Performance Evaluation of Solar Home Systems in Hot Climate Condition: mc-si PWM versus a-si MPPT Charge Controller System

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1 ก ก ก ก 2549 Performance Evaluation of Solar Home Systems in Hot Climate Condition: mcsi PWM versus asi MPPT Charge Controller System Wuthipong Suponthana 1, *, Nipon Ketjoy 2, Wattanapong Rakwichian 2 1 PhD Candidate, School of Renewable Energy Technology, Naresuan University, Phitsanulok 65000, Thailand 1 Leonics Co., Ltd. Bangkok Thailand Tel: ws@leonics.com 2 School of Renewable Energy Technology (SERT), Naresuan University, Phitsanulok 65000, Thailand Tel: Fax: sert@nu.ac.th Abstract In solar electrification system charger controller is one the balance of system (BOS) equipments that has major impact to the performance of the system especially in small system like Solar Home System (SHS). Thailand's megaproject 200,000 SHS's consist of two types of PV module assemblied or made in Thailand, multicrystalline silicon PV (mcsi) and amorphous silicon PV (asi), which use different type of charge controller. The system with multicrystalline silicon (mcsi) PV module use PWM or OnOff charger controller while amorphous silicon (asi) PV module use DCDC converter or DCDC converter with maximum power point tracker (MPPT) charge controller. The energy yield performance of these two systems could be different which make the daily energy produced to be consumed by user of these system are different. The over consume of energy in SHS users lead to system unbalanced and system fail finally. In this paper two SHS's, mcsi PV module with PWM charger controller and asi PV module with MPPT charge controller are set up in Bangkok with other BOS equipment which consisted of deep cycle battery, modify sine wave inverter and resistive load, to evaluate energy yield performance of two type of SHS's in hot climate condition such as Thailand. 1. Introduction Small AC solar electrification system such as Solar Home System (SHS) consists of five major components which are PV module, charger controller, inverter, battery and load. Among all of them charger controller is the component that have to be selected suitable with the type of PV module and the battery for h i g h e s t e n e r g y y i e l d f o r m P V m o d u l e. Thailand megaproject 200,000 SHS's uses two types of PV module, mcsi and asi, which have different output voltage at maximum power (V mp ). The msi (36 cells) has suitable V mp to charge 12 V dc battery thus pulse width modulation (PWM) or On Off charger controller is usually selected to use with the SHS using mcsi PV module. The asi has higher V mp which is not suitable to charge 12 V dc battery thus SHS with asi PV module requires voltage conditioner such as DCDC converter to change V mp to suitable voltages for charging 12 V dc battery. Since this type of charge controller has to convert DC voltage which mean the input part of the charge controller from PV module and the output part of the charger controller to battery are separated thus the good charge controller integrates the Maximum Power Point Tracking (MPPT) algorithm into the input part which is called MPPT charge controller. In this paper we will evaluate the performance of MPPT and PWM charge controller working with their suitable PV modules. Two SHS's, mcsi PV module with PWM charger controller and asi PV module with MPPT charge controller are set up in Bangkok with other BOS, deep cycle battery, modify sine wave inverter and resistive load, to evaluate performance of two type SHS's in hot climate condition such as Thailand. The performance evaluation model introduced by IEA PVPS TASK2 is applied to evaluate result data [1,2]. 2. Solar charge controller There are several types of charger controller designed to use with PV power system in the market. According to the charge controller survey by Photon Energy magazine among 38 manufactures and over 260 models of charge controller only 3 types of algorithms used in these charge controller, OnOff (series or shunt), PWM and MPP charger controller [2]. OnOff algorithm which may be series or shunt regulation uses electronics control to switch PV output connecting to the battery and uses battery voltage to control the switch to be turn on and turn off. The switch will turn on and connect PV to battery when battery voltage is lower than set point and the switch will t u r n o f f a n d d i s c o n n e c t P V f r o m b a t t e r y. * Corresponding author

