Lynne Kiesling
Wired has some interesting, short articles right now on power technologies: wireless power transmission by converting electricity to lasers or microwaves and then re-converting them to electricity, broadband over power lines (BPL) deployment, and the effect of wind farms on wildlife, particularly birds in the Altamont Pass in California.
Here’s one question I have about the wireless power technology: how much energy does it take to transform the power to microwaves and back? Is any lost in the transmission? Those two questions make a big difference in how energy efficient the technology is, as well as whether it’s likely to be economically feasible.
Allow me to speculate…
A problem with microwave radiation is that it tends to spread out as it travels unless highly focused, almost like a laser. For communication this isn’t such a big deal, as your antenna needs only to be able to receive the information encoded in the signal as opposed to recouping all of the energy sent by the transmitter. But if you’re transmitting energy, your collector needs to capture as much of the beam as possible. So, the alternative would seem to be highly focused radiation with moderate-sized gathering equipment. The less focused the radiation, the larger the receiver needs to be. So, we would expect to see a highly concentrated microwave beam to practically carry a large amount of energy. Well, now you have a problem because the radiation is dangerous and will be cooking birds, and perhaps other things, all along its path. Actually I have no idea whether there are practical ways to concentrate a microwave beam enough for this to have any practicality as a real transmission alternative across open spaces.
Lasers can stay highly focused in a vacuum, but it has proven difficult to reliably send large amounts of energy through the real atmosphere. Otherwise, the Star Wars defense system might have been practical.
D.O.U.G.,
Years ago, Arthur D. Little proposed a system of solar satellites which would collect energy in geosynchronous orbit and deliver the energy to earth as a tightly focused microwave beam. Then, someone remembered the Raytheon’s original name for the first microwave ovens – “RadarRange”.
It would not take much focusing error, particularly from orbit, to begin applying heat where it wasn’t needed or wanted. I also suspect the microwave approach would rapidly turn a cold fog into a warm fog.
Current high voltage power transmission incurs losses of approximately one percent. I seriously doubt that the conversion to and from microwave energy could approach this level of loss.
D.O.U.G. too
Looking at my bookshelf:
From ‘Colonies In Space’, T.A. Heppenheimer, 1977 Warner Books, we have in Chapter 3: Power from Space, pg41, the following-
Efficiency:
Amplitron dc-to-microwave:
~90%: (demonstrated?)
Schottky diodes + antenna arrays microwave-to-dc:
54%: demonstrated
60-70%: theoretical
Later in the chapter, pg45, the author puts the cost of a ground-orbit-ground power relay system as equivalent to that of conventional power transmission lines over several thousand miles, and puts the transmission cost as low as seven tenths of a cent per kilowatt hour.
Somewhat antiquated numbers, but something to work with.
There’s also a forward by Ray Bradbury “E is for Electricity, KP is for Knowledge Problem”
OK, I made that title up.
I should say, the atmosphere is almost perfectly transparent to microwaves, no number given in Heppenheimers book, but it’s low.
So, if we assume 99% transmission for each pass through the atmosphere, then a back of the envelope calculation for best overall system efficiency is –
e = 0.62% ( 0.9 * 0.99 * 0.99 * 0.7 )
The original Solar power station author, Peter Glaser, did indeed work for Arthur D. Littleton.
NASA did a lot of work on this stuff in the 1970’s and 1980’s as well – I can remember piles of reports in my college library. I imagine some of that is on line by now.
As an aside, the beam energy level for power transmission is not anywhere near high enough to melt tanks or set a city on fire, as sometimes is seen in “Sim Earth”.