Water-side Economizer Research Articles

Development of near-optimal advanced control sequences for chiller plants with water-side economizers in U.S. Climates (ASHRAE RP-1661)

Various advanced control sequences for chiller plants with water-side economizers (WSE) have been proposed in literature, but the evaluation and optimization of those controls is limited. It is possible to maximize energy savings by selecting different sequences and related parameters based on the plant configuration, load, and climate. This paper addresses this gap by developing near-optimal advanced control sequences for chiller plants with WSEs. First, advanced control sequences for chiller plants with WSEs are categorized into condenser water, chilled water, and hybrid controls and representative sequences from each category are identified. Next, 504 different scenarios are optimized. These scenarios represent all possible combinations of two plant configurations, a constant or variable load profile, three advanced control sequences, and seven optimization parameter combinations in six climate zones. The results show the recommended near-optimal sequences can reduce energy consumption by up to 15% relative to the baseline depending on the configuration, load profile, and climate. Specifically, the CW-CHW sequence is recommended for the majority of systems because it is often the most energy efficient and/or reduces the runtime of chillers. The methodology in this paper provides practical guidance for achieving energy savings through near-optimal control of chiller plants with WSEs.

Energy-saving and economic analysis of the data center cooling system using magnetic bearing chillers under different climate conditions

Magnetic bearing chiller has excellent Coefficient of Performance, but the high initial investment makes it challenging to find a balance between energy-saving efficiency and economic potential. This paper proposes two energy-saving retrofits with water-side economizers, using efficient centrifugal chillers (Plan 1) and magnetic bearing chillers (Plan 2), respectively, to replace the existing cooling system of a case data center in Nanchang. Energy models of three cooling systems are developed to analyze the energy efficiency and performance. Compared to the existing cooling system, the operation time of Full free cooling and Partial free cooling for Plan 1 and Plan 2 reach 2489 and 1324 h, respectively. Result in energy savings of 63.19 % and 64.54 %, and power usage effectiveness reduce 0.341 and 0.349, respectively. Because the annual chiller COP of Plan 2 is 6.7 % higher than that of Plan 2, Plan 2 saves additional 3.96 % annual energy consumption compared to Plan 1. Furthermore, the energy-savings and economy of the retrofit plans in five typical climate cities in China are discussed. The energy savings rates for Plan 1 and Plan 2 are 61.4%–64.7 % and 62.1%–65.6 %, respectively, and their dynamic payback periods are 2.02–5.15 years and 2.16–5.6 years.

Research on a Plan of Free Cooling Operation Control for the Efficiency Improvement of a Water-Side Economizer

Research on a Plan of Free Cooling Operation Control for the Efficiency Improvement of a Water-Side Economizer

Climate zones for the application of water-side economizer in a data center cooling system

Water-side economizers have been widely adopted in response to the growing energy consumption in data centers. Although many studies have recognized the energy-saving effects of water-side economizers under different climate conditions, their water utilization is still unclear. This study combines energy savings and water consumption to assess the applicability of water-side economizers in 34 cities in China under two sets of chilled water supply and return water temperatures. The energy savings when the supply and return water temperature is 21/27 °C are 3.17 times higher than when it is 12/18 °C. Nine cities can reduce water consumption while pursuing energy savings. To further quantify the applicability of water-side economizers, this work introduces a new indicator, water to energy saving ratio, and utilizes K-means clustering to conduct zoning studies on different cities. The results show that the applicability of water-side economizers in different regions can be divided into four zones. In Zone I, the applicability of water-side economizers is the lowest, and it is recommended that they be combined with high-efficiency heat transfer technologies. Zone II comprises 12 cities, with moderate applicability of water-side economizers. In Zone III, there is a need to balance the relationship between water and energy. Zone IV has the lowest energy and water consumption, with an average of 23% water resource savings compared to Zone I. However, attention should be paid to preventing the freezing of cooling towers in winter.

