# Opportunities for Integrating Underground Railways into Low Carbon Urban Energy Networks: A Review
## 1 Introduction
### The United Kingdom  
The UK has one of the lowest shares of renewable energy used to provide heating and cooling in Europe, with only 7% of its demand being met with renewables, as shown in Figure 1. This demonstrates the challenge that will be faced in the coming years as the UK aims to decarbonize its economy and reduce GHG emissions by 80% by 2050 in comparison to 1990 levels, with the latest report by the Committee on Climate Change (CCC) asking for a new set of policies to achieve a more ambitious target of net-zero GHG emissions by 2050 [9]. The heating and cooling industry will play an important role in achieving that goal, particularly as heating and cooling account for approximately a third of carbon emissions and around half of the energy consumption in the UK [10]. In order to stimulate low carbon heating, the UK government set in place the Renewable Heat Incentive (RHI) in 2011, a financial refund linked to the quantity of heat generated from renewable sources. However, this incentive will expire in March 2021, and it is still not clear what the Government's plans are in terms of encouraging low carbon heat sources in the next decades, which will be a key driver to decarbonize the energy sector in the UK.  
Sustainable heating is also a problem at a city level in the UK. In London, nearly 90% of the heating sources used to warm buildings are gas-fired boilers, posing a threat not only in terms of climate change  
Figure 1: Shares of renewable energy sources used for heating and cooling for the EU and its 28 member countries in 2016 [8].  
but also for air pollution [11]. London's 3.5 million homes produce one third of the capital's total GHG emissions and nearly 75% of the energy consumed by homes is used to provide space and hot water heating. Meanwhile, workplaces account for approximately 40% of the city's emissions, with half of their energy demand being related to heating [11]. The Greater London Authority (GLA) has a bold plan to make London a zero carbon city by 2050, which is aligned with the latest recommendation from the CCC for the national emission reduction target. It is clear that, if London and the UK want to decarbonize their energy systems, their heat generation needs to be transformed to be based on low carbon heating alternatives.  
For a metropolis such as London and other cities in the UK and around the globe, waste heat recovery systems represent a unique opportunity to make use of the surplus heat that is wasted in a variety of urban infrastructures, as has been identified in [9; 11]. In addition to using low carbon heat, these systems unlock the potential to generate and distribute energy locally through district energy networks, which increase the efficiency of the overall energy system, helping to reduce the carbon footprint associated with heating while also tackling fuel poverty. This paper aims to explore the opportunities of waste heat recovery in the urban environment, with particular focus on recovering heat that is generated in underground railway tunnels, and its distribution through district heating networks (DHNs), showing how the combination of these technologies can become a key tool for changing the provision of heating in the smart cities of the future.