the risk of leaks, deformations, or failures caused by thermal cycling. The size of the expansion tank is determined by the volume of HTF that needs to be accommodated during thermal expansion. The design should consider the maximum expected temperature difference and the coefficient of thermal expansion of the HTF. To prevent overpressurization, expansion tanks often incorporate a pressure relief valve that opens if the pressure exceeds a certain threshold, allowing excess HTF to be released safely.

4.4. Power Tower – Solar Field

At the heart of these power plants is the solar field array of heliostats, which plays a crucial role in concentrating sunlight onto a central receiver. Heliostats are mirror-like devices that track the sun’s movement on the horizontal and vertical axis to reflect sunlight onto a central receiver located atop a tower. These precision-engineered mirrors are the backbone of Power Tower CSP plants. They are designed to accurately follow the sun’s path throughout the day, ensuring maximum energy capture.

Figure 6: Typical components of a Heliostat, http://www.powerfromthesun.net/Book/chapter10/chapter10.html.

Mirror Modules: Mirror modules are the heart of heliostats. They are precision-engineered mirrors designed to efficiently capture and reflect sunlight toward the central receiver. These modules are typically constructed with high-performance glass or metal substrates, coated with a specialized reflective material, such as silver or aluminum. The choice of material is critical to achieving the highest possible reflectivity and durability. Mirror modules are engineered to withstand harsh environmental conditions, including wind, dust, and extreme temperatures, to ensure they maintain their reflective properties over the long term.
Support Structure: Heliostat mirror modules are mounted on robust support frames, which are responsible for holding the mirrors securely in place and facilitating their precise movement. These support frames are typically made of durable materials like steel or aluminum, designed to provide stability and withstand mechanical stresses. The frames must allow for accurate alignment of the mirror modules and accommodate the range of motion required to track the sun’s position throughout the day.
Torque Tube/Tracking Assembly: The Tracking Assembly serves as the backbone of the heliostat structure, connecting the support frame to the drive mechanism. The torque tube is engineered for durability and designed to minimize friction, allowing for smooth and precise tracking of the sun’s movement. Precision tracking is essential to keep the mirrors aligned with the sun. Heliostats use advanced tracking systems. Dual-axis trackers, such as those used in Heliostats, follow the sun’s east-west movement, as well as the sun’s north-south movement, to esnure as much sunlight is focussed on the central receiver at the top of the tower.
Drive mechanism: The drive mechanism is responsible for adjusting the orientation of the heliostats in real-time to ensure that sunlight is continuously focused on the central receiver. Heliostats employ advanced tracking systems to precisely follow the sun’s path. These systems rely on motors, gears, and sensors to achieve high levels of accuracy, maximizing energy concentration at the central receiver. Typically, the sun’s path can be pre-programmed based on a series of geometric calculations considering the time of day, day of the year, the lattitude and longitude of the CSP plant, and the realtive position of the heliostat to the central receiver.
Pilon: The pilon, also known as the pole or pedestal, is the structural support that connects the heliostat assembly to the ground. It plays a crucial role in ensuring the stability and safety of the heliostat array. Pilons are engineered to withstand the forces exerted on the heliostats, including wind loads and the weight of the mirror modules. The design and construction of pilons are critical considerations in the overall structural integrity of the solar field array.

4.5. Power Tower – Heat Collector/absorber

The central position of the receiver offers a universal advantage to collect all energy at one location and save on transport networks. The central receiver system can be designed as either an external or cavity-type receiver. In an external receiver, the working fluid flows through tubes that are mounted on the outside of the receiver. In a cavity-type receiver, the concentrated solar radiation from reflectors incident into the cavity of the receiver through an aperture. The cavity-type receiver maximizes the capturing of sun rays and minimizes the radiative losses to the environment. The design of the central receiver system must take into consideration several factors, including the working temperature range, heat transfer efficiency, freezing protection, expansion and contraction, heat exchanger, control system, and working fluid. The type of fluid used in the central receiver system can vary, but it is typically a synthetic oil or molten salt that is heated in the receiver and then circulated through a series of pipes to transfer the heat to the power block.

