Bus Air Conditioning Research Articles

Fault detection and diagnosis of electric bus air conditioning systems incorporating domain knowledge and probabilistic artificial intelligence

The air conditioning systems in electric city buses usually operate in rapidly changing ambient conditions and are more likely to suffer from mechanical faults. Although many fault detection and diagnosis (FDD) methods have been developed for building air conditioning systems, they are difficult to be applied to bus air conditioners since its operation is highly dynamic and fault-free data are usually unavailable. Therefore, this paper proposes an FDD method for electric bus air conditioners to tackle the above issues. First, the method identifies faults in an unsupervised manner by comparing selected features among a group of peer systems. Then, considering the features are influenced by the operating conditions, Gaussian process regression (GPR) models are established to find the relationships between each feature and its influential parameters. The probabilistic nature of the GPR is used to differentiate predictions with large uncertainty, which are then excluded from FDD. In this way, robustness of the method is evidently improved. Finally, fault indexes are defined to detect and diagnose mechanical faults. We applied the method to a group of air conditioners in a city bus fleet. Results showed that it can effectively identify refrigerant undercharge and indoor and outdoor fan problems with low false positive/genitive rates. Also, the method is highly robust and not sensitive to the faulty systems in the bus fleet.

A Comprehensive Assessment of the Refrigerant Charging Amount on the Global Performance of a Transcritical CO2-Based Bus Air Conditioning and Heat Pump System

To mitigate the contemporary environmental challenges and to respect the regulations on the progressive ban of hydrofluorocarbons (HFC), natural fluid CO2 was selected as an ideal refrigerant alternative in the transportation domain. In this study, the optimal CO2 charging amount and the refrigerant distribution in a bus air conditioning/heat pump system were analyzed in detail. The results showed that there was a plateau (so named by the best charging amount) of the CO2 charging amount in which the system performance reached an optimal value and maintained it nearly unchanged during this plateau while the performance declined on both sides of the plateau. In addition, the ambient temperature was found to have little effect on the determination of the refrigerant charging plateau, while the refrigerant distribution was affected by the ambient temperature to some extent. Due to the large thermal load and thermal inertia on a bus, the data and conclusions obtained are different from those of ordinary electric small passenger vehicles. This article aims to discover some quantitative parameters and lay a theoretical foundation in this field which is lacking relevant research. Through the research based on the GT-Suite simulation platform, we simulated the transcritical CO2 cycle applied on a bus, and the performance changes of the bus system (COP 1.2–2.2, refrigerating capacity 9.5–18 kW) under different charging rates (3–8 kg) were obtained.

Effects of alternative refrigerants on the ejector dimensions for single and dual ejectors enhanced bus air conditioning system

One of the most critical criteria influencing the performance of an ejector enhanced vapor compression refrigeration system is the ejector size. R134a was chosen as the refrigerant in the majority of the research studies since it is widely used in practical applications. The use of R134a refrigerant, on the other hand, is restricted under the Kyoto Protocol due to its high global warming potential (GWP). In this study, an investigation was conducted to evaluate the differences in ejector dimensions that may occur when alternate refrigerants such as R1234yf, R1234ze(E), or R600a were used in the ejector enhanced bus air conditioning system. The cycle is analyzed thermodynamically to determine the state points. The diameters and lengths of the ejectors were determined using continuity, energy, and momentum equations. It can be observed that the kind of refrigerant has a significant impact on the size of the ejector. It is also determined that the R1234yf has the closest size to the R134a, and it can be deduced that utilizing a dual ejector decrease the overall size of the ejector, hence eliminating technical concerns in the ejector production process.

