# Opportunities for Integrating Underground Railways into Low Carbon Urban Energy Networks: A Review
## 2 District Energy
### District Heating  
Heat networks represent an interesting alternative to deliver cost-effective low carbon heat, as they enable the use of a wide range of low carbon sources, including renewable and waste heat, and are able to incorporate new buildings and heat sources as the network grows, being particularly attractive in urban areas, where concentrated demands minimize distribution costs and heat losses. As they allow heat generated from different sources to be used, DHNs also avoid a lock-in to technologies that may become obsolete in future decades. This feature provides a key mechanism to allow a smooth transition from current fossil fuel based heating to future low carbon thermal grids.  
The recovery of urban waste heat represents a low carbon alternative to conventional heating methods that is often only feasible when district heating is deployed, as it allows the waste heat to be efficiently distributed to where end users are located. Using locally produced heat lowers the cost of heating when fuel prices are high or environmental taxes or fees apply, cutting costs related to heat provision and therefore helping to reduce fuel poverty [14]. In the UK, heat networks supply only around 2% of the overall heating demand [15], while in London, approximately 6% of the energy demand is supplied via district systems [11]. However, national and local authorities have recognized the importance of heat networks and are planning to promote the use of DHNs as a key future policy. The CCC [9] has indicated that low-carbon district heating will play an important role in decarbonizing the heating of buildings, helping to achieve the proposed net-zero GHG emission target by 2050, while the London Environment Strategy [11] has already set a target of meeting 15% of the capital's energy demand using district schemes and renewable sources.  
Historically, the evolutionary trend amongst district heating has been towards lower operating temperatures, higher energy efficiency and lower use of fossil fuels as heat sources. Over the years, district heating has evolved significantly, from 1 st Generation District Heating (1 GDH), categorized by using steam as the heat carrier [14], to recent 4 th Generation District Heating (4 GDH) and 5 th Generation District Heating and Cooling (5 GDHC). The 4 GDH is defined by low temperature water networks based on the use of renewable and waste heat sources, being linked to electrical grids to form smart energy systems [16]. As for 5 GDHC, it not only involves smart coupling of sectors, but also the provision of both heating and cooling through the same network operating at very low temperatures, with negligible thermal losses and a high energy efficiency [17]. These two generations will define the future of district energy, which will increasingly involve the use of renewable and waste heat as the main sources of energy, shifting the heating sector towards decarbonization, smart coupling and greater efficiency. In cities, the operation of several infrastructures generates significant amounts of heat, making waste heat based district heating a particularly interesting opportunity in urban areas. The following section aims to explore the benefits and challenges of exploiting waste heat from a variety of sources in the urban environment.