Solar Capturing: the Solar – to – Products approach
1. Introduction ‘Solar Capturing’
The focus of this programme, which is part of the NWO proposition for the ‘Topsectors’ Energy and
Chemistry, is on the direct or indirect storage of solar energy in chemical bonds. This introduction
describes the rationale behind the programme in the context of the Topsectors Energy and Chemistry but
also in relation to the Topsector Agri&Food and the TKI BBE (Topconsortium voor Kennis- en Innovatie
Biobased Economy). In paragraph 2 the Solar-to-Products approach is described in more detail.
1.1 Economy, fossil raw materials, Bio-Based Economy and Solar Capturing
Before the Industrial Revolution, energy was obtained from renewable sources, such as biomass, wind
and water power. The limitations of these traditional sources saw their replacement by fossil fuels as the
Industrial Revolution progressed, beginning with coal and then crude oil and more recently natural gas.
This replacement has been so complete and effective that the energy sector in our industrialised society
entirely depends on abundant, cheap energy from fossil resources. The change from biological resources
to fossil resources during the progression of the Industrial Revolution was not limited to the energy
sector, however. Developments in the chemical industry saw new materials manufactured from fossil
materials increasingly replace materials from biological origin in for example construction, fibres and
textiles.
Their usefulness as a source of primary energy carriers and manufacturing building blocks means that
fossil raw materials play a major role in our economy and although in principle fossil raw materials are
finite, there are still reserves for a very long time to come. The drawbacks of the large-scale use of fossil
raw materials are clear however. The societal challenges linked to these problems urgently demand a
transition from an economy based on fossil resources to an economy based on renewable energy and the
replacement of fossil resources by renewable raw materials. In such an economy, agriculture can become
the provider of biomass. Another approach is the production of chemical compounds from carbon dioxide
and water via the direct route using solar energy, or the indirect route using electricity from renewable
energy.
The carbon in biomass and other CO 2 based raw materials is renewable when it is obtained via the
conversion of carbon dioxide and water driven directly or indirectly by solar energy. The urgency of the
societal challenges described above make a transition to a new energy- and production infrastructure
necessary. These transitions ask for generation of new knowledge, technology development, innovation,
political consistency and very large investments. Also the ethical aspects and the societal acceptance of
the changes brought about is necessary.
1.2 Solar Capturing and the Topsectors Energy, Chemistry and Agri&Food’
For the closely related economic sectors of energy and the chemical industry, fossil fuels and
hydrocarbons will remain important resources for the coming decades, although their large-scale use has
major disadvantages. On the supply side, fossil fuels are sensitive to price fluctuations and their supply is
vulnerable to geo-political tensions in the global market. Independent of these more obvious economic
problems, the use of these fossil carbon sources is associated with the release of carbon dioxide,
resulting in a long term, slow increase in atmospheric carbon dioxide levels. The changes in our
atmosphere have two major negative consequences: ocean acidification and climate change, both of
which are predicted to have major consequences for the ecology of the Earth and the security and wellbeing of the human population of the Earth as a whole. From a societal perspective solutions for those
problems are urgently needed.
A Bio-Based Economy (BBE) is an economy in which fossil raw materials are replaced by raw materials of
a non-fossil (biological) origin. This is especially relevant for that part of the economy in which materials
with a carbon skeleton play a role. For those parts of the economy using raw materials not derived from
either freshly harvested or fossilised biological organisms, for instance metals and minerals, the link to
fossil fuels is via energy use during processing.
a.
