Energy is the most ingredient for the growth of a civilization. The more energy we can harness and use; the better are our living conditions. Throughout our history we’ve been on the quest for energy. We’ve progressed from using muscular power of other animals, burned wood, boiled steam and now use fossil fuels.
The past three decades have seen a rapid economic growth in India. This growth is being powered by coal & fossil fuels. The downside of a coal powered growth is a suicidal destruction of our habitats due to climate change. We have already started feeling the detrimental effects of pollution, global warming. Things can only get worse from here. Hence, there is an urgent need to find a source of clean limitless energy to meet our aspirations of being a developed country.
SPACE SOLAR POWER
The Sun is the solar systems cleanest energy source. It is the only functional Nuclear Fusion reactor. It has operated for several billion years and has been the source of near limitless energy on earth. Some of the popular renewable energy sources such as hydroelectric and wind are derived from the Sun.
On the earth’s surface, Solar Power is limited; the sun shines for only half a day; cloud cover, rains and time of day limit the amount of solar energy available. But in the Geo-Synchronous orbit; 36,000KM above our heads; there are no day/night cycles, there are neither clouds nor rains and the solar flux is about 10 times higher than on the earth’s surface. This fantastic energy from the sun is referred to as Space Solar Power.
The space solar power concept entails capture of this energy in space and beaming it to earth using microwave beams. This concept was first developed by Dr.Peter Glaser[i] and has since been studied rigorously by various space agencies and individuals[ii],[iii]. All of these studies have pointed out the technical feasibility of the concept. But this system is yet to be launched due to cost and other factors.
Problems preventing the immediate use of Space Solar Power
Existing Space Solar Power concepts have not been launched due to the following limitations in their designs:
- A mass of 10,000-20,000 tons to produce 1GW of energy.
- High satellite launch cost. Launching 1Kg to Geo-Synchronous orbit using PSLV/GSLV costs ~₹6,00,000/-. Launching a 20,000 ton satellite would cost ₹1,20,00,00,00,00,000 (12 lakh crores). In comparison, an ultra large coal power plant costs ₹12,600/- crores.
- Regular repair, replacement and maintenance of fragile, massive space structures.
- High cost of space hardware. E.g.: Solar cells, high power microwave transmitters.
From the list of limitations, it is obvious that a big decrease in launch costs and space hardware costs are required to make space solar power satellite a reality.
Space solar power concepts have evolved rapidly during each energy crisis. The first version of the satellite entailed building a monolithic structure with a mass of over 100,000 tons. Such a gargantuan proposal could not gather support due to the enormous economic cost of such a project.
The most practical Space Solar Power concept is the SPS-ALPHA (Solar Power Satellite via Arbitrarily Large Phased Array) by Artemis Innovation[iv]. It breaks down the Space Solar Satellite into 8 discrete repeatable modules which are much easier to launch, assemble and replace. In its current form, the SPS-ALPHA would weigh around 20,000tons. This is still very heavy. A practical Space Solar Satellite must be lighter in order to be feasible.
In this article, we present the concept of a Space Solar Satellite called CASSP, which is built using a product of nanotechnology; Single Walled Carbon Nanotubes. It is substantially lighter and cheaper. By adding a 9th discrete module called communication module, the Space Solar Satellite would double up as a powerful communication satellite thus generating significant additional revenues.
Nanotechnology is the field of engineering that deals with objects, at least one of whose dimensions is of the order of a nanometer (10-9m). This is close to the size of an atom. This finesse in understanding allows engineers to create new materials and components that exhibit properties that are desirable for us.
SINGLE WALLED CARBON NANOTUBE (SWCNT)
One of the most famous products of Nanotechnology research are Carbon Nanotubes. They are tiny tubes of Carbon atoms that are about 1,00,000 times smaller than a human hair. They were first identified by Dr.Sumio Ijima of Japan in 1991[v].
There are 2 types of Carbon Nanotubes: 1. Single Walled Carbon Nanotubes (SWCNT) and 2. Multi Walled Carbon Nanotubes (MWCNT). As their names suggest. An SWCNT is made of a Single tube of Carbon atoms as shown in figure 1. Depending on their diameters, SWCNT exhibit either semiconducting or metallic properties. Their properties can be precisely known and are the subject of this article.
SWCNT have attracted a lot of attention due to their incredible material properties. They are the strongest material known to mankind[vi], they are excellent electrical and heat conductors[vii] [viii], are the best field emitters, are inert and radiation resistant[ix]. This makes SWCNT a perfect material for use in space applications.
