Solar photovoltaic fed dual input LED lighting system with constant illumination control

Abstract

A dual input LED lighting scheme with constant illumination is proposed in this paper. The scheme employs a photovoltaic array as the first input and a battery as the second one. A microcontroller is programmed to operate a changeover switch as well as a DC-DC converter for uninterrupted and constant illumination in work place. The scheme is suitable for conference halls, laboratories, clean rooms, marriage halls, theaters, etc. The complete modeling, design and experimentation of the proposed scheme are explained and the economic viability of the scheme is justified.

Keywords

photovoltaic / LED light / dual input / P&O algorithm

Introduction

Solar photovoltaic (PV) power is a pollution free, sustainable resource of energy that can be harnessed domestically [ 1]. By the end of 2014, the total installed solar photovoltaic capacity throughout the world has increased to about 178 GW as compared to 40 GW in 2010. The global capacity addition of solar PV in the year 2014 is alone estimated to be 40 GW [ 2]. Another positive aspect is that the solar PV systems are achieving grid parity in many countries around the world due to recent technology advancements. In the present scenario, the growth of PV will not retard even if the subsidies and allowances for PV installation are withdrawn [ 3]. On the other hand, due to rigorous researches, the stand alone PV systems are attaining good reputation. Backup supply units, remote area power supplies, street lights are some of the popular stand alone systems [ 4]. Recently, the interior lighting system powered by solar PV attracted many researchers and lighting firms as a promising field. Thanks to the advancements in LED technology with continuous reduction in price, LEDs are replacing conventional light fittings in industries, offices, commercial establishments, automobiles and domestic applications [ 5– 7]. The illumination of LED light is directly proportional to the current. The major drawback with LED strings is the cumulative voltage drop in a string and current sharing problems related to both the LED’s exponential voltage-current characteristics and the negative temperature coefficient of the LED’s forward voltage drop [ 8]. The constant-current LED driver is widely used in LED products but it suffers from large number of components, more loss and high cost [ 9– 11]. However, a constant power control of LED driving [ 6] is shown to be a promising method.

This paper proposes a novel solar PV powered LED lighting scheme suitable for closed environments such as conference halls, theaters, clean rooms etc, where lighting facility is required even during daytime. In the proposed scheme, under full bright sun light condition, the PV feeds the LED lighting system at constant power through a DC-DC converter. During cloudy conditions, when ample sun light is not available, a stiff source, such as a battery, feeds the LED. The changeovers from the PV to battery as well as constant power control in the LED are controlled using a PIC microcontroller. In the closed loop mode, the microcontroller senses the open circuit voltage of the PV array, and if there is sufficient sun light, it activates the DC-DC converter employing the perturb and observe (P&O) algorithm. In the absence of solar irradiation, the microcontroller performs the changeover action to battery and then the DC-DC converter is operated using the proportional-integral (PI) control action for rater LED illumination. The complete design of the scheme and closed loop operation are illustrated with relevant measured waveforms. Further an economic feasibility of the scheme is also evaluated.

System description

The block diagram of the proposed system is shown in Fig. 1. The solar photovoltaic (PV) module is designated as the main source of power and in case of low or no solar irradiation, a battery bank or a rectified AC supply will act as a supplementary power source to the system. The DC-DC converter is to provide a constant power to the LED lamp. The two sources, viz. the PV and the battery are connected to the DC-DC converter using a single pole double throw switch (SPDT) as shown in Fig. 1. The input source is a PV module and it is well established in the literature that the current-voltage (I-V) characteristic is nonlinear. The load in this paper is the LED lamp and the task is to maintain a constant power at the load side, regardless of the variation in solar irradiation. Therefore, it is essential to choose a power converter which can effectively combine the nonlinear PV source to the constant power load. Hence, a buck-boost converter is selected as the power electronic interface, since it can adjust the output voltage/current above or below the input voltage/current. In the closed loop operation, the microcontroller continuously checks the open circuit voltage of the PV system to gauge the quantum of solar irradiation. With sufficient sun light, the SPDT is closed to the PV panel and the DC-DC converter is operated through the P&O algorithm for constant power output. With little or no solar irradiation, the controller turns off the SPDT switch to the battery and now the output power regulation is conducted by employing PI control action.

For the power control, the output voltage across the LED module is measured and is given as one of the inputs to the multiplier IC. The other input to the multiplier IC is the current flowing through the LED module. Now, the multiplier IC gives the output power (P_o) of the LED module. The controller calculates the error as the difference between P_SET and P_o, and is given as

(1) e(t)=P_{\mathrm{SET}}-P_o(t).

