Elsevier

Energy and Buildings

Volume 43, Issue 12, December 2011, Pages 3646-3656
Energy and Buildings

A fault-tolerant and energy efficient control strategy for primary–secondary chilled water systems in buildings

https://doi.org/10.1016/j.enbuild.2011.09.037Get rights and content

Abstract

Primary–secondary chilled water systems with decoupled bypass in building air-conditioning often cannot work in healthy condition as desired in practical operation, due to excess secondary flow demand resulting in deficit flow in the bypass line. This paper presents a fault-tolerant and energy efficient control strategy for secondary chilled water pump systems to solve this operation and control problem providing enhanced operation performance and energy efficiency of chilled water systems. The strategy employs the flow-limiting technique that ensures the water flow of secondary loop not exceed that of the primary loop while still maintaining highest possible delivery capacity of cooling to terminals. The strategy is also integrated with a differential pressure set-point optimizer to determine the optimal set-point. The performance of this strategy is evaluated in a simulated real-life environment representing the chilled water system in a super high-rise building by comparing it with two conventional control strategies. Results show that the proposed strategy can effectively eliminate the deficit flow at both starting and normal operation periods. The energy saving in the secondary chilled water pumps can be up to over 70% and 50% at system starting and normal operation periods respectively compared with the other strategies.

Highlights

► A fault-tolerant control strategy for secondary chilled water pump systems. ► The strategy eliminates deficit flow in bypass line and enhances energy efficiency. ► A flow-limiting technique ensures secondary loop flow not exceed that of primary loop. ► It still maintains highest possible delivery capacity of cooling to terminals. ► Over 50% and 70% of chilled water pump energy saving at starting and normal operation.

Introduction

Over the last decades, primary–secondary chilled water systems have been widely used in commercial buildings. In a typical primary–secondary chilled water system, the primary constant speed pumps ensure the chillers operate with constant flow rate, and the secondary variable speed pumps vary the flow rate according to the cooling demands of the terminals. It is an energy efficient configuration when compared with a constant flow system [1]. While in real application, most of the primary–secondary systems, from time to time, cannot work as efficient as expected due to the excess secondary flow demand, which causes deficit flow problem (i.e., the required flow rate of secondary loop exceeds that of the primary loop). The excess return water flow rate will flow through the bypass line and mix with the main supply chilled water, resulting in increased temperature of water supplied to building and thus higher flow demand from terminals. Since the cooling coils are selected to produce a temperature rise at full load that is equal to the temperature differential selected for the chillers. The flow rate of secondary loop should be therefore equal to that of the primary loop under full load condition and should be less than that of primary loop under part load condition. When the deficit flow problem exists, the temperature differential produced by the terminals will be much lower than its design values, which is known as low delta-T syndrome [2], [3], [4], [5]. Kirsner [2] pointed out that low delta-T chilled water plant syndrome exists in almost all large distributed chilled water systems.

The deficit flow may cause a series of operational problems, such as the high supply water temperature, the over-supplied chilled water, and the increased energy consumption of the secondary pumps. If such phenomenon cannot be eliminated, a vicious circle in the secondary loop may be caused. It means that, when the deficit flow occurs, the mixing of the return chilled water to the supply chilled water results in higher temperature of chilled water supplied to the terminal air-handling units (AHU). The increased temperature in the supply chilled water consequently leads to an increased chilled water flow rate which further worsens the deficit flow. The deficit flow will not disappear until the flow rate in the primary loop is increased greatly (e.g., an additional chiller is switched on).

Many possible reasons of the deficit flow problems have been studied in previous studies [6], [7], [8]. Taylor [7] pointed out that some causes can be avoided, such as improper set-point or controls calibration, the use of three-way valves, improper coil and control valve selection, no control valve interlock, and uncontrolled process load, etc. While some causes cannot be avoided, such as reduced coil effectiveness, outdoor air economizers and 100% outdoor air systems.

In the last two decades, some researchers and experts in the HVAC field have devoted considerable efforts to deal with the low delta-T syndrome and deficit flow problem [9], [10], [11]. Among the studies, Kirsner [2] stated that the standard primary–secondary chilled water design cannot solve the low delta-T syndrome and a new paradigm with variable-flow primary pumps should be adopted for chilled water design. Fiorino [10] indicated strongly that a higher delta-T can be achieved by proper application of cooling coils, controls systems, distribution pumps, and piping systems. Up to 25 practical methods are recommended to achieve high chilled water delta-T ranging from component selection criteria to configurations of distribution systems.

Besides the approaches concerning the design and commissioning, the implementation of check valve on the bypass line has attracted more attention. The check valve actually is a one way valve, which only permits the water flow direction from the supply side to the return side and avoids the flow at the reverse direction in the bypass line. Kirsner [4] analyzed the advantage the use of check valve in the bypass line and thought that installing check valve in the bypass line is a cheap and a simple improvement to primary–secondary design of chilled water plants that allows a plant to deal with low delta-T syndrome. Avery [5] presented a case study that installing a check valve in the bypass line of a primary–secondary chilled water system that serves a HVAC system and meets industrial requirements. The results showed that the secondary supply water temperature stabilized at 7 °C and the annual chiller power consumption decreased by 20% per year. On the contrary, the disadvantages of check valve application arouse the concerns of researchers [12], [13], [14], [15]. Taylor [12] pointed out that the secondary pumps will be deadheaded if the primary pumps are off and chiller isolation valves are closed while the secondary pumps are on. Rishel [15] stated that check valve may not be the answer to the low delta-T syndrome under some situations, like the system utilizing the energy storage system or water side economizers.

