# Superinsulation Materials for Energy-Efficient Train Envelopes
###### Abstract
The actual retrofit of such a train, including the economic aspects, is described by [PERSON] [13]. The author notes the difficulty to recuperate the cost of such a retrofit solely via the value of the energy savings and finds a payback time of 24 years, the main cost being the work to install the insulation. The situation is somewhat different for the construction of a new train, as some types of insulation must be installed anyway, thus there is no large difference whether a higher insulation thickness or a better performing material is used. Comparing the thermal envelope of trains to those of buildings, it is interesting to note that trains operating in heating-dominated climates such as Central Europe typically have U-values between 1.5 and 2.5 W/(m\({}^{2}\cdot\)K) [13; 14], whereas those for modern buildings are usually in the range between 0.15 and 0.3 W/(m\({}^{2}\cdot\)K) [15]. This difference exists in spite of comparable comfort requirements in buildings and passenger trains, especially for long-distance trains. However, train envelopes are under much stricter space constraints than building envelopes. This is because the outer dimensions of a train are usually limited by the track width of the railway system, by tunnels or the dimensions of train stations. Judging from discussions with train manufacturers and operators, the lower thermal performance of train envelopes compared to buildings seems to be due to four main reasons: (i) the space constraints mentioned above, (ii) structural connections in the train envelope that create a significant amount of thermal bridges, (iii) the less stringent comfort requirements in short-distance transportation (e.g., urban trains) and (iv) the considerably less stringent legal or normative requirements on maximal allowed U-values for trains. For buildings, the optimal U-value in terms of overall energy and CO\({}_{2}\) footprint over the lifetime of the building is around 0.10 to 0.2 W/(m\({}^{2}\cdot\)K), depending on the insulation material and heating system [16]. For such a building, increasing the insulation thickness can result in an increase in embodied energy, which is not offset by the energy savings during operation in the building's whole lifetime anymore--especially when low CO\({}_{2}\) heat sources are used, as is more and more common nowadays, such as heat pumps driven by low CO\({}_{2}\) electricity. Hence, the U-values of modern buildings in many heating-dominated countries in Central Europe or Scandinavia are close to the energetic optimum. Train envelopes, on the other hand, are far from these elevated performance standards. Thus, an improvement of the train envelope insulation has strong potential for considerable energy savings of HVAC during train operation with only relatively small increases in embodied energy, reducing the overall energy consumption and ecological footprint over the train's lifetime. Thermal superinsulation materials are characterised by a thermal conductivity of 20 mW/(m\(\cdot\)K) or less and have been established in the building sector for applications with space constraints in the last 20 years. With thermal superinsulation, it is possible to achieve two to four times the thermal resistance compared to conventional insulation materials at the same insulation thickness. Alternatively, very thin building envelopes can be built that still insulate just as well as conventional, thick envelopes. Superinsulation materials are used for inner-city buildings where space is at a premium and for the retrofit of existing buildings, especially for historic ones [17]. Additionally, architectural details such as window reveals, roller shutter housings or dormer window walls are common applications [18]. However, superinsulation materials are usually significantly more expensive than conventional insulation materials for the same level of performance. With the space constraints in trains, superinsulation materials are ideal to achieve a superior thermal performance in the thermal envelope of trains.