Metering System - define the points of measurement
From Electrical Installation GuideThe point of measurement is the combination of the meter and its location. To define the points of measurement in the building, we need to answer to the following questions:
- What are the data to compute the performance metrics
- How to get the data and where to get these data ?
What are the data to collect ?
For each performance metric, the needed data can be split into 2 categories  :
- Static data such as Building area, rating or efficiency of equipment. These data are usually used to normalize measurements for benchmark comparisons
- Dynamic data such as energy consumption, temperatures, flows …with the corresponding recording frequency. The recording frequency depends on the project goals and could be yearly, monthly, daily or time series
The recording frequency depends on the project objectives:
- for energy usage analysis or energy consumption alarming: measurement each 10’ or 15’ is necessary to get the load curve
- for energy sub-billing or bill auditing: the recording frequency shall be compatible with the tariff structure (once a day could be enough for constant tariff but not sufficient for several tariff slots per day)
- for cost allocation or building energy performance benchmarking: once a day is enough
Method of measurement and meters locations
The above dynamic data lead then to identify the Point of Measurement in the building by selecting the method of measurement and the meters location.
Especially for existing building, practicality of measurement should be verified according to the utilities distribution (electrical architecture and wirings, possibility to separate lighting and small power consumptions, gas distribution, accessibility to water meter, ..) and possible physical locations.
Methods of consumption measurement
There are different methods to measure consumptions, described in the CIBSE guide GIL 65 “Metering energy use in new non domestic buildings” . We tried to complete this list and to describe when each method is relevant or not.
The method of measurement shall be selected according to the project goals (especially the desired accuracy), the estimate budget and the operation conditions. The result of this step is the list of necessary meters to be installed in the building.
- Direct metering: measure directly the consumption trough electric power meters, gas meters, oil meters, heat meters or steam meters…
Direct metering shall be selected in the followings cases :
- For loads of major consumptions or total building consumptions.
- When the measure is used for sub-billing for tenant areas as it requires a class 1 or 0,5 accuracy.
- When other data such as electric power quality data have to be measured for the same energy use. That the reason why disturbing or interruption sensitive loads shall be identified at a early stage when designing the metering system.
- Hours run meter: for load of type constant power, measuring the number of operating hours enables to compute the corresponding consumption (ex : fans without VVD, lighting, …). Knowing the rating power indicated on the equipment plate is not always sufficient as the load factor has to be taken into account to estimate correctly the consumption. For existing buildings, the load factor can be known with a power measurement trough a portable meter. As soon as there is load control (eg lighting control with occupancy sensor), it becomes very difficult to estimate the load factor and this method is no more valid.
- Indirect metering : readings from indirect meters can be used to evaluate energy consumption such as the example here below :
Most of the time, estimation accuracy depends on the equipment data (such as the boiler efficiency here above) that can change or evolve with time, therefore these data have to be checked regularly.
- By difference: Two direct metering can be used to determine a third measurement by difference. Such method should only be used if the two others measurements are acquired trough direct metering.
- By data analysis: it is possible with one measurement to break down different energy uses or area consumptions by knowing how the building operates. For example, in a hotel knowing that during the night most of the consumption in the floors is of type common use (corridor lighting …), the electrical consumptions of the floor during this time slot represents the lighting consumption of the common area.
- Estimation: Calculations can be used to estimate consumptions of small power loads thanks to ratio (e.g. kWh/m²) given in some standards or by knowing the occupant behavior. For example, plug-in lighting consumption can be estimated trough number, power rating and scheduled of lamps.
Locations of meters
Location of the meter is determined according to what energy flow they need to measure in the schematic diagram but other criteria should also be taken into account such as:
- Practicality of the measurement
- Needs of local visualization: meters shall be placed so that they can be read easily by the building operator. Typically this will be the plant room, main electrical room, boiler room or control room.
Special consideration for electrical meter location
- Feeder or incoming instrumentation for electrical switchboard ?
For electrical meters, it is generally advisable to instrument the feeders of the main LV switchboard instead of the incomers of the sub-distribution switchboards in order to:
- reduce the lengths of communication cables,
- get an overview of consumptions of all feeders of the Main LV switchboard in the switchboard room
- use protection devices that embed metering. It will avoid external meter and CT installation, reduce cabling and increase switchboard spare capacity.
However, some reasons can lead to instrument the incomer of a sub-distribution switchboard such as:
- the lack of room in the main switchboard for existing buildings
- multi tenant building with sub-billing. The meters should then be installed in each floor/tenant panelboard, so that the tenant can have access to this meter.
- an electrical distribution with Busbar Trunking System in order to get an easy access to the meters and generally because of lack of room in the tap-off unit.
- Integrated or independent metering panelboard
The metering devices can be installed inside the switchboard or in an external metering panelboard when it is requested or when space is limited in the switchboard for existing buildings. This can have an impact on the type of meter to select (embedded or separated metering)
- ↑ Measured Success : Constructing Performance Metrics for Energy Management - John Van Gorp