Traditionally, people get most of their energy from carbon based fuels like wood and fossil fuels. All are polluting and the fossil fuels are non-renewable. There are at least three non-carbon sources of thermal energy. They are Solar, Geothermal and Nuclear.
Solar Thermal Energy
People probably dried themselves and their belongings with sunshine. Later, they started drying their food. When I was a boy, an Italian neighbor had a large garden. In late summer he began drying his red peppers by hanging them from his clothes lines. Sun dried fruit is considered superior to oven dried fruit.
A modern application is water heating. Solar water heaters became available around 1900. The first solar water heaters were simply black painted tanks mounted on a roof.
These have improved over the years. Some are passive and depend on thermal siphoning to move fluid.. Others are active and use pumps. When used in areas where freezing occurs, an anti freeze liquid must be used to collect heat from the sun. This heat is transferred to the potable water through a device called a heat exchanger. They were first mandated in Israel and are now popular in many parts of the world.
This type of water heater is quite common in many sunny areas where freezing does not occur. It is a passive system. The water circulates by itself because hot water rises over cold water. No heat exchanger is required.
Because of its simplicity, it pays for itself in fuel savings quickly.
This is a more complicated system used in colder climates. The collectors on the roof are glazed, insulated boxes. Inside are copper pipes painted black and filled with antifreeze. The antifreeze is circulated through a heat exchanger with a pump. The collectors take turns heating the potable water.
Although these were cost-effective for a while, the price of copper has gone up so much that they are no longer popular.
Swimming Pool Heaters
Swimming pools need only lower temperature so they can be heated with collectors made of plastic. The water is moved by the thermosiphon effect.
They are very competitive with gas heat and heat pumps.
Large-scale Water Heating
Large-scale water heating can be accomplished with many mirrors that track the sun. Each reflects the sunlight to a boiler mounted on a tower. Steam from the boiler runs a gas turbine. The turbine drives an electric generator to produce electricity from sunshine.
These are typically built in dessert areas.
A popular solar cooker is made by Haines. It is inexpensive and portable. The cooking pot is surrounded by clear plastic. It is held off the base so sun reaches the bottom as well as the top and sides. It heats water quickly,
thus it is good for pasteurizing water. Water does not need to be boiled to be pasteurized. It need only be held at 150 degrees Fahrenheit (65.5 Celsius) for 5 minutes.
An inexpensive way of indicating pasteurization by using the North Star Design’s WAPI. This is sealed clear plastic tube with some green wax inside. It is placed in the container that holds the water that is being pasteurized. The wax melts at 150 F and drops to the bottom of its tube. It can be reused by turning the tube over.
There are three kinds of geothermal energy.
First, areas like Iceland have pools of very hot water. These have been tapped for hot tubbing and space heating.
Second, Icelanders are also drilling deep wells that reach the earth’s molten magma. They will produce steam to power gas turbines and electric generators.
Third, in other areas, underground temperatures are between the extremes of summer and winter temperatures on the surface. Water is pumped from more than 20 feet in the ground where it is heated or cooled.
Solar energy is generated by nuclear fusion in the sun. In 1938, Otto Hahn and an assistant invented nuclear fission of heavy elements in his laboratory. In 1939, it was explained by Liese Meitner and her cousin.
The Americans picked up on this and with the help of Jewish refugees, built the first Atomic Bombs in 1945.
Following the war, various kinds of nuclear reactors have been developed. Typically they heat water to produce steam. The steam is used to run gas turbines. These, in turn, drive electric generators.
Nuclear reactors produce excess heat as they generate electricity. Sometimes the cooling water is run through greenhouses to heat the space and improve the growth rate of fruit and vegetables being raised.