Copyright 1983 by ASME

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-139 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published In an ASME Journal. Released for general publication upon presentation. Full credit should be given to ASME, the Technical Division, and the author(s). Papers are available from ASME for nine months after the meeting. Printed in USA. Copyright 1983 by ASME Economics of High-Performance Regenerators 1983 ASME Gas Turbine Conference, Phoenix, Arizona By S. J. Valentino, Senior Project Engineer, AiResearch Manufacturing Company, Torrance, California ABSTRACT Industrial energy conservation has become a topic of increasing concern in recent years since energy costs began to escalate rapidly. No longer is it adequate for users and project designers to evaluate equipment on first cost; instead prime consideration must be given to the costs of operation, maintenance, and availability. In the gas turbine area, considerable effort has been put forth to increase the cycle efficiency by heat recovery methods. While the technique is not new, use of veryhigh-effectiveness regenerators specifically designed to minimize maintenance downtime has increased in recent years. The resulting fuel savings offer a payback of 1 to 2 years; this payback, in conjunction with low maintenance, has made the use of high-performance equipment economically attractive. This paper presents fuel savings and evaluation techniques for several heavy-duty gas turbines, together with design features employed to achieve minimum downtime. Field experience on a number of installations also is presented. INTRODUCTION Energy conservation is quickly becoming more than a passing consideration in large projects. For many companies energy costs are now the fastestrising portion of operating costs. The problem facing industry is twofold: conservation of a basic resource, and the impact of spiralling fuel costs on profits. Alleviation of the problem requires increased efficiency in fuel-consuming equipment. While there are several ways of improving the gas turbine cycle thermal efficiency, a regenerator of very high effectiveness is an attractive candidate. With today's gas turbines it can improve cycle efficiency by 20 to 40 percent. Also, waste heat recovery equipment now can be designed to operate for many years without special maintenance. BENEFITS OF REGENERATION Regenerators increase gas turbine cycle efficiency. The benefits are clear when some examples are examined. The performance of several turbines in simple cycle and regenerated cycle are compared in Table 1. The performance shown (i.e., the heat rate converted to cycle efficiency) is based on medium-effectiveness regenerators. The term effectiveness, as applied to regenerators, is the ratio of the increase in air temperature as it passes through the regenerator to the difference in inlet air and gas temperatures. Thus, if the compressor discharge temperature were 500 F and the turbine exhaust temperature were 1000 F, a 100-percenteffective regenerator would heat the air discharge to 1000 F before this air enters the burner. Figure 1 illustrates how regenerators of higher effectiveness will further improve cycle efficiency. This curve shows the improvement for a nominal 15,000-hp turbine where the heat rate was 7410 Btu/hp-hr with an 81-percent regenerator. Performance then improves to 7260 Btu/hp-hr with an 85 percent unit, and to 7120 Btu/hp-hr with a 90- percent regenerator. When compared to the simplecycle heat rate of 9530 Btu/hp-hr, this is a 33.9 percent in improvement in cycle efficiency and a like benefit in fuel economy. The amount of fuel conserved is significant when one considers the large number of turbines in use. A single gas turbine in the 15,000-hp class can save up to 300,000,000 cu ft of gas per year. (SI conversions are listed at the end of this paper.) The following example clearly demonstrates this savings. With the data previously discussed, the fuel savings can be readily calculated: Power Simple cycle 14,600 hp Regenerative cycle 14,000 hp Heat rate Simple cycle Regenerative cycle 9530 Btu/hp-hr 7120 Btu/hp-hr Annual hr of operation = 7500 hr Lower heating value of fuel = 1000 Btu/cu ft

