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UNIVERSITY OF CALIFORNIA
Santa Barbara
An Analysis of Compressed Air Energy Storage:
Scope in California and Potential Greenhouse Gas Emissions from Residual Methane
by
Asami Osato
A senior thesis submitted for the degree of
Bachelor of Science
in
Environmental Studies
Thesis Advisor:
Brandon Kuczenski, Ph.D.
UCSB Institute for Social, Behavioral, and Environmental Research
May 2015
ii
Abstract: AB 2514 requires 1,325 MW of energy to be stored in California by 2020. Currently the best options for California are Pumped Hydro Energy Storage (PHES) and Compressed Air Energy (CAES) Storage. PG&E owns and operates Helms PHES, and is testing a CAES plant in San Joaquin Valley. This CAES plant is the first to implement the reuse of a depleted natural gas reservoir. The potential GHG emissions from the reuse of a gas reservoir could be significant, because residual methane will be released into the atmosphere as air is cycled in and out of the reservoir. This thesis analyzes the potential of CAES in California and the overall GHG emissions if more CAES plants were built. The volume of GHG emissions from residual methane in potential CAES plants is not negligible (6.13 * 108, 1.53 * 109, 4.60 * 109 grams for a 300MW plant based on 0.4, 1, and 3 percent methane concentrations). However, implementing CAES plants in California is beneficial in the aggregate. With an unknown volume of residual methane entrained in the reservoir, the point at which CAES becomes more GHG-efficient than a conventional natural gas plant can vary: 5 years for the 0.4 percent scenario, 7 years for the 1 percent scenario, and 13 years for the 3 percent scenario. Given that the Germany and Alabama’s CAES plants have been running for 37 and 24 years respectively, it is clear that new CAES projects are likely to outlive its breakeven point.
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Acknowledgements: I would like to thank the UC Santa Barbara community, faculty and staff who provided me with guidance and support this past year. In particular, I would like to thank Professor Simone Pulver who provided me with help and guidance throughout the entire process. I would also like to thank Professor Mel Manalis and Kyle Meisterling for pointing me in the right direction. I would especially like to thank Brandon Kuczenski for all of his help. I could not have done this without you! Thank you for being patient with me and giving me smart advice throughout this entire process. I would also like to thank Dena Parish for introducing me to Joe Sutton, who is the reason for my initial interest in CAES. Joe, thank you for answering all of my questions and keeping me updated on the project! I would like to thank my family, friends, and everybody who helped me get this done. Han, thank you for your patience and support through the entire process. Jen, thanks for always being there!
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Table of Contents Chapter 1. Introduction………………..…………………………………….2 Chapter 2. Literature Review………………………………………………..9 Chapter 3. Background………………..…………..……...……..………….15
I. California’s Energy Laws………..…………..…..………..……….15 II. The Grid…………..…………..…………..…………..…………16 III. Overgeneration………………..…………..…………..……....…..17 IV. Energy Storage Solutions…………………..…………..………….…22
Chapter 4. Analysis of CAES
I. Explanation of the Technology……………………………………..25 II. Scope in California…………………………………………….….26 III. Residual Methane Problem……………………………………..….28 IV. Overall GHG Emissions Comparison…………………….…………..31
Chapter 5. Conclusion…………..………………………………………….36 References………….……….……….……….……….……….…………...37 List of Figures Figure 3-1. The Duck Curve and overgeneration risk………………….………………..18 Figure 3-2. Generation mix showing overgeneration with different scenarios………….21 Figure 4-1. Oil, gas and geothermal fields in California…….…………………………..27 Figure 4-2. Cumulative GHG emissions over time (0.4% methane scenario). ………….34 Figure 4-3. Cumulative GHG emissions over time (1% methane scenario)………….….35 Figure 4-4. Cumulative GHG emissions over time (3% methane scenario)……….…….35 List of Tables Table 3-1. 2014 Overgeneration Statistics…………………………………………….…19 Table 3-2. Projected Overgeneration Statistics…………………………………………..20 Table 4-1. Volume of Residual Methane (different scenarios) ……….……….……..….31 Table 4-2. Residual Methane Global Warming Potentials (different scenarios) …….….31 Table 4-3. CAES Construction Emissions (different scenarios) ……….……….………33
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Ch. 1 Introduction and Overview
I. The Problem
Pumping air underground to produce electricity sounds strange, at first. However
scientists have been coming up with creative ways to store energy, and the newest project
in California is another example. A Compressed Air Energy Storage (CAES) plant uses
renewable energy to compress air into an underground reservoir. When the energy is
needed, the pre-compressed air is released from the reservoir to spin a turbine that
generates electricity. This energy is needed during peak hours, when people come home
and turn on their lights, cook on their stove, and take hot showers. During off-peak hours,
the rest of the day and night, the energy generated from solar (in the middle of the day)
and wind power (at night) cannot be used. Energy storage allows use of energy that is
otherwise wasted, so this technology has the potential to lower energy costs for customers
as well as reduce GHG emissions from energy use.
But what does this new development mean for the environment? Throughout
history, we have supported projects that would increase efficiencies and help the
economy, but we do not consider the environmental effects until it’s too late. Pumped
Hydro Energy Storage (PHES) is one example. PHES plants are used all over the
world—they utilize two large reservoirs with different elevations. Energy is generated
when water is passed from the higher reservoir to the lower reservoir, and the water is
pumped back upstream for later generation. In California, the Helms PHES plant has
been in operation for thirty years. Recent integration of renewables into the energy
system, “the grid,” has allowed renewable energy to be used to pump water back
upstream during off-peak hours. However, new PHES plants are not being built in
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California because of its harmful effects on the surrounding ecosystem, affecting
especially fish populations.
