Heat demand in districts

The heat demand describes the amount of thermal energy needed for heating. Usually, the heat demand is related to a heating period, e.g. one year (annual heat demand). This is usually given in kWh or MWh is given. For buildings, the area-related specific heating demand is often given in kWh/m² per year. Three different types of heat demands can be distinguished:

• Space heating
• Domestic hot water
• Process heat

For the planning of heat supply systems, the space heat and domestic hot water demands are particularly relevant.

Heating demand for space heat

The energy required to provide space heat is directly dependent on the weather. In years with severe, cold winters, the heating demand is higher than in years with mild winters. To determine an average heating demand, measured consumption data can be subjected to a weather adjustment based on degree days.

Heating demand for domestic hot water

The heat demand for domestic hot water preparation does not directly depend on weather conditions. Therefore, for demand calculations, it is often assumed that the demand for domestic hot water remains constant throughout the year and between different years. However, analyses show that the demand for domestic hot water varies seasonally and is slightly higher in winter than in summer. Due to the demand for domestic hot water, districts have a constant demand for heat throughout the year. This is of great importance for district energy systems: On the one hand, it must be taken into account that with a central heat supply (with district heating) the relative heat losses in summer can be very high. On the other hand, waste heat from cooling demands in summer can be used to cover domestic hot water demands. This is particularly promising for 5GDHC networks, as low-cost air conditioning (space cooling) can be provided at low costs.

Process heat demand

In addition to space heat and domestic hot water, another heat demand type is relevant for some building types: Process heat. Process heat is mainly used in industrial and commercial buildings. The temperature requirement ranges from 30 - 300 °C. Typical examples are bakeries, which require up to 50% of their heat for baking processes. But laundries, restaurants or hospitals also have a relevant share of process heat. A large proportion of these additional heat demands results in increased electricity demands for the building. In the nPro tool, process heat requirements that are covered electrically are added to the electricity requirement, since these do not have to be provided via a heating network.

Share of domestic hot water demand in total heat demand

The ratio of domestic hot water demand to total heat demand (sum of space heat and domestic hot water demand) is a characteristic parameter for buildings. While the heat ratio is rather small in old buildings with low insulation standards, it increases steadily with increasing insulation standards. The reason for this is that the domestic hot water demand differs only slightly in buildings with different insulation standards; the heating demand for space heat, however, decreases strongly with improved insulation standards. For old single-family houses built before 1918, the share of domestic hot water demand in the total heat demand is less than 10 %, whereas for new buildings it may well be 50 %. This means that in a building with a high insulation standard, half of the heating demand is due to domestic hot water heating. In hotels, the share of domestic hot water tends to be higher than in single-family homes, since the consumption of domestic hot water is higher. A similar situation can be observed for hospitals. The higher occupancy density compared to residential buildings leads to an increased demand for domestic hot water.

Sources

1. Leitfaden Energienutzungsplan, Bayerisches Staatsministerium für Umwelt und Gesundheit, 2011.
2. Leitfaden Nahwärme, Frauhenhofer UMSICHT. ISBN: 978-3-8167-5186-1.
3. Fachbuch: Recknagel - Taschenbuch für Heizung + Klimatechnik 2017/2018.

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