08Solar Fuels I

Solar Fuels I

Kristin Persson

3106 Etcheverry

LBNL 33-143D

384 HMMB

[email protected]

What are solar fuels

Substances that store the suns energy as useful chemical energy allowing for reasonable rate conversion into other forms of energy

Earliest form of energy source except food : combustible biomass

Common forms: H2, ethanol, methane, ammonia

Solar Fuels Solar fuels are substances that store solar energy in the form of usable chemical energy. Candidates should: Possess adequate energy density

Exhibit flexible conversion to heat, electrical or mechanical

energy

Capable of being produced, transported and stored efficiently

Be environmentally friendly enabling sustainable incorporation of conversion products into global circulation in biosphere

http

://ww

w.ch

almers.se/ap

/EN/research

/che

mical-p

hysics/

C. Graves et al. / Renewable and Sustainable Energy Reviews 15 (2011) 1

http://www.ccisolar.caltech.edu/

1800 H2 electrolysis;

Nicholson and Carlisle

First combustion engine used H2

1807 de Rivaz engine featured spark ignition and H2 as fuel

The compressed hydrogen gas fuel was stored in a balloon connected by a pipe to the cylinder

Oxygen was supplied from the air by a separate air inlet

Manually operated valves allowed introduction of the gas and air at the correct point in the cycle

Solar Fuels

Photolysis Electrolysis Thermolysis

biomass Water and CO2 splitting biofuel

Exotic hybrids

Process that converts sunlight, water and/or CO2 into some version of carbohydrates and oxygen

Direct combustion Convert into fuels Integrating

bacteria

Biomass and solar fuel

Pellets big in US and Europe

There is uncertainty to what degree making heat or electricity by burning wood pellets contributes to global climate change, as well as how the impact on climate compares to the impact of using competing sources of heat. Factors in the uncertainty include the wood source, carbon dioxide emissions from production and transport as well as from final combustion etc.

Efficiency of biomass

For actual sunlight, where only 45% of the light is in the photosynthetically active wavelength range, the theoretical maximum efficiency of solar energy conversion is approximately 11% Plants do not absorb all incoming sunlight (due to reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels) Hence, not all harvested energy is converted into biomass, which results in a MAXIMUM overall photosynthetic efficiency of 3 to 6% of total solar radiation.

At 8.5-15 oven dry tons/hectare/year:

energy potential is 7-12 TW

Biomass potential In reality, its worse: solar to biomass conversion (total cycle) is inefficient (0.3%)

Land with crop production potential, 1990:

25 Tera m2

Cultivated Land, 1990: 9 Tera m2

Additional land needed to support 9 billion people in 2050: 4 Tera m2

Remaining land available for biomass energy: 13 Tera m2

Possible/likely this is water resource limited

25% of U.S. corn in 2007 provided 2% of transportation fuel

Biomass conversion to liquid fuel

Fraction that can be used in a sustainable way depends on: Emissions, economics and efficiencies of conversion of biomass into usable gas

and liquids Social issues - about one-sixth of humans are undernourished (competition

between food and biofuel) 30%40% of net produced biomass already used as food, feed, fiber, and fuel,

which corresponds to ~ 10-18% of the actual worlds primary energy supply.

Even with enhanced conversion efficiencies for biofuel production and large-scale use of biomass waste, clearing sludge, animal wastes, and wood it will be hard to provide ALL the required amounts of renewable chemical energy necessary to fuel an industrializing world economy and to cover the needs of todays world population and that of the future.

http

://solarw

ind

greenen

ergy.blo

gspo

t.com

/20

11

/10

/bio

fuel-facts.h

tml

The reason for the current focus on using biomass for liquid fuels rather than for direct combustion is that coal is abundant and inexpensive, it is less expensive to transport (per joule) and it burns with higher energy efficiency and less ash than biomass.

Estimated ~3x1021 J of chemical energy stored in usable photosynthetic biomass per year.

Topics

Photolysis

Electrolysis

Thermolysis


Exotic hybrids


bacteria

Lysis (/lass/; Greek lsis, "a loosing" from lein, "to unbind")

Many Roads to Hydrogen

Courtesy of Harry Atwater

Solar Fuels Production & Use

http://www.rsc.org/ScienceAndTechnology/Policy/Documents/solar-fuels.asp

Chemical Feedstocks


Common intermediate: syngas In principle, syngas can be produced from any hydrocarbon feedstock, including: natural gas, naphtha, residual oil, petroleum coke, coal, and biomass.

Syngas is a crucial intermediate resource for production of hydrogen, ammonia, methanol, and synthetic hydrocarbon fuels. Syngas is also an intermediate in producing synthetic petroleum for fuel or lubricant via the FischerTropsch process and methanol to gasoline process.

Syngas is combustible but with less than half the energy density of natural gas.

