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Net Energy Analysis of Solar Fuel Device
Presented by Pei Zhai
The work was supported by LBNL and JCAP
GCEP workshop, Stanford 03/31/15
• Free sunshine • How to harness it directly? – many ways-- water heater, photovoltaics,
bio-fuel
A relatively new technology— Solar Fuel Device
Other names: Artificial photosynthesis, Artificial leaf, Solar water splitting, and
Photo-electro-chemical (PEC) device
Principles
Simply speaking, one device combining two steps:
1) converting sunlight to electric energy (electron-hole)
2) converting electric energy to hydrogen (reduction of H+ by electrons)
4h+ + 2H2O 4H+ + O2(g) photoanode
4e- + 4H+ 2H2(g) photocathode
Research centers worldwide
Institutes, universities and industries involved
• U.S.A.– MIT ($9.5 million)，Caltech, Berkeley ($116 million) Joint Center of Artificial Photosynthesis (JCAP) (2010 DOE energy hub)
• Europe– EU (€ 4 million), Netherlands (€25 million)，UK
• Asia—Japan (14 billion), Korea, Singapore
Why Net Energy Analysis (NEA)?
• Renewable energy technologies: purpose is to harness free energy
• Net energy = energy out - energy in
• Bottomline requirement: Net energy is positive
• Energy in :
direct (fabrication) and indirect (embodied in materials)
Primary energy requirement to produce PEC device
Energy content of hydrogen
Input Output
Net energy = Output - Input
PEC
Contributions and limitations of NEA
Context– renewable energy technologies
• Fundamental requirement for renewable energy technologies
• Good ‘Entry Point’ into life cycle thinking
• Should not mask the other impact assessments (land use, toxicity release)
• Could be one of many metrics to help decision-making
Conceptual structure of PEC device (solar fuel)
Photo-anode micro-wire
Membrane
Photo-cathode micro-wire
Chamber
Glass cover
Photo-anode
Membrane
Photo-cathode
Pipes and other components (not included)
Note: figures are not in real scale
Catalyst
Catalyst
Defining system boundary and functional units
Materials
Fabrication
Photoelectrodes
Catalysts
Photoelectrode fabrication
Encapsulation material
PEC device LCA boundary
LCA method
Scope: materials and fabrication processes
Functional units: MJ per m2 PEC
MJ per kg Hydrogen
Goal: Net primary Energy
Membrane
Other processes
Catalyst deposition
Membrane fabrication
Other materials
Life Cycle Assessment of emerging technologies --Challenges and opportunities
• Opportunities– • Help scientists to have a big picture of their research • Point out some energy intensive components which they may
not have realized • Minimize the negative environmental impacts even at very early
stage of R&D • Challenges– • Few available data or literatures • Dynamics and uncertainties (material and experimental
procedures always change, making assumptions of future) • Interpretation of the results (never single point, always a range)
Assumptions for Lower, Medium and Higher cases
Category Component Lower case Medium case Higher case
Material choices
Photocathode Si Si Si
Photoanode WO3 WO3 GaAs
Catalysts for photocathode
Co Pt Pt
Catalysts for photoanode
No catalyst No catalyst Pt
Encapsulation PVC PVC Polycarbonate
Thickness of chamber
3 mm 5 mm 7 mm
Thickness of membrane
30 um 50 um 70 um
Fabrication Thermodynamic efficiency
70% 50% 30%
Details of calculation (medium case)
• Embodied energy in materials
• Primary energy use in fabrication
Materials
Fabrication
Photoelectrodes
Catalysts
Photoelectrode fabrication
Encapsulation material
PEC device LCA boundary
LCA method
Scope: materials and fabrication processes
Functional units: MJ per m2 PEC
MJ per kg Hydrogen
Goal: Net primary Energy
Membrane
Other processes
Catalyst deposition
Membrane fabrication
Other materials
Materials Thickness (nm)
Mass (g/m2)
Energy
intensity (MJ/g)
Embodied
energy (MJ/m2)
Si 2000A 4.7 0.2 5.2
WO3 20B 0.1 1.1 0.02
Pt 1.5 0.03 279 9.0
A: It is an equivalent thickness converting from Si wire array which has 2.8 µm of diameter, 50 µm of length and 7 µm of lattice spacing. B: It is an equivalent thickness converting from WO3 wire array which has 70 nm of diameter, 4 µm of length and 0.5 µm of lattice spacing.