California’s Renewables Portfolio Standard (RPS) requires utility companies to
procure 33% of their electricity from renewable sources by 2020. Solar and wind power
are the major renewable energy sources in California and they are great for combatting
climate change, but the intermittent nature of these energy sources make it difficult to
incorporate into the grid. In order to make better use of these renewables and comply
with the RPS, California utilities are beginning to focus on energy storage projects that
could allow the solar and wind power that is generated during off-peak hours, to be used
during peak hours, allowing indirect use of renewable energy throughout the day.
Successful energy storage projects have the potential to increase grid efficiency,
reliability, and sustainability. It could also allow California to run mostly on solar and
wind power (Cavallo, 2007), while reducing the need to build more fossil-fueled
generation that depletes natural resources and pollutes the air (Fthenakis, 2009; Lund,
2009).
AB 2514, introduced in 2010 and put in effect in 2013 by the CPUC (CPUC),
requires 1.325 GW of energy to be stored by 2020 by the three IOUs in California:
Pacific Gas and Electric, Southern California Edison, and San Diego Gas and Electric.
The Pacific Gas and Electric Company (PG&E) was given $50 million dollars to start the
initial work of investigating the feasibility of CAES technology, which will be the first of
its kind in California. Construction is already in progress at King Island in the San
Joaquin Valley, and the project is expected to begin commercial use in 2020. Other
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CAES plants exist in the United States, but the success in California will allow others to
have a further understanding of the technology and its feasibility in the state.
This thesis presents an analysis of the potential of CAES in California and will
take an in-depth look at the environmental implications of the project, specifically GHG
emissions. This project will be the first to utilize a depleted natural gas reservoir to store
compressed air. Too often we disregard the negative externalities associated with major
energy projects; this thesis seeks to understand how the technological and environmental
costs of CAES compare to those of other energy projects.
II. Rationale
Intellectual
Previous literature has shown the benefits of energy storage and its ability to
increase use of renewable energy while decreasing use of fossil fuels. Many scholars
suggest that PHES or CAES are the most cost-efficient, compared to other options
including batteries, flywheels, and others (Ibrahim, 2008; Kouskou, 2014; Mahlia, 2014).
This is partly due to the relatively low financial costs of construction and maintenance
(Fertig and Apt, 2011; Kouskou, 2014). In addition, these technologies have been already
implemented in various locations and have been proven to be beneficial. CAES and
PHES are also known to have high efficiency rates at about 70 percent (Ibrahim, 2008).
Case studies and models have shown that energy storage can lower GHG emissions and
they can be beneficial for power producers (Clearly et al, 2013).
However, the literature tends to overlook environmental consequences, focusing
on technology cost and characteristics like reliability and efficiency. There are some
studies on the physical effects of PHES systems on lake ecosystems, but the studies focus
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on the science and fail to explain what the effects could mean for humans. There is
disagreement among literature regarding which would be a more sustainable option.
Because each ecosystem is different, the effects are therefore unique and it is difficult to
compare CAES and PHES costs as a whole. In addition, due to the use of a depleted
natural gas reservoir as a storage medium, there is no literature on the global warming
potential-related impacts of this new type of CAES.
Policy
The California RPS requires 33 percent of our energy to come from renewables.
However solar and wind power have the potential to power the entire state, and the costs
of these technologies are going down (Cavallo, 2007; Fthenakis, 2009). The amount of
fossil fuels burned by the United States can be harnessed in just forty minutes via
sunlight (Cavallo, 2007). In addition, Governor Brown suggested in his 2013 State of the
State address that the penetration of renewable energy in California will exceed the 33
percent RPS, and the California Solar Initiative will result in 3,000 MW of solar
infrastructure installed by 2016 (E3, 2014). Solar power is expected to account for nearly
half of California’s renewable energy production by 2020, according to CPUC
projections (E3, 2014). However, potential renewable energy inputs are wasted; solar
power is generated in the middle of the day when sunlight is strongest and wind power is
generated in the middle of the night. Because of the nature of the grid, energy cannot be
stored and must be immediately used or cut off in order to keep the energy level balanced
and prevent blackouts. Energy storage will allow this extra renewable energy to be used,
in place of energy generated from conventional power plants that release GHG into our
atmosphere.
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As California population steadily increases, our energy demand will increase. In
order to prevent further release of gases that contribute to climate change, we must shift
towards more efficient, cleaner energy sources. By harnessing the otherwise wasted
energy, storage can decrease our overall energy generation (Mahlia, 2014) as well as
decrease the need for new transmission lines (Denholm, 2009). Building more
transmission lines can lead to decreased stability of the grid, and construction involves
more environmental damage. In addition, the use of fossil fuels releases GHG that are
changing our planet’s climate (IPCC, 2014). By storage of energy generated from
renewables for later use, we can divest from fossil fuel usage.
III. Thesis Statement
AB 2514 made energy storage a requirement for California, and the best options
for California with current technology are PHES and CAES. PHES is known to have
many detrimental effects on ecosystems not only during construction, but during the
operation phase. PHES changes lake levels and other characteristics of the habitat,
including temperature and turbidity. Construction of PHES plants is extremely expensive
due to the nature of the technology, which usually requires construction of a reservoir.
This involves land clearing and damages the surrounding environment.
