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The Earth is a physical system with an energy budget that includes all  incoming energy and all  outgoing energy. The planet is approximately in equilibrium, so the sum of the gains is approximately equal to the sum of the losses. In order to use more total energy than presently and without raising the Earth’s average temperature, that additional energy must be compensated for by using only sources that would otherwise have been wasted. For example, fusion power is a possible source of new power and might exceed Earth’s energy budget, whereas increasing our use of solar voltaic energy is compensated by simultaneously cooling the land beneath the solar panels.

Incoming energy

The total rate at which energy enters the Earth’s atmosphere is estimated at 174 petawatts. This flux consists of:

• solar radiation (99.97%, or nearly 173 petawatts)
  This is equal to the product of the solar constant, about 1,366 watts per square metre, and the area of the Earth’s disc as seen from the Sun, about 1.28 × 1014 square metres, averaged over the Earth’s surface, which is four times larger. (That is, the area of a disc with the Earth’s diameter, which is effectively the target for solar energy, is 1/4 the area of the entire surface of the Earth.) The solar flux averaged over just the sunlit half of the Earth’s surface is about 680 W m−2
  This is the incident energy. The energy actually absorbed by the earth is lower by a factor of the co-albedo; this is discussed in the next section.
  Note that the solar constant varies (by approximately 0.1% over a solar cycle); and is not known absolutely to within better than about one watt per square metre. Hence geothermal, tidal, and waste heat contributions are less uncertain than solar power.
• geothermal energy (0.025%; or about 44to 47terawatts)
  This is produced by stored heat and heat produced by radioactive decay leaking out of the Earth’s interior.
• tidal energy (0.002%, or about 3 terawatts)
  This is produced by the interaction of the Earth’s mass with the gravitational fields of other bodies such as the Moon and Sun.

• waste heat from fossil fuel consumption (about 0.007%, or about 13 terawatts) The total energy used by commercial energy sources from 1880 to 2000 (including fossil fuels and nuclear) is calculated to be 17.3×1021 joules.

There are other minor sources of energy that are usually ignored in these calculations: accretion of interplanetary dust and solar wind, light from distant stars, the thermal radiation of space. Although these are now known to be negligibly small, this was not always obvious: Joseph Fourier initially thought radiation from deep space was significant when he discussed the Earth’s energy budget in a paper often cited as the first on the greenhouse effect.

Outgoing energy

The average albedo (reflectivity) of the Earth is about 0.3, which means that 30% of the incident solar energy is reflected into space, while 70% is absorbed by the Earth and reradiated as infrared. The planet’s albedo varies from month to month and place to place, but 0.3 is the average figure. The contributions from geothermal and tidal power sources are so small that they are omitted from the following calculations.

• 30% of the incident energy is reflected, consisting of
• 6% reflected from the atmosphere
• 20% reflected from clouds
• 4% reflected from the ground (including land, water and ice)

Earth’s longwave thermal radiation intensity, from clouds, atmosphere and ground

• The remaining 70% of the incident energy is absorbed:
• 51% is absorbed by land and water, and then emerges in the following ways:
  23% is transferred back into the atmosphere as latent heat by the evaporation of water, called latent heat flux
  7% is transferred back into the atmosphere by heated rising air, called Sensible heat flux
  15% is transferred into the atmosphere by radiation
  6% is radiated directly into space
  19% is absorbed by the atmosphere (16% by the air, 3% by clouds).

The Earth and its atmosphere are also radiant energy sources themselves. The atmosphere absorbs 90% of the energy radiated by the Earth, and radiates its own energy, 50% back towards the ground and 50% into space.

When the Earth is at thermal equilibrium, the absorbed and radiated energy are equal: 70% of the incident solar energy = 50% of the atmosphere’s radiation + 11% of the land+water radiation + 9% of the cloud’s radiation.

World human energy consumption for an  year is 15 terawatts . Each day, 89,000 terrawatts of solar radiation (energy) reaches the earth. In a year, this totals almost 32.5 million terawatts. Doing the math, 15 terawatts is a really, really, small percentage  of the energy the sun sends our way.  According to  calculations of Mark Z. Jacobsen ,a professor at Stanford, we would need: 3.8 million (5-mega watts) wind turbines; 720,000 (0.75-mega watts) wave devices; 5,530 (100-mega watts) geothermal plants; 900 (1300-mega watts) hydro plants; 490,000 (1-mega watts) tidal turbines; 1.7 billion (3-kilo watts) roof PV systems; 40,000 (300-mega watts) solar PV plants; and 20 (300- mega watts) concentrated solar panels plants.

Source(s): Digital Geography Wikipedia

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