How the Arizona Solar Tower Actually Would Work

Slashdot recently posted an article on a massive project, currently in the land acquisition and planning stages, to create a massive solar thermal tower in the Arizona desert using a pretty clever and unique design. Rather than using water, glycerin, or even molten sodium as a working fluid, the design is based around air and uses what amounts to greenhouses to power the plant. A field of greenhouses heating air surrounds a massive tower, with turbines in the base of the tower. If they had hired a better PR guy, the greenhouses might even be referred to as heat farms.

Unfortunately, the article does the usual bit of hand-waving about how the system actually works, claiming it's based on "temperature differences" between the hot air and the upper atmosphere. While I'm pretty sure this is, charitably speaking, not wrong, it is only a small part of a larger and more complicated concept. I'm going to endeavor to explain it: the power plant appears instead to be designed around a clever application of the stack effect.

In brief, the stack effect can be explained by looking at a static equilibrium of a stack, or, if you like, chimney, cooling tower, or any other somewhat cylindrical hollow structure full of hot gas. Hot gas, as might be predicted by every gas law you've ever seen, is less dense than cool gas. If we assume that the stack is sealed against air, then all things being equal a stack emitting gas hotter than the ambient air will have lower pressure at its base than ambient air pressure at the datum - the column of air above it is less dense. At the same time, if the stack is sufficiently high, the hot gas at the top will still have enough of a pressure differential with the air at that height to flow out. If it isn't high enough, additional energy has to be added to the gas, which is why most industrial installations have induced draught systems (with fans in the stack) or forced draft systems (fans at the air intake) to add enough pressure difference without having to build a huge stack.

Now suppose we drill a hole in the bottom of the stack. Since there is a pressure differential caused by the stack effect, air will intrude into the stack without any additional energy input.

That's basically what this plant is trying to do: take advantage of that pressure differential to run a few turbines. The tower has to be huge, to maximize the column of hot air overhead to create the maximum pressure differential at the bottom. In addition, since the turbines at the base will actually decrease the exit pressure of the stack gas stream, the stack has to be high enough that when this air reaches the top, it will have more pressure than the surrounding air. Ground-level air from the ambient area will spontaneously flow into the apertures provided on the outside of the solar heat farms, which will warm up the air so that when it flows into the stack it is about as dense as whatever prompted the original driving force.

I think this is a really cool idea. The article mentioned that it would be able to operate under most weather conditions. I believe this, since you can probably take one or more of the turbines offline to give a smaller pressure differential at the base between turbine intake and exit to adjust for varying heat input. I'm pretty certain the only thing necessary to start it up is the provision of an initial hot gas stream in the stack, probably from flaring some natural gas.

Two things bother me though. The first is the expense: a 200MW power plant that can only operate during daylight hours is costing $750 million to build. For that price, you could build a 1 GW top-of-the-line supercritical water-based coal plant. Sure you'd need to buy the fuel, but coal is not all that expensive. I don't have any information about this deal they have with the SoCal Power Authority, so I can't say much else, but it definitely looks to be a little shaky.

The second bit that bothers me is that the Slashdot posting mentioned that food could be grown in the greenhouse if a water source could be found. This was not mentioned in the article, thankfully, because it is a really fucking stupid idea. I'll grant that you might use only the outer parts of the greenhouse solar heat farming complex for growing crops - no one in their right mind would want 90degC air for their growing environment. But even excepting that ambient temperatures in Arizona are already hot enough for most plants. It's the lack of water and soil that really hurts. So why bother growing in a greenhouse if you've already got the temperatures you need?

And let's just suppose that this design is copied and put somewhere that doesn't have soil or water problems. You're still going to need a whole honking lot of water. This system is designed not just to heat, but to circulate. A conventional greenhouse retains water by being a relatively closed system. An open one would literally evaporate all of your water away by continually replacing your hot, humid air with dry air from the outside, which would proceed to warm up, suck up moisture, and leave. Recovery of the water couldn't be done unless you either liked it salty (through a salt dehumidifier) or wanted to make your entire power plant pointless (by cooling the air so water condenses). Growing crops in an environment like that would be insane without unlimited water. And besides, making this plant in a dry area has a secondary benefit: avoiding corrosion. Why'd the designers want to give that up?

Economic growth and penises

A new paper out of a Swedish university gives new meaning to the phrase "stimulus package." Apparently, penile length is strongly correlated with economic growth between 1960 and 1985. After a healthy rise, bigger is not apparently that much better, as the correlation is U-shaped and growth potential drops off at average lengths greater than 16cm. It gets better: the authors then attempt to speculate on a possible mechanistic explanation for the macroeconomic effects.

I couldn't make shit up this crazy if I tried.

The Energy-Water Nexus

I've been thinking a lot recently about water issues (which may or may not be related to work). There's been a lot of talk recently about an emerging "Energy-Water Nexus" that threatens future economic growth in the United States. Essentially, it follows from the observation that water supply and energy supply are largely interdependent. This makes a good deal of sense. Thermal electric power generation uses huge amounts of water, for example, to reject heat to the environment at the lower end of the thermodynamic cycle. Vast quantities of water are required for coal mining, gas extraction, and oil production. In turn, surface water and groundwater must be transported or extracted, and saltwater desalinated, for use in industry or in the home.

At first glance, this doesn't seem to be a problem - not particularly at least. After all, the volumes of water we extract have a far lower energy intensity than the water intensity of energy. Four things speak against that simplistic viewpoint - one of them an emerging trend.

First and foremost, the amount of water used in industry faces some pretty stiff competition. More than 70% of the water used worldwide is used in agriculture. Globally, industrial uses - including thermal power generation - account for maybe 16%. This might seem obvious, but for all the energy we expend on purifying and extracting water, only a little bit is going back to extracting and producing more energy.