Evaluation of energy performance and ecological benefit of free-cooling system for data centers in worldwide climates

Water-side economizer (WSE) is an effective free-cooling solution aiming to enhance the natural cooling duration and in turn reduce the cooling-energy consumption of data centers. However, the system's performance is constrained by the limited understanding of its control mechanisms and the influence of local climatic conditions on its free-cooling capabilities. To overcome these limitations, a thermodynamic model of the WSE system with control strategies is provided and its adaptability performance in diverse worldwide locations is systematically evaluated. Modelica-based models of chiller plants and control strategies are firstly developed, and then the energy consumption and ecological benefit of the WSE system are investigated via large-scale simulations in worldwide climates. The performance indicators and application potential of the system are demonstrated through visualization maps. The analysis reveals significant energy savings potential of the WSE system, with the capacity to reduce annual cooling energy consumption of data centers by 4.4–33.7 % in hot to marine zones, and even by a remarkable 62.6 % in plateaus and cold zones. Furthermore, it is observed that every 1 °C increase in outdoor air temperature can reduce energy efficiency ratio by 1.3–8.7 % while simultaneously increase PUE by 0.3–1.0 %. The ecological benefit results revealed that the WSE system can effectively harness larger quantities of stable free-cooling resources in mixed and cool climate zones, exceeding its capabilities in hot and warm regions. These results offer valuable insights for the impact of climate on the energy and ecological benefits of WSE system for data center, which can be helpful to guide the design or retrofit of such cooling systems in specific locations, presenting an immense opportunity to maximize cooling-energy savings.

Model-based data center cooling controls comparative co-design

This article presents a comparative simulation-based control logic design process. It uses the Control Description Language (CDL) and the ASHRAE Guideline 36 high-performing building control sequences with the Modelica Buildings Library (MBL) to demonstrate a comparative analysis of two control designs for a data center chilled water plant. Details include a description of the closed-loop plant and control design methodology, including sizing and parameterization, base and alternative (Guideline 36) control logic with software implementation structure, and outline of the simulation experimentation process. The selected control designs are paired with comparable chilled water plant configurations. The models include a chiller, a water-side economizer, and an evaporative cooling tower. The plant provides cooling at 27ºC zone supply air temperature to a data center in Sacramento, CA. The comparative simulation results examined the impacts of a selected control logic detail, and present an example model-based design application. Overall, the simulation results showed a 25% annual and a 18% summer energy use reduction for alternative controls. This shows that simulation-based control logic design performance evaluation can improve energy efficiency and resilience aspects of system controls at large.

Comparative study on different energy-saving plans using water-side economizer to retrofit the computer room air conditioning system

An energy efficient, environmentally friendly and economical retrofit plan of traditional computer room air conditioning system is necessary to reduce energy consumption of data centers. In this study, two energy-saving retrofit plans, a computer room air handler system with water-side economizer (Plan 1) and a loop thermosyphon system with water-side economizer (Plan 2), are compared with an air-cooled computer room air conditioning system (existing cooling system) in a data center. To analyze and compare the energy efficiency, environmental and economic potential, energy models of the existing cooling system and retrofit plans are established. Under different set points (including supply chilled water temperature and supply air temperature) and 2.08–6 kW average electric power of racks, the free cooling periods of Plan 1 and Plan 2 reach 3878–5867 h and 4541–7661 h, respectively, which lead to the annual energy consumption of Plan 1 and Plan 2 decreases by 27.6–47.3% and 53.4–63.6%, respectively. Due to the lower annual energy consumption of main equipment (including chillers, chilled water pumps, cooling water pumps and terminal equipment), Power Usage Effectiveness of Plan 2 is 0.049–0.090 lower than that of Plan 1, and Global Coefficient of Performance of Plan 2 is 23.3–67.3% higher than that of Plan 1. Furthermore, the annual CO2 emissions of Plan 1 and Plan 2 are 337.3–557.1 tons and 599.8–1106.9 tons less than those of the existing cooling system at different operating stages respectively. Finally, the dynamic payback periods of Plan 1 and Plan 2 are 4.11–5.42 years and 1.64–3.57 years.

