Projected images of the Solar Hourglass. Image: Land Art Generator Initiative.

By Heba Hashem

October was an eventful month for Santiago Muros Cortés. The Argentina-based architect impressed the jury in Copenhagen with his plan for a Solar Hourglass that would use a solar beam instead of sand as a trickling material. The announcement was also an honour to the CSP industry, given that the design was selected out of 300 entries proposing a variety of renewable energy technologies.

As the first-place winner, Cortés received a prize award of US$15,000 from LAGI, an initiative that encourages artists, architects, scientists, and engineers to design public art installations that double up as renewable energy generators.

"Unlike any other power plants out there, the Solar Hourglass was born to become an inseparable component of the urban landscape,' says Cortés. "It's only through social and cultural integration that we can incorporate sustainable power-generating infrastructure into our everyday lives, thus making the public aware and appreciative of this process."

Although the competition awards do not guarantee a commission for construction, the idea is certainly not precluded. In fact, LAGI treats all submissions as a portfolio they can present to worldwide investors interested in creating sustainable statement.

"CSP benefits greatly from economies of scale, and the Solar Hourglass is at the small end of the spectrum of solar power tower-type CSP plants. Still, there are some other working examples of CSP at these scales and we do believe that it would be very feasible," Elizabeth Monoian, cofounder of LAGI, told CSP Today.

Beam-down technology

Relying on the beam down tower technology that has been prototyped and tested at Masdar Institute of Science and Technology, the Solar Hourglass is envisaged as a solar central receiver comprising small flat mirrors that concentrate solar power on a storage tank.

In the design, these mirrors (heliostats) are arranged parabolically on top of the upper bulb of the hourglass and reflect the solar heat onto a cone-shaped set of smaller mirrors, which concentrate the reflections and shoot them down the neck of the installation.

"Parabolic heliostats would increase the effectiveness by concentrating the first reflection to a smaller point and enabling the consistent vertical beam that is the focal point of the piece," explains Elizabeth Monoian.

The concentrated beam of solar heat then reaches a receiver, which is coated with a special absorber surface that maximizes the transfer of heat to the heat transfer fluid (HTF) inside the receiver. Consisting of molten nitrate salt, the HTF is heated to temperatures over 600 ºC, and is later transferred to a heat exchanger where water is turned into steam that runs a turbine generator.

Because a small percentage of the produced steam is released back to the neck of the hourglass, the solar beam becomes visible to the public. That way, the structure would send an optimistic message to those who visit it – that there's still time to make things right environmentally – while the 1,960sqm parabolic set of heliostats would concentrate enough heat to produce 6.2 MW. The big question now is – how easy would it be to build such a project?

What the experts think

According to Steven Meyers, researcher at the University of Kassel in the Institute of Thermal Energy Engineering, with a special focus on the solar heat integration into industrial processes, the Solar Hourglass is an interesting new development for the Beam Down technology, and CSP in general.

"On aesthetics alone, it looks quite smooth and sleek, certainly not what is normally associated with power plants, and would serve as an interesting landmark for solar thermal energy."

As a former research engineer at Masdar's Laboratory for Energy and Nano Science (LENS), Meyers was directly involved with the thermal and optical modeling of the beam down solar thermal plant at Masdar. When looking at the technical aspects of Cortés's design, however, he identifies several concerning topics, which his team experienced while working on the Beam Down plant at Masdar, and which are likely to be encountered with the Solar Hourglass.

"The first is the complicated optics of multiple mirror reflections, which becomes increasingly difficult to control with every surface interaction. When this is coupled to the entire mirror arrangement being suspended in the air by wires, this may further increase the difficulty of concentrating the solar radiation, since the optics are subject to wind loads".

Second, and linked to the first, is the issue of stray light and safety. "In one of the pictures, the hourglass design depicts visitors coming within a couple meters of the final 'light beam' before it enters the receiver". This, Meyers stresses, will not be possible as it would pose a serious safety hazard.

"The concentrated light will be so bright that people even standing on the perimeter will have a difficult time seeing the center focal point. Also, the amount of thermal energy coming from this beam will be felt many meters from the center, unsafe for people to come close to it."

During the operation of the Beam Down at Masdar, which was at best a 100 kW system, one twentieth of the size of the proposed project, Meyers explains that they had to wear strong sunglasses around the central beam; always staying 2-3 meters away. While both are not impossible problems to solve, they will require engineering development to decrease the risk.

Potentially feasible

Commenting on the feasibility of constructing the power plant, Meyers says it would depend on what the objectives are. "If the goal is to raise awareness of CSP and solar energy as a whole, this may be interesting concept to explore."

It would be difficult, however, to realize the project in its current form, he suggests. "The biggest hurdle is the sheer size of the upper half of the hourglass, which contains all of the heliostats and secondary concentrator."

"At this size, the solar equipment alone will weigh more than 60,000 kg, assuming a conservative 30kg/m2, excluding the support structure, which will also be quite heavy. Then, all of this must be connected to 4 ground mounted posts, by thick steel cables."

Indeed, from a technological viewpoint, there will be levels of sophisticated engineering that must be dealt with due to the creative design. As for the proposed thermal energy storage (TES) mechanism, it seems fairly plausible, according to Meyers, as molten salt is a well-tested and proven TES medium; hence, there would be no issues apart from the fact that people would be in close proximity to the system.

Geographically, the project would naturally perform best in the countries already well known for CSP technology, such as the US Southwest, Spain, Chile, South Africa, Australia, and certain parts of the MENA region.

But due to the unique shape of the collector field, which will have higher than normal optical losses at lower elevation angles, Meyers points out that the greatest annual performance would be witnessed in projects closest to the equator.

"Something smaller scale with more technical thought could potentially be realized. It would certainly be an interesting piece of artwork installed in a park or a public space, and for this, a smaller plant would be more feasible. I do applaud the designers for a good job creating an innovative design".