Direct Expansion System Research Articles

Performance evaluation of a central air conditioning system using condensate as a discharge line cooler

The energy consumption by central air conditioning (A/C) in a building is very high and efforts to reduce this energy consumption is ongoing. This study investigates the use of condensate produced by the evaporator inside the air handling unit (AHU) to cool the compressor discharge line. A central A/C with a direct expansion (DX) system and a compressor capacity of about 25 HP which uses R22 as a working fluid was used. The discharge line has a diameter of 15.9 mm wrapped with a 520 mm long heat exchanger with diameter 50 mm. The condensate produced by the AHU is passed to the heat exchanger that envelops the discharge line. The use of discharge line cooler could reduce the input power from 18.21 kW to 17.54 kW. On the other hand, the cooling capacity increased from 46.44 kW to 49.11 kW. This means that discharge cooler reduces the input power by 3.8 % and increases the cooling capacity by 5.7 %. As a result, the COP increases by 9.8 %, from 2.5 to 2.8. The method developed is not complex and only simple modification is required, thus an improvement of the performance of a central A/C system can be achieved.

Comparative analysis: Exergetic and economic assessment of LNG cold energy power generation systems with different cold utilization methods

Harnessing the cold energy inherent in LNG transportation processes can significantly mitigate energy wastage. Employing an innovative incremental analysis methodology, this study scrutinizes six LNG cold energy power generation systems, featuring a newly proposed parallel and cascade combined cycle (PAC) system. A novel approach that setting the minimum pressure within the systems higher than atmospheric levels has been adopted, for the optimal working fluid selection. The net power output (Wnet) of the direct expansion (DC) system registers at −129.51 kW, while both the Single-Stage Organic Rankine Cycle (SORC) and Combined Cycle (CC) systems yield Wnet of 2868.46 kW and 3081.46 kW with R32 as working fluid. Despite the lower available exergy extraction of the working fluid, the CC system outperforms due to its superior efficiency in converting pressure exergy into power output. Sensitivity analysis suggests the optimal Tcon in CC is limited by the normal boiling point temperature (NBPT) of working fluid, while that in SORC remains unrestricted. The maximum Wnet of two-stage Parallel Combined Cycle (PCC) and Cascade Combined Cycle (CCC) can reach 3291.65 kW and 4268.78 with the optimal working fluid combinations R32 + propane, and ethane + propane. The reason Wnet of CCC outperforms is the cascade utilization of LNG cold exergy enables the working fluid in its second stage obtains significantly 9.49 times higher amount of exergy compared to PCC. Through sensitivity analysis, while Tcon1 mostly predominates the performance of PCC, both Tcon1 and Tcon2 exert substantial influence on CCC. For the three-stage PAC, its Wnet can reach the highest 4700.82 kW with ethane + propane + propane. It is because the PAC not only utilizes LNG’s cold exergy in a cascaded manner, but also obtains the exergy from working fluid. According to economic analysis, the PAC system exhibits an advantage with the highest annul total net income (ATNI). And the CC emerges as a cost-effective choice if the irreversible losses and Wnet are not considered. According to innovatively explores about the impact of direct expansion and changes in NG outlet pressure on working fluid selection and economic feasibility, it shows the optimal combinations remain the same, and the systems incorporating a direct expansion component with lower NG outlet pressure demonstrate a more economically advantageous solution.

Experimental Investigation of R404A Indirect Refrigeration System Applied Internal Heat Exchanger: Part 1—Coefficient of Performance Characteristics

In this study, the performance characteristics of an R404A indirect refrigeration system (IRS) applied to an internal heat exchanger (IHX) is evaluated for supermarkets and hypermarkets. In a direct expansion system, R404A is the primary refrigerant and R744, a brine, is the secondary fluid. While there are abundant studies analyzing the theoretical performance of IRS, experimental research on IRS is lacking, and there are no papers that address the results of changes in the IHX in detail. In this study, the results achieved by modifying various parameters are experimentally evaluated to provide fundamental data for designing the optimal IRS. In the main results, looking at the trend of the increase in IHX efficiency, the change is very minimal when the efficiency is above 50%, so it is ideal to apply an IHX efficiency of about 50% considering economics and COP, etc. Applying the results in this study enables the operation and maintenance of IRSs as an eco-friendly system by achieving energy efficiency through optimizing the system coefficient of performance and securing economic feasibility by minimizing the R404A charging amount of the refrigeration cycle. To serve supermarkets and hypermarkets, R744 as a secondary fluid may help to realize an ecologically friendly, compact IRS system with a high heat transfer coefficient that can operate at low temperatures (−35 to 5 °C).