Because CAES is mostly underground, it has minimal surface level impacts and
less of an overall impact on the ecosystem. The impact on local species is nowhere as
significant as the PHES plant. Because the reservoir already exists, CAES construction
costs are lower. CAES utilizes a depleted natural gas reservoir, making use of an existing
resource. However, CAES technology requires burning of additional fuel during power
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generation, and the reuse of a natural gas reservoir can lead to potentially significant
methane emissions.
IV. Research Strategy
This thesis will be an analysis looking at the feasibility and potential of CAES in
California, based on the project in San Joaquin County, the first CAES plant in
California. The project is currently being tested by the Pacific Gas and Electric Company
(PG&E), where I had the opportunity to intern last summer. This thesis will involve a
three-part analysis:
1) Overgeneration is a phenomenon that the state of California has already begun to
experience. This overgenerated energy is potentially storable energy for CAES.
Overgeneration statistics from 2014 will be reported, as well as projected statistics
based on higher RPS in California where solar energy is abundant. Information on
2014 statistics will be from CAISO. A 2014 report by E3 will be used to report
potential overgeneration based on different RPS scenarios.
2) Based on the characteristics of depleted gas reservoirs that are usable for CAES,
the total volume of potential CAES will be quantified. This information will come
from a scoping study done by EPRI in 2008.
3) A discussion of GHG emissions in detail will follow. Since the actual volume of
residual methane in the reservoir is unknown, the volume of residual methane will
be calculated as a percentage of the volume of compressed air in the reservoir.
The volume of residual methane in the reservoir is a fixed volume that will be
released once, but the amount of time it takes for all of the entrained methane to
be released is unknown. Assuming that the CAES plant cycles air in and out of
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the reservoir each day, all of the entrained methane will be released in the first
year. The methane released from this process will be added onto the total
construction emissions of the CAES plant.
V. Overview
Chapter two will be a literature review of existing conversation of environmental
impacts of energy storage technologies, with a focus on PHES and CAES. Chapter three
will be a background chapter with an overview of current energy policies in California,
an explanation of how the grid works and the need for energy storage. Current energy
storage technologies will also be explained. Chapter four will be an analysis of CAES: an
explanation of the technology, the potential scope in California, the residual methane
problem with GHG calculations, and a comparison of overall GHG emissions of CAES
compared to conventional natural gas plants. Chapter five will be a conclusion chapter.
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Ch. 2 Literature Review
The benefit of energy storage technologies is the increased reliability on clean
energy. Both PHES and CAES systems result in lower GHG emissions when compared
with conventional plants, but PHES is known to have financial and environmental
concerns. CAES is a newer “hybrid” plant that requires some GHG emissions during air
decompression, but has a much smaller surface impact compared to PHES.
I. Energy Storage Options
Many energy storage technologies are currently being researched and under
development. Lead batteries are an obvious energy storage option that has been used for
many years, but have never been used on a large scale. Lead batteries cannot store large
amounts of energy in a small volume (Ibrahim et al, 2008), so a battery that could store
energy for Californians would require the creation of an extremely large battery—which
would be very expensive. In addition, lead batteries are not durable and do not have long
life spans (Ibrahim et al, 2008). Flywheels, a promising technology that utilizes kinetic
energy, are still being developed (Mahlia, 2014; Ibrahim, 2008; Kousksou, 2014).
Thermal energy storage is another option, but has not been researched in detail (Ibrahim
et al, 2008; Kousksou, 2014). Out of the storage options, CAES and PHES are the most
developed and cost-efficient when considering resource use, power generated,
maintenance, and efficiency (Ibrahim et al, 2008; Kousksou, 2014; de Boer, 2014).
However, there is some controversy as to which is the superior option. Some
argue that because PHES plants have the potential for a higher storage capacity (Helms
has a capacity of over 1,000 MW; the CAES plant would have 300 MW), it would be
more cost-beneficial.
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II. CAES and PHES Benefits
Both PHES and CAES have shown the potential to reduce overall GHG emissions
from the energy sector due to increased use of clean energy (Clearly et al, 2013;
Denholm, 2004; Greenblatt, 2007; Salgi, 2008). PHES has been in use for decades; they
have proven to make use of renewables during off-peak hours for peak hour generation.
CAES has shown the same benefits through computer models and case studies in Europe
(Clearly et al, 2013; Salgi, 2008). The technologies are also ideal for grid incorporation
with fast ramping rates (the rate that a generator can increase/decrease output) (Fertig,
2011; Ibrahim, 2008; Kousksou, 2014; Mahlia, 2014) as well as quick start up and shut
down processes (Mahlia, 2014). This means both CAES and PHES can quickly deliver
large amounts of energy at one time when necessary. Compared with a conventional
power plants that generally only converts 40% of power to electricity (Najjar and
Zaamout, 1998), both technologies are quite effiecient, with 70-75% average efficiencies
(Dene et al, 2010; Ibrahim et al, 2008). With these characteristics, storage would allow
millions of dollars that would have been used to instal peak load power plants, to be
saved (Najjar and Zaamout, 1998).
III. PHES Costs
Construction
For PHES to work, two large reservoirs with different elevations are required. The
larger the reservoirs, and higher the difference in elevation, the larger the storage capacity
and higher the power potential (Mahlia, 2014). Naturally, these sites do not exist—
finding suitable geologic formations is very difficult (Denholm, 2004; Fthenakis, 2009;
Kim, 2012; Kousksou, 2014; Ibrahim, 2008). PHES would require construction of at least
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one reservoir, which is extremely costly both monetarily and environmentally (Kim,
2012; Yang, 2011). The process would involve land clearing, which is the biggest threat
to biodiversity. Land clearing also releases GHG because of vegetation removal.
