SOLAR POWERED ELECTROLYSIS OF WATER FOR THE PRODUCTION OF HYDROGEN FUEL

Thomas Dastalfo (tjd46@pitt.edu)

INTRODUCTION

We only have one Earth to call our home, and we need to take care of it. Our current methods of producing energy, mainly through the burning of fossil fuels, are not sustainable, which means that we will eventually run out of these fuel sources. Also, these fossil fuels are putting massive amounts of pollution into our air, oceans, and ground. This is leading to global warming and health problems for human and animal populations. Even today, nearly 138.5 million people in the US alone live in areas where air pollution levels are sometimes dangerous to breathe. [1] The solution to this problem is to produce our energy through renewable and clean means. Amongst the most promising of renewable energy sources is solar energy. Every day, the sun shines about 120,000 terawatts of electricity onto the Earth, which is about 10,000 times more energy than all of mankind uses in a day. [2] The problem is harnessing and storing that energy. I will focus on the problem of storing that energy. Typically, sunlight is captured by photovoltaic (PV) panels, and is usually stored in batteries, or just put straight onto the electric power grid. An effective method of storing this energy is to use the electric current produced by the solar panels to perform electrolysis and separate water into its constituent elements: hydrogen and oxygen. The hydrogen produced can then be stored and used later in hydrogen fuel cells where it is combined with oxygen to produce two products: electricity and water. Therefore it is a 100% harmful emissions free fuel. This is why scientists and engineers agree that if solar powered electrolysis can be made efficient enough to be practical, it may be the answer to our clean, sustainable energy source to make mankind a more sustainable species. I think this innovation is tremendously important because we want to leave behind a clean, sustainable, and habitable world for our children so that they may have bright futures. I believe that through further developing solar panel electrolysis, we can leave our children a world in which all of their energy needs are satisfied simply by the sunlight that falls on Earth.

PROBLEMS WITH OTHER FORMS OF SOLAR STORAGE

Solar panels are a great form of capturing 100% renewable energy, but it must be taken into account that they are not 100% efficient at capturing sunlight and turning it into electricity. They are actually not even close to 100% efficient. Typical PV panels are only about 15% efficient at converting sunlight to electricity, and it also must be taken into account that they only get 6-8 hours of direct sunlight per day, and that’s only on sunny days. [3] This is why we need to develop the most efficient forms of storing the energy that they produce. Current common methods of storing electricity produced by solar panels include expensive and limited battery systems, storing through mechanical methods, and by putting power straight into the grid. The problem with battery storage of power is that these batteries are expensive, eventually lose their effectiveness after a certain amount of charges, and are limited in the amount of charge they can hold. In order to buy the newest Tesla Powerwall batteries necessary to store and put out about 16 kWh (kilowatt hours) of power continuously, about enough to power a residential home under reasonably high demands, it would take eight Tesla Powerwall batteries, which would cost about $45,000. However, Tesla makes a more powerful home battery to be used to store energy from solar systems, but the problem with this one, which can store 10 kWh of power, is that it’s only meant to have its charge cycled no more than 50 times a year, or it will start to degrade. [4] These are the problems that plague modern battery storage options for solar systems, even the most cutting edge ones. Another method of storing solar energy is through mechanical means, most notably through the flywheel. The premise behind storing energy in a flywheel system is that the flywheel is spun at extremely high rotational speeds in a nearly frictionless system so that it can store energy in the form of rotational mechanical energy, that can then be taken from the flywheel and used to directly power mechanical systems or power a generator to produce electricity on demand. The problem with this system is the limitations of the materials that make up the flywheel itself. To store more energy in a flywheel, the flywheel either has to gain mass, rotate faster, or both. The problem with it gaining mass is that the system has to become larger and more expensive, therefore less practical. The issue with rotating faster is that eventually at extremely high rotational speeds the flywheel begins to pull itself apart, due to centripetal acceleration. [5] Finally, the other common form of storing energy from solar panels is not to store it at all, but rather to put it onto the electrical grid as soon as it is produced. This is good for the fact that it means that we need to burn less fossil fuels to keep the grid powered, but it’s impractical. On a sunny day, solar panels are producing max output at around noon and putting that energy straight onto the grid. However at noon, the energy demand on the grid may be low, but it may be higher at night when people get home from work. This causes a problem, because now there is not enough power on the grid to supply the increased demand. This can lead to brownouts, or an unintentional drop in voltage in an electrical grid, or blackouts, a total failure of an electrical grid. [6]

