Coal–Solar Hybrid Power Plant

Mills [41,43] explored the concept of combining solar with conventional coal-fired power generation. This approach offers a route to combining renewable energy with inexpensive stable output from existing (or new build) thermal generation assets. In suitable locations, solar radiation can be harnessed and used to raise steam that can be fed into an existing conventional coal-fired power plant (a coal- solar hybrid). In such a system, solar thermal energy can be used to produce high- pressure and high-temperature steam that can be integrated into an existing power plant’s steam cycle in several ways such that power output is boosted and/or coal consumption reduced.

Most existing solar thermal designs operate at ~300°C-400°C, lower than that of a typical modern coal-fired power plant (operating at 500°C or more). Thus, the temperature of the steam from the solar field is not high enough, and further heat must be provided before it can be fed to the plant’s steam turbine(s). Feeding steam produced by the solar collection system directly into the main turbine can increase the overall efficiency of the plant by making the best use of the steam output from the solar field. However, the conditions of the steam generated by this must be matched to the coal-fired steam turbine cycle—this can be an engineering challenge [41, 43]. Alternatively, solar thermal energy can be used to heat the feedwater prior to entering the boiler. In a conventional steam power plant, as feedwater enters the feedwater heater, steam is extracted from the steam turbine to heat it. When solar heat is added to the feedwater, less steam is extracted from the turbine; this reduces coal input, increases the unit electrical output, or both.

Mills [43] points out that potentially, there are a number of points where steam generated from solar power could be fed into a conventional coal-fired power plant. This will be influenced partly by the type of solar collection system employed, as these tend to operate at different temperature ranges. Thus, candidate locations for steam from trough-based collectors include feedwater heating, low-pressure cold reheat, and high-pressure steam between the evaporator and superheater. In the case of power towers, possible locations are similar to those from troughs, but would also allow higher temperature admissions to hot reheat or main steam circuits. Possible locations from systems using Compact Linear Fresnel Reflectors include feedwater heating and low-pressure cold reheat [43]. Thus, the main applications are preheating of boiler feedwater, additional preheating of feedwater downstream from the top preheater, and the production of intermediate pressure (IP) steam or main steam [43]. According to Mills [43], the merit order of the various hybridization modes is: (a) solar preheating of high-pressure feedwater, (b) additional solar preheating of feedwater, (c) solar heating of low-pressure feedwater, and (d) solar production of high- and intermediate-pressure steam .

Some existing coal plants are particularly well suited to hybridization as they already allow operation in boost mode. As a turbo generator often has a capacity margin, this is achieved by closing the highest pressure steam extraction (but with an efficiency penalty). To date, feedwater heating has been the focus of several projects [41,43,45-47]. On coal-based power plants, cycle efficiency is improved by preheating feedwater before it enters the boiler. The preheating is carried out through the use of a train of preheaters that extract steam from the turbine at various pressure levels. By replacing the highest pressure steam extractions with solar steam (fully or partially), water preheating can be maintained while expanding more steam through the turbine, thereby boosting its power output [41,43].

According to Mills [43], it is anticipated that some developing countries will build concentrated solar power (CSP) plants as well as new coal-fired units; they may already operate the latter. Thus, there are a significant number of potential sites, both existing and newly built, in countries that benefit from a good supply of solar energy [43]. The incorporation of solar energy into an existing coal-fired power station has the potential to increase overall plant efficiency, reduce coal demand and CO, emissions, plus minimize the problem of solar power’s variability. At night, or when solar intensity is low, the output from the coal plant can be increased accordingly, allowing the combination to operate on an uninterrupted basis, 24 hours a day. When adequate solar intensity resumes, the coal plant can be ramped down once again. Alternatively, the increased steam flow produced by the solar boiler can be fed through the existing steam turbine, boosting output (so-called “solar boost”). This method of incorporating solar energy will cost less than an equivalent standalone CSP plant as many of the systems and infrastructure of the coal plant, such as steam turbine and grid connection, are already in place. A stand-alone plant requires all such systems in order to function. The levelized cost of energy (LCOE) from a coal-solar hybrid will be lower than that of a stand-alone CSP plant and be able to compete with that produced by PV systems [41, 43].

