EPFL's District Heating Heat Pump - Performance Analysis for DIfferent Working Fluids

The present case study is multi-criteria performance analysis of EPFL’s heat pump for different working fluids. The heat pump is based on a 2-stage vapour compression cycle which is modelled using the software Belsim VALI for 7 low-GWP refrigerants. The simulation of the heat pump over different typical periods allowed to estimate the yearly mean COP of the heat pump. R-600a, R-600 and R-717 are the refrigerants showing the higher thermal efficiencies (COP = 5.5-5.7) and thereby also the higher environmental and economical performances. According to the TEWI indicator, the leakages of low-GWP refrigerants are negligible compared to the CO2-eq. emissions from the electricity generation. The decrease in the isentropic efficiency when the heat pump does not work at its maximum capacity strongly affect the COP. Therefore it is recommended to implement a heat storage system to increase the number of operating hours without isentropic efficiency decrease.

Evaporation du réfrigérant HFC 407C sur des tubes lisses ou à surface améliorée.

Daniel Favrat, John Richard Thome, Olivier Zürcher

The substitution of CFC refrigerants in refrigeration systems, heat pumps and organic Rankine cycles for heat recovery, requires good methods for predicting heat transfer of substitute fluids. The measurements in the LENI test facility (concentric tubes with water flowing in a countercurrent flow) with HFC 407C, HFC 134a, HCFC 123, HFC 404A and HFC/HCFC 402A provide a new data band for new refrigerants, and allow a coherent comparison with old refrigerants CFC 11, CFC 12, CFC/HCFC 502 and with existing correlations. The existing correlations were found to be inadequate. Because of this work, an improved flow pattern map and flow boiling model were developed, which resulted in a substantial progress in the accurate predict of heat transfer in plain, horizontal tubes for refigerants without oil. The Kattan et al. (1996, [6]) correlation was programmed to calculate and compare predicted heat transfer coefficients to the new HFC 407C test data. The flow pattern map proposed by Kattan et al. (1996) was also programmed and compared to flow regimes observed for HFC 407C. It predicted the HFC 407C flow pattern data quite accurately. The original objective of the HFC 407C measurements was the validation of the Kattan et al. (1996) correlation applied to a zeotropic refrigerant blend. Local flow boiling heat transfer coefficients were measured for HFC 407C evaporating inside a microfin and plain tube. In addition, microfin heat transfer augmentation relative to a plain tube was investigated. The presence of oil in the evaporator had a complex effect on heat transfer coefficients. Local flow boiling heat transfer coefficients were measured for refrigerant HFC 407C ester oil mixtures (Mobil EAL Arctic 68). A new thermodynamic approach for modeling mixtures of zeotropic refrigerant blends and lubricating oils was also developed. A very high accuracy, straight vibrating tube type of density flowmeter was used to measure oil concentrations of flowing HFC 407C oil mixtures. The test covered oil concentration from 0.5-5 wt.% oil. A method was proposed to predict thermodynamic and transport properties of the refrigerant-oil mixtures. In addition, a first empirical approach was proposed for predicting heat transfer for boiling of refrigerant-oil mixtures. Based on this work, the following phenomena important to the problem were identified that require further investigation: • oil retention in the evaporator tubes that was found and clearly proved in the present project. • the formation of foam in the HFC 407C/oil mixture (but not in the previous HFC 134a/oil tests) •the effect of oil on flow patterns, in particular stratified and stratified-wavy flows.

1996

Comparison Between a Single Stage and a Two Stage Air to Water Domestic Heat Pump with one Variable Speed Compressor.

Daniel Favrat

Replacing conventional boilers by air to water heat pumps represents a major market opportunity and a contribution to more sustainable heating approaches. However, such retrofit situation impose tough technical requirements which must be met in order to demonstrate reasonable efficiencies at high temperature lifts together with a moderate heat rate decrease at lower air temperatures. During the last decade major improvements in heat exchanger and compressor efficiencies have permitted significant COP increase in single stage domestic heat pumps, bringing the technology to a point where further progress can only be achieved with more complex cycles. Two stage cycles common in large heat pump or refrigeration systems have not been exploited so far in the domestic heat pump range, although they potentially meet some of the requirements mentioned above. This paper describes the results obtained with a laboratory prototype allowing operation with both single and two stage with economizer cycles, as well as the speed control of one of the two compressors. At A-7/W50 the two stage configuration outperformed the single stage by up to 15% in terms of COP and up to more than 70% in terms of heat supply. Exergetic efficiencies (discounted for fan power and defrosting) up to 49% at A-10/W60 have been measured.

1997

Theoretical and Experimental Study of a Thermally Driven Heat Pump Based on a Double Organic Rankine Cycle

Jonathan Demierre

Nowadays, one of the main alternatives for a more rational use of energy in heating applications is the heat pumping technologies. The market is dominated by two kinds of heat pump systems: the electrically driven vapor compression heat pumps (EHP), which are the most widely used in residential heating applications and the thermally driven heat pumps (TDHP), that are usually based on a sorption process. In this thesis, theoretical and experimental investigations of a concept of thermally driven heat pump, based on the coupling of a vapor compression heat pump cycle and an organic Rankine cycle, are presented. The system is named here as ORC-ORC. The studied concept uses a single stage centrifugal compressor directly coupled to a single stage radial inflow turbine. The shaft is rotating on gas bearings, which allows the system to be oil-free. Like most of the other TDHP's, this system has the advantage to work with a variety of fuels or heat sources like wood pellets, natural gas, solar heat, geothermal heat or waste heat. The concept studied in this work is a gas red system for space heating and domestic hot water production in small residential buildings. A systematic approach has been used to evaluate, in term of energy efficiency, the potential of ORC-ORC systems and to propose optimal design specifications at different levels of the design process. The method is based on the optimization which allows to identify the best configurations at each design step with respect to the designer choices. This approach is divided in three steps. In the first step, a model of the complete system has been developed based on a process integration approach. This step allows to quickly determine whether the system is potentially attractive or not, for given conditions, before going deeper in the design process. The results show that, in general, it is advantageous for the ORC evaporation to be supercritical. In the second step, based on the process integration results and heuristic rules, a suitable system heat exchanger network has been generated and modeled. The predicted values of the coefficient of performance (COP) at design conditions, for the different situations, are in the range 1.30 and 2.19. In a last step, the off-design characteristics of the compressor-turbine unit are considered, in order to evaluate the COP as well as the heating capacity range that can be covered with a specific compressor-turbine unit design. For this purpose, a model of the compressor-turbine unit has been developed. Experimental investigations have been carried out with R134a as working fluid. A first simple test rig has been developed to test the selected ORC evaporator in supercritical conditions. The measurements have allowed to calibrate and to validate an in-house supercritical evaporator simulation tool. An ORC-ORC prototype has been developed and built. Commercially available equipment has been used, except for the compressor-turbine unit that has been designed especially for this application. The targeted operating point, with an HP evaporation temperature of 0 °C and a condensing temperature of 35 °C, has not been reached, because of experimental difficulties which caused some delay. A maximum temperature difference of 20 °C between the HP evaporation and the condensation has been achieved, which corresponds to a compressor pressure ratio of 1.9. Measurements have been performed with compressor-turbine unit rotational speeds up to 171 krpm. The theoretical investigations shows that the proposed ORC-ORC concept is an interesting alternative of thermally actuated heating device for small residential buildings. Concerning the experimental investigations, although a number of problems have been encountered during the tests, it has been demonstrated that the proposed concept is feasible with today's technology.