Renewable Energy Global Innovations features: Small pyramidal textured ultrathin crystalline silicon solar cells with double-layer passivation

Significance Statement

Over the past few decades, crystalline silicon (c-Si) solar cells have dominated the global photovoltaic market with above 90% share. Although the module cost has been reduced in the past tens of years, expensive cost is still a main obstacle for c-Si solar cells to compete over fossil-fuel based energy. At present, commercial c-Si solar cells are typically about 180 µm in thickness, however, the cost of the silicon material alone accounts for up to 40% of total module cost. Therefore, to develop ultrathin c-Si photovoltaics like less than 20 µm becomes an alternative technology roadmap to significantly drive down the costs with the advantage of more cost-effectiveness.

Recently, Wensheng Yan’s group reported highly efficient ultrathin c-Si solar cells, where the Si base thickness is as thin as 16 micrometers and the Si is on a thick conductive metal substrate. In this work, they adopt small pyramids for the front surface texture via the photovoltaic (PV) industry compatible chemical wet-etching method with the advantages of low cost and large-scale texturing. In addition, they use the double-layer passivation to reduce surface recombination and thereby achieve higher conversion efficiency. As a result, the best efficiency of 16.4% was reported. This research result shows a great potential for the PV industrial applications via the technology transferring.

About The Author

Wensheng Yan is currently working at Karlsruhe Institute of Technology of Germany as a group leader of nanostructured photovoltaics. His current research interest includes 1^st, 2^nd, and 3^rd generation silicon solar cells. He was the recipients of highly prestigious Australian Renewable Energy Agency (ARENA) Postdoctoral Fellowship (2012) and Humboldt Research Fellowship for Experienced Researchers of Germany (2016). At present, he serves as regular reviewers for more than 20 international academic journals.

As part of international cooperation motivation, he is guest professors of Chongqing University of Posts and Telecommunications of China and Three Gorges University of China, respectively, for the project cooperation in the area of photovoltaics.

About The Author

Xinyu Tan is currently a professor at College of Materials and Chemical Engineering, China Three Gorges University. She received her PhD in condensed physics from Huazhong University of Science and Technology in 2008. She worked as a postdoctoral research fellow at Tsinghua University and visiting scholar in Michigan University in 2012 and 2015.

His current research interest includes silicon and carbon based solar cells. She was the recipients of prestigious excellent postdoctoral fellow of Hubei Province (2016) and High-level Talents in the New Century of Hubei Province (2014). Her research interests include development of optoelectronics and solar cell materials and more than 100 papers have been published.

Reference

Xinyu Tan, Wensheng Yan*, Yiteng Tu, and Can Deng, “Small pyramidal textured ultrathin crystalline silicon solar cells with double-layer passivation”, Optics Express, 25, 14725 (2017).

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Renewable Energy Global Innovations features: Mesoporous single-crystal-like TiO2 mesocages threaded with carbon nanotubes for high-performance electrochemical energy storage

Significance Statement

The commercial lithium ion batteries based on graphite anode is known for its low rate performance.  Titanium oxide appears to be a promising building block for electrodes used in lithium-ion storage owing to its superior charge storage capability, environmental benignity and low cost. Unfortunately, titanium oxide has poor ionic and electronic conductivity as well as substantially low lithium-storage capacity. This poses numerous challenges towards achieving high performance devices.

Several researchers have devoted their efforts explore various polymorphs, including titanium oxide-bronze, rutile and anatase phases. Titanium oxide-bronze exhibit faster lithiation/delithiation kinematics and higher capacity as compared to rutile and anatese. However, it preparation requires harsh and complex conditions making it less attractive for large-scale preparation.

Another promising approach is through structure and morphology control.  Including composite architectures based on low-dimensional titanium oxide;  Core-shell morphologies such as carbon coated titanium oxide nanoparticles and cable like titanium oxide carbon nanotubes. The low-dimensional titanium oxide offers shortened ion diffusion length and the conductive inclusions enable effective electron transport granting these composites high electrochemical storage performance. However, the titanium oxide nanocrystals are assembled around the conductive moieties that may disassemble easily from the conductive networks and lead to rapid capacity fading. More importantly, previous works with low dimensional crystals often demonstrate very low tapping density (<0.2 g cm³) which would not be feasible in real applications.

Researchers led by professors Yunfeng Lu and Hexing Li at university of California Los Angeles and in collaboration with scientists at Shanghai University of Electric Power and Shanghai Normal University, developed a nanocomposite architecture prepared by in-situ growth mesoporous single-crystal-like titanium oxide particles threaded through by carbon nanotubes. Their work is now published in Nano Energy.

