A heliostat field is a large array of mirrors that track the Sun and reflect sunlight toward a receiver on top of a central tower. It matters because it concentrates a wide area of solar energy into one small target, creating temperatures high enough to produce steam or heat a thermal storage material. This makes solar energy useful not only when sunlight is bright, but also when heat can be stored for later electricity generation.
Heliostat fields are a major technology in concentrated solar power plants.
Key Facts
- Power from sunlight on one mirror is P = I A, where I is solar irradiance and A is mirror area.
- Reflected useful power is approximately P_useful = I A η, where η includes mirror reflectivity, tracking accuracy, and optical losses.
- A heliostat mirror must aim so its surface normal bisects the angle between the Sun direction and the receiver direction.
- Concentration ratio is C = A_field / A_receiver, comparing total mirror area to receiver area.
- Thermal energy stored is Q = m c ΔT, where m is mass, c is specific heat, and ΔT is temperature change.
- Electrical output depends on efficiency: P_electric = η_total P_solar_collected.
Vocabulary
- Heliostat
- A heliostat is a sun-tracking mirror that reflects sunlight toward a fixed target such as a tower receiver.
- Receiver
- A receiver is the part of a solar tower that absorbs concentrated sunlight and converts it into thermal energy.
- Solar irradiance
- Solar irradiance is the power of sunlight arriving per square meter, usually measured in watts per square meter.
- Concentrated solar power
- Concentrated solar power is a method of using mirrors or lenses to focus sunlight and produce heat for electricity generation.
- Thermal storage
- Thermal storage is the process of saving energy as heat, often in molten salt, so it can be used later.
Common Mistakes to Avoid
- Pointing each mirror directly at the tower is wrong because the mirror must be angled to reflect incoming sunlight into the receiver using the law of reflection.
- Ignoring mirror losses is wrong because real mirrors do not reflect 100 percent of sunlight and tracking errors, dust, and atmospheric absorption reduce delivered power.
- Treating the Sun as fixed during the day is wrong because heliostats must continuously rotate in two axes to keep the reflected beam on the receiver.
- Assuming a larger field always increases efficiency is wrong because mirrors farther from the tower can suffer greater cosine losses, blocking, shading, and atmospheric losses.
Practice Questions
- 1 A heliostat has a mirror area of 25 m² and sunlight irradiance is 900 W/m². If the optical efficiency is 0.72, how much useful power reaches the receiver from this mirror?
- 2 A heliostat field has 6000 mirrors, each with area 20 m². If the average useful power delivered per square meter is 600 W/m², what total thermal power reaches the receiver?
- 3 Explain why a heliostat mirror is not simply aimed straight at the receiver, and describe how the Sun direction affects the mirror angle during the day.