Spath and Dayton NREL report 2003

Hydrogen flow ; every node needs innovation


Solar Fuel as Energy Storage and Distribution

C. Jooss, H. Tributsch, Solar Fuels (Chap. 47) in Fundamentals of Materials for Energy and Environmental Sustainability,D. Ginley and D. Cahen Eds, Cambridge Univ. Press (2012)

Materials Science and Engineering of Clean Energy [3.70]

Drivers & Disincentives CO2 capture Water splitting to generate H2 Use of solar energy CO2 conversion to CO H2 + CO conversion to HC feedstocks HC feedstock conversion to

Fertilizer Plastics Pharmaceuticals Synthetic fuel for transport

What role do the following play & which will likely be most critical ? Regulations Incentives Public opinion Politics International relations Education

Competition Installed infrastructure Experience Inertia Culture Corporate culture

Environmental issues Investment culture Trade regulations Taxes Rate of innovation Specific inventions

A Race for Efficiency, Low-cost and Durability

Requires sustainable mass production of solar fuels. The storage of light and electrical energy in molecular chemical bonds easily stored and

transported. Chemical energy easily converted to other useful forms of energy.

Requires interconversion of simple molecules H2, H2O, CH4 and CO2, a continuing challenge. Bottleneck: durable and inexpensive catalysts for photo-induced and/or thermally assisted conversion of various redox pairs with catalyst for O2 reduction the central issue.

http://people.ucalgary.ca/~cberling/research.html

Solar Fuel Characteristics


Hydrogen as Fuel High energy density/mass (141 MJ/kg vs 45.7 MJ/kg for gasoline): Good for weight-limited transportation (space). Abysmal volumetric energy

Low energy density/volume, liquid boils at 20.27 K Explosive mixture 4-74% vol in air Clean emissions, only small amount of NOx 70 million metric tons produced per year, nearly all from fossil fuels By steam reforming of methane or natural gas (700-1100C) Followed by water gas shift reaction: 130 C

Ammonia


Ammonia as Fuel NH3 cheaply produced from hydrogen or syngas via Haber Bosch process

N2 + 3 H2 2 NH3 (H = 92.22 kJmol1)

Catalyst: iron promoted with K2O, CaO, SiO2, and Al2O3

N2 (g) N2 (adsorbed) N2 (adsorbed) 2 N (adsorbed) H2(g) H2 (adsorbed) H2 (adsorbed) 2 H (adsorbed) N (adsorbed) + 3 H(adsorbed) NH3 (adsorbed) NH3 (adsorbed) NH3 (g)

500 million tons (453 billion kilograms) of nitrogen fertilizer produced per year, mostly in the form of anhydrous ammonia, ammonium nitrate, and urea. 35% of world natural gas production is consumed in the Haber process (~12% of the world's annual energy supply)

http://en.wikipedia.org/wiki/Haber_process

Can be combusted in benign manner (but must handle NOx)

Used as fuel in alkaline and solid oxide fuel cell

Can be stored as liquid (8 kbar/RT) or in metal ammine complexes but highly toxic.

Use as hydrogen storage medium.

Methane


Methane as Fuel CH4 is a nature gas and provides ~30% of US energy requirements Greenhouse warming potential = 72 x that of CO2 which makes containment an issue Produced from syngas, coal gasification, pure CO2 and biomass conversion

Methane from CO2:

Water gas shift

+ CO methanation

Sabatier method : 250-500 C @ 5-10 bar

Conversion efficiencies: electrolysis (to get H2) + methanation= 46-86% reduced by 15% using atmospheric CO2. Alternatively use CO2 dissolved in sea water containing 140 x that in atmosphere and extract via electrolysis.

Two-reaction process:

Up to 95% efficiency achieved

Ethanol


Ethanol as Fuel

Ethanol, C2H5OH: volatile, colorless liquid Combustion: water, CO2, NOx Biofuel with production increased from 17-74 billion liters between 2000-2009

leading to 5% share of fuel for cars. Resultant shortages in corn and sugar cane for food purposes. Water limited Reductions in greenhouse emissions relative to gasoline ~18-28% but sometimes

increased! Depends on feedstock, transport, and conversion process

A more sustainable ethanol production via hydrogenation of CO2 e.g. with Rh based catalyst

Microbial fermentation of sugar/distillation/dehydration

Methanol


Methanol compared to diesel

ExxonMobils Methanol-to-Gasoline (MTG) Technology for Coal-to-Liquids Project

Methanol is used directly as a fuel in race cars and has been proposed as the energy carrier of choice in the methanol economy parallel to the hydrogen economy FT produces synthetic crude that must be refinedit does not directly produce fuel The MTG process produces gasoline directly, no further refining required.