Assembled from sub-10 nm anatase nanocrystals, single-crystal secondary particles in micrometer scale with mesoporous features and threaded by carbon nanotubes (CNTs) was synthesized by one step sovolthermal method. No further high temperature heating required.

The electrochemical performance of the composite was characterized by using coin-type cells with loading of 2-3 mg/cm². From the galvanostatic charge/discharge curves, the composite achieved 260 mAh g^-1 at 0.5C which is among the highest capacities of the reported TiO₂ anode. A relatively short voltage plateau (phase charge) with a long slopping “tail” (200 mAh g^-1) indicated a high surface charge contribution. Even at an extremely high rate of 30C, it still delivered 120 mAh g^-1.  The researchers also investigated the composite performance at extended voltage window from 0.005-2.7V. Such lower voltage was often avoided due to formation of solid electrolyte interface (SEI) and further lithium insertion would challenge the robustness of the active materials. With sub-10nm building nanocrystals, the composite achieved 440 mAh g^-1 at 50mA g^-1. As seen from the cyclic voltammetry diagram, such “extra” capacity below 1V main came from surface contribution. Above 1V to 2.7V, the composite still maintained the reversible anatase phase confirmed by the in-situ X-Ray Diffraction. Overall, high rate charge discharge at 2000 mA g^-1  for 1000 cycles was demonstrated with negligible capacity fading. The scanning electron microscopy after cycling confirmed the structure was maintained

By threading single crystal -like  titanium oxide mesocrystals with carbon nanotubes, the research team was able to realize high rate anode composite for lithium ion batteries. The composites with iso-orientated primary nanocrystals has successfully enhanced the rate and cycling performance. Importantly, the tapping density (1.12 g cm³) is much closer to the real applications which was often ignored in nano-scale engineering. This method might bring some guidelines for the functional materials and device towards highly efficient energy storage systems.

Tapping-Density-Comparison

Synthesis-Scheme

Design-Stratagies

About The Author

Yiting Peng obtained her Ph.D. degree under the co-supervision of Professor Hongbin Geng in Harbin institute of technology and Professor Yunfeng Lu in University of California, Los Angeles at 2013. She is now a Lecturer at Shanghai University of Electric power.

Her research mainly focused on composition and architecture design towards supercapacitors, lithium-ion batteries, advanced Lead-acid batteries.

About The Author

Zaiyuan Le received his BASc degree in Materials Science and Engineering from University of Toronto (Canada) in 2012. He is currently a Ph.D. candidate under supervision of Prof. Yunfeng Lu in Chemical and Biomolecular Engineering at University of California, Los Angeles.

His research interests lie in titanium oxide based lithium and sodium based energy storage, including rechargeable batteries and hybrid capacitors.

About The Author

Meicheng Wen received his M.S. degree in physical chemistry from Shanghai Normal University under supervision of Professor Hexing Li and Associate Professor Dieqing Zhang in 2013 and his Ph.D. degree in engineering from Osaka University under the supervision of Professor Hiromi Yamashita in 2016. He is currently a specially appointed Assistant Professor in the Division of Materials and Manufacturing Science at Osaka University.

About The Author

Dieqing Zhang received her PhD (2010) in environmental chemistry from The Chinese University of Hong Kong under Prof. Jimmy C. Yu. Currently is an Associate Professor in Department of Chemistry at Shanghai Normal University. She is now engaged in the design and fabrication of novel and efficient photocatalysts for nitric oxide oxidation/reduction, hydrogen production and CO₂ reduction, etc.

About The Author

Zheng Chen received his Ph.D. at UCLA in 2012 under the supervision of Prof. Yunfeng Lu in the Department of Chemical and Biomolecular Engineering. From 2013-2016, he was a postdoctoral associate working with Prof. Zhenan Bao in Chemical Engineering and Prof. Yi Cui in Materials Science and Engineering at Stanford University.

Currently as Assistant Professor at Department of NanoEngineering at UCSD, his research focuses on functional polymers, nanostructured materials and hybrids for applications in electrochemical energy, flexible devices and sustainable environment.

About The Author

Hao Bin Wu received his BS degree in chemistry from Fudan University (China) in 2010. He obtained his Ph.D. degree in materials science from Nanyang Technological University (Singapore) under the supervision of Professor Xiong Wen (David) Lou in 2015.