Operation
The biggest environmental effect of PHES is fish entrainment (Anderson, 2007).
Fish are affected both from getting stuck in turbines and frequent changes in water
elevation. Because the large fish would be the most heavily affected, this would disturb
existing ecosystem balance (Anderson, 2007). This also changes nutrient levels in the
lake—there would be an increased amount of “internally recycled” phosphorus
(Anderson, 2007); fueling algal growth. This could be significant or not, depending on
the ecology of the lakes as well as the existing food web (Anderson, 2007). In addition,
PHES processes change the temperature of the water. When water travels through pipes
downstream or upstream, some of the energy is lost as friction, which significantly heats
up the water (Bonalumi, 2012; Anderson, 2006). The implications of this on lake
ecosystems are unclear, because it depends on the existing ecosystem. In addition, PHES
“amplifies trends that are expected to occur” due to climate change (Bonalumi, 2012).
The effect of a warmer year on an ecosystem can be heightened due to this added
warming of the lakes from PHES.
Also, PHES creates an ecosystem corridor connecting two reservoirs that has not
existed before, so the increased temperature and changing nutrient levels will affect both
reservoirs as well as downstream watersheds (Bonalumi, 2012). This new corridor will
also lead to introduced species in the lake (Bonalumi, 2012). However, not all introduced
species become invasive, so the effects of this will also depend on the ecosystem.
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Because of the indirect impacts on lake ecology and species, there has not been
much development of PHES in recent years. “While there is considerable commercial
interest in the USA, only one PHES plant has been submitted a final environmental
impact statement to the regulatory authorities” (Deane et al, 2010).
IV. CAES Costs
Construction
Unlike PHES, there are many sites in the United States that are suitable for CAES
operation (Fthenakis, 2009; Greenblatt, 2007; Mahlia, 2014). CAES would not require
extreme land manipulation or creation of a new reservoir because existing depleted
natural gas reservoirs or salt domes could be used. 75% of the United States has these
formations that could be potential CAES sites (Mehta, 1992). Some scholars say CAES
would have the lowest capital cost out of all energy storage technologies that exist today
(Fertig, 2011; Kousksou, 2014). Because CAES demands less construction than PHES,
the costs could be significantly cheaper.
However, there is no literature discussing potential GHG emissions resulting from
the reuse of a gas reservoir. Existing CAES plants utilize salt caverns instead of depleted
natural gas reservoirs. The Environmental Assessment for this project mentions the
potential harm of this type of reservoir. The Environmental Assessment states,
“preliminary reservoir modeling indicates that air withdrawn from the reservoir is likely
to produce some amount of residual natural gas in the extracted flow. The data suggest
the concentrations of natural gas should be relatively low; however, model refinement is
still in progress and the modeling entails some assumptions and uncertainties” (p. 20).
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The Environmental Assessment calls the methane emissions “short-term” and
insignificant based on CEQ thresholds (NETL, 2014).
Operation
Because CAES is mostly underground, there would be minimal surface impact
compared to PHES (Cavallo, 2007; Kousksou, 2014; Salgi, 2008). However, CAES
requires additional fuel for heating during decompression of air (Denholm, 2004; Kim,
2012; Kousksou, 2014; Mahlia, 2014). When air expands through the turbine, the change
in pressure causes freezing which could stop generation. In order to generate 1 kWh of
energy, “0.7-0.8 kWh of electricity needs to be absorbed during off-peak hours to
compress the air, as well as 1.22 kWh of natural gas during peak hours (retrieval)”
(Ibrahim, 2008). Due of this additional fuel input, CAES is a not a completely
“renewable” source of energy; it is a “hybrid” plant that would release some GHG when
burning fuel for heat, as well as during transportation of fuel to the plant.
V. Conclusion
Many articles review the existing energy storage options and recommend PHES
or CAES. It is clear that out of the options that are available today, the two are the most
viable options for large-scale storage systems. According to existing literature, CAES has
an overall lower surface impact and may have a lower impact on the environment.
However, CAES technology requires additional fuel inputs. Also, when air is cycled in
and out of the reservoir consistently, all of the leftover methane in the reservoir will
eventually be pumped out from underground and into the air. The combination of the
total residual methane emitted from the reservoir as well as the emissions from burning
fuel could mean high GHG emissions. If more CAES plants were built, in California,
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they will likely utilize depleted natural gas reservoirs, and the overall global warming
impact could be significant.
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Ch. 3 Background
I. California’s Energy Laws
Due to global climate change, California lawmakers have set ambitious standards
to increase reliability on clean energy sources. Following the Industrial Revolution,
Carbon-intensive resources like coal and oil have been extracted from the ground and
combusted in the air, which has dramatically increased the amount of Carbon in the air,
in the form of Carbon Dioxide. Carbon Dioxide is a GHG that traps heat in the
atmosphere, contributing to drastic shifts in the global climate.
California’s Renewable Portfolio Standard (RPS) requires 33 percent of
California’s energy to come from renewable sources by the year 2020. The program was
originally developed in 2002 with Senate Bill 1078, which required 20 percent renewable
energy by 2017. In 2006, Senate Bill 107 shortened the deadline to 2010. Senate Bill 2 in
2011 increased the goal to 33 percent renewables by 2020. This number will likely
continue to increase as we reach closer to our goals—goals like 40-51 percent by 2030
have been suggested (E3, 2014).
