Heat engines use a difference in temperature to do work, all else being equal. Obviously the temperature in a near vacuum as you have in space is very cold and a few degrees above absolute zero in deep space. Without the protection of an atmosphere as we have on earth solar radiation could create high temperatures on non-reflective surfaces especially if you focus solar radiation from a larger area using polished reflective surfaces onto that non-reflective surface.
You also need some way for thermal energy to be removed from a heat engine and released into its environment. One way you could do this in space is by relying on thermal radiation by having a very hot point on the satellite that would radiate EM energy over a wide direction much like a light bulb at a frequency and rate related to the temperature of that point . That would probably be quite impractical and silly.
One idea that might be worth investigating is whether you could use the thermal energy from focusing solar radiation in space on one location on a satellite to provide the energy needed to pump a laser on that satellite. “The sun has been used to pump lasers.” (http://en.wikipedia.org/wiki/Laser_pumping, 2/10/2008). If you could do that then you could aim that laser at another relay satellite and move the energy to where it could eventually be changed into electrical potential and used. Such a satellite would have a high efficiency and it could be used to extract usable energy from a wider spectrum of solar radiation as compared with using photovoltaic solar panels. Such solar collector satellites would probably also be cheaper to mass produce once the technology matures.
A solar collector satellite might use similar ideas that are used for high temperature terrestrial Concentrated Solar Power stations. A number of mirrors or reflective surfaces focus a large area of solar radiation onto a relatively small surface that is thus raised to a high temperature. Some liquid medium is used to transport the thermal energy in that collector to a heat engine where that energy can be converted into other more useful forms such as electricity. On a solar collector satellite most of the thermal energy could be used to pump a laser.
Sodium was proposed as a high temperature medium with a high heat capacity for use in nuclear power reactors so maybe a similar kind of medium could be used in solar collector satellites. You could even use a heat engine to pump the medium from the collector to the laser in the satellite so that when thermal energy is used up in the laser the on-board heat engine pumps hotter sodium or whatever around to the laser and returns the cooler medium to the collector, and so on. The laser would perhaps be fired in pulses so with such a feedback system the on-board pump may also pulse the flow of the medium through the satellite, which might seem a little strange to start off with. There would also have to be handshaking protocols with relay satellites using parallel low power signal lasers so that the high energy lasers only fire when it is safe to do so. It might also be an idea to make sure that the solar collector and relay satellites have quite a few safety features to make sure that a damaged satellite would not be able to fire its high powered laser.
These solar collector satellites would need to be compact when being launched into space so you could imagine the reflective panels opening up on the sattelite like petals of a flower once they are in position.
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4/10/2008
This approach might be most suitable for harnessing solar energy in space.
The solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth’s atmosphere in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter (W/m²),[1] though this fluctuates by about 6.9% during a year (from 1412 W/m² in early January to 1321 W/m² in early July) due to the earth’s varying distance from the Sun, and by a few parts per thousand from day to day.
http://en.wikipedia.org/wiki/Sunlight, 4/10/2008
A circular area on a plane perpendicular to the sun’s rays with a radius of just over 15 meters would catch an average of about a megawatt of solar radiation. The actual reflective surfaces could be thin and light. Depending on the orbit, a solar collector satellite with reflecting panels could provide a constant supply of energy. The efficiency would have to be determined experimentally, although I suspect it may be quite high compared with PV solar cells and even compared with heat engines on earth.
Over the course of a year the average solar radiation arriving at the top of the Earth’s atmosphere is roughly 1366 [1] watts per square meter (see solar constant). The radiant power is distributed across the entire electromagnetic spectrum, although most of the power is in the visible light portion of the spectrum. The Sun’s rays are attenuated as they pass though the atmosphere, thus reducing the insolation at the Earth’s surface to approximately 1000 watts per square meter for a surface perpendicular to the Sun’s rays at sea level on a clear day.
The actual figure varies with the Sun angle at different times of year, according to the distance the sunlight travels through the air, and depending on the extent of atmospheric haze and cloud cover. Ignoring clouds, the average insolation for the Earth is approximately 250 watts per square meter (6 (kW·h/m²)/day), taking into account the lower radiation intensity in early morning and evening, and its near-absence at night.
http://en.wikipedia.org/wiki/Insolation, 4/10/2008
Solar power installations on earth have much less energy available to them and the efficiency of PV cells is quite low while heat engines have their efficiency limited by operating around livable temperatures, nearly 300K above absolute zero. It is certainly worth testing this idea. One problem might be efficiently extracting energy from the laser beams from these solar collector satellites.