Figure 7: A – External Receiver/Absorber, B – Cavity Receiver/Absorber
A – https://www.solarpaces.org/star-receiver-could-cut-tower-csp-cost-11/, B – Tiryaki, Gürcan & Camdali, Unal. (2017). ENERGY AND EXERGY ANALYSIS OF A SOLAR POWER TOWER SYSTEM WITH A CAVITY RECEIVER.

Absorber tubes are the core elements of the central receiver. They are an intricate network of tubes designed to capture concentrated sunlight. These tubes are typically made of materials like Inconel Alloy 625 / Haynes Alloy 230 with a black Pyromark 2500 coating, ensuring efficient heat absorption. The absorbed solar energy heats the molten salt circulating within the tubes, initiating the energy conversion process.

Inlet and outlet headers are essential components for the efficient circulation of the molten salt through the absorber tubes. The inlet header distributes the molten salt evenly to the absorber tubes, while the outlet header collects the heated molten salt , which has absorbed solar energy. These headers play a crucial role in maintaining uniform flow and heat distribution within the central receiver.

Molten salt circulation pumps circulate the primary heat transfer fluid (molten salt) through the solar receiver to heat it up and to either feed the solar steam generator, store the energy during the high sun radiation hours (cold salt pumps), or deliver it after the sunset (hot salt pumps). These pumps must be robust, capable of withstanding high temperatures, and designed for the specific properties of molten salt. Efficient circulation ensures that the captured thermal energy is effectively stored and later used to generate electricity.

4.6. Boiler system

At the core of every CSP plant lies the power block, a complex system engineered to efficiently convert the captured thermal energy from solar concentration into electrical power. The Rankine cycle is the fundamental thermodynamic process underlying the power block’s operation. It constitutes a closed-loop cycle that facilitates the conversion of thermal energy into mechanical work and, subsequently, electrical power.

The Rankine cycle involves four principal stages: isentropic compression of the working fluid (typically steam), adiabatic expansion of the working fluid through a turbine, isobaric condensation of the working fluid in a condenser, and isentropic pumping of the working fluid. In CSP applications, the Rankine cycle is tailored to utilize the concentrated solar energy for heating the working fluid, thereby initiating the energy conversion process. The first step in this conversion process is exchanging the heat absorbed by the HTF (either synthetiic oils or molten salt) with water to generate steam.

At CSP plants, the boiler system stands as an essential facet of thermal energy conversion. It plays a critical role in harnessing and elevating the temperature of the working fluid to facilitate the Rankine cycle’s operation and ultimately produce electricity.

In the economizer, which is situated at the entrance of the boiler system, the residual heat in the exhaust gases from the steam turbine is used to preheat the incoming feedwater. This process significantly reduces the amount of thermal energy required to reach the desired working fluid temperature. By optimizing the use of exhaust heat, the economizer enhances overall plant efficiency.

The main boiler represents the core of the boiler system. Here, the pre-heated feedwater is turned into steam. This is done by exchanging heat between the HTF and the pre-heated feedwater in shell-and-tube heat axchangers.

The superheater is located downstream of the main boiler. Its role is to further elevate the temperature of the steam beyond its saturation point. This superheating process ensures that the steam remains in a vapor state throughout its passage through the Rankine cycle, preventing any condensation that may impair turbine efficiency. Superheated steam possesses increased energy content and is essential for driving the steam turbine efficiently.

Finally, in a two-tubeine system, a reheater may be used. In a conventional Rankine cycle, after the high-pressure steam exits the high-pressure turbine stage, it is directed to the condenser, where it is condensed back into a liquid. However, in some power plants, especially those with large turbines and high-pressure boilers, a re-heater is added to the cycle to enhance efficiency. The re-heater in the Rankine cycle increases the cycle’s efficiency by allowing for more energy extraction from the working fluid. It reduces the moisture content of the steam and prevents it from undergoing excessive expansion, improving overall turbine performance and power plant efficiency.