Performance Enhancement of Electric Bus Air Conditioning System by Heat Pipe Equipment (Experimental Study)

Performance Enhancement of Electric Bus Air Conditioning System by Heat Pipe Equipment (Experimental Study)

Experimental study on the heating performance of transcritical CO2 heat pump for electric buses

In this paper, a transcritical heat pump using CO2 as the refrigerant was developed, and the heating performance test experiment was carried out in the standard bus air conditioning performance enthalpy difference laboratory to explore the influence of outdoor temperature, compressor speed and electronic expansion valve (EEV) opening on the performance of the heat pump system. Operating at −25 °C/20 °C (outdoor temperature is −25 °C, indoor temperature is 20 °C), the heating capacity (Q) of the CO2 heat pump is 10.5 kW, the coefficient of performance (COP) is 1.24, indicating that the system can also effectively save energy under ultra-low temperature conditions. As the compressor speed rises, the COP of system drops sharply, so the compressor operating frequency should be reduced as much as possible under the premise of satisfying the Q and outlet air temperature (Ta.out). Increasing the opening of the EEV can effectively reduce the compressor exhaust temperature (Tcom.out) and pressure(Pcom.out) to ensure the safe and stable operation of the system.

Experimental Thermodynamic Investigation on the Refrigerant Charge in a Transcritical CO2 Electric Bus Air Conditioning System

Due to its considerable impact on climate, bus air conditioning systems are being pushed to take a new and sustainable path. Electric buses relying on transcritical CO2 air conditioning units are perceived to be eco-friendly and future-proof solutions to achieving such a target. However, in order to have highly efficient air conditioning systems, the CO2 charge needs to be optimized. In this paper the energy and exergy-based analyses were performed to investigate the effect of normalized refrigerant charge on the system performance by using a test rig of a transcritical CO2 air conditioning unit for an 8 m electric bus. Results showed that the normalized refrigerant charge range of 0.248~0.336 was recommended in order to ensure the maximum coefficient of performance (COP). In addition, in sufficient charge conditions, the optimal COP, cooling capacity and exergy efficiency were 1.716, 18.97 kW and 29.79%, respectively, under the standard refrigeration condition of 35 °C/27 °C. As the ambient temperature rose from 35 °C to 40 °C, the COP, cooling capacity and exergy efficiency decreased by 16.03%, 10.90% and 12.22%, respectively. Furthermore, the exergy efficiency was found not to be sensitive to slightly insufficient charge, whereas overcharge was observed to be even beneficial to exergy efficiency under the condition of ensuring the maximum COP. In addition, insufficient refrigerant charging seriously affected the irreversible losses in the indoor and outdoor heat exchangers, whereas slight overcharge had little effect on the component exergy efficiency. Finally, the need to improve the CO2 compressor efficiency to enhance the system performance was revealed.

LPT (Local Public Transport) electric buses: innovative solutions to reduce the energy absorption of auxiliaries

The focus of the study is the analysis and sizing of the air conditioning system on board the vehicle which operates in thermal transient for the duration of the typical mission, using “fan coil” type air conditioning devices powered by a hot or cold heat vector fluid loaded into “electro-thermal” charging station and the related ground cogeneration plant (at the BUS terminus). For winter air conditioning, it was assumed that a heat transfer fluid at a temperature of 90 ° C could be stored on board. This value allows the use of water as a heat transfer fluid without pressurizing the systems, minimizing costs and supply problems, and is compatible with the characteristics of ICE cogeneration systems. For summer air conditioning, it was assumed that ice or fluid at a temperature of -20 ° C could be loaded. Also in this case it was decided to operate with a common fluid, such as a solution of water and salt (e.g. calcium chloride CaCl2). A comparative analysis of two solutions was carried out: the first involves the standard solution with BUS air conditioning system with heat pump powered by traction batteries charged by the grid and the second one analyzes the use of a high temperature Fuel Cell in a trigenerative configuration with refrigeration unit absorption fueled by the fumes of the FC. The trigeneration plant produces 100 kW of electricity via SOFC (Solid Oxide Fuel Cell) and, through an exchanger, water at 95 ° C which is stored on board the BUS for winter heating and feeds an absorption refrigeration machine that produces fluid at a temperature of -20 ° C for summer conditioning. For the two solutions, the savings of non-renewable primary energy and the reduction of GHG emissions compared to the standard solution (recharging the electric BUS from the network with on-board air conditioning powered by the traction batteries) are calculated through a WTW analysis. Consumption and emissions of electric powertrains, potentially for consumption and zero emissions, are closely linked to the methods of production and distribution of electricity.

