Cerro Dominador CSP power plant, largest concentrated solar thermal power plant in South America

Located in the north of Chile in the Atacama Desert (the driest one in the world) Abengoa’s “Cerro Dominador” Solar Power Plant project will be the largest concentrated solar thermal power plant and the first for direct electricity production in South America. Its location has one of the highest irradiation paces on earth, with an average of global horizontal irradiation of 2400 MWh/m2/ year, making it a privileged location for the use of solar power. With a nominal capacity of 110 MW it will rely on molten salt heat storage to have up to 17.5 hours of thermal energy storage, which would allow it to generate electricity 24 hr a day. This will be the first plant its kind to operate in South America and it represents both a challenge and a technological development opportunity for Abengoa.
The project is part of Chile’s renewable energy program which intends to have 20% of electricity generation from renewable sources by 2025.

The plant will use a central tower design in which a field of sun tracking mirrors, or “heliostats”, surrounds a central receiver at the top of a tower and the sun is reflected from the heliostats to the receiver. (CSPWorld.org , 2014)

Cerro Dominador CSP plant 3D drawing (CSP-World , 2014)

Abengoa’s heliostat design is patented and its new model, the ASUP 140, has better performance and lower cost than previously produced heliostats. (Abengoa, 2012) The area of the mirror is quite large compared to other manufacturers which generally use heliostats with an area between 1 to 20 m2.

One of the innovative aspects of this project is its long time heat storage capacity of 17.5 hrs. This will allow the plant to operate 24/7, overcoming the issues associated with predictability and reliability of solar energy, which is one of the main challenges for the penetration of solar power. The plant is intended to operate as the first “base load” solar power plant in the world. (Helioscsp.com, 2014)

The plant will operate in a steam Rankine cycle, extracting heat from the hot molten salt tank to generate steam which is used for driving a 110 MW turbine. (Parkinson, 2014) The power block portion of the plant is no different than a fossil fuel powered thermal plant, the only difference is that the steam is generated in a steam generator that extracts heat from the hot molten salt tank.

The projects form part of Chile energy development strategy for 2030, in which a national renewable energy program is delineated and goal of 20% renewable electricity generation is set for 2025. (Ministerio de Energía, 2012) This strategy attempts to promote Chile’s economic and technological development as well as reducing Chile’s dependency in coal and natural gas, which currently account for 63% of electricity generation. (Camara Chilena de la Construccion, 2013)

As part of Chiles’ renewable energy program the Chilean parliament passed a law in 2013 that obliges power generators to supply 20% of their energy from renewable sources by 2025, which is achieved by bidding agreements with renewable energy generators. This guarantees that all the energy produced will be purchased by the power operator. (Centro de Energías Renovables , 2013)

The project will also aid in fulfilling Chile’s international commitments to reduce in 20% its carbon dioxide emissions by 2020. (Obaid, 2014)

Being the first CSP project to be built in Chile, Abengoa’s Cerro Dominador solar power plant stands out due to its size and large heat storage time which is said to allow the plant to work continuously 24 hrs a day. This represents a significant advantage since the plant will operate as the first “base load” solar power plant in South America. The privileged solar resource conditions of the north of Chile are very favourable to the project and have so far been relatively unexploited, and it is likely that the success of this project will catalyse further development of concentrated solar power in the region.