Topsector Energy
In relation to the ‘Topsector Energie’ the importance of BBE is mainly related to fuels for mobility
(transport via land, water and air) and seasonal storage of energy from electricity in times when more
electricity is produced than is used. With an increasing amount of renewable energy (wind and solar) in
the energy infrastructure the storage problem becomes increasingly important. Furthermore energy from
biomass is an important technological option to reach the 2020 greenhouse gas reduction goals agreed
within the European and Dutch context. Because of the limited efficiency of oxygenic photosynthesis, the
limited availability of biomass in the Netherlands and the increasing prices of biomass on the global
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market, bio-energy for large-scale production of electricity is not an option for the long term. The use of
biomass for small-scale production of energy carriers and heat, preferably in combination with use at or
near the site of production, fits very well in the frame of system-integration in the context of a transition
of the energy infrastructure.
In our model of a future energy infrastructure based on an increasing contribution from renewable
sources, flexibility becomes a key ingredient to accommodate the intermittent character of sun and wind.
Considering the mix of decentralized and centralized sources, like solar panels at local households and
large offshore wind parks, the amount of electrical power generated during the day can fluctuate strongly
in time and per region (with occasional power peaks that can exceed demand many times).
Therefore an integrated set of technologies with appropriate regulation and legislation is necessary to
make a robust energy system relying on renewables, incorporating amongst others:
integration of decentralized and centralized sources of sustainable energy to provide a
guaranteed base power supply, both for industrial and domestic needs.
smart grids solutions to match supply and demand, but also coupling of energy systems based
on different energy vectors (like electricity, heat and chemicals),
storage concepts with different criteria for response time, discharge time and capacity,
socio-techno aspects involving network management, CO 2 and energy policy.
To guarantee a base power supply throughout the year, more renewable sources are being installed to
increase the overall capacity. In return that will also result in more periods with excess power, especially
during sunny (summer) and windy (winter) days. In particular the amount of surplus electricity that
cannot be quickly buffered by smart grids and electrical (or other) storage concepts, can result in
detrimental grid instabilities.
In the optimal case this surplus should be stored for later times when there is a shortfall of renewable
energy. Moreover, effective storage will also allow the full installed capacity of sustainable energy
sources to be fully used, rather than having to limit (curtail) the output from these energy sources. That
puts, however, stringent requirements on the storage technologies. They should be able to respond
quickly and accurately if they satisfactorily buffer power fluctuations from solar or wind based sources of
energy.
Figure 1. The niches of the various storage technologies shown with respect to storage capacity
and discharge time. (CAES = compressed air energy storage) [source DNV KEMA]
In the mix of energy storage solutions developed (figure 1), storage in chemical bonds is recognized as
the prime candidate for long-term (seasonal) and large-scale storage of renewable energy. When the
chemical energy storage is aimed for electricity generation at a later time, the round-trip efficiency
comes into play. This overall efficiency is for chemical storage far lower (H 2 : >40 % and CH 4 : > 34%)
than that of electrical storage (batteries  >90%) 1. In addition, stability and costs also have to be
taken into account for the duration of storage if a fair comparison is to be made between the costs of
these storage technologies. Flywheels and batteries, for example, are too expensive to be considered for
seasonal storage, while chemical storage is too inefficient for storage shorter than (typically) a day.
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“Joint EASE/EERA recommendations for a European energy storage technology development roadmap towards 2030” (June
2013). http://www.ease-storage.eu/Technical_Documents.html
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Figure 2. Plot of the volumetric and gravimetric energy density of several energy carriers
to show the potential for CO 2 -neutral hydrocarbons (including system volume and
weight, such as battery package and storage tanks). [Crabtree, Physics World (2009)]
The capacity for energy storage in chemicals is further expressed by the high energy density in both
volume (MJ/m3) and mass (MJ/kg), especially for hydrocarbon fuels (see figure 2). That is also the main
reason why energy transport over large distances proceeds today via liquid chemicals in container ships
and trucks, rather than a costly electrical (super-)grid spanning continents. Notwithstanding in aviation,
where it is the question whether innovation in electrical storage will evolve towards lightweight batteries
that can compete with the energy density of liquid (bio-)fuels.
