Oil Migration on Single and Two Stage Heat Pump Systems (Calibration Tests and Process Design Phase 1)

The subject of this project is to develop a high performance air water heat pump which is able to be introducted in the retrofit market. The system requirements of this market are heating temperatures of 65°C, at ambient temperatures descending until -12°C. To provide higher base performance ratings, the choice of the thermodynamic system is a two stage heat pump with economizer exchanger. This type of heat pump has improved performances over a large range of application. For heat output adjustment it can be switched to different heating modes (single stage and two stage). For the two stage cycle, the lubrification of the compressors is a major aspect to be studied. A new online measurement method is applied to evaluate the oil quantity which migrates through the cycle. The absorption spectre of the refrigerant - oil mixture is analysed with a Fourier Transform Infrared (FTIR) spectrometer and oil quantity can be extracted with high precision. (The analysis method has still to be refined). The infrared spectometers also allow to control the composition of the refrigerant mixture. A single capture of the spectre needs about 30 seconds. The measurements have to be referred to a regularly updated background spectre, which requests to vacuumize and clean the measurement cell. The oil migration is simultaneously measured with a high precision densitymeter. The output signal allows a real time follow of the oil quantity and the transmitter will give reliable results after calibration over the application range. Any change of the refrigerant composition cannot be captured by the densitymeter. The resulting density shift would therefore not be attributed correctly to the amount of oil. For the analysis of oil distribution in the whole heat pump, the measurement method to determine the oil level in the compressor crankcase, and the observation of the oil retention in the evaporator are also discussed in this project. The heat pump design has been made, based on simulations of the thermodynamic cycle and using existing parts in the laboratory, such as the evaporator and the condenser. The design aim is to avoid oil trapping and to ensure oil transport in the vapour lines of the heat pump. free download : www.waermepumpe.ch/fe/berichte.html

Efficient air-water heat pumps for high temperature lift residential heating, including oil migration aspects

Michele Zehnder

This thesis presents a system approach with the aim to develop improved concepts for small capacity, high temperature lift air-water heat pumps. These are intended to replace fuel fired heating systems in the residential sector, which leads to a major reduction of the local greenhouse gas emissions. Unfavorable temperature conditions set by the existing heat distribution systems and by the use of atmospheric air, as the only accessible heat source, have to be overcome. The proposed concepts are intended to cover the total application range and to provide the full heat demand without any additional (electric) heat supply. A systematic approach, using a multi objective optimization tool, has been applied to evaluate possible alternate refrigerants, which perform best, regarding to the system COP and the specific heat output. All the optimal refrigerant blends are composed by flammable refrigerants and a potential increase of the COP of 8% (compared to the commercial blend R-407C) has been determined. These potential improvements highly depend on the acceptance to use flammable refrigerants, as can be shown by this evaluation. The standard concepts of small capacity heat pumps suffer from a restricted application range and show highly decreasing performances (COP and provided heat) at extreme operating conditions. From the examination of the thermodynamic cycle, different improved concepts are proposed and are assembled in a generalized super-configuration. The most promising concepts have been built as prototype units and have been tested in laboratory. These concepts are: a) the two-stage compression cycles with economizer heat exchanger or b) with economizer flash tank at the intermediate pressure level, c) the booster-compressor setup, d) the one-stage compression cycle including a new developed hermetic compressor, which enables intermediate injection of saturated vapor flow and e) a small capacity auxiliary cycle for liquid subcooling. In all these concepts the application range could be extended, by achieving reduced discharge temperatures and the heat rate provided at extreme operating conditions could be substantially increased (20%-35%, +100% for the booster setup), using the same compressor and evaporator size. System COP has been improved (~5%) over a large application range, and specifically in high temperature lift operating conditions. The experimental evaluation reveals the major problem of unbalanced oil migration in two-stage compression cycles (except for the booster concept). An extensive evaluation has been applied, to analyze the oil migration in two-stage compression heat pumps. A new developed measurement technique, using a Fourier Transform Infrared Spectrometry combined with a high pressure supporting ATR cell, has been calibrated (with a lower detection limit at 0.2% - 0.4%, and a sensitivity of 0.1%) with the used refrigerant-oil mixtures (R-134a/POE oil and R-407C/POE oil). It has been applied to perform on-line oil concentration measurements during two-stage and one-stage steady stage and during transitory operating modes. A generalized steady state simulation model has been developed including namely the new developed compressor model with an intermediate injection port, considering the geometrical flow path of the tested prototype compressors. A flow map based extensive heat transfer model is integrated into a finned tube evaporator model and taking into account oil effects on the heat transfer. General models of plate heat exchangers and for capillary tube expansion devices complete this modular simulation model, on which the concepts of the super-configuration can be calculated and some parametric analysis has been performed. An in house developed fluid interface module includes the fluid properties calculation program Refprop and allows to define new mixtures or to use the large number of predefined refrigerants.

EPFL2004

Wärmepumpe mit Zwischeneinspritzung bei Scrollkompressoren.

