by John Whitney AIA
For some time now, the intermittent nature of both solar and wind renewable energy generation systems has been a thorn in the side of the clean energy industry. Coal, natural gas, and nuclear advocates have trumpeted the fact that generation capacity factors for their power plants range from 85% to 90% while solar and wind numbers are in the 20% to 35% range. Over and over we get hammered with the question: What happens when the sun goes down and the wind stops blowing?
Well, as renewable energy supporters have been correctly responding, the simple two-part reply is: Wind turbines continue to generate electricity at night and peak electricity demand kicks in during the day when the sun is shining. Furthermore, because renewables are a relatively small portion of their power generation fleets, utilities have been able to effectively incorporate their intermittent generation into the mix. Even as renewable generation contributions continue to increase it can be argued that a smarter grid, with fine-grained response capability, will be able to effectively absorb and integrate intermittent power sources.
There is no doubt, however, that intermittency issues will need to be dealt with more completely as the renewable share of power generation increases. Photovoltaics (PV) are currently forced to depend on batteries and the ability of the grid to absorb their intermittent generation. But, battery storage technology is still developing, inefficient, and costly, and while eventual development of a comprehensive smart grid will provide the tools to manage much of the problem, the capacity of our current grid has very real limits.
The good news on this horizon comes from recent developments in thermosolar concentrated solar power (CSP). We can now expect effective CSP capacity factors to be increased to the 75% range with technologies that are current and cost effective. In 2010, Italian utility Enel brought on line a 5 MW thermosolar CSP demonstration facility that integrates the two most promising 24/7 baseloading technologies: Molten salt energy storage (MSES) and combined-cycle natural gas power generation. The Enel Archimede plant can operate on stored solar thermal energy alone for several hours. When the thermal reservoir is exhausted a natural gas combined-cycle generator kicks in as needed to meet the relatively light late night and early morning power demand. The result is a reliable baseload 24/7 power plant with around-the-clock generation capability.
Molten salt energy storage (MSES) is often described as ‘solar salt’ batteries. Unlike lithium-ion chemistry-based batteries, MSES is an incredibly efficient and cost-effective thermal energy storage system. Particularly well suited for thermosolar CSP power plants, MSES retains over 90% of captured heat for up to 24 hours, allowing generation of electricity around the clock in an elegant, economical, and environmentally benign manner.
Several stand-alone MSES CSP power plants have been designed and built recently by the Sener Group. These facilities go a long way to solving the solar capacity problem. Sener’s first MSES facility, built for the ACS Group/ Cobra outside of Grenada, Spain in 2009, is the 50 MW Andasol-1 CSP plant. Andasol-1 uses MSES with 7.5 hours of effective heat storage. This plant has a capacity factor rating of 41%, an impressive increase from the thermosolar CSP industry median of 18%. However, while this allows continued power generation for hours after sunset it still doesn’t get us to the Holy Grail of baseload 24/7 solar power generation.
That goal for a stand-alone thermosolar CSP plant will be achieved near Seville at Sener’s Gemasolar power plant; a baseload 24/7 19 MW CSP tower plant with 15 hours of molten salt energy storage. Gemasolar is scheduled to open in mid-2011 with a remarkable capacity factor of 75%, coming close to coal, nuclear, and natural gas capacities.
Several utility-scale MSES CSP plants are in the pipeline in the U.S., although none are as ambitious as the Gemasolar plant. Solar Reserve plans to start construction in 2011 on the 110 MW Crescent Dunes MSES CSP plant near Tonopah, Nevada. And, in December 2010, U.S. Energy Secretary Steven Chu announced that a $1.45 billion federal loan guarantee had been finalized for the Abengoa Solar 250 MW Solana project in Arizona, the world’s largest parabolic trough concentrating solar plant. Much like the Andasol-1 plant, both of these CSP projects employ MSES with 7 to 8 hours of thermal storage that will allow them to reach 41% capacity factors.
The other technology used by Enel at the Archimede demonstration facility to accommodate CSP intermittency is essentially a peak-load stand-by combined-cycle natural gas generator. Only instead of ramping up to provide peak-load power at mid-day it ramps up every night to pick up demand that the CSP plant cannot meet. Several companies have expanded on this concept and are combining CSP power plants with an existing natural gas or coal-fired facilities. No molten salt storage system is needed and these hybrid CSP plants work with less costly conventional CSP steam generation technologies.
