Energy-Efficient Three-Wheel Bleedless Electrical Environmental Control System for a Passenger Aircraft

The airline sector plays an essential part in our daily lives because it is the primary mode of air transportation that satisfies the human desire to travel long distances more quickly. However, these benefits are associated with several adverse effects. The aviation industry is estimated to produce 628 megatons of carbon dioxide (CO₂) annually. Environmental considerations are of prime importance in the aerospace sector, and the accent particularly stresses low-carbon emission solutions for modern aircraft. Apart from generating thrust, aircraft engines also supply power to various aircraft systems through hydraulic, pneumatic, electrical, and auxiliary systems [1]. An aircraft comprises a highly intricate system with numerous energy-consuming subsystems. Fuel is the sole power supplier in a typical aircraft, and the majority of fuel energy is used for propulsion. Aircraft engines are responsible for producing the necessary propulsion, as well as supplying electrical, hydraulic, and pneumatic power for equipment such as avionics, flight controls, flight decks, cabin entertainment, and environmental control systems (ECSs) [2]. Among all nonpropellant users in commercial aircraft, ECSs consume the most engine-bleed air [3,4].

The ECS helps to maintain a pleasant atmosphere for passengers, crew members, and avionics. The cabin pressure regulates the temperature and humidity inside the aircraft. As technology in the aviation industry continues to advance, aircraft are now equipped with a variety of sophisticated equipment, including high-power electronics, electronic warfare systems, radar, and avionics. An ECS consists of a set of devices in an aircraft responsible for controlling the internal environment to ensure that the payload (goods, life, and humans) is maintained within a safe range of temperature, pressure, and composition. According to the information, secondary power systems utilize between 3% and 5% of the power generated by the engine, with ECS responsible for 75% of the overall consumption. However, there are various ECS architectures, each with system weight, cost, and reliability. There must be a balance between the cost of the system, the amount of bleed air required, and passenger comfort.

The vast majority of contemporary airliners, such as the Boeing B747/767 and Airbus A330/380, depend on engine-bleed air to function their air-conditioning systems [5]. The bleedless ECS is the name given to the system in which the air extraction source is changed from the engine compressor to the electric compressor with the assistance of an electric motor. This system replaced bleed air with ambient and ram air; however, all other equipment remained the same. The vast majority of ECS designs utilize a vapor cycle to decrease the temperature of the filtered air circulated within the cabin, prior to its mixing with fresh air from the air cycle. As environmentally friendly refrigerants such as R-1234yf (2,3,3,3-tetrafluoropropene) and R-1233zd(E) (1-chloro-3,3,3-trifluoropropene) are easily accessible, a vapor-compression refrigeration system is also a viable option [6].

Several researchers have thoroughly assessed the environmental conditions of cabin air by gathering extensive data through simulations. Khalil [7] employed ansys fluent 17.0 to simulate a mixing air ventilation system for the Airbus 340-600 (A340-600) ECS and investigated indoor air quality by utilizing computational fluid dynamics to assess the influence of temperature and humidity. Jennions et al. [8] verified and validated the Boeing B737-800 passenger aircraft ECS component level using SESAC-simscape ECS Simulation. The study results indicate that the ambient atmosphere surrounding the aircraft has an effect on both the temperature needed for the passenger air-conditioning outlet. Chowdhury et al. [9] presented a pack simulation model and a novel in situ ground test facility for investigating the performance of a Boeing 737-400 (B737-400) aircraft's pack under various operating modes, including the impact of the trim air system. The collected data are utilized to verify and validate the SESAC-simscape ECS simulation framework. The research conducted on ECS pack operation in real-world settings adds to scientific understanding and aids in creating a comprehensive simulation framework for system-level fault diagnostics. Chowdhury et al. [10] built upon their previous research by conducting additional simulations to assess the impact of temperature control valve and pack valve malfunctions on pack simulation accuracy at varying severity levels. The study examined the effects of these malfunctions on key performance indicators such as temperature, pressure, and mass flow. Jennions and Ali [11] examined the possibility of the pack causing functional failure in the ECS. If the ECS control system's decline goes unnoticed, it can lead to shutdowns and expensive maintenance. This research delves into the critical fault modes in the pack, paying particular attention to the primary and secondary heat exchangers. Yin et al. [12] utilized ansys simplorer to evaluate the dynamic responses of the MD-82 aircraft's ECS in conjunction with simulations of the cabin thermal environment. The ECS received a proportional integral derivative (PID) control from fluent, which effectively regulated the temperature within the cabin. The PID controller ensured that the temperature remained constant, maintaining a deviation of only 0.6 K from the target while providing uniform heating and cooling.

