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Sandfall

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A rather beautiful new piece of technology from, appropriately, Sandia.
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Source: photoree.com

 

 

 

 

Concentrating thermal solar power – CSP or CST – has been the perpetual bridesmaid to solar PV. Just as it´s about to go commercial, the price of solar PV drops again and the projects are shelved. CSP is growing, but at a fraction of PV´s headlong pace. Brightsource, the leading commercial player in the US, only predicts the global market to reach 30 GW by 2020. (One prediction for PV at the same date is 330 GW.) CSP has one big thing going for it: storage. As you start with a hot fluid, it´s fairly easy to add a thermal heat reservoir at the point of production, The Spanish company Gemasolar has demonstrated 24-hour electricity generation with hot salt storage coupled to a solar tower. The Australian grid operator AEMO reckons that this isn´t optimal, as there´s very little demand for electricity at 3 am, and 9 hours of storage will get you comfortably through the dark evening (pdf report, Appendix 2). Still, a 9-hour Gemasolar clone remains expensive.

The likeliest way to bring costs down is to go hotter. Currently operational CSP plants use water/steam as the working fluid, and standard Rankine-cycle steam generators. The PS10 solar power tower near Seville, the largest currently operational, has a steam working temperature of only 275 deg C at the receiver outlet. Brightsource´s Ivanpah tower in California, the largest under construction, will run hotter at 560 deg C.

The Rankine cycle isn´t very efficient. Most current steam generators are at 35-42%. This is a very mature technology and it´s very hard to improve on it, especially with relatively small 100MW rigs. To push up efficiency, the favoured way is to go hotter. Partly because of the basic physics of heat engines; partly because above 800 deg C or so you can switch to or add a Brayton cycle, aka a gas turbine. Feed the still hot gas turbine exhaust into a conventional steam generator, and you have a combined-cycle generator plant. These are routinely used in natural gas plants and regularly achieve efficiencies over 60%.

A solar tower can easily reach such temperatures at the focal point. Large research parabolic solar furnaces  get over 3,500 deg C. Long before the theoretical limit of 6,000 deg – the temperature of the Sun´s surface – you would run out of materials to hold targets in, and you are left with a fun but completely useless fixed death ray. The problem is capturing the heat with a non-magical material and transferring it to your heat engine.

One candidate is an alveolated ceramic block fixed in the receiver, with blown air as the working fluid. The EU went into this in the last decade but the scheme hasn´t made it into precommercial pilots. I suspect the problem isn´t the ceramic but the air: perhaps it´s not dense enough to support high rates of heat transfer.

Enter Sandia with a different idea: grind the (EU´s?) refractory ceramic into sand and use a sand-air mixture as the working fluid. Capturing the heat is beautifully simple: the sand just falls in a curtain past the receiver window and is heated up to 1000 deg C. It is collected at the bottom of the tower in a heat exchanger which feeds air or steam into the turbine. The sand has a lot of thermal inertia so it could be its own storage, depending on cost. The working fluid´s higher temperature and density translates to a more compact and efficient plant with – they hope – a much lower cost of generation.

falling particle_smThis photo shows a demonstration receiver. Clearly this is still an early-day lab rig. The aim is a complete operational design by 2017, and they still have to overcome a host of problems, including machinery (pumps, bucket elevators) that will survive a diet of red-hot abrasives. More sketches here. Via CleanTechnica.

Still, it´s very pretty idea. Gravity is as cheap and reliable a way of moving stuff around as you can possibly get. Give them more money!

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Is it frivolous to judge technological innovations on aesthetic grounds, as I´m doing here? It would be to make it the only criterion. But form follows function. The human aesthetic response is surely part of Daniel Kahneman´s automatic, instinctive, efficient mental System 1, even if it´s educated by conscious learning and reflection. The ev-psych argument that it´s partly based on utility is very strong. Beautiful people have the characteristics of biologically good reproductive partners. Beautiful landscapes of the Arcadian type correspond to good environments for hunter-gatherers. It´s not a stretch that this extends to artefacts as well as images: a beautiful house is one you would like to live in, an iPhone is a tool you enjoy using. So our aesthetic reaponse to engineering reflects an instinctive judgement that it´s fit on a range of practical dimensions: transparent, controllable, economical, compact, stable, reliable.

Of course the aesthetic reponse has other triggers than fitness for use. We are also attracted to environments – mountains, storms,  deserts of sand and snow  – that are extremely hostile; animals like tigers and dinosaurs that are (or would be) very dangerous; and representations of violence and terror in tragedies and action movies, embedded and distanced in a moral narrative. Why this should be so is a puzzle. Aristotle´s theory of catharsis about the last category look fishy, but I don´t know of a better one. The Romantics distinguished between the Sublime (the Alps, Gothic cathedrals, King Lear) and the Beautiful (girls, flowers, songs). There´s something in this, but as far as I can tell the subjective sensation is the same. Maybe there´s a connection with the paradoxes of sexual selection: features like the peacock´s tail or behaviour like the pronking of gazelles that are worse than useless, but demonstrate to potential mates a male´s superior fitness as obstacles he has successfully overcome. (Sexual selection is not gender-neutral.) Perhaps we admire Gothic cathedrals because of the sheer crazy prowess of building such unnecessarily tall and airy structures out of stone and weak mortar. In any case it doesn´t look as if my aesthetic response to the sand CSP concept is driven by any of these factors.

Early-stage R&D is necessarily based on inadequate information. It´s likely to fail. The decision to fund or not is a poker bet based on the few cards in your hand. It´s sound to pay attention to your instincts as well as your reason – though as information accumulates during the game or project, reason should exert greater control. The aesthetic response is one such input from instinct. So don´t believe in the beautiful idea as Dirac advised – but do give it a chance.