Currently he works with Professor Yunfeng Lu as a Postdoctoral Scholar at University of California, Los Angeles. His research interests focus on synthesis and applications of nanostructured and hybrid materials for electrochemical energy storage and conversion, including rechargeable batteries, electrochemical capacitors and electrocatalysis.

About The Author

Prof. Hexing Li received his doctor degree from Fudan University. Now, he is working as the president of Shanghai University of Electric Power and the director of Chinese Education Ministry Key Laboratory and International Joint Laboratory on Resource Chemistry, as well as an Associated Editor of Appl. Catal. B Environ.

His research interest is photocatalysis for environmental cleaning and thermocatalysis for green chemistry. Up to now, more than 395 papers and 3 monographs have been published. His H-index is 59.

About The Author

Yunfeng Lu obtained his Ph.D. degree under the supervision of Professor C. Jeffrey Brinker in University of New Mexico at 1998. He became a Brown Chair Professor at Tulane University in 2005 and now he is a Professor at University of California, Los Angeles.

His research interest focused on composition and architecture design towards energy storage and conversion, including supercapacitors, lithium and sodium-ion batteries, lithium metal batteries, flow batteries, intermediate-temperature fuel cells, and effective methane conversion.

Reference

Yiting Peng, Zaiyuan Le, Meicheng Wen, Dieqing Zhang, Zheng Chen, Hao Bin Wu, Hexing Li, Yunfeng Lu. Mesoporous single-crystal-like TiO2 mesocages threaded with carbon nanotubes for high-performance electrochemical energy storage. Nano Energy, volume 35 (2017), pages 44–51.

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Renewable Energy Global Innovations features: Insight into stall delay and computation of 3D sectional aerofoil characteristics of NREL phase VI wind turbine using inverse BEM and improvement in BEM analysis accounting for stall delay effect

Significance Statement

Various aerodynamic load computation approaches for undertaking the aeroelastic analysis of wind turbines has been developed. Blade Element Momentum, for instance, is applied frequently owing to its fast and simple nature. This method provides the loads along the blade span, torque and the amount of power generated by the turbine from the wind speed, aerofoil attributes and blade geometry. Wind turbine blades are designed with aerofoil cross sections; therefore, the Blade Element Method demands aerofoil characteristics as a function of the angle of attack in order to calculate the forces.

Real time flow over the turbine are in axial and radial directions. However, only axial flow is taken into consideration when using 2D aerofoil characteristics while radial flow is normally neglected. This creates high discrepancies of the computed aerofoil characteristics and forces of the CFD or Experiment analysis with the values calculated from the Blade Element Momentum code implementing the 2D aerofoil attributes. This unpleasant agreement is due to stall delay and is evident in most stall-regulated turbines.

Professor E.Y.K. Ng and his PhD student Ijaz Fazil at Nanyang Technological University in Singapore discussed the aerofoil characteristics as well as its various extrapolation methods for an array of angles of attack. They conducted Unsteady RANS (URANS) CFD analysis on two-bladed NREL Phase VI wind turbine and the measurements were compared with the experimental outcomes of the NREL/NASA test for confirmation. Their research work is now published in Energy.

The NREL phase VI rotor adopted for in their study was a two blade stall regulated wind turbine and the blades were tapered and twisted. The rotational speed was 72rpm for all wind speeds.. They conducted the URANS analysis for different inlet wind speeds by implementing the sliding mesh approach. The authors measured pressure at 18 radial locations including the five radial locations considered in experimental measurements.

There was a rise in the lift coefficient along the blade span when compared to the 2-dimensional aerofoil attributes for the same angle of attack owing to the span wise flow. This is the stall delay. This effect if effective at inboard sections and it reduces gradually towards the blades’ tips. Apparently, there is no clear explanation for stall delay but the authors believed that when stall occurred, separated flow on the suction side of the blade, appeared to rotate along with the blade, and experienced centrifugal force. The centrifugal force caused the separated flow to move in a radial manner towards the tip. This span wise flow also permitted Coriolis force to act towards the trailing edge, and therefore, resulted in stall delay.

The research team also discussed in their paper the reasons for the over prediction of the Blade Element Momentum analysis on using existing stall delay correction models. The Blade Element Momentum with or without the stall delay models couldn’t predict the correction distribution of aerofoil characteristics along the span of the blade in order to contemplate the impact of the stall delay.

The study proposed the use of the 3D aerofoil characteristics which is computed using Inverse BEM method in the Blade Element Momentum to take care of the stall delay. A good agreement with the aerofoil characteristics distribution along the span of the blade was realized in the current method.

