Solar energy is a virtually inexhaustible and freely available energy source. More sunlight (~1.2 X 105 TW) falls on the earth's surface in 1 h than is used by all human activities in 1 year globally. The sun is earth's natural power source, driving the circulation of global wind and ocean currents, the cycle of water evaporation and condensation that creates rivers and lakes, and the biological cycles of photosynthesis and life. It is however a dilute energy source (1 kW/m2 at noon, Chapter 2); about 600-1000 TW strikes the earth's terrestrial surfaces at practical sites suitable for solar energy harvesting.27 Covering 0.16% of the land on earth with 10% efficient solar conversion systems would provide 20 TW of power,28 nearly twice the world's consumption rate of fossil energy and an equivalent 20,000 1-GWe nuclear fission plants. Clearly, solar energy is the largest renewable carbon-free resource amongst the other renewable energy options.
Consider the total amounts possible for each in the light of the 14-20 TW of carbon-free power needed by 2050. Table 5 provides a summary;26 clearly the additional energy needed per year over the 12.8 TW fossil fuel energy base is simply not attainable from biomass, wind, nuclear and hydroelectric options. The answer to this supply dilemma must lie with solar energy. Chapter 2 provides an overview of the solar energy resource with particular emphasis on the solar spectrum.
Solar energy can be harnessed in many ways25 but three routes of particular relevance to the theme of this book rely on electrical, chemical, and thermal conversion. Thus the energy content of the solar radiation can be captured as excited electron-hole pairs in a semiconductor, a dye, or a chromophore, or as heat in a thermal storage medium. Excited electrons and holes can be tapped off for immediate conversion to electrical power, or transferred to biological or chemical molecules for conversion to fuel. Solar energy is "fixed" in plants via the photosynthetic growth process. These plants are then available as biomass for combustion as primary fuels or for conversion to secondary fuels such as ethanol or hydrogen. All of these possibilities are addressed in more detail in the Chapters that follow.
While there is tremendous potential for solar energy to contribute substantially to the future carbon-free power needs, none of the routes listed above are currently competitive with fossil fuels from cost, reliability, and performance perspectives. Photovoltaic solar cells have been around for decades and have been widely deployed in space vehicles. Terrestrially, their utilization thus far has been limited to niche applications or remote locales where less expensive electricity is not available. Costs for turnkey installations were 6-10 times more expensive in 1999 for solar electrical energy than for electricity derived from coal or oil. The present cost of photovoltaic (PV) modules is ~$3.50/peak watt. Considering the additional balance of system costs (land, maintenance, etc.) this translates to an energy cost of ~$0.35/kWh. The target at present is ~$0.40/peak watt corresponding to electricity at $0.02/kWh or H2 produced by PV hybrid water electrolyzers at $0.11/kWh. Major advances in electrolyzer technology could bring this hydrogen cost to $0.04/kWh,29 which is about the present cost of H2 from steam reforming of natural gas. These issues are further elaborated in Chapters 2, 3, and 9. A cost goal of $0.40/peak watt requires solar photovoltaic conversion at a total cost of $125/m2 combined with a cell energy conversion efficiency of ~50%. Such combinations of cost and efficiency require truly disruptive photovoltaic technologies. Many such approaches are being actively pursued in research laboratories around the world. A critical discussion of outstanding issues, including dispelling the seven myths of solar electricity may be found in Refs.25, 29, and 30.
The economic outlook for the other two solar approaches is not much rosier, at least at present. Solar fuels in the form of biomass produce electricity and heat at costs that are within the range of fossil fuels, but their production capacity is limited. The low efficiency with which plants convert sunlight to stored energy means large land areas are required. To produce the full 13 TW of power used by the planet, nearly all the arable land on earth would need to be planted with switchgrass, the fastest growing energy crop. Artificial photosynthetic systems, however, are more promising (see next Section) and these are discussed in Chapter 6. Solar thermal systems provide the lowest-cost electricity at the present time, but require large areas in the Sun Belt in the U. S. and continuing advances in materials science/engineering.
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