What is the cost (in the UAE) using PV technology to produce 1 Kg of hydrogen gas?
Hydrogen gas has numerous industrial uses including the production of ammonia through the Haber process, fertilizers and hardening of oils (Gupta, 2008). Hydrogen gas is an environmental-friendly source of sustainable energy because its combustion does not produce air pollutants. Additionally, it is a sustainable source of energy because its production uses water, whereas its combustion liberates water thus leading to the recycling of water. However, the amounts of hydrogen present in ordinary air are insufficient to cater for the uses of the gas. Therefore, there is a need to produce it on a large scale. One viable method of hydrogen production is the electrolysis of water.
Photovoltaic (PV) units facilitate photoelectrolysis where light energy is used to decompose water into oxygen and hydrogen gas. The total electricity requirement to produce 1 kg of hydrogen is 50 kWh/kg (Mazloomi & Gomes, 2012). This amount of energy translates to $1.5 per kg of hydrogen going by the current cost of solar energy in the UAE of 3.00 U.S. cents ($0.03) per kilowatt-hour (Parkinson, 2016).
How much solar energy is required to produce 1 Kg of hydrogen with current PV and electrolyzer technologies?
Theoretically, electrolyzers work at 100% efficiency leading to minimal losses of energy to the surroundings. However, it has been reported that industrial electrolyzers with the highest efficiencies operate at 65% the theoretical values (Bozoglan, Midilli, & Hepbasli, 2012). According to the Florida Solar Energy Basics (2016), the electrical energy required to produce 1 kg of hydrogen is 51 kWh when using such an electrolyzer and the lower heating value of hydrogen. Therefore, the same amount of solar energy (51 kWh) is required.
How much energy can be derived from 1 Kg of hydrogen if converted back to electricity via a fuel cell?
A fuel cell combines hydrogen and oxygen from the air to create electrical energy that is used to drive other processes. The chemical reaction that occurs in fuel cells is as follows:
H2 →2H+ + 2e– (Corbo, Migliardini, & Veneri, 2011).
The total electric output from the combustion of hydrogen in a fuel cell is 237.13 kJ for every mole of hydrogen. Therefore, the total energy output from 1 kg of hydrogen is 118,565 kJ/Mol or 32.934 kWh because 1 kJ/Mol is equivalent to 0.000277778 kWh.
The above energy is the theoretical yield at about 83% efficiency. However, the actual yield possible considering the inefficiencies of most fuel cells is less than 32.934 kWh. The World Energy Council (2016) approximates the efficiency of most fuel cells to be at 58%.
Therefore, the actual energy attainable from a fuel cell is calculated by the equation (58×32.934 kWh)/83= 23.01 kWh.
With current efficiencies and costs of solar panels, is it feasible in this region?
The inadequacies of PV cells and electrolyzers increase the theoretical amount of energy required to produce hydrogen gas from water. On the other hand, the current efficiencies of fuel cells lead to significant losses of energy to the environment thus reducing the electrical energy obtained from burning of hydrogen. Therefore, the process is not viable in the region unless the efficiencies of the production and combustion processes are improved.
The efficiency of all components of the system
The entire system of generating hydrogen and burning it to produce energy constitutes two main systems: the PV system and fuel cells.
The efficiency of fuel cells is calculated by finding the difference in the actual energy used and theoretic energy requirement as a percentage of the calorific content of 1 kg of hydrogen gas as follows.
Calorific content of 1 kg hydrogen= 39 kWh
Actual energy used in generating 1 kg of hydrogen= 51 kWh
Efficiency= (Theoretical input-actual input)/theoretical output×100
= [(39-51)/39] ×100
= -30.76%
The efficiency is a negative value, which implies that the process is inefficient.
The efficiency of the fuel cell is calculated by finding the actual yield as a percentage of the theoretical energy yield.
Actual energy yield= 23.01 kWh
Theoretic energy yield= 32.934 kWh at 83% efficiency
100% efficiency = (32.934×100)/83
=39.68
Efficiency of the fuel cell realized = (23.01)/39.68)×100
= 57.98%
The efficiency of fuel cells can be increased to 85% if the liberated heat is harnessed and put to better uses.
References
Bozoglan, E., Midilli, A., & Hepbasli, A. (2012). Sustainable assessment of solar hydrogen production techniques. Energy, 46(1), 85-93.
Corbo, P., Migliardini, F., & Veneri, O. (2011). Hydrogen fuel cells for road vehicles. New York: Springer.
Florida Solar Energy Basics. (2016). Hydrogen basics: Solar production. Web.
Gupta, R. B. (2008). Hydrogen fuel: Production, transport, and storage. CRC Press: Boca Raton.
Mazloomi, K., & Gomes, C. (2012). Hydrogen as an energy carrier: prospects and challenges. Renewable and Sustainable Energy Reviews, 16(5), 3024-3033.
Parkinson, G. (2016). Solar prices plunge to new lows as Dubai auction nets under 3c/kWh. Web.
World Energy Council. (2016). Fuel cell efficiency. Web.