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Absorption cooling system uses district heating and solar low temperature sources

EnEff:Wärme - Forschung für energieeffiziente Wärme- und Kältenetze
Prototype of the 50-kW absorption cooling system

Prototype of the 50-kW absorption cooling system

© TU Berlin

Settlement summary

Project status Projektstatus: Phase 3Realisation
Location Marchstrasse 18, 10587 Berlin, Germany
Project plan As part of research work completed to date, an absorption cooling system with a cooling capacity of 50 kW was developed and largely optimised under operating conditions. At the same time, the potential uses of heating networks as cooling networks were investigated. For demonstration purposes, various buildings were equipped with one of the systems. Currently, a 160-kW functional model is being developed. It is to be tested in autumn/winter 2011 at TU Berlin and will be demonstrated in summer 2012 in buildings. A joint revision of the 50-kW and 160-kW functional models upon completion of the project will prepare the systems for market entry.
Developer, organizer TU Berlin, ZAE Bayern, Vattenfall Europe Wärme AG
Project themes

Project description


Vattenfall Europe Wärme is one of the future users of high-efficiency cooling from district heating; Technische Universität Berlin provides expertise and develops thermally-driven cooling technology. In this research project, both partners are developing and testing new thermal cooling systems. Based on the state of the art, the university applied the latest findings from system technology and hydraulic structures to create a cooling system which reduces the investment-specific cooling generation costs by half to two-thirds and which doubles the electrical efficiency compared with the state of the art. This cooling system consists of an absorption cooling system, a recooling system and a common control strategy. A key part of the project is the redevelopment of absorption cooling systems in the 50-320-kW output range in a partially modular design. They are designed for integration in inner-city building structures with minimal costs. The demonstration systems can also be optimised in real field applications, thus proving the great potential of this technology.


While air conditioning was previously often viewed merely as an added comfort and frequently deemed superfluous, it is becoming increasingly common in commercial areas due to stricter health and safety requirements and new building designs with translucent facades. In the hotel and restaurant industry, air-conditioned function rooms and restaurants are already considered essential in some cases for successful business operation, especially during hot summers.

In order to meet the growing demand for cooling, new low-energy technologies are required due to climate protection obligations and the high dependence of the EU on imported energy. Absorption cooling based on solar thermal energy and combined heat and power generation (CHP) will play an important part in this. These systems eliminate the need of further compression-based systems and can replace existing systems. Consumption of electricity and energy, and CO2 emissions are also reduced. In addition, supplying cooling using district heating as an energy source increases CHP generation in the summer. This allows it to replace less energy-efficient electricity generation on the market and makes an additional contribution to saving energy and reducing emissions.
In the project which started in 2008, absorption cooling systems in the 50-320-kW output range were developed to reduce, in a partially-modular design, production costs while maintaining high thermal and electrical operating efficiency. These systems were developed by TU Berlin and ZAE Bayern, measured at TU and field-tested by energy supply company Vattenfall Europe Wärme. During the project, systems for feeding and distributing the cooling in the building using the existing heating distribution system were also designed and tested.


In 2009, a production sample system with a cooling capacity of 50 kW was designed and developed. In early 2010 it was manufactured by an industrial supplier and has been undergoing measurements since April 2010 at TU Berlin to determine the efficiency and process. Since 2010, a wide variety of experience has been gathered in approx. 5,000 operating hours.
The new cooling system is highly flexible in its use of district heating supply structures. The system can be operated with significantly varying volume flows through a temperature range from 60 °C to over 100 °C. On the one hand, various district heating networks require specific conditions for the temperature level and available volume flow, which can all be reached with the system. On the other hand, this knowledge can be used to optimise the return temperature to the district heating network, even in partial load operation. In principle, the system can even be operated at a very low inlet temperature (to 60 °C). Throughout the entire load range, the spread of the drive temperature can be increased by 35% by reducing the volume flow from 0.9 l/s to 0.6 l/s with just a 10% reduction of the cooling capacity. By contrast to the current state of the art, where the performance of an absorption cooling system is controlled with a variable inlet temperature of the drive, this system can be operated specifically in the output range between 50 and 10 kW with a constant spread and variable volume flow. This creates new potential, in particular in connecting motor CHP plants and absorption cooling systems. In solar operation, the cooling system can be kept operational even with low irradiation by varying the volume flow, thus increasing the solar coverage.

