1. The solar-mains hybrid lamp

In the April 4, 2024, issue of EDN, the design of a solar mains hybrid lamp (HL) was featured. The lamp receives power from both a solar panel and a mains power supply to turn on an array of LED lamps. Even when solar power is widely variable, it supplies a constant light output by dynamically drawing balanced power from the mains supply. Also, it tracks the maximum power point very closely. 

Wow the engineering world with your unique design: Design Ideas Submission Guide

1.1 Advantages

The advantages of the HL are as follows:

1. It utilizes all the solar power generated and draws only the necessary power from the grid to maintain constant light output.
2. It does not inject power into the grid; hence, it does not contribute to any grid-related issues.
3. It uses a localized power flow with short cabling, resulting in negligible transmission losses.
4. It uses DC operation, resulting in a simple, reliable, and low-cost system.
5. Generated PV power is utilized even if the grid fails, thus acting as an emergency lamp in the event of a grid failure during the daytime.
6. It has a lengthy lifespan of 15 years with minimal or no maintenance, resulting in a good return on investment.

1.2 Disadvantages

The limitations of the HL are as follows: 

1. It does not provide light if the grid fails after sunset.
2. Solar power is not utilized outside of office hours or on holidays.

As mentioned above, the HL’s utility can be fully realized in places such as hospitals, airports, and malls, as it can be used every day of the week.

In offices that are open for work only 5 days per week, the generated PV power will be wasted on weekends and outside of office hours (early mornings and evenings). 

For such applications, to fully utilize the generated PV power, a battery backup scheme is proposed. It is designed as an optional add-on feature to the existing HL. The PV power, which would otherwise go to waste, can now be stored in the battery whenever the HL is not in use. The stored energy can be utilized instead of mains power on workdays to reduce the electricity bill. In cases where the grid fails, it will work as an emergency lamp. 

2. Battery backup block diagram

The block diagram of the proposed scheme is shown in Figure 1. It consists of a HL having an array of 9 LED lamps, A1 to A9. Each HL has five 1-W white LEDs connected in series, mounted on a metal core PCB (MCPCB). For more details, refer to the previous article, “Solar-mains HL.” Here, the HL is used as is, without any changes. 

The PV voltage (Vpv) is supplied through a two-pole two-way switch S1 to the HL. Switch S1A is used to connect the PV panel to either the lamp or to the battery. As shown in the figure, the PV panel is connected to the battery through an Overvoltage Cutoff circuit. This circuit disconnects PV power when the battery voltage reaches its maximum value of Vb(MAX). 

A single-pole two-way switch S2 is used to select either MAINS or BAT to feed power to the VM terminal of the HL. When S2 is in the BAT position, battery power is fed through the undervoltage trip circuit. Whenever the battery voltage drops to the minimum value Vb(MIN), the HL is disconnected from the battery. Switch S1B is used to disconnect the battery/mains power to the HL when S1 is in the CHARGE position.

Figure 1 The proposed add-on battery backup system for HL.

Note: This simple battery cutoff and trip circuit has been implemented to prove the concept of battery backup using the existing HL. In the final design, the Overvoltage Cutoff circuit should be replaced with a solar charge controller, which will track the maximum power point as the battery charges. Readily available off-the-shelf solar charge controllers could be used. The selection of a solar charge controller is given in Section 5.

Here are the lamp specifications: 

1. Solar PV panel: 30 Wp, Vmp = 17.5 V, Imp = 1.7 A
2. Adapter specifications Va = 18 V; Current 2 A
3. Lead Acid Battery: 6 V 5 Ah. (3 batteries connected in series)
4. Battery nominal voltage Vb = 18V, Vb(MAX) = 19 V, Vb(MIN) = 17 V
5. Lamp power output: 30 W

3. Overvoltage and undervoltage circuits 

The circuit diagram of the battery Overvoltage Cutoff and Undervoltage Trip is shown in Figure 2. Three lead-acid batteries (6 V, 5 Ah) connected in series are used for storing solar energy. The battery is connected to the solar panel Vpv through a P-channel MOSFET T1 (IRF9540). The Schottky diode D1 (1N5822) is connected in series to prevent the battery from getting discharged into the solar panel when it is not producing any power. 

