excess energy in renewable electricity generation - koziel, luo, wang, yang

7/23/2019 Excess Energy in Renewable Electricity Generation - Koziel, Luo, Wang, Yang

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Excess Energy in Renewable Electricity Generation

Mark Koziel, Yuting Luo, Claire Wang, Fan Yang

University of Chicago

I. Introduction

Fossil fuels are the largest source of carbon dioxide eissions in the !orld, and their

"roduction and consu"tion usually brings heavy "ollutants to the air and !ater #$%& 'ence,

there is gro!ing concern about the health and environental i"acts of electricity "roduction

fro fossil fuels, and this has led to the fast develo"ent of rene!able energy generation&

(ong all rene!able resources used for electricity "roduction, global su""ort is strongest for

solar and !ind, !hich have been gro!ing )uickly in recent years and co"rised *&+ of total

U- electricity generation in +./$ #*, 0%&

'o!ever, the high variability and interittency of these resources create challenges for grid

o"erators trying to anage and dis"atch their generation& While "o!er grids tend to "roduce the

right aount of electricity at the right tie, the uncontrollable variation in !ind1solar generation

akes it difficult to consistently and reliably eet electric deand #2%& 3n the other hand, due to

the fact that the develo"ent of !ind1solar fars usually out"aces the develo"ent of

transission infrastructure, "o!er grids ust also deal !ith transission congestion !hile

dis"atching electricity fro these sources& With an econoic axiu generation out"ut set by

the deand and transission constraints, any excess energy that exceeds this constraint cannot

be acce"ted by the grid 4 it is stranded energy #+%& Moreover, because the increental generation

cost of rene!able energy is zero and the storage cost is high, !henever there is stranded energy,

it is !asted 5or curtailed6 and not dis"atched& 7o!er grids call this curtailmentor dispatch down&

8n this "a"er, !e ex"lore the excess energy in !ind and solar fars& We collect data on

!ind1solar generation and deands, as !ell as curtailent or dis"atch do!n, fro both M8-3

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5Midcontinent 8nde"endent -yste 3"erator6 and C(8-3 5California 8nde"endent -yste

3"erator6& 9y looking into these data, !e find that a large aount of !ind1solar generation gets

stranded and !asted because of the reasons entioned above&

We further ex"lore the "ossible solutions for excess rene!able energy, including different

ty"es of batteries, hydro "u"ing, co"ressed air, and so on& 9y co"aring the cost of different

ethods, !e conclude that :esla 7o!er"ack at a "rice of ;+*.1kWh is the chea"est o"tion for the

storage of rene!able energy& Considering this battery "rice and the sall "ortion of excess

energy that actually needs to be stored, storage costs are not a large "ortion of costs for

rene!ables&

3verall, des"ite the issue of excess energy, rene!able electricity "roduction certainly has the

"otential to becoe co"etitive !ith fossil fuel sources&

II. Background

We chose to study solar "hotovoltaics and !ind because they are on track to be the ost

significant rene!able sources of energy& :he fraction of U- electricity generated by !ind and

solar "o!er has gro!n ra"idly in the recent "ast, and is "redicted to continue gro!ing for the

foreseeable future& 9et!een +../ and +.//, there !as a three


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electricity before +.=.& :his !ould cut +>. illion etric tons of carbon eissions, e)uivalent

to taking *@ illion cars off the road #//%&

:he !ind energy sector is gro!ing continuously, but not at a steady rate& :he total annual

generation of U- !ind fars in +./$ !as ore than =. ties greater than in +..., "roducing a

total of />/2@/... MWh 5A/>. illion MWh6 #/+,/=%& 'o!ever, cuulative !ind "o!er

ca"acity in the U- gre! by +> in +./+, but only by + in +./=& :his greatly diinished

gro!th rate !as largely due to the late extension of the Bene!able lectricity 7roduction :ax

