SUSTAINABLE CHILLED WATER SYSTEMS
Making Our Chilled Water Plants More Efficient
July 10, 2007
by
Thomas Hartman, P.E., Principal
The Hartman Company
755 County Road 247
Georgetown, Texas 78628
Until recently, few designers or manufacturers thought variable speed belonged in chiller plants. Adding VFDs to chillers and permitting water flows to change were not usually recommended. Now that has all changed. Nearly all in the industry embrace the concept of chiller plants in which all the chillers, pumps and tower fans are variable speed. Such plants are called "all-variable speed" plants. Whatever the reasons for the change in attitude, simply configuring a plant as all-variable speed without careful consideration of equipment selection and operation may not yield energy and cost savings. Both designers and plant managers involved in chiller plant design and operation need to consider how to most effectively integrate these new technologies into their plants and distribution systems because they offer enormous cost reductions but only when correctly designed and operated. Integrating an all-electric, all-variable speed chilled water system utilizing configuring and control technologies derived from new principles developed specifically for variable speed can cut the total plant energy nearly in half compared to a state of the art conventional plant. Furthermore electric all-variable speed configurations are less costly, simpler to operate and maintain, and save even more energy when compared to hybrid plant alternatives. Since energy accounts for more than half the life cycle cost of a typical chiller plant, new all-variable speed technologies offer a very substantial reduction in overall plant life cycle cost.
A schematic of a typical all-variable speed chiller plant is shown in Figure 1 below. There are no substantial differences in plant equipment (chillers, condenser pumps and cooling tower) layout between an all-variable speed chiller plant and a typical conventional plant. Although it is not shown, a header pumping arrangement on chilled and condenser water systems works equally as well with an all-variable speed chiller plant just as with constant speed plants. Notice in this figure that there is only one stage of pumping and no decoupling of the distribution from the chilled water generation circuit. Incorporating determinant flows is an important element of new all-variable speed technologies so decoupling lines are not employed. Also, the relative sizing and selection of the equipment in the plant is likely to be different than in conventional plant configurations. But the real differences between a constant speed plant and an all-variable speed plant are the use of variable speed drives to operate all equipment and the DDC optimization control network that automatically coordinates the operation of the equipment under all operating conditions. Such automatic network control may or may not be employed in a conventional plant, but effective automatic control is a necessary ingredient for the success of an all-variable speed chiller plant.
Figure 1: All-Variable Speed Chiller Plant Configuration.
Evaluating Benefits of All-Variable Speed Chiller Plants
To evaluate this new approach to chiller plant design and operation, designers or plant managers must first get their arms around the potential benefits possible by building or converting their existing chiller plants or new plant designs to all-variable speed. Figure 2 below shows the average annual operating efficiency for plants with electric motor driven centrifugal chillers, which make up the majority of larger chiller plants. The efficiencies shown in Figure 2 are based on the total energy use of the plant including chillers, condenser water pumps, tower fans, and all chilled water pumping. Most existing plants have been shown to operate at or above 1.0 kW/ton. The most modern conventional plants that keep their equipment updated and are well maintained may operate in the "Fair" range from 0.85 to 1.0 kW/ton average annual efficiency. Those that also have effective efficiency monitoring and automatic optimized control systems may dip into the "Good" range, but a conventional constant speed plant that shows an annual average operating efficiency less than 0.8 kW/ton is a rarity, limited to those plants with exceptionally efficient equipment and good automatic optimization controls.
