Based on a study by the by the European Solar Thermal Electricity Association and Greenpeace International (Concentrating Solar Power: Outlook 2009), it would only take 2% of the solar energy from the Sahara Desert to supply the world’s electricity needs. Unfortunately, current solar technologies are too expensive and slow to produce — and lack the efficiency — to make such massive installations practical.
Current concentrated photovoltaic systems only collect electrical energy and dissipate the thermal energy to the atmosphere. Photovoltaic thermal concentrators (Figure 1) generate electrical energy with an efficiency upwards of 25%, while simultaneously generating reusable heat for heating, water desalination or adsorption cooling with an efficiency of about 50%. The overall efficiency of such systems is, therefore, higher than 75%. Unfortunately, these devices are expensive, and the materials are not available in sufficient amount for use in flat panels.
Scientists at the IBM Research-Zurich lab and the IBM Egypt Nanotechnology Center, however, may have found a solution in a high concentration photovoltaic thermal (HCPVT) system. The system is capable of concentrating solar radiation at 2,000 times and converting 80% of the incoming radiation into usable energy. The scientists have built a prototype HCPVT system that 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 on to several microchannel liquid-cooled receivers with triple-junction photovoltaic chips. Each 1-cm by 1-cm chip can convert 50W on average over a typical eight-hour day in a sunny region.
The system is made affordable by replacing 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 use of 2000-fold concentrated solar radiation reduces the number of necessary cells further but causes a power density above 150 W/cm². This power density parallels that of highperformance processors, so that similar advanced cooling solutions as are needed.
Water Desalination And Cool Air
The entire receiver combines hundreds of chips and provides 25 kW of electrical power. The photovoltaic chips are mounted on microstructured 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 direct-cooling solution, with small pumping power, is inspired by the hierarchical branched blood-supply system of the human body. 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. Hot-water-cooled silicon microchannel coolers (Figure 2) have extremely low thermal resistance, which allows cooling 100°C photovoltaic cells with almost equally hot water (90°C).
Instead of throwing away the heat, it’s used for processes such as absorption cooling and desalination. The water heats salt water that then passes through a porous membrane distillation system, where it is vaporized and desalinated.
So the microchip receiver (MCR) provides electrical energy for the grid as well as hot water for running an adsorption cooler or a membrane distillation (MD) setup. Apart from the reflection losses of primary mirrors (10 to 15%), little energy is lost.
Benefits
With such a high concentration and a 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 (LCOE) will be less than 10 cents per kWh. For comparison, feed-in-tariffs for electrical energy in Germany are currently more than 25 cents per KWh and production cost at coal power stations are around 5 to 10 cents per KWh.
The HCPVT system can also provide air conditioning by means of a thermaldriven adsorption chiller. An adsorption chiller is a device that converts heat into cooling through 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. The use of active water cooling enables the realization of larger receivers and higher solar concentrations, which means lower energy costs and higher electrical efficiencies.
Scientists envision the HCPVT system providing sustainable energy and potable water to locations around the world. Remote tourism locations are also an interesting market, particularly resorts on small islands since conventional systems require separate units, with consequent loss in efficiency and increased cost.
Such systems could provide 30 to 40 liters of drinkable water per square meter of receiver area per day, while still generating electricity with a more than 25% yield or 2 kW hours per day — a little less than half the amount of water the average person needs per day. But large installation could provide enough water for entire towns.
Challenges
A drawback of PV systems is that they produce electricity only when the sun shines. However, storing low-grade heat is 100 to 1,000 times cheaper than storage of electrical energy and allows a predictable output of the thermal product (cooling, water, and electricity), which increases its relative value. The wide distribution and use of electrical energy only started after the introduction alternating current transformers because the latter enabled a low-loss conversion from transmission current to usable current. This led to the replacement of heat for transporting energy. Heat can be transformed into electricity, but so far there are no transformers from heat to heat. Therefore, we need a better adsorption heat pump (heat transformer) technology. This is being developed in a parallel project [1]. Moreover, there is no standardized heat-transport grid comparable to the electricity grid. This technology is evaluated economically in the context of solar power plants and compared with electricity transport [2]. Once these devices are available, heat transport fits seamlessly into an overall energy capturing, storage, transport and use system much better than an electrical grid alone.
The Next Step
The next step planned is the development of a faceted mirror that is 100 m² in size and a 500 cm² large receiver with state-of-the-art system efficiency. Scaling such HCPVT systems to terawatt sizes is not limited by the availability of materials [3]: the primary materials used are concrete, steel and glass. Semiconductors or rare earths consumption and life-cycle issues are reduced by the product of concentration and efficiency factor (approximately 5000-fold). Moreover, existing clean and mass-production-compatible industrial processes from the computer, construction and automotive industries can be used for building HCPVT systems. Because of the high cost pressure, a central issue in the solar industry is the availability of inexpensive mirrors. New methods to build large mirrors with high-concentration factors in a cost-efficient way are currently being evaluated to achieve grid parity in hot and dry climates.
HCPVT systems with adjustable electrical output deliver electrical energy on demand despite irradiation fluctuations and store excess energy thermally so that it is available at night, which increases the return of solar power plants and permits stable grids even with a large share of solar power plants. SPW
References
[1] Al-Mogbel, A., et al., “A Simulation Study for Solar Adsorption Air-Conditioning in Saudi Arabia,” Proc. Saudi HVAC Confex Conf. Feb. 11 – 13, 2013.
[2] Garcia-Heller, V. et al., “Economics of heat re-use in solar power stations,” EUPVSEC, 2013.
[3] Depuyt, B., et al., “Scalability of CPV towards Multi- Gigawatt Deployment,” CPV8, AIP 1477, 376–382 (2012).
By: Bruno Michel, Stephan Paredes, Chin Lee Ong, Werner Escher, Rami Ghannam, Gilles Maag and Aldo Steinfeld
SPW Contributors from IBM Research
The IBM Research team includes Michel, Paredes, Ong, Escher and Ghannam. The ETH Zürich Institute of Energy Technology team includes Maag and Steinfeld.
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