Saket Vora

The following report was written by me for an assignment in the CEE 272 – Grid Integration of Renewables class at Stanford University. The prompt called for us to consider how renewable energy sources can contribute to a de-carbonized electricity sector. I chose the concentrated solar power technology to study.

Introduction

A significant driver of global climate change is the high atmospheric presence of greenhouse gases, named for their ability to allow solar radiation to the Earth’s surface but trapping it inside the troposphere. Carbon dioxide is a greenhouse gas and one whose atmospheric concentration has grown rapidly over the past two hundred years. Recent years have seen an increased push towards reducing mankind’s contribution of carbon dioxide emissions, and the electricity sector is a significant portion of this – at 2,400 million metric tons per year (MMtCO₂e/year), comprises 40% of the United States total CO₂ emissions (1). De-carbonizing the electricity sector will require greater number of renewable sources to come online, and the challenges of integrating these intermittent and distributed sources will have to be tackled. This report examines one renewable energy source, concentrating solar power (CSP), and will discuss its use, utilization, costs of implementation, and challenges to overcome.

Background

Concentrating solar power generation utilizes a simple method for the generation of steam. Several arrays of mirrors reflect sunlight, concentrating it on an absorber containing a fluid or thermal mass. The heated fluid or mass is then used to boil water into steam, which drives a turbine and then a generator. There are three leading forms of CSP. The first places parabolic trough shaped mirrors (a U-shaped arrangement) with a tube containing synthetic oil running along its focal point, and the oil can be heated up to 400 ^oC (2). The second is parabolic dish mirror on which a collector is placed at its focal point. In this configuration, a Stirling engine is the collector, and the heat is converted into electricity at each dish. The third approach utilizes a central tower with mirrors arranged radially around it. The mirrors direct sunlight to a thermal mass (molten nitrate salts, for example) and heat it to temperatures 600^oC.

The central tower approach. Photo linked from sustainabledesignupdate.com.

Parabolic Dish with Stirling engine. Photo linked from altenergystocks.com

Parabolic trough-shaped approach. Photo linked from CNET's Green Tech site.

Of the three, parabolic trough-style CSP plants are the most popular. The parabolic dish approach requires 2-axis tracking and requires expensive mirror designs, but the central tower scheme is being re-examined due to greater ease at maintaining the heated fluid loop and higher temperatures possible for the thermal mass. When a thermal mass such molten salts are used, CSP plants gain the ability to continue producing electricity at night or when lower levels of sunlight are available. When generation exceeds demand, the extra heat is stored in the thermal mass for later use. This approach works only for a limited time, on the order of a few days.

Performance and Costs

The median capacity size for existing CSP plants appears to be between 10MW and 20MW, with the average skewed higher due to a small number of large facilities sized at 80MW+. The worldwide average of capacity factors for CSP plants is 19%, without using thermal storage (3). With thermal storage technologies that allow electricity generation without light present, capacity factors are estimated to increase to between 33% and 48%, depending on the type of CSP scheme and storage. The National Renewable Energy Laboratory estimates capital costs of $2M to $5M per MW to construct a plant (2), and CSPs enjoy small operating and maintenance costs compared to fossil fuel plants. Currently, the price per kilowatt-hour for CSP varies between 13 and 18 cents, but some newer plants intend to use compact linear Fresnel reflectors (CLFRs) and hope to reduce capital costs by 20% (4). In optimum situations, CSP is targeted to drop to 9 cents per kWh (5).

Concentrated solar power electricity output can match a standard power curve more closely than wind power. This is especially true if solar thermal masses can be used effectively, so that CSP plants can continue to provide steady electricity for the few extra evening hours as the sun sets. As it can be inferred in the above description of CSP, this scheme is a very low CO₂ way of producing electricity. Routine maintenance of the mirrors includes cleaning with water and fixing breakages. The molten salts used as thermal masses are not sources of carbon emissions. Carbon emissions are incurred during the construction of a CSP plant, but a short build time of 12 to 24 months allows for an energy payback time of around 7 months (3). As far as overall grams of CO₂e/kWh across a plant’s lifetime, CSP ranks 2^nd, just below wind but ahead of every other renewable energy source. Thus, the deployment of concentrated solar power plants contribute to de-carbonizing the electricity sector.