Figure 2: Chiller Plant Total Energy Use Spectrum
Now consider the range of annual efficiencies for an optimized all-variable speed chiller plant composed of the same basic equipment but operated entirely with network optimized variable speed control. Such all-variable speed plants generally operate at an average annual energy efficiency of from less than 0.5 to 0.6 kW/ton, depending on the climate, the equipment employed, the configuration, and the load profile of the load served. This means that the potential reductions for converting a well maintained and operated chiller plant to an all-variable speed chiller plant can be expected to amount to between 25% and 50% or more of the current plant energy use. The resulting annual dollar energy savings amounts to a low of $30 to over $100 per installed ton, depending on the length of the cooling season, the load profile and the electric rate. For those considering hybrid plants containing absorption or engine driven chillers, consider that all-variable speed electric cooling will nearly always be vastly more economical to operate and much less costly to maintain. Unless waste heat is available (and in many cases, even if it is), optimized all-variable speed chiller plant technologies make hybrid chiller plants obsolete. If demand limiting is mandated by rate structure or emergency cooling is needed in the event of a power failure, these can be accomplished far more cost effectively with a standby or emergency electric generator to power some of the cooling equipment. Such an approach offers better economy and greater load shaping flexibility than incorporating non-electric chillers in the plant.
All-Variable Speed for New Chiller Plants
For those with new plants in the planning or design stage, these potential energy reductions make consideration of switching to an optimized all-variable speed configuration an imperative. The marginal costs for incorporating optimized all-variable speed technologies into a new plant are very small. Variable speed chillers cost very little more than their constant speed alternatives. Applying variable speed drives to tower fans is cheaper than implementing two speed motors, and locating variable speed drives at the pumps can be less expensive than using regular starters housed in motor control centers. Payback on any extra investment required to configure a new plant as an all-variable speed chiller plant will usually be well under a year.
Conversion of Existing Plants to All-Variable Speed
Although the substantial energy reduction from all-variable speed technologies always justifies incorporating all-variable speed into a new plant, some analysis may be required to see if it can be justified as an upgrade for existing plants in good condition and efficient operation. For these plants, there are two paths to such a conversion that need to be considered. First, if the life cycle savings do justify it, an immediate retrofit involving adding variable speed drives to all the chillers, pumps and tower fans and implementing optimization controls should be considered. If not, a second path to consider is replacing chillers with variable speed chillers as each approaches the end of its useful life. In this path an upgrade of variable speed condenser pumps and tower fans along with network controls may be initiated immediately or as the new variable speed chillers come on-line.
The second approach is a fallback. It usually carries the same low marginal cost as that of planning a new plant because this path waits until the chillers reach the end of their useful life and have to be replaced anyway. If an immediate retrofit conversion to all-variable speed cannot be accomplished due to budget limitations, then the "conversion at time of replacement" should become the upgrade plan for the chiller plant.
Determining which upgrade path to follow requires an analysis of the savings potential of the conversion and the costs involved for an immediate retrofit. The most effective means of calculating the savings potential is to determine the present energy costs for the plant, estimate the present plant efficiency, and then apply the estimated efficiency improvement a conversion would bring. Such an evaluation can become more involved if the electric rate structure is dominated by a high demand charge. It may take some time to assemble the data necessary for this analysis, but it is not really complicated. It is also surprising how many resources are available to assist your efforts.
Once this energy savings estimate has been made, the next step is to estimate the costs involved in implementing a retrofit that will achieve the estimated savings. For plant owners or operators, the most direct route may be to call in manufacturers and contractors knowledgeable in the new all-variable speed technologies. With a frank discussions it is possible to get reliable budget pricing for adding the variable speed drives and necessary controls and making any other changes that may be necessary for a retrofit to all-variable speed operation. It should be noted that effective optimization control software modules are packaged technologies that are readily available at low cost. The packages include complete control sequences, performance verification, and the support necessary to ensure the plant performs as expected. With some effort, it is not too difficult to arrive at a total upgrade cost estimate by assembling the cost of upgrading the chillers, pumps and tower fans to variable speed and adding a digital control system, if none presently exists, and the necessary control software and support.
Another approach is to have outside help to assist in this analysis. However, caution is advised. Because all-variable speed technology is relatively new, many engineers are not knowledgeable about all-variable speed plant operation, but few are eager to acknowledge this reality. It takes an experienced firm to do justice to such an analysis. Chiller manufacturers are now developing computer programs that can help with the savings portion of the analysis, and a manufacturer may be able to recommend a firm to assist with other aspects of the analysis.
