Fixed-speed Compressors Research Articles

Effects of simultaneous soft faults in a reversible and variable-speed air-to-water heat pump

The widespread adoption of air-to-water heat pumps is driven by their high heating efficiency, with some systems offering reversibility for cooling applications. Despite their benefits, these systems are susceptible to soft faults such as undercharge, overcharge, and outdoor unit airflow restrictions (condenser fouling in cooling mode or evaporator fouling in heating mode). Detecting and addressing these faults before they escalate into hard faults is critical. While previous experimental studies have focused on residential heat pumps with imposed faults, most were conducted on air-to-air systems, primarily in cooling mode and with fixed-speed compressors. This study addresses the gap by investigating a reversible air-to-water heat pump employing R290 and a variable-speed compressor. The experimental work consists of imposing faults of undercharge, overcharge, condenser and evaporator fouling, individually and in combination. The investigation analysis considers the system’s behavior under these faults in cooling and heating modes and how the control strategy (subcooling or superheat) and a compressor suction accumulator affect it. In cooling mode, the combination of 85% undercharge and 50% condenser fouling worsens the EER up to 39% if the superheat is controlled and up to 10% if the subcooling is controlled. In heating mode with subcooling control, 85% undercharge combined with a 50% evaporator fouling can worsen the COP by 32%. However, there are cases where those same fault levels only impact the COP by 4% due to the charge requirements of the condition and that the excess charge is stored in the compressor suction accumulator. As compressor speed can significantly affect the refrigerant charge requirements of the system, this work shows its potential for fault detection, particularly for undercharge. These findings can provide a basis for developing Fault Detection and Diagnosis methodologies for reversible air-to-water heat pumps with variable-speed compressors.

Energy Performance of Room Air-Conditioners and Ceiling Fans in Mixed-Mode Buildings

Studies show that people can tolerate elevated temperatures in the presence of appreciable air movement (e.g., from using ceiling fans). This minimises the use of air-conditioners and extends their set-point temperature (Tset), resulting in energy savings in space cooling. However, there is little empirical evidence on the energy savings from using ceiling fans with Room Air-Conditioners (RACs). To address this gap, we analysed the energy performance of RACs with both fixed-speed compressors and inverter technology at different set-point temperatures and ceiling fan speed settings in 15 residential Mixed-Mode Buildings (MMBs) in India. Thermal comfort conditions (as predicted by the Indian Model for Adaptive Comfort-Residential (IMAC-R)) with minimum energy consumption were maintained at a set-point temperature (Tset) of 28 and 30 ∘C and a fan speed setting of one. Compared with a Tset of 24 °C, a Tset of 28 and 30 °C resulted in energy savings of 44 and 67%, respectively. With the use of RACs, a configuration with a minimum fan speed was satisfactory for an optimal use of energy and for maintaining the conditions of thermal comfort. In addition, RACs with inverter technology used 34–68% less energy than fixed-speed compressors. With the rising use of RACs, particularly in tropical regions, the study’s outcomes offer a significant potential for reducing space-cooling energy consumption and the resultant greenhouse gas (GHG) emissions.

Correlation to predict conditions that lead to liquid-flooding at compressor start-up as a function of evaporator size and fluid properties

Refrigeration cycles with fixed speed compressors cycle on and off to modulate their time averaged cooling capacity. While the cycles are turned off, refrigerant can migrate from the condenser into the colder evaporator. The next start-up process can then pull liquid refrigerant out of the evaporator and into the compressor causing liquid flooding. This phenomenon results in a loss of latent heat for cooling, can undercool the lubricant, and may cause damage to the compression mechanism. The geometric and thermophysical conditions that lead to liquid flooding are poorly understood. Experimental data collected for four refrigerants and three heat exchangers was analyzed for flooding conditions. A correlation was fitted to the experimental data, predicting the experimentally determined charge threshold for flooding with an average deviation of 6.6% and a maximum deviation of 16.7%. Repeating the tests with other heat exchanger types and smaller or larger compressors is suggested as important future work.