Credit 57:C6 #/$%& -ince the gro!th of !ind "o!er de"ends on the "resence or absence of

federal subsidies, its iediate future in the United -tates is uncertain&

III. Wind Curtailment in MISO

8n this section, !e "resent !ind curtailents collected fro Midcontinent 8nde"endent -yste

3"erator 5M8-36& M8-3 has ore than /+ DW of !ind ca"acity and a "eak deand of @> DW

#/%& 9efore +.//, they used anual curtailents to reduce the out"ut of !ind generation& 8n id

+.//, they launched the Eis"atchable 8nterittent Besource 5E8B6 "rotocol that re)uires !ind

"lants to bid into the energy arket #+%& -ince then, the excess generation in !ind "lants is

autoatically dis"atched do!n&

Figure / sho!s the total energy anually curtailed1do!n


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Figure /& M8-3 estiated anual curtailent versus dis"atched do!n

We also looked into the excess !ind generation in +./$ #=%, !hich is dis"atched do!n as

interittent resources in an autoatic "rocess& -ince anual curtailent is no longer a aHor

"art of curtailed generation, !e do not sho! it in the figure& Figure + sho!s that M8-3 !ind

generation and E8B dis"atch do!n in +./$ follo! siilar trend, and these t!o "arts constitute

the total available !ind energy& E8B dis"atch do!n is about $&+ 4 >&= of the total available

!ind energy, !hich eans nearly /1/. of !ind generation is stranded and curtailed because of

oversu""ly and transission congestion&

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Figure +& M8-3 !ind generation and E8B dis"atch do!n in +./$

We calculated the detailed nubers of the seasonal !ind utilization, as sho!n in :able /&

-iilar to the "revious figures, !ind energy is consued least in suer and ost in s"ring&

:here is ost energy dis"atched do!n in s"ring, !hich takes about 2++DWh 1 5///0/DWh I

2++DWh6 A 0&/ of the total available !ind energy& (s for the !hole year, the ratio of E8B

dis"atch do!n to total available energy is +=.>DWh 1 5=>@>+DWh I +=.>DWh6 A *&0&

:able /& M8-3 !ind generation and E8B dis"atch do!n in +./$

Wind Generation GW!" #IR #is$atc! #own GW!"

%otal Mont!ly #aily %otal Mont!ly #aily

Winter //..> =00@&== /++&=/ 0*$ +/>&.. 2&+2

-"ring ///0/ =2+.&== /+/&=+ 2++ +$.&02 2&>*

-uer 0/>+ +.0.&02 02&+. =2/ /+=&02 $&.=

Fall /.0=/ =*$=&02 //0&>+ *0/ />2&.. 0&/0

:otal =>@>+ =+$>&*. /.0&>. +=.> /@+&== 0&=+

:o understand these nubers, !e co"are the to the electricity generation fro fossil

fuels& 'ere are our assu"tions

:he energy density of coal is +$ MJ1kg&

:he efficiency of "o!er "lant boilers is @*&

:he efficiency of stea turbines is $*&

:he efficiency of electricity generators is @>&

:he efficiency of electricity generation fro coal is @* $* @> A $+& -ince the

daily stranded !ind generation is 0&=+DWh, the aount of coal that is needed to generate sae

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aount of electricity is 0&=+DWh $+ +$ MJ1kg A ++*2 tons& 8f coal "rice is ;*.1ton, the

cost of ++*2 tons of coal is ;//+,>*.&

I&. Excess Energy in C'ISO

8n order to calculate the levelized cost of electricity generation 5LC36 for solar "o!er, !e

started by finding a value for the ca"ital cost "er unit of energy in ters of ;1W for a utility sized

solar far& :he :o"az -olar FarNs construction began in +./+ and !as co"leted by the end of

+./=& 8t is a **. MW ca"acity solar far that is currently the largest solar far in the !orld& :he

installation of the far cost ;+&$ billion #+=%& :hat eans that the ca"ital cost is e)ual to ;+&$

billion divided by **. MW, !hich leaves a "rice of ;$&$1Wca"& 8gnoring the co"lexities of the

necessity of borro!ing oney, !e si"ly ulti"lied this cost by t!o !hich brings us to

