under the sun

This article includes the situation in Singapore and the comparison with other countries.

Singapore being tropically located with vast amount of sunny days, hasn’t there been any consideration to tap this free energy for residence consumption, i.e. installing solar (photovoltaic) panels on external walls of high rise apartments, tapping this energy for use in cooling system (air-cons), etc.

 The Sun’s energy is an enormous and constant energy resource, but because of the earth’s protective atmosphere only a small amount of the total energy produced by the sun reaches earth. Astronomers have determined that the sun’s energy has remained relatively constant over the last century and this “solar constant” will continue to be 1.36 kilowatts per square metre (+-3.5%) for about the next four billion years. The incident solar radiation (insolation) received at any particular location on the Earth’s surface may vary between 0 and 1.05 kilowatts per square metre depending on the latitude, the season, the time of day, and the degree of cloudiness.

Solar energy has always been an alternative renewable energy source. There are only two primary disadvantages to using solar power: amount of sunlight and cost of equipment. The amount of sunlight received by external walls of a high-rise building varies greatly depending on direction, time of day, time of year and weather conditions. Hence, solar equipment are usually installed on the roof and not on the vertical walls. While solar energy technologies have made huge technological and cost improvements, they are still more expensive than traditional energy sources.

Compared with subtropical desert regions, Singapore receives less than two-thirds of the solar radiation and much lower sunshine values. Mean daily sunshine hours for he different months of the year vary from 33 to 55 per cent of the maximum possible. February, March and July with 6.2 hours of bright sunshine receive the highest and November and December with 4.5 and 4.4 hours, respectively, the lowest. On the average, solar radiation is at maximum in February (484.4 milliwatt-hr per square centimetre) and March (489.6 milliwatt-hr per square centimetre) while a secondary maximum is experienced in September (438.6 milliwatt-hr per square centimetre) thereafter drop below 400 milliwatt-hr per square centimetre in November (376.3 milliwatt-hr per square centimetre) and December due to the more frequent occurrence of overcast skies during the end of the year period. 

Solar energy can be used primarily in three ways: passive solar heat, active solar heat, and photovoltaics (also called solar cells or PV). The first two energy sources involve collecting the heat produced by the Sun for use in heating living or working space, or hot water. Photovoltaics uses the light produced by the Sun (or any light source) to generate electricity directly. Sunlight striking a photovoltaic or solar cell causes a voltage and current to be created in a semiconductor that can be used just like the electrical energy from a battery or DC generator.

In US, a typical residence can be served with a 4 kilowatt (kW) photovoltaic system. Because Photovoltaic system produces power only when the Sun is shining, an independent residential system must include batteries to store energy when the Sun is not shining. Often a residential system includes a backup generator to provide additional assurance that power will always be available. PV system requires sufficient roof area to provide the necessary power output for the house. The cost of a complete residential system, independent of the utility system, is about US$25,000 to US$40,000.

For solar cooling, there are both active and passive cooling techniques that can be used. A passive solar system uses the sun-facing walls or windows of a house as collector and natural means of heat transfer. The principle behind active solar cooling is much the same as for a gas-fired refrigerator. Active systems are expensive and technically elaborate and so are not generally used in homes.

references: http://www.science.edu.sg/ssc/detailed.jsp?rtid=375&type=6&root=2&parent=2&cat=21

 mindy