Turbine inlet cooling enhances the economics of combined-cycle systems without, and even better with, duct firing.
By Dharam V. Punwani, Avalon Consulting and Craig M. Hurlbert, Turbine Air Systems
A basic flaw of all combustion turbines (CTs) is that hot weather derates their capacities. The impact of this fundamental flaw is 10 percent to 35 percent of the rated/nameplate output capacity, which is always rated at 59 F as specified by the International Standards Organization (ISO).
As ambient temperature increases, power demand and electricity prices typically increase too. Most utilities depend on gas turbines (both simple and combined-cycle) to meet some or all peaking generation needs. Unfortunately, the CT output decreases when it is most needed.
In combined-cycle, cogeneration and combined heat and power systems, a rise in ambient temperature not only reduces the CT power output, it also reduces the total thermal energy available in the CT exhaust gases. This reduces the output capacities of the equipment downstream of the CT for production of additional power or for providing heating and cooling (using absorption chillers) needs of single or multiple facilities.
On hot summer days, when electricity prices are high but the output of a combined-cycle system is derated, there are two options for boosting the system output: Turbine inlet cooling (TIC) or duct firing.
It simply cools the inlet air to the compressor of the combined-cycle system. TIC can mitigate the economical and environmental impacts of this fundamental flaw. The benefits were recently discussed in another Power Engineering magazine article, “Unearthing Hidden Treasure,” written by the authors of this article and published in the magazine’s November 2005 issue, page 62.
Duct firing, on the other hand requires injection of a fuel, such as natural gas or waste gas downstream of the CT exhaust to increase the temperature and mass flow rate of the exhaust gases. Duct firing allows production of more and/or higher pressure steam in the heat recovery steam generator (HRSG). The additional and/or higher pressure steam produced in the HRSG allows more electricity to be produced in the steam turbine downstream of the HRSG. Duct firing cannot be retrofitted unless spare capacities of HRSG and steam turbine are already built in the plant when initially constructed.
This article discusses the benefits of TIC for combined-cycle systems with or without duct firing. The information in this article is based on an independent report prepared in 2003 for Turbine Air Systems Ltd.
System Analysis Approach
Various combined-cycle configurations, designed for ambient conditions of 77 F and 50 percent relative humidity (RH) were used for simulating performance at the summer conditions of 95 F and 40 percent RH. This ambient condition was selected because it is typical of the ambient condition when summer power enhancement is most beneficial.
The selected design conditions are typical for a combined-cycle plant in intermediate service, meaning it is neither a base-loaded plant nor a peaker. This plant can be expected to operate approximately 4,000 hours per year. The TIC systems, on the other hand, are typically designed for summer conditions.
Future users may opt to design the balance-of-plant equipment, such as the HRSG and steam turbine generator (STG), for a defined CT inlet air temperature of 50 F. Such optimization would provide maximum output and efficiency during the summer months when a chiller system is expected to operate. The downside of such a system would be a slight loss of performance during “shoulder” hours and winter operations (if any). Most merchant plants were originally designed for baseload or intermediate-dispatch service, but are now operating almost exclusively as summer peakers, making it practical to design the balance-of-plant for a CT with an inlet temperature of 50 F and the main cooling tower for a temperature of 95 F dry bulb (DB) and 75 F wet bulb (WB).
A 2x1 GE 7FA (PG7241FA) gas turbine combined-cycle power plant, typically referred to as a STAG 207FA, was used in this study. The HRSG and other plant hardware are generic and designed for the gas turbine exhaust conditions. The steam turbine is generic, but an attempt was made to conform to standard GE D-11 sizes available in the market. All models were designed using Thermoflow’s GTPro software. (This software was chosen because of its wide acceptance among power project developers, not only for initial screening studies, but also for detailed plant design.) The GTMaster software was used to simulate summer conditions. The Thermoflow PEACE option in the software was used to size the hardware and estimate the cost of the equipment as well as the facilities’ construction and installation costs. The four types of systems simulated and their estimated costs are shown in Table 1.
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It is expected that the GE F-class CT study results are representative of results for all advanced gas turbines such as the Westinghouse W501FD, the Alstom GT24, the Mitsubishi Heavy Industries 501F, and the Siemens V84.3A, and all of these turbines’ 50 Hz counterparts. Moreover, the general concepts of this study can be reasonably extrapolated down to older E-class non-reheat cycles, as well as up to G/H class advanced steam-cooled cycles. All models are designed using the same set of assumptions. (These assumptions are not listed in this article but are available upon request from the authors.) Even though only the chiller-based approach is discussed here, other TIC technologies could be analyzed using the same approach.
It is important to note that in setting up the GTPro models, certain aspects of plant performance were “fixed,” such as STG last stage blade length and cooling tower approach and range. However, other design parameters were allowed to “float” to find the best economic and technical results. The most important of these features is the heat transfer surface of the HRSG.
It was important to allow the GTPro program to find the best heat transfer surface area because as inlet temperatures change, so do the exhaust temperatures. Changes in exhaust temperatures change the approach for the HRSG’s superheater section. Moreover, as the inlet temperature changes, significant changes in exhaust mass flow occur. To attain the highest possible steam cycle output in the chilled cases, the HRSG needs to be optimized for higher mass flow and lower exhaust temperatures. This is a critical design factor in optimizing the combined cycle for chiller operations. An HRSG not designed for chiller operations will clearly under perform when compared to a properly designed HRSG.