SWCNT based products that use these properties have been successfully developed in laboratories around the world[x]. Their path to markets has been limited due to high cost of SWCNT. The main reason for the high cost of Single Walled Carbon Nanotube has been the notorious difficulty in purifying the material. This problem was solved recently by NoPo Nanotechnologies (NoPo) through the development of a proprietary continuous process that produces very high quality nanotubes at very low cost.
This breakthrough in production has made nanotube based electronic products practically viable. A Solar panel made of NoPo SWCNT would cost less than 2 US$ per square meter in Nanotube costs. Since carbon nanotubes can be semi-conducting or metallic based on their diameter. A complete solar cell could be fabricated using only SWCNT. Nanotubes are inherently radiation resistant. So there is no need for additional coatings. Due to their high tensile strength, they could withstand micro-meteor impacts with minimal additional reinforcements.
The Carbon Nanotube based Space Solar Power (CASSP) concept builds on top of the highly practical modular SPS-ALPHA concept. CASSP replaces some of the modules of SPS-ALPHA with much lighter SWCNT derivatives.
Carbon Nanotube Solar Cells
A p-n junction diode exhibits the best possible photovoltaic effect. Single walled Carbon Nanotube are the only material that exhibit behaviour of an ideal diode. A 0.8nm diameter SWCNT with a band gap of 1.4eV would show peak absorption in the solar band. Such a cell is calculated to have an efficiency exceeding 35%[xi]. This cell would produce 2W/gram and a square meter of it would weigh just 230grams including mechanical supports!
The theoretical Carbon Nanotube Solar Cell is compared with a commercial Space Solar cell in Table 1.
|Commercial Space Solar Cell||SWCNT Solar Cell|
|Base Material||GaInP/GaAs/Ge on Ge substrate||Metallic SWCNT/Semi conducting SWCNT|
An SPS-ALPHA Solar cell Module using existing technology is estimated to weigh 15-20Kg for a 4m diameter Hexagon pod. An SWCNT solar cell module occupying the same area would weigh ~3kg and generate 6.3kw of energy.
The Hexbus module that provides support for the solar cells as proposed in SPS-Alpha weighs 25Kg and is made of a hollow tubular structure to provide rigidity and wiring lines. Due to the flexibility of the CASSP solar modules. This can be fabricated with 3-5mm thick and 20mm wide aluminium (Alloy 1060H12) flats which would weigh just 3Kg for a 4m diameter pod.
Carbon Nanotube Electron Field Emitter
Microwaves are generated by passing electrons through specially designed cavities within a magnetic field. Microwave generation was first perfected during the 2nd World War. Most of the commercial Microwave generators continue to use the same designs.
The most critical components of a Microwave generator is a thermionic electron emitter. This consists of a filament that is heated to over 2000K. Electrons thus generated are accelerated into a cavity by a large Voltage of several kilovolt (kV). The hot electrons rapidly heat up the Microwave generator, thus requiring continuous active cooling. This heating results in low efficiency while operating continuously.
SWCNT are the best electron emitters. Their emission starts at Voltages of just 1-2V/µm. They can sustain currents of >150A/cm2 [xiii] making them an ideal material to make microwave generators.[xiv] This could lead to a higher efficiency. The low heat generation would allow for lighter composites to be used to fabricate the Microwave tube cavity.
Usage of Carbon Composites in place of steel would lead to a factor of ten reduction in weight. The cold emission would reduce heating substantially and minimize cooling requirements. Assuming a factor of 5 reduction in weight and a nominal 10% increase in efficiency to 60%. The weight per Watt of energy transmitted is (1g/W).
These field emitters have been used commercially as electron sources for portable X-Ray diffraction equipment.
|Solar Power Generation Module||15-20Kg||~3Kg|
|Reflector & Structural Beam||~125-150Kg||-NA-|
|Wireless Power Transmission Module||~50Kg||~10Kg|
|Propulsion Control Module||50-500Kg||10-100Kg|
ECONOMIC VIABILITY OF CASSP
CASSP is intended to be used in areas where energy costs are very high and worth paying for. Islands are one such area. As a case study, let’s consider Andaman & Nicobar Islands of India. These islands are strategically located between mainland India, Singapore, Australia, Malaysia and Thailand. It has a huge potential to be a major trading hub but is held back due to lack of any energy resource.
Almost all the energy on the islands is produced by burning diesel and wood.[xv] This would be a perfect test bed for deploying space solar power. The cost of energy generation on these islands is between US$0.5-US$1/Unit and is directly subsidized by the Government of India.
