1、4782 Techno-Economic Analysis of a Large-Scale Rooftop Photovoltaic System Manohar Kulkarni, PhD, PE Member ASHRAE ABSTRACT This paper presents the optimization process of a grid- connected photovoltaic (Pv) system, which is intended to replace a large-scale thermal solar system on the rooftop of a
2、federal ofice building. A PV energy conversion model is described. Based on this model, array surface tilt angle and array size are optimized. The optimization method is based on maximizing the utilization of the array output energy and, at the same time, minimizing the electricitypowersold to thegr
3、id. An efectiveness factor is introduced that takes into account both of these parameters. The array configuration and the output parameters are determined by comparing several PV modules. A 43.2 kWPVsystem is designed, and operational problems such as harmonic efects and anti-islanding are discusse
4、d. Finally, the system performance is simulated, and through economic analysis it is shown that the cost of PV system can be recouped in 13 years under the current renew- able energy incentive program by the state of Illinois. INTRODUCTION In the past two decades photovoltaics has developed into a m
5、ature technology and has become acceptable worldwide. As a promising renewable energy resource, photovoltaic tech- nology enjoys substantial government supports in research and application in several major industrial countries. The international competition, along with years of experience in manufac
6、turing, research, and development, has resulted in improved photovoltaic module efficiency, cost reduction, and productivity increase. According to Holihan (2003), the worldwide photovoltaic shipment increased four times in the 1990s and reached a peak megawatt generation of 201 MWp in 1999, while t
7、he price of photovoltaic modules has dropped Xiangyang Gong from $3O/Wp in 1970s to $5/Wp now. Although this price is somewhat acceptable, the cost of an entire system still remains relatively high compared with traditional power generation technology. The high cost necessitates that the design para
8、m- eters, such as surface tilt angle and array size, should be opti- mized. A grid-connected photovoltaic system eliminates the need for a battery storage bank, resulting in considerable reduction of the initial cost and maintenance cost. The photo- voltaic system instead uses the grid as a bank whe
9、re the excess electric power can be deposited and when necessary also with- drawn. When the photovoltaic system is applied in buildings, the PV modules usually are mounted on rooftop and facade, which can reduce the size of the mounting structure and land requirements. This paper presents the design
10、 optimization processes for a large-scale rooftop photovoltaic system, which will be used to retrofit the existing thermal solar system on the roof- top of the federal office building in Carbondale, Illinois. This building was constructed in 1978 as one of only three proto- type thermal solar buildi
11、ngs in the United States. The roof of this building was designed to have a slope of 42“ to maxi- mize the reception of solar radiation. After 17 years of oper- ation, the hydronic thermal solar system on the building rooftop was shut down in 1995. Hundreds of thermal solar modules and the racks stil
12、l remain on the roof. As shown in Figure 1, the thermal solar modules are arranged on three roof zones of the building. The southwest array is the largest one, with dimensions of 14.33 m (47 ft) by 28.96 m (95 ft). The northwest array is the smallest having dimensions of 10.67 m (35 ft) by 21.34 m (
13、70 ft). Dimensions of the east array are 14.63 m (48 ft) by 21.34 m (70 ft). Study of the Manohar Kulkarni is a professor and chair of the Department of Mechanical Engineering at the University of North Dakota, Grand Forks, N.D. Xiangyang Gong is a PhD student in the Department of Mechanical Enginee
14、ring at Texas A + c;(ep3 i- p( - “Z“p31 , (2) where 7 is the monthly average hourly radiation per unit area *3=( 2 -w) (8) (7) 2 i + cos(e ) on a horizontal surface. It is estimated from the monthly aver- age daily radiation data given in Table 1. The monthly average difise fraction id/ was develope
15、d by Erbs et al. (1982) as = 1.317-3.023K,+3.372$- 1.7693, The instantaneous diffuse fraction ZdlI is a function of k, as explained by Duffie and Beckman (1991). The long-term probability distribution of kt is a known function of Et, defined as xt = ?I, which can be determined by daily radiation dat
16、a listed in Table 1. The integrals in Equation 5 have been eval- (3) i where Kt is the average ratio of the horizontal solar radiation to the extraterrestrial radiation. The parameter R, in Equation 2 is the ratio of beam radi- ation on the aperture plane to that on a horizontal surface, Cis the con
17、centration ratio, Op is the array tilt angle with respect to the horizontal, and p is the ground reflectance. This concludes the explanation of all parameters in Equation 2 and the term c in Equation 1. Now the parameter in Equation 1 is the average energy conversion efficiency weighted in proportio
