ASHRAE 4701-2004 A Comparison of Electrical- and Thermal-Load-Following CHP Systems《电和热负荷跟踪热电联产系统的比较》.pdf
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1、470 1 A Comparison of Electrical- and Thermal-Load-Following CHP Systems Ali A. Jalalzadeh-Azar, Ph.D, P.E. Member ASHRAE ABSTRACT Realization of the full benefits of implementing the combined heat and power (CHP) concept in buildings hinges upon optimum CHP system integration, sizing, and operation
2、 in parallel with, or independent oJ the electric utility grid. This realization necessitates assessment of the appropriate CHP designioperation possibilities and selection of the best candi- date for a given application. Electrical- and thermal-load- following CHP models are certainly among such ca
3、ndidates. This paper is essentially an extension of a previous study on a grid-independent, electrical-loadfollowing CHP system for a hypothetical ofice building. The objectives usthis study are to evaluate the thermodynamic performance of a thermal- load-following CHP system for the same building a
4、nd to compare the results with those of the previous study. Included in the scope of the current work are (1) a parametric analysis addressing the influence of the subsystem efficiencies on the total primary energy consumption, (2) an evaluation ofjrst- law efficiencies at two levels: CHP system and
5、 overall system, (3) an estimation of net monthly electricity import/export, and (4) an assessment of how electric utility efficiency affects the overall system energy consumption. The parametric analysis demonstrated the positive and significant responsiveness of the total primary energy consumptio
6、n to improvements in the eficiencies of the on-site power generation and building electrical systems for the ther- mal-loadfollowing model. A similar finding was also echoed by the previous work on the electrical-load-following CHP The net monthly export of electricity for the thermal-follow- ing-mo
7、del) occurred during the peakcooling months, when the building thermal loads are the highest. While an increase in the efficiencies of the on-site power generation and electrical equipment reduced the net monthly import of electricity, the effects of such a measure with the absorption cooling system
8、 were the opposite. However, the issue of an optimum balance between export and import of electricity can only be addressed through an economic assessment, which is not within the scope of this work. The scenarios adopting more efficient absorption cooling showed a stronger sensitivity to the electr
9、ical utility eficiency. The thermal-load-following CHP model was found to be superior to the other previously studied model from thefirst- law thermodynamic standpoint. The monthly average CHP eficiency of this model was higher and comparatively much less sensitive to seasonal variations. The therma
10、l-load-follow- ing model offered a higher overall system effiency fuel utili- zation) as well. U INTRODUCTION This study presents a thermodynamic analysis of a ther- mal-load-following combined heat and power (CHP) system for a hypothetical commercial building in Atlanta, Georgia, USA. The current w
11、ork is an extension of a previous paper (Jalalzadeh-Azar 2003), which examined a grid-independent electrical-load-following CHP system. Common to both stud- ies are the hypothetical building, the HVAC system, and the CHP subsystem technologies. However, the CHP system of the current study does not o
12、perate independent of the electric grid (Figure 1). Because thermal energy demand is the crite- rion for sizing and operation of this thermal-load-following CHP system, the recoverable heat from the on-site power generation is fully utilized. In this model, an exchange of elec- tricity with the grid
13、 takes place when a mismatch occurs between the on-site power supply and the actual demand. The Ali A. Jalalzadeh-Azar is a senior engineer at the National Renewable Energy Laboratory, Golden, Colo. 02004 ASHRAE. 85 Primary Exhaust 4 fieat Exchanger Power AbS. Gen. Chiller Fuel - Air -c I Gas Space
14、Servke Heater H.W. - - Bld . Elec. Loads Bldn. Thermal Loads Other equip. Service hot water Elec. Grid 2- Note: The service hot water system is equipped with an auxiliary gas-fired burner. Figuve 1 Schematic of baseline CHP system. main underlying assumptions for the current work are: (1) the electr
15、ical energy generated in excess of the demand is always exported to the utility, and (2) the export of electricity propor- tionately displaces the primary energy consumption at the central plants. Although export of electricity from customers to utilities is not currently prevalent, incentives and p
16、rograms, such as net metering programs (U.S. DOE 2003a; Wan 1996), will promote such a practice. This study addresses only the out any economic assessment. This study encompasses a parametric analysis similar to that of the previous study. The purpose of this analysis is to examine the effects of im
17、proving the performance indices of the subsystems on the overall efficiency of the thermal-load- following CHP and to compare the results with those of the electrical-load-following model. The subsystems considered for this analysis are on-site power generator (gas turbines), absorption cooling, and
18、 building electrical equipment. The results of this parametric assessment include the total energy consumption for the building, the energy required for on-site power generation, and the waste heat utilization. In reporting these results, the energy quantities are normalized with respect to the corr
19、esponding values of the baseline CHP system. One of the differentiating facets of this study is the defi- nition and calculation of first-law efficiencies for the CHP system and the overall system, which incorporates the CHP system, electric grid, and auxiliary thermal-energy supply units. The defin
20、ition of the overall system efficiency adopted in this study is applicable to both models with certain simpli- fications, as will be discussed later. The overall system effi- ciency, which is reflective of total fuel utilization, is significant primarily because it is applicable to all methods of CH
21、P implementation and accounts for all forms of energy consumption. Regardless of the CHP model (electrical- or thermal-load-following) or the size of a CHP system relative 8 energy aspects of the CHP systems under consideration with- to the building loads, this overall efficiency can provide useful
22、information to the engineers and end users. Focusing only on the CHP efficiency, as opposed to the overall system effi- ciency, will not address the relative impact of the CHP imple- mentation on the overall building energy performance. In fact, adoption of similar macro-level measures is perhaps im
23、pera- tive in promulgation of CHP-related energy policies and incentives. A major challenge in estimating the overall CHP/grid system efficiency for a building is the lack of accurate data on the local electric utility efficiency at any given time. In addi- tion, as more advanced power cycles are in
24、stalled, the effi- ciency of the central power plants continues to increase. The impact of varying the efficiency of the electric utility on the overall system performance has been addressed in this paper. BUILDING DESCRIPTION The baseline building under consideration is a hypothet- ical office buil
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