4.7. Steam Turbine

Steam turbines in CSP plants play a pivotal role in converting concentrated solar thermal energy into mechanical work, which is then transformed into electricity. These turbines operate under unique conditions due to their dependence on solar heat as the primary energy source.

There are certain key characteristics of CSP plant steam turbines that differentiate them from those in coal-fired plants. Firstly, CSP plants typically operate at lower temperatures compared to coal-fired plants. The steam generated in CSP plants is at temperatures ranging from 250°C to 565°C (482°F to 1,049°F), depending on the specific technology used. This lower temperature range is a result of the limitations imposed by the concentrated solar heat source.

Furthermore, due to the lower temperature of the steam, CSP plant turbines operate at moderate pressure levels. The pressure in CSP turbines is usually lower than that in coal-fired turbines, which can reach much higher levels due to the significantly higher temperature of steam generated by burning coal. These lower temperatures and pressures in CSP plants can impact overall efficiency. To compensate, CSP plant designers often implement advanced technologies such as two-stage turbines or reheating to optimize energy extraction from the steam.

4.8. Condenser

The condensing and cooling of the steam exiting the low-pressure turbine is critical to plant efficiency and operation. There are two major methods used for cooling at CSP plants: recirculating (evaporative) wet-cooling and dry-cooling^[37].

Wet-cooling technology uses water as the cooling fluid, absorbing the latent heat of condensation from the steam exiting the low-pressure turbine. There are two types of wet-cooling technologies: (i) Once-through cooling- water is extracted from a source, used to cool the steam in a condenser and returned to the source to replenish the water abstraction, albeit with water at an elevated temperature. (ii)Recirculating, evaporative cooling, where cooling water is circulated between a cooling tower and a condenser, but the warm cooling water evaporates, forming water vapour in the atmosphere.

Figure 8: Once-through and recirculating evaporative cooling, https://doi.org/10.1016/j.renene.2018.08.033.

Once-through cooling has never been used for CSP because of the lack of adequate water resources in high DNI areas. Recirculating wet-cooling (hereafter referred to as wet-cooling only) is, however, very prevalent, with almost 80% of all operational plants using this cooling technology. This is due to the lower capital cost of wet-cooling technology and greater efficiency, compared to dry-cooling. Furthermore, compared to recirculating wet-cooling, the reduced efficiency of dry-cooling results in a larger solar field required to maintain the power output, at higher capital costs.

Wet-cooling is very effective because the heat from the steam is rejected to the air through the evaporation of the cooling water. Therefore, compared to dry-cooling, the wet-cooling process is less affected by variations in ambient air temperature, since the evaporative cooling is dependent on wet bulb temperature. As a result of this, wet cooling uses almost 10 times more water than dry-cooling. There are two major mechanisms of water loss in wet-cooling. (i) Evaporative cooling of the warm water leaving the condenser in the cooling towers, as mentioned earlier, is the primary heat transfer method. This, in turn, results in the concentration of minerals each time water is lost to the atmosphere. (ii) In addition, dilution is required to prevent the cooling water from becoming saturated with minerals; which will result in scale formation and reduce cooling efficiency. Dilution is achieved by adding fresh cooling “makeup water”, and rejecting the higher concentration cooling water, known s “blowdown”, thereby continuously limiting mineral saturation and its consequences.

Dry-cooling, as mentioned before, does not use water as the cooling fluid, but air. Instead of the steam being cooled in a condenser by cooling water, it is cooled in what is called an air-cooled condenser (ACC). Here, the steam passes through a bundle of tubes, and ambient air is pulled (or pushed) through this to absorb the heat. This means that the effective cooling that can be achieved is dependent on the dry-bulb temperature of the air, which is always higher than the wet-bulb temperature in dry, arid conditions, where CSP is most prevalent. Dry cooling requires minimal water only for cleaning of the condenser tube bundles. This cleaning is carried out at fixed intervals to prevent external fouling on the tubes and ensure efficient operation. As discussed, the drawback of dry-cooling is its higher capital costs and negative impact on plant efficiency. In light of the fact that CSP is considered the most expensive RET, this extra addition to overall cost and levelised cost of electricity (LCOE) is a major limiting factor for its use at CSP plants. Another inherent drawback is that ambient temperatures are highest on days of high solar irradiation, resulting in the highest efficiency losses on days that are supposed to be the most productive.