Solar collection systems normally operate using focusing mirrors or similar that track the sun’s path and concentrate solar radiation onto a central point(s) where the heat is transferred to a heat transfer fluid. This is then used to raise steam that is fed into the coal plant and expanded through conventional steam turbines. As noted, the heat generated can be fed into the water/steam circuit of the coal plant to boost power output and/or reduce coal demand. Steam can be injected at several possible places in a conventional Rankine cycle. Rankine cycle is used in the vast majority of conventional steam-based thermal power plants where an operating fluid is continuously evaporated and condensed.

As pointed out by Mills [41, 43], steam turbines in most power plants have an excess capacity margin that allows increased output. By incorporating solar-derived steam, the amount of steam bled off the turbine for feedwater heating (which creates an efficiency penalty) can be reduced. This makes more steam available for expansion through the turbine, increasing its output (boost mode). The main modifications required to the coal-fired plant’s steam cycle are often limited largely to the heat recovery steam generator (HRSG) which must be capable of handling the steam coming from the solar steam generation system.

In recent years, a number of coal-solar hybrid projects have been developed or proposed; some have focused on solar boost via feedwater heating, whereas others have adopted alternative hybridization configurations such as solar boost with superheated (SH) steam fed into a cold reheat pipe and coal saving with additional feedwater preheating after the power plant’s top preheater [43, 48]. Clearly, any solar-based systems can only operate effectively in locations where the daily level of sunshine is adequate. A further requirement for coal-solar hybrids is the availability of suitable land, close to the existing power plant, needed for the solar collection system. Up to several thousand hectares may be required. Usually, CSP technology involves concentrating sunlight onto a receiver that contains a heat transfer medium, either oil-based fluid or a molten salt. Several types of solar collection devices are available commercially, and the choice will be influenced by factors such as land availability. The solar collector field can comprise 30%-50% of the cost of a CSP plant.

Around the world, in recent years the application of CSP has grown steadily. Many industry observers consider that the technology can provide a route for harnessing the huge solar resource with potentially better dispatchability than via PV cells. Also, required CSP can be built at lower costs, if it is integrated with conventional fossil fuel power plants, by “coal-solar hybridization” process. The hybridization will allow a path to implementing and maturing the technology, at lower cost than for new greenfield installations. An additional bonus of hybridizing is that by eliminating part of the coal feed, plant emissions can be reduced. Under some circumstances, the addition of a renewable energy source to a coal-fired plant (“greening” existing coal-fired assets) could allow access to feed-in tariffs or other forms of subsidy. Compared to conventional fossil fuel-fired power plants, the cost of electricity produced from solar power remains high. However, potentially, solar augmentation could provide the lowest cost option for adding solar power to an existing generation fleet [41, 43, 45-48].

Some existing coal plants are particularly well suited to hybridization as, assuming that the turbo generator has the corresponding capacity margin, they already allow a “boost mode” by closing the highest pressure steam extraction. Hybridizing in this manner could provide a power boost without extra coal consumption. If the solar potential exceeds the turbine’s extra capacity, coal saving is possible. In current coal-solar hybrid plants, solar steam feeds only the highest pressure preheater, but other hybridization concepts could be adopted and combined to increase the solar share, especially on greenfield projects [43]. Such solar boosters increase capacity and energy generation without extra coal consumption, and with virtually no other extra cost than that of the solar field [43, 45-48].

A. Advantages of coal-solar hybridization

Depending on the particular circumstances, the main advantages cited by Mills [41, 43] for coal-solar hybridization are:

• The higher initial investment is balanced by reduced fuel consumption or increased power output.

  • • Combining the two technologies allows “greening” of existing coal-fired power assets.
  • • Hybridization can provide both dispatchable peaking and base load power to the grid at all times. CSP coupled with conventional thermal capacity (with or without thermal storage) can offer that capability.
  • • Hybrid technologies could help meet renewable portfolio standards and C02 emissions reduction goals at a lower capital cost than deployment of standalone solar plants. Capex (Captial expense) is less for the same capacity.
  • • Siting solar technology at an existing fossil fuel plant site can shorten project development timelines and reduce transmission and interconnection costs.
  • • Solar thermal augmentation can lower coal demand, reducing plant emissions and fuel costs per MWh generated.
  • • Solar augmentation can boost plant output during times of peak demand. According to US studies carried out by Electric Power Research Institute (EPRI), potentially, a solar trough system could provide 20% of the energy required for a steam cycle.
  • • Hybridization will reduce the level of coal and ash handling, reducing load on components such as fabric filters, pulverizing mills, and ash crushers.

