Eco-friendly HVAC units look to keep rail cool

As temperatures rise, so too does our dependence on air conditioning. The problem is that conventional heating, ventilation and air conditioning (HVAC) systems, which use artificial refrigerants, produce climate change causing greenhouse gases.

Answering this problem is a new generation of eco-friendly HVAC systems. Unlike traditional units, which utilise HydroFluoroCarbon (HFC) refrigerants, these new systems use natural refrigerants like CO2 or propane. In doing so, they not only reduce carbon emissions, but also energy consumption. It is this ability to reduce both emissions and energy that make eco-friendly HVAC units particularly attractive to rail.

Unfortunately, installing them is easier said than done. One challenge is the limited space and weight restrictions of passenger vehicles. Not only will any new HVAC unit have to be installed within the space already available, it also cannot add any additional weight to the vehicle. Furthermore, because the energy consumption and total lifecycle costs of HVAC systems are influenced by the train’s design, air ducts, and maintenance requirements, the system itself must be designed with the close collaboration of the HVAC supplier, vehicle integrator and train operator.

To address these challenges, the Europe’s Rail Joint Undertaking delivered several prototype HVAC systems that are based on heat pumps and that use CO2 as a natural refrigerant.

A promising HVAC technology

The prototypes are based on extensive research that evaluated the most suitable HVAC technologies and eco-friendly refrigerants for railway applications. Several criteria were considered, including costs, energy consumption, weight, maintainability, and impact on global warming.

From this comprehensive assessment, heat pumps using CO2 as a refrigerant were identified as the most promising HVAC technology.

Other key findings included:

• Designing and operating a new HVAC unit will require specifications from both the train vehicle builder (e.g. volume of ventilated compartment, location of HVAC unit, ambient conditions) and the HVAC supplier (e.g. maintenance documentation, operation manuals, interface documents, drawings).
• The high pressure and possible leakage of the refrigerant presents a risk to the health of passengers and maintenance staff.
• Standardisation could complicate the development and installation of a new HVAC unit.
• While the use of heat pumps and natural refrigerants will reduce energy consumption, this reduction will come with an increase in weight, capital and maintenance costs.

Meeting actual needs

To ensure that the prototype HVAC units met the actual needs of rail operators, DB and SNCF provided a list of requirements previously used when procuring HVAC systems.

The two railway operators, together with train vehicle builders, went through the requirements specific to systems using CO2 as a refrigerant.

These requirements were carefully discussed with HVAC suppliers, who assessed whether they could meet the requirement, whether the requirement caused problems, or whether the requirement was impossible. They also identified any requirements that although could pose problems for the prototypes, should not be an issue for a series production.

New generation HVAC prototypes

Based on the in-depth research and the specifications elaborated by DB and SNCF, several HVAC unit prototypes were developed and tested. Each unit was based on a heat pump for cooling and heating and used CO2 as a natural refrigerator.

Wabco Faiveley HVAC prototype

Measurements of air volume flow, over-pressure tests, and testing of cooling and heat pump performance of the Wabco Faiveley prototype were conducted in a climatic measuring chamber where ambient conditions such as temperature and humidity could be varied.

The test results were compared with results from an existing HVAC system using R134a refrigerant.

The prototype was also subjected to vibration tests (per EN61373, category 1, grade A) to qualify for installation on or in a railway car body.

Key findings

• The expected and required cooling capacity can be achieved with a slightly increased energy consumption at the design point (compared to conventional unit).
• Heating capacity increases with improved minimum ambient temperature.
• In terms of efficiency, although the prototype has higher peak consumption, there is a potential for lower annual consumption due to an enlarged heat pump operating range.
• The prototype’s dimensions are the same as the R134a unit.
• Although weight increased by 12%, this was less than expected and mainly caused by the compressor and accumulator.

After successful completion of the lab tests, the prototype was installed on one end vehicle of a DB single-deck regional train. For comparison, a state-of-the-art HVAC unit using R134a refrigerant was installed on the other end-vehicle of the same train. The train then operated a regular passenger service in southwest Germany.

Although both HVAC units operated smoothly, there were gaps in the data acquisition of the prototype. Furthermore, a valve failure meant the prototype had to be removed earlier than planned. Because the energy consumption of the two units could not be compared in the field, tests were instead conducted in a climate chamber.

Key findings:

• Both the prototype and conventional units were able to meet the comfort conditions defined in EN 14750.
• Annual energy consumption of the prototype unit is 7% lower than the consumption of the reference unit.
• No significant differences are expected for the maintenance of the prototype compared with the conventional unit.

Conclusions

The innovative HVAC prototype with natural refrigerant and heat pump demonstrated its suitability in normal train operations. With these field tests, a technology readiness level 7 (system prototype demonstration in operational environment) was achieved.

Knorr-Bremse prototype

The two prototypes developed by Knorr-Bremse were subjected to lab tests to verify and prove safety and conformity with applicable laws, standards and the functional specifications. All new components underwent shock and vibration tests in accordance with EN61373, while electromagnetic compatibility tests were conducted in accordance with EN50121-3-2. Functional and performance tests were conducted in a climate chamber.