;>&>1Wca"& Looking at the data for the :o"az -olar Far for out"ut energy in the onth of July,

!e obtain a ca"acity factor of =. #+=%& :his eans that the true ca"ital cost can be found by

dividing the value ;>&>1Wca" by &=. !hich raises the cost to ;+@&==1Wtrue& :he total "o!er

"roduced over the lifetie of a solar cell is found by ulti"lying +* years by >20. hrs1year and

dividing by /... to obtain a value of +/@ kWh1Wtrue& We can no! use these t!o nubers to

obtain the LC3 for ca"ital costs& We divide ;+@&==1Wtrue by +/@ kWh1Wtrue to obtain a value

of /=&=@ cents1kWh& :he total o"erating and aintenance 53OM6 costs for solar "o!er end u"

being /&* cents1kWh& :his eans that the total LC3 for solar "o!er adds u" to /$&>@

cents1kWh& :his is the nuber !e !ill use in our calculations as the cost of electricity generation

for solar "o!er, but lo!er nubers !ill also be used as future estiates for the "rice of solar&

:he "rice of solar has been ra"idly decreasing and that change !ill continue to add to the

viability of solar "o!er as an econoically co"etitive energy source&

:he calculations necessary to find the LC3 for !ind "o!er are siilar to those used for

solar "o!er& For the ca"ital cost "er unit of energy in ters of ;1W, !e looked at utility


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!ind fars in the Western United -tates and that "rice turned out to be ;+&/1Wca" #/=%& We

ulti"ly this cost by t!o for the sae reasons as before& :his leaves us !ith a "rice of

;$&+1Wca"& Wind in California is not the strongest !ind in the country, but it still yields a

ca"acity factor of +* #/=%& :his brings the true ca"ital cost to ;/0&>1Wtrue& We find the total

"o!er "roduced over the lifetie of the !indill by ulti"lying +. years by >20. hrs1year and

dividing by /... to obtain /2*&+ kWh1Wtrue& Just as before, !e can use these t!o nubers to

find the total ca"ital costs for !ind "o!er& When !e divide the t!o nubers by each other, !e

obtain a value of @&*@ cents1kWh& :he 3OM costs for !ind "o!er are estiated at /&/

cents1kWh& :his brings the total LC3 for !ind "o!er to /.&0@ cents1kWh& :his nuber !ill be

used in the calculations as the cost of electricity generation for !ind "o!er&

For our calculations !e utilized data fro C(8-3 #+$%& We took the electricity load, !ind

out"ut, and solar out"ut data for the entire onth of July +./$& We then averaged those data to

obtain daily averages for the onth& We chose the onth of July because on average that is the

onth !ith the highest electricity load, eaning that energy generation ust also be higher to

eet deand& 8n order to "ro"erly do these calculations one !ould have to exaine the largest

dis"arities in !ind and solar generation as co"ared to electricity load& For exa"le, solar

energy generation in the !inter is uch lo!er Hust as is load, but !e !ould have to find the exact

nubers of those decreases& :he "ur"ose of our calculations is si"ly to exaine the outlook for

rene!able energy !hen load is greatest& :he energy storage re)uireents in these calculations

!ould be the lo!est "ossible, es"ecially if !ind energy is the "redoinant rene!able energy

source, since !ind is uch ore variable than solar insolation on any given day& :he first thing

!e did !as gra"h the electricity load, !ind out"ut, and solar out"ut data& We then scaled u" the

!ind and solar out"uts so that the aount of energy !as e)uivalent to the electricity load& Fro

this gra"h, it can be seen that the !ind energy out"ut is lo!er than the load in the iddle of the

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day and higher than the load at night, !hile the o""osite is true for the solar energy out"ut& :he

aounts bet!een the curves are the aount of energy that !ould need to be stored for this

technology to be used& :his aount for solar energy is around =+.,... MW, !hile !ind energy

totals about /=.,... MW& :his eans that solar energy !ould be uch ore de"endent on

energy storage, and !ould have to be chea"er than !ind energy to be viable& (nother i"ortant

thing to note is that even in a syste using /.. rene!able energy, it !ould not be necessary to

store the full aount of electricity used over the course of the day, !hich reduces battery costs&

