Net Energy Analysis of Solar Fuel Device

Transcription

1 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

2 Free sunshine How to harness it directly? many ways-- water heater, photovoltaics, bio-fuel

3 A relatively new technology Solar Fuel Device Other names: Artificial photosynthesis, Artificial leaf, Solar water splitting, and Photo-electro-chemical (PEC) device

4 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 + + 2H 2 O 4H + + O 2 (g) photoanode 4e - + 4H + 2H 2 (g) photocathode

5 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

6 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) Input Primary energy requirement to produce PEC device PEC Output Energy content of hydrogen Net energy = Output - Input

7 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 decisionmaking

8 Conceptual structure of PEC device (solar fuel)

9 Pipes and other components (not included) Glass cover Photo-anode Membrane Photo-cathode Chamber Photo-anode micro-wire Catalyst Membrane Photo-cathode micro-wire Catalyst Note: figures are not in real scale

10 Defining system boundary and functional units

11 PEC device LCA boundary Materials Photoelectrodes Catalysts Membrane Encapsulation material Other materials Fabrication Photoelectrode fabrication Catalyst deposition Membrane fabrication Other processes LCA method Goal: Net primary Energy Scope: materials and fabrication processes Functional units: MJ per m2 PEC MJ per kg Hydrogen

12 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)

13 Assumptions for Lower, Medium and Higher cases Category Component Lower case Medium case Higher case Material choices Fabrication Photocathode Si Si Si Photoanode WO3 WO3 GaAs Catalysts for photocathode Catalysts for photoanode Co Pt Pt No catalyst No catalyst Pt Encapsulation PVC PVC Polycarbonate Thickness of chamber Thickness of membrane Thermodynamic efficiency 3 mm 5 mm 7 mm 30 um 50 um 70 um 70% 50% 30%

14 Details of calculation (medium case) Embodied energy in materials Primary energy use in fabrication

15 PEC device LCA boundary Materials Photoelectrodes Catalysts Membrane Encapsulation material Other materials Fabrication Photoelectrode fabrication Catalyst deposition Membrane fabrication Other processes LCA method Goal: Net primary Energy Scope: materials and fabrication processes Functional units: MJ per m2 PEC MJ per kg Hydrogen

16 Photo-electrodes and catalysts Photo-electrodes (Si, WO3); catalyst (Pt) Materials Thickness (nm) Mass (g/m 2 ) Energy intensity (MJ/g) Embodied energy (MJ/m 2 ) Si 2000 A WO 3 20 B Pt 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 WO 3 wire array which has 70 nm of diameter, 4 µm of length and 0.5 µm of lattice spacing.

17 PEC device LCA boundary Materials Photoelectrodes Catalysts Membrane Encapsulation material Other materials Fabrication Photoelectrode fabrication Catalyst deposition Membrane fabrication Other processes LCA method Goal: Net primary Energy Scope: materials and fabrication processes Functional units: MJ per m2 PEC MJ per kg Hydrogen

18 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

19 PEC device LCA boundary Materials Photoelectrodes Catalysts Membrane Encapsulation material Other materials Fabrication Photoelectrode fabrication Catalyst deposition Membrane fabrication Other processes LCA method Goal: Net primary Energy Scope: materials and fabrication processes Functional units: MJ per m2 PEC MJ per kg Hydrogen

20 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

21 PEC device LCA boundary Materials Photoelectrodes Catalysts Membrane Encapsulation material Other materials Fabrication Photoelectrode fabrication Catalyst deposition Membrane fabrication Other processes LCA method Goal: Net primary Energy Scope: materials and fabrication processes Functional units: MJ per m2 PEC MJ per kg Hydrogen

22 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)

23 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)

24 Catalysts Pt depostion Electron-beam deposition requires high vacuum environment (8e-04 Pa) Source: McKone, J. R. et al. (2011).

25 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)

26 PEC device LCA boundary Materials Photoelectrodes Catalysts Membrane Encapsulation material Other materials Fabrication Photoelectrode fabrication Catalyst deposition Membrane fabrication Other processes LCA method Goal: Net primary Energy Scope: materials and fabrication processes Functional units: MJ per m2 PEC MJ per kg Hydrogen

27 Membrane fabrication Main process is heating at 140 C Source: Spurgeon, J. M. et al. (2011)

28 PEC device LCA boundary Materials Photoelectrodes Catalysts Membrane Encapsulation material Other materials Fabrication Photoelectrode fabrication Catalyst deposition Membrane fabrication Other processes LCA method Goal: Net primary Energy Scope: materials and fabrication processes Functional units: MJ per m2 PEC MJ per kg Hydrogen

29 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

30 Break-down results of primary energy requirement

31 MJ/m Materials Fabrication (medium case, error bars showing lower and higher cases)

32 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.

33 Equation-- Primary energy requirement in MJ to produce 1 kg of hydrogen MJ/kg MJ/m2 Determined by Solar-to-Hydrogen STH efficiency Longevity (years)

34 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)

35 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 3% (-)74 3% % % % Net energy MJ of 1 kg of H2

36 Uncertainties could affect results 1. STH efficiency and longevity 2. thermodynamic models efficiency 3. material choice, chamber layer thickness Which uncertainties have higher effect?

37 Base point MJ/kg (medium case, 10% efficiency and 10 years)

38 Discussions The most energy intensive process is the fabrication of photoelectrodes 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)

39 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

40 Following up works by my colleagues at LBNL

41 More information Tomorrow 1:15pm, Jeff Greenblatt will talk more about large-scale application and early technology appraisal Joint Center of Artificial Photosynthesis Lawrence Berkeley National Lab CarbonCycle 2.0 Initiative

42 Thank you and would like to take any comment or suggestion Pei Zhai