The comparison of extracted CL (at 18 locations) from CFD analysis using inverse BEM and Distribution of CL along the blade span from BEM analysis on using existing stall delay models.

Comparison of extracted CL (at 18 locations) from CFD analysis using inverse BEM and distribution of CL along the blade span from proposed BEM analysis.

About The Author

Ijaz Fazil received his Bachelor of Engineering in Mechanical Engineering from Anna University, India in 2008. He has scored 93.94 percentile in GATE 2008. He received his Master of Science in Mechanical Engineering from National University of Singapore, Singapore in 2010. He commenced his Ph.D. study on Computational Fluid Dynamics (CFD) analysis of rotor and wake aerodynamics of wind turbine at Nanyang Technological University, Singapore, in August 2011. To date, he has presented 3 conference papers and has 1 journal publication. His research interests are –CFD, Wind Turbine Aerodynamics, Building Physics, Thermal Comfort and Fire Modelling. LinkedIn profile.

About The Author

EYK Ng obtained his Ph.D at Cambridge Univ. and a faculty in NTU. He is the Editor-in-Chief for the ISI Journal of Mechanics in Medicine and Biology for dissemination of original research in all fields of mechanics in medicine and biology since 2000;

He is the Founding Editor-in-Chief for the ISI indexed Journal of Medical Imaging and Health Informatics. His main area of research is human physiology, biomedical engg; computational fluid dynamics and numerical heat transfer. Ng has had more than 275 ISI journal articles and 100 conference papers and 13+1 books published including “Compressor Instability with Integral Methods” (2007); “Cardiac Pumping and Perfusion Engineering” (2007); “Imaging and Modelling of Human Eye” (2008); “Distributed Diagnosis and Home Healthcare, D2H2 v.1 and 3” (2009, 2012); “Performance Evaluation in Breast Imaging, Tumor Detection and Analysis” (2010); “Computational Analysis of Human eye with Applications” (2011); “Multimodality Breast Cancer Imaging” (2013); “Human eye imaging and modeling,” “Image Analysis and Modeling in Ophthalmology,” “Ophthalmology Imaging and Applications” (2013, 2014), “Bio-inspired Surfaces and Applications” (2016); “Computation and Mathematical Methods in Cardiovascular Physiology” by WSPC (2017, in-press) and “Application of Infrared to Biomedical Sciences” (2017). He is an invited keynotes speaker for more than 15 international scientific confs./workshops. 15 of his papers have been adopted as references in Singapore Standard (SS 582: 2013) and ISO/IEC 80601-2-59:2008.

He is also presently serving as panel member for the Biomedical Standards Committee, Singapore. The co-inventor of three USA patents on multiple analytical software classifier programs to identify the different stages of breast cancer development using thermal data with Cyrcadia Health, Inc. system, he further explores the use of IR in the field of ophthalmology for early detection of health abnormality.  Here is his publication list  and CV . 

Reference

Ijaz Fazil Syed Ahmed Kabir, E.Y.K. Ng. Insight into stall delay and computation of 3D sectional aerofoil characteristics of NREL phase VI wind turbine using inverse BEM and improvement in BEM analysis accounting for stall delay effect. Energy, volume 120 (2017), pages 518-536.

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Renewable Energy Global Innovations features: Design technique to improve the energy efficiency of a counter rotating type pump-turbine

Significance Statement

The development of new renewable energy sources such as wind, hydro and tidal powers and their related studies has been on the raise since the adoption of laws on carbon dioxide which are presently stringent. Great mileage has already been achieved in the exploitation of these noble resources. However, it is difficult to control the amount of energy generated due to the precincts presented by natural conditions. The application of the pump-turbine has emerged as one of the key power stabilization technique that offers the advantages of minimizing energy loss and controlling the power generated. In depth studies have hitherto led to the development of the counter-rotating type pump-turbine which is a novel concept in the field of pump-turbines. Conversely, the energy efficiency of this counter-rotating type pump-turbine calls for thorough design review for its’ maximization since it is characterized by intrinsic internal flow patterns.

With a professor Toshiaki Kanemoto of Kyushu Institute of Technology, Jin-Hyuk Kim and his colleagues at Korea Institute of Industrial Technology proposed a study to address the design scheme in order to improve the energy efficiency of the counter rotating-type pump turbine based on its operating mode. They hoped to achieve this by developing more methodical and rational design techniques that would ensure the improved energy efficiency of the system. The research work is published in Renewable Energy.

