Implementation of a scroll booster compressor into a single-stage air-water heat pump for peak winter day

This paper shows an implementation of a scroll booster compressor mounted at the suction line of a single-stage air-water heat pump. The compressor selected to be used as booster is a beltdriven scroll compressor initially dedicated to automotive refrigeration. For the tests reported here, it is driven by an electrical motor and has been modified to obtain a lower built-in volume ratio. The computations for those modifications have been made with our own software called GeoScroll. Tests of the heat pump at A-11/W65 show a large improvement of the heat output, which is doubled (raising from 7.39 kW, in the normal single-stage configuration, up to 15.7 kW) while the COP decreases by less than 10% (from 2.08 to 1.89). This performance degradation is not due to bad performances of the booster compressor, that are at the level expected, but to a decrease of the evaporation pressure (1 bar, which is equivalent to 11 K), as the evaporator has not been designed for such heating powers. Even with working conditions outside its design range, the heat pump behavior with the booster has been proved to be very stable (the experiments were performed over few weeks). Furthermore, no oil accumulation has been noticed in the booster compressor, which is favorable to an integration of the device into existing heat pumps, currently in service. Indeed, it can be easily included into existing single-stage heat pumps with few adaptations. The concept is attractive in comparison to backup with heating devices, used in case of low temperatures, with a far better exergy efficiency and with contribution to the reduction of the electric demand peak.

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

Integrated design, optimization and experimental investigation of a direct driven turbocompressor for domestic heat pumps

Jürg Alexander Schiffmann

Heating and cooling will more and more rely on heat pumping in order to enable a more rational use of energy. The further development of domestic and small industrial multi-stage heat pumps with enhanced performance is impaired by the presence of oil mixed with the refrigerant and its migration through the hermetic loop. Oil-free directly driven small-scale turbomachinery has been identified as a technology that will allow a significant performance surge in refrigeration and heat pump applications. A proof of concept compressor unit has been designed, built and successfully tested, demonstrating the technical feasibility of such a system. Directly driven turbocompressors are composed of several components, involving different specialization fields like compressor aerodynamics, bearing and rotordynamic design as well as the electric layout of the motor and its drive. Fragmentation is the commonly used procedure for designing complex and interdisciplinary systems. The splitting into submodules certainly simplifies the design process of the individual components, it leads, however, to conservative designs and makes it more complex to keep track of the mutual interactions between the submodules. Only an integrated and simultaneous design of the complete system ensures an optimum solution. The models of the different components have been linked together to build a global system, enabling its integrated optimization. Compared to a unit that has been designed using fragmented design, the overall efficiency could be increased by 16 points by using the proposed integrated design procedure. In order to be able to predict the system's performance, losses, critical speeds and stability margins the different components have been modeled in a modular and generic way. As the system operates with a vapor close to the saturation line, both the impeller and the gas-bearing models include real gas effects. The bearing model additionally incorporates rarefaction and clearance distortion effects resulting from centrifugal growth and thermal influence. Models for calculating the windage losses occurring in the bearings and in the gap between the rotor and the stator of the electric motor have been introduced as well. In order to enable the calculation of the whirl speeds and the corresponding stability margins, a rotordynamic model has been developed specifically for gas bearing supported rotors. The model has been extended with a forced response module that allows to predict the orbits of the rotor as a function of a given unbalance and vice-versa. A vapor phase test rig for measuring the compressor performance has been built. The oil free gas bearing supported radial turbocompressor with a tip diameter of 20 mm could be tested to speeds up to 210 krpm, reaching pressure ratios higher than 3.2 and powers of 1.7 kW. Internal isentropic compressor efficiencies in excess of 80% have been measured. A detailed comparison between the measured and the predicted compressor map shows excellent agreement.

EPFL2008

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.