Electricity peak load reduction of a refrigerator using phase change material

Phase change material (PCM) has become a topic of study in thermal management, as it has a high-energy storage density. Most refrigerators have a fixed-speed compressor, which operates randomly (on/off) during their cycle. This leads to a high-power peak of electricity when the refrigerators start at the same time. Therefore, this study aims to investigate the reduction potential of the power peak and energy consumption of a refrigerator using PCM as a heat absorber. Nine testing conditions were performed by using water as the internal thermal load, at 1, 2, and 3 kg. Besides, each internal thermal load condition was varied with a PCM mass of 500, 1,500, and 2,500 g. The necessary data, such as the internal temperature and power of a refrigerator and the outside ambient temperature, were collected. The results show that the mass of PCM significantly affected the peak power of the refrigerator and energy consumption. Moreover, the results clearly show that the PCM can be used to reduce the peak power and energy consumption of the refrigerator. The correlation of the power, related to the mass of the internal heat load, and the mass of PCM was also reported. The conclusion of this study is that the PCM is suitable for reducing the peak power and energy consumption of a household refrigerator.

Dynamic modelling of the steady state and load processing operation of a domestic refrigerator cooled through natural convection

A dynamic model of a domestic refrigerator is developed in this study and is used to analyse both the steady state (cyclic) and warm load cooling (load processing) operation. The compressor is modelled semi-empirically using loss parameters, while the capillary tube is modelled by incorporating both friction and momentum pressure drop. Heat exchangers (condenser and evaporator) are modelled as tubes divided into finite volume cells. The capillary tube-suction line heat exchanger is treated by connecting part of the capillary tube to the suction line via a heat resistor. The cabinet model consists of submodels for walls, air and shelves connected in a network to allow for thermal interaction. All the components are then arranged in a cycle, and cyclic operation is validated at 32 °C using a fixed speed compressor. A sensitivity analysis is then carried out by varying the compressor speed. In the second part of the study, the model is modified to include the effect of placing a warm load of water in the refrigerator and simulating the subsequent processing operation. This is validated using an experiment in which 20 L of water at 27 °C is placed inside the Fresh Food compartment. Results show that the power consumption value is captured well, with the average power consumption deviating less than 10% for all tests. The energy consumption is within ±5% for all tests. For load processing, the deviation in water temperature was a maximum of 1.2 °C for the whole cooling period.

Energy-Saving Potential of Thermal Diode Tank Assisted Refrigeration and Air-Conditioning Systems

Lowering the condensing temperature of the Refrigeration and Air-conditioning (RAC) system has been proven to effectively increase the system’s Coefficient of Performance (COP). This paper revolves around evaluating the energy-saving generated by applying a Thermal Diode Tank (TDT) in the RAC systems. The TDT is a novel invention, which is an insulated water tank equipped with gravity heat pipes. If the TDT was placed outdoors overnight, its inside water would theoretically be at the minimum ambient temperature of the previous night. When the TDT water is used to cool the condenser of RAC systems that operate during the daytime, a higher COP of this TDT assisted RAC (TDT-RAC) system could be achieved compared with the baseline system. In this study, a steady-state performance simulation model for TDT-RAC cycles has been developed. The model reveals that the COP of the TDT-RAC cycle can be improved by 10~59% over the baseline cycle depending on the compressor types. The TDT-RAC cycle with a variable speed compressor can save more energy than that with a fixed speed compressor. In addition, TDT-RAC cycles can save more energy with a higher day/night ambient temperature difference. There is a threshold tank size for a given TDT-RAC cycle to save energy, and the energy-saving can be improved by enlarging the tank size. A desk-top case study based on real weather data for Adelaide in January 2021 shows that 9~40% energy could be saved by TDT-RAC systems every summer day on average.

ART.I.CO. (ARTificial Intelligence for COoling): An innovative method for optimizing the control of refrigeration systems based on Artificial Neural Networks

Advanced control methods proved they effectiveness in reducing energy consumption of refrigeration systems equipped with a variable-speed compressor, but they could not be suitable for fixed-speed compressors, which are usually controlled by a simple ON/OFF logic with a mechanical thermostat, which does not allow to optimize the performance of such devices. Hence, a novel control method based on the use of Artificial Neural Networks to optimize the operations of refrigeration systems equipped with a fixed-speed compressor is proposed. This technique uses an Artificial Neural Network, which stem from a three-step process, able to provide the ON/OFF control loop with the optimal hysteresis value accordingly to the requirement of the user, in terms of set-point temperature and optimization priority, and the ambient temperature. The proposed control method was encoded in a microcontroller to test its effectiveness with a refrigeration system. The results of the experimental tests demonstrated the great potential of this approach showing a reduction of energy consumption of 6.8% and 2.2% with no stored material and ambient temperatures of 25 °C and 32 °C, respectively. Then, the introduction of 45 kg of stored material led to energy savings up to 13.4% and 6.6% with ambient temperatures of 25 °C and 32 °C, respectively. Furthermore, it was evidenced that door openings and pick-and-place operations can reduce the positive effect of this approach, reducing the energy saving to 3.7%. The results show that Artificial Neural Networks can be successfully applied to optimize the ON/OFF control loop of refrigeration systems, considering both plug-in and built-in solutions.

