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.

Feasibility and Costs of II-VI Materials in Multijunction Solar Cells

Solar energy is often hailed as the successor to fossil fuels as the planet’s main source of energy. However, solar cells face various issues affecting widespread adoption including prohibitive costs and low energy conversion rates. Currently, multijunction solar cells are the most efficient. These cells are able to boost conversion rates by employing different junctions of semiconductors that utilize different wavelengths of light. The most commonly used semiconductors belong to the III-V group due to perceived advantages over II-VI group semiconductors. Garland et al (2011) argue that II-VI semiconductors are both more efficient and less expensive than III-V semiconductors. Results from models and initial experimentation indicate that II-VI solar cells are 3–4% more efficient than III-V solar cells.—Alan Hu

Garland, J. W., T. Biegala, M. Carmody, C. Gilmore, and S. Sivananthan. Next-generation Multijunction Solar Cells: The Promise of II-VI Materials. Applied Physics Letters 109, 102423(2011).

Garland and colleagues of EPIR Technologies project III-V and II-VI solar cell output with the commonly accepted “standard” model put forth by Xu et al. (2010) in an earlier paper. The model uses a beta coefficient that takes such factors into account as the dimensions and doping (the molecular makeup) of the semiconductor. The beta is calculated through a best fit line describing modeled data and figures from real world performance of the latest III-V solar cell. Projected efficiency is then computed by dividing the sum of the junction outputs by the power input. The researchers supplement their projected figures with real world figures generated through actual experimentation. Garland and colleagues also grew II-VI semiconductors and collected empirical data on the solar cells.

The results from both projections and empirical observation supported the argument in favor of II-VI solar cells. Calculated efficiency for a III-V solar cell under one sun, a measure of sun intensity, was 43.7% whereas the figure for a II-VI solar cell was 49.7%. Empirical observations agree with these results: III-V solar cells were observed to have achieved 38.6% under one sun whereas II-VI solar cells achieved 44.5% under the same conditions.

The study also argues that II-VI solar cells could bring about significant reductions in manufacturing cost. The authors claim that due to the sturdy nature of silicon wafers used in the production of II-VI solar cells and lower costs of growing II-VI crystals, nearly all associated costs of creating II-VI semiconductors are lower than those of creating III-V semiconductors. Specifically, molecular beam epitaxy (MBE), which is a process for growing crystals, can be replaced by a cheaper production line method of production due to the nature of II-VI materials.

The lower cost of II-VI solar cells means that medium concentration photovoltaics can be used instead of high concentration photovoltaics. III-V solar cells were relatively costly; as such, it was cheaper to have fewer solar cells and instead have a system of mirrors that concentrated sunlight onto a small area of solar cells. This meant the costs of a solar energy field were increased by the installation of such tracking systems. The cheaper II-VI solar cells allow for a relatively larger area of solar cells and less complicated tracking systems.

The search for a commercially viable alternative to fossil fuels continues. As long as solar cells are more expensive and less effective than existing energy sources, the widespread adoption of solar energy is unlikely. However, incremental advances in photovoltaic technology are gradually cheapening the cost of solar energy. The improvement of 3–4% in energy conversion rates brought about by the use of II-VI semiconductors is one small step toward a greener future.

D. Xu, T. Biegala, M. Carmody, J. W. Garland, C. Grein, and S. Sivananthan. Proposed monolithic triple-junction solar cell structures with the potential for ultrahigh efficiencies using II–VI alloys and silicon substrate. Applied Physics Letters. 96, 073508(2010).

Enabling Greater Penetration of Solar Power via the Use of CSP with Thermal Energy Storage

Due to concerns about dwindling fossil fuel reserves and climate change, and to the falling costs of solar photovoltaic (PV) energy, solar power is being increasingly integrated into power grids. Unfortunately, several factors involving the composition of current grids limit solar power’s ability to be fully utilized. One such limitation is that the peak hours of solar availability do not coincide with the peak hours of demand. Another limitation is the current grid’s inflexibility, or inability to increase or reduce output from conventional energy sources to coincide with variable fluctuations in solar output. A solution to this problem has been proposed in the form of thermal energy storage (TES) deployed with concentrating solar power (CSP) (Denholm and Mehos 2011). This technology complements PV by storing otherwise wasted solar capacity as heat, which can be stored and used as output during periods where PV generation is minimal. Furthermore, PV with CSP stabilizes solar output to a firmer, more dependable source less subject to random fluctuations. This allows for solar to replace a portion of the grid currently occupied by conventional energy generation, and thus furthers the percentage of potential energy output due to renewables.—Donald Hamnett