The current installed capacity of the islands is 35MW and this can barely meet the energy requirements for lighting. The annual energy consumption in kilowatt-hours is 306 million units. The Annual energy bill is ~US$153-306 million USD.
Assuming a 50MW initial CASSP satellite to meet the energy requirements on the island. The potential energy consumption is 438 million units. The potential revenues per annum are US$219-438 million.
From Table 1. The cost of SWCNT based Solar panels for generating 50MW of energy is USD 5 million.
Each CASSP module is capable of delivering 6.3kW. Due to losses in microwave conversion. We must produce 40% more power than what is received on earth. So this system would require ~11200 solar generator modules weighing 33.6 metric Tons.
From the previous discussion on Carbon nanotube field emitters. A Carbon nanotube powered Klystron would weigh 1/5th of a commercial source at 1g/W of energy radiated. A 50MW Klystron would weigh 50 metric Tons. A magnetron producing 2kW costs ~US$50. Assuming a 5-10x cost increase for Klystron. This would cost ~USD125-250 per kW of energy transmitted. The Klystron Amplifier would cost ~12.5 million USD.
Interconnects, propellant, micro-robots etc…are expected to weigh an additional 30 metric Tons. This brings the total satellite weight to around 113.6 metric Tons.
Ground systems are expected to cost ~US$50 million. They would be deployed in small diverse clusters in order to meet energy needs of each individual island.
The system would be launched abroad an emerging Heavy lift launcher such as SpaceX Falcon Heavy which is expected to launch in 2015[xvi]. From various estimates, a single launch on SpaceX heavy would cost US$135 million. The total launch cost is therefore US$270 million.
Thus the total cost for deploying CASSP is US$337.5 Million. With expected revenues in the range of US$219 – 438 million. CASSP can pay for itself within 2 years of operation.
SPIN-OFF REVENUE SOURCES
A CASSP satellite would be located in the geosynchronous orbit. It would already have a powerful microwave transmitter to beam energy. By adding additional signal processing hardware, the satellite could be used as a powerful communication satellite with minimal additional cost. A communication satellite with a 25kW rated power costs about $300million to build and launch. An SPS satellite could relay these communication signals along with power transmission by piggybacking on the main signal. This would provide a dual use of both data and power further enhancing the value of the satellite. A CASSP satellite with a communication satellite module could generate annual revenues of more than US$120Million.[xvii]
A space solar power satellite is more realistic than ever due to the emergence of Carbon nanotechnology. A satellite can be built, launched and made economically viable using technologies that have just begun emerging from laboratories. The most feasible deployment of SPS would be on an island with a large growth potential. If we were to invest in this technology, we could realize its fruits before the end of this decade. The biggest proponent of Space Solar Power was our former president Dr.A.P.J.Abdul Kalam. Launch of the first satellite before 2020 would be a fitting tribute to his efforts in making India a developed country.
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[xiv] Kenneth B. K. Teo and others, ‘Microwave Devices: Carbon Nanotubes as Cold Cathodes’, Nature, 437 (2005), 968–968 <http://dx.doi.org/10.1038/437968a>.
[xv] ‘New Page 2’ <http://electricity.and.nic.in/> [accessed 15 June 2015].
[xvi] ‘Falcon Heavy | SpaceX’ <http://www.spacex.com/falcon-heavy> [accessed 15 June 2015].
[xvii] ‘Satellite Industry Report Shows Satellite Industry Growth of 7% in 2012 – SpaceRef Business’ <http://spaceref.biz/organizations/satellite-industry-association/satellite-industry-report-shows-satellite-industry-growth-of-7-in-2012.html> [accessed 15 June 2015].
Gadhadar Reddy is co-founder and CEO of NoPo Nanotechnologies (I) Private Limited. At NoPo, he successfully developed technology to produce highly consistent, high quality Single Walled Carbon Nanotubes which has enabled real world products that can make use of the materials’ incredible properties. The technologies being developed at NoPo are all designed so that they can be used in the construction of a Space Solar Satellite. He strongly believes that Space Solar Power is the stepping stone needed for human’s to tap resources from space.
He is a space enthusiast at heart. He has a diploma in Space Sciences, a Bachelor’s degree Electronics & Communication Engineering, a Master’s in Molecular Sciences & Nanotechnology (USA) and thinks of his startup as his PhD project.
This is derived from an award winning paper, “CARBON-NANOTUBE BASED SPACE SOLAR POWER (CASSP)” authored by Gadhadar, Narayan Prasad and Divyashree which won the 4th International Space Solar Power Competition.