Pompe à chaleur bi-étagée à haute performance.

The objectives of the present work is to theoretically and experimentally demonstrate the advantages of a two-stage cycle equipped with an economizer and single variable speed compressor for heat pumps using air as cold source. Furthermore, an efficient deicing device based on the two-stage concept is discussed in this paper. A two-stage cycle numerical model has also been developed allowing fast determination of the heat pump’s steady-state operating range as a function of the principal design variables. The experimental validation has been carried out using two successive heat pump biPAC1 uses a two-stage cycle with an economizer and a variable speed compressor with a mid-stage casing pressure (two scroll compressors, the first one with a 30 to 110 Hz frequency range). The second heat pump biPAC2 uses a two-stage cycle with a two-phase injection (limitation of the distillation effect of the non-azeotropic refrigerant in the economizer) for further optimization. Furthermore, another first stage compressor type has been used (piston compressor with a frequency range of 45 to 65 Hz) as well as a new evaporator with an enhanced tube surface and a new plat condenser. The experiments using the biPAC2 have resulted in a 70% increase of the condenser’s power and a 14% increase of the performance coefficient for an operating regime L-7/W50 with the HCFC 22 refrigerant. The good performances of the biPAC2 have been confirmed through further testing using HFC407C, the de-icing cycle and following the normalized test case EN255. These results have been compared to the three best commercial heat pumps. Two-stage cycle results show an excellent exergetic efficiency for the extreme temperatures (50% at L-10/W60). The simulations based on the experimental results are given for a temperature range between L-8/W50 and L-12/35 for typical household requirements (degrees-day of Zurich). the numerical simulations lead to an average annual performance coefficient (with de-icing) of 3.9 in a monovalent operation.

Wärmepumpe mit Hilfskreislauf zur Kondensatunterkühlung, Phase I.

Daniel Favrat, Michele Zehnder

In this report different air-water heat pump cycles under the operating conditions of the retrofit market and in the low heating power range are examined. The already existing heat pump models show excessive power and efficiency loss at very low ambient temperatures. Due to the variations of the source temperatures the heat pump system has to be improved over a relatively wide application range. The existing perfomances are insufficient and new solutions have to be found. The introduction of a supplementary compressor, as well as the efficient use of the liquid subcooling are two examples, which will determine the selection of the cycles (see pages 4 - 7). By simulation of the suggested cycles, like the heat pump with the auxiliary liquid subcooling cycle, as well as the two stage cycles (with intermediate expansion or intermediate injection) it can be shown that these cylces operate with an improved seasonal coefficient of performance up to 10% - 15% (see table 4.4). Heat pump cycles with more than one compressor permit a better adjustment of the heat output. The seasonal coefficient of performance of these heat pump cycles is calculated with the temperature distribution of the climate in Zurich and a given heat curve. The cycle components are simulated on a purely theoretical base, with the exception of the compressors characteristics and inlcuding the real heat output from the heat pump. The cycles are calculated with pure refrigerant and with zeotropic mixtures. The results with propane show equivalent seasonal coefficients of performance on all suggested cycles. The heat pump cycle with liquid subcooling shows good performance in the middle range of the air temperatures. The main compressor will then work at its best conditions. At low external temperatures the two-stage cycles with two serial connected compressors, show very good performance. And at low temperature differences between the air and the heating water single-stage heat pump are the best with the actual compressor types. With zeotropic refrigerant mixtures (R407C, R290/R600a), the advantages of the proposed cycle with liquid subcooling in the condensing part are not significant and the losses, due to the low performance of the auxiliar compressor. will lower the coefficient of performance under the level of the other two-stage cycles (see table 4.3). The simulation results show that several solutions can be considered for this application. An evaluation of the costs and the resulting technical problems can only be mentioned at this time.

1998

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

Dynamic Modeling and Simulation of Two-stage Compression Heat Pumps with Flash Economizer

Jean-Baptiste Carré, Daniel Favrat, Simon Gailledrat, Samuel Henchoz

Multistage heat pumps are one of the most efficient technologies for delivering heat at temperatures below 100°C, Thanks to their fairly high exergy efficiency. In the recent years the development of radial compressors mounted on gas bearings have unlock the possibility of a downsizing of multistage heat pumps to the capacity range of domestic applications. Such small scale multistage heat pumps are expected to have increased efficiency at nominal and part load conditions, to be fully hermetic and to be more compact thanks to their oil free operation and the use of dynamic machines. As a result of an industrial collaboration, an experimental two-stage heat pump with flash economizer was built and tested in a previous work. It used radial oil free compressors arranged on a single shaft rotating on gas bearings. A dynamic simulation model has been developed in order to better understand the behavior of the aforementioned prototype, as well as to preliminary test control strategies. As a result, a whole range of dynamic models, including the evaporator, condenser, flash economizer, and compressors were developed using the modeling and simulation software gPROMS. They rely on simple two and three zone models for the evaporator and condenser respectively. Zero dimensional models were used for the compressors, two different types of which were implemented. One is a volumetric compressor with a fixed built-in volume ratio, connected to an asynchronous electric motor the other one is a variable speed radial type based on a simplified compressor map deduced from experimental data. As much as possible, the aforementioned models have been inspired from components available in the base library of gPROMS. The results showed that a two stage heat pump based on quasi constant speed volumetric compressors poses problem at low temperature lift, when the top part of the cycle is degenerated, making it close to a single stage machine. The version with the dynamic compressors does not exhibit any problem of degeneration of the cycle, while having a better performance and load characteristic curve. Both systems showed an insensitivity of the performances to the charge of refrigerant due to the buffering effect of the flash economizer. As a conclusion, a simple dynamic model was developed for simulating various types of two-stage heat pumps with flash economizer. It confirmed the potential of performance improvement of radial compressors based heat pumps over volumetric ones.

2013

Dynamic Modeling and Simulation of Two-stage Compression Heat Pumps with Flash Economizer

Jean-Baptiste Carré, Daniel Favrat, Simon Gailledrat, Samuel Henchoz

2013

Wärmepumpe mit Zwischeneinspritzung bei Scrollkompressoren.

Daniel Favrat, Michele Zehnder

2000

Wärmepumpe mit Hilfskreislauf zur Kondensatunterkühlung, Phase I.

Daniel Favrat, Michele Zehnder

1998