Strategies to improve chiller plant performance, efficiency
Learn how to design chilled water systems that meet the thermal comfort demands and achieve operational and energy efficiencies
- Learn about the impact of pumping schemes and plant optimization of chilled water systems.
- Understand how and when to consider a waterside economizer.
- Review how and when to deploy a heat recovery chiller.
For many buildings, the chilled water system provides tremendous potential for creating energy savings. However, because of the role the chilled water system plays in thermal comfort of the building occupants, those potential energy savings strategies are not always pursued in favor of traditional approaches. It is possible to design chilled water systems that meet the thermal comfort demands of the building and achieve operational and energy efficiencies that can significantly decrease ongoing operational costs.
Chilled water distribution
The chilled water distribution system must be evaluated before a new chiller plant design or existing chiller plant upgrade can be finalized. There are several factors to consider including:
- Existing or proposed design delta T, or lower water return temperatures.
- Maximum and minimum chilled water supply temperatures.
- Type of chilled water system control valves, installed or proposed (three-way or two-way valves).
- Significant pressure drop differences in the chilled water piping distribution loops.
- Terminal equipment, proposed or installed.
The impact of these criteria will guide the chilled water plant production decisions and the most efficient pumping arrangement.
The most common types of chiller plant pumping arrangements are constant flow, primary-secondary variable flow and variable primary flow systems. For the vast majority of chilled water plants, the energy efficiency of the plant can be maximized by varying the pumping capacity to match the required thermal load. When the pumping capacity matches the thermal load, it increases the temperature difference between the chilled water supply temperature and chilled water return temperature.
This is known as the chilled water system delta T, and the higher the delta T, the lower the pumping energy required for the system. Increasing the temperature difference between the chilled water supply and return takes full advantage of the total capacity of the chillers; variable primary flow systems typically have a lower first cost than primary-secondary variable flow systems.
Upgrading an existing constant flow or primary-secondary flow chilled water plant to a variable primary flow chilled water plant that is connected to a distribution system with three-way valves would result in a constant flow system with a low delta T, for a large range of the chilled water plant’s operation. Providing a variable flow chilled water plant that is connected to a chilled water distribution piping network with two or more substantially different pressure drops could result in significantly less pump energy savings and the potential for the existing control valves leaking by in the lower pressure drop chilled water loop.
Alterations in the existing distribution system are required in many chiller plant upgrades and they should not be overlooked in the proper design of an upgraded plant. Changing the three-way control valves to two-way control valves and evaluating the use of two-way pressure independent control valves will solve many of these distribution issues. The existing chilled water coils were likely not selected to perform with the 2019 edition of ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings required 15°F temperature difference between entering and leaving water temperature.
Evaluating the existing chilled water coils at varying chilled water supply temperatures is required to determine if the coils must be replaced or what temperature differences can be achieved with the existing coils (see Figure 1).
Once the chilled water distribution parameters are understood, the chilled water pumping arrangement can be designed. A variable primary flow pumping system is typically the most energy-efficient system and provides the benefit of fewer pumps in the system. Operating the variable primary pumps in parallel to match the optimum efficiency point on the chilled water distribution system curve is an effective way to minimize the system pumping energy.
Several pump manufacturers offer sensorless pumps with integral variable frequency drives that have the pump curves implanted in the pump VFD, and can operate single or multiple pumps at the most efficient point on the system curve. These pumps are a very cost-effective way to limit the number of field mounted sensors and controls while minimizing pump energy usage.
Variable flow condenser water systems are also a way to reduce the total pump energy used in the chilled water plant. Care must be taken when reducing the flow in a condenser water system to avoid suspended solids from settling out in the system. Minimum flow rates are important to maintain in the cooling towers to ensure that the cooling tower fill remains fully wetted. Minimum flow rates must also be maintained within the condenser section of the chiller. Even with the potential concerns, variable flow in the condenser water system is still a viable option and can further reduce the overall kilowatt per ton of chiller water produced throughout the entire range of plant operation.
Chiller plant optimization
Optimization is the action of making the best or most effective use of a situation or resource. What this means for a chilled water plant, as dictated by ASHRAE Standard 90.1 and the International Energy Conservation Code, is controlling the associated equipment, whether new or existing, to operate as efficiently as possible and ultimately consume the least amount of energy, while meeting the building needs. There are different levels of optimization currently being applied in the industry ranging from simple sequencing of the equipment to the installation of electrical usage metering to enable system adjustments in real time through software.
Currently, some controls manufacturers integrate plant optimization into their standard control package. This is typically limited to inputting project specific equipment performance data into the control software, which will, in turn, sequence a specified number of chillers, cooling towers and pumps based on operational “sweet spots” to meet building load. This could also include using control sequences such as pump differential pressure reset and optimum start controls for systems using setback control.
