*2.6. Solar Thermal Systems*

The Linear Fresnel Reflector (LFR) technology is based on an array of linear mirror strips that concentrate the solar rays coming from the sun onto a linear fixed receiver. The advantage of this technology is the construction of large absorber tubes. Secondary reflectors are mounted above the receiver to reflect the solar irradiation that does not impact directly on it from the primary reflectors. This fact can considerably increase the optical efficiency of the system.

The concentrated radiation reaching the receiver tube heats the liquid circulating through it and converts the solar radiation into useful heat. Water or thermal oil is used as the working fluid. Furthermore, a selective coating layer is applied to the absorber tube to improve solar absorption. LFRs can effectively generate heat at temperatures between 50–400 °C. [40].

Collectors are manufactured by bending a reflective sheet material into a parabolic shape. In operation, the receiver tube moves at the same time as the parabolic collector due to a solar-tracking system. Besides, its surface is covered with a selective coating with high absorbance for solar radiation but low emittance for thermal radiation losses.

Parabolic Trough Collectors (PTC) are widely used in process heat application in the industry and it is also the most mature technology for Heliostat Field Collectors to generate heat for processes or solar thermal electricity generation. The PTCs reach high temperatures up to 400 °C with better efficiency than LFRs [40]. To improve efficiency of the process, a thermal oil with good properties is used instead of water as the working medium. A glass cover tube around the receiver tube reduces the convective heat loss from the receiver. Besides, the space between the glass cover and the receiver tube can be evacuated to achieve lower convective heat losses. However, small transmittance losses occur when the reflected light pass through the glass to reach the absorber.

The operating costs are determined as follows, depending on size and investment [41]:


Both solar thermal systems require a steam accumulator and provide steam for the baseload.
