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© TU Berlin
New generation of compact chillers
Projektinfo 07/2012

Bee and Bumblebee in comparison.
© TU Berlin

The absorption chillers currently available on the market are heavier and larger than the new models.
© TU Berlin
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Cooling with heat

The economic and ecological success of many CHP plants, district heating networks and large-scale solar power systems very much depends on the utilisation of heat outside of the heating periods. An increasingly interesting market for low-temperature heat is cooling and air-conditioning buildings as well as generating process cooling energy with thermally driven chillers. Scientists from Berlin and Bavaria have developed a new generation of particularly compact, efficient absorption chillers with small capacities for cooling and heating operations.

Absorption chillers are considered to be very robust. A disadvantage, however, is that the units often have very large dimensions. This means that it is often not possible to retrofit them in basements or only with considerable effort. Furthermore, most commercially available absorption chillers are designed for high output ranges from 300 kW upwards – too large for decentralised use in many buildings.

In a research project conducted by the Technische Universität Berlin in cooperation with Vattenfall Europe Wärme AG, scientists from the university working together with ZAE Bayern and further partners from research and industry have extended the output range downwards. The focus was not just on their weight and dimensions but also on removing other hurdles preventing the use of absorption chillers. For example, the systems currently available are relatively expensive and present considerable demands in terms of the recooling. The researchers also want to improve the temperature difference of the drive heat. In many chillers that use district heat, the heat transfer medium only cools by around 10 °C. This means that a large proportion of the usable heat at a very usable temperature level is therefore circulated unnecessarily and also requires a high pump capacity.

Bee and Bumblebee – two model systems

A 50-kW plant, called ‘Bee’ by the researchers, has now passed comprehensive tests and undergone the subsequent optimisation work. Two of these systems have been undergoing practical testing in buildings since the middle of 2011. Based on the experience with Bee, a larger ‘Bumblebee’ system has also been developed with a 160 kW cooling capacity. This has also already completed initial testing and meets the requirements with a rated operational output of 168 kW.

The single-level absorption chillers work with water as the refrigerant and lithium bromide as the absorber. With this pairing of materials, it is sufficient to use the drive temperatures that are provided by district heating systems and solar power systems. Cold water temperatures as low as 5 °C can be generated. When suitably incorporated into heating systems, the chillers can also work as heat pumps for heating buildings. In contrast to most absorption chillers that have been commercially available until now, Bee and Bumblebee are modularly built as twin-container systems. With difficult transport routes they can be disassembled, transported and reassembled on site. “However, our aim was not only to achieve good access through doorways and a low transport weight,” explains Project Head Stefan Petersen, “our development goals also included keeping the investment and operating costs as low as possible with a high system efficiency.”

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On the test rig

From Easter 2010 to the end of 2011, an initial function model for the 50-kW system achieved around 8,000 operating hours on a test rig at the TU Berlin. The chiller can be operated with significantly varying volume flows through a temperature range from 55 °C to over 100 °C. That is particularly significant in terms of its use in district heating networks. Firstly, different district heating networks provide their own specific framework conditions in terms of the temperature level and available volume flows. These were all achieved by the system. Secondly, reducing the volume flow with partial loads makes it possible to optimise the return flow temperature in the district heating network.

With existing commercially available systems, the output is controlled by varying the inlet temperature of the drive. The new systems enable three additional options. For example, if the volume flow of the district heating is reduced from 0.9 l/s to 0.6 l/s, this reduces the cooling capacity by just 10 % across the entire load range. However, the spreading in the drive temperature increases by more than 35 %. With variable volume flows, the entire output range between 50 and 10 kW can be achieved with a constant temperature difference. This creates new potential, in particular when connecting engine-driven CHP plants to absorption chillers. In solar operation, the chiller can be kept operational even with low insolation by varying the volume flow, thus increasing the solar coverage. The two other options take into consideration the control of the cooling water temperature and the volume flow. In particular, this helps to reduce the bypass flow consumption.

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TU Berlin, IET

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ZAE Bayern - Würzburg

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BINE-Projektinfo 07/2012
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