The next level of optimization is through standalone software packages, which operate in the background using proprietary algorithms and work in conjunction with the building management system. This typically involves the installation of electrical energy usage meters for real time data collection in determining equipment sequencing as well as implementing predictive actions based on the software algorithms.
Equipment manufacturers are also starting to include aspects of optimization into their onboard controls as well. For example, a centrifugal chiller with multiple compressors having the ability to stage them on and off based on operating at the lowest kilowatts per ton possible.
From an owner’s perspective, implementing some form of chilled water plant optimization can be appealing for a couple different reasons. For example, referencing strategies in ASHRAE 90.1, this could mean using pumps with integral VFDs for a variable flow system or using chilled water reset in a system with integrated waterside economizer as described in the section below. There is the obvious reduction in energy usage, which directly translates to dollars saved with the utility company.
Optimization is also appealing because it tends to prolong the life of the installed equipment. To truly understand the benefits of chiller plant optimization, it is recommended to complete a baseline analysis of the existing system or new installation to help validate the benefits to system performance. Establishing a baseline is an important aspect of this process especially as it relates to return on investment as there is a premium associated with chilled water plant optimization.
An important aspect to note is owner and plant operator buy-in to the software to allow it to operate as intended. For example, in a scenario where two chillers are operating, the software may sequence three chilled water pumps online where traditionally there may only be two. This would happen because three pumps operating at a lower frequency may use less energy that two pumps operating at 60 hertz. Scenarios like this can be difficult for operators to accept after operating in a more traditional way for many years.
The best results from optimization are achieved when all of the system equipment is sized appropriately to meet the actual chilled water demand and not over or undersized. It is common that equipment in older chilled water plants were selected based on the peak load and not the total operating range of the plant. Those plants were often designed as constant volume systems, so a load study that considers the actual program of the building is recommended before sizing a plant upgrade and/or replacement.
The load study for a new building is easier to achieve. Understanding the actual building load so that equipment can be right-sized is critical. This allows the software to sequence the equipment so it can operate most efficiently for longer periods of time throughout the year, thus providing a greater overall percent reduction in energy usage.
Waterside economizer uses the evaporative cooling capacity of the cooling tower to produce cold water that is exchanged through a heat exchanger to provide chilled water that offsets the need for mechanical cooling. In climate zones without significant year-round high relative humidity, integrated waterside economizers can provide significant energy savings by reducing the hours of operation of chillers and by reducing the chiller load during hours when 100% economizer isn’t possible.
The benefits of waterside economizers increase with warmer chilled water supply temperatures, so they pair especially well with hydronic systems such as radiant cooling, chilled beams and dedicated outdoor air system fan coil boxes, where air-side economizers are either not applicable or not feasible.
In other scenarios where traditional air-side economizers are not ideal, such as climate zones where an outside air economizer would introduce too much dehumidification load or mission critical data centers where excessive outside air may reduce the interior relative humidity too low, waterside economizers may be used to achieve significant savings. Like all heating, ventilation and air conditioning system selections, it is important to understand the impact on all systems together, including building enclosure, building massing, load profile and occupant comfort expectations.
When waterside economizers are optimized alongside each of these influencing systems, then the potential benefits of waterside economizing only increase (see Figure 2).
Traditional chilled water systems
Traditional chilled water systems producing 42°F to 44°F chilled water will be limited in how many hours they can take advantage of 100% waterside economizer, especially when the engineer has specified a traditional cooling tower approach of 6°F to 7°F and required a plate and frame heat exchanger with its 1°F to 2°F approach. This may leave the system able to operate at 100% economizer mode only when wetbulb temperatures are at or below 36°F. A traditional chilled water design approach in a building with high internal loads, such as an office building results in a low percentage of operating hours that can be used for 100% economizer mode.
Although cooling tower cost goes up as the cooling tower approach decreases, each project team should evaluate the cost benefit analysis to select close approach towers in the 2°F to 3°F range. This increases the number of full economizer hours and will further reduce the operating hours on the chillers and their corresponding energy use.
Mild temperature chilled water systems
The real beauty of waterside economizers is on display when they are paired with mild temperature chilled water systems. Instead of operating in the 42°F to 44°F range, these systems tend to operate around 54°F to 58°F and supply radiant cooling systems, chilled beams or sensible only DOAS fan coil boxes. Typically, these systems are working in parallel with a DOAS system, which is handling dehumidification with a direct expansion system or standalone low-temperature chilled water coil supplied by a separate system.
As radiant systems, chilled beams and DOAS fan coil boxes are designed for sensible cooling only, they do not require low-temperature chilled water and in fact don’t want chilled supply water temperatures which could result in condensation. So, the elevated chilled water temperatures are ideal. These increased supply water temperatures greatly increase the available hours for 100% waterside economizer, showing economizer hours with a traditional approach cooling tower (see Figure 3).