2 TABLE 1 THERMAL EFFICIENCY* Turbine Simple Cycle Efficiency, Percent Regenerator Cycle Efficiency, Percent Regenerator Effectiveness, Percent General Electric frame 3 General Electric frame Canadian Westinghouse CW352 *Efficiencies were determined from heat rates at International Standards Organization conditions (see 1981 Gas Turbine World Performance Specs, ref. 1). Data are used for discussion purposes; specific performance data and information should be obtained from turbine manufacturers SIMPLE-CYCLE HEAT RATE = 9,530 BTU/HP-HR - HEAT RATE This yield is important when gauging the benefit of using high-, medium-, or low-effectiveness units on an overall project basis, especially on large projects that employ from 10 to 30 gas turbines, where enormous amounts of fuel can be saved. ECONOMIC BENEFITS In evaluating the monetary benefits of regenerators, the simple payback and the internal rate of return (IRR) methods will be used. (The latter also is known as discounted cash flow, or DCF.) EFFECTIVENESS, PERCENT Figure 1. Heat rate vs regenerator effectiveness for 15,000-hp gas turbine Annual fuel consumption (simple cycle) = 14,600 hp x 9530 Btu/hp-hr x 7500 hr/yr = x Btu/yr = x 10 9 cu ft/yr Annual fuel consumption (regenerative cycle) = 14,000 hp x 7120 Btu/hp-hr x 7500 hr = x Btu/yr = x 10 8 cu ft/yr Annual fuel savings Simple Payback Method Simple payback is a quick screening method that is widely used in industry. The payback period is the number of years until the gross annual savings equal the capital investment; however, in this method the time value of money, the tax benefits, and the benefits accrued beyond the payback period are not considered. Acceptable payback periods vary widely with individual companies. Typically the payback period for industrial applications is 2 to 3 years. Since the cost of fuel is changing, the savings can be calculated on a unit basis and the savings can be evaluated for various fuel costs. Using the fuel savings from the earlier example, one can calculate the monetary benefit for the regenerated 15,000-hp gas turbine as follows: Annual fuel savings Natural gas lower heating = value = 295,900,000 cu ft 1,000 Btu/cu ft = x x 10 8 Fuel cost (nominal) $1 per million Btu = 295,900,000 cu ft/yr Such fuel conservation also is achieved by the larger gas turbines, as evidenced by the efficiency improvement of the 30,000-hp Class CW352 gas turbine shown in Table 1. In the example, the additional savings obtained by using the high-effectiveness unit is 30,400,000 cu ft of gas per year per turbine. Annual savings = x 10 8 Btu/cu ft x = $295,900 cu ft x 1 x 10 3 $1 x 10-6 Btu Figures 2 and 3 show the annual fuel savings at several fuel costs for 15,000- and 30,000-hp turbines. Staying with the example of the 15,000-hp turbine, the annual fuel savings for fuel at $3/million Btu is $887, F-37368

3 32 CC 128 O B c/112 z IL 0 0 HOURS OPERATION = 7500 HRS/YR FUEL COST CONSTANT 40 o Err0. BTU WAIF Mrd /I Ail "7 l 11 53/106 BTU 44211"P YEARS Figure 2. Fuel savings for a nominal 15,000-hp gas turbine 6 YEARS Figure 3. Fuel savings for a 30,000-hp class gas turbine Assuming a representative incremental capital investment increase of $1,500,000 for a regenerative cycle installation, the payback period would be less than 2 yr. Payback period = Incremental Capital Investment Annual Fuel Savings Payback period = $1,500,000 = 1.69 yr $887,700/yr The payback period at other fuel costs can be determined quickly from Figures 2 and 3. The payback period of less than 2 yr, even with the moderate $3/million Btu fuel, makes the regenerative cycle an attractive investment. Internal Rate of Return (IRR) Method Once it has been determined that the investment has an acceptable payback period, it should be further evaluated using a more rigorous method in which the time value of money and taxes is considered. Frequently the internal rate of return (or discounted cash flow) method is used. This is a very powerful tool in that it can consider incremental investments, accelerated depreciation, dissimilar periods of investment, inflation, and other factors that an investment analyst may wish 10 3 to evaluate. Here some brief definitions of financial terms that relate to regenerator applications are in order: Capital Expenditure--The incremental investment in a regenerator and installation, which is expected to have a multiperiod economic life. Cash Flow--The incremental increase in revenue resulting from fuel savings with the improved cycle efficiency of regenerated gas turbines. Present Value--The value in today's dollars of discounted future fuel savings, thus: Future Sum at Year n Present Value = n (1 + i) where i is the annual interest or present value rate (decimal value). Internal Rate of Return--The interest rate that relates the discounted future fuel savings to the incremental capital investment. This rate makes the after-tax present value of the fuel savings equal to the expenditure. Stated another way, it is the equivalent interest rate returned on an investment over its economic life. Discount Rate--The compound interest rate used to determine present value of future cash flow (through fuel savings). The discount rate is a subjective factor established by a company and may differ widely between firms. Typically, it is set at 15 to 25 percent after taxes as a minimum. The evaluation procedure requires that the present value of each of the net annual fuel savings for the economic life of the equipment be calculated at various interest rates until the sum of the present values equals the incremental capital expenditure. It is an iterative process that is tedious if done manually, but lends itself readily to a simple program for a calculator or computer. The complexity arises in that the annual savings may vary due to inflation and other cost factors. However, to look at the benefit of regeneration from this more meaningful approach and to demonstrate that regeneration is in fact a desirable economic consideration, the same example for simple payback will be used. The fuel savings will be considered on the basis that the fuel cost is constant at $3 per million Btu for the economic life of the equipment, and also on the basis that fuel will escalate at a rate of 8 percent per year. The 8 -percent fuel escalation is a conservative value compiled from data received from several gas pipeline companies. First, with the fuel cost constant, the calculation is simplified in that savings also will be constant. From the previous example of annual fuel savings of $887,700, net cash flow can be calculated. For simplicity, straightline depreciation will be used: Incremental capital expenditure $1,500,000 Annual fuel savings 887,700 Yearly depreciation (over 20 yr) 75,000 Composite tax rate (U.S.) 50 percent F-37369