It could also avoid the requirement to upgrade fabric filters or electrostatic precipitators (ESPs).

  • • Solar input could provide some level of mitigation against difficult coal contracts, such as wet coal, fines, and variable coal quality.
  • • The majority of solar plant components could be sourced locally, helping boost local economies.
  • • Rapid deployment—depending on size and configuration, hybrid plants could be completed in less than two years from notice to proceed.
  • • Hybridization could be used to extend the lifespan of existing thermal facilities—for example, where regulatory changes require a coal-fired plant to reduce emissions or face closure.
  • • Hybridization could avoid certain limitations and restrictions applied to new greenfield site projects.
  • • Hybrid plants will benefit from the general cost reductions that CSP technology is achieving. Many of these will also be directly applicable to hybrid plants.

B. Disadvantages of coal-solar hybridization

Although coal-solar hybridization can provide some benefits, according to Mills [41, 43] there are obvious criteria that must be met:

  • • The location must receive good solar intensity for extended periods, both on a daily and yearly basis. This is not always the case.
  • • A suitable area of land close to the existing thermal power plant is required.

It must meet certain criteria in terms of hectares available, topography, and issues such as shading.

  • • The land will no longer be available for other purposes such as agriculture.
  • • A solar add-on will require capital investment.
  • • There will be additional costs for operation and maintenance of the solar component, such as mirror washing.
  • • The scale of most coal-solar hybrid projects has so far been low, mainly because these have been retrofits at existing coal-fired plants. Practical issues have tended to limit the solar contribution to ~5%. A new pow'er plant, designed and built based on the hybrid concept from the outset, could possibly accommodate up to 30%-40% solar share.

Each potential project brings its own combination of advantages and disadvantages. Although there are certain areas that will be common to all, there are various factors that will be specific to each individual site—projects need to be examined on a site- by-site basis. The addition of a thermal storage system, used to store excess solar heat harvested during daylight hours, can be important for solar thermal power generation. When demand dictates, heat can be reclaimed and used to raise steam. This is a major advantage over PV-based generation as it is easier to store large amounts of heat than electricity. The ability to reclaim heat at night means that electricity from a plant incorporating thermal storage can usually be considered dispatchable, whereas that from a PV plant is not.

According to Mills [43], not all coal-solar hybrids would necessarily opt to include thermal storage. The main reason for its addition is to increase the capacity factor, which increases the utilization of the pow'er plant and thereby improving the overall economics. Usually, around half of the cost of a stand-alone CSP plant comes from the pow'er island, with the balance from the solar equipment. In a hybrid plant, the pow'er block is shared with the coal-fired portion and is therefore already available. Consequently, there may be less incentive to add thermal storage to a hybrid. However, adding storage can increase the utilization of the solar portion, making it easier to control and improve the plant’s reliability. The viability of a coal-solar hybrid w'ill be influenced by numerous site-specific factors, and the addition of thermal storage is likely to depend on the specific project. Various challenges remain for the large-scale deployment of coal-solar hybrid systems—these may be political, technical, or financial. From a purely practical point of view, any solar-based power generation system needs a consistent source of sunlight of adequate intensity, and this clearly limits possible locations. Although arguably less important for PV systems, it is crucial for those based on solar thermal technology. Coal-solar hybrids require land close to the power plant for the solar collection system, possibly up to several thousand hectares. However, land immediately around a pow'er plant is sometimes unattractive for other purposes, so may be readily available. A possible complication is that, often, coal-fired plants are located near a source of water needed for cooling. This means that they are rarely located in arid high-desert sites that are well suited to solar thermal applications [41,43].

It appears that any outstanding technical problems associated with hybridization can be overcome. However, depending on the individual circumstances, the biggest issue may simply be economic viability. In some locations, hybrids may have to compete directly with other forms of power generation such as natural gas-fired plants. At the moment, gas prices in some parts of the world remain low, and this will undoubtedly make investment in hybrids more uncertain [47]. However, there are situations where alternatives are much more limited, and here, coal-solar hybrids could find useful niche markets. More details of this concept are described in detail in an excellent reports by Mills [41,43].

 
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