Figure 3. Storage of electrical energy in chemical bonds couples the electricity grid to the gas
infrastructure. First step is the production of syngas that can also be used as feedstock to
chemical industries or for liquid fuels synthesis. Active CO 2 capture both at point sources and
from air is essential in this scheme to close the carbon cycle. [Image courtesy of DIFFER]
Via Power-to-Gas (P2G) and Power-to-Liquid (P2L) schemes, sustainable electricity can be converted to
chemicals and that links the electricity grid to the gas and liquids infrastructure, see figure 3. By doing
so, the vast storage capacity of the well-connected infrastructure for fossil-based hydrocarbons comes
available (e.g. an estimated 552 TWh of storage capacity exist in NL gas grid alone).
Moreover as shown in figure 3, these schemes can also be considered a route to bring sustainability to
other non-electrical sectors, such as energy transport, mobility and chemical industries. This aspect
makes the potential for CO 2 -neutral hydrocarbon fuels as energy vector even more compelling.
The P2G and P2L conversion schemes will probably only become economically viable when they are
aligned to the availability of cheap surplus renewable solar energy. In the longer run the competition will
be between land use and efficiency. In view of likely tax regulation on CO 2 pollution to mitigate climate
change, other economic drivers will give an incentive for (chemical) industries to make their processes
more and more sustainable.
Currently, the goal for sustainable P2G technologies is to close the gap with the current costs of
hydrogen generation by steam reforming using fossil natural gas: typically < 1 €/kg H 2 . Alternative
technology for hydrogen generation focuses on H 2 O splitting, predominantly using electrolysis systems.
But also the route via CO 2 splitting is investigated knowing that H 2 can readily be obtained via CO using
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the water-gas-shift reaction: CO + H 2 O gives H 2 + CO 2
(with the option to
recycle the CO 2 ). Further use of H 2 to synthesise hydrocarbon fuels obviously needs active CO 2 capture
to provide a carbon source, which today is already possible for < 0.040 €/kg CO 2 using point sources and
< 0.4 €/kg CO 2 when sourced from the air. However, quantitative analyses of energy and climate
impacts of producing synthetic hydrocarbon fuels from CO 2 have revealed that using existing
technologies in P2G applications requires much more energy than existing fuels, due to the poor overall
efficiency performance of existing (indirect) technologies, and only with access to new technological
developments and innovation that allows to eliminate redundant conversion steps and exploit benefits
from synergy at all scales, the P2G and P2L line can be brought to full potential 2.
b. Topsector Chemistry
In relation to the ‘Topsector Chemistry’ it has to be realised that both the energy- and chemical industry
sectors use large amounts of fossil resources as a primary energy source. The traditional chemical
industry is very energy intensive and additionally the petrochemical industry uses raw material from
fossil origin as the starting point for the production of a range of organic chemical products. Therefore,
‘greening’ the chemical industry via the use of raw materials of recent biological origin is an option for
both the short and the long term. In addition, the growing availability of renewable electricity calls for an
“electrification” of chemical processes and a potential new role of electrosynthesis in relation to the
conversion of bio-based platform chemicals to valuable products. Recently ECN and TNO initiated a
programme to electrify the chemical industry with the aim of reducing energy costs and creating new
high-quality products 3. One of the areas of research is electrosynthesis, i.e. directly converting
electricity into chemicals using existing technologies such as electrochemistry, membrane technology and
separation technology.
The objective of greening the chemical industry means that the interface between the chemical industry,
agriculture and food- and feed industries can be a cradle for innovation when it comes to bio-based raw
materials. A great diversity in molecular structures is present in biomass and can potentially be used in
industrial processes after bio-refining and chemical and biotechnological processing. The (energy) costs
of separation and purification can be high, however, and will be a determining factor in the actual
development of added value production chains. In the recent Roadmap for Catalysis in the Netherlands
‘Catalysis – Key to a Sustainable Future’ this problem is mentioned and one of the objectives is to
combine catalysis and separation. From that perspective the use of micro-organisms like in the
fermentation industry can be interesting because in theory micro-organisms can be modified in such a
way that they function as living catalysts with membranes that secrete the product into the environment.