Daniel Favrat, Michele Zehnder

This project is evaluating a heat pump cycle with midstage injection at scroll compressors. The injected refrigerant is evaporated partially by an internal heat exchanger (fig. 2.1). A glycol-water heat pump with 12 kW heat capacity at B-5/W50 was built and tested in the laboratory at several conditions (B- 5/0/5 at W35/50 and B-10/W60). The refrigerant is R407C. For the theoretical analysis the exact geometry of the injection wholes is determined. The results are discussed in the hypothesis of a monovalent heating implementation in the market of retrofitting fuel based heating systems. Measurements at stationary test conditions with injection show an increase of the heat output by 15% compared to the values without injection (fig 4.2 and 4.5). The heat output is not affected by the position of the manually regulated injection valve. The end of compression temperature can be controlled by the amount of injected refrigerant. The application range for this heat pump is enlarged and at the point B-10/W60 the maximum system temperature has been reduced from 110°C to 75°C. The coefficient of performance (COP) with and without injection is comparable at the major part of the tested points (fig. 4.1 and 4.4). The only available compressor type with the possibility of injection has a geometry which is not adapted for saturated vapor. The resulting reduction in performance is equivalent to the gain by the better thermodynamic concept. The optimum COP is reached with an injection rate issued by a thermostatic valve control. With higher injection rate, the COP is decreasing linearly and a reduction up to 15% is achieved at B-10/W60. Two injection configurations (the injected refrigerant is separated before and after the economizer exchanger) have been compared and show equivalent results. The simulation model has been developed considering the test results. For the compression phase during injection a correction factor has been introduced. The simulation can be used to analyse of the qualitative behavior. A high potential of performance improvement can be achieved by reducing the injection pressure level (table 5.2). Shifting the position of the injection wholes in higher pressure regions does not affect the performances but can keep the compression curve away from the saturation limit.

2000

Analyse de la migration d’huile dans les pompes à chaleur mono- et biétagées

Daniel Favrat, Michele Zehnder

The Laboratory of Industrial Energy systems has evaluated a series of high performance air-water heat pump concepts with the aim to retrofit existing fuel fired boilers for residential hydronic heating systems. The results are published within the framework of the Swiss Retrofit Heat Pump project, which is conducted by the Swiss Federal Office of Energy. The two-stage heat pump cycle has shown the best efficiency prospects. Oil migration between the upper stage and lower stage compressors is the main obstacle for commercialization in the low heat rate range. The oil mass concentration is measured by the Fourier Transform Infrared Spectrospopy (FTIR) method with an attenuated total reflection cell (ATR). This method was calibrated on a separated test loop and compared to measurements of mass-sampling and of density by a Coriolis effect densitymeter. Infrared analysis gives a precision of < 0.1% of oil concentration and the detection limit was identified at about 0.3%. Density measurements cannot be used in the ordinary temperature range of the liquid line in the heat pump. Mass sampling and the FT-IR measurements give corresponding results. An air-water heat pump (10 kWth, R-407C) was build in the laboratory to perform comparison tests in the single-stage and two-stage heating modes. The main elements of the heat pump are: two scroll type compressors, a brazed plate condenser, a finned tube evaporator, an economizer heat exchanger and two electronic expansion valves. Defrosting is done by cycle inversion with a 4-way reversing valve. Measured improvements on the two-stage heating mode of 30% for the COP and 24% for the heat output comparing to the corresponding test point (A-7/W50) in the single stage configuration have been shown. Important reductions of the maximum system temperature at the outlet of the compressors enable an extended range of application (including the point A-12/W65). In the two-stage heating mode an oil transfer rate of 5 - 10 g/min from the high pressure stage to the low pressure stage compressor was detected in all tested conditions, which is strongly limiting the continuous service of the heat pump. For long term running tests an additional balancing system composed of an oil pump and an oil vessel has been installed. The circulating oil concentration is between 0.2% and 0.4% for the two-stage configuration as measured with the FT-IR method. In single-stage mode the concentration is lower than 0.2% and only the mass sampling technique provided reliable results. Sensibility analysis in changing the regulation parameters did not give concluding results in order to design a stable configuration of the two-stage heat pump. In absence of integrated two-stage scroll compressors intermediate solutions have to be considered. These consist in the implementation of a booster compressor or the implementation of an oil separator at the outlet of the high pressure stage compressor coupled with a oil return management or a periodic shut down of the heat pump with external balancing of the oil level in the crankcase. An adapted heat transfer model for refrigerant-oil mixtures was developed, in order to investigate the theoretical impact of oil in the evaporator.

2003

Analyse de la migration d’huile dans les pompes à chaleur mono- et biétagées

Daniel Favrat, Michele Zehnder

2003

Efficient air-water heat pumps for high temperature lift residential heating, including oil migration aspects

Michele Zehnder

EPFL2004

Wärmepumpe mit Zwischeneinspritzung bei Scrollkompressoren.

Daniel Favrat, Michele Zehnder

2000