Denholm, P., Mehos, M, 2011. Enabling greater penetration of solar power via the use of CSP with thermal energy storage. National Renewable Energy Laboratory. 1–28.

Paul Denholm, Mark Mehos, and colleagues at the National Renewable Energy Laboratory used a REFlex model to predict the ability of CSP to increase grid flexibility and solar penetration in the Southwestern United States. This model compares the hourly load with renewable resources to calculate energy curtailment, based on the grid’s flexibility, or ability to change generator output to accommodate variable renewable energy sources. To determine the limits of PV, the researchers used weather data from 2005 and 2006 in the System Advisor Model (SAM), which converts the data into hourly PV output. These data were in turn used to model the interaction between solar and wind generated energy, using simulated data for 2005 and 2006 from the Western Wind and Solar Integration Study (WWSIS). Simulations of energy generation per hour were conducted over a year and the four-day period, April 7-10, was displayed as the paper’s example. The area under the curve was split into different sections representing contributions from the various energy sources. Simulations were run for the current PV system with an assumed 80% flexibility, and a PV with CSP system. In the latter example, CSP was added to the REFlex simulation using SAM produced hourly generation values.

The simulation under a PV-only scenario showed that a significant proportion of annual PV production, 5%, is curtailed due to a lower energy demand, since energy demand is low when PV output is high, and other generators cannot slow output down at fast enough ramp rates. In this situation, the PV curtailment and cost increase exponentially as the percentage of energy from PV increases. Clearly, this poses an obstacle to switching to an increasingly renewable grid because it puts a limit on the proportion of solar energy that is practical. In order for this PV to be utilized, a more flexible grid is required. When the researchers included CSP into the REFlex model, the data suggested that flexibility improved and curtailment reduced.

The CSP model was based on wet-cooled trough plant technology (Wagner and Gilman 2011). In this scenario, solar energy that would have otherwise been curtailed during low load hours was instead stored thermally during the day. At the same time, PV energy was incorporated into the grid; however, as PV lessened in the night hours and load increased, the stored CSP energy was run through the grid. Energy that would otherwise have been wasted during low load hours could now be stored and used during peak hours. This decreased the annual solar curtailment to less than 2%. Also, it increases the solar energy contribution to 25%, 15% PV and 10% CSP.

Denholm and Mehos also found that CSP/TES helped to lower the baseline amount of conventional energy needed, because CSP allows for solar energy to be utilized at all hours. This result has cascading effects, as the implementation of CSP lowers the minimum conventional generation requirements. CSP allows for additional solar power to be added at much lower marginal curtailment rates. Lowering the minimum load that relies on conventional energy opens up greater probabilities of increased solar and wind components. The researchers, through their complex grid analysis, determined that CSP technology has the ability to both increase solar efficiency and the possibility of the implementation of greater variable energy inputs.

Photovoltaic Technology in Regions of Low Solar Irradiation: A Broad Assessment of Environmental Impact

Whereas most lifecycle assessments (LCAs) use one-dimensional indicators and only apply to areas of high solar irradiation, Laleman et al. (2011) used both one-dimensional indicators and the multi-dimensional Eco-Indicator 99 (EI 99) to conduct a broad assessment the environmental impact of various photovoltaic (PV) technologies employed in areas of low solar irradiation such as Canada and Northern Europe.  Furthermore, they used these same indicators to compare PV systems to other sources of electricity production.  The authors found the energy payback time of PV systems to be less than 5 years, and the global warming potential to be approximately 10 times lower than a coal plant and 4 times higher than a nuclear power plant or wind farm.  The authors obtained significantly different results using EI 99 compared to one-dimensional indicators, and thus stressed the importance of carefully evaluating a combination of different environmental impact assessment approaches.—Lucy Block
Laleman, R., Albrecht, J., and Dewulf, J., 2011. Life Cycle Analysis to estimate impact of residential photovoltaic systems in regions with a lower solar irradiation. Renewable and Sustainable Energy Reviews 15, 267-281.

