December 04, 2008
Solar Thermal Cheaper Than Solar Photovoltaics
Regulations requiring utilities to use more renewable power provides an incentive for the construction of solar thermal electric power generation sites. Solar thermal has a cost advantage over silicon-based photovoltaics.
Costing about 18 cents a kilowatt-hour at present, solar thermal power is roughly 40% cheaper than that generated by the silicon-based panels that sit on the roofs of homes and businesses, according to a June report by Clean Edge Inc. and the Co-op American Foundation. Analysts say improved technology and economies of scale should help lower the cost of solar thermal to about 5 cents a kilowatt-hour by 2025. That would put it on par with coal, the cheap but carbon-spewing fuel that generates about half the nation's electricity.
Should we attach much credence to cost projections for solar thermal? Why expect that it can become that cheap? I see photovoltaics (PV) has having greater potential for cost reductions because PV is simpler in operation. So I expect PV will eventually cost less than solar thermal.
I don't mind if PV ultimately wins, but in the meantime I'd prefer that our California policies favor the current best of breed. That is, if we are going to spend public monies and tax credits, favor solar-thermal. I want my tax dollar spent on the practical and not the dream.
I'm not sure why one needs to do only PV or solar thermal. PV cells are essentially black (like solar thermal units), and they work more efficiently when cooler. One could easily produce units that generated both heat and electricity simultaneously.
The problem with that is that heat flows downhill; A solar thermal system is going to have the black surface as hot as possible, because you want that solar heat to be at a relatively high 'potential' for ease of use. Smaller storage masses, things like that.
What you want is solar thermal combined with something like the solar thermionic systems. Of course, they require high concentration levels to work well.
I think the deciding issue will be storage. As we know, the problem with solar is that the sun provides no power at night, and little or none in some weather.
If home storage, presumably in batteries, proves both cheap and practical then rooftop PV is likely to become very common.
But if massive battery banks do not pan out I expect utilities will prefer solar thermal. They will store heat in some medium for use when the sun does not provide power.
For truly large installations an advantage of solar thermal is that there is no connection between the mirrors and the generators. In contrast, PV requires many cells to be wired together with some intricacy. Wind farms have the connection problem also, every single generator must somehow be connected to a central point.
But, thermal or PV, the grid will remain almost universal. The question will be what portion of KWH a given user buys from it.
One more time. Costs cannot be compared until you spec out the system. It must include all the little stuff like land, support towers and frames, roof reinforcements, and the big stuff like transmission lines, storage devices, over capacity to build up inventory for days when it is cloudy and nights. Just knowing the generating cost is no where near enough information.
Given sufficiently low cost then PV doesn't need storage capability up to the point where it is supplying all the electricity used during the day. PV would have to become extremely cheap to reach that point.
Also, storage takes many forms. One can store heat cool in water or lead. If or when PV becomes really cheap using electricity to generate heat or cooling will make sense. Though with less light during winters PV has limits on how much warming it can provide.
There's not a single price point that PV has to compete at. Wholesale electricity costs vary by time of day and day of year. PV can compete during the day before it has to compete at night.
Randall: I'm a little perplexed by your reply. I said nothing about various mediums for storing heat. Recently I notice some interest in phase changing salts again. That was used forty years ago. I have no idea what problems arise with salt as a storage medium. But IMO homes will use batteries for the storage problem or they won't have storage.
I thought the question was about utility costs of Thermal v. PV. And I gave an answer. The utilities will be better off with thermal unless batteries arrive to provide low cost storage. Low cost battery storage might tip the utility toward PV.
I don't expect such batteries soon enough to allow utilities to meet mandated percentages of renewable power. Typically those are around 10% or 15% within a decade.
But suppose that low cost storage does arise. That would also promote battery storage at the home and there would be a tremendous demand for them in cars. Meanwhile Thermal could cruise on with the utilities storing heat not electricity.
The duality is not reflexive; Solar Thermal seems unlikely to become the electrical producer in the home even it provides lower cost at the utility.
So we enter a political realm. Regulators and politicians will favor home owners over utilities. Hence PV will be subsidized for the home and so will batteries. But the same regulators and politicians will have little incentive to prefer PV over Thermal at the utilities. From the utilities they will want renewable energy at the lowest cost; that seems likely to be Thermal.
I endorse Fat Man on this. Infrastructure, land costs, and requirements are paramount. If we knew for sure that the national grid would be totally upgraded, and when, that would affect every generating project and every cost estimate. But we don't know even that.
Everyone is guessing. Some guesses are very sophisticated. They are still guesses. About all we know is that solar works a lot better at lower latitudes. We don't know what any method will cost by 2020. For that matter it is almost impossible to say what they cost now because of subsidies, mandates, and closely held proprietary cost information.