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December 24, 2016
NREL Raises Rooftop Photovoltaic Technical Potential Estimate
(Please note: This story comes from the National Renewable Energy Laboratory. Copyright concerns are nil.)
http://www.nrel.gov/news/press/2016/24662
[font face=Serif][font size=5]NREL Raises Rooftop Photovoltaic Technical Potential Estimate[/font]
[font size=4]New analysis nearly doubles previous estimates and shows U.S. building rooftops could generate close to 40 percent of national electricity sales[/font]
March 24, 2016
[font size=3]Analysts at the Energy Department's National Renewable Energy Laboratory (NREL) have used detailed light detection and ranging (LiDAR) data for 128 cities nationwide, along with improved data analysis methods and simulation tools, to update its estimate of total U.S. technical potential for rooftop photovoltaic (PV) systems. The analysis reveals a technical potential of 1,118 gigawatts (GW) of capacity and 1,432 terawatt-hours (TWh) of annual energy generation, equivalent to 39 percent of the nation's electricity sales.
This current estimate is significantly greater than that of a previous NREL analysis, which estimated 664 GW of installed capacity and 800 TWh of annual energy generation. Analysts attribute the new findings to increases in module power density, improved estimation of building suitability, higher estimates of the total number of buildings, and improvements in PV performance simulation tools.
The analysis appears in "Rooftop Solar Photovoltaic Technical Potential in the United States: A Detailed Assessment." (PDF) The report quantifies the technical potential for rooftop PV in the United States, which is an estimate of how much energy could be generated if PV systems were installed on all suitable roof areas.
To calculate these estimates, NREL analysts used LiDAR data, Geographic Information System methods, and PV-generation modeling to calculate the suitability of rooftops for hosting PV in 128 cities nationwide-representing approximately 23 percent of U.S. buildings-and provide PV-generation results for 47 of the cities. The analysts then extrapolated these findings to the entire continental United States. The result is more accurate estimates of technical potential at the national, state, and zip code level.
"This report is the culmination of a three-year research effort and represents a significant advancement in our understanding of the potential for rooftop PV to contribute to meeting U.S. electricity demand," said Robert Margolis, NREL senior energy analyst and co-author of the report.
Within the 128 cities studied, the researchers found that 83 percent of small buildings have a suitable location for PV installation, but only 26 percent of those buildings' total rooftop area is suitable for development. Because of the sheer number of this class of building across the country, however, small buildings actually provide the greatest combined technical potential. Altogether, small building rooftops could accommodate up to 731 GW of PV capacity and generate 926 TWh per year of PV energy-approximately 65 percent of the country's total rooftop technical potential. Medium and large buildings have a total installed capacity potential of 386 GW and energy generation potential of 506 TWh per year, approximately 35 percent of the total technical potential of rooftop PV.
"An accurate estimate of PV's technical potential is a critical input in the development of regional deployment plans," said Pieter Gagnon, an engineering analyst of solar policy and technoeconomics at NREL and lead author of the report. "Armed with this new data, municipalities, utilities, solar energy researchers, and other stakeholders will have a much-improved starting point for PV research and policymaking, both regionally and nationwide."
"It is important to note that this report only estimates the potential from existing, suitable rooftops, and does not consider the immense potential of ground-mounted PV," said Margolis. "Actual generation from PV in urban areas could exceed these estimates by installing systems on less suitable roof space, by mounting PV on canopies over open spaces such as parking lots, or by integrating PV into building facades. Further, the results are sensitive to assumptions about module performance, which are expected to continue improving over time."
Technical potential is an established reference point for renewable technologies. It quantifies the amount of energy that can be captured from a particular resource, considering resource availability and quality, technical system performance, and the physical availability of suitable area for development-without consideration of economic factors like return on investment or market factors such as policies, competition with other technologies, and rate of adoption.
