Differential Pressure Set-point Research Articles

Optimal control for district heating systems based on imbalance analysis of pipe networks

In district heating (DH) systems, the transmission and distribution process occupy large amount of energy consumption. Hydraulic and thermal imbalance in pipe networks often lead to uneven heating, affecting user comfort and resulting in energy losses. To solve this problem, this study investigates the root causes of hydraulic and thermal imbalances in DH systems. Key factors have been identified such as initial flow rates, initial loads, and positions in the network of end-users. And the relationship between hydraulic and thermal imbalances have been analyzed. Building on this analysis, we further propose a novel strategy for differential pressure set-point adjustment. This strategy is developed based on a quantitative understanding of the flow rate changes and positions of end-users. It aims to rectify imbalances efficiently, thus enhancing the overall performance of the network. The effectiveness of the proposed strategy is validated through comparative analysis with the conventional constant differential pressure (CCDP) control strategy and an exist variable differential pressure (VDP) control strategy. Results demonstrate significant improvements in network performance, including enhanced stability and reduced energy losses. The proposed strategy reduces the average pressure difference in the main pipe by up to 10.9 %, improves the supply and return temperature difference by 10.9 %, and maintains valve openings above 70 % during peak periods. Additionally, it achieves 23.5 % energy savings in transmission and distribution, highlighting its potential to enhance system performance and efficiency. In addition, this strategy is more in line with the actual situation of end-user demand change for valve adjustment.

Variable Differential Pressure Control Strategy for Variable Water Flow Air Conditioning Systems

In large-scale air conditioning water systems, variable water flow (VWF) control strategies are frequently utilized to conserve energy. This paper presents a variable differential pressure (DP) set-point control strategy for VWF air conditioning systems based on the pipeline characteristic curve. This strategy bifurcates the most unfavorable loop into two segments: the equivalent main pipe (EMP) and the most unfavorable terminal branch pipe (MUTBP). Initially, the impedance of the EMP is obtained by curve fitting the measured values of the water supply and return main pipes (WSRMP), as well as the MUTBP. Subsequently, by calculating the disparity between the DP of the actual pipeline and the DP of the EMP, and comparing it with the DP of the MUTBP, the optimal working condition point for pipeline operation can be identified. Finally, a theoretical calculation is conducted on a typical air conditioning water system. This adjustment strategy achieves an energy-saving rate of 15.27%, 12.10%, and 11.50%, respectively, under the three adjustment conditions of closing the nearest terminal, the middle terminal, and the most unfavorable terminal, as compared with the constant DP set-point control strategy of WSRMP. This strategy boasts fewer control devices, a simple control system, and better operability and engineering applicability than other strategies.

Reinforcement learning control strategy for differential pressure setpoint in large-scale multi-source looped district cooling system

Large-scale multi-source looped district cooling (MLDC) systems are expected to be a promising solution to support decarbonization goals and combat climate change owing to their advanced structure. Differential pressure control is one of the most common methods to improve the energy and hydraulic performance of MLDC systems. Traditional rule-based control (RBC) and model-based control may not be appropriate for large-scale systems because of the need for expert knowledge and accurate models. Reinforcement learning control (RLC) has attracted considerable research attention owing to its efficiency and flexibility. However, very little is known about RLC in large-scale heating, ventilation, and air conditioning (HVAC) systems and complex issues. Therefore, this study employs a reinforcement learning technique to optimize the differential pressure setpoints of multiple cold sources, which can achieve energy savings and fulfill hydraulic head demands simultaneously. In this study, a Modelica model of a large-scale MLDC system in Beijing was developed as a virtual environment. The Modelica-Python co-simulation testbed for RLC was then implemented. The results show that RLC can save up to 12.99% of the annual distribution energy consumption while achieving a good hydraulic performance. As an information base for a variety of stakeholders, this study offers a reinforcement learning solution that can improve the operating performance of large-scale HVAC systems.

Modelling and control of district heating networks with reduced pump utilisation

District heating systems (DHS) provide thermal energy to a range of consumers. Hence, an adequate sizing of the key elements involved in the energy supply system and their management are critical. Pumps and valves are essential components of a DHS as they ensure hydraulic operating conditions are met for the energy distribution process. To achieve this, a hydraulic system is typically controlled by defining a differential pressure set-point at a critical location in the network. However, a good understanding of the dynamic behaviour of the hydraulic system during the diverse operating conditions is required for its efficient control and to maximise its performance. This paper presents a control strategy based on suitable dynamic models of the hydraulic system. These non-linear models enable the simulation of the behaviour of mass flow rate, pressure drops in pipes, power consumption of the pump and the heat delivery to meet the thermal loads. Control system design is carried out in MATLAB, and the designed controller is verified with Apros—a commercial process simulation software. It is shown that the hydraulic behaviour of a DHS is well described by the dynamic models presented. In addition, the designed control scheme reduces the electricity consumption of pumps compared with a conventional mass flow rate controller based on a look-up table and a differential pressure valve.