Thermodynamic Analysis of an Intercity Bus Air-Conditioning System Working with HCFC, HFC, CFC and HC Refrigerants

The purpose of this study is to perform thermodynamic analysis of an intercity bus air-conditioning (ICBAC) system working with hydrochlorofluorocarbon (HCFC), hydrofluorocarbon (HFC), chlorofluorocarbon (CFC) and hydrocarbon (HC) refrigerants. For this aim, an intercity bus with a busload of 56 passengers, which is thought to motion in the Adana province of Turkey, was selected. R404A, R410A, R502, R507, R22, R290, R134a, R500 and R600a were selected as different HCFC/HFC/CFC/HC refrigerant types in the ICBAC system. Useful and reversible works of compressor, coefficient of performance (COP), exergy efficiency and exergy destructions of the entire ICBAC system and its each sub-unit were obtained. The results showed that the performance of the ICBAC system was significantly influenced by changing the refrigerants. However, R600a refrigerant based on hydrocarbon was found as a refrigerant to have minimum total exergy destruction, maximum exergy efficiency and maximum COP compared to other refrigerants used in the ICBAC system with the same amount of cooling load and the same climatic condition.

Performance improvement potentials of low global warming potential refrigerants for intercity bus air conditioning system

In this study, theoretical investigation of two evaporator ejector refrigeration system was carried out based on energy and exergy analysis using R134a and low global warming potential refrigerants (namely R1234yf and R1234ze(E)). In order to perform the analyses, a simulation model was developed and then the influence of different parameters on the COP, COP increase rate and exergy destructions were discussed for each refrigerant. The model was validated with experi¬mental data for R134a and later used to predict the behavior with R1234yf and R1234ze(E). It was found that the highest exergy destruction occurs in the R1234yf system and the increase rate in the COP with respect to the conventional system by using ejector as an expansion device are 15% for R134a, about 17% for R1234yf and about 15% for R1234ze(E), respectively.

Thermodynamic Analysis of Bus Air Conditioner Working with Refrigerant R600a

Refrigerant R134a (tetrafluoroethane) is widely used in automotive air conditioners and has a good performance. However, since refrigerant R134a still has a high global warming potential (GWP), this refrigerant must be replaced with environmentally friendly refrigerants such as R600a (isobutane), which is a natural fluid, no effect on the climate, inexpensive and readily available. In this study, thermodynamic analysis of a bus air conditioner working with refrigerant R600a was performed. Adana province of Turkey was chosen as the study region. Firstly, a bus used for inter-city passenger transportation with a passenger capacity of 56 persons was selected. Cooling Load Hourly Analysis Program (HAP) was used to determine the hourly cooling load capacity of the selected inter-city bus model. Useful and reversible works of compressor, coefficient of performance (COP), exergy efficiency and exergy destructions of the bus air conditioning system were obtained and evaluated in detail.

“We will now be working together to develop innovations in bus air conditioning systems”

“We will now be working together to develop innovations in bus air conditioning systems”

An Experimental Study on a Bus Air Conditioner to Determine its Conformity to Design and Comfort Conditions

The air conditioning system for a bus should be selected considering a number of parameters, including passenger capacity, local climatic conditions, and fuel consumption. It is possible to determine whether a selected air conditioning system provides desired performance through testing. This study examines how to verify experimentally whether a bus air conditioning system meets design and comfort requirements. An experimental study was conducted on a prototype bus and was tested when driving on the Adana-Ceyhan highway in Turkey. The internal and external temperatures, evaporator inlet and outlet temperatures and relative humidity values were measured. Thermal sensation values were calculated by using empirical correlations given by ASHRAE. Furthermore, the instantaneous cooling load of the bus was obtained according to the experimental data, and the results are compared with the calculated cooling load of the bus by using the radiant time series method provided by ASHRAE. With respect to the obtained results, the selected air conditioning system conformed to design and comfort requirements.