In addition, inorganic (nanostructured) materials with multi-functionality of separation and catalysis are
also under development.
c. Topsector Agri&Food
Presently the 'Topsector Agri&Food' can be the provider of biological resources for the food-, feed-,
chemistry- and energy sectors. In this sector there is also a lot of experience with the use of enzymes for
processing of biomass and production of bio-based products.
From the BBE perspective the replacement of fossil raw materials by raw materials from biological origin
(see figure 4) followed by large-scale bioprocessing are attractive approaches for both high volume/low
price and low volume/high price chemical production. Raw materials of biological origin can be produced
by large-scale agriculture, aquaculture and by using micro-organisms. For platform chemicals high
productivity agriculture is an attractive route delivering bio-materials for industrial use in addition to their
use as food and feed, while for more specific molecules production in algae and micro-organisms might
be a more attractive approach. Over the years there has been the tendency to maximize the
food/"waste" ratio of crops; However, given the new methods that already exist or can be developed for
processing the "waste" part, new opportunities have arisen to increase the amount of "waste" biomass,
without jeopardizing the food production part.
2
3
Energy and Climate Impacts of Producing Synthetic Hydrocarbon Fuels from CO2, Environ. Sci. Technol., 48, 7111-7121
https://www.tno.nl/media/5813/electrification_of_chemical_industry.pdf
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Figure 4. Producing chemical building blocks from biomass and fossil resources.
The direct or indirect use of solar energy to produce a broad range of chemical compounds whose carbon
skeleton is derived from carbon dioxide and water is a very attractive long term solution. This should be
the long term strategy for both the energy and chemical industry sectors. In the longer term (bio)molecular design and synthesis of existing or completely new materials, products and processes opens a
broad range of new possibilities. The scientific strength of Dutch science in the physical, chemical,
(micro)biological and social sciences domains form the basis of these developments which can increase
the Technology Readiness Level further to cause more economic development.
2. The Solar – to - Products approach
From the global perspective, the ultimate target is a transformation from an economy based to a very
large extent on energy and materials derived from fossil resources, via a bio-based economy in which
fossil resources are gradually replaced by resources of recent biological origin to an economy driven by
mainly solar energy in which material cycles are closed: a circular economy. From the greenhouse gas
emission perspective such a transition is urgently needed. It will, however, certainly not be an immediate
change but rather an evolutionary process in which the different economies (fossil based, bio-based and
circular) will co-exist for a considerable time to come. The main reason for this is that energy transitions
easily take 30 years and the envisaged transition process will be very complex. Additionally an energy
transition will ask for very large investments in fundamental sciences, engineering, institutional change
and empowerment of people.
Solar energy can be converted to and stored in the chemical bonds of, in principle, a very broad range of
molecules starting with carbon dioxide and water as feedstock materials. Contrary to common belief,
light to chemical conversion is not determined by insolation, the total amount of solar radiation energy
(measured in Wm-2) that is collected per unit of time, but by the total number of (solar) photons that are
absorbed per unit time (measured in mol m-2 s-1) *. Many photochemical processes have close to 100%
conversion of absorbed photons to the initial reaction product, but the efficiency with which the energy of
these photons is conserved in these products must be less than 100% in any useful system – ‘less’,
however, need not mean ‘low’, and the challenge is to operate close to the thermodynamic limits.
The target for science and engineering is to develop efficient routes to energy carriers (fuels and
storage), platform chemicals and specialty chemicals (greening chemistry) and to develop the technology
needed to turn theoretical possibilities into reality. Inspiration for technology development can be found
in photocatalytic approaches using inorganic semiconductor materials and nano-structured solar cell
materials. But also in (micro)biological systems used since antiquity to produce food and feed, and to
further process the primary products of agriculture (for example making beer or wine). In more physical
terms, these goals can be translated into the desire to devise new technologies, or improve existing
systems (such as the biological ones), so that they operate with the highest light-use efficiency in terms
of both the quantum yield of end product formation and the energy content of that product.
