SET2011, 10th International Conference on Sustainable Energy ...

NuRER 2012 – III. International Conference on Nuclear & Renewable Energy Resources İstanbul, TURKEY, 20-23 May 2012

USE OF HYBRID STORAGE TO ENHANCE CHARGING PERFORMANCE OF STAND-ALONE SOLAR PV SYSTEM Bin-Juine Huang1, Po-Chien Hsu1, Je-Wei Chang1, Wei-Min Tseng1, Ruey-Her Yen1 1

Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan 106 Abstract

A hybrid storage design using a lead-acid battery and a super-capacitor (SC) with a charging control system was proposed in the present study to enhance the charging performance of stand-alone solar PV system. Experimental setup for four solar systems (130Wp PV, 100Ah/12V battery, 50W LED) with different SC was carried out for field test. The single-day performance comparative test of 4 PV systems shows that the highest increase of PV power generation using SC(288F) is 37%. The long-term comparative test results shows that the increase of PV power generation for system with SC(288F) is 25% for HT>15 MJ/m2 day. The performance results of solar PV charging system using SC at different weather show that the increase of PV power generation is >15% in clear weather (HT>15 MJ/m2 day) for SC >58F.

Keywords: Stand-alone solar PV, solar charging, super-capacitor charging 1. Introduction The battery charging performance in a stand-alone solar PV system affects the PV system efficiency and the load service time. The New Energy Center of National Taiwan University has been devoted to the development of a PWM charging technique to continue charging the lead-acid battery after the overcharge point to increase the battery storage capacity [1]. To avoid overcharging of battery, a charging control is employed after the overcharge point to control the charging current using PWM technique such as to maintain a fixed battery voltage. This however will reduce the efficiency of PV power generation since PV power generated during the off periods of PWM cannot be utilized. One of solutions is to increase the battery installation capacity such that the battery will never reach overcharge point. This will increase battery cost and weight. The other solution is to shut down the charging process when the battery reaches the overcharges point. This has been widely used in practice but will reduce the charged energy of the battery. A hybrid storage design using a lead-acid battery and a super-capacitor (SC) with a charging control was proposed in the present study to enhance the solar PV charging performance of stand-alone solar PV system. The use of capacitor with lead-acid battery has been used very often for the purpose of discharge with high instantaneous power output to drive motors etc. The use of capacitor or SC in charging process especially solar charging has not been theoretically or experimentally studied yet. The present study carries out a comparative outdoor test to find out the actual increase in solar PV power generation with the hybrid storage design. 2. Experimental setup 2.1. Design of solar PV charging system Four solar PV charging systems with different SC was designed and installed for field test. Figure 1 shows the system configuration. 50W LED luminaires was used as the load for lighting in all the systems. Three kinds of SC (25F, 58F, 288F) were used. The design of the 4 solar PV systems is listed in Table 1. 130Wp PV module and 100Ah/12V lead-acid battery were used (Figure 2). The charged energy was discharged to the LED load for lighting at night and the battery voltages of the 4 system were kept about the same level at the end of discharge.

Figure 1. Solar PV charging system with SC and lead-acid battery. 

E-mail of Corresponding Author: [email protected]

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controller

Lead-acid battery Super-Capacitor

Figure 2. Four experimental setup Table 1. Experimental setup System super-capacitor 16V solar PV module lead-acid battery LED luminaire

C_0F E_25F E_58F E_288F no 25F 58F 288F 130Wp (SEC-130G6M) polycrystalline 100Ah/12V 50W/12V

