Energy Efficient Chillers

A chiller is a machine that removes heat from a liquid via a vapour-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. Chillers for cooling in different applications (home and industrial), for different temperature environments (low, medium and high), with different types (air, water, ammonia etc) and different shapes (process, scroll, screw etc.) have become a significant part of consumers’ lives. These devices in whatever shape and size are becoming indispensable with the changing life style – and putting a great pressure on the energy consumption/utilization – thus the environmental sustainability. There is a great need to go far energy efficient chillers. This article presents a brief summary about the need, development status and tips for making the use of chillers more energy efficient and environment friendly.

Energy consumption

Chillers often represent a plant’s single largest electric load. A 600-ton chiller can draw 0.547 kW/ton, which, at a rate of $0.08 per kWh, adds up to more than $200,000 per year in operating costs. Factor in fouled tubes, leaking refrigerant, changes in water temperature and a myriad of other variables and operating costs can escalate by 8 to 10 percent an additional $20,000 per year. Operating a chiller at its peak performance saves energy as well as maintenance costs.

It is estimated that chillers consume 20 percent of the total electrical energy generated in North America, making it very important to reduce consumption in this area. Chillers are the single largest user of electricity in most commercial and institutional facilities. In many cases, they are the single largest user of any form of energy in buildings. As energy costs continue to increase, conservation strategies have become crucial. There are a number of steps we can take – that not only will improve the operation of the chiller system but also will increase overall system energy efficiency.

Efficiency requirements

Chiller efficiencies have improved significantly during the past 10 years, mostly as a result of new refrigerants and microprocessor controls, as well as improved compressor and motor design. However, hightech chillers have narrower tolerances, so that service and upkeep are more crucial than ever for keeping them operating at peak performance. In the past, the most commonly used measure of chiller performance was fullload efficiency rating. Engineers and managers often used the efficiency rating, expressed in kilowatts per ton (kW/ton), to determine the most efficient chiller for a particular application. While the rating did indicate a chiller’s relative efficiency, it had a serious drawback. It was only valid for a chiller operating at full load.

As the load on a chiller falls, so does its operating efficiency. Most chillers operate at full load for about 1 percent of their annual operating hours. That means that for 99 percent of a chiller’s operating hours, it operates at less than its full-load efficiency.

This situation creates two problems. First, in making the purchase decision at least in part on different chillers’ full-load operating efficiency, a manager might not have selected the most efficient chiller for the application. Second, estimating a chiller’s annual energy use based on the full-load efficiency rating will result in seriously low estimates. Managers now have a better estimate of chiller performance that can correct both of these problems.

Comparison

In recent years, chillers have generally become more energy efficient, even though at the same time most of them use low ozonedepleting refrigerants with lower heat-transfer capacities. Today’s designs are 10-30 percent more efficient than previous generations of equipment. In most cases, chillers can be expected to become even more efficient over the next decade because of improved microprocessor control, superior flow and temperature measurement, and the introduction of larger heat exchangers. Different chiller designs have different areas of strength, although some are inherently more efficient in terms of energy units consumed per unit of chilled water produced.

• Water-cooled chillers, which use water to remove the heat, are twice as energy efficient as air-cooled chillers, which use the flow of outside air to remove heat from the chiller.
• Newer chillers often have variable frequency drives whereas in older systems, cooling tower fans run continuously at full speed, causing excess energy and money to be wasted.

Useful strategies

• Purchasing the most efficient chiller is only the first step in minimizing energy costs.
• Once installed, an organisation must maintain the chiller properly if it is to perform as efficiently as possible. In most cases, any savings produced by cutting back on chiller maintenance will be more than offset by increased energy costs.
• Maintenance and engineering managers looking for ways to improve the energy efficiency of their buildings start by improving the efficiency of chillers.
• Managers have two primary options to improving chiller performance: replacement and maintenance. Today’s replacement chillers offer managers benefits in both performance and operating efficiency, making any chiller more than 10-15 years old a replacement candidate.
• Selecting the most appropriate chiller for a particular application. Slight differences between different models can result in rather large differences in the annual energy requirements for cooling a facility.
• Managers must have enough information to determine – if reduced annual energy costs of one model are enough to justify what might seem like a significant difference in first costs.
• The best way to improve chiller compressor motor efficiency is by upgrading motor control from constant- to variable-speed using a Variable Frequency Drive (VFD). Rising energy costs and electric utility rebates for VFD upgrades, coupled with falling prices for large horsepower VFDs, can reduce investment payback times to as little as one year. Managers also should consider the option of equipping existing and replacement chillers with variablespeed drives. Conventional chillers operate at a constant speed. As the load decreases, vanes in the inlet to the compressor close, reducing the chiller’s output. This change also reduces the chiller’s energy use, but the decrease in energy use does not directly match the decrease in the chiller’s output. A variable-speed drive reduces the output of the chiller by reducing the speed of its compressor. As the chiller speed decreases, so does the compressor’s energy use. When evaluated on an annual basis, the average energy savings produced by the variable-speed drive is about 30 percent, providing a rapid return on the investment.
• One feature that has improved energy efficiency of today’s chillers is the performance of heat-transfer surfaces within chillers. But the performance of these surfaces requires that they be clean and free of buildup, including sludge, corrosion, algae, and scale. Even a very thin layer deposited on the surface can result in a significant decrease in efficiency. To keep these surfaces clean, technicians should clean the condenser tubes annually. Evaporator tubes require cleaning only every three or four years, since they function in a closed system that has limited exposure to contaminants.
• New materials are now being developed that are a step change in the amount of water they can hold. These materials are known as Metal-Organic Framework (MOF) compounds. Some of the recent ones will adsorb more than four times as much water as silica gel. As the name implies, these materials are made of various metals, such as zirconium or chromium, bound together with a loose web of carbon atoms, so that there is a much greater surface area with sites where water molecules can be bound. At present, such materials will work well for a few cycles of adsorption and its reverse, desorption but a commercial adsorption chiller needs to keep up this performance for thousands of cycles without requiring a change of the MOF. The first materials are undergoing such tests, and if they succeed we might soon see commercial air conditioning using 10% of the energy required for today’s units.

Additional tips for improving chiller efficiency

• Lowering the temperature of the water entering the condenser can improve the chiller’s efficiency.
• Flow rate must be regulated closely, because too low a flow rate reduces chiller efficiency, leading to laminar flow.
• The amount of cooling that any chiller can provide depends on how much refrigerant it moves through the compressor per unit time. It’s important to maintain the proper level of refrigerant.
• Leaks, as well as air and moisture, decrease efficiency and system reliability.
• For efficient starter and motor operation, check the safety and sensor calibrations on microprocessor controls.
• A modern and intelligent technology is to control the large power demands of screw compressors with the frequency control network, using inverter electronic devices.
• Prepackaged retrofit for centrifugal compressor chiller also said to reduce required maintenance and extend operating life. This type of package is most effective retrofitted to centrifugal compressor chillers, but can also be applied to other chiller types, including those with reciprocating, rotary-scroll, or screw compressors.

Acknowledgement

The use of information retrieved through various references/sources of internet in this article is highly acknowledged.