Essential to the Solar-to-Product concept is that the energy required to drive the chemistry comes
directly or indirectly from sustainable sources:
•
indirect routes use renewable electricity as input
•
direct routes use solar energy as input
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Figure 5. Schematic representation of the direct and indirect Solar Products approach
The Solar-to-Product approach requires the development of novel materials (e.g. effective light capturing
materials, and electrocatalysts) and materials combinations which are robust and abundantly available.
Novel nano-structured device architectures need to be designed based on detailed kinetic, energetic and
coherent understanding of light-to-charge and charge-to-chemical conversions.
Solar-to-Products technology requires a multi-disciplinary approach, which includes (bio)chemistry,
physics, (micro-)biology, systems biology, synthetic biology and materials and engineering science, to
answer the basic research questions on the different length and time scales and to develop a large-scale
technology which is an enormous challenge as well. In the end, the success of the generation of e.g.
solar fuels as a CO 2 neutral solution which can replace the present fossil-based energy infrastructure
depends on whether it will offer the potential to be applied on the TeraWatt scale.
The direct and indirect routes towards solar fuels have different timelines for application. The Technology
Readiness Levels (TRLs) are: artificial photosynthesis 1-3, electrolysis 1-7, plasmolysis 1-5, modification
of plants and micro-organisms 1-5. The focus of this program is on the TRLs 1-4.
3. Call for proposals
The NWO program Solar to Products is structured along three sub themes for which breakthrough
multidisciplinary research proposals are solicited. These three subthemes are:
•
•
•
The indirect route (paragraph 3.1)
The direct route, natural (paragraph 3.2.1)
The direct route, artificial (paragraph 3.2.2)
These themes are inter-linked and reflect the multi-disciplinary character of the research into using solar
energy to drive conversion of CO 2 and H 2 O to the formation of chemical building blocks.
3.1 The indirect route
The indirect route, i.e. using renewably generated electricity to produce platform molecules has a
relatively short time path towards application. However, the approaches using these routes in which
electrochemistry plays a key role, are not yet sufficiently energy- and cost-effective, mainly due to the
use of non-abundant expensive, and inefficient (electro-)catalytic materials for the electrodes. Electrodes
can be based on nano-structuring classical metallic or metal oxide electrodes, or involve bio-inspired
(immobilized) molecular complexes. In addition, and also important in view of the MW-GW scale that is
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envisioned, the device integration and cost-efficient manufacturing approaches need to be investigated
and developed. Research subjects which could be addressed in this theme are for example:
•
Novel electrocatalysts which are based on earth abundant materials and directly convert CO 2
into small organic molecules (methanol, methane, ethanol). Gain fundamental understanding of
the activity, selectivity and stability.
•
Novel electrocatalysts based on earth abundant materials for water oxidation. Although a
classical topic, it is still a very crucial step to improve the electrolysis of water. The hydrogen
produced can be used in/integrated with many industrially relevant catalytic processes, including
CO 2 based methanol synthesis, Fisher Tropsch, etc.
•
Novel electrocatalysts based on earth abundant materials to convert (biomass-based) platform
molecules into high(er)-value chemicals. Electrocatalysis of the conversion of poly-ols, sugars,
HMF, lignine (derivatives), etc, for both oxidation and reduction.
•
Novel polymer membranes concepts for PEM based electrolysers and their cost efficient
synthesis
•
Heat integration and co-catalysis for high pressure solid oxide electrolysis
•
Novel innovative approaches such as e.g. electrocatalysis for the conversion of nitrogen into
ammonia
This list of possible research topics is certainly not complete. It should be noted that proposals should
make clear how to develop the optimized systems (electrolysers) and evaluate process conditions using
the proposed materials, with the focus on energy efficiency and costs.
