Variable Speed Pumps Research Articles

Common fault effects on a natural refrigerant, variable-speed heat pump

The European heat pump market is growing every year as European legislation moves towards the use of natural refrigerants that have less global warming potential than conventional refrigerants. Even though heat pumps are very efficient, the appearance of hard-to-detect faults increases energy consumption. The present study emulated some of the most common faults on a variable-speed heat pump charged with propane. The faults tested were evaporator fouling, compressor valve leakage, liquid line restriction and refrigerant overcharge. The effect of these faults on performance and several heat pump features are described. Evaporator fouling was the fault with least COP degradation and compressor valve leakage with the most (7% and 56% COP reduction, respectively). The results are compared with the results from a comparable study. The heat pump covered the demand generated by nearly all the faults at the expense of increasing the power consumption. The features of each fault are summarized in a chart, which could be used as table for diagnosing faults.

A hierarchical gray-box dynamic modeling methodology for direct-expansion cooling systems to support control stability analysis

In this paper, a gray-box dynamic modeling approach for direct-expansion cooling systems is presented. The overall approach incorporates a multi-stage training procedure that consists of 1) identification of component sub-models from quasi-steady-state performance data, 2) system model integration with estimation of refrigerant charge and 3) fine tuning of thermal capacitances of the evaporator and condenser to capture the system dynamic responses. Compared to traditional physics-based models, the proposed modeling approach has advantages including reduced engineering efforts in the model development phase, improved computational efficiency and enhanced prediction accuracy. The modeling method was validated using a 3-ton variable-speed heat pump and proved to be capable of accurately predicting the system transient behaviors over a wide range of operating conditions. The established dynamic model was then applied for control stability analysis, with a specific goal of determining a proper control execution time step. The case study results showed that the stable control execution time step could change significantly, from 3 sec to 19 sec, as the operating conditions and control settings vary, and a proper selection of the execution time step is critical to ensure stable and reliable operations.

Comparative Study of Energy Consumption and CO2 Emissions of Variable-Speed Electric Drives with Induction and Synchronous Reluctance Motors in Pump Units

This study carried out a comparative analysis of indicators of electricity consumption and CO2 emissions for four-pole induction motors (IMs) of efficiency classes IE3 and IE4 with a rated power of 2.2–200 kW in a variable speed pump unit. In addition, innovative IE4 converter-fed synchronous reluctance motors (SynRMs) were evaluated. The comparison was derived from the manufacturer’s specifications for the power drive systems (PDSs) at various rotational speeds and loads. The results showed that the emission indicators for IE3 class motors were significantly worse compared with IE4 class motors for low power ratings, which make up the vast majority of electric motors in service. This justifies expanding the mandatory power range for IE4 motors to at least 7.5–200 kW or even 0.75–200 kW, as it will dramatically contribute to the achievement of the new ambitious goals for reducing greenhouse gas emissions. In addition, the operational advantages of IE4 SynRMs over IE4 IMs were demonstrated, such as their simpler design and manufacturing technology at a price comparable to that of IE3 IMs.

Study on vibration characteristics of hydraulic planetary transmission considering engine and torque converter excitation

Large-size vehicle loaders are mainly used in various construction projects with complex working environment, which has a serious impact on the internal vibration of the whole system. In this study, the vibration characteristics of hydraulic planetary transmission of vehicle loader considering the influence of the external excitation are evaluated theoretically and experimentally. The output torque of the four-stroke six-cylinder diesel engine is obtained by force calculation of crank and connecting rod mechanism. The equivalent stiffness and damping of the torque converter are solved according to the torsional dynamic equation of the torque converter. The trapped oil area, oil pressure, and flow rate of gear meshing part are extracted to obtain the trapped oil pressure of variable-speed pump. Then, the multi-degree-of-freedom gear system dynamic model is established to calculate the meshing force, which is applied to vibration response analysis model by using mode superposition method. Lastly, the vibration response test is performed on the experimental prototype to verify the calculation method. The conclusion shows that the generation principle of external excitation and its calculation method in this paper are feasible in the analysis of dynamic characteristics of hydraulic planetary torque converter.