Following the successful completion of lab tests, the prototypes were installed on a Bombardier vehicle widely used by DB for regional and commuter services. For comparison, another coach on the same train was equipped with state-of-the-art HVAC units using R134a refrigerant.

The test coaches ran a commercial service in Bavaria, during which test data was collected and analysed, comparing the results from the innovative prototype with the corresponding results of the state-of-the-art HVAC unit. Specific attention was given to performance, energy consumption, maintainability and reliability.

Key findings:

• There were no passenger complaints related to the two vehicles, confirming that both HVAC systems (prototype and conventional) can achieve a satisfactory level of comfort.
• Both HVAC systems can maintain comfort conditions in the vehicles (in heating and cooling modes) as required by EN 14750.
• The energy consumption of the prototype is about 7% lower than that of the conventional unit.^[1]
• The efficiency of the prototype is higher for ambient temperatures below 23°C, whereas the conventional HVAC unit is more efficient in cooling mode at higher temperatures.
• There was no difference in the reliability of the two systems, both ran fault-free during the entire field test period.
• The prototype’s higher pressure means precaution must be taken to prevent the CO2 circuit from leaking (particularly as to bolted connections).
• Maintenance requirements for the prototype are expected to be slightly higher (10 to 20%) due to the added complexity (e.g. heat pump).

Conclusion

The innovative HVAC unit with natural refrigerant and heat pump demonstrated its suitability for applications on railway vehicles. A technology readiness level 7 (system prototype demonstration in operational environment) has been achieved.

Average prototype performance

(compared to conventional HVAC system)

• • Global warming impact -99.93%
  • Expected Capital cost +10 to 20%
  • Energy cost -5 to +10%
  • Maintenance cost +10 to +20%
  • Reliability 0%
  • Weight +10 to +20%
  • Volume +0 to +5%

Capital and maintenance costs are stated as percentage of the overall cost of the train. The energy consumption is stated per year (not per km) and covers the energy for passenger operation (16 h/day), standstill (4 h/day) and overnight parking (4 h/day).

Towards an eco-friendly HVAC system for rail

Field tests of the prototype HVAC units demonstrated that, for the climatic conditions of Central Europe, it is possible to achieve the same thermal comfort with systems using natural CO2 refrigerant.

However, doing so will require the completion of several additional steps:

• Further optimisation of the control algorithms to reduce energy consumption in heating mode (heating with electrical resistors vs. heating with heat pump).
• Development of refrigerant charge control solutions, particularly for CO2 technology requiring high pressure in the system.
• Additional pre-standardisation work on control interfaces and train control and monitoring systems.

There is also the potential to investigate other sources of natural refrigerants, such as propane, that could offer even lower energy consumption than CO2.

Regardless of any additional steps, there is a clear opportunity for railway operators, rail vehicle builders and HVAC suppliers to use these findings to develop a new generation of eco-friendly HVAC systems tailored to the unique needs of the rail sector.

 Recommendations for standardisation

To ensure that standardisation doesn’t become an obstacle to the introduction of innovative, eco-friendly HVAC units, the Europe’s Rail Technical Demonstrator offers several recommendations.

• Pre-standardisation work for HVAC classes should be integrated into a European Standard, such as CEN/C 256/SC 03/WG08 ‘Air conditioning, Heating and Ventilation Functions’.
• Mechanical Interfaces: Although the pre-standardisation work on mechanical interfaces is not ready for inclusion in a European standard, the following standards address the topic:
  • CEN/C 256/SC 03/WG08 ‘Air conditioning, Heating and Ventilation Functions’
  • Eurospec WG HVAC Systems. Scope of the voluntary standard: “Specification for air conditioning of railway vehicles”. The specification is an add-on to the specification of Interoperability (TSI) and the Euronorms.
  • ISO/TC 269/SC 02/WG 2 HVAC Systems
• Electrical interfaces: Recommendations are made for the medium voltage and low voltage (battery voltage) supply of HVAC systems on mainline trains and on LRVs.
  • 400 V 3 ph AC 50 Hz power supply is the preferred choice for all vehicles except LRVs.
  • 110 V DC is the preferred choice for low voltage (battery) supply of all vehicles except LRVs.
  • 24 V DC is the preferred choice for low voltage (battery) supply of LRVs.
• The recommendations for supply voltages should be integrated into a European Standard. The following standards and specifications could address these topics:
  • CENELEC CLC /TS 50534 ‘Railway applications – Generic system architectures for onboard electric auxiliary power systems’
  • CENELEC EN 50533 ‘Railway applications – Three-phase train line voltage characteristics’
  • EuroSpec WG HVAC Systems

[1] Energy consumption could not be determined using field test data because the two coaches did not always run in the same train consist. Instead, consumption was established through tests in a climatic chamber using representative duty cycles defined by DB.