Environmental and energetic impact of refrigeration systems using CO2 (R744): A Cuban case study

Despite the extensive use of CO2 in food retail refrigeration units, it has never been implemented, tested or simulated in Cuba. This work aims to evaluate by simulation a Booster and Parallel compression system in twelve Cuban cities using two different semi-empirical approaches to model the compressors and the gas cooler/condenser. The compressors are modeled with a semi-empirical model and the gas cooler/condenser is modeled with a lumped model based on the ε-NTU method, discretizing the gas cooler in a one-zone model and the condenser into a three zones model. According to the results, the Booster and Parallel average consumptions are 640 MWh and 540 MWh, respectively. Both systems are less efficient than the conventional R404A refrigeration system. The lower capital costs of transcritical systems make them cost-effective solutions compared to the conventional system. Moreover, the Parallel system is an economically feasible alternative compared to the Booster one. The payback period of the Booster system is 10 to 12 years by considering the environmental and operating costs savings respect the Direct eXpansion System, while for the Parallel system the payback period is between 8 and 8.5 years. Both transcritical systems are excellent environmental alternatives to the R404A conventional system.

Integration of solar thermal collectors and heat pumps with thermal energy storage systems for building energy demand reduction: A comprehensive review

Solar energy, coupled with innovative technologies, holds the promise of propelling buildings towards net-zero and carbon neutrality. In this regard, this review explores the integration of solar technologies, heat pumps, and thermal energy storage systems to reduce building energy demand. It thoroughly examines various types of solar thermal collectors (STCs), including both concentrating devices like compound parabolic concentrators and parabolic troughs, as well as non-concentrating designs such as flat plate and evacuated tube collectors. It examines innovative strategies for enhancing STC performance, such as alternative profiles for compound parabolic concentrators and the insertion of thermal fins in absorber tubes. Furthermore, the review categorizes solar-assisted heat pumps (SAHPs) into indirect expansion (IDX) and direct expansion (DX) systems, describing their advantages and limitations. It performs a comparison between IDX-SAHPs and DX-SAHPs, explaining their effectiveness and applicability. Eventually, the review explores thermal energy storage materials, categorizing them into sensible heat storage, latent heat storage, and thermochemical heat storage (TCHS). It discusses the advantages and disadvantages of each category, as well as notable materials within them. A comparative examination between open and closed systems for TCHS further enriches the discussion. Throughout the review, recent advancements and key findings from relevant studies are summarized in separate tables, offering valuable information into practical strategies for sustainable building energy systems. Regarding the provided sections, this review plays a crucial role in introducing scholars to practical methods employed in recent years to integrate the STCs with heat pumps and thermal energy storage systems.

A two-step identification framework for building thermal dynamic based on high fluctuation performance data: A case of office with variable refrigerant flow system

Thermal dynamics model plays a crucial role of advanced control in the intelligent building. Grey box method is widely used in the modeling of building thermal dynamics based on the operation data of air-conditioner. The performance of direct-expansion (DX) system fluctuated severely during its operation, which leads to the difficulties in identification and evaluation of the building thermal model. In this paper, a two-step identification framework for second-order thermal resistance-capacity network of building based on DX system data is proposed. A case study is conducted on an office building equipped with variable refrigerant flow system in Foshan, China. The results show that the proposed framework can identify the parameters rationally. The model achieves high accuracy for both indoor temperature (Mean absolute errors of 0.74°C) and cooling load prediction (Mean absolute errors of 4453.86W) in a short time interval of 10 minutes. Additionally, the best complexity of the grey-box model for case building and the data acquisition interval is determined. Finally, an improved set of evaluation metrics for thermal load evaluation is proposed and discussed.