18、n to the solar radi- ation. It can be evaluated by the following equation (Siegel et al. 1981; Clark et al. 1984): 1 IC,MAX 2 7i = l,rlp, 1 - Nia- Tr) - -“(l - VIpt) 1 IcP(c)dI, O CU (4) I Here 7, is the monthly average hourly temperature, which is estimated in terms of the monthly average daily tem
19、perature based on the model developed by Erbs et al. (1983). The parameters qr and qpt are array reference energy conversion efficiency and efficiency of the power tracking equipment, respectively. The other terms in Equation 4 are explained in the uated by Liu and Jordan (1 960). kr,max I ksP(k,)dk
20、, = - 0.1551 + 0.9226/(, (9) O kr,mux 1 k;cf)2P(k,)dk, = k,(0.2769 + 0.3184P(zc)dzc = u$; J k;P(k,)dk,+a21; O O 2 Id J k;ff)“(k,)dk, + a31; J k, (7) P(k,)4 O O where kt is the ratio of horizontal solar radiation, Zto the extra- terrestrial radiation, Z,. Parameters a, a2, a3 are constants for given
21、hour and month. They can be evaluated by Equations 6 through 8. 436 ASHRAE Transactions: Research The diffuse radiation approaches the array surface from all unobstructed angles, while direct radiation strikes the array surface from only one angle. Since the atmospheric constitu- ents scatter a port
22、ion of the total beam radiation from the sun, some diffuse radiation always exists even when the sky appears very clear. The orientation of a surface on earth is defined by two angles: the surface tilt or slope angle and the surface azimuth angle. The surface tilt angle indicates how far up from the
23、 hori- zontal a given surface is sloped, while the azimuth angle denotes how the surface is located relative to the true north- south and east-west coordinates (due south represents an azimuth angle of O“, due east is -90“, north is 180“, and west is 90“). A horizontal surface receives the maximum d
24、iffuse radiation but only a minimum reflected radiation. When a south-facing surface is tilted up from horizontal, the amount of diffuse radiation received decreases. However, the receipt of radiation reflected off the ground increases. For the federal building in Carbondale the azimuth angle of the
25、 building eP is zero. The incidence angle of the sun 4 depends on the geographical location (latitude 3746and longitude 89“ 14) and time of the year. The aim of design opti- mization is to determine the optimal tilt angle lP for each season in order to obtain maximum output from the PV array. Accord
26、ing to Mathew Buresch (1 983), the optimal tilt angle for a south-facing surface equals the sites latitude. This surface would receive the optimum amount of direct-beam solar radiation over the entire year. If the maximum solar energy is expected to be received during the winter months, the surface
27、tilt angle should approximately equal the latitude angle plus 11“; consequently, the best tilt angle during the summer months is the sites latitude angle minus 11“. When compared with the latitude angle of Carbondale, 37“46, it can be observed that the roof angle of 42“ for the federal building was
28、designed for maximizing solar energy received in fall and winter. In winter the radiation strength becomes lower. At the same time, the thermal solar collectors lose a significant amount of heat to the environment. The conversion efficiency of the thermal solar collectors becomes lower. However, for
29、 a PV system this angle may not be the best because the effi- ciency of PV modules increases when the ambient tempera- ture decreases, as in winter. In addition, the diffuse and reflected components of solar radiation were neglected in the above analysis. It should be noted that tilting a surface up
30、 from horizontal decreases the di fise radiation and increases the reflection received from the ground, which makes an optimum tilt angle for PV different from the angle for solar-thermal. Using the climatic data presented in Table 1 in the photo- voltaic analysis software developed by Klein and Bec
31、kman (2001), the PV electricity output from a 954.6 m2 array at different tilt angles from 22“ to 48“ can be calculated. The PV outputs are plotted in Figure 2 and Figure 3 for illustrating the effect of tilt angle on monthly and yearly PV output, respec- tively. Figure 2 The effect of tilt angle on
32、 monthly PV output. 138000 137000 136000 - 135000 5 134000 +On Rak I j-eff Peak, to the grid fetches about 4 $kWh, while the electricity bought from the grid costs almost 9 $/kWh (considering tax and the demand charge) in the Midwest. Moreover, the average photo- voltaic electricity generation cost
33、is about 12 $/kWh. There- fore, the optimal PV system is one where the array output matches the electricity loads very well and the electricity sold to the grid is minimized. Thus, the load profile of the building and power output profile of the proposed PV array should always be studied in combinat
