Better use of district heating with the eXergy machine
District heating is popular in residential and commercial properties, hospitals and many other buildings, but efficiency problems occur time and again in practice: Excessively high return temperatures with low heat consumption reduce the efficiency of the district heating supply.
Particularly in summer during long hot water circulation phases, i.e. times without hot water tapping, the return flow can easily rise above the value required by the district heating operator. However, a low return temperature should be aimed for so that there is a large temperature spread between the flow and return at the district heating transfer station - then heat generators can work efficiently and the network can transport more energy. In order to achieve this ideal state - the large temperature spread between the flow and return - there is the eXergy machine (eXm).
How does the eXergy machine work?
- The eXm is an electrically powered Water-to-water heat pump, designed for source temperatures up to approx. 55 °C and for large temperature strokes. The eXm is connected to a buffer cylinder (heating cylinder).
- The eXm accesses the buffer tank via two circuits:
- A circuit draws water from the middle segment of the storage tank, heats it via the condenser and feeds it into the hot storage tank area.
- The other circuit also takes water from the middle storage tank area, cools it down at the evaporator of the eXm and feeds it into the lower, cold area of the storage tank. This creates a cooler return flow to the district heating connection station.
- A sufficiently dimensioned eXm can always create an ideal temperature stratification in the cylinder - regardless of consumption behavior and the corresponding operating states.
Advantages of the eXergy machine for district heating
| Advantage | Description |
|---|---|
| More customer connections possible | Essential for district heating providers: Low return temperatures are favorable for the network, they increase efficiency and may allow more customer connections without having to create new generation and network capacities for the new connections. |
| Better temperature stratification in the storage tank | Less mixing (e.g. with low load and long domestic hot water circulation operation), more stable flow and return temperatures. This ensures that the district heating customer's consumers are supplied in the best possible way, resulting in a high level of customer satisfaction. |
| High efficiency of the eXm (high COP) | When used in a building supplied with district heating, the eXm provides around 5 kWh of heat for every kilowatt hour of electricity. Thanks to the improved temperature stratification in the buffer cylinder, it does not need to be topped up with district heating as often during circulation operation. |
Example: Schlossstrasse / Boll residential complex (Vechingen, Switzerland)
This reference project in Switzerland illustrates the advantages:
- Initial situationA district heating network was already heavily utilized. Nevertheless, a residential complex with 72 apartments and commercial premises was to be supplied. Conditions: A flow/return temperature of approx. 75/85 °C or 35/40 °C was to be provided.
- MeasuresThe hot water supply was converted. Hydraulic instantaneous water heaters were installed and the old domestic hot water storage tanks were converted into buffer storage tanks (heating storage tanks). The eXm manages the temperature stratification in the two buffer tanks. It ensures a hot flow to consumers (the hydraulic instantaneous water heaters and the high-temperature heating circuits) and a cool return flow to the district heating transfer station.
- ResultThe connection of the additional apartments was possible without any relevant repercussions for the previously existing district heating customers.
Technical requirements
In order for the eXm to develop its potential, it is important:
- A suitably sized buffer tank (or storage system) must be available or installed in which the temperature stratification can be created.
- The eXm output should match the existing heat demand and distribution structure.
The eXergiemaschinen team will be happy to help you with dimensioning.
Conclusion
The eXergy machine is an effective tool for making district heating networks more efficient, even when the network is reaching its performance limits. The eXm helps to reduce costs as it can keep return temperatures low and thus better utilize heat sources and network capacities. Furthermore, network losses are reduced due to the lower return temperatures.
FAQs on the eXergy machine with district heating
The eXm works with two circuits that are connected to a heating cylinder: One circuit draws water from the middle storage tank area, heats it and feeds it into the upper area as hot supply water. At the same time, another circuit feeds water via the evaporator, cools it down and feeds it into the lower, cold storage tank area, where the return flow is connected to the district heating network. In this way, the return flow remains cool even under unfavorable consumption conditions. As the two circuits have separately controllable pumps, the „cool return“ function can be prioritized for the district heating application. When the eXm runs and how the pumps are controlled is determined by the control system depending on the actual temperatures in the buffer tank.
The eXm is available in many different performance classes: Small models for wall mounting are available with 3 and 5 kW thermal output, floor models of the eXm are available with nominal heat outputs of 5, 10, 20, 40 and 100kW available.
Yes, both new connections and existing systems benefit from the integration of the eXm. The prerequisites are the presence of a buffer storage tank (heating storage tank), sufficient space for the eXm and its connections and a suitable power supply.