4.9. Thermal storage system

Thermal energy storage systems are the backbone of CSP plants, enabling the harnessing of solar energy even when the sun isn’t shining. These systems address the intermittency of solar energy, providing a stable and reliable power supply, day or night. TES systems store excess thermal energy generated during sunny periods and release it when needed, enhancing the dispatchability of CSP plants.

Molten salt systems are among the most widely used TES technologies in CSP plants. They involve heating a mixture of sodium nitrate and potassium nitrate to a high temperature, usually around 565°C (1,049°F). The hot molten salt is stored in insulated tanks and can retain its heat for several hours, allowing for on-demand electricity generation.

Solid-state TES technologies involve the use of solid materials, such as ceramics or concrete, with high heat capacity to store thermal energy. These materials absorb and release heat during charging and discharging cycles, offering long-term storage solutions.

Pressurized steam storage is a TES technology that leverages high-pressure steam as the energy storage medium. It offers unique advantages and challenges compared to other TES systems, such as molten salt or thermal oil. During the charging phase, excess thermal energy generated by the CSP plant is used to heat water, creating high-pressure steam. The pressurized steam is then stored in specially designed tanks that can withstand the high pressure and prevent heat loss to the environment. When electricity generation is required, the stored high-pressure steam is released through a pressure reduction system. As the steam expands, it passes through a steam turbine, driving a generator to produce electricity.

An advantage of pressurized steam is its quick response, making this system responsive to grid demand fluctuations. Furthermore, unlike other TES systems that require a separate heat transfer fluid, pressurized steam storage uses water directly, simplifying the system. However, high-pressure steam systems may experience higher heat losses due to the greater temperature differences with the surroundings. The duration for which pressurized steam is also limited by the high pressures required and the losses that increase with larger storage vessels.

The two-tank molten salt storage system stands out as a reliable and efficient TES technology in CSP plants. This system comprises two separate tanks—one for storing hot molten salt and another for cold molten salt. During the charging phase (when excess thermal energy is available), hot molten salt from the solar field is pumped into the hot tank, where it is stored until needed. When electricity generation is required, the hot molten salt from the hot tank is circulated through a heat exchanger, transferring its thermal energy to a working fluid, typically water, to produce steam. The steam is then directed to a steam turbine for electricity generation.

After the hot molten salt has transferred its energy, it becomes cooler and is pumped back into the cold tank for reuse in subsequent cycles. This maintains a constant temperature differential between the two tanks, ensuring efficient energy storage and release. The two-tank molten salt system offers reliability and flexibility, allowing CSP plants to store energy for several hours or even into the night. Its ability to maintain consistent temperatures enhances overall system efficiency.

5. Conclusion

Concentrating solar power offers a unique advantage over other renewables in the form of its capacity to store energy thermally. In recent projects, large solar photovoltaic plants have been built in conjunction with CSP in order to offset the higher capital costs associated with it and leverage the lower (and declining) costs of large-scale PV. This adds some complexity in the form of power electronics required for the high-voltage conversion of DC to AC electricity. Regardless of how CSP is paired with other renewables, its value lies in unlocking stable, dependable, clean energy from these technologies, since it can offer the baseload or dispatchable supply that weather-dependent PV and wind simply cannot offer. CSP offers a truly practical alternative to fossil-based energy supply while still utilizing many of the same sub-systems and components from these legacy generation technologies. In fact, CSP offers value to other industries in the form of process heat, which PV and wind can only offer by coupling them to inefficient heating cycles, resulting in energy losses. This is what makes CSP even more well-positioned than conventional renewables since CSP can not only offer a stable clean energy supply, but it can also benefit various sectors that rely on heat. One such sector is the fossil fuel sector itself, in the form of solar-enhanced oil recovery and process heat for refining and petrochemical products. This will be discussed in length in a follow-up article, building on the details covered in this introduction to concentrating solar power.