First, the research team analyzed the influence of each design variable on the system performance based on the results of the performance evaluation on the test sets, as the regression analysis for the 2^k factorial design was performed. Furthermore, the respective model, whose efficiency was improved according to the operating mode, was then produced. Finally, they evaluated the performance of each generated model using numerical analysis.

The researchers observed that the range of error for numerical analyses and experimental results was smaller hence presenting a high reliability. By employing the design of experiment technique, performance improvement of the counter rotating type pump turbine was observed. The team also observed that the results of the regression analysis in accordance with the implementation of the 2^K factorial design confirmed that those design variables associated with the front runner (four blades) had a significant impact on the performance in the turbine mode. On the other hand those design variables associated with the front impeller (five blades) had a significant impact on the performance in the pumping mode. It was also confirmed that unstable flow components observed in the base model were suppressed through an analysis on the internal flow field. The team agreed that the overall performance of the system improved due to the suppression of these irregular flow components.

Herein, the preliminary design technique with the trade-off relation between both pumping and turbine modes of the counter-rotating type pump-turbine have been presented. A comparison between the standard centrifugal pump-turbine design and the axial type pump-turbine design show that the high specific speed is exceedingly useful in terms of efficiency at especially the condition of high flow rate and low head.Furthermore, the counter-rotating type is possible to make the compact size design in comparison with the general design with the same power, and consequentially is useful for the operation in the region of low head.

About The Author

Jin-Hyuk Kim received his Ph.D. degree in Thermodynamics and Fluid Mechanics at Inha University, Korea, in 2013. He was a postdoctoral researcher in Faculty of Engineering at Kyushu Institute of Technology, Japan, in 2013. He is currently a senior researcher at Korea Institute of Industrial Technology (KITECH) and an assistant professor at University of Science and Technology (UST), Korea.

His research interests are turbo machinery (fans, compressors, pumps, turbines, and pump-turbines) designs and developments; steady and unsteady numerical analyses; optimization methods; flow measurements and experimental techniques.

About The Author

Young-Seok Choi received his B.S. degree from Seoul National University in 1988, and his M.S. and Ph.D in Mechanical Engineering at the same university in 1990 and 1996, Korea, respectively. He is currently a principal researcher in KITECH and a Professor in Advanced Energy & System Technology at the UST (University of Science and Technology). Dr. Choi is a Director of KSFM (Korean Society for Fluid Machinery).

His research interests are turbo machinery (fans, compressors, pumps, turbines, and pump-turbines) designs and developments; steady and unsteady numerical analyses; optimization methods; flow measurements and experimental techniques.

About The Author

Toshiaki Kanemoto received his Doctor of Engineering from Tokyo Metropolitan University, Japan, in 1985. He is currently a research professor in Institute of Ocean Energy at Saga University, Japan, and an Emeritus professor at Kyushu Institute of Technology, Japan, and a consultant professor at South China University of Technology, China, and a visiting professor at Kanagawa University, Japan.

His research interests are turbo machinery (wind power unit, hydroelectric units, ocean stream power units, etc.) designs and developments.

Reference

Joon-Hyung Kim, Bo-Min Cho, Sung Kim, Jin-Woo Kim, Jun-Won Suh, Young-Seok Choi, Toshiaki Kanemoto, Jin-Hyuk Kim. Design technique to improve the energy efficiency of a counter rotating type pump-turbine. Renewable Energy volume 101 (2017) pages 647-659.

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Renewable Energy Global Innovations features: Solvent-assisted microstructural evolution and enhanced performance of porous zinc oxide films for plastic dye-sensitized solar cells

Significance Statement

Recently, dye-sensitized solar cells have been attracting considerable attention. This can be highly attributed to their low fabrication cost and possibilities of several practical indoor uses such as, the replacement of transparent conducting glass for plastic substrates. Their lightness and flexibility place them as best candidates for use in mobile devices and curved surfaces. During fabrication, the metal oxide semiconductor films used are generally heated to very high temperatures so as to obtain the effective connections among semiconductor particles for better electron transport properties. A challenge is therefore presented in achieving high performance dye-sensitized solar cells since the processing of semiconductor films using plastic substrates demand application of low temperatures.

A team of researchers at Keio University in Japan, Hitomi Ohashi, Manabu Hagiwara and Shinobu Fujihara proposed a study that focused on the post chemical treatments of the low temperature-processed zinc oxide films on plastic substrates through immersion in water-ethanol solvents. They aimed at improving the cell performance; specifically, in the short circuit current density and the power conversion efficiency. Their work is now published in Journal of Power Sources.

