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Parabolic Trough Solar Collectors (Ultimate Guide)

Parabolic Trough Solar Collectors are the most mature CSP technology and one of the most prospective solar concentrating technologies available.

This guide will discuss the basics of parabolic trough solar collectors, their advantages and disadvantages, and how they work. It will also analyze their design, technological aspects, and applications.

What is a Parabolic Trough Solar Collector?

Parabolic trough solar collectors are curved mirrors that focus sunlight onto a pipe filled with fluid.

Parabolic trough mirrors

The fluid is heated to high temperatures, which then creates steam to power a turbine and generate electricity. Parabolic trough solar collectors are often used to generate electricity in large-scale solar power plants.

Parabolic trough solar collectors are the most common type of solar thermal collectors as they offer a ton of benefits.

The trough shape of the collector allows it to capture a larger area of the sun’s radiation than a flat panel. The parabolic shape also focuses the radiation onto a small area, which increases the heat transfer and makes better use of solar energy.

The glass reflector around the trough helps to direct more sunlight onto the absorber and also protects the absorber from the elements.

Parabolic Trough Solar Collector Design

Parabolic trough solar collectors consist of a curved mirror that reflects sunlight onto a tube filled with fluid.

The fluid is heated by the sun and then used to heat water or another fluid to produce steam. This steam can then be used to generate electricity or heat buildings.

Absorber/Receiver

The receiver is the component-oriented at the center of the solar reflector dish to reflect and concentrate sun radiations. The size of the receiver should be optimized to minimize the shadowing effect.

To reduce the mass needed to track the sun, the mass of the receiver should also be optimized. Lastly, the receiver should also be connected to the tracking system to monitor the sun along with the dish.

Parabolic dish reflector

The parabolic dish reflector is the primary design component of a parabolic trough dish collector.

When it comes to the reflector dish, the arrangement is crucial. Primarily, the dishes can be arranged continuously or as distinct elements (mirrors) to assume the shape of a parabolic.

It’s important also that the parabolic dishes are arranged as close as possible to:

  • Reduce collisions and protect the mirrors
  • Protect the collectors
  • Minimize the heat transfer fluid pipe
  • Reduce maintenance
  • Reduce parasitic pumping capacity

The solar reflector dish should be mounted to the dual-axis solar tracker so that it’s always oriented to the sun’s direction.

As mentioned above, the primary role of mirrors in a parabolic trough solar collector is to reflect the sun’s radiation and focus it onto the receiver.

Mirrors consist of reflective layers and protective layers that protect the reflective layers from abrasion and corrosion.

There’re 3 main types of mirrors used in parabolic trough solar collectors:

Silvered glass mirror

The silvering on the back of the mirror helps to reflect more light, making it the best type of mirror to use in a PTSC because it reflects the most light.

Silvered glass mirrors are also less likely to tarnish over time, meaning they will look shiny and new for longer.

Anodized sheet aluminum reflectors

Anodized sheet aluminum is also a good choice for radiation reflection because it’s less expensive than other types of reflectors.

Additionally, anodized sheet aluminum reflectors have relatively high reflectivity, and are strong and durable making them an effective choice for solar thermal applications.

Silvered polymer reflectors

Silvered polymer reflectors use several layers of polymers with an inner layer of pure silver.

Silvered polymer reflectors can reflect up to 95% of the light that hits them, meaning improved efficiency. The other good thing with silvered polymers is that they’re easily and readily available.

Reflector tracking system

A solar tracking system is a device that orients a solar parabolic trough collector toward the sun. This increases the collector’s efficiency by keeping the collector in the sun’s path and exposing it to the maximum amount of sunlight possible.

Solar tracking systems can be either active (motorized) or passive.

Active systems use a motor (electronic signal conversion) to rotate the collector. There are two main types of active trackers: closed-loop and open-loop.

Closed-loop trackers are more common and are basically a set of motors and sensors that keep the mirrors aligned with the sun. Open-loop trackers rely on feedback from the sun to maintain alignment.

Passive systems rely on the sun’s movement (thermosiphon effect) to rotate the collector.

Passive trackers have no motors, gears, or any other moving parts. They rely on the natural direction of the wind to turn the turbine and generate power.

This type of tracker is typically less expensive than an active tracker, but it’s not as efficient for use in parabolic trough solar collectors.

The support structure

The parabolic trough solar collector supporting structure is made up of three parts:

  • The main support
  • The frame
  • The receiver brackets

The main support is the vertical metal bar extending from the collector to the ground. This primary supporting structure is made up of steel or aluminum tubes welded together to form a metal bar. The metal bar is then bolted to the ground.

The frame is the metal grid that the mirrors attach to. They help maintain the rigidity and the parabolic shape of the system. This component also acts as a channel to transmit the torque from the tracking system to the collector system.

The receiver brackets are the metal brackets that attach to the reflector mirrors and hold them in place.

To reduce losses between the receiver located at the center of the parabolic trough and the bracket holding it in place, an insulating material is used between them.

Generally, the supporting structure houses the parabolic collector components, the tracking system, and the motor that powers it. It helps in the alignment of the solar reflector with the sun.

