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  • Concentrating the power of 2000 suns

Photonics Spectra
Jul 2013
A new photovoltaic system could harness the power of 2000 suns and convert a whopping 80 percent of incoming light into useful energy – providing sunny, remote locations with electricity, fresh water and cooler air at lower prices than conventional devices allow.

Just 2 percent of the solar energy from the Sahara Desert could meet the world’s electricity needs, according to a study from the European Solar Thermal Electricity Association and Greenpeace International. But current solar technologies are too expensive and slow to produce; they also require rare-earth minerals and lack the efficiency to make such massive installations practical.

Rendering by Airlight Energy of the prototype high-concentration photovoltaic thermal system under development by an international collaboration of researchers. The prototype system uses a large parabolic dish attached to a tracking system that determines the best angle based on the position of the sun. The entire receiver combines hundreds of chips and provides 25 kW of electrical power.

In Switzerland, scientists at IBM Research and ETH Zurich, both of Zurich; Airlight Energy of Biasca; and Interstate University of Applied Sciences Buchs NTB of St. Gallen have developed a lower-cost solution to harness the power of 2000 suns; funding will be under a three-year, $2.4 million grant from the Swiss Commission for Technology and Innovation. The high-concentration photovoltaic thermal (HCPVT) system features an inexpensive design and achieves a cost-per-aperture area of less than $250 per square meter – three times lower than comparable systems – and a levelized cost of energy of less than 10 cents per kilowatt-hour.

The prototype HCPVT system uses a large parabolic dish composed of many mirror facets attached to a sun-tracking system, which positions the dish at the best angle to capture the sun’s rays. The rays reflect off the mirrors onto several microchannel liquid-cooled receivers with triple-junction photovoltaic chips – each 1 x 1-cm chip can convert between 200 and 250 W, on average, over a typical 8-hour day in sunny locations.

The prototype system under development. The photovoltaic chips are mounted on microstructured layers that pipe liquid coolants within a few tens of microns off the chip to absorb the heat and draw it away 10 times more effectively than with passive air cooling. T = temperature; p = pressure.

“We plan to use triple-junction photovoltaic cells on a microchannel-cooled module which can directly convert more than 30 percent of collected solar radiation into electrical energy and allow for the efficient recovery of an additional 50 percent waste heat,” said Bruno Michel, manager of advanced thermal packaging at IBM Research. “We believe that we can achieve this with a very practical design that is made of lightweight and high-strength concrete, which is used in bridges, and primary optics composed of inexpensive pneumatic mirrors – it’s frugal innovation, but builds on decades of experience in microtechnology.”

The entire receiver combines hundreds of chips to provide 25 kW of electrical power. The photovoltaic chips are mounted on microstructured layers that pipe liquid coolants within a few tens of microns off the chip to absorb the heat and draw it away 10 times more effectively than with passive air cooling. The coolant maintains the chips at almost the same temperature for a solar concentration of 2000 times and can keep them at safe temperatures up to a solar concentration of 5000 times.

The direct-cooling solution with very small pumping power was inspired by the hierarchical branched blood supply system of the human body and has been tested by IBM scientists in high-performance computers, including Aquasar. An initial demonstrator of the multichip receiver was developed in a previous collaboration between IBM and the Egypt Nanotechnology Research Center.

“The design of the system is elegantly simple,” said Andrea Pedretti, chief technology officer at Airlight Energy. “We replace expensive steel and glass with low-cost concrete and simple pressurized metallized foils. The small high-tech components, in particular the microchannel coolers and the molds, can be manufactured in Switzerland, with the remaining construction and assembly done in the region of the installation. This leads to a win-win situation where the system is cost-competitive and jobs are created in both regions.”

ETH Zurich will develop the solar-concentrating optics. “Advanced ray-tracing numerical techniques will be applied to optimize the design of the optical configuration and reach uniform solar fluxes exceeding 2000 suns at the surface of the photovoltaic cell,” said professor Aldo Steinfeld.

A prototype of the HCPVT system is currently being tested at the IBM Research lab in Zurich. Several prototypes will be built in Biasca and Rüschlikon, Switzerland, as part of this collaboration.

Current concentrating photovoltaic systems collect electrical energy and dissipate thermal energy into the atmosphere. The HCPVT system aims to eliminate the overheating problems of solar chips while also repurposing the energy from thermal water desalination and adsorption cooling.

To capture the medium-grade heat, IBM engineers used an advanced technology developed for water-cooled high-performance computers such as Aquasar and SuperMUC to absorb heat from the processor chips. This heat was repurposed to provide space heating for the facilities.

Instead of heating a building, the system’s 90 °C water will be used to heat salty water, which will be passed through a porous membrane distillation system, where it is vaporized and desalinated. This process could provide 30 to 40 liters of drinkable water per square meter of receiver area per day while generating electricity with a more than 25 percent yield, or 2 kilowatt-hours per day – a little less than half the amount of water the average person needs per day, according to the United Nations. A larger installation could provide enough water for a town.

The system also could provide air conditioning by means of a thermal-driven adsorption chiller. Such devices, with water as working fluid, could replace compression chillers, which stress electrical grids in hot climates and contain working fluids that are harmful to the ozone layer.

The HCPVT solution could provide sustainable energy and potable water in locations around the world; it could also find use in remote tourism locations on small islands, since conventional systems require separate units, with consequent lost efficiency and increased cost.

The prototype is now being tested at IBM Research-Zurich. Additional prototypes will be built in Biasca and Rüschlikon, Switzerland, as part of the collaboration.

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