Assessing the performance limits of a variable-speed residential heat pump system

Most of the ducted split-type heat pump systems in the world feature fixed-speed compressors and fans. To meet forthcoming minimum energy rating requirements, reduce operational costs of the heat pump, and increase environmental sustainability, the efficiency of heat pump systems must be improved. Variable-speed equipment offers significant advantages for load modulation and has the ability to increase the seasonal performance significantly. Additionally, novel electrical motor technologies, such as permanent magnet (PM) motors can reduce the power consumption of the drives by up to 25-55% compared to the widely used permanent split capacitor (PSC) motor.In this study, a ducted fixed-speed heat pump system with a cooling capacity of 17.6 kW and a seasonal coefficient of performance (SCOP) rating of 4.11 is analyzed to quantify the theoretical performance limits that could be obtained by increasing the motor efficiency of the condenser fan, the evaporator fan and the compressor. Furthermore, the performance increase by replacing the fixed-speed components with variable-speed equipment is evaluated. A detailed charge-sensitive system simulation model that was previously developed by the authors and validated using experimental data obtained with the same unit was employed to carry out parametric studies to evaluate the impact of variable-speed compressor and indoor/outdoor fans on the SCOP rating.The study showed a 0.9% improvement of SCOP with every percentage increase of compressor motor efficiency. The dependence of the evaporator fan and condenser fan motor efficiency on SCOP was lower, but considering the generally low motor efficiencies of these components, a noticeable efficiency gain can be achieved. In variable-speed equipment, the motor efficiency of the component at minimum operational point was the significant factor on SCOP. The motor efficiency at intermediate and maximum speed had only marginal effects on the SCOP. This showed the importance of equipment with high motor efficiencies at low speeds for ducted split-system heat pumps. The change of the fixed-speed system to a fully variable-speed heat pump resulted in a high gain of SCOP, proving the superiority of a variable-speed system. Overall, the model predicted an SCOP gain of 35% when changing the system to a fully variable system with state-of-the-art high efficient motors.

A robust gray-box modeling methodology for variable-speed direct-expansion systems with limited training data

• A robust training method against limited training data is proposed for DX units. • The methodology identifies the most influential parameters to estimate. • More estimation parameters result in degraded parameter accuracy. • Model prediction accuracy is highly dependent on training data size. Compared to physics-based models, the gray-box modeling approach has clear advantages including reduced engineering efforts in the model development phase, higher computational efficiency, improved accuracy and reduced uncertainties. In this paper, we present a gray-box steady-state modeling methodology for variable-speed direct-expansion systems that is robust against limited training data. The methodology incorporates a dual-stage training scheme, where component models are identified separately, and then integrated through continuity constraints to establish a system model trained on a higher hierarch. To further improve model reliability and robustness, the training methodology incorporates two commissioning techniques, i.e., significance ranking and de-correlation, to down-select the estimation parameters according to the quality of the training data. To demonstrate the efficacy, the proposed modeling methodology was applied to a 3-ton variable-speed heat pump. Qualities of the identified models were evaluated in terms of parameter accuracy and model prediction accuracy against variable training data sizes. Robustness of the methodology was demonstrated in achieving reasonable model accuracies with limited training data.

Two Current-Based Methods for the Detection of Bearing and Impeller Faults in Variable Speed Pumps

The growing number of variable speed drives (VSDs) in industry has an impact on the future development of condition monitoring methods. In research, more and more attention is being paid to condition monitoring based on motor current evaluation. However, there are currently only a few contributions to current-based pump diagnosis. In this paper, two current-based methods for the detection of bearing defects, impeller clogging, and cracked impellers are presented. The first approach, load point-dependent fault indicator analysis (LoPoFIA), is an approach that was derived from motor current signature analysis (MCSA). Compared to MCSA, the novelty of LoPoFIA is that only amplitudes at typical fault frequencies in the current spectrum are considered as a function of the hydraulic load point. The second approach is advanced transient current signature analysis (ATCSA), which represents a time-frequency analysis of a current signal during start-up. According to the literature, ATCSA is mainly used for motor diagnosis. As a test item, a VSD-driven circulation pump was measured in a pump test bench. Compared to MCSA, both LoPoFIA and ATCSA showed improvements in terms of minimizing false alarms. However, LoPoFIA simplifies the separation of bearing defects and impeller defects, as impeller defects especially influence higher flow ranges. Compared to LoPoFIA, ATCSA represents a more efficient method in terms of minimizing measurement effort. In summary, both LoPoFIA and ATCSA provide important insights into the behavior of faulty pumps and can be advantageous compared to MCSA in terms of false alarms and fault separation.

