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Building energy systems for heat networks in districts
On this page, you find a description of the different building energy systems that can be used in district calculations.
General information
Two different types of heat supply can be distinguished:
- Heat supply via the heat network, and
- heat supply via decentralized heat pumps (air-source heat pump or ground-source heat pump) without connection to a heat network.
If the heat supply is provided through the heat network, as much energy as possible is utilized from the heat network. However, due to low network temperatures, it may not be possible to directly cover all heat demands through the heat network. In this case, decentralized units (booster heat pump, electric heater, etc.) must be installed in addition.
Figure 1: Overview of heat supply solutions at building level
In the nPro-Tool, all kinds of district heating networks can be simulated: Conventional district heating, 5GDHC networks, as well as other low-temperature concepts.
Calculation of the COP (coefficient of performance)
The following assumptions are made when calculating the COP of the heat pumps:
- When the COP is calculated according to the Carnot formula, a maximum possible COP of 8 is assumed for heat pumps, and a maximum EER of 12 is assumed for chillers. This limit is necessary because the COP can become infinitely high for small temperature differences when calculated according to Carnot.
- The return temperature for space heating is at least of 20 °C.
For the building heat pump, a heat source temperature is assumed, which is ...
- ... the average of the supply and return temperatures of the network (if the heat pump uses the heat network as heat source).
- ... the extraction temperature from the ground (if it is a geothermal heat pump).
- ... the ambient air temperature (if it is an air-source heat pump).
Calculation of the heat demand covered by the heat network
The share of the heat demand that is covered by the heat pump and/or electric heater (and not directly with the heat network) is calculated for each hour of the year as follows:
- If the supply temperature (of the space heating or domestic hot water demand) is lower than the supply temperature of the heat network minus a temperature difference (e.g. 2 K, can be set by the user), then the heating demand is completely and directly covered by the heat network.
- If the return temperature of the heating system is smaller than or equal to the supply temperature of the district heating network minus the temperature difference, the share covered by the central heat pump/electric heating rod of the building for space heating is as follows $$A_{Heat pump/Electric heating rod} = \frac{ T_{Supply temp. heating} - (T_{Supply temp. heat network} - \Delta T_{water-water}) } { T_{Supply temp. heating} - T_{Return temp. heating} }$$ and for tap water $$A_{Heat pump/Electric heating rod} = \frac{ T_{Tap temp.} - (T_{Supplytemp. heat network} - \Delta T_{water-water}) } { T_{Tap temp.} - T_{Cold water temp.} }$$ The heat transfer gradient \(\Delta T_{water-water}\) can be defined by the user.
- In all other cases, the heat demand is completely covered by the building's heat pump/electric heating rod.
Calculation of decentralized heating of domestic hot water
The share of the domestic hot water demand \(A_{dec}\) that is covered decentrally is calculated as:
$$A_{dec} = \frac{ T_{Tap temp.} - T_{Preheat temp.} }{ T_{Tap temp.} - T_{Cold water temp.} }$$Here, \(T_{Preheattemp.}\) describes the preheating temperature of the domestic hot water after the central heat pump/electric heating rod and before the water is heated to the final tap temperature. The preheating temperature is defined by the user.
System configurations
There are currently 24 different heat supply configurations available for individual buildings that can be used in district calculations.
Heat network without decentralized domestic hot water heating
In the following configurations, the building is connected to a heat network. The domestic hot water is heated centrally.
Table 1: Heat network without decentralized domestic hot water heating
| Configuration | Description |
|---|---|
|
The heating demands are covered directly by the heat network via a heat exchanger (heat transfer station). This assumes that the network temperatures are sufficiently high. |
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Heat demands that can be covered directly by the heat network are covered directly via a heat exchanger. The heat pump uses the heat network as heat source and covers the heat demands that cannot be covered directly by the heat network due to insufficient network temperatures. |
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Heat demands that can be covered directly by the heat network are covered directly via a heat exchanger. The electric heater covers the heat demands that cannot be covered directly by the heat network due to insufficient network temperatures. |
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Heat demands that can be covered directly by the heat network are covered directly via a heat exchanger. The heat pump uses the heat network as heat source and covers the heat demands that cannot be covered directly by the heat network due to insufficient network temperatures. The peak load is covered by an electric heating rod. |
Heat network with partial decentralized domestic hot water heating
In the following configurations, the building is connected to a heat network. The domestic hot water is preheated centrally and raised to the tap temperature decentrally.
