Central energy plants — consisting of chillers, boilers and associated towers, pumps, and accessories — require a large capital investment and are energy-intense. The goal of these plants is to provide a reliable source of heating and cooling for a building or group of buildings, while minimizing the costs for energy, operations, maintenance, and equipment replacement.

To achieve a reliable, efficient plant, there are many decisions to be made including careful selection of equipment as well as plant layout and pumping strategy. Proper selection and design of the controls system can help to dramatically improve efficiency, and through integration, can also help to manage reliability and maintenance. There are several key elements to keep in mind when designing controls for a central plant.

Efficiency

The goal of the plant is to provide the required output Btus (chilled water, hot water, steam, etc.) using the minimal amount of input energy (electricity, natural gas, fuel oil, etc.). An efficient plant starts with chillers and boilers that are highly efficient, but this is not enough. To optimize the plant, control sequences are needed that look at the plant as a whole.

Examples of optimization strategies include use of variable flow pumping at minimum static pressure, measuring plant load and running the most efficient equipment to meet the load, efficient tower operation, and resetting condenser water based on plant load and outdoor air conditions. Each of these strategies needs to be carefully developed for the plant, implemented, commissioned, and documented. Integration is also key to efficiency, since data from VFDs, flow meters, chillers, and boilers can all be used to calculate and document plant efficiency in real time.

Reliability

Central plants require a high level of reliability. To achieve this, most plants are designed with a certain level of redundancy. Controls can help assure reliability in several ways. The most basic is to utilize sequences that monitor for faults and automatically start backup equipment. For example, in the event of a pump failure, a back-up pump should automatically be brought on.

Beyond this, controls algorithms and analytics can be used to monitor key parameters to look for problems before they become serious. For instance, monitoring the approach temperature between the refrigerant and the evaporator at a specific load is a good indicator of tube fouling. Integrating data from key equipment including chillers, boilers, VFDs, etc., is key to being able to do good monitoring and improve reliability.

Maintainability

Finally, in order to properly maintain the plant assets, good maintenance procedures are required. Controls and integration can provide critical data to assist in developing and supporting a maintenance plan.              

Central plant design is both challenging and rewarding. Proper design of the controls and integration to key equipment is one of the best options to improve not only efficiency, but also reliability and maintainability. ES