Optimal short-term water-energy dispatch for pumping stations with grid-connected photovoltaic self-generation

Increases in the energy costs of irrigation water pumping facilities puts the economic sustainability of recent investments in the modernization of farms at risk. To address this problem, it is essential to apply renewable technologies for the production of electricity, and photovoltaic energy is particularly attractive due to its lower cost and recent technological advances. The aim of this research is to develop a mathematical techno-economic dispatch model that optimizes the hourly schedule of pumping equipment subject to electrical and hydraulic constraints to minimize the weekly operating costs of a real pumping station. The resulting model is formulated as a mixed-integer nonlinear programming problem that determines the optimal hourly combination of pumping equipment and available resources to meet water and energy needs. The proposed model comprises fixed and variable speed pumps, a grid-connected photovoltaic plant, and two water ponds for internal regulation and storage. The results verify that the combination of self-consumption photovoltaic facilities and variable speed drives make it possible to maximize the percentage of self-consumed energy up to 99.41% during the month with the highest demand for water. In this case, the pumping station reduces its energy costs by 21.56%, in addition to improving water management.

Trajectory Tracking Control of a Direct-drive Variable Speed Pump Control System with a Non-linear Model Compensation

Trajectory Tracking Control of a Direct-drive Variable Speed Pump Control System with a Non-linear Model Compensation

A New Model Predictive Control Method for Eliminating Hydraulic Oscillation and Dynamic Hydraulic Imbalance in a Complex Chilled Water System

To enhance the energy performance of a central air-conditioning system, an effective control method for the chilled water system is always essential. However, it is a real challenge to distribute exact cooling energy to multiple terminal units in different floors via a complex chilled water network. To mitigate hydraulic imbalance in a complex chilled water system, many throttle valves and variable-speed pumps are installed, which are usually regulated by PID-based controllers. Due to the severe hydraulic coupling among the valves and pumps, the hydraulic oscillation phenomena often occur while using those feedback-based controllers. Based on a data-calibrated water distribution model which can accurately predict the hydraulic behaviors of a chilled water system, a new Model Predictive Control (MPC) method is proposed in this study. The proposed method is validated by a real-life chilled water system in a 22-floor hotel. By the proposed method, the valves and pumps can be regulated safely without any hydraulic oscillations. Simultaneously, the hydraulic imbalance among different floors is also eliminated, which can save 23.3% electricity consumption of the pumps.

Stability and dynamic analysis of doubly-fed variable speed pump turbine governing system based on Hopf bifurcation theory

The stability of a doubly-fed variable speed pump-turbine governing system (VSPTGS) under the condition of small power disturbance was investigated. First, a novel model of the VSPTGS in turbine mode and pump mode is proposed based on the electromechanical transient model of the doubly-fed induction generator (DFIG). Then, the algebraic stability criteria of the system in turbine mode and pump mode are derived using Hopf bifurcation theory. Furthermore, the stability domain of the speed controller parameters is obtained when the power controller parameters are determined, and the dynamic behavior of the system is analyzed using bifurcation diagrams and numerical simulation. Finally, the power controller parameters are varied to study the influence of these parameters on the stability domain. The results show that Hopf bifurcation of a VSPTGS is subcritical. When the parameters of the power controller are fixed, the stability domain in pump mode is larger than that in turbine mode. The stability in turbine mode only depends on the speed controller parameters, whereas the stability in pump mode depends on both the speed controller parameters and the power controller parameters.

Design of Energy Saving Controllers for Central Cooling Water Systems

Since fuel prices account for approximately 40% of a ship’s operating costs, shipping companies worldwide have made significant efforts to save energy on board such as introducing new technologies or machine operation methods. Many ship operators have adopted an advanced control system using a variable-speed pump and/or an optimizing control system of a three-way valve on the outlet side of the central cooling system. It is often considered that the best way to control a central cooling system is to integrate the two control systems. However, when applied in practice, there is a frequent uncontrollable phenomenon in which the three-way valve is opened to its minimum and the variable-speed seawater pump is operated at its maximum, resulting in a large amount of energy consumption. Therefore, in this study, the speed of the variable-speed seawater pump is set to the minimum, and the feed-forward controller is adopted for the three-way valve control system. The input variable of the feed-forward controller is the Main Engine load, and it is designed to directly control the bypass openness with the three-way valve controller. Using this design, it was demonstrated that the variable-speed seawater pump was operated at a minimum and energy was saved.