Online differential pressure reset method with adaptive adjustment algorithm for variable chilled water flow control in central air-conditioning systems

Central air-conditioning systems predominantly operate under partial load conditions. The optimization of a differential pressure setpoint in the chilled water system of a central air-conditioning system leads to a more energy-efficient operation. Determining the differential pressure adjustment value based on the terminal user’s real-time demand is one of the critical issues to be addressed during the optimal control process. Furthermore, the online application of the differential pressure setpoint optimization method needs to be considered, along with the stability of the system. This paper proposes a variable differential pressure reset method with an adaptive adjustment algorithm based on the Mamdani fuzzy model. The proposed method was compared with differential pressure reset methods with reference to the chilled water differential temperature, outdoor temperature, and linear model based on the adjustment algorithm. The energy-saving potential, temperature control effect, and avoidance of the most unfavorable thermodynamic loop effects of the four methods were investigated experimentally. The results indicated that, while satisfying the terminal user’s energy supply demand and ensuring the avoidance of the most unfavorable thermodynamic loop, the proposed adaptive adjustment algorithm also decreased the differential pressure setpoint value by 25.1%–59.1% and achieved energy savings of 10.6%–45.0%. By monitoring the valve position and supply air temperature of each terminal user, the proposed method exhibited suitable online adaptability and could be flexibly applied to buildings with random load changes.

Influence of Pipeline Operating Pressure on Value Setting of Automatic Control Valves at Different Pressure Drop Rates

When a natural gas pipeline ruptures, the adjacent upstream and downstream automatic control valves (ACV) should close quickly to prevent leakage or explosion. The differential pressure set point (DPS) at each valve location is the main criteria for value setting in ACV actions. If the DPS is not properly adjusted, the ACV may mistakenly close or it may not take any actions at a proper time. In this study, the effect of characteristic parameters such as pipeline operational pressure (POP) and pipeline pressure drop rate (ROD) due to rupture or a major leak was experimentally investigated on DPS. 25 different conditions with the double set of the mentioned typical characteristic parameters were chosen. In each condition, the differential pressure (DP) was measured over a period of 180 s by statistically analyzing the experimental results, so 25 maximum DP values (DPSs) were obtained. The DPS rises by an increase in ROD or a decrease in POP. Because of using nitrogen gas instead of natural gas for safety reasons and the uncertainties, the DPS results can be practically applied by adding a safety factor of 15%. Finally, the diagram of DPS with respect to ROD and that of non-dimensional DPS (DOP) versus non-dimensional ROD (RTP) were provided for different POP’s.

Experimental values for adjusting an automatic control valve in gas pipeline transportation

When a natural gas pipeline ruptures, the adjacent automatic line control valves (ALCVs) should close quickly to prevent leakage or explosion. The differential pressure set point (DPS) at each valve location is the main criteria for value setting in ALCV action. If the DPS is not properly adjusted, the ALCV may mistakenly close or it may not take any action at proper time. This study focused on the DPS values prediction for setting ALCV installed on a gas pipeline with 1mm orifice diameter. The effect of characteristic parameters such as pipeline operational pressure (POP) and pipeline pressure drop rate (ROD) due to rupture or major leak was experimentally investigated on DPS. Twenty-five different conditions with double set of typical mentioned characteristic parameters were chosen. For each condition, the differential pressure (DP) was measured over 180 seconds by analyzing the experimental values. Therefore, 25 maximum DP values (DPSs) were obtained. The DPS increases by increase in ROD or decreasing POP parameters. Because of using nitrogen gas instead of natural gas due to safety reasons, the DPS results can be practically applied by adding a safety factor of 15%. The diagram of DPS with respect to ROD and non-dimensional DPS (DOP) versus non-dimensional ROD (RTP) was provided for different POPs.

