Molly Ayala
Passenger buses use air conditioning systems to cool, dehumidify, and filter the air within the passenger compartment. These systems are powered by a drive belt directly coupled to the crankshaft of the propulsion engine or a separate power source. The compressor, condenser, evaporator, and controls are the main components of a bus air conditioning system. The compressor is responsible for compressing the refrigerant gas and increasing its temperature and pressure. The design and characteristics of the compressor play a vital role in the performance and efficiency of the air conditioning system. The current solutions for HVAC systems in electric buses are not efficient, with about 30% of battery power used for heating and cooling. This has led to the adoption of gas-powered HVAC systems by over 40% of e-bus manufacturers.
| Characteristics | Values |
|---|---|
| Purpose | Cooling, dehumidification, and filtration of the air within the passenger compartment of a bus |
| Components | Compressor, condenser, evaporator, and controls |
| Types | Main-engine driven, sub-engine, electrically driven, and all-electric |
| Energy Efficiency | Varies depending on the type and design of the compressor; scroll compressors are known for their high energy efficiency |
| Environmental Impact | HVAC systems can increase on-road CO2 emissions and limit the range of electrified powertrains |
| Customization | Custom options are available based on factors such as bus size, cooling capacity, and energy efficiency |
| Challenges | Current HVAC solutions for electric buses are not efficient enough, utilizing about 30% of the battery power for heating and cooling |
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What You'll Learn
Electric bus air conditioning systems
There are several factors to consider when choosing an air conditioning system for an electric bus. Firstly, the cooling needs of the bus should be calculated based on the size of the bus, climatic conditions, and maximum expected passenger capacity to ensure adequate cooling. Secondly, energy efficiency is important to minimise the impact on the battery life of electric buses. Electric buses have limited space and weight capacity, so a light and compact system is preferred without compromising cooling capacity. It is also crucial to check the power requirements of the air conditioner, including voltage and current specifications, to ensure the bus's electrical system can handle the load without causing electrical problems or overloading the system. Additionally, a low-noise air conditioner can provide a quieter ride for passengers.
Several companies specialise in manufacturing electric bus air conditioning systems, such as Guchen, TKT, and Thermo King. Guchen's electric bus air conditioner uses R407C, a commonly used refrigerant in the industry, known for its good cooling capacity and thermodynamic performance. TKT, a leading manufacturer of bus air conditioners, offers customisations for various types of electric buses, including carrier buses, minibuses, and shuttle buses. Thermo King provides a complete range of all-electric bus HVAC systems for standard diesel-driven buses, with a focus on improving indoor air quality (IAQ) and mitigating airborne health risks.
The impact of bus air conditioning systems on energy consumption and CO2 emissions has been studied. It was found that HVAC energy demands could increase on-road CO2 emissions and limit the range of electrified powertrains. However, there are efforts to reduce CO2 emissions from road transport, and electric buses are becoming more popular due to their environmental benefits, lower operating costs, and quiet operation.
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HVAC system optimisation
The transition to sustainable urban mobility is seeing a shift towards electric buses. The heating, ventilation, and air-conditioning (HVAC) system on a bus is a primary auxiliary load and can significantly affect the efficiency and driving range of the vehicle. Therefore, optimising the HVAC system is crucial to improving the overall performance of electric buses.
HVAC systems are responsible for the cooling, dehumidification, and filtration of the air within the passenger compartment of a bus. The system works by transferring the heat from the passenger compartment to the outside air. This transfer of heat is facilitated by a medium with high heat transfer capabilities, such as a refrigerant. The refrigerant, under varying pressures, can perform different heat-absorbing functions. The compressor, a key component of the HVAC system, controls the operation of the refrigerant and circulates it through the system.
Optimising the HVAC system in electric buses is essential to reducing energy consumption and increasing the driving range. Advanced dynamic models have been developed to simulate and optimise the energy consumption of buses under real operating and extreme conditions, with a focus on the HVAC system. One study found that optimising the control parameters of the HVAC system and prioritising the heat pump as the primary heating source resulted in significant reductions in specific energy consumption for city buses.
Another study utilised a steady-state model to optimise the energy-comfort trade-off of HVAC systems in electric city buses. This approach revealed small deviations in the HVAC system's power demand and achieved thermal comfort compared to dynamic simulations. The steady-state model also allowed for the comparison of different HVAC system designs based on year-round performance evaluations. Furthermore, this method can be used to extract setpoints for online controllers to achieve close-to-optimal performance without predictive information.
In conclusion, optimising HVAC systems in buses, especially electric buses, is crucial to improving energy efficiency and reducing greenhouse gas emissions. By utilising advanced modelling and simulation techniques, such as dynamic and steady-state models, significant reductions in energy consumption and improvements in system performance can be achieved. These optimisations not only benefit the operational efficiency of buses but also contribute to the broader goal of sustainable urban mobility.