Analysis of bus air conditioning system by finite elements method (ANSYS)

Air conditioning systems used for the cooling of vehicles to create a uniform temperature distribution in terms of providing thermal comfort to people is an important parameter. From this point, realization of appropriate computer simulations of these systems during the design phase provides a significant convenience to the success of the design. In this study, determination of the time required to reduce the bus indoor initial temperature at  310 K ( 37 °C) of a bus for 49 passengers including the driver to  293 K ( 20 °C) gradually with a uniform temperature distribution and the temperature, pressure and velocity distribution were analyzed using finite elements method (ANSYS). In finite element program, developed for this purpose indoor thermal load of 49 people including the driver in the bus has been calculated by using thermal mannequin modeling, the internal conditions were integrated to real conditions. As a result of the temperature pressure and velocity distribution graphics obtained at 5 minutes intervals depending on real conditions case scenario in ANSYS program, it was determined that in about 15 minutes the indoor temperature distribution of the bus has reached to  293 K value uniformly

Exergy analysis of an inter-city bus air-conditioning system

In this paper, exergy analysis was implemented to improve the inter-city bus air-conditioning (AC) system design. Firstly, an inter-city bus with a passenger capacity of 56 people was selected and then its hourly cooling load capacity was determined with the help of cooling load hourly analysis program (HAP). Analyses were performed for different air mixing ratios (MRs) and seasons. Coefficient of performance (COP), exergy efficiency (ψ) and exergy destructions (Ėxdest) of whole system and its each sub-units were evaluated. Results demonstrated that the atmospheric air temperature and air MR substantially affect the inter-city bus AC systems' performance. According to the results obtained for July, when air MR is 0.5, maximum exergy destruction value was calculated as 6.96 kW at compressor. Exergy destruction values were computed as 2.78, 2.61 and 0.99 kW for evaporator, condenser and expansion valve, respectively.

Transient evaluation of a city bus air conditioning system with R-445A as drop-in – From the molecules to the system

A method for calculating the thermophysical properties of the blend R-445A is presented. The data were then employed in a transient simulation of an R-134a air conditioning system in a city bus operating with R-445A to evaluate the blend under transient boundary conditions. The cumulated COP of the reference R-134a system, derived from the cumulated cooling capacity divided by the cumulated compressor power, is ca. 3.9. Using R-445A as drop-in, the cumulated COP is ca. 8% lower, and also the maximum cooling capacity is reduced, whereas the high pressure and the compressor outlet temperature increase. Furthermore there is a higher risk of ice formation at the evaporator. Selective leakage of R-445A may cause a COP increase and a maximum cooling capacity decrease over the operation time.

Transcritical organic Rankine vapor compression refrigeration system for intercity bus air-conditioning using engine exhaust heat

T-ORVC (Transcritical organic Rankine vapor compression) refrigeration system for intercity bus air-conditioning using engine exhaust heat is investigated. Wet refrigerant R134a and dry refrigerant R245fa are considered as refrigerants. Typical efficiencies for pump, turbine and compressor utilized in literature (namely LV (low values), MV (medium values) and HV (high values)) are compared for the influence on COP (coefficient of performance) of the system analyzed. The results are strongly dependent upon the efficiencies. COPH (coefficient of performance for heat energy) used for the heating of the refrigerant, coefficient of performance for electrical energy (COPW) needed for the pump and coefficient of performance for both heat energy and electrical energy (COPT) needed together for the heating of the refrigerant and for the pump (COPT) are calculated. Besides the isentropic efficiencies, mechanical, electromotor and electric energy production efficiencies are also taken into account. Using MV efficiencies, COPT values at 8 MPa pressure can be as high as 0.54 and 0.66 at 180 and 200 °C temperatures for R134a and R245fa, respectively. COPW values are very high and therefore electrical energy need is very low as compared to exhaust heat energy need.Assuming 30 kW of cooling load for an intercity bus and under different engine loads, all heat exchanger properties are determined.The results for a cooling load of 30 kW show that using T-ORVC (organic Rankine vapor compression cycle) system, air-conditioning of the intercity buses can be realized using waste heat energy of engine exhaust gases.

Design of Bus Air-Conditioning Control System Based on PIC18F458

In this paper, a kind of efficient air-conditioning control system is proposed. PIC18F458 is adopted as main control unit of the system , which collects all kinds of analog singles such as temperature, humidity and so on, then convert them to digital by the A/D itself. After disposing and calculating each analog signal the system drives corresponding actuator to work to adjust temperature, humidity and cleanness of the air in bus, making passengers satisfied with air quality in bus.