An experimental study of the effective parameters on automatic line-break control valves action in natural gas pipelines

The automatic line-break control valve (LBV) is a robust self-contained pipeline monitoring system which is widely used to prevent gas releasing upon rupture or large leakage event in the oil and gas pipelines. The differential pressure set point (DPS) at each valve location is the main criteria for value setting in LBV action. If the DPS is not properly adjusted, the LBV may mistakenly close or it may not take any action at proper time. In this study, the effect of characteristic parameters such as orifice diameter (OD), pipeline operational pressure (POP) and pipeline pressure drop rate (ROD) due to rupture or major leak was experimentally investigated on DPS. 100 different conditions with triple set of typical mentioned characteristic parameters were chosen. For each condition, the differential pressure (DP) was measured over 180 s by statistically analyzing the experimental results. Therefore, 100 maximum DP values (DPSs) were obtained. The attainable time to DPS depends only on OD and is independent of the POP and ROD and decreases by increasing the OD. The DPS increases by increase in ROD or decreasing OD or POP parameters. Because of using nitrogen gas instead of natural gas due to safety reasons and the uncertainties, the DPS results can be practically applied by adding a safety factor of 10%–15%. Finally, the diagram of DPS with respect to ROD was provided for different POPs and ODs. The diagram can be used as a basis for designing and regulating of LBVs installed on gas transportation pipelines.

Experimental investigation and control optimization of a ground source heat pump system

This paper presents an experimental investigation and control optimization of a ground source heat pump (GSHP) system. A set of experimental tests was first carried out to examine the performance of the GSHP system under different ground loop differential pressure set-points and different configurations of the ground heat exchangers (GHEs). A model-based optimization strategy was then developed to determine the optimal operating speed of the variable speed pumps in the ground loop system, in which a hybrid optimization technique integrating a performance map-based near-optimal strategy and the exhaustive search method was used to search for the optimal solutions of the optimization problem. The results from the experiments showed that, for the GSHP tested, the coefficient of performance (COP) of the heat pump units increased while that of the whole GSHP system decreased with the increase of the ground loop differential pressure set-point. The GSHP system with GHEs in the parallel operation outperformed that in the series operation. Using the proposed model-based strategy, energy savings of 8.0% during the heating period, and 9.0% during the cooling period can be achieved in comparison to a rule-based strategy with two-stage control for variable speed pumps.

Experimental study of hydraulic stability for variable water volume air conditioning System

The hydraulic stability of the variable water volume air conditioning system has a significant impact on the thermodynamic stability, service life of the pipe network and energy consumption. The hydraulic stability of variable water volume air conditioning system was investigated by means of experiments. An air conditioning system with variable water volume is served as the platform, and then the changes of the passive branches’ hydraulic characteristics are studied in valve action cycle of the active branch under the following three conditions: without pump control, constant differential pressure (DP) control or variable differential pressure set-point control. The results show that the branch with the small flow and high-pressure drop has the good hydraulic stability. Under the pressure difference control, the hydraulic stability of the system is improved obviously, the energy-saving effect of the pump is better, and the energy saving of the chilled water pump at least 20.8% when closed any branch valve. This study provides a reference for the design of variable water volume air conditioning systems with good hydraulic stability.

An optimal differential pressure reset strategy based on the most unfavorable thermodynamic loop on-line identification for a variable water flow air conditioning system

The variable differential pressure (DP) set-point control strategy has become the main energy-saving measure for central air conditioning water system variable volume adjustment. In optimal operation control, it is critical to use the right reference object to implement global, accurate, and online control. In mainland China, the most unfavorable loop is typically used as the reference object for variable DP set-point control in the majority of cases of variable volume air conditioning system adjustment. However, its concept and feasibility require further clarification. First, this paper analyzes the difference in the characteristics of a most unfavorable loop under air conditioning design work conditions versus under operational conditions. Under design operational conditions, the referenced most unfavorable hydraulic loop only reflects the distribution of the pipe network's hydraulic characteristics, whereas under system operational conditions, the variable volume adjustment strategy should be chosen according to each user's load requirement, i.e., a most unfavorable thermodynamic loop. Then, this paper provides definitions of the most unfavorable thermodynamic loop and online identification method and proposes a variable DP set-point optimal reset strategy based on the maximum valve position loop's optimal reset valve position domain and best valve position domain. Finally, the variable DP set-point control of a primary pump variable volume air conditioning water system in an industrial factory is used as an example to conduct a test studying 6 variable DP or constant DP control strategies, including the method proposed in this paper. The test results show that the most unfavorable loop-based variable DP control strategy saves 47–58% water pump power consumption (up to 985kWh electric power consumption per day) when compared with the conventional variable DP control strategy, whereas the most unfavorable thermodynamic loop-based variable DP control strategy is more advantageous in ensuring the overall cooling energy supply for each user.

