7.3.1: Solar Towers Molten Salt Heat Storing Technology

In the solar tower CSP technology, all sunlight is focused on a single bulk absorber. An alternative method is to use linear absorbers in the form of a long pipes running over a light-reflecting troughs. The geometry of such system is depicted in the Fig. \(\PageIndex{1}\).

From introductory physics courses one should remember that a spherical concave mirror (one that you get by “cutting a slice of a spherical shell” does not have a perfect focal point. If one needs a mirror that really focuses light rays into a single point, one has to use a parabolic mirror. For them who do not remember that material too well, a short video will surely help to refresh their memories.

Figure \(\PageIndex{1}\): Left: The geometry of a parabolic trough system, with linear absorbers. Right: A cross- section of a trough, with the absorber running along the “focal line” of the parabolic reflector (source: Wikipedia).

There is a similar story with trough-shaped mirrors. To make a simple trough, one may take a thin-walled pipe and cut a “slice” out of it (but not by cutting in the direction perpendicular to the pipe axis – then one gets a ring! – the cutting must be parallel to the pipe’s axis. From such a operation, one gets a “cylindrical trough”. Which is not good, because a concave mirror made of a cylindrical trough suffers the same problem as a spherical mirror: its focus is not perfect. In order to get a trough mirror focusing all incident rays along a perfect line, the trough profile must not be a circle, but a parabola. Fortunately, it’s not a big deal to make such a trough

– in YouTube one can find many video-instruction how to make a parabolic trough reflector in one’s garage (for instance, this one – it’s combined with a good “theoretical background”). People use backyard parabolic trough mirrors for a variety of purposes – e.g., for “solar cooking”, water heating, or even for running miniature steam turbines to make their own electricity.

In real parabolic trough power plants the absorbers have the form of two concentric pipes. The outer pipe is made of glass. The inner pipe is the actual absorber. There is a small gap between the outer and the inner pipe – it’s evacuated so that the absorber tube is thermally insulated by the vacuum layer. Inside the absorber tube a heat-collecting fluid is circulated

Figure \(\PageIndex{2}\): The structure of the absorber tube (a.k.a. the “receiver”). The outer glass tube (1) and the “vacuum jacket” (2) in between prevents contact of the absorber tube with air. If there were not such shielding and the absorber tube had a direct contact with surrounding air, most of the heat deposited to the absorber would be “captured” by air molecules and taken away by convection. There is no way of completely eliminating losses of heat from the absorbers, but the vacuum insulation keeps them at a reasonably low level. Inside the glass tube there is the actual absorber – a metal tube (3) with black outer surface. A cooling liquid ( 4 – e.g., silicon oil) circulates in the absorber tube and next passes through a “steam generator” where it transmits the heat it has collected to steam which then drives a turbine (aop).

Individual troughs can be combined to make large arrays, covering many acres. Such power plants may have the same heat-storing capability as the tower-type Crescent Dunes plant discussed in the preceding Section.

Figure \(\PageIndex{3}\): A scheme of a tower-type solar power plant with molten-salt energy storing capability. 1 – the tower, 2 – the receiver, 3 – pumps, 4 – steam generator (i.e., a heat exchanger), 5 – exhaust steam condenser with cooling water runing through it, 6 – steam turbine, and 7 – a generator of “green” electric power (aop).

Figure \(\PageIndex{4}\): Parabolic troughs may be oriented parallel to the north-south direction – as the sun moves across the sky, the trough symmetry plane’s must remain parallel to the impinging rays, so that the trough inclination varies accordingly (the situation illustrated by the image). Another possible trough orientation is parallel to the east-west direction – then the inclination of the trough’s symmetry plane is set to the angle at which sunlight impinges at noon at the given day – and it remains constant constant throughout the day (aop).

When Sun moves across the sky, in tower-type CSP plants each heliostat has to be individually moved. In the trough-type plants the troughs also have to be moved, but the motion is much simpler. If the long axis of the trough is perfectly aligned with the north-south direction, the only motion needed to always keep the reflected sun rays precisely focused on the absorbers is a “rocking motion”, as shown in Fig. \(\PageIndex{4}\). It requires a simpler mechanism than that needed in a heliostat for following the Sun position in the sky. It is also a great convenience that all troughs in a solar field must assume the very same position with respect to Sun – so that a single electronic device may control the motion of all troughs. Controlling the motion of ten thousand heliostats in the solar field of a tower-type CSP plant, where each heliostat has to be individually aligned, must be much more difficult a task! The list of the largest CSP plants in the world can be found in this Web page. As can be seen, in the US the parabolic trough technology is the dominant one, with 1350 MW nameplate capacity installed, whereas the combined power of the tower-type facilities is 517 MW. The proportion may change, though, if the announced plans of building a “mammoth” 1600 MW Sandstone Solar Energy Project are implemented. So, it’s difficult to say about the two technologies, which one is “better”. Probably, as is often the case, the competition between the two technologies will be resolved by market economy mechanisms.