Propane heat pumps, waste heat recovery, refrigerant charge and energy optimisation were some of the most talked about research topics at the 11th IIR Gustav Lorentzen Conference on Natural Refrigerants. Several research papers presented working hypotheses on optimising system performance and results from an European FP7 project called Next Heat Pump Generation Working With Natural Fluids (NxtHPG), which aims to develop 5 high efficiency heat pump prototypes working with natural fluids.
A propane water-to-water heat pump booster for sanitary hot water production: seasonal performance analysis of a new COP optimised solution
The paper by M. Tammaro, C. Montagud, J. M. Corberán, A. W. Mauro and R. Mastrullo (Università degli Studi di Napoli Federico II) analyses the performance of a water-to-water prototype electrical heat pump (EHP) for domestic hot water (DHW) production up to 60 ºC using propane as working fluid.
The study aims to identify optimal condensation pressures at different working conditions. The study's new concept is to let the system condense at a pressure that is fixed at each operating condition to maximise the COP.
The design and control of the prototype had to be selected depending on the conditions at the boundaries: the demand or daily load profile of the end user (DHW consumption per person), the choice of the appropriate size of components for the EHP, and the heat storage and hydronics.
Heat pump main characteristics:
49 kW nominal heating capacity when heating water from 10 to 60°C in the condenser and working with water entering the evaporator at 20°C.
- Plates condenser 1400 W·K-1
- Plates evaporator 9700W·K-1
- Scroll Compressor 170 cm3 displacement and 2900 rpm
The end user considered in the study was a hotel in Strasbourg, France, which is one of the three reference climates in Europe according to European Regulation (EU) 812/2013. In a hotel, climate differences within regions and seasonal variability have a negligible effect on the consumption of a DHW. The most relevant factors in terms of consumption are that the higher the number of rooms and the occupancy rate, the higher the consumption.
The results showed that the performance of the EHP is affected not only by the temperature level of the source of energy available (evaporator inlet temperature) but also by the rest of the system characteristics, such as the volume of the tank and the share of the tank that is kept heated at the delivery temperature, which will also depend on the coupling to the demand (control logic).
Results obtained also led to the conclusion that in order to achieve high system efficiency, there is a need to choose appropriate control strategies for each specific end user.
Design and analysis of R290 high capacity air to water heat pump with small diameter finned tube heat exchanger
In this paper, authors E. Navarro-Peris, J. Gonzalvez-Macia, J.M. Corberan and S. Filippini (Universitat Politècnica de València) compared theoretical performances of an air-to-water inverter heat pump of 50 kW for the production of hot water and for heating applications. The prototype heat pump works with R290 and uses a finned tube heat exchanger of 5 mm instead of 7 mm of diameter.
The theoretical system also included:
- Braze plate heat exchangers
- Rotary screw compressors
- Oil extraction valve and oil changing systems
The analysis compared differences in terms of COP and refrigerant charge. The results have shown that 5 mm evaporators can significantly reduce the cost of the system, as well as reduce the refrigerant charge of the evaporator by up to 30% and the total refrigerant charge of the heat pump by up to 1 kg without a significant reduction in COP (the best COP was reached at 170g refrigerant charge). These results can be obtained with the same COP and almost the same capacity in heating mode.
Researchers concluded by explaining that the results they had obtained were preliminary results. The next step will be to identify the best design and configuration for a 130g propane charge. They will test, in the field, a second prototype incorporating 5 mm tube heat exchangers. Another of their working hypothesis will be that an increase of the row separation will improve the COP without increasing the total size of the heat exchanger.
Project NxtHPG: Next generation heat pumps working with natural fluids
J. M. Corberan and C. Montagud (Universitat Politècnica de València) explained that hydrocarbons are a known efficient alternative to HFCs in heating and cooling applications but suffer from a lack of public trust due to their flammability. To comply with the regulation applying to household applications, the whole system refrigerant charge needs to be less than 150g.
Academia and manufacturers are working on optimising hydrocarbon heat pumps to decrease the flammability of the system by:
- Reducing the size of the systems to allow a reduction of the refrigerant charge
- Decreasing the amount of oil in the compressor and/or using another type of oil more appropriate for propane (PAG oil instead of POE that has less solubility)
With that objective in mind, the European Union has founded the Next Generation Heat Pump Technologies project coordinated by the Universidad Politècnica de Valencia (UPVLV) as part of the 7th framework programme. The project associates component manufactures (DANFOSS, DORIN, ALFA LAVAL, LU-VE), heat pump manufacturers (CIAT, ENEX) and research groups (UPVLC, ENEA, UNINA, EPFL, KTH, NTNU) in order to assess several issues related to the application of natural refrigerants (mainly hydrocarbons and CO2) in heat pumps at a commercial level. The European Heat Pump Association is in charge of the dissemination activities.
The general objective of the project is the development of a set of five high efficiency (10-20% SPF improvement) and high capacity heat pumps working safely with natural refrigerants, together with a set of improved components and auxiliary devices. This includes providing the necessary know-how to key European companies in the industry. The developed technology would lower carbon emissions (20% lower TEWI) compared to current HFCs/HFOs system.
Read the article on hydrocarbons21.com