NREL's work was supported by funding from the Energy Department's Office of Energy Efficiency and Renewable Energy in support of its SunShot Initiative. The SunShot Initiative is a collaborative national effort that aggressively drives innovation to make solar energy fully cost-competitive with traditional energy sources before the end of the decade. Through SunShot, the department supports efforts by private companies, universities, and national laboratories to drive down the cost of solar electricity to $0.06 per kilowatt-hour. Learn more at energy.gov/sunshot.
NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.
###
Visit NREL online at www.nrel.gov[/font][/font]
[font size=4]New analysis nearly doubles previous estimates and shows U.S. building rooftops could generate close to 40 percent of national electricity sales[/font]
March 24, 2016
[font size=3]Analysts at the Energy Department's National Renewable Energy Laboratory (NREL) have used detailed light detection and ranging (LiDAR) data for 128 cities nationwide, along with improved data analysis methods and simulation tools, to update its estimate of total U.S. technical potential for rooftop photovoltaic (PV) systems. The analysis reveals a technical potential of 1,118 gigawatts (GW) of capacity and 1,432 terawatt-hours (TWh) of annual energy generation, equivalent to 39 percent of the nation's electricity sales.
This current estimate is significantly greater than that of a previous NREL analysis, which estimated 664 GW of installed capacity and 800 TWh of annual energy generation. Analysts attribute the new findings to increases in module power density, improved estimation of building suitability, higher estimates of the total number of buildings, and improvements in PV performance simulation tools.
The analysis appears in "Rooftop Solar Photovoltaic Technical Potential in the United States: A Detailed Assessment." (PDF) The report quantifies the technical potential for rooftop PV in the United States, which is an estimate of how much energy could be generated if PV systems were installed on all suitable roof areas.
To calculate these estimates, NREL analysts used LiDAR data, Geographic Information System methods, and PV-generation modeling to calculate the suitability of rooftops for hosting PV in 128 cities nationwide-representing approximately 23 percent of U.S. buildings-and provide PV-generation results for 47 of the cities. The analysts then extrapolated these findings to the entire continental United States. The result is more accurate estimates of technical potential at the national, state, and zip code level.
"This report is the culmination of a three-year research effort and represents a significant advancement in our understanding of the potential for rooftop PV to contribute to meeting U.S. electricity demand," said Robert Margolis, NREL senior energy analyst and co-author of the report.
Within the 128 cities studied, the researchers found that 83 percent of small buildings have a suitable location for PV installation, but only 26 percent of those buildings' total rooftop area is suitable for development. Because of the sheer number of this class of building across the country, however, small buildings actually provide the greatest combined technical potential. Altogether, small building rooftops could accommodate up to 731 GW of PV capacity and generate 926 TWh per year of PV energy-approximately 65 percent of the country's total rooftop technical potential. Medium and large buildings have a total installed capacity potential of 386 GW and energy generation potential of 506 TWh per year, approximately 35 percent of the total technical potential of rooftop PV.
"An accurate estimate of PV's technical potential is a critical input in the development of regional deployment plans," said Pieter Gagnon, an engineering analyst of solar policy and technoeconomics at NREL and lead author of the report. "Armed with this new data, municipalities, utilities, solar energy researchers, and other stakeholders will have a much-improved starting point for PV research and policymaking, both regionally and nationwide."
"It is important to note that this report only estimates the potential from existing, suitable rooftops, and does not consider the immense potential of ground-mounted PV," said Margolis. "Actual generation from PV in urban areas could exceed these estimates by installing systems on less suitable roof space, by mounting PV on canopies over open spaces such as parking lots, or by integrating PV into building facades. Further, the results are sensitive to assumptions about module performance, which are expected to continue improving over time."
Technical potential is an established reference point for renewable technologies. It quantifies the amount of energy that can be captured from a particular resource, considering resource availability and quality, technical system performance, and the physical availability of suitable area for development-without consideration of economic factors like return on investment or market factors such as policies, competition with other technologies, and rate of adoption.