References 

1. Abengoa. (2012). ABENGOA Annual Report 2012. Retrieved from abengoa.com: http://informeanual.abengoa.com/web/2012/en/actividades/areas_de_actividad/produccion-industrial/negocio-solar/
2. Camara Chilena de la Construccion. (2013). Matriz Energética y Generación Eléctrica: Una mirada Internacional [Power Generation and Energy Matrix: An International Look]. Camara Chilena de la Construccion.
3. Centro de Energías Renovables . (2013, September). Congreso aprobó Ley de Fomento al Desarrollo de las ERNC 2025 [Congress approved Development Promotion Act of NCRE 2025] . Retrieved from Centro de Energías Renovables (CER): http://cer.gob.cl/blog/2013/09/congreso-aprobo-ley-de-fomento-al-desarrollo-de-las-ernc-2025/
4. CSP-World . (2014). Abengoa kicks-off construction for 'Cerro Dominador' CSP plant in Chile. Retrieved from csp-world.com: http://www.csp-world.com/news/20140710/001342/abengoa-kicks-construction-cero-dominador-csp-plant-chile
5. Helioscsp.com. (2014). Abengoa considers upgrading Concentrated Solar Power (CSP) project in Chile to 200 MW. Retrieved from helioscsp.com: http://www.helioscsp.com/noticia.php?id_not=2492
6. Kable Intelligence Limited. (2015). Cerro Dominador Solar Power Plant, Atacama Desert, Chile. Retrieved from power-technology.com: http://www.power-technology.com/projects/cerro-dominador-solar-power-plant-atacama-desert/
7. Ministerio de Energía. (2012). Estrategia Nacional de Energía 2012 - 2030 [National Energy Strategy 2012 - 2030]. Retrieved from http://www.minenergia.cl/estrategia-nacional-de-energia-2012.html
8. Obaid, A. (2014, September). Chile asume siete compromisos, incluida disminución del 20% de CO2 [Chile takes seven commitments, including 20% reduction of CO2]. Retrieved from www.latercera.com: http://www.latercera.com/noticia/tendencias/2014/09/659-597142-9-chile-asume-siete-compromisos-incluida-disminucion-del-20-de-co2.shtml
9. Parkinson, G. (2014). Abengoa to build 110MW solar tower storage plant in Chile. Retrieved from RenewEconomy.com: http://reneweconomy.com.au/2014/abengoa-to-build-110mw-solar-tower-storage-plant-in-chile-24839

Solar fuel from two step termochemical cycles

With the increasing need to reduce CO2 emissions to attenuate the effects of global warming there is a significant role of renewable energy sources to achieve this goal. However, issues of variability of energy sources and unpredictability are still barriers for integration of renewables. In this regards, energy storage and dispatchability issues prevent further penetration of renewable power sources (Bader et al., 2013).

One promising approach to solve this is the production of “solar fuels” (Hydrogen and syngas). This is, the use of solar energy to produce synthetic fuels via thermochemical cycles. This way solar heat can be stored in the chemical bonds of chemicals that can be used to produce fuels or other useful transportable and storable chemicals (Chueh et al., 2013). One advantage of this approach is that the fuels produced can be easily incorporated into current energy distribution system.

The main ways that have been explored to produce fuels from solar energy are the splitting of water and carbon dioxide to generate hydrogen, carbon monoxide or a mix of both (syngas)(Chueh et al., 2010). This offers the added benefit of possible CO2 sequestration. Splitting water and carbon dioxide can be done in a single high temperature step, however this requires temperatures over 3000 degrees and it poses the problem of separating oxygen from hydrogen and/or carbon monoxide at a high temperature (otherwise the resulting mix is explosive). 

A more sophisticated approach is the use of two step solar thermochemical cycles based on the reduction and oxidation of a metal oxide. In this cycles a metal oxide is reduced at a high temperature (provided by concentrated solar radiation) producing an oxygen deficient material. Then the metal oxide is re-oxidized with water or carbon dioxide capturing the oxygen from them and releasing hydrogen or carbon monoxide. This approach has the advantage of requiring lower temperatures and avoiding the problem of  separating oxygen from hydrogen and/or carbon monoxide at a high temperature (Furler et al., 2012). 

Diagram of a two step metal oxide-redox water/CO2 sppliting thermochemical cycle (Energy&
Environmental Science, 2012)

However there still several problems associated with this cycles that need to be overcome. So far the efficiencys achieved are the low, with record solar to fuel efficiency being 1.7% . During the reduction step a low oxygen partial pressure is required which is usually achieved by using and inert gas sweep. This reduces the efficiency due to the energy penalties of heating the gas and maintaining its flow. Another issue is that due to the high temperature required to drive the reactions, conduction and re-radiation heat losses become significant, which also penalizes the efficiency of the cycle (Ermanosky et al., 2013). Finally there also issues with the cyclability and stability of the metal oxide materials, some metal oxides tend to volatilize at high temperature and also tend to sinter which reduces the surface area slowing down the reaction kinetics (Furler et al. 2014).

