The recovery of waste heat in DH allows the reduction of fossil fuels consumptions and therefore of GHG emissions while increasing the access to clean, affordable, and secure energy.

Ideally, to foster the integration of waste heat and renewable energy in district heating (DH) systems, the city should present a dense built environment and buildings with a centralized heating system that can be easily converted into a DH substation. Refurbished and adaptable buildings would be easier to be connected to a DH network at low temperature and reduced distribution temperatures enables the utilization of a wider range of possible technologies and heat sources to be used. Therefore, an adequate built environment will make thing easier for waste heat recovery and renewable energy sources. This can be influenced by the conformation of the territory under study and thus by the presence or not of waste heat sources in the proximity of heat demands or already existing distribution networks.

Also, the role of decision-making authorities, such as policy makers and cities’ mayors, is very relevant, with a well-made spatial planning and with a long-term vision. So that they can make appropriate choices and investments, it is essential that they have a comprehensive view of the urban energy system. They should be aware of what is the potential of DH in their area of interest and what are the benefits that can be reached, such as reduced greenhouse gases emissions, independence from fossil fuels and carbon-based energy markets, sustainability, security of supply, etc. Moreover, it is essential a good communication among all the involved parties, thus policy-makers, DH operators, owners of waste heat sources such as industries and district users, thus citizens. They should also try to create a favourable environment for DH uptake by considering the competitiveness of DH with respect to other heating alternatives, and thus avoiding, for example, to incentivize fossil-fuelled based technologies such as CHPs.[1]

 Regarding the legal framework, the model of ownership that has demonstrated to be the most effective for DH system is the municipal/cooperative model. Denmark, where DH market share is among the highest in the world, can serve as an example in this sense. In 2019, municipally and cooperatively owned district heating companies supplied approximately 93% of the heat sold in Denmark, with the remaining 7% given by private companies [2].

 [1]  K. Lygnerud, E. Wheatcroft, and H. Wynn, “Contracts, business models and barriers to investing in low temperature district heating projects,” Appl. Sci., vol. 9, no. 15, 2019, doi: 10.3390/app9153142.

[2]  K. Johansen and S. Werner, “Something is sustainable in the state of Denmark: A review of the Danish district heating sector,” Renew. Sustain. Energy Rev., vol. 158, no. December 2021, p. 112117, 2022, doi: 10.1016/j.rser.2022.112117.

One of the main aspects that can hinder the implementation of district heating based on waste heat recovery and renewables is the lack of information and capacity to generate an effective large-scale and long-term urban planning, that can be able to take into considerations the local nature of district heating together with its international and global implications. This double nature of DH (local and global) makes it difficult to comprehensively analyse and assess its potential.

A research gap is identified in methodologies and instruments able to comprehensively analyse and assess district heating potential. Indeed, the existing broad energy system models cannot represent the complexity of an urban area, and in the main time the outcomes of the existing detailed local models cannot be reflected in the higher scale of analysis.

Together with urban planners, a lack of knowledge of the other involved parties, such as owners of waste heat sources, DH operators and users, is to be stressed. They may not be aware of the opportunities district heating can bring and this can lead to a not favourable opinion of DH: waste heat owners such as industrial companies can be uninterested in being part of a DH systems and citizens may consider their connection to a district heating network not convenient. The divergent view that heat owner and DH companies can have is another big obstacle.

Another barrier for DH implementation and diffusion is therefore the lack of a well-established regulatory framework, which can push towards a fully decarbonized heating system and that can regulate the market in order to achieve energy equity and security of supply.

Other obstacles of technical nature may be:

• The lack of waste heat sources in the proximity of the existing DH network or of the area to be heated
• The impossibility to recover the available heat because of not adequate temperature levels
• The temporal mismatching between heat availability and heat demand

The fluctuating and not controllable nature of renewable sources and other waste heat sources that can may not ensure a constant supply of heat.

The main problems that can arise in DH networks based on waste heat recovery may be due to an improper handling of the systems. Indeed, what has been generally noticed is that even if a system has been well designed and built, it can bring adverse impacts in the energy system and in the society due to errors in operations. The latetrs can be addressed to both DH operators and costumers. For example there can be temperature levels, mass flow rates and pressure values along the distribution and transmission network that are far from the design conditions. One example is the EU-EIT Climate-KIC project called Merezzate+ (https://www.merezzateplus.it/). 

