Heat Recovery Systems Vol. 4, No. 5, pp. 341-345, 1984 0198-7593/84 $3.00 + .00 Printed in Great Britain Pergamon Press Ltd

E N E R G Y C O N S E R V A T I O N A N D C O G E N E R A T I O N

I N T H E D A I R Y I N D U S T R Y

E. NIEUWLAAR and B. DE VRIES State University of Groningen, Department of Chemistry, Vrije Studierichting, The Netherlands

Abstract--In this article we report on detailed simulation calculations of gas turbine--waste heat boiler systems in a dairy factory in relation to alternative energy conserving investment opportunities. Pay-back periods and over-all energy savings are presented. For all combinations of energy conserving investments and cogeneration systems, normalized overall energy consumption and additional capital expenditure are calculated. It turns out that the attractiveness of cogeneration significantly decreases with ongoing energy conservation and that the gap between the firm-oriented economic optimum and the national energetic optimum becomes smaller when energy conserving measures are taken.

I N T R O D U C T I O N

In the last decade the prospects for industrial energy conservation and combined heat and power generation (or cogeneration) have drastically changed. In 1973 industrial energy consumption in The Netherlands was expected to show a doubling before 1990. Authoritative estimates put the technical-economical conservation potential at, at most, 7.50./0 . Nowadays, most experts agree that annual energy consumption in The Netherlands will increase by 25~ at most during the period 1980--2000. Recently, a group of industrial energy managers has presented a Research and Development Programme on Rational Energy Use in Industry to the government, in which the conservation potential at pay-back periods of less than 6 yr is estimated to be 30~o. The potential for cogeneration capacity in industry is according to recent official reports at least 2000 MWe on top of the already existing 1100 MWe. Indeed, perspectives have changed.

One of the insights that has emerged, is the need for an integrated approach of process energy conservation and cogeneration systems on the basis of accurate heat and electric power use patterns. The present research focuses on this integration, using economic production theory, a computer simulation model and detailed process data from a dairy factory in the northern part of The Netherlands. The research has been started in 1981 and has recently been finished. It has been financed in the context of the National Energy Research Programme.

T H E C O M P U T E R S I M U L A T I O N

We confine ourselves to a gas turbine for electric power generation. The turbine exhaust gases are led to the waste heat boiler for steam generation. Auxiliary boilers are installed if necessary. All equipment uses natural gas as a fuel. From manufacturing and engineering firms a set of reliable technoeconomic data has been obtained. With help of statistical analysis of these data, capital cost equations have been estimated. Fuel inputs for gas turbine, waste heat boiler and (auxiliary) steam boilers are calculated from engineering sub-models. Thus, representative cost and efficiency estimates can be derived for gas turbines of any size. To check the validity of this approach, we calculate the sensitivity for key parameters like gas turbine compression ratio and inlet temperature and we repeat the calculations for the actual machines. It turns out that for larger sizes there is a rather large discrepancy between predicted and actual performance.

From the available data on steam and electric power use we generate an annual demand pattern. For the dairy factory for which we present some results here, hourly values for process steam and electricity consumption at twelve locations during one week are at our disposal. With additional information on production levels and outside temperature, we have been able to construct an annual demand pattern. To reduce the number of calculations, this hourly pattern has been condensed into an electricity/steam (E/S) matrix. The (i,j)-element of this matrix contains the

341

342 E. NIEUWLAAR and B. DE VRIES

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Fig. 1. Three dimensional representation of the demand pattern for electricity and steam in a dairy factory.

number of hours during which electricity resp. steam demand are within the :bounds of interval: i resp. j. Figure 1 is a graphical representation of this matrix for the dairy factor)':

To evaluate cogeneration we calculate for each electricity/steam demand combination the amount of kWhe produced by the gas turbine, and the amount of kWhe that has to be exchanged with the grid. From the E/S-matrix this is summed up to annual values. These are compared with the reference situation in which steam is separately generated and all electricityis bought from=the utilities. The results are presented in the firm-oriented measure of the pay-back ~ r iod and in the national measure of overall energy savings. Overall in this context refers to the ~system of both the factory and the central electricity generation system.

In practice there are some constraints on operating the gas turbine-waste heat :boiler(gt-whb) system. We neglect start-stop and grid-exchange aspects. As to the mode of opel ration we discern:

full-load operation: the gt either runs at full load or not at all. Heat dumps are: forbidden: auxiliary boilers match steam demand/whb supply, and

steam resp. electricity following: the gt,whb system matches within certain technical limits the hourly steam resp. electricity demand.