There is a second, unrelated argument for making and publishing an aesthetic judgement. Most R&D projects fail. Indeed most human projects of any ambition do too: most scientific papers and novels are unread, most films are scarcely watched, most competitors in a sporting or electoral competition lose. But while generally failing, we can all still occasionally achieve good work. The relegated football team can score a fine goal in a match it loses. I suggest that it´s therefore a social and cultural duty to praise areté wherever we see it. Success, which is often due to luck, is a very crude and misleading proxy for excellence.

Comments

  1. Anonymous says

    (Edit)

    Just a note: All heat engine cycles are inefficient at low temperature differences, not just the Rankine cycle. it’s a basic rule of thermodynamics at work. So I wouldn’t fault the Rankine cycle for this.

    Talk to the oil refineries. IIRC, hot sand plays a major roll in them as a heat transfer medium.

  3. Maynard Handley says

    (Edit)

    Unrelated to this tech, but regarding beauty. A large part of what is claimed to be beautiful appears to be some combination of learned and going along with the crowd. The Alps and Gothic Cathedrals are historically notorious examples of this, of course, as are Impressionism and Cubism.
    But more relevant examples are technological. There is beauty, IMHO, in something like the curves of three highways stacked above each other intersecting in mid Los Angeles, and in the river of light one sees flowing down them; however there are people who claim (I assume, in at least some cases sincerely) to be unable to see anything there but ugliness, I assume because of their feelings about private transportation, human footprint and so on.

    Point is, perhaps some of your feelings about this product are driven by more than aesthetics, given that I’ve never heard you wax on about the beauty of gas turbines or microchips, and you’re simply misidentifying your emotions (like a kind of synesthesia)?

    • James Wimberley says

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      I´ve written about microchips (here and here), so I am interested. You are right that I do not have an aesthetic preference for ARM processors over Intel´s – just a policy one, because they save energy, and a tribal, because they are British-designed. I don´t understand enough about the technology for the aesthetic instinct to cut in. Any computer user can see that Windows is bloated and kludgy without reading a line of code, but that doesn´t work for processors. In the rather pointless blog wars between ARM and Intel fans, a lot of the partisanship seems to be be driven by a David-and-Goliath aesthetic preference: for power on the Intel side, sneaky skill on the ARM one.

      Gas turbines? Too unremarkable to comment on. The basic design dates back a hundred years, the principle two hundred. I like stayed suspension bridges too.

    • James Wimberley says

      (Edit)

      PS: you really think that Gothic cathedrals are valued merely out of fashion? Since they were built in the 13th century, they have been officially approved of for around 550 out of 750 years. Since they were looked after during the period unfashionability (1600-1800), it´s reasonable to think that the classical aesthetic canons did not have universal purchase on the population.

      Two counter-examples. The great Umayyad mosque in Cordoba, in a style entirely alien to Christian Spain, has generally been well looked after by its new owners. The great exception was the construction of an ordinary Renaissance cathedral in the 16th century – 300 years after the Reconquista in 1235 – in the middle, breaking up the vista of columns. The project was pushed through by a triumphalist faction of the local clergy, against widespread opposition. When Charles V saw the result of the plans he had approved, he realized he had made a bad mistake and the clerics had ¨destroyed something unique in all the world¨. At the other end of the Mediterranean, when the Ottoman Turks took Constantinople in 1453 they converted Hagia Sophia, with more structural respect, into a mosque. Indeed its great central dome became the new model for Ottoman first-division mosques (Sultan Ahmed and Blue Mosques).

      The very best stuff speaks universally and over cultural chasms.

      • James Wimberley says

        (Edit)

        PPS: I concede that beauty of the paradoxical Sublime type is more likely to go in and out of fashion. It´s less stable and possibly less universal than the utility-linked Beautiful type. King Lear was provided with a happy ending in the 18th century. The mediaeval Arabs translated all the Greek science they could get their hands on, but not the tragedies.

  4. paul says

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    Oh, and just a technical nit: nothing fundamental stops you from getting temperatures higher than the surface of the sun. You need to stop being a black-body absorber/emitter, and you might lose some efficiency doing so, but it’s not impossible.

    • James Wimberley says

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      This CSP technology doesn´t mean that, because it´s not ready. Survival requires either very strong carbon reduction policies – stronger than Germany´s 80% cut by 2050 – more or less everywhere, or a technological revolution in energy production and use. So far the revolution is going better than the policy. Wind and especially solar PV deserve that over-used accolade, disruptive. They are already having an impact on utilities´ bottom lines and plans quite out of proportion to their contribution to the energy mix, because they wreck the baseload/peak load production paradigm and (in the case of solar) centralised distribution.

      They are also shifting the political economy of energy. Wind and solar have been winning the political fights: PTC renewal in the USA, the failed attempt by the FDP to stop the Energiewende in Germany, the successful movement in Georgia, supported by the Tea Party, to force Georgia Power to accept distributed solar, the Japanese solar FIT, and above all the repeated raising of targets in China. The setbacks in Spain and the Czech Republic to solar (not wind) are insignificant in the balance.

      Since wind and solar are still getting cheaper, they will get more influential. Take the worst case. Tony Abbott´s denialist government in Australia is dismantling not only Labour´s climate change policy but the institutions for formulating one. But I predict that Abbott will not find it politically desirable to stop the rooftop solar boom. So as long as Miao Liansheng continues to sleep in Yingli´s offices and greet its workers as they come in at 6 am, we have a chance.

      • NCG says

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        Again with the good news: ” the successful movement in Georgia, supported by the Tea Party, to force Georgia Power to accept distributed solar, …”

        Darned if that isn’t the first good thing I’ve ever heard about them. You learn something every day!!!

        Thanks again. My hat is off.