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Making Smart Lighting Cost Effective with Energy Harvesting

Lighting places specific demands on power consumption in industrial applications, and finding efficient ways to manage the power and make lighting systems more effective is an increasingly important element in reducing operating costs.

Using smart lighting can significantly cut energy bills across an organization or a location, but the cost of installing and maintaining the sensor and wireless network can outweigh the savings. However, energy harvesting is now becoming a viable technique to help industrial systems designers reduce power. As the power consumption of wireless transceivers and microcontrollers falls, they can now be used with the low but constant power available from harnessing the energy from the environment.

Using solar or thermal energy can be used to power sensors and wireless networks, coordinating and controlling lighting across a wide area without having to retrofit miles of cabling. These systems do use batteries, but adding the small, incremental cost of energy harvesting brings large benefits in operational management, allowing the sensor nodes to be placed alongside lighting in inaccessible areas and eliminating the need to replace the batteries. With hundreds or even thousands of lights in an industrial environment, with many inaccessible except by crane, this is a key savings.

While it is not viable to use the milliwatts generated by energy harvesting to power LED lights, this ‘free’ power from the environment can be used to drive sensors that can reduce or even shut down lighting when it is not needed. This technology can also power a wireless link which can optimize the lighting across a manufacturing floor or industrial environment, ensuring the most efficient use of the energy and reducing costs.

Enabling the lighting only when someone is nearby, either by using a proximity or movement sensor or a notification from a central server, can significantly reduce the power requirement and thus reduce the overall operating costs.

Energy harvesting facilitates this capability, and can be added to an industrial lighting system without having to install cabling, as the nodes can be placed directly wherever they are required. As lighting is spread all over an area, this can save on the material and installation of miles of cabling and provide a quick and easy technique for retrofitting a control system.

This can also work in conjunction with the design of the environment for what is called ‘daylight harvesting’. This makes best use of the available daylight, reducing the artificial lighting to provide a consistent environment. This requires close control of the lighting systems, often linked back to software on a server that uses the layout of the facility. Energy harvesting technology can provide an easy way to retrofit a wireless link into the lighting system to provide the necessary control.

While this may seem complex, the average saving from using ‘daylight harvesting’ is 24%, and in some industrial case studies was found to be as high as 80%. This provides a considerable reduction in the energy costs.

In a strange way, the energy of the lights themselves can be used to power the sensors and the wireless network. The eZ430-RF2500-SEH from Texas Instruments is a complete Solar Energy Harvesting development kit to help create a perpetually powered wireless sensor network based on the ultra-low-power MSP430 microcontroller (Figure 1).

The Solar Energy Harvesting module includes a high-efficiency 2.25 x 2.25 inch solar panel optimized for operating indoors under low-intensity florescent lights, which provides enough power to run a wireless sensor application with no additional batteries. Inputs are also available for external energy harvesters such as thermal, piezoelectric, or another solar panel.

Figure 1: The Energy Harvesting development kit from Texas Instruments.

The system also manages and stores additional energy in a pair of thin-film rechargeable EnerChips, capable of delivering enough power for over four-hundred transmissions. These act as an energy buffer that stores the energy while the application is sleeping and has light available to harvest. The batteries are environmentally friendly and can be recharged thousands of times. They also have a very low self-discharge, which is vital for an energy harvesting system.

The eZ430-RF2500 is used to run the energy harvesting application. It is a complete USB-based MSP430 wireless development tool and provides all the hardware and software necessary to use the microcontroller and 2.4 GHz wireless transceiver. It includes a USB debugging interface that allows for real-time, in-system debugging and programming for the MSP430, and provides the interface to transfer data to a PC from the wireless system.

The integrated temperature and RF signal strength indicators can be used to monitor the environment, and external sensors can be used to collect additional data to support an industrial lighting system.

The core technology behind the module is the photovoltaic or solar cell that converts ambient light into electrical energy. The energy from the solar cell must be converted, managed, and stored. This is handled by the EnerChip, a small DIP-mounted board on the Solar Energy Harvesting Module. A boost converter is used to increase the voltage from the solar cell to a sufficient level to charge the thin-film battery and run the rest of the system.

The Charge Control block continuously monitors the output of the boost converter, and if this falls below the voltage needed to charge the EnerChip, the charge controller disconnects the converter from the system to prevent back powering it in low light conditions. The Power Management block prevents the EnerChip from discharging too deeply in low-light conditions or under abnormally high current loads. It also ensures that the load is powered up with a smooth power-on transition, maintaining the performance in low light conditions.

Microchip’s XLP 16bit Energy Harvesting Development Kit also uses solar energy for a wireless node to control an industrial lighting system. It uses Microchip’s 16-bit PIC microcontroller, which has a sleep current below 20 nA and brown-out reset down to 50 nA. The system is able run effectively from the small amounts of power available from the solar cell that is using the internal light to generate power.

The development board has onboard temperature sensors, data EEPROM, a potentiometer, a watch crystal, LEDs, and an expansion connector for external modules that support an RF transceiver and SD/MMC Cards.

Figure 2: Microchip’s XLP 16-bit energy harvesting development kit.