Optimal diameter of district heating pipe network based on the hybrid operation of distributed variable speed pumps and regulating valves

The district heating system that uses the hybrid operation of distributed variable speed pumps and regulating valves can effectively reduce the pressure level at the far-end of the heating network, while retaining the energy saving advantages of the distributed variable speed pump system and ensuring the safe operation of the heating network. In this study, pipe network optimization based on genetic algorithm was used to determine the pipe diameter required for the hybrid operation of distributed variable speed pumps and regulating valves. A mathematical model was established to evaluate the economic efficacy based on the annual equivalent cost. The zero-pressure difference point was evaluated to optimize the pipe diameter. A real pipe network was assessed to evaluate the efficacy of the method, and a sensitivity analysis of pressure constraints and economic factors was conducted. Our results show the system of hybrid operation of distributed variable speed pumps and regulating valves can reduce the annual equivalent cost by about 10.35% compared to the conventional central circulating pump system. In addition, the annual energy consumption due to heat loss and pump operation is only 0.36% higher than that of the conventional central circulating pump system.

Flexibility of energy and water management in pressurized irrigation systems using dynamic modeling of pump operation

Variable water demands in growing season, spatial altitude difference in hydrant points, incompatibility in irrigation time, crop pattern alternation and the other environmental factors are among the most important dynamic factors affecting the operation of pumping stations in an irrigation system. Pumping stations could be effectively operated using a dynamic or time-dependent approach. In this study, the performance of an agricultural pumping station, which will be equipped with variable speed pumps, was analyzed. The station is located in an agricultural area in Qazvin Province, northern Iran. The dynamic model of the pumping station was developed for simulating five defined operation scenarios. The results showed that using variable speed pumps is capable of reducing energy consumption up to 67 %, in comparison with current constant speed modes. The ratio of energy consumption for pumped water was determined equal to 0.37 kwh/m3 in variable speed mode, implying up to 45% reduction in comparison with use constant speed pumps.

Techno-economic performance of photovoltaic-powered air-conditioning heat pumps with variable-speed and fixed-speed compression systems

The present study aimed at finding which kind of the two available commercial heat pump types (i.e. the variable-speed and fixed-speed compression systems) would be the appropriate option to become fully photovoltaic (PV)-powered, especially for the office building sector. First, behaviors of the heat pumps are determined based on experimental thermodynamic analyses using T-s (temperature-entropy) cycles. Results reveal that the variable-speed compression system shows up to 400% higher efficiencies under partial loads. The minimum PV systems required for these two systems for 8 working hours (9:00–17:00) in the Tehran climate are designed. The results show that the fixed-speed compression system requires a minimum size of a PV power system twice that of the variable-speed compression counterpart. With the half-sized PV system, the variable-speed compression air-conditioner provides 64% and 88% of the heat and cold of the fixed-speed counterpart, respectively. The economic evaluation shows that it provides 50% and 16% extra cooling and heating compared to those of the fixed-speed counterpart per invested US $. Hence, applying variable-speed compression heat pumps powered by PV systems would be one of the most promising methods for peak-shaving, especially for high cooling demands and in the office building sector.

A stochastic load demand-oriented synergetic optimal control strategy for variable-speed pumps in residential district heating or cooling systems

For a residential district heating or cooling (RDHC) system in the “part-time part-space (PTPS)” service mode, the thermal load demands of end users are notably stochastic.To reducing the energy consumption of centralized variable-speed pumps (VSPs) in RDHC systems, this study presents a stochastic load demand-oriented synergetic optimal control strategy for VSPs in RDHC systems. In this strategy, the spatial and temporal distribution of the stochastic load demands (SLDs) in a residential district is predicted using the stochastic occupant behavior (SOB) method. The operation regulation of the RDHC system is mathematically described based on graph theory. A synergetic optimization mathematical model is developed based on each end user’s SLD as well as the operation regulation characteristics of the RDHC system. This synergetic optimization mathematical model is subsequently solved using particle swarm optimization (PSO) algorithm. Thus, an optimal hourly pump frequency ratio is obtained. A case study is performed to comprehensively compare the proposed synergetic optimal control (SOC) strategy with the conventional constant pressure difference control (CPDC) strategy. The results show that for the same load demands, the energy consumption of the pumps throughout the heating season under the SOC strategy is 62.9% lower than that under the CPDC strategy.

