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Introduction

1.1 Energy and Membranes

Energy crisis, water and food shortage, environmental pollution, and so on are important problems of human beings for the next 50 years. Energy crisis is the single most important problem because of an increase in energy usage from the technological development and its influence on energy toward water, food shortage, and environmental pollution. According to International Energy Outlook 2021 (International Energy Association 2021) from the United States Energy Information Administration (EIA), the world energy consumption will grow by nearly 50% between 2020 and 2050. Most of this growth comes from regions where strong economic growth is driving demand, particularly in Asia. Demand for all fuels increased, with fossil fuels meeting nearly 70% of the growth for the second year running. As a result, global energy‐related CO2 emissions rose by 1.7% to 33 gigatons (Gt) in 2018. Coal use in power generation alone surpassed 10 Gt, accounting for a third of total global CO2 emission. Most of that came from newly built coal‐fired power plants in developing countries [1]. As the amount of energy consumption increased enormously, in particular, after the Second World War, there was a worldwide issue on energy consumption and production that raised intensive environmental issues (see Figure 1.1). The largest portion of the energy sources are fossil fuels such as coal, oil, and natural gas that cover more than 80% of all the energy sources.

To solve the energy crisis, various efforts have been made to economically produce energy and use it efficiently. From fossil fuels to renewable energy, various kinds of resources have been utilized to produce energy. Fossil fuels are hydrocarbons, primarily coal, petroleum, or natural gas, generated from buried combustible geologic deposits of organic materials that have been converted to petroleum, coal, and natural gas by heat and pressure in the earth's crust over hundreds of millions of years. Because of their origin, fossil fuels have a high carbon content, resulting in a high heat content. According to the Statistical Review of World Energy from British Petroleum in 2020, the main primary energy sources worldwide consisted of coal (24%), petroleum (33%), and natural gas (27%), amounting to an 84% share for fossil fuels in primary energy consumption in the world [2]. Fossil fuel is the most economic resource to produce energy; however, it is required to reduce the share of fossil fuel due to emission of CO2 inducing global climate change. Currently, technology innovation is applied as a means to decrease the environmental impact of coal combustion by increasing the boiler efficiency, co‐combustion with biomass or carbon capture, utilization, and storage [3]. Nuclear power is one of the candidates to replace fossil fuels owing to its low price for energy production. However, it is hard to completely replace fossil fuels with nuclear power due to its drawbacks such as uncertain accidents and the production of radioactive nuclear wastes. Especially, the three nuclear disasters (that is, Three Mile Island in 1979, Chernobyl in 1986, and Fukushima in 2011) caused by human mistakes and/or natural disasters such as flood and earthquake make it difficult to expand the number of nuclear power plants. Renewable energy including solar, wind, and geothermal energy has been developed during the last decades with an infinite potential; however, they require further research to overcome their limitations such as low energy density, intermittent energy production, and so on. As such, each energy resource has strengths and possibilities; however, it also has weaknesses to be solved.

Figure 1.1 Global primary energy consumption by source.

Source: Our World in Data based on Vaclav Smil (2017) and BP Statistical Review of World Energy

Let us briefly look at the conversion of fossil fuels to electrical energy. Thermal power plants use coal, petroleum, and natural gas as fuel. According to the species and phases of the fuel, transportation of the fuel to the plant and post‐ and pre‐treatment of the fuel are different resulting in the application usage. However, the principle of generating electricity from fossil fuels is the same. The steam is produced at a high pressure in the steam boiler after burning fuel in boiler furnaces. Superheated steam then enters into the turbine and rotates the turbine blades. The turbine is mechanically coupled with alternator that its rotor will rotate with the rotation of turbine blades. After entering into turbine, the steam pressure suddenly falls and the corresponding volume of the steam increases. After imparting energy to the turbine rotor, the steam passes out of the turbine blades into the condenser. In the condenser, cold water is circulated with the help of a pump which condenses the low‐pressure wet steam. This condensed water is further supplied to a low‐pressure water heater where the low‐pressure steam increases the temperature of this feed water and it is again heated at high pressure. The chemical energy in fossil fuels is converted to thermal energy through carbon and hydroxide combining with oxygen during the combustion. Heat energy boils pressurized water to steam, and the steam travels through a mechanical turbine, causing it to rotate converting thermal energy to electric energy, i.e. electricity.

Pulverized coal (less than 5 cm) is burned in a boiler where water boils to steam. Converted steam is used to operate turbines for electronic generators. Compared to thermal power plants using other fuel types, coal requires a specific fuel processing and ash disposal. Flue gas from the combustion of fossil fuels contains carbon dioxide (CO2) and water vapor, as well as pollutants such as nitrogen oxides (NOx), sulfur oxides (SOx), and, for coal‐fired power plants, mercury, traces of other metals, and fly ash. Solid waste ash from coal‐fired boilers should be removed. Gas‐fired power plants, which burn natural gas (mainly methane) to generate electricity, produce a quarter of the world's electricity; however, they are also a significant part of global greenhouse gas emission resulting in global warming.

As the majority of current global primary energy relies on fossil fuels, the energy system is the source of approximately two‐thirds of global CO2 emissions. As methane and other short‐lived climate pollutant emissions are believed to be severely underestimated, it is likely that energy production and use are the source of an even greater share of emissions.

Meanwhile, membrane technology, which has been able to be utilized with various kinds of energy application, has been developed to solve the water issues and the problems of energy production. For example, in energy production using fossil fuels, membrane technology can be utilized to reduce the environmental impact from the emission of carbon dioxide. Membranes can separate the carbon dioxide from the ash inducing environmental issues and prevent them from being discharged to the outside. Moreover, membrane technology is also used as an integral part of the energy production in various ways. For example, in a fuel cell application, membrane performance determines the overall system performance including power density and long‐term stability of the fuel cells. In energy production using salinity gradients and osmotic pressure, membranes enable the harvesting of blue energy from the ocean, induced by the chemical potential difference using the diffusion difference of water and salt.

1.2 Brief History of Membrane Technology

Membrane technology has been known for a long time. The first report on gas separation was made by Mitchell [4], who observed the separation of gases through membranes. He noticed that gases had different permeabilities in experiments using rubber balloons exposed to various gas atmospheres. Graham [5, 6] reported in 1829 about different rates of permeation in experiments where a wet pig bladder inflated when stored in CO2 environment. Fick established the theoretical equations for diffusion through membranes. Graham proposed the concept of solution–diffusion as the permeability process in polymeric membranes in 1866 [7]. Daynes [8] established a time lag method to determine diffusion coefficients employing the unsteady‐state solution of Fick's second law to mathematically determine the diffusion coefficient from the extrapolation of the steady‐state flux to the time axis. Barrer and Rideal [9] noted the temperature‐dependent Arrhenius equation of gas permeability and diffusion of gases. Although most of these scientific advances have enlightened the membrane community, historic advances in gas and liquid separation membranes occurred in 1960 with the discovery of asymmetric cellulose acetate membranes with a selective layer thickness <1 μm for reverse osmosis by Loeb and Sourirajan [10–12]. Before this discovery, dense cellulose acetate membranes separated water and salts but were too thick to produce sufficient water flux to meet the commercial needs [13]. The production of asymmetric cellulose acetate membranes spurred the commercialization of reverse osmosis membrane and later led to gas separation membrane [14, 15]. A thin polyamide layer of 0.2 μm thickness can be coated on top of a microporous polysulfone support layer with a thickness of 200 μm to produce a thin‐film composite membrane for reverse...