Second, the amount of water we use is increasing, and pressure is being put on the energy side of things. With population growth comes increased water consumption. That much is obvious; what is less obvious are the first order effects on agriculture. Furthermore, with economic growth, power consumption and thus water devoted to power consumption rises linearly. To put that in perspective, we can compare the global statistics I cited above to the ones for the United States: Fully 53% of our water is used in industry, of which a whopping 49% (that's about 92.5% of what's used in industry as a whole) devoted to power generation. With economic growth in China and India, and soon Africa, starting, water stress suddenly is a whole lot closer to reality.

Third, all water issues are, with rare exception, local issues. Water isn't evenly distributed, and the weight of water makes it difficult to transport, so local areas must find their own solutions to water issues. Some places are already running short of water resources for industry. Two big examples are the middle Yellow River region in China and the Jamnagar industrial district in Gujarat, India. In the former, rampant industrial overinvestment has lead to water-guzzling factories being shut down and heavy rationing instituted. In the latter, overreliance (no pun intended*) on groundwater has caused water tables to fall to dangerously low levels - farmers outside the area have to drill 30ft deeper wells every year, and what they get is increasingly saline because of contamination from seawater.

Fourth, the emerging trend is even more water intensity in our energy consumption, with the rise of biofuels and biochemicals. Irrespective of subsidies, the economics of high oil prices have driven the adoption of these new technologies, almost all of which are water intensive. Consider that the processing of one gallon of corn ethanol requires 30 gallons of water consumed in the most advanced plant in the United States. A more typical case is 300-600 gallons.


So there's a darned good reason that a lot of the power and chemical process industry is starting to care about water issues again.

* The megacomplex of refinery and chemical plants in Jamnagar is owned by Reliance Heavy Industries. Yeah, probably no one got that.

Humectants and BEEF

I cut most of the beef out of my diet about two years ago; that said, I'll admit that I still have a weakness for the occasional steak. And on the subject of steak, I recently found out something cool about biofuels and animal feed.

Animal feed supply is one of the biggest industries in the United States. Two of the biggest sources of animal feed, particularly for slaughter cattle, are DDGS (dried distiller's grains and solubles) and soy flour. DDGS is the dried byproduct of fermentation from grains, these days typically ethanol but including some beer manufacturers. It's such a huge source of feed that some ranches are now relocating to be near ethanol plants to take advantage of wet distiller's grains (WDG), which have a shorter shelf life but are cheaper since they don't need to be dried.

Soy flour is the other main source of feed; a tiny, TINY proportion of commercially grown soybeans goes to people. The remainder is crushed for its oil, with some having protein extracted by hexane washing, and is toasted and fed to cows. But powdered soy flour isn't directly edible, so typically a feedlot will add a humectant - a moisturizing agent - to make it possible for cows to eat.

Over the past 50 years, this was typically yellow grease. Yellow grease is nasty stuff - it's essentially what comes out of the frialator at restaurants. Most food outlets pay for their used oil to be disposed of by professional retrieval companies, which then sell it on the market. In recent years, yellow grease prices have gone up because it turns out it's an excellent feedstock for biodiesel. Well, not so excellent, since it's got catalyst poison making up 50% of its mass, but there are ways to get around that (no joke). Point is, it's cheaper than virgin oil by a lot, so the biodiesel manufacturers that are smart have equipped themselves to deal with this heavy feedstock.

At first, feedlot owners weren't too happy about that. But recently, feedlot owners have discovered that raw glycerin, a byproduct of the biodiesel process that contains about 50% water, lots of glycerin, and other contaminants, can be relatively cheaply processed without glycerin concentration - the energy-intensive step - to be a humectant for soy flour. And get this - turns out the cows like it even better, and it's cheaper than yellow grease anyway. The biodiesel manufacturers I talked to are currently giving it away for five and a half cents a gallon.

Considering I spent a good two years of my time at MIT trying to find a home for raw glycerin, this makes me very happy. Also it's great not to have my ribeye cut contain recycled Mickey D's. But more importantly, it's now providing an important secondary revenue source for biodiesel manufacturers, letting more of them continue producing even when diesel prices are low.

So I herd you liek screwing over Indonesia

Reading the latest edition of World Ethanol & Biofuels report makes me seriously wonder if Europe just wants to shoot itself in the foot. Among many other things, it just reimposed tariffs on imported biodiesel - not just a small one, but 400 Euros per tonne - to punish American manufacturers re-exporting through Canada, which apparently counts as European. There are no words in the English language to describe how stupid this is. The US has spare biodiesel capacity, to the tune of hundreds of millions of gallons per year, but almost no market. Europe has a ridiculous market - and the US already exports regular diesel to them - but somehow it seems fixated on preserving a few, low-skilled jobs, damn the consequences.

Let's get this straight: they allow imported oil with a nominal tariff, and imported American diesel with a nominal tariff, and have no problem with biodiesel, ostensibly because it reduces carbon emissions and stuff. But on the other hand, they let dozens of plants in the US than can produce this green fuel idle, to "preserve" uneconomical jobs in Europe... and spend more carbon to import the caustic soda needed... and don't build any spare capacity... and the net effect on the US biodiesel industry is called "rape." For the poor Indonesian biodiesel industry, which can't export biodiesel to the largest biodiesel market, yet has cheaper soda (from Australia), and is forced to instead export low value-added palm oil instead in exchange for slashing and burning its rainforests, this state of affairs is the same as for the US biodiesel industry, except with the words "no lube" appended.





And above it all European citizens are now complaining that biodiesel is too expensive.

I WONDER WHY?