# How Much Solar Energy Can We Harvest?

INTRODUCTION

The sun delivers about 7000 times more energy than we currently consume globally. However, we cannot cover the whole surface of the Earth with solar energy collectors. How much of this energy can we collect? Will it be enough to replace fossil fuels?

An obvious application of solar energy is to produce electricity. Global consumption of electricity is increasing. How much electricity could we produce if we used solar energy.

I have carried out calculations for various regions of the world using various values for the time when the sky is clear. These calculations allow me to estimate the minimum and the maximum of solar energy delivered by the sun in various parts of the world.

SOLAR ENERGY POTENTIALS

We have around 4 billion hectares of land in the world, which is not used for anything. Suppose that we could use up to 10% of this land to accommodate solar cells. How much energy would we expect to harvest?

To answer this question, we have to consider the minimum and the maximum annual clear sky irradiance (the intensity of solar radiation) measured in the number of watts per square metre. We also have to correct the calculations for the minimum and maximum of the annual average sky clearance. Using tabulated data (IPCC 2001), I have calculated the minimum and maximum solar energy potentials, P, i.e. the minimum and maximum energy delivered by the Sun to various regions of the world per year and globally. Results are presented in Table 1. Using the last two values in this table we can calculate that on average 10% of the so far unused global land area receives from the sun 32,227 exajoules of energy per year [EJ/y]. The minimum value is 15,393 EJ/y. Compare it with 463 EJ/y, which was the global consumption of energy in 2005 or with 645 EJ/y, the projected global consumption of energy in 2020 (see The Little Green Handbook).

Table 1. Solar energy potentials assuming that 10% of unused land area can be covered by solar cells A — Total surface area of the currently unused land in billion hectares (Gha). R — Annual clear sky solar irradiance (the intensity of solar radiation in watts per square metre (W/m2)). f  — Annual fraction of time when the sky is clear. P — Solar energy potential (the energy delivered by the Sun to the 10% of the currently unused land) expressed either in in trillion watt-hours per year (TWh/y) or in exajoules per year (EJ/y). Example:Assuming that we can use 10% of the currently unused land area of 0.5940 Gha in North America for solar cells, the minimum solar energy available in this fraction of the total area would be 1813 exajoules per year [EJ/y] and the maximum 7418 EJ/y depending on the annual irradiance (R) and the fraction of the time the sky is clear (f). As a point of reference, our global annual consumption of energy in 2005 was 463 EJ/y.

Another way to calculate the average annual solar potential is to use the average solar radiation intensity of 342 W/m2 and consider that on average only 58% of it reaches the Earth's surface (see Solar Radiation). If we use these figure we shall find that 10% of the so far unused land area receives on average 24,605 exajoules of energy per year [EJ/y], which is close to the previously calculated average potential. The corresponding figure expressed in units that can be compared with our global consumption of electricity is 6,834.808 TWh/y.

The amount of energy we can harvest and use depends on the efficiency of solar cells. Solar cells' efficiency is improving but let us assume the lowest value of only 10%.

Our global consumption of electricity in 2005 was 15,182 TWh/y (see The Little Green Handbook). However, 9,541 TWh/y of electricity was produced by fossil fuels and 2,555 TWh/y by nuclear power, or the total 12,096 TWh/y. Assuming the lowest solar potential and the lowest efficiency of solar cells we can calculate that we could produce 35 times more electricity than produced by fossil fuels and nuclear power. The additional advantage is that solar power is clean and last practically forever.

Even if we used only 1% of unused land area we could produce nearly 4 times more electricity than we produce using fossil fuels and nuclear power. I should remind that this is the lowest limit. With better efficiency of solar cells and a higher average irradiance we could produce more electricity. The surplus of solar energy could be used to replace fossil fuels in transportation and reduce further our emissions of carbon dioxide.

Copyright You may use the information contained in this article as long as you refer to it as Nielsen, R. 2005, 'How Much Solar Energy Can We Harvest?', http://home.iprimus.com.au/nielsens/.

For additional information and a discussion of all critical global trends shaping our future see The Little Green Handbook.

Reference

IPCC 2001, Summary for Policy Makers: Climate Change 2001: Impacts, Adaptation, and Vulnerability, IPCC, Geneva, Switzerland.