Table 2: Heat network with partial decentralized domestic hot water heating
| Configuration | Description |
|---|---|
 |
The heating demands are covered directly by the heat network via a heat exchanger (heat transfer station). The domestic hot water is first preheated centrally (with the heat network) and then raised electrically to the tap temperature (e.g. by instant water heaters). |
 |
The heat pump uses the heat network as a heat source and covers the heat demands that cannot be covered directly by the heat network due to insufficient network temperatures. The domestic hot water is first preheated centrally (heat pump) and then raised electrically to the tap temperature (e.g. by instant water heaters). |
 |
The electric heater covers the heat demand that cannot be met by the heat network due to insufficient network temperatures. The domestic hot water is first preheated centrally (heating rod) and then raised electrically to the tap temperature (e.g. by instant water heaters). |
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The heat pump uses the heat network as a heat source and covers the base load of the heat demand if this cannot be covered directly by the heat network due to insufficient network temperatures. The peak load is covered by an electric heating rod. The domestic hot water is first preheated centrally (heat pump/heating rod) and then raised electrically to the tap temperature (e.g. by instant water heaters). |
Heat network with fully decentralized domestic hot water heating
In the following configurations, the building is connected to a heat network. The domestic hot water is heated decentrally.
Table 3: Heat network with fully decentralized domestic hot water heating
| Configuration | Description |
|---|---|
 |
The heat demand is covered directly by the heat network via a heat exchanger (heat transfer station). The entire domestic hot water demand is covered decentrally by electricity, e.g. by instant water heaters or under-sink devices. |
 |
The heat pump uses the heat network as a heat source and covers the heat demands that cannot be covered directly by the heat network due to insufficient network temperatures. The entire domestic hot water demand is covered decentrally by electricity, e.g. by instant water heaters or under-sink devices. |
 |
The electric heater covers the heat demand that cannot be met by the heat network due to insufficient network temperatures. The entire domestic hot water demand is covered decentrally by electricity, e.g. by instant water heaters or under-sink devices. |
 |
The heat pump uses the heat network as a heat source and covers the base load of the heat demand if this cannot be covered directly by the heat network due to insufficient network temperatures. The peak load is covered by an electric heating rod. The entire domestic hot water demand is covered decentrally by electricity, e.g. by instant water heaters or under-sink devices. |
Ground-source heat pump
In the following configurations, the entire heat demand is covered by a ground-source heat pump and/or heating rod. The building is not connected to a heat network.
Table 4: System configurations for heat supply in the building by geothermal heat pump
| Configuration | Description |
|---|---|
|
The geothermal heat pump covers the entire heating demands of the building. |
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The geothermal heat pump covers the base load of the heat demand. The peak load is covered by an electric heater. |
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The geothermal heat pump covers the entire heating demands of the building. The entire domestic hot water demand is covered decentrally by electricity, e.g. by instant water heaters or under-sink devices. |
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Geothermal heat pump covers the base load of the heat demand. The peak load is covered by an electric heating rod. The entire domestic hot water demand is covered decentrally by electricity, e.g. by instant water heaters or under-sink devices. |
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The geothermal heat pump covers the entire heating demands of the building. The domestic hot water is first preheated centrally (heat pump) and then raised electrically to the tap temperature (e.g. by instant water heaters). |
 |
The geothermal heat pump covers the base load of the heat demand. The peak load is covered by an electric heating rod. The domestic hot water is first preheated centrally (heat pump/heating rod) and then raised electrically to the tap temperature (e.g. by instant water heaters). |
Air-source heat pump
In the following configurations, the entire heat demand is covered by an air-source heat pump and/or heating rod. The building is not connected to a heat network.
Table 5: System configurations for heat supply in the building with air-source heat pump
| Configuration | Description |
|---|---|
|
The air-source heat pump covers the entire heating demands of the building. |
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The air-source heat pump covers the base load of the heat demand. The peak load is covered by an electric heating rod. |
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The air-source heat pump covers the building's entire heat demand. The entire domestic hot water demand is covered decentrally by electricity, e.g. by instant water heaters or under-sink devices. |
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The air-source heat pump covers the base load of the heat demand. The peak load is covered by an electric heating rod. The entire domestic hot water demand is covered decentrally by electricity, e.g. by instant water heaters or under-sink devices. |
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The air-source heat pump covers the building's entire heat demand. The domestic hot water is first preheated centrally (heat pump) and then raised electrically to the tap temperature (e.g. by instant water heaters). |
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The air-source heat pump covers the base load of the heat demand. The peak load is covered by an electric heating rod. The domestic hot water is first preheated centrally (heat pump/heating rod) and then raised electrically to the tap temperature (e.g. by instant water heaters). |
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