Sustainability and operational performance assessment of supermarket air-conditioning architectures using secondary fluids and slurries

Secondary loop architectures and phase change material (PCM) slurries could reduce the environmental impact of refrigeration and air-conditioning systems by lowering significantly the amount of primary refrigerants. However, there is a barrier to their industrial development due to the lack of studies on their behaviour in operational scenarios. The present work analyses the behaviour of different secondary loop systems in an industrial context, here the air conditioning of a supermarket in France. For this purpose, a generic approach, developed in previous work, is proposed to assess their potential of adoption. This modelling approach is able to describe the sustainability and operational performance (energy, environmental, economic, social) of refrigeration systems. After validation of the model on real architecture and components, five standard or new architectures were tested: centralized direct expansion system; secondary loop system with ethylene glycol water; secondary loop system with ice slurries, TBPB hydrate slurries or CO2 hydrate slurries. The main results of this study show that new secondary systems using hydrate slurries have better cooling energy performance than direct expansion or classical brine / ice slurry secondary systems. Moreover, even including pumping power, the life cycle climate performance of secondary loop systems is much better than that of direct expansion systems. However, the total cost of ownership of direct expansion systems is lower than that of secondary systems, but with little differences for smaller pipes. Finally, trade-offs between several performances can be proposed. For example, some architectures with suitable pipe diameters could meet TCO/LCCP-based and CAPEX/pumping-based trade-offs.

Ammonia heat pumps for district heating: thermo-economic optimization analysis based on evaporator geometry for either direct expansion or chilled water configurations

Electric heat pumps are recognized as a key technology for decarbonization and have received increasing policy support in several countries over the last few years. These devices offer a highly efficient form of electric heating and could make an important contribution to the transition to a low carbon future, especially in case of district heating systems. Within this context, this paper presents a thermo-economic optimization analysis of a 7.35 MW electric heat pump system employed for district heating purposes and using ammonia as working fluid. The implemented code gathers multiple sub-models calibrated ad-hoc from real data or manufacturers’ datasheets. The heat exchangers are simulated by considering phenomenological equations and well-known heat transfer coefficient and pressure drop prediction methods. The optimization analysis is performed at constant condensation temperature that guarantees in/out hot water temperatures of 30/62 °C and is focused on the matching between compressor and evaporator heat exchanger, by considering either a direct expansion system or a chilled water heat pump. Both the coefficient of performance (COP) and the set-up costs are considered as optimization performance indicators for the construction of the Pareto front.

Performance Analysis of an R744 Supermarket Refrigeration System Integrated with an Organic Rankine Cycle

The energy and economic performance of a transcritical R744 booster supermarket refrigeration system with and without parallel compression and integrated with an organic Rankine cycle (ORC) was investigated. The results obtained were compared with those of a transcritical R744 booster supermarket refrigeration system with and without parallel compression and those of a conventional R404A direct expansion (DX) system. Nine different locations, namely Copenhagen (Denmark), Paris (France), Athens (Greece), New Delhi (India), Phoenix and Miami (US), Madrid (Spain), Bangkok (Thailand) and Riyadh (Saudi Arabia), were considered. It was discovered that the ORC is effective only at ambient temperatures higher than 27 °C when operating without parallel compression and 28 °C when operating with parallel compression. By using the heat recovered from the gas cooler to fuel the ORC, the latter was found to be capable of covering between 4% and 24% of the electricity demand of the R744 system in warm and hot climates (without parallel compression). The simple payback period of the additional investment associated with the ORC was found to be between 1.4 and 2.5 years in warm climate locations, while the same was found to be less than about 0.5 years in locations experiencing hot climatic conditions.