데이터 센터의 공냉식 프리쿨링 냉동기 에너지 효율 분석

The number of data centers and the amount of data processed worldwide has exploded. However, the growth rate of energy consumption in data centers has shown a different pattern (from 194 TWh in 2010 to 205 TWh in 2018) which is attributed the spread of uninterruptible power supply (UPS) and increase in cooling efficiency.SUP(1)/SUP The decrease in electric power consumption due to the increase of such high-efficiency data centers can be confirmed through previous studies.SUP(2)/SUP There are various cooling methods that benefit from the increase in semiconductor performance in data centers.SUP(3)/SUP Recently, domestic data centers have also introduced systems that employ free cooling. In this study, the energy performance of an air-cooled free-cooling chiller and a general air-cooled chiller, in which water-side economizer (WSE) and a chiller are integrated, was compared based on historical data in Seoul, Korea. In addition, the power consumption and the cold water conditions of the data center were compared between the chillers. Finally, the energy efficiency was analyzed accordingly.

Model-based predictive control optimization of chiller plants with water-side economizer system

A chiller plant with a water-side economizer (WSE) system is an environmental technology that utilizes the low-grade natural cooling source to optimize the cooling supply and reduce the energy consumption. Many studies focused on the real-time optimization of conventional chiller plants, but few researchers aimed at optimizing chiller plants with a WSE system. There is also a lack of optimization platforms that can provide a systematic evaluation of the energy saving potential of WSE under different optimization scenarios. Therefore, this study proposes a model-based predictive control (MPC) method to optimize multiple parameters of the WSE system. First, WSE system models and an optimization platform were developed using the equation-based Modelica modeling method. An actual real-world chiller plant system was considered to validate the chiller plant models. Then, the energy saving potential was demonstrated by considering 15 simulation cases, implementing three optimization strategies and five optimization frequencies. The obtained results indicated that the hourly optimization strategy can achieve the maximum energy saving potential of around 14.3% compared to the baseline model, followed by the daily optimization, with a reported 12% energy savings. The difference in energy consumption between the other three optimization frequencies was found to be small. The largest chiller plants’ energy efficiency ratio was 7.04 when performing hourly optimization for multiple parameters, which is 16.7% higher than that obtained by running annually optimization for a single parameter. Although multi-parameter optimization can save more energy than single-parameter optimization, it is time-consuming and requires extensive computational resources. The proposed MPC optimization method can provide a reference for engineers to select appropriate optimization parameters and frequency to be used for applications in practice.

Resilient cooling through geothermal district energy system

Decarbonization and resilience to heat waves have recently become high priorities for building and district energy systems. Geothermal coupled district heating and cooling systems that operate a water loop near ground temperature gain increasing adoption to support decarbonization. In these systems, vapor-compression machines, distributed in the energy transfer stations, lift the temperature up or down to the needs of the particular building. In principle, these systems can provide low-power, free cooling from the geothermal bore field during heat waves when electricity is often scarce. However, the performance of such a resilience operation mode and its implication on the energy system configuration and the sizing of the bore field and HVAC equipment is not yet understood. Consequently, we are assessing their resilience, power use and design implications under a scenario of a heat wave on five working days during which chillers are switched off to reduce electrical consumption. Our analysis is based on high-fidelity, coupled dynamic models of district energy, building-side HVAC and actual control logic, with whole building energy simulation used to assess thermal conditions in a 2004 vintage multi-zone office building in Chicago, IL. The results show that relying only on waterside economizer cooling, the indoor thermal conditions can be maintained in a tolerable range for the majority of the building zones with half the electrical energy compared to standard chiller operation. Thermal comfort in the hottest zones can be further improved by oversizing the cooling coil. However, the waterside economizer has significant implications on the system configuration and sizing: The geothermal bore field needs to be sized about 30% larger than the upper limit of the range observed for conventional geothermal systems. Nevertheless, if a central chiller plant is added, the bore field can be downsized to the typical design range. The latter configuration still allows compressor-less cooling during the heat wave with peak power reduced by 60% compared to the standard design and chiller operation. • Geothermal coupled energy systems improve resilience of buildings during heat waves. • Demonstrated acceptable thermal comfort in typical office building during heat wave. • Integrated waterside economizer in ETS of ambient loop district energy network. • Used model-based HVAC and controls co-design for economic and reliable operation.

