Ammonia in small systems: testing a prototype

The aim of the project was to design a heat pump system extracting heat from a secondary loop with the heat delivered to a hydronic system able to supply both space and domestic hot water heating. In this type of design the relative high temperature increase of ammonia during compression served as an advantage, as it was used to generate domestic hot water at temperatures above the saturation temperature in the condenser. The design was focused on minimising the charge of refrigerant in the system, which was achieved by using compact heat exchangers and by designing the system without a receiver. Moreover, all tubing was selected with the smallest suitable diameter, and the system layout was made so as to get short tubing distance.

The final prototype

Following problems with oil return, the system was re-designed so that the evaporator could be fed alternatively from the top or the bottom, and to include an oil separator whose use was found to make no major difference to the charge or the performance of the evaporator. Other changes introduced at this stage included:

• Introduction of a combined receiver and sightglass in the liquid line: This unit designed and made at the Royal Institute of Technology, enabled to 1) see any changes in the colour of the ammonia, indicating wear of the system; 2) determine if the charge was sufficient or too large; and 3) enhance controlling of the expansion valve.
   
• Introduction of a strainer in the liquid line: As blockage of the expansion valve had occurred on some occasions, a filter (strainer) was introduced in the liquid line just before the expansion valve.
   
• Several types of expansion valves tested: Different types of expansion valves were tested, both manual and electronic. The most successful, in the setup, was the small electronic valve from Carel (E2V).
   
• Different heat exchangers tested: Tests run using different types of heat exchangers found that prototype heat exchangers designed at the Royal Institute of Technology performed at least as well as the plate heat exchanger when used as a condenser while its performance as an evaporator was the best among all the types of heat exchangers tested.
   
• PAG oil was loaded into the compressor: This considerably enhanced the evaporator performance and resulted in the system been able to run stably by resolving the oil return problem.

Test results

The results presented are for the final prototype with an ammonia charge of 100g, using the miscible PAG oil and aluminium microchannel heat exchangers as condenser and evaporator and a plate heat exchanger as desuperheater.

The COP of the system working under these conditions was found to be in the range 3.6 to 4.3. Furthermore, under the tested conditions, the hot water flow rate out of the desuperheater was about 1.5 litres per minute at 63 °C, a high enough temperature to ensure killing of bacteria such as Legionella. In addition, in all of the tests with the final prototype, the ammonia charge was about 100g, while the capacity was about 9 kW, giving a charge of 11g refrigerant per kW heating capacity.

Background & future plans

The SHERHPA (Sustainable Heat and Energy Research for Heat Pump Applications) project evaluated capacity, costs, safety, environmental performance and efficiency of heat pump applications using natural refrigerants and was coordinated by GRETh (Groupement pour la recherche sur les Echangeurs Thermiques) and EHPA (European Heat Pump Association) with financial support from the EU 6th Framework Programme. It involved 19 small and medium-sized companies and 10 research institutes in the area of heat pump manufacturing, energy and control from 13 countries.

Although the SHERHPA project has now ended the Royal Institute of Technology plans to continue the work on small capacity ammonia systems using the experience and components developed during the project. A planned heat pump prototype will use the prototype mini-channel heat exchangers and an open compressor, together with a synchronous permanent magnet motor.