We hear a lot and talk a lot these days about solar energy. But we rarely stop to consider the fact that what we call solar energy actually consists of two types of energy: heat and light. There are different types of systems for collecting and converting each type into useful forms: photovoltaics (PV), which convert the photons directly into electricity, and solar thermal systems to capture the heat for various uses. Is it possible to combine both?
That’s exactly what a new system developed by IBM in partnership with Airlight Energy, ETH Zurich, and Interstate University of Applied Sciences Buchs NTV will do. The group won a $2.4 million grant from the Swiss Commission for Technology and Innovation to develop a low-cost, high-concentration photovoltaic thermal (HCPVT) system. Contributor Bruno Michel of IBM’s Zurich Research Lab, may have been inspired by work he did previously on zero-emission data centers, which proposes using hot water rather than air to cool the servers.
The benefit of doing that is that the hot water could then be used to heat the buildings. Indeed, the technology being used here is similar to what was used in IBM’s water-cooled supercomputers. As we shall see shortly, that heat can also be used to cool the buildings and provide fresh water.
The system is set up much like a concentrating solar power (CSP) system, with parabolic reflectors that increase the intensity of the sun’s rays by a factor of 2,000. But instead of superheating a fluid that is then used to drive a turbine, as CSP systems generally do, this system uses hundreds of triple-junction PV cells set in liquid-cooled micro-channel receivers to produce electricity directly.
This type of system is known as concentrating solar photovoltaic or CPV. But the micro-structured channels used here are the key to the system’s impressive efficiency. They are configured only microns away from the PV chips, thereby providing cooling that is 10 times more effective than traditional passive air cooling.
This arrangement can maintain the chips at a steady operating temperature over a range of input conditions from the nominal 2,000 to 5,000 times focusing gain in solar intensity. By optimally maintaining the temperature of the PV cells, their efficiency will reach 25 percent. When combined with the further utilization of the heated cooling water, this yields a remarkably high system efficiency of 80 to 85 percent.
The system will be sized to produce 25 kW of electricity. Given the intense rays of sunlight being focused onto it, the cooling liquid, which maintains the cells at a stable 100 degrees C, must remove a lot of heat — enough heat, in fact, to power a desalination process at essentially no extra charge.
The researchers are hoping that a system like this, which they claim can produce 2 kWh and 30 to 40 liters of drinking water per square meter per day, will be ideal for places like the Middle East and Africa. They believe these systems can be built primarily from low cost materials and that their “elegant simplicity” will make them a good fit for developing regions. The cost is estimated to be one-third that of comparable systems. The system not only provides electricity, drinking water, and cooling to those that desperately need all three, but can also bring jobs as well, with all but a few high-tech components being produced locally.
To put this in perspective, a study by the European Solar Thermal Electricity Association and Greenpeace International found that an area consisting of 2 percent of the Sahara Desert can produce enough electricity to meet the needs of the entire world.
Desalination is widely used in areas like Saudi Arabia, where there is lots of energy and salt water but little fresh water. Until recently, the predominant approach was reverse osmosis, because it uses less energy and is therefore less expensive than thermal desalination. But now that renewables are entering the picture, thermal desalination, which essentially distills the water by heating then cooling it, is becoming cost-competitive.
Saudi Arabia’s Saline Water Conversion Corporation (SWCC), which is responsible for 18 percent of the world’s desalinated water output, has pledged to eventually convert all of its plants to run on solar power. The Al-Khafji solar desalination project is expected to begin production this year with an output of 30,000 cubic meters per day, enough for 100,000 people.
The IBM system will provide cooling by means of an absorption chiller, an alternative form of refrigeration that uses heat, rather than mechanical energy, to pressurize the working fluid, which, in this case is water, rather than Freon or other fluorocarbons.
Absorption chillers are well-suited to solar applications since they produce the most cooling when the sun is shining, which is when they are needed the most.
CHP is not new. It is being used increasingly in many large scale applications to squeeze more useful energy from sources as diverse as coal, natural gas, and biomass. CHP systems often have twice the thermal efficiency of the underlying electric generation process.
The idea of using solar as the basis for a CHP system may have first been introduced by Cogenra Solar, which installed a 75 kW solar CHP system in 2011 at a Hydroponics facility in Santa Rosa California. The system produces 15 kW of electricity and 60 kW of thermal energy from a rooftop installation of 36 photovoltaic-thermal (PVT) receivers. By taking advantage of heat that would otherwise be wasted, Cogenra, which was founded three years ago with funding from Khosla Ventures, is using as much as 75 percent of the energy from the sun rather than the 15 to 20 percent that conventional PV systems use.
Cogenra is now breaking into the international solar cooling market as well, working with Johnson Controls and their York absorption chillers. These systems also use PV panels on a single-axis tracker that concentrates the sun’s energy by a factor of 10. A water chamber collects the heat, which is then piped into the chiller. According to Cogenra’s website, “Using high temperatures in a cooling system may be counter-intuitive, but that’s the way absorption chilling and chillers work. In contrast to conventional mechanical chillers, absorption chillers use heat to evaporate fluid contained in a low pressure environment. That circulating cooler fluid then captures heat from its surroundings, be it another stream of flowing liquid or the air in the open space of a room, thereby cooling it.”
Unlike the IBM system, which uses water as the working fluid for its low cost and simplicity, Cogenra, which primarily serves industrial and commercial customers, uses a more conventional refrigerant, lithium bromide. The systems use industry-standard crystalline silicon PV cells, which provide 15 percent electrical conversion efficiency. These cells are then embedded in an all-black piping system where a mixture of anti-freeze and distilled water is circulated, carrying the heat off to the absorption chiller where it is used to evaporate the coolant. Overall, the CPV-cooling combination achieves a thermal efficiency in the 60 percent range.
Cogenra is now also developing an energy storage solution using a grant from the California Energy Commission. The proposed system will be based on hot water that can be stored in tanks and then run through low-temperature turbines at night or during times of cloud cover. Such a system will squeeze even more energy from the sunshine that falls freely on the planet every day.