Just coming on-line in 2011 is the Florida Power & Light 75 MW hybrid CSP co-location solar power facility near Indiantown, FL. The FPL Martin Plant is the first hybrid solar facility in the world to directly connect to an existing combined-cycle natural gas power plant to offset fossil-fuel usage. When the sun is up, the CSP plant keeps the turbines running and allows the natural gas facility to reduce fuel consumption. At night, the natural gas plant ramps up to meet need.
This hybrid technology also works with existing coal-fired power plants. Completed in 2010, the Colorado Integrated Solar Project incorporates thermal energy from a parabolic-trough concentrating solar plant with the steam cycle of Unit 2 at the Xcel Energy Cameo Generating Station, located east of Grand Junction, Colorado. The 4 MW CSP Cameo Plant is being used to test the viability of this hybrid technology with the plant’s aging 44 MW coal-fired generator.
The Cameo Plant is not the first example of this hybrid co-location technology. In 2004 Ausra developed the world’s first solar thermal power collector system for coal-fired power augmentation. Ausra’s Compact Linear Fresnel Reflector solar steam generator technology began testing operations at Macquarie Generation’s 2,000-MW Liddell Solar Thermal Station in New South Wales, Australia, initially generating 1 MW of solar-generated steam. In 2008, Ausra completed construction of its Phase II, 9 MW solar thermal steam system expansion. This project has been so successful that it has been reported that a second 9 MW hybrid CSP plant, to be built by Novatec Solar, will start construction at the Liddell site in 2011.
Both Ausra and Novatec Solar have the capability to develop hybrid co-location CSP power plants for either natural gas or existing coal-fired generation plants. An interesting side note that should give us a sense of how potentially profitable this development may be was the news in April 2010 (pre-Fukushima disaster) that French nuclear energy giant Areva SA had purchased U.S.-based Ausra in order “to become a world leader in concentrated solar thermal” power. Areva has predicted that the global use of solar-thermal power will grow by about 30-fold this decade, a forecast that spurred the world’s largest maker of nuclear reactors to buy Ausra. Areva estimates that the technology will be installed on plants with 20 GW of power potential by 2020. That compares with about 625 megawatts today, according to Bloomberg New Energy Finance data.
One of the great things about our rapidly evolving cleantech arena is that there always seems to be another game-changing development on the horizon. In the thermosolar CSP hybrid gas turbine space that game changer might be the Solugas Project, a CSP tower/ heliostat demonstration project that introduces direct high temperature solar heating into the Brayton topping cycle of a natural gas combined cycle (CC) power plant.
The 5 MW Solugas Project is a joint effort with Abengoa Solar S.A. (CSP solar tower), Turbomach S.A. (CC gas turbine), and German Aerospace Center DLR + GEA Technika Cieplna (solar receiver design and construction). Partially backed with European Union funds, the project kicked-off on November 1, 2008 and is scheduled to last 54 months.
The design of the system is quite elegant. Serial connection of high temperature (up to 1000ºC) pressurized solar air receivers with the combustion chamber of a gas turbine results in a solar hybrid operation that can compensate for any irregularities in solar conditions. Depending on the system configuration and operation strategy, the solar component of the hybrid power plant power production can be between 40% and 90% of power generation. In terms of thermodynamic efficiency, it is anticipated that the Solugas project will be vastly superior to any currently available thermosolar application for electricity generation.
Like other hybrid CSP power plants, the Solugas hybrid CSP Brayton cycle natural gas CC power plant has guaranteed baseload 24/7 dispatchable power independent of meteorological conditions and time of day. However, it also has the following additional advantages:
• High cost reduction potential due to remarkably high conversion efficiency.
• Low environmental impact due to low water consumption. The gas turbine Brayton cycle turbine requires no cooling water and the interconnected natural gas combined cycle configuration requires up to 70% less cooling water than conventional natural gas power plants.
• Reduced land usage due to high solar energy conversion efficiency, reducing collector area and land use.
Results from this demonstration project should be available in 2013.
John Whitney AIA is a registered architect with over 25 years of experience in all aspects of project development, design, management, and construction. As Co-Founder and President of Taylor-Whitney Architects, his significant focus was on higher education clients and sustainable energy. As the Founder and Principal of the Clean Energy Action Project, he now provides consulting services that include master planning, feasibility studies, and research in the realm of cleantech and renewable energy. John is currently coordinating a solar PV database research project for AASHE (Association for the Advancement of Sustainability in Higher Education).
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