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By David Connolly, Susana Paardekooper, and Kenneth Hansen, Aalborg University
It is much more difficult to explain the concept of energy efficiency for district heating. Heat savings are simple; they reduce demand so you no longer need to supply the heat. District heating does not reduce the demand for heat and in fact, it is often argued that it actually increases demand due to the additional losses in the pipes. So how does it improve energy efficiency? The key to this is that district heating enables you to use sustainable energy that otherwise could not be utilised, simple! District heating creates new connections between different types of sustainable energy, and a more interconnected energy system. Integrating excess heat with district heating Today, excess heat comes from three primary applications: power plants, industry, and waste incineration. Power plants are typically designed to produce electricity with an efficiency of approximately 40%; the remaining energy is usually exhausted from the power plant as heat into the surrounding sea, lake, or river. Similarly, during many industrial processes, such as a fuel, chemical, or metal production, large quantities of heat are also thrown away as a by-product. Waste incineration is a form of waste-treatment during which large quantities of trash are collected and burned in a furnace, producing heat and electricity as by-products. All of these forms of excess heat already exist in the energy system today, and are in different ways expected to be part of our future. The quantity of excess energy in Europe from these sources, which is effectively wasted, was quantified in Heat Roadmap Europe. Figure 1 presents the energy flow in Europe’s energy system today in three different steps: • Primary energy supply: the total energy required for the system before conversion. This means before the losses of transformation, for example while producing electricity or refining fuel. • Final consumption: the ‘useful’ energy produced for the consumer, before the losses in the grids and networks. • End use: the total amount of energy actually used by the consumer as a final product. By looking at these three distinct steps separately, Figure 1 indicates that there is more energy lost, primarily in the form of heat during the production of electricity (i.e. step 1 to step 2), than the total heat demand in all of Europe’s buildings and industry combined (i.e. step 3). In other words, there is more excess heat available today than would be required to heat every building in Europe. Wasting this energy is not energy efficient nor does it fit in the concept of a sustainable energy system.
Traditionally, the energy system was divided into three very distinct sub-sectors: electricity, heating, and transport. A division like this was sensible since each of these sectors operated very independently of one another: power plants provided electricity, boilers provided heat, and engines provided transport. Due to this historical division, many of the methods and tools developed to analyse the energy system also evolved based on this isolated sub-sector approach. However, the benefits of district heating are only visible when a more holistic approach is utilised, since it exists across multiple sectors rather than within one. This is especially true when it comes to energy efficiency. In the Heat Roadmap Europe studies [1]–[4] (www.heatroadmap.eu), a holistic energy systems approach is utilised to quantify the benefits of energy efficiency on the demand and supply side of the heat sector: district heating being one of the key changes on the supply side. This holistic approach enables us to quantify the benefits that occur when sectors and systems are connected in new ways in the energy system. Doing so revealed ground-breaking and significant results: the energy reductions and carbon dioxide savings due to energy efficiency on the supply side of the heating sector are similar to those that are feasible due to energy efficiency on the demand side (i.e. via heat savings). Therefore, it is essential that energy efficiency measures on the supply side, like district heating, are considered with equal importance for decarbonisation as measures on the demand side, like heat savings. The term ‘energy efficiency’ is often associated with a reduction of the consumer’s demand in the heat sector. It triggers the image of an old building getting the equivalent of a cosmetic makeover, as its old single-glazed windows are replaced with a modern triple-glazed equivalent. Sealing the walls and roof with new insulation will also secure the building from a cold breeze in the winter months. There is no doubt that these measures are essential and will play an essential role in the transition to a sustainable energy system, but they are not the only solution. In the past, heat savings have dominated the discussions around energy efficiency, but things are changing and now the role of energy efficiency on the supply side is starting to become a key part of the debate, which primarily relates to district heating in the cities and heat pumps in the countryside.
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