          Ruben Laleman, Johan Alrecht, and Jo Dewulf of Ghent University in Belgium used lifecycle data from the Ecoinvent database (v2.0) to assess the environmental impact of six different PV technologies under conditions of low solar irradiation (900-1000 kWh/m2/year).  As opposed to only using a one-dimensional indicator such as Cumulative Energy Demand (CED), Energy Payback Time (EPT), or Global Warming Potential (GWP), as many other authors conducting LCAs do, Laleman et al. compared environmental impact findings of these one-dimensional indicators to the multi-dimensional EI 99.  The authors also compared their findings of PV environmental impacts to the impact of other electricity sources such as hard coal, natural gas, and the Belgian electricity mix.
          The authors’ findings of PV technology’s environmental impact for the one-dimensional indicators—CED, EPT, and GWP—were comparable to previous literature conducted on the subject, but their findings for the EI 99 had very little correlation with the one-dimensional indicators (at most 22%).  Therefore, they stress the importance of employing a multi-dimensional indicator, especially alongside one-dimensional indicators, in order to give the most nuanced picture possible of environmental impacts.
          Besides assessing environmental impact for various environmental indicators—mineral extraction, fossil fuels, respiratory effects, ozone layer  depletion, ionizing radiation, climate change, carcinogenics, land occupation, ecotoxicity, and acidification and eutrophication—EI 99 categorizes those indicators into three main dimensions: human health, ecosystem quality, and the depletion of non-renewable resources, and creates three different “perspectives”—i.e., three different ways to deal with the subjective process of weighting and normalizing results based on different rankings of preferences, values, and attitudes.  The three perspectives are Hierarchist, Egalitarian, and Individualist.  The Hierarchist represents the view of the “average scientist” who is presumed to follow the IPCC’s (International Panel on Climate Change) assessment reports on the effects of climate change, balance short- and long-term concerns, and bases her views on consensus.  The Egalitarian greatly values ecosystem quality, considers the very long term—another way of saying she is concerned with sustainability—and is highly risk-averse, potentially resulting in overestimation of risks.  The Egalitarian is prone to consider all possible negative environmental effects of a phenomenon like climate change as definite.  This view contrasts with that of the Individualist, who only considers “proven” effects (as opposed to effects based on consensus but around which there remains some doubt).  The Individualist does not place any importance in fossil fuel depletion; rather, she only considers the depletion of minerals relevant.  Furthermore, the Individualist’s perspective lies within a short-term time frame, whereas the Egalitarian thinks in terms of a very long time frame.  Laleman et al. emphasize the need to clarify and outline these different perspectives in LCAs employing EI 99 so as not to cause serious misinterpretations, and for clarity’s sake they also include unweighted results.
          First, the authors evaluated environmental impact using one-dimensional indicators for the following six PV technologies: Cadmium Telluride (CdTe), CuInSe2 (CIS), ribbon Si, multi crystalline Si (multi c-Si), mono crystalline Si (mono c-Si) and amorphous (a-Si).  The newer technologies are the CdTe, CIS and ribbon Si.  Using the same figure for yearly energy output and the same conversion coefficient for electricity generation efficiency, the authors’ calculations for CED and EPT indicators are proportional to one another.  Whereas CED measures total energy required to construct the PV system over its lifetime, EPT measures the amount of time until the PV system produces more energy than was required for its construction.  In these analyses, the newer technologies were found to be more efficient than older ones, requiring less than 30,000 megajoules equivalent per kilowatt-peak (kilowatt-peak [kWp] is a measure of solar energy output under laboratory conditions; a standard home installation is considered to be 3 kWp in this study) for their construction.  All PV types had an EPT of less than 5 years in low irradiation conditions.  CdTe, CIS and ribbon Si EPTs were about one year less than those of the other PV systems, though this difference decreased as irradiation conditions increased.  In high solar irradiation regions like Spain, EPTs were only 2­–3 years.