For these reasons, it must be strong enough to hold all the components of the system irrespective of the weather conditions.

Working fluid/ working gas

The working gas of a parabolic trough solar collector is the fluid that flows through the collector and absorbs the heat from the sun.

The working gas in the tube is heated by the sun and flows to a heat exchanger, where it transfers its heat to water, which is then used to generate power.

The most common working fluids for parabolic trough solar collectors are water and air, but other fluids such as helium can also be used.

The main advantage of using water as the working fluid is that it’s a renewable resource. The disadvantage is that water can evaporate, which can cause the system to lose efficiency. Air has the advantage of being non-volatile, but it is less efficient than water because it can only absorb a limited amount of heat.

Helium is an excellent working fluid for parabolic trough solar collectors, provided the operating temperature is 700K and below.

Heat engine

The heat engine is an external combustion engine that heats and cools a working gas sealed in a cylinder. It converts thermal energy to kinetic energy.

A heat engine can also be operated independently by using the heat transfer fluid from the receiver. The most common types of heat engines for parabolic trough solar collector systems are the Stirling engine and the Brayton cycle engine.

How does a Parabolic Trough Solar Collector Convert Sun Power to Electrical Energy? (The Working Principle)

The mirrors of a parabolic trough solar dish focus the solar radiation onto a receiver mounted onto the central location of the parabolic dish along with a heat engine. The heat engine contains pipes that carry a working gas.

When the thermal energy is transmitted to the receiver, the working gas inside the heat engine pipes expands, driving pistons that in turn move a crankshaft that drives an electricity-producing generator.

The energy conversion that occurs in a parabolic trough solar collector are as follows:

Solar energy →Thermal energy →Kinetic energy →Electrical energy

Applications of Parabolic Trough Solar Collectors

Concentrating PVs

A parabolic trough solar collector can be used as a concentrating photovoltaic (PV) system.

In this type of system, concentrated sunlight is used to generate electrical power. The collector consists of a parabolic reflector that focuses the sun’s energy onto a small area. This focused energy is then used to generate electrical power using PV cells.

The curved surface of a parabolic trough collector is used to collect and focus sunlight onto a small area of PV cells. This increases the amount of power that the PV cells generate and can increase the system’s efficiency by up to 50%.

Water Desalination

The use of parabolic trough solar collectors for water desalination has been gaining attention in recent years due to its potential to provide a low-cost and environmentally friendly desalination process.

Parabolic trough solar collectors are also very efficient at converting sunlight into heat.

This makes them an excellent choice for water desalination, as the heat can be used to evaporate the water. What’s more, PTSCs can withstand extreme weather conditions, making them a good choice for locations that experience a lot of sun or wind.

Water Decontamination

Parabolic trough solar collectors can also be used for water decontamination. The high temperature of the water produced by the collector can be used to kill bacteria and other microorganisms in the water. This makes the water safe to drink or use for other purposes.

Solar water and air heating

The use of parabolic concentrated solar thermal for water and air heating is becoming more common. In this type of solar thermal collector, a curved mirror reflects sunlight onto a receiver tube located at the mirror’s focal point.

The concentrated sunlight heats the fluid inside the tube to a very high temperature. The thermal energy is then tapped, allowing it to be used in water or air heating.

Solar drying

Parabolic concentrated solar drying is a process that uses concentrated solar energy from the system to dry food and other products. The process can be used to dry food products, agricultural products, solid wastes, and other materials.

Concentrated solar drying offers a number of advantages over traditional drying methods. It’s a very efficient way to dry food and other products, and it does not produce any harmful emissions.

Solar air conditioning

One potential advantage of PTSC over other types of solar thermal collectors is that it can achieve higher temperatures. This makes it suitable for applications such as air conditioning, which requires a high-temperature source of energy.

By using the sun to generate cool air, these systems can reduce energy costs by up to 50%.

The Concentration Factor of Parabolic Trough Solar Collectors

The concentration factor is the ratio between the collection area and the receiver area of a radiation collector.

It’s a dimensionless geometric quantity used to calculate a radiation receiver’s efficiency. The concentration factor is also known as the collection efficiency or the capture ratio.

Parabolic trough collectors have a medium concentration ratio between 10 and 100. This means that they can achieve a concentration of energy output higher than linear Fresnel collectors but lower than the dish Stirling system.

Is the Parabolic Trough Solar Collector an Active or Inactive Technology?

Solar technologies can be classified into either active or active. Passive solar technologies are generally less expensive to install, but they also work less efficiently. They include window insulation, thermal mass, and solar shading.

Active solar technologies are more expensive to install, but they are more efficient and can provide more power.

They require external components such as pumps and a fluid system to convert sun energy into thermal or non-thermal energy. Solar thermal collectors, including parabolic trough collectors, fall under this category.

Some of the external components used in a parabolic trough solar collector that make it an active solar technology include a solar tracking system and fluid transport system.

Parabolic Trough Solar Collectors Manufacturing Errors

A few errors may occur in the collector during manufacturing and normal operation. PTSC errors are a threat to the concentration ratio and, consequently, the optical efficiency.

