Simulation of an Ammonia-Carbon Dioxide Transcritical Cascade Refrigeration System

To improve human society in a sustainable way, energy efficiency and environmental protection have emerged as essential concerns. As a result, employing low-quality thermal energy is viewed as a prudent move with obvious advantages for energy efficiency [1]. The thermally activated absorption refrigeration system is a competitive technology for recycling engine waste heat for heating, cooling, and power generation. However, engine waste heat can be inconsistent and shift based on usage [2]. The cooling capacity is sensitive to engine output, making single-stage absorption chillers difficult to use to achieve lower refrigeration temperatures, particularly when storage temperatures are below 30℃. Chlorofluorocarbons and hydro chlorofluorocarbons have significant global warming or ozone depletion potentials [3], which makes them harmful to the environment if used. Thus, in recent years, the search for ecologically friendly, naturally occurring chemicals to utilize as substitute refrigerants has gained momentum. As a natural gas, carbon dioxide (CO₂) is one of the few refrigerants that is not harmful if accidentally released. Thermo-physically, it's unparalleled, and it's also a near-perfect working medium [4]. The industrial refrigeration market is expected to reach $25 billion by 2025, driven by rising consumer demand, innovative refrigeration systems, and government assistance for cold chain infrastructure improvements. This growth affects various sectors and our daily lives [5]. The high energy consumption of refrigeration systems and the unfavorable environmental effects of many refrigerants, such as their high global warming potential (GWP), have made it challenging to control the rapidly expanding refrigeration sector [6]. For this reason, boosting the efficiency of refrigeration systems and switching to low global warming potential (GWP) refrigerants are two important avenues for reducing the negative effects of refrigeration systems on the environment and making substantial carbon cuts [7]. Refrigeration technology has had far-reaching effects on human productivity and survival. It sees widespread use in everyday life, commerce, and manufacturing. The absorption refrigeration system (STARS) and the common single-stage compression refrigeration system (STCRS) are the two primary types of refrigeration technology. STCRS is necessary for air conditioning, food storage, human survival, and transportation [8]. Applications like fast freezing and storing frozen food require a high compression ratio, a considerable temperature difference in the heat exchanger, or relatively low temperatures in the evaporator (between -40 and -50℃). Both the volumetric efficiency and coefficient of performance (COP) of the STCRS will decrease as a result of the high output temperature and pressure of the refrigerants [9]. STARS transforms waste heat into high-quality cold energy for cooling processes, but refrigeration systems are limited at low evaporation temperatures. CRS is proposed for more manageable refrigeration temperatures and is used in cryogenics, instrument cryopreservation, and hypothermic medicine. However, as temperature differences between heat sources and cold energy increase, the coefficients of performance and economy decrease [10]. Supermarkets also frequently make use of this type of refrigeration for storing food, transporting food, and cooling the store. The device is versatile enough to function with a low evaporation pressure at ambient temperatures and a moderate condensation pressure at higher temperatures [11]. Another popular CRS has two circuits: a high-temperature cycle (HTC) and a low-temperature cycle (LTC). This system is known as a two-stage vapor-compression cascade refrigeration system (CCRS) [12]. The two circuits are linked together by a heat exchanger that operates as both the evaporator and condenser for the low-temperature circuit (LTC). The two-stage CRS has greater cooling capacity and a higher COP [13] due to its optimized design compared to the single-stage STRS. Since a drawback of two-stage CRS is its greater reliance on electrical power, a heat-and-power-driven compression-absorption cascade refrigeration system (CACRS) has been developed. With a compression system at LTC and an absorption system at HTC, this refrigeration system is linked in a cascade. The lowest evaporation temperature is -170℃ [14], and the two subsystems employ different kinds of refrigerants. Reviewers of the literature on the transcritical cascade CO₂-propylene system compare the COP for subcritical and transcritical cycles with different refrigerant combinations and operational conditions in Table 1. It has been found that the use of heat rejected at the gas cooler increases the COPsys of CO₂-propylene transcritical cycles operating under similar conditions compared to subcritical cascade cycles [15]. It has been established that both NH₃ (R717) and CO₂ (R744), two natural refrigerants, are highly effective refrigerants, with R717 being more suited to large manufacturing systems and R744 being more suited to the commercially viable trade sector [16]. These tests show that a high COP can be achieved when the R744 refrigerant is used in the low-temperature cycle (LTC) and the R717 refrigerant is used in the high-temperature cycle (HTC) [16]. Due to its extreme toxicity, R717 should not be sold to the public. A high ambient temperature reduces R744's effectiveness. When used together in a single refrigeration system, such as in a cascade arrangement, the two natural refrigerants provide benefits and ensure safe operation that neither could on their own [17]. The R717/R744 combination has been extensively used throughout CCRS testing. Generally, R717 and R744 have been used as the refrigerant combination in CCRS-related research. These tests show that a high COP [18] can be achieved when the R744 refrigerant is used in the low-temperature cycle (LTC) and the R717 refrigerant is used in the high-temperature cycle (HTC).