When you pair these systems with close approach towers, you can see dramatic increase in hours of full economizer mode. This brings the total hours available for full economizer up over 80% of hours in Oakland, Calif. (see Figure 4).
Advanced waterside economizer strategies
Besides selecting close approach towers, there are several other strategies that can be deployed to increase waterside economizer hours, reduce chiller hours and possibly eliminate the need for compressor cooling all together. The first strategy is a chilled water supply temperature reset control sequence (ASHRAE 90.1-2019 Part 188.8.131.52), which should be deployed on all waterside economizer systems.
In this scenario, the BMS monitors all cooling valve positions. As soon as all chilled water valves are less than 100% open, the BMS will linearly reset the chilled water supply temperature upward until the first valve must open 100% to satisfy the local load. This can result in significant increased hours with full economizer, especially in buildings with high-performance enclosures and most buildings in the shoulder seasons, when envelope loads are low.
Additionally, waterside economizer systems pair well with thermal energy storage systems, especially mild temperature systems serving sensible only cooling systems. Thermal energy storage systems maximize the use of nighttime charging of the storage tanks when outside wetbulb temperatures are at their lowest, allowing for low cost chilled water production using nighttime off-peak power rates. If the building has been designed to be a low-load, high-performance building, teams may be able to install sufficient thermal storage to remove the need for chillers altogether to meet the sensible building load.
Although the typical thermal storage medium is water (or ice for low-temperature chilled water systems), recent research from the University of California, Berkeley’s Center for the Built Environment has shown significant flexibility in mass-radiant cooling systems to support load shifting through controls manipulation alone and the inherent thermal mass of the slab. That flexibility has shown that in some instances, active cooling into the slab may shift upward of 12 hours separation from the time of peak load in the space, while still keeping the space operative temperature with the comfort range expected by ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy.
Adding ceiling fans into the space, which with modest air-speeds support thermal comfort even up to 78°F room setpoints may increase that load shifting flexibility even more, potentially allowing 100% of cooling hours to be met with full waterside economizer.
Heat recovery chillers
Heat recovery chillers can provide energy savings in facilities where there is a need for simultaneous heating and cooling, such as hospitality and health care facilities. While six-pipe, dual-condenser heat recovery chillers are available, this discussion focuses on four-pipe, single-condenser heat recovery chiller applications.
A standard water-cooled chiller operates to remove heat from a chilled water loop and transfers that heat into a condenser water loop. The heat is then rejected from the condenser water loop to the outdoors by a cooling tower. The waste heat that is normally rejected to the outdoors can be recovered and used in applications where heat is required, such as heating domestic water or terminal reheat.
A heat recovery chiller is designed to provide both heating hot water and chilled water. The waste heat that is removed from the chilled water loop is captured in a hot water loop that is used for heating. When specifying a heat recovery chiller, it is important to consider the baseline heating and cooling load profiles of the building to properly size the heat recovery chiller.
When considering a heat recovery application, always select the lowest practical heating temperature to meet the needs. Space heating systems are normally designed at 140°F supply water temperature. Typically, heat recovery chillers are designed to provide hot water for space heating at 105°F to 110°F. To accommodate this lower water temperature, terminal reheat systems can be designed to operate with 110°F water when specified with higher capacity, multiple row heating coils.
Another application such as service water preheating normally uses heat recovery water temperatures of 85°F to 95°F. Selecting the lowest practical heating temperature reduces the chiller lift and results in the chiller operating more efficiently.
Heat recovery chillers can be very effective in health care facilities. Hospitals typically have large variable air volume air handling units that provide cooling and dehumidification and deliver air at a temperature of approximately 55°F. To help with infection control, clinical spaces within health care facilities are required to have minimum air change rates. As a result of minimum air change rates, rooms are often provided with more air than is needed for cooling the space. To counter this overcooling, terminal reheat is required. As a result, reheat energy has historically been one of the largest end uses of energy in a hospital, representing 25% to 30% of the total annual energy usage depending on the climate zone.
A heat recovery chiller that is sized to provide the terminal reheat load during summer operation can offset the reheat load entirely while also providing chilled water and reducing the demand on the main chiller plant. During winter operation, the heat recovery chiller can operate to meet the process cooling loads of the hospital while also providing hot water to reduce the demand on the boiler plant. Essentially, the building owner gets heat energy at virtually no cost because it is a byproduct of the cooling process.
Chiller plant design can have a significant impact on the ongoing operating costs of a building. Strategies such as chiller plant optimization, water side economizer and heat recovery chillers can create positive results by improving overall plant efficiency and reducing energy costs. The type of building, climate and load profile are contributing factors into whether one or all of those strategies should be considered.