Life Cycle Assessment of a Vanadium Redox Flow Battery

Subscriber access provided by Kaohsiung Medical University

Energy and the Environment

Life cycle assessment of a Vanadium Redox Flow Battery Selina Weber, Jens Peters, Manuel Baumann, and Marcel Ralf Weil Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02073 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

Environmental Science & Technology

1

Life cycle assessment of a Vanadium Redox Flow

2

Battery

3

Selina Weberac, Jens F. Petersac*, Manuel Baumannbc and Marcel Weilbc a

4 5

b

Helmholtz Institute Ulm (HIU), Karlstr. 11, 76133 Karlsruhe , Germany

ITAS, Institute for Technology Assessment and Systems Analysis, Karlstr. 11, 76133 Karlsruhe,

6 7 8

Germany c

Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany *

Corresponding author. [email protected]

9

ABSTRACT. Batteries are one of the key technologies for flexible energy systems in future. In

10

particular, vanadium redox flow batteries (VRFB) are well suited to provide modular and

11

scalable energy storage due to favourable characteristics such as long cycle life, easy scale-up

12

and good recyclability. However, there is a lack of detailed original studies on the potential

13

environmental impacts of their production and operation. The present study fills this gap by

14

providing a comprehensive life cycle assessment of a representative VRFB. Transparent and

15

comprehensive inventory data are disclosed as a basis for further environmental studies. VRFBs

16

are found to be promising regarding the assessed impact categories, especially at high energy-to-

17

power (E/P) ratios. On the other hand, significant impacts are associated with the vanadium

18

pentoxide production, why the origin and processing of the vanadium bearing ores is a key for

19

further reducing the environmental impacts associated with the VRFB manufacturing. While the

ACS Paragon Plus Environment

1


Page 2 of 29

20

lower efficiency of the VRFB is a disadvantage in comparison to e.g. lithium-ion batteries (LIB),

21

its recyclability is significantly higher. In this sense, the importance of taking a cradle-to-cradle

22

life cycle perspective when comparing very different battery systems can be highlighted for

23

further research on this topic.

24 25

TOC Art

26

KEYWORDS. Environmental assessment; stationary energy storage, renewable energy, system

27

analysis.

28 29

1. Introduction

30

Batteries are gaining importance for supporting increasingly flexible energy systems based

31

mainly on renewables. Among the different available technologies, redox flow batteries (RFB)

32

are particularly suited for stationary systems, providing modular and scalable energy storage for

33

a variety of applications. The liquid electrolytes of RFBs are stored in tanks and pumped through

34

the stacks (containing the battery cells) where the electrochemical reactions take place. A major

35

advantage is that both the stacks (determining the power rating of the battery) and the tanks

36

(determining the amount of electrolyte and thus the storage capacity) can be dimensioned

37

independently.1 This allows designing RFB individually to fit the specific energy and power


2

Page 3 of 29


38

requirements of a given application. The most studied and currently most promising RFB

39

technology are vanadium redox flow batteries (VRFB). A VRFB has the advantage of using the

40

same element on both sides of the battery cell, what avoids the very challenging problems of

41

cross-contamination of the electrolytes.2 Moreover, VRFBs do not suffer from permanent self-

42

discharge,3 show a fast response, ambient temperature operation and long cycle and service life.4

43

Since the ongoing transition towards renewable energies is driven by environmental concerns,

44

also the installation and use of battery storage systems should aim at minimising associated

45

environmental impacts. However, no detailed study on the potential environmental impacts of

46

VRFB production and operation yet exists. While several techno-economic assessments of

47

VRFB modules have already been published,4–10 there is a lack of detailed original life cycle

48

assessment (LCA) studies of this battery type. The reference work in the field is based on very

49

rudimentary and largely unquantified data and dates back to 1999.11 All other literature related to

50

the environmental impacts of VRFB is at least partially based on this extremely simplified LCA

51

study or does not disclose comprehensive and transparent inventory data.12–16 Consequently,

52

there is an urgent need for an up-to-date and comprehensive life cycle assessment (LCA) based

53

on thoroughly modelled inventories as basis for future works in the field.

54

2. Materials and Methods

55

2.1. LCA framework

56

Life Cycle Assessment (LCA) is a standardized method to assess the potential environmental

57

impacts of goods, products or services over their entire life cycle.17–19 Accordingly, the present

58

study follows a cradle-to-cradle approach, considering all stages of the lifecycle of a VRFB,

59

namely the production, use phase and the end-of-life treatment. However, the focus of the


3


Page 4 of 29

60

inventory modelling is set on the cradle-to-gate stages i.e., the production of the VRFB

61

components involving raw materials extraction, material processing and product manufacturing.

62

The functional unit (FU) refers to the provision of 1 MWh of electricity by the battery over the

63

20 year lifetime of a hypothetical renewables support application. This includes the impacts of

64

battery production (and all upstream processes until the mine), replacement of battery

65

components, battery end-of-life (EoL) handling, and the impacts associated with the electricity

66

‘lost’ during charge/ discharge due to internal inefficiencies. As electricity sources, wind and

67

photovoltaic (PV) installations are considered, plus the average German grid mix for comparison

68

purpose. Environmental impacts are quantified following the CML method, considering the

69

following impact categories: global warming potential (GWP), human toxicity potential (HTP),

70

acidification potential (AP) and depletion of abiotic resources (ADP).20 For these, high relative

71

impacts were identified out in a recent review of LCA studies on LIB.21 The life cycle modelling

72

and assessment is carried out using openLCA 1.622 and ecoinvent 3.3 (cut-off model) as

73

background LCI database.