Previous research has addressed the power optimization of the ECS. Liu et al. [13] investigated the ECS thrust-specific fuel consumption and analyzed the impact of bleed air and electric power on it. Their findings, based on an energy efficiency evaluation at the task level, demonstrated that the vapor cycle can reduce fuel consumption while still maintaining a high cooling capacity. Duan et al. [14] employed a component-level parameter decomposition algorithm in the optimization of multiobjective aircraft ECS. To decouple and solve the model, a three-iteration algorithm was implemented, comprising heat exchangers, power balance constraints, and a pack. Liu et al. [15] optimized the aircraft's ECS thermodynamically by minimizing the fuel energy consumption rate (FECR), shaft power intake, and loss due to propulsive power. By decreasing the bleed-air inlet parameters and eliminating the compressor power, the FECR optimization cases were reduced by 11%.

Wang et al. [16] enhanced the efficiency of an aircraft thermal system by incorporating heat exchangers into several subsystems, including fuel-cycle, liquid cooling, air cycle, and vapor-compression refrigeration. Zhao et al. [17] conducted a study on the high-pressure water separation air cycle under five off-design conditions, comprising compression ratio, expansion ratio, heat exchanger efficiency, turbine efficiency, and compressor efficiency, at altitudes ranging from 0 to 10,000 m. Their findings revealed that these five parameters are critical for ECS design. Liping et al. [18] studied 31 flights to predict civil aircraft operative cabin temperatures and found that uncomfortable thermal phenomena were observed on some flights, particularly continental flights of shorter duration.

To eliminate the use of bleed air, it is necessary to use electrically driven compressors as a means of supplying air to the ECS. Schettini et al. [19] proposed the development of an electric aircraft onboard system modeling platform for conducting energy management research. The platform utilized a matlab/simulink simulation system to create and validate electrical energy management logic, resulting in a reduction in the maximum power request by 32% in the case study. Yang et al. [20] proposed an experimental study on an energy-efficient ECS aimed at an electrically operated ECS of the Boeing 787 Dreamliner, using the enthalpy method to reclaim energy from air that has been exhausted from the pack. This method resulted in a 66% reduction in power consumption, increased the coefficient of performance (COP) from 0.30 to 0.90, and enabled turbochargers to recoup shaft power from 2 to 10 kW.

Previous research has investigated the development of ECS models for various air cycle systems (ACSs) designs. To enhance the efficiency of these cycles, ACS has devised numerous configurations, including the simple cycle, the bootstrap cycle, the three-wheel cycle, and the four-wheel bootstrap cycle. Over the past few decades, the evolution of ECSs has progressed from double-wheel designs, including the fundamental concept proposed by Hu and You [21], Ma et al. [22], and Li et al. [23] and the bootstrap type proposed by Ordonez and Bejan [24], Pérez-Grande and Leo [25], Vargas and Bejan [26], Yoo et al. [27], Zhao et al. [17], and Li et al. [28], to three-wheel Tu and Lin [29] and Bejan and Siems [30], four-wheel types proposed by Jiang et al. [31], and conventional models with bleed air to electric models with bleedless.

