(Text by Antonio Alvaro Camargo from Right Energy published by GESEL at UFRJ November 18,2021)
Hydrogen is the lightest and most abundant element in the visible universe.
Hydrogen is the lightest and most abundant element in the visible universe. On earth it practically does not exist in its pure form under normal conditions of temperature and pressure: the gas H2. Likewise, H is widely available in the biosphere as it is found in H2O (water), biomass (48% C; 6% H; 45% O), natural gas (CH4), petroleum (CXHY), etc.
Hydrogen is an element of conflicting manners.
Being the smallest atom, its molecule is very small and light (1/14,36 of the density of air) and its storage as a gas for vehicle use needs to be done at a pressure between 700 bar and 800 bar, a condition in which it will have some tendency for leaking. As H2 in the open-air burns at 2000°C in an invisible flame, imagine an accident with a hydrogen-powered vehicle inside a tunnel? Or even a leak in a poorly ventilated garage? Technology will solve this, but its adoption will not be trivial.
Energy density is the biggest challenge to the use of hydrogen.
Nothing has more energy per unit mass than hydrogen: around 120 MJ/kg, against 44 MJ/kg for gasoline and 47 MJ/kg for natural gas (LHV). On the other hand, the energy density of hydrogen is very low by any other metric: as a gas it is 10.8 MJ/Nm3 against 36.6 MJ/Nm3 for natural gas. As a liquid at (-253°C), H2 has 8.5 MJ/liter against 32.3 MJ/liter for gasoline and around 35.2 MJ/liter for diesel and Jet Fuel. A liter of liquid hydrogen (-253°C) has 24% of the energy of a liter of Jet Fuel, showing the enormous challenge, for example, of using it to decarbonize medium and long-haul aviation. All LHV values.
1) BioEthanol as a source of GH2 via steam reforming with catalysts
While bioethanol has some advantages as a vehicle for storing and transporting green hydrogen – it is non-toxic, liquid at room temperature and already has infrastructure for largescale production and distribution in the US, Brazil, India and some other places – only 13.04% of its mass is hydrogen against 17.65% in ammonia and 25% in biomethane. Probably the most suitable method for producing H2 from 1 liter of bioethanol is its catalytic reforming in the presence of water vapor, carried out at temperatures between 500°C and 780°C. This reaction that requires the addition of external energy can be summarized by the formula C2H5OH + 3H2O >< 2 CO2 + 6H2. It is not the objective here to discuss this reaction, which depends on many alternative catalysts, types of reactors and temperature at which it occurs. But we are not far from reality if we state that in general only 60 % to 67% of the hydrogen in ethanol will effectively be recovered through steam reforming for further direct use or for storage.
That means, on average, four hydrogen atoms out of the six available in the ethanol molecule will be extracted. Crucial to remember here that sugar cane is a natural solar panel that energizes the photosynthesis process that still depends on nutrients and a lot of water. Ethanol’s attractiveness as a route to GH2 will largely depend on its environmental footprint. C4 plants, adapted to high temperatures, high CO2 absorption and less water loss, such as sugarcane, corn, and elephant grass, transform light into aboveground biomass with high efficiency. But even then, only 0.85% to 0.95% of the energy in solar light in the end will be fixed as energy in the resulting biomass. In the case of sugarcane, an average of 0.90%-0.95% of the energy in sunlight ends up being stored in aboveground biomass. As sugars make up around 1/3 of the dry biomass of the whole sugarcane plant above ground, only 0.20% of sunlight turns into ethanol.
The yield of GH2 when made from sugarcane ethanol from Brazil.
According to CONAB’s spreadsheet for the 2019/2020 sugarcane season, the state of SP produced 16,489 billion liters of ethanol from 2.555 million hectares, resulting in a productivity of 6,453 L ethanol/ha-year. This volume of ethanol at an average density of 0.803 kg/L in the production mix between hydrated and anhydrous forms (2:1) results in a mass production of 5,182 kg ethanol/ha-year. As hydrogen accounts for 13.04% of the mass of ethanol, then we have 675.9 kg of H2/ha-year that, through catalytic steam reforming that recovers around 65% of the hydrogen in ethanol, results in 440 kg of H2/ha-year.
2) The production of GH2 from electrolysis of water powered by solar PV electricity
The solar radiation index GHI1 in the mid to northern sectors of the state of São Paulo, in areas where sugarcane plantations thrive (near cities like Sertãozinho and Ribeirão Preto) is around 5,400 Wh/m2-day. This average solar irradiance and an abundance of water not surprisingly make the state of São Paulo the largest sugarcane producer in Brazil. A photovoltaic solar plant built in this same area of the state, with total installed capacity of 1000 kWp (dc) from fixed panels and built on 1 hectare of land will produce around 1,580 MWh/year of electricity.
