Energy Management Controls in Cooling Plants

20 Jan 2018

With cooling systems accounting for maximum energy consumption, a monitoring and controlling system can help reduce costs and consumption.

In India, the building sector accounts for approximately 35% of the total energy consumption and is growing at a rate of 8% annually (Bureau of Energy Efficiency). Commercial buildings in India account for nearly 8% of the total electricity supplied by the utilities. Electricity usage in the sectors has been growing at about 11%–12% annually. Building energy consumption has seen an increase from14% in the 1970s to nearly 33% in 2004/05. Electricity use is primarily from lighting, space conditioning, HVAC (heating, ventilation, and air-conditioning) appliances and equipment (including IT equipment), and water heating (Bureau of Energy Efficiency, Scheme for BEE Star Rating for Buildings - BPO, December, 2009). As the maximum energy-saving potential primarily lies in HVAC or cooling systems, making cooling systems as energy efficient as possible is, therefore, an important component of reducing building operating costs. This article addresses the energy management controls in centralized, chilled water plants using an energy management system (EMS). This task essentially consists of minimizing the energy consumption of cooling systems, while preserving comfort in the buildings.

Energy Management System: An EMS is an intelligent HVAC automation system which controls and monitors the cooling plant equipment and their operations. It commands the equipment on the basis of multiple algorithms considering the demand cooling load, ambient conditions, and indoor thermal conditions. These algorithms are developed specifically for each application. The control system collates and calculates various types of output signals, manages loading and unloading of multiple chillers, and includes a logic-based control of related chilled water pumps, condenser water pumps, and cooling towers. Various energy-optimization strategies can be customized for output control, which will further improve the efficiency of a plant room and save operating costs for users. It can be customized as per the user requirement and can be integrated with models like adaptive thermal comfort. EMS allows one to program a cooling plant to suit your climate and geographical location exactly as per your requirement at that point of time, and automatically adjusts itself to save electricity. Mainly, the following controls are recommended under EMS:

Chilled Water Supply Temperature Control

Mostly in retrofitting exercises, it is found that chillers are operated at constant chilled water temperature regardless of the cooling load. However, modern chillers can vary the chilled water supply temperature as the cooling load varies. By allowing the chilled water temperature to float up during low-cooling load periods, savings of about 1% per degree temperature rise is typical. In other words, raising the chilled water supply temperature by an average of 5 °F will save about 5% in chiller energy. To do this, the chiller must somehow sense when there is reduced load. When the ambient temperature is lower, chilled water set point temperature can be raised correspondingly for achieving energy savings. Also, chilled water return temperature is used as an indicator for reduced demand load (lower return temperatures indicate that lesser heat is being extracted from the building).

Condenser Water Temperature Control:

Typical cooling tower schemes maintain a constant condenser water temperature (usually at 90 °F) by varying the flow rate of air. As the outdoor wet-bulb temperature decreases, the condenser water temperature can be decreased which can lead to energy savings. Broadly, lowering the condenser water temperature by an average of 5 °F will save about 2.5% savings in chiller energy consumption.

Chiller Sequencing:

Chiller sequencing ensures that the most efficient combination of chillers is operating and the system is operating at or near maximum efficiency. During part load conditions, it ensures that the plant is operating on best system part-load value. Along with chiller sequencing, an EMS generates alarm for critical conditions.

Cooling Tower Staging Control:

Cooling towers are an essential part of water-cooled cooling plants; the efficient use of cooling towers plays a crucial role in the energy-savings operation of the entire chiller plant system. Though the energy utilization of cooling towers is a small proportion of total chiller plants, the cooling tower control method directly impacts the operating efficiency of chillers. Advanced staging control adaptively determines the operating number of cooling towers according to outdoor weather conditions and system running status. The control method also avoids the surge of chillers due to high load.

Integration of Variable Frequency Drives (VFDS)

Applications of VFDs are very well known for achieving energy savings. VFDs integrated with EMS on constant speed chillers and on constant speed cooling tower fans produces huge energy savings.

Installation of Smart Energy Meters:

Smart energy metres can be installed at all the load feeders to reduce manual interventions. It can log the electrical parameters and compile the same at a single user location. This data can even be accessed from a remote location.

EMS takes accurate decisions on when the air conditioning system shall be on and off by using its algorithms to avoid unnecessary energy consumption. The control algorithm has a self-learning mode to learn the facility’s air conditioning load pattern and modify the operations of the equipment. Advanced algorithms are inbuilt in the module to mitigate the Delta-T syndrome which is common in a constant primary variable secondary plant. It is a self-diagnosing technology which identifies areas of energy wastage in the system. It results in huge energy savings and reduces human errors in operation. It calculates real time iKw/TR for the plant room and determines the deviation from design parameters. It records precise energy endues data and provides finer insight into the energy consumption trends of plant. As per the industry claims, integration of an EMS with the above-mentioned controls can produce energy savings, varying from 10% to 60% on a case-to-case basis.