TECH 581 – Solar Energy Systems Summer 2009 Module 6-1 – Solar Photosynthesis

Photosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight.

Green plants, microbes and algae have employed photosynthesis to capture energy from sunlight and convert it into electrochemical energy.

Photosynthetic organisms convert around 100,000,000,000 tonnes of carbon into biomass per year.

The general equation for photosynthesis is :

n CO2 + n H2A + photons → (CH2O)n + n O2 + n A

carbon dioxide + electron donor + light energy → carbohydrate + oxygen + oxidized electron donor

Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:

2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is a redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and also the electrons needed to convert carbon dioxide into carbohydrate, which is a reduction reaction.

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Solar Energy Systems/ J.P. Agrawal / M6_1 - page 3 Photosynthesis occurs in two stages.

Stage I: light-dependent reactions energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls.

In plants: the proteins are held inside organelles called chloroplasts, In bacteria: the proteins are embedded in the plasma membrane.

Stage II: light-independent reactions, use the products in stage I to capture and reduce carbon dioxide.

Light-independent Reactions:A part of light energy gathered by chlorophylls is stored in the form of adenosine triphosphate (ATP).The rest of the energy is used to remove electrons from a substance such as water. These electrons are then used in the reactions that turn carbon dioxide into organic compounds.

In the light reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron.

This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH.

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This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH.

This process creates a proton gradient across the chloroplast membrane; its dissipation is used by ATP synthase for the concomitant synthesis of ATP.

The chlorophyll molecule regains the lost electron from a water molecule through a process called photolysis, which releases a dioxygen (O2) molecule.

The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:

2 H2O + 2 NADP+ + 2 ADP + 2 Pi + light → 2 NADPH + 2 H+ + 2 ATP + O2

Not all wavelengths of light can support photosynthesis.

The photosynthetic action spectrum depends on the type of accessory pigments present. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.

This process creates a proton gradient across the chloroplast membrane; its dissipation is used by ATP synthase for the concomitant synthesis of ATP.

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Light-independent or dark reactions:

the enzyme RuBisCO captures CO2 from the atmosphere and in a process that requires the newly

formed NADPH, called the Calvin-Benson Cycle, releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is:

3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

The overall process of photosynthesis takes place in four stages.

1. The first, energy transfer in antenna chlorophyll takes place in the femtosecond [1 femtosecond (fs) = 10,−15 s] to picosecond [1 picosecond (ps) = 10−12 s] time scale.

2. The next phase, the transfer of electrons in photochemical reactions, takes place in the picosecond to nanosecond time scale [1 nanosecond (ns) = 10−9 s].

3. The third phase, the electron transport chain and ATP synthesis, takes place on the microsecond [1 microsecond (μs) = 10−6 s] to millisecond [1 millisecond (ms) = 10−3 s) time scale.

4. The final phase is carbon fixation and export of stable products and takes place in the millisecond to second time scale. The first three stages occur in the thylakoid membranes.

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

Artificial Photosynthesis

The research being done can be split up in two approaches:1) photoelectrochemical cell 2) dye-sensitized solar cell

Photoelectrochemical cellResearch is being done into finding catalysts that can provide the photosynthesis itself and catalysts that can convert sunlight to carbohydrates. For the first type of catalysts, nature usually uses the oxygen evolving complex. Having studied this complex, researchers have made catalysts such as blue dimer to mimic its function, but these catalysts were very inefficient. Another catalyst was engineered by Paul Kögerler, which uses four ruthenium atoms.The carbohydrate-converting catalysts used in nature are the hydrogenases. Catalysts invented by engineers to mimic the hydrogenases include a catalyst by Cédric Tard, the rhodium atom catalyst from MIT, and the cobalt catalyst from MIT.

Disadvantages:Artificial photosynthesis cells (currently) last no longer than a few years (unlike PV and passive solar panels, for example, which last twenty years or longer).

Advantages:Dye-sensitized cells can be made at one-fifth of the price of silicium cells.The solar energy can be immediately converted and stored, unlike in PV cells, for example, which need to convert the energy and then store it into a battery (both operations implying energy losses). Furthermore, hydrogen as well as other carbon-based storage options are quite environmentally friendly.

