Ultra-Low Temperature District Heating

In our showcase, we highlight the key points to consider when designing and planning ultra-low temperature district heating networks. For this purpose, we use our heatbeat Digital Twin, and demonstrate how we can use dynamic simulations to provide detailed insights for better design of ultra-low temperature district heating networks at an early stage.

Ultra-low temperature district heating - A brief overview

Ultra-low temperature district heating uses one or more central heat sources at a low temperature level. These can be, for example, geothermal fields, wastewater systems or other low-temperature waste heat sources. The heat is distributed at a low temperature level (mostly between 0 and 20 °C) in a district heating network. Decentralized heat pumps in the buildings efficiently provide space heating and domestic hot water. Often, the ultra-low temperature district heating network can be used for passive cooling of the buildings in summer.

Advantages of ultra-low temperature district heating are:

• Low network losses or even gains from the soil
• Utilization of low-temperature heat that cannot be used otherwise
• Central development of investment-intensive sources (e.g., geothermal heat)
• Use of inexpensive uninsulated pipes
• Individual building requirements (e.g., temperatures) can be met
• Electrification of heat supply and thus possibility to use locally generated renewable electricity (e.g., from PV systems)
• Ultra-low temperature district heating networks are often eligible for funding (e.g., 4th generation district heating or federal funding for efficient heat networks, see our newsletter on BEW funding)

Design and planning of ultra-low temperature district heating networks

For our showcase, we use a fictitious district. The district consists of 100 residential buildings (80 single-family houses, 20 terraced houses or small multi family houses) with a total gross floor area of 22,000 m². All buildings have a high insulation standard and thus low heat losses. Decentralized heat pumps are planned in all buildings, which will be used for heating purposes as well as for domestic hot water generation. Buffer tanks are installed in the buildings for this purpose.

Sources for ultra-low temperature district heating

The most important prerequisite for the successful implementation of ultra-low temperature district heating is the possibility of exploiting a central source. In our projects, the following sources have already been utilized:

• Geothermal energy (probes and geothermal collectors)
• waste water
• Waste heat from data centers
• ice storage
• ambient air
• lake water

In our example, the ultra-low temperature district heating network uses geothermal energy. We assume that the source has a maximum extraction capacity of 250 kW. The temperature of the source is between 3 °C and 18 °C, it varies throughout the year and is modeled as a source in the Digital Twin, see Figure 1. To cover peak loads, a central heat pump is also installed, which uses the ambient temperature as a source and is coupled with a buffer tank (6 m³). The electricity of the heat pump can be provided with an own photovoltaic system (~150 m² for about 29 kWPeak) or purchased from the public power grid.

Building demands

The Digital Twin collects all building properties and uses them to create individual building models to determine the dynamic heating and cooling requirements as well as the energy demand for domestic hot water preparation. In addition to storage tanks, we also take into account distribution losses within the buildings (e.g., in apartment buildings). The dynamic energy demand and the expected heating capacity of the buildings is essential for the dynamic evaluation of the energy system. More information on this approach can be found in our first showcase on a conventional heating network.

Dimensioning of the ultra-low temperature district heating network

In contrast to conventional heating networks, where the aim is to achieve the highest possible temperature difference between supply and return, the temperature differences in ultra-low temperature district heating networks are significantly smaller. Usually, the temperature difference is between 3 and 8 K. It depends on the maximum possible spread of the decentralized heat pumps. This leads to high volume flows and large pipe diameters. Several points must therefore be taken into account for dimensioning. One important aspect is that the buildings do not cover their entire heat demand from the network, but the decentralized heat pumps only use the network to obtain the source heat, and thus a smaller proportion of the heat demand is also covered by the electricity used by the decentralized heat pumps.

The temperature differences between the ground and the thermal network are also often small. Due to this small temperature difference, insulation of the installed pipes can often be avoided in projects with ultra-low temperature district heating, so that more cost-effective pipes made of plastic can be used. In our Digital Twin, we consider both the actual temperature differences present and the operation of the heat pump when dimensioning the individual pipe segments. For our example, we have determined the dimensions of the pipe network as shown below.

We also take the pressure states in the thermal network into account in the design. Especially when using flexible plastic pipes, only low absolute pressures are allowed. Our Digital Twin simulates the pressure states at every point in the network. Assuming a worse-point control, the following animation shows how the pressures in the flow (red) and return (blue) change over the course of a day with a high heat load:

How large are the heat losses/gains through uninsulated pipes?

Ultra-low temperature district heating networks often use uninsulated plastic pipes. As a result, the interaction with the surrounding soil is much more pronounced than in conventional heat networks. The following figure shows the energy balances between the energy center, ultra-low temperature district heating network and the buildings. Part of the heat demand is covered by the electrical work of the heat pump. A part (175 MWh) is absorbed by the interaction with the ground, so that only 530 MWh of the original heat demand has to be covered by the energy center. This example shows that the interaction with the ground in ultra-low temperature district heating networks must not be neglected.

Which annual performance figures do the decentralized heat pumps achieve?

For the energy and economic evaluation of the district energy system, the decentralized heat pumps must also be considered. These are not only dependent on the demand behavior of the buildings, but also on the source temperature (i.e., the temperature of the network). This can vary depending on the size of the network and the position of the building in the network. For the two buildings b0004 (very close to the energy hub) and b0091 (far from the energy hub), the graph below shows the supply temperatures present at the building. Noticeable are the temperature gains in the summer months when demand is low and ground temperatures are high.

This can also be seen in the following plot of the COP values, which is shown for the two buildings and all operating times of the heat pumps over the course of the year. In comparison, the more distant building has a 0.2 better seasonal COP.

How is the operation of the sources divided?

In our example, the geothermal energy provides the base load during the heating period. During this time, the peak loads are covered by the heat pump. For this purpose, either the storage tank can be used, which was previously charged by the heat pumps using the self-generated PV electricity, or the heat pump is used directly with electricity from the public supply network. In order not to exceed the permissible full load hours of geothermal energy (e.g. according to VDI 4640), the heat demand in summer is covered by the heat pump. In summer, the heat demand is lower (only domestic hot water demand) and the solar potential is higher, so that in the period between April and October almost the entire energy demand can be provided CO₂-free.

How do the pumps of the supply need to be designed?

A ultra-low temperature district heating network also makes requirements on the hydraulics. Often, these small networks have only low-pressure losses and thus low absolute pressures (which are also specified by the use of plastic pipes). However, the low temperature differences in the network result in high volume flows. By determining the dynamic plant characteristic curve, we can make early statements about the hydraulics in the energy hub. The following figure shows the dynamic plant characteristic curve at the energy hub. With the help of this system characteristic curve, the pumps in the system can be designed, especially given the fact that a water-glycol mixture is often used, this is also relevant early in the planning.

As this example shows, the holistic evaluation of ultra-low temperature district heating requires the consideration of numerous mutually influencing aspects. And since the key to efficient and economical operation of the network over a long lifetime is laid in the early planning phases in particular, it is worthwhile to take a detailed look at the operation of the system at this early stage. Our heatbeat Digital Twin helps to evaluate the operation in the Digital Twin already in this early phase and to optimize the design of the system accordingly.

In addition, we provide them with all simulation results individually in high resolution via our web portal, so that they can use them for their individual calculations in the long term.

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If you have any further questions regarding this presentation or see potential for cooperation, we are looking forward to hearing from you: