Just Released! “Energy, Biology, Climate Change”

FrontCover6x9 white border 72dpi EBCC2015

Our newest book, published on May 6, 2015 and available at Amazon.com for $19.95.

The focus of this book is the interactions between energy, ecology, and climate change, as well as a few of the responses of humanity to these interactions. It is not a textbook, but a series of chapters discussing subtopics in which the authors were interested and wished to write about. The basic material is cutting-edge science; technical journal articles published within the last year, selected for their relevance and interest. Each author selected eight or so technical papers representing his or her view of the most interesting current research in the field, and wrote summaries of them in a journalistic style that is free of scientific jargon and understandable by lay readers. This is the sort of science writing that you might encounter in the New York Times, but concentrated in a way intended to give as broad an overview of the chapter topics as possible. None of this research will appear in textbooks for a few years, so there are not many ways that readers without access to a university library can get access to this information.

This book is intended be browsed—choose a chapter topic you like and read the individual sections in any order; each is intended to be largely stand-alone. Reading all of them will give you considerable insight into what climate scientists concerned with energy, ecology, and human effects are up to, and the challenges they face in understanding one of the most disruptive—if not very rapid—event in human history; anthropogenic climate change. The Table of Contents follows: Continue reading

Optimization of tilt angle for solar panel: Case study for Madinah, Saudi Arabia

In the search for a feasible alternative to conventional energy production, solar power is a proposed solution with a lot of upside, as the sun delivers a massive amount of energy to the Earth daily.  Roadblocks to the implementation of solar and other renewable include cost, infrastructure, and efficiency.  Improving in any of these areas will serve to hasten the transition to solar power.  Efficiency can be further broken down into the categories of technology and logistics.  While extensive research is needed to develop new materials to improve on the modern components of solar panels, the logistics of giving the panels the best opportunity to harness the sun’s power are not as worrisome.  We currently have an extensive amount of data as to which locations receive the most light, and how to place panels to fully use it.  One method of improving solar efficiency is using tilted solar panels.  Conventional flat solar panels are less efficient, because the sun hits them at an angle, decreasing the potential surface area of the panels.  By tilting the panels to hit the sun’s rays head on, we are able to fully utilize a panel’s surface area.  The first tilted panels have been installed to get the most utility out of the sun based on annual averages, at an angle approximately equal to their latitude.  However, as Benghanem discovered, a month-to-month angular model can significantly increase the panels’ efficiency.—Donald Hamnett
Benghanem, M., 2010. Optimization of tilt angle for solar panel: case study for Madinah, Saudi Arabia. Elsevier, doi:10.1016/j.apenergy.2010.10.001.

The amount of solar energy incident on a panel is a complex calculation, and is variable based on the time scale used.  It is a function of local radiation climatology, the orientation and tilt of the collector surface, and ground reflection properties.  Orientation and tilt is the factor that we have control over.  Though modeling this situation is not new, the existing models make an assumption that yields incorrect values.  The current models assume that sky radiation is isotropically distributed at all times; in other words, it is uniformly distributed in all directions.  This is not the case on our planet, so Benghanem’s model arrives are a more sophisticated expression for solar energy potential.  He developed an estimate based on anisotropic modeling methods, using horizontal solar radiation data from meteorological databases.  The sky-diffuse radiation can be expressed as the ratio of the average daily diffuse radiation on a tilted surface, to that on a horizontal surface.  This relation is composed of values for the following variables: daily beam radiation incident on a horizontal surface, extra-terrestrial daily radiation incident on a horizontal surface, daily ground reflected radiation incident on an inclined surface, ratio of average daily beam radiation incident on an inclined surface to that on a horizontal surface, and the surface slope from the horizontal.  Other factors included in the overall analysis were tracking the sun’s movement throughout the day to keep the panels perpendicular to the sun’s radiation, the zenith angle, which requires use of solar time and hour angle as opposed to the human established time-zone times.  The cosine of the zenith angle can be calculated as a function of hour angle, latitude, and solar declination.
This analysis confirmed that the optimum tilt angle is indeed different for each month of the year, and that the yearly optimum tilt angle equals the latitude of the site.  At Madinah site, which was the example modeled, the winter months (December-February) required an angle of 37 degrees, the spring months (March-May) required an angle of 17 degrees, the summer months (June-August) required an angle of 12 degrees, and the autumn months (September-November) required an angle of 28 degrees.  Overall, using yearly averages as opposed to monthly averages lost about 8% of the energy that would have been produced using monthly averages for the site at Madinah.

Modeling and Experimental Validation of a New Hybrid Photovoltaic Thermal Collector.

Photovoltaics and solar thermal collectors are often competing technologies used to harness the energy of the sun.  Photovoltaic (PV) systems convert solar energy directly into an electric current whereas solar thermal collectors harness the heat of solar radiation to generate electricity in a process similar to the one used in conventional power plants.  Both systems have advantages and drawbacks.  Photovoltaics generate lower conversion rates under direct sunlight and experience a decrease in efficiency when the temperature of the solar cell increases.  Current technology also does not allow electricity to be stored for later use.  Solar thermal collectors reach relatively high temperatures under a clear sky which allows them to reach higher conversion rates.  Thermal energy can be efficiently stored in the form of fluid storage tanks or molten salts.  However, solar thermal collectors generate little power under diffused light conditions while photovoltaics can continue operating under such weather.  The weaknesses and advantages of both systems complement each other and there have been attempts to integrate the two systems into a hybrid cell.  Touafek et al. (2011) have attempted to create precisely such a hybrid photovoltaic thermal collector (PVT collector).  The PVT collector consists of a photovoltaic layer placed adjacent to a thermal collector unit.  As such, excess heat that decreases the efficiency of the photovoltaic cell will be transferred to the solar thermal unit, which can generate electricity with the heat.  Touafek et al. present the results of their modeling and experimentation on a prototype hybrid PVT collector.—Alan Hu
Touafek, K., Haddadi M., Malek A. 2011. IEEE Transactions on Energy Conversion 26, 176-183.

            Touafek at the Unit of Applied Research in Renewable Energy, Haddadi at the Algerian National Polytechnic University, and Malek at Renewable Energy Development Center at Algeria constructed a prototype PVT collector and created a theoretical framework used to calculate various indicators.  The PVT collector prototype consists of two main components: the first is the photovoltaic section of the system and the second is the thermal collector.  The photovoltaic section of the system consists of three layers in the following order: tempered glass, a PV cell, and Tedlar (used as a backsheet to protect the PV system).  Immediately adjacent to the Tedlar layer is thermal collector which essentially is a system of pipes that pumps cooling fluid.  Any excess heat generated by the PV system is absorbed and transferred by the pipes and coolant. 
Toaufek et al. develop a numerical model that attempts to take into account all sources of heat that may affect the PVT collector, including heat from ambient air, from the ground, from the sky, from solar radiation absorbed by the glass, and from other sources.  Also included in the model is the transfer of heat between components of the PVT collector.  For example, heat transferred from the glass to the PV cell through conduction is considered in the calculations.  Toaufek et al. are able to isolate the temperature of the solar cell and the fluid through the model.  The two figures are important as the goal of the PVT collector is to minimize the temperature of the PV cell and to maximize the temperature of the fluid.
The researchers use the temperature of the solar cell and the temperature of the fluid to judge the optimal thickness of the pipe material.  Thicknesses of 0.01, 0.02, 0.03, 0.04, and 0.05 m were tested and the thickness that maximized fluid temperature and minimized solar cell temperature.  Toaufek et al. continue to present calculations from the model and from experimentation to validate the accuracy of their model.  The paper claims a total efficiency of 80%, though it is unclear if this refers to EQE or thermal efficiency.  Further research and experimentation is recommended in the conclusion.

Meta-Analysis of Estimates of Life Cycle Greenhouse Gas Emissions from Concentrating Solar Power

A life cycle assessment (LCA) is a method of predicting environmental impacts of renewable energy technologies.  The advantage of renewable energy plants is that they do not emit significant amounts of greenhouse gases (GHGs); however, over their lifespan they do impact the environment with GHG emissions.  One such renewable energy source is concentrating solar power (CSP).  Analysts have conducted LCAs of the three main CSP technologies: parabolic trough (trough), power tower (tower), and parabolic dish (dish).  The current LCAs in regard to CSP technology have relatively high variability caused by a range of factors that include, “the type of technology being investigated, scope of analysis, assumed performance characteristics, location, data source, and the impact assessment methodology used” (Heath and Burkhardt 2011).  Gavin A. Heath and John J. Burkhardt from the National Renewable Energy laboratory aimed to conduct a meta-analysis which reduced the variability in CSP LCAs through a method called harmonization.  Harmonization takes several already published reports and aims at establishing more consistent methods and assumptions between them.  As part of the larger LCA Harmonization Project of the United States’ National Renewable Energy Laboratory, this meta-analysis will be used clarify estimates of central tendency and inform future decision making in regard to CSP technology as a whole, though the estimates for specific plants will vary due to their deviation from these generic estimates.—Donald Hamnett  

Heath, Garvin A., Burkhardt, John J., 2011. Meta-Analysis of Estimates of Life Cycle Greenhouse Gas Emissions from Concentrating Solar Power. National Renewable Energy Laboratory.

The three major LC phases for typical CSP plants used in the study are upstream, operational, and downstream.  Upstream processes include extraction of raw materials, materials manufacturing, component manufacturing, site improvements, and plant assembly.  Operational processes include manufacture and transportation of replacement components, fuel consumption in maintenance/cleaning vehicles, on-site natural gas combustion, and electricity consumption from the regional power grid.  Downstream processes include plant disassembly and disposal or recycling of plant materials.  In searching the English literature of environmental impacts of CSP, Heath and Burkhardt found 125 references, 13 of which provided sufficient numerical analyses yielding 42 GHG estimates (19 trough, 17 tower, 6 dish).  This was trimmed to 36 for the first-level harmonization process because of how few dish estimates were given.  The second level harmonization process, requiring more complete input and assumption documentation, used five trough studies with five estimates.  The first “light” level of harmonization, at a gross level, adjusts emissions estimates proportionally by comparing them to consistent values for performance characteristics, creating a common system boundary.  The parameters chosen for this level are solar fraction, direct normal irradiance, lifetime, solar-to-electric efficiency, global warming potentials, auxiliary natural gas consumption, and auxiliary electrical consumption.  The input-intensive harmonization method used GWIs, measures of the mass of GHGs emitted from the production of common materials and from other activities.
            In the light analysis, the most effective harmonization parameter was solar fraction, independently reducing the interquartile range (IQR) for trough CSP emissions by 85%.  The factor that had the biggest impact on central tendency was auxiliary electrical consumption, independently increasing the median of trough emissions by 50%.  Cumulatively, the light harmonization parameters decreased the trough IRQ by 69% and increased the median by 76%.  For tower systems, the IQR and median were reduced by 26% and 34%, respectively.  The more intensive GWI harmonization for the five trough CSP estimates assessed further reduced the median by an additional 6% and increased the range by 5%.  When pooled with the 14 trough estimates not assessed in the GWI analysis, the IQR decreased by an additional 9%.  These data and GHG emission estimates for CSP technology provide decision makers with a more thorough and complete view on the factors involved in the integration of this technology.

Effect of Electron Donor Concentration on Power Conversion

Organic photovoltaics (OPVs) operate by separating two electrodes with a layer of organic polymeric material that generates a current between the two ends of the diode. Initial designs of OPVs used a single layer of organic material that, when struck by sunlight, generates excitons due to electrons vacating the Highest Occupied Molecular Orbital (HOMO) and filling the Lowest Unoccupied Molecular Orbital (LUMO). Due to the existing difference of charge between the two ends of the circuit, the excitons flow to the positive electrode, establishing a current. This design, however, yielded extremely low power conversion rates due to the weakness of the electric field established by the electrodes. Bilayer OPVs, which had electron donor and acceptor layers of organic material, were subsequently developed to address this issue. The two layers of the bilayer OPV generated themselves electrostatic forces that helped generate a stronger electric field. Bilayer OPVs still yielded low power conversion rates, however, due to the discrepancy between the diffusion length of excitons and the length of the polymer layer required to absorb enough sunlight. Bulk heterojunction (BHJ) cells combine the donor and acceptor layer of the bilayer OPV into a single bulk layer. This design solves the issue of matching exciton diffusion length with the length of the polymer layer and has so far yielded the highest power conversion rates of any OPV. An central question in growing BHJ cells is the question of what acceptor/donor ratio is optimal for achieving higher conversion rates. Zhang et al. (2011) have demonstrated through their experimentation that low donor concentration (under 10%) and the presence of molybdenum oxide (MoOx) can yield relatively high conversion ratios across a diversity of donor materials.—Alan Hu

Zhang, M., Wang H., Tian H., Geng Y., Tang C. 2011. Bulk Heterojunction Photovoltaic Cells with Low Donor Concentration. Advanced Materials 23, 4960–4964.

