Solar energy is essential to life on Earth. The energy from the sun keeps the Earth warm and supports life. The energy of wind also comes from the sun. Solar radiation heats up air. Warm air rises and surrounding air move in to fill its place. This movement of air is wind. The kinetic energy of wind can be used to generate electricity. Solar heating is also essential to the water cycle. Solar radiation heats up water in the sea, causing it to evaporate. When the water vapour cools and condenses, it falls as rain. When rainwater forms rivers or flows down from high places, the gravitational potential energy of the water can be used to generate electricity.
The amount of solar energy is unevenly distributed around the Earth and varies with time. Regions close to the equator receive much more sunlight than those at higher latitudes. In places of high latitudes, much more solar radiation is received in summer than in winter. The amount of cloud in the sky also affects the solar energy received in a place. We can use the heat from the sun for many purposes, for example, heating water, warming buildings, cooking food and providing a warm environment in greenhouses.
In Hong Kong, solar water heaters are used to heat water in some government properties. In Shek Kwu Chau Treatment and Rehabilitation Centre, solar water heaters are used to provide hot water for forty showers per day. Solar water heaters are also used in Sheung Shui Slaughterhouse to provide hot water for washing down the slaughter hall after slaughtering. At Shing Mun Valley swimming pool, solar water heaters are used as one of the energy sources for heating the pool. Most solar water heaters have two main parts: a solar collector and a storage tank. The collector absorbs radiation from the sun and heats the water circulating through it. The heated water is then stored in the storage tank for use as needed.
Solar collectors are the key component of solar water heaters. A typical flat-plate collector (Fig. 5) consists of an insulated metal box with a glass or plastic cover to let in sunlight. The collector inside has a large surface area and is usually dark in colour to facilitate the absorption of sunlight. The plate absorbs sunlight and warms the water passing through the flow tubes. A large number of flow tubes are used to increase the rate of heat conduction.
Besides heating water, solar energy can also be used to warm indoor air. In a typical solar heating system, water (or an antifreeze solution) is pumped through a collector. The heated water is stored in a well-insulated storage tank. The water can then be distributed throughout the house to warm the air.
Solar energy can also be used to heat water or food. Fig. 7 shows a homemade solar water heater. It mainly consists of a reflector and a metal container for holding water. The aluminium foil on the curved surface of the reflector concentrates sunlight onto the container. The container is painted black to enhance the absorption of radiation. The container is placed inside a sealed glass bottle to reduce heat loss through conduction and convection. The following video shows you the operation of this homemade solar water heater:
Do you want to construct a solar water heater on your own and see how well it works? You can do it in the following activity.
Solar cookers are particularly useful in developing countries where fuels for cooking are scarce. In these countries, people mainly rely on burning wood to cook food. However, the forests in developing countries are shrinking by more than 15 million hectares a year. Besides, the smoke from burning wood causes respiratory infections which kill at least 5 million young children each year. Solar cooking, which does not require fuel and produces no smoke and no pollution, is a perfect solution for these problems. In fact, research has found that 36% of the world's fuelwood needs could be saved by using solar cookers [1]. Therefore, promoting solar cooking could save millions of trees.
Solar heating keeps the Earth warm. But solar heating is not the only factor that affects the temperature on Earth. In fact, the Earth's atmosphere also plays an important role in keeping the Earth warm. Tiny amounts of greenhouse gases in the atmosphere help retain the heat on the Earth's surface. The greenhouse gases function like the glass of a greenhouse to keep the earth warm. A warming trend of about 0.5 oC has been observed on the average global temperature since the late 19th century. This observed trend is called global warming. Scientists generally agree that enhanced greenhouse effect contributes to the global warming phenomenon, although it is difficult to determine to what extent. The enhanced greenhouse effect is caused by emission of greenhouse gases like carbon dioxide, nitrous oxide, methane etc through human activities like the burning of fossil fuels, deforestation and agricultural and industrial activities. To understand more about greenhouse effect and global warming phenomenon, take a look at the following animation.
The enhanced greenhouse effect is mainly due to release of carbon dioxide and other greenhouse gases into the atmosphere. If we can burn less fossil fuels, use more renewable energy sources like solar energy, reduce deforestation and the emission of greenhouse gases in agricultural and industrial activities, we should be able to reduce the enhanced greenhouse effect and possibly help reduce global warming. The issues related to global warming and greenhouse effect are still controversial. References [2] and [3] may help you learn more on these issues.
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