Economic Considerations
Which of these CSP technologies is the most attractive to a potential ISCC developer? Once the technology options are identified, a detailed economic analysis must be performed to determine the levelized cost of electricity for the specific site under consideration, including capital costs, O&M costs, and performance data for all the different operating scenarios. This analysis must look at different MWth solar inputs to the combined cycle and different solar technologies that generate different steam conditions.
Also, site data must be carefully examined to quantify the energy contribution of the solar facility that will define the overall performance characteristics of the ISCC. Data available from the National Renewable Energy Lab (NREL) give hourly dry bulb temperature, relative humidity, and solar insolation for many sites. Software programs, such as the NREL Solar Advisory Model (SAM), help analyze a particular plant configuration. Figure 7 illustrates the results of one such preliminary analysis. It shows average solar thermal energy production in MWth for January and August for each hour of the day.
7. Cyclical solar energy. These are typical solar energy production curves for a summer day and a winter day. The shape of the curves will be specific to the site selected. Source:Bechtel Power
Figure 8 illustrates the performance characteristics of our hypothetical 2 x 1 combined-cycle plant modified to accept 100 MWth of solar energy input in the form of HP saturated steam converted into electricity at essentially the bottoming cycle efficiency. The bottom line in the figure is the "no solar" case used for comparison with a conventional combined cycle.
8. Mixing gas and solar. The potential additional power generated by an ISCC is illustrated here. The bottom line represents the case where the solar field produces no thermal energy, which is equivalent to a conventional combined-cycle plant. Source: Bechtel Power
An advantage of solar energy over other forms of renewable energy is that it produces energy when it is most needed: during peak times of the day and year. "Time of delivery" pricing, where energy payments vary with time of day and attempt to mirror the cost of generation, can greatly help a solar facility. For example, some Pacific Gas & Electric Co. power purchase agreements include time of delivery pricing that values energy produced during "super-peak" periods (from June through September between 1 p.m. and 8 p.m. Monday through Friday) at rates almost double those at any other time of year. The available pricing structure must obviously be included in an economic analysis to assess the viability of any hybrid solar plant configuration.
Options for Integrating Solar with Rankine Cycles
Many of the same issues confronted when integrating solar technology with a combined cycle are similar to those encountered when integrating solar technology with Rankine cycles — with one notable exception: All of the electrical power produced in a conventional steam plant is produced by burning fossil fuels. This fact can be very advantageous for a hybrid cycle in which solar energy displaces fossil fuel energy. In addition, boiler efficiency typically increases slightly as boiler load is reduced, so using solar energy to reduce boiler load to save fuel is an added advantage.
Integration with Rankine cycles will only be touched on here, as most integration applications to date have focused on ISCC.
Medium-Temperature Solar Technology. A typical subcritical Rankine cycle power plant has turbine throttle steam conditions of ~16.6 MPa (2,400 psia) and 538C (1,004F). As with a combined cycle, medium-temperature solar technology can generate saturated or slightly superheated steam injected upstream of the superheater sections of the boiler. However, integration of solar steam into the boiler proper is a much more complicated proposition than in a combined cycle due to the higher gas temperatures involved and complex boiler fuel controls.
Most solar energy options assume the steam produced by a CSP system will heat feedwater to displace turbine extraction steam to the feedwater heaters (Figure 9). Reducing or eliminating extraction steam to feedwater heaters appears to be the most practical application for medium-temperature solar integration, as it avoids complex issues of integration with the boiler and its controls.
9. Medium temperature solar integrated Rankine cycle. Source: Bechtel Power
High-Temperature Solar Technology. Power tower technology can be used to generate superheated steam for injection into the main steam line to the turbine. The same amount of cold reheat steam can be extracted and reheated in the solar field (Figure 10). This approach minimizes potential integration problems with the conventional steam boiler.
10. High-temperature solar integrated Rankine cycle. Source: Bechtel Power
Low-Temperature Solar Technology. Options for integrating low-temperature solar technology are limited primarily to generating steam or heating feedwater to reduce the turbine extraction steam to feedwater heaters.
Controls and Transient Behavior
Any solar hybrid cycle must include an in-depth analysis of the transient behavior of the entire system, including the solar steam source. Because there is no field experience with utility-scale ISCC, the true complexity of the problems associated with normal start-up, normal and emergency shutdown, and load transient behavior (to name a few scenarios) must be carefully analyzed.
The final dynamic model will be of an integrated system capable of predicting steam temperature and pressure variations during steady-state and transient conditions. The controls engineers can then develop the necessary controls and identify the instrumentation to protect the equipment and provide for operator safety. Finally, the complete system must be cost-optimized to minimize field installation and O&M costs over the plant’s lifetime.
-—Dave Ugolini (dugolini@bechtel.com) and Dr. Justin Zachery (jzachary@bechtel.com) are senior principal engineers for Bechtel Power Corp. Hyung Joon Park hjpark@bechtel.com) is a financial analyst for Bechtel Enterprises. Dr. Zachery is also a POWER contributing editor.
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The IGSPP uses the waste heat from the Gas Turbine Unit (GTU} to supplement solar heat from Parabolic Solar Collector Array (PSCA) in order to augment power generation in the steam turbine unit. In this design, the gas turbine unit waste heat is used for feed water preheating, to generate additional steam, and for steam superheating and solar energy is generally used for direct steam generation into PSCA. This combination does not reduce the solar energy source to negligible role as most integrator of large fossil-fuelled power plant but places both sources on approximately the same level and allows the power plant to operate independently of the solar field. The plant operates during sunny periods at full integrated mode of operation with an increasing in solar steam generation in solar field and feeding the surplus high voltage electricity of steam turbine unit into Electrical Power Grid (EPG). Whereas superheated solar and fossil steam production in the plant is delivered to steam turbine unit for electrical power generation and utilizing the exhaust gases of GTU in modified heat recovery steam boiler. While during cloudy periods and at night the IGSPP operates as a conventional Combined Cycle Power Plant (CCPP) integrated with EPG. The modular arrangement of IGSPP also facilitates power generation dispatching because the GTU can be operated independently (with or without the Steam Turbine Unit (STU)) if part of the STU is down for maintenance or if at night less than the CCPP total capacity is required. This may give a higher efficiency for small loading than if the total capacity was operated. Integration of GTU in this manner allows the power plant to operate near full load efficiency more often and improving the net annual solar-to-electric efficiency. As a result of the solar input is not lost waiting for the STU to start up, and because the average turbine efficiency will be higher since the turbine will always be running at 50% load or above.
[1] Hussain Alrobaei,2006, Integrated Gas Turbine Solar Power Plant/ The Energy Central Network/ energycentral.com/centers/knowledge/whitepapers