Development and demonstration of a method to detect refrigerant charge level for variable refrigerant volume systems

Unlike traditional direct expansion systems, variable refrigerant flow (VRF) systems have longer routing, more complicated setup, and sophisticated controls and require much more refrigerant for proper operation. Proper refrigerant charge becomes more critical to their operations. Intensive studies have investigated refrigerant charge faults for VRF systems. Due to the daunting complexity, few studies use knowledge-based fault detection and diagnosis (FDD) methods. Most of them use data-driven FDD methods to study this fault. Although they seem promising, these methods can only handle single faults and are not scalable or interpretable. This paper proposes a decoupling-based FDD method to detect and diagnose improper refrigerant charge in VRF systems. This method was originally proposed by our previous work for rooftop units (RTUs) and split systems, which has been tested and validated by extensive studies. Due to the differences between VRF and RTU, the original method – virtual refrigerant charge sensor (VRC) for VRF is modified, analyzed, and investigated in detail. Data filter criteria were implemented to eliminate noisy data. The modified VRC, when combined with a data filter, was tested on two systems in the field. The results demonstrated its effectiveness in detecting an undercharged system, specifically a 30 % undercharge.

Optimal design of direct expansion systems for electricity production by LNG cold energy recovery

The aim of this paper is the investigation of cold energy recovery of liquefied natural gas (LNG) in regasification terminals achievable through direct expansion techniques. After an overview of the state of the art related to the LNG system and the technologies proposed for power recovery, multi-pressure direct expansion solutions with internal heat recovery (multi-pressure DE) are analysed. The purpose is to identify the highest level of recoverable energy and the optimal design parameters, considering technological constraints such as pressure and minimum end-expansion temperature. The multi-pressure DE solutions prove to be innovative power recovery methods because, with a relatively low use of seawater, it is possible to save regasified LNG and generate electrical power, valorising the energy invested in liquefaction. An estimate of recoverable power in operating terminals is carried out. Configurations with cryogenic or non-cryogenic turboexpanders are discussed. Using the proposed solution with non-cryogenic turboexpander and distribution pressure of 3.5 MPa, 56 kJ for each kg of LNG can be produced, which can become more than 150 kJ/kg with cryogenic turboexpanders. For long-distance distribution, cryogenic multi-pressure DE cycles can recover around 100–120 kJ per kg of LNG. The presented techniques can allow recovery of up to and over 30% of the maximum specific work available in LNG regasification.

On the Use of Ultrasonic Flowmeters for Cooling Energy Metering and Sub-Metering in Direct Expansion Systems

The Energy Efficiency Directive (EED, Directive 2012/27/EU) has made mandatory the installation of individual metering systems in the case of buildings with centralized heating/cooling and hot water sources (multi-apartment and multi-purpose buildings), provided it is economically and technically feasible. Individual metering of heating/cooling systems is mainly based on thermal energy meters (TEM), which are widely used for direct metering in heating applications. On the other hand, direct metering of energy consumption in cooling systems still represents a challenge, given the different types of cooling units and the lack of regulations from the technical and legal points of view. In this context, this paper briefly overviews the available centralized cooling systems and the possible solutions for metering and sub-metering, which depend on the specific application. Vapour Compression Refrigeration (VCR) systems are spreading worldwide for air conditioning applications. Particular attention has been paid to the direct metering of cooling energy and specifically to refrigerant flow rate measurement, which represents a critical issue because of the small-diameter pipes and the different thermodynamic properties of the fluid used. Thus, an experimental campaign has been developed and carried out in order to compare a clamp-on ultrasonic flowmeter with a more accurate Coriolis one in a direct expansion (DE) system. The experimental tests have been performed at two different temperature conditions, showing a relative error in the mass flow rate measurements within ±10%.

