The secret to low-water-use, high-efficiency concentrating solar power

Manyreaders have expressed interest in learning more about the waterconsumption of concentrating solar power and how measures to reduce itmight impact system efficiency and cost.  After my recent CSP post, “World’s largest solar power plants with thermal storage to be built in Arizona,” Michael Hogan wrote in the comments (here)about a low-water-consuming cooling system he had experience with.  Iasked Hogan, a long-time power industry executive and currently thePower Programme Director for the European Climate Foundation (bio here), to write a longer piece for Climate Progress.  Here is what he put together, with links and figures (click to enlarge).

figure1 small The secret to low water use, high efficiency concentrating solar power

EXECUTIVE SUMMARY:  If concentrating solar power (“CSP”) is a core climate solution,indirect dry cooling systems (also known as “Heller” systems) will be acrucial enabling technology, since large-scale CSP will be located indesert regions. US power companies have long favored directdry cooling systems for fossil plants, probably because of the visualimpact of Heller systems.  But Heller systems have longexperience in certain regions and will probably play an important rolein the success of large-scale CSP.  This is due to their higherefficiency, smaller footprints, quieter operation, lower maintenance,higher availability, and more flexible site layout.  Heller systems canreduce water consumption in a CSP plant by 97% with minimal performanceimpact.  The height of the cooling towers should be less of anissue in remote desert locations, especially since the central tower inpower tower facilities will be of comparable height.

Concentrating solar thermal power plants (“CSP”) have been identified a number of times in Climate Progress as a core climate solutiondue to their almost unique potential to replace coal as the dominantsupplier of baseload and/or firm dispatchable capacity to the world’spower grids.  It is said that CSP could represent 3 of the 12-14 wedges in the 450ppm solution –- 20-25% of global mitigation potential. I concur wholeheartedly with that view, and I applaud CP for itsefforts to educate readers on the singular challenges of eliminatingcoal-fired power production at scale.   But if CSP is a core climatesolution, dry cooling technologies, and in particular Heller systems,will be a crucial enabler (see note at the end regarding the status ofthe name “Heller” system).

One of the concerns often cited about CSP is water consumption,particularly because the technology’s reliance on direct normalinsolation means that it is most economically located in desertregions.  Because most CSP systems rely on Rankine cycle steamturbine-generators to produce electricity, they face the samerequirements as fossil-fired power plants for condensing large volumesof saturated steam back into boiler feedwater. (Parabolic dish systemsuse Stirling or Brayton engines to produce useful energy, each of whichhas its own advantages and disadvantages)  Where an abundant and cheapsupply of water is available, the most efficient way to accomplish thisis by evaporation (or “wet cooling”), which is what produces the largeplume of water vapor one often sees rising from power stations. Convective cooling using ambient air (“dry cooling”) requires highercapital costs and can reduce plant performance, and thus planners offossil plants have sought to locate them close to adequate supplies ofcooling water whenever possible.

In the desert areas where CSP will thrive, the consumption of largeamounts of water by conventional wet cooling systems is clearlyunsustainable.  Dry cooling alternatives will be required, and CSP willhave to demonstrate its commercial viability despite the capital costand performance penalties this will entail.  Fortunately this is aneminently manageable problem.

figure2 small The secret to low water use, high efficiency concentrating solar power

[Acronyms: “LEC” = levelized electricity cost; “O&M” = operation & maintenance]

Deutsches Zentrum fur Luft- und Raumfahrt e.V. (“DLR”), a German government research agency, presented a studyin 2007 comparing a particular dry cooling technology, the Hellersystem, with wet cooling for CSP plants in Spain and in the Californiadesert (see figures above).   Water consumption was reduced by 97%, andthe performance impact was quite minimal.  Indeed the impact onperformance in the higher desert temperatures of California wasoverwhelmed by the benefits of better annual insolation.  They alsonoted that the potentially negative impact of high daytime temperaturesis mitigated by the use of thermal storage, which uses energy collectedduring peak daytime insolation to produce electricity when temperaturesare considerably lower.  One interesting aspect of the DLR study wastheir focus on Heller systems over more familiar (at least in the US)direct dry cooling systems, and that is worth a closer examination.