Currently, about 20 percent of California’s energy comes from renewable power
(CAISO, 2015). Renewable sources that qualify under the California RPS include solar
thermal, solar, wind, small hydro (less than 30MW), biogas, biomass, and geothermal
(CAISO, 2015). In addition to renewable resources, California’s energy comes from
natural gas and coal imports, large hydro, thermal, and nuclear (CAISO, 2015). In 2013,
the statistics were as follows: Natural Gas – 44.31%, Nuclear – 8.84%, Coal – 7.82%,
Large Hydro – 7.76%, Oil – 0.01%, Renewables – 18.77%, 12.49% unspecified (QFER,
2015).
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AB 2514 was introduced in 2010 and put in effect in 2013 by the CPUC (CPUC,
2015). The bill requires 1.325 GW of energy to be stored by 2020 by the three IOUs in
California: Pacific Gas and Electric, Southern California Edison, and San Diego Gas and
Electric.
Under this bill, energy storage is defined as “commercially available technology
that is capable of absorbing energy, storing it for a period if time, and thereafter
dispatching the energy” (AB 2514). In addition, the technology must follow one of the
following guidelines: 1. Use mechanical, chemical, or thermal processes to store energy
for a later time, 2. Store thermal energy for heating or cooling at a later time, 3. Use
mechanical, chemical, or thermal processes to store energy generated from renewable
resources for a later time, or 4. Use mechanical, chemical, or thermal processes to store
energy generated from mechanical processes that would otherwise be wasted for delivery
at another time (AB 2514).
This bill was created with three goals in mind by California lawmakers:
optimization of the grid, which includes peak reduction, contribution to reliability needs,
deferment of transmission and distribution upgrade investments; integration of renewable
resources; reduction of GHG emissions to 80 percent below 1990 levels by 2050, which
is a California goal.
II. The Grid
Think of the energy grid like a big pool with multiple hoses adding water, and
some that suck out water. Too much water and it will overflow, but too little water and
the pool doesn’t have enough to support itself. In order to maintain the water level, the
amount of water going in and out of the pool has to be managed in real time.
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On the grid, blackouts happen when the pool of energy falls out of the narrow
range. If there are too many hoses adding water to the pool, some inputs are blocked off
to stop the flow of water. And if increased outputs are expected, extra hoses have to be
ready. In order for extra energy to be available for dispatch immediately, the sources
must be already running.
Peak and Off-Peak
Energy demand throughout the day varies, and the fluctuations of demand
throughout the day produce peak hours and off-peak hours. Peak hours occur when
energy demand from consumers is relatively high, and off-peak hours are when energy
demand from consumers is relatively low. Generally, peak hours occur around 5PM to
8PM, when consumers come home from work to cook, watch TV, charge their phones,
turn on the lights, and use their personal computers.
Because renewable energy relies on natural resources, they suffer from
intermittency problems—they are only available at certain times of the day. Solar is
generated during the middle of the day when the sun is out. Wind is mostly generated in
the middle of the night. The problem with intermittency is that solar is generated
primarily during off-peak hours, when consumers are mostly at work or in school. When
wind energy is generated at night, the demand for energy is also low because consumers
are asleep. Because solar and wind energy are mostly generated during off-peak hours,
they can be wasted sources of energy. This extra energy is called overgeneration.
III. Overgeneration
Overgeneration is a phenomenon that occurs when supply of energy exceeds
energy demands. Overgeneration is defined as “when more electricity is supplied than is
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needed to satisfy real-time electricity requirements” (CAISO, 2013). There are two main
times when overgeneration occurs. The first occurs when the ISO prepares to meet
upcoming peak demands. Many existing energy resources require a long time to come on
line, but they need to be ready for the peak demand. Because of this slow ramping rate,
these energy sources must be producing at a certain level before peak hours, even when
the electricity is not needed.
The second occurs because of intermittent renewables, which can be explained by
the duck graph below (CAISO, 2013). The supply of energy due to increased renewable
generation exceeds the demands during off-peak hours, so that extra supply is cut off
from the grid. Because the ISO must balance supply and demand throughout the day,
overgeneration must be mitigated. Energy storage can be the solution for the second type
of overgeneration that occurs due to intermittencies.
Figure 3-1.
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Note. The duck curve shows energy demand throughout the day and overgeneration risk. Reprinted from Fast Facts (p. 3), CAISO, 2013: Folsom, CA. Copyright 2013 by CAISO. CAISO: 2014 Overgeneration Statistics
Due to the high availability of solar and wind and existing infrastructure in
California, overgeneration actually occurred four times in 2014 (S. Greenlee, personal
communication, April 2, 2015). Table 3-1 shows these statistics.
Table 3-1.
Date Wind Curtailment
(Max MW)
Solar Curtailment
(Max MW)
February 19 114 148
March 7 123 0
April 12 174 25
April 27 485 657
According to recent studies conducted by CAISO, overgeneration is expected to
increase with a higher RPS. A 40 percent RPS in 2024 will have 13,402 MW curtailed (S.
Greenlee, personal communication, April 2, 2015). Although not law yet, the governor
has called for 50 percent by 2030, which will likely see more overgeneration by
renewables.
E3 Report: Projected Overgeneration
E3’s January 2014 report investigates the challenges of implementing a higher
RPS in California. According to their study, which utilizes their Renewable Energy
20
Flexbility (REFLEX) Model on ECCO’s ProMaxLT production simulation platform, the
biggest challenge is overgeneration.