Simulation Results
The simulation results shown in Table 1 illustrate the four systems’ maximum power output capacities when ambient summer conditions are 95 F and 40 percent RH. Figure 1 illustrates the effect of power augmentation technology on the net incremental capacity enhancement at the subject ambient conditions. It shows that the simultaneous use of TIC and duct firing results in the maximum net capacity enhancement of 127,133 kW over that for the base case.
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Total installed costs (blended capital costs) for each of the four systems are listed in Table 1. Based on the power output capacities of these systems at the summer ambient conditions discussed in the previous section, unit cost of these systems can be calculated to be $470, $445, $433, and $414 per kW, respectively. (Because the volume of data generated by the study is enormous, this article discusses only the summaries and analyses. The back-up data are available from the authors upon request.)
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Figure 2 shows a comparison of the unit electricity cost of the base plant (Case 1) with the incremental unit cost for the enhanced capacity provided by the three power augmentation technologies discussed earlier: TIC (Case 2), duct firing (Case 3) and TIC with duct firing (Case 4).
The results in Figures 1 and 2 show that combined use of TIC and duct firing not only provides the most capacity enhancement, it also provides the incremental capacity at the lowest capital cost.
The effect of TIC and duct firing on the system heat rates and incremental heat rates at the ambient summer conditions discussed earlier are shown in Table 2.
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The results in Table 2 show that compared to the base case, TIC with chillers and duct firing increases system heat rates. Total system heat rate is the highest for duct firing (Case 3). Incremental heat rate is also the highest for duct firing: 8,839 Btu/kWh. Compared to the base case, it represents an increase of about 39 percent. The incremental heat rate for TIC system is an increase of only about 12 percent.
TIC versus Duct Firing
Often during the conceptual design phase a developer or engineer might first decide to install duct firing and later consider adding TIC. The decision to add TIC after adding duct firing might seem inconsequential from a timing perspective. But it is not. In fact, the timing does matter from an analysis standpoint. There are certain MWs in the total turbine output that can be traced to either TIC or duct firing. However, once one technology is chosen, those MWs are attributed to the first decision. The second technology is then evaluated on the basis of incremental power increase as demonstrated in Tables 3 and 4.
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Tables 3 and 4 show the incremental values attributed to chilling and duct firing depend on the order in which they are evaluated. In the scenario where chillers are considered first, the incremental capacity is nearly 54 MW; but when considered after firing, the increase is just more than 49 MW. There seems to be approximately 5 MW “lost” in the process. This represents approximately 10 percent of the capacity output enhancement by TIC with chillers, making the TIC scenario seem less optimal. From a heat rate perspective, this 10 percent is being lost from the steam cycle, where the additional exhaust flow provided by chilling should be providing “free” MWs at the STG. On the other hand, duct firing is credited with more than 78 MW when considered first, but its credited with only 73 MW when considered second. Again, approximately 5 MW are “missing” in the process.
Due to high fuel prices and much higher heat rates, duct firing should be used only after the TIC chillers are operating at full capacity. Figure 3 shows the typical dispatch order for the S207FA plant studied. Based on the preferred dispatch order, the proper order of evaluation should be TIC first and duct firing second.
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Although this article does not discuss all TIC technologies and does not provide economic analysis of plant operations (spark-spread and capital recovery), estimates show that the highly augmented plants enjoy superior economic returns because of the on-peak electric demand and energy rates during summer. Increased returns are assured through maximum output during the lucrative summer peaking season and through operational flexibility that allows the owner to react to changing economic conditions.
Conclusions
On an ordinary summer day (95 F and 40 percent RH), a typical combined-cycle system designed for ambient conditions of 77 F and 50 percent RH, can be modified to increase capacity. The system analysis presented here shows the following:
- TIC with chillers increases the net output capacity by about 12 percent at a cost of only about 50 percent of that for the uncooled basic system.
- Maximum capacity enhancement of about 28 percent is achieved by the combined use of TIC and duct firing. It is also the least cost option and achieves capacity enhancement at a cost of only about 42 percent of that for the uncooled basic system.
- Incremental heat rate for duct firing is about 24 percent higher than that for the TIC with chillers and therefore, should be used only after fully utilizing the TIC system.
- The plant owners’ goal should not be the “lowest first cost,” but rather, the “best optimized cost.”
- TIC and duct firing are not necessarily “either/or” technologies; in fact, they work best together.
- TIC, without or with duct firing, provides incremental peaking capacity at an attractive cost.
Authors
Dharam V. Punwani is the president of Avalon Consulting Inc. (www.avalonconsulting.com), located in the Chicagoland area (Naperville, Ill.). Avalon provides technical and economic evaluations related to turbine inlet cooling and cogeneration (combined heat and power) systems. Prior to founding Avalon Consulting in 1996, he was vice president of technology development at the Institute of Gas Technology, where he worked for nearly 30 years. He is a former chairman and the current executive director of the Turbine Inlet Cooling Association. He can be reached at 630-983-0883 or dpunwani@avalonconsulting.com
Craig M. Hurlbert is the president and chief operating officer of Turbine Air Systems (www.tas.com), the leading provider of packaged cooling systems for the commercial and energy industries. Hurlbert’s career includes service as the head of business development in Latin America for North American Energy Services, general manager of GE’s Contractual Services business, the president and CEO of the PIC Energy Group, and general manager of P2 Energy. Hurlbert is the current chairman of Turbine Inlet Cooling Association (www.turbineinletcooling.org.)
Authors’ note: This article in no way reflects the opinions of the Turbine Inlet Cooling Association and is written totally independent of the Association.
