2. June 2014

Turbo Compressors Enable Lighter Satellite

For the cooling of electronics in future satellites, heat pumps might be the solution. They can be built to be more efficient, lighter and compact thanks to Celeroton’s turbo compressors. This was proven in a series of experiments with ground-breaking results.

Satellite sensors and electronics generate a lot of waste heat. To prevent them from overheating, the heat must be emitted to space with radiators. Until now, the radiator temperature always had to be below the systems to be cooled in order to allow for the heat to flow out. Radiator temperatures of around 50 °C lead to bulky and heavy radiators.

A unit increase in radiator temperature results in a quadrupling of the radiated energy per surface area. In order to build future satellites lighter and more compact, heat pumps should be applied for the cooling of the electronics. With this, the radiator temperature can be set much higher, which results in savings of space and weight.

Here, Celeroton comes into play: With ultra-high-speed turbo compressors such heat pumps can be built much lighter, more compact and saving more energy than with conventional drive solutions.

In 2012, the National Aerospace Laboratory (NLR) of the Netherlands contacted Celeroton to make benefit of this potential. Following this, a series of feasibility tests were carried out in a concept study.

For use in satellites Celeroton’s compressors had the following requirements:

Low weight and small volume
The overall weight is critical for transporting cargo to space. Therefore, all system parts must be realized as compact and lightweight as possible.
State-of-the art compressors, allowing for a cooling capacity of 10 kW, weight approximately 40 kg, while special aviation compressors weigh approximately 19 kg. NLR required an overall weight of the compressors, including electronics, of less than 2 kg!

High temperature differences
In order to achieve a sufficient cooling capacity the heat pump system has to ensure an increase of the radiator temperature from 45 °C to 100 °C. Comparable state-of-the art compressors are designed for much lower maximum temperatures of 65 °C.

High efficiency
Besides the energy used for the transport into space, high attention must also be paid the energy consumption during operation since the satellite’s solar collector area is limited. NLR targeted a Coefficient Of Performance (COP) of at least two.

Low vibrations
For the design of the compressor, care had to be taken not to generate additional vibrations in the satellites. The goal was a vibration value of less than 0.3 N, whereas conventional compressors with a similar size generate approximately 40 N.

In the beginning of the project 12 potential refrigerants were examined closely. In order to comply with the high energy requirements of NLR, low losses had to be guaranteed. This is only possible with low pressure values, which is why fluids like ammonium and CO2 could not be used. This led to selection of Isopentane (R601a).

Calculations showed that the gas pressure, for the required heating, had to be increased from 1.76 to 7.2, which equals a pressure ratio of 4.1. To achieve this, a system with three subsequent turbo compressors with a nominal speed of 190,000 rpm each was designed. As a basis for the first compressor, the existing turbo compressor CT-17-700 with a 3D-impeller was chosen. For the second and third stage, new systems with 2D-impellers were designed and built. For the operation of the compressors, three Celeroton standard CC-75-500 converters were used.

The overall system (see figure) was tested with a series of measurements at NLR. The goal of these measurements was to validate the system performance (pressure ratio, power, temperature, etc.). With a COP of 2.6 and an isentropic efficiency of 68% at an evaporator power of 5.1 kW the requirements were clearly exceeded.

System with 3 compressors

Recently, the project was successfully completed: With the outstanding efficiency, Celeroton could impressively prove the applicability of its turbo compressors for the cooling of future satellites: more information.

To make the system effective in a real application, a subsequent project will be necessary.