Following a comprehensive literature review, it has been observed that the current research on three-wheel bleed ACS simulation models primarily focuses on steady-state and overall ECS performance analysis. However, no research has been conducted on bleedless three-wheel ACS. It has been discovered from the literature that bleed-air extraction to power the ECS affects engine performance, resulting in increased fuel consumption; thus, it is an essential candidate for optimization. ACS provides numerous advantages, including its lightweight and compact design, reliable performance, and cost-effective maintenance. Air is a free, nontoxic, nonflammable, environmentally friendly, and leak-free refrigerant. ACSs can fulfill ventilation, pressurization, and cooling requirements. The efficiency of these systems is gauged by their COP, which is notably low. Due to this low COP, traditional ACSs have had difficulty finding commercial success. The inefficiency of the compressor, expander, and recuperative heat exchanger has been a significant factor in the poor performance observed. Improved component design technologies in the future could enhance COP. As the aerospace sector has started to work on making aviation technology more sustainable, there has been much focus on electrifying aircraft systems to make them more energy and fuel efficient. A more electric aircraft (MEA) resolves this challenge by running the ECS on electric power instead of on bleed air. MEA emphasizes the significance of electrical power for aircraft systems other than propulsion, which can help reduce audible aircraft noise, improve energy efficiency, and decrease CO₂ and nitrogen oxide (NO_x) emissions. An electrically driven ECS (EECS) can save more energy and fuel than a conventional ECS, and this study can be expanded to consider these benefits.

This research proposes specifically replacing the current three-wheel bleed-air-driven high-pressure water separation subsystem (HPWS) cycle of ECS onboard the Airbus 320 (A320) civil aircraft with air cycle of EECS. matlab was employed to develop the system component model of the ECS to validate the accuracy of the data from the A320. This study draws on the research conducted by Peng [32], which utilized a conventional bleed-air A320 three-wheel ECS as a reference. The evaluation of the proposed EECS system was carried out by comparing it to a current bleed-air system with respect to cabin requirements, including fuel penalty and energy consumption.

The passenger aircraft's ECS, illustrated in Fig. 1, comprises multiple subsystems, including the ACS, temperature control system, and ventilation control system. These are the crucial elements of the ECS, functioning as a cabin pressure control system (CPCS), with specialized CPCS valves regulating the cabin outflow to maintain the necessary cabin pressure, air distribution system, and associated valves [33,34]. An air cycle system is the most vital part of the ECS in an aircraft and controls airflow by controlling the temperature, pressure, and humidity; it is also known as a cooling pack. The Joule–Brayton cycle in reverse forms the basis for the heat cycle of an open system. In civil aircraft, the compressed bleed air extracted by a conventional ECS originates from the fifth or seventh phase of engine compression based on the available pressure. This bleed air has the highest temperature of approximately 500 K and Mach number ranging from 0.6 to 0.8. The hot air is cooled in the heat exchangers before being circulated inside the pack. Prior to being circulated within the pack, the heated air is cooled in heat exchangers.

The process of drying and sanitizing the air from the passenger compartment is conducted in the air-mixing module, where it is subsequently combined with fresh air obtained from the upstream air cycle mixing module. Airflow valves A, B, and C are then utilized to strategically circulate this blended air through the diverse compartments of the aircraft, including the cockpit, passenger compartments, electrical device compartment, and cargo compartment. Finally, the air is expelled outside the aircraft through air outflow control valve D, maintaining comfortable temperatures and pressures in each compartment.

A three-wheel bleed-air HPWS cycle of ECS that utilizes bleed air to power both the compressor and fan uses a turbine. The primary components of this ECS, as illustrated in Fig. 2, comprise the primary heat exchanger (P-HX), secondary heat exchanger (S-HX), air cycle machine (ACM), reheater (R-HX), condenser (C-HX), and water separator (WS). The introduction of ram air, represented by thick arrows connecting Nodes 1–4 in Fig. 2, is shown to be introduced into the ECS via a diffuser. The bleed air is indicated by thin arrows extending from Nodes 12–21.

The P-HX unit is employed to cool bleed air from the fifth and seventh stages of the engine compressor by utilizing ram air. In contrast, the S-HX, R-HX, and C-HX are utilized to cool the air subsequent to it being compressed by an ACM compressor. High-pressure air from the condenser was directed into the WS, where the centrifugal force from the whirling vanes removed 95% of liquid water. Avionics require a relative humidity of less than 50%; therefore, water extraction is necessary to prevent ice formation at the turbine outlet and keep the air dry. Before the air is sent into the turbine, it undergoes a process of dehumidification and reheating to reduce its temperature and pressure to the appropriate levels required for optimal performance. These two airstreams are the inputs for the ECS and contain energy inputs for the system. While the aero-engine propulsive power is sacrificed to pressurize the ram airstream, the engine directly powers the bleed airstream. The temperature of the expanded and cooled bleed airstream at the conclusion of the process is indicated by Node 21.

