Researchers have announced a collaboration to develop an affordable photovoltaic (PV) system capable of concentrating solar radiation 2000 times and converting 80% of the incoming radiation into useful energy. The system can also provide desalinated water and cool air in sunny, remote locations where they are often in short supply.
A three-year $2.4m (CHF2.25m) grant from the Swiss Commission for Technology and Innovation has been awarded to scientists at IBM Research; solar power technology supplier Airlight Energy of Biasca, Switzerland; ETH Zurich (Professorship of Renewable Energy Carriers) and Interstate University of Applied Sciences Buchs NTB (Institute for Micro- and Nanotechnology MNT) to research and develop an economical high-concentration photovoltaic thermal (HCPVT) system.
Based on a study (‘Concentrating Solar Power: Outlook 2009’ published by Greenpeace International, SolarPACES and the European Solar Thermal Electricity Association, technically it would only take 2% of the solar energy from the Sahara Desert to supply the world’s electricity needs. Unfortunately, existing solar technologies on the market are too expensive and slow to produce, require rare-Earth minerals and lack the efficiency to make such massive installations practical, say the researchers.
The prototype HCPVT system uses a large parabolic dish, made from a multitude of mirror facets, which are attached to a sun tracking system. The tracking system positions the dish at the best angle to capture the sun's rays, which then reflect off the mirrors onto several micro-channel liquid-cooled receivers covered with 1cm x 1cm triple-junction photovoltaic chips, each of which can convert 200-250W, on average, over a typical eight hour day in a sunny region.
The entire receiver combines hundreds of chips and provides 25kW of electrical power. The chips are mounted on micro-structured 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 5000x.
The direct cooling solution (with very low pumping power) is inspired by the hierarchical branched blood supply system of the human body and has already been tested by IBM scientists in high-performance computers, including Aquasar. An initial demonstrator of the multi-chip receiver was developed in a previous collaboration between IBM and the Egypt Nanotechnology Research Center.
Graphic: Schematic of HCPVT system.
“We plan to use triple-junction photovoltaic cells on a micro-channel-cooled module which can directly convert more than 30% of collected solar radiation into electrical energy and allow for the efficient recovery of an additional 50% waste heat,” says Bruno Michel, manager, advanced thermal packaging at IBM Research. “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,” he believes.
“We replace expensive steel and glass with low-cost concrete and simple pressurized metalized foils,” says Airlight Energy’s chief technology officer Andrea Pedretti. “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,” he adds. “The system is cost competitive and jobs are created in both regions.”
The solar concentrating optics will be developed by ETH Zurich. “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,” says professor Aldo Steinfeld.
With such a high concentration and a radically low-cost design, scientists believe they can achieve a cost per aperture area below $250 per square meter (three times lower than comparable systems). The levelized cost of energy will be less than 10 cents per kilowatt hour (kWh). For comparison, feed-in-tariffs for electrical energy in Germany are currently still larger than 25 cents per kWh, and production costs at coal power stations are 5-10 cents per kWh.
Water desalination and cool air
Current concentration photovoltaic (CPV) systems only collect electrical energy and dissipate the thermal energy to the atmosphere. With the HCPVT packaging approach, researchers can both eliminate the overheating problems of solar chips while also repurposing the energy for thermal water desalination and adsorption cooling.
To capture the medium-grade heat, IBM scientists and engineers are using technology they developed for water-cooled high-performance computers, including Aquasar and SuperMUC. With both computers, water is used to absorb heat from the processor chips, which is then used to provide space heating for the facilities.
“Microtechnology, as known from computer chip manufacturing, is crucial to enable such an efficient thermal transfer from the photovoltaic chip over to the cooling liquid,” says Andre Bernard, head of the MNT Institute at NTB Buchs. “By using innovative ways to fabricate these heat transfer devices, we aim at a cost-efficient production,” he adds.
In the HCPVT system, instead of heating a building, the 90°C water will be used to heat salty water that then passes through a porous membrane distillation system where it is vaporized and desalinated. Such a system could provide 30-40 liters of drinkable water per square meter of receiver area per day, while still generating electricity with a more than 25% yield (2kWh per day) - a little less than half the amount of water the average person needs per day according to the United Nations. A large installation could provide enough water for a town.
The HCPVT system can also provide air conditioning by means of a thermal driven adsorption chiller (a device that converts heat into cooling via a thermal cycle applied to an absorber made from silica gel, for example). Using water as the working fluid, adsorption chillers can replace compression chillers, which stress electrical grids in hot climates and contain working fluids that are harmful to the ozone layer.
Scientists envision the HCPVT system providing sustainable energy and potable water to locations around the world including southern Europe, Africa, the Arabian peninsula, the southwestern part of the USA, South America, and Australia. Remote tourism locations are also an interesting market, particularly resorts on small islands, such as the Maldives, Seychelles and Mauritius, since conventional systems require separate units, with consequent loss in efficiency and increased cost.