NREL's work was supported by funding from the Energy Department's Office of Energy Efficiency and Renewable Energy in support of its SunShot Initiative. The SunShot Initiative is a collaborative national effort that aggressively drives innovation to make solar energy fully cost-competitive with traditional energy sources before the end of the decade. Through SunShot, the department supports efforts by private companies, universities, and national laboratories to drive down the cost of solar electricity to $0.06 per kilowatt-hour. Learn more at energy.gov/sunshot.
NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.
###
Visit NREL online at www.nrel.gov[/font][/font]
December 20, 2016
[font size=2]Keywords: Climate change, climate sensitivity, global warming.[/font]
[font size=3]1. INTRODUCTION
Human activities are altering Earth’s atmospheric composition. Concern about global warming due to long-lived human-made greenhouse gases (GHGs) led to the United Nations Framework Convention on Climate Change |1| with the objective of stabilizing GHGs in the atmosphere at a level preventing “dangerous anthropogenic interference with the climate system.”
The Intergovernmental Panel on Climate Change |IPCC, |2|| and others |3| used several “reasons for concern” to estimate that global warming of more than 2-3°C may be dangerous. The European Union adopted 2°C above pre - industrial global temperature as a goal to limit human-made warming |4|. Hansen et al. |5| argued for a limit of 1°C global warming (relative to 2000, 1.7°C relative to pre - industrial time), aiming to avoid practically irreversible ice sheet and species loss. This 1°C limit, with nominal climate sensitivity of ¾°C per W/m² and plausible control of other GHGs |6|, implies maximum CO₂ ~ 450 ppm |5|.
Our current analysis suggests that humanity must aim for an even lower level of GHGs. Paleoclimate data and ongoing global changes indicate that ‘slow’ climate feedback processes not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments, may begin to come into play on time scales as short as centuries or less |7|. Rapid on-going climate changes and realization that Earth is out of energy balance, implying that more warming is ‘in the pipe-line’ |8|, add urgency to investigation of the dangerous level of GHGs.
A probabilistic analysis |9| concluded that the long-term CO₂ limit is in the range 300-500 ppm for 25 percent risk tolerance, depending on climate sensitivity and non-CO₂ forcings. Stabilizing atmospheric CO₂ and climate requires that net CO₂ emissions approach zero, because of the long lifetime of CO₂ |10, 11|.
…[/font][/font]
Target atmospheric CO2: Where should humanity aim? - by James Hansen et al.
This is the origin of the goal of 350 ppm.
http://dx.doi.org/10.2174/1874282300802010217
[font face=Serif][center][font size=1]The Open Atmospheric Science Journal, 2008, 2, 217-231[/font][/center][br][hr][font size=5]Target Atmospheric CO₂: Where Should Humanity Aim?[/font]
[font size=4]James Hansen*,1,2, Makiko Sato1,2, Pushker Kharecha1,2, David Beerling3, Robert Berner4, Valerie Masson-Delmotte5, Mark Pagani4, Maureen Raymo6, Dana L. Royer7 and James C. Zachos8[/font]
[font size=2]1NASA/Goddard Institute for Space Studies, New York, NY 10025, USA
2Columbia University Earth Institute, New York, NY 10027, USA
3Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
4Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA
5Lab. Des Sciences du Climat et l’Environnement/Institut Pierre Simon Laplace, CEA-CNRS-Universite de Versailles Saint-Quentin en Yvelines, CE Saclay, 91191, Gif-sur-Yvette, France
6Department of Earth Sciences, Boston University, Boston, MA 02215, USA
7Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459-0139, USA
8Earth & Planetary Sciences Dept., University of California, Santa Cruz, Santa Cruz, CA 95064, USA[/font]
[font size=1]Abstract: Paleoclimate data show that climate sensitivity is ~3°C for doubled CO₂, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO₂ for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO₂ was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO₂ fell to 450 ± 100 ppm ; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO₂ will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO₂ forcings. An initial 350 ppm CO₂ target may be achievable by phasing out coal use except where CO₂ is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO₂ is not brief, there is a possibility of seeding irreversible catastrophic effects.[/font]
[font size=2]Keywords: Climate change, climate sensitivity, global warming.[/font]
[font size=3]1. INTRODUCTION
Human activities are altering Earth’s atmospheric composition. Concern about global warming due to long-lived human-made greenhouse gases (GHGs) led to the United Nations Framework Convention on Climate Change |1| with the objective of stabilizing GHGs in the atmosphere at a level preventing “dangerous anthropogenic interference with the climate system.”