Improving the performance of a commercial absorption cooling system by using ejector: A theoretical study

Absorption cooling systems (ACS) have lower coefficients of performance (COP) compared to direct expansion (DX) cooling systems. Nevertheless, ACS offers a green alternative to typical DX systems. In this study, a numerical model was developed for the commercial low-capacity Robur® absorption cooling system (RACS). The model was developed based on mass, concentration, and energy balance equations, in addition to heat transfer equations. The model results were validated against experimental data available in the literature for the same cooling unit yielding a good agreement. Hence, to improve the COP of the RACS, a vapor ejector was introduced between the generator and the condenser. An improvement of 70.6% in the COP was obtained at the design condition. A parametric analysis was implemented to study the significance of the key parameters in the RACS performance. It was found that the increase in the ambient temperature not only increased the activation temperature, but it also decreased the COP and increased the circulation ratio (CR). Consequently, in hot environments, lowering the evaporator temperature is recommended to avoid the need for higher CR. Optimizing the nozzle throat and the mixing tube diameter improves the ejector performance, and hence the RACS performance, as long as the ejector operates under critical conditions. Finally, the absorber coil was found to have the most significance on the RACS performance in comparison with the rectifier coil and the refrigerant heat exchanger.

Selection of HVAC technology for buildings in the tropical climate case study

In this paper, the integral performance indicator for selecting HVAC systems (ACPI) is applied to select the Air Conditioning (AC) system with better results in their life cycle for a typical building in Barranquilla, Colombia. The AC systems under study were: direct expansion system (Split and VRF), air-cooled system (VAC and VAV), and water-cooled system (Chiller). Of the six selection criteria of ACPI, the energy consumption index (EC) and thermal comfort (TC) were calculated through energy simulation of buildings, the regulations were used for IAQ and carbon emissions (CE), and were estimated the investment costs (IC) and operating and maintenance costs (OMC). The best-performance AC system for the building was the air-cooled AC system got the best ACPI values between 25 % and 22.4 %, followed by the water-cooled system with 22.3 %, and finally the direct expansion systems between 16.8 % and 13.5 %, influencing in this level of priority the IAQ and IC criteria. This study contributes to introducing in Colombia a multifactorial analysis to select the more proper AC system technology according to the building and location conditions.

Experimental evaluation of energetic and exergetic enactment for exotic evaporative to expansion-type edifice-coolers

Evaporative cooling is a natural, environment-friendly, and economical method of air-conditioning involving minimal energy consumption. However, the wider dependency on the climatic condition, lower cooling capacity, lesser effectiveness, excess humidification, etc., limits the evaporative coolers' global acceptance. Therefore, multi-staging of evaporative coolers or hybridization with any conventional direct expansion system is recognised as a solution for building air-conditioning. Accordingly, a three-stage cooling test rig comprising indirect and direct evaporative coolers followed by a direct expansion system is indigenously developed and efficacy of seven single to multi/hybrid modes is experimentally evaluated. Rigorous experiments are conducted during the entire summer of Hyderabad, India, from April to July of 2019 by operating the test rig from 20th to 26th of each month from 8:00 AM to 8:00 PM with an interval of one hour. Based on the acquired experimental data, energy, exergy, heat transfer and environmental analyses are carried out. The three-stage cooling combination exhibits the highest wet-bulb effectiveness reaching a maximum of 300% with the lowest supply air temperature of 16 °C. Multi-staging provides an average sensible cooling load reduction ranging from 15.21% to 65.04% on the direct expansion cooling system. Moreover, the two-stage indirect-direct evaporative cooler exhibits excellent exergy efficiency with sufficient cooling capacity reaching 90%. The DEC-DX, IEC-DEC and IEC-DEC-DX cooling combinations exhibit superior SEER magnitudes over entire cooling season indicating effectiveness of evaporative pre-cooling.

Multi-objective optimization of a novel combined parallel power generation system using CO2 and N2 for cascade recovery of LNG cryogenic energy

Liquefied natural gas (LNG) is a kind of clean fuel with high energy density, and a large amount of cold energy will be released during gasification, which should be recycled for energy conservation. A novel combined parallel power generation system respectively using CO2 and N2 as working fluid is proposed for cascade utilization for LNG cryogenic energy. The solar heating system is introduced to improve the overall performance, and the obstacle of discontinuous operation of the solar system is solved by using two serial turbines with different inlet parameters. Thermodynamic analysis and multi-objective optimization are conducted for the novel system, and performance comparison has been made with combined N2 and LNG system with or without regenerator, combined CO2 and LNG system, LNG direct expansion system. Results show that the novel parallel power generation could realize the cascade utilization of LNG cold energy with less exergy loss and better overall performance compared to the other four systems, and it is more efficient than most ORC systems with less complex configurations. The optimal average net power of the novel system is 12.4816 MW, and the energy efficiency and exergy efficiency are 17.25% and 29.90%, respectively.

