Chicago, being the “windy” city that it is, is no stranger to temperature extremes. The climate can get pretty harsh – winter seasons can get pretty chilly, while summers can definitely become scorching hot.
Nonetheless, it does not mean that it is a terrible place to live in. Thanks to various cooling systems, and a couple of cooling centers, Chicagoans can seek relief during those hot summer months. One of the most notable initiatives to combat the heat is the use of chiller plants.
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An Introduction to Chicago Chiller Plants
While HVAC systems are responsible for regulating the comfort levels of most indoor environments, a chiller plant act as a centralized cooling system which provides cooling for a building or a number of buildings. Additionally, it provides a portion of the air conditioning by HVAC systems. According to the Energy Star website, around 39% of buildings which are more than 100,000 square feet in size have a chilled-water system.
Although chiller plants sound like a new innovation, the concept is not actually new. Records have shown that the ancient Romans were already using cooling for their indoor environments. However, they did not use a centralized plant but instead used water to run through their buildings’ walls in order to cool down the temperature inside.
By the 19th century, people started experimenting in an attempt to create a modern airconditioning system by using electric fans to blow air across cold surfaces. Unfortunately, such systems did not really make it to the spotlight due to their massive costs and inefficiency.
It was not until 1922 that a centrifugal chiller was invented by Willis Carrier, allowing the public to gain easy access to airconditioning.
Although chillers, likewise known as condensing units, are used primarily for airconditioning purposes, chiller plants, on the other hand, are made of several mechanical equipment. Cool air is distributed throughout the building by using a distribution system composed of a metal ductwork for transporting the air and a fan for pushing it.
Nowadays, most chiller plants also come with a variable-speed drive capable of running several condensing units at once which is more efficient than just turning all of them on or off. In some cases, cooling towers are used for cooling the air before it can even enter the chiller.
How Efficient Are They?
According to the Department of Energy, 10-15% of the energy which is consumed by buildings are allocated for airconditioning. Even though condensing units are becoming more and more efficient with the introduction of newer technologies, it is still possible to save more energy by reducing the size of the plant along with improving the distribution systems.
Considering that chillers are among the largest consumers of energy in a building, it can have a massive impact on operational costs. Therefore, monitoring the chillers in a plant is important in order to know how efficient the system is.
Calculating a chiller’s efficiency is actually simple by measuring its CoP or Coefficient of Performance.
Basically, it is the ratio between the chiller’s refrigeration effect and the amount of electrical energy required to produce it. These two units are measured in Kilowatts (kW). Let’s have an example:
A chiller produces 3,000kW of cooling which is equivalent to 10,236,423 BTUs/h while requiring an electrical energy of 500kW to produce such an effect. The CoP can be calculated using this formula:
Cooling effect / electrical energy requirement
Thus, using the given formula:
3,000kW / 500 kW = 6. The Coefficient of Performance, in this case, is 6 which only means that for every 1kW of electrical energy used, 6kW of cooling is produced.
Keep in mind, however, that a chiller’s CoP will depend on its cooling load, not to mention each chiller has its own different efficiency. Chiller manufacturers will normally provide the CoP design data to make a performance comparison between chillers quicker and easier.
Chiller Plant Optimization and Its Impact on the Buildings in Chicago
Since chiller plants are often referred to as the “heart” of an HVAC system in buildings, it is important to optimize them in order to achieve better performance. Aside from a chiller plant being the main cooling source, it uses a lot of a building’s energy needs, resulting in massive power costs.
For example, in a typical Chicago hotel, the HVAC system can use up to 50% of the total electrical load, and a chiller plant takes up a huge portion of that.
One of the real challenges of chiller plants is that a lot of them do not run at their designed temperature differential. This is the difference of temperature between the chilled water return (delta T) and the chilled water supply. Chiller plants running at less than optimal levels are actually common, and in most cases, they have a way lower delta T than what is necessary. This is caused by several factors such as the method used for controlling the equipment and the way the airside systems operate.
Optimizing a chiller plant is made possible by starting with the chilled water pumps and chillers. The most efficient chiller plant arrangement used is the variable-primary chilled water system as it uses only a single set of pumps for distributing water throughout the building. Another efficient arrangement is primary-secondary which uses two sets of pumps for the chillers and building systems.
It is of utmost importance to control the equipment such as the pumps, chillers, and cooling towers properly, especially when combined with a quality control system. A quality system allows coordination of all aspects of the system and grants access to the internal data of the chiller.
Doing this allows data to be used as an input for creating an effective control program. By having a deep understanding of how chiller operates, it is possible to control the chiller plant in the most effective manner possible. If not enough expertise is available to efficiently control the plant, hiring a reliable controls partner is the most ideal option to achieve optimization.
This is how chiller plants work not just in Chicago but throughout the states in general. The optimization process allows buildings to provide efficient cooling amidst the hot climate in the state without taking up too much of its electrical load. But of course, that can only be achieved by hiring the right people for the job.