Multi-ejector R744 booster refrigerating plant and air conditioning system integration \u2013 A theoretical evaluation of energy benefits for supermarket applications

The multi-ejector rack is the most promising technology to push the so-called \u201cCO2 equator\u201d further south and improve the global energy efficiency of R744 supermarket refrigeration systems. This paper theoretically compares the energy consumption of a CO2 refrigerating plant equipped with a multiejector unit with that of a R404A direct expansion system (DXS), of a conventional CO2 booster configuration and of two CO2 solutions using parallel compression. The energy benefits related to the adoption of low temperature (LT) overfed display cabinets were also assessed. Furthermore, various scenarios involving different sizes of the supermarket, integration and capacity of the air conditioning (AC) system and efficiency of the parallel compressors were investigated. The evaluations were carried out by considering different locations in Southern Europe. The results showed that, as a function of the selected boundary conditions, energy savings ranging from 15.6% to 27.3% could be accomplished with the multi-ejector concept over DXS

A comparative study to investigate two configurations of a two-stage evaporator in a CO2 heat pump chiller

This study is carried out to investigate the performance of a CO2 heat pump chiller that provides heating and cooling simultaneously. For heating applications, hot water is produced at 90°C; while for cooling applications, chilled water is produced at 4°C. The evaporator utilized for chilled water production is a novel two-stage evaporator. Two configurations have been proposed to implement this two-stage evaporator in the CO2 system. One configuration is termed GFES: the first stage is fed by gravity; the second stage is fed by ejector suction. The other configuration is termed EDES: the first stage is fed by the ejector discharge; the second stage is fed by ejector suction. The primary goal of this study is to compare the performances of these two evaporator configurations and their effects on the system’s overall performance. To fulfill this objective simulation models are developed in Modelica. Results show that the COPs are nearly the same for both configurations. In addition, it is observed that the integration of the two-stage evaporator enhances the overall performance as the cooling capacity is shared between the two stages and the suction pressure of the compressor is elevated by utilizing the ejector.acceptedVersio

Experimental evaluation of CO2 ejector cooling system for supermarkets at tropical zones

Cooling system for supermarkets, now-a-days in developing countries like India, mainly use man-made synthetic refrigerants/mixtures such as R134a, R404A and R410A (HFCs). These fluorinated refrigerants possess outrageous impact on the environment/ambient due of their high GWP and ODP. The EU F-gas regulation is formulated recently and came into force in 2014 in order to restrict the use of synthetic refrigerants for various HVAC applications. Under this regulation, it is also illegal to vent any such synthetic refrigerants to the atmosphere during the system servicing or end-of-life decommissioning. Sudden phase down of HFCs forced the present scenario to identify a potential replacement of these synthetic refrigerants. The influence of the same is comparatively high for developing countries. In the present study, the performance of a multi-ejector based supermarket cooling system is experimentally evaluated using natural refrigerant carbon dioxide (CO2). 33 kW cooling capacity of the system which is capable of maintaining three simultaneous different temperatures such as -29 °C for freezing, -6 °C for refrigeration and 7-11 °C for air-conditioning is examined at high ambient temperature context (up to 46 ºC gas cooler outlet temperature). The maximum COP of the supermarket cooling system appeared as 4.2. It is also observed that the maximum exergy efficiency of system is 0.316 obtained corresponding to 3.2 PIR of the system

An experimental study of an ejector-boosted transcritical R744 refrigeration system including an exergy analysis

The field of refrigeration witness a massive transition in the supermarket with a strong focus reflected on energy consumption. The use of ejector allows for overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. In this study, a detailed experimental work and exergy analysis on the R744 transcritical ejector cooling system was investigated. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The impact of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature and receiver pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destruction were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provide a maximum pressure lift of 9.51 bar. Moreover, it was found out that the overall available work recovery potential increased by rising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the exergy consumed and destruction were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature. © 2021 Elsevier LtdacceptedVersio

Performance improvements of supermarket R744 systems by pivoting compressor arrangements

The adoption of the EU F-Gas regulation 517/2014 and the development of the Multi Ejector concept have led CO2 to take center stage as one of the preferred solutions in several applications, at the expense of synthetic refrigerants. Despite the expected significant energy saving in warm climates using the Multi Ejector, the increase in investment costs and level of complexity would hinder its spread. In this work a numerical and experimental campaign have been performed to explore the implementation of “pivoting” compressors, i.e. a technique that enables the medium temperature (MT) and parallel (IT) compressors in a booster system to be interchangeable according to cooling loads, ambient conditions and ejector capacity. The novel configuration presented in this work helps to downsize the installed compressor capacity in ejector-supported systems while maintaining all the benefits due to the ejector. The basic version of the solution is based on: i) MT and low temperature (LT) compressors, ii) high pressure controlled through a Multi Ejector both during summer and winter conditions. The tests performed in the laboratory proved how the “pivoting” solution is beneficial to attain a higher degree of flexibility with more compact systems while maintaining the efficiency and justifying economically the ejector implementation. An additional solution called LT “pivoting”, i.e. connecting LT compressors either to MT or IT compressors, proved to be particularly useful for energy saving.Performance improvements of supermarket R744 systems by pivoting compressor arrangementsacceptedVersio

Cold thermal energy storage in solid-liquid transition of carbon dioxide: Investigating the possibility

