1. INTRODUCTION: FUTURE ENERGY NEEDS
Mankind has recently enhanced its living standard and its population in an explosive way. In fact, the human population quadrupled and primary power consumption increased 16-fold during the 20th century [1]. The consumption of energy, food, and material resources is predicted to increase 2.5 fold in the coming 50 years. As a result of our efforts for better life, we have come to face, in this 21st century, serious global issues threatening our safe life or even our existence itself on our mother planet earth. These are issues such as global warming, environmental degradation, declining nutrition on land and sea from rising CO2, and rapid decrease of fossil reservoir. Since the living standard and the population of developing countries are increasing continuously, the demand of energy will be several times larger than that of today’s requirement by the time of the half way of this century.
In 2000, the world had 6.1 billion human inhabitants. This number could rise to more
than 9 billions in the next 50 years as shown in Fig.-1. This future population increase is mostly due to very rapid increase in less developed countries although the number in more developed countries will be almost constant (about 1 billion) or rather decrease [2].
Fig.-1 World Population Prospects [2]
The explosive increase in the human population inevitably requires an exponential increase in the consumption of energy, food, and material resources. One primary power source at present comes from fossil fuels such as oil, coal and natural gas. However, the fossil fuels have two serious factors which prevent them from being used for a long term as primary power source. One is their limited amount that does not last long if used with the same or higher pace than that of today (Fig.-2). The other is their negative feature of emitting carbon dioxide, one of the green house gases, which causes the global warming.
Fig.- 2 Pattern of Global Energy Dependence [3]
Fig.-3 Atmospheric carbon dioxide monthly mean mixing ratios. Data prior to May 1974 are from the Scripps Institution of Oceanography (SIO, blue), date since May 1974 are from the National Oceanic and Atmospheric Administration (NOAA, red). A long term trend curve is fitted to the monthly mean values [4]
Atmospheric CO2 has increased from 275 parts per million (ppm) before the industrial era begun to 379 ppm in March 2004 as shown in Fig.-3. Some scientists suggest that it will pass 550 ppm this century. Climate models and paleoclimate data indicate that 550 ppm, if sustained, could eventually produce global warming comparable in magnitude but opposite in sign to the global cooling of the last Ice Age [5].
Global energy demand continues to grow along with worldwide concerns over fossil fuel pollution, the safety of nuclear power and waste, and the impact of carbon-burning fuels on global warming. As a result sustainable energy sources like solar, wind, hydropower, biomass, geothermal, hydrogen, ocean thermal, tidal power etc are drawing prime attention, out of which solar power is the most promising one. Terrestrial solar power has too many limitations like atmospheric attenuation, daily and seasonal variation, and affects by climate conditions etc. To overcome these limitations concept of Solar Power from Space is getting momentum, which was first proposed by Czech-American engineer Peter Glaser as a solution to the oil crises of the 1970s [6]. Solar Power from Space is a proposed concept to place a gigantic solar power station in space orbiting around the earth that uses microwave power transmission to beam solar power to a very large antenna on earth where it can be used in place of conventional power sources.
2. SPACE SOLAR POWER (SSP) vs TERRESTRIAL SOLAR POWER (TSP)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
The SSP concept arose because space has several major advantages over earth for the collection of solar power. Space is free from day-night cycle, atmosphere, clouds, dust, rain, fog and other climatic changes, so it would receive 30% more intense and at least eight times more sunlight than that of at ground constantly and continuously unaffected by the weather. In geosynchronous orbit it would receive sunlight almost 24 hours a day hence avoiding the expensive storage facilities necessary for earth-based solar power systems. Since earthâs axis is tilted, it would be in earthâs shadow only for 70 minutes maximum at late night when power demands are at their lowest, during 42 days near the equinoxes [7] as shown in Fig.-5.
Fig.-5 Daily duration of eclipses as a function of the date [7]
3. SSP: SYSTEM DESIGN AND TECHNOLOGIES
The SSP system is composed of a space segment and a ground power receiving site (Fig.-6). Space segment consists of mainly three parts; solar energy collector to convert the solar energy into DC (Direct Current) electricity, DC-to-microwave converter, and large antenna array to beam down the microwave power to the ground. Ground power receiving site uses a device called rectenna (rectifying antenna) to receive and rectify the microwave power beam. The rectenna system converts the microwave power back to DC power which is then converted to conventional AC (Alternating Current), and is connected to existing electric power networks.
Assuming typical values for efficiencies like 15% for solar panels to convert solar energy into DC, 70% conversion rate in the space segment from DC to microwave, 90% beam (power) collection efficiency, and 80% conversion rate for rectenna from microwave to DC in ground segment, the estimated over-all efficiency is approximately 7.5 %. With such efficiency a SSP space segment would be of size of about 50 km2 (5 km x 10 km) to generate 5 GW DC power on earth (Fig.-6).
Fig.-6 : Reference Model: 5 GW GEO based Space Solar Power Station Designed by US Department of Energy (DOE) and NASA in 1979 [8]
3.1 -SOLAR CELL: EFFICIENT STRUCTURES
In the very near future, breakthroughs in nanotechnologies promise significant increase in solar cell efficiencies from current 15% values to over 50% levels. That might decrease required size of space segment by about 3 fold. Author proposes Metal-Metal junction cavity solar cell which theoretically promises to increase solar-electric conversion efficiency many folds.
A cavity of metal m2 (work function W2) with thin polish of metal m1 (work function W1, W1 <W2 , Fig.-7) on inner surface, with a pin hole is kept at the focus of the solar concentrator coinciding the pinhole and focus. Pinhole is covered with transparent glass to protect inner polish of cavity from atmospheric reaction. Such cavity behaves as metal-metal junction solar cell (termed as M-M cavity solar cell) with various features (described below) leading to enhancement of solar-electric conversion efficiency.
· The major loss in usual structures is the reflection loss (about 30%) but in M-M cavity solar cell once ray enters in cavity, undergoes multiple inner reflecti
s.sankar
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