Power Engineering

Solar Steam Boosters for Coal-fired Power Plants

The 25 MWt, 5 MWe Kimberlina solar field near Bakersfield, Calif.

By Milton Venetos and Dr. William (Bill) Conlon, Areva Solar

As electricity demand in the U.S. Southwest continues to climb amid the uncertainty over the future costs of carbon emissions, coal-fired power plant owners and operators continue to seek ways to expand their electricity production without increasing their emissions and fuel costs. Thanks to the region’s reliable solar insolation, solar solutions are available now.

One cost-effective, proven solution is solar steam augmentation from concentrating solar power (CSP) technology. An example of this is the Areva solar compact linear Fresnel reflector (CLFR) solar steam generator (SSG). These generators can be integrated with existing coal-fired power plant equipment in a relatively short time, to generate high-pressure saturated or superheated steam that can be injected into a plant’s steam cycle, helping to generate additional electricity. These systems may also integrate with natural gas-fired, combined-cycle power stations to boost capacity without added emissions.

Solar Steam Augmentation: What It Is

Solar steam can augment existing steam systems, ranging from thermal power stations to industrial process applications, to increase output, reduce emissions and fuel costs, and generate renewable energy credits. Integration with existing steam systems is a straightforward way for electric utilities and steam plant operators to use solar energy and leverage their assets.

Figure 1 illustrates how the CLFR solar steam generator functions. Rows of nearly flat mirrors rotate as the sun traverses the sky to reflect and concentrate solar energy onto an elevated receiver. The Areva Solar receiver is an ASME Section I Boiler, which directly generates high-pressure superheated steam, unlike competing systems that use intermediate heat transfer fluids. A typical Areva SSG is about 1,640 ft (500 meters) long by 140 ft (43 m) wide by 60 ft (18 m) tall, giving each SSG a footprint of 5.3 acres (2.15 hectares), and making it the most land-efficient CSP option. The solar steam generator is laid out with its length running north to south. An electric motor drive system moves the reflectors to track the sun in one dimension as it moves from east to west.

Each SSG is rated at 9 MW thermal (MWt), or about 3 MW electric (MWe), generating gross annual thermal energy of 39,325 MMBtu (11,525 MWh) from its footprint. Earlier versions of the CLFR technology delivered relatively low temperature steam, but recent advances by Areva at their Kimberlina solar thermal energy plant have led to superheated, high-pressure steam conditions that match the needs of power customers. Currently, the Areva system can deliver solar steam temperatures up to 750 F (400 C) at 1,535 psia (106 bara); by 2011 temperature and pressure are expected to increase to 900 F (482 C) at 2,400 psia (165 bara).

Areva’s solar boilers are the first and only solar boilers to receive the American Society of Mechanical Engineers (ASME) “S” stamp. Areva Solar also received the National Board Certificate of Authorization “NB” to register its solar boilers.

Applying Solar Steam Augmentation

Applying CLFR technology as an augmentation to an existing coal-fired power plant is fairly simple. The SSGs replace extraction steam used for feedwater heating. The steam that is not extracted continues to expand through the steam turbine to produce more electricity without increasing emissions. Figure 2 illustrates this process.

Replacing some or all of the extraction steam for the final high-temperature or top feedwater heater is possible because many coal plant designs enable this feedwater heater to be partially or fully bypassed, to make more power at a lower efficiency. However, when solar steam is added, the plant can operate at full efficiency, producing additional power without burning additional coal.

Areva Solar has demonstrated this system with a 9 MWt/3 MWe pilot project at the Liddell Power Station in New South Wales, Australia. This was among the first solar/coal-fired booster project in the world and it employed an earlier version of the Areva CLFR technology, which had a lower peak power rating. Two SSGs are interconnected to the top feedwater heater of one of the four 500 MWe units at that station. These SSGs have delivered steam to the power plant since April 2008. A 44MWe booster facility using Areva Solar CLFR is being planned in Australia and several other CSP firms are working on augmentation facilities around the world.

Heat balance diagrams (shown in Figures 3 and 4) illustrate the opportunity and means to add solar steam to a supercritical, 750 MWe coal-fired power plant.

Figure 3
Figure 4

Figure 3 shows the design point heat balance for a roughly 750 MWe coal-fired power plant operating without solar steam augmentation. In most coal-fired power plants, the steam cycle includes a series of feedwater heaters that extract steam from different stages of the steam turbine to preheat the feedwater to the boiler. The highest temperature (or “Top” feedwater heater) is labeled “Top Heater” in Figure 4. In this particular power plant, the top heater can be partially bypassed to run the plant in an overload mode, which increases the plant’s power output but also reduces its efficiency. Since the top heater can be bypassed, there is an opportunity to augment the plant’s output with solar steam fed to the top heater, to produce the power of the overload mode at nearly the efficiency of the normal operating mode. In Figure 4, the extraction flow to the top heater has been reduced from 68.9 to 24.5 kilograms per second (kg/s), which increases the power plant’s output from 744.4 MWe to 781.8 MWe, but reduces its efficiency from 45.5 percent to 44.3 percent. A CLFR solar field that would supply to the top heater with 44.4 kg/s of steam would restore efficiency to nearly 45.5 percent and provide most of the additional power without burning more coal.

An alternative integration strategy for older, lower-pressure subcritical coal-fired plants would be to supply highpressure saturated steam from the solar field to the boiler’s evaporator outlet or drum. Potentially, this alternative offers larger fuel savings by displacing greater amounts of fossil steam with solar steam than in the bypassed feedwater heater case. Detailed studies of the target plant would be required to determine the feasibility and actual performance of such an application.

Performance for a Hypothetical Coal-fired Plant

Table 1 (on page 178) provides estimated performance data for a hypothetical 600 MWe coal-fired power plant in Phoenix, Ariz. This system is designed to supply steam to the plant’s top feedwater heater, as shown in Figure 2 (page 190).

With the assumptions outlined in Table 1, the solar field would perform as shown in Table 2.

Tables 3 and 4 show the average steam production per month per hour and average gross electrical production per month per hour, respectively.

Benefits of CLFR Solar Steam Augmentation

Using CLFR technology for solar steam augmentation enables operators to reduce fuel consumption and emissions, and to do so most productively during times of peak demand. Its small footprint (illustrated in Figure 5), scalability, modularity, ability to be sited on grades up to 3 percent, rapid time to deployment and use of water rather than an heat transfer fluid all provide advantages over other solar augmentation options.

Depending on which states are considered to comprise the U.S. Southwest, the area hosts 27 to more than 80 coal-fired power plants, producing between 10,000 and 40,000 MW of electricity. Solar augmentation of existing coal-fired power plants using Areva Solar’s CLFR concentrating solar power technology offers those facilities a cost-effective strategy to quickly boost that capacity with solar energy, while reducing fuel, emissions, and operation and maintenance costs.

Authors: Bill Conlon is vice president, engineering at Areva Solar, responsible for solar steam systems design. He received degrees in Nuclear Engineering from Rensselaer Polytechnic Institute, including a Ph.D. in boiling heat transfer and two-phase flow. Mr. Venetos leads development of Areva Solar’s proprietary performance modeling systems. He holds a Master of Science in Mechanical Engineering from Stanford University and a Bachelor of Science in Mechanical Engineering from Worcester Polytechnic Institute.

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