E3’s model looks at four scenarios: (1) 33 percent RPS, (2) 40 percent RPS, (3)
50 percent RPS Large Solar, and (4) 50 percent RPS Diverse. In all cases, overgeneration
is a problem—especially with solar energy. Even at 33 percent, the study showed
significant amounts of overgeneration by solar resources. With increases in RPS
percentage, the study shows increased overgeneration.
Table 3-2.
Note. 2030 Overgeneration statistics for the 33%, 40%, and 50% RPS Large Solar Scenarios. Reprinted from Investigating a Higher Renewables Portfolio Standard in California (p. 14), E3, 2014: San Francisco, CA. Copyright 2014 by E3.
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Figure 3-2.
Note. Generation mix calculated for an April day in 2030 with the (a) 33% RPS, (b) 40% RPS, and (c) 50% RPS Large Solar portfolios showing overgeneration. Reprinted from Investigating a Higher Renewables Portfolio Standard in California (p. 13), E3, 2014: San Francisco, CA. Copyright 2014 by E3.
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IV. Energy Storage Options
Below are some of the key characteristics that define the importance of current
energy storage technologies:
Efficiency: the amount of energy that is put into the system over the amount of energy
that comes out. This has to do with the energy loss involved with electricity conversion.
Ramping: the amount of time a system takes to go from zero to full power output. A fast
ramping rate means the system starts up quickly, and is flexible.
Duration: the length of time a system can provide power at its rated capacity.
Longevity: the length of time a system can be used, or the lifecycle of a system. The
longer its lifecycle, the more useful and the more cost-beneficial the system is.
Cost per kWh: the amount of money it takes to produce a unit of energy. This is
especially important for comparing different systems.
A. Compressed Air Energy Storage (CAES)
CAES relies on a huge underground reservoir to store compressed air that will be
released later to generate electricity. Air is compressed into the underground
reservoir during off-peak hours. When the energy is needed, the compressed
energy is released, and during decompression the air spins a turbine that generates
electricity. What makes this technology ideal for bulk storage are the fast ramping
rates (usually less than 10 minutes) and the ability to undertake frequent start-up
and shut-down (Gonzalez et al, 2004). The advantage of this technology is the
low capital costs (60-125 dollars per kWh) (Higgins, 2014).
B. Pumped Hydro Energy Storage (PHES)
23
The first PHES plant was constructed in Italy and Switzerland in 1890. Presently
PHES provides over 90 GW of storage around the world (Schoppe, 2010). PHES
utilizes two giant reservoirs at different heights. Water is the storage medium, and
when water is released from the upper reservoir to the lower reservoir, it spins a
turbine that generates electricity. The higher the difference in height of the two
reservoirs, the greater the potential energy there is. Later, the water is pumped
from the lower reservoir back to the upper reservoir. The main drawback of this
technology is the high construction cost, and there are not many sites that are
ideal for the system. In addition, the high environmental damage caused by both
construction and operation of these plants, have caused a halt to this technology
being considered in the United States.
C. Flywheels
Flywheels have been experimented since the 1950s. The technology utilizes
inertial energy by accelerating the rotor inside, and maintaining the energy in the
system (Mahlia, 2014). They have a low maintenance cost, long lifespan, and high
efficiency, which make it the perfect model of energy storage. However, issues
with this technology have not made it a possible storage system yet. Initial capital
costs are very high, and they are unable to provide power for long durations
(Higgins, 2014).
D. Thermal
Thermal energy storage can be defined as the temporary storage of thermal energy
at high or low temperatures (Kouskou et al, 2014). Advantages include high
efficiency and low costs. However, they only last 25 years (Higgins, 2014). It is a
24
relatively simple technology and is a popular source of energy for heating and
cooling, but has not been for other large-scale applications.
E. Batteries
Several types of batteries have been used for decades, but only on a small scale.
Batteries are quite efficient, have ramping rates of seconds, and can last thousands
of years (Higgins, 2014). The energy storage capacity of batteries is increased by
increasing the size of the battery, but a large-scale battery would be very costly.
Flow batteries can cost over 500 dollars per kWh, and Lithium-Ion batteries can
cost anywhere from 900-6200 dollars per kWh (Higgins, 2014).
25
Ch. 4 An Analysis of CAES
I. CAES Technology
A conventional CAES plant is not a completely “renewable” source of energy.
During compression of air, the air heats up because of how quickly the air is compressed.
In order to prevent fires or explosions, water is used to keep the air at a moderate
temperature. During decompression, the rapid expansion of air causes freezing. In order
to prevent freezing of the air in the reservoir, fuel is burned.
Advanced Adiabatic (AA) CAES and Gas Turbine (GT) CAES are two other
CAES technologies that work similarly, but differ in the way the air is heated during
decompression. AA CAES captures the heat lost by the system during decompression and
stores it for use during decompression. This method is free of carbon emissions and
technologically advanced, utilizing thermal energy storage. Simulations of this
technology have been done, but it has not been utilized yet on a large scale. GT CAES
similarly is a carbon emission-free system, but integrates wind energy. Excess wind
production is used to drive the compressors, but the heat is not captured from this stage.
The air during decompression is preheated using gas turbine exhaust energy, which is
recovered using a recuperator. This system is a carbon-free source, but relies on wind
energy, which has intermittency problems.
Seismology
According to the Environmental Assessment, there have been public concerns
about the possibility of earthquakes caused by the project. However, as this project
injects and withdraws air from the reservoir, it maintains a balance between the amount
26
of fluid injected and withdrawn. Because the balance is maintained with the original gas
content in the reservoir, this project will not induce seismic events (NRC, 2012).