Growth and Utilization

Estimates for the total worldwide technical potential for concentrated solar power varies widely between 630GW and 4700GW (4), which is converted to 1.05 to 7.8 PWh/year using a capacity factor of 19% (3). However, the current installed capacity for CSP as of 2008 is approximately 400MW, mostly in California, USA. Australia, Spain, and Israel are also now fielding CSP plants. If we focus on the state of California, a study by Simon & McCabe showed a technical potential of 1 million MW across 16 counties.[1] ⁽⁶⁾ Indeed, California is home to the most number of concentrated solar power plants in the world, one of them being the Solar Energy Generating Systems (SEGS) which is the largest solar generating facility in the world for a combined 354MW of generation spread across nine plants.

There is tremendous potential for the CSP sector to grow. First, there is no shortage of suitable untapped land areas[2], and its suitability to the Southwest and Midwest regions of United States lends itself to easier land use permitting. A number of new plants, by companies such as Ausra, BrightSource, Stirling Energy Systems, are slated to be online after 2010. More states are following California’s lead and signing renewable energy requirements for electric power providers

One critical aspect that must be addressed is the additional high-voltage transmission lines to effectively deliver the electricity from these CSP plants to the main grid. CSP does not have the high degree of spatially distributed generation points that commercial or residential photovoltaics have, and by utilizing a traditional steam engine (for parabolic-trough and central tower CSP schemes) the problem of frequency-matching to the grid is avoided (photovoltaics require an inverter to convert its DC output to AC).

Navigant Consulting published a report in 2008 regarding the federal Investment Tax Credit (ITC) as an incentive to encourage construction of new solar thermal plants. It found 75% to 85% losses in market size for the US CSP market if the current ITC was significantly reduced. (7) Since up to 60% of a CSP plant’s capital cost is from equipment, the CSP sector relies heavily on manufacturing. Furthermore, if the current ITC incentive is renewed, it could “spur an additional 34,000 jobs and 287,000 job-years of employment between 2009 and 2016.” (7) Until there is a proper price on carbon that factors in the externalities of fossil fuels into coal and petroleum electricity production, incentives like the ITC will be critical to ensure short-term continuation and growth of CSP plants.

Challenges to Overcome & Conclusion

The issue of energy storage is still one that needs a solution vetted for long-periods of time. Improving thermal masses so that they can be used longer during diminished lighting conditions will be critical to helping CSP plants more smoothly integrate with the grid and to be seen as not just a very low carbon approach to electricity generation, but a desirable one at well. As further experience is gained with more CSP plants in a variety of locations, costs should decrease and become a viable alternative to current fossil fuels prices.

References

1. Greenblatt, Jeffery, et al. Clean Energy 2030. www.google.com. [Online] Google, November 20, 2008. [Cited: 1 16, 2009.] http://knol.google.com/k/-/-/15x31uzlqeo5n/1#references.

2. Leitner, Arnold. Fuel from the Sky: Solar’s Power’s Potential for Western Energy Supply. RDI Consulting, National Renewable Energy Laboratory. 2002. NREL/SR-550-32160.

3. Review of solutions to global warming, air pollution, and energy security. Jacobson, Mark Z. Stanford : RSC Publishing, December 1, 2008, Energy & Environmental Science. DOI: 10.1039/b809990c.

4. Sims, Ralph E.H. and Schock, Robert N. IPCC 4th Assessment Report – Energy Supply. s.l. : IPCC, 2007.

5. Mills, David and Morgan, Robert. Solar thermal power as the plausible basis of grid supply. Ausra, Inc.

6. Simons, George and McCabe, Joe. California Solar Resources. Energy Research & Development Division, California Energy Commission. 2005. CEC-500-2005-072-D.

7. Frantzis, Lisa, et al. Economic Impact of Extending Federal Solar Tax Credits. Solar Energy Research and Education Foundation. s.l. : Navigant Consulting, 2008.