74

2.2. VRFB model (manufacturing phase)

75

Conceptually, the VRFB system is divided into two subsystems: a power sub-system

76

comprising all components related with the stack and the battery cells (determining the power

77

rating), and an energy sub-system comprising the electrolyte and associated components

78

(determining the storage capacity). All remaining components are assigned to the periphery. The

79

considered VRFB system shows a rated power of 1 MW and a nominal energy capacity of 8.3

80

MWh. The principal battery layout is mainly derived from published literature, majorly a recent

81

publication on stationary VRFB.5 The dimensions of the different parts provided there allow for

82

calculating the battery composition on a mass basis. The tabulated mass balance and further


4

Page 5 of 29


83

details of the modelling approach are provided in Section 1.1. of the Electronic Supporting

84

Information (SI).

85

The considered 8.3 MWh system requires an electrolyte volume of 202 m3 and, assuming a

86

void volume of 50%, a tank capacity of 300 m3. For a nominal power of 1 MW, two stacks with

87

a total of 156 cells are required. A single cell consists of a membrane, a bipolar plate, two carbon

88

felt electrodes, and a polyvinylchloride (PVC) cell frame. These individual cells are joined

89

(‘sandwiched’) in a steel frame to form stacks, with the amount of cells per stack determining the

90

system voltage. The system power is determined by the active area i.e., the size of the bipolar

91

plate and membranes within the stacks. Thus, the power output of a VRFB can be designed by

92

adjusting the amount of stacks (doubling the stack number doubles the power output) or by

93

increasing the stack size. Each electrolyte circuit requires two centrifugal pumps and pipes with a

94

length of 30 meters from the tanks to the cells (calculated based on a simple schematic battery

95

system layout), plus another 5 meters of pipes per stack. According to the stack geometry, an

96

average 14 meters of cable are required per stack. The peripheral components comprise one heat

97

exchanger, the inverter and battery management system (also process control system; PCS).

98

Figure 1 shows the composition of the modelled VRFB representing the cell and stack

99

components. The high importance of the electrolyte becomes evident, being this the by far

100

dominating component in terms of mass share. The electrolyte is stored in tanks made of glass

101

fibre (at least one for the positive and one for the negative side), and pumped through the stacks

102

by centrifugal pumps. The electrolyte composition is taken from the technical data sheet of a

103

commercial manufacturer (Gfe GmbH),23 with a vanadium concentration of 1.6 mol / L and a

104

sulphate concentration of 2 mol / L. As membrane material, the standard material Nafion® from

105

E.I. DuPont De Nemours & Co. (DuPont) is used.24 Sulfonated polyetheretherketone (sPEEK)


5


Page 6 of 29

106

represents a promising alternative to Nafion® membranes and is assessed additionally for a

107

sensitivity analysis. The electrodes of VRFBs are generally made of carbon-based felts based on

108

polyacrylonitrile (PAN). Bipolar plates are made from compound materials consisting of

109

synthetic graphite in combination with polypropylene as a binder. Each cell is surrounded by an

110

extruded PVC frame. The individual cells are combined into stacks by the help of gaskets and a

111

steel structure (stack frame) consisting of two double T-profiles that correspond to the

112

dimensions of the stack. Each stack is completed by two copper current collectors. Gaskets seal

113

the individual cells to prevent leakages of the electrolytes, requiring longevity and resistance to

114

acid corrosion. The currently most promising gasket material is FKM, but also EPDM and

115

silicone are suitable.25 All these components are modelled explicitly and in full detail within this

116

work. Currently, no large-scale VRFB production industry yet exists, why reliable data on these

117

processes are difficult to obtain. This applies to all stages of the battery’s life cycle, like charge-

118

discharge efficiency and its possible change over lifetime, the lifetime of battery components and

119

their recyclability and re-usability (especially for the electrolyte and its re-conditioning for

120

subsequent use), but also the modelling of battery components like e.g., the electronics and

121

cables or cooling systems. This requires the use of reference processes, expert judgements and

122

assumptions, increasing thus the modelling uncertainties. In order to provide maximum

123

transparency and reproducibility in this regard, the comprehensive life cycle inventory data

124

(LCI) for each step of the manufacturing process and the corresponding assumptions and data

125

sources are disclosed fully in the SI (Sections 1.2.-1.4.). The production of the VRFB is assumed

126

to take place in Germany, using the corresponding electricity mixes. By contrast, the vanadium

127

pentoxide (V2O5) for the electrolyte is assumed to be produced in South Africa, being this one of

128

the main countries of origin of V2O5.26


6

Page 7 of 29


129 130

Figure 1. Composition (in wt. - %) of a VRFB module with an energy capacity of 8.3 MWh and

131

a rated power of 1 MW

132

2.3. Vanadium production (upstream processes)

133

The vanadium pentoxide production process is modelled based on information from a mine

134

operator in South Africa.27 This process represents vanadium obtained from titanomagnetite

135

ores, the principal source of industrial V2O5. The ore is processed in a first step to produce

136

vanadium containing pig iron, which can further be processed in an electric arc furnace to steel

137

and a vanadium bearing slag (as a by-product of steel manufacturing). From this slag with an

138

elevated vanadium content of around 25%, vanadium pentoxide can be extracted via acid

139

leaching processes.28,29 This includes the grinding of the slag, roasting and the subsequent

140

leaching with ammonium sulphate and sulphuric acid. The resulting ammonium polyvanadate is

141

then converted into high-purity vanadium pentoxide via roasting, while the residual slag is

142

landfilled.28 Figure 2 depicts the process chain, indicating important process steps and the


7


Page 8 of 29

143

corresponding material flows. The detailed LCI data for the vanadium pentoxide process steps

144

can be found in Section 1.3.1. of the SI.

145

146 147

Figure 2. Flowchart of the manufacturing of vanadium pentoxide

148

2.4. LTO Lithium-Ion Battery model (reference battery)

149

LTO type lithium-ion batteries (lithium-iron-phosphate based cathode with lithium titanate

150

anode) are considered to be one of the principal competitors of VRFB. This is basically due to

151

their high cycle lives, making them especially applicable for stationary applications where

152

energy densities are less crucial than in the mobility sector.12 The LTO battery model is based on

153

published literature, majorly the work by Bauer (2010) and a recent publication on stationary

154

batteries.30,31 The battery configuration is adapted to match that of the VRFB in terms of storage

155

capacity. The inverter (not considered in the underlying publication) is assumed to be identical to

156

that of the VRFB. As for the VRFB, no secondary infrastructure (buildings, foundations, etc.) is


8

Page 9 of 29


157

accounted for. More details and the complete LCI data of the LTO battery can be found in the SI

158

(Section 1.5.).