4 Cash flow (constant fuel cost) Annual savings $ 887,700 Less depreciation 75,000 Taxable income $ 812,700 Less taxes (50 percent) 406,350 Net savings $ 406,350 Add depreciation 75,000 Net cash flow $ 481,350 Since the cash flow is constant, the rate of return can be approximated by using interest tables for annuities. However, to facilitate the calculation, the series of curves in Figure 4 were developed for the case where savings are a constant flow stream; from the curves for a 20-yr economic life, the rate of return is 32 percent--well above the acceptable rate of 15 to 25 percent. In a world of increasing costs, however, the inflationary effect on fuel prices should be accounted for. With the escalated fuel cost, the analysis is more complex and therefore only the computer results are summarized in Table 2. The IRR for both the 8-percent and 0-percent escalations are compared in Table 2. If all other factors remain unchanged, the 8-percent/yr fuel cost increases the rate of return from about 32 percent to 42 percent. The computations in Table 2 were based on the following conditions: Capital investment = $1.50 x 10 6 Fuel savings = x 10 6 cu ft of gas Fuel cost = $3/10 6 Btu Life cycle = 20 yr Tax rate = 50 percent New U.S. tax laws allow further advantages in the form of investment tax credits and accelerated depreciation. These new regulations can increase the discounted cash flow rate of return to values greater than 45 percent. In most cases, a gas turbine that can be regenerated will result in an attractive investment return. INSTALLATION REQUIREMENTS A regenerative cycle gas turbine will require combustion air piping outside the turbine, exhaust gas ducts, an exhaust stack, and a pad on which to mount the equipment. Figure 5 is a plan-view layout of an installation with the additional equipment shown shaded. The layout is for a side-mounted regenerator (with horizontal exhaust flow). A turbine with upward exhaust flow would require the same equipment, except that an exhaust stack may not be required because the regenerator is mounted overhead and exhaust flow is sufficiently above ground to prevent any undesirable effects. Photographs in Figures 6 and 7 show a typical installation of each type. DESIGN CRITERIA a 246. Figure 4. Rate of return percentage To make the regenerative cycle viable and attractive, turbine manufacturers specified a set of essential characteristics for regenerators. TABLE 2 INTERNAL RATE OF RETURN AT 0- AND 8-PERCENT FUEL ESCALATION, $ MILLION Fuel Escalation at 0 percent Fuel Escalation at 8 Percent Fuel savings Depreciation <0.234> <0.179> Taxable income Taxes <1.266> <1.321> Net income Depreciation Cash flow Internal rate of return, percent F-37370

5 TURBINE During the initial design phase of the new highperformance regenerator, the manufacturers Garrett was working with established the following criteria: Reliable Design--The unit must operate for its design life and cyclic operation with no scheduled maintenance. High Performance--The effectiveness of the regenerator must be at least 5 percentage points greater than existing equipment. High-Temperature Operation--The regenerator must operate with an exhaust gas temperature as high as 1170 F for some models and must have low corrosion characteristics at high temperatures. Figure 5. Regenerator-to-engine interface Modular Design--The units must be designed in modular form and require minimum installation effort. Operation--The unit must operate unattended, require no special personnel skills, and require no special equipment for maintenance. PERFORMANCE OBJECTIVES ACHIEVED After working closely with turbine engine manufacturers, all of the performance criteria were eventually met. The high-effectiveness regenerator is now capable of 120,000 hr of operation and 5000 undelayed starts without scheduled maintenance. Also, an 85- to 90-percent effectiveness has been achieved with 3 - to 3.5-percent total pressure loss. To meet the requirements for high-temperature operation and low corrosion, stainless steel was selected as the heat exchanger material. OPERATING EXPERIENCE Figure 6. Side-mounted regenerator installation Once the high-performance regenerators were developed and offered, the problem of user acceptance of regenerators had to be faced. Through 1982, a total of 48 regenerators of this type will have been shipped. Of the 48 shipped, 44 are operational; the remainder will be operational during These units have accumulated a substantial number of operating hours with no field problems. Table 3 summarizes field operation as of September 1, TABLE 3 OPERATIONAL SUMMARY AS OF SEPTEMBER 1, 1982 Regenerators installed 44 Sites 28 Cumulative hr 285,000 High-time unit hr 20,600 Cumulative starts 2,350 Figure 7. Overhead-mounted regenerator installation Starts in single unit 28 0 F

