WIRELESS POWER TRANSMISSION

BACKGROUND (1899-1990)
Nikola Tesla
l1856-1943
lInnovations:
–Alternating current
–Wireless power transmission experiments at Wardenclyffe.
Wardenclyffe
1899

Able to light lamps over 25 miles away without using wires
High frequency current, of a Tesla coil, could light lamps filled with gas (like neon).
lWorld War II developed ability to convert energy to microwaves using a magnetron, no method for converting microwaves back to electricity
l1964 William C. Brown demonstrated a rectenna which could convert microwave power to electricity.
l1968’s idea for Solar Power Satellites proposed by Peter Glaser
–Would use microwaves to transmit power to Earth from Solar Powered Satellites
lIdea gained momentum during the Oil Crises of 1970’s, but after prices stabilized idea was dropped
–US Department of Energy research program 1978-1981.
lConstruct the satellites in space
–Each SPS would have 400 million solar cells.
lUse the Space Shuttle to get pieces to a low orbit station
lTow pieces to the assembly point using a purpose built space tug (similar to space shuttle).
Advantages over Earth based solar power
lMore intense sunlight
lIn geosynchronous orbit, 36,000 km (22,369 miles) an SPS would be illuminated over 99% of the time
lNo need for costly storage devices for when the sun is not in view
–Only a few days at spring and fall equinox would the satellite be in shadow.
Problems
lIssues identified during the DOE study
–Complexity—30 years to complete
–Size—6.5 miles long by 3.3 miles wide
lTransmitting antenna ½ mile in diameter(1 km).
NASA Fresh Look Report
lSPS could be competitive with other energy sources and deserves further study
lResearch aimed at an SPS system of 250 MW
lWould cost around $10 billion and take 20 years
lNational Research Council found the research worthwhile but under funded to achieve its goals.
lPossible power generation of 5 to 10 gigawatts.
“If the largest conceivable space power station were built and operated 24 hours a day all year round, it could produce the equivalent output of ten 1 million kilowatt-class nuclear power stations.

Microwave Power Transmission
From the Satellite
lSolar power from the satellite is sent to Earth using a microwave transmitter
lReceived at a “rectenna” located on Earth
lRecent developments suggest that power could be sent to Earth using a laser.
Rectenna
“An antenna comprising a mesh of dipoles and diodes for absorbing microwave energy from a transmitter and converting it into electric power.”
lMicrowaves are received with about 85% efficiency
lAround 5km across (3.1 miles)
l95% of the beam will fall on the rectenna.
Rectenna Design
lCurrently there are two different design types being looked at
–Wire mesh reflector
lBuilt on a rigid frame above the ground
lVisually transparent so that it would not interfere with plant life
–Magic carpet
lMaterial pegged to the ground.

l10 MW satellite delivering microwave power
–Will not be in geosynchronous orbit, instead low orbit 1100 km (683 miles)
–Much cheaper to put a satellite in low orbit
–200 seconds of power on each pass over rectenna.
Power to Mobile Devices
lIf microwave beams carrying power could be beamed uniformly over the earth they could power cell phones
lBiggest problem is that the antenna would have to be 25-30 cm square.
lSince a low orbit microwave beam would spread less, the ground based rectenna could be smaller
lWould allow collectors on the ground of a few hundred meters across instead of 10 kilometers
lIn low orbit they circle the Earth in about every 90 minutes.
ISSUES
lWould require a network of hundreds of satellites
–Air Force currently track 8500 man made objects in space, 7% satellites
lWould make telecommunications companies into power companies.
RELIABLITY
lGround based solar only works during clear days, and must have storage for night
lPower can be beamed to the location where it is needed, don’t have to invest in as large a grid.
lA network of low orbit satellites could provide power to almost any point on Earth continuously because one satellite would always be in range.
lPossible health hazards
–Effects of long term exposure
–Exposure is equal to the amount that people receive from cell phones and microwaves
lLocation
–The size of construction for the rectennas is massive
CONCLUSION
lMore reliable than ground based solar power
lIn order for SPS to become a reality it several things have to happen:
–Government support
–Cheaper launch prices
–Involvement of the private sector.