T1 is controlled using comparator CMP1 of IC1 (LM393). The battery voltage is sensed using the potential divider R6 and R7. The reference to the comparator non-inverting pin (3) is generated from a +12-V power supply implemented using the IC2 (LM431) shunt regulator. If the battery voltage is lower than the reference voltage, the CMP1 output (pin 1) is high. This turns on transistor T3, which turns on T1. The green LED_G indicates that the battery is being charged.

Figure 2 The circuit diagram of Overvoltage Cutoff and Undervoltage Trip circuits.

The battery is connected to the load through MOSFET T2 (IRF9540). T2 is controlled using comparator CMP2 of IC1. The battery voltage is sensed using the potential divider R14 and R15, and is connected to the non-inverting terminal (Pin 5). The reference voltage is connected to the inverting terminal (Pin 6). 

So long as the battery voltage is higher than the reference, the CMP2 output remains high. This drives transistor T4, which turns on T2. When the battery voltage drops below the reference, T2 is turned off, thus disconnecting the lamp load. LED_R indicates the battery voltage is within the Vb(MIN) and Vb(MAX) range.

Figure 3 shows the PCB assembled according to the circuit diagram in Figure 2. The connections for the solar panel Vpv, battery Vb, and battery output Vb+ (through the MOSFET T2) are made using three 2-pin screw terminals. 

Figure 3: The assembled PCB for battery overvoltage cutoff and undervoltage trip circuit.

Figure 4 shows the interconnections of the battery charger circuit with the HL.

Figure 4 A top view of the interconnections of the battery charger circuit with the HL.

The modes of operation of this circuit are captured in Table 1. When S1 is in the CHARGE position, the PV voltage is supplied to the batteries for charging. In this mode, the position of S2 does not affect the charging process. 

When S1 is in the PV position, the HL turns ON. Using S2 we can select either mains power or battery power.

Table 1 Operating modes of the battery backup circuit: battery charging, hybrid with mains power, and hybrid with battery power. 

4. Integration and testing

Figure 5 shows the integration of the battery protection circuit with the HL and three batteries. The cable from the PV panel is connected to the 2-pin screw terminal labeled as Vpv. Three 6-V batteries in series are connected to the screw terminal Vb. A DC socket labeled Va is mounted for plugging into the adapter pin. In the photograph, S1 is in CHARGE position, so the battery is being charged using PV power. In this case, the position of S2 is irrelevant and will not affect the charging process.

Figure 5 An image of the circuit in Battery Charging mode. The green LED indicates the battery is being charged from the PV panel. The red LED indicates battery power is available for use.

Figure 6 shows the HL turned on using PV power and a battery. In this case, S1 is in the PV position, and S2 is in the BAT position. Note that the LED lamp array (A1 to A9) is facing downwards. On the HL PCB, there are nine red and nine green indicator LEDs. Each pair of LEDs represents 11% of the total power. The photograph shows four green LEDs are ON, which means 44% of the power is coming from solar. The remaining 55% of power is being drawn from the battery. The green and red LED combination changes as the sunlight varies. 

Figure 6 The lamp in Hybrid mode. Four green LEDs indicate 44% of the power is coming from the PV panel. Five red LEDs indicate 55% of the power is being drawn from the battery.

5. Design Example of a 90-W HL with battery backup

Here, the design of a 90-W HL with a battery backup is proposed. The nominal working voltage selected is 48 V. 

