Ultra-Efficient and Cheap Solar System Enabled by Microchannel Cooling

The water-shortage problem in many areas can be solved by cheaply desalinating abundant quantities with this technology: The Swiss Commission for Technology and Innovation has awarded Swiss scientists a $2.4 million (2.25 million CHF) grant to develop an affordable photovoltaic system capable of concentrating solar radiation 2,000 times and converting 80 percent of the incoming radiation into useful energy -- yielding a system that will provide desalinated water and cool air in sunny, remote locations where they are often in short supply, on top of the affordable clean electricity.

The prototype HCPVT (High Concentration PhotoVoltaic Thermal) system uses a large parabolic dish, made from many mirror facets, which are attached to a sun-tracking system. At the heart of the system are several novel microchannel-liquid-cooled receivers with triple junction photovoltaic chips. Each chip can convert 200-250 watts, on average, over a typical eight hour day in a sunny region.

The entire receiver combines hundreds of chips and provides 25 kilowatts of electrical power in the current configuration. The photovoltaic chips are mounted on micro-structured layers that pipe liquid coolants within a few tens of micrometers 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 almost at the same temperature for a solar concentration of 2,000 times and can keep them at safe temperatures up to a solar concentration of 5,000 times.

The direct cooling solution with very small pumping power is inspired by the hierarchical branched blood supply system of the human body and has been already tested by IBM scientists in high performance computers, including Aquasar.

"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," said Andre Bernard, head of the MNT Institute at NTB Buchs. "And by using innovative ways to fabricate these heat transfer devices we aim at a cost-efficient production."

"We plan to use triple-junction photovoltaic cells on a micro-channel 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, 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 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 metalized 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. 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 2,000 suns at the surface of the photovoltaic cell," said Aldo Steinfeld, Professor at ETH Zurich.

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, which is 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 cost at coal power stations are around 5-10 cents per KWh.

Water Desalination and Cool Air

Current concentration photovoltaic systems only collect electrical energy and dissipate the thermal energy to the atmosphere. With the HCPVT packaging approach, scientists can both eliminate the overheating problems of solar chips while also repurposing the energy for thermal water desalination and adsorption cooling.

In the HCPVT system, instead of heating a building, the 90 degree Celsius 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 percent yield or two kilowatt hours per day. This is a little less than half the amount of water the average person needs per day according to the United Nations, but 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. An adsorption chiller is a device that converts heat into cooling via a thermal cycle applied to an absorber made from silica gel, for example. Adsorption chillers, with water as working fluid, 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, Arabic peninsula, the southwestern part of the United States, 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.

Funding has been awarded to scientists at IBM Research; Airlight Energy, a supplier of solar power technology; 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 the economical High Concentration PhotoVoltaic Thermal (HCPVT) system.

A prototype of the HCPVT system is currently being tested at IBM Research - Zurich. Additional prototypes will be built in Biasca and Rueschlikon, Switzerland as part of the collaboration.