Parabolic dishes re-enter the fray
1st August 2018
Parabolic dish designers claim their revamped designs can generate solar electricity competitively, unlike their predecessors.
By Zara Maung
Once dismissed as commercially unviable, parabolic dishes are starting to show signs of reemergence in the CSP industry. Designers of a new breed of parabolic CSP dishes claim to have addressed the inefficiencies of previous models and created dishes with the potential to generate cost effective utility scale electricity.
Whereas one such company, Thalis, has updated the dish’s original method of generating electricity – the Stirling engine – others are using CSP dishes to produce steam, which can be used to drive a centralized turbine.
Steam v. Stirling
Dish engineers ZED Solar’s dish thermal system operates as a Direct Steam Generation system, with distributed steam generation at each dish in the solar field. The steam generated at each dish is piped into a molten salt heat storage. Heat can then be extracted as required to drive a steam turbine and generate electric power on demand.
“With a high sun concentration ratio of over 2000 sun, each dish can produce steam at the required steam conditions ranging up to 700 ºC and up to 130 bar pressure,” claims Zaafir Waheed, CEO of ZED Solar.
“Our dish thermal systems achieve an ultra-high net solar to thermal power efficiency of 88%. That means that 88% of the total incoming solar energy exits the solar park as heat energy,” Waheed says.
Another CSP dish contender is Sunrise CSP’s Big Dish, claiming to be the world’s largest solar concentrating dish, with highly accurate optics, 2-axis tracking and highest-efficiency conversion of sunlight to thermal energy.
Sunrise CSP’s dishes are modular, coming in 400kW thermal units. The dish and receiver technology produce super-heated steam for industrial applications such as electricity generation and high-temperature process heat. In 2017, Sunrise CSP and the Australian National University (ANU) concluded an exclusive world-wide license agreement for the ANU’s new generation super-heated steam receiver.
Figure 2. Sunrise CSP bets on the Big Dish
“We can deliver a working fluid, like steam or air, at any temperature through to more than 1,700 oC,” says Artur Zawadski, CEO of Sunrise CSP.
The Thalis designed CSP dish system uses a Stirling Engine 3kW system, that is currently upgraded to 7 kW, and with up to 12 hours Thermal Energy Storage System. The company claims its free piston Stirling engines are distinct from traditional kinematic Stirling engines that have inherent life and reliability limitations imposed by their lubricated mechanical system and sliding seals.
Projects in the pipeline
The new parabolic dishes are currently at the pilot stage. ZED Solar has set up a pilot project in Dubai for dish thermal systems, to demonstrate that they are suitable for power generation as well as for process heat for a range of applications.
Meanwhile, Sunrise CSP are currently in the power market in China, India and elsewhere, looking at small scale tri-generation systems (power and heat for multiple uses needing one to many dishes) through to 100MW+ grid-connected electricity generation solutions.
Thalis is the technology provider for two CSP projects to be implemented in 2018 – funded by the EU’s NER300 program – comprising a 110MW project in Greece and a 50MW in Cyprus, with a total budget of €485 million.
Although CSP dishes have generated some interest in the electricity industry, solar experts hold reservations about the feasibility of the technology at utility scale.
Chuck Andraka of Sandia National Labs asserts, “Dishes for low-temperature heat (several hundred degrees) do not make sense. You are spending considerable money to bring the energy to a point (create exergy), and then you essentially disperse that energy (destroy exergy) to heat a working fluid moderately (instead of heating a smaller amount of material to a very high temperature.”
“Dishes cost more but give you more in terms of concentration ratio. This concentration is effectively used for very high temperatures (over 700 ºC). Lower temperatures do not justify the concentration costs.”
Eduardo Zarza Moya, technical coordinator at CIEMAT-Plataforma Solar de Almería, also questions the use of steam generation. “The transport of high pressure steam has some significant technical constraints, including the O&M problems associated with the swivel joints required to allow the thermal expansions and the movement of the dish concentrator, the pressure drop in the piping connecting each dish to the common power block and thermal losses in the piping, thus limiting the final overall efficiency.”
“What these companies are promising is similar to what previous companies have unsuccessfully offered in the past and therefore I need to see a commercial system running under real environmental conditions for at least five years. For me this would be the real proof of concept.”
ZED Solar remains determined to prove its technology, however, claiming that there are no more than 5% heat losses from piping in its entire solar park.
“Swivel joints are used in parabolic trough systems as well and also need to allow expansion and movement, the pressures are the same,” says Waheed.
“We have a dedicated process piping team in the company who have developed and tested our piping systems over a period of 4 years. Combined with this we have also conducted a multi-year effort on shading studies and land optimization, which has allowed us to tremendously shrink the size of our solar field, thereby drastically reducing the total piping length and associated losses.”
He says the dishes’ cavity type receivers convert concentrated solar energy into heat energy to the working fluid with an efficiency of around 97%.
“No one in industry or academia has previously done anything close to the holistic approach we have taken to dish parks.”