The Intergovernmental Panel on Climate Change |IPCC, |2|| and others |3| used several “reasons for concern” to estimate that global warming of more than 2-3°C may be dangerous. The European Union adopted 2°C above pre - industrial global temperature as a goal to limit human-made warming |4|. Hansen et al. |5| argued for a limit of 1°C global warming (relative to 2000, 1.7°C relative to pre - industrial time), aiming to avoid practically irreversible ice sheet and species loss. This 1°C limit, with nominal climate sensitivity of ¾°C per W/m² and plausible control of other GHGs |6|, implies maximum CO₂ ~ 450 ppm |5|.
Our current analysis suggests that humanity must aim for an even lower level of GHGs. Paleoclimate data and ongoing global changes indicate that ‘slow’ climate feedback processes not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments, may begin to come into play on time scales as short as centuries or less |7|. Rapid on-going climate changes and realization that Earth is out of energy balance, implying that more warming is ‘in the pipe-line’ |8|, add urgency to investigation of the dangerous level of GHGs.
A probabilistic analysis |9| concluded that the long-term CO₂ limit is in the range 300-500 ppm for 25 percent risk tolerance, depending on climate sensitivity and non-CO₂ forcings. Stabilizing atmospheric CO₂ and climate requires that net CO₂ emissions approach zero, because of the long lifetime of CO₂ |10, 11|.
…[/font][/font]
July 22, 2016
How about that!? Hydrogen may be an energy source after all!
Oceans May Be Large, Overlooked Source of Hydrogen Gas
https://nicholas.duke.edu/about/news/oceans-may-be-large-overlooked-source-hydrogen-gas[font face=Serif][font size=5]Oceans May Be Large, Overlooked Source of Hydrogen Gas[/font]
July 20, 2016
[font size=3]DURHAM, N.C. -- Rocks formed beneath the ocean floor by fast-spreading tectonic plates may be a large and previously overlooked source of free hydrogen gas (H₂
, a new Duke University study suggests.
The finding could have far-ranging implications since scientists believe H₂ might be the fuel source responsible for triggering life on Earth. And, if it were found in large enough quantities, some experts speculate that it could be used as a clean-burning substitute for fossil fuels today because it gives off high amounts of energy when burned but emits only water, not carbon.
Recent discoveries of free hydrogen gas, which was once thought to be very rare, have been made near slow-spreading tectonic plates deep beneath Earth’s continents and under the sea.
“Our model, however, predicts that large quantities of H₂ may also be forming within faster-spreading tectonic plates -- regions that collectively underlie roughly half of the Mid-Ocean Ridge,” said Stacey L. Worman, a postdoctoral fellow at the University of Texas at Austin, who led the study while she was a doctoral student at Duke’s Nicholas School of the Environment.
…[/font][/font]
http://dx.doi.org/10.1002/2016GL069066
July 20, 2016
[font size=3]DURHAM, N.C. -- Rocks formed beneath the ocean floor by fast-spreading tectonic plates may be a large and previously overlooked source of free hydrogen gas (H₂
, a new Duke University study suggests.
The finding could have far-ranging implications since scientists believe H₂ might be the fuel source responsible for triggering life on Earth. And, if it were found in large enough quantities, some experts speculate that it could be used as a clean-burning substitute for fossil fuels today because it gives off high amounts of energy when burned but emits only water, not carbon.
Recent discoveries of free hydrogen gas, which was once thought to be very rare, have been made near slow-spreading tectonic plates deep beneath Earth’s continents and under the sea.
