Lowering the Cost of EnergyOctober 1, 2009 By: Loïc Moreau, LEM Sensors
A submetering system that couples highly accurate energy measurements to wireless technologies enables effective energy monitoring for energy-conscious companies, allowing them to reduce both their carbon footprint and their utility burden.
In light of the increasing global environmental and economic pressures or individuals and companies to limit their energy use and otherwise minimize their carbon footprint, and while governments debate what must be done at an international level, forward-looking companies are exploring ways to become more energy efficient. The hurdle to becoming more energy conscious is awareness; nationally we may be aware that there are energy issues, but a lack of information can hinder direct action to improve that situation.
Utility companies provide meters to monitor usage, but the meters installed often provide only coarse-grained details about usage and are intended to allow the utility company to bill the customer. Submetering, a relatively recent development in the power industry, enables companies and large consumers to monitor their energy usage more accurately, and thus target their energy conservation and their use-reduction efforts.
Utility companies are currently not providing submetering to all end users; the energy consumer must provide the initial investment. Getting a return on that investment may be measured through the money saved, but the greater the investment the longer the return. This is particularly true for the majority of installations, which are complex and require a period of downtime to install and commission an energy monitoring system.
The Wi-LEM Energy Monitoring System
By harnessing developments in wireless telemetry, LEM has developed a range of energy management products that work together to create an energy management system. The system is simple to install, requires almost no commissioning, and (thanks to the type of wireless mesh networking used) is easy to maintain and extend, if necessary.
The monitoring devices accurately measure energy usage and communicate the information wirelessly. This minimizes the cost and inconvenience of installation.
The Wireless Local Energy Meter (Wi-LEM) system (Figure 1) comprises several nodes; the minimum configuration requires an Energy Meter Node (EMN) and a Mesh Gate. The EMN gathers the data and is wirelessly connected to the Mesh Gate, which provides the interface to the PC for storing and processing the data gathered.
Making Measurements. An EMN (Figure 2) provides energy measurement and is powered directly from the measured voltage, removing the need for an additional power source at installation. The in-line connection for voltage measurement is the only intrusive part of installation, as current measurements are made using a split-core current transducer. The EMN draws 2 W of power and is capable of making active, reactive, and apparent energy measurements through the voltage and current transducers. The transducers measure up to 300 or 480 V and 2000 A, respectively. Active energy is the energy that is actually being consumed and billed for. Reactive energy and apparent energy are for information purposes and will not be billed for.
Figure 2. The energy meter node (EMN) with its split-core current transducers
This accuracy is possible because of the split-core transducer's specially developed packaging and electronics that remove the inaccuracies caused when a split-core current transducer is installed, allowing the few microvolts detected by the transducer to be accurately processed. While the EMN complies with the IEC 62053-21 standard for accuracy in energy measurements, it exceeds the minimum accuracy of 1% accuracy at 10% nominal power. Measurements made by LEM engineers show the EMN is capable of making measurements with 1% accuracy at just 1% of nominal power.
These data are then passed wirelessly to the Mesh Gate (Figure 3) for off-line processing. The Mesh Gate can manage up to 200 EMNs and acts as the brain of the network, defining the path that data take. The wireless protocol used is based on the 802.15.4 standard and uses Millennial Net's communication layers.
Figure 3. The Wi-LEM mesh gate
Utility companies predominantly use the active energy measurement for billing, since this measure reflects the energy actually being used. Being able to measure your own active energy use enables better power management and more accurate billing. When utility companies bill, they take two elements into account:
- the actual energy consumption, measured in kilowatts used over time (kWh),
- the peak demand—the maximum kWh used—is the highest level of consumption allowed and stated in the contract with the utility. If you consume more energy than your peak demand, you will be billed accordingly and you will incur a large penalty. Utilities use the penalty to minimize the risk that many customers will be at peak demand at the same time, forcing the utilities to find additional energy to keep up with the demand.
Reactive energy identifies where power is being used, based on the power factor. For AC electrical systems the power factor is the ratio of the active power flowing to a load to the apparent power (defined as the product of the root-mean-square voltage and current) and is a value between 0 and 1. It is ideal to have a power factor of ~1. This value will change as each powered device is brought online. Many utility companies now bill on a power factor basis, meaning it is essential to know when your energy usage is approaching a limit set by your utility company. This is not easy to achieve using conventional utility meters.
Wireless Communication. Typically, submetering measurements would be communicated through the energy monitoring system using wired connections, which increases the cost and complexities of installation. With the Wi-LEM system, all communications are handled wirelessly.
Mesh networking addresses the problems of creating a reliable wireless network in a typical industrial environment. Mesh networks operate using multiple wireless nodes that are self-discovering, so that if the route between any two nodes becomes compromised—by a new metal cabinet being installed, for instance—the network automatically reorganizes itself to find an alternative route. This approach has been standardized under the IEEE 802.15.4 protocol, a standard that specifies the physical layer (PHY) and media access control (MAC) for low-rate wireless personal area networks. ZigBee and WirelessHART are both based on the standard. With the Wi-LEM system, LEM has used the Modbus protocol running on the IEEE 802.15.4 network, on top of which it has implemented its own proprietary stack for secure communication between Wi-LEM nodes (Figure 4). This guarantees robustness and security within a Wi-LEM network, while leveraging the strengths of the industry standard Modbus protocol, and making full use of the self-discovery and self-healing properties of the IEEE 802.15.4 mesh networking standard. Once at the Mesh Gate, the data—which already conforms to the Modbus standard—is passed to a PC through a standard RS-232/422 Modbus RTU connection, allowing the PC to store the data in a database for later analysis.
To make the mesh network as robust as possible, LEM has also produced Mesh Nodes; simple wireless repeaters that need no configuration. Once powered, they automatically join the network and act as additional routing paths for signals in areas with increased electromagnetic interference (EMI), such as buildings that are constructed from reinforced concrete, or those that contain a large number of metal cabinets.
Additional Measurements. The Wi-LEM submetering system also includes the Wi-Pulse and Wi-Zone Nodes. Most modern gas and water meters now provide a standardized pulse output, complying with the 62053-31EN (European norm) international standard. Connecting the meters to a Wi-Pulse node can provide additional data for the energy saving program manager. The Wi-Zone Nodes provide environmental information—ambient temperature and relative humidity—that can also provide insight into how energy is being used within the facility.
Submetering is becoming a far more important area of energy management. We believe that the Wi-LEM system enables businesses and facilities to take proactive steps towards minimizing their energy consumption.
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