Like all cooling systems, the cooling capacity of the system is inversely proportionally dependent on the recooling temperature. Consistent avoidance of thermal bridges on the process side and an innovative heat transfer layout allow the system to establish excellent thermal conditions (COP) even at high recooling temperatures of up to 45 °C. Until now, approximately 40 °C were considered the upper limit for operation. The expansion of this limit in conjunction with the constantly higher COP now allows dry recooling systems to be used in central and northern Europe as well.

The trend towards controlled pumps in building services technology now permits significant potential reductions in secondary electricity consumption even when using thermally-driven cooling systems – in particular during the majority of operating hours in partial load conditions. For this, it is important that the cooling systems have stable control properties depending on changes of the volume flow and permit stable system behaviour via a broad range of volume flows.

Figure 6 shows the relationship between the cooling water temperature/volume flow and the resulting cooling output. The absorber and capacitor of the new system allow partial load volume flows of up to approx. 20% of the rated volume flow in serial flow with virtually constant operating quality. At approx. 25% of the volume flow, the cooling capacity remains at approx. 60% of the basic capacity with constant cooling water inlet temperatures. At the same time, the hydraulic energy required in the recooling circuit is reduced by 98%! This is the basis for new efficient control strategies to optimise the drive temperature and secondary consumption for the targets of tomorrow even with a prescribed cooling load.


Motivated by the rapid commissioning and positive results in the test setup, a decision was made in April 2010 to equip a building with a 50-kW system in the same summer. A Vattenfall Wärme AG office building in Berlin-Lichtenberg, Germany was selected. Due to the location, the district heating supply temperature in the summer is 75-80 °C. The property has a peak demand of approx. 42 kW of cooling capacity. For recooling, the cooling system was designed using a tabletop cooling unit with a rated output of 102 kW under design conditions. The system has been in continuous operation since June 2011.

The new cooling system can significantly reduce the investment costs in this output class. Systems with investment costs of EUR 200 – EUR 240/kW are the target in the 50-320-kW output range. For this purpose, a new 160-kW system is currently being developed at TU Berlin and ZAE Bayern. A prototype of this system is to be tested at TU Berlin from November 2011 on.

Energy characteristics

The table shows an overview of characteristics of absorption cooling systems already available on the market (before) and the current development status (after). The market analysis is based on the evaluation of eight systems by five different manufacturers in the 40-200-kW output range. The seasonal energy efficiency ratio (SEER) for the electrical and thermal costs refers to current best practice implementations. Systems implemented to date generally have far lower figures. The electrical costs incorporate all necessary components of the cooling system, including the recooling system, measurement and control technology and hydraulic supply.

Energy characteristics

before potential after unit
Minimum drive temperature 75,00 50,00 55,00 °C
Maximum exhaust heat temperature (system inlet) 40,00   46,00 °C
Volumetric power density 24,00   44,00 kW Kälte / m³
Rated COP 0,70 0,80 0,78 kW Kälte / kW Wärme
Thermal SEER 0,65   0,78 kWh Kälte / kWh Wärme
Electrical SEER 12,00   20,00 kWh Kälte / kWh Strom

Additional information:

  • Test building: Vattenfall Wärme AG office building at Syringenplatz in Berlin Lichtenberg, Germany
  • Overview of the energy characteristics of both cooling system types
  • Variation of the hot water volume flow and temperature
  • Cooling power and recooling temperature: comparison with an older system
  • Variation of the cooling water mass flow and temperature
  • COP via partial load, cooling water variation