The sun can help coal fired power plants burn less coal by pre-heating the water used to make high pressure high temperature steam during periods when the sun is shining. In other words the sun would do part of the work of producing high pressure/ high temperature steam and in this manner the overall hybrid solar/coal power plant would use less coal than a coal only power plant would need to produce the same amount of electric power.

A significant cost of any solar thermal plant is the complex of boilers, steam generators, condensers and cooling towers in which water is boiled to produce high pressure steam that drives turbines transforming mechanical energy into electric energy and that recovers as much of the low pressure low temperature steam as possible in order to re-cycle it through the power cycle again. Roughly forty percent of the upfront capital cost of a standalone solar thermal electric power facility is in building the steam powered thermal electric portion of the facility that is critical in transforming the concentrated thermal energy that has been produced by the solar arrays into electricity. Maintaining and operating this portion of the power producing facility is also a significant part of the on-going operating costs.

In comparison the large arrays of solar troughs (or mirrors focused on a solar tower) that collect and concentrate the sun’s heat amount to around 50-60 percent of the total cost of a standalone solar thermal plant.

Leverage the Existing Electric Energy Producing Infrastructure of Current Coal Fired Thermoelectric Power Plants to Lower the Cost of Solar Thermal Power

By pairing solar thermal collecting arrays with existing or proposed coal fired thermoelectric power plants this very large cost – that would be necessary in a standalone solar thermal plant — can be avoided, in this manner significantly reducing the per kilowatt hour cost of the solar portion of the hybrid solar/coal plant. Various estimates have calculated that solar thermal energy could be between 30 to 50 percent cheaper by hitching a ride on existing thermoelectric power plants. This would bring the cost of solar thermal electric energy below 12 cents per kilowatt- hour or even lower making it competitive with existing grid power.

Another advantage is that because coal fired thermoelectric power plants operate at a higher temperature the overall efficiency of energy conversion from thermal energy into mechanical and hence electric energy operates at this higher level of efficiency. Existing solar thermal designs operate at around 400°C versus 500°C or higher typical in large thermoelectric plants. By pairing solar power with the existing power plant the solar contribution to the overall energy output also operates at this higher energy conversion efficiency of around 45% versus the 38% typical in existing solar thermal electric plants.

As Hank Price, director of technology at Abengoa Solar said, “It’s potentially the most cost-effective way to get significant solar power on the grid”.

The energy contributed by the sun reduces the amount of coal that the power plant needs to burn and in fact achieves a comparable reduction in green house gas emissions as would be achieved by a standalone solar power facility of similar size, but at a much lower initial cost.

Abengoa Solar and Xcel Energy Announce Solar/Coal Hybrid Power Project

Abengoa Solar, a large Spanish utility scale solar power producer and Xcel Energy, Colorado’s largest electrical utility, have begun modifying the Cameo coal plant near Grand Junction in Colorado so that portion of the energy needed to heat water is provided by the sun. This is a demonstration project and the solar contribution will be small (somewhere around 3%), but could easily be scaled up by adding more mirrors. As much as ten or fifteen percent of the total energy needs of existing coal fired power plants that are suitably sited in sunny regions and with enough surrounding land to build the arrays could come from the sun.

Not a Panacea, but it is a Low Hanging Fruit that Can Help Build out the Solar Thermal Industrial Base and Help Reduce Global Greenhouse Gas Emissions in the Meantime

Anything to do with the continued mass burning of coal is anathema to many environmentalists and many find it hard to pair the notion of renewable energy with these green house gas spewing fossil fuel dinosaurs. But once one gets past the initial instinctive distaste it is an idea that makes good sense for those coal fired plants that are sited in sunny regions and that have enough surrounding area to support the solar collecting fields. This will not change the world by any stretch of the imagination, but it can help at the margins making some of our country’s (and the world’s) coal fired power plants somewhat less polluting and fossil fuel consuming than they currently are. Additionally these solar-coal hybrids could help sunny regions meet their greenhouse gas reduction targets.

Additionally, because of the major cost savings – in the solar portion of the total power system — of these solar-coal hybrids, it will make it easier to justify adding solar power to the grid. If major utilities begin to retrofit existing coal fired power plants in suitable areas with solar power assists then the entire solar thermal industry manufacturing base will be propelled from a marginal small scale position into becoming a much larger producer.

As this happens, economies of scale and attendant improvements in the manufacturing process and solar thermal technology (such as in the glass for the mirrors and so forth) will help to create a strong solar thermal industrial base that will then be able to stand on its own and lower its own costs to grid parity for stand alone systems.

In addition in areas where there are abundant low grade geothermal resources, in many places in the American West for example, a geothermal-coal hybridized power plant could use less coal. As with the solar-coal hybrid the geothermal resources would be used to preheat the water for the boilers saving coal.

© 2009, Chris de Morsella. All rights reserved. Do not republish.

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Author: Chris de Morsella (146 Articles)

After a decade performing as a lead guitarist for rock bands, Chris de Morsella decided to return to the career his uncle mentored him in as a youth....Software Engineering. Since that time he has thrown himself into his work. He has designed a compound document publishing architecture for regulatory submissions capable of handling very large multi-document FDA regulatory drug approval submissions, for Liquent, a division of Thompson Publishing. At the Associated Press, Chris worked with senior editors at facilities around the world, to develop a solution for replacing existing editorial systems with an integrated international content management solution. He lead the design effort at Microsoft for a help system for mobile devices designed to provide contextual help for users. Chris also helped to develop the web assisted installer for LifeCam2.0, the software for Microsoft’s web cam and developed late breaking features for the product He also served with the Rhapsody client team to redesign and build a major new release of Real Networks Rhapsody client product. His most recent assignment has been Working with the Outlook Mobile Time Management team for the next release of Outlook Mobile for the SmartPhone. Chris' interests are in green building and architecture, smart grid, the cloud, geo-thermal energy, solar energy, smart growth, organic farming and permaculture. Follow Chris on Twitter.

  • Jerry Toman

    it were produced at a higher one by this slight-of-hand. The Second Law of thermodynamics “knows better”.

    Said differently–“you can’t make a silk purse out of a sow’s ear.”

    Indeed, a small amount of “extraction” steam for preheating BFW could be saved, but the overall efficiency would be reduced and capacity reduced below the nameplate. A small amount of coal burning would be saved but I doubt that the cost of a retrofit would ever pay out for just 8-10 hours a day of effective sunlight.

    It would be better to maintain the purity of the solar array and design it for its own process conditions in a “stand-alone” entity.

    If, however, an existing plant were to be retrofitted for carbon capture, one could consider a solar array for regeneration of the absorbent solution used to capture the CO2.

    The best way to increase the efficiency of a coal,nuclear or any thermal plant is to utilize the waste heat normally rejected to cooling water as input to an Atmospheric Vortex Engine, which could operate 24/7, not just during the day.


    • chris

      How does this violate the second law of thermo dynamics? Heat harvested from the solar thermal field would be exchanged with the relatively cold water feeding the coal plant’s boilers, pre-heating it — perhaps under pressure — to some degree. This “work” will then not need to be done by burning coal. Coal will still be burned to produce the high pressure steam, but it will start with already boiling water as the feedstock into the high pressure boilers. There may be technical engineering reasons why it would be hard to retrofit the existing boiler systems to be adapted to utilize a pre-heating unit, but I fail to see how this violates the second law of thermo-dynamics, total entropy increases.

      What is being described — as I understand it is a dual loop system being retro-fitted onto the relatively small number of coal thermoelectric plants that are sited in suitable locations. An outer pre-heating loop powered by the sun when solar energy is available (and possibly also from stored solar heated hot molten salt during ordinary periods of night), but that also has an auxiliary means of pre-heating the input water — brings it up to boiling temperatures (possibly under high pressure and hence hotter). Then this pre-heated fluid is fed into the inner loop where it is turned into very high pressure steam from the heat obtained by burning coal.

