If you want more solar BTUs for your buck, track the sun. This generally means the array is on the ground since very few homes are set up for rooftop tracking. Some commercial buildings are suitable, but once you get up in the air, even one story, the wind load becomes a factor, and engineering an array (and the building itself) to stand up to the wind will probably be a money loser.
There is another way to get more output from the same amount of space and that is by concentrating the light. PV panels are ‘one-sun’ arrays: what you see is what you get. Concentrators (many suns) come in a few different shapes and sizes, and they can magnify what the sun has on offer by a factor of several thousand. It is not that difficult to build a dish (it looks like a big DirecTV saucer, or one of those beasts that searches the heavens for signs of life), but at some point, cost becomes an issue, in fact, the issue.
Laying strips of glass or aluminum and inch or so wide (3 or 4 cm) on a parabolic frame can give you a dish with a ‘concentration ratio’ on the order of 5,000 to 10,000 suns. The temperature at the focus (which can be smaller than a golf ball) would probably approach that of the surface of the sun. I’m not sure what materials would endure such conditions, and I wouldn’t recommend standing around to see what happens if you got it wrong.
Regardless of the materials used, the cost to do such a thing would certainly be more than the value of the output, so it could be ‘technically sweet’ (in Oppenheimer’s ironic, make that Plutonic, phrase) but of no practical use. You don’t need a thousand suns to boil water.
Kramer Junction, CA ( 354 MW -- Built 1984 - 1990)
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Troughs, however, are another shape that concentrates sunlight (around 60 times), and here we have something that is going to make a difference. Whereas dishes provide ‘point focus’ concentration, that is they bring 100s and 1,000s of suns onto a very small surface, troughs provide ‘line focus’ as the light bounces off the surface onto a pipe or tube of some type that runs along the entire array parallel to that reflective surface. They are easy to build and require no new thinking. The new thinking is that there is a better way to make use of 60 suns; high temperature troughs used to make power need very expensive pipes or tubes which run corrosive and flammable liquids through them.
FPL Martin solar thermal troughs for pre-heated steam
To date, this technology has been used to make steam in order to spin a turbine, just as burning gas does. From the steam outflow to the power plant nothing changes. In the not too distant future, I expect that hot water and low temperature steam will be used for space heating and air conditioning, perhaps even desalination. The array pictured above was built by Florida Power and Light (Next Era Energy) to pre-heat steam for injection into the turbine cycle at this 1.15 GW natural gas power plant. It is generous to call it a hybrid since the solar component is little more than a rounding error, but the decision to heat water with sunlight is not an error.
The AC load, which accounts for nearly 20 percent of all U.S. household electricity use, is also the leading source of utilities’ pain in places like California and Texas because as a hot summer afternoon drags on, millions of people turn on their AC units, nearly all at once, and even though you only need the capacity for the hundred hottest hours in the course of a year, without those mostly idle plants, the system would buckle and you could add total darkness to sweat as the evening wears on. The price of power at those times skyrockets. Furthermore, transformers can buckle and wildfires can wipe out just about anything, even your rooftop PV.
I know what you’re thinking: Renewables are fickle and the grid will not be as reliable when the percentage of power from such sources rises. On the contrary, the Germans and the Danes have a much higher penetration rate, and only a tenth of the power outages as the U.S. Nonetheless, it will be interesting to see how the system operates on the morning of August 21, when the moon blocks out the sun as it crosses the U.S.
Troughs are ready for their close-up
It is worth noting that 40 percent of our (U.S.) primary energy is used to make heat. Troughs can make it for less than the cost of natural gas today, and you will never have to wonder about the cost of the fuel five or ten years from now. In fact, they can make it for less than the lifetime cost of the inverters alone in a PV array. Another great thing about trough technology is that, like PV, it scales from a small unit you can put next to your garage to massive arrays like the Kramer Junction field shown earlier. Heliostats, the mirrored assemblies that independently point at a central tower do not scale down. (There are other issues worth noting at another time that will limit if not knock out power towers from making inroads.) Related: Macquarie: OPEC Deal To Collapse In 2018
So, if troughs are such a brilliant idea, why hasn’t it happened yet? Thanks to Germany’s Energiewende and China’s manufacturing muscle and will, financing became available for all things photovoltaic, ie, solar electricity. Even ten years ago, when PV cost around $5 per Watt, it seemed like a fantasy to suggest the price would ever fall below a dollar. Today the ‘China price’ of a PV panel is around 40 cents; and a recent bid in India came in below a dollar for the entire system. This is getting ‘too cheap to meter,’ and too cheap for conventional alternatives to matter.
As a result, 3-cent solar power is blooming throughout the world; the electricity wars are over and Renewables (wind included) have won. It’s all over but the shouting, and of course the scaling, which takes years but cannot be stopped, rather like melting ice structures in Antarctica.
There has been no such undertaking when it comes to heat. After all, isn’t heat supposed to be bad? Ironically, though this is not the majority view, solar heat and hot water will do as much to mitigate climate change and sea level elevation as anything we can do, including slapping a price on Carbon.
(Click to enlarge)
Like hundreds of thousands of windmills in the Midwest before rural electrification, look to individuals to drive this technology and these applications forward. In addition to providing AC, using steam from troughs feeding chillers, district heating, using summer heat stored in the ground, will help small towns and dense residential areas, assuming they left some greenbelts nearby on which to place the collectors. As for big cities, it is hard to see how they will ever pay their way when it comes to energy matters.
(Click to enlarge)
And so, with news that a trillion tonnes of ice have broken off the Larsen Shelf in Antarctica, you may think we, as a species, are in hot water, maybe even doomed. Think again. In fact, this threat from rising seas is coming from cold water; hot water will keep it from rising too high; hot water will save the world. Cheap troughs are the way we are going to get it. You heard it here first. Steam from a mirror, fancy that!
By Henry Hewitt for Oilprice.com
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