159

2.5. Battery recycling (end-of-life)

160

The batteries are dismantled after the end of the application lifetime (20 years) and recycled.

161

Due to the very limited data availability, a simplified approach is used for the end-of life model

162

of the batteries, based on an adaptation of existing ecoinvent datasets. According to the chosen

163

cut-off model, all impacts associated with the recycling process are allocated to the battery

164

system, while the recovered products are available free of burden. For quantifying the potential

165

benefits of the battery recycling, the cradle-to-cradle impacts of the battery system are calculated

166

twice, once using the average market mix of the individual materials for the production of the

167

batteries, and once using majorly recycled raw materials (closed loop recycling). Since the

168

battery materials cannot be recovered by 100%, a minor share of material has still to be sourced

169

from the markets, even when assuming a maximum use of recycled raw materials, with this share

170

being determined by the recycling efficiency. The difference in results between the system based

171

on recycled and the one based on average market materials is then the potential reduction of

172

environmental impacts due to recycling. This can be considered a temporal mismatch (recovered

173

materials are available 20 years after battery production), but would be a valid assumption for an

174

established battery industry. The recycling processes themselves are modelled based on

175

ecoinvent processes, which are adapted to the specific compositions of the different batteries.

176

The LTO battery installation is dismantled mechanically and the racks and tray housings

177

separated from the battery cells. Steel parts, cables, waste electronic components and waste

178

plastic are send to separate specific recyclers, using ecoinvent data for recycling of these

179

components. The recycling of the battery cells themselves is modelled based on ecoinvent,


9


Page 10 of 29

180

assuming 50% hydrothermal and 50% pyrometallurgical recycling. A general recycling

181

efficiency of 90% is assumed for the basic metals steel and copper and for lithium in the

182

hydrometallurgical route, while the remaining fraction of the battery cells is sent to waste

183

treatment by incineration.32,33 Since no LTO-specific recycling processes exist yet, this leads to a

184

comparably low recovery of materials from these cells. This reflects the current state-of-the art,

185

where LTO batteries would be processed in the same recycling line as other LIB, while in future

186

more specific processes might be used, increasing recovery.34 For all metal parts that can be

187

readily dismantled by mechanical separation on a macro-scale, a higher recycling efficiency of

188

95% is assumed. This applies also for all components of the VRFB, which can entirely be

189

dismantled mechanically. The electrolyte does not degrade significantly and can be re-used for

190

battery applications, but probably requires some re-processing and purification.35 For this

191

purpose, the electricity input required for re-balancing the electrolyte is accounted for, estimated

192

based on electrochemical calculations. Since the actual purification steps required for

193

regenerating a 20 year old electrolyte are yet unknown, this is only a rough approximation and

194

associated with elevated uncertainties. More details about the end-of-life modelling and the

195

inventory tables are provided in Section 3 of the SI.

196

2.6. Battery operation (use-phase)

197

The environmental impacts associated with battery operation are related to internal losses

198

(charge-discharge efficiency), maintenance activities and battery replacements due to limited

199

battery lifetime (10 years for stack components and 20 years for electrolyte, see also Table S.29

200

of the SI). The service provided by the batteries is renewables support (RS) i.e., balancing

201

demand from the grid and daily electricity generation. Since the source of electricity (and thus

202

the environmental impacts associated with its generation) play a relevant role for the total


10

Page 11 of 29


203

impacts of the battery systems, two different renewable energy sources are considered:

204

Electricity from wind turbines and from PV installations. Additionally, the average German grid

205

mix is used as alternative electricity source for comparison purpose (partially fossil based

206

electricity). The battery charges electricity from these sources and dispatches it to the grid in

207

times of lower generation and higher demand. A detailed model of the application is out of the

208

scope of this study, why the same simplified load profile is used for all considered electricity

209

sources. It requires an average 1.12 cycles per day over 20 years i.e., a total of 8176 charge-

210

discharge cycles over the application lifetime.12 For the same reason, no degradation effects (e.g.,

211

dropping efficiency along lifetime, energy input for electrolyte re-balancing) are considered in

212

the use-phase model. More details about the use phase modelling, underlying assumptions and

213

the complete inventory data can be found in Section 2 of the SI.

214 215

3. Results 3.1. Battery manufacturing (cradle-to-gate)

216

Figure 3 shows the contribution of the different VRFB components to the total impacts of

217

battery manufacturing (including upstream processes, without using recycled material) for the

218

four assessed impact categories.


11


Page 12 of 29

219 220

Figure 3. Break down of environmental impacts from VRFB manufacturing to battery

221

components and upstream processes without the use of recycled materials. GWP = global

222

warming potential, HTP = human toxicity potential, AP = acidification potential, ADP = abiotic /

223

mineral depletion potential. Numbers on top of the bars indicate the absolute manufacturing

224

impacts per MWh of electricity delivered by the battery over lifetime.

225

GWP. The battery components associated with the electrolyte (electrolyte, tanks and

226

corresponding upstream processes) make up more than 90 % of the total global warming

227

impacts, while the stack and periphery components contribute only 5 and 2 %, respectively.

228

When looking at the electrolyte production in detail, it becomes clear that the major sources of

229

GWP impacts can be found in the upstream processes i.e., the recovery of the V2O5 from the


12

Page 13 of 29


230

vanadium bearing slag and the prior vanadium bearing cast iron production from the titanate

231

ores.

