New Way to Approach Overall Consumption
By Loïc Moreau
Saving energy is an important issue from both an environmental and energy cost perspective. The key question is: How can a substantial and sustainable reduction of energy consumption be achieved? The most credible solution is to establish an understanding of how users consume their energy and make them responsible for it. Sub-metering is an extremely powerful tool in measuring energy consumption. A new generation of wireless sub-metering electrical components — Wi-LEM or wireless local energy meters — dramatically reduces the total cost of ownership and makes the ROI for a sub-metering project much faster, especially for existing buildings. Wi-LEM makes universal sub-metering possible and in turn can be used to make energy users responsible for their consumption.
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Figure 1: Wi-LEM comprises a constellation of several components. The platform consists of energy meter nodes, mesh gates, and mesh nodes.
Seven Good ReasonsElectrical sub-metering provides powerful information about how, where, and when electrical energy is used. This enables energy managers to make important decisions that will save electrical energy and improve efficiency. Electrical sub-metering can be used effectively by a wide variety of energy users including manufacturing companies and industrial plants, which can use sub-metering to assign energy costs to individual departments or product lines, thereby identifying energy as a true product cost that can be managed and optimized. It can also be used by managers of office buildings and other commercial properties, who can use sub-metering to allocate accurately both electrical energy and re-bill costs to tenants.
Seven compelling reasons to install electrical sub-metering are examined below.
1. To identify performance problems in processes and equipment
Sub-metering can help identify sources of energy loss including plugged heat exchanger coils in chiller plants, clogged inlet filters on air compressors, the wearing of (or loss of) lubricant motors, load bearings, or gear boxes, and control failures that cause equipment to run continuously or at inappropriate times.
2. To determine equipment and system efficiency
Is the energy efficiency of installed office or manufacturing equipment acceptable? If electrical sub-meters are positioned strategically on circuits that feed key pieces of equipment, energy managers can develop powerful energy metrics (statistics or benchmark values) for evaluating the performance of installed equipment.
3. To audit before and after energy use for projects intended to improve efficiency
Energy efficiency projects are often not viewed as central to a company's business. Sub-metering can supply two types of supporting evidence to help justify these projects. First, before a project, it can gather measured data that quantifies energy savings opportunities. And second, after a project, it can gather measured data that verifies the expected rate of savings.
4. To discover opportunities for potential energy efficiency improvements
Sub-metering can help pinpoint energy savings opportunities by answering important questions. For instance, who is using the most energy and how are they using it? Information gathered from electrical sub-meters can allow an energy manager to focus on the biggest savings opportunities in each building or process area. Another important question involves comparing the energy savings to something. An ongoing benefit of electrical sub-metering is sound, detailed documentation of a building's or facility's historical energy use patterns. Having a solid database of previous energy use can increase confidence in projections of energy savings.
5. To allocate energy costs to specific departments or processes
Some common methods of estimating energy allocation (based on square meters of floor space, number of workers or occupants, or the capacity of the electrical supply circuits) have the disadvantage of spreading energy savings from one area throughout an entire facility and thereby providing no incentive for departments within a facility to reduce their own energy use. A sub-metering system provides data that can be used to analyze and allocate energy cost information.
6. To assign accountability for energy users
Simply making energy efficiency a factor in each manager's annual performance evaluation can shave several percentage points from a company's overall energy expenditure. One way to compare the relative energy efficiency performance of various departments is to analyze energy use per unit manufactured or per tenant occupant.
7. To verify the accuracy of utility bills
Few bills are taken on faith as much as electric power bills. Given the many thousands of dollars paid for energy every year by the owners and operators of most buildings and facilities, identifying even small accounting errors can quickly recoup sub-metering costs.
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Table 1: By measuring active and reactive energy, maximum current, and minimum voltage among other parameters, the EMN provides much more information than a traditional sub-meter.
The second and major part of the project cost is network installation and commissioning. To achieve accurate energy monitoring, the most effective method is to position electrical sub-meters as close as possible to the end use of the energy — i.e. per floor, per group of usage (HVAC, lighting, compressors), per department, or basically almost everywhere throughout the building. Based on the existing wired solution, this also means longer installation time and greater cost and often significant disturbances for workers such as wall drilling. These cumbersome installation requirements are particularly problematic for facilities with customers. In most countries, the building will be used all week, which means that installation is restricted to nights or weekends, which increases labor costs even more.
The New GenerationWi-LEM, an innovation in the world of electrical sub-metering, has been designed to respond to the emerging requirements of the energy efficiency and energy service market. It comprises a constellation of several components as shown in Figure1. The platform consists of several elements: energy meter nodes, mesh gates, and mesh nodes. Let's review each below.
An energy meter node (EMN) is the sensing component used to measure various parameters. By measuring (see Table 1) active and reactive energy, maximum current, and minimum voltage among other parameters, the EMN provides much more information than a traditional sub-meter. Measurement is done within a time interval programmable from 5 to 30 minutes. In order to reduce hardware costs and simplify installation, the EMN uses a pre-wired split core current transformer. As calibration is done at the factory, the accuracy of the complete acquisition chain is guaranteed and conforms to many industry standards. To achieve the equivalent level of accuracy with a traditional current transformer and sub-meter, the developer would need to use a class 0.5 accuracy, which represents higher cost devices, especially for split core current transformers. A variety of EMNs are available for 120 and 240 VAC.
The mesh gate (MG) is the second part of the platform. It is a stand-alone gateway that manages the wireless network in total transparency for the user. The MG is always aware of the network configuration (i.e. which EMNs are connected and what is the best route to transmit the data). The wireless network is a mesh configuration based upon the 802.15.4 ZigBee standard, which has proven robustness in industrial and commercial environments. It can manage up to 240 EMNs and stores the latest data transmitted that the PC software can request by using a MODBUS RTU protocol through a serial interface RS 232 or RS 485.
The mesh node (MN) is the last part of the platform — a simple repeater that extends the range of the network by acting as an intermediate point and can be added to the network without any need for additional configuration or programming. Communication distance between EMN and MG is traditionally limited to 82 feet line of sigh; however, by using an MN, it means that whatever the topology of the site, the Wi-LEM can always be deployed, which is not the case for point-to-point networks. Also, because the number of MNs is not limited, redundancy is possible, which brings flexibility and robustness to the network.
Loïc Moreau is a technical expert with LEM, a specialist in the measurement of electrical parameters. More information is available by contacting LEM USA Inc., Milwaukee, WI, at 800-236-5366 or visiting www.lem.com.