Smart energy for a cool planet: The opportunities of concentrated solar power
Giovanna Di Sauro
Although some do not believe in global warming in spite of recent storms, tornados, droughts, and the general moody weather globally, it is acknowledged by many experts that global temperatures are bound to rise. According to Concentrated Solar Thermal Power – Now!, a joint report by the European Solar Thermal Industry Association, the European Solar Thermal Industry Association, IEA SolarPACES, and Greenpeace International, temperatures have already risen by 0.6 ºC during the past century (global mean) and even if all greenhouse gas emissions were stopped immediately, we are already facing a temperature increase of about 1.2 ºC.
Many, even when not skeptical about the existence and the factors affecting global warming and climate change, find themselves wondering whether there exists an immediate, economically advantageous, and practical alternative to oil. For years there has been talk about investing in the development of technology targeting renewable energy sources, but in practice there seems to have been little success — or maybe we are just mistaken.
There have been experimental Concentrated Solar Power (CSP) systems that have been tested around the world in the last 15 years. This technology is relatively new, and innovation is being pushed forward rapidly in the field of concentrated solar power. The International Energy Agency endorses an international technology program called SolarPACES, involving 30 countries in the developed and developing world, encouraging research and development, as well as successful marketing of existing CSP technologies.
CSP plants work to convert the energy of the sun into heat, which can then be used to generate electricity through traditional systems such as generators and steam turbines. The heat can also be stored using a variety of media, such as molten salts which can then be used to keep the power plant running at times when direct sunlight is not available. CSP technologies can be scaled up to a current maximum of 200 megawatts or down to provide village power of about 10 kilowatts, according to the amount of energy needed.
This is possible because CSP plants can take different forms and shapes: they can be based on the use of parabolic reflectors that heat an internal pipe, parabolic dishes, or be structured as spectacular power tower plants. Power towers use sun-tracking mirrors called heliostats, which are able to focus sunlight to a receiver located at the top of the tower. The heat can then be used to heat molten salts or air, depending on the plant’s needs and design.
The real breakthrough of CSP, however, is not the fact that it is a practical technology exploiting a renewable energy source that will be inexhaustible for the next six billion years; rather, it is its potential competitiveness with oil and natural gas in a very close future. Regardless of the need to reduce carbon emissions, we live in a world hungry for energy. Together with the developed nations, China and India have now started the hunt for fossil fuels and gas to feed their growing energy needs. In the meantime, the sun blazes the Sahara with the energy equivalent of 1.5 million barrels of oil for every square kilometre. According to German scientists Dr. Gerhard Knies and Dr. Franz Trieb, covering only 0.5 per cent of hot deserts with CSP plants would provide enough energy for the entire world, as well as help produce desalinated water. At the moment, electricity coming from oil costs about 8.09 cents per kilowatt/hour (kWh) in the U.S. Given the pace of technological advances in the CSP field, and the leveling of production costs due to the scale-up of production of CSP-related technologies, the cost of CSP-produced solar energy is expected to fall to about seven cents per kWh in the medium term and eventually to a mere five cents per kWh.
CSP plants are already operating in some of the hottest and sunniest parts of the U.S. In Boulder City, Nevada, Solargenix installed a 350-acre thermal power plant that is expected to be providing enough energy to cover the needs of about 40,000 households in 2007. The panels, developed by SCHOTT, an international technology group, have an anti-reflective glass coating, new steel coating in the absorber tubes, and show a series of improvements that will allow this plant to successfully absorb and convert 96 per cent of the direct sunlight it receives into usable energy. Similar projects are being developed in California, where some of the earliest CSP U.S. experimental plants were built.
Not only the U.S., but Europe, North Africa, and the Middle East could also harvest the potential of their coastal territories and deserts if political leaders started to consider the creation of a unified, high-voltage electricity grid to allow the transport of energy produced there to the rest of the region. Although this would result in some energy losses due to electrical conversion and transport (about three per cent), the whole region could provide, in an environmentally friendly way, for all its energy needs by 2050.
CSP truly is the cheapest and the largest bulk producer of solar electricity in the world, and there already exist initial agreements and global market initiatives that are working as venues for communication, the development of new technologies, and the support of future implementations. The technologies are available, but policies must be implemented, nationally and internationally, such that the potential of this technology can be exploited for the benefit of all.
A way of doing this would be to impose nationally and internationally binding targets for renewable electricity. This has already been planned or put into practice in some states. In California, for example, on August 21, 2006, Governor Schwarzenegger signed a bill that will make the state the third-largest producer of solar power after Japan and Germany. This bill will, among other things, require homebuilders to provide solar power to buyers by 2011, which will allow homeowners to sell excess electricity back to the grid.
Public-private partnerships are also going to have a significant role. Although many of the technologies will be implemented by private companies, economical and legislative government support will be necessary to allow CSP to be truly competitive with other energy sources such as oil and gas. However, the fate of CSP and other future energy sources is ultimately in the hands of the implementers, especially national governments, international organisations, and economic communities, and of their people.
Shapes and forms of concentrating solar power
(an excerpt from SolarPACES.org)
Parabolic trough systems
The sun’s energy is concentrated by parabolically curved, trough-shaped reflectors onto a receiver pipe running along the inside of a curved surface. This energy heats oil flowing through the pipe and the heat energy is then used to generate electricity in a conventional steam generator. A collector field comprises of many troughs in parallel rows aligned on a north-south axis. This configuration enables the single-axis troughs to track the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes. Individual trough systems currently can generate about 80 megawatts of electricity. Trough designs can incorporate thermal storage — setting aside the heat transfer fluid in its hot phase — allowing for electricity generation several hours into the evening. Currently, all parabolic trough plants are “hybrids,” meaning they use fossil fuel to supplement the solar output during periods of low solar radiation. Typically a natural gas-fired heat or a gas steam boiler/reheater is used; troughs also can be integrated with existing coal-fired plants.
Power tower systems
A power tower converts sunshine into clean electricity for the world’s electricity grids. The technology utilises many large, sun-tracking mirrors (heliostats) to focus sunlight on a receiver at the top of a tower. A heat transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine-generator to produce electricity. Early power towers (such as the Solar One plant) utilised steam as the heat transfer fluid. Current U.S. designs (including Solar Two) utilise molten nitrate salt because of its superior heat transfer and energy storage capabilities. Current European designs use air as a heat transfer medium because of its high temperature and its good handability. Individual commercial plants will be sized to produce anywhere from 50 to 200 MW of electricity.
Parabolic dish systems
Parabolic dish systems consist of a parabolic-shaped point focus concentrator in the form of a dish that reflects solar radiation onto a receiver mounted at the focal point. These concentrators are mounted on a structure with a two-axis tracking system to follow the sun. The collected heat is typically utilised directly by a heat engine mounted on the receiver moving with the dish structure. Stirling and Brayton cycle engines are currently favoured for power conversion. Projects of modular systems have been realised with total capacities up to five MWe [megawatts of electrical output]. The modules have maximum sizes of 50 kWe [kilowatts of electrical output] and have achieved peak efficiencies up to 30 per cent net.