# How the solar tower works?

The solar tower is another way to use sunlight as heat source to produce steam and spin a turbine. Can produce electricity even during night.

Click in button below to know the operation of other thermal solar plants.

### Control system

Many flat mirrors in ground called heliostats reflect sunlight and concentrate light in a solar tower’s receiver.

Heliostats have servomotors or stepper motors to be positioned in right angle. Reflecting the maximum of sunlight for entire day.

Motors are controlled by microcontrollers or computers. To calculate Sun’s position, must know elevation $\theta_{el}$, zenith $\theta_{Z}$ and azimuth $\theta_{A}$ angles.

This is the equation to calculate elevation angle.

$sin (\theta_{el})=sin\delta\cdot sin\phi+cos\delta\cdot cos\omega\cdot cos\phi$

$\theta_{el}=arcsin(sin\delta\cdot sin\phi+cos\delta\cdot cos\omega\cdot cos\phi)$

• $\delta$ is declination angle, formed by Equator’s plane and direction of Sun’s ray.

Equation to calculate this angle, n is the day of year, goes from 1 to 365.

$\delta =23,45\frac{\pi}{180}sin[2\pi(\frac{284+n}{36,25})]$

• $\phi$ is latitude angle.
• $\omega$ is hour angle, formed by Sun’s ray and meridian of local time. It is negative from six until 11:59 and becomes positive at afternoon, varies 15º for each hour.

The hour angle is calculated as shown below. Where $t$ is the local time in hours.

$\omega =15(t-12)$

Knowing the coordinates of solar tower, positions of Sun and heliostat, are calculated the angles heliostats must have to direct light to receiver all day. Cameras and light sensors also are used to know Sun’s position.

### Producing electric energy

Inside receiver, there is a heat exchanger, where occurs change of physical state from liquid to gas to turn a turbine. Liquid pumped up is water, liquid sodium or molten salt, whose composition is 60% sodium nitrate ($NaNO_{3}$) and 40% potassium nitrate ($KNO_{3}$). In receiver, temperature is between 500 and 1000ºC.

How this power plant can produce electric energy without Sun? If the chosen fluid is molten salt, can be stored in tanks with atmospheric pressure, keeping heat to keep turbine working in Sun’s absence. Salt is used to produce steam to turn the turbine.

Converting from Fahrenheit $T_{F}$ to Celsius $T_{C}$, hot salt is at 565,5ºC and cooler one at 287,7ºC. Relation between two measures is:

$T_{F}=\frac{9}{5}T_{C}+32$