Experts undecided on BHK’s hybrid CSP plant design
While few would doubt Babcock-Hitachi’s capacity for innovation, its decision to blend linear Fresnel and power tower technology in one plant has perplexed some observers.
By Jason Deign
Babcock-Hitachi’s decision last year to pursue an innovative hybrid CSP plant design has raised eyebrows among industry experts.
Sources contacted by CSP Today have voiced concerns over whether the design, which combines linear Fresnel and power tower technologies in a single plant, will be as effective as the Mitsubishi Hitachi Power Systems (MHPS) subsidiary expects.
"From their website it appears that they are using the linear Fresnel system as a pre-heater for the power tower,” says Dr Thomas Mancini, principal at TRMancini Solar Consulting.
“Since a linear Fresnel system is less efficient than a power tower, its cost would have to be much less than the cost of adding heliostats to the power tower field for this to make sense. Frankly, I don't see where the advantage lies."
Babcock-Hitachi KK (BHK) unveiled the design last September after winning a contract from the Japanese Ministry of the Environment to “develop and verify technologies for enhancing measures to reduce carbon dioxide emissions,” according to a press statement from MHPS.
BHK is due to build a ‘verification facility’ at MHPS’s Yokohama Works in Japan, “with the start of test operations of the new system slated for 2016,” The Asahi Shimbun reported.
The developer said its Hybrid Solar Tower System “utilises a Fresnel-type solar collector through the water evaporation stage, and subsequent saturated steam heating is performed by concentrating sunlight on to a receiver in the small tower's upper section with heliostats.”
The announcement indicates that BHK is proposing to use linear Fresnel as a pre-heater for cost-reduction reasons.
Cost advantage
The technology is generally considered to be the cheapest of mainstream technologies, so in theory using it to make up part of the overall plant could indeed reduce costs. But it is also the most inefficient form of CSP, which could negate any cost advantage.
Although it is not stated in the press release, the hybrid plant will presumably use the Hitz Super Low Profile Fresnel (HSLPF) technology developed by Hitachi Zosen for a demonstration plant completed in 2013 at Al-Jubail, in the Kingdom of Saudi Arabia.
“In contrast to conventional Fresnel reflectors, which have a flat or fixed-curve shape, HSLPF reflectors can be controlled in both angle and curvature so as to follow the sun’s position and collect light efficiently,” announced Hitachi Zosen in a press release.
“This improves the solar concentration ratio from 50 times, a challenge of conventional Fresnel reflectors, to as much as 70 times, equivalent to parabolic troughs.”
Overcoming the traditionally low efficiency of linear Fresnel will be critical if the hybrid design is to succeed. And concerns over linear Fresnel are heightened by the fact that it has yet to achieve the kind of success that parabolic trough and, more recently, power tower have enjoyed.
While some analysts insist it could predominate in the smaller CSP plant segment, the fact remains that no major developers are now backing linear Fresnel.
The last one to do so, Areva, pulled out of the market last year after failing to make a go of the technology it acquired from Ausra of Australia. But it is not only the use of linear Fresnel that has CSP watchers guessing about the BHK design.
“I’d like to know what kind of storage they are considering for verification in the pilot plant,” comments KTH Royal Institute of Technology solar thermal power R&D engineer Rafael Guédez.
Heat storage
The BHK announcement mentions “element verification of the high-temperature heat storage system” but says nothing of what technology would be used. That could be critical to the success of the design, Guédez believes.
“Without the storage, the system would have no competitive advantage over PV,” he says.
Other points in the announcement raise further questions. “They are talking about producing saturated steam, which might not be the optimum for the power cycle,” says Guédez. “After that, though, they mention that the tower will act as a super-heater.
“I don’t know what they are talking about, exactly. Perhaps they are thinking of storing saturated steam, but maybe that wouldn’t be the most efficient, either. It will be interesting to see what their pilot plant ends up as.”
Neither BHK nor MHPS have ever built a commercial CSP plant before, although Mitsubishi, like Hitachi, has at least had some exposure to the industry.
In 2011 Mitsubishi bought a 15% share of Acciona Termosolar, the business controlling Acciona’s four operational CSP plants in Spain, with a total capacity of 200MW.
The move followed a 2009 memorandum of understanding “that sets forth a comprehensive strategic alliance with Acciona to jointly explore the opportunities in the field of CSP, photovoltaic, wind generation and other environment-related areas,” Mitsubishi said.
Mitsubishi has also applied to build a 50MW plant in Maan, Jordan, using an as-yet unannounced technology.
“I think Mitsubishi is trying to explore a lot of opportunities,” says Ilaria Besozzi, business development manager at the Swiss CSP developer Airlight Energy.