          The GWP measures quantity of greenhouse gases emitted over the lifecycle of a PV system.  As with CED and EPT indicators, the GWP indicator showed the three newer PV technologies, along with multi c-Si, to have less impact than the three older ones (approximately 5000 kg of CO2 equivalent compared to approximately 6000 kg of CO2 equivalent).
          The EI 99 results differed significantly from the one-dimensional indicators.  Using the Hierarchist perspective, CdTe was found to have the highest impact score, and greatly exceeded the scores of the other newer technologies (450 compared to 317 and 353).  A breakdown of impact scores according to individual environmental indicators shows that most impact originates from fossil fuels and respiratory effects.  The authors note that reducing the energy input of PV production will decrease the impact related to fossil fuel extraction, respiratory effects, climate change, acidification and carcinogenics as they all relate to one another. 
          In order to compare the environmental impact of PV technology to other sources of electricity, the authors selected the multi c-Si system, as it has the largest market share.  They employed both a pessimistic (20 year) and optimistic (30 year) lifespan estimate for the PV system, and using both GWP and EI 99 indicators they compared the impact for 1 kWh (kilowatt-hour) produced by the various electricity sources.
          The GWP analysis showed PV electricity to have a markedly lower impact than fossil fuel based sources (even with an expected lifespan of 20 years, the PV’s GWP was 0.12 kg of CO2 equivalent per kWh (kgCO2-eq/kWh) compared to 0.53 for natural gas).  The Belgian mix is surprisingly low, at 0.33 kgCO2-eq/kWh, due to the high (55%) proportion of nuclear energy contribution.  The GWP of PV electricity was found to be approximately four times higher than nuclear and wind and ten times lower than coal (the authors claim their impact assessment for nuclear takes into account the impact of radiation on human health).
          The EI 99 results for compared environmental impact across electricity sources varied greatly depending on the perspective used.  Because the Individualist perspective does not “value” fossil fuel extraction as having an environmental impact, the mineral extraction associated with PV construction is weighted very highly and thus the total impact of PV is very high for the Individualist compared to the Egalitarian and Hierarchist perspectives.  Since PV technology requires a significant level of aluminum, iron, and copper, the Individualist finds PV to be much more impactful than natural gas, whereas the Egalitarian and Hierarchist find natural gas to be significantly more impactful.  In the unweighted category of ecosystem quality, PV is about twice as impactful as natural gas (and both are small compared to coal).  In the category of human health and resource depletion, PV impacts are negligible, natural gas impacts are small, and coal impacts are high.  Though a comparison of mineral ore extraction across electricity sources show that PV requires a relatively large amount of mineral ore, an EI 99 assessment of overall resource depletion shows mineral extraction associated with PV to be negligible compared to the fossil fuel extraction required for other electricity sources.  With regards to the issue of mineral ore required for PV construction, the authors indicate that the removal of the aluminum frame used for PV panel installations would greatly reduce overall environmental impact, and they recommend an efficient recycling program for the ores. 

          The authors conclude that PV systems have a relatively low  environmental impact even in areas of low solar irradiation, especially compared to fossil fuel based sources of electricity, though mineral extraction requirements should be taken into consideration.  Lifetime energy production ranged from 4-6 times lifetime energy consumption, and could reach 12 times lifetime energy consumption in sunny regions.  Lifetime greenhouse gas emissions were significantly lower than fossil fuel based sources of electricity production.  The EI 99 analysis showed that when fossil fuels were considered to have a negative impact on the wellbeing of future generations, PV systems were found to be less impactful than natural gas, coal, and the Belgian electricity mix.  The weighting step of EI 99 analysis greatly affected results, making the Individualist perspective consider PV more impactful than natural gas—as the authors point out, many would consider the large weight the Individualist assigns to mineral extraction to be illogical or irrational in this case.  The authors suggest this implies a need for great care and consideration of complexities in conducting a LCA.  Furthermore, due to low correlation of EI 99 results with one-dimensional indicators, Laleman et al. recommend the use of various indicators for a thorough and comprehensive LCA.