Modelica-based modeling and simulation of district cooling systems: A case study

While equation-based object-oriented modeling language Modelica can evaluate practical energy improvements for district cooling systems, few have adopted Modelica for this type of large-scale thermo-fluid system. Further, to our best knowledge, district cooling modeling studies have yet to include hydraulics in piping networks alongside plant models featuring realistic mechanical systems and controls. These are critical details to include when looking to make energy and control improvements in many physical system installations. To fill these gaps, this study released new open-source district cooling models at the Modelica Buildings Library and leveraged these models for a real-world case study at the University of Colorado Boulder. The site includes six buildings connected to a central chiller plant featuring a waterside economizer. Several energy saving strategies are pursued based on the validated model, including control setpoint optimization, equipment modification, and pump setpoint adjustments. Results indicate that a combination of the studied measures can save the campus annually 84.6 MWh of energy, 8.9% of electricity costs, 58.0 metric tons of carbon dioxide emissions, while the waterside economizer cuts down chillers’ run times by 201 days/year, reducing maintenance costs and extending chiller life.

Open-source Modelica models for the control performance simulation of chiller plants with water-side economizer

There are several cooling mode control sequences for chiller plants with water-side economizers adopted in industry and academia, and it is widely known that this supervisory control significantly affects energy consumption; however, there is a lack of a modeling resource to allow multiple control sequences to be evaluated systematically under different settings, such as system configurations, load types, and climate locations. To fill this gap, this paper develops open-source Modelica models to simulate the control and energy performance of multiple cooling mode control sequences for chiller plants with water-side economizers. These models allow users to develop and test their advanced control sequences for chiller plants with water-side economizers for their target climates and system configurations. To demonstrate how these models can be utilized, a chiller plant with an integrated water-side economizer is simulated using two advanced cooling mode control sequences, two cooling load types, and six climates, for a total of 24 simulation cases. This study revealed that the energy saving potential varied from 8.6% to 36.8% for constant load profiles in all of the considered climates, and from 6.3% to 25.8% for variable load profiles in most of the climates. Results also showed that the developed system models are able to capture transient control details and reveal counterintuitive energy performance.

A Study on the Energy Reduction Measures of Data Centers through Chilled Water Temperature Control and Water-Side Economizer

The degree of integration of IT devices and consumption of cooling energy are consistently increasing owing to developments in the data center industry. Hence, to ensure the smooth operation and fault prevention of IT devices, the energy consumption of cooling systems has increased, leading to active research on improvements in cooling system performance for reducing energy consumption. This study examines the reduction in cooling energy consumption using a simulation by applying chilled water control and a water-side economizer (WSE) system to enhance the cooling system efficiency. The simulation results showed that the energy consumption was reduced by 1.8% when the chilled water temperature was set to 11 °C in a conventional system and by up to 19.6% when WSE was also applied. Furthermore, when the changes in chilled water temperature were applied for efficient operation of WSE, the energy consumption was reduced by up to 30.1% compared to that in conventional energy systems.