232

The mining and vanadium extraction activity takes place in South Africa, where the average

233

electricity mix is based heavily on coal, causing the high contribution of electricity consumption

234

along the V2O5 production chain. In fact, the electricity demand for the vanadium production is

235

responsible for 46 % of the total GWP. The mining of the magnetite ore contributes another 9%

236

to the total GWP, of which the major share (6%) are caused by petrol demand during the mining

237

operations. Apart from that, the quicklime used in the pig iron and steelmaking process

238

contributes a total of 8.5 % to the GWP. Other important drivers for the GWP impacts are the

239

demand for process heat and liquid oxygen, but also direct CO2 emissions from the iron- and

240

steelmaking processes.

241

HTP. For the human toxicity potential, the main contributor is the production of the electrolyte

242

again, but with a significantly lower share of only 55 %. Also here, the vanadium pentoxide and

243

thereof the slag production is the main driver (50 % of the total HTP). Within these, impacts are

244

driven principally by the electricity demand, followed by the mining operation and the treatment

245

of the slag. Apart from the electrolyte production, the production of the current collector (21 %)

246

and the inverters (5%) show important contributions to the overall HTP, both mainly due to the

247

required copper.

248

Another 10% of the HTP impacts are caused by the electrolyte tank, with the two precursor

249

materials glass fibre and polyester resin contributing 7 % and 3 %, respectively. Minor

250

contributions stem from periphery components like cables, electronics and heat exchanger,


13


Page 14 of 29

251

principally due to copper, precious metal mining for electronics and stainless steel production.

252

All other components of the VRFB together contribute less than 3 % to the total HTP.

253

AP. The battery components associated with the electrolyte make up more than 95 % of the

254

total acidification impacts. The stack and periphery components have only minor contributions to

255

the overall AP impacts (4 % and 2 %). When looking at the electrolyte production in detail, the

256

major contributions stem from the upstream processes along the V2O5 production chain. The

257

highest are direct emissions from the V2O5 production (53%), where the roasting of the

258

vanadium bearing slag prior to acid leaching causes significant SO2 emissions. Installing an

259

exhaust gas scrubber for SO2 recovery might reduce these substantially. However, since no

260

further information is available in this regard, this is not considered (conservative model).

261

Significant contributions are associated also with the electricity demand in the V2O5 production

262

chain (27%), followed by the mining operation (5%). From the stack and periphery (together

263

only 4% of the total AP) components, the current collector and electronic components show the

264

highest contributions (3%), mainly due direct emissions from copper and precious metal mining.

265

ADP. As for the other impact categories, the electrolyte is the most important driver for abiotic

266

(mineral) resource depletion (93%). Here, the extraction of the resources titanium dioxide and

267

vanadium (both contained in the mined magnetite ore) are mainly decisive for the ADP. The

268

extraction of titanium dioxide accounts for 51 % and the vanadium extraction for 21 % of the

269

overall ADP. This distribution is a direct result of the allocation of impacts between the two by-

270

products according to economic criteria. While this is in line with current practice and ecoinvent

271

principles, it can certainly be discussed whether titanium needs to be accounted for as depleted

272

resource when it is not further used in the process but left on the site as tailing residue.36 Besides

273

the vanadium extraction, sulphuric acid is one of the major driver for the impacts in this category


14

Page 15 of 29


274

(7%) as ingredient for the electrolyte and another 4% in the acid leaching of the vanadium slag;

275

accounted for under ‘V2O5 production - others’ in Figure 3. The high ADP associated with

276

sulphuric acid is also an allocation effect: A minor share of the (ecoinvent) market mix for

277

sulphuric acid is sourced from mining processes, where it is obtained as a by-product from

278

emission abatement (SO2 recovery). Since it is a marketable by-product, part of the resource

279

depletion impacts are allocated to the sulphuric acid, causing its comparably high contribution to

280

ADP. Besides the electrolyte, relevant contributions stem from the copper required for current

281

collectors and electronics (4%), the glass fibre for the electrolyte tank (1 %) and the

282

hexafluoroethane required for Nafion® and FKM synthesis. All other components of the VRFB

283

system show only minor contributions.

284

3.2.Whole life cycle (cradle-to-cradle)

285

Figure 4 shows the cradle-to-cradle comparison (whole life cycle) of the environmental

286

impacts obtained for the VRFB with those of the LTO for the four considered impact categories,

287

both for three different electricity sources: electricity from wind turbines, from a photovoltaic

288

installation, and the average German grid mix. The right side of each sub-figure shows the

289

impact using recycled materials, while the left side displays the impacts without recycling (using

290

virgin material). Replacement refers to additional battery required due to ageing during the 20

291

year lifetime of the installation and can be considered additional manufacturing impacts due to

292

limited lifetime.


15


Page 16 of 29

293 294

Figure 4. Environmental impacts per MWh of electricity provided over lifetime, broken down to

295

life cycle stages. GWP = global warming potential; HTP = human toxicity potential; AP =

296

acidification potential; ADP = abiotic depletion potential, elements

297

When disregarding the potential benefit of battery recycling (use of virgin materials; left six

298

bars in each sub-figure in Fig.4), the manufacturing impacts dominate the total impacts in all

299

assessed categories if renewable energy is used as electricity source. In spite of its higher


16

Page 17 of 29


300

nominal energy density (38 vs. 26Wh/kg i.e., a lower battery mass is required for providing the

301

same storage capacity), the LTO battery shows, per MWh of electricity provided over lifetime,

302

higher impacts from the manufacturing stage except for AP.