“Our model, however, predicts that large quantities of H₂ may also be forming within faster-spreading tectonic plates -- regions that collectively underlie roughly half of the Mid-Ocean Ridge,” said Stacey L. Worman, a postdoctoral fellow at the University of Texas at Austin, who led the study while she was a doctoral student at Duke’s Nicholas School of the Environment.
…[/font][/font]
How about that!? Hydrogen may be an energy source after all!
March 28, 2016
Hydrogen or batteries for grid storage? A net energy analysis
http://dx.doi.org/10.1039/C4EE04041D[font face=Serif][font size=5]Hydrogen or batteries for grid storage? A net energy analysis[/font]
Matthew A. Pellow,*a Christopher J. M. Emmott,bc Charles J. Barnhartd and Sally M. Bensonaef
Energy Environ. Sci., 2015,8, 1938-1952
DOI: 10.1039/C4EE04041D
Received 22 Dec 2014, Accepted 08 Apr 2015
First published online 08 Apr 2015
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
[font size=3]Energy storage is a promising approach to address the challenge of intermittent generation from renewables on the electric grid. In this work, we evaluate energy storage with a regenerative hydrogen fuel cell (RHFC) using net energy analysis. We examine the most widely installed RHFC configuration, containing an alkaline water electrolyzer and a PEM fuel cell. To compare RHFC's to other storage technologies, we use two energy return ratios: the electrical energy stored on invested (ESOIe) ratio (the ratio of electrical energy returned by the device over its lifetime to the electrical-equivalent energy required to build the device) and the overall energy efficiency (the ratio of electrical energy returned by the device over its lifetime to total lifetime electrical-equivalent energy input into the system). In our reference scenario, the RHFC system has an ESOIe ratio of 59, more favorable than the best battery technology available today (Li-ion, ESOIe = 35). (In the reference scenario RHFC, the alkaline electrolyzer is 70% efficient and has a stack lifetime of 100 000 h; the PEM fuel cell is 47% efficient and has a stack lifetime of 10 000 h; and the round-trip efficiency is 30%.) The ESOIe ratio of storage in hydrogen exceeds that of batteries because of the low energy cost of the materials required to store compressed hydrogen, and the high energy cost of the materials required to store electric charge in a battery. However, the low round-trip efficiency of a RHFC energy storage system results in very high energy costs during operation, and a much lower overall energy efficiency than lithium ion batteries (0.30 for RHFC, vs. 0.83 for lithium ion batteries). RHFC's represent an attractive investment of manufacturing energy to provide storage. On the other hand, their round-trip efficiency must improve dramatically before they can offer the same overall energy efficiency as batteries, which have round-trip efficiencies of 75–90%. One application of energy storage that illustrates the tradeoff between these different aspects of energy performance is capturing overgeneration (spilled power) for later use during times of peak output from renewables. We quantify the relative energetic benefit of adding different types of energy storage to a renewable generating facility using |EROI|grid. Even with 30% round-trip efficiency, RHFC storage achieves the same |EROI|grid as batteries when storing overgeneration from wind turbines, because its high ESOIe ratio and the high EROI of wind generation offset the low round-trip efficiency.
[hr][font size=4]Broader context[/font]
The rapid increase in electricity generation from wind and solar is a promising step toward decarbonizing the electricity sector. Because wind and solar generation are highly intermittent, energy storage will likely be key to their continued expansion. A wide variety of technology options are available for electric energy storage. One is a regenerative hydrogen fuel cell (RHFC) system that converts electricity to hydrogen by water electrolysis, stores the hydrogen, and later provides it to a fuel cell to generate electric power. RHFC systems are already operating in several dozen locations. In this net energy analysis, we compare the quantity of energy dispatched from the system over its lifetime to the energy required to build the device. We find that, for the same quantity of manufacturing energy input, hydrogen storage provides more energy dispatched from storage than does a typical lithium ion battery over the lifetime of the facility. On the other hand, energy storage in hydrogen has a much lower round-trip efficiency than batteries, resulting in significant energy losses during operation. Even at its present-day round-trip efficiency of 30%, however, it can provide the same overall energy benefit as batteries when storing overgeneration from wind farms.
