In this study, a novel dual-nozzle ejector enhanced vapor-compression cycle (DEVC) for solar assisted air-source heat pump systems is proposed. In DEVC, the use of the dual-nozzle ejector for recovering the expansion losses is a very promising approach to improve the cycle performance. A mathematical model of the DEVC is developed to predict its performance under specified operating conditions. The simulation results indicate that for the range of given operating conditions, the coefficient of performance (COP) and the volumetric heating capacity of the novel cycle using refrigerant R410A are theoretically improved by 4.60–34.03% and 7.81–51.95% over conventional ejector enhanced vapor-compression cycle (CEVC), respectively. The results imply that the solar-air source heat pump systems could take advantage of the best features of the DEVC. The potential use of DEVC therefore deserves further experimental validation. It is expected that this new cycle will be beneficial to developing dual-source coupled heat pump applications.
Heat pump systems can offer energy-efficient and environmentally friendly methods for various industrial, commercial and residential heating applications. During the past years, the extensive heat pump research and development have been under way in the world [1]. The most common type of heat pump systems is based on the vapor-compression cycle, which can use the solar, ambient air and geothermal energies as heat sources, respectively [2]. Lohani and Schmidt [3] modeled and compared the energy and exergy flow for a space heating system with different heat generation plants and found the ground source heat pump heating system is better than air source heat pump or conventional heating system. Choi and Chung [4] measured the performance of a heat pump unit with variation of secondary fluid flow rate and compressor speed and the results are applied to design the Ground-coupled heat pump system. Nam et al. [5] suggested an estimation method to determine the thermal and hydraulic properties of the ground and the results can be used to design the heat exchanger of energy pile system base on geotechnical investigation. However, solar, ground and air-source have different temperature level and characteristic, and the purely single source heat pump is faced with different problems when it comes to practical application for heating applications. Thus, various dual-source coupled heat pump systems have draw considerable attention and have been an interesting subject for researchers in recent years [6], [7], [8], [9], [10], [11] and [12]. Among them, solar assisted air-source heat pumps have great advantages in improving the heating performances [13]. When the solar energy is available, the heat pump systems can absorb heat from the ambient air and solar energy by two evaporators with two different evaporator temperatures simultaneously. In this case, the heat pump systems raise the thermal energy gained from two sources to a higher temperature level, and thus there are reasons for expecting higher performance for such systems than conventional air-source heat pumps. In addition, the solar-air source heat pump systems allow for efficient operation over a wider range of seasons and weather conditions, and for more hours throughout the day.
Commonly, the single air source heat pump typically operates on a conventional vapor-compression cycle (VC). To achieve high heat pump efficiency, one of various efforts needs to be devoted to improving the vapor-compression cycle performance. One way is to apply an ejector expansion device in the vapor-compression cycle. In this case, the ejector could recover the expansion losses and increase the cycle efficiency. Previous studies have focused primarily on application of the conventional ejector with one nozzle in the vapor-compression cycle for performance improvement [14], [15], [16] and [17]. Actually, the ejector can be also applied in a vapor-compression cycle based dual-source coupled heat pump system to make the heat pump operate very efficiently. For this purpose, we present a novel vapor-compression cycle with dual-nozzle ejector for dual-source coupled heat pumps, such as a solar assisted air-source heat pump [11] and [18]. In this dual-nozzle ejector enhanced vapor-compression cycle (DEVC), the ejector equipped with two nozzles has the advantage of a very efficient expansion losses recovery. Moreover, the use of dual-nozzle ejector in the cycle not only may operate the heat pump with two heat sources at the same time, but also may improve the heat pump performances. In the present work, the performances of the proposed cycle DEVC are studied with simulations. The object of this study is to present an evaluation of potential performance improvements of the DEVC for its application in solar assisted air-source heat pump systems.
This paper proposes a novel dual-nozzle ejector enhanced vapor-compression cycle for solar assisted air-source heat pump systems. In the theoretical study, a mathematical model is developed to predict the performance of the DEVC. The simulation results have indicated that the discharge temperatures of the compressor in DEVC can stay below 100 °C at all given operating conditions, and DEVC can perform well for heat pump heating applications compared to the conventional ejector enhanced vapor-compression cycle. However, the selection of proper mass flow rate allocation ratio for the DEVC in accordance with dual heat sources conditions is very important in order to maintain high cycle performance and reliability in the DEVC heat pump system. It may be concluded that solar assisted air-source heat pump systems could take advantage of the best features of the DEVC. Of course, to accomplish this, more intensive experimental study on the characteristics of dual-nozzle ejector and cycle performance is still required. In addition, the geometry design of the ejector depends on relevant operation condition and mass flow allocation ratio. Thus, proper dual-nozzle ejector geometry should be designed according to the operation condition and allocation ratio φ range in practice. And further theoretical works also need to solve this problem in next step. In general, it is expected that the new cycle presented here will be very useful to the investigators dealing with solar assisted air source heat pump systems.
