Parabolic trough collectors are solar energy machines that use curved mirrors to concentrate sunlight onto a long receiver pipe. They matter because they can collect much more heat than a flat surface of the same width. This heat can be used to make steam, spin a turbine, and generate electricity.
Large solar fields of troughs are used in sunny regions where direct sunlight is strong and reliable.
The mirror has a parabolic cross section, which means incoming sunlight parallel to the mirror axis reflects toward a focal line. A dark receiver pipe sits along that focal line and carries a heat transfer fluid such as synthetic oil or molten salt. The hot fluid flows to a heat exchanger, where water is boiled into steam for a generator or stored for later use.
Tracking motors slowly rotate the trough during the day so the sunlight keeps focusing on the pipe.
Key Facts
- A parabolic trough focuses parallel sunlight onto a focal line, not a single point.
- Mirror law of reflection: angle of incidence = angle of reflection.
- For a parabola y^2 = 4fx, the focal length is f.
- Thermal power collected can be estimated by P = ηIA, where η is efficiency, I is solar irradiance, and A is mirror aperture area.
- Useful heat added to the fluid is Q = mcΔT.
- Parabolic trough plants usually need direct normal irradiance because clouds and haze reduce focused sunlight.
Vocabulary
- Parabolic trough
- A long curved mirror shaped like a parabola that concentrates sunlight onto a receiver pipe.
- Focal line
- The straight line where parallel incoming sunlight is concentrated after reflecting from a trough mirror.
- Receiver pipe
- A pipe located at the focal line that absorbs concentrated solar energy and heats a flowing fluid.
- Heat transfer fluid
- A liquid or gas that carries thermal energy from the receiver pipe to a heat exchanger or storage system.
- Solar tracking
- The controlled movement of a solar collector so it stays aligned with the Sun during the day.
Common Mistakes to Avoid
- Placing the receiver pipe anywhere above the mirror is wrong because only the focal line receives the strongest concentration of reflected sunlight.
- Treating all sunlight as equally useful is wrong because parabolic troughs mainly use direct sunlight, while diffuse light from cloudy skies cannot be sharply focused.
- Ignoring heat losses is wrong because the receiver pipe loses energy by radiation, convection, and conduction, reducing the useful output.
- Confusing temperature with energy is wrong because a high temperature does not by itself tell how much heat is transferred without mass, specific heat, and temperature change.
Practice Questions
- 1 A parabolic trough has a mirror aperture area of 120 m^2, receives solar irradiance of 850 W/m^2, and has an efficiency of 45%. Estimate the useful thermal power using P = ηIA.
- 2 A heat transfer fluid with mass 300 kg and specific heat 2200 J/(kg·°C) is heated from 180°C to 300°C. How much thermal energy is added using Q = mcΔT?
- 3 Explain why a parabolic trough collector must rotate during the day to keep heating the receiver pipe effectively.