Solar Fuel Production all uphill


Topics

Photolysis

Electrolysis

Thermolysis


Exotic hybrids


bacteria

Thermolysis

Direct Thermolysis

Slightly easier for CO2 than for H2O Still very challenging, multi-step reaction schemes Necessity of separating O2/H2 in final product

Thermochemical Cycles

Integrated solar capture and fuel production

Oxygen and fuel produced in separate steps

Challenges due to structural change & volatilization (vapor pressure)

Fuel largely limited to hydrogen

Although a large number of thermochemical cycles have been proposed, only a small number have been demonstrated experimentally and none are commercialized.

or CO2

or CO

General considerations

Adv. Energy Mater. 2014, 4, 1300469

Highest possible efficiency = 1 Tlow/Thigh

Reactor design and operation, a point

of diminishing returns exists with respect to T high ,

above which thermal radiation losses dominate

1773 K (1500 C) is a realistic upper limit

Thermodynamic Analysis

Greduction(Tr)= HMOx1 HMOx-TTr(SMOx1 +0.5 SO2 SMOx) 0

Ggas splitting(Tgs)= HMOx HMOx-1HH2O-TTgs(SMOx+SH2-SMOx1 SH2O) 0

or

Ggas splitting(Tgs)= HMOx HMOx-1 HCO2-TTgs(SMOx+SCO-SMOx1 SCO2) 0

MOxMOx-1+ 0.5O2 MOx1+ H2OMOx+ H2 or MOx1 + CO2MOx + CO

Assume that we can ignore the difference in entropy between the two solid states ; common practice

Thermo contd

Set Hred = HMOx1 HMOx

Greduction(Tr)= Hred 0.5TTrSO2 0 (1)

Ggas splitting(Tgs)= -Hred HH2O-TTgs(SH2SH2O) 0 (2)

or

Ggas splitting(Tgs)= -Hred HCO2-TTgs(SCOSCO2) 0 (3)

Setting equations to zero gives the necessary condition for favorable reactions

Adding (1) + (2) or (3) eliminates the only solid materials dependent quantity Hred

Thus the only quantity that really matters is the one we neglected - the difference in solid state entropy !

Meredig and Wolverton 2009

No practical reaction below 1350 K

All thermo with no solid state

materials input


No practical reaction below 1800 K ; exp finds 1350 K Lowering pressure for reduction cycle can help gas release

Materials may be highly volatile or suffer irreversible degradation at very high TR temperatures 2000 K and kinetic processes will be very slow at very low GS temperatures < 1000 K

Entropic Considerations

Bringing entropy back into the

picture we find that

Sred = SMOx1 SMOx 0

makes the reactions more favorable

at lower temperatures

However, for most simple oxides, Sred is negative.


Which binary oxides have favorable

entropy change ?

According to

computational

considerations; no

binary oxide falls in the

optimal gap. A few of

the better candidates

are already considered:

Fe-O, ZnO and CeO2


Considered Systems

metal-substituted ferrites (magnetite and spinel) and ceria

1/MFexO4 1/ [MFexO4] + 1/2O2 (M = Fe, Co, Ni, or Zn)

1/[MFexO4] + CO2 1/ MFexO4 + CO

1/CeO2 1/CeO2+1/2 O2 1/CeO2+CO2 1/ CeO2 + CO

Systems with large oxygen cycling under largely homomorphic constraints Other considerations: reaction thermodynamics, radiation losses and emissivity, vaporization losses and corrosion, reaction kinetics, microstructural stability, phase stability and/or formation of multiple phases, and interactions with support materials influence the choice of materials:

Ceria cycle

Ceria thermodynamics and phase diagram well-known

High melting temperature 2500 C, non-volatile

HT Homomorphism important

HT structure is

the same for

large change in

oxygen content

Kinetics ; morphology

Oxygen diffusion even at high temperatures needs to be facilitated by high surface areas and foam-like morphology Ceria is better than most : D ~ 10-5 -10-4 cm2/s 600 C

Ferrites

Magnetite (Fe3O4), this reduction can proceed as far as an overall composition of Fe3O3.1 before a phase boundary is reached where metallic Fe forms

Compared to ceria, ferrites have a much greater reduction capacity at a given temperature

However, very deep reductions cannot be achieved using Fe3O4 without heating in excess of the melting point of the reduction product FeO, which presents serious challenges for practical implementation.

This problem can be addressed by substituting up to 50% of the Fe(II) cations in the octahedral sites with a different divalent metal, which lowers the temperature required to reduce the material and simultaneously raises the melting point. Metal-substituted ferrites include MnFe2O4 , CoFe2O4 , NiFe2O4 , and ZnFe 2 O 4 ,

Efficiency

Despite interesting materials idea

Todays reactors have abysmal performance; 0.5-2.5 %

Losses by parasitic reactions, O non-reactivity, gas separation, T radiation

08Solar Fuels I

Documents

Transcript of 08Solar Fuels I