First, the research team began by fabricating the porous zinc oxide films of two kinds of macroscopic morphologies on the indium tin oxide-coated polyethylene naphthalate substrate at low processing temperatures for use in the plastic dye-sensitized solar cells. They then immersed the fabricated films into water-ethanol solvents at a temperature of about 90 ^°C where crystal growth of the zinc oxide was anticipated. Afterward they conducted short-circuit and power conversion efficiency tests on the films.

It was observed that by immersing the films into the water-ethanol mixture, its microstructure was greatly modified. This was due to the growth of the constituting zinc oxide particles and also due to the evolution of the inter-particle connection owing to the solvents composition. The change in the microstructure could therefore be explained by the dissolution and re-precipitation of zinc oxide with the Ostwald ripening, depending on the solvent composition. They also observed that plastic dye-sensitized solar cells using the immersed zinc oxide films with the flower-like particles and the densely packed nanoparticles exhibited a power conversion efficiency of 3.9 and 4.1%, respectively.

The cell performance is largely improved especially in the short-circuit current density together with the power conversion efficiency. The immersion effect is more remarkable for the cell using the densely packed zinc oxide films. The plastic film-type fabricated dye-sensitized solar cells are hereby presented to be more economical from the fabrication and performance point of view as opposed to their transparent conducting glass-type counterparts. In fact, the plastic N719-sensitized zinc oxide cell commands a conversion efficiency as high as 4.1%.

About The Author

Hitomi Ohashi received Master of Engineering in Integrated Design Engineering from Keio University, Japan, in 2016. She was a winner of GGRN Student Poster Award at 11th International Conference on Ceramic Materials and Components for Energy and Environmental Applications (Vancouver, 2015). Now she works at one of the largest chemical companies in Japan.

About The Author

Dr. Manabu Hagiwara is a research associate in the Department of Applied Chemistry of Keio University, Japan. He received the B. Eng. and M. Eng. degrees in 2009 and 2011 from Tokyo Institute of Technology, Japan. He received the Ph.D. degree in Materials Engineering from Tokyo Institute of Technology in 2013. He has been conducting research on the development of functional oxides using chemical synthesis methods.

The main focus of his current research is on synthesizing novel lead-free ferroelectric and piezoelectric oxides, especially those with bismuth-based compositions, and on understanding the structure-property relationships in them. His research interest also includes fabrication of nano/meso-structured oxide materials for thermoelectric and electromagnetic applications.

About The Author

Dr. Shinobu Fujihara received his Ph.D. in Molecular Engineering from Kyoto University, Japan, in 1995. He has been working at Keio University since then and now is Professor. He is also a CerSJ Fellow (The Ceramic Society of Japan). He was a Visiting Scholar at RWTH Aachen University of Technology, Germany, from 2001 to 2002.

His main research fields include sol-gel processing of nanomaterials, optical design and synthesis of inorganic luminescent materials, and nanostructural control of metal oxide semiconductors for photoelectrochemical devices.

Reference

Hitomi Ohashi, Manabu Hagiwara, Shinobu Fujihara. Solvent-assisted microstructural evolution and enhanced performance of porous zinc oxide films for plastic dye-sensitized solar cells. Journal of Power Sources volume 342 (2017) pages 148-156.

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Renewable Energy Global Innovations features: Early exploration of Aluto geothermal field, Ethiopia (History of discovery well LA-3)

Significance Statement

The southern sector of the Ethiopian Rift Valley hosts the Aluto Geothermal Field, located between Lakes Ziway and Langano. Initial exploration studies until 1978 by the Ethiopian Geological Survey outlined two separate large areas (> 30 km2 each) with active thermal manifestations. One area, exhibiting fumaroles and steaming ground, occupies the greater summit region (> 2000 masl) of the inactive Aluto volcano, a strato-volcano comprising a complex pile of rhyolitic domes and pyroclastic flows. The second area with significant surface discharges covers the lower southern flanks near the N shores of Lake Langano where hot springs and weak fumarole activity discharge up to 20 MW anomalous heat around the greater Oitu Bay area (c. 1600 masl).

A UNDP- sponsored mission in 1978 proposed deep exploration drilling of several geothermal prospects in the Southern Rift Valley and obtained pledged financial support by the Ethiopian Govt, the EEC, and UNDP. During 1980 a second review meeting selected the Aluto prospect as its first target. A conceptual model was used to locate the first two deep well sites assuming that a convective, liquid dominated, high-T system occurs beneath Oitu Bay. The model was based on observed low resistivity values of rocks at intermediate depths and the geochemistry of discharged fluids. Deep drilling was supervised by a NZ consulting team (GENZL). Results and progress were reviewed by a small group of project engineers and earth scientists with the first author (MPH) acting as consultant on secondment from the University of Auckland.