The new ECS, which is a schematic of the proposed energy-efficient three-wheel bleedless EECS, is illustrated in Fig. 3. The proposed system incorporates an electric compressor (E-COMP), which contrasts with the conventional bleed-air-driven ECS. Similar to the cooling packs of A320, fresh air was compressed in an E-COMP instead of engine-bleed-air extraction. The compressor is powered by electric power from the engine shaft, which is more efficient than bleed-air extraction. Therefore, ECS optimization aims to reduce engine power consumption while maintaining the same output.

The bleedless ACS system intakes low-pressure ambient air that subsequently undergoes compression process a-12 to increase its temperature and pressure through E-COMP. This air then follows the same processes as the traditional bleed ACS method. As seen in Processes 13–14 of the P-HX system, the air is cooled in the S-HX before being further compressed by the ACM compressor. The air then goes through the R-HX and C-HX where it is condensed in Processes 16–17–18. The R-HX's cold side sources dry air that undergoes cooling and pressurization during Processes 19–20, which involve the use of the turbine. Finally, in Processes 20–21, the cold air is directed toward the cold side of the C-HX, where it is set to the desired air temperature and pressure. All these above processes follow the adiabatic/isentropic compression and expansion as shown in Fig. 4.

This portion provides a summary of mathematical models that depict the operational performance of both conventional and electric ECSs during steady-state cruise mode. The inlet conditions and initial parameters were sourced from the literature (Table 1) to calculate the outlet conditions. The mathematical model independently simulated each component, and the primary energy equations were used to determine the outlet temperatures and pressures. Furthermore, the methods used to determine the fuel mass penalty are explained in detail.

Mathematical models for ACSs rely on assumptions about fluid properties, flow state, and process pipe conditions to improve accuracy.

• High-altitude air is often dry and behaves like an ideal gas due to its constant specific heat capacity.

• The analysis was carried out assuming the system was at steady-state, disregarding the transformation of kinetic energy.

• To minimize the risk of pressure loss, the pipes in the current system design are insulated and relatively short in length. It should be noted that this design assumes the disregard of pipe pressure losses.

• The HXs did not take into account the consequences of longitudinal heat conduction or the nonuniformity of flow.

• Given that air is commonly employed as the working fluid in HXs, changes in fluid properties are generally disregarded.

In an ACM with three wheels, the turbine drives both the compressor and fan, which are mounted on the same shaft. No heat is lost or gained between the air and its surroundings in ACM; therefore, it is considered adiabatic. Equations (10) and (11) describe the thermodynamic model for ACM.

Subscripts 1 and 3 signify the initial and final air values, while Subscript 2 denotes the water spray values.

The ECS penalizes aircraft efficiency in four ways: system weight, engine-bleed air, shaft power, and ram air intake aerodynamic drag. The SAE AIR1168/8 [36] includes a fuel mass penalty calculation guide. Fuel penalties caused by the system weight were not considered because the commercial ECS component weight data were inaccessible. The remaining penalties show ECS energy savings.

The calorific value $(Hv)$ of Jet A-1 fuel for civil aircraft is 43.15 × 10⁶ J/kg, aircraft lift-to-drag ratio $(K)$ is 20.8, coefficient of combustion (⁠$ec$⁠) is 0.98, specific heat of fuel (⁠$cp,f$⁠) is 1.130 kJ/kg K, the coefficient of drag (⁠$cd$⁠) is 1.12, turbine inlet temperature (⁠$TTurb,i$⁠) of the engine is 1200 K, and the thrust-specific fuel consumption (⁠$SFCth$⁠) is 14.4 × 10⁶ kJ/N s.

