Pompe à chaleur biétagée à haute performance. Phase 2: Dégivrage par énergie à basse exergie.

The capacities of performance improvements of low and mid-size heat pumps are studied in this project. For the new buildings market, the components of the heat pump match already well the requirements for high performance and monovalent heating. Minor improvements are to be expected on inverter driven compressors and on the integration of the domestic water in the heat pump cycle. This study is focused on the retrofit market (replacement of existing oil burners). For this case the existing air-water heat pumps are not economical and performance decrease rapidly at low air temperatures. A two-stage heat pump with two phase injection and an economiser vessel has been built in collaboration with Termogamma. It has been compared to a single-stage configuration (Pompe à chaleur biétagée à haute performance, Phase I). The measurements at steady state conditions show significant improvements on heat output and coefficient of performance for the refrigerants R22 and R407C. In phase 2 of this project the tests are focused on a new defrosting scheme using energy stored at low exergy in the economiser. This method is compared to the reversed cycle defrosting at critical conditions A2/W50/93r.h. Both defrosting methods have equivalent performance reduction. Relative heat output reduction is -12% COP is reduced by -4% comparing to the steady state conditions (evaporator not frosted). Combining the two methods, reversed cycle and economiser, the overall performances are further improved (heat output -7% and COP -3%). The complete cycle length was limited by oil migration effects. The mechanisms of oil migration in two-stage heat pumps will be examined in a starting project at the Laboratory for Industrial Energy Systems (LENI). Several types of refrigerants are compared in the single-stage and two-stage case by simulation. The simulation includes real compressor and exchanger characteristics. The results show the important decrease of the coefficient of performance and the reduction of the heat output at low air temperatures. At higher temperatures the single stage mode is more adapted for the existing compressors. COP crossing occurs between 0°C and 5°C external air temperature. For the retrofit application, traditional heat pump design cannot be explored efficiently in the case of monovalent heating. This two stage concept is very promising, but oil migration problems have to be solved for commercialisation. The refrigerant cycle can be simplified, according to the refrigerant type used. The economiser did not show the expected improvement for the defrosting cycle. The major part of the energy potential could not be explored in the tests. For an air-air heat pump the increased refrigerant charge can be justified with the important improvements on the defrosting (only hot gas defrosting can be implemented in this case).

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

Potential of refrigerant based district heating and cooling networks

Samuel Henchoz

Urban areas represent an ever increasing challenge in terms of energy use and environmental impact associated with it. Projections from the United Nation expect this number to reach 66% by 2050. In Europe 40% of the final energy consumption and 36% of the emissions of greenhouse gases are caused by buildings. The building sector is also one of the area where there is a great potential of reduction of the greenhouse gases emissions and of the dependence on fossil fuels. Among the measures that can help realise this potential, efficient energy conversion technologies supplying thermal energy services could play an important role. The present thesis aims at demonstrating the potential of a new type of district network, capable of delivering indistinctly cooling and heating services using a two-pipe network and in which the transfer of energy across the network is done by exploiting the evaporation/condensation of a refrigerant. The main research question is: "Is it possible to build and operate safe, reliable, energy efficient and economically profitable district heating and cooling networks that use a refrigerant as a transfer fluid?" The demonstration followsthree axes, the first being a thermoeconomic analysis. Thisanalysis focuses on a test case area in Genevaâs city centre where 5 variants of refrigerant based district heating and cooling networks, one cold water network and the mix of conversion technologies currently in use are compared on the basis of their energy and exergy performances and on the economic profitability. Considerations on economic uncertainty, safety and technical issues are also included in the analysis. The key findings are: â¢ All the variants of network can potentially reduce the final energy consumption of over 80% as compared to the current situation. â¢ All the variants of network have rather similar exergy efficiencies comprised between 39.5% and 45%. â¢ The most profitable variant uses CO2 as a transfer fluid and an open cycle CO2 heat pump at the central plant. It costs initially between 27 and 35 mio â¬ , reaches break-even in 4 to 6 years and the net present value after 40 years is comprised between 40 and 80 mio â¬. â¢ A cold water network is the second best option, although more expensive initially and thus less profitable, it has several advantages in terms of safety and availability of components. â¢ The CO2 variants exhibit a much better compactness than the cold water network. The second axis is the design, construction and testing of a lab scale refrigerant network. First a description of the design process and of the test facility is provided. It is followed by a presentation of the results of the test campaign. The tests aimed at demonstrating the practical feasibility of the concept, mostly by assessing the controllability of the network. Overall the good behaviour of the test facility and its ability to be smoothly and automatically controlled could be demonstrated, which further improved confidence in the practicality of the concept. The third axis is the development of dynamic models of simulation. These models are described in the present manuscript. They include, heat exchangers, pipes, pumps and valves. A short comparison between experimental and simulation results is also provided. The comparison between experiment and simulation showed that at their current stage of development the models cannot simulate accurately enough a refrigerant based network.

EPFL2016

Wärmepumpe mit Zwischeneinspritzung bei Scrollkompressoren.

Daniel Favrat, Michele Zehnder

2000

Potential of refrigerant based district heating and cooling networks

Samuel Henchoz

EPFL2016

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

Daniel Favrat, Michele Zehnder

2003