How commissioning can aid long-term optimised performance
Efficient management of building energy performance requires access to current, meaningful data about the performance of building-services systems. Elio Galluzzi of SAV Systems explains why the solution begins with commissioning.
In commercial buildings it is actually quite rare for the energy manager to have an accurate picture of how the building-services systems are performing. There may be a rough overview based on the performance of central plant, but this is not the same as understanding performance on a zone-by-zone level.
This is because it is difficult to access data that provides a meaningful picture of performance. Yet without such data it is virtually impossible to understand how the building is performing currently and to maintain optimum performance through the lifecycle of the assets.
In its TM39 and TM46 guides, CIBSE recommends the use of automatic monitoring and targeting (AM&T) to understand how energy is being used through effective metering of appropriate parameters. In the case of heating or cooling systems the key parameters are flow rates and the temperature differential (∆T) between the flow and return temperatures of hydronic heating and air-conditioning systems.
It is equally important to understand how changes such as improving the thermal performance of the building envelope or variation in staffing densities affect these parameters. An inherently flexible design that enables easy adjustment of the systems — should they need to be re-aligned to a zone’s changing requirements — is therefore also important.
Understanding these performance parameters presents a number of challenges.
At the design stage of a speculative building, for example, information is available about the whole building — but usually not how it will be divided up. Consequently, an appropriate level of sub-metering of heating and cooling systems will be needed for billing of tenants or departments.
Metering each system centrally will not provide sufficient data to fully understand performance at zone level, whereas metering at each terminal unit (fan coil, chilled beam etc.) will provide too much data and be expensive to implement. Another option is to put meters on each floor, but this will not facilitate billing on floors shared by several tenants/departments.
Clearly the ideal solution is to meter the terminal units in zones, with each zone being served by a commissioning module.
All of the criteria described above can be met by using commissioning modules that incorporate electronic flow measurement and the capability to log energy data.
The commissioning modules themselves provide a centralised distribution hub for a group of terminal units such as fan coils or chilled beams. Ideally, each commissioning module should include spare ports so that additional terminal units can be added in the future if required. Plus, if connections are made using flexible pipework, then any future reconfiguration can be accommodated easily and cost-effectively.
To measure performance, commissioning modules can be fitted with electronic flow-measurement devices to enable continuous monitoring of flow rates. These must be MID (Measuring Instruments Directive) approved if they are to be used for billing.
The commissioning modules will also continuously monitor the ∆T across all terminal units to ensure efficient operation and support maintenance of the system.
One way of measuring flow rates is to use a turbine energy meter, which uses a mechanical turbine within the pipework to measure flow rate. These are responsive and accurate but prone to wear of mechanical parts and subject to damage by debris in the water.
An alternative is an ultrasonic transit time flowmeter, which measures the time taken for an ultrasonic signal to travel from one sensor, across a pipe, to a second sensor. With no flow, the transit time would be equal in both directions. With flow, sound will travel faster in the direction of flow and slower against the flow. Flow rates are therefore determined by comparing upstream and downstream measurements.
Of these two types of energy meter, an MID-approved ultrasonic meter should be preferred as it deals better with debris and allows the energy meter to be placed on the primary circuit of the module. These also have a longer lifespan.
When measuring just flow rates you are relying on certain specifications and equations to know how the system is run. If you measure and monitor the ∆T you can get a better understanding of how the energy transfer is performing. When commissioning modules are used, the system can be set up so that any underperformance of a terminal unit is flagged up. The system can then be interrogated remotely, by measuring the ∆T across the various terminal units.
Using the capabilities of commissioning modules in this way and acting on the data obtained through AM&T, a regime of continuous commissioning can be maintained, where the hydronic systems are re-commissioned to reflect any changes in the building.
This ability to respond to ‘change and churn’ is essential in meeting the objectives of the ‘soft landings’ approach that will ensure buildings continue to perform as designed throughout their life. The key is to ensure that the approach to the initial commissioning of the systems enables AM&T and ongoing commissioning.
Elio Galluzzi is product manager for commissioning modules at SAV Systems.