Drilling of the first deep well (LA-1) near the lake shore started in 1981 and reached a final depth of c 1.3 km; a bottom hole T of 85 deg C was encountered. T-logs showed that fluids with elevated temperatures (up to 72 deg C) were only found in the upper 150 m. The results refuted the original model of a high T-system with convective upflow beneath Oitu Bay but indicated shallow lateral (advective) outflows from a high-T reservoir beneath the Aluto strato-volcano.

The 2^nd well (LA-2) was shifted to the western foothills of Aluto dome , c. 10 km to the NW of LA-1 , close to a large fumarole where coherent low resistivity rocks at intermediate depth had also been found throughout the area.. Well LA-2 was completed in late 1982 and encountered 105 deg C at bottom hole. Mineralogical studies of cuttings and cores showed that high-T, alteration minerals (epidote for example) can be found in the well. This finding confirmed that the local low resistivity structures, used in part for locating the first two wells, can reflect some old (palaeo-fluid) alteration effects not only recent thermal alteration. The palaeo-temperature structure in LA-2 could be interpreted in terms of a conductive heating-and cooling cycle lasting over a period of c. 0.14 Ma (using a one-dimensional analysis).

The project proceeded with the search for a third well site within the high standing region around Aluto dome where scattered steaming ground had been observed although repeat- resistivity surveys had not found a coherent, low resistivity pattern in the upper 300 m. The project opted to use c. 50 to 70 meter temperature-gradient (TG) holes to explore high- standing hot grounds. Using an intersection pattern of hot grounds and fault-controlled NNE trending steam vents, well LA-3 (1920 masl) could be sited. Another logistic problem had to be overcome, namely pumping (drill-) water via a 7 km long pipeline from Lake Ziway (1640 masl) to the upper slopes of Aluto Dome. After its completion, drilling of LA-3 well was started in February 1983. It was drilled to c. 2.1 km depth, encountered 320 deg C fluids at the bottom, and discharged freely – it became the discovery well of the Aluto prospect.

The Aluto study has shown that siting of exploratory wells over inferred permeable fault structures of volcanic geothermal systems, based in part on resistivity surveys, becomes an uncontrolled exercise if targets are not associated with coherent manifestations. Thermally altered volcanic rocks at Aluto are caused by present-day convective and advective hot fluids but also by palaeo-thermal outflows through now cooler rocks. The Aluto example shows that its likely hot and productive area cannot be assessed from the extent of low resistivity anomalies at intermediate depths if  palaeo-resistivity structures are present. The work has been published in Geothermics.

About The Author

Manfred P Hochstein

Dipl. Geophysiker, TU Clausthal, Germany
Dr. rer. nat. Münster, Germany (PhD)
Fellow of the Royal Society of New Zealand

Born in 1932 in Germany, I started my career by specialising in Geophysics, Geology and Physics when receiving my diploma and doctoral degrees in 1959 and 1962 respectively at German universities. Post-graduate studies followed (1962-1964) in the US and Antarctica (Antarctic Service Medal 1963) prior to joining the Department of Scientific and Industrial Research (DSIR), Wellington (NZ) as a geophysicist in 1964.

I continued to work as:

• Superintendent of the Geophysical Survey, Geophysics Division (DSIR) from 1966 until 1972.
• Associate Professor at the University of Auckland teaching Applied Geophysics at the Geology Department (1973-1997).
• Founding member of the UNDP- sponsored Geothermal Institute (1978) to provide graduate courses for earth-science and engineering students (mainly from developing countries). Director of Geothermal Institute (affiliated with the Geology Dept. and School of Engineering, University of Auckland) from 1980 until 1994; continuation with part- time teaching at the Institute from 1998 to 2008.
• Invited as consultant to assess UNDP sponsored geothermal projects (Chile, El Salvador, Nicaragua, Kenya, Ethiopia, P.R of China and Tibet) between 1967 and 1988; participation in NZ – Indonesia bilateral geothermal aid projects (1972-1978); consultancy work for UNEP projects (Kenya, Rwanda, Tanzania) since 2012.
• Consultant since 1988 for geothermal projects in New Zealand, Indonesia, El Salvador, Philippines, and Zambia; involvement in sub-contracted studies for Castlerock Consulting (World Bank-sponsored), assessment of geothermal prospects in Indonesia and Vanuatu (2010 and 2011).