Shape error

The geometric shape of a solar trough collector is parabolic. The sun’s rays are focused at the focal line of the parabolic trough.

The error in the shape of the focal line will cause an error in the shape of the solar collector. This, in turn, will cause an error in the amount of energy collected by the solar collector.

Slope error

The accuracy of a parabolic collector is dependent on the smoothness of the mirror surface. Any ripples or bumps in the mirror will cause a deviation in the rays, resulting in an inaccurate energy collection. This is known as slope error.

Receiver deviation error

The receiver deviation error is a measure of the misalignment between the receiver and the focal line.

This misalignment can be caused by a number of factors, including the movement of the receiver or the target, atmospheric conditions, and reflections.

Specularity error

Specularity error is an optical error caused by light reflection from a mirror. The error is due to the imperfection of the reflection and results in a distortion of the radiations.

The magnitude of the specularity error depends on the surface roughness of the mirror and can be minimized by polishing the mirror surface.

Tracking deviation

Solar tracking error occurs when the collector is not perfectly pointed at the sun. This can happen for several reasons, including incorrect installation or alignment, misalignment of the reflectors, or shading from nearby objects.

As a result of this misalignment, the solar energy system will not be as efficient as expected.

Frame deformation

The collector’s frame should be subjected to a normal loading at all times. However, if the load is essentially by self-weight, torsional loads, and wind exceeds the limit, it may deform it.

The parabolic trough solar collector should be designed to resist this deformation.

Durability and maintenance of Parabolic Trough Solar Collectors

Parabolic trough solar collectors are the most efficient type of solar collector. They are also one of the most expensive to install and maintain.

Durability is a crucial concern with parabolic trough solar collectors. The curved mirror surface is susceptible to damage from hail, wind, and debris. The collector must therefore be able to withstand high winds and extreme weather conditions.

Parabolic trough solar collectors’ maintenance and cleaning practices are essential to ensure the system is running at peak performance.

Dust, dirt, and other particulates will slowly build up on the mirror surface over time. This will cause the mirror to become less efficient at capturing and reflecting sunlight. It’s important to clean the mirror surface regularly to prevent this from happening.

In addition, the optical elements and tracking system must be kept clean and free of debris.

The reflector must be appropriately aligned with the receiver to achieve the maximum solar energy conversion. Any misalignment will cause a decline in the efficiency of the system.

Pros of Parabolic Trough Solar Collectors

Ecologically friendly with low emissions

PTSCs have low emissions to the environment during the system’s life cycle. This makes them a more environmentally friendly option than other types of solar collectors.

Easy to manufacture and maintain

Parabolic trough solar collectors are the most common type of solar thermal collectors.

They have a simple design and are therefore relatively low-cost to manufacture. They also have lower maintenance and operating costs than other types of solar collectors.

Solar energy is one of the most promising sources of renewable energy. In particular, parabolic trough solar collectors have a long lifespan and are very reliable.

Long lifespan

One of the main benefits of using parabolic trough solar collectors is their long lifespan.

These collectors can last for up to 25 years, which is significantly longer than other types of solar collectors. That’s not to mention that they’re also very reliable and require very little maintenance in the long run.

Cons of Parabolic Trough Solar Collectors

Sun tracking is a must

The angle of the sun affects the output of solar collectors. This is particularly relevant to solar collectors that use mirrors, such as parabolic troughs.

If the sun’s angle changes too much, then the output of the solar collector will be reduced. This is because the mirror will not be able to reflect sunlight onto the receiver at the correct angle.

Sun tracking must be used to maintain the optimum angle of the sun.

The cost of electricity is high

Although parabolic troughs are one of the cheapest CSP technologies, the cost of electricity from parabolic trough solar collectors is still twice as expensive as electricity from conventional sources.

Construction and installation costs can be high

The cost of construction and installation of a parabolic trough collector can be high. The cost of the reflector material, receiver, support structure, and the labor required to construct the reflector can add significantly to the system’s total cost.

The good thing is, the cost of a solar trough system can be offset by the savings on energy costs.

How Hot Can a Parabolic Trough Solar Collectors get?

When the sun radiations fall onto the receiver, the thermal energy is transmitted, making the component very hot. Generally, the temperature can fall in the range of 750°C-1000°C.

Does the Government Support Parabolic Trough Solar Collectors?

Concentrating solar power is a clean and renewable energy source that has the potential to provide significant amounts of energy for the United States.

The DOE is committed to supporting the development of CSP technologies, and it has continuously funded projects intending to subsidize the cost of electricity from parabolic troughs.

Summary

The parabolic trough solar collector is the most common type of solar thermal collector. It has been in use since the 1880s.

A PTC system consists of a parabolic reflector that concentrates sunlight onto a receiver tube. The tube contains a fluid that is heated by the sun. The heated fluid is then used to generate heat or electricity.

A primary advantage of PTC systems is their maturity. A number of PTC systems have been in operation for many years, and the technology has been refined over time. This leads to a high degree of reliability and low maintenance costs.

The use of parabolic trough solar collectors is growing rapidly worldwide as the technology becomes more efficient and affordable. As solar energy becomes more prevalent, parabolic trough solar collectors are likely to continue to grow.