303

The impacts from the use phase are caused by the electricity dissipated due to internal

304

inefficiencies of the batteries. For wind and PV installations, these are associated majorly with

305

the production phase of the turbines and solar panels, why PV installations usually show higher

306

impacts per kWh of electricity than wind parks.12 For the grid electricity, fossil power plants,

307

above all coal and lignite, are the major sources of environmental impacts causing high

308

contributions to GWP, AP and HTP. Only for ADP, a different profile can be observed. Here, the

309

lowest impacts are obtained for the grid mix. This is typical for renewable energy technologies

310

that rely to a significant share on functional metals and thus cause higher mineral resource

311

depletion impacts than thermal power plants.37,38 Due to the lower charge-discharge efficiency of

312

the VRFB, its use phase impacts are significantly higher. For comparably ‘clean’ electricity from

313

wind turbines, this is less relevant, and the VRFB scores better than the LTO in the majority of

314

the assessed categories. However, with electricity associated with higher environmental impacts

315

(like PV or grid electricity), the result is reversed due to the increasing relevance of internal

316

losses. Especially for the (partially) fossil based average grid mix, the higher efficiency of the

317

LTO is becomes decisive. For both battery types, ageing effects can also increase the internal

318

losses within the cells over lifetime and thus the impacts from the use-phase. Since no data is

319

available in this regard, this is not considered further. The replacement of battery cells / stacks

320

along the assumed 20 year operation time of the storage system plays only a minor role due to

321

the high lifetime of both batteries. Only for HTP, the stack exchange for the VRFB shows a

322

significant contribution, mainly due to the copper required for the current collectors. However,


17


Page 18 of 29

323

the current collector plate could be recovered easily during dismantling, and when taking into

324

account the potential for using recycled materials (right sides in Figure 4), its impact is reduced

325

drastically.

326

The benefit of recycling (displayed as a reduction of impacts when using recycled material) is

327

significant in all cases and higher than the additional environmental impacts caused by the

328

processing of the waste streams (‘EoL’-phase in Figure 4). In general, the EoL processes for the

329

LTO battery show higher impacts due to the much more complex process required for treating

330

the highly integrated battery cells by hydrometallurgical and pyrometallurgical processes. The

331

VRFB in comparison can be dismantled by mechanical processes and the different components

332

can then be recycled directly as mono-fraction materials, reducing expenses for EoL treatment

333

and increasing the amount of recoverable materials. This leads to a drastic reduction of impacts

334

for the VRFB in all assessed categories. In consequence, when considering the benefit of battery

335

recycling, the VRFB scores better than the LTO battery in all cases as long as renewable energy

336

is charged. This highlights the importance of the end-of-life stage for the overall impacts of

337

batteries and the relevance for a design for recyclability of battery systems, even if this might

338

lead to lower technical performance (e.g., energy densities).

339

When comparing the results with those obtained by Hiremath et al.14, the most extensive recent

340

study in the field, these obtain values of 15 – 135 kg CO2-eq cradle-to-gate emissions per MWh

341

delivered by the battery, depending on the use profile of the VRFB. Yet, their calculation is a

342

direct recompilation of the results provided by Denholm & Kulcinski,15 who provide a value of

343

40 kg CO2-eq; close to the values obtained in the present study (38.2 kg CO2-eq /MWh). The

344

reference study by Rydh11 obtains a GWP impact of 8.15 kg CO2-eq / MWh provided over

345

lifetime, significantly lower. However, their very simple inventory does not model upstream


18

Page 19 of 29


346

processes and assumes the vanadium to be available free of burden, thus underestimating the true

347

impacts of the electrolyte. Another recent study16 only provides aggregated results (single score)

348

and no documentation of their inventory model, why it cannot be used for comparison.

349

4. Discussion and sensitivity analysis

350

The inventory data for the VRFB basis for this assessment are a hypothetical model of a

351

current state-of-the-art VRFB. Data from many sources are compiled for the inventory model

352

and numerous assumptions and simplifications are required for this purpose, why the

353

associated uncertainties are high and need to be considered when interpreting results.

354

Providing uncertainty ranges for the values provided in the inventory tables would help to

355

estimate their influence on the results. However, for being able to provide a meaningful

356

uncertainty distribution, a certain minimum of data points is required, while often only a single

357

value is available. Thus, we do not provide uncertainty ranges, but disclose the full inventory

358

data in the SI, allowing for further development of the inventory model and successive

359

inclusion of further data for future uncertainty analysis. Applying the standard uncertainty

360

estimation process according to ecoinvent,39,40 we obtain a σ2 of 21 for energy and material

361

inputs, resource demand and the most relevant emissions. The indicators that contribute most

362

to the total uncertainty are reliability (because of frequent use of assumptions for modelling)

363

and completeness / sample size (modelling is based on a limited amount of datasources and

364

small sample sizes).

365

4.1. Battery layout / (E/P) ratios

366

Comparing the default VRFB system with an energy-to power (E/P) ratio of 8:1 (in the

367

following using integer values for the E/P ratios for better readability) to systems with lower E/P


19


Page 20 of 29

368

(4:1) and higher E/P (10:1) allows giving recommendations about the most favourable system

369

configuration under environmental aspects (Figure 5). With increasing E/P ratio, also the mass

370

share of the energy components increases (91%, 95% and 97% for E/P ratios of 4/1, 8/1 and

371

10/1, respectively). Since, on a mass basis, the power components (stack) show relatively higher

372

environmental impacts than the electrolyte and tanks, lower impacts are obtained with high E/P

373

ratios. For very high E/P ratios, the impacts associated with the VRFB production would

374

asymptotically approach those of the electrolyte. In consequence, the most favourable

375

applications for VRFB under environmental aspects would be the provision of high storage

376

capacities but comparably low power rating, showing the lowest impacts per kWh of storage

377

capacity.

378


20

Page 21 of 29


379 380

Figure 5. Comparison of the results for the battery production (no use of recycled materials) of

381

three different energy-to-power ratios (4:1, 8:1 and 10:1) and contribution of the battery

382

components to the environmental impacts per category. GWP = global warming potential, HTP =

383

human toxicity potential, AP = acidification potential, ADP = depletion of abiotic resources

384

4.2. Membrane materials

385

The membrane plays a key role in any VFRB system. Apart from GWP impacts from its

386

production, it is also a major cost driver.2 Among the variety of potential membrane materials

387

available for VRFBs, one of the most promising alternatives to the common Nafion® membrane

388

material is sulfonated polyetheretherketone (sPEEK). In order to evaluate the potential

389

environmental benefits or drawbacks of this material, Figure 6 displays the characterization


21


Page 22 of 29

390

results for a sPEEK based VRFB stack relative to an identical stack using the standard material

391

(Nafion®). More details of the modelling and the corresponding LCI can be found in the SI

392

(Section 1.2.1. and 1.2.2).

