Smart-Grid Designs Solve Low-Power Riddles

By Ellen Konieczny

Imagine that you go to your kitchen to get a drink and pass your home’s energy-usage monitor. Due to a recent heat wave, you see that your energy usage is already at what it usually is for the entire month. Yet you’ve still got one week left in your billing cycle. To keep the bill low, you turn your A/C thermostat up a degree and make a mental note to not keep lights on unnecessarily.

The next day, the weather is more comfortable. You log in from work and turn the A/C off completely. Such capabilities are not farfetched, thanks to plans to roll out smart-grid networks across the globe (see Figure 1). In fact, some utility companies have already tested these technologies. For such two-way communications to be realized on a grand scale, however, the infrastructure, smart meters, and millions of wireless devices involved will need to consume minimal power.

Fig. 1: The various aspects of the smart grid and how they will be connected. (Courtesy of Ember Corp.)

Emmanuel Sambuis, general manager for the metering business at Texas Instruments, says water and gas meters now require 20-year operation from the same battery. In some devices, the requirements are now as much as 25 to 30 years—particularly in areas where batteries are particularly difficult to access or where there are so many devices that changing out batteries can become expensive. In some extreme cases, companies have been developing energy-scavenging solutions that require no batteries at all.

What’s changing, however, is the addition of low-power communications technology inside of even home-area-network products, such as in-home displays and intelligent thermostats. Generally, such low-power communications are RF-based. In the case of power-line communications, regulations also apply and force the energy consumption to be minimal. To raise energy efficiency in e-metering applications, TI has developed an SoC microcontroller that integrates all metering functionality onto a single chip, with ultra-low-power operation so that only simple voltage regulation is required for a complete solution. The MCU provides direct device operation from a 3V supply with the CPU and ESP active at only 2.5 mA. During a power outage, the device can operate in standby mode at 1.1 µA with the real-time clock function active.

It is essential to keep in mind that smart-grid devices will most likely be asleep for the majority of the time. “The biggest challenge is in enabling the battery-operated devices to not be awake for long periods as well as for them to be able to join the network, acquire and process any data as quickly and efficiently as possible, and go back to sleep, said Skip Ashton, senior vice president of engineering at Ember Corp. “Designing technology–radio, processor, and networking software–which enables devices to do that reliably and securely is the crux. A user of a battery-operated device expects instant operation and control when they are using it but long battery life when they are not. This type of ‘instant-on’ capability requires coordination of the radio as well as the software controlling the devices.”

Thankfully, standards bodies like the ZigBee Alliance (www.zigbee.org) include low-power operation as a critical goal as they develop their protocols. For suppliers implementing the protocols and hardware, however, Ashton emphasizes it is important to view the technology offering as a “system” that includes hardware and software. “A tightly integrated platform, which has been developed from the ground up to work together to deliver excellent performance, efficiency in code size, and processing of security and application data, lends itself to better resolve the challenge of minimizing power consumption and extending battery life. Although the standards can prescribe a certain level of behavior, different suppliers can innovate within the standard to improve performance,” Ashton says.

In addition to running IEEE 802.15.4/ZigBee wireless, for example, the MeshConnect modules and integrated circuits (ICs) from California Eastern Laboratories promise to get good range out of a very low-powered device (Fig. 2). According to David Cohen, director of marketing, and Rich Howell, director of business development, the MeshConnect modules and ICs put out +7 dBm power out native (i.e., without using an external power amplifier). The MeshConnect technology delivers standby mode at less than 0.3 µA.

Figure 2: With sleep-mode power consumption below 1 µA, the MeshConnect Extended Range Module offers extended battery life

Although such products are impressive innovations on their own, they are only part of a bigger picture. A successful smart grid will require close collaboration between providers of communication devices (RF transceivers and processors, for example), providers of communication software, and designers of communication systems. In addition, success will largely depend on advances in signal processing.

“Smart-grid technology developers look to advanced digital and analog signal-processing technology to power next-generation energy infrastructure,” said Ronn Kliger, energy group director at Analog Devices. “By leveraging ICs optimized for a range of smart-grid applications—from energy-metering solutions to dynamic, grid-integrated management and communication systems—developers are able to design intelligent systems that promote energy efficiency and management flexibility.”

Along with energy-metering ICs, the firm offers RF, power-line carrier communication, power management, and digital signal processing in support of smart-grid applications.

Measurement capabilities also will need to be fine-tuned, as standby or “vampire” power poses a clear threat to the smart grid’s low-power-consumption efforts. Standby power results from electronic devices that are plugged into wall sockets, such as TVs, DVD players, cell phones, and answering machines. Whether they are on or off, they consume power 24 hours a day. It is difficult to accurately measure the power that they actually use, however, which is why they must be accounted for in the smart grid. A number of companies have developed ways to measure even the smallest amounts of energy usage. For example, Teridian claims to provide accuracy of +/-0.5% over a 2000:1 dynamic range.

Of course, the most frightening specter haunting power consumption in smart-grid devices may be standby current. This issue becomes especially critical for the finest silicon technology nodes, where transistor leakage current starts to dominate. Leakage power already poses a significant problem at advanced process nodes, and the problem increases with density at advanced nodes. The dynamic power dissipation arising from high-frequency switching of the tens of millions of transistors directly impacts aspects like battery life, packaging and cooling costs, form factor, and reliability.

All of the major EDA companies are now advising SoC developers to consider low power as part of the architecture rather than something implemented later in the flow, and entire flows are becoming power-aware and power optimized. That goes for the process as well as the components. Third-party IP vendors such as Virage Logic, ARM and Synopsys are now standardized on low power versions rather than splitting their product lines between IP geared for low power and performance, and even in the FPGA space, where concern for power was either an afterthought or non-issue, all of the major vendors are now offering lower-power solutions. Actel has even developed chips that rival the power consumption of some of the most advanced ASICs.