Experimental Study on a Novel Optimal Differential Pressure Reset Method for Online Application in Chilled Water System

This paper firstly define most unfavorable thermodynamic loop to reflect the dynamic situation of terminal cooling load distribution, then present its online identification method that can easily applied for optimal reset of differential pressure. We use the optimal domain of maximum valve position among whole braches in chilled water system to calculate the variation of differential pressure setpoint, and control the pumps according to the optimal setpoint. We applied presented method to a VWF retrofit project in a Suzhou factory building and evaluate its energy saving effect through its equipped building energy management system. Experimental results showed that presented method can achieve 22%-53% pump power consumption savings compared to traditional control method. During the test period, the average level of valve position of presented method was highest among all tested control strategies. The optimal reset method of differential pressure presented in this paper is an energy efficient, high stability and easily online applied control method.

Experimental analysis of the thermal stability of the pressure control method for a variable flow air-conditioning water system

In this study, we investigate the stability of a pipe network for differential pressure (DP) control of a variable flow air-conditioning chilled water system. First, the hydraulic properties of the pipe network and the thermal properties of the terminal air-conditioning equipment are presented, and the thermal stability is defined to serve as an indicator of the impact that flow regulation in the active branches has on the energy supply capacity of the passive branches. An evaluation indicator of the thermal stability is also developed for online applications. Second, dynamic changes in the hydraulic and thermal properties of the passive branches of a primary pump variable flow dual loop air-conditioning water system were experimentally studied during an active branch regulating valve action stroke cycle using an uncontrolled hydraulic pump, constant DP control and variable DP set-point control. The feasibility and energy savings of various DP control strategies were evaluated in terms of the thermal stability and energy savings for hydraulic pumps. The results show that using a variable DP set-point control strategy produced significant energy savings; however, the thermal stability must be considered when setting the step size in adjusting the DP set-point.

Evaluation of Differential Pressure Setpoint of Chilled Water Pumps in Clean Room HVAC Systems for Energy Savings in High-Tech Industries

Variable-frequency speed control technologies are widely used in chilled water pumps in clean room heating, ventilation, and air-conditioning (HVAC) systems in high-tech industries to reduce energy consumption. Pumps with variable frequency drivers work based on the reading from the differential pressure sensor placed between the chilled water supply and return lines before the secondary and tertiary pumps. The determination of the differential pressure setpoint must take pipeline topology and material, chilled water flow demand, heat loading, and system equipment aging into account so as to provide operating flexibility for energy conservation while ensuring the secure operation of the clean room. To enhance clean room energy efficiency, a new methodology is proposed in this paper for determining the proper chilled piping pressure setpoint for the pumps. The mathematic model of head loss required for generic secondary and tertiary chilled networks widely used in clean rooms is derived using the Hazen-Williams formula principle. The field testing results confirm the validity of the proposed approach through a practical clean room HVAC system for electronic semiconductor manufacturing, and the energy savings and investment benefits due to the use of the proper differential pressure setpoint are determined.

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

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.

On-line optimization control method based on extreme value analysis for parallel variable-frequency hydraulic pumps in central air-conditioning systems

This paper proposes a class of theoretical models based on parallel hydraulic pump characteristics and on-line control optimization methods for variable-flow hydraulic pumps in central air-conditioning systems, which includes the optimized allocation of parallel variable-frequency hydraulic pump speed ratios and the optimized number of operating units. It is suitable for use in fast load variations and for on-line control of variable-flow central air-conditioning systems that use a variable differential pressure setpoint control strategy. This study first establishes a model of parallel variable-frequency hydraulic pump characteristics and uses the extreme value analysis method to investigate the optimal combination of speed ratios of multiple parallel variable-frequency hydraulic pumps. We then explore the optimal number of operating units of parallel variable-frequency hydraulic pumps that utilize the same hydraulic operating point. Using these two methods and monitoring parameters such as the total flow rate and total head of parallel hydraulic ptumps, this study proposes an optimization control method that can perform on-line decision-making for the optimal number of operating hydraulic pumps based on automatically calculated operational zones. The proposed method was applied to a plant with a primary pump variable-flow air-conditioning system that uses a variable differential pressure setpoint control strategy. The actual application shows that this method obtained electricity savings of 15.8% and 4% compared with single-pump operation scheme and dual-pump operation scheme, respectively, for the hydraulic pumps. The method is universal and simple for on-line implementation, which is suitable for control optimization of chilled-water pumps in a variable-flow air-conditioning system.