Free cooling potential for Brazilian data centers based on approach point methodology

Data centers cooling loads may reach about 40% of their electricity demands. The number of that type of facility is continuously growing around worldwide and actions addressing energy saving are welcome to rationalize their operation. Free cooling appears as an effective solution to achieve energy savings, and the present work proposes a methodology for assessing data center free cooling operation based on the heat exchanger approach point. The methodology can be applied to both airside and waterside economizer modes. The proposed methodology is applied to fourteen Brazilian state capitals that represent all country climatic conditions. Free cooling environmental air temperature and humidity criteria are based on literature review and on ASHRAE thermal guidelines. Results show that cities such as Curitiba, São Paulo, Porto Alegre and Brasília can potentially operate in both airside and waterside economizer modes for more than 3000 h per year, even considering conservative data center thermal operating limits. Facilities placed on North and Northeast regions can only run under free cooling conditions if operational parameters are to be relaxed to consider more flexible operating limits. It is shown that indirect economizers are potentially more suitable to operate under ASHRAE recommended limits. The proposed methodology is shaped for evaluating free cooling hours during system conception.

Model-based optimization of free cooling switchover temperature and cooling tower approach temperature for data center cooling system with water-side economizer

Free cooling is commonly used in data centers to reduce cooling system energy consumption using a water-side economizer (WSE). In this system, cooling mode is switched when wet bulb temperature (Twb) reaches switchover temperature (Tsw), and the cooling tower fans are controlled to maintain a specified cooling water approach temperature (Tapp). Typically, both Tsw and Tapp values are fixed. However, under part load conditions, Tsw and Tapp can be optimized to maximize the system performance and save energy. Currently, there are few appropriate methods available to optimize Tsw and Tapp in partial-free cooling mode for data centers with WSE. Thus, this study presents a model based methodology to optimize Tsw and Tapp in different cooling modes. The proposed methodology is verified through a case study based on data of a real operating data center. Comparison between the calculated optimal Tsw and Tapp with that derived by simulation demonstrates the satisfactory accuracies of the proposed method. The case study illustrates that when Tsw and Tapp are optimized, the cooling system energy consumption can be saved by 10% when cooling load ratio is 0.6.

Effect of climate conditions on the thermodynamic performance of a data center cooling system under water-side economization

This paper evaluates the potential of water-side economizers in the refrigeration system of data centers under different climate conditions. Due to the wide range of conditions along the country (Desert, Mediterranean, Temperate rainy and Tundra climate), Chile is selected as case study. The number of hours per year in which economization is possible is estimated using the data base of 22 weather stations along the country. The refrigeration system is modeled in steady state through a set of thermodynamic equations simultaneously solved using the Engineering Equation Solver (EES). The system performance is evaluated by calculating the Coefficient of Performance (COP) and the Water to Energy Ratio (WER). The latter is a new metric proposed to compare the volume of water required by the system to save 1 MWh of cooling energy. The thermodynamic analysis shows that the chiller decreases its energy usage if water-side economizers are implemented in favorable climates such as cool-summer Mediterranean with winter rain (Csc), temperate rainy (Cfb) and tundra (ET) climates. Here, the use of economizers allows a monthly increase in the COP of 50 to 120% (compared to the conventional operation) and an annual average COP ranging from 7.8 to 9.7. These climates also offer an additional gain of lower water requirements, with annual WER values ranging from 11 to 17 m3/MWh. Desert climates, on the other hand, prevent implementing economizers, offering the lowest annual average COP values (5.6–5.7). In climates in which the complete economization is impossible, the partial use of economizers allows a monthly increase in the COP of 10 to 45%. The costal influence decreases the system performance, reducing the COP and increasing the WER.

