Air conditioning and energy metering

Energy metering is the topic of the day for all trades, including air conditioning. This task is mainly assigned to the building management system (BMS) supplier. In the following article, the main principles used in energy data acquisition are described, and we focus on the cooperation between the air conditioning supplier and the BMS supplier.

Why measure?

Before the data acquisitoin system specification starts, we should know what will the measured values be used for. It may be e.g. energy costs allocation at office and residential buildings. If there is a single building user, such as at a production plant, the data may be useful for management accounting (variable costs calculation) or for identification of weak points and reducing of the overall energy consumption of the plant. The meter readings may also be a major data source for operator who uses them for EPC (Energy Performance Contracting) projects.

Depending on the purpose of use of the measured data, the measuring points (places where the meters are installed) must be specified. Some of the nodes may not be populated by meters at all, and their consumptions may be acquired using virtual meters, or calculations using other physical meters. Usually, various auxiliary or less important consumptions are measured like this.

 virtualni_meric

Virtual meter M4 = M1 – M2 – M3

When deploying virtual meters, transport losses must be taken into account (especially at heat meters) as well as inaccuracy of physical meters. If the physical meters entering the calculation are of different ranges, the resulting virtual meter may not „work“ correctly. Note that for cumulated values (kWh, m3, …) the measuring error also cumulates, and in the course of time the virtual meters may show surprisingly errorneous values.

Readouts and communication

It is apparent that manual readouts (entering values into tables or portable terminals) are uncomfortable, time consuming, and potential source of errors, especially at systems with high amount of meters. If the system contains about 10 or more meters, the designer usually specifies a communication bus for readouts. In this case a close cooperation with the air conditiong or heating system supplier is required, as this is most probably the supplier of the meters as well. The BMS designer should specify the communiation interface. This is mostly the M-Bus. If the meters provide pulse outputs, such as water meters, it is advisable to supply them together with a M-Bus converter. The converters may be detachable, which simplifies meter replacement in case of calibration and service.

Some energy meters (e.g. Siemens UH50) provide the M-Bus communication card as an extra accessory. The card is not part of the meter itself. Those add-ons may make the whole supply more expensive. That’s why it is good to check if the meters are specified according to the BMS designer’s requirements to other suppliers.

The electrical energy meters usually also provide the M-Bus interface. Smarter devices, which show higher harmonic frequencies or provide monthly reports, may talk over Modbus / RS485. Modbus and M-Bus, although of similar names, are two completely different standards, both eletrically and regarding the communication protocol. Just to note, they can not share the same data bus.

M-Bus addressing

Let’s make a short stop at the M-Bus meter addressing principles. They are of high importance for information exchange between the air conditioning supplier and the BMS supplier.

The meters use double addressing: primary and secondary. These addressing types are mutually independent and different readout systems may use different addressing types. It is possible to scan both address types on the M-Bus using numerous service tools (programs), both generic or supplied by the respective meter manufacturers.

Primary addressing

Every meter on the bus must have a unique address ranged 1…250. The address is defined either by buttons in the meter setup (this is used at electricity and heat meters which have a display and buttons for data readout), or using a service program and a M-Bus to RS232/USB communication converter (this can be found e.g. at water meter converters which do not have displays nor buttons, e.g. Actaris Cyble M-Bus). If the BMS supplier requires primary addressing, it is good to order preaddressed meters with self-adhesive stickers carrying the addresses. The addressing may also follow later at the site, but it is less comfortable and not so easy to manage.

Secondary addressing

When using secondary addresses, the communication with the meter is different. A secondary address is a 8-digit number, which is allocated to the meter at the production, and (mostly) can not be changed afterwards. It is a good practice that the secondary address is identical with the serial number of the meter; then it can be easily recognized at the meter label, cover or display. Again, there is no rule without an exception: for example if the M-Bus communication module is to be ordered separately from the meter. Then the table of the secondary addresses of the meters must be kept together with the meter list.

 megatron

Heat meter with its serial number (and secondary M-Bus address at the same time) of 65328349. Source: Siemens

Water meters with a separate M-Bus module may have the secondary address printed on a label which is attached to the communication cable. In this case the label has to be kept accessible, it must not let be hidden by the piping insulation.

When to use primary and secondary addressing?

The BMS designer together with the BMS or meter readout supplier usually decides.