The key motivation and objectives of this study are to attempt a thermodynamic analysis of an NH₃-CO₂ transcritical cascade system using internal heat exchangers to assess the system's effectiveness. Operating and design parameters have been altered to optimize the intermediate temperature and examine the implications for system performance. Additionally, it is contrasted with other refrigerant mixes' efficiency and NH₃-CO₂'s performance in transcritical cascade systems.

Table 1. Evaluation of COPsys utilizing CO₂–propylene with COP of other cascade systems (cooling only) [15]

A parametric analysis of different gas cooler exits temperatures (32, 34, 36, 38, and 40 degrees Celsius) and evaporation temperatures (-25, -35, -40, -45, -50, and -55℃). The ideal condensing temperature of a cascade heat exchanger in a CO²-NH³ cascade refrigeration system is also determined for different cascade heat exchanger temperature differences (2, 4, and 6℃). To make the thermodynamic analysis simpler, the following premises are made:

 The cascade heat exchanger outlet, HT cycle outlet, and LT cycle outlet evaporator have saturated refrigerants.

 Irreversible adiabatic compression with 0.8 isentropic efficiency.

 Negligible pressure and heat losses in the piping systems or system components.

 Evaporators, gas coolers, and cascade heat exchangers transmit heat isobarically.

 Evaporators, gas coolers, and cascade heat exchangers transmit little ambient heat.

 The high-temperature evaporator receives all the heat that is rejected by the low-temperature condenser.

Mass and energy balances on every one of the system's components were performed during simulations of each refrigeration system using the ASPEN HYSYS software program. The refrigeration system was assumed to function in steady-state circumstances with low kinetic, pressure drops, and potential energy losses through the components of the system, as well as heat and friction losses. These assumptions formed the basis for the thermodynamic analysis of each system. The refrigeration system's performance coefficient is expressed as follows using the first law of thermodynamics.

Following are the compressor power requirements for the HT cycle:

$\dot{W}_{\text {comp } 1}=\dot{m}_{\mathrm{H}}\left(h_2-h_1\right)$        (1)

The HT gas cooler's heat transfer rate is expressed as follows:

$\dot{Q}_H=\dot{m}_H\left(h_2-h_3\right)$           (2)

The HT internal heat exchanger's energy balance results in:

$\left(h_1-h_6\right)=\left(h_3-h_4\right)$       (3)

Energy balance at the cascade condenser can be used to calculate the mass flow ratio:

$\dot{m}_{\mathrm{H}}\left(h_6-h_5\right)=\dot{m}_{\mathrm{L}}\left(h_8-h_9\right)$      (4)

The HT compressor's isentropic efficiency can be expressed as:

$\eta_c=\frac{\left(h_{2 s}-h_1\right)}{\left(h_2-h_1\right)}$     (5)

The following equation illustrates how effective the HT internal heat exchanger is:

$\varepsilon=\frac{\left(T_1-T_6\right)}{\left(T_3-T_6\right)}$     (6)

For the LT cycle, the compressor power consumption is provided by:

$\dot{W}_{\text {comp2 }}=\dot{m}_{\mathrm{L}}\left(h_8-h_7\right)$     (7)

The following factors determine how quickly the LT evaporator absorbs heat:

$\dot{Q}_{\mathrm{I}}=\dot{m}_{\mathrm{L}}\left(h_{12}-h_{11}\right)$        (8)

The internal heat exchanger in the LT has the following heat balance:

$\left(h_9-h_{10}\right)=\left(h_7-h_{12}\right)$     (9)

The LT propylene compressor's isentropic efficiency can be represented as follows:

$\eta_c=\frac{\left(h_{8 s}-\mathrm{h}_7\right)}{\left(\mathrm{h}_8-\mathrm{h}_7\right)}$         (10)

The following equation represents the LT internal heat exchanger's efficiency:

$\varepsilon=\frac{\left(T_7-T_{12}\right)}{\left(T_9-T_{12}\right)}$        (11)

The definitions of the cooling and heating COPs are as follows:

$\operatorname{COP}_{\mathrm{LT} \text { cooling }}=\frac{\dot{Q}_{\mathrm{L}}}{\dot{W}_{\text {comp2 }}} ; \operatorname{COP}_{\mathrm{HT} \text { cooling }}=\frac{\dot{Q}_{\mathrm{L}}}{\dot{W}_{\text {comp1 }}}$      (12)

