Hybrid solutions that combine small photovoltaic (PV) panels with industrial grade rechargeable Lithium-ion (Li-ion) batteries provide long-term power for remote wireless devices connected to the Industrial Internet of Things (IIoT). In this article we outline key factors to consider for implementing them.
While the vast majority of wireless devices connected to the IIoT are powered by primary (non-rechargeable) lithium batteries, solar powered energy harvesting is playing an increasingly vital role in making wireless connectivity a viable option. The need to periodically replace batteries, especially for applications that require extended operating life in hard-to-access locations and extreme environments, has been a deterrent to the wider acceptance of wireless sensors.
The combination of Li-ion batteries and solar PV, two highly synergistic technologies, offers a well-proven and cost-effective solution for applications that draw relatively high amounts of average daily current, enough to prematurely exhaust a primary lithium battery.
Solar PV/Li-ion battery hybrid technology is already being utilized to power systems including everything from GPS sensors and asset trackers to environmental monitoring systems, smart agriculture (monitoring moisture, temperature, and rainfall), marine buoys, machine to machine (M2M), and systems control and data automation (SCADA) applications. This article provides detailed information for deciding which applications would benefit most from hybrid power and the important factors that need to be considered in their design.
First Steps in Planning your Installation
Generally speaking, the battery should be able to provide enough energy storage capacity to power a remote wireless device for a minimum of 5 days on a single battery charge. Otherwise, you could run the risk of a temporary system shutdown during periods of continuous rain or heavy clouds.
When calculating your panel size, you must plan for the worst months (typically December in the Northern Hemisphere). You also need to take into account regional variations in average daily solar energy capture. For example, certain areas in Washington State receive the equivalent of just one hour of full sun per day during winter months, so the PV panel may need to be oversized to compensate for these low amounts of average daily sunlight. Conversely, the American Southwest has sunny desert regions that receive up to 4 times more average daily solar energy during the winter months, thus enabling the use of smaller and less expensive PV panels in these areas.
Design for your Anticipated Maximum Load
Numerous variables can influence the daily power consumption of a solar-powered energy harvesting device, including: the types of sensors used; steady state power consumption while the device is in standby mode; the self-discharge rate of the battery; and the average amount of current drawn during active mode. For example, humidity, gas, motion, and light sensors can draw current in the 1 – 2mA range, GPS sensors can draw 10 – 25mA, while a cellular-based radio may require up to 1A in active mode.
How to Minimize your Load
Various methods can be used to minimize energy consumption, including: the use of low-power microprocessors and components; reducing the clock rates of processors and controllers; utilizing back-off strategies to reduce communication strength; and deploying a low-power communication protocol such as Cat-M1, NB-Iot, LoRa, SigFox, ZigBee, and Wireless HART, to name a few.
Although reducing the frequency of data collection and transmission also conserves energy, it could result in compromised product performance and loss of data integrity. This trade-off can typically be avoided by specifying a larger PV panel and/or using a greater number of Li-ion batteries for energy storage.
On the other hand, many remote wireless devices can operate so efficiently that the use of energy harvesting technology may not even be required. In fact, the majority of industrial grade remote wireless devices that draw low average daily current are powered by bobbin-type lithium thionyl chloride (Li-SOCl2) batteries that can operate for up to 40 years without replacement.
Consider the Operating Environment
Solar/Li-ion battery hybrids must be ruggedly designed to withstand worst-case environmental conditions to ensure reliable performance. PV panels designed for use in extreme environments should be rated to operate for a minimum of 10 years using high-efficiency cells constructed of the highest quality materials available. If not, the performance of the PV panels could deteriorate from UV damage, delamination, or corrosion. Consumer grade Li-ion batteries are not designed to operate or recharge at extreme temperatures, so industrial grade Li-ion cells would be required under those conditions.
The Charging Circuit
The charging circuit needs to be designed to optimize the PV panel/battery combination. The voltage and current available from the panel has to be sufficient to charge the battery under varying solar conditions, temperature, angle of the panel towards the sun, and varying load level. For example, a PV panel rated at 6V, 1A will produce an open circuit voltage as high as 7V, but it will drop when connected to a load. It will continue to fluctuate with solar conditions and temperature: for example, the higher the temperature, the lower the PV voltage. However, the voltage and current needed to charge the battery will remain relatively constant. Therefore, the PV panel must be chosen such that its lowest possible output voltage will be sufficient to charge the battery. The charging circuit should be designed to optimize the charging voltage and current to achieve maximum efficiency of the energy transfer.
