Category: Photovoltaics

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.

Solar Energy: The Unstoppable Transformative Technology

As most readers of this blog will know solar energy comes in two broad categories: photovoltaics (PV) and concentrated solar power (CSP). The latter category includes concentrated solar thermal power (as in parabolic troughs, …) and concentrating photovoltaics (CPV). This blog will focus on PV; concentrated forms of solar energy will be discussed in a subsequent blog.

PV is a now a well-known and widely deployed form of renewable energy in which radiation from the sun is converted directly into electricity via panels of solar (or PV) cells. They can be roof-mounted or ground-mounted, as shown below, or used in many other ways to provide smaller amounts of electricity to handheld calculators, roadside telephones, battery chargers, remote microwave relay stations, solar lanterns, water pumping, and numerous other applications. It is a modular technology that can be scaled up in kW size as needed. It also lends itself to integration with various building and other materials – e.g., as roof tiles, building facades, blankets, clothing, and other flexible materials. There is an extensive and rapidly growing literature on PV – one hardly knows where to start. One useful starting point I would recommend is
http://wwww.eia.gov/kids/energy.cfm?page=solar_home-basics-k.cfm
Another useful source of information is the web site of the Solar Energy Idustries Association: http://www.seia.org

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Roof-mounted PV

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Ground-mounted PV

There are two energy technologies that I consider transformative (some people prefer the term ‘disruptive’), i.e., they change the way we generate and use electricity. These are fuel cells, which use hydrogen as a ‘fuel’ to generate electricity, water and heat (and will be discussed in a future blog), and PV, the focus of this blog.

PV is transformative because it can be used wherever the sun is shining (e.g., in space to power satellites and space stations, and even on Mars to power robotic vehicles), it can generate power where it is needed without the need for power lines, it is modular, and its cost is coming down significantly as more and more PV is manufactured. Our infrastructure is already highly dependent on PV – think about satellites used for wireless telephony and GPS, and terrestrial PV that increasingly is supplying electricity to individual homes and businesses as well as utilities.

i would also note that our use of terrestrial PV is only beginning. An industry that started in 1973 in the U.S. (PV had been used earlier for space applications) now employs more than 120,000 people in the U.S., will add more than 4 gigawatts (yes, I said gigawatts) in the U.S. alone in 2013, on top of 8.5 GW already installed in the U.S. and 102 GW worldwide. Global additions in both 2011 and 2012 totaled 31 GW, and PV today is, annually, a multi-billion dollar industry and growing.

The above discussion clearly indicates that PV is an unstoppable energy technology, as the German electric utilities have learned and U.S. utilities will eventually learn as well. The problem that PV presents to utilities is its decentralized nature and the fact that PV generation is maximum at peak periods of electricity demand when utilities are used to charging higher than average kWh prices. If this peak demand on the utility systems is reduced by home- or business-generated electricity then utility revenues are adversely affected based on current utility business models.

It seems clear that this business model will have to change, and, based on experience, that utilities will resist this change as long as they can. The German utilities faced this problem first because the German government introduced a feed-in-tariff (FiT) for PV in the 1990’s, stimulating a massive deployment of PV in Germany ever since. Today Germany leads the world in PV deployment with about 30 GW installed. I would even note that on one very sunny summer day last year more than half of Germany’s electrical demand was met by PV. When faced with this reality German utilities got into the PV business and are now even offering energy storage services to the German public.

The U.S. federal government has not yet seen fit to offer a FiT to the American public but several states are taking the lead in stimulating PV and other renewable energy use. U.S. utilities are clearly behind the German curve and some are resisting the new PV reality by making hookup to the grid unnecessarily complicated, by proposing extra charges for homes that install PV and battery storage systems, and not incorporating PV into their own generating systems. This will change, hopefully sooner rather than later, as utilities take advantage of these new business opportunities.