Simultaneous use of air-side and water-side economizers with the air source heat pump in a data center for cooling and heating production

Due to the development of information technology, energy consumption of data centers is increasing continuously and quickly. Therefore, finding solutions for minimizing the energy consumption of data centers and reducing the greenhouse gas emissions is essential. In this paper, a combined free cooling and waste heat recovery system is proposed to reduce the electrical energy consumption of a data center in Mashhad and to save the natural gas fuel consumption of its adjoining office building, as a case study. Two types of economizers including air-side economizer (ASE) and water-side economizer (WSE) systems are used for free cooling as well as an air source heat pump (ASHP) used as a waste heat recovery system. In fact, in this study, the effect of simultaneous use of the air-side economizer (ASE), water-side economizer (WSE) and air source heat pump (ASHP) is investigated for improving the energy performance index of the data center. For this reason, a thermo-economic-environmental analysis is performed to study the performance of the proposed system. The results show that the proposed system have the electricity and natural gas saving potentials of more than 250 MWh/year and 15,000 m3/year respectively, which leads to 267 ton/year of CO2 emission reduction. Furthermore, it is found that the Power Usage Effectiveness (PUE) index can decrease down to 2.07, indicating an improvement of 16% in the calculated energy efficiency of the data center. Finally, economic analysis shows that payback period of the proposed system will be 2.5 years.

Equation-based object-oriented modeling and simulation of data center cooling systems

In this paper, we introduce a newly developed open source data center package in the Modelica Buildings library to support modeling and simulation of cooling and control systems of data centers. The data center package contains major thermal and control component models, such as Computer Room Air Handler, Computer Room Air Conditioner, models of different subsystem configurations such as chillers with differently configured waterside economizers, as well as templates for different systems. Two case studies based on this package are performed to investigate the performances of the cooling and electrical system under normal conditions and emergency situations such as a blackout: one is for a data center powered by conventional energy, and the other is for a data center powered by renewable energy. Simulation results show that the dynamic modeling and multi-domain simulation in the Modelica-based tool make it convenient for users to investigate both normal and emergent operations for conventional and renewable data centers.

Energy and economic assessment of major free cooling retrofits for data centersin Turkey

Mechanical cooling is responsible for a significant fraction of the energy consumption of data centers (DCs). Free cooling systems take advantage of ambient conditions to reduce the need for compressor-based cooling. This study utilizes thermodynamic models of major free cooling systems such as the direct air-side economizer (ASE), indirect air-side economizer (IASE), indirect evaporative cooler (IEC), and indirect water-side economizer (WSE) integrated with the existing cooling infrastructure of a typical 1 MW IT load DC. Proposed models utilize hourly weather data of various cities in Turkey to compute annual energy consumption and cost-saving potentials of each free cooling method with respect to the baseline DC with both open aisle (OA) and enclosed aisle (EA). Results confirm the energy-saving potential by IEC leading to less than 10 % of annual chiller hours across Turkey and less than 1 % in half of the ten cities studied, especially in EA DCs. However, despite greater energy-saving potential, IEC has more extended payback periods of 1.5 to 3.7 years due to the high capital investment compared with those of ASE and WSE with less than 1.4 years.

Hybrid geothermal heat pumps for cooling telecommunications data centers

The technical and economic performance of hybrid geothermal heat pump (GHP) systems supplying year-round cooling to small data centers were analyzed and compared to air-source heat pump (ASHP) cooling systems. Numerical TRNSYS models were used to simulate the operation of five configurations that included GHPs, ASHPs, air-cooled heat exchangers – dry coolers (DCs), and/or waterside economizers (WSEs). The models were validated against data measured from an experimental GHP cooling system in Ithaca, NY, USA. The average coefficient of performance (COP) and the total cost of ownership (TCO) of the optimized systems were evaluated and compared to a base case ASHP cooling system. The results showed that the GHP systems had higher lifetime average COPs than the ASHP system, while the TCOs of the GHP systems were slightly higher than the ASHP system. The reductions in cooling capacity and performance of the geothermal systems due to subsurface temperature increases were calculated and showed that the addition of a DC or WSE mitigated capacity reductions and improved lifetime performance. The economic performance of the GHP systems relative to ASHP systems were analyzed using weather data from several locations across the U.S., with sensitivities to soil conductivity, price of electricity, and cost of drilling the geothermal wells quantified. GHP systems performed best in colder climates with greater temperature variability both seasonally and diurnally.