[hr]
…
[font size=4]5 Conclusion[/font]
Energy storage in hydrogen is a technically feasible option for grid-scale storage, and is already in pilot demonstrations. Because of its low round-trip efficiency, it may be overlooked in spite of its potential advantages, such as high energy density and low rate of self-discharge. In order to examine the potential benefits and drawbacks of hydrogen as a grid-scale energy storage technology, we apply net energy analysis to a representative hypothetical regenerative hydrogen fuel cell (RHFC) system. We introduce and apply a method to determine the energy stored on invested (ESOIe) ratio of a reference case RHFC system.
We find that the reference case RHFC system has a higher ESOIe ratio than lithium ion battery storage. This indicates that the hydrogen storage system makes more efficient use of manufacturing energy inputs to provide energy storage. One reason for this is that the steel used to fabricate a compressed hydrogen storage cylinder is less energetically costly, per unit of stored energy, than the materials that store electric charge in a battery (electrode paste, electrolyte, and separator). However, lithium ion batteries remain energetically preferable when considering the operation of the system, as well as its manufacture, due to their higher round-trip efficiency (90%). This is reflected in the overall energy efficiencies of the two storage technologies: the overall energy efficiency of a typical lithium ion battery system is 0.83, compared to 0.30 for the reference case RHFC system. This highlights that in spite of its relatively efficient use of manufacturing energy inputs, the round-trip efficiency of a RHFC system must increase before it can provide the same total energy benefit as other storage technologies. Higher RHFC round-trip efficiency relies on improved electrolyzer and fuel cell performance.
When storing overgeneration from wind turbines, energy storage in hydrogen provides an energy return similar to batteries, in spite of its lower round-trip efficiency. The aggregate EROI of wind generation augmented with RHFC storage is equal to that of the same wind facility augmented with lithium ion battery storage, when up to 25% of the electricity output passes through the storage system. For spilled power from solar photovoltaics, storage in hydrogen provides an EROI that is slightly higher than curtailment, though lower than batteries. As with other storage technologies, energy storage in hydrogen coupled to wind generation provides an overall EROI that is well above the EROI of fossil electricity generation.
…[/font][/font]
Matthew A. Pellow,*a Christopher J. M. Emmott,bc Charles J. Barnhartd and Sally M. Bensonaef
Energy Environ. Sci., 2015,8, 1938-1952
DOI: 10.1039/C4EE04041D
Received 22 Dec 2014, Accepted 08 Apr 2015
First published online 08 Apr 2015
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
[font size=3]Energy storage is a promising approach to address the challenge of intermittent generation from renewables on the electric grid. In this work, we evaluate energy storage with a regenerative hydrogen fuel cell (RHFC) using net energy analysis. We examine the most widely installed RHFC configuration, containing an alkaline water electrolyzer and a PEM fuel cell. To compare RHFC's to other storage technologies, we use two energy return ratios: the electrical energy stored on invested (ESOIe) ratio (the ratio of electrical energy returned by the device over its lifetime to the electrical-equivalent energy required to build the device) and the overall energy efficiency (the ratio of electrical energy returned by the device over its lifetime to total lifetime electrical-equivalent energy input into the system). In our reference scenario, the RHFC system has an ESOIe ratio of 59, more favorable than the best battery technology available today (Li-ion, ESOIe = 35). (In the reference scenario RHFC, the alkaline electrolyzer is 70% efficient and has a stack lifetime of 100 000 h; the PEM fuel cell is 47% efficient and has a stack lifetime of 10 000 h; and the round-trip efficiency is 30%.) The ESOIe ratio of storage in hydrogen exceeds that of batteries because of the low energy cost of the materials required to store compressed hydrogen, and the high energy cost of the materials required to store electric charge in a battery. However, the low round-trip efficiency of a RHFC energy storage system results in very high energy costs during operation, and a much lower overall energy efficiency than lithium ion batteries (0.30 for RHFC, vs. 0.83 for lithium ion batteries). RHFC's represent an attractive investment of manufacturing energy to provide storage. On the other hand, their round-trip efficiency must improve dramatically before they can offer the same overall energy efficiency as batteries, which have round-trip efficiencies of 75–90%. One application of energy storage that illustrates the tradeoff between these different aspects of energy performance is capturing overgeneration (spilled power) for later use during times of peak output from renewables. We quantify the relative energetic benefit of adding different types of energy storage to a renewable generating facility using |EROI|grid. Even with 30% round-trip efficiency, RHFC storage achieves the same |EROI|grid as batteries when storing overgeneration from wind turbines, because its high ESOIe ratio and the high EROI of wind generation offset the low round-trip efficiency.