Zhang et al. at the University of Rochester and the Changchun Institute of Applied Chemistry grew cells composed of a sequence of indium tin oxide (ITO), molybdenum oxide (MoOx), TAPC:C60/C70, bathophenanthroline (Bphen), and aluminum (Al). As the researchers were primarily interested in measuring the effect of the BHJ layer TAPC:C, the dimensions and composition of the other layers were kept constant: ITO (90nm), MoOx (2nm), Bphen (8nm), Al (100nm). Researchers then modeled and tested the cells for indicators including the short circuit density, voltage, fill factor, power conversion efficiency, calculated hole mobility, and calculated electron mobility. Hole and electron mobility, the speed at which an electron can move through a semiconductor, were calculated with Bässler’s model.

The experiments yielded results that indicated a low concentration of donors was optimal. Voc increased as donor concentration decreased until 5%, below which Voc began to drop quickly. The same was found to be true for Jsc, which increased until donor concentration dropped below 5%. Similarly, the fill factor peaked at 0.53, which occured when donor concentration was between 5% and 10%. High donor concentrations, on the other hand, yielded poorer results: when TAPC concentration was 50%, fill factor was 0.29. External quantum efficiency (EQE) also peaked with a low TAPC concentration. At TAPC 5%, EQE was about 60% while at TAPC 50%, EQE was only 10%.

Researchers also found that the commonly used HOMO/LUMO gap rule did not directly affect Voc as long as donor concentrations were sufficiently low. Instead, the Voc was largely determined by the Schottky barrier, a difference in charge formed between a metal and semiconductor, created between the MoOx and BHJ layers.

Zhang et al. (2011) achieved an impressive conversion ratio of 5.23% in their tests. They concluded that a low donor concentration maximized efficiency in BHJ OPVs and recognized the importance of the MoOx for generating the electric field

Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts

Solar photovoltaic cells (PV) provide a renewable method of both generating electric current and storing fuels as hydrogen gas. The existence of this technology offers the hope of replacing our increasingly scarce fossil fuels with a dependable, infinite alternative; however, the monetary cost of manufacturing these cells has blocked their implementation. Traditionally, the PV process required the use of “prohibitively expensive light-absorbing materials [e.g., (Al)GaAs and GaInP], and/or fuel-forming catalysts (e.g., Pt, RuO2, IrO2), and strongly acidic or basic reaction media, which are corrosive and expensive to manage over the large areas required for light harvesting” (Reece et. al 2011). The aim of Reece and his colleagues is to mimic the photosynthetic process with earth-abundant materials under neutral pH conditions. They were successful in achieving this goal for both wired and wireless solar photo-electrochemical systems (PEC), replicating leaf photosynthesis to fix hydrogen and oxygen gas. For the wired system they were able to obtain an efficiency of 4.7%, while the wireless system had an efficiency of 2.5%. The continual development of this technology offers greater potential for solar energy to replace traditional sources due not only to necessity, but due to market factors as well.—Donald Hamnett

Steven Y. Reece, et al.,Wireless solar water splitting using silicon-based semiconductors and earth-abundant catalysts. Science 334, 645.

The specific compounds Reece and his colleagues discovered that make this process work are: a cobalt oxygen evolving catalyst (Co-OEC), a nickel-molybdenum-zinc hydrogen evolving catalyst (NiMoZn), and a triple junction amorphous silicon (3jn-a-Si) interface between the catalysts which is coated with indium tin oxide (ITO). In the wired case, the NiMoZn catalyst was deposited on a nickel mesh substrate, which was wired to the 3jn-a-Si electrode. In the wireless case, the NiMoZn catalyst was deposited directly onto the silicone electrode’s adjacent stainless steel surface. Baseline values for the 3jn-a-Si cell were obtained by operating the cell in a three-electrode voltammetry setup. Under a no-light condition, the cell had a current of less than .05 mA/cm2, a low value. Upon illumination of 1 sun (100mW/cm2) of air mass (AM) of 1.5, the cell produced a current of .39 mA/cm2 at a potential of 0.55V when using a 1M potassium borate electrolyte (pH 9.2). To increase the efficiency in this example, they added the 0.25 mM Co2+(aq) catalyst. The Co-OEC increased the current to 4.17 mA/cm2. Bubbles at both electrodes indicated the formation of oxygen and hydrogen gas. The experiment went on to study the effect of differing thicknesses of Co-OEC films on the electrodes’ surfaces. It was found that at a 5 minute deposition time, the thin (85 nm) film created photoanodes with optimum performance. There is a give and take between Co-OEC presence and performance of the cell, because though it increases activity, the built up film also blocks incident radiation. When they tested the wireless cell, it was found to be stable for 10 hours. It was discovered that the stability of the wireless cell is dependent on the type of conductive oxide barrier layer used.

