Quantum Power Conversions (P) Ltd, Bangalore, India | sales@qpcpower.com | www.qpcpower.com

What is a Solar Power Conditioning Unit (PCU) how is it different from an Inverter+ solar charger? A Solar PCU is an electronic unit which consists of the following functional units.

1. Solar charger 2. Inverter 3. Grid (mains utility) charger. 4. Output selector mechanism 5. Battery bank 6. Control algorithm

Solar charger The solar charger is that part of the PCU which converts the solar energy collected by the solar photovoltaic Panel (SPV) to charge the battery. There are two main types of solar chargers- PWM type and the MPPT type.

a. In the PWM type solar charger, the SPV Panel is directly connected to the battery by means of an electronic switch. Since the SPV panel is a current source, it delivers charging current at a voltage equal to the battery voltage plus the drop across the switching element. This is a very effective and low cost charger. The drawback in this method is that the SPV Voltage is controlled by the battery voltage. Hence, it does not operate at the peak power point of the SPV Panel and so utilization is low. Different technologies are used for the switching the panel to the battery. Combination of MOSFET and diode is most common. In a Zero drop type switching, MOSFET instead of diodes and so the losses are very negligible hence it has very high conversion efficiency.

b. In a MPPT type solar charger, an electronic conversion unit changes the voltage of the solar panel to the battery level. Here, the panel voltage is not dependent on the battery voltage level. So the panel is operated at its peak power point. Hence utilization of the panel is maximized. Drawback of such schemes is in the costs and hence is practical only for larger systems.

Quantum Power Conversions (P) Ltd, Bangalore, India | sales@qpcpower.com | www.qpcpower.com

Inverter The inverter is the heart of the Solar PCU. It is that part of the PCU that converts the DC voltage from the battery to alternating current (ac) power to the output. Different technologies exist for inverter. In the transformer based inverter, the dc-ac inversion happens at low voltage of the battery and then this is stepped up to 220V ac using a transformer. This technology is being replaced world over by High efficiency Switched Mode Power Supply (SMPS) based inverter. Here the dc voltage is first transformed to high voltage dc using a High frequency SMPS. The high voltage DC is then inverted. Grid (mains utility) charger. The grid charger is an auxiliary charger in a Solar PCU. Its function is to charge the battery from the grid power when solar charge is inadequate. Different technologies using SMPS type off-grid charger or the SCR based phase controlled charger, or the transformer based charger are in the market. Some inverters itself are bi-directional and have the ability to charge the battery in the reverse direction from the ac grid. Such designs reduce costs but have a major drawback in the fact that it takes considerable amount of time for the power flow to change direction. So in case of abrupt grid failure, there is a considerable delay for the power flow to reverse direction from the grid battery to that of battery to ac output. This delay causes the connected appliance to stop and re-start causing operational disturbances. Depending on the design this delay can be up to few seconds. Any attempt to reduce this changeover time can have high reliability issues.

Output selector mechanism When working in the solar mode, the ac output from the PCU will be coming from the inverter. But when solar energy is inadequate the appliances will be connected to operate from the Grid supply. This is accomplished by the use of a relay.

Battery bank The Battery bank stores the solar charge for use by the inverter. In a typical day, the incident solar energy keeps varying depending on cloud formation, shadow, and time of day and so on. The ac appliances connected to the inverter also have its own variation in loading pattern. Hence is essential to have a battery bank which acts as a buffer and a storage element for smooth and continuous operation of the inverter. Control algorithm The control algorithm is what distinguishes a solar PCU from an ordinary Inverter + solar charger. It can allot priority and optimally select the source of charging either from solar of grid or both. It can also select the source of ac output to be either from the inverter or the grid. It can operate with different priority modes like Solar-Battery-Grid or as Solar-Grid-Battery. In the Solar-Battery-Grid mode of operation the solar charger first charges the battery and also supports the inverter to power the appliances. Once the solar energy stops coming, the PCU starts consuming the stored energy from the battery. Once the stored energy is consumed to certain level, the inverter stops and appliances are shifted to the grid utility by the output selector. This type of operation is suitable for installations where grid power is generally available. The main focus here is to save electricity bill cost.

Quantum Power Conversions (P) Ltd, Bangalore, India | sales@qpcpower.com | www.qpcpower.com

In the Solar-Grid-Battery mode of operation, the solar charger first charges the battery and also supports the inverter to power the appliances. Once the solar energy stops coming, the appliances are shifted to the grid utility by the output selector. The battery is not discharged in this case till there is a grid failure. This type of operation is suitable for installations where grid supply is irregular and the user will want to use the grid power as long as it is available. The battery is kept reserved for use when the grid fails. At the same time, in daytime, the loads will work in solar power. The main focus here is to have un interrupted power supply. The efficiency of the inverter plays an important role in deciding the overall cost of the system. For example, in the case of a transformer based Inverter, the efficiency is usually about 70 percent. (Transformer based designs of higher efficiency using extra large size transformer to reduce operating loss comes at high cost) Whereas in a SMPS based high frequency Inverter, the efficiency is about 90%. This 20% difference directly impacts the cost of installation for the same output performance. For example, the output performance from a transformer based PCU with 1000Wp SPV installed can be achieved by about 800Wp SPV which uses a SMPS based inverter. In addition, every day there is a loss of 20% energy.

Many solar PCU in the market today are a hurriedly made modification of transformer based normal backup inverter using old and inefficient technology. In any backup inverter, the grid charger comes on as soon as the grid input is turned ON. So it will happen that both solar charger and grid charger turns ON in daytime. Such PCU cannot differentiate between the content of charge from solar and grid. When the battery voltage reaches a certain level, the output change to inverter and it could very well be discharging the battery which was actually charged by grid charger! In such cases, users have found that instead of reducing electricity bill cost, the reverse has happened!

I short it is important to choose the right type of technology for a solar PCU. Not doing so, the user can be frustrated at not getting the desired benefits.