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source: New System Converts Sun?s Energy into Hydrogen Fuel - SciTech Daily
It is known as a dye-sensitized photoelectrosynthesis cell, or DSPEC, and it generates hydrogen fuel by using the sun’s energy to split water into its component parts. After the split, hydrogen is sequestered and stored, while the byproduct, oxygen, is released into the air.
“But splitting water is extremely difficult to do,” said Meyer. “You need to take four electrons away from two water molecules, transfer them somewhere else, and make hydrogen, and, once you have done that, keep the hydrogen and oxygen separated. How to design molecules capable of doing that is a really big challenge that we’ve begun to overcome.”
Meyer had been investigating DSPECs for years at the Energy Frontier Research Center at UNC and before. His design has two basic components: a molecule and a nanoparticle. The molecule, called a chromophore-catalyst assembly, absorbs sunlight and then kick starts the catalyst to rip electrons away from water. The nanoparticle, to which thousands of chromophore-catalyst assemblies are tethered, is part of a film of nanoparticles that shuttles the electrons away to make the hydrogen fuel.
However, even with the best of attempts, the system always crashed because either the chromophore-catalyst assembly kept breaking away from the nanoparticles or because the electrons couldn’t be shuttled away quickly enough to make hydrogen.
To solve both of these problems, Meyer turned to the Parsons group to use a technique that coated the nanoparticle, atom by atom, with a thin layer of a material called titanium dioxide. By using ultra-thin layers, the researchers found that the nanoparticle could carry away electrons far more rapidly than before, with the freed electrons available to make hydrogen. They also figured out how to build a protective coating that keeps the chromophore-catalyst assembly tethered firmly to the nanoparticle, ensuring that the assembly stayed on the surface.
With electrons flowing freely through the nanoparticle and the tether stabilized, Meyer’s new system can turn the sun’s energy into fuel while needing almost no external power to operate and releasing no greenhouse gases. What’s more, the infrastructure to install these sunlight-to-fuel converters is in sight based on existing technology. A next target is to use the same approach to reduce carbon dioxide, a greenhouse gas, to a carbon-based fuel such as formate or methanol.
“When you talk about powering a planet with energy stored in batteries, it’s just not practical,” said Meyer. “It turns out that the most energy dense way to store energy is in the chemical bonds of molecules. And that’s what we did – we found an answer through chemistry.”
Related Studies:
Leila Alibabaeia, et al., “Solar water splitting in a molecular photoelectrochemical cell,” PNAS, vol. 110 no. 50, 20008–20013; doi: 10.1073/pnas.1319628110 Solar water splitting in a molecular photoelectrochemical cell
Hanlin Luo, et al., “A Sensitized Nb2O5 Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell,” Chem. Mater., 2013, 25 (2), pp 122–131; DOI: 10.1021/cm3027972 A Sensitized Nb2O5 Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell - Chemistry of Materials (ACS Publications)
Source: University of North Carolina at Chapel Hill
I would like to find the most efficient developed so far.
The engine does not need to have a high energy density - that is amount of power produced for the size of the engine.
But I have not seen versions of the Sterling - other than toys and a small one used on a solar collector dish.
It seems that the implementation of the engine do not attain very high efficiencies.
Frankly in looking at the wiki article and others I really don't understand how it could be very efficient.
There doesn't seem to be enough time in the cycle for the air in the closed cycle to change temperature.
(wiki- His unique closed-cycle air engine design[12] in which application it is now generally known as a "regenerator")
it seems that in the Beta type the piston head is loose and with the air flowing back and forth the differential of the hot/cold is minor and the total power rather negligible?
There seems to be a large gap between the theoretical and the practical efficiencies.