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Energy efficiency
The air conditioning system in a bus works by cooling, dehumidifying, and filtering the air within the passenger compartment of the vehicle. This process involves the use of a refrigerant that absorbs and removes heat from the passenger compartment, which is then circulated outside the bus. The compressor, evaporator coil, and condenser are key components in this process.
To improve energy efficiency, bus operators can employ various strategies. One approach is to use a combination of electric power and auxiliary fossil fuel heating systems. Buses with diesel-fuelled auxiliary heating systems consume less power than those that rely solely on batteries for temperature control. Additionally, efficient driving habits can improve energy regeneration. For instance, using regeneration while braking can achieve up to 35-40% regeneration, compared to only 5% when the mechanical brake is activated.
Efficiency tests for electric buses have become common, with manufacturers conducting range tests to showcase their models' performance on a single charge. These tests consider various factors, including ambient temperature, charging conditions, and the impact of heating and cooling systems on energy consumption.
The UITP's Working Group on Thermal Comfort and Energy Efficiency has developed a toolkit to help bus operators enhance energy efficiency and passenger comfort. This toolkit provides a comprehensive understanding of electric bus performance in various climates and offers strategies to balance energy efficiency with passenger comfort.
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OEM and after-market systems
The global shift towards sustainable transportation has made electric buses popular choices for eco-friendly mobility solutions. Electric buses are equipped with efficient air conditioning systems, offering passengers a refreshing and climate-controlled environment while reducing emissions and environmental impact.
Bus air conditioning systems are classified into groups based on AC compressor drive types. The three primary types of bus air-con systems according to their drive systems are:
- Standalone systems with an individual internal combustion engine dedicated to driving the AC compressor. This is ideally suited for medium to long-distance coach buses, offering consistent performance and reliability.
- Electric compressors, where the AC compressor is directly powered by a drive belt connected to the bus's propulsion engine. This is a compact solution suitable for vans, minibusses, and city buses with limited space.
- All-electric buses, where the power to operate the electric compressor is supplied by the car battery. This system offers benefits such as reduced fuel consumption, zero emissions, minimal energy use, and easy thermostatic control.
OEM and aftermarket systems vary, from system installations that tie into existing OEM dashboard HVAC systems to stand-alone complete air conditioning systems. The components that make up an A/C system are the compressor (mounted on the engine or a separate power source), dash evaporator (driver area), controls (switches, thermostats, electronics), hanging or flush evaporators, and rooftop or skirt-mounted condensers.
Manufacturers are continually updating or adding new system configurator links through their dealer networks, so it is important to work with your dealer to find the cooling system that is specific to your vehicle’s cooling needs. Aftermarket offerings now emphasize energy efficiency, smart technology integration, and easy installation to meet customer demands.
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Challenges of HVAC in electric buses
Electric buses are the fastest-growing part of the EV market, with a compound annual growth rate of over 100% since 2013. This growth is largely due to new environmental concerns and legislation. However, electric buses face challenges with their Heating, Ventilation, and Air Conditioning (HVAC) systems.
The HVAC system is the primary auxiliary load in an electric bus and significantly affects the efficiency and driving range of the vehicle. The system can consume up to 30% of the battery energy, limiting the bus's mileage. In extreme external temperature conditions, the energy consumption per kilometre can increase by up to 56% due to the HVAC system. This issue causes "range anxiety" for bus drivers, who worry that operating the HVAC system for long periods will deplete the bus's battery before the end of their route.
There is currently no "golden standard" for HVAC architecture in electric buses, and manufacturers are adopting different approaches for heating and cooling. HVAC energy demands could increase on-road CO2 emissions and limit the range of electrified powertrains. The heating and cooling of electric vehicles can be powered either by electricity or fuel. Electric-powered cooling is obtained with Vapour Compression Cooling (VCC) or heat pumps, while electric-powered heating can be achieved through Positive Temperature Coefficient (PTC), heat pumps, a combination of PTC and heat pump, or a water heater. Over 40% of electric buses adopt a fuel-powered heating solution.
To overcome the limitations of short driving ranges, dynamic wireless charging (DWC) facilities can be installed on dedicated bus lanes to charge electric buses while they are in motion. Additionally, a preference-inspired (P-ins) mechanism can be used to manage multiple goals, including thermal comfort, indoor air quality (IAQ), and system energy efficiency. This mechanism guides the agent towards an optimal control policy with a high convergence rate, reducing energy consumption without compromising thermal comfort and IAQ.
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Frequently asked questions
Passenger buses have AC electricity through the use of air conditioning systems. These systems are powered by electricity and work by using a compressor to circulate refrigerant through the system, cooling and dehumidifying the air within the passenger compartment of the bus.
There are three main types of air conditioning systems used in passenger buses: main-engine driven, sub-engine, and electrically driven. The type of system used depends on factors such as bus size, cooling requirements, and power supply capabilities.
When choosing the right air conditioning system for a passenger bus, it is important to consider the specific needs of the bus, such as bus size, cooling capacity, and energy efficiency. It is also important to work with a dealer or manufacturer to find a system that is compatible with the vehicle's existing systems and that meets the specific cooling needs of the bus.
