Ammonia in small systems: challenges & compressors

Despite its excellent thermodynamic and transport properties, resulting in energy-efficient systems with little environmental impact, ammonia has not been used in small capacity systems. Under the EU SHERHPA project, a 9 kW water-to-water heat pump with a small ammonia charge was developed. This project provided useful insights on issues such as the opportunities, challenges and availability of components suitable for small ammonia systems. Björn Palm from KTH Sweden reports on these issues in the latest IEA Heat Pump Newsletter, summarizing key drivers and barriers for the use of ammonia in small-scale systems:

Reduction of refrigerant charge

Since the recognition of chemical refrigerants’ negative effect on the ozone layer and contribution to global warming, there has been a growing interest in the use of natural refrigerants. Given that traditionally ammonia has been used in large commercial and industrial applications, an opportunity to increase its application range to cover also small capacity systems arises. One important aspect to consider when designing ammonia systems is the reduction of the total refrigerant charge in the system as an important step in increasing safety.

Ammonia’s incompatibility with copper

Given that ammonia is strongly corrosive to copper, it has not until recently been possible to design hermetic compressors for ammonia, as the electric motor, usually having copper windings, would be in contact with the refrigerant. In addition, most small and medium-size refrigeration and heat pump systems using CFCs, HCFCs or HFCs have been built using copper tubing. As a consequence, the refrigeration industry has been divided into those dealing primarily with halogenated hydrocarbons using copper tubing and hermetic or semi-hermetic compressors, and those using ammonia in large-capacity systems with steel tubing and open compressors.

Lack of components for ammonia small capacity systems

The greatest obstacle for the commercial introduction of small capacity ammonia systems has been the lack of components specifically designed for these applications. , The first step during the SHERHPA project was hence to identify components designed for other applications which would also be suitable for a water-to-water ammonia heat pump both for space heating and domestic hot water production. The 10 kW system would also be designed to be gastight and to operate with a minimum amount of refrigerant.

Compressors available for small capacity ammonia systems

Three different available compressor type options for the design of the heat pump were identified:

• Open compressors: Open compressors designed for ammonia are available from a few manufacturers even for small sizes. One example is the F2 compressor from Bock, a model which was actually chosen for the project’s pre-prototype. This compressor is designed for ammonia, and the heating capacity, when run at the lowest allowable speed, is about 7 kW.
   
• Separating hood compressors: In this type of compressor, the electrical windings of the stator of the compressor motor are located on the outside of a pressure-proof hood (a thin gas-tight shell), while the rotor, which has no windings but only permanent magnets, is located inside the shell. This means that there are no electrical connections or shaft from the refrigerant side to the outside, thus minimising the risk of refrigerant leakage. Also, there is no direct contact between the refrigerant and the windings.
  Although this type of motors has been in applications where the windings need to be shielded from a harsh environment, it is not widely used in the refrigeration industry. The project identified one Austrian manufacturer of such compressors for refrigeration and heat pump applications, covering a range from about 3 kW to about 21 kW of cooling capacity.
   
• Aluminium motor compressors: A third option is to find a hermetic compressor designed with aluminium instead of copper in the motor windings. The electrical insulation of the wires in the motor would also have to be chosen to be compatible with ammonia. The project identified Mayekawa as a manufacturer of this type of compressor. The Mayekawa motor is compatible with ammonia by using aluminium in the motor windings and by the use of Teflon® for electrical insulation of the windings. The scroll type compressor, featuring a cooling capacity from 5 to 15 kW, is connected directly to the motor, which runs at 3600 r/min at 60 Hz.