6 The performance of the units has met all the design requirements, and in each case the estimated savings have been verified by the users. In most applications the selection of the new highperformance regenerator was based on economic analysis of life-cycle costs, which showed a rapid payback and a good rate of return. SUMMARY The benefits of energy conservation are selfevident; as one major producer succinctly stated, "What we do not burn we can save; what we save, we can sell." Considering savings of 300,000,000 cu ft/yr for a single gas turbine, the savings for a large project will have a significant impact on the available resource. The economic benefits also are compelling: a payback of less than 2 yr, and 40-percent discounted cash flow rates of return. Although other forms of energy conservation such as bottoming cycles also can be applied to gas turbine installations, the regenerator offers the lowest cost, minimum maintenance, and an efficiency comparable to other systems. ACKNOWLEDGEMENT The author wishes to thank Mr. S. J. Lee, engineering analyst, AiResearch Manufacturing Company, Torrance, California, for his valuable technical assistance. REFERENCES (1) di Biasi, V., ed., "Mechanical Drives," Gas Turbine World Performance Specs, 1981, Vol. 10, No. 6, Dec. 1980, pp (2) American Society of Mechanical Engineers, A High- Effectiveness Regenerator Design Concept, ASME 78-GT-78, (3) American Society of Mechanical Engineers, Selection of a High-Efficiency Regenerator for Pipeline Gas Turbines, ASME 77 - GT - 39, (4) American Society of Mechanical Engineers, Designing Reliability Into High-Effectiveness Industrial Gas Turbine Regenerators, ASME 79-GT-99. (5) Brigham, E. F., and Weston, J. F., Managerial Finance, Dryden Press, 5th ed., SI CONVERSIONS Power: 30,000 hp = kw 15,000 hp = kw 14,600 hp = kw 14,000 hp = kw Heat rate: 9530 Btu/hp-hr = x 10 6 J/kwh 7410 Btu/hp-hr = x 10 6 J/kwh 7260 Btu/hp-hr = x 10 6 J/kwh 7120 Btu/hp-hr = x 10 6 J/kwh Lower heat value: 1000 Btu/cu ft = x 10 6 J/m3 Energy: 1 Btu = J x Btu/yr = 1.10 x 10 6 GJ/yr x Btu/yr = x 10 6 GJ/yr Volume: 300,000,000 cu ft/yr = x 10 6 m3 /yr 295,900,000 cu ft/yr = x 10 6 m3 /yr RESPONSE TO REVIEWERS' COMMENTS Prior to its publication this paper was sent to the ASME technical committee for review and comment. The committee's comments (paraphrased here) are answered below: 1. COMMENT: Discuss the eventual economic impact of repair or replacement of the regenerator and the added maintenance costs during the initial payback period. RESPONSE: The added maintenance cost during initial payback period for the type of regenerator being proposed is negligible. Field experience with the number of units shown in Table 3 indicates that little or no maintenance is required during the initial payback period. Although the intent was to keep the paper general relative to regenerators, the regenerator described in References 2, 3 and 4 is designed to operate for a period of approximately 20 years without any scheduled maintenance. However, for purposes of economic evaluation, as confirmed by various pipelines, $5,000 per year can be included for inspection. This small amount has little effect on the overall economic evaluation "x 10 9 cu ft/yr = x 10 6 m3 /yr x 108 cu ft/yr = x 10 6 m3 /yr F

7 2. COMMENT: Is the cost of the additional site area required for regenerators factored into the overall economic equation? RESPONSE: The cost of additional site areas for regenerators is not factored in the initial equation. This is because in most cases, the site area is not considered by the user. In many instances no additional area is required because the regenerators can be installed above the gas turbine. However, should the cost of this area prove to be significant, it would have to be included in the overall economic equation. 3. COMMENT: Address any restricted range of operations (i.e., offshore platform) due to the increased equipment size and weight and their impact on the cost of the installation. RESPONSE: Regenerated gas turbines are wellsuited to specialized installations such as offshore oil-drilling platforms. Regenerators referenced in this paper are relatively compact and lightweight compared to older designs. (In addition, these new regenerators are specifically designed for the coastal environment encountered in offshore platforms.) The installation of a regenerator on a platform, particularly if the regenerator is installed above the gas turbine, may require some additional structure to strengthen the platform base. However, most platforms already can accommodate the additional weight. If some special provisions have to be made, the additional cost would have to be factored into the original payback equation. For example, if additional structure costing $200,000 had to be added to the platform, the payback period would increase from 1.69 years to 1.92 years, still an attractive investment. F