1. INTRODUCTION: FUTURE ENERGY Wants
Mankind has recently enhanced its living standard and its population in an explosive way. In truth, the human population quadrupled and primary power consumption increased 16-fold throughout the 20th century [1]. The consumption of energy, food, and material resources is predicted to improve 2.5 fold in the coming 50 years. As a result of our efforts for far better life, we have come to face, in this 21st century, serious global problems threatening our secure life or even our existence itself on our mother planet earth. These are problems such as global warming, environmental degradation, declining nutrition on land and sea from rising CO2, and rapid decrease of fossil reservoir. Because the living standard and the population of developing countries are growing continuously, the demand of energy will be many times bigger 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 much more
than 9 billions in the next 50 years as shown in Fig.-1. This future population boost is mostly due to quite rapid enhance in much less developed countries despite the fact that the number in more developed countries will be practically constant (about 1 billion) or rather decrease [2].
Fig.-1 World Population Prospects [2]
The explosive improve in the human population inevitably demands an exponential increase in the consumption of energy, food, and material resources. 1 primary power source at present comes from fossil fuels such as oil, coal and natural gas. Nevertheless, the fossil fuels have two severe factors which stop them from being employed for a long term as primary power source. One is their limited quantity that does not last long if utilized with the identical or higher pace than that of these days (Fig.-2). The other is their negative feature of emitting carbon dioxide, 1 of the green home gases, which causes the global warming.
Fig.- 2 Pattern of Global Energy Dependence [3]
Fig.-three Atmospheric carbon dioxide monthly mean mixing ratios. Data prior to Could 1974 are from the Scripps Institution of Oceanography (SIO, blue), date since Could 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) prior to 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 1. Terrestrial solar power has too a lot of limitations like atmospheric attenuation, every day and seasonal variation, and affects by climate conditions etc. To overcome these limitations idea of Solar Power from Space is getting momentum, which was very first proposed by Czech-American engineer Peter Glaser as a answer 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 quite large antenna on earth where it can be used in location of conventional power sources.
2. SPACE SOLAR POWER (SSP) vs TERRESTRIAL SOLAR POWER (TSP)
The SSP idea arose because space has numerous key advantages over earth for the collection of solar power. Space is totally free from day-night cycle, atmosphere, clouds, dust, rain, fog and other climatic changes, so it would receive 30% much more intense and at least eight times far more sunlight than that of at ground continuously and continuously unaffected by the weather. In geosynchronous orbit it would obtain sunlight virtually 24 hours a day hence avoiding the costly storage facilities needed for earth-based solar power systems. Since earth’s axis is tilted, it would be in earth’s shadow only for 70 minutes maximum at late night when power demands are at their lowest, throughout 42 days near the equinoxes [7] as shown in Fig.-5.
Fig.-5 Daily duration of eclipses as a function of the date [7]
three. SSP: SYSTEM DESIGN AND TECHNOLOGIES
The SSP system is composed of a space segment and a ground power receiving website (Fig.-6). Space segment consists of mainly three parts solar energy collector to convert the solar energy into DC (Direct Present) electricity, DC-to-microwave converter, and huge antenna array to beam down the microwave power to the ground. Ground power receiving web site uses a device known as 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 Present), 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 Created by US Department of Energy (DOE) and NASA in 1979 [8]
three.1 -SOLAR CELL: EFFICIENT STRUCTURES
In the really near future, breakthroughs in nanotechnologies promise significant enhance in solar cell efficiencies from present 15% values to over 50% levels. That may possibly decrease needed size of space segment by about three fold. Author proposes Metal-Metal junction cavity solar cell which theoretically promises to boost 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 several 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