(1) Global Horizontal Irradiance
As a reference for H2 production we will pick numbers from Siemens’ Silyzer 300 water electrolysis system – which can produce up to 2,000 kg of H2/hour from a plant efficiency index declared by the company above 70% based on the LHV of Hydrogen. From these numbers, a hectare with a 1 MWp photovoltaic system can produce an average of 1,422 MWhe/year across the 25 years of the plant’s lifespan. Applying efficiency of 70% we will have 995 MWhe equivalent to 3.584 million MJ = 29,700 kg of H2/ha-year.
Verdict:
440 kg/ha-year of GH2 from ethanol versus 29,700 kg/ha-year from solar PV
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Now, how green will “GREEN HYDROGEN” really be?
Ethanol
In February 2010, the US EPA (Environmental Protection Agency) published an important review of its RFS (Renewable Fuel Standard), called RFS-2, which is perhaps still the most complete study on the life cycle of Brazilian ethanol.
In it, the LCA (life cycle analysis) of ethanol in Brazil produced only by fermenting sugars (without the production of additional cellulosic ethanol from sugarcane straw) underwent an important change for the better. The study indicated that the real reduction in emissions (kg CO2e) of our ethanol compared to the life cycle emissions of standard American gasoline in 2005 would be 61% and not the 44% of the initial assessment of 2008, contested by Brazilians in the sector and by Embrapa. The new reading made our ethanol classified by the EPA as “Advanced Renewable Fuel”, which reduces emissions by more than 50% compared to gasoline, an index that corn ethanol does not reach. Brazil’s 61% renewable ethanol ranks near the top of the world biofuels scene, second only to used vegetable oil and animal fat. But the 61% also remember that there is no 100% renewable energy by any means.
For GH2 from ethanol we will have emissions that add up to 3,865/44We know that 1 hectare of sugarcane that produces an average of 5,180 kg of ethanol per year will result in 440 kg of H2. The carbon contained in this ethanol will produce 9,909 kg of CO2, 39% of which is non-renewable, that is, 3,864 kgCO2/year.
For GH2 from ethanol we will have emissions that add up to 3,865/440: 8.78 kgCO2e/kg of H20
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Photovoltaic Solar System
An LCA of high-efficiency solar panels (20.1%) was developed by a team from the Brookhaven National Laboratory of the US DOE, the Center for Life Cycle Analysis at Columbia University and SunPower, the manufacturer of the studied panels.
In general terms, the “cradle-to-grave” LCA considered the primary energy used in all phases of production in the Philippines – from mines to decommissioning – of 248,652 photovoltaic panels of 1.62 m2 and 327 watts dc. The energy matrix in the Philippines at the time of the study – representative of Asia – was made up of coal (25.9%); oil (8.0%); gas (32.2%); hydro (16.2%) and geothermal (17.7%), which resulted in a total of 281 kgCO2e/m2 of solar panel manufactured. According to the study, the SunPower panel returns the energy used in its manufacture in the Philippines in 1.4 years.
Considering the 1,000 kWp solar plant in Sertãozinho, in the state of SP, using the SunPower panel (20.1%), we arrive at a total of 3,058 panels of 327 Wp and 4,954 m2 of panel area, resulting in 1.392 million kgCO2e emitted in its manufacturing. It is fair to distribute the total emissions over the 25 years of operation of the solar plant, which results in an average of 55,680 kCO2/year for an average production of 29,700 kg H2/ha-year of GH2 over 25 years. Remember that the plant’s solar electricity production drops by 0.5% per year between the 1st year of operation and the final 25th year, therefore the use of this 29,700 kg/ha-year number and not the 33,000 kg H2/ha-year achieved just in the 1st year of the life of the solar plant.
For the FV it results in 55,680 kCO2e-year/29,700 kg H2 = 1.87 kgCO2e/kg of H2.
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CONCLUSION
Two distinct processes – on the one hand, the steam reforming of sugarcane ethanol obtained via fermentation and, on the other hand, the hydrolysis of water from photovoltaic solar electricity – both operating from the same source of primary energy – the 1,972 kWh/m2- year (5,400 Wh/m2-day) provided by the sun – produce very different results when it comes to the production of green H2. From a land use perspective, using solar PV instead of ethanol for green H2 production in a simple way reduces the environmental impact by over 67X! This is the rate of area reduction that the photovoltaic route allows in relation to sugarcane ethanol to achieve the same production of green H2. These numbers can be seen in the following table.
In terms of emissions, there is also an advantage of PV, but this difference must be viewed with caution because only the impact of panel manufacturing was considered in the case of solar H2, while the EPA study for emissions resulting from our ethanol considers practically the entire chain of resources used in its production.
But real CO2 emissions from both processes based on solar energy lose significance when the
difference of land requirement is 67 times apart! The ethanol route is simply not a choice if
environmental footprint is one of the main criteria for public policy decision.
