The Solar Resource

Solar energy is energy produced within the body of the Sun and then radiated into space. This energy from the Sun is generated during nuclear fusion reactions that take place inside the body of the star, within the core, a region that occupies about one-sixteenth of its volume. Within this region the temperature and pressure are high enough to promote fusion reactions between protons (hydrogen nucleii), which produce helium nucleii, at the same time releasing large quantities of energy. This emerges in the form of high-energy gamma radiation.

The temperature inside the core of the Sun is around 15,000,000 K, and the amount of energy produced is 3.86×10²⁷ Joules/second, which corresponds to the conversion of 600 million tonnes of hydrogen each second. However, because of complex interactions, the gamma ray photons carrying this released energy can take over a million years to pass through the outer layers of the Sun toward the surface. By the time they reach the Sun’s surface, the temperature has cooled to 6000 K, the temperature that can be registered by measurement from the Earth. The energy emitted from the surface of the Sun amounts to an average of close to 230 million W for each square meter. While this may appear extreme, the conditions found within the Sun are similar to many stars in our galaxy (the Sun is classified as a yellow dwarf).

The energy emitted from the surface of the Sun is mostly in the form of ultraviolet, visible, and infrared radiation, as well as massless, chargeless particles called neutrinos. The energy is radiated in all directions and its intensity diminishes as the distance from the Sun increases—that is, as the density of solar rays becomes smaller. At the distance of the Earth from the Sun, which averages 149,600,000 km, the amount of solar radiation passing through a square meter perpendicular to the direction of the Sun’s rays is, according to the most recent estimates, 1361 W/m².¹ This figure is commonly known as the solar constant.

At the point at which the radiation from the Sun reaches the Earth, before it enters the Earth’s atmosphere, the light waves fall mostly within the wavelength range of 200 nm (ultraviolet) to 2500 nm (infrared). At this stage its composition is roughly 56% infrared radiation, 36% visible radiation, and 7% ultraviolet. The remaining 1% is found mostly at longer wavelengths.

Not all of this radiation reaches the surface of the Earth, as illustrated in Fig. 2.1. Some of it will be scattered by dust and molecules within the atmosphere. Scattering of this sort is a random process, sending the radiation in all directions. Some will be scattered back into space while the remainder will fall toward the Earth’s surface as diffuse radiation. More radiation is reflected back into space by clouds, and these play an important role in regulating the temperature of the atmosphere and of the Earth’s surface.

Another part of solar radiation is absorbed by atoms and molecules in the atmosphere. Nitrogen and oxygen absorb short wavelength ultraviolet radiation, thereby blocking radiation with a wavelength shorter than 190 nm. Oxygen molecules can split into atoms by absorbing short wavelength radiation, leading to the production of ozone, which absorbs slightly higher wavelength ultraviolet radiation. Meanwhile, water vapor, carbon dioxide, and molecular oxygen also absorb in the infrared region of the spectrum. When all these effects are taken into account, the flux of direct radiation that reaches the Earth’s surface is roughly 1050 W/m² perpendicular to the direction of the radiation. When the additional scattered radiation reaching each square meter at ground level is added to this, the total flux is 1120 W under optimum conditions. By this stage the composition of the radiation is roughly 50% visible radiation and 47% infrared (Table 2.1).

Other factors will also modify the intensity of radiation reaching the surface of the Earth. One important factor is the rotation of the Earth. When the Sun is directly overhead at midday, the radiation has the least distance to pass through the atmosphere before it reaches any specific area of the ground. However, as the Earth rotates that same area on the ground away from midday, the distance through the atmosphere that the rays must pass increases. At the point at which the Sun appears to set from that point on the Earth, and no direct radiation reaches the surface there, the distance through the atmosphere that the last rays must travel is at its maximum. Beyond sunset there is no further direct radiation, but there is still scattered radiation that continues through evening until night falls and no sunlight remains.

All these factors reduce the amount of energy reaching the Earth’s surface. As a consequence, the average solar flux on each square meter of the Earth—taking into account the fact that each square meter is in darkness for part of the time—is 170 W/m². The highest flux, found near the Red Sea in the Middle East, is 300 W/m². This is less than one quarter of the flux equivalent to the solar constant.

The effects of the atmosphere on solar intensity are quantified in terms of a factor called the Air Mass (AM) factor. AM0 corresponds to the intensity of the sunlight at the edge of the Earth’s atmosphere. AM1 represents the intensity of sunlight at the surface of the Earth when the Sun is directly overhead, at the zenith, without any cloud cover. It therefore represents the attenuation caused by the column of air between a square meter on the Earth’s surface and a square meter at the edge of the atmosphere. A standard intensity that is used when testing solar cells is AM1.5. This is the typical solar intensity found at midday in many of the main population centers of the Earth. It corresponds to 1000 W/m².

