Chilled water plants, with VFDs on chillers, chilled water pumps, condenser water pumps, cooling towers and automatic flow valves, and a CMS/ BMS are fully automated but most likely not optimised. Typical building automation systems control equipment through fixed set points and schedules. These systems have limited knowledge of the actual equipment being controlled and how their performance changes at different operating conditions. Let’s take the following example: A BAS controls a cooling tower to a fixed 78 degrees F (25 degrees C). Chillers are sequenced on once they reach full capacity, the chilled water set point is adjusted based on a fixed time of day schedule and chilled water pumps are operated to maintain a fixed pressure. Typical strategies, such as the aforementioned, have been proven to consume as much as 75% or more energy at part load when compared to a CPECS (Central Plant Energy Control System) or fully optimised plant, all as a result of the typical system not having the calculation horsepower to make complex equipment performance calculations.
A CPECS software system has complete knowledge of compressor, tower and pump performance characteristics, which it uses in real time to modulate control levels to all VFDs and provide the maximum level of system performance while respecting chiller flow, temperature limits and occupant comfort. The result is a chilled water central plant that operates in synchrony to deliver the highest possible total performance. Unlike other static optimisation strategies, a CPECS has the ability to self-correct chiller, tower and pump performance maps such that regardless of wear and tear, inaccuracies in manufacturer’s data or off-design conditions, your plant will always run at peak efficiency. A CPECS goes past the central plant and out to the heat exchangers or AHUs, continuously scanning specific data points in order to balance central plant performance with air side performance without sacrificing occupant comfort. Any time a variable-speed chiller plant operates at a capacity less than its maximum, it provides a huge opportunity for optimisation of set points and flows without compromising on occupant comfort or process temperatures. ASHRAE studies conclude that air conditioning applications operate at part load over 96% of the time. The CPECS web interface displays actual plant performance in real time to the operator placing upmost focus on efficiency. A CPECS typically has the ability to remotely warn of equipment failures or poor efficiency via a built-in email server. Each installation deploys with a full enterprise SQL database that resides on the site (all data owned and under control of the plant owner). Performance is directly related to cooling tower temperature and flow rate. An increase of 1degree F (0.56 degrees C) in condenser water inlet temperature may impact chiller performance by as much as 2.8%. Accurate cooling tower control and an optimised total system energy approach is essential in an efficient chiller plant. Typical cooling tower control neglects tradeoff between fan energy and chiller energy at part load.
▶ A CPECS achieves optimisation through:
• Optimised cooling tower control and sequencing
• Chiller sequencing, which seeks lowest kW for the capacity • Variable speed, variable set point chilled
• Optimised VFD condenser water pump control
• Optimised pump sequencing
• Chilled water reset based on actual HEX or AHU demand
▶ CPECS features include:
• Energy report generation and condition monitoring • Seven-year onsite and offsite historical data
• Internet remote monitoring and control
• Chilled water flow monitoring as standard
• Open programming language and open protocol
• Easy integration into the existing BAS network
• Carbon usage reporting
Better Technology and Greater Savings
A CPECS typically is able to achieve annual total plant operating efficiencies of 0.48kW/tonne (7.3 COP) and better. These results far exceed today’s energy efficiency code requirements and defy conventional thinking. Each plant is installed with its own flow, electrical and temperature metering, which enables the plant owner to view plant efficiency in real-time. In addition to electrical savings, reductions of up to 10% in tower water consumption can be realised. Also, CPECS plants have connectivity to Modbus RTU, Modbus TCP/IP or BacNet.
A CPECS allows immediate evaluation of savings via the use of a real-time baseline calculation. Baseline performance can be programmed as a 90.1 code compliant plant or a custom baseline that evaluates actual performance against a pre-retrofit value. The CPECS web interface, unlike any other chiller optimisation software, delivers to the end user an actual performance baseline and, most importantly, a target performance indicator. Other “optimisation” providers and programs cannot deliver a real-time performance target; as a result, performance shortcomings are not realised until too late.
A CPECS has a user-friendly graphical interface and can be viewed from anywhere through the internet. A CPECS Virtual panel can also be installed in any chilled water plant and overlaid on the existing BAS or CPM system; the CPECS system will operate for three months and allow you to see what your savings would be with the CPECS software operating the chilled water plant versus the current system. In other words, the predicted savings can be proven and a determination can be made based on the savings to have a fully optimised CPECS system installed in the plant.
The writer is the Managing Partner of Al Shirawi US Chiller Services. He can be contacted at email@example.com
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.