**1.2. Solar energy**

**1.1. Sources of energy**

352 Titanium Dioxide - Material for a Sustainable Environment

71%, respectively.

period of time is needed to deposit more crude oil.

Energy makes change; it does things for us. The cars along the road as well as the boats over the water can be moved by energy. A cake in the oven and the ice frozen in the freezer also are been done using energy. It plays our favorite songs on the radio and lights our homes. Energy enables our bodies grow and make our minds to think. Scientists define energy as the ability to do work. Energy sources are classified into renewable and nonrenewable sources as shown in **Figure 1**. Most of our energy is extracted from nonrenewable energy sources. Coal, petroleum, natural gas, propane, and uranium are nonrenewable energy sources. The name of nonrenewable energy sources has appeared because their supplies are limited. Petroleum, for example, was formed millions of years ago from the remains of ancient sea plants and animals. A long

Renewable energy sources include biomass, geothermal energy, hydropower, solar energy, and wind energy. They are replenished in a short time, so they are called renewable. Electricity is mainly produced from renewable energy sources. Sunlight, or solar energy, can be used directly for heating, lighting, generating electricity, and many other applications [1, 2]. Indirectly, sun's heat also creates winds, whose energy is used to generate electricity by wind turbines, which is called wind energy. Plants are grown by sunlight. Biomass is the organic matter which is composed of plants. It can be used, for example, to produce electricity and transport fuels. Furthermore, geothermal energy absorbs the Earth's internal heat and can be used for a large variety of applications such as building heating and electricity production. Ocean energy draws on energy of the ocean waves, currents, and tides which can under favorable conditions be used for the production of electricity. Global energy consumption consists of six primary sources: 44% petroleum, 26% natural gas, 25% coal, 2.5% hydroelectric power, 2.4% nuclear power, and 0.2% non-hydro renewable energy [3]. Between 1990 and 2010, worldwide consumption of petroleum, coal, and natural gas is enhanced by 22, 27, and

**Figure 1.** Renewable and nonrenewable energy sources. From Ref. [4]. Reprinted with permission from WordPress.

Solar energy develops a large variety of techniques, including photovoltaic systems, concentrating solar power, passive solar systems, solar hot water, and solar process heat [5].

Conversion of the light energy into electricity is the main use of photovoltaic solar cells and has been existing for a long time; many applications like solar panels on satellites and smaller uses as calculator or watch energy supply, which is familiar for everyone, enable them to be famous. The solar energy is instituted on the solar cell (photovoltaic) effect, which allows the conversion of light (photons) to electricity (electrons).