Optimization Methodology for Estimating Pump Curves Using SCADA Data

Water distribution systems (WDSs) deliver water from sources to consumers. These systems are made of hydraulic elements such as reservoirs, tanks, pipes, valves, and pumps. A pump is characterized by curves which define the relationship of the pump’s head gain and efficiency with its flow. For a new pump, the curves are provided by the manufacturer. However, due to its operating history, the performance of a pump deteriorates, and its curves decline at an estimated rate of about 1% per year. Pump curves are key elements for planning and management of WDSs and for monitoring system efficiency, to determine when a pump should be rehabilitated or replaced. In practice, determining pump curves is done by field tests, which are conducted every few years. This leaves the pump’s performance unmonitored for long time periods. Moreover, these tests often cover only a small range of the curves. This study demonstrates that in the era of IoT and big data, the data collected by Supervisory Control And Data Acquisition (SCADA) systems can be used to continuously monitor pumps’ performance and derive updated pump characteristic curves. We present and demonstrate a practical methodology to estimate fixed and variable speed pump curves in pumping stations. The proposed method can estimate individual pump curves even when the measurements are given only for the pumping station as a whole (i.e., total flow, pumping station head gain). The methodology is demonstrated in a real-world case study of a pumping station in southern Israel.

Hydraulic design and operation of variable-speed pumps as the water–energy saving strategies in pressurized irrigation systems

Low efficiencies of irrigation pumping stations usually stem from their improper designing, operation, and maintenance of such systems. In the present study, the water and energy losses and the pressure variations of a pressurized irrigation system were investigated through analysing the characteristic curves of the pumps and assessing the discharge and pressure heads needed by the system during the irrigation season (i.e. the system demand curves). Variable-speed pumps can be adjusted to the system demand, making them more efficient than constant-speed pumps while bringing down water and energy consumption. Given this assumption, the suitability of variable-speed pumps to save water and energy in a 100 hectares apple and peach orchard, located in Isfahan Province, central Iran, was investigated. The pressure head, the pump efficiency, and the water and energy losses of the orchard’s irrigation system designed based on variable-speed pumps were calculated and compared with those of common constant-speed systems. The results showed that using variable-speed technology in designing pumping stations not only increases the efficiency and reduces unnecessary pressures but also can result in optimal water use and, depending on operational condition, 44–54% reduction in energy consumption can be achieved, which is significant in national scale.

Use of Fixed and Variable Speed Pumps in Water Distribution Networks with Different Control Strategies

The control system’s point is to bring the pumping curve close to the set-point curve. That concept is essential for proper design of a pumping station. An adequate design is focused not only on selecting the total number of pumps and the type of control to use (flow or pressure), but it also is important to determine the optimal number of fixed speed pumps (FSPs) and variable speed pumps (VSPs) for each flow rate. This work discusses the most common methods and procedures for control systems on a design of pumping stations with a proposed methodology. This methodology consists of expressing the characteristics of the pumping curve and the set-point curve in a dimensionless form so that the methodology is standardized for any pump model and set-point curve. These formulations allow us to discuss how the characteristic of a pump and the set-point curve of the network influence the optimal number of FSPs and VSPs in energy terms. In general, the objective of this work is to determine the most suitable total number of pumps in a pumping station design and to determine the optimal pumping configuration in every flow rate, thus the consumed energy would be the minimum. Additionally, this methodology develops an expression to estimate the performance of a frequency inverter when a VSP operates at different rotational speeds. This work will be applied to different study cases, and the obtained results allow us to question several usual procedures for pumping control system. In general, it can be concluded that the number of pumps of a pumping system cannot be inferred in a simple form without a deep analysis of a control system.

Alternative operating modes to reduce the load shedding in the power system of El Hierro Island

The activation of the load shedding programs in the six isolated power systems belonging to Canary Archipelago has not been uncommon in last years. These power systems have been increasing the penetration of renewable power generation technologies, which are mainly decoupled from the grid through a power electronic converter and not being able to respond naturally to frequency drops. Common Under Frequency Load Shedding programs are activated when a relevant incident, as the outage of a generation unit, takes place. This work analyses the benefits of alternative control strategies in one of the Canary Islands (El Hierro island) in terms of reducing the need of load shedding activation. These control strategies are the variable speed wind turbines inertial contribution to frequency regulation and the use of Pelton turbines as synchronous condensers. Both strategies do not involve a significant loss in the efficiency or in the available wind energy. Additionally, the participation of variable speed pumps in the frequency regulation, control strategy recently adopted in El Hierro power system, has been included in the analysis. To achieve these objectives, a complete dynamic mathematical model of this power system has been delivered. Representative generation schedules that fulfil several real combinations of wind generation and load were generated and used to carry out the study. The results obtained show the effectiveness of the alternatives control strategies proposed. The amount of load to be shed is reduced or even avoided in many scenarios, but the three alternative control strategies present their own particularities.