Metering and sub-metering of energy and resource use is a critical component of any proper and professional comprehensive O&M program. Metering for O&M and energy efficiency refers to the measurement of quantities of energy delivered. These would include kilowatt-hours of electricity, tonne-hours of chilled water, pounds of steam and gallons of water. Metering may also involve identifying times of use for the various energy sources, the instantaneous demand for energy as well as identifying energy use for a collection of buildings, individual buildings, rooms or specific equipment, such as a chilled water plant, chillers, pumps and towers.
I have been lucky enough to have worked in this industry for over 40 years and have been able to observe the best practices from around the world, including the United States, Central and South America, Mexico, the Caribbean, Europe and the MENA region. An important aspect of my observation is that although most facilities are enthusiastic about the desire to track and save energy and to benchmark and improve the operation, they sell themselves short by collecting data that are not as accurate as they should or could be or. A key reason for this is that many do not employ the best-of-the-best metering technology to collect the data.
WHAT TO METER
Meters can be installed to measure data for several objectives:
- For performance evaluation and benchmarking of a building or chilled water plant. Measured data are used to establish baseline energy use, monitor changes in consumption and to share information with managers and staff.
- For billing purposes. These meters should meet the profile of ‘utility revenue grade’ accuracy, though, and be installed with metering accuracy instrument transformers.
- For measurement and verification of a building or building systems. Measured data are used to identify inefficient operations and validate proper operation of systems during commissioning and on an ongoing basis.
- To initiate demand side management programmes. Measured data are used to trigger demand-limiting logic, such as setting thermostat setpoints up a few degrees and limiting the fan static pressure in VAV systems.
Energy metering has a variety of applications for facility managers, plant managers and energy managers. Metering provides the information that, when analysed, allows the building operations staff to make informed decisions on how to best operate mechanical and electrical systems and equipment. The decisions will ultimately affect energy costs, equipment costs and overall building performance. The necessity to control costs, diagnose equipment malfunction, allocate usage and set efficiency goals are all increasingly important reasons for energy and water metering; however it should be as accurate as possible, otherwise, the real benefits of metering will not be realized. With the escalating volatility of energy and water rates, these needs are becoming even more important.
It is, however, important to keep in mind that meters are not an energy efficiency or energy conservation technology. Meters and their supporting systems are devices that provide building owners, managers and operators data that could be used to:
- Reduce energy/utility use
- Improve overall building operations
- Improve equipment operations. How the metered data are used is critical to a successful metering programme. Depending on the type of data collected, the following can be achieved…
- Verification of utility bills
- Comparison of utility rates
- Proper allocation of costs or billing of reimbursable tenants
- Demand response or load shedding, when purchasing electricity under time-based rates
- Measurement and verification of energy project performance
- Benchmarking building energy use
- Identifying operational efficiency improvement opportunities and retrofit project opportunities
- Usage reporting and tracking in support of establishing and monitoring utility budgets and costs, and in developing annual energy reports
Ultimately, the business case for metering energy or utility use is based on the anticipated benefits to the site. Most of the metered data uses, listed above, will result in energy cost savings that can be used to justify the cost to purchase, install and operate the metering system. The degree of cost savings realised depends on the unit cost of the energy or utility being saved and on the effectiveness with which the site analyses the data and acts upon its findings and recommendations. But other potential benefits should also be considered as part of the metering business case.
Examples can include:
- Supporting efforts to attain Energy Star and/or LEED-EB (Leadership in Energy and
- Environmental Design – Existing Buildings) certifications
- Prolonging equipment life, thus reducing capital investment requirements, and improving its reliability by verifying the efficient operation of equipment
With the benefits in mind, let’s take a close look at the metering equipment options available…
The most basic kW/power meters provide energy- and demand-related information. More sophisticated meters provide information on power quality, capture events, log and store data, display data through a local screen and communicate with or control other devices or systems. Meters can be grouped into several categories based on their capabilities, with revenue grade meters, advanced energy meters and sub-meters being three among them.
Power meters are also rated in terms of accuracy. Metering accuracy should be a minimum of one per cent, where used for billing purposes. Meters with accuracy classifications better than one per cent are readily available at reasonable cost. ANSI Standard C12.10, Code for Electricity Metering lists metering accuracy requirements and applications. Revenue grade meters are readily available and are cost competitive. Revenue grade and metering accuracy class instrument transformers should be specified for any chiller and chilled water plant meter programme. The total accuracy of any meter installation depends on the accuracy of the meter and also the accuracy of the instrument transformers.
All electric power meters, except Class 100 and 200, require inputs from CTs, PTs or both. Metering accuracy transformers with the proper burden ratings should be used for all installations. Instrument transformers complying with requirements and ratings stipulated in ANSI Standard C57.13 should always be specified. Relaying class instrument transformers are not suitable for use in metering circuits, where billing and revenue accuracy is required. Split-core CTs should never be installed, where revenue grade accuracy readings are desired.
A common problem with power meters after installation is data measurement errors or gaps in data, due to insufficient current flow to the meter. This may be the result of the installation of CTs with a primary ratio that is too large for the actual load. This problem is avoided by specifying CTs that have primary ratios based on expected demand and having a rating factor of RF2.0 (200% of the primary rating).
I would strongly recommend for any building, chilled water or District Cooling plant that only utility grade/revenue grade meters be utilised. I have seen far too many buildings, chilled water and District Cooling plants that will track chiller kW consumption by extracting the data from the microprocessor-based chiller system. This is totally unacceptable and will provide data that is not as accurate as it should be. The microprocessor-based system is not a revenue grade power meter. I have seen chiller-based data being off by as much as +/-5%, when verification testing is performed.
Metering individual chillers, chilled water plants or total building chilled water energy usage requires the measurement of three variables: Entering Water Temperature (TE, ºF), Leaving Water Temperature (TL, ºF) and Flow Rate (GPM)
From these, energy usage can be calculated. However, flow measurement can be quite inaccurate, depending on the type of meter, calibration and how it is installed. In addition, temperature measurement accuracy also varies by sensor type and calibration. In any chilled water application, sensor accuracy relative to each other is very significant. For instance, if one sensor reads 1ºF high while the other is 1ºF low, the energy calculation can be 20% off. I have seen sites, where it is off by more than this. Again, the chiller-based temperature sensors should never be used for the metering of chillers energy – they are not as accurate as required for this function.
The writer is the Managing Partner of Al Shirawi US Chiller Services. He can be contacted at firstname.lastname@example.org
CPI Industry accepts no liability for the views or opinions expressed in this column, or for the consequences of any actions taken on the basis of the information provided here.