2 ENETT PWM algorithm which may be PWM series or PWM shunt regulation uses electronics control switch to turn on and off control switch in variable frequency, 500 Hz to 1 khz or higher with a variable duty cycle to maintain battery charging voltage close to the set point [4]. The PWM algorithm allows battery to be charged at the state near fully charge in precise voltage and p r o d u c e l e s s h e a t. MPP algorithm which may be buck or boost regulation separates controlling of received PV module power and battery charging power by using DC to DC converter thus the voltage level of V mp and battery voltage can be different. At the input of DC to DC converter attached to the output of PV module the PV maximum power searching algorithm called maximum power point tracking (MPPT) is embedded into the controller. The algorithm capable to search for the current and voltage at the present solar radiation that allow PV to supply maximum power by adjusting a current at input of DC to DC converter to get maximum output power. The DC to DC converter will convert input power form PV at maximum point to suitable voltage for charging battery. 3. AC SHS in Thailand The AC SHS system installed in Thailand's megaproject consists of PV module 120 W p, charge controller with charge current 10 A, modify sine wave output inverter 150W, deep cycle battery 125 Ah 12 V dc. The installation sites are in remote area of hot and humid environment locate in Thailand between latitude 5 45'N to 20 30'N and longitude 97 20' E to ' E. The installed system consists of two types of PV modules w h i c h a r e f o l l o w i n g : 1. mcsi PV module with 36 cells type, power rating 120 W p, V m p a r o u n d 1 7 V d c. 2. asi PV module, power rating 40 W p x 3, power rating 120 W p, V m p a r o u n d 4 4 V d c. These two types of PV module require different type of charge controller to charger 12V dc battery in the system. The configuration if these two systems are shown below (Figure 1 and Figure 2). Figure 1. Solar Home System with mcsi PV module and OnOff charge controller Figure 2. Solar Home System with asi module and MPPT charge controller 4. Performance evaluation The parameters usually used to assess the performance of stand alone system are the following [1,2]: Y A = E A /P o (kwh/kw p.d) : array yield (1) Y R = H t /G STC (kwh/kw p.d) : reference yield (2) Y f = E PV /P o (kwh/kw p.d) : final yield (3) E PV = E L /(1+E BU /E A ) (kwh) : PV energy consumed (4) L C = Y R Y A (kwh/kw p.d) : capture losses (5) L S = Y A Y f (kwh/kw p.d) : system losses (6) PR = Y f /Y R : Performance ratio (7) When: P o : Peak Power (W p ) H t : Mean daily irradiation in array plane (kwh/m 2.d) G STC : Reference irradiation at STC (1 kw/m 2 ) E A : Array output energy (kwh/d) E L : Energy to loads (kwh/d) E BU : Energy from backup system (kwh/d)

3 ENETT Performance Ratio (PR) is the parameter used in representing the capability of energy production potential of the system. Higher PR is mean better system design and lower PR value mean production loss due to design or technical problem [5]. The value of PR is load consumption dependent which means that not suitable design between PV and load may lead to low PR value. IEA TASK 2 has introduced additional indicators called "Used Factor" (UF) which defined as: enough storage for energy produced by PV module on the next operating day. UF= E A / E POT (8) In Stand Alone Systems E POT is determined by measuring the PV array current even during disconnections [2]. η SYS = Y f /Y A : system efficiency (9) η PROD = E POT / P o.y R : production efficiency (10) PR/UF = η SYS.