The accuracy of the conventional bleed-air ECS model was validated using the ECS results for the A320 passenger aircraft [32]. The component-level temperature profiles of the A320 ECS with the matlab modeled three-wheel bleed air and bleedless air streams with HPWS are presented in Fig. 6. The temperature profiles in both cases were significantly different owing to the different temperature drops in each component. The EECS operates at low pressure and temperature. In contrast, a conventional ECS works on the fifth/seventh stage engine compressed air, which extracts energy from the engine and affects its propulsive efficiency. Figure 7 shows the pressure profiles of the same system. In comparison to the conventional ECS, the EECS operates under conditions of lower pressure and temperature. Working at low pressure means that less compressor work is required to drive the electric compressor over the engine compressor because the conventional ECS works on compressed engine air and the EECS works on ambient air, which saves energy.

The bleedless ECS is known to function at a lower temperature and pressure than the bleed ECS, which translates to a reduced energy requirement for operation. It is essential to recognize that three-wheel ECSs with bleed and bleedless configurations have distinct outlet temperature and pressure values, which are influenced by their respective energy input sources. The bleed air ECS transfers power extraction from the engine compressor to the E-COMP, resulting in lower energy consumption.

The extra fuel demand of each system was determined by employing the formulas presented in Sec. 3.5. For conventional and electric ECSs, Fig. 8 compares the average fuel mass penalty per hour, whereas Fig. 9 shows the same for missions lasting between 5 h and 15 h for every ECS. The conventional ECS has fuel mass penalties caused by the bleed and ram air intake, whereas the EECS has a penalty caused by the shaft power off-take and ram air intake. In addition, the bleed-air system works at high pressures and temperatures, whereas the bleedless system works at low temperatures and pressures. The cooling of high-temperature bleed air requires more ram air flow in the P-HX and S-HX, which results in a high drag in the ram air scoops, which also contributes to the extra fuel penalty. The graphs indicate that the EECS incurs a much smaller penalty in fuel mass, which increases only slightly even for longer missions. Compared with the conventional ECS, the EECS results in a 50% reduction in the fuel mass penalty every hour during the mission.

The greater fuel mass penalty for the EECS over the conventional system is largely due to the ram air intake, while the bleed-air cycle ECS experiences a much higher penalty due to the bleed-air off-take, compared to the bleedless air cycle ECS, which experiences a lower penalty due to the shaft power off-take as shown in Fig. 10. Accordingly, considering the jet A-1 fuel emission factor of 3.15 kg CO₂/kg of fuel and an average mission time of 15 h, it is estimated that 3938.28 kg CO₂ per conventional bleed system and 1957.56 kg CO₂ per bleedless system, respectively. Thus, the EECS saves more fuel than the conventional ECS by removing bleed air. Therefore, the energy saved using the EECS was calculated. The necessary extra energy for both conventional and new ECS systems can be calculated by multiplying the fuel penalty by the specific exergy of the fuel, as shown in Table 2. This can be done to determine the required additional energy for each system. The specific exergy of the fuel can be used to calculate the extra energy needed for both conventional and new ECS systems.

Figure 11 illustrates the performance comparison of bleed and bleedless ECSs, the COP increased from 0.13 to 0.27 for the bleedless system at a given input data of the bleed and fresh air mass flowrate, ram air mass flowrate, and Mach number [15]. Considering the total energy consumption from all sources is logical. The energy from bleed and fresh air (E_bleed/fresh air), ram air (E_ram air), and the extra weight of the heat exchangers (E_HX,m) all contributed to the overall FECR of ECSs. The entry of direct ram air into the ECS occurs at ambient temperature and pressure, but it presents a specific level of resistance. It is propelled by the fan, which is powered by bleed air, and the core engine's propulsive power. The energy consumption of the fan is already accounted for in the bleed air, while the partial propulsive power overcomes the ECS diffuser resistance.

Additionally, the ECS nozzle accelerates the ram air to generate specialized propulsive power. The ram air system requires a significant amount of energy to operate, but it generates propulsive force by utilizing heat exchangers and fans, resulting in a negative energy expenditure. Additionally, after optimization, both the ram air and bleed-air systems become more energy efficient.

