A recent review on ventilation and cooling of underground high-voltage cable tunnels

The rapid progression in the current economic cities and the push for societal advancement are driving the development of tall, slim structures that require underground utility tunnels. These tunnels, known as underground services tunnels, are built beneath urban areas and house a variety of main pipelines for gas, heating, and electricity. They serve as underground walkways, high-voltage lines, drainage systems, and water supply networks. The construction of these underground tunnels is essential for facilitating the future growth of buildings and infrastructure without disrupting the streets where utilities were traditionally located in older city layouts. The increase of the quantity of deeply buried extra-long tunnels has increased quickly, leading to geothermal hazards emerging as a significant issue in engineering projects. This research has studied many approaches for ventilation or cooling of underground cable tunnels. Approximately 90% of the utilized techniques consisted of air ventilation, encompassing mechanical, natural, or a combination of both approaches. It is important to consider that this particular method is more commonly employed in regions with higher weather conditions similar to Egypt. Nonetheless, this approach efficiently sustains satisfactory indoor temperatures surrounding cables, guaranteeing that they stay below 40 °C. Nevertheless, in the particular climate region of Egypt, where outdoor temperatures can soar more to 40 °C, it became crucial to subject the air to cooling before it is supplied into the tunnel. The impact of the mist (FOG) system on tunnel cooling was also examined. However, it has been discovered that the humidity levels in tunnels rise excessively, despite the high initial cost involved, particularly in lengthy tunnels. Consequently, it is not regarded as a viable method for cooling cables. Its sole application lies in serving as a fire suppression system for cable tunnels. Approximately 10% of the conducted research employed water cooling, which involved circulating cold water through pipes located next to the cables in rows. As a result of convection heat transfer, the air surrounding the cables in the tunnel is effectively cooled. It was found from this study that mechanical air cooling (ventilation) is the most efficient way of dealing with heat dissipated from cables by convection inside tunnels.