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The Secret to Low-Water-Use, High-Efficiency Concentrating Solar Power
Joe Romm, 30 Apr 09

Many readers have expressed interest in learning more about the water consumption of concentrating solar power and how measures to reduce it might 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.  I asked Hogan, a long-time power industry executive and currently the Power 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).

EXECUTIVE SUMMARY:  If concentrating solar power (“CSP”) is a core climate solution, indirect dry cooling systems (also known as “Heller” systems) will be a crucial enabling technology, since large-scale CSP will be located in desert regions. US power companies have long favored direct dry cooling systems for fossil plants, probably because of the visual impact of Heller systems.  But Heller systems have long experience in certain regions and will probably play an important role in the success of large-scale CSP.  This is due to their higher efficiency, smaller footprints, quieter operation, lower maintenance, higher availability, and more flexible site layout.  Heller systems can reduce water consumption in a CSP plant by 97% with minimal performance impact.  The height of the cooling towers should be less of an issue in remote desert locations, especially since the central tower in power 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 solution due to their almost unique potential to replace coal as the dominant supplier of baseload and/or firm dispatchable capacity to the world’s power 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 its efforts to educate readers on the singular challenges of eliminating coal-fired power production at scale.   But if CSP is a core climate solution, dry cooling technologies, and in particular Heller systems, will be a crucial enabler (see note at the end regarding the status of the name “Heller” system).

One of the concerns often cited about CSP is water consumption, particularly because the technology’s reliance on direct normal insolation means that it is most economically located in desert regions.  Because most CSP systems rely on Rankine cycle steam turbine-generators to produce electricity, they face the same requirements as fossil-fired power plants for condensing large volumes of saturated steam back into boiler feedwater. (Parabolic dish systems use Stirling or Brayton engines to produce useful energy, each of which has its own advantages and disadvantages)  Where an abundant and cheap supply of water is available, the most efficient way to accomplish this is by evaporation (or “wet cooling”), which is what produces the large plume of water vapor one often sees rising from power stations.  Convective cooling using ambient air (“dry cooling”) requires higher capital costs and can reduce plant performance, and thus planners of fossil plants have sought to locate them close to adequate supplies of cooling water whenever possible.

In the desert areas where CSP will thrive, the consumption of large amounts of water by conventional wet cooling systems is clearly unsustainable.  Dry cooling alternatives will be required, and CSP will have to demonstrate its commercial viability despite the capital cost and performance penalties this will entail.  Fortunately this is an eminently manageable problem.

[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 study in 2007 comparing a particular dry cooling technology, the Heller system, with wet cooling for CSP plants in Spain and in the California desert (see figures above).   Water consumption was reduced by 97%, and the performance impact was quite minimal.  Indeed the impact on performance in the higher desert temperatures of California was overwhelmed by the benefits of better annual insolation.  They also noted that the potentially negative impact of high daytime temperatures is mitigated by the use of thermal storage, which uses energy collected during peak daytime insolation to produce electricity when temperatures are considerably lower.  One interesting aspect of the DLR study was their 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 where necessary -– “direct” air cooling (usually called an “air-cooled condenser” or “ACC”) and “indirect” air cooling (often referred to as the “Heller system”, after Laszlo Heller, the Hungarian thermodynamics professor who pioneered this approach in the 1950s).  In ACC systems, the saturated steam from the steam turbine exhaust is carried directly to a very large array of A-framed fin-tube bundles, where large mechanical fans force air over the tubes, convectively condensing the steam.

ACC system

In Heller systems, the steam is condensed by spraying water directly into the exhaust flow in a ratio of about 50:1 (called “direct contact jet condensing”), creating a large volume of warm water, some of which is pumped back to the boiler as the working fluid and the rest of which is pumped to bundles of tubes arrayed at the base of a natural-draft hyperbolic cooling tower.  The warm water circulating around the base of the tower and the cooler air at the top of the tower, combined with the tower’s hyperbolic shape, stimulate a powerful updraft that draws ambient air over the tube bundles, thereby convectively cooling the water 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 are none in the US.  This is probably because the much lower auxiliary power requirements of Heller systems come with the visual impact of a large hyperbolic cooling tower (typically 150m high and 120m in base diameter), often a difficult sell given that most fossil power stations are located in the vicinity of the populated demand centers they’re intended to serve.  The auxiliary power required to run an ACC system is roughly twice the power required run a Heller system, and the Heller system is considerably quieter, but these have apparently been considered prices worth paying for the lower profile (a typical ACC system can be 40m high), particularly when it was cheap coal-fired power.  Simple lack of familiarity could be another factor in the hidebound world of US power utilities.

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

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

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

They also note that the footprint of an ACC system is larger than that required for a Heller system, though specific data is not offered.  Overall system efficiency of a Heller system is in the range of 2% better than an ACC system.  That performance improvement meant one thing in a fossil power plant in the bad old days of cheap dirty power, but when it means 2% less land area covered by solar collectors, and lower auxiliary consumption of much more costly power, it takes on a much greater significance.  The same sources note that since the Heller systems are mechanically far simpler than ACC systems, maintenance is much less of an issue and system availability is significantly greater.  In the remote areas where these plants will be located, and given the large land areas over which they will spread, these are far more significant considerations than they were for compact fossil power plants located close to the populations they served.  Another factor noted in these sources is that an ACC must be located next to the steam turbine it serves, because of the cost of transporting saturated steam over any distance, whereas the Heller system has much more flexibility in where the cooling tower is located.  This will be much more important to CSP, where one can envision clusters of power tower complexes in a given area each with its own steam turbine, than it was with fossil plants.  And finally, the feature that most worked against Heller systems in US fossil plant applications – visual impact – should be far less of an issue in remote desert sites, especially with solar power tower complexes where the central towers will likely be of similar height.

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

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

This piece originally appeared in Climate Progress.

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I am happy to see it pointed out that large amounts of water are not needed for solar power. Thanks.

There seem to be two design decisions here: whether to use direct or indirect condensation, and whether to use a cooling tower or forced cooling. Only 2 of the possible 4 combinations are discussed. Lets say I've decided I will use a cooling tower. What advantage is there to using the Heller system over placing a direct condensing system in my tower? (Other than less steam piping, I get that).

I see one advantage to direct: The steam is hotter than the water will be in the Heller system, so the fluid to air heat exchanger will be smaller, and hence cheaper.

Posted by: Bart Hibbs on 1 May 09

Isn't there a way to deal with the salts in piped in ocean water? If so, the benefits would obviously be enormous. On such a scale as needed, this shouldn't reflect much of difference in price, possibly even cheaper?

Also, the many thousands of square miles that (we should all promote that) need to be covered with mirrors stats can NOT BE BULLDOZED! (Sorry solar troughs!)

As for water for cooling, the Brayton cycle uses less since it is more efficient (higher temps from larger mirror fields).

Posted by: Robert Bernal on 6 May 09

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