In 2023, the Dutch Ministry of Infrastructure and Water Management launched an action plan to make temperature-controlled transport more sustainable. A key knowledge gap was the contribution of transport refrigeration units (TRUs) to overall emissions. To close this gap, TNO conducted an extensive measurement program on TRU emissions in real-world operations.
Monitoring TRUs in practice
Eight TRUs were monitored for a total of 2,050 days, with operating times ranging from 231 to 1,787 hours. Six units were modern Stage V diesel engines, while two represented older standards (Stage II and IIIA). On average, TRUs ran 900–1,000 hours per year with the diesel engine active, although usage varied widely depending on cooling demand and whether units were connected to the grid or equipped with start-stop systems.
Key emission findings
- NOx emissions averaged 36 g/hour, significantly lower than the earlier assumption of 100 g/hour. This leads to a revised national estimate of 1.8 kilotons NOx per year, down from 4.6 kton.
- CO₂ emissions averaged 5.5 kg/hour, in line with previous estimates, contributing about 0.3 Mton annually.
- PM emissions from one Stage V TRU ranged from 340 to 825 mg/hour, depending on load, but remained within regulatory limits. Current data is too limited to adjust national PM factors.
An anonymized dataset of 850 additional TRUs confirmed that newer units typically operate twice as many hours as older ones, mirroring trends in truck usage.
Comparing TRUs and trucks
Despite being small engines, TRUs can have a disproportionate impact:
- During city deliveries, a TRU may emit one-third of the total NOx from a Euro VI truck–trailer combination. In some cases, the TRU emits as much or more NOx than the truck itself.
- For CO₂, TRUs contribute roughly 15% of emissions during urban operations.
- While PM emissions per hour are similar to Euro VI trucks, emissions per kWh are over ten times higher, largely because TRUs below 19 kW are not required to have diesel particulate filters.
Implications for policy and practice
The study provides more accurate, evidence-based emission factors for TRUs, which will be used in national modelling. However, it also highlights regulatory blind spots: Stage V emission standards are relatively weak for engines with a power rating of less than 19 kW, allowing TRUs to emit more pollutants than modern trucks.
With legislation tightening and consumer demand for sustainable logistics increasing, the findings underline the urgency to strengthen emission standards, promote electrification of TRUs, and expand measurement campaigns—especially on particulate matter.
In short, while the overall emission picture is now clearer, TRUs remain a significant challenge on the road to zero-emission logistics.