April 19, 2009 US utility PG&E has this week announced it is seeking regulatory approval for a power supply deal that could see it buying power generated by solar satellites within seven years. If the proposal gets approval from regulators in its home state of California it will agree to a power purchase deal that from 2016 would see PG&E buy 200MW of renewable power over a 15-year period from space solar technology startup Solaren Corp.

We recently reported on plans announced by a company called Space Energy, Inc to create and launch a Space-Based Solar Power (SBSP) prototype satellite into low earth orbit (LEO). The company quoted test results from a recent experiment conducted for a Discovery channel documentary as proof that the wireless power transmission technology works. Having conducted our own research into the results of this test we discovered less than 1/1000th of 1% was successfully transferred between the transmitter and receiver locations only 148 kilometers (92 miles) apart.

Solaren was formed in 2001 by a team of satellite engineers and space scientists to build a space energy company to generate and distribute electricity at competitive prices from Space Solar Power (SSP) stations in geosynchronous orbit. Solaren currently consists of about ten engineers and scientists, but plans to grow to more than 100 over the next twelve months.

In an interview with PG&E's Next 100 blog, Solaren chief executive Gary Spirnak said that the underlying technology for the project was “very mature”. Spirnak admits that a Space Solar Power station has never been built before and put into geosynchronous orbit but says the underlying technology is based on communications satellite technology and is therefore very mature. For over 45 years, satellites have collected solar energy in earth orbit via solar cells, and converted it to radio frequency (RF) energy for transmissions to earth receive stations. This is the same energy conversion process Solaren uses for its SSP plant.

Goldstone wireless test

While we agree communication satellite technology is “very mature” we still have to question the proposed wireless power transmission technology as this has in fact never been demonstrated at any distance of more than 1.6 Kilometers (1 mile). The 'world record' for wireless power transmission over any distance at high efficiency was conducted back in 1975. Using a NASA deep space tracking dish at the Goldstone Deep Space Communication Complex, 30 kw was transmitted over 1.6 km (1 mile) at 82.5% efficiency. More recently in 1997 at Grand Bassin on Reunion Island 10 kw was successfully transmitted over 1 km (0.625 mile). It's quite a leap of physics to go from a 1.6 km test to Geostationary orbit at 36,000 km (22,500 miles). That and the fact there is not a single commercial application of long range wireless power transmission. Such a wireless power system may provide a solution to the current demands to expand the worlds electric power grids to accommodate new renewable sources of energy from wind and solar that are not always conveniently located near population centers.

Solaren claim that solar energy from space has an efficiency advantage. From space, solar energy is converted into radio waves, which are then beamed to Earth. The conversion rate of the RF waves to electricity, they claim, is in the area of 90 percent, citing U.S. government research efforts. The conversion rate for a typical earthbound nuclear or coal-fired plant, they claim, is in the area of 33 percent.

Free-space path loss

The conversion rate they are actually referring to is the maximum achieved in a lab for converting radio waves that land on the receiving antenna (called a retenna or rectifying antenna) back into usable energy. As all these 'lab' tests are conducted over extremely short range so they do not take into account free-space path loss which is proportional to the square of the distance between the transmitter and receiver and is also proportional to the square of the frequency of the radio signal. A radio signal 'spreads out' when it leaves the antenna according to Inverse-square law and as much as possible has to be captured by the antenna aperture at the receiving end to achieve high efficiency. Any energy that spreads out and does not make it to the receiving antenna is considered a loss of efficiency. Wireless power experiments to date have tired to get around these limits by using extremely focused antenna such as parabolic dishes and co-phased arrays but they still have yet to deal with loss due to distance as all such successful tests have only been over a mile or less.

The closest comparison to what Solaren envision is a DirecTV satellite. These transmit 250 watt TV signals down to earth with a satellite that is primarily solar powered. However, the TV signal that reaches the ground is very weak and direct to home services like DirecTV are only possible after the development of cheap mass produced low noise block down converters (LNB) for the receiving dish. Also, the DirectTV signals are beamed across the whole country to all its subscribers, while with the Solaren service for PG&E, the signal would be tightly focused, aimed at a receiving station in Fresno, Calif.

PG&E has not provided any funding itself for the Space based solar proposal and will only pay for energy once it is delivered. California law mandates that Pacific Gas & Electric must use renewable sources to produce 20% of its electricity from 2010 and beyond. To that end they have signed power supply contracts with many new renewable energy firms including announced plans with BrightSource to purchase up to 1,000 MW of solar thermal power. Some of these deals could be considered speculative as the companies have yet to raise funding or selected locations for their plants.

Solaren is now seeking “billions” in funding to design, build and launch their solar satellite into space, to operate the satellite and deliver the electricity to PG&E's grid. Having a large public utility such as PG&E interested in buying energy from Solaren may help in raising those funds from investors.

Paul Evans