Several solution are currently being tested to overcome these limitations. Different metal oxide compositions and morphologies have been developed in an attempt to improve reaction kinetics. The use of heat recovery from the inert gas sweep is also a promising solutions and thermodynamic analysis suggest that with heat recuperation efficiencys of up to 30% could be achieved. The use of a carbonaceous feedstock during reduction allows to reduce the temperature and increase gas yields, however it is a less sustainable option due to the use of the carbonaceous feedstock.

Summarizing, it can be said that two step metal oxide redox solar thermochemical cycles appear as a promising pathway to achieve a sustainable energy system in the future. However this technology is still very much in development and several issues need to be solved before its economic viability.

References 

1. Bader, R., Venstrom, L. J., Davidson, J. H., & Lipiński, W. (2013). Thermodynamic Analysis of Isothermal Redox Cycling of Ceria for Solar Fuel Production. Energy Fuels(27), 5533-5544.
2. Chueh, W. C., & Haile, S. M. (2010). A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO2 mitigation. Philosophical Transactions of the Royal Society A, 368(1923), 3269-3294.
3. Chueh, W. C., Falter, C., Abbott, M., Scipio, D., & Furler, P. (2010). High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria. Science(330), 1797-1801.
4. Chueh, W. C., McDaniel, A. H., Grass, M. E., Hao, Y., Jabeen, N., Liu, Z., . . . Gabaly, F. E. (2012). Highly Enhanced Concentration and Stability of Reactive Ce3+ on Doped CeO2 Surface Revealed In Operando. Chemistry of Materials, 24(10), 1876–1882.
5. da Silva, A. L., & Assaf, J. M. (2007). Activity of Cu/CeO2 and Cu/CeO2-ZrO2 for low temperature water-gas shift reaction. Natural Gas Conversion VIII, 167, pp. 213–218. Natal.
6. Ermanoski, I., Siegel, N. P., & Stechel, E. B. (2013). A New Reactor Concept for Efficient Solar-Thermochemical Fuel Production. Journal of Solar Energy Engineering(135).
7. Furler, P. (2014). Solar Thermochemial CO2 and H2O Splitting Via Ceria Redox Reactions. ETH Zurich. Zurich: Ph.D. Thesis, ETH Zurich.
8. Furler, P., Scheffe, J., Gorbar, M., Moes, L., Vogt, U., & Steinfeld, A. (2012). Solar Thermochemical CO2 Splitting Utilizing a Reticulated Porous Ceria Redox System. Energy Fuels(26), 7051-7059.
9. Furler, P., Scheffe, J., Marxer, D., Gorbar, M., Bonk, A., & Steinfeld, U. V. (2014). Thermochemical CO2 splitting via redox cycling of ceria reticulated foam structures with dualscale porosities. Physical Chemistry Chemical Physics, 16, 10503-10511.
10. Furler, P., Scheffea, J. R., & Steinfeld, A. (2012). Syngas production by simultaneous splitting of H2O and CO2 via ceria redox reactions in a high-temperature solar reactor. Energy & Enviromental Science, 5, 6098-6103.

  

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Perovskyte solar cells: an emerging technology that promises high efficiency and low manufacture cost.

Over the last 10 year a perovskite based solar photo-voltaic cells have emerged as a promising technology that could offer high energy conversion efficiency and low manufacture cost. Compared to other emerging PV technologies the efficiecy of perovskite solar cells has increased much more rapidly. The first perovskite solar cell in 2006 had an efficiency of 2.6%, while this year efficiency of 20.1 % have already been reported (Nrel.gov, 2015). Other technologies have taken decades to reach 20% efficiency. 

Record PV cell efficiency over time (click to enlarge). Note the rapid increase for perovskyte cells (yellow circle with red border) compared tt other technologies (Nrel.gov, 2015).  

This has made perovskites one of the hottest topics in PV research. While on 2009 a total of 50 publications where made on this topic, on 2014 more than 400 publications were made and as of May 2015 more than 300 publications were made (Ossila.com, 2015). 

What exactly is a perovskite solar cell?

Perovskite solar cells are any cell with a perovskite material light absorbing layer. Perovskite materials are any material with the crystal structure of calcium titanium oxide (CaTiO3). The basic composition is given by the formula ABX3 :

–A = An organic cation  (eg. methylammonium (CH3NH3)+)

–B = A big inorganic cation  (usually lead(II) (Pb2+))

–X3= A slightly smaller halogen anion (usually chloride (Cl-) or iodide (I-))

The perovskyte crystal structure, note that both unitary cells are equivalent (Ossila.com, 2015).