Development of accurate and widely applicable compressor performance maps

The compressor performance map provides critical information, including mass flow rate, power consumption, capacities, and operating current. The map can be applied to an alternative efficiency determination method (AEDM) for HVAC&R products. However, most of the existing maps have a low prediction accuracy under low ambient temperature heating modes, as such conditions were not thoroughly considered when maps were generated. This paper provides a comprehensive study of a fixed speed compressor performance under a wide range of operating conditions, including the AHRI 210/240 standard conditions. The test matrix was developed based on realistic heat pump system operating conditions. We then quantified the existing performance map’s prediction inaccuracy, particularly under the low ambient temperature heating modes. Driven by the significant deviation at lower ambient temperature conditions, a comprehensive performance map was generated based on the test results. We also found that one polynomial equation could simultaneously fulfill the needs for predicting the compressor performance under both the cooling and heating modes. Furthermore, the accuracy has improved significantly from 63% to 2% regarding the relative maximum prediction errors over existing performance maps. It suggests that to increase prediction accuracy when using the AEDM, one must provide a suitable superheat model in the capacity simulation of the heat pump systems with the fixed speed compressor.

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.

Evaluation of the Thermal Performance and Energy Efficiency of CRAC Equipment through Mathematical Modeling Using a New Index COP WEUED

As the world data traffic increasingly grows, the need for computer room air conditioning (CRAC)-type equipment grows proportionally. The air conditioning equipment is responsible for approximately 38% of the energy consumption of data centers. The energy efficiency of these pieces of equipment is compared according to the Energy Standard ASHRAE 90.1-2019, using the index Net Sensible Coefficient Of Performance (NetSCOP). This method benefits fixed-speed compressor equipment with a constant inlet temperature air-cooled condenser (35 °C). A new method, COP WEUED (COP–world energy usage effectiveness design), is proposed based on the IPLV (integrated part load value) methodology. The IPLV is an index focused on partial thermal loads and outdoor temperature data variation for air intake in the condenser. It is based on the average temperatures of the USA’s 29 major cities. The new method is based on the 29 largest cities worldwide and with data-center-specific indoor temperature conditions. For the same inverter compressor, efficiencies of 4.03 and 4.92 kW/kW were obtained, using ASHRAE 90.1-2019 and the proposed method, respectively. This difference of almost 20% between methods is justified because, during less than 5% of the annual hours, the inlet air temperature in the condenser is close to the NetSCOP indication.

Impact of Compressor Drive System Efficiency on Air Source Heat Pump Performance for Heating Hot Water

A standard Polish household with a central heating system powered by a solid fuel furnace was chosen as a case study. The modular Air Source Heat Pump (ASHP) was used to heat the hot water outside the heating season. In this article comparative studies of the impact of the compressor drive system used on the energy efficiency of the heat pump have been carried out in operating conditions. The ASHP heating capacity and coefficient of performance (COP) were determined for the outside air temperature in the range from 7 to 22 °C by heating the water in the tank to a temperature above 50 °C. For the case of a fixed speed compressor, average heating capacity in the range 2.7−3.1 kW and COP values in the range 3.2−4.6 depending on the evaporator supply air temperature were obtained. Similarly, for the inverter compressor, the average heating capacity in the range of 2.7−5.1 kW was obtained for the frequency in the range of 30–90 Hz and COP in the range 4.2−5.7, respectively. On cool days, the average heating capacity of the heat pump decreases by 12%. For the simultaneous operation of two compressors with comparable heating capacity, lower COP values were obtained by 20%.

Multi-Response optimization applied to a mechanically assisted reed valve of a hermetic reciprocating compressor

• novel valve concept to improve variable speed compressors. • simultaneous improvement of efficiency, cooling capacity and reliability. • significant reduction of the valve impact velocities. Reed valves are widely used in hermetic reciprocating compressors for domestic refrigeration. They are crucial components in terms of efficiency, cooling performance and reliability of the compressor. While reed valves already cause a significant proportion of the thermodynamic losses in fixed speed compressors, they cause even more challenges in variable speed compressors. Especially in variable speed compressors, a further improvement of the reed valve dynamics requires the consideration of a new valve concept. In this work, a new and cost-effective concept of a mechanically assisted suction reed valve is introduced. Simulation based response modelling and a multi-response optimization approach are applied to systematically optimize the design. Simulations between 1500 rpm and 5000 rpm indicate the strengths and weaknesses of individual optimized design variants over a wide compressor speed range. Calorimeter and valve dynamics measurements show considerable improvements of the COP and the valve impact velocity.