393 394

Figure 6. Environmental impacts associated with the production of a 1 MW VRFB stack using

395

different membrane and gasket materials in relation to the default battery design

396

The use of sPEEK instead of Nafion® as membrane material decreases the environmental

397

impacts in all of the assessed categories. Especially for GWP and ADP, the overall impacts

398

caused by the battery stack production can be reduced by 36% and 19%, respectively, while for

399

HTP and AP the differences are comparably small. The reason for the high GWP reduction

400

potential is majorly the influence of the tetrafluoroethylene in the production of the Nafion®

401

membrane, which makes a great contribution to the GWP. For a sPEEK- based VRBF stack, the

402

production of the membrane contributes only 13 % to the total GWP associated with its


22

Page 23 of 29


403

production (in comparison to 45% for the Nafion®-based stack). Of these 13%, 12% stem from

404

the dinitrogen monoxide emitted during the sPEEK membrane production, while other

405

components show only minor contributions. For ADP, the high reductions achievable when

406

substituting Nafion® by sPEEK can be attributed majorly to the avoidance of hexafluoroethane

407

required for the Nafion® synthesis process. However, some major limitations and uncertainties

408

of this analysis have to be taken into account. First, the energy consumption of the sPEEK

409

membrane production process is estimated based only on the shaping and polycondensation

410

process steps. Other process steps are neglected due to lack of data and the total energy demand

411

is therefore probably underestimated. On the other hand, the dinitrogen monoxide emissions of

412

the sPEEK synthesis process, modelled based on laboratory data,41 are relatively high (see SI,

413

Section 1.2.2. for more details). In an industrial plant, these would most probably be converted

414

into nitrogen and water by an ammonia washing process. This is not taken into account, why the

415

GWP of the sPEEK membrane is probably overestimated. Hence, the use of sPEEK membrane

416

can be considered a promising alternative to the widely used Nafion® membrane under

417

environmental aspects. Yet, the replacement of standard membranes against the alternative

418

membrane is not technically trivial and requires further research. Impacts on battery performance

419

and lifetime aspects have to be considered in order to avoid a burden shifting, e.g. reduced

420

production impacts against higher impacts during operation due to reduced lifetime or higher

421

inefficiencies.

422

4.3. Gasket materials

423

Gaskets are important components that can determine the lifetime of a VRFB stack. Leakages

424

due to fail of gaskets can lead to oxidation or deterioration of the electrolyte and to the failure of

425

the whole VRFB. Different materials for gaskets are available, namely FKM (fluoroelastomer),


23


Page 24 of 29

426

EPDM (ethylene propylene diene monomer) and silicone.25 FKM gaskets are most common and

427

therefore considered as the standard material for the base-case configuration in the previous

428

assessments. The effect of using alternative gasket materials (namely EPDM and silicone) on the

429

characterization results for the 1MW VRFB stack are provided in Figure 6.

430

While EPDM and silicone show very similar environmental impacts (except for ADP, where

431

silicone scores slightly better), they both have the potential to significantly reduce the GWP

432

impacts associated with the VRFB stack (up to 30 %). This is mainly due to avoidance of

433

tetrafluoroethylene required for the FKM gasket production, responsible for the relatively high

434

environmental impacts of this material. While HTP and AP impacts of EPDM and silicone

435

production are higher than for FKM, this is negligible for the results of the whole stack in these

436

categories, where membrane and gaskets contribute only very minor shares (Figure 5). For ADP,

437

a notable reduction (7% and 10% for EPDM and silicone, respectively) can be observed, which

438

is attributable (similar as for substitution of the Nafion® membrane) to the avoidance of

439

hexafluoroethane.

440

Still, also for the sensitivity analysis the limitations and uncertainties associated with this have

441

to be taken into account. For the modelling of the LCI of the silicone and EPDM gasket

442

production, the standard ecoinvent data records are used without any application-specific

443

changes. Thus, the production process might vary considerably depending on the excipients

444

used. Apart from that, the impacts of the different gasket materials on battery performance and

445

lifetime need to be evaluated for a more comprehensive assessment of alternative gasket

446

materials.

447

4.4. Performance parameters and recyclability


24

Page 25 of 29


448

While VRFBs show low energy densities and low efficiencies, their long lifetime and good

449

recyclability make them a promising alternative for stationary energy storage. This is especially

450

true for low power applications, since the environmental impacts of VRFB are decreasing for

451

higher energy-to-power (E/P) ratios. In comparison with a comparable stationary lithium-ion

452

battery (LTO-type), the VRFB shows lower environmental impacts in three of the four assessed

453

categories (per MWh of electricity provided over lifetime and using wind power for charging).

454

Considering also the potential benefit of battery recycling, the VRFB outscores the LTO battery

455

clearly in all categories due to its easy recyclability (it is less integrated than LIB cells and easily

456

be separated into mono-fractions on macro-scale) . This shows the relevance of recyclability for

457

the environmental performance of batteries, which cannot be disregarded for battery

458

assessments.

459

However, its charge-discharge efficiency is significantly lower (75% vs. 90% for the LTO),

460

and when using electricity with higher environmental impacts per MWh, the highly efficient

461

LTO battery can become the better choice. In fact, already with electricity from a German PV

462

installation (instead of wind electricity), the advantage of the VRFB is reduced significantly, and

463

for (partially) fossil based electricity, the LTO battery scores better. A thorough assessment of

464

the different battery types within different applications considering the actual charge-discharge

465

requirements, their influence on battery lifetime and the actually charged electricity could

466

therefore be a highly relevant topic for future works.