2.2. Charging controller The solar PV charging controller was designed according to the feedback structure of Figure 3 [1]. To charge the battery to its full capacity, a three-stage charge algorithm was utilized. Phase 1 is to directly charge the battery from solar PV until the battery voltage reaches its overcharge point. Usually, the battery is charged in full speed without controlling the charging current in Phase 1. Only 50–80% state of charge (SOC) can be achieved at Phase 1. Phase 2 is to maintain the battery voltage at the overcharge point to replenish the remaining storage capacity. Phase 3 is to reduce and fix the battery voltage to maintain 100% SOC. In both Phase 2 and Phase 3, the charging current generated from PV needs to be reduced in order to maintain at a setting voltage. A feedback control system as shown in Figure 3 is developed using a PWM technique to regulate the charging current and fix the battery voltage in Phase 2 and 3. A metal-oxide-semiconductor-field-effect transistor (MOSFET) is used to switch the charging current (on/off) from solar PV via a PWM signal. The average charging current Iave can be controlled by regulating the dutycycle Duty in order to fix the battery voltage. The controller C(s) was designed with robust properties to prevent voltage overshoot [1]. The whole control system was implemented on a PIC microprocessor with measurement of solar PV power generation, battery current and voltage, solar radiation, ambient temperature etc. IT Solar PV Gpv(s) Ipv Vo

+

e -

Controller C(s)

Iave=Duty x Ipv

MOSFET Duty

Lead-acid battery GLA(s)

Vbat

Super-capacitor Gsc(s) Measurement Figure 3. Feedback structure of battery charging control [1]. 3. Test Results Figure 4 shows the single-day performance comparison for PV charging system with or without SC (288F). It is clearly seen that the use of SC increases the PV power generation by 21.8%. The energy charge of battery will increase 31% if using 3-Phase charging algorithm (without SC), as compared with the charging terminated completely at the overcharge point. The PV charging with SC (288F) will double the increase of energy storage, or 64% compared with the charging with 3-Phase charging algorithm.

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Figure 4. Single-day performance comparison for 288F SC. Figure 4 shows that the energy charge of battery will increase 31% if using 3-Phase charging algorithm (without SC), as compared with the charging terminated completely at the overcharge point. The PV charging with SC (288F) will double the increase of energy storage, or 64% compared with the charging with 3-Phase charging algorithm. Figure 5 is the comparative test results for the system with SC 288F. It is seen that the increase of PV power generation is 25% for HT>15 MJ/m2 day. The highest increase is 37%. The long-term comparative test results for 4 systems are shown in Table 2.

Figure 5. Long-term comparative test results for SC 288F.

date 2010/12/17 2010/12/18 2010/12/19 2010/12/21 2010/12/22 2010/12/23 2010/12/24 2010/12/27

Table 2. Long-term field test results increase of power generation Solar compared to the system without irradiation SC, % 2 HT, MJ/m SC SC SC day 288F 58F 25F 14.98 15.92 15.23 7.04 7.24 14.17 9.90 15.71

16.3 19.4 8.1 -1.8 -0.4 10.4 -0.1 16.5

14.7 18.0 9.7 -0.7 1.1 11.1 1.2 15.0

-6.4 -7.1 -6.9 1.2 1.4 -5.1 -0.4 -7.7

PV power generation without SC, Wh/day 440 454 497 231 247 460 350 397

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NuRER 2012 – III. International Conference on Nuclear & Renewable Energy Resources İstanbul, TURKEY, 20-23 May 2012 2010/12/28 2010/12/29 2010/12/31 2011/1/05 2011/1/18 2011/2/22 2011/2/26 2011/2/27 2011/2/28 2011/3/05 2011/3/13 2011/3/14 2011/3/17 2011/3/20 2011/3/30 2011/4/01 2011/4/02

12.51 16.68 13.34 7.34 15.0 14.1 19.8 19.0 18.4 13.52 15.41 14.81 12.39 16.47 16.00 21.35 19.20

16.5 15.8 14.8 2.4 12.0 24.0 29.5 24.5 21.8 9.3 23.1 12.1 13.0 26.3 37.0 36.5 26.1

13.7 12.1 12.1 2.9 9.4 16.6 18.9 14.4 13.5 9.5 12.9 11.4 7.2 12.2 12.8 21.3 15.6

-5.2 -4.3 -5.1 -5.7 -5.3 -2.8 0.7 1.5 2.2 2.3 0.5 3.2 2.3 3.2 19.9 14.5 11.6

430 461 436 355 364 434 471 528 539 439 446 483 376 465 419 464 514

The performance results of solar PV charging system using SC at different weather conditions is summarized in Table 3. It is seen that the increase of power generation is >15% in clear weather (solar irradiation HT>15 MJ/m2 day) for SC >58F. Table 3. Increase of PV power generation at different weather System