Distribution System Issues
A critical part of any chilled water system is the chilled water distribution system. Though chilled water distribution systems vary widely in size and configuration, problems associated with these systems are quite universal: low delta T, inability to fully load chillers, inadequate flow in sections of the distribution system, and excessive pumping pressure requirements at peak cooling demand conditions. One or more of these problems plague nearly all chilled water distribution systems. Figure 3 below shows a typical primary-secondary variable flow distribution system. In smaller systems the primary loop and the secondary distribution pumps may all be located in the plant. In a large building complex, the secondary loop may extend throughout the campus and tertiary distribution pumps may be located in the individual buildings served by the distribution system. Actual configurations may have more or less distribution circuits and usually will have multiple pumps at each pumping station. However, the lessons discussed here are generally scalable and are easy to apply to a wide variety of distribution systems.
Figure 3: Typical Chilled Water Distribution System Configuration.
In Figure 3, the primary chilled water pumps (PCHWP1 through 3) are typically constant speed pumps and the secondary chilled water pumps (SCHWP1 - 3) are variable speed pumps. The primary pumps are cycled on and off with the chiller each serves, and the speed of the secondary pumps is modulated to meet a differential pressure setpoint as measured at the end of the distribution circuit each serves. A decoupling line shown in the figure permits flow in either direction at the end of the primary circuit since the "stepped" primary flow will nearly always be different than the continuously variable secondary flow. This system is widely employed, but has two inherent problems that lead to low delta T and poor performance:
1. When primary flow is greater than secondary flow, low delta T in the primary circuit results from the recirculating primary chilled water through the decoupling line and directly back to the chillers. The lower than expected return chilled water temperature makes it impossible to fully load the on-line chillers because the primary pumps are fixed flow. This wastes energy and if it occurs at peak conditions, it robs the plant of capacity.
2. Whenever secondary flow exceeds primary flow, flow reverses in the decoupling line and is mixed to degrade the supply water temperature. This reduces the cooling capacity of the loads in distribution circuits closest to the decoupling line. The result is greatly increased flow in those circuits and reduced delta T, which also robs the system of its full capacity capabilities.
Because primary and secondary flow is almost never exactly balanced and actual delta T always varies somewhat from design, one of the two problems is almost always at play in such systems, both of which can reduce the design delta T of the system and both of which make it difficult to operate the system effectively at full capacity. A number of solutions have been proposed to correct this problem, but such "cures" often destroy the system's ability to meet the cooling load requirements. One popular method of correcting low secondary circuit delta T problems is shown in Figure 4.
Figure 4: Diagram of a Typical Delta T "Enhancement"
While the Figure 4 diagram or some variation of it is often touted as a cure for low delta T, it much more often has disastrous effects on system operation. The idea is that the diverting valve on each secondary (or tertiary in some cases) circuit return (sometimes a mixing valve is used on the chilled water supply) will modulate some return water back to the pump anytime the return temperature is below design. It is reasoned that the elevated supply temperature will raise the return temperature and ensure that the design delta T from the circuit is maintained at all times. However, this fix rarely has the desired results. When air is the medium being cooled, return chilled water temperature is much more affected by entering air temperature than chilled water supply temperature. Raising the chilled water supply temperature thus has little effect on return chilled water temperature, but it does profoundly reduce cooling coil capacity, especially latent cooling capacity. As the supply chilled water temperature rises, load valves open further and flow in the circuit increases dramatically, often without a significant increase in the return water temperature and usually with a reduction in cooling effect. Thus, when the scheme shown in Figure 4 is installed on a distribution circuit, one poorly operating load in the circuit can severely compromise the capacity of all loads in the circuit. In large systems it is also possible at times to have the flow reversal such that return chilled water from the mains travels to the supply header through the diverting or mixing valve. Thus the Figure 4 "fix", and the many schemes that are similar to it, do not fix system operation at all. Instead, it is a "poison pill" to chilled water distribution systems.