ZigBee's Smart Energy 2.0 Profile Brings New Capabilities and Design Challenges

By Lee H. Goldberg

Contributed By Hearst Electronic Products


Thanks to its ability to transport rate, demand, and load management messages to and from the Smart Grid across a wide variety of wired and wireless media, the soon-to-be finalized Smart Energy 2.0 (SEP2.0) Application Profile promises to be a key element of residential energy management systems. Capable of passing energy-related messages across nearly any wired or wireless home area network (HAN), SEP2.0 will allow the next generation of smart appliances, HVAC, lighting, and energy management systems to talk with each other – and interact with the Smart Grid. (Figure 1) 
The Smart Energy 2.0 profile
Figure 1: The Smart Energy 2.0 profile makes it possible to create an IP-based HAN that can manage every aspect of a home’s energy consumption and production across both wireless and wired media.

The first half of this article will provide an introduction to the Smart Energy 2.0 protocol, its capabilities and its applications. This will be followed by a discussion of what it will take to implement SEP2.0 in real-life applications and the challenges embedded systems designers will face.

SEP2.0 in brief

The Smart Energy Profile (SEP) 1.0/1.1 was originally developed to allow ZigBee low-power wireless networks to support communication between smart meters and products that that monitor, control, and automate the delivery and consumption of electricity and other utilities. Like most ZigBee profiles, it consists of a set of Application Objects and an Application Support Sublayer (APS) that reside on top of the ZigBee Pro Network layer protocol stack (Figure 2). They work in conjunction with the device-dependent ZigBee Device Object (ZDO) code to support a collection of application-specific features and commands, known as a Function Set. Depending on the application it supports, a ZigBee device can be associated with one or more Function Sets.

The ZigBee Smart Energy 1.x Application Profile

Figure 2: The ZigBee Smart Energy 1.x Application Profile accesses its 802.15.4 MAC/PHY via the ZigBee Pro protocol stack. (Courtesy of the ZigBee Alliance).

The SEP1.x profile’s Function Sets are primarily intended to support the functional requirements of smart meters used by electric, gas and water utilities to manage their distribution networks, automate their billing processes, and communicate with customers’ energy management systems. The smart meter also acts as a communication gateway between the utility and the customer. By enabling the exchange of messages about pricing, demand response, and peak load management, the smart meter allows residential and commercial customers to cut their utility bills while helping the utilities make better use of their resources.

The Smart Energy 2.0 Profile was created in response to the need for a single protocol to communicate with the growing universe of energy-aware devices and systems that are becoming common in homes and commercial buildings. As a result, the Function Sets currently defined under SEP2.0 were expanded to include:
  • Demand Response & Load Control
  • Directed Messaging
  • Public Messaging
  • Price
  • Pre-Payment
  • Metering
  • Mirroring
  • Plug-in Electric Vehicle
  • Distributed Energy Resource Management
  • Billing
One or more of these Function Sets can be used to implement one of the Device Types defined in the SEP2.0 profile. At present, this list includes:
  • In-Premises Displays
  • Smart Thermostats
  • Load Controllers
  • Meters (including non-revenue grade submeter)
  • Plug-In Electric Vehicles (PEV)
  • Smart Appliances
  • Premises Energy Management Systems (PEM)
  • Range Extenders
  • Energy Services Interfaces (ESI)
  • Pre-Payment Terminals
  • Inverters
In addition, SEP2.0 replaces the ZigBee Pro protocol stack used by SEP1.x with the ZigBee IP stack, which uses the 6LoWPAN protocol to encapsulate the proprietary ZigBee packet structure within a compressed IPv6 packet. At the Transport layer, IP packets bearing messages containing standard ZigBee command and data packets are exchanged using the HTPP and TCP protocols (Figure 3).

The ZigBee IP protocol stack a)

The ZigBee IP protocol stack b)

Figure 3: The ZigBee IP protocol stack (a) encapsulates ZigBee’s proprietary packet structure (b) within compressed IPv6 packets. When used in combination with the Smart Energy 2.0 Application profile, it provides a media-independent interface between the network and MAC layers of the stack that allows communication across nearly any IP-based network. (Courtesy of the ZigBee Alliance.)

The ZigBee IP stack also creates a clean interface at the transport layer that allows SEP2.0 packets to be carried by nearly any IP-based network technology. The most recent draft version of the SEP2.0 profile includes support for communication across ZigBee and 802.11 Wi-Fi wireless networks, as well as power line communication (PLC) networks which use the new HomePlug GP “green PHY.” Future versions of SEP2.0 will include support for sub-GHz ZigBee PHYs (802.15.4g) and other popular network technologies such as Ethernet and multimedia over coax (MoCA).

Applications

The new realities of the Smart Grid will create a demand for SEP2.0-enabled devices by both service providers and consumers. Among the first products to hit the market will be Energy Service Portals (ESPs) which connect the utility’s communication infrastructure and the HAN. Typically provided to consumers by the utility, these portals will use the SEP2.0 and Energy Services Interface (ESI) Application Profiles to provide a bridge between the SEP1.x protocol used by most of today’s smart meters and the home’s IP-based network. Depending on the particular utility, the ESP will be a dongle that resides within the smart meter, a stand-alone gateway (Figure 4), or a module within the home’s existing broadband access equipment (a DSL/cable modem or fiber ONT).
The Energy Service Portal
Figure 4: The Energy Service Portal serves as a bridge between the utility’s communication network and the HAN. (Courtesy of the ZigBee Alliance).