("Theoretical thermal efficiency equals that of the hypothetical Carnot cycle - i.e. the highest efficiency attainable by any heat engine. However, though it is useful for illustrating general principles, the ideal cycle deviates substantially from practical Stirling engines.") =the theory
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The design challenge for a Stirling engine regenerator is to provide sufficient heat transfer capacity without introducing too much additional internal volume ('dead space') or flow resistance. These inherent design conflicts are one of many factors which limit the efficiency of practical Stirling engines
---------- i don't understand the red bit --
Stirling engines, especially those that run on small temperature differentials, are quite large for the amount of power that they produce (i.e., they have low specific power). This is primarily due to the heat transfer coefficient of gaseous convection which limits the heat flux that can be attained in a typical cold heat exchanger to about 500 W/(m2·K), and in a hot heat exchanger to about 500–5000 W/(m2·K).[56] Compared with internal combustion engines, this makes it more challenging for the engine designer to transfer heat into and out of the working gas. Because of the Thermal efficiency the required heat transfer grows with lower temperature difference, and the heat exchanger surface (and cost) for 1 kW output grows with second power of 1/deltaT. Therefore the specific cost of very low temperature difference engines is very high. Increasing the temperature differential and/or pressure allows Stirling engines to produce more power, assuming the heat exchangers are designed for the increased heat load, and can deliver the convected heat flux necessary.
what is specific cost?
does someone know how to an example with numbers of the "1 kW output grows with second power of 1/deltaT"
Carbon-neutral fuels are synthetic hydrocarbons. They can be produced in chemical reactions between carbon dioxide, which can be captured from power plants or the air, and hydrogen, which is created by the electrolysis of water using renewable energy. The fuel, often referred to as electrofuel, stores the energy that was used in the production of the hydrogen The most energy-efficient fuel to produce is methanol,[citation needed] which is made from a chemical reaction of a carbon-dioxide molecule with three hydrogen molecules to produce methanol and water.All synthetic hydrocarbons are generally produced at temperatures of 200–300 °C, and at pressures of 20 to 50 bar. Catalysts are usually used to improve the efficiency of the reaction and create the desired type of hydrocarbon fuel. Such reactions are exothermic and use about 3 mol of hydrogen per mole of carbon dioxide involved. They also produce large amounts of water as a byproduct.[
Since carbonic acid in seawater is in chemical equilibrium with atmospheric carbon dioxide, extraction of carbon from seawater has been studied.[20][21] Researchers have estimated that carbon extraction from seawater would cost about $50 per ton.[5] Carbon capture from ambient air is more costly, at between $600 and $1000 per ton
Researchers have also suggested using biomass as a carbon source for fuel production. Adding hydrogen to the biomass would reduce its carbon to produce fuel. This method has the advantage of using plant matter to cheaply capture carbon dioxide. The plants also add some chemical energy to the fuel from biological molecules. This may be a more efficient use of biomass than conventional biofuel because it uses most of the carbon and chemical energy from the biomass instead of releasing as much energy and carbon.
Nighttime wind power is considered the most economical form of electrical power with which to synthesize fuel, because the load curve for electricity peaks sharply during the warmest hours of the day, but wind tends to blow slightly more at night than during the day. Therefore, the price of nighttime wind power is often much less expensive than any alternative. Off-peak wind power prices in high wind penetration areas of the U.S. averaged 1.64 cents per kilowatt-hour in 2009, but only 0.71 cents/kWh during the least expensive six hours of the day.[12] Typically, wholesale electricity costs 2 to 5 cents/kWh during the day.[22] Commercial fuel synthesis companies suggest they can produce gasoline for less than petroleum fuels when oil costs more than $55 per barrel.[23]
The U.S. Navy estimates that 100 megawatts of electricity can produce 41,000 gallons of jet fuel per day and shipboard production from nuclear power would cost about $6 per gallon. While that was about twice the petroleum fuel cost in 2010, it is expected to be much less than the market price in less than five years if recent trends continue. Moreover, since the delivery of fuel to a carrier battle group costs about $8 per gallon, shipboard production is already much less expensive.[24]
The George Olah carbon dioxide recycling plant operated by Carbon Recycling International in Grindavík, Iceland has been producing 2 million liters of methanol transportation fuel per year from flue exhaust of the Svartsengi Power Station since 2011.[27] It has the capacity to produce 5 million liters per year.[28]
Capturing CO2 directly from the air or extracting carbonic acid from seawater would also reduce the amount of carbon dioxide in the environment, and create a closed cycle of carbon to eliminate new carbon dioxide emissions.[2] Use of these methods would eliminate the need for fossil fuels entirely, assuming that enough renewable energy could be generated to produce the fuel. Using synthetic hydrocarbons to produce synthetic materials such as plastics could result in permanent sequestration of carbon from the atmosphere.[12]
Sunshine heats the air beneath a very wide greenhouse-like roofed collector structure surrounding the central base of a very tall chimney tower. The resulting convection causes a hot air updraft in the tower by the chimney effect. This airflow drives wind turbines placed in the chimney updraft or around the chimney base to produce electricity. Plans for scaled-up versions of demonstration models will allow significant power generation, and may allow development of other applications, such as water extraction or distillation, and agriculture or horticulture.