467

ASSOCIATED CONTENT

468 469

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. It provides the complete LCI data in tabulated form


25


470

explicitly for reproducibility purposes and additional descriptions of the underlying modelling

471

approaches and assumptions.

Page 26 of 29

472 473

AUTHOR INFORMATION

474

Corresponding Author

475

* Dr. Jens F. Peters. Helmholtz Institute Ulm (HIU), Karlstr. 11, 76133 Karlsruhe, Germany. E-

476

mail: [email protected]; Phone: +49-721-608 28177.

477

ACKNOWLEDGMENT

478

The authors acknowledge the funding of the Helmholtz Society within the SCI Program Storage

479

and Cross-Linked Infrastructures.

480

ABBREVIATIONS

481

ADP, Abiotic Depletion Potential, minerals (impact category); AP, Acidification Potential

482

(impact category); E/P, Energy-to-power ratio; EPDM, ethylene propylene diene monomer

483

(gasket material); EoL, End-of-Life; FKM, Fluoroelastomer (gasket material); FU, Functional

484

Unit; GWP, Global Warming Potential (impact category); HTP, Human Toxicity Potential

485

(impact category); LCA, Life Cycle Assessment; LCI, Life Cycle Inventory; LTO, Lithium

486

Titanate Oxide (Lithium-Ion battery chemistry); PAN, Polyacrylonitrile; RFB, Redox-Flow

487

Battery; sPEEK, Sulfonated polyetheretherketone (membrane material); VRFB, Vanadium

488

Redox-Flow Battery.

489

REFERENCES

490 491

(1)

Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Gostick, J. T.; Liu, Q. Redox Flow Batteries: A Review. J. Appl. Electrochem. 2011, 41 (10), 1137.


26

Page 27 of 29

492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537


(2)

(3)

(4) (5)

(6) (7)

(8)

(9)

(10) (11) (12)

(13)

(14)

(15)

(16)

(17)

(18)

Minke, C.; Turek, T. Materials, System Designs and Modelling Approaches in TechnoEconomic Assessment of All-Vanadium Redox Flow Batteries – A Review. J. Power Sources 2018, 376, 66–81. Cunha, A.; Brito, F. P.; Martins, J.; Rodrigues, N.; Monteiro, V.; Afonso, J. L.; Ferreira, P. Assessment of the Use of Vanadium Redox Flow Batteries for Energy Storage and Fast Charging of Electric Vehicles in Gas Stations. Energy 2016, 115, 1478–1494. Electrical Power Research Institute (EPRI). Vanadium Redox-Flow Battery: An In-Depth Analysis; Palo Alto (CA), 2007; Vol. 3. Minke, C. Techno-ökonomische Modellierung Und Bewertung von Stationären VanadiumRedox-Flow-Batterien Im Industriellen Maßstab, 40th ed.; Cuvillier Verlag: Göttingen, 2016. Noack, J.; Wietschel, L.; Roznyatovskaya, N.; Pinkwart, K.; Tübke, J. Techno-Economic Modeling and Analysis of Redox Flow Battery Systems. Energies 2016, 9 (8). Viswanathan, V.; Crawford, A.; Stephenson, D.; Kim, S.; Wang, W.; Li, B.; Coffey, G.; Thomsen, E.; Graff, G.; Balducci, P.; Kintner-Meyer, M.; Sprenkle, V. Cost and Performance Model for Redox Flow Batteries. J. Power Sources 2014. Zhang, M.; Moore, M.; Watson, J. S.; Zawodzinski, T. A.; Counce, R. M. Capital Cost Sensitivity Analysis of an All-Vanadium Redox-Flow Battery. J. Electrochem. Soc. 2012, 159 (8), A1183–A1188. Zeng, Y. K.; Zhao, T. S.; An, L.; Zhou, X. L.; Wei, L. A Comparative Study of AllVanadium and Iron-Chromium Redox Flow Batteries for Large-Scale Energy Storage. J. Power Sources 2015, 300, 438–443. Ha, S.; Gallagher, K. G. Estimating the System Price of Redox Flow Batteries for Grid Storage. J. Power Sources 2015, 296, 122–132. Rydh, C. J. Environmental Assessment of Vanadium Redox and Lead-Acid Batteries for Stationary Energy Storage. J. Power Sources 1999, 80 (1), 21–29. Baumann, M. J.; Peters, J. F.; Weil, M.; Grunwald, A. CO2 Footprint and Life Cycle Costs of Electrochemical Energy Storage for Stationary Grid Applications. Energy Technol. 2017, 5, 1071–1083. Arbabzadeh, M.; Johnson, J. X.; De Kleine, R.; Keoleian, G. A. Vanadium Redox Flow Batteries to Reach Greenhouse Gas Emissions Targets in an off-Grid Configuration. Appl. Energy 2015, 146, 397–408. Hiremath, M.; Derendorf, K.; Vogt, T. Comparative Life Cycle Assessment of Battery Storage Systems for Stationary Applications. Environ. Sci. Technol. 2015, 49 (8), 4825– 4833. Denholm, P.; Kulcinski, G. L. Life Cycle Energy Requirements and Greenhouse Gas Emissions from Large Scale Energy Storage Systems. Energy Convers. Manag. 2004, 45 (13-14), 2153–2172. Unterreiner, L.; Jülch, V.; Reith, S. Recycling of Battery Technologies – Ecological Impact Analysis Using Life Cycle Assessment (LCA). Energy Procedia 2016, 99, 229– 234. EC-JRC. ILCD Handbook: General Guide for Life Cycle Assessment - Detailed Guidance; European Commission - Joint Research Centre. Institute for Environment and Sustainability: Ispra, Italy: EC-JRC - Institute for Environment and Sustainability, 2010. ISO. ISO 14040 – Environmental Management – Life Cycle Assessment – Principles and Framework.; International Organization for Standardization: Geneva, Switzerland, 2006.