Energy saving in six doors walk-in cooler

Two Identical six doors walk-in coolers are installed with the fixed speed compressor and the digital scroll compressor. Both compressors have almost the same cooling capacity of 8 kW at the evaporating temperature of -6.7 °C and the ambient temperature of 32 °C. The heating loads from the envelope, the water bottles, and the air change during the door closing and opening of both coolers are the same. The temperature of the room in front of the cooler doors is controlled at 25 °C. They are set to have a similar average room temperature and a similar cool down. Both units are run 24 h per day for 30 days during which the ambient temperature and the power consumption are recorded. The results show that for Thailand weather, yearly saving is 20% for the digital scroll as compared with the fixed speed compressor.

Energetic Assessment of an Electronic and a Thermostatic Expansion Valve for a Variable Capacity Compressor

In this study, a chiller system with an electronic expansion valve (EEV) and a thermostatic expansion valve (TXV) were compared from an energy saving view under variable and fixed speed compressor. The experimental chiller system under study has a variable speed scroll compressor, EEV and TXV, air cooled condenser and auxiliary equipments. The R134a was used in the compressor as a refrigerant. Although in variable speed chiller systems usually EEVs are used, in this study a TXV was also used to compare its performance. The results of the experimental studies show that the chiller system with EEV has 30% lower superheat value and 6% lower power consumption than the chiller system with TXV under variable speed operation. From the fixed speed compressor view, the chiller system with TXV performs 1.7% lower superheat value than the chiller system with EEV and, the chiller system with EEV has 8.5% higher compressor power consumption than TXV.

An electronic expansion valve modeling framework development using artificial neural network: A case study on VRF systems

Electronic expansion valves (EEV) are widely used in variable refrigerant flow systems (VRF) to control the mass flow rate of each indoor unit. EEV model is used to predict the mass flow rate through an EEV. While the power-law correlation method has been used to build the EEV model so far, Artificial Neural Network (ANN) methods have been adapted to model the EEV with a fixed speed compressor thanks to its higher accuracy. However, the EEV is typically working with the variable speed compressor in a VRF system. In addition, the parameters used in ANN modeling could be further optimized. The objective of this study is to develop an EEV modeling framework and optimize the input parameter number and hidden neuron number. We presented the framework through an EEV model development for a VRF system. For these, we used the field test data, applied a principal components analysis approach in optimizing the ANN input parameter number, and investigated the proper number of hidden neurons and appropriate transfer function pairs. We found the performance of the ANN model would not improve much as the number of input parameters and the number of hidden neurons reached a threshold. Only three transfer function pairs are suitable for this case in total nine groups we studied. Our ANN model has the average absolute deviation of 2.2%. The framework can be easily applied to EEV modeling in other VRF and air condition systems with necessary data support.

Characterisation of efficiency losses in ground source heat pump systems equipped with a double parallel stage: A case study

This work focuses on methods to characterise the efficiency losses in existing ground-source heat pump (GSHP) installations under part-load conditions. Efficiency losses were identified and quantified in a GSHP by analysing key parameters measured and recorded by its monitoring system. The heating/cooling production facility comprises a vertical borehole heat exchanger field and a GSHP equipped with a double parallel stage and fixed-speed compressors within. The methodology was based on the EN-14825 standard and other recent works. However, the particularity of having two parallel stages motivated the creation of a modified theoretical approach, since the original methodology was only applicable to single-stage heat pumps. To obtain significant results, minute-resolved data acquired over time periods with almost constant thermal load (quasi-steady state conditions) proved necessary. The study concluded that the coefficient of performance can suffer reductions as high as 9% exclusively due to part-load operation. Most importantly, it demonstrated that the actual capacity of the GSHP as a whole shows an effective reduction ranging between 20% and 24% with respect to the declared capacity of the heat pump alone. Moreover, weekly-averaged data collected during 3 complete cool and warm seasons point to a progressive degradation from the date of commissioning. Finally, Ground Loop Design software (v2016) was used to validate the methodology. Consistent agreement was found between simulated and measured seasonal performance.