The following equation represents the system's overall COP:

$\mathrm{COP}_{\text {sys }}=\frac{\dot{Q}_{\mathrm{L}}+\dot{Q}_{\mathrm{H}}}{\mathrm{W}_{\text {comp2 }}+\mathrm{W}_{\text {comp1 }}}$   (13)

Only particular enthalpy words can now be used to express the system's COP, as in:

$\begin{aligned} & \operatorname{COP}_{\text {sys }} =\frac{\left(h_6-h_5\right)\left(h_{12}-h_{11}\right)+\left(h_2-h_3\right)\left(h_{\mathrm{g}}-h_9\right)}{\left(h_8-h_7\right)\left(h_8-h_9\right)+\left(h_8-h_9\right)\left(h_2-h_1\right)}\end{aligned}$    (14)

where,

$\dot{Q}_{\mathrm{L}}$ and $\dot{Q}_{\mathrm{H}}$: The low and high temperature evaporators cooling loads in kW, respectively.

$\dot{W}$: The total power required by each system's fans, lighting, compressors, and other electrical components, expressed in kW. Therefore, in the current analysis, it has been assumed that the isentropic efficiencies of the LT and HT compressors are the same and constant. The aforementioned formulas provide the compressor power consumption for the HT cycle.

$\dot{m}_{\mathrm{H}}$: Mass flow rate of NH₃ (kg/s)

$\dot{m}_{\mathrm{L}}$: Mass flow rate of CO₂ (kg/s)

COP: The coefficient of performance

A cascade refrigeration system's key performance metrics include cooling, heating, and system COP. The ratio of the desired output (cooling or heating capacity) to the energy input, or COP, is a measure of a system's energy efficiency (power consumption). Better efficiency is indicated by higher COP values. As the evaporation temperature rises, the cooling coefficient of performance often declines. This is to be expected because cooling at higher evaporation temperatures causes the system to work harder. In a different way, the COP of R717 cycle rises with a higher evaporating temperature because it works better at higher temperatures just like it does in heating application. In the end, the COP of the system indicates the desirable zone of system operation, where heating and cooling processes are evenly balanced for maximal effectiveness. The heat transfer capacities of evaporator and the cooling/heating section of the system depend on the transfer rates of these two components. A system, which is more efficient in moving heat between refrigerant and the media being chilled or heated, has higher thermal conductivity. The evaporator and gas cooler are effective in transferring heat at a higher rate, due to the increased flow rate of the mass. This has been due to the higher heat transfer that can be achieved with the more flow rate of the refrigerant. The trade-off between achieving further gains in mass flow rate and the increased power consumption, bottlenecks, and other practical limitations would become apparent at some point. What emerges more clearly when evaluating the overall effectiveness of the cascade is the power consumption matter because it should be the aspect to be focused on. It immediately affects the system's running costs and environmental effects. As might be predicted, maintaining the necessary pressure ratios requires more power when the compressor is running at higher rates.

A refrigeration cascade prototype consisting of CO₂ and NH₃ was designed, built, and tested, as well as the cooling in stationary facilities as part of this contribution was done. In the case study analyzing the thermodynamic behavior of a CO₂-NH₃ transcritical cycle system was conducted. Concerning the system’s coefficient of performance (COP), the temperature of cascade heat exchanger, and the mass flow ratio for LT and HT cycles among the most important parameters to be experimented with are mentioned in the study. The parameters in question are the TE for affected water, the discharge pressure (DP) and the output temperature of the gas cooler (TC). The following assertions are supported by the results of the study: Attention has turned to the first ever use of the innovative refrigerant NH₃ in a transcritical cascade refrigeration system.

 (1) Liquid CO₂ and NH₃ transcritical cycle offer better efficiency than subcritical NH₃ cascade cycles, which means they provide better system performance. A transcritical R717/R744 cascade system carries the flag as the most energy-efficient solution for applications that involve heating and cooling, with sustainability as the cherry on top. It adjusts on mass flow rates, power consumption and compressor speeds in order to maximize COP (coefficient of performance) but yet save power consumption. The system offers the benefits of both refrigerants, and its variables including mass flow rates and compressor speeds are simultaneously optimized. This approach can be used to develop more environmentally friendly cooling and heating solutions because it not only saves energy but also reduces the environmental impact thus helps reduce greenhouse gas emissions. This research provides valuable information that can be used to operate the system and design future ones, thus encouraging the application of energy-, resource-, and environment-friendly refrigeration systems in various industry and commerce sectors.