COGENERATION: THE PRESENT SITUATION

Figure 2 presents the results if the present steam and/or electricity demand is t o b e met at:the full,load of operation (solid lines). The graph at the upper left shows the pay-baCk period for increasing size, that is: rated capacity of the gas turbine. The lower left c u r v e shows _the gt-kWe-production and the part of it that can be used on-site. The graphs show flint there i s an optimal gas turbine size, above which the investment becomes unattractive and the benefits of a larger scale are offset by a decrease in load factor and an increase in the numbe r of kWhe to be sold to the grid.

The graphs a t the right side put the results in an overall, national persp~tive.-The u p ~ r right curve shows the investtnent per m ~ of overall saved natural gas for inca~aaing gas turbine sizei The lower right curve indicates the amount of overall saved natural gas. Evidently, t ~ optimum is now. at much larger gt-size because the losses at the central electricity generation are-~xpti¢itly a¢cotmted for.

To give an impression of the sensitivity of the results for varying gt,paratr~ters (compression ratio, turbine inlet temperature) and for the mode of operation (steam:foUow~),i the rcsultsare given in the same figure (dashed lines). Other parameters: kWhe- and kWe.pric¢, n ~ l ~ gas price, investment cost a,o. turn out to be less influential within the respective u ~ r t a i n t y intervals.

As an illustration of how the gas turbine is operated, Fig. 3. shows annual duration Curves for electricity demand, gt-production and back.delivery. . . . .

PROCESS ENERGY CONSERVATION

In the dairy industry about 30% of energy use is for concentrating and drying mitk and whey into milk and whey powder. In the dairy factory for which energy use data a rea t our disposal,

Energy conservation and cogeneration 343

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Fig. 2. Results for the cogeneration evaluation in the present situation. The mode of operation is full-load (solid line) or steam-following (dotted line). For the full-mode', results are given for low values of the compression ratio/inlet temperature (4 resp. 1040 K) and for high values (9 resp. ! 150 K). See the text for

explanation.

cheese, milk powder and whey powder are the main products and water evaporation in a four-stage evaporator with thermocompression and drying consuming unit processes. We confine ourselves to the evaporator, for which an increase in the number of stages, the use of mechanical recompression and inverse osmosis are the energy conserving investment options. Figure 4 indicates the specific capital and energy requirements of these alternatives for 20 and 30 ton h -1 water evaporated. The drawn lines are isoquants, based on regression-analysis of the available data.

In consultation with the factory managers we have chosen four alternative investment projects for further evaluation. These are: installation of a five-, a six- or a seven-stage evaporator or a mechanical recompression unit. From the specific steam and electricity requirements the new energy use patterns are calculated. These significantly differ from the present patterns, especially if

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Fig. 3. Annual duration curves for electricity demand (D), steam-following gt-production (S) and back-delivery to the grid (B).

344 E. NIEUWLAAR and B. DE VRIES

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Fig. 4. Specific capital and energy requirements for evaporators m the dairy industry. Steam and electricity are converted into rn ~ natural gas equivalent (90 m 3 ton-~ steam, 0.3 m 3 kWhe).

mechanical recompression is introduced because then the steam use of 0.08-0.15 ton/ton water evaporated is almost completely substituted for electric power. Thus, one expects a changing perspective for cogcneration.

COGENERATION: THE FUTURE SITUATION

The results of the cogeneration system evaluation after energy conservation has been realized, are shown in Fig. 5. The vertical axis indicates the overall energy consumption in the factory, normalized to its present value. The open points indicate the decrease in energy consumption for the four previously mentioned energy conservation projects. For each of them the pay-back period is in the order of 2.4 yr. The drawn curves give the calculated energy and investment values for cogeneration schemes for a range of gas turbine sizes (cf. Fig. 2.). The point of minimum pay-back period ( 0 ) and the point of maximum over,all energy conservation ( I ) are each indicated. The upper curve refers to cogeneration in the present situation; the other four refer to cogeneration after the corresponding energy conservation project is introduced.

The results emphasize the strong interaction between energy conservation measures and (the evaluation of) cogeneration projects and underline the need for an int©grated approach, The curves in Fig. 5 show, that the gas turbine-waste heat boiler system quickly becomes tess attractive when more thorough energy conservation is pursued. In the case of the mechanical recompression (lower curve) the pay-back period for the gt-whb has increased to at best 4.7 yr. The reason is the

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Fig. 5. Overall energy conservation vs investment for cogcneration with and without process energy conservation.

Energy conservation and cogeneration 345

combined effect of changing E/S-patterns and of a smaller gas turbine size. For the total project of both mechanical recompression and cogeneration, the pay-back period is still as low as 3.3 yr. There is a distinct difference between the points of minimum pay-back period and the point of maximum over-all energy conservation. For the more thorough process of energy conservation the difference between these two becomes smaller and, in the case of mechanical recompression, vanishes altogether.

Acknowledgement--The research is supported by the Project Bureau Energieonderzoek (T.N.O.) and by the Netherlands Organisation for the advancement of pure research (Z.W.O.)