In my last article "How about Overheated Metros – a New Theory explained", I have discussed the effect of the hot underground network of metro during the summer. In this article, I am suggesting some possible solutions based on my research for overcoming the issues which are not solved for centuries.
There are many heat sources in Metro systems. However, only those that are seasonal are under consideration in this paper. Indeed, it is worth noting that, in the cooler weather the combined effect of the non-seasonal heat sources is insufficient to heat trains to a temperature in which customers would feel comfortable and so in winter these heat sources need to be augmented by the addition of heating from train saloon heaters.
However, there are two heat sources that increase in summer: outside ambient air and solar irradiation. The former is an important and notable seasonal variant, but whilst ambient air temperature varies throughout the daily cycle, it does not dramatically change over a 30 minute period. It cannot, therefore, explain the cause of the considerable increase in train temperatures on the surface often achieved over that same time period, so this paper will focus primarily on the role of solar irradiation.
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! In this link, under the “Measurement” section it explains that “…direct sunlight at Earth's surface when the Sun is at the zenith (directly above) is about 1050 W/m2”.
This is generally supported by the monitoring and other evidence presented in my previous paper “Cooling the Tube – On Ice till 2030”, particularly the links on Page 2, and also by the fact that in the summer of 2018 my pale colored sandstone patio was as red-hot as the sand on a Mediterranean beach. However, the sun’s power is also affected by the Angle of Incidence, which reduces its intensity when it is at shallower angles. For example, at a 45⁰ angle of incidence, although solar radiation can cover a 40 percent greater area, it is then 30 percent less intense than when at its maximum angle of incidence of 90⁰.
Consequently, we also need to consider how this changing intensity might affect trains. Prof. Piercarlo Romagnoni and Prof. Fabio Peron of the Università Iuav di Venezia produced a factsheet that examines the temperature impact solar radiation has on thermal insulation materials in roofing applications. Although some train roofs are not insulated, this factsheet gives an indication of the potential external skin temperatures. One test they undertook was on a roof sheet formed of a curved sandwich panel:
Whilst this is not exactly how a train roof is constructed, I would submit the external skin of a train roof would absorb similar levels of solar irradiation and reach similar temperatures. The maximum temperatures on curved roofs in more moderate climate zones like Venice can reach 67°C. Summer ambient air temperatures in Venice can reach 40°C, so the surface temperature of the aluminum external skin of this sandwiched panel is potentially around 25°C above ambient, which is akin to the difference between train rail temperatures and ambient air temperatures.