      Other interesting technologies such as earth (or ground) sourced heat pumps for example also employ a dual loop system; the outer ground loop pre-heats cold air (or cools it) bringing it within a narrow range of temperatures that saves considerable portion of the overall energy the inner loop heat pump needs to expend in order to raise (or lower) the outside ambient air in order to service the building heating and cooling needs.

      There are also quite a number of industrial processes in existence (or that should be in existence) that recover “waste” heat in similar dual loop systems.

      This is definitely not the BIG answer — there is no single BIG answer for our energy future — and this idea is colored by the coal industries own narrow PR self interest. It is not a long term answer, but it could serve as a bootstrap to help propel the solar thermal industrial base out of the garage and into becoming a mainstream large scale and maturing industrial sector.

  • Jerry Toman

    Nothing would please the coal companies more than to embed their plant with a huge solar array to “legitimize” and make permanent their pollution.

    The fact of the matter is that you cannot take heat produced at one temperature and “boot-strap” it so it behaves “as if” it were produced at a higher temperature. This is “sleight-of-hand” pure and simple.

    • chris

      Jerry, This post is touching on a provocative idea that causes discomfort in me as well. It is true that attaching solar thermal fields onto coal plants could be just a green washing agenda by the coal industry now that the clean coal PR campaign has begun to wither under closer public scrutiny.

      However even if this is the case, how sweet it would be if the coal companies helped nurse an US domestic industrial base for the manufacture and build out of solar fields by doing so. The large solar fields required would entail large substantial and sustained large orders for the types of ultra-clear glass and reflective mirrors, for the molten salt or other thermal carrier systems be it molten salt, hot oil or some other thermal carrier, for the highly insulated heat collecting pipes or solar tower targets, for the specialized pumps etc., for the controllers (the active solar orientation systems, and the expertise and infrastructure to assemble these component products into completed and working solar thermal fields.

      Our nation currently is grossly lacking this infrastructure and if the coal sector pays the big money to build it out — even if their motives may be impure — it will have the benefit of leaving us with a solar thermal field manufacturing and assembly base in place. A base that could then easily grow out to begin building pure solar thermal systems as the cost structure has changed due to economies of scale and the acquired expertise

  • Jerry Toman
  • Jerry Toman

    Before discussing “minor details”, I would like to address the strategic implications of amalgamating solar thermal, or any other “renewable” technology with coal.

    Why would anyone truly committed to low-carbon alternatives wish to ally their technology with coal, the “worst-of-the-worst” with respect to carbon emissions as well as other types of pollutants? If any fossil fuel could be a candidate, they’d be looking at tying the renewable to natural gas which has half the carbon footprint of coal.

    Yea, maybe “fracking” to liberate the gas may result in contamination of somebody’s drinking water or a few streams here and there, but that’s solvable– nothing compared to what mountain-top-removal does, and that’s only the beginning of the misery produced by the “coal-train”, ending up with the emission of GHGs.

    Among “semi-renewable” sources, there’s geothermal–a much better candidate than coal for a symbiotic relationship to produce power, since many geothermal sources are too weak to produce high efficiency.

    From a power producer’s perspective, why would they want to integrate solar with coal, except to the extent that it would be a good PR device, and allow them to “fudge the numbers” to make solar appear to be a larger fraction that it really is to meet renewable mandates.

    Unless one is talking about a complete shutdown of the coal plant, with a new solar farm making use of the existing machinery of the coal plant’s steam cycle (should be considered) we aren’t talking about a very large “penetration” of the solar fraction of the primary heat into the total–maybe 10% at most. By 5% you would run into significant design constraints and the operability would start to go down. By 10% penetration, it is likely that the plant would be on the ragged edge of operability

    Secondly, it would never be applied to an existing supercritical or future ultra-supercritical steam plant which could produce power with efficiencies in the high forties (based on HHV). These are just to capital intensive and too finely tuned to consider integrating a “peak” supplier, such as solar, into the process flow scheme. Too risky.