Various design solutions should be considered for the charge circuit, including maximum power point tracking (MPPT) chargers or linear regulators. Whatever solution you deploy, be sure to capture the smallest amounts of solar energy possible, down to 1mA, while not restricting your maximum charging rate during peak periods. An MPPT charger automatically adjusts the voltage and current levels so that the solar power is used most efficiently under all conditions. A common mistake, however, is to establish a maximum power point for the charge circuit that is too high. On high temperature days, if the panel voltage drops too far below the maximum power point of the circuit, the power to the battery could drop to zero.
When designing the charging circuit, it's important to consider the total daily estimated energy output capacity of the PV panel, which is determined by its surface area and its efficiency. For example, monocrystalline cells typically operate at 19% efficiency, producing about 0.12 watts per square inch (0.019 watts per square centimeter). Back contact monocrystalline cells such as those made by SunPower, LG, and others can achieve over 22% efficiency by moving all or most of the contact points (which appear as thin silver lines on the solar panel) to the back, thus creating greater surface area on the PV panel to capture incremental amounts of added energy.
Choosing the Panel
Every PV panel requires some form of protective coating such as glass, urethane, ETFE, Epoxy, or PET. Glass offers high UV resistance but results in added weight and increased profile and has a risk of shattering. Urethane is highly UV resistant and is designed to last up to 10 years. ETFE is lightweight but will have a shorter life span than either glass or urethane. Epoxy or PET are not recommended for industrial applications.
Requirements for voltage, energy storage, custom coatings, and size/shape constraints could impact your choice between an off-the-shelf or custom-designed solution. In addition, the panel needs to be mounted on some form of substrate, including aluminum, aluminum-plastic-aluminum, or PCB plastic. The PV/Li-ion hybrid assembly can be mounted to the surface of the powered device (if the angle to the sun is appropriate) or mounted separately, if necessary.
Consider the Application
When deploying an industrial grade solar/Li-ion hybrid in a remote location that has an extreme environment, it is essential to specify a properly sized battery that can satisfy the device's anticipated energy consumption requirements, including enough reserve storage capacity to handle the worst-case scenario for extended periods of low sunlight. Extended battery life is also critical for deployments in remote, hard-to-reach locations, where the cost of labor to replace a battery will far exceed the cost of the battery itself.
Consumer grade Li-ion batteries are ill-suited for long-term industrial deployments, as they are designed to only operate for a maximum of 5 years and 500 recharge cycles. Consumer Li-ion cells also have a limited temperature range (-20°C to 60°C) and cannot deliver the high pulses required for loads such as two-way wireless communications.
Industrial grade Li-ion batteries are now available that can operate for up to 20 years and 5,000 full recharge cycles, while also featuring an extended temperature range (-40°C to 90°C) and the ability to operate and recharge at extremely cold temperatures. These ruggedly constructed batteries can also handle the 15A pulses and 5A continuous discharge current needed to support two-way wireless communications and remote shut-off.
Figures 1 – 3 give a sense of the variety of possible applications for Solar PV/Li-ion battery hybrids.
The cattle in Figure 1 can move from sun to shade even on a sunny day, so the battery on their collar has to hold its charge through different weather conditions as the herd travels. The CattleWatch hub collars have built-in photovoltaic panels that harvest solar energy and store it in industrial grade Li-ion rechargeable batteries that have to deliver the high pulses required to power remote satellite-based communications between the in-herd mesh network and the rancher.
The parking meters in Figure 2 have to face all kinds of weather conditions but at least they remain stationary. The solar-powered parking meters provide municipalities with valuable real-time data and revenue streams. The incorporated industrial grade Li-ion battery provides a highly efficient and economical solution that operates for up to 20 years to minimize long-term maintenance costs.
The device in Figure 3 faces severe environmental challenges, as it can be either moored or floating freely in an ocean. The Spoondrift Spotter system is a low-cost, solar-powered ocean wave measurement and tracking device that transmits real-time data, for example, to fishermen. It is not only in a hard-to-reach location but has to deliver the pulse currents required by its radio transmitter. Its battery will keep it running even in absolute darkness for about six days.
The Bottom Line
When planning your IIoT system it is important to consider how you intend to power your remote sensors right from the beginning. Too often, all the attention goes to designing the system and the power sources are neglected until it's too late. You should make sure you can feed the worst-case load and consider how to minimize the energy drain. It's also important to design the PV panel for the application-specific environment.
When specifying a solar/Li-ion hybrid, it pays to think long-term and consider investing a little more initially to ensure a more ruggedized solution that can last as long the device. The future pay-off could be significant, including a lower total cost of ownership, improved reliability, and greater customer satisfaction.