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With the advancement of science and technology and the improvement of people’s material living standards, there is not only an increasing demand for energy, but also greater concern regarding whether the use of energy causes environmental pollution. Energy sources such as hydrogen, solar energy, biomass energy, and fuel cells are clean energy sources, and their resources are extremely abundant; therefore, they are ideal energy sources for the future. 1 Hydrogen energy: Hydrogen energy is an ideal and highly promising secondary energy source. Hydrogen energy has many advantages: its raw material is water, and the resources are not limited ； Hydrogen burns at a fast reaction rate and has a high heat of combustion ； An even more prominent advantage is that the product of hydrogen combustion is water, which does not pollute the environment; it is the cleanest fuel. Furthermore, hydrogen energy has a wide range of applications and strong adaptability. However, for hydrogen to become a widely used energy source, it is crucial to address issues such as affordable hydrogen production technologies, as well as safe and convenient methods for storage and transportation. Currently, the main industrial methods for producing hydrogen are the water-gas method and water electrolysis. Since both of these methods require energy and still depend on fossil fuels, they are not ideal. The most promising method currently considered is water photolysis, which involves the decomposition of water into hydrogen and oxygen using solar radiation in the presence of a catalyst. Photocatalysts come in various forms, including metal oxides, semiconductor electrodes, and lower plants such as blue-green algae. Currently, the method of water photolysis using various photocatalysts is still in the experimental stage. A major breakthrough is expected by the end of this century or the beginning of the next. Hydrogen has a low density, which makes it difficult to store. For example, at a pressure of 15 MPa, a 40 dm3 steel cylinder can hold only 0.5 kg of hydrogen. Liquifying hydrogen requires a great deal of energy, and the container must be insulated, which is very unsafe. The most promising method for storing hydrogen is the use of solid metal hydrides. For example, the lanthanum-nickel alloy LaNi5 can absorb hydrogen to form a metal hydride, and when heated, LaNi5H6 can release hydrogen. LaNi5 alloy can undergo repeated hydrogen absorption and release over a fairly long period of time, and it has a high hydrogen storage capacity; 1 kg of LaNi5 alloy can store more than 15 g of hydrogen at room temperature and a pressure of 250 kPa. In addition to lanthanum-nickel alloys, there are various other alloys that can store hydrogen (see Section 5.2.3). Research and development are currently underway to further improve hydrogen storage performance, making it a safe, convenient, and economical method for storing hydrogen. . At present, many efforts related to the utilization of hydrogen energy are still in the experimental stage. Hydrogen energy can be used to generate electricity, provide heat, and supply power. It can replace almost all existing energy sources, and it boasts the advantages of being efficient, clean, and quiet—advantages that these other energy sources lack. For example, liquid hydrogen has been used as an energy source in satellites and spacecraft. In 1980, China developed its first hydrogen-powered vehicle. In 1985, the former Soviet Union also conducted tests on vehicles that used a mixture of hydrogen and gasoline as fuel, employing Ti, Fe, and V alloy hydrides. Adding about 4% hydrogen by mass to gasoline could result in a 40% reduction in fuel consumption, as well as a 90% decrease in CO levels in the exhaust gases. It is expected that in the near future, hydrogen energy will not only be widely used as a power source for cars, airplanes, ships, trains, etc., but will also become an important energy source in industry and daily life. 2 Solar energy: Solar energy is an inexhaustible, pollution-free source of energy. Solar energy comes from nuclear reactions in which hydrogen fuses into helium. It is estimated that the Earth’s surface receives a huge amount of energy from the Sun, totaling 5×1021 kJ per year; even one ten-thousandth of this amount could meet the world’s current energy needs. However, solar energy is affected by day and night, seasons, geography, and climate, and its energy density is low; therefore, how to effectively collect solar energy is a highly important issue in the utilization of solar energy. There are mainly three ways to collect and utilize solar energy: photothermal conversion, photovoltaic conversion, and photochemical conversion. Photo-thermal conversion typically involves using concentrators or collectors to convert solar radiation energy into thermal energy. Used for heating hot water for industrial or domestic use, heating systems, air conditioning, as well as drying crops and raising fish in warm water. However, the materials of the concentrators or collectors need to be improved to enhance heat collection efficiency. Photoelectric conversion is the direct conversion of solar radiation energy into electrical energy. A device that converts solar energy directly into electrical energy using the photoelectric effect is called a solar cell. There are various types of solar cells; depending on the semiconductor materials used in their construction, they mainly include silicon cells, cadmium sulfide cells, gallium arsenide cells, etc. The manufacturing process for solar cells is complex, the production costs are high, and they are also subject to limitations related to the supply of semiconductor materials. Currently, solar cells are still limited to within 1 kW. With the improvement of solar cell manufacturing technology, photoelectric conversion will be a more practical way to utilize solar energy. It can be used as a small power source, or it can be utilized to build large-scale, high-power solar power plants; it is particularly suitable for constructing such plants in deserts, mountainous areas, islands, and other similar locations. It will surely pave the way for humanity to make large-scale use of solar energy. -Chemical conversion is the direct conversion of solar radiation energy into chemical energy. The photosynthesis of green plants is a photochemical conversion process. To date, it is the most important and fundamental way of utilizing solar energy. However, the photochemical conversion achieved through green plants cannot yet be fully controlled by humans. Therefore, studying various fully controllable photochemical conversion methods is also one of the major research topics in the world today. In recent years, it has been discovered that certain substances can directly absorb solar energy to undergo chemical reactions that produce high-energy substances, while the reverse reaction is an exothermic reaction. Therefore, this effect can be utilized to store light energy in chemical substances, which can then be released when needed. Our country is rich in solar energy resources, especially in the inland plateau areas; for example, the energy density in Tibet is (6.3–10)×105 J/(cm2·a), while other regions have a value of (4.6–5.9)×105 J/(cm2·a). Solar ovens in Tibet can reach 240°C in January. Solar furnaces, solar cookers, etc. have been widely used in some areas of our country facing energy shortages. People have also envisioned the creation of “cosmic solar stations.” These stations operate in space along specific orbits, and since they are not hindered by Earth’s atmosphere when receiving solar energy, their efficiency is much higher than that of systems on the ground. Figure 4.7 shows the conceptual design for this power station – the “synchronous space station” design. The synchronous space station is in an orbit parallel to the equator, at a distance of 3.56×104 km from Earth, and orbits the Earth in sync with it, maintaining a constant distance from ground receiving stations. On the space station, solar energy is converted into electrical energy by several solar cells; this electrical energy is then transformed into microwaves using a microwave generator, which are subsequently transmitted in a focused form to ground reception stations. The ground receiving station then converts the microwaves into electrical energy. Such power plants will be able to meet the electricity supply needs of a modern large city. If several \"synchronized space stations\" operate in orbit, they will provide a continuous supply of electricity for humanity. At present, the utilization of solar energy is still in its initial stage, and it can only be used as an auxiliary energy source in the near future. But in the long run, solar energy is humanity’s most fundamental source of energy, lasting as long as the sun itself. Once the technologies for utilizing solar energy undergo fundamental changes, it is inevitable that solar energy will replace nuclear power on a large scale, thereby fundamentally changing the way humans use energy. Biomass energy and biogas – energy derived from waste. Biomass energy is the energy stored within living organisms, resulting from green plants converting solar energy into chemical energy through photosynthesis. Under the action of sunlight, the chloroplasts in plants convert water, carbon dioxide, and inorganic salts into simple small molecules, which are then combined to form more complex large molecules such as sugars, proteins, and fats. Energy is stored in the form of ATP (adenosine triphosphate). Each mole of ATP stores about 50 kJ of energy. Therefore, utilizing biomass energy is an indirect way of using solar energy. The resources of biomass energy on Earth are extremely abundant; currently, the amount of organic matter produced globally each year through photosynthesis is around 200 Gt, which corresponds to an energy amount of 3×1021 J. Around the world, 27 Mt of waste is generated every day; the discharge of various types of wastewater amounts to as much as 450 Gt per year, and the amount of human and animal excrement exceeds several billion tons each year. All of this waste can be potential resources for biomass. It has been determined that the calorific value of municipal waste is similar to that of lignite; approximately 2 tons of waste are equivalent to 1 ton of coal. Forestry and its waste, as well as agricultural waste, are also vast reservoirs of biomass resources. However, biomass used as an energy source accounts for less than 1% of its total biomass, yet the energy it provides constitutes 14% of the world’s total energy consumption. The traditional method of obtaining energy from biomass is direct combustion, which results in a low efficiency in utilizing biomass energy and causes environmental pollution. Therefore, it is necessary to change traditional energy usage methods and utilize biomass conversion technologies to significantly improve energy efficiency. Using biomass fermentation to produce biogas is the most rational and effective method for converting biomass energy. Biogas is a combustible gas produced through aerobic and anaerobic decomposition of waste materials such as urban garbage, sewage, sludge, human and animal excrement, crop straws, and weeds. Its main components are: 55%–65% CH4, and 35%–45% CO2. Biogas is an efficient, inexpensive, and clean energy source. The residues from fermentation can also be utilized comprehensively as fertilizers, feed, etc. At the same time, it can also maintain the balance of natural ecosystems in a relatively reasonable manner. Many in North America and Western Europe **have long been using municipal wastewater, garbage, etc. to produce biogas, which is then used as a source of energy. Recently, the French company Rolca has developed a waste biogasification device that can continuously process household waste, sewage, sludge, industrial organic waste, agricultural waste, etc., without producing smoke or unpleasant odors. While treating waste, it also recovers valuable energy in the form of biogas, as well as organic fertilizers with high utility value. For example, a method has been developed in England to rapidly convert household waste into pollution-free fuel. It involves putting waste, metal nickel catalyst, and light oil into a high-pressure reactor; by maintaining a temperature of 350°C, after 10 minutes, they are converted into fuel, carbon dioxide, and water. And this transformation takes millions of years to occur in nature. The fuel produced by this method contains no sulfur or oxygen, and its combustion does not cause acid rain. Through thermal cracking, it is also possible to produce low-molecular-weight products such as gasoline, thereby transforming urban waste treatment plants from mere consumption-oriented facilities into production units that generate direct economic value. It is evident, then, that waste energy holds great potential.