Notes

The sun is the source of almost all the energy on​ Earth's surface. Energy from the​ sun, or solar​ energy, is absorbed by plants and used as fuel by virtually all organisms. Because fossil fuels contain energy from the remains of​ organisms, they also contain energy originally from the sun.
2. Solar energy does more than provide energy for the organisms on Earth. The source of energy that drives the water cycle is the sun. The flow of streams on​ Earth's surface is part of the water cycle.​ Fast-moving rivers are often used to generate electricity. Since the water in the rivers results from the​ sun-driven water​ cycle, the energy produced by the flow of rivers can be traced to the sun. The sun also produces uneven warming of​ Earth's surface. These temperature differences cause winds to blow.​ Therefore, wind energy is also driven by energy from the sun. The sun is imperative to many natural processes on Earth.
3. The sun obtains its energy through thermonuclear fusion. In this​ process, high temperatures in the​ sun's core cause hydrogen nuclei to​ fuse, or​ join, forming helium nuclei. As each helium nucleus​ forms, a loss of mass occurs. This lost mass is converted to the heat and light energy of the sun.
4. Earth receives less than one billionth of the energy produced by the sun. Much of the solar energy that reaches Earth is in the form of visible light and infrared radiation. Recall that energy cannot be created or destroyed. It​ can, however, be changed in form. When light energy from the sun strikes​ objects, the energy in the light is absorbed by these objects. The absorbed energy is changed to​ heat, or thermal energy. There is an example of transformation of energy. Solar energy can be harnessed and used to generate heat and electricity. In some​ places, solar energy is replacing​ nuclear, biomass, and fossil fuels as an energy source.
5. The use of solar energy has many advantages. It is​ free, clean, and nonpolluting. Although you must buy the equipment for capturing and storing solar​ energy, the cost may be recovered by the savings in fuel bills over many years.
6. One drawback to solar energy systems is that the energy source is not constant. There is limited sunlight on cloudy or rainy​ days, and no sunlight at night. The amount of sunlight on any given day is variable.​ However, solar energy collected during the day can be stored. Another drawback to solar energy is that devices for harnessing and storing solar energy are not very efficient. The size and cost of the equipment may outweigh the benefits. But as technology​ progresses, the use of solar energy is becoming more ​cost-effective.
7. If the energy of sunlight is used directly as a source of light or​ heat, the energy use is described as passive. In passive solar​ heating, the​ sun's energy is​ collected, stored, and distributed naturally in an enclosed dwelling. You have experienced passive solar heating if you have ever walked through a greenhouse or sat inside an enclosed car on a sunny day. Passive solar energy is not used to produce electricity. But it can reduce the need for electricity or fuels by providing an alternative source of heat. Passive solar energy therefore helps reduce the use of fossil and nuclear fuels. It may also help in conserving other sources of energy.
8. Passive solar heating of homes is mostly accomplished through​ well-planned building design and construction. In North​ America, houses that make optimum use of passive solar heating have large windows that all face south.​ South-facing windows can gather the greatest amount of the​ sun's energy for the longest number of daylight hours. For optimal energy​ gathering, a house should be situated so it is facing south. In​ addition, these homes contain building materials and furnishings that best absorb solar energy. Examples of such materials are​ stone, brick, and concrete.
9. Buildings that make use of passive solar energy often have​ glass-enclosed areas. A​ glass-enclosed area functions much like a greenhouse. The area generally has walls and floors made from​ dark-colored, light-absorbing materials such as concrete and brick.
10. Notice the​ large, dark,​ water-filled barrel in the glass enclosure in Figure 17.1. Such barrels are used as​ energy-absorbing and storage structures. During the​ day, the barrels absorb energy and the water inside becomes heated. At​ night, when temperatures become​ cooler, the heat energy in the water is returned to the air in the room. At​ night, the heat energy in the water is dispersed. To get the greatest benefit from passive solar​ heating, good insulation is needed to prevent the escape of the captured energy from the house.​ Thick, heavy curtains or shutters also help prevent heat from escaping from a room when there is no sunlight.
11. In existing​ homes, passive solar energy may not be practical as a primary energy source. Such homes may require costly renovations or major construction to obtain the greatest benefit from solar heating.​ However, when new homes are​ built, passive solar heating should be considered. The University of Saskatchewan in Canada built a house using passive solar heating. During one​ year, the house saved​ $1,350 in fuel bills compared to an average house. Passive solar heating has the potential to significantly reduce fuel bills.
12. An active solar energy system has a greater capacity for storing and distributing energy than a passive system. In active solar​ heating, devices are used to​ collect, store, and circulate heat produced from solar energy. In a passive​ system, the building itself and its position act as the system. An active​ system, however, uses​ tubes, tanks,​ fluids, pumps,​ fans, and other devices to collect and distribute energy. These pieces of equipment can release the​ sun's energy to the system long after the sun has set. There are many components in an active solar energy system.
13. Buildings that use active solar energy gather the​ sun's energy by using solar collectors. Most solar collectors are mounted on the roofs of houses or buildings where they can capture a maximum amount of the​ sun's energy. Placement of the collector on roofs also saves space on the ground. It is more advantageous to place solar collectors onto roofs than on the gound.