C_0F

E_25F

E_58F

E_288F

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Solar irradiation : 0 ~ 5 MJ/m day (raining) Average energy generation (Wh/day) 72 74 74 70 Average increase 0.00% 2.50% 2.55% -3.59% Solar irradiation : 5 ~ 10 MJ/ m2day (cloudy) Average energy generation(Wh/day) 255 2578 254 250 Average increase 0.00% 0.86% -0.46% -1.84% Solar irradiation : 10 ~ 15 MJ/ m2day (partly-cloudy) Average energy generation(Wh/day) 423 429 464 474 Average increase 0.00% 1.38% 9.71% 12.04% Solar irradiation : 15 ~ 20 MJ/ m2day (clear) Average energy generation(Wh/day) 490 498 560 612 Average increase 0.00% 1.64% 14.4% 25.0%

The present study has shown that the use of hybrid storage in stand-alone solar PV system really increase solar PV power generation. The larger super-capacitor (288F) has shown to have the best performance for the solar PV systems tested (130Wp PV, 100Ah/12V battery). The total energy storage of 288F SC is estimated to be around 2 Wh which is negligible as compared to the main lead-acid battery (100Ah/12V for about 1,200Wh). The supercapacitor acts as a dynamic energy storage buffer which will level out the dynamic effect of solar irradiation. This probably makes the charging current to the lead-acid battery reduced and the rate of battery voltage rise is slowed. The Phase 1 charging becomes longer. As the charge goes into Phase 2 and 3, the PWM current to the battery controlled by the feedback control system provides another dynamic effect for the charging process. The test results of Table 4 show that the use of super-capacitor will reduce the resistance of lead-acid battery. This may explain why SC will help the solar PV charging process. Table 4. Internal resistance and charging power of lead-acid battery at different time With super-capacitor Without super-capacitor time (4/23) lead-acid battery charging power lead-acid battery charging power resistance (Ω) (W) resistance (Ω) (W) 8AM 9.99 17.4 11.53 14.2 10AM 6.30 28.6 6.98 24.7 12PM 3.21 59.1 4.93 35.9 4. Conclusions A hybrid storage design using a lead-acid battery and a super-capacitor (SC) with a charging control system was proposed in the present study to enhance the charging performance of stand-alone solar PV system. Outdoor experiments for four solar systems (130Wp PV, 100Ah/12V battery, 50W LED) with different SC were carried out. The single-day performance comparative test of 4 PV systems shows that the highest increase of PV power generation using SC(288F) is 37%. The long-term comparative test results shows that the increase of PV power

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generation for system with SC(288F) is 25% for HT>15 MJ/m2 day. The performance results of solar PV charging system using SC at different weather show that the increase of PV power generation is >15% in clear weather (HT>15 MJ/m2 day) for SC >58F. The use of SC to enhance the charging process is due to the battery resistance drop caused by the dynamic effect of capacitor. More theoretical researches are needed to investigate the PV charging phenomena using the hybrid storage device. Acknowledgement This publication is based on work supported by Award No. KUK-C1-014-12, made by King Abdullah University of Science and Technology (KAUST), Saudi Arabia. References [1] Huang B.J, Hsu P.C, Wu M.S, Ho P.Y, “System dynamic model and charging control of lead-acid battery for stand-alone solar PV system” Solar Energy 84, 822– 830(2010). Nomenclatures Ac total solar cell area of PV module, m2 Cbat cumulated battery charge, Wh Ibat charging current of battery, A Ipv current generated by PV module, A IT instantaneous solar radiation on PV surface, W/m2 HT daily-total solar radiation on PV surface, MJ/m2day Vbat charging voltage of battery, V Vo overcharge point of battery, V

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