    We are thus talking about using and converting an aging plant with below average efficiency–probably in the mid-thirties. With CSP, this degree of efficiency can already accomplish this, as you admitted.

    This is getting longish–I hope to address the remaining issues in a follow-up.

    Summary: Confuses the information, makes pact with the “devil”, solar penetration limited to roughly 5% of total thermal capacity, too risky to integrate into future “ultra-supercritical” designs of coal plants which are being considered to compete with natural gas ICCPs in terms of efficiency.

    • chris

      Solar/coal hybrid is a hold your nose proposition — as I mentioned in the post — and it is quite likely that the coal lobby will try to use even minuscule solar field supplemental add ons to existing plants to try to obtain massive green washing bennies in the public mind and to confuse the public understanding (or misunderstanding of the issues involved). I agree with and share your concerns in this regard; however Abengoa Solar is one of the world’s largest and most committed solar companies and it is partnering with Xcel Energy to build the solar field for this pilot solar/coal hybrid — Abengoa can hardly be characterized as not being committed to the growth of solar energy.
      Perhaps you are correct that it will only be applied to aging and marginal plants and will never be incorporated into higher efficiency high temperature super critical plants. However even if this niche application of solar thermal onto solar/coal hybrid plants manages to reduce coal usage by 5 to 10 percent it is worth looking at, both because at this point every improvement — even at the margins — that reduces our green house gas emissions is important and because the currently minnow sized solar thermal industry needs to start laying down large scale fields one after the other in order to build out the industrial/research base and accumulated know how it needs in order to become truly viable as an base line energy provider.
      I still do not see why a dual loop system is technically or even economically infeasible for supercritical steam plants. The outer low temperature loop could be driven by the sun (or stored solar energy) when possible and could be fired by burning fossil energy when needed. In either case the low temperature outer loop would provide a partially heated feed stock of high pressure hot water into the inner super critical loop, which would operate independently of the outer one.
      I think your idea of re-cycling the existing steam cycle plant from old coal (or other fossil fired) thermal plants to be “fired” by the heat of the sun is an idea worth investigating and could in some cases lower the overall capital expenditure needed in order to build a large solar thermal plant. Solar thermal power is also being mated with natural gas fired plants in solar/natural gas hybrids.
      Finally it bears mentioning that if coal had to pay the true costs involved with its extraction, transport and burning it would be a far more expensive energy source than our misleading and tragically incomplete cost accounting system currently assigns to it.

  • Jerry Toman

    You and I are not the only ones having concerns about integrating solar with coal:

    To “piggyback” solar onto a dirty coal plant sends the wrong message, IMO. It is similar in some ways to the PHEV–which enjoys greater range than pure EV at the expense of greater vehicle weight. The perception is that this is beneficial when in reality is simply means that more weight is added on to the EV, which causes it to consume more KWhs than it otherwise would.

    Concerning the Second Law, I will give you that, at low penetration of the solar heat component, it is possible to “free up” capacity in the radiant and HT convection section of the furnace for more HT heat absorption and marginally greater efficiency than stand-alone thermal.

    But the greater expense of the feedwater preheater using fluids at lower temperature, as compared to standard design (steam extraction) would tend to offset much of the efficiency gain, IMO. JJT

    • chris

      It is in some ways a deal with the devil metaphorically speaking. I wonder what Abengoa’s strategic reasoning is for entering into this project — or was it driven purely by short term tactical reasons i.e. improve the bottom line in a tough market going through a period of retrenchment. XCel itself is an interesting vertically integrated power company — their current generating capacity is still dominated by coal and natural gas, but they are the nation’s number one utility for wind power with 3000 megawatts of wind energy capacity in service system-wide in 2008. As an aside they are leaders in the move to building out the smart grid as well. Perhaps there is some forward thinking going on in XCel Energy. But then perhaps it is really just geenwashing as well.

      Solar thermal hybridization of gas plants makes sense for the similar reasons — piggy backing on the existing thermal-cycle plant (although gas turbines are of a very different design than the steam cycle of coal plants) . I believe Abengoa is pursuing this solar-gas hybridization in a facility in North Africa.