The controller produces PWM pulses to drive MOSFET, so that a constant power is made available for the LED module.

PV alone supplies LED, P_PV>P_SET

When the PV alone supplies the LED, i.e., the power required by the LED, P_SET is less than or equal to the PV power, P_PV, the P&O algorithm is used to track the set power from the PV array. The P & O method has a simple feedback structure and fewer measured parameters. It operates by periodically perturbing (i.e., incrementing or decreasing) the PV terminal voltage and comparing the PV output power with that of the previous perturbation cycle. If the perturbation leads to an increase (decrease) in array power, the subsequent perturbation is made in the same (opposite) direction [ 12, 13]. In this proposed system, the voltage and current from the PV array are sensed at every instant and the instantaneous power is calculated. Accordingly, the duty ratio is adjusted and drives the DC-DC converter to deliver the constant power output. The flowchart, for clarity, is given in Fig. 2.

Insufficient solar irradiation, P_PV<P_SET

The closed loop output power regulation, when the solar irradiation is insufficient, is illustrated in Fig. 3. The SPDT switches to the battery in this case. The control strategy is to sense the output power, P_o and adjust the duty ratio of the DC-DC converter by using a PI control action. The design of the controller is based on the small signal model of the buck-boost converter [ 14]. By employing the pole placement technique [ 15], the control parameters are computed as k_p = 0.03 and k_i = 0.6.

Experimental results

To validate the proposal, a 12 W prototype dual input LED lighting system was fabricated in the laboratory. A suitable code in MPLAB is developed for the P&O algorithm and the PI control and downloaded to the PIC16F876A controller. The experiments were conducted on a full bright day on May 25, 2015 at Tiruchirappalli, Tamilnadu India at 14:50, when the solar irradiation and ambient temperature were 5.2 kWh/(m²•d) and 41°C respectively. The measured results are depicted in Figs. 4 and 5. Figure 4 shows the variations of reference power of 12 W (P_set), output power (P_o), PV output power (P_PV) and battery power (P_bat). Initially, the PV supplies the power to the LED and as can be seen, the output power increases to set the value through the P&O algorithm. Then PV panel is artificially covered to simulate the cloudy condition and it can be seen that the LED power drops to zero. The SPDT switch is now turned off to the battery. However, the PI control process immediately restores the LED power. The transient of this system clearly demonstrates the success of the proposed algorithm. Figure 5 displays the voltage and current waveforms of the PV and the battery respectively. A photograph of the hardware developed in the laboratory is given in Fig. 6.

Economic analysis

The economical analysis of the system is very essential to decide the feasibility of the project. For the economic feasibility test, a 10 m × 10 m conference hall, as exhibited in Fig. 7 is considered. To provide an average illumination of 300 lx in this hall, at about 0.80 m of working plane height, ten 40 W LED modules are required. The assumptions and variables used for the lighting design of the conference hall are tabulated in Table 1. For the specifications given in Table 1, the three dimensional light distribution is computed using the CGLux software and is given in Fig. 8. The cost wise breakup of the components is presented in Fig. 9. The LED lights and the PV modules share the major percentage of the total cost. The total cost of the project is estimated as Rs. 76000.

The economic computations for the conference hall of 10 m × 10 m are listed in Tables 2 and 3. The economic feasibility is measured in terms of simple payback, net present value, internal rate of return, and etc., of which, simple payback is of prominent value. The simple payback from energy savings is the ratio of total installation cost to the energy savings per year [ 16]. The cost of energy savings is calculated by considering the utility grid tariff for powering the same lamps. In this case, the payback is calculated as 8.4 years. Considering the long life time of LEDs as compared to other lights, the life time savings will be very high and attractive. The life time savings calculation is shown in Table 4.

The solar irradiation data of the site, electricity tariff, government subsidies, system maintenance, PV module degradation factor etc., are the other key parameters to be taken into account while performing the economic calculations [ 17]. It may be noted that, apart from the cost savings, the system reduces carbon and green house effect which lead to sustainable development.

Conclusions

A novel scheme for dual input LED lighting scheme suitable for interior lighting has been designed and presented. A new power management system is proposed which includes maximum utilization of PV power leading to reduced energy bills. The design of closed loop scheme is explained and evaluated through measurements. The economic analysis presented shows the feasibility of the new proposal.

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