1.1. Heating, ventilation and cooling (HVAC)

Heating and cooling in buildings represent a significant amount of the energy consumption of public transport companies and great potential reductions can be achieved in this field, especially in older depots, workshops and stations.

Heating adjustments

Depots and workshops are required for stabling and maintaining buses and rail vehicles. In order to guarantee a certain level of comfort for employees, these must be heated during winter and sometimes cooled in summer. Heating is often done with radiant gas heaters, but these installations are rarely optimised to function only during working hours. Proper adjustment of heating systems can have a major impact on energy use.

Office buildings are usually better insulated, but their energy use can also be reduced by optimising the heating parameters. The cost of doing this is relatively low, as it does not require any investment. All it requires is a few staff hours spent analysing the property, defining the proper functioning of the heating system and carrying regular controls. Awareness campaigns can help to achieve optimum results (see Influence and Communicate section).

Ventilation control

Ventilation is required in bus depots and workshops to ensure that gas exhausts are correctly evacuated outside of the buildings and to prevent any poisoning of staff. These facilities are usually fitted with an automatic ventilation system comprising extractors controlled by automatic timers. Ceilings may also be equipped with manual openings, in case the ventilation system is out of order. Unfortunately, ventilation systems are often not properly configured, which can result in significant heating losses.

After the definition of a working time schedule during which ventilation must be provided, the system’s settings can be adjusted.

Automatic fast-closing doors

Public transport vehicles enter and leave depots and workshops several times a day. Most of the time, the doors of the building remain open once a vehicle has left and must be closed manually. Installation of fastclosing doors enables a reduction in the time needed for opening and closing the doors, thus reducing heating losses. A sensor detects the vehicle in front of the doors and opens or closes them automatically. In addition to reducing the heating and cooling needs, an initiative like this can also avoid additional manual work risks related to the opening and closing of the doors.

Adiabatic cooling

The heat released by the IT equipment significantly increases the temperature in technical rooms. Above a specific temperature, this equipment stops working properly. As a result, technical rooms need to be ventilated and cooled either through natural or mechanical ventilation.

Air conditioning requires expensive maintenance, increases energy use, raises the temperature in the adjacent locations (a metro station for example) and dries the air. They also use refrigerants that can cause GHG emissions through leakage and are often significant energy consumers.

Adiabatic cooling is a concept that involves decreasing the temperature through evaporative cooling. A circulating pump takes the water to the top of a filter media, which absorbs the water and is moistened. Then a ventilator passes the air through the filter media. The exchange between the air and the water causes the water to evaporate. As the water evaporates, the temperature decreases and outgoing air from the unit becomes cooler and less dry than the incoming air.

Fig. 28 – Evaporative cooling principle

In cold periods, the cooling works only through mechanical ventilation. In hot periods, the system works through adiabatic cooling and the thermostats control the opening of the water inlet valve and the opening of the circulating water pump. The use of an adiabatic cooling system can meet several objectives:

  • Recover outside dry and hot air to cool technical rooms
  • Refresh the technical rooms without releasing heat into the adjacent rooms
  • Consume less electricity
  • Reduce direct and indirect greenhouse gas emissions
  • Reduce dust loading in the technical rooms
  • Decrease the electricity bill and the maintenance costs.

Combined Heat and Power Heat

Combined Heat and Power (CHP) is a technique to generate electricity and heat simultaneously. Most systems producing electricity produce heat as a byproduct, and this heat is usually lost, decreasing the global efficiency of the system. CHP is the principle of recovering this otherwise lost energy. The heat recovered can be used to heat a depot or an office building and to produce hot water. The system must be sized according to the thermal needs of the building. If the system produces more energy than the building needs for internal use, electricity can be sent back to the main electrical grid.

The system consists of a gas engine, which is connected to an alternator generating electricity. To maximise the gain, the heat produced by the system is recovered at two levels:

  • Recovery of the heat through an exchanger heating the water flow. This water flow is sent to the radiators and air heaters of the heating system
  • Recovery of the exhaust gas heat. This lowtemperature heat is recovered through condensing and directed to the low-temperature circuit. This heat is finally sent to air heaters and a heat plate exchanger, for domestic water  production.
Fig. 29 – Principle of the combined generation of electricity and heat

Fig. 30 – Comparison of the efficiency between a traditional system and a Combined Heat and Power system. Source: US Department of Energy.


The CHP process improves the global system efficiency and offers large cost savings, as it consumes less primary energy than the separate production of heat and electricity. Compared to conventional power plants, CHP energy savings vary between 15 and 40%. For public transport companies operating large buildings, the use of CHP is highly recommended, as the return on investment is higher than for smaller buildings.

Green roofs

In summer, when temperatures are high and the sun is shining, buildings’ roofs can become very hot and increase the overall temperature inside. The same happens in wintertime when the roof gets cold and has an impact on the temperature inside of the building. A green roof is a vegetative layer grown on a rooftop. It provides shade and removes heat from the air through evapotranspiration.

Green roofs can be installed on a wide range of buildings, from industrial facilities to offices. While the initial costs of green roofs are higher than those of conventional materials, companies are able to offset the difference through reduced energy use and the longer lifespan of green roofs compared with conventional roofing materials. The implementation of green roofs is recommended for the public transport sector, because many buildings have extensive roofs.