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 240

10000

20000

30000

40000

50000

60000

70000

80000

90000

California Energy Demands and Renewable Energy

Solar Energy !"#!" $%&' Ele("ri(i"y )oad $%&'

*d+!s"ed Solar Energy !"#!" $%&' &ind Energy !"#!" $%&'

*d+!s"ed &ind Energy !"#!" $%&'

,ime of Day $-o!r'

Energy $%&'

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Figure =& C(8-3 energy deands and rene!able generation 5historical data6

:he next thing that !e did !as to take our "reviously calculated solar and !ind energy

"rices, along !ith the "rice for the :esla 7o!er"ack, as it has the greatest "otential to be able to

scale u" infinitely !ithin the next fe! years& With these nubers, !e !ere able to change the

"ercentages of !ind and solar energy being used along !ith the aount of storage that these

technologies re)uired& 8n the gra"h belo! is the /.. !ind and /.. solar that !as seen

"reviously, along !ith the "lot of a syste running on >. !ind and +. solar, !hich !e found

through trial and error to be the o"tial ratio& :his is due to the fact that !ind energy generation

"rices are so uch chea"er than solar energy generation "rices that !ind energy is favored even

in California, !here solar insolation is highest& 'o!ever, solar energy is still useful, since it

akes u" for the large energy deficit that !ind generation leaves in the iddle of the day&

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 240

10000

20000

30000

40000

50000

6000070000

80000

90000

#"imal *mo!n"s for California Renewable Energy

Ele("ri(i"y )oad $%&' *d+!s"ed Solar Energy !"#!" $%&'

*d+!s"ed &ind Energy !"#!" $%&' Combined Energy !"#!" $%&'

,ime of Day $-o!r'

Energy $%&'

Figure $& C(8-3 energy deands and rene!able generation 5latest data6

:he electricity "rices for this o"tial ratio today !ould be //&@@ cents1kWh, !hich is

only slightly ore ex"ensive than the average "rice of electricity in the United -tates& :his "rice

is definitely underestiated, as !ind energy has very large fluctuations in generation, soeties

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u" to a factor of five during soe "arts of the day& :his eans that either ore storage !ould be

re)uired than the calculations for these averages suggest, or ore average energy generation

!ould be re)uired, or soe cobination of both, !hich inevitably drives the "rice of rene!ables

higher than the o"tial "rice found above& :hese calculations !ere done using technologies

available today, but "rices for these technologies have been dro""ing ra"idly, es"ecially for solar

and battery storage technologies& 9ecause of this, !e redid the calculations using a value of

;/&>1W, !hich is the forecasted "rice of solar !ithin five to ten years #+*%& :his translates to solar

"o!er energy generation being 0&.@ cents1kWh, !hich is lo!er than the average "rice for

electricity generation in the United -tates today& :he costs of generating !ind energy are not

declining as )uickly, so it !as not necessary to calculate an adHusted "rice for !ind& :aking into

account energy storage along !ith current !ind energy generation "rices, !e found the o"tial

ratio in this ne! scenario to be 2. solar and =. !ind& Wind energy is still a significant

"ortion because of energy storage costs& With these nubers, the "rice of electricity turns out to

be @&=2 cents1kWh, !hich is still belo! the current "rice of non


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co"ared to the :esla 7o!er"ack& 'ydro"o!er storage is also a chea" o"tion that is able to

"rovide soe energy storage& 8t is not infinitely scalable, but it should still be used to its

axiu "otential& With these technologies, curtailent of rene!able energy !ould be

obsolete, as it !ould be chea"er to si"ly store excess energy rather than !asting soe of the

ca"acity of the rene!able technologies raising the cost "er energy unit generated& ven though a

full s!itch to rene!able energy is not currently "ossible, it ay !ell be "ossible !ithin a

generation as these technologies continue to i"rove&

&. (ossible Solutions

(s discussed in the "revious section, the daily excess energy is as uch as /.2kWh, and the