Since my retirement in 1998, I continued with consulting work for the NZ Dept of Conservation (monitoring thermal manifestations), undertaken through UniServices, the commercial arm of the University. I became a sub-contracted consultant for the AECOM Group (Auckland and Jakarta) and  continued with own research work assisted by  IGNS (Wairakei) staff.

During the next decade (up to 2010), I received invitations to visit geothermal projects in Indonesia and China. My visit of Indonesian prospects on  Banda Arc Islands (2008) was sponsored by PGE (Pertamina, Jakarta) – it was followed by teaching of a refresher course at the 2009 annual Geofisika Indonesia Conference in Yogyakarta and a Peer Review of the Wayang-Windu  Field  (Java), contracted through the AECOM (Auckland ) group. I visited China as invited speaker at the Intern. Geothermal Conference in Beijing (2002) and the UNESCO sponsored Conference on Coal Fire Research (Beijing 2006). During this period, I became Guest Editor of a special issue of ‘Geothermics’, covering Indonesian geothermal developments (2008) and joined the IESE research group at the Univ. of Auckland as a ‘research scientist’.

Consultancy work in 2011 involved a review of the Jailolo prospect (Moluccas) for StarEnergy, reviews of heat-loss studies at Muaralabuh, Rantau Dedap, and Rajabasa (Sumatra) for Supreme Energy involving also ‘hands–on’ teaching. Similar work in 2012 covered a Peer Review of low T systems in Zambia (for Kalahari GeoEnergy Ltd), a review of the Karisimbi project (N Rwanda) for IESE/UniServices  Auckland, and due diligence assessments of the Ijen Caldera prospect (Java) for AECOM (Jakarta) and a small, high T prospect in NZ (Reporoa) for AECOM (Auckland).

Consulting in 2013 was taken up by a review of the Tampomas geothermal prospect (Java) for IESE, a review of the Silali prospect in Kenya (for UNEP, Nairobi) and a review of the Tangkuban Perahu project for Indonesian Power (IP) under an IESE/UniServices contract. PGE (Pertamina Jakarta) invited me and other IESE staff to conduct a 2-week professional refresher course for their geothermal staff at Bandung in August. Professional work in 2014 involved continuation with the IP (Tangkuban Perahu) project and tendering for a Tendaho (Ethiopia) project. During the year, I completed a review of the Kinigi (Rwanda) project for UNEP (Nairobi) and an appraisal of the first phase of the Baturraden (G.Slamet) project in Java under contract with AECOM (Jakarta).

In 2015 I undertook a review of the Banten (Java) geothermal prospect for a PIP (Indonesian  Govt.) loan assessment, contracted by Castlerock Consulting. I became a member of the IGA Geothermal Resources and Reserves Committee and reviewed old assessments of Nicaraguan geothermal resources by private consultants. Later during the year, exploration reports of two Mexican geothermal prospects, a Univ. of Mexico project, were assessed under contract with the newly formed IESE (Auckland) group. A review of two Ethiopian prospects (Tulu Moye and Gedemsa) for the Auckland AECOM group completed the year’s activity. Work on the Mexican projects and reviews of Baturraden reports continued into 2016 until May when I reviewed several Tanzanian geothermal projects for UNEP (Nairobi). Recently, I was invited as guest speaker and (short course) lecturer at the  Universitas Indonesia, Jakarta (Oct 2016). In 2016 I was awarded the annual ‘Patricius Plakette’ by the German Bundesverband Geothermie in Essen (Germany).

My own research studies during my professional activities during the last 20 yrs involved the assessment and delineation of natural resources (geothermal, ground water, and mineral resources) and studies of regional geological/geophysical topics (sedimentary basins, volcanic and tectonic structures, glaciology, seismological research). I have maintained good contact with many ex-students both in NZ and overseas who still ask me to review some of their manuscripts. My research has led to the publication of c. 100 papers and book articles in refereed journals and c. 100 articles in the Proceedings of geothermal conferences and workshops (references listed in ‘Google Scholar’ and ‘Research Gate’). I am still a member of the Editorial Board of the Geothermics journal, a life honorary member of the NZ Geothermal Association, and since 1992 a Fellow of the Royal Society of NZ.

Reference

Manfred P. Hochstein, Befekadu Oluma, Hagen Hole. Early exploration of the Aluto geothermal field, Ethiopia (History of discovery well LA-3). Geothermics volume 66 (2017) pages 73–84.

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