All these studies demonstrate that low delta-T syndrome and deficit flow problem widely existed in the primary–secondary chilled water system and the elimination of this problem can improve the energy efficiency of the chilled water system. However, most of the studies pay more attention to analyzing the possible causes and solutions of this problem from the view of design and commissioning. In practice, even the HVAC systems were properly designed and well commissioned, deficit flow still cannot be completely avoided in the operation period due to some disturbances, e.g., improper control. There are no reliable, robust and secure solutions that can eliminate deficit flow in real applications. The research associated with proper control of secondary pumps to eliminate deficit flow and low delta-T syndrome for real applications is missing. Further more, many of the proposed solutions from the viewpoint of design might be only feasible to be adopted in new systems, while solutions from the viewpoint of operation and control are practical and preferable for the large number of existing systems suffering from the deficit flow and low delta-T syndrome.

Therefore, this paper aims at developing a fault-tolerant and energy efficient control strategy for secondary pumps to eliminate deficit flow problem of the primary–secondary chilled water systems concerning both operating efficiency and tolerance to unhealthy and faulty balances of chilled water systems. The performance of this strategy is validated in the case studies on a simulated dynamic system constructed based on a real system in a supper-high-rise building in Hong Kong.

Section snippets

Formulation of the fault-tolerant control strategy

In previous studies, a number of researchers have paid great efforts on the energy efficient control and operation of variable speed pumps to enhance their energy efficiencies [16], [17], [18], [19], [20], [21]. However, when the deficit flow occurs, these control strategies cannot handle this problem, resulting in the fact that the chilled water system is hard to be controlled as the anticipation with high robustness and energy efficiency in real applications. The developed fault-tolerant

Test platform

It is hard to compare various control strategies in real air-conditioning systems due to its extreme complexity. The proposed fault-tolerant control strategy was validated and evaluated using a dynamic simulation platform representing a typical chilled water system for a building, as shown in Fig. 6. This is a typical primary–secondary chilled water system, in which two water cooled centrifugal chillers with rated cooling capacity of 7230 kW are installed to generate the chilled water of 7 °C at

Performance tests and evaluation of the fault-tolerant control strategies

In order to compare the control performance and energy efficiency of the chilled water system before and after the utilization of this proposed fault-tolerant control strategy (Strategy #3), two conventional control strategies are used for comparison, as shown in Table 2. In the first conventional control strategy (Strategy #1), secondary pumps are controlled to maintain a fixed differential pressure of the remote loop, which is the upper limit of the differential pressure set-point constrains

Conclusions

A fault-tolerant control strategy for secondary chilled water pumps is developed not only for eliminating the deficit flow but also for enhancing the energy efficiency of the chilled water distribution systems. This fault-tolerant strategy employs the developed flow-limiting technique that is activated when deficit flow tends to occur and eliminates it by resetting the differential pressure set-point for pumps control. This strategy also integrates optimal differential pressure set-point that

Acknowledgements

The research presented in this paper is financially supported by a grant (PolyU5308/08E) of the Research Grant Council (RGC) of the Hong Kong SAR, a grant of The Hong Kong Polytechnic University. The work is also supported by Sun Hung Kai Real Properties Limited.

References (22)

  • Z.J. Ma et al.

    Energy efficient control of variable speed pumps in complex building central air-conditioning systems

    Energy and Buildings

    (2009)
  • S.W. Wang

    Intelligent Buildings and Building Automation

    (2010)
  • W. Kirsner

    Demise of the primary–secondary pumping paradigm for chilled water plant design

    HPAC Engineering

    (1996)
  • J.P. Waltz

    Variable flow chilled water or how I learned to love my VFD

    Energy Engineering

    (2000)
  • W. Kirsner

    Rectifying the primary–secondary paradigm for chilled water plant design to deal with low ΔT central plant syndrome

    HPAC Engineering

    (1998)
  • G. Avery

    Controlling chillers in variable flow systems

    ASHRAE Journal

    (1998)
  • McQuay

    Chiller Plant Design: Application Guide AG 31-003-1

    (2002)
  • S.T. Taylor

    Degrading chilled water plant delta-T: causes and mitigation

    ASHRAE Transaction

    (2002)
  • T.H. Durkin

    Evolving design of chiller plants

    ASHRAE Journal

    (2005)
  • D.P. Fiorino

    Achieving high chilled-water delta Ts

    ASHRAE Journal

    (1999)
  • D.P. Fiorino

    How to raise chilled water temperature differentials

    ASHRAE Transactions

    (2002)
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