Nature provides us with abundant resources. Over 90 percent of the Earth’s crust is composed of silicate minerals, making silicon its second most abundant chemical element. The properties of silicon and its derivatives make them suitable for buildings and roads construction.
They are also key components in the electronics and semiconductors that have been changing our lifestyles.
Energy, on the other hand, is one of the most popular topics of the 21st Century, as the way we use energy is causing worrisome climate changes. During the recent 21st annual Conference of the Parties (COP21), the inclusion of a 1.5 degrees Celsius limit was seen a major victory for island nations, such as Singapore, as well as the developing countries. To these countries, the impact of climate change due to greenhouse gases, manifested as temperature anomalies and rising sea levels, are becoming increasingly intolerable and threatening. By placing the 1.5 degrees Celsius limit alongside the legally binding goal to hold global temperatures“well below 2°C above pre-industrial levels”, COP21 explicitly spelt out the decarbonising effort with an increasing use of renewable energy.
Nature provides us with abundant solar, wind and other renewable energy resources, which in fact are sufficient to support the human needs, the challenge is in affordably harvesting these energy resources are challenging and costly. One of the most challenging technical issues associated with the use of renewable energy, such as solar and wind, is the intermittent nature of energy supply which prompts the need for energy storage. Hence, sourcing for an appropriate energy carrier, especially for those off-the-grid applications such as automobiles, aircrafts and ship propulsions, are extremely challenging if we were to electrify these vehicles.
Hydrogen, a clean energy carrier, is the most abundant chemical element in the universe, making up 75 percent of normal matter by mass and more than 90 percent by number of atoms. Unfortunately, hydrogen gas is very rare in the Earth’s atmosphere, though it is the third most abundant chemical element on the Earth’s surface – mostly in the form of chemical compounds such as hydrocarbons and water.
One cubic metre of water contains about 111 kilogram (kg) of hydrogen, and if hydrogen is used in a typical fuel cell car, 1 kg of hydrogen would cover a range of 100 km. In other words, splitting hydrogen from one cubic metre of water using any form of energy – for example, solar photovoltaic – would allow a fuel cell car to cover a driving range of 11,100 km. When hydrogen gas is oxidised chemically in a combustion engine or electrochemically in a fuel cell system, it produces pure water as a by-product, emitting no carbon dioxide.
Figure 1 shows two important facts on energy density and on the evolution of energy used in human society. It is a fact that the state-of-the-art lithium ion battery is puny compared to fossil energy in both gravimetric and volumetric energy density, hence the use of lithium ion battery for massive renewable energy storage posed a question mark.
On the other hand, human civilisation has dramatically improved due to the discovery of energy, from wood to coal to oil and now we use natural gas more than ever. The evolution of energy shows increased hydrogen content in fuels in a log-linear manner, according to a study of the International Institute for Applied Systems Analysis in Austria. One would expect that by the end of this century, the use of renewable energy will dominate the energy market and that hydrogen will be a natural choice and an ideal energy carrier for off-the-grid applications.
Practically, hydrogen can be split from seawater using massive offshore wind energy through high-temperature solid oxide electrolysers. This technology not only promises high energy conversion efficiency, but it is also inert to sodium chloride (salt) poisoning of the hydrogen electrode when the right materials are used in electrolyser cells. Hydrogen gas produced offshore can be piped to the shore for various applications, including direct use in automobiles, conversion to electricity through fuel cell systems, or injection into the natural gas pipeline to form hythane (hydrogen plus methane), which increases the heating value of natural gas for power plants or even home cooking.
This scenario paints a picture for hydrogen as a new energy vector in human society. However, one of the drawbacks of hydrogen gas is its low volumetric energy density; Compressing the gas to achieve a desired density at high pressure is necessary.
Alternatively, hydrogen can be stored in solid form as metal hydride, such as magnesium hydride or in liquid form as methylcyclohexane (MCH), but more research is needed before these can be practically used.
Like many other energy sources, hydrogen also poses some safety concerns, but its associated risks are no more than those of fossil energy. Despite hydrogen having a wide flammability range, it is more diffusive and buoyant, which means that it can be dispersed more easily and rapidly than other energy sources after an accidental release.
In summary, we are not short of energy as nature provides us with more energy than necessary. It is our choice to adopt more affordable renewable energy with help from financial institutions, especially for the poorer countries. The important message is rational energy end-use, rather than strictly efficiency especially from the perspective of energy storage.
DR CHAN SIEW HWA
Dr Chan Siew Hwa is a professor in the School of Mechanical & Aerospace Engineering (MAE) at the Nanyang Technological University (NTU). He is concurrently holding two appointments as the Co-Director of the Energy Research Institute at NTU (ERI@N) and the Deputy Director of the Maritime Institute at NTU (MI@NTU). He is also a non-Executive Director of Maz Energy Pte Ltd, where he provides technical advice to the Board since 2004.