In addition, this project is located in the Central Valley of California where the
faults have been inactive for the past 1.5 million years (NETL, 2014). It is highly unlikely
that earthquakes will occur at this project site.
Existing Plants
Two other CAES plants exist today, in Alabama and in Germany. Both of these
projects use salt caverns as a storage medium. Salt caverns are shapeable using
freshwater to dissolve the salt, and they have been proven to be reliable reservoirs.
However this process is long and expensive (Connolly, 2009). Germany’s plant in
Huntorf was the first CAES plant to be built, commissioned in 1978. Two salt caverns
with a total volume of 310,000 cubic meters with a capacity of 290 MW for 3 hours.
Alabama’s McIntosh salt dome is a 560,000 cubic meter storage reservoir that provides
110 MW of power for 26 hours. This plant, commissioned in 1991, uses a recuperator to
reduce the fuel consumption used for reheating during decompression, by 25 percent
(Mahlia, 2014).
II. Potential in California
In California, salt domes are not available due to its geology (EPRI, 2008).
PG&E’s CAES plant will utilize a depleted natural gas reservoir to store compressed air,
and is the first CAES plant that will reuse an already existing reservoir. The success of
this project will allow other utilities in California to consider this technology, due to the
abundance of depleted natural gas wells in the state.
27
In a scoping study done for the Department of Energy by EPRI, over 128 sites
were identified as possible CAES locations in California. Combined, these sites were
estimated to have over 1.8 Giga Tons of storage capacity (EPRI, 2008). Most of the gas
fields are located in Northern California, in the Sacramento Basin between Modesto and
Red Bluff (EPRI, 2008). AB 2514 requires 1,325 MWs of storage—all of this could be
potentially stored by CAES based on this study.
Figure 4-1.
Note. Oil, gas and geothermal fields in California. Reprinted from Compressed Air Storage (CAES) Scoping for California (p. 59), EPRI, 2008: Palo Alto, CA. Copyright 2008 by EPRI.
28
Picking the Site
More than 120 depleted natural gas reservoirs were surveyed by PG&E in order to
find a location for this project (NETL, 2014). The key properties PG&E looked at
included porosity, permeability, discovery pressure, size, oil production history, operating
status. Other characteristics included sand thickness, storage rights, mineral rights, depth,
and water drive (NETL, 2014).
III. Residual Methane Problem
Gas reservoirs become “depleted” when they are abandoned by oil companies
because the cost of extraction becomes more expensive than the return. Once they are
abandoned they may be plugged with cement to prevent leakage, but this is a very
expensive process and is not done all the time, nor is it required for oil companies to do
so. A recent study has also proven that cement does not actually prevent methane from
slowly leaking out of the reservoir (Kang, 2014).
When a depleted natural gas reservoir is used as a storage medium, all of the
methane that is entrained in the reservoir will eventually be pumped out as air is cycled in
and out of the reservoir. Because the methane is expected to be pumped out of the
reservoir in the first year, this is a “construction” issue (NETL, 2014). However, the
problem with methane is its potency in the atmosphere. It is known to be 25 times more
potent than carbon dioxide over the long-term (100 years), and 72 times more potent in
the short-term (20 years) (IPCC, 2005).
A. Model
This model estimates the global warming potential of CAES in California, based
on AB 2514 requirements. The GHG emissions are from construction and operation of
29
the plant. The operational GHG emissions are well studied, and include the emissions
from natural gas combustion for heating. The construction impacts include the actual
emissions from construction of the plant, and the emissions from residual methane.
The total methane emissions that will be emitted during the operation of CAES
will be calculated. Although the amount of time it will take for the methane to escape the
reservoir is unknown, the methane will end up in the atmosphere, so it is important to
account for.
1. Residual Methane based on Volume of Compressed Air
There is no information regarding the actual volume of methane leftover in the
reservoir, so this number is estimated based on the capacity of compressed air in the
reservoir. The total volume of residual methane in the reservoir was calculated by
multiplying the estimated percentage of methane left in the reservoir by this volume.
For reference, the Ideal Gas Law is used to estimate the size of the reservoir:
PVreservoir = nRT
Rearrangement yields:
Vreservoir = nRT/P
Then, residual methane is calculated:
Vmethane (scf) = Vair (scf) * Percentage of methane in compressed air
Multiple scenarios are used to calculate total residual methane. Scf is converted to moles
then grams of methane. The size of the reservoir in scf and the methane entrainment
scenarios are provided by PG&E.
30
2. Methane released from AB 2514
In California, natural gas reservoirs are abundant and would likely be the storage
medium for CAES plants. To calculate the potential total methane emissions if all MWs
required by AB 2514 were stored in the form of CAES, a simple ratio calculation is done:
Vtotal = Vmethane * statuatory capacity / plant capacity
3. Potential GW Impact
The global warming impact is measured by converting the total methane
emissions to carbon dioxide equivalent emissions. The emission factor used is from a
report by the Intergovernmental Panel on Climate Change (IPCC).
GWP = Vtotal * emission factor
Overall Model:
Total residual methane emitted = Quantity of methane per cubic feet of reservoir *
Reservoir volume / Plant capacity * Total installed capacity in California
B. Results
1. Reservoir Size
Using the Ideal Gas Law with P = 15.853 MPa, n = 1.198 moles/scf * 8 bscf, R =
8.3141, T = 321.9 Kelvin, Vreservoir = 1.6 million m3 = 57.1 mcf.