Though there is still work to be done to increase the efficiency and stability of these PEC cells, Steven Reece and his team has succeeded in replicating the photosynthetic functions of a leaf. Additionally, the fact that they have done so in a relatively neutral pH means that hydrogen and oxygen fuels could be generated without a membrane, as the two gases are very insoluble at a neutral pH. Lastly, the most notable aspect of this research is that they have replicated photosynthesis with low-cost, earth-abundant materials. The continued research in this area may very well spark an energy revolution, as the decreased overhead will incentivize investment in solar technology

Operating Lifetimes of Organic Photovoltaics

Organic photovoltaics (OPVs) are a type of solar cell that has recently been receiving much research.  Though such polymer solar cells have low conversion efficiencies when compared with expensive nonorganic multijunction solar cells, OPVs are also much cheaper to produce and can be inexpensively manufactured in large quantities.  Low conversion efficiencies is not the only drawback of OPVs; organic materials are much more vulnerable to degradation from environmental factors, and the lifetimes of OPVs must be seriously considered as part of the cost benefit analysis.  Peters et al. (2011) present conclusions from their study of the efficiency decay of two types of OPVs: the well-studied P3HT and relatively new PCDTBT devices.  The study concludes that the less researched PCDTBT devices decayed at a slower rate than P3HT though the P3HT had, in general, a higher absolute conversion efficiency figure.  Researchers expect that further development of PCDTBT solar cells will raise their conversion efficiency while maintaining their long lifetime.—Alan Hu
Peters, C., Sachs-Quintana, I., Kastrop, J., Beaupre, S., Leclerc, M., McGehee, M., 2011. High Efficiency Polymer Solar Cells with Long Operating Lifetimes. Advanced Energy Materials 1, 491–494.

Peters et al. at Stanford University and University of Laval test the lifetimes of P3HT and PCDTBT devices by exposing an experimental group of the two types of solar cells to a standardized environment.  Initial device efficiencies of P3HT and PCDTBT respectively were 4 ± 0.05%. and 5.5 ± 0.15%.  Since UV radiation is known to cause defects in polymer solar cells and that commercial OPVs are likely to carry UV blockers, an LG sulfur plasma lamp, which emits very little UV radiation, was used as a source of light.  The solar cells were kept in a dark room for a week after fabrication before being placed under the lamp.  The cells were then aged at maximum power point for 4400 hours under one-sun intensity at 37ºC.  Light intensity was calibrated with a National Renewable Energy Lab-certified silicon photodiode.  Both light intensity and temperature were measured every 5 seconds; current-voltage curve data was collected every hour.  The researchers defined burn-in of solar devices as the short period of exponential loss in efficiency after initial use.  Lifetime ends by convention when the efficiency of the device has dropped below 80% of its initial value.
Results from the experiment show that PCDTBT solar cells had lower efficiency ratios immediately after burn-in but suffer from less degradation than P3HT.  PCDTBT had a burn-in period of about 400 hours after which its efficiency figures remained relatively stable.  The VOC and fill factor of PCDTBT in particularly remained practically flat for 4000 hours after the initial burn-in.  P3HT devices on the other hand experienced a roughly 10% drop in efficiency per 1000 hours. This was caused by a simultaneous decrease in both VOC  and JSC­.
            A burn-in demarcation was set at 1300 hours and the lifetime of each type device tested was found with a linear regression.  Assuming 5.5 hours of one-sun intensity per day and 365 days per year, the average lifetimes of PCDTBT and P3HT solar cells were found to be 6.2 and 3.2 years respectively. As such, PCDTBT solar cells demonstrated a clear advantage over P3HT solar cells in terms of durability; one particular PCDTBT device was so remarkably stable that it was projected to have a lifetime of 11 years.
            A laser beam-induced current map, a test used to determine if a portion of the solar cell has lost effectiveness, shows that both P3HT and PCDTBT showed no loss of device area after 200 hours of aging.
            PCDTBT solar cells experience a more dramatic loss of efficiency over the burn-in period but are far more stable than P3HT solar cells after this initial period.  P3HT cells currently hold a conversion efficiency advantage over PCDTBT cells though considering the nascent stage of research into the PCDTBT polymer, PCDTBT cells have the potential for large advances in efficiency.  The researchers predict that as PCDTBT cells become more optimized, both longevity and greater efficiency can be achieved in the inexpensive OPVs.