The Energy Service Portal’s application functions can also be integrated into a Home Energy Management System (Figure 5) that is connected to all energy-related devices within the home including washer/dryers, dishwashers, HVAC systems, and PHEV charging stations. Such a device can run automatic demand response programs that react to utility messages according to user set policies, as well as load analysis software that can provide the user with a detailed analysis of their real-time energy consumption pattern and behavior. The functions required to manage, buy, and sell the two-way electricity flows available from electric vehicles and rooftop photovoltaic systems can also be incorporated into the energy manager or implemented as a separate device.
A ZigBee-enabled home energy management system
Figure 5: A ZigBee-enabled home energy management system can employ multiple Application Profiles to provide unified control of all home energy systems. In this example, the system uses the Smart Energy (SE) profile to pass the utility’s load management and demand response messages to the home’s major loads and energy sources. The Home Automation (HA) and RF for Consumer Electronics (RF4CE) profiles are used to communicate with Smart Appliances, lighting systems and other consumer-controlled products. (Courtesy of Freescale Semiconductor.)

Energy-aware homes will also employ a large number of end-point applications such as smart thermostats, in-home energy displays (IHDs), and tablet-based control panels that use SEP2.0-enabled ZigBee or Wi-Fi radio links to communicate with the home’s ESI and other elements of its energy management system (as shown in Figure 5). SEP2.0-capble end points can also be implemented with a power line communication PHY to provide a network interface in Smart Appliances, grid-aware solar inverters, and PEV charging systems.

Designing with SEP2.0

If you are considering equipping your design with SEP2.0 capabilities or upgrading an existing product to support SEP2.0, the good news is that the ZigBee Alliance has created well-defined provisions for interoperability with, and upgrade paths from, the earlier SEP1.x standard. In addition, there is no significant bump in processing requirements, although the key generation and exchange functions in the new security layer may be tough for 8-bit devices to handle unless they have some sort of hardware security accelerator core. The bad news is that the SEP2.0 profile and the applications it supports require MCUs with significantly more memory (both flash and RAM) than is required for most SEP1.x applications.

Storing the code for a SEP1.x stack, a small application profile and a simple user application (such as an energy service interface) requires roughly 160 Kbytes of flash in a typical MCU, plus 10-12 Kbytes worth of RAM. While there is no firm consensus on what it will take to the implement the same functionality on an MCU running SEP2.0, some developers think it may require as much as 256 Kbytes of flash and 24-32 Kbytes of RAM. Some applications which involve “sleeping devices” may require as little as 160 Kbytes of flash and 8 Kbytes of RAM.

Until recently, the most common approach to implementing an SEP2.0 network node will be to use a dedicated MCU to support the MAC/PHY functions and another processor to run the Application Profiles and other application software. Freescale’s two-chip SEP2.0 solution is based on a part that is pin-for-pin compatible with the MC13224, but has a new ROM firmware version specifically for ZigBee Pro applications and contains sufficient flash memory (128K) to support the SEP2.0 stack. The SoC integrates a 32-bit ARM7 processor, a ZigBee radio, an encryption accelerator, and a large array of analog and digital I/O (Figure 7). It has sufficient memory and processing power to run the full SEP1.0 stack, but Freescale advises that for SEP2.0 applications, it should only run the MAC/PHY portion of the stack and run the rest of the profile and any application software on a separate 32-bit Kinetis ARM host processor, which has more memory, an optional floating-point processor, and I/O configurations that include metrology front ends, LCD display drivers, and an Ethernet port (Figure 6).

Freescale wireless MCU

Figure 6: Freescale offers a wireless MCU integrating an ARM-7 processor and a complete 802.15.4 radio.

Energy Micro’s versatile low-power MCUs are also finding applications in Smart Energy/smart meter applications. Their EFM32G890F128 Gecko MCU pairs a 32-bit ARM-Cortex M3 with 128 Kbytes of flash, a 16 Kbyte RAM, and autonomous I/O elements that collect and transmit data while the Gecko’s CPU remains in a low-power sleep state. The Gecko MCUs’ ability to work well with limited energy resources makes them an excellent choice for battery-powered smart utility meters and remote sensor nodes.

Texas Instruments’ two-chip SEP2.0 solution consists of the cc2533 (click for info on the cc2533 eval kit), which combines an RF transceiver, with a single-cycle 8051 compliant CPU to run the MAC/PHY portion of the SEP2.0 stack. The application processor, one of TI’s ARM7-powered Stellaris 9000 series MCUs runs the remainder of the SEP2.0 stack and any relevant application code. Most members of the Stellaris 9000 family include a fully-integrated Ethernet MAC, a CANbus port, a USB OTG/Host Device and enough memory and processing power to implement many simple Smart Energy applications such as In-Home Displays (IHDs), Energy Service Portals (ESPs), and network end points for Smart Appliances (Figure 7).

An In-Home Display (IHD)a) cc25x MCU

Figure 7: An In-Home Display (IHD) implemented using a two-MCU solution (a). This design uses a cc25x MCU to support the MAC/PHY functions and a more powerful application processor to support the SEP2.0 profile and application code necessary to run the display (b). (Courtesy of Texas Instruments.) 


Summary

As the market matures, more highly-integrated, single-chip SEP2.0 solutions are beginning to emerge. One example is Texas Instruments’ soon to be released (volume production in late 2012) cc2538, a single-chip solution that integrates an IEEE 802.15.4 (2.4-GHz) radio, an ARM Cortex-M3 processor, dedicated Smart Energy 2.0 hardware security acceleration, and enough flash and RAM to run the ZigBee IP stack, the SE2.0 profile, and simple application software for products such as smart meters for electricity, gas, or water and in-home displays.

For more information

Check out these TechZone articles to learn more about Smart Energy networks and related topics:

Rapidly Integrating ZigBee Smart Energy
Wireless Connections Inside the Home Will Make the Smart Grid Smarter
MCU Solutions for 6LoWPAN Home Automation Networks
Designing Intelligent Appliances for the Smart Grid
Low Power Wireless for Smart Homes

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