Heat Pumps and Cold Heat Networks for an Environmentally-Friendly Energy Supply

Two individuals in a lab looking at a tablet Copyright: © Peter Winandy

The Institute for Energy Efficient Buildings and Indoor Climate is researching key technologies for the transition to clean heat in the building sector.



Dirk Müller

Head of Institute, Vice Dean


+49 241 80 49760



The heating of buildings and the provision of domestic hot water are significant contributors to CO2 emissions and energy consumption in Germany. Therefore, Germany has to implement many technical measures in the coming years to achieve its climate targets. Two key elements on this path are replacing gas and oil boilers with heat pumps and feeding renewable energy from wind and solar sources into the power grid. Systematically electrifying the heat supply with heat pumps has a double benefit: Heat from the environment is raised to a usable temperature level with heat pumps in a resource-saving manner, while renewable energies in the power grid can be used directly for heat generation. In inner cities, heat networks are a particularly good option. They take waste heat from industrial applications or data centers, incorporate geothermal and solar thermal energy, and thus, save a considerable amount of energy. Here, heat pumps as so-called large-scale heat pumps, especially in combination with combined heat and power plants, play a key role.

In order to create sustainable energy systems, the heat and power supply must be planned together. Electricity can be produced on the roof of buildings, then used directly in the building or stored temporarily. In the future, electric vehicles will be part of the domestic energy system due to their battery capacity. Peak demand in the high and extra-high voltage electrical grids can be avoided through grid-serving operations. At the same time, requirements from industry, the commercial, retail, service sectors, and households must be considered as a whole to identify any shortages at an early stage. This complex interaction can only be mapped using mathematical methods for both system design and operations. Optimizing energy systems in existing buildings and neighborhoods with heat pumps and heat networks is a continuous research focus of the Institute for Energy Efficient Buildings and Indoor Climate.

  Individuals in a Lab Copyright: © Peter Winandy

Optimizing Heat Pumps

In many buildings, heat pumps can replace the existing boilers. Supplementary measures on the building envelope are helpful, as lower temperatures in the heating cycle increase the efficiency of heat pumps. Modern heat pumps can also be used for cooling applications. Both functions require tailor-made dimensioning of the heat pump, to keep the costs for heating and cooling as low as possible and ensure sustainable operation. The design process of heat pumps and the connected storage systems is a challenging task due to the dynamic operating conditions. It is important to find the best compromise between capital and operating costs. If heat pumps are oversized, a secondary (less efficient) heater may be necessary during operations. If the heat pump is undersized, however, capital costs and the number of start-ups increase, leading to additional losses and less inefficient operation. In both cases, operating costs would increase. The Urban Energy Lab 4.0 – Refrigerant Lab operates test stands for heat pumps and individual components. In addition, research is conducted on the best refrigerants and interactions between refrigerant cycles and heat pump systems aiming to improve heat pump efficiency.

Heat pumps must not be a disturbance either inside or outside buildings. Each compression heat pump has a compressor with rotating parts and each outdoor air source heat pump also uses a fan to efficiently capture heat from the environment. In order for the technology to gain wide acceptance, these components must operate quietly. Residential areas have high requirements for efficiency and maximum noise levels, especially at night. The acoustic environment must also be monitored so that the heat pump adjusts its operating volume to the environment.

To achieve maximum efficiency and produce the minimum amount of emissions, a heat pump should generate a lot of heat when the temperature of the heat source is as high as possible and the temperature of the heating cycle is as low as possible. At midday, electricity from the building's own photovoltaic system can be used. In addition, the system benefits from a higher outdoor temperature and thus, operates at higher efficiencies. Anticipatory operations and intelligent charging reduce heating costs and increase the usage of renewable energy. Besides adapting to the demand, applications are created that are transferred into practice based on self-learning algorithms from machine learning methods. Additional potential can be tapped through forward-looking operations. In the future, heat pumps will recognize whether a technical problem is foreseeable in the following days or weeks.

Cold District Heating Networks for Neighborhoods

Cold heat networks are one option for a sustainable heat supply. They provide supply infrastructures that can operate through heat pumps exclusively based on electricity or cogeneration. The basis for cold heat networks is water-to-water heat pumps in the connected buildings, which generate the necessary temperature for heating systems and domestic hot water. Connected to such a heating network, a multitude of buildings and energy sources can thus be hooked up and an optimal energy balance within the neighborhood can be achieved.

Another advantage is that the same network infrastructure can be used for heating and cooling. For this purpose, pipelines are usually operated at one hot and one cold temperature level at least. If a heat pump is used for heating, the warmer conductor is used as the heat source and the cooled water is fed to the PTC thermistor. The resulting cooling power cools the colder conductor. This energy can in turn be used for cooling in other buildings.

Flexibility in temperature levels and generally lower temperatures allow a variety of renewable heat and waste heat sources to be incorporated into cold heat networks as supply options. Cold heat networks are operated at temperature levels that are close to ambient temperature. This allows geothermal energy, solar thermal energy, and outdoor air heat pumps to be used efficiently and the heating and cooling supply to be almost completely free of fossil fuels. Waste heat from wastewater, river water, or data centers can also be used.

At the same time, the requirements for planning and operating the supply infrastructure are increasing. Considering the potential of energy balancing, we cannot consider our work done by designing the entire system for a fixed heat demand and must instead evaluate the temporal patterns of potential heating and cooling demands. For this purpose, we at the Institute for Energy Efficient Buildings and Indoor Climate are developing methods to support the design and operations of cold heat networks. The "TransUrban.NRW" Energy Transition Living Lab aims to apply this technology in four neighborhoods in North Rhine-Westphalia. An interdisciplinary consortium of energy system providers, startups, and science covers all steps from planning to operation.