(2) A cascade heat exchanger connects two single-stage systems that employ NH₃ (R717) for chilling in the HT cycle and CO₂ (R744) to condense NH₃ in the LT cycle to form the cascade refrigeration system. In order to increase system performance, the LT cycle's waste heat is also put to use for heating.

(3) The analysis of the thermodynamics of each system was conducted under the supposition that the refrigeration system operates under steady-state conditions, that pressure drops, the transfer of kinetic and potential energy through the system's components, as well as the loss of heat and friction, are all negligible.

(4) The system's coefficient of performance approaches 5.770 and 4.739, respectively, when the temperature rises from 32 to 40℃. The same is true for COP, which decreases as the gas cools.

(5) It can be noted that there is a positive relationship between gas cooler temperature and COP of cooling; at 32℃, COP of cooling is approximately 0.601, and at 40℃, COP of cooling is 0.680.

(6) That the system's COP remains constant while the evaporator's temperature changes. At -25℃ and -55℃, the system's COP reaches a value of 6.136. When the temperature of the evaporator changes, the results for COP of heating and COP of cooling are the same. The COP of heating is 5.522 at -25℃ and -55℃, whereas the COP of cooling is 0.6144.

(7) There is no doubt that the system's COP, heating, and cooling are all impacted by the rising discharge pressure. When the discharge pressure is 8000kPa, the system's coefficient of performance (COP) is 0.788; however, when the discharge pressure is increased to 12000kPa, the COP decreases to 0.614, indicating a negative correlation between the discharge pressure and the system's COP.

(8) It can be noted that there is a positive relationship between gas cooler temperature and COP of cooling; at 32℃, COP of cooling is approximately 0.5942, and at 40℃, COP of cooling is 0.6803.

In conclusion, the simulation results give important information on how the transcritical R717/R744 cascade system functions. The consideration of COP's corresponding with cooling, heating, and system coefficients finds the trade-off between the cooling and heating efficiency and puts a suggestion on the choice of the optimal operating conditions. Heat transfer rate and power consumption investigation prove crucial areas, where, ensuring for system efficiency, we will be able to go out with useful information to be considered in the system design and optimization. With different parameters the simulation of a plant suggests that the right balance of settings is necessary in order that performance and efficiency can be at their peak. With the transcritical cascade refrigeration applications being demonstrated through simulation results, these systems can now be designed to serve either the cooling or heating purposes.

The transcritical R717/R744 cascade system simulation and modelling can be enhanced in a number of ways, all of which will contribute to a more full and accurate representation of the system's behaviour in the future:

• Improve the precision of working models, such as, compression units, heat exchangers and evaporators. Include the departure effect from ideal behaviour and more complex thermodynamic models that will contribute to the correct representation of real-life performance.

• Conduct extensive laboratory experiments to support the confirmation of the simulation results. In order to enable direct comparison with the simulation, gather information on various system attributes and operating environments. This validation will raise confidence in the model's precision and point out any inconsistencies that require correction.

• Extend the simulation to handle transient operation, according to transient analysis. To comprehend the system's dynamic behaviour and control mechanisms, examine the system's response to start up, shutdown, and transient events.

• Utilize multi-objective optimization approaches to determine the ideal trade-off between power usage, cooling COP, heating COP, and other important performance indicators. Finding Pareto-optimal options for system design and operation will be aided by this method.

• Examine how the cascade system performs when using different refrigerants. Examine novel, low-GWP refrigerants to see if they can replace R717 and R744 and if they can adhere to changing environmental requirements.

• Heat Exchanger Design Optimization: Increase heat transfer rates while reducing pressure drops in the heat exchanger design. To increase total system efficiency, take into account cutting-edge geometries and materials.

• Implement cutting-edge control techniques to improve the system's performance in real-time. The system's functioning can be modified via dynamic control techniques in response to shifting external conditions and load requirements.

• Including More Advanced Thermodynamic Models: To enhance the precision of the refrigerant property data, think about incorporating more sophisticated thermodynamic models, such as equations of state and sophisticated mixing procedures.

• Analysis of Transient Heat Transfer Phenomena: In order to properly define the system reactions to dynamic load situations, analyzing the transient heat transfer radiation in the evaporator and the gas cooler is necessary.

• Energy storage technology integration as a measure to comprehensively enhance system effectiveness and load flexibility is another aspect that needs to be considered.

• Environmental analysis is the first step in determining the climatic consequence, conducting a life cycle assessment research covering the things like energy consumption, refrigerant leaking as well as manufacturing emissions.

The transcritical R717/R744 cascade simulation and modelling will be more reliable, accurate, and adaptable by considering these improvements, which will make decisions making easier during design and efficiency management of cooling and heating systems made up of natural gas that does not harm the environment.