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
25、ding in Atlanta, Georgia, which is identical to the building incorporated in the previous study (Jalalzadeh- Azar 2003). This assumption is necessary to facilitate the performance comparison of the two CHP models: electrical- and thermal-load-following. This baseline building has a floor area of 180
26、0 m2 (19,500 fi2) and an average main ceiling height of 3.9 m (13 fi.). The total area of the windows and glaz- ing comprises about 25% of the total wall area. The average heat transfer coefficient of the building envelope is approxi- mately 0.60 W/m2C (0.1 1 Btu/h.ft2.“F). The design cooling and he
27、ating capacities ofthe KVAC system are approximately 245 kW (837,000 Btu/h) and 170 kW (582,000 Bhuh), respec- tively. The building operates from 8:OO a.m. to 7:OO p.m., seven days per week, during the entire year. The temperature setpoint during the operating period is 24C (76F) for cooling and 22C
28、 (72F) for heating. The night setup and setback are 27C (80F) and 20C (68“F), respectively. The HVAC system resumes the daytime control mode at 6:OO a.m. Examination of the monthly building loads depicted in Figure 2 reveals that (1) the cooling load of the building is the dominant thermal load, (2)
29、 the cooling load is considerably influenced by the external heat gain, and (3) the service hot-water load comprises only a small portion of the total thermal load, particularly in the summer months. As with the previous study, the air-handling units of the cooling system are assumed to be equipped
30、with temperature- controlled economizers for outside air intake with a setpoint of 18C (65F). With the CHP system in place, the peak elec- trical power demand for building equipment is approximately 45 W/m2 (4.15 W/ft2), of which about 20 W/m2 (1.78 W/ft2) is for internal lighting. This relatively l
31、ow electric demand results from the use of thermally activated cooling and heat- ing systems. The specific electrical and thermal loads of the building (loads per unit floor area) remain unchanged in the course of parametric analysis. 86 ASHRAE Transactions: Research CHP SYSTEM DESCRIPTION The basel
32、ine CHP system configuration shown in Figure 1 is similar to that of the electrical-load-following, grid-indepen- dent model analyzed in the previous paper. The system consists of gas-fired microturbines for power generation, an absorption chiller, a hot water space heating system, and a service hot
33、 water unit. As discussed in that paper, in order to achieve higher part- load efficiencies, three equally sized microturbines are imple- mented to facilitate staged operation. (For the baseline building, the total on-site power generation capacity is about 81 kW.) The efficiencies and operating set
34、point temperatures of the CHP subsystems are identical to those of the grid-independent system of the previous study (Table 1). In contrast, the system of the current paper has a number of differentiating features: t 36 - 32 Building Thermal Loads - O Hot Water I Heating Load ) O Cooling Load flfl I
35、 % 24 fl Il Il A Power Generation Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month of Year Absorption Cooling Elec. Equip. Figure 2 Thermal loads of building. Scenarios It interacts with the electric utility grid in import and export modes. Its power generation system is sized so that the exhau
36、st heat can meet the peak thermal energy demand for space cooling and heating. Although the exhaust thermal energy of the cooling and heating systems is available for the service hot water unit through a cascaded arrangement, no direct heat recovery from the exhaust gas of the power generation syste
37、m is allocated to this unit (Figure 1). An auxiliary gas-fired burner meets any shortfall of the thermal energy availability. Operating Exhaust Temp. Temp. Efficiencv OC (OF) Efficiencv OC (OF) Efficiencv The reason for not including direct heat recovery from the power generation system for the serv
38、ice hot water is that the related thermal load is very small compared to the space cool- ing and heating loads. (The annual thermal energy input for the service hot water system is determined to be about 5% of the total input for all of the thermally activated systems. For the baseline CHP, at least
39、 60% of the required heat input for the service hot water is supplied through the indirect waste heat utilization.) Furthermore, allowing direct heat recovery for the service hot water unit will lead to frequent cycling and ineffi- cient, small part-load operation of the power generation system. Ref
40、. CHP i1* METHOD OF ANALYSIS 260 (500) I COP* I 94 (200) For the performance assessment of the CHP system, the hourly cooling, heating, and electrical loads of the building are first determined via a simulation program, Energy-10 (NREL- LBNL 2002). The hourly simulation results are then used as PG 1
41、 PG2 PG3 Table 1. Scenarios for Parametric Analyses 1.2 q* 260 (500) COP* 94 (200) * 1.4 q* 260 (500) COP* 94 (200) * 1.2 TI* 538 (1000) COP* 94 (200) * AC 1 AC2 AC3 r* 260 (500) 1.2 COP 94 (200) i1 538 (1000) 1.4 COP“ 182 (360) 9* 538 ( 1000) 1.8 COP* 182 (360) * BE 1 * q 260 (500) COP* 94 (200) 1.
42、2 * ASHRAE Transactions: Research a7 input data in another program for analytical evaluation of the CHP system under different scenarios. The key equations used in the analysis are described below. Energy Consumption The total primary energy consumption for a given time period, from time tl to t2, i
43、s the sum ofthe hourly fuel supplied to the on-site power generators, the auxiliary burner (for the hot waterheater only), and the fuel corresponding to the power imported from the grid (i.e., the primary energy input at the central plant). - t2 Qtotai fuel, i - i=t, (? . gen fuel + Qaux.fuei + Qcen
44、t . piantfuel li= (1) i = 1, 3 Ep .gen + Qaux .load I Egrid qp .gen, over11 qburner qgri i i i( = i, where the time span of tl to t2 represents a month of the year or the entire year for the results presented in this study. In Equation 1, Qtotalfuel is the time rate of total primary energy input, Ep
45、 ,gen is the on-site power output, and Egrid is the net electrical power input from the gnd, all of which represent hourly averages. The net grid power import, bgr;d, is deter- mined as follows: Egrid = toial-Ep.gen Note that when power is exported to the grid, grid takes on a negative value, indica
46、ting a displacement of energy use at the central plantis). In Equation 1, qp ,gen, represents the efficiency of the on-site power generation, which is a function of the inlet air conditions and the fraction of the actual power output to the power capacity, as described in the earlier paper (Jalalzad
47、eh-Azar 2003). The parameter qgrid is the efficiency of the delivered electrical energy that accounts for all losses, including those associated with fuel conversion at the central plant, transmission, and distribution. This efficiency is taken to be 30%. Power Generation Provided that the on-site p
48、ower generation system is acti- vated and modulated to meet the thermal load of the building, the power output is related to the required thermal output (exhaust heat) as follows: (3) Because the building in this study is treated as a single zone, the cooling and heating systems do not operate simul
49、ta- neously. Therefore, the required exhaust thermal energy for the thermal-load-following CHP is approximated by (4b) Qheat .load Texh - ?re for heating. Qexh,req?dZ ( )( ?) qh . CHX Texh - h In these equations, TJ is the operating fluid temperature of the absorption cooling generator, TL h is the operating fluid temperature of the space heating system, and Trefis the refer- ence temperature. Note that TJc and Tjh essentially represent the temperatures of the fluid entering the primary heat exchanger in the cooling and heating modes, respectively. The symbols tHx,
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