Designing Smart Energy Devices

Homeowners must find new ways to conserve energy, and utilities need to deliver enhanced services and manage peak demand. Wouldn't it be helpful if utility companies and consumer devices could carry on a continuous background dialog about energy supply and consumption through wireless networking? They can, using the ZigBee Smart Energy (SE) profile.

SE is the newest application profile of the ZigBee wireless network standard. It defines the behaviors of secure, easy-to-use wireless communications between utility companies and home area network (HAN) devices. ZigBee SE offers developers an open standard for designing devices that serve the fast growing advanced metering infrastructure (AMI) and HAN sectors. The technology enables two-way communications between common household devices and the utility companies' back-end IT infrastructure. The profile benefits consumers by allowing them to cost-effectively manage energy consumption, using automation and near real-time information, while having the ability to choose interoperable products from a diverse range of manufacturers.

This article will explain what the ZigBee SE profile is, how it accelerates engineering design, and what issues engineers need to consider when designing HAN products based on the ZigBee SE profile.

Why Smart Energy?
Utilities in a number of regions are facing increasing demand while being handicapped by their inability to build power plants in a timely manner. Peak power usage has been growing faster than general power usage. Although peak consumption typically occurs <100 hr. per year, it can be 20% higher than general usage. Figure 1 shows one utility company's load-duration curve of peak usage.


Figure 1. A load-duration curve showing one utility company's peak usage

The types of loads to be reduced during peak demand periods include air conditioning, pool pumps or spa equipment, water heaters, smart appliances, and other such devices. A few hours offset in the operation of these devices is usually not noticeable to the homeowner, but such measures can significantly reduce peak power consumption.

Meanwhile, studies in the U.S. and overseas indicate that if you show homeowners their electricity usage on a real-time basis, they will find a way to reduce consumption by 15% to 20%. To tap the power of informed consumers and head off looming energy shortfalls and ever-higher demand spikes, utilities and regulators are developing Smart Energy networks.

Profile Development
A ZigBee Smart Energy profile group was formed to specify how such a home communication network would operate. The group consisted of representatives from utilities, equipment providers, home automation device manufactures, and technology suppliers. It developed use cases, mapped them to specific devices in the home, and created messages and commands required to support Smart Energy functionality. Once the profile was developed, it was implemented and evaluated in a series of tests during the early months of 2008, culminating in the certification of 19 ZigBee Smart Energy-compliant products in May 2008.

The process of writing, developing, and testing the specification ensures the specification can be implemented and will interoperate among different manufacturers and different devices. The group used the tests to modify and improve the specification to ensure clarity and interoperability.

What Is a Smart Energy Network?
Figure 2 shows a typical Smart Energy network, within which are actually two communications networks. The first is the neighborhood area network (NAN) that provides a link from electricity meters to the utility's data centers. The second network is the HAN. The HAN is formed by the energy services portal (ESP), which is typically a meter but could be a broadband gateway or other device that has connectivity to the utility data centers. The ESP serves as the connection between the HAN and the NAN and provides utility data to the home. The ESP also performs a security function by letting only authorized devices on the network. Because your utility consumption data is private, utilities do not want unauthorized devices on the network.


Figure 2. A typical Smart Energy network that actually combines two communications networks—a neighborhood area network and a home area network

Within the home, there is a range of devices that can be used to provide Smart Energy functionality. These include:

  • An in-home display (IHD) to provide electricity pricing, usage history, and utility messages
  • Metering devices to show the homeowner consumption data
  • A thermostat to provide demand response and load control, with the home heating or cooling system
  • Individual load-shedding devices that may be installed on window air conditioners, pool pumps, water heaters, or other devices
  • Smart appliances that react to pricing data or demand response messages to reduce their load
  • A prepayment terminal for use in areas where consumers can pre-purchase their electricity as a credit on their meter

Each of these devices can subscribe to particular messages from the electric meter. For example, a load-control device subscribes to load-control messages. When the meter is notified of a load-control event from the utility network, it sends a load-control message to the device. The device responds that is has received the message and signals whether it will participate in the event. At the start of the load-control event, the device indicates that the event has started; it sends a second status message at the end of the event. Through these simple messages, your thermostat set point, for example, may have changed from 72°F to 76°F for 3 hours on a hot summer afternoon to reduce air conditioning usage, and the device has notified the utility that it participated in the event. (Demand response events have an opt-out functionality to allow homeowners to indicate they do not want to participate in a particular event.)

