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District Heating: Definition and Operation

District heating is a long-distance heat transport system for heating, cooling and hot water domestic production

District heating represents an innovative and continuously evolving solution for building heating that aims to safeguard the environment through centralized production of thermal energy, also from renewable sources.

This approach reduces environmental impacts and promotes large-scale energy efficiency. It is a system of remote heat transportation that can be used for heating, cooling, and producing domestic hot water.

When evaluating the opportunity to adopt district heating, it is essential to consider some technical aspects of the system before proceeding with the connection. Initially, it is important to note that district heating can be used in any type of building, whether new or existing, equipped with centralized or autonomous systems, even integrated with other renewable thermal sources, such as solar thermal.

One fundamental element is the heat exchanger, which facilitates the exchange of thermal energy between two fluids: the first being the hot water produced by the centralized boiler, while the second is the liquid used in the domestic heating system. The dimensions of this device are determined based on the required thermal power.

District Heating: What Is It

The term district heating refers to a technology that allows the transportation of heat from the production site to the utilization site.

It is a simple, less polluting, economical, and safe system for conditioning buildings, especially residential ones, which can be used for heating, cooling, and producing domestic hot water.

How a District Heating System Is Made

A district heating system is mainly composed of three elements:

  • thermal plant responsible for heat production, usually in cogeneration with electrical energy;
  • supply network that transports heat to the final consumer;
  • return piping system that returns the cold fluid to the plant.

District Heating: How It Works

When talking about district heating, it is immediately understood that the heat is not generated in the same place where it is used.

In fact, it is produced in one or more thermal plants and from there, through a distribution network, it is sent to the connected users. These plants produce hot or superheated water, usually at temperatures of 90° and 120°C.

The functioning of district heating is based on cogeneration plants where heat is produced and then distributed through the distribution network to individual housing units.

To understand the potential of district heating, it is necessary to delve into its operation, which can be schematized as follows:

  • in cogeneration plants, a liquid is produced and distributed, usually water, at a temperature of 80°C-90°C or 120°C-130°C when superheated. Heat can be produced through boiler plants (using fossil fuels, biomass, or waste-to-energy), heat pump systems (utilizing geothermal energy, for example), or solar thermal;
  • the liquid is transported through a network of pipes (primary distribution network) to the buildings connected to the district heating system;
  • once it reaches its destination, the liquid releases the heat to the building’s system, which can use it to heat the premises and domestic water;
  • the heat transfer fluid, cooled after releasing heat to the buildings, returns to the plant to be brought back to the maximum temperature and start the cycle again.

Heat Sources of the District Heating Plant

There are several heat sources used in district heating systems:

  • water-to-water heat pumps: this system transfers heat from groundwater to the water used in district heating through the use of electrical energy;
  • cogeneration: it is the combined production of electricity and heat from a single energy source through combustion;
  • waste-to-energy from municipal solid waste: this technology converts the heat generated by the combustion of municipal solid waste into thermal energy;
  • heat recovery from industrial processes: involves using the residual low-temperature heat produced by industrial processes;
  • geothermal source: harnesses heat from the ground at different temperatures (low, medium, and high);
  • renewable sources: among these are solar thermal, biomass, biogas produced from landfills or processing waste, and other similar sources.

Heat Distribution Network

The heat distribution system can be:

  • direct: a single hydraulic circuit connects the production plant with the terminal units, i.e., the heating bodies (radiators, radiant panels, etc.) of the user;
  • indirect: there are two separate circuits, in contact with each other through a heat exchanger located near the user.

Configurations of District Heating Networks

Based on the arrangement of the pipes and the shape of the network, we can distinguish 3 types of networks:

  • branched: composed of a main pipeline from which additional secondary lines branch off, directly transporting the heat transfer fluid to the users;
  • looped: the heated heat transfer fluid is sent from the plant, reaches the user, and then returns to the plant. This configuration allows the network to be fed in both directions, making it a flexible system easily expandable to accommodate additional users in the future;
  • meshed: consisting of pipes forming a series of interconnected closed circuits. This is the ideal configuration in terms of heat regulation and distribution but entails higher installation costs compared to the other two types.

Designing a District Heating Network

Designing a district heating network requires a series of well-defined phases:

  • identification of the area;
  • analysis of users and estimation of the district heating thermal demand;
  • sizing of the cogenerator;
  • selection of the site for the production plant;
  • layout and sizing of the network;
  • choice of the plant type;
  • simulation of operation;
  • energy and environmental balance;
  • economic analysis.
District Heating Design

District Heating Design

Area Identification

The characteristics that the area intended for district heating must have in order for the project to be successful are:

  • good building density, with multi-story buildings of a volume greater than 2-3,000 m3;
  • presence of centralized heating systems.