• Article - Waste heat Italy https://reader.elsevier.com/reader/sd/pii/S2666955221000083?token=D2C0967BA30FF3777EC07B334A94AC47F10036A2ABE6216A5F40F120B95DBADFB63252689C0A016D0F226DCEE25EC8E8&originRegion=eu-west-1&originCreation=20220412092127
• https://eurec.be/waste-heat-is-a-sustainable-source-for-district-heating-networks/

EU projects:

• Reuse heat https://www.reuseheat.eu/
• Rewardheat https://www.rewardheat.eu/en/
• SoWhat https://sowhatproject.eu/

DNSH:

The environmental goals that should not be armed according to the DNSH principle are six:

1. Climate change mitigation
2. Climate change adaptation
3. Sustainable use and protection of water and marine resources
4. Transition to the circular economy, including waste prevention and recycling
5. Prevention and reduction of air, water and soil pollution
6. Protection and restoration of biodiversity and health of ecosystems.

District heating’s features makes it a technology that can intrinsically lead to the fulfilment of these principles. Indeed, it is able to efficiently distribute recovered waste heat otherwise wasted, with a consequent reduction in primary energy (and fossil fuel) consumption, and avoided greenhouse gases emissions. In this sense it can mitigate the climate change and it is also adaptable to integrate a wide range of renewable energy sources. Since it is a large-scale infrastructure with a strong local nature, DH is potentially able to properly exploit the resources available on the territory. With a well-done spatial and energy planning, the exploitation of natural resources such as soil and water can be done without causing an increased pollution of them. And in the same way, this can be done for the protection of the ecosystem.

Moreover, being an infrastructure that connects many different energy consuming sectors, namely the heating sector (with the buildings), the power sector (with the large-scale heat pumps and the CHP plants), the industry, the telecommunication network (with datacenters), etc., DH also has a central role in sector coupling and circular economy.

Again, it must be stressed that all these intrinsic benefits that DH can bring can be reduced or even reversed if a proper spatial planning and a good energy strategy are not done. Its strengths may become problems if not well addressed, both in the planning and in the operating phase.

Specific identified risks that may infringe the DHNS principle is the noise pollution that can be envisaged as a consequence of the market uptake of heat pumps, both in large-scale and small-scale applications.

Instruments and activities to be developed and investigated to foster district heating uptake, especially with low temperature distribution networks based on waste heat and renewable sources, are:

• Models and methodologies able to address the intermediate level of analysis and to fill the identified research gap between large-scale and small-scale levels.
• Capacity building of all the involved parties to assure them a proper overview on DH and waste heat recovery. There could be tutoring, dissemination and communication activities, for example between researchers and DH operators, between DH operators and users, and between policy-makers and urban planners.

Also users should be trained in how to properly operating their heating system, for example by well setting in-building temperatures. A way to promote their good behaviour can be the introduction of reduced tariffs in case they guarantee a temperature on the return pipe of the distribution network that is below a certain threshold. This is indeed a practice implemented in Denmark.

• Proper urban spatial planning, definition of long-term strategies, policies and market regulatory framework. It is important that both investors and customers are incentivized to behave properly, without conflicting actions that can damage one another. 
• Introduction of incentives to make investments more attractive and affordable and/or the introduction of taxes or increased costs for CO2 emitting technologies. [1]

[1] K. Lygnerud, E. Wheatcroft, and H. Wynn, “Contracts, business models and barriers to investing in low temperature district heating projects,” Appl. Sci., vol. 9, no. 15, 2019, doi: 10.3390/app9153142

From the case study illustrated in [4] it can be stressed that at the basis of a more detailed research for DH potential assessment stands the strong interest showed by the local administration in transitioning towards a greener and more sustainable energy mix to supply thermal energy in the existing DH network. The knowledge and the spatial characterization of the urban area then highlighted favourable local conditions for the uptake of waste heat-based DH.

[4] G. Spirito, A. Dénarié, V. F. Cirillo, F. Casella, J. Famiglietti, and M. Motta, “Energy mapping and district heating as effective tools to decarbonize a city: Analysis of a case study in Northern Italy,” Energy Reports, vol. 7, pp. 254–262, 2021, doi: 10.1016/j.egyr.2021.08.147