Heat Pump driven by a Small-Scale Oil-Free Turbocompressor – System Design and Simulation

Small-scale oil-free turbocompressors driven by high-speed electric motors and supported on refrigerant vapor bearings represent a promising technology to increase the performance of domestic heat pumps. Gas bearings enable high rotational speeds with low mechanical losses. Furthermore, turbocompressors can reach high compression efficiencies and offer a compact and lightweight design. This paper evaluates the performance of a heat pump using a small-scale radial turbocompressor rotating on gas bearings. The turbocompressor has been optimized for R134a and achieves pressure ratios ion the order of 1.5 to 3.5 at rotational speeds of 160 to 280 krpm with isentropic efficiencies of up to 75%. The impeller diameter is 15.2 mm. A system model of the heat pump has been programmed in the Engineering Equation Solver (EES) software. The model uses effectiveness-NTU models for the heat exchangers and polynomial fit approximations for the non-dimensional compressor map data. The heat pump produces 4.0 kW of 30°C water from 10°C ground heat with a predicted COP of 8.1 and 53.4% 2nd law efficiency. Simulation results, the design of the turbocompressor impeller, as well as the layout of the experimental setup are presented. First experimental measurement results will be expected in the beginning of 2017. The system serves as a proof of concept before stepping forward to a two-stage heat pump cycle with two turbocompressors reaching heat sink temperatures of 55 to 65°C.

Two-Stage Heat Pump using Oil-Free Turbocompressors – System Design and Simulation

Adeel Javed, Jürg Alexander Schiffmann

The combination of multi-stage heat pump cycles with small-scale oil-free turbocompressor technology running on gas bearings could be a promising way to increase performance in domestic and commercial heat pumps. This paper presents a novel two-stage heat pump system with two heat sources at two different temperature levels using two separate turbocompressors rotating on gas bearings optimized for R134a. The system allows integration of unused heat sources, e.g. solar thermal or waste heat, into heat production with a minimal loss of exergy. The cycle comprises an evaporator for the first heat source, a condenser as heat sink, an open economizer with integrated heat exchanger for the second heat source, and a tube-in-tube suction line heat exchanger (SHX) in the high-pressure for superheating and subcooling. The aim of this study is to evaluate theoretically the performance of this heat pump cycle using a system model programmed in the software EES (Engineering Equations Solver). The simulation assumes steady-state, negligible pressure drops and heat losses, and adiabatic expansion processes. The superheating in the evaporator and the SHX is 5°C, and there is no subcooling in the condenser. The heat exchangers are modeled using effectiveness-NTU models. At the design point, the heating capacity of the condenser is set to 6.5 kW and provides hot water of 55°C. The first heat source is brine of 5°C. The second heat source is water of 30°C and has been designed to provide up to 30% of the total condenser heat capacity. The two turbocompressors are designed specifically to meet the heat pump design point. Presently, one-dimensional (1D) compressor maps are used in the heat pump model. Simulation results show that coefficient of performance (COP) improvements of 20% to 30% are achievable, depending on the source temperature levels of the heat pump cycle and the amount of second heat source added to the system. The COP increases with higher source temperatures, higher second heat source capacity, and lower sink temperature. The pressure ratios are defined by the imposed temperature levels. The mass flow rate of the refrigerant in the first stage is mainly determined by the second heat source capacity, and in the second stage by the heat capacity of the condenser. In future work, this novel heat pump concept will be tested experimentally.

2016

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

Two-Stage Heat Pump using Oil-Free Turbocompressors – System Design and Simulation

Adeel Javed, Jürg Alexander Schiffmann

2016

Wärmepumpe mit Zwischeneinspritzung bei Scrollkompressoren.

Daniel Favrat, Michele Zehnder

2000