As summer temperatures rise, concern rises also over the rising temperatures in the underground environment of overcrowded metro systems that include above and below ground sections and have no sub-surface cooling systems. Following on from his previous papers, Calvin Barrows, makes the case for painting trains with high-performance solar reflective paint and fitting low emissivity glass windows as a cost effective method of addressing the issues retrospectively in older metro systems.
Even though we all experience the effect of sunshine in summer, there does not appear to be more than a superficial appreciation of just how powerful solar irradiation can be. When the sun is at its zenith, direct sunlight at earth surface is about 1050 W/m2. This is supported by monitored and observational evidence. The power of the sun is also affected by the angle of incidence, which reduces its intensity when it is at shallower angles. For example, at a 45° angle of incidence, although solar radiation can cover a 40% greater area, it is then 30% less intense than when at its maximum angle of incidence of 90°. Consequently, how this changing intensity might affect trains also needs consideration.
Professors Piercarlo Romagnoni and Fabio Peron of the Università Iuav di Venezia produced a factsheet which examines the temperature impact of solar radiation on thermal insulation materials for roofing applications. Although some train roofs are not insulated, this factsheet gives an indication of the potential external skin temperatures.