The steady-state performances of conventional and electric ECSs were simulated numerically using matlab, and the results were compared. It was determined that the HPWS method effectively improves the performance of the S-HX by spraying water collected from the WS, which lowers the temperature of the ram air inlet. EECS requires no alteration to the circuitry of the existing ECS and it operates at low pressures and temperatures, which leads to the development of compact components. Based on an analysis of the fuel penalty for each mission, the EECS was 50% more efficient than the conventional ECS. The different factors that contribute to these expenses include the high ram air intake for ram air scoops, the ability to collect bleed air, the off-take shaft power, and the additional thrust needed to balance the system's weight. The EECS experiences a significant amount of drag owing to its ram air intake for the heat exchanger and the electrical power system of the compressor. This study demonstrated that the remaining fuel penalties were still effective in conserving fuel in the EECS. The estimated energy requirements are 3.59 MJ for the conventional ECS and 1.78 MJ for the EECS. Accordingly, considering the jet A-1 fuel emission factor of 3.15 kg CO₂/kg of fuel and average mission time of 15 h, it is estimated that 3938.28 kg CO₂ per bleed-air system and 1957.56 kg CO₂ per bleedless system, respectively.

It was discovered that the penalty associated with bleed-air off-take from the engine was the primary factor leading to high fuel penalties for conventional ECS. Although the EECS had a higher penalty for ram air intake, the shaft power off-take was significantly lower than the bleed-air off-take. Consequently, a conventional ECS requires significant extra fuel consumption to function correctly.

The research presented in this article has been made possible with the support and permission of the Birla Institute of Technology and Science (BITS), Pilani Campus, Rajasthan.

There are no conflicts of interest.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

$f$ =

Friction coefficient

g =

Acceleration due to gravity (m/s²)

h =

Enthalpy (kJ/kg)

$k$ =

Geometric factor (m)

$m$ =

Mass flowrate (kg/s)

$r$ =

Latent heat of vaporization (kJ/kg)

$t$ =

Dry bulb temperature (°C)

$C$ =

Heat capacity (kW/K)

$D$ =

Aerodynamic drag (N)

$K$ =

Aircraft lift-to-drag ratio

$L$ =

Length (m)

$P$ =

Pressure (kPa)

$Q$ =

Cabin heat load (kW)

$T$ =

Temperature (K)

$cd$ =

Coefficient of discharge

$cp$ =

Constant pressure-specific heat (kJ/kg K)

$cp,a$ =

Dry air-specific heat (kJ/kg K)

$cp,f$ =

Specific heat of fuel (kJ/kg K)

$cp,v$ =

Specific heat of the vapor (kJ/kg K)

$cp,w$ =

Specific heat of water (kJ/kg K)

$cp,wv$ =

Water vapor-specific heat (kJ/kg K)

$ec$ =

Coefficient of combustion

$hv$ =

Saturated vapor enthalpy (kJ/kg)

$hw$ =

Saturated liquid enthalpy (kJ/kg)

$mw$ =

Mass flowrate of water from water separator (kg/s)

$qm,bl$ =

Bleed-air mass flowrate (kg/s)

$qm,f,P$ =

Extra fuel consumption (kg/s)

$DH$ =

Hydraulic diameter (m)

$Hv$ =

Fuel calorific value (J/kg)

$Pt$ =

Total pressure (bar)

$Pv$ =

Water vapor partial pressure (bar)

$Wb$ =

Bleed-air off-take extra fuel flowrate (kg/h)

$Ws$ =

Shaft power off-take extra fuel flowrate (kg/h)

$Wr$ =

Ram air intake extra fuel flowrate (kg/h)

$SFCP$ =

Power-specific fuel consumption (kg/N s)

$SFCth$ =

Thrust-specific fuel consumption (kg/N s)

$ω$ =

Specific humidity (g/kg of dry air)

$ωT$ =

Total specific humidity (g/kg of dry air)

amb =

Ambient

c =

Cold side

cab =

Cabin

$h$ =

Hot side

$i$/In =

Inlet

o/Out =

Outlet

min =

Minimum

$p$ =

Pressure

ram =

Ram air

Turb =

Turbine

$εHX$ =

Effectiveness of heat exchanger (%)

$η$ =

Efficiency (%)

$ηc$ =

Compressor efficiency (%)

$ηd$ =

Diffuser efficiency (%)

$ηt$ =

Turbine efficiency (%)

$v$ =

Aircraft speed (m/s)

$ρ$ =

Density (kg/m³)

$τ$ =

Mission time (h)