Two basic types of dry cooling systems have long been employed wherenecessary -– “direct” air cooling (usually called an “air-cooledcondenser” or “ACC”) and “indirect” air cooling (often referred to asthe “Heller system”, after Laszlo Heller, the Hungarian thermodynamicsprofessor who pioneered this approach in the 1950s).  In ACC systems,the saturated steam from the steam turbine exhaust is carried directlyto a very large array of A-framed fin-tube bundles, where largemechanical fans force air over the tubes, convectively condensing thesteam.

figure3 small The secret to low water use, high efficiency concentrating solar power

ACC system

In Heller systems, the steam is condensed by spraying water directlyinto the exhaust flow in a ratio of about 50:1 (called “direct contactjet condensing”), creating a large volume of warm water, some of whichis pumped back to the boiler as the working fluid and the rest of whichis pumped to bundles of tubes arrayed at the base of a natural-drafthyperbolic cooling tower.  The warm water circulating around the baseof the tower and the cooler air at the top of the tower, combined withthe tower’s hyperbolic shape, stimulate a powerful updraft that drawsambient air over the tube bundles, thereby convectively cooling thewater before it is returned to the condenser.  Both are closed systems.

Heller system [Acronyms: “CW” = cooling water; “DC” = direct contact]

While the Heller system has been widely used elsewhere, there arenone in the US.  This is probably because the much lower auxiliarypower requirements of Heller systems come with the visual impact of alarge hyperbolic cooling tower (typically 150m high and 120m in basediameter), often a difficult sell given that most fossil power stationsare located in the vicinity of the populated demand centers they’reintended to serve.  The auxiliary power required to run an ACC systemis roughly twice the power required run a Heller system, and the Hellersystem is considerably quieter, but these have apparently beenconsidered prices worth paying for the lower profile (a typical ACCsystem can be 40m high), particularly when it was cheap coal-firedpower.  Simple lack of familiarity could be another factor in thehidebound world of US power utilities.

The Electric Power Research Institute has kicked off a comparative studyof indirect dry cooling (due to be completed in mid 2010), on thetheory that it is the most economic dry cooling solution forlarge-scale thermal applications.  The prospect of large amounts of CSPbeing built in the world’s deserts calls for a reconsideration of therelative merits of these two approaches, since it would require drycooling to be deployed in a different application and to a far largerextent than has ever been the case.

Three Bechtel engineers published a paper in 2005 (Digital ObjectIdentifier reference DOI:10.1115/1.1839924) (originally presented at anAmerican Society of Mechanical Engineers conference in 2002) thatcompared cooling technologies for combined-cycle gas power plants. They cited the following comparison of installed costs for variouscooling systems, including ACC and Heller.

[Acronyms: “WSAC” – wet-surface air condenser]

They also note that the footprint of an ACC system is larger thanthat required for a Heller system, though specific data is notoffered.  Overall system efficiency of a Heller system is in the rangeof 2% better than an ACC system. That performance improvement meant one thing in a fossil power plant inthe bad old days of cheap dirty power, but when it means 2% less landarea covered by solar collectors, and lower auxiliary consumption ofmuch more costly power, it takes on a much greater significance.  Thesame sources note that since the Heller systems are mechanically farsimpler than ACC systems, maintenance is much less of an issue andsystem availability is significantly greater.  In the remote areaswhere these plants will be located, and given the large land areas overwhich they will spread, these are far more significant considerationsthan they were for compact fossil power plants located close to thepopulations they served.  Another factor noted in these sources is thatan ACC must be located next to the steam turbine it serves, because ofthe cost of transporting saturated steam over any distance, whereas theHeller system has much more flexibility in where the cooling tower islocated.  This will be much more important to CSP, where one canenvision clusters of power tower complexes in a given area each withits own steam turbine, than it was with fossil plants.  And finally,the feature that most worked against Heller systems in US fossil plantapplications – visual impact – should be far less of an issue in remotedesert sites, especially with solar power tower complexes where thecentral towers will likely be of similar height.

I should note that as a senior executive of the private powercompany InterGen in the late 1990s I oversaw the deployment of a Hellersystem on our 2,400 MW gas-fired combined cycle plant in Adapazari,Turkey (see below), which is still the world’s largest installation ofan indirect dry cooling system and continues to work extremely well.  Itrace my enthusiasm for the technology to that personal experience.

One final note on the term “Heller” system.  A German engineeringcompany, GEA, appears to own the trademark rights to the name “Heller”,which they acquired when the bought EGI, the Hungarian company thatcommercialized indirect dry cooling systems.  Indirect dry cooling is ageneric technical solution that is often referred to as “the Hellersystem”.  I have no affiliation with GEA.

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