      What would be more interesting in my opinion is the idea that you mentioned i.e. the decommissioning of a coal fired power plant — located in a suitable area for solar thermal power — that re-fitted the existing plants steam cycle infrastructure to make it suitable for use with a solar thermal power source, especially if the solar collecting fields were over-sized to charge thermal batteries so that the plants hours of operation could be extended beyond the hours of core sunshine and the plant could deliver power when it was needed.

      In general however anything that can extract more usable electric energy from existing thermoelectric power plants will lead to lower overall carbon emissions. For example tacking on AVE technology onto a large coal fired plant could extract more work out of the heat and by doing so reduce the need for yet more power plants (or the need to keep dirty older ones online).


  • Jerry Toman

    Indeed, Chris–adding on a bottoming cycle (AVE) that can take advantage of the much colder “heat sink” of the upper troposphere could easily add about a third more generating capacity to an existing coal fired plant without burning any more coal. However, the same holds true for a CSP plant, where up to 50% more, electricity could be generated from a plant with less than 30% thermal efficiency (70% of incident solar heat rejected to CW or otherwise lost to the ambient)

    While a few modern coal plants could be retrofitted while taking down some of the older, inefficient ones initially, I really think we’ve got to get rid of at least half of our coal-based emissions within 10 years, if for no other reason than to be an example for the rest of the world–which will not act until the US does.

    I guess there’s a long shot that some CO2 from coal could be captured and stored, but only by a few plants that happen to be in the right place.

    • chris

      Hopefully someday — soon — enough decision makers will become educated about AVE technology and convinced of the promise it holds to put some serious venture capital into some large scale pilot demonstration plants.

      Carbon capture is largely an exercise in fraudulent messaging and greenwashing — IMO. The reason I am of this opinion is because of the physics of it all. Burning coal, which for practical purposes is almost pure carbon, produces carbon dioxide as it’s main effluent gas. Carbon dioxide gas has an atomic weight of 44 (12 from carbon and 32 from oxygen). Thus in order to calculate the resulting mass of CO2 that is produced by burning coal we need to multiply the mass being burned by a factor of 44/12 (or 3.7), which is the atomic weight of the CO2 divided by the atomic weight of the carbon constituent of the gas. Thus for every ton of coal that is burned around 3.7 metric tons of CO2 is produced. CO2 is a gas at ambient temperatures and as a gas it occupies far more volume than the original original volume of the coal. This is because it is a gas. In fact, one ton of CO2 occupies 556.2m³ of volume at sea level pressure and room temperature [see: for formula]

      Anthracite Coal has a specific density of roughly around 1.5 — so a meter cubed of anthracite weighs roughly 1.5 metric tons and when burned produces around 5 and 1/2 metric tons of CO2, which is 3059 meters cubed of CO2!

      Even if all of the CO2 can be captured, which itself is debatable where will all of this gas be stored? Minuscule amounts of the world total could hypothetically be stored in a few geologic repositories, but the available geologic repositories will never be able to store more than some vanishingly small portion of the total CO2 produced — even if it was pumped in at very high pressure.

      Thus any gas based sequestration is clearly impossible. I have heard that under the high pressure regimes existing on the sea floor it might be possible to store dry ice CO2 — although the pressures would have to be above 5,000 atmospheres in order for CO2 to remain in its solid phase. I just don’ think it is energetically possible to turn CO2 gas into CO2 ice and doubt that even the abyssal plain has a high enough pressure at its bottom to keep CO2 stable in its solid form (prehaps as a supercritical fluid). But just the energy required for this even if it is technically possible rule it out. Chemical sequestration based on forming carbonates requires the the mineral resource that has the capacity to absorb CO2 to produce a carbonate. Again nothing I have seen seems even remotely likely to fit the bill.

      Carbon sequestration is a fraud… a PR exercise meant to confuse the public and politicians with an always hypothetical promise that someday perhaps CO2 will be able to be sequestered — so therefore there is no reason why we should not continue to burn the stuff.