34、ion. Two years of utility bills (2000 and 2001) for the federal building were collected. The electricity data were obtained from these bills. The hourly normalized electricity demand (kWWh) during the on-peak and off-peak periods is shown in Figure 5. 438 ASHRAE Transactions: Research Figure 5 shows
35、 that the electric load reaches a peak in July when the on-peak load is 40.07 kWhh and off-peak load is 48.21 kWhh. The load reaches the lowest point in either March or November. It is reasonable because natural gas is used for both heating and domestic hot water boilers in this building. Therefore,
36、 the load is high in summer and low in spring and fall. As discussed earlier, the array size should be optimized according to the electric load profile of the building. Hourly PV output should match the electric load to increase the economic benefit of the investment. The output of the PV system cha
37、nges hourly as well as loads. The electricity gener- ated by PV may be used by the building totally or partially. In order to study the utilization fraction of the PV system, an effectiveness factor is defined. This effectiveness factor is equal to the array output that is utilized by the building o
38、ver total PV array output, as shown in the following equation: 365/ 24 n=o n=o fe = 365, 24 , n=o Here?= 0.945) and 540 m2 ve= 0.848), the elec- tricity sold to the grid is kept to a minimum while ensuring that the peak power output of the array is a bit larger than the load (power sold to the grid
39、is positive). The roof of the federal building has three zones available for PV array installation (Figure 1). Their areas are 228 m2, 3 13 m2, and 41 5 m2, respectively. The combination of any two areas would be 541 m2, 643 m2, and 728 m2. There are two feasible options in choosing array areas. One
40、 is using only the southwest rooftop (95 x 47 ft, 415 m2) for PV array and the other is to use both the northwest rooftop (70 ft x 35 fi, 228 m2) and east rooftop (70 fi x 48 ft, 313 m2) for PV array. The second option, with a combined array area of 540 m2, is a reasonable choice for array area beca
41、use some surplus power output during the peak hour can be made by taking advantage of the large roof area available. However, 540 m2 is the sum of the east roof zone (228 m2) and northwest roof zone (3 13 m2). This means that two arrays will need to be built, one on the east roof zone and the other
42、on the northwest roof zone. Due to the different dimensions of the two roofs, the array outputs, such as power, voltages, and currents, will certainly be different and two separate systems will be needed. Although there are no theoretical problems, the system cost, wire cost, and mainte- nance cost
43、will certainly be higher than a sole array. When the hourly load and output of the 540 m2 array were studied, the result indicated that the peak power output is higher than load. Based on the above analysis, it is recom- mended that a single array on the southwest roof zone (95 ft x 47 ft, 415 m2) s
44、hould be used. SYSTEM DESIGN AND CONSIDERATION In the current photovoltaic market, BP Solar, ASE Amer- icas, Shell (Siemens) Solar, Solarex, and United Solar account 440 3504 3000 - - 2500 2000 $1500 1000 500 O -500 . _. _. - . x Jan Feb Mar April May Jun Jul Aug Sep Ckt Nov Cec Month t 643Iw2 -8- 5
45、5Om“Z -A- %OW2 , -?t+ 500W2 -+- 45Om“Z + 415W2 - 35Om“Z -4-313W2 - 2282 Figure 9 Electricity sold to grid ut different array ureas (tilt angle = 42“). for more than half of the US photovoltaic market. The types of the modules vary from several watts to hundreds of watts. The sizes vary from several
46、square inches per module to 3,500 square inches per module. For large array areas, a big module is the best choice. They have advantages in installation and maintenance. Array Configuration Four large PV modules in the current market were compared in order to set up an array of 415 m2 (95 ft x 47 ft
47、). The results are shown in Table 3. According to the National Electric Code, 600 V is recommended as the upper limit for open circuit voltage in photovoltaic system. Further, 150 A is recommended as the maximum acceptable short current for a 45 kW inverter. Based on these two standards and consider
48、ing the available roof, Module 1 is recommended for building the array, A 45 kW is selected for the system. Grid-Connected Photovoltaic System A grid-connected photovoltaic system was designed as shown in Figure 10. The chief components in this system are: 288 modules of type 1, a 45 kw inverter (ou
49、tput 208 V), an isolation transformer (208V/480V), an array combiner, DC/ AC disconnect switches, electric meters, etc. The DC currents generated by the PV array are organized by a combiner to form a more powerful DC current. This current flows to the inverter and is converted to 208V AC current. The voltage of the AC ASHRAE Transactions: Research II II Figure 1 O Scheme of grid-connected photovoltaic system. current rises to 480 V after the isolation transformer and then is connected to a 480 V building bus by a disconnection switch. The meters in the sys