27


538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583

Page 28 of 29

(19) ISO. ISO 14044 – Environmental Management – Life Cycle Assessment – Requirements and Guidelines; International Organization for Standardization: Geneva, Switzerland, 2006. (20) Guinée, J. B.; Gorrée, M.; Heijungs, R.; Huppes, G.; Kleijn, R.; Koning, A. de; Oers, L. van; Wegener Sleeswijk, A.; Suh, S.; Udo de Haes, H. A.; de Bruin, H.; van Duin, R.; Huijbregts, M.. Handbook on Life Cycle Assessment. Operational Guide to the ISO Standards. I: LCA in Perspective. IIa: Guide. IIb: Operational Annex. III: Scientific Background.; Kluwer Academic Publishers: Dordrecht, 2002. (21) Peters, J. F.; Baumann, M. J.; Zimmermann, B.; Braun, J.; Weil, M. The Environmental Impact of Li-Ion Batteries and the Role of Key Parameters – A Review. Renew. Sustain. Energy Rev. 2017, 67, 491–506. (22) GreenDelta GmbH. OpenLCA 1.6 http://www.openlca.org/ (accessed Sep 5, 2017). (23) GfE Metalle und Materialien GmbH. Vanadium Electrolyte Solution 1.6 M http://www.gfe.com/produktbereiche/vanadiumchemikalien/anwendungen/energiespeicher/ (accessed Sep 3, 2017). (24) The Chemours Company. Nafion N115, N117, N1110 Ion Exchange Materials https://www.chemours.com/Nafion/en_US/index.html (accessed Aug 24, 2017). (25) Eisenhuth GmbH & Co. KG. Komponenten für Redox-Flow-Batterien, Brennstoffzellen und Elektrolyseure https://eisenhuth.de/de/komponenten-fuer-redox-flow-batterienbrennstoffzellen-und-elektrolyseure/ (accessed Aug 7, 2017). (26) US Geological Survey. Mineral Commodity Summaries 2016; US Geological Survey, 2017. (27) Evraz Highveld Steel. Process Overview http://www.evrazhighveld.co.za/ processoverview.asp (accessed Sep 3, 2017). (28) Bauer, G.; Güther, V.; Hess, H.; Otto, A.; Roidl, O.; Roller, H.; Sattelberger, S. Vanadium and Vanadium Compounds. Ullmanns Encycl. Ind. Chem. 2000, 38, 119–126. (29) GfE Metalle und Materialien GmbH. Vanadium-Recycling: hochreine Vanadium Chemikalien: GfE http://www.gfe.com/know-how/vanadium-sourcing/ (accessed Sep 3, 2017). (30) Peters, J. F.; Weil, M. Aqueous Hybrid Ion Batteries – An Environmentally Friendly Alternative for Stationary Energy Storage? J. Power Sources 2017, 364, 258–265. (31) Bauer, C. Ökobilanz von Lithium-Ionen Batterien; Paul Scherrer Institut, Labor für Energiesystem-Analysen (LEA): Villigen, Switzerland, 2010. (32) Fisher, K.; Wallén, E.; Laenen, P. P.; Collins, M. Battery Waste Management Life Cycle Assessment; Final Report for Publication; Environmental Resources Management (ERM), 2006. (33) Buchert, M.; Jenseit, W.; Merz, C.; Schüler, D. Ökobilanz Zum „Recycling von LithiumIonen-Batterien“ (LithoRec); Endbericht LithoRec; Öko-Institut: Darmstadt, Germany, 2011. (34) Tang, W.; Chen, X.; Zhou, T.; Duan, H.; Chen, Y.; Wang, J. Recovery of Ti and Li from Spent Lithium Titanate Cathodes by a Hydrometallurgical Process. Hydrometallurgy 2014, 147-148 (Supplement C), 210–216. (35) Roznyatovskaya, N.; Herr, T.; Küttinger, M.; Fühl, M.; Noack, J.; Pinkwart, K.; Tübke, J. Detection of Capacity Imbalance in Vanadium Electrolyte and Its Electrochemical Regeneration for All-Vanadium Redox-Flow Batteries. J. Power Sources 2016, 302 (Supplement C), 79–83.


28

Page 29 of 29

584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602


(36) Peters, J. F.; Weil, M. A Critical Assessment of the Resource Depletion Potential of Current and Future Lithium-Ion Batteries. Resources 2016, 5 (4), 46. (37) Graedel, T. E. On the Future Availability of the Energy Metals. Annu. Rev. Mater. Res. 2011, 41 (1), 323–335. (38) World Bank. The Growing Role of Minerals and Metals for a Low Carbon Future; World Bank Publications; World Bank, 2017. (39) Althaus, H.-J.; Doka, G.; Heck, T.; Hellweg, S.; Hischier, R.; Nemecek, T.; Rebitzer, G.; Spielmann, M.; Wernet, G. Ecoinvent Report No. 1 - Overview and Methodology. In Sachbilanzen von Energiesystemen: Grundlagen für den ökologischen Vergleich von Energiesystemen und den Einbezug von Energiesystemen in Ökobilanzen für die Schweiz.; Frischknecht, R., Jungbluth, N., Eds.; Swiss Centre for Life Cycle Inventories: Dübendorf, Switzerland, 2007. (40) Weidema, B. P.; Wesnæs, M. S. Data Quality Management for Life Cycle Inventories—an Example of Using Data Quality Indicators. J. Clean. Prod. 1996, 4 (3), 167–174. (41) Gebert, M. Benchmarking-Methodik Für Komponenten in PolymerelektrolytBrennstoffzellen; Reihe Energietechnik; Schriften des Forschungszentrums Jülich Band 30; Forschungszentrum Jülich: Jülich, Germany, 2004.


29

Life Cycle Assessment of a Vanadium Redox Flow Battery

Recommend Documents