Improved control strategy for fixed-speed compressors in parallel system

Control systems for parallel compressor trains consist of a discharge pressure, anti-surge and load-sharing regulation. Control of a parallel compressor system has proven to be challenging because the control targets usually exhibit control interactions between the different control loops. To decouple this control interference, Mitsubishi Heavy Industries has developed an advanced feed-forward control structure for parallel fixed-speed compressor systems. However, operation in the presence of an unpredictable disturbance presents a few technical challenges for this structure. Most of these problems result in poor load sharing and then operation in the recycle mode in order to protect the system from surge conditions. Moreover, an anti-surge control delay occurs when operating under a low load. To overcome these problems, an improved control structure that incorporates an additional discharge flow controller signal and a nonlinear signal calculator for anti-surge valve control is proposed. To demonstrate the benefits of the proposed method, the control performance of the conventional control structure is compared to that of the proposed control structure. The target process is a parallel fixed-speed compressor system for a fuel gas supply plant. The improved control structure allows for desired load-sharing control, which is not possible with the conventional control structure. The improved control structure manages to exit the anti-surge control region, whereas the conventional control structure activates the interlock system.

Flexible and stable heat energy recovery from municipal wastewater treatment plants using a fixed-inverter hybrid heat pump system

Among many options to improve energy self-sufficiency in sewage treatment plants, heat extraction using a heat pump holds great promise, since wastewater contains considerable amounts of thermal energy. The actual heat energy demand at municipal wastewater treatment plants (WWTPs) varies widely with time; however, the heat pumps typically installed in WWTPs are of the on/off controlled fixed-speed type, thus mostly run intermittently at severe part-load conditions with poor efficiency. To solve this mismatch, a specially designed, fixed-inverter hybrid heat pump system incorporating a fixed-speed compressor and an inverter-driven, variable-speed compressor was developed and tested in a real WWTP. In this hybrid configuration, to improve load response and energy efficiency, the base-heat load was covered by the fixed-speed compressor consuming relatively less energy than the variable-speed type at nominal power, and the remaining varying load was handled by the inverter compressor which exhibits a high load-match function while consuming relatively greater energy. The heat pump system developed reliably extracted heat from the treated effluent as a heat source for heating and cooling purposes throughout the year, and actively responded to the load changes with a high measured coefficient of performance (COP) of 4.06 for heating and 3.64 for cooling. Moreover, this hybrid operation yielded a performance up to 15.04% better on part loads than the single inverter operation, suggesting its effectiveness for improving annual energy saving when applied to highly load-fluctuating real WWTPs. To improve the overall efficiency of the heat recovery system, although the heat pump is the largest energy-consuming component, taking up 56.0–68.5% of the total energy, new efforts to develop a novel design are also needed to make the heat exchanger more energy-efficient.

Fuzzy multivariable control of domestic heat pumps

Poor control has been identified as one of the reasons why recent field trials of domestic heat pumps in the UK have produced disappointing results. Most of the technology in use today uses a thermostatically-controlled fixed speed compressor with a mechanical expansion device. This article investigates improved control of these heat pumps through the design and evaluation of a new multivariable fuzzy logic control system utilising a variable speed compressor drive with capacity control linked through to evaporator superheat control. A new dynamic thermal model of a domestic heat pump validated using experimental data forms the basis of the work. The proposed control system is evaluated using median and extreme daily heating demand profiles for a typical UK house compared with a basic thermostatically-controlled alternative. Results show good tracking of the heating temperature and superheat control variables, reduced cycling and an improvement in performance averaging 20%.

Control of multi-evaporator air-conditioning systems for flow distribution

Modern air-conditioners incorporate variable-speed compressors and variable-opening expansion valves as the actuators for improving cooling performance and energy efficiency. These actuators have to be properly feedback-controlled; otherwise the systems may exhibit even poorer performance than the conventional machines which use fixed-speed compressors and mechanical expansion valves. In this paper, a control strategy with flow distribution capability is proposed for multi-evaporator air-conditioners to accommodate different thermal demands in different rooms. The structure in the control strategy is based on a low-order, linear model obtained from system identification. To determine appropriate control parameters, theorems regarding stability of the closed-loop system are given. Moreover, by performing steady-state analysis on the control system and utilizing characteristics of the identified system parameters, one can analytically explain the mechanics of flow distribution. Experiments indicate that the proposed strategy can successfully regulate the indoor temperatures regardless that the reference settings for respective rooms are different and the settings are switched in the middle of the control process.