THE OPTIMAL FORM OF STORING SOLAR ENERGY: HYDROGEN GAS

Unfortunately, hydrogen gas doesn’t exist naturally on Earth. Hydrogen gas is produced by us through a chemical reaction called electrolysis. Electrolysis of water is a chemical reaction that uses an electric current to split water molecules into its constituent elements, hydrogen and oxygen. The products of the reaction come out in gaseous form. The formula for the chemical reaction of the electrolysis of water is as follows: 2H2O + electric current = 2H2 + O2. The electrical current is supplied to the water through an anode and a cathode that is supplied electrical current by the solar panel when the solar panel is struck by sunlight. The hydrogen that is produced from electrolysis can then be stored in tanks under a pressure of 5,000 to 10,000 psi, and transported anywhere to be pumped into hydrogen fuel cells to fuel a chemical reaction that produces electricity and water. The energy content of stored hydrogen fuel is very high. Hydrogen has a power density of 33.3 kWh/kg as compared to diesel fuel, which has a power density of 11.9 kWh/kg, which is right around where all of the other hydrogen competitors fall. This means that a 1kg sample of compressed hydrogen gas contains about three times as much energy as a 1kg sample of liquid diesel fuel. [7] In addition to the high density of energy in stored hydrogen gas, when hydrogen gas in reacted in a fuel cell and converted to electricity, the only byproduct is pure water. Almost any other fuel source, such as diesel, emits toxic gases such as carbon monoxide and other greenhouse gases into the atmosphere whenever it is converted to into thermal energy. Compare this to hydrogen and it is clear that hydrogen is a very powerful fuel source that is also incredibly ecologically friendly. Another benefit of hydrogen gas is that it can be transported anywhere in its compressed form, much like gasoline, by trucks, boats, or through pipelines, and then converted to electricity via hydrogen fuel cells when it arrives at its location. To transport comparable amounts of energy in the form of electricity without the use of an already established power grid, would take massive batteries that would be incredibly expensive and very heavy. A cutting edge Tesla battery that can store 7 kWh of electricity costs about $7000 and weighs 100 kg. [8] 1 kg of compressed hydrogen gas has 33.3 kWh of electricity stored within the bonds of its molecules, and costs only as much to transport as the pressurized metal container that holds it and the fuel of the vehicle transporting it. This shows hydrogen’s promise as a fuel that can be used in developing parts of the world that don’t have reliable long distance electric grids set up.

CURRENT PROBLEMS WITH THE SOLAR POWERED ELECTROLYSIS METHOD

Current efforts to make solar powered electrolysis a widespread technique have encountered a few problems, mostly efficiency and cost. Currently, the highest efficiency that can be achieved is about 5%. [9] That means about 5% of all of the solar energy that hits the solar panel is stored in the hydrogen gas that is produced through electrolysis. A cheap, conventional solar panel has an efficiency of about 15% at converting sunlight to electricity, so the reason that the efficiency is so much lower when converting this electricity into hydrogen is because energy is lost in the wires and in the electrolyzer. The electrolyzer is the device in which water and current is pumped into in order to produce the desired hydrogen gas. The reason for inefficiencies in the electrolyzer is because electricity is lost into the water because of a lack of a perfect catalyst, which is a chemical or nanoparticle that lowers the energy required to split water molecules.

SOLUTIONS TO CURRENT PROBLEMS WITH THE SOLAR POWERED ELECTROLYSIS METHOD

Many solutions have been proposed to improve the efficiency of converting solar energy into hydrogen, and therefore improve the cost effectiveness and viability of this method of energy production and storage. A large problem is the amount of energy lost when transferring the electric current from the solar panel to the electrolyzer. The idea was proposed that that inefficiency could be cut out of the solar panel if the panel is placed directly in the electrolyzer, and electrical current comes straight off of the solar panel through a nanoparticle catalyst coating and into the water. [10] The problem that this method faces is that the material used to make solar panels cannot function underwater, when submerged in water these materials become unstable and degrade. So, the solution was to create a nanoparticle coating for these cells that would protect these solar cells from water damage, would be reactive to sunlight, and could effectively transmit charge into the water in order to split the water. In TU Delft’s study, they combined a PV solar panel with a nanoparticle coating of bismuth vanadate, with an additional added concentration gradient of tungsten atoms. This experiment yielded 5% efficiency on the first attempt, and it is believed that an efficiency of 10% can be eventually achieved. [9] This is a very good efficiency for the amount of sunlight energy that strikes the solar panel being directly stored in the produced hydrogen gas. Another way that the efficiency of the electrolysis of water is being improved is through the use of solar panels that match the exact voltage needed by the electrolyzer. There is energy that is lost in power converters, DC to DC converters and inverters, which are used to convert the voltage being output by the solar panel to the exact voltage needed by the electrolyzer. In a new design the power converters are taken out altogether, and solar panel systems that output exactly the same voltage at peak output that is needed by the electrolyzer are being used. This is increasing the efficiency of hydrogen production and also taking expensive power converters, which are around $100/kW, out of the design, increasing cost efficiency. [11] A final method being used to increase efficiency of solar systems that produce hydrogen gas, is the use of excess energy that is produced during peak solar input hours to produce hydrogen and be stored in that way. This makes sense, because solar electrolysis to produce hydrogen is in its infancy right now, and is currently not incredibly efficient. Also, this method makes solar power systems much more practical, because the production of hydrogen during hours of peak solar input allows the solar system to turn this hydrogen into electricity at night when the system is no longer harvesting solar energy and putting it directly onto the grid, so that these systems can provide linear energy output 24 hours a day, and have stored energy on demand to meet increased grid needs during peak demand hours.

CONCLUSION

It is truly one of the grand engineering challenges that face us today, to make solar energy a more viable form of energy production. The topic of producing hydrogen gas as a method of storing electrical energy through the use of solar powered electrolysis is such a hotly researched topic among scientists and engineers because of its enormous potential. If we can make this technology even 50% efficient, and make it cheap enough for the average consumer to afford, then we can make huge progress towards becoming a 100% sustainably powered civilization. Just imagine a world with no air pollution, no electricity bills, and no wars fought over oil or any other energy resources. Just clean, free energy for everyone. This is the reality that I believe the technological advancement of solar powered electrolysis for the production of hydrogen gas can bring to us.

References

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