In general, systems with small number of meters mostly use primary addressing. This means that if a meter is to be changed, the new meter must be set the same primary address as the old meter, and if possible, the old read value must be set as the new starting value. This is easier for the BMS supplier as no changes in the readout software are required. At small sites, the meter bus is usually connected to an automation station rather than to a separate meter readout system. The engineer would have to modify the application software. If the primary addresses are maintained, the meter exchange can well do without these modifications.

Systems with hundreds or thousands of meters need another solution for this task: in a separate meter readout system there is a function “meter replacement” which can assign a new meter (with a new secondary address) to a particular metering point. The system mostly copes also with data consecutiveness – the measured value increases monotonously even when the new meter starts cumulating from zero. The meter management using secondary addresses is also easier for the supplier, as he needs not to deal with primary addressing and keep the reference table between the meter serial number and its primary address.

Requirements on the technology supplier

The technology supplier is, here, the company which supplies and installs the respective meters. Exchange of data between this firm and the BMS supplier is the major condition for successful commissioning of the system.

The first weak point is labelling. All meters should be labelled – same as all peripherals – with a sticker carrying the position according to the project, and if possible, with a note what actually the device is metering.

Example:

Wrong: “1S25” – we do not have an idea if the meter is located in the room named 1S25, or if it measures some energy which is consumed in this room.
Right: “CW 1S25 Bicycle washer” – here we at least can see that this is a cold water meter, indicating water consumption in the bicycle washing room (whatever the room number may be).

It must be noted that the BMS data will be available to the facility manager. His or her room numbering may be very different from that which was used by the construction firm (and which is also taken over by the other suppliers). If a meter is labeled ambiguously or wrongly in the building management system, the BMS supplier would have to rename the whole “project – application software – database – SCADA” chain.

The main output thus should be a meter table which may look e.g. like this:

tabulka_mbus

Table of secondary addresses of meters in a residential house. Left column: the appartment name.

A frequent problem may be that the installers jot down the serial numbers of the meters, which may not necessarily be the secondary addresses (see above). Then all the measuring points must be visited again. In this context it should be pointed out hat all meters should be accessible at all times (installation shafts with doors across the whole section, service apertures), identifiable (labels, ideally also outside of the service apertures if design permits, symbols in floor plans), and the label data should be legible (meters installed with displays facing an open space rather than walls, thermal insulation).

Change management

There is no site with no changes. Activities such as adding / removing a meter, changing its name, new function of the supplied room, or exchanging of the meter for another type should be communicated to all the concerned parties in writing (per e-mail) as soon as possible. In the advanced stages of contruction it is not enough to send out just the updated table, because searching for changes in it would be too much demanding.

Requirements for data output

The way of presentation of the acquired data in the BMS system is essential. If the energy manager would have to copy the values from the screen into his notebook on a monthly basis, it would be little or no progress against manual readouts. The data formats and way of communicating is specified by the facility manager. However, the BMS supplier may become a subcontractor of the building technology supplier, who is responsible to the facility manager for those issues. It is advisable to have a written agreement in the technical description on how the meter data shall be handed over. Usually, a table with exported metering point names and consumptions for a particular date is enough, which most of the BMS programs are able to provide.

Some customers require also data pre-processing, such as plausibility check, i.e. checking if the values make (physical) sense. For instance, at water or energy consumption data it is expected that this month’s value is equal or greater than the last month’s value. Meter values for distribution branches may be added together and the sum compared with the energy source output – the consumed energy values should not be bigger than the consumed energy, etc. If also history data shall be taken into consideration, it may be a task for an energy management system, such as ContPort [2].

The more complex the installed technologies, the more complicated the cost allocation may be. One of the case studies can be found in [3]. But, however inaccurate and nonobjective this measurements may be, it proved that the mere existence of a cost allocation system makes the building users behave more efficiently.

At energy metering, more than at the other parts of a BMS, it shows that only if the assignments are 100 % completed the whole system makes sense. Mistakes like swapped measuring points may be lasting for years, and discovered only after customer claims. This kind of problems is mostly caused by poor communication between the subcontractors and can be avoided easily. Only a properly installed and operated data acquisition system may be a motivation to savings and bring ideas on optimization of technologies.

 

[1] https://elektro.tzb-info.cz/merici-a-regulacni-technika/9539-instalace-komunikacni-sbernice-m-bus-1-cast
[2] https://domat-int.com/en/about-system/energy-management-system
[3] https://vetrani.tzb-info.cz/vetrani-bytovych-domu/12645-rozuctovani-nakladu-na-vetrani-v-systemech-vav-cav