Cymbet’s Solar Energy Harvester provides power for a range of controllers. The harvester uses a high-efficiency solar panel suitable for indoor industrial lighting and captures, and manages and stores energy in two Cymbet EnerChip thin-film rechargeable energy storage devices.

Microchip and Cymbet have worked together to develop Energy Conscious software algorithms for use on the 16-bit microcontroller. The monitored information can be reported to the PC user interface via a USB connection. This allows the system designer to balance energy collection and energy use in a ‘daylight harvesting’ scenario to reduce the power consumption as far as practicable.

Silicon Labs also has a Solar Energy Harvesting Board that uses a thin film battery with a lifetime of over fifteen years or 7000 mAh. This allows the wireless node to have a very thin profile with a battery height of 0.17 mm to fit into lighting housings.

The board consists of a wireless sensor node and an EZRadioPRO USB Dongle using a Silicon Labs Si4431 radio operating at 919.84 MHz and powered by the solar energy harvesting power supply. The leakage current of the energy harvesting supply is 3 µA and is cancelled out by as little as 50 lux shining into the solar cell, allowing the energy harvesting supply to power the system for approximately 7 days in a dark environment and still be available for control and switching, or indefinitely if there is a light source, either indoor lighting (200 lx) or outdoor lighting (10,000 lx).

Another approach is to make use of thermal energy. The EDK312 evaluation kits from EnOcean enable the system designer to draw power from temperature differences using a Peltier effect device (Figure 3). With industrial environments often having significant heat generated, the Peltier device can be used to generate a 10 mV voltage difference for every degree of temperature difference. This can then be used to power the STM 312 RF module operating at 868 MHz, as well as lighting sensors.

Figure 3: The EnOcean EDK312 thermal energy evaluation kit.

The kit comes with a pre-defined feature set for energy management, data acquisition, data processing, and wireless data transmission. User specific applications can be implemented based on the Dolphin application programming interface.

Advanced Linear Devices provides a Micropower Step-up Low Voltage Booster Module to interface a wide range of energy harvesting devices to batteries so they can be used with sensors and wireless links.

The EH4295, part of the EH4200 Series of Micropower Step-up Low Voltage Boosters, is a self-powered voltage-booster module that converts a low DC voltage input to a higher AC or DC voltage output suitable for many low-power energy harvesting applications using photodiodes, thermoelectric or electromagnetic generators as the input source (Figure 4). The EH4295 does not need a separate power supply to operate, and it derives its power directly from the low input voltage energy harvesting source, starting at as low as 2 µW, which enables an on-board self-starting oscillator.

Figure 4: The EH4295 step-up low voltage booster from Advanced Linear Devices.

The EH4295 features nominal input impedance of 950 Ω, making it suitable for many different energy generating sources and also applicable for trickle-charge applications such as a battery charger or a super-cap charger, including situations where the energy input is not well controlled or regulated.

The EH4295 self-starting oscillator oscillates at a natural frequency of about 400 Hz, which depends on the source impedance, the source voltage, the loading at the output and the resonating components on board the EH4295.

At the core of the EH4295 is a MOSFET array that is designed and developed for this application. An on-board transformer that couples to a dedicated MOSFET array forms the heart of the self-starting oscillation circuit. The oscillator waveform is coupled to a transformer inside the module that provides an AC output signal, limited in amplitude by the output loading. A typical output loading is a full wave rectifier that can handle AC inputs over 20 V and input power as limited by the output of the EH4295.

When coupled to the EH4295, the energy-generator source internal impedance and the EH4295 input impedance form a network where the energy-generator source starts to deliver power to the EH4295. As soon as the internal oscillation threshold power level is reached, oscillation begins, and energy transfer is initiated. Typically this power level is less than 10 µW for the EH4295, and varies across different models and units. Hence the EH4295 is excellent for high efficiency, low power applications where the minimum operating power range is very low, and where otherwise wasted energy cannot be captured and stored in a battery pack or capacitor storage bank using other means.

As input energy builds up at the energy-generator source, the amount of power transferred also changes accordingly. The maximum power rating of the EH4295 limits its power handling capability, but does allow an external secondary DC-DC converter to take over at some higher power point. The AC output generated by the on-board oscillator enables the EH4295 to support other switching circuits to convert at a higher voltage and power level.

For many energy-harvesting applications, the EH4295, combined with EH300 Series Energy Harvesting Modules, offers a simple and efficient solution when used with a low-voltage, low-energy generating source which only delivers sporadic intermittent amounts of input power. The combined EH4295 and EH300 Series Modules can ramp from zero output power to useable levels for operating many remote sensor networks and circuits requiring DC supply voltages in the 1.8 V to 6.8 V range. The boosted AC or DC output voltage levels can also be used to generate a reference DC output to drive or to initiate other electronic circuits such as external Power Step-up DC-DC converters requiring DC supply voltages over 1.0 V in order to operate.

Using energy harvesting technology in an industrial lighting system can bring significant benefits. Solar power can drive a wireless node and sensor to provide an infrastructure for ‘daylight harvesting’ coordinating and switching the lighting systems to support and boost the natural lighting. Thermal energy can also be used to power sensor nodes that can switch lighting off when not required, similarly reducing the power consumption. These techniques can reduce both the cost of running an industrial lighting system and the significant cost of implementing and retrofitting such a control system.