η PROD (11) Normally in the system EPOT is not monitor thus UF could not determine IEA TASK 2 also introduce another coefficient called "Production Factor" (PF) which use the parameter at STC specify by manufacture to determine it as follows. PF = E A / (Po.H t /G STC ) = Y A /Y R (12) PR/PF = η SYS = Y f /Y R (13) 5. Test system configuration The test systems consist of two SHS's. The msi PV module 123 W p with PWM charge controller, 150 Watt modify sine wave inverter, 125 Ah 12 Vdc battery and 100 watt linear load. The a Si PV module 64 W p x 2 = 128 W p with MPPT charge controller, same model of inverter and battery as shown in the Figure 3. The recorder is set to record solar radiation, ambient temperature, msi and asi module temperature as environmental data. Both msi and asi PV is measured value of module output voltage and current every 5 seconds as well as voltage and current that the systems charge and discharge the batteries. AC energy consumed by load is also recorded. The systems are set up by exposing mcsi and asi PV module for 4 weeks before start recording the data to be analyzed. The Batteries in both systems are fully charge before initial starting and are forced to deliver stored energy to 100 watt linear load through inverter until the battery voltage reaches low voltage cut off (10.5 V dc ). The batteries are prepared to have Figure 3. Test system configuration 6. Result of evaluation At the installation location near the equator where the ambient temperature is high as exhibit in Figure 4. The daily average module temperature measure at the back panel of asi PV module is higher than temperature measured at the back of mcsi PV module. The energy produced by asi PV module is higher than energy produced by mcsi PV module when they are exposed to produced energy form the same solar irradiation as shown in Figure 5 in the value of Y A which is electrical energy produced in one day per W p of the installation. To evaluate the daily energy yield which has different solar irradiation. The indicative value of Y A /Insolation is created. This value indicates that the energy yield per W p of install systems per kwh of solar irradiation in one day (insolation) produced from a Si PV module is higher than produced form mcsi about 18% in the 5'th week after installing and decrease to 10% in the 21'st week after installing. The trend line show that the percentage of this different is decreasing and stable at about 10% as shown in Figure 6. The efficiency of the system as indicate in equation (9) is calculated and shown in the Figure 7. The system efficiency is the result of energy produced form PV module and energy deliver to load which has energy loss in charger controller which clearly seen that the loss in MPPT charger (17%) is higher that PWM charger (7%). The PWM charger is just a switch which at the time it is turn on it pass current form PV directly to battery but MPPT need to convert voltage form 65 V dc to 14 V dc which mean it need DC to DC converter to convert high DC voltage to lower DC voltage and in process of DC to DC converter it has some loss (may be more than 10 % in small lower power DC to DC converter, energy loss in battery which is around 20%, energy

4 ENETT loss in inverter for conversion form DC 12 Vdc to 220 V ac which is around 17%. Solar Irradiation (kwatt/m 2 ) Solar Irradiation & Ambient Temperature Insolation 6.36 kwh/day/m Ambient Temperature (Degree C) show that PF of system with asi and mcsi are related to ambient and module temperature. The higher temperature is lead to higher PF of asi module than mcsi. PR of the tested systems are also has result in the same direction beside PR has other performance of BOS which make the PR of csi higher than asi. Both SHS systems have PR around 0.47 to 0.55 which is considering high PR for stand alone system. The PR can represent that systems deliver all energy produced form PV to load and empty the batteries in the system before receiving energy produced by PV in the next morning :21:41 07:00:01 08:00:01 09:00:01 10:00:01 11:00:01 12:00:01 Time 13:00:01 14:00:01 15:00:01 16:00:01 17:00:01 17:35:51 17:35:56 Charge Controler/Battery/Inverter/System Efficiency Solar Irradiation Ambient Temperature csi Panel Temperature asi Panel Temperature % 2% Figure 4. Daily solar irradiation and ambient temperature 90.00% BOS Efficiency 80.00% % Ya (Wh/Wp) and Insolation (kwh/day/m 2 ) Ya (Wh/Wp), Insolation (kwh/day/m 2 ) Average Temperature (Degree C) Average Temperature (Degree C) Efficiency (%) 70.00% 60.00% System Efficiency 1% 50.00% 40.00% % 30.00% % MPPT Charge Controler with asi PWM Charge Controler with csi % Battery Efficiency of asi System Battery Efficiency of csi System asi System Efficiency csi System Efficiency % Inverter Efficiency % Different of System Efficiency 0.00% 