Development of novel control strategy for multiple circuit, roof top bus air conditioning system in hot humid countries

A novel control strategy to improve energy efficiency and to enhance passengers’ thermal comfort of a new roof top bus multiple circuit air conditioning (AC) system operating on partial load conditions is presented. A novel strategy for automatic control of the AC system was developed based on numerous experimental test runs at various operating conditions, taking into account energy saving and thermal comfort without sacrificing the proper cycling rate of the system compressor. For this task, more than 50 test runs were conducted at different set point temperatures of 21, 22 and 23 °C. Fanger’s method was used to evaluate passenger thermal comfort, and the system energy consumption was also calculated. A performance comparison between that of the conventional AC system and that of the newly developed one has been conducted. The comparison revealed that the adopted control strategy introduces significant improvements in terms of thermal comfort and energy saving on various partial load conditions. Potential energy saving of up to 31.6% could be achieved. This results in a short payback period of 17 months. It was found from the economic analysis that the new system is able to save approximately 20.0% of the life cycle cost.

Thermoeconomic optimization for a finned-tube evaporator configuration of a roof-top bus air-conditioning system

This paper presents a methodology of a design optimization technique that can be useful in assessing the best configuration of a finned-tube evaporator, using a thermoeconomic approach. The assessment has been carried out on a direct expansion finned-tube evaporator of a vapor compression cycle for a roof-top bus air-conditioning (AC) system at a specified cooling capacity. The methodology has been conducted by studying the effect of some operational and geometrical design parameters for the evaporator on the entire cycle exergy destruction or irreversibility, AC system coefficient of performance (COP), and total annual cost. The heat exchangers for the bus AC system are featured by a very compact frontal area due to the stringent space limitations and structure standard for the system installation. Therefore, the current study also takes in its account the effect of the variation of the design parameters on the evaporator frontal area. The irreversibility due to heat transfer across the stream-to-stream temperature difference and due to frictional pressure drops is calculated as a function of the design parameters. A cost function is introduced, defined as the sum of two contributions, the investment expense of the evaporator material and the system compressor, and the operational expense of AC system that is usually driven by an auxiliary engine or coupled with the main bus engine. The optimal trade-off between investment and operating cost is, therefore, investigated. A numerical example is discussed, in which a comparison between the commercial evaporator design and optimal design configuration has been presented in terms of the system COP and evaporator material cost. The results show that a significant improvement can be obtained for the optimal evaporator design compared with that of the commercial finned-tube evaporator that is designed based on the conventional values of the design parameters

Theoretical and experimental simulation of roof-top bus multiple-circuit air-conditioning system performance

Many air-conditioning (AC) systems are designed to operate at maximum cooling capacity regardless of the variation in the daily cooling load. At low loads, the conditions can be uncomfortably cold and the overcooling is an unnecessary waste of energy. To address these two issues, a multiple refrigeration circuit concept is proposed and applied to a roof-top bus AC system. A two-circuit model is proposed for a standard bus size in which each circuit has two evaporators of equal sizes arranged in parallel and installed on each passenger row, respectively. This means that each passenger row is served by two different evaporators sharing a common heat exchanger box. Depending on the cooling load, this concept allows one or both circuits (compressor motors) to be switched on and during either modes, it also allows one or more sets of evaporator blowers to be switched on. A steady-state computer model has been developed to simulate the performance of the proposed two-circuit AC system. A two-circuit air conditioner is also designed to form a roof-top bus AC system, fabricated, and installed on to an experimental rig. The experimental data are used to validate the computer model. The validation is on the system thermal performance and on the evaporator air outlet conditions (dry bulb temperature and relative humidity) at different modes of system operation, either at full or partial cooling loads. The simulated results gave satisfactory agreement with those obtained from the experimental work. Maximum absolute deviations are within the range of 19.3 per cent, although most of the simulated results are less than a 10 per cent range from the experimental ones, which validates the computer program. The paper describes the modelling work carried out and the results obtained are presented in comparison with the experimental data.