Category: Solar energy

Solar Power Satellite Systems: A Viable Option?

In earlier blogs I’ve commented on Solar PV and Concentrating Solar Power. Here I will comment on Solar Satellite Power Systems (SSPS). As proposed, such systems would use electricity generated by a collection of solar PV panels in geosynchronous orbit (i.e., an orbit above a fixed point on earth) to power a microwave generator. The generated microwaves would be beamed through the atmosphere to a ground-mounted receiver (‘rectenna’) that would convert the microwaves to electricity that would be distributed to consumers via the terrestrial grid.

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This concept first received NASA attention and review in the 1970’s which raised a number of issues which still remain problematic. A small group of SSPS enthusiasts still promote the technology but broad support is lacking.

The obvious advantage of SSPS is its access to unimpeded radiation from the sun without the interference of clouds or atmospheric absorption and scattering. This is partially offset by the need for the microwaves to pass through the atmosphere to the rectenna but presumably a microwave frequency would be chosen with minimal atmospheric absorption. It should also be noted that every step of SSPS is technically feasible and well established – solar conversion to electricity, microwave generation, microwave transmission through air, microwave collection and conversion to electricity, and grid transmission.

Personally, I am not a supporter of federal investment in the technology for the following reasons:

– putting anything into orbit is expensive, very expensive, and until these costs are reduced significantly SSPS will not be cost competitive.
– economics dictate that large SSPS concentrations (100’s to 1,000’s of MWe’s) be placed in orbit. One suggestion I recall is to place a 10 GWe unit in geosynchronous orbit to supply the electrical needs of New York City. In my opinion this is crazy – putting all your eggs in one highly vulnerable basket.

These vulnerabilities include exposure to higher-than-usual radiation levels in space which will shorten expected equipment lifetimes, possibility of collisions with space debris and micrometeorites, ordinary technical failures (with a lot of electricity potentially at risk), and vulnerability to sabotage/attack in the event of international tensions.

– aircraft will need to avoid the beams passing through the atmosphere to avoid any possible impacts to humans from exposure to relatively high strength microwave signals. Birds will be another potentially impacted species.

– the large land areas required for rectennas which would ideally be located in close proximity to cities with large electricity demand.

So, is SSPS a viable option for future electricity supply? Not in the near- to mid-term in my opinion. Long-term may be a more optimistic story. Solar PV costs are now much lower than they were just a few years ago and going down, radiation resistance of solar cells and microwave generating equipment may be improved, the cost of insertion into geosynchronous orbit will hopefully come way down, and small SSPS units (100-300 MWe) may become practical to be considered. The other problems would remain, and terrestrial competition from other renewable electric technologies will increase.

In a time of limited federal budgets R&D investment in SSPS does not strike me as a prudent use of government funds. Nevertheless, I recognize that SSPS has its core of ardent supporters (several came to my office while I worked at DOE) and I hope some of them will comment on this blog with their own views.

Concentrating Solar Power: A Viable Option For Desert Regions

It has been reported for more than 2000 years that Archimedes used mirrors to concentrate sunlight and set Roman ships afire during the seige of Syracuse in 213BC. While much evidence has been presented to refute this claim, it is probably too powerful a legend to die. Nevertheless, the legend supports the saying heard often in the early days of modern solar energy that if solar had been a weapon of war it would have been fully developed by now.

Following the Arab Oil Embargo of 1973-74 and increased U.S. interest in energy issues, the U.S. Department of Energy started a concentrating solar power project called Solar One. It involved hundreds of ground-mounted reflecting mirrors, called heliostats, that followed the sun and directed their sunlight to a water receiver at the top of a 400-foot centrally-located tower.

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The heated water was converted into steam and fed into a steam-turbine electricity generator. Construction of Solar One was completed in 1981 and was operational from 1982 to 1986. It was then redesigned to incorporate molten salt (60% sodium nitrate; 40% potassium nitrate) as the thermal collection and storage medium and relabeled Solar Two. The redesign was needed to address the instability of Solar One when sunlight was disrupted by passing clouds. Solar Two was successfully tested at 10MWe using molten salt but operation was eventually discontinued in the mid 90’s when the industry was unwilling to share further development costs with DOE. The Solar Two tower was eventually demolished in 2009; the heliostats are now being used for astronomy research.

CSP also comes in two other ‘flavor’s, parabolic trough and dish-Stirling, both of which are discussed in the attached PowerPoint (‘Concentrating Solar Power’) that I presented in 2010 to a meeting of utility executives. I did so for two reasons, to make sure the executives were familiar with CSP (which had been of limited visibility for a number of years) and to catch up on the current state of the technology which was beginning to reappear.

Concentrating Solar Power

I will end this blog by emphasizing one of CSP’s major advantages over intermittent renewable energy sources such as PV and wind – it comes with storage. The major barriers to its greater utilization are its requirement for unscattered (direct normal) radiation (you can’t focus scattered sunlight), cost, and the need for cooling (water or air). Deserts, which usually have few clouds and therefore little scattering of sunlight, are natural venues for CSP power plants. Unfortunately, deserts are also known for their lack of water. These issues are discussed in the PowerPoint.