Conventional district heating grids usually work with centralized heat generation and supply very different consumers with high transmission temperatures. Many grids have grown over decades. Successive expansions of the pipe grid, newly connected residential areas and dynamic commercial development have led to heterogeneous grids in many places. They supply new development areas with low-energy houses as well as older housing estates, office areas, industrial estates and energy-intensive production plants. As the transmission temperature is generally matched to the heat demand of the largest consumer, this means that the entire distribution grid is operated at high temperatures. A transmission temperature of 100-120 degrees in load operation is the rule rather than the exception in such grids.

Temperature reduction by mixing loop

In order to be sustainable, district heating grids must meet two key requirements: They must supply consumers as efficiently as possible, i.e. in line with demand, and be able to use renewable energies and locally available waste heat. In many places, this is not yet the case. A comparatively simple and effective way of upgrading existing heterogeneous grids is to use intelligently controlled temperature zones.

Temperature zones are useful wherever consumers with similar heat requirements can be grouped together (depending on type and location) and supplied via a separable grid section. In this way, sub-grids with different flow temperatures can be set up that are adapted to the respective heat demand, for example for a district with low-energy houses, a residential area with existing buildings and a mixed or commercial area.

The conventional way of lowering the grid temperature in sub-zones is to use heat exchanger stations. However, these require a great deal of installation work and entail both pressure losses and considerable operating costs. A newer approach is to adjust the flow temperature zone by zone using mixing loops - a principle that is also used on a smaller scale in heating systems of buildings. Water is taken from the cooler return flow and added to the flow by means of a pump. Thanks to intelligent control, the flow temperature can not only be statically lowered to a setpoint, but also dynamically adapted to changing loads. In contrast to a conventional heat exchanger, the pump-based solution can also compensate for possible pressure losses in the system and thus ensure consistent operation.

The advantages are obvious. Lower temperatures mean lower operating costs, lower CO2 emissions and up to 30 percent less heat loss. In addition, the pipe system is relieved, there are fewer leaks and the service life of the grid increases. Finally, with (partial) grid temperatures below 90 degrees, low-grade heat from geothermal and solar thermal energy as well as waste heat from industrial processes or data centers, for example, can also be used.

Real-time data for optimizing operations

Intelligent control is crucial for the efficient operation of the temperature zone. The Grundfos solution is equipped with a controller that compares the real-time data supplied by the system, e.g. including flow and return temperatures and various operating parameters, with specified setpoints such as minimum temperature or differential pressure. In this way, the system controls the speed of the pump and thus the feed from the return flow. No management system is required, as the temperature optimization unit also works as a standalone solution. Visualized operating data and setpoints can be easily accessed via the Grundfos iGrid Cloud.

The term 'iGrid' (intelligent grid) stands for a concept with which district heating grids can be optimized even more comprehensively. A central component is the iGrid temperature optimizer, a special controller unit that uses real-time data from SCADA systems to calculate the optimum flow temperature in the district heating grid. With access to current weather data and a 24-hour weather forecast, weather-led adjustment is also possible. The system also recognizes consumption patterns over the course of the day, week and year in order to raise the setpoint for the flow temperature in good time during morning peak loads on weekdays, for example.

Another controller unit, the iGrid pressure optimizer, can be used to reduce pressure and energy consumption in the grid with the help of differential measuring points and special algorithms. For temperature and pressure optimization, existing sensors can be integrated and new measuring points can be set up. Grundfos offers wireless building and shaft measuring points for this purpose, which are supplied with the necessary energy via a thermoelectric generator and do not require a connection to the power grid. The controller units for temperature and pressure optimization work with Grundfos' own iGrid Cloud, which is NIS2 (Network and Information Security Directive) compliant. The data can be accessed either directly via cloud or by connection to an existing SCADA system.

Positive experience

Digitally controlled temperature zones via a mixing loop are attracting growing interest. In Denmark, which is considered a model country for district heating, a number of grid operators are already relying on the iGrid technology. One example is the district heating grid in Gentofte, a suburb of Copenhagen. Here, the solution was used to reduce the flow temperature across the board. The grid has a heat requirement of around 9,000 MWh per year and was typically operated with a flow temperature of 79 degrees before the modernization, and up to 95 degrees at peak load. By lowering the flow temperature to an average of 60 degrees, heat losses were reduced from 2,750 MWh to 1,950 MWh, i.e. by 24 percent, which corresponds to the heating requirements of more than 30 normal residential buildings. Despite the additional energy requirement of 14 MWh for pump operation, the modernization saves 47 tons of CO2 per year, and the payback period for the modernization project is well below the usual calculations for district heating grids.

There is also positive experience with the Grundfos solution in Germany. The first pilot system in Germany went into operation in Krefeld. The local district heating grid supplies around 1,700 consumers and provides 95 MW of heat on cold days. Depending on the time of year, the grid is operated with a flow temperature of up to 120 degrees. The temperature optimization is used in a section that supplies a mixed area with several schools, a supermarket and single and multi-family homes with a total load of around 4.5 MW of heat. The temperature optimization system, designed as a cabinet solution, is installed between main grid and the relevant section and connects flow and return in order to lower the flow temperature in the section. The control unit determines the required setpoints on the basis of external parameters such as flow and return temperatures and the system operating data.