"eak "o!er excess is /.$MW& :herefore, !e are looking for an energy storage syste that has a

huge "o!er and energy ca"acity, !hile reains as chea" and effective as "ossible&

Bechargeable 9atteries #/0, /2%

9atteries are the ost develo"ed electric energy storage technology, and can serve as a

benchark for other kinds of technologies, since the :esla Motors co"any is "roducing and

selling rechargeable batteries on a large scale&

:he "rice offered by the co"any is described by the "rice "er ca"acity 5;1kWh6&

'o!ever, !hat really atters to us is the "rice "er energy stored& :herefore, !e need to kno! the

lifetie of the batteries and estiate the cycles it can endure, based on the assu"tion that each

battery run one cycle in a day& 5Usually, "roducts have a cycle life and a lifetie& 8n these

calculations, !e choose the shorter one and convert the units to cycles&6 :he "rice "er energy

stored e)uals the "rice "er ca"acity divided by lifetie cycles, !hich is close to the "rice of

electricity& 9atteries re)uire the least to !ork of all the technologies, as !ill be sho!n later in

this section& (lso, the volue of each battery is co"arably sall, so it !ill not re)uire too vast

a s"ace& :herefore, battery storage technology is realistic&

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-u"erca"acitors #/>, /@%

While batteries store electrical energy in the for of electrocheical energy, ca"acitors

!ork in a ore direct !ay !ithout converting the energyP hence, they are ore efficient&

'o!ever, they are often uch ore ex"ensive and re)uire a uch larger s"ace, as they are

liited by their lo! energy density and ca"acity 5.&/$kWh co"ared to /.kWh of :esla 7o!er. 7lausible

&I. Summary and #iscussion

8n this "a"er, !e a""roached the "roble of storing !ind and solar energy fro a ostly

theoretical stand"oint& We did not investigate the feasibility of integrating interittent energy

sources into the grid 4 "o!er systes are not currently e)ui""ed to accoodate high levels of

interittent generation, and ight need to de"loy a ore flexible fossil


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Fro our investigation, !e concluded that the "roble of storing excess energy eans

that it is not currently feasible to "o!er the United -tates only on !ind and solar, even before

taking into account lack of transission infrastructure and other factors& 'o!ever, !ith ne!ly

eerging storage technologies, it is not i"ossible to iagine a grid running on only interittent

energy sources, es"ecially since the "rices of both generation and storage have been steadily

declining&

&II. 'cknowledgements

3ur thanks to lisabeth Moyer and (ndre! Malone, for all they have done for us in D3-

+$2.*, and also to Kristian Buud, the senior anager of M8-3, for his generous hel" in our

"roHect&

&III. Re)erence

#/% Lori 9ird, Ja)uelin Cochran, and Qi Wang, Wind and -olar nergy Curtailent 7racticesG,

RBL U?8D 3ctober /2, +./$&

#+% Lori 9ird, Ja)uelin Cochran, and Qi Wang, Wind and -olar nergy Curtailent x"erience

and 7ractices in the United -tates, :echnical Be"ortG, RBL1:7


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#//% Judee 9urr, Lindsey 'allock, and Bob -argent, -tar 7o!er :he Dro!ing Bole of -olar

nergy in (ericaG, Roveber +., +./$&

#/+% Mark 9olinger and Byan Wiser, +./+ Wind :echnologies Market Be"ortG, (ugust +./=&

#/=% Mark 9olinger and Byan Wiser, +./= Wind :echnologies Market Be"ortG, (ugust +./$&

#/$% Union of Concerned -cientists, 7roduction :ax Credit for Bene!able nergy,htt"11!!!&ucsusa&org1cleanXenergy1sart

excess energy in renewable electricity generation - koziel, luo, wang, yang

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