Life cycle assessment of an experimental solar HVAC system and a conventional HVAC system

Solar heating ventilating air conditioning systems are useful tools to meet the objectives of the European Commission in terms of sustainability in buildings, since their use can reduce the environmental impact, including CO2 emissions, due to their low energy consumption. In order to quantify the improvement that in environmental terms the use of this type of system could entail, in this work it was carried out (a) a comparative life cycle assessment of a solar heating ventilating air conditioningsystem based on evaporative cooling and desiccant wheel with a conventional direct expansion system; and (b) an analysis of feasible modifications of the desiccant wheel based system and their influence on the life cycle analysis results. The experimental desiccant wheel based system showed a slightly higher environmental performance than the conventional direct expansion based system, between 2% and 10%, for the 3 impact categories evaluated: human health, ecosystem quality and resource consumption. When weight optimisation and the reuse of materials were considered, the environmental performance of the experimental based system became even up to between 22% and 50% higher than that of the conventional direct expansion based system. That involved a 60 % reduction in climate change potential indicator, which mainly was influenced by CO2 emissions.

Effect of HVAC’s Management on Indoor Thermo-Hygrometric Comfort and Energy Balance: In Situ Assessments on a Real nZEB

This paper proposes the analysis of real monitored data for evaluating the relationship between occupants’ comfort conditions and the energy balance inside an existing, nearly zero-energy building under different operational strategies for the heating, ventilation, and air-conditioning system. During the wintertime, the adaptive comfort approach is applied for choosing the temperature setpoint when an air-to-air heat pump provides both heating and ventilation. The results indicate that in very insulated buildings with high solar gains, the setpoint should be decided taking into consideration both the solar radiation and the outdoor temperature. Indeed, when the room has large glazed surfaces, the solar radiation can also guarantee acceptable indoor conditions when a low setpoint (e.g., 18.7 °C) is considered. The electricity consumption can be reduced from 17% to 43% compared to a conventional setpoint (e.g., 20 °C). For the summertime, the analysis suggests the adoption of a dynamic approach that should be based on the outdoor conditions and differentiated according to room characteristics. Considering the indoor comfort and the maximization of renewable integration, the direct expansion system has better performance than the heat pump; this last system should be integrated with a pre-handling unit to be energy convenient.

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.

Climatic Effect on the Exergetic Performance of Conventional to Hybrid Evaporative Coolers With Varying Dead-State Temperatures in India

Abstract A comprehensive exergy, exergo-economic, and sustainability assessment of seven conventional to hybrid air-conditioning systems comprising direct and indirect evaporative coolers with direct expansion system and their several combinations integrated into an eight-story domestic building for five different cities corresponding to arid, semi-arid, humid subtropical, tropical wet and dry, and tropical wet climatic zones across India are investigated based on simulation output from energyplus. The exergetic performances are reported for varying dead state temperatures ranging from 5 °C to 40 °C, while the saturated humidity ratio and pressure at system outlet are two other dead state properties. The results reveal that the specific exergy of moist air and exergetic efficiency decreases with increasing dead state temperature and becomes least at a dead state temperature near American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) comfort temperature of 23 °C. In arid, semi-arid, and humid subtropical climates, the three-stage evaporative cooling system exhibits the lowest exergy destruction of 100 J/kg1 and the highest exergy efficiency of 90% at a dead state temperature of 40 °C. The two-stage direct evaporative-direct expansion cooling system exhibits superior exergy efficiency in tropical wet and dry and tropical wet zones. Furthermore, the Grassmann diagram based on the climate of Hyderabad indicates that the three-stage cooling system is energetically and exergetically optimum with exergy destruction of 28.86%.