Areas of new construction or urban redevelopment are inherently optimal for the implementation of a district heating network. During the urbanization of new areas, the laying of pipes is facilitated, and the timing of connections is less uncertain because user acquisition can be defined in an aggregated manner with construction companies.

User Analysis

Once the area is identified, data related to the buildings is collected:

  • age, building typology, volume, and use (residential, tertiary, etc.);
  • number of existing heating systems, categorized by type (centralized or autonomous) and fuel;
  • fuel consumption for at least the three previous years;
  • ownership regime.

Next, the estimation of the thermal demand in the area is carried out, which can be done through 2 methods:

  • inferred based on fuel consumption data;
  • reconstructed based on building characteristics (surface area-to-volume ratio, glazed surface area, type of insulation, etc.), type of use, climatic conditions of the location (degree days), regulatory standards (energy class of the building).

Special users, such as hospitals, large public buildings, sports centers, shopping malls, or industries requiring process heat, require a more in-depth specific study.

The next step is the estimation of district heating penetration, segmenting the thermal demand based on the following user characteristics:

  • ownership regime (private or public);
  • age of existing systems;
  • system type;
  • fuel used.

Cogenerator Sizing

Sizing the cogenerator in a district heating system is a delicate phase, as it involves a series of technical, economic, and financial considerations, including selling electricity under the best conditions.

An important aspect is the strategy to adopt for operation, taking into account variations in thermal load and the tariff bands established by the Electricity Authority based on the time of day and week.

An effective method to determine the optimal cogenerator size is to develop a diagram illustrating the thermal power required by the system throughout the year, in relation to the number of hours during which this power is needed, organizing the data from maximum to minimum.

In common situations, the construction of the diagram can be simplified by making basic assumptions about daily and weekly variations, focusing on an average power value for each month. The cogenerator size can be defined to ensure a sufficient number of profitable operating hours for the system, generally at least 4000 hours per year.

Selection of Production Plant Site

The first step in choosing the site is the verification of the feasibility of connecting to existing plants, i.e., the possibility of recovering heat from industries, incinerators, or power plants. The selection of the production plant site must meet the following objectives:

  • minimize environmental impact (emissions, noise) for residents;
  • minimize the average length of the heat transport path through the heat transfer fluid from the plant to users (preferably positioning the plant at the most central point of the area considered);
  • minimize energy input supply costs (especially to be evaluated in the case of sources such as biomass and geothermal).

Layout and Sizing of the Network

The network structure can be divided into:

  • primary network: the main trunk, laid in public soil under the road surface;
  • secondary network: direct connections to individual users and sections crossing private properties.

The different user acquisition methods identified during the thermal demand assessment phase for district heating influence the extension of the secondary network (for example, offering incentives for connections can promote the growth of the secondary network).

Correctly calculating the sizes of the primary network (pipe diameter) is crucial as it significantly affects the overall cost of the system.

The network sizing depends on various parameters, such as:

  • thermal power resulting from the evaluation of existing thermal loads and future expansion forecasts;
  • temperature difference between the heat transfer fluid at the inlet and outlet (which can be hot water, superheated water, steam, or heat transfer liquids).

Choice of Plant Type

Before defining the plant configuration, a preliminary choice regarding the energy source to be used must be made. Once the source is established, there are numerous technologies available for plants (for a detailed overview, refer to Annex I). The currently most widespread choice is gas cogeneration.

Factors influencing the choice include:

  • size of the district heating system;
  • required temperature level in heat distribution;
  • level of economic priority given to the sale of electricity.

Operation Simulation

Once all necessary parameters are defined, the simulation of the system’s operation as hypothesized is carried out. Simulating the operation for a typical year generates the following results:

  • fuel consumption;
  • electricity production;
  • electricity sold to the grid;
  • heat production (from cogeneration and integration);
  • heat supplied to users;
  • emissions.

Energetic and Environmental Balance

The purpose of the energetic and environmental balance is to quantify the achievable energy savings and reduced emissions thanks to the implementation of the district heating system compared to traditional decentralized production.

Although it can sometimes be challenging to evaluate such data accurately, the first step is to analyze the conventional systems that the district heating system will replace, both in terms of fuel consumption and emissions generated. Subsequently, these results are compared with those resulting from the simulation of the district heating system’s operation.

Economic Analysis

The economic analysis is the decisive verification phase for the actual implementation of the project. The main cost items of a district heating system project are:

  • distribution network;
  • production plant;
  • fuel;
  • plant maintenance and management;
  • heat transmission network maintenance and management.

District Heating: Pros and Cons

The use of district heating offers several advantages. In particular: