Review4th Generation District Heating (4GDH): Integrating smart thermal grids into future sustainable energy systems
Introduction
The design of future sustainable energy systems including 100 percent renewable systems is described in a number of recent reports and studies including [1], [2], [3], [4], [5], [6]. Such systems are typically based on a combination of fluctuating renewable energy sources (RES) such as wind, geothermal and solar power together with residual resources such as waste and biomass on which we may expect increasing pressure due to environmental impact and future alternative demands for food and material. For example, biomass resources in Europe are small compared to the European energy balance [7]. In order to ease the pressure on biomass resources and investments in renewable energy, feasible solutions to future sustainable energy systems must involve a substantial focus on energy conservation and energy efficiency measures.
District heating infrastructures have an important role to play in the task of increasing energy efficiency and thus making these scarce resources meet future demands. District heating comprises a network of pipes connecting the buildings in a neighbourhood, town centre or whole city, so that they can be served from centralised plants or a number of distributed heat producing units. This approach allows any available source of heat to be used. The inclusion of district heating in future sustainable cities allows for the wide use of combined heat and power (CHP) together with the utilisation of heat from waste-to-energy and various industrial surplus heat sources as well as the inclusion of geothermal and solar thermal heat [8], [9], [10], [11], [12], [13], [14]. In the future, such industrial processes may involve various processes of converting solid biomass fractions into bio(syn)gas and/or different sorts of liquid biofuels for transportation fuel purposes, among others [15], [16].
Future district heating infrastructures should, however, not be designed for the present energy system but for the future system. One of the future challenges will be to integrate district heating with the electricity sector as well as the transport sector [17]. In the following, such a future system will be referred to as a smart energy system, i.e. an energy system in which smart electricity, thermal and gas grids are combined and coordinated to identify synergies between them in order to achieve an optimal solution for each individual sector as well as for the overall energy system [18]. A transition from the current fossil fuel- and nuclear-based energy systems into future sustainable energy systems requires large-scale integration of an increasing level of intermittent renewable energy. This also entails a rethinking and a redesign of the energy system. In smart energy systems, the focus is on the integration of the electricity, heating, cooling, and transport sectors, and on using the flexibility in demands and various short-term and longer-term storage across the different sectors. To enable this, the smart energy system must coordinate between a number of smart grid infrastructures for the different sectors in the energy system, which includes electricity grids, district heating and cooling grids, gas grids and different fuel infrastructures.
A number of recent studies [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], including Heat Roadmap Europe [19], [27], come to the conclusion that district heating plays an important role in the implementation of future sustainable energy systems. However, the same reports also emphasise that the present district heating system must undergo a radical change into low-temperature district heating networks interacting with low-energy buildings as well as becoming an integrated part of smart energy systems.
The development of future district heating systems and technologies involves energy savings and conservation measures as an important part of the technology [31]. The design and perspective of low-energy buildings have been analysed and described in many recent papers [32], [33], including concepts like energy efficient buildings [34], zero emission buildings and plus energy houses [35], [36]. However, such papers mostly deal with future buildings and not the existing building stock which, due to the long lifetime of buildings, is expected to constitute the major part of the heat demand for many decades to come. Some papers address the reduction of heat demands in existing buildings and conclude that such an effort involves a significant investment cost [37]. The share of currently existing buildings in the building stock is expected to remain high for many years. No study has been found which identifies how to completely eliminate the heat demand in existing buildings within a reasonable time frame. In the European Commission's strategy [38] for a competitive, sustainable and secure “Energy 2020”, the need for “high efficiency cogeneration, district heating and cooling” is highlighted (p. 8). The paper launches projects to promote, among others, “smart electricity grids” along with “smart heating and cooling grids” (p. 16). In recent state-of-the-art papers [39], [40], [41] and discussions [42], the specific requirements of future grids have been discussed and such future district heating technologies have in some cases been named 4th Generation District Heating Technologies and Systems (4GDH). The purpose of this paper is to define the concept of 4th Generation District Heating and thereby contribute to the understanding of the need for research and development of this future infrastructure and related technologies.
Section snippets
The first three generations of district heating and cooling
The first generation of district heating systems used steam as the heat carrier. These systems were first introduced in USA in the 1880s. Almost all district heating systems established until 1930 used this technology, both in USA and Europe. Typical components were steam pipes in concrete ducts, steam traps, and compensators. Today, such systems using steam can be considered an outdated technology, since high steam temperatures generate substantial heat losses and severe accidents from steam
The future 4th generation of district heating
Recent studies have investigated the feasibility of district heating in terms of implementing a sustainable energy system based on renewable energy and including substantial reductions in the space heating demand [25], [45], [46]. The studies conclude that the role of district heating is significant, but that district heating technologies must be further developed to decrease grid losses, exploit synergies, and thereby increase the efficiencies of low-temperature production units in the system.
Summary and definitions
The purpose of this paper has been to define the concept of 4th Generation District Heating (4GDH) including the concept of smart thermal grids. The paper has described the historical development of district heating systems in terms of three generations and afterwards identified the future challenges for the district heating technology of reaching a future renewable non-fossil heating and cooling supply as part of the implementation of overall sustainable energy systems.
On such a basis, the
Acknowledgement
The work presented in this paper is a result of the research activities of the Strategic Research Centre for 4th Generation District Heating (4DH), which has received funding from the Danish Council for Strategic Research. We especially wish to thank all our colleagues within the 4DH centre for helpful comments and fruitful discussions.
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