According to the types of energy sources, energy resources are renewable and non-renewable.

Figure 45 Energy resources

1.1.1. Non-renewable energy sources

Non-renewable energy sources (oil, natural gas and coal) are fossil resources formed over millions of years from the remains of plants and animals, trapped by layers of rock and sand beneath the earth’s surface. Over time, under the action of high temperature and pressure, they have turned into coal, oil (oil) or natural gas. The extraction of these fossil fuels is associated with disturbance of the natural environment – changes in the terrain, destruction of arable land, accumulation of quarry materials, soil destruction and others. These fuels are limited in quantity and will be depleted (for example after 70-130 years). As time goes on, their price will rise. Their combustion creates a significant carbon footprint, which leads to an increase in the greenhouse effect.

• Coal has been formed for millions of years. They are combustible black or brown-black sedimentary rocks with a large amount of carbon and hydrocarbons (over 50% by weight and over 70% by volume of coal is carbon).
• Oil is a mixture of hydrocarbons formed by plants and animals that lived millions of years ago. Crude oil is a fossil fuel and exists in liquid form in underground basins or reservoirs, in small spaces in sedimentary rocks or near the surface. It is obtained by pumping.
• Natural gas contains various compounds, but its most important component is methane (CH4). Natural gas has a higher calorific value than oil and is significantly richer in energy than coal. It is used for the needs of the chemical, glass, porcelain and earthenware and other industries, as well as by some thermal power plants.
• Nuclear is the energy that is released during nuclear fission or fusion. It is an extremely rich source of energy (one gram of uranium-235 provides more energy than 3.5 tons of coal), is environmentally friendly, does not pollute the air. Today, nuclear energy provides about 15% of the world’s electricity, saving more than two billion tonnes of carbon dioxide or 20% of global CO 2 emissions each year. Despite all these advantages, the emitted radioactive rays are dangerous for the life of organisms and humans. However, environmental hazards can also arise from improper storage of radioactive waste.

1.1.2. Renewable energy sources
Renewable energy is obtained from sources that are considered to be naturally renewable or practically inexhaustible – sunlight, wind, tides, geothermal energy, etc.

Figure 46 Energy sources
In 2008, about 19% of global energy consumption came from renewable sources (including about 13% from traditional biomass (firewood and others, mainly burned for heating) and 3.2% from power plants). Renewable sources – small hydropower plants (small channel hydropower plants), modern biomass, wind power plants, solar and geothermal power plants, biofuels – provide another 2.7% of energy consumption, and their share is growing rapidly.
• Wind energy is the kinetic energy of air masses in the atmosphere. It becomes a useful form of energy – mechanical (wind is used to drive sailboats, to pump water for irrigation or to drive windmills) or to produce electricity through wind turbines. Wind energy is clean, without harmful emissions.
• Solar energy. In Bulgaria, one of the sunniest countries in Europe, we can not boast of rational use of light, unlike from countries with a more unfavorable climate than ours. Two solar panels can cover half of the hot water needs (55 ° C) of a family of four, consuming 200 liters per day. The investment pays off in about 5 years. Solar energy has high hopes. For the last 35 years the price of photovoltaic modules has decreased a hundred times – from $ 76.7 / W to $ 0.74 / W. Some European countries, which are oriented towards 100% renewable energy, rely heavily on solar energy.
• Hydropower (in the form of kinetic energy, temperature differences or osmotic energy) can also be used. Because water has about 800 times the density of air, even a small flow of water can generate significant amounts of energy. Hydropower plants convert the kinetic energy of water into electricity. In Bulgaria there is a special interest in the construction of small channel hydropower plants.
• Geothermal energy is used in heating and cooling buildings. The temperature of the soil and water is almost constant at a depth of 5 m – about 10 ° C, which makes it warmer than the outside air in winter and colder than it in summer.
• Bioenergy is obtained from plant or animal biomass. Plants photosynthesize and as a result of the formed organic substances grow and create biomass, which can be used directly as fuel or for biofuel production. Biofuels biodiesel and bioethanol, which are produced from crop biomass, can be burned in internal combustion engines and boilers. When biofuels are burned, the chemical energy stored in them is released.
• Wood is the most ancient source of energy used by humans for heating and the production of various products. With the development of technology, it has been replaced by fossil fuels, which are characterized by higher calorific value, easier extraction and use. It was found that burning 1 ton of wood emits 26.5 times less ash, 30 times less SO2 and 5 times less N2O2 compared to coal. For Bulgaria economic interest are fast-growing tree species such as poplar, willow, acacia and others. We have extensive experience in the selection of fast-growing species for the production of construction and technological wood. This is an objective prerequisite for providing planting material for the establishment of energy plantations.
Bioenergy covers the use of solid, liquid and gaseous fuels.

Figure 47 Bioenergy
Gaseous biofuel. In the most typical case, the biomass produces biogas with a content of 50-87% methane, 13-50% CO2, minor impurities H2 and H2S. After purification of the biogas from CO 2, biomethane is obtained. Biomethane is a complete analogue of natural gas, the only difference being the origin. Liquid biofuel. Sunflower crops, rapeseed, sugar cane, palm oil, etc. are used as raw materials. Virtually every type of organic raw material, rich in hydrocarbons and available in large quantities, is suitable for the production of biofuels – for example, in Spain a project is being developed for the use of orange peel for production. The highest yields (7 to 13 times higher than those of sugar cane) are obtained from the use of algae. Solid biofuels are primarily wood for combustion, including in the form of firewood, wood briquettes and other types such as organic household waste. This type of biofuel is most common in developing countries due to the minimal investment required to use it. They are also very common in Bulgaria. The combustion of solid biofuels is very harmful to human health when carried out in open hearths and in old solid fuel stoves without filter systems. Combustion releases carbon dioxide, sulfur dioxide and fine dust particles. Advantages of renewable energy.
Renewable energy is also called “green energy” because it is extremely important for sustainable development. Its advantages can be considered in 4 directions:
1. Reduced impact on climate change – RES have a lower carbon footprint during construction and almost zero during operation.
2. Accessibility and security of energy supply – sun, wind and water are available almost everywhere and are free. Through them, each country can be independent of energy imports from other countries.
3. Long-term economic benefits: the tendency of RES prices to decrease and of fossil fuels to increase. 4. The population approves of environmentally friendly energy. There is no most preferred type of renewable energy source. Depending on the local climatic conditions, one or another species is the most suitable. However, given the limited time of energy production, the best solution is to combine several types of renewable sources so that the disadvantage of each can be offset by the advantages of another.
Source: [17] http://education.ecofund-bg.org/