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Artificial Photosynthesis…

Dye-sensitized solar cell

Research is also being done into a streamlined form of photosynthesis that breaks water into oxygen and hydrogen. This process is called photoelectrolysis. This process is the first stage of plant photosynthesis (the light-dependent reaction).

Carbon dioxide is not required in this approach.

The hydrogen released in artificial photosynthesis (stage 1) could be used in hydrogen engines to generate energy without pollution.

The light-independent reaction (also known as the Calvin-Benson cycle) is the second stage of plant photosynthesis, which converts carbon dioxide into glucose. Glucose is stored energy for a plant's growth and repair.

It has been suggested that such a process replicated on an industrial scale could help to counter global warming. Specifically, the light-independent reaction of photosynthesis could be used to "mop up" excessive amounts of carbon dioxide in the atmosphere.

Again, however, such a process would ultimately require a source of energy, just as plant photosynthesis does.

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http://www.voiceofprogress.com/?p=1080 artificial leaf

``artificial photosynthesis.'‘ tries to imitate the elaborate process of photosynthesis.

For a long time scientists have been trying to develop an artificial version of photosynthesis that can be used to produce liquid fuels from carbon dioxide and water. Unlike fossil fuels the liquid fuel from artificial photosynthesis will be carbon-neutral and will not contribute to global warming.

The process converts carbon dioxide (CO2) and water (H2O) into oxygen and carbohydrates such as glucose, the sugary fuel that powers our bodies.

The fundamental idea is to create an artificial leaf i.e. a single platform light-harvesting system that can capture solar photons and catalytic systems that can oxidize water.

Heinz Frei, a chemist with Berkeley Lab’s Physical Biosciences Division, has discovered that nano-sized crystals of cobalt oxide can effectively carry out the critical photosynthetic reaction of splitting water molecules.

Green plants perform the photo-oxidation of water molecules within a complex of proteins called Photosystem II, in which manganese-containing enzymes serve as the catalyst. Manganese-based organometallic complexes modeled off Photosystem II have shown some promise as photocatalysts for water oxidation but some suffer from being water insoluble and none are very robust.

More on: Scientists seek to make energy as plants do

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Frei and Jiao were looking for purely inorganic catalysts that would dissolve in water and would be far more robust than biomimetic materials.

Frei said. “In earlier work, we found that iridium oxide was efficient and fast enough to do the job, but iridium is the least abundant metal on earth and not suitable for use on a very large scale. We needed a metal that was equally effective but far more abundant.” Frei further stated, “Photooxidation of water molecules into oxygen, electrons and protons (hydrogen ions) is one of the two essential half reactions of an artificial photosynthesis system – it provides the electrons needed to reduce carbon dioxide to a fuel.

Effective photooxidation requires a catalyst that is both efficient in its use of solar photons and fast enough to keep up with solar flux in order to avoid wasting those photons. Clusters of cobalt oxide nanocrystals are sufficiently efficient and fast, and are also robust (last a long time) and abundant. They perfectly fit the bill.”

In the beginning Frei and Jiao tested micron-sized particles of cobalt oxide. They found the particles were inefficient and not nearly fast enough to serve as photocatalysts. However, when they reduced the size of the particles from being micron-sized to being nano-sized the situation changed dramatically. “The yield for clusters of cobalt oxide (Co3O4) nano-sized crystals was about 1,600 times higher than for micron-sized particles,”.

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Frei said, “and the turnover frequency (speed) was about 1,140 oxygen molecules per second per cluster, which is commensurate with solar flux at ground level (approximately 1,000 Watts per square meter).”

The researchers used mesoporous silica as their scaffold, growing their cobalt nanocrystals within the naturally parallel nanoscale channels of the silica using a technique known as “wet impregnation.” The best performers were rod-shaped crystals measuring 8 nanometers in diameter and 50 nanometers in length, which were interconnected by short bridges to form bundled clusters.

The bundles were shaped like a sphere with a diameter of 35 nanometers. According to Frei in addition to the catalytic efficiency of the cobalt metal the major factor behind the enhanced efficiency and speed of the bundles was their size.