5.1 HL specs

The specifications for the HL design are as follows: 

1. Solar Panel Specifications: Power = 30 Wp, Vmp = 17.5 V, Imp = 1.7 A
2. Number of Solar Panels connected in series: 3
3. Solar Array Voltage: Vpv = 3 x 17.5 = 52.5 V; Voc = 60 V
4. Number of LEDs in each MCPCB (A1 to A9): 15 white LEDs of 1 Watt each.
5. Forward voltage of LED: 3.12 V
6. Voltage across each lamp (A1 to A9): 15 x 3.12 = 46.8 V
7. Current through LED lamps: 0.2 A (selected) 
8. Current limiting resistor [1]: R1 to R9 = (52.5 – 46.8)/0.2 = 28.5 Ω (select 27Ω/2W)
9. Adapter specifications: 48 V, 2 A

As stated earlier, this lamp can be used without a battery backup in facilities that are open all seven days a week. In these applications, the solar power generated is fully utilized, so the cost of this lamp is minimal. The deployment of a large number of such lamps can significantly reduce the electricity bill. 

However, in offices that operate 5 days a week, the power generated during weekends goes to waste. In cases where another load can utilize the available PV power on weekends, such as a pump, vacuum cleaner, or a battery that needs charging, the PV panel’s output can be connected to that load. This way, we can still use the HL as is. However, if there is no other load that can utilize the PV power, then we must resort to battery backup.

5.2 Battery selection

The battery selection can be as follows: 

1. Lithium-ion Battery: 13S (13 cells in series), Nominal voltage 48 V
2. Battery voltages: Vb(MIN) = 42 V, Vb = 46.8 V, Vb(MAX) = 54.6 V
3. Energy storage capacity (24 Ah):  48 x 24 = 1152 Wh
4. Solar energy generation per day: 90 W x 6 hrs = 540 Wh
5. Battery storage: 1152 Wh / 540 Wh = 2.1 or 2 days  

5.3 Solar charge controller specs

A wide range of solar charge controllers is available on the market. To select a suitable charge controller, the following specifications are provided as guidelines:

1. Battery Type: Li-ion, Life-Po4
2. Nominal Voltage: 48 V
3. Controller type: MPPT
4. Maximum output current: 5 A
5. Protections: Battery reverse polarity, solar panel reversal, short circuit protection, battery overvoltage cutoff, battery low voltage trip.

Note that the open-circuit voltage (Voc) of the solar array is 60 V; therefore, the selected components should have a voltage rating greater than 60 V. 

This design is for a 90-W HL; however, higher-wattage lamps can also be designed. In that case, the lamp MCPCB selected should have a higher power rating. Alternately, the number of MCPCBs can be increased to around 16. This way, the array can be arranged in a 4×4 layout. With an increased number of arrays, both the hardware and software of HL have to be upgraded. 

It may be possible to connect two MCPCBs in parallel to increase the lamp power. However, in this case, the two MCPCBs should have a matching LED array forward voltage. This will ensure equal division of lamp current. 

5.4 Scheduling

The design shown here uses manual switches which can be replaced with semiconductor switches. In this case, the operation of the HL can be automated with a weekly programming cycle. On weekdays, it will work in hybrid mode. In this mode we can either select mains power or battery power. The duration of battery power consumption can be planned to ensure that battery is available for charging during weekends. 

6. Storing the HL’s excess energy

The solar-mains HL proposed earlier, provides constant light irrespective of the sunlight conditions. It is a very cost-effective design and can be deployed in large numbers to reduce electricity costs. However, if it is not used on all 7 days of the week, then the solar power gets wasted. To avoid any power wastage, a battery backup system has been proposed here as an add-on feature. Using batteries, the excess solar energy can be stored. The battery backup makes this lamp work as an emergency lamp, also during grid failures.  

Vijay Deshpande recently retired after a 30-year career focused on power electronics and DSP projects, and now works mainly on solar PV systems.

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The post A battery backup for a solar-mains hybrid lamp  appeared first on EDN.

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