Industrial freezing is an energy-intensive process which is growing due to the increasing demand. This is exerting stress on electrical grids, especially at peak hours. To tackle this issue, thermal energy storage has received attention; however, there is a gap in terms of suitable materials for thermal energy storage with temperatures below -40 °C commonly needed in these applications. In this paper, the solid-liquid phase change of carbon dioxide has been conceptually considered for thermal energy storage in a special type of heat exchangers known as pillow plate heat exchangers. Characteristically, these heat exchangers can withstand very high pressures which is a technical requirement for carbon dioxide thermal energy storage. This paper discusses the potential system layout and challenges ahead of this technology, along with the proposal for further investigation to verify the concept.Cold thermal energy storage in solid-liquid transition of carbon dioxide: Investigating the possibilityacceptedVersio

Cold thermal energy storage for industrial CO refrigeration systems using phase change material: An experimental study

Refrigeration systems in industrial food processing plants are large users of electric energy and often show high peak power consumption. Cold thermal energy storage (CTES) technology integrated into refrigeration systems can reduce the peak power requirement and achieve peak shifting by decoupling the supply and demand of the refrigeration load. This paper presents the design and performance of a CTES unit consisting of a pillow plate heat exchanger (PP-HEX) immersed into a low-temperature phase change material (PCM) as the storage medium. It is one of the first experimental investigations featuring a large-scale technical solution that allows for coupling the evaporation and condensation processes of the refrigeration system with the melting and solidification of a low-temperature PCM in the same heat exchanger. The charging and discharging performance of the plates-in-tank CTES unit was extensively tested using CO as the refrigerant and a commercial PCM with phase change temperature of -9.6 °C. The charging time was found to be mainly affected by the refrigerant evaporation temperature, while the discharging rate and discharged energy over the cycle was higher when increasing the refrigerant condensing temperature. Using a plate pitch of 30 mm resulted in the highest mean discharge rate and total discharged energy over the cycle with 9.79 kW and 17.04 kWh, respectively. The flexible CTES-PCM unit can be adapted to fit several refrigeration load characteristics and temperature levels by changing the PP-HEX geometry and type of PCM used as the storage medium.publishedVersio

Multiejector CO2 cooling system with evaporative gascooler for a supermarket application in tropical regions

In the present study, the performance of a 33 kW multiejector trans-critical CO2 cooling system is experimentally evaluated for a supermarket application with/without internal heat exchanger and evaporative cooling. In order to enhance the overall performance of the system for tropical regions, the testing is carried out at high ambient temperature (up to 46 °C) with 5 cm, 10 cm and 15 cm cooling pad thickness arrangements. The experimental results clearly projects that the evaporative cooler capacity reaches a maximum of 10 cm pad thickness. However, a minor improvement is observed in terms of Coefficient of Performance and Power Input Ratio beyond 10 cm pad thickness. Maximum improvement in COP with internal heat exchanger and evaporative cooler is 11% and 40% respectively. On the other hand, a maximum reduction in the system Power Input Ratio with internal heat exchanger and evaporative cooler is 8.5% and 26% respectively. However, a minor enhancement in Coefficient of Performance and Power Input Ratio of 4% and 6% are observed respectively with 15 cm cooling pad thickness. Furthermore, a comparative analysis is carried out with the existing and present experimental study to project the compatibility of an evaporative condenser in the ejector based CO2 cooling system. From the study, it is evident that the evaporative cooling arrangement for the gascooler of the CO2 system is suggested as a potential solution to the supermarket application at a high ambient temperature context. © 2021 Elsevier LtdacceptedVersio

Simultaneous implementation of rotary pressure exchanger and ejectors for CO2 refrigeration system

Natural refrigerant CO2 has become a viable choice for refrigeration units. The CO2 systems are working efficiently on land-based facilities, and their demand is increasing for offshore applications, e.g., cruise ships and fishing vessels, due to their environment-friendly nature and compactness. The investigated application of the CO2 system in this work is a single-stage system for air conditioning and a two-stage system for provision refrigeration at high heat rejection temperatures. The CO2 transcritical cycle allows operating in higher ambient temperatures and in a colder climate with significant heat recovery. However, the system efficiency decreases in higher ambient conditions due to the high-pressure ratio and expansion losses. Therefore, ejectors are implemented to boost the cycle efficiency at high heat rejection temperature conditions. The pressure exchanger (PX) device recently came up and claimed to be an option to recover expansion work in CO2 systems. PX is already in use for reverse osmosis (RO) desalination units to recover pressure work from the high pressure reject concentrate to low-pressure seawater. This work theoretically investigates the implementation of a CO2-PX for transcritical CO2 systems combined with ejectors and compressors. The energy efficiency of alternative system configurations is evaluated for various operating conditions.Simultaneous implementation of rotary pressure exchanger and ejectors for CO2 refrigeration systemacceptedVersio

Performance evaluation of high-temperature heat pump systems for hot water and steam generation in food processing

A large amount of fossil fuels is consumed for hot water and steam generation in food processes such as smoking, scalding, drying, pasteurization, sterilization, cleaning, and cooking, which currently rises both economic- and environmental concerns. At the same time, there is a considerable number of low-grade waste heat available, often from associated cooling processes. High-temperature heat pumps (HTHPs) are considered as a promising solution for steam boilers replacement and waste heat recovery owing to their high energy efficiency and sustainability. In the present study, the performance of three heat pump systems — a trans-critical CO2 heat pump system, a cascade propane-butane heat pump system and an ammonia-water absorption-compression heat pump (ACHP) system, for hot water and steam production in food processing is evaluated based on different application scenarios. In all the scenarios and temperature lift levels, the ACHP exhibits the best thermal performance with moderate pressure levels and low-pressure ratios. When applying the ACHP for both the cleaning and cooking processes, the achievable energy saving rate can reach 79%. Further, the optimal HTHP system architectures for different application scenarios are discussed.acceptedVersio