As the world faces an impending dearth of fossil fuels, most immediately oil, alternative sources of energy must be found. 174 PW worth of energy falls onto the top of the Earth's atmosphere in the form of sunlight which is almost 10, 000 times the total amount of energy used by humans on Earth, as taken from all sources, oil, coal, natural gas, nuclear and hydroelectric power combined. If even a fraction of this could be harvested efficiently, the energy crunch could in principle be averted. Various means for garnering energy from the Sun are presented, including photovoltaics (PV), thin film solar cells, quantum dot cells, concentrating PV and thermal solar power stations, which are more efficient in practical terms. Finally the prospects of space based (satellite) solar power are considered. The caveat is that even if the entire world electricity budget could be met using solar energy, the remaining 80% of energy which is not used as electricity but thermal power (heat) still needs to be found in the absence of fossil fuels. Most pressingly, the decline of cheap plentiful crude oil (peak oil) will not find a substitution via solar unless a mainly electrified transportation system is devised and it is debatable that there is sufficient time and conventional energy remaining to accomplish this. The inevitable contraction of transportation will default a deconstruction of the globalised world economy into that of a system of localised communities.
Keywords: photovoltaics, Graitzel cells, dye-sensitized solar cells, solar energy, solar power, quantum dots, thin film cells, satellites, peak oil, concentrating solar power, thermal solar power, space-based solar power
1.1 The problem of energy
The world now uses a grand total of 18 TW (18 terawatts) of energy which amounts to 568EJ (568 exajoules=5.6 x [10.sup.20]J) over the period of a year, as provided from all fossil fuels combined (oil, gas and coal), plus nuclear and hydro (hydroelectric) power along with the meagre fraction of the total that is provided by renewables (1). The breakdown (2) of these various contributions is given in Table 1, and is presented more visually in Figure 1 as a bar chart. One sees immediately that almost two-fifths of total energy is derived from crude oil, and about one quarter each from coal and natural gas. Nuclear and hydro-power each contribute around 6%, while renew- able energy from all sources, wind, wave, geothermal, wood, solar, etc. amount to less than 1%. These figures refer specifically to the year 2004, when the world got through 471 EJ of energy, and while the relative proportion of each contributing energy source has remained fairly constant, it is clear that at a current 568 EJ, the demand placed on these energy sources by humans rises relentlessly (Figure 2). There are two aspects to be considered: firstly, the fossil fuels and uranium too (used for nuclear power) are in finite supply, since they were laid down by the forces of geology millions of years ago, and evidence is that their production will peak within the near future. Crude oil in particular is predicted to peak at some point during the next 5 years. According to Figure 3, we are at the peak of oil production now-"peak oil", beyond which world supplies of oil will decline forever (3). The "oil-age" is coming to an end. There is much debate over just how much coal and gas there is left to be garnered, but the evidence is that the easily extracted forms of both will plateau in production within this century. Secondly, the C[O.sub.2] produced by burning fossil fuels is thought to contribute to global-warming and climate-change (4,5). Thus both on grounds of resource-limitation and carbon emissions, alternative, ideally renewable sources of energy are needed.
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2. Solar energy
Figure 4 summarises the quantity and fate of solar radiation striking the top of the Earth's atmosphere (1,2). …