Building a Smart Energy Device
Now that an SE profile is available, engineers can confidently build these in-home devices (see sidebar "Seven Steps to Implementing a ZigBee SE-Certified Product"). The specification defining what the devices are required to do (and the optional functionality) is available on the ZigBee Alliance Web site. A number of companies are adding the functionality to their existing equipment or building new products targeted at helping homeowners conserve energy and reduce their electric bill.

The process of developing a ZigBee-compliant product begins with a ZigBee-certified stack that provides the underlying networking, security, and management functionality. A list of certified stacks is available on the ZigBee Web site, and each of the companies offers development kits for engineers.

Starting from the base SE specification and developing all the application software in-house is a more difficult development path. Because the SE profile defines the messaging and behavior of the in-home devices, technology providers are creating tools and sample applications to simplify development of such devices. The companies also provide hardware reference designs for the basic integration of chipsets.

For example, Figure 3 shows the AppBuilder from Ember Corp., which was one of the first ZigBee-compliant platforms. The tool provides a graphical interface for a software developer to specify the particular device type and functions from the SE profile. Once the features and options are selected in AppBuilder, source code is generated to start the project. The development team can modify or extend the source code to suit their particular needs. Many companies want to extend the functionality with manufacturer-specific features to provide a competitive edge. Such modifications are acceptable as long as the mandatory SE functions for the particular device are compliant.


Click for larger image Figure 3. Ember’s AppBuilder graphical interface, which enables software developers to specify SE profile-defined functions for Smart Energy devices (Click image for larger version)

Once a design team has developed the hardware and software, the components can be tested in-house against the sample applications to validate functionality. After testing is completed, the product can be certified as SE-compliant by sending it to one of the two ZigBee certification laboratories.

The initial Smart Energy profile was developed with a particular set of home devices and energy controls in mind. However, companies are looking at ways to expand the profile with new functionality for electric cars, micropower generation units, and advanced home controls. Also, engineers are developing software applications that present and manage home energy usage more intelligently, whether the user is at home or away.

If you or your company has an innovative addition or extension to these systems, start with the existing SE profile and software and add the new extension. Soon you can be helping your customers reduce their electricity bills, avoid outages, and preserve the environment.

Seven Steps to Implementing a
ZigBee SE-Certified Product
  1. Select a supplier and ZigBee-certified platform to use. The key decision here is to use either a single ZigBee system-on-chip that integrates a programmable microprocessor, radio, network protocol stack, and memory or a ZigBee network co-processor that the manufacturer can partner with their microprocessor of choice.


  2. Purchase a development kit containing hardware and software that supports the ZigBee SE profile. Most leading ZigBee platform vendors provide development kits designed for their hardware and software. Some provide certified ZigBee SE profile reference application software as part of the solution.


  3. The hardware engineering team uses a reference design to build prototypes of the product they aim to build.


  4. The embedded software team modifies ZigBee SE sample code to accommodate their hardware design and company-specific extensions, such as a unique method of demand response to minimize electric loads of an air conditioner while maintaining comfort levels.


  5. The teams integrate and test the software and hardware.


  6. The product is sent to a ZigBee Alliance testing house for certification of SE compliance. The ZigBee Alliance—ZigBee's standard governing body—has sanctioned certification testing by two qualified international test service providers: National Technical Services Inc. and TÜV Rheinland Group.


  7. If it passes, the product is ready for production and can bear the "ZigBee SE-Certified Product" designation on its packaging.