The solar thermal energy heating and cooling systems, discussed in a previous post, operate at low temperatures. Therefore, they are only suitable to power and provide cooling at a low scale. The high-temperature mechanism to collect solar energy for power generation can utilize solar sterling dishes (a type of parabolic solar thermal energy collector, but the mirror is made in the shape of a dish) or solar towers. The major components of solar towers are shown in Fig. 1, and explained in the following section.
Major components of the solar towers
There are six major components of the solar towers, which are mentioned below in sequential order.
Solar radiation collectors-Mirrors/reflectors
The solar energy from the Sun is is reflected to the receiver by the means of mirrors or solar reflectors. These reflectors are designed in such a way as to give maximum possible reflection without absorption and transmission. These reflectors have solar tracking enabled and they reflect the maximum solar radiation to the receiver.
The solar tower is an elevated structure, which has working fluid (usually molten salt) in its top portion. The radiation from reflectors is concentrated on the top part of the solar tower where the working fluid is present. This concentrated radiation melts the salt which then transfers the heat energy to the water turning it into vapor. Since the tower operates on molten fluid, the solar tower can generate electricity even when low solar radiation is available, if the salt is in molten state. The solar towers also provide power after sunsets, which is one of the main advantages of this technology.
The steam tank stores the generated steam from the solar tower before sending it to the turbine.
The turbine rotates as the steam is passed to this unit from the steam tank. The steam from the turbine is then passed to condenser unit.
The condenser unit turns the steam from turbines back into liquid state, which is again passed to solar tower.
The generator unit is present just after the turbine unit, and due to the rotation of the turbine, it generates electricity. This generated electricity can then be transferred to the grid or it can be utilized elsewhere.
Efficiency calculation of solar towers
The efficiency of the solar tower is linked to four efficiencies. These four efficiencies are optical efficiency, receiver efficiency, mechanical efficiency, and efficiency of the generator. The optical efficiency (ηo) is the fraction of solar radiation that is converging on the receiver. The receiver efficiency (ηr) is the fraction of converged solar radiation that is turned into heat energy. The mechanical efficiency (ηm) is the efficiency of the Carnot engine formed by the solar tower. Lastly, the generator efficiency (ηg) is the efficiency of the generator unit (which can combine the efficiency of turbine and generator). In the mathematical expression, the efficiency of the solar tower (ηST)can be written as Eq. 1.
where Sa is the absorbed solar flux, Sl is the lost solar flux, and Si is the incident solar flux. The ηc is the Carnot engine efficiency, which is the maximum efficiency that can be achieved by a solar tower system. The Ts and Tr are the temperatures of the heat sink and receiver, respectively.
The Si is equal to the multiplication factor of solar flux S (which is 1000 W/m2), Ns (solar concentration), and A (collection area). The term absorbed solar flux (Sa) is equal to the multiplication factor of optical efficiency, absorptivity (α), and Si. The Sl is equal to the multiplication factor of emissivity (ε), collection area (A), Boltzmann constant (σ), and Tr raised to the power of 4. For an ideal case the absorptivity, emissivity, optical efficiency can all be taken as unit (1), then the Eq. 1 can be rewritten as Eq. 4, given below.