Credit: SolarGIS © 2015 GeoModel Solar

Nexus Renewables plans to launch a pre-commercial prototype with mirror surface 4,046m2 at the end of May, targeting the industrial and localized micro-grid markets, Friesth said.

There are a number of potentially significant CSP markets, such as India, Israel or Italy, where space is at a premium.
Plans seen by CSP Today reveal a unique linear parabolic reflector design which Friesth says will be combined with a Stirling engine-based generation block and a range of thermal storage options.

The company’s ‘compact linear parabolic reflector’ system features two absorbers per row of troughs. These, along with two curved reflectors above them, maximise the output of the system by trapping heat usually lost by primary absorber reflection, convection and overshoot.

“Trough systems have attempted to reduce their convection losses with a glass-encased absorber system...yet amplified thermal input is only transmitted to 60-70% of the absorber surface as there is no reflector on top,” Nexus says in its documentation.

This design means the system needs only two-thirds of the area of a comparable-sized parabolic trough system, or half the land required by a two-axis PV array, to achieve the same output, according to the company.

The solar field will be connected to a scalable Stirling engine made by Ricardo, a high-performance manufacturer, and will use a range of heat transfer fluids, depending on the specific CSP application required.

“Stirling engines in a back-to-back combined cycle configuration will reduce convection and flow resistance losses and allow simple build methods,” documentation from Nexus says.

The developer proposes using molten salt for high-temperature applications (up to 585ºC), such as Combined Heat and Power or storable and dispatchable energy generation.

Corn oil would be used for temperatures of up to 230ºC and applications such as thermal augmentation and Integrated Solar Combined Cycle plants (ISCCs).

Source: Nexus, 2015

Low-temperature applications

Finally, for low-temperature applications such as cold storage, quick freezing and refrigeration the heat transfer fluid would be water. While Nexus says the technology could be scaled to grid-level, the more obvious initial applications are in distributed power and process heat.

“With the thermal storage capable of 5 MWh+ of storage on a residential system and 2.5 KWh of Stirling engine generated power, it can be a microgrid,” explained Friesth.
The technology could be used by independent power producers on a commercial and utility scale but might also provide peaker power capability.
“With a central control network the technology can form the basis for true demand response and distributed energy on demand, based on storage,” Friesth said.

Following a trial of the 4,046m2 demonstration model, Nexus will produce in the second quarter of this year a commercial demonstrator boasting of 12,000m2 solar field and 1.8 MWth. Full commercial launch is planned in 2016.

Finn Hansen, a former managing director of wind turbine manufacturer Vestas, is acting as chief technology officer for Nexus.
The firm cites the US Department of Energy and Sandia National Labs among its lenders.

Some CSP firms have succeeded in establishing themselves with niche applications for industrial process heat, such as GlassPoint, a solar steam generator maker in the oil recovery space, and Rackam, which manufactures a parabolic solar concentrator.
However, a number of smaller start-ups are still finding it hard to make an impact in the industrial and distributed grid markets.

Distributed power

CEWA Technologies, for example, is marketing a point concentrator dish design for distributed power. Dish technologies are a favoured format for distributed power applications because they can be deployed on a modular basis.

However, reliability problems have dogged manufacturers that have pursued this design, particularly when they have tried to use Stirling engines. Infinia, Stirling Energy Systems and Wizard Power all designed dish systems that used Stirling engines. All ended up filing for bankruptcy.

Today only a handful of developers, such as Innova in Italy and Ripasso in Sweden, continue to favor Stirling technology.

Other CSP suppliers have focused on linear models closer to the Nexus solar field concept.
Focused Sun, for example, uses linear Fresnel mirrors with the addition of PV cells in the absorber to maximise efficiency.
“On the solar concentrator, we reduce costs by using sandwich fabrication,” Shawn Buckley, founder of Focused Sun, said.

Meanwhile Protarget of Germany and Global CSP are both marketing modular variants of the well-established parabolic trough approach to solar collectors.

Commercial interest in these products has thus far been small and Dr Eduardo Zarza Moya, of Spain’s Almeria Solar Platform questioned the level of efficiency that can be achieved through Nexus’ compact linear parabolic reflector system.

“I get the impression that the system might have a low optical performance because in the morning and afternoon it will have a high incident angle, which will reduce the amount of direct solar radiation it can take advantage of.”

In response Friesth said each compact linear reflector row has its own mount axis offset to allow greater tracking accuracy.
“Using compact linear designs, incorporating the second absorber row, allows greater reflector focus and reduced shadow over a standard linear reflector system,” he said.