0.00% Figure 7. Efficiency of BOS component and System efficiency Different of System Eficiency (%) Production Factor (PF) and Performance Ratio (PR) Insolation asi 48 Vdc csi 12 Vdc asi Average Module Temp csi Average Module Temp Average Ambient Temp Figure 5. Energy produced by asi PV module and mcsi PV module Ya/Insolation (Wh/Wp)/(kWh/day/m 2 ) Ya/Insolation Insolation and Percentage of Different between asi and csi asi 48 Vdc csi 12 Vdc 2 1 Insolation (kwh/m2/day) and % of Different Production Factor (PF) asi Production Factor (PF) csi Production Factor (PF) asi Performance Ratio (%PR) csi Performance Ratio (%PR) asi Module Temperature csi Module Temperature Average Ambient Temperature Figure 8. Production Factor and Performance Ratio of the systems Performance Ratio (%PR) Insolation % Different 0.10 Power (% Different) Linear (asi 48 Vdc) Linear (csi 12 Vdc) Energy Delivered to Load and Yf Figure 6. Comparison of Y A /Insolation between asi and csi and the different in percentage The Production Factor (PF) and Performance Ration (PR) of the experiment systems are calculated by recorded data according to equation (12) and (7) respectively. PF which is the energy produced per reference energy that the module should produce at STC (Y R ) and PR which is the energy delivered to load per reference energy (Y R ) is shown in Figure 8. The result Energy to Load (kwh/day) asi Energy to Load csi Energy to load Yf of asi System Yf of csi System Figure 9. Energy Deliver to load of the asi and mcsi PV module systems Yf ((kwh/day)/wp)

5 ENETT The final value of the systems that are in the interest of the use is the amount of energy deliver to load which is shown in the Figure 9. The data indicate that asi deliver higher amount of energy to load comparing to system with mcsi but if we consider Y f which is the energy deliver to load per W p of installation the Y f of both system are almost same. 7. Conclusion The asi PV module with MPPT charger controller system can produce higher energy per W p of installation power than mc Si PV module with PWM charger controller system installed at same location in hot climate condition where ambient temperature is high. Due to some losses in BOS components which is used by asi system which are loss in MPPT charger controller, loss in battery charging and discharging and loss in inverter the output energy per installed W p deliver to load from both system are almost same. To increase opportunity to get higher energy yield to load from asi PV module system the loss from MPPT charge controller which major loss is created by DC to DC converter part in the charger should be reduced. This loss is higher when the different between input voltage received from PV and nominal battery of the system is wider. To reduce this loss and increase efficiency for gain higher benefit from MPPT algorithm the loss on DC to DC converter part should be reduced by narrow down the different between V mp from PV and the nominal battery voltage which can be done by selecting asi PV module with lower V mp or make lower PV output system voltage or increase system battery voltage. The percentage of difference between energy yield of PV systems with asi and mcsi PV is reduced when systems are installed and operate in longer time which indicate some value of the degradation of energy production of different type of PV module. The result show non stable of energy reduction on the tested systems form asi PV system higher than mcsi PV system which could not used to make clear conclusion and the monitored and recorded should be done for at least two years after installation for final result. 4. Sandia National Laboratories, Batteries and Charge Control in StandAlone Photovoltaic SystemFundamentals and Applications, January 15, Mayer, D. and Heidenreich, M., Performance analysis of stand alone PV systems form a rational use of energy point of view. 6. Vervaart, M.R. and Nieuwenhout, F.D.J., Solar Home System: Manual for the design and modification of Solar Home System component, January References 1. International Energy Agency, Operational performance, reliability and promotion of photovoltaic systems, Report IEAPVPS T2 03:2002, International Energy Agency, Analysis of Photovoltaic Systems, Report IEAPVPS T201:2000, Photon Energy magazine Jan 06 issue