“We suspect that the comparatively very large internal area of these 35 nanometer bundles (where catalysis takes place) was the main factor behind their increased efficiency,” he said, “because when we produced larger bundles (65 nanometer diameters), the internal area was reduced and the bundles lost much of that efficiency gain.”

Now the real challenge for the researchers will be to integrate the water oxidation half reaction with the carbon dioxide reduction step in an artificial leaf type system. Frei said, “The efficiency, speed and size of our cobalt oxide nanocrystal clusters are comparable to Photosystem II. When you factor in the abundance of cobalt oxide, the stability of the nanoclusters under use, the modest overpotential and mild pH and temperature conditions, we believe we have a promising catalytic component for developing a viable integrated solar fuel conversion system. This is the next important challenge in the field of artificial photosynthesis for fuel production.”

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Researchers at the University of California at Berkeley have engineered a strain of pond scum that could, with further refinements, produce vast amounts of hydrogen through photosynthesis.The work, led by plant physiologist Tasios Melis, is so far unpublished. But if it proves correct, it would mean a major breakthrough in using algae as an industrial factory, not only for hydrogen, but for a wide range of products, from biodiesel to cosmetics.The new strain of algae, known as C. reinhardtii, has truncated chlorophyll antennae within the chloroplasts of the cells, which serves to increase the organism's energy efficiency. In addition, it makes the algae a lighter shade of green, which in turn allows more sunlight deeper into an algal culture and therefore allows more cells to photosynthesize."An increase in solar conversion efficiency to 10 percent ... is thought to be enough to make the mass culture of algae viable," says Juergen Polle, a former student of Melis’ who now does research on algae at the City University of New York, Brooklyn.Polle points out that Melis has probably already reached that 10 percent threshold. But further refinements are still required before C. reinhardtii farms would be efficient enough to produce the world’s hydrogen, which is Melis’ eventual goal.Currently, the algae cells cycle between photosynthesis and hydrogen production because the hydrogenase enzyme which makes the hydrogen can’t function in the presence of oxygen. Researchers hope to further boost hydrogen production by using genetic engineering to close up pores that oxygen seeps through, making the end product more cheaply

Solar Energy Systems/ J.P. Agrawal / M6_1 - page 12 Melis’ truncated antennae mutants are a big step in that direction. Now Seibert and others (including

James Lee at Oak Ridge National Laboratories and J. Craig Venter at the Venter Institute in Rockville, Maryland) are trying to adjust the hydrogen-producing pathway so that it can produce hydrogen 100 percent of the time.A bigger challenge, and one that’s further down the road to solving, is improving the efficiency of the hydrogenase itself."Right now the electron chain that goes into the system should produce a lot more hydrogen than comes out, and we don’t know what’s causing the bottleneck," says Seibert. "More basic research is needed to better understand exactly what’s happening in there." Seibert also points out that there are plenty of naturally occurring hydrogenases in microbes, most of which haven’t been studied and some of which might be much more efficient than the one used by C. reinhardtii.Whether or not scientists can find solutions for those two problems will have a lot to do with realizing the vision of a hydrogen-powered economy based on algae farms in desert areas.But algae can do a lot more than produce hydrogen. They are already used widely in the cosmetics industry to produce key chemicals used in make-up and perfume. And pharmaceutical companies have long viewed algae as a potential way to produce drugs in a cheap and environmentally friendly manner.Some algae are also viewed as an ideal source for biodiesel because they can produce oils at a much higher rate than other plants (which can then be converted into vehicle fuel without adding any carbon dioxide to the environment).For all these applications, Melis’ antenna-truncated algae should be a major breakthrough, allowing higher rates of production and thus making the end product more cheaply

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This was the last lecture of this course.

I tried to introduce basics and many important topics of solar energy systems. It is not possible to depths of all of them. I have also not been able to add all aspects either.

But I hope you have an idea of this subject now.

I enjoyed delivering the lectures and hope that now you may have an area to explore on your own.

This is certainly a subject of immense importance to our mankind.

Please submit all the due items and enjoy your summer, whatever left of it.