Photo-electrodes and catalysts
Photo-electrodes (Si, WO3); catalyst (Pt)
Materials
Fabrication
Photoelectrodes
Catalysts
Photoelectrode fabrication
Encapsulation material
PEC device LCA boundary
LCA method
Scope: materials and fabrication processes
Functional units: MJ per m2 PEC
MJ per kg Hydrogen
Goal: Net primary Energy
Membrane
Other processes
Catalyst deposition
Membrane fabrication
Other materials
Membrane
• Nafion®-- Perfluoro-sulfonic acid (PFSA)
• Very few data from any database or literature review
• PE (polyethylene) as a proxy
• The cost of PSFA is 19 times of PE, we assume the energy intensity of PSFA is 19 times of PE
• Primary energy of PFSA is estimates as 139 MJ/m²
Materials
Fabrication
Photoelectrodes
Catalysts
Photoelectrode fabrication
Encapsulation material
PEC device LCA boundary
LCA method
Scope: materials and fabrication processes
Functional units: MJ per m2 PEC
MJ per kg Hydrogen
Goal: Net primary Energy
Membrane
Other processes
Catalyst deposition
Membrane fabrication
Other materials
Chamber and glass cover
• Using the most common plastic and glass which are PVC and coated flat glass
• Data for primary energy in materials are from a LCA database -- Ecoinvent 2.2
• Total primary energy is 534 MJ/m²
Materials
Fabrication
Photoelectrodes
Catalysts
Photoelectrode fabrication
Encapsulation material
PEC device LCA boundary
LCA method
Scope: materials and fabrication processes
Functional units: MJ per m2 PEC
MJ per kg Hydrogen
Goal: Net primary Energy
Membrane
Other processes
Catalyst deposition
Membrane fabrication
Other materials
Photo-cathode--Si wire array growth
• Main step: vapor-liquid-solid (VLS) growth
• Growth environment: 1000 °C
Figure : p-Si wire array from (Boettcher S.W., et.al. 2011)
Photo-anode--WO3 wire array growth
• Main step: vapor-liquid-solid (VLS) growth
• Growth environment: 1000 °C
Figure : WO3 wire array from (Cao B., et.al. 2009)
Catalysts – Pt depostion
• Electron-beam deposition requires high vacuum environment (8e-04 Pa)
Source: McKone, J. R. et al. (2011).
Energy use for Electrodes micro-wire array growth
Thermo-dynamic models
Heating: Eh= mass * specific heat * (T-T0)
Vacuum pumping: Ev= P0 *V * Ln (P/P0)
Materials
Fabrication
Photoelectrodes
Catalysts
Photoelectrode fabrication
Encapsulation material
PEC device LCA boundary
LCA method
Scope: materials and fabrication processes
Functional units: MJ per m2 PEC
MJ per kg Hydrogen
Goal: Net primary Energy
Membrane
Other processes
Catalyst deposition
Membrane fabrication
Other materials
Membrane fabrication
• Main process is heating at 140 °C
Source: Spurgeon, J. M. et al. (2011)
Materials
Fabrication
Photoelectrodes
Catalysts
Photoelectrode fabrication
Encapsulation material
PEC device LCA boundary
LCA method
Scope: materials and fabrication processes
Functional units: MJ per m2 PEC
MJ per kg Hydrogen
Goal: Net primary Energy
Membrane
Other processes
Catalyst deposition
Membrane fabrication
Other materials
For other ancillary processes, data are adjusted from PV industry
• Environmental control 200 MJ/m²
• Water pumping 31 MJ/m²
• Miscellaneous chemicals 15 MJ/m²
Break-down results of primary energy requirement
(medium case, error bars showing lower and higher cases)
0
200
400
600
800
1000
1200M
J/m
2
Materials Fabrication
Functional unit is important for LCA study
Now the results of ‘energy in’ are in MJ/m² –>
Because the ‘energy out’ MJ/kg in hydrogen –>
in order to calculate Net energy
• Need to convert MJ/m² to MJ/kg,
• That brings more uncertainty
• Because need to know performance parameters: efficiency and longevity of PEC
• It is in early-stage, we could only assume a range.
Equation-- Primary energy requirement in MJ to produce 1 kg of hydrogen
Determined by Solar-to-Hydrogen STH efficiency
MJ/kg
MJ/m2
Longevity (years)
Results-- MJ to produce 1 kg of hydrogen
Lower left part of the figure below the black line has negative net energy, (e.g. if efficiency is 3% and longevity is 8 years, net energy is zero)
Scenarios of future achievement
Different STH efficiency and longevity combination would lead to different net energy
STH Efficiency
Longevity (year)
Primary energy requirement MJ
Energy content MJ of 1 kg H2
Net energy MJ of 1 kg of H2
3% 5 194 120 (-)74
3% 8 120 120 0
5% 10 58 120 62
10% 10 29 120 91
10% 30 10 120 110
Uncertainties could affect results
1. STH efficiency and longevity
2. thermodynamic models efficiency
3. material choice, chamber layer thickness
Which uncertainties have higher effect?
Base point -- 29 MJ/kg (medium case, 10% efficiency and 10 years)
Discussions
• The most energy intensive process is the fabrication of photo-electrodes
• Now, the method requires very high temperature 1000 °C • In future, it is possible to adopt other methods like chemical-etching.
• Chamber material costs 20% energy • Point to the direction of designing with less chamber material
• Electrodes and catalyst materials cost < 1% • From energy analysis perspective, no worries
• The key parameters to determine the net energy balance device
STH efficiency, longevity and fabrication thermal-efficiency • This study points out the bottom-line requirement (positive net
energy)
Publication and acknowledgement
• Publication: Net primary energy balance of a solar-driven photoelectrochemical water-splitting device, Energy and Environmental Science, 2013 (For all the citations used in this presentation, please refer to this publication)
• Co-authors: Sophia Haussener, Joel Ager, Roger Sathre, Karl Walczak, Jeffery Greenblatt, Thomas McKone
• Funding agency: DOE—LBNL and JCAP
Following up works by my colleagues at LBNL
More information
• Tomorrow 1:15pm, Jeff Greenblatt will talk more about large-scale application and early technology appraisal
• Joint Center of Artificial Photosynthesis
http://solarfuelshub.org/
• Lawrence Berkeley National Lab
– CarbonCycle 2.0 Initiative
http://carboncycle2.lbl.gov/