3.2 The direct route
Several overall processes for direct Natural or Artificial routes are illustrated in Figure 5 (see above).
Proposals can be submitted focusing on one or more of the current limitations of such processes. A
successful proposal needs to address the critical (scientific) hurdles, and needs to discuss the energy
efficiency and photon efficiency (number of photons per CO 2 molecule captured and converted), which
the authors envision to achieve in creating a chemical feedstock or fuel. In the following two paragraphs
the research topics are specified.
3.2.1 The direct route, natural
‘Photosynthetic cell factories’ have been engineered to bypass biomass formation, and to create productformation pathways for the direct conversion of CO 2 into e.g. ethanol, lactic acid and butyraldehyde.
Plants can also be engineered to be a direct source of specialty chemicals and fine chemicals. Food and
non-food use for plants can be combined to create synergies in agricultural development that improve
the food supply and accelerate the development of a bio-based economy. However, for this approach to
become economically competitive with fossil-based production processes, yield and efficiency need to be
further increased. Among others, proposals can be focused on i) strain selection of micro-organisms (e.g.
Algae) with high growth rate, thermal resistance, optimal pH range, etc, including research aiming to
acquire detailed knowledge of the constituting enzymes, ii) systems and synthetic microbiology
(modeling, experiment and computational analysis), iii) optimizing photo-bioreactor design, considering
solutions for efficiency loss originating from temporal fluctuations in light intensity, and iv) energy
efficient land use.
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3.2.2 The direct route, artificial
Many artificial inorganic, organo-metallic, and hybrid materials (e.g. MOFs) have been proposed to be
effective in (photo)catalytic conversion of CO 2 and H 2 O to fuel-like molecules. Furthermore, various
reactor configurations have been validated, including photo-electrochemical cells based on tandem
materials, as well as micro-reactor platforms. The critical hurdle is to overcome the low yield due to
scavenging of charge carriers in multi-electron photochemical conversion. A real breakthrough is still
necessary to approach yields of interest to the chemical industry. The following list of topics for research
is not complete, but might serve as inspiration. Proposals can focus on improvement of i) catalytic
functionality for multi-electron transfer to the CO 2 molecule, ii) materials that make efficient use of the
overpotential of water oxidation, iii) interfaces for tandem (electrodes), minimizing ohmic losses, and iv)
cell/reactor design, with focus on mass transport (CO 2 , H+, O 2 , hydrocarbon products), optimization of
light efficiency (e.g. by wavelength manipulation and using quantum information), and product
separation.
4. Summary
The direct or indirect use of solar energy to produce a broad range of chemical compounds whose carbon
skeleton is derived from carbon dioxide and water is a very attractive long-term solution for development
of sustainable energy and chemistry. In the longer term (bio-)molecular design and synthesis of existing
or completely new materials, products and processes opens a broad range of new possibilities for
economic development. An integrated approach involving the Topsectors Energy, Chemistry and
Agri&Food will be a scientifically sound starting point for reaching long term targets. Changing carbon
dioxide from a burden into a valuable resource is highly desired in a societal context.
This proposition of NWO to the Topsectors Energy and Chemistry is based on the Dutch scientific
strength in (surface-)physics, (bio-)chemistry, nanotechnology, (micro-)biology, systems biology,
synthetic biology, mathematics, materials, engineering and agriculture. However, the technology
readiness levels are currently not high enough to enter the industrial R&D stage. The focus of this NWO
programme is on closing this technology gap, by addressing fundamental research needs with clear
application objectives on the horizon.
This document was prepared by the so called Solar to Products “writing group”. This group was
composed of the following members:
Rietje van Dam, chair
Huub de Groot
Marc Koper
Guido Mul
Richard van de Sanden
Peter-Paul Schouwenberg
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