Even with all the attenuation and absorption effects discussed above, the quantity of solar energy reaching the Earth is enormous. The total solar flux reaching the disk of the Earth is 1.08×10⁸ GW, while the total amount of energy that falls onto the Earth from the Sun each year is 3,400,000 EJ, or between 7000 and 8000 times global primary energy consumption; the solar energy falling on the Earth in one hour would supply current annual energy demand. Only a tiny proportion of this would be required to provide the 5000 GW to 6000 GW of electricity generation currently available across the globe. It is for this reason that solar energy is considered to be the best and most valuable source of renewable energy, and the resource most capable of displacing fossil fuels in a carbon-emission-free world.

As already noted, the solar radiation that reaches ground level on the Earth is of two types: direct radiation and diffuse radiation. The latter is produced when direct radiation is scattered and reflected as it passes through the atmosphere. Vegetation can absorb both types of radiation, allowing photosynthesis and plant growth to continue under all light conditions. Planar solar cells can also absorb both direct and diffuse solar radiation. However, solar thermal power plants and concentrating solar photovoltaic technologies both require direct solar radiation in order to operate effectively. This limits their application to regions where there is low average annual cloud cover. When there is no cloud cover, between 80% and 90% of the direct radiation passing through the atmosphere will reach the surface as direct radiation.

Intermittency

While the solar resource is vast, it is also intermittent. The Sun shines during the day but it does not do so at night. This diurnal variation means that solar input can usually only be relied on for part of the 24-hour daily cycle (see Fig. 2.2). In addition, the intensity of the solar radiation falling at any given point will vary by the minute and by the hour depending upon the time of day and weather conditions, such as the amount of cloud cover, as shown in Fig. 2.3.

This means that on its own solar energy cannot provide a continuous source of power anywhere in the world. In order to make energy continuously available, either another source of energy must also be provided to back up the solar power, or a means of storing solar energy is needed. Both approaches are in use today.

While many grids have significant solar inputs, it is rare that the solar energy will be sufficient to supply all users on the grid, so there will normally be a range of other sources of electricity available, some renewable—such as wind energy—and others based on fossil fuels. A range of energy storage systems are also capable of storing solar energy when the Sun is not shining. There is limited use of these on grid systems today, but they are much more common in off-grid solar power systems where batteries are charged with solar energy during the day for use overnight. The development of cheap storage systems is expected to be one of the key technologies that will enable all renewable technologies—especially solar and wind power—to provide a large proportion of the global electricity supply in the future.

Distribution of Solar Energy

The orientation of the Earth with respect to the Sun means that solar energy that falls on the Earth is not evenly distributed. More falls on land and sea close to the equator, while less falls in the more northerly and southerly regions of the Earth, toward the poles. Global solar energy distribution maps show that the regions with the highest levels of solar irradiation are found across Africa and the Middle East into Iran, Afghanistan, and parts of India; in Australia; in the southern regions of North America, particularly Mexico and the southwestern United States; and in some parts of South America, including Brazil and the western coastal desert regions. This can be seen in Fig. 2.4, which shows the global long-term average horizontal radiation across the globe. Global horizontal radiation is the total amount of solar irradiation, both direct and scattered, that falls on a defined area at ground level. Fig. 2.5 shows a similar map of the global direct radiation.

The solar distribution affects planning and strategizing for a future in which fossil fuels are no longer widely used to generate electricity, and solar energy becomes a major resource replacement. In the United States, for example, the use of the southwestern region (which has high insolation) as a major source of solar electricity generation to supply the whole the country has been discussed. Europe has limited solar resources but close by, across the Mediterranean Sea, North Africa has vast quantities of solar energy. This energy could be captured using solar power plants, and then the electricity generated could be shipped through transmission lines across the Mediterranean Sea to Southern Europe. Proposals along these lines have also been discussed.

Other major energy-consuming regions will also be looking at solar potential. India has some regions with good solar resources, particularly in the northeast, and these could be exploited in an effort to replace coal, which is currently the main source of electricity in the country. China also has the potential to generate power from solar energy in the country’s southwestern region, and in some more central regions too.

To put solar potential into perspective, consider a group of solar thermal power plants built in the U.S. state of California during the late 1980s and early 1990s. These power plants were pioneering solar plants, and used solar reflectors to capture the Sun’s energy and the heat to drive a steam generator. The solar stations—which incidentally are still in operation in the second decade of the 21st century—were designed on the basis that the site could provide 2725 kWh/m²/year, or 22.75 GWh/year for each hectare of land. Assuming that this incident energy could be converted with around 10% efficiency (modern solar plants hope to achieve better than this), then 2 million hectares or 20,000 km²—a land area of roughly 150 km by 150 km—would be able to supply all the electricity for the entire United States.