There might have to be a number of stages to transport the energy from space to earth. You could imagine a number of high altitude drone aircraft that could take a laser from space-based satellites, change the energy into a more suitable frequency and then beam that down through denser atmosphere to a power station on the ground. With regards to weather conditions, if some power stations were under clouds or inaccessible from the sky then you could plan to send the energy to another power station connected by the grid to that inaccessible power station and then transmit the energy via conventional transmission lines.
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A solar collector satellite could best be described by analogy as a funnel that collects the solar radiation over a certain area and where that energy is focused onto a small area, perhaps with parabolic reflective surfaces. The main problem is designing a laser that could be pumped by the kind of energy that would be available on such a satellite.
Gas Dynamic Laser (GDL) is laser based on differences in relaxation velocities of molecular vibrational states. The laser medium gas has such properties that an energetically lower vibrational state relaxes faster than a higher vibrational state, thus a population inversion is achieved in a particular time.
Pure Gas dynamic lasers usually use a combustion chamber, supersonic expansion nozzle and CO2 as a active laser medium in mixture with nitrogen or helium. Gas dynamic laser could be however pumped not only by combustion, but by any adiabatic expansion of gas. Any hot and compressed gas with appropriate vibrational structure could be utilized.
It can be hard to tell at a quick glance whether something on the internet is genuine or a hoax. But perhaps you could use the energy from solar collectors in space to heat and compress a gas for use in a high powered laser. Such a laser system would go through a cycle similar to a thermodynamic cycle in a heat engine. The laser gas medium is heated by the concentrated solar energy then it could be compressed to raise its temperature further; the gas could then be discharged to create the laser beam if this approach is feasible. The spent laser gas medium could then be removed from the laser cavity, moved back to where it can be heated by the concentrated solar energy, compressed once again, and so on – once again, if such a cycle were possible. The collected solar energy would be used to heat the gas and to provide the energy through heat engines or whatever to do the mechanical work and maintain the satellite. The cycle has to conserve all of the laser gas medium so the laser beam would have to be pulsed; discontinuous but periodic, unlike combustion gas lasers. There might be other ways to go about it. All the viable options need to be properly researched and tested.
And finally, this is very early in the development of these kinds of space-based energy systems so at first they may not look that impressive. Obviously you would aim to have solid state or at least a static gaseous laser medium. If you could have a solid state laser medium that would lase within a high temperature range that would be acheivable on such a satellite, such a laser could perhaps be used to pump another laser medium.
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You could probably design a compact reflective collector surface with a frame design similar to compact umbrellas and with light reflective and sturdy foil to span the foldable frame. It doesn’t have to be perfect in design, it just has to reflect most of the solar radiation onto a central location on the satellite.
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6 December 2008
It may prove difficult to line up the relvenat satellites, but another option for moving energy around in space is to have a satellite with something like a large lens in between a solar collector satellite and a relay satellite. A laser or microwave beam will become diffuse the further it has to travel. You could have a series of relay satellites located at a suitable distance, before a laser beam becomes too diffuse and spreads so that a lot of the energy is wasted. Another option is to let the laser beam become diffuse and spread after travelling over a longer distance and then place a largish ‘lens’ satellite somewhere in the middle between the transmitter and receiver satellites. If you could also direct the beam in a slightly different direction, as you would with a mirror in optics, that would be even better. You could have far greater distances between relay satellites with such a scheme and the laser beam when it reaches the receiver relay satellite could be focused, narrow and crisp. It may be very difficult to line up the relevant satellites unless you use relatively stable locations such as Lagrangian points as nodes or unless you can also direct the beam in a slightly different direction. Perhaps it could be used as a way to send energy to remote probe spacecraft if the idea about sending energy between satellites with lasers is feasible. Perhaps a space probe to the outer solar system could leave a few of these lens-like satellites at specific locations as it travels further away from the sun. There might then be only a few times a year when the satellites line up and we could beam some extra energy to the probe, but that might be enough to keep it going for decades far beyond the outer edges of the solar system.
These ‘lens’ satellites could also serve like an infrastructure of communications satellites to relay software commands to probes, as well as energy, and information back to earth. Such a system for the solar system beyond earth’s orbit around the sun could make exploration probe missions easier and more systematic. If energy can be relayed to probes they could be built larger and packed with more equipment while the missions could last longer.
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