07-02-2013, 11:30 AM
SOLAR ENERGY THROUGH SOLAR SPACE STATIONS
SOLAR ENERGY.doc (Size: 445 KB / Downloads: 41)
ABSTRACT:
Conventional energy sources will last only up to the middle of the next century due to enhancement in living standard and explosive population growth. The current primary energy source –fossil fuel threatens global environment by emitting CO2, which is responsible for green house effect. Solar Power from Space (SSP) promises clean and everlasting energy supply for the growth of the mankind. With the state-ofart technologies yet developed, SSP is feasible but not economic. Key issues regarding SSP are launch cost, efficiency of solar cells, wireless power transmission, heavy investment, maintenance etc. Yet proposed models suggest to place space segment in GEO, LEO or at lunar surface, which are either economically infeasible due to huge launch cost or require long setup time. This paper presents a mutual collaboration based solar power model to overcome launch cost, which involves three GEO satellites and two earth stations. Next major breakthrough required is efficiency of solar cells, to reduce launching weight and size for SSP to be economic. Yet stateof- art solar cells have efficiencies only about 15%. Author proposes Metal-Metal junction cavity structure solar cell, which theoretically promises to increase solarelectric conversion efficiency many folds. Rest of the paper discusses fetter’s analysis about economic feasibility of SSP, issues regarding microwave power transmission and influences of SSP on environment and existing life.
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 . 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 .
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. In this paper we proposes Metal-Metal junction cavity solar cell which theoretically promises to increase solarelectric conversion efficiency many folds. A cavity of metal m2 (work function W2) with thin polish of metal m1 (work function W1, W1 <W2 , Fig.-4) 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 reflections till completely absorbed.
• Quantum efficiency curve for all types of single incidence photoelectric conversion
processes w.r.t wavelength as shown in Fig.-5. Due to this, usual structures are not efficient over whole spectral range. There is not such a problem in M-M cavity cells as all the photons are completely absorbed.
WIRELESS POWER TRANSMISSION:
Power transmission from the satellite to the rectenna on ground is made by microwave beam using DC to microwave converters like Klystron, in ISM (Industrial
Scientific and Medical) band at 2.45 GHz or 5.8 GHz emitted from the spacetenna with retrodirective beam control capability. The bandwidth of beam is quite narrow since an essentially monochromatic wave is used without modulation. Spacetenna directs the microwave power beam to ground based rectenna site, which transmits a pilot signal to enable spacetenna to locate rectenna site. The spacetenna is constructed as a phased-array antenna, which is a large number of dipoles attached to the same flat surface. Each one is fed energy from the transmitter. The relationship between spacetenna diameter (D1), rectenna diameter (D2), wavelength (ë) and spacetenna to rectenna distance (S) is given by (Fig.-7).(D1*D2)/(e*s)=2.8
A typical size of the rectenna site is 4 km in diameter for the transmitting antenna diameter of 1km operating at 5.8 GHz. Under these conditions, 93% of the transmitted power is collected. The peak microwave power density at the rectenna site results in 27 mW/cm2 if the Gaussian power profile on the transmitter is assumed. The beam intensity pattern has a non-uniform distribution with a higher intensity in the center of the rectenna and a lower intensity at its periphery as shown in Fig.-8. It is noted that the safety requirement for the microwave power density for human is set to 1mW/cm2 in most countries. The power density satisfies this requirement at the periphery.
CONCLUSIONS:
This paper has examined the aspects of space-based power for the earth, the significance, the need and possible solutions and finds that
• Present energy-supply practices cannot continue indefinitely
• Economic development and a growing population will, by the middle of the next century, combine to require at least twice the energy we use today.
• Importation of solar energy from space could make up the shortfall in an environmentally-benign way.
• Initial studies and technology demonstrations have shown that space-based energy supplies are technologically feasible but not competitive yet with current state-of-art technologies.
• Author proposes cost effective mutual collaboration based model for solar power.
• Paper presents Metal-Metal junction cavity structure solar cell, which theoretically promises to increase solar-electric conversion efficiency many folds.
The conclusion is that space-based power can provide the necessary new source of clean energy for earth in the next century to loosen our dependence on fossil fuels. An international effort will be needed for any space-based system and many segments of the world's business and technical communities must be involved.