2. Residual Methane
PG&E estimated that 1 to 3 percent of the air released will be methane, with their
model estimating 0.4 percent of 8 billion scf (J. Sutton, personal communication,
February 9, 2015). Table 4-1 shows results for a 0.4 percent, 1 percent, 3 percent, and a
worst-case 5 percent scenario. This CAES plant has a capacity of 300 MW, and AB 2514
requires 1,325 MW of energy stored by 2020.
31
Table 4-1.
Percentage Grams of Residual Methane in
300 MW plant
Total grams of methane based on
AB 2514 (1,325 MW)
0.4 % 6.13 * 108 2.71 * 109
1 % 1.53 * 109 6.77 * 109
3 % 4.60 * 109 2.03 * 1010
5 % 7.67 * 109 3.39 * 1010
3. Global Warming Potential of Residual Methane
Methane has a GWP of 25 (IPCC, 2005). Table 4-2 shows the total grams of
carbon dioxide equivalent emissions for the four scenarios above.
Table 4-2.
Percentage Global Warming Potential (g CO2e)
0.4 % 6.77 * 1010
1 % 1.69 * 1011
3 % 5.08 * 1011
5 % 8.47 * 1011
IV. Overall GHG Emission Comparison
A. Model
In order to compare GHG emissions over time, yearly emissions were calculated
for both CAES and Natural Gas combustion. Cumulative GHG emissions for CAES will
be compared with Natural Gas operational emissions.
32
PHES has high GHG emissions during construction, but the emissions highly
depend on the existing environment. Some systems will require one reservoir to be built;
others require two reservoirs to be built. In addition, most of the impacts of PHES are
ecosystem and biodiversity-related.
1. CAES Emissions
Construction emissions per year were given in the Environmental Assessment.
Because construction will last less than a year, the construction emissions are fixed. The
residual methane is expected to be pumped out of the reservoir within the first year, so
this will be a construction impact. These actual construction emissions are added to
residual methane emissions for a total construction impact. Residual methane emissions
from previous calculations will be used.
Total construction emissions = Actual construction emissions + Residual methane
emissions
Operation emissions are provided by the project’s Environmental Assessment.
2. Natural Gas Emissions
Because natural gas plants have already been constructed, construction emissions
are not added to total emissions. Operational emissions per MWh are provided by the
EPA.
3. Overgeneration
The E3 report estimates overgeneration per year in 2030, based on a 33 percent
RPS, 40 percent RPS, and 50 percent RPS. 2030 estimates based on a 33 percent RPS
will be used. However the overgeneration per year in intervening years is unknown. It is
assumed that overgeneration starts at zero and grows at a constant rate until it reaches
33
190 GWh/year in 2030. This model assumes that only overgenerated energy will be used
to power CAES, and that all of the overgenerated energy can be stored by CAES each
year.
Overall Model:
CAES Emissions = Operation emissions (gCO2e/MWh) * Overgeneration (Mwh) +
Construction emissions (gCO2e)
Natural Gas Emissions = Operation emissions (gCO2e/MWh)
B. Results
1. CAES Emissions
Actual construction emissions = 3.27 * 108 g CO2e (NETL, 2014)
Table 4-3 shows the total construction emissions after the residual methane has
been added, based on different scenarios.
Table 4-3.
Percentage of
Residual Methane
Residual Methane
emissions (g CO2e)
Total construction
emissions (g CO2e)
0.4 % 6.77 * 1010 6.81 * 1010
1 % 1.69 * 1011 1.70 * 1011
3 % 5.08 * 1011 5.08 * 1011
Operation emissions = 2.51 * 105 g CO2e/MWh (NETL, 2014)
2. Natural Gas Emissions
The burning of natural gas requires 7.45 * 105 g CO2e/MWh (EPA, 2015).
34
3. Overgeneration per Year
The E3 report estimates 190 GWh/year of total overgeneration in 2030 based on a
33 percent RPS. Increase in overgeneration per year until 2030 is 1.27 * 104 MWh/year.
4. CAES vs. Natural Gas
Figure 4-1, 4-2, and 4-3 show the cumulative GHG emission comparison based on
the 0.4%, 1%, and 3% scenarios, respectively.
Figure 4-1.
The point when CAES is more GHG-efficient than Natural Gas is the year 2020.
35
Figure 4-2.
In this case, the point when CAES is more GHG-efficient than Natural Gas is the year 2022.
Figure 4-3.
In this case, the point when CAES is more GHG-efficient than Natural Gas is the year 2028.
36
Ch. 5 Conclusions
The volume of GHG emissions from CAES plants is not negligible. However,
implementing CAES plants in California is beneficial in the aggregate. With an unknown
volume of residual methane entrained in the reservoir, the point at which CAES becomes
more GHG-efficient than a conventional natural gas plant can vary: 5 years for the 0.4
percent scenario, 7 years for the 1 percent scenario, and 13 years for the 3 percent
scenario. Given that the Germany and Alabama’s CAES plants have been running for 37
and 24 years respectively, it is clear that new CAES projects are likely to outlive its
breakeven point.
However, even the parameters are changed (i.e. more efficient natural gas
combustion, or less efficient CAES operation), the initial residual methane volume is the
most significant because the operational emissions are low. Therefore it is important to
account for the residual methane in the reservoir before CAES construction begins.
Overgeneration is a problem in California, and will become a bigger problem as
more solar infrastructure is built and California’s RPS is increased. Energy Storage is a
potential solution to the problem, and CAES is a good option. There is lots of potential
with CAES in California. Compared with PHES, which has a huge impact on the existing
impact and results in habitat degradation, CAES is the better option.
37
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