Power output depends primarily on two factors: collector area and chimney height. A larger area collects and warms a greater volume of air to flow up the chimney; collector areas as large as 7 kilometres (4.3 mi) in diameter have been discussed. A larger chimney height increases the pressure difference via the stack effect; chimneys as tall as 1,000 metres (3,281 ft) have been discussed.[1]
Model calculations estimate that a 100 MW plant would require a 1,000 m tower and a greenhouse of 20 square kilometres (7.7 sq mi). A 200 MW tower with the same tower would require a collector 7 kilometres in diameter (total area of about 38 km²).
The solar cyclone distiller[49] could extract atmospheric water by condensation in the updraft of the chimney. This solar cyclonic water distiller with a solar collector pond could adapt the solar collector-chimney system for large-scale desalination of collected brine, brackish- or waste-water pooled in the collector base.[50]
A solar updraft power station would require a large initial capital outlay, but would have relatively low operating cost.[3]
Capital outlays would be roughly the same as next-generation nuclear plants such as the AP-1000 at roughly $5 per Watt of capacity.
n energy tower (also known as a downdraft energy tower, because the air flows down the tower) is a tall (1,000 meters) and wide (400 meters) hollow cylinder with a water spray system at the top. Pumps lift the water to the top of the tower and then spray the water inside the tower. Evaporation of water cools the hot, dry air hovering at the top. The cooled air, now denser than the outside warmer air, falls through the cylinder, spinning a turbine at the bottom. The turbine drives a generator which produces the electricity.
The greater the temperature difference between the air and water, the greater the energy efficiency. Therefore, downdraft energy towers should work best in a hot dry climate. Energy towers require large quantities of water.
I think the wiki author is correct in questioning the 70% of Carnot limit.
Actually the Carnot limit is not the question here it is the energy capture in relation to the water pumping costs.
---------------for downdraft tower-------
Zaslavsky and other authors estimate that depending on the site and financing costs, energy could be produced in the range of 1-4 cents per kWh, well below alternative energy sources other than hydro. Pumping the water requires about 50% of the turbine's output. Zaslavsky claims that the Energy Tower would achieve up to 70-80% [3] of the Carnot limit. If the conversion efficiency turns out to be much lower, it is expected to have an adverse impact on projections made for cost of energy.
Projections made by Altmann[4] and by Czisch[5][6] about conversion efficiency and about cost of energy (cents/kWh) are based only on model calculations[7], no data on a working pilot plant have ever been collected.
Actual measurements on the 50 kW Manzanares pilot solar updraft tower found a conversion efficiency of 0.53%, although SBP believe that this could be increased to 1.3% in a large and improved 100 MW unit.[8] This amounts to about 10% of the theoretical limit for the Carnot cycle. It is not unreasonable to expect a similar low conversion efficiency for the energy tower, in view of the fact that it is based on a similar principle as the solar updraft tower.
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you can see how placing tower and skirt over the shallow seawater - say Australian coast
in daytime the seawater evaporates up the tower (= solar updraft tower) and goes to condensation tank.
At night the tank water is sprayed in but outlets are to irrigation of land.
