Dish Stirling: Made for MENA?
With no need for wet cooling and exceptionally high efficiency rates and concentration ratios, could
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By Heba Hashem in Dubai
CSP sites typically consume large amounts of water for cooling, up to 500 million gallons a year, or about 800 gallons per megawatt hour. Using air instead of water for the cooling process, as in the case of Dish Stirling technologies, can reduce water consumption by as much as 97%, according to The European Water Platform.
For the Middle East and North Africa (MENA), where water is becoming as valuable as oil, and where direct normal irradiance is favourably high, the Dish Stirling technology presents a viable option, whether it is to power the region’s hundreds of desalination plants, or to be integrated within upcoming solar parks, such as Dubai’s 1GW solar project that will deploy both PV and CSP systems.
Unlike standard CSP plants that are based on wet cooling systems, the Dish Stirling uses dry cooling in most constructions, enabling electrical supply in arid regions. Dish Stirling compares favourably to parabolic trough technology.
A wet-cooled parabolic trough consumes 1,000 gallons of water per MWh, and has a peak solar efficiency of about 21.5% and concentration ratios of between 50 and 90. By contrast, Dish Stirling uses only 4.4 gallons of water per MWh and has demonstrated a world-record solar-to-electric conversion efficiency of 31.25% with concentration ratios of nearly 3,000 – the highest of all CSP systems.
Despite the clear advantages of Dish Stirling technologies, parabolic troughs are currently the most commercialized, accounting for 94% of installed CSP plant capacity worldwide.
“Dish Stirling technology has not reached yet the maturity to be installed at a commercial level and exists only at a demonstrative scale for the moment. Some Dish Stirling plants are planned in the U.S., but no existing plant is registered so far in dry desert regions”, Elena Dufour, EU Energy Policy and Research Engineer at the European Solar Thermal Electricity Association tells CSP Today.
Desert installations
In 2010, the world’s first pilot Stirling CSP project – the Maricopa plant – was commissioned by the beleaguered Tessera Solar and Stirling Energy Systems (which filed for bankruptcy last September), near Phoenix in Arizona. Like the Middle East, Arizona is well-known for its hot summers, harsh desert climate and potential water shortage.
Using 60 of SunCatcher’s dishes, the 1.5MW CSP plant does not use water for electricity generation or cooling cycles, and uses minimal water for site operations and washing its mirrors. This would be an ideal cooling method for Middle East-based CSP projects.
Still in the game is Italian CSP technology developer Innova, which recently launched Trinum; the first thermodynamic CSP system equipped with a small size Stirling free piston engine that can generate electric energy, hot water and cold air.
“The version of Trinum that is currently marketed generates both electricity and thermal energy which are redirected to the grid. Innova is working on a stand-alone version that would be able to dissipate heat automatically and allow installations in remote, hostile environments like deserts”, says Francesco Guidetti, CEO of Innova Technology Solutions.
“We are currently exploring options within Gulf countries. At the same time, we need to increase production capacity as demand is much higher than what we expected”, Guidetti adds.
Scaling up deployment
Australia’s Wizard Power has also had a “number of discussions related to entry into the Middle Eastern market”, having developed the world’s largest paraboloidal dish solar concentrator – the 500m2 Big Dish – which can be fitted with Stirling engine receivers, among other receiver technologies.
“Dish Stirling technology comprises single units and is therefore not appropriate to be used for large power plants. They would be nevertheless suitable for a modular deployment”, states Dufour. Wizard Power’s technology, however, has been designed for large-scale, usually grid-integrated developments.
Lisa Robey, the company’s marketing manager, explains: “A typical steam-based Big Dish power plant consists of a large field of Big Dishes connected to a central power block via an insulated piping network. Therefore, energy loss in transmission of feed water and superheated steam in the piping network is minor, making very large solar fields of thousands of dishes feasible, enabling single plants of gigawatt capacity.
Wizard Power’s Big Dish power plants are generally no smaller than 50MW and reach the best economies of scale when they are over 100MW. As a result, our technology is designed for large-scale, usually grid integrated developments”.
Hybridisation challenges
Generally, dish or engine systems use heat engines, and thus have an inherent hybrid capability – the ability to operate on either solar energy or fossil fuels, or both. For Dish Stirling systems, however, the addition of a hybrid capability is a challenge. According to the U.S. Department of Energy’s Solar Dish Engine Report, the external, high temperature, isothermal heat addition required for Stirling engines is in many ways easier to integrate with solar heat than it is with combustion heat.
Geometrical constraints make simultaneous integration even more difficult. As a result, “costs for Stirling hybrid capability are expected to be on the order of an additional $250/kW in large-scale production. These costs are less than the addition of a separate diesel generator set, for a small village application, or a gas turbine for a large utility application”, the DOE report states.
To simplify the integration of the two heat input sources, the first Science Applications International Corporation (SAIC) and Sterling Thermal Motors (STM) hybrid Dish Stirling systems will operate on solar or gas, but not both at the same time.
Although the cost of these systems is expected to be much less than a continuously variable hybrid receiver, their operational flexibility will be substantially reduced. System efficiency, based on higher heating value, is expected to be about 33% for a Dish Stirling system operating in the hybrid mode.
Financial and technical constraints
Dish Stirling technology continues to exhibit cost disadvantages; its levelized cost of electricity is higher than other CSP options. However, U.S. developers are confident that these restrictions can be overcome, via mass production and by building thousands of single installations in large areas, with capacities ranging from 800 - 1 000MW.
Technical weaknesses also exist, such as heat-to-electricity conversions that require moving parts, which result in maintenance needs. For such conversions, a centralised approach would be better than the decentralised concept. According to the International Energy Agency’s SolarPACES, reliability improvement of Dish Stirling engines is a main thrust of ongoing work, where deployment and testing of multiple systems enables faster progress.
Dish Stirling systems have traditionally targeted high-value remote power markets, but the industry is increasingly interested in pursuing larger, grid-connected markets. In 2010, Sun & Wind Energy estimated there were about 2MW of operational dish/Stirling capacity worldwide and 1MW under construction. It also estimated 2,247MW to be in planning. This suggests that rapid development once technology breakthroughs and cost reductions are achieved is imminent.
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