[hr][font size=4]Broader context[/font]
The rapid increase in electricity generation from wind and solar is a promising step toward decarbonizing the electricity sector. Because wind and solar generation are highly intermittent, energy storage will likely be key to their continued expansion. A wide variety of technology options are available for electric energy storage. One is a regenerative hydrogen fuel cell (RHFC) system that converts electricity to hydrogen by water electrolysis, stores the hydrogen, and later provides it to a fuel cell to generate electric power. RHFC systems are already operating in several dozen locations. In this net energy analysis, we compare the quantity of energy dispatched from the system over its lifetime to the energy required to build the device. We find that, for the same quantity of manufacturing energy input, hydrogen storage provides more energy dispatched from storage than does a typical lithium ion battery over the lifetime of the facility. On the other hand, energy storage in hydrogen has a much lower round-trip efficiency than batteries, resulting in significant energy losses during operation. Even at its present-day round-trip efficiency of 30%, however, it can provide the same overall energy benefit as batteries when storing overgeneration from wind farms.
[hr]
…
[font size=4]5 Conclusion[/font]
Energy storage in hydrogen is a technically feasible option for grid-scale storage, and is already in pilot demonstrations. Because of its low round-trip efficiency, it may be overlooked in spite of its potential advantages, such as high energy density and low rate of self-discharge. In order to examine the potential benefits and drawbacks of hydrogen as a grid-scale energy storage technology, we apply net energy analysis to a representative hypothetical regenerative hydrogen fuel cell (RHFC) system. We introduce and apply a method to determine the energy stored on invested (ESOIe) ratio of a reference case RHFC system.
We find that the reference case RHFC system has a higher ESOIe ratio than lithium ion battery storage. This indicates that the hydrogen storage system makes more efficient use of manufacturing energy inputs to provide energy storage. One reason for this is that the steel used to fabricate a compressed hydrogen storage cylinder is less energetically costly, per unit of stored energy, than the materials that store electric charge in a battery (electrode paste, electrolyte, and separator). However, lithium ion batteries remain energetically preferable when considering the operation of the system, as well as its manufacture, due to their higher round-trip efficiency (90%). This is reflected in the overall energy efficiencies of the two storage technologies: the overall energy efficiency of a typical lithium ion battery system is 0.83, compared to 0.30 for the reference case RHFC system. This highlights that in spite of its relatively efficient use of manufacturing energy inputs, the round-trip efficiency of a RHFC system must increase before it can provide the same total energy benefit as other storage technologies. Higher RHFC round-trip efficiency relies on improved electrolyzer and fuel cell performance.
When storing overgeneration from wind turbines, energy storage in hydrogen provides an energy return similar to batteries, in spite of its lower round-trip efficiency. The aggregate EROI of wind generation augmented with RHFC storage is equal to that of the same wind facility augmented with lithium ion battery storage, when up to 25% of the electricity output passes through the storage system. For spilled power from solar photovoltaics, storage in hydrogen provides an EROI that is slightly higher than curtailment, though lower than batteries. As with other storage technologies, energy storage in hydrogen coupled to wind generation provides an overall EROI that is well above the EROI of fossil electricity generation.
…[/font][/font]
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