Comparative study of various hydrogen production methods for vehicles.

Abstract

Hydrogen as an energy carrier is a promising candidate to store green energy and it has a potential to solve various critical energy challenges. Although, hydrogen is a clean energy carrier, the possible negative impacts during its production cannot be disregarded. Therefore life cycle analyses for various scenarios have been investigated in this study. In this thesis, a comparative environmental assessment is presented for different hydrogen production methods. The methods are categorized on the basis of various energy sources such as renewables and fossil fuel. For the fossil fuel based hydrogen production, steam methane reforming (SMR) of natural gas is studied. Renewable based hydrogen production includes electrolysis using sodium chlorine cycle. Electrolytic hydrogen production is also compared using different cells such as membrane cell, diaphragm cell and mercury cell. Wind and solar based electricity is also used in electrolytic hydrogen production. Furthermore, vehicle cycle is studied on the basis of available literature to compare the hydrogen vehicle with gasoline vehicle. The investigation uses life cycle assessment (LCA), which is an analytical tool to identify and quantify environmentally critical phases during the life cycle of a system or a product and/or to evaluate and decrease the overall environmental impact of the system or product. The LCA results of the hydrogen production processes indicate that SMR of natural gas has the highest environmental impacts in terms of abiotic depletion, global warming potential, and in other impact categories. The abiotic depletion for SMR is found to be 0.131 kg Sb eq. which is the highest among all methods. The second highest abiotic depletion value comes under electrolysis using mercury cell which is 0.00786 kg Sb eq. However, thermodynamic results suggested that SMR is the efficient method of hydrogen production because the amount of hydrogen energy produced as output in the system is larger than any other method. The energy efficiency of the system in this method is about 76.8% and the exergy efficiency is about 72.4%. In terms of vehicle cycle comparison, it is found that the gasoline vehicle appears to be the largest contributor in energy consumption and GHGs emissions. The energy consumption of gasoline vehicle is three times higher than hydrogen vehicle. Moreover, GHGs emissions of the hydrogen vehicle are 8% of the gasoline vehicle.