For perovskite PV cells the two materials that have proven to be most successful are: 

-Methylammonium lead trihalide (CH3NH3PbX3) (Bandgap 2.3 eV - 1.6 eV) and, 

-Formamidinum lead trihalide (H2NCHNH2PbX3) (Bandgap 2.2 eV - 1.5 eV).

So why have these material drawn so much attention?

Perovskites have several advantages that make then suitable for high efficiency solar cells. First of all their light absorption coefficient is very high and it increases with temperature making them good light absorbents (Green et al., 2014). This light absorption spectra can also be tuned by changing the halide content of the perovskite which is a big advantage to tune the material for specific applications (Jun Hong, 2013). 

Variable absorption coefficient for different composition perovskites (Eperon et al., 2014) 

They exhibit large carrier diffusion lengths, making them suitable for thin film applications (Hodes,  2013). Their open circuit voltage is also high, resulting in cells with high efficiency (Ball et al., 2014). Finally the manufacture methods for perovskites are simple wet chemistry methods, such as spin coating or vapor deposition, that require only common lab equipment and are easy to scale. This makes the fabrication process simpler and lower in cost. 

Flexible perovskite solar cell fabricated by low temperature deposition (http://www.materialsviews.com/new-processing-methods-flexible-perovskite-solar-cells/)

However, there are some challenges that perovskite cells still need to overcome. Perovskite cells are unstable and degrade rapidly under moisture and UV radiation conditions. This means that full encapsulation of the cell is required (D’Innocenzo et al., 2014). Another drawback is that the composition of the perovskite has lead in it, a toxic metal. There have been attempts to replace the lead with other elements but they have not been successful and lead still offer materials that produce the highest efficiency cells (Baker, 2014). 

Summarizing we can say that perovskites are a promising material that has showed rapid development and efficiency increase. Perovskites combine high performance parameters (diffusion length, absorption coefficient, Voc) and potential for low cost manufacture. However challenges like the use of lead and rapid degradation still need to be overcome for commercialisation of this technology. 

References 

1.Baker, Joel R. 2014. “Perovskite Solar Cells.”

2.Ball, James M., Michael M. Lee, Andrew Hey, and Henry J. Snaith. 2013. “Low-Temperature Processed Meso-Superstructured to Thin-Film Perovskite Solar Cells.” Energy & Environmental Science 6: 1739. doi:10.1039/c3ee40810h.

3.Collavini, Silvia, Sebastian F Vçlker, and Juan Luis Delgado. 2015. “Understanding the Outstanding Power Conversion Efficiency of Perovskite-Based Solar Cells,” 9757–59. doi:10.1038/srep00591.

4.D’Innocenzo, Valerio, Giulia Grancini, Marcelo J P Alcocer, Ajay Ram Srimath Kandada, Samuel D Stranks, Michael M Lee, Guglielmo Lanzani, Henry J Snaith, and Annamaria Petrozza. 2014. “Excitons versus Free Charges in Organo-Lead Tri-Halide Perovskites.” Nature Communications 5: 3586. doi:10.1038/ncomms4586.

5.Eperon, Giles E., Samuel D. Stranks, Christopher Menelaou, Michael B. Johnston, Laura M. Herz, and Henry J. Snaith. 2014. “Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells.” Energy & Environmental Science 7 (3): 982. doi:10.1039/c3ee43822h.

6.Green, Martin A, Anita Ho-baillie, and Henry J Snaith. 2014. “The Emergence of Perovskite Solar Cells” 8 (July). Nature Publishing Group. doi:10.1038/nphoton.2014.134.

7.Hodes, Gary. 2013. “Perovskite-Based Solar Cells” 342 (October): 317–19.

8.Nrel.gov, 'NREL: National Center for Photovoltaics Home Page', 2015. [Online]. Available: http://www.nrel.gov/ncpv/

9.Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells Nano Letters (2013), 13(4), 1764-1769 CODEN: NALEFD; ISSN: 1530-6984;