تجزیه و تحلیل عملکرد از چرخه های GAX ترکیبی پیشرفته

The objectives of this paper are to develop advanced hybrid GAX cycles (HGAX) using NH3–H2O by combining absorption and vapor compression cycles, and to perform parametric analysis of system pressures and component sizes for performance enhancement. Four different HGAX cycles are developed—Type A (Performance improvement), Type B (Low temperature applications) Type C (Reduction of desorption temperature) and Type D (Hot water temperature applications). A compressor is placed between the evaporator and the absorber in Type A and Type B, and placed between the desorber and the condenser in Type C and Type D. It is found that the COP can be improved by 24% compared with the standard GAX cycle (in Type A) and the evaporation temperature of as low as −80 °C can be obtained from the HGAX cycle (Type B). In Type C, the maximum desorption temperature can be reduced down to 164 °C. Therefore, the corrosion problem, which becomes severe at higher temperature 200 °C, can be completely removed. The maximum desorption temperature for the standard GAX cycle ranges 190–200 °C. In Type D, the hot water temperature of as high as 106 °C could be obtained. Therefore, Type D can be applied for space heating and panel or floor heating applications.

The internal heat exchange due to the temperature glide of NH3–H2O mixture provides the fundamental basis for the Generator Absorber heat eXchange (GAX) cycle [1]. The GAX cycle essentially appears to be a single stage configuration. However, it provides a higher coefficient of performance (COP) than any other single effect cycle due to the temperature overlap between the generator and the absorber. Fig. 1 shows the fundamental concept of the GAX cycle–temperature overlap. The dotted lines represent the single stage cycle of the low pressure side while the solid lines represent the GAX cycle. As the absorber pressure increases, the corresponding absorber temperature increases in the ammonia–water absorption cycles. The temperature ranges partially overlap between the absorber and generator as the absorber exit temperature increases. The “overlapped” heat is internally transferred from the absorber to the generator leading to a higher COP. This overlapped heat is an attractive characteristic of the GAX cycle using NH3–H2O, which can not be realized in LiBr–H2O absorption systems. The term “generator” is replaced by “desorber” from now since the term “desorber” is academically proper for a binary mixture such as NH3–H2O. Many papers have been found on the standard GAX cycle [2], [3], [4], [5], [6] and [7]. Recently, the GAX cycle is adopted in many applications such as space heating, space cooling and refrigeration. It can be also combined with a vapor compression process to obtain a higher COP or to obtain a lower refrigeration temperature. This cycle was called GAX hybrid cycle [8]. Kang et al. [9] developed an advanced GAX cycle for utilization of waste heat which was called WGAX cycle. They reported that the generator outlet temperature could be reduced to 172 °C with a higher COP of the WGAX cycle than that of the standard GAX (SGAX) cycle. They presented that the corrosion problem in the standard GAX cycle at higher Td than 200 °C could be solved by adopting the WGAX cycles with a comparable COP. Kang et al. [10] developed an environmentally friendly GAX cycle using NH3–H2O for panel heating applications which was called the PGAX cycle. For space heating applications, 45 °C of coolant is high enough since room temperature is typically about 26 °C. However, 65 °C of coolant is required for panel heating in which the coolant flows through pipes under the wall or floor. The PGAX cycle can be operated in three different modes with just one-hardware—cooling, space heating and panel heating applications. Full-size image (6 K) Fig. 1. Concept of the GAX cycle. Figure options The objectives of this paper are to develop the advanced GAX cycles named Hybrid GAX (HGAX) cycles, and to study the effect of key parameters on the cycle performance and the evaporation temperature. Four different advanced HGAX cycles are developed—Type A (Performance improvement), Type B (Low temperature applications) Type C (Reduction of desorption temperature) and Type D (Hot water temperature applications). A compressor is placed between the evaporator and the absorber in Type A and Type B, and placed between the desorber and the condenser in Type C and Type D. In Type A and Type B, the evaporator pressure and the absorber pressure are controlled according to its application purpose. In Type C and Type D, the condenser pressure and the generator pressure are controlled according to its application purpose.

This study developed four different advanced HGAX cycles by controlling the pressures of evaporator, absorber, condenser and desorber depending on its applications—performance improvement, low temperature applications, reduction of desorption temperature and hot water temperature applications. The following conclusions are drawn from the present study. 1. It is found that the COPc in the HGAX-Type A increases as high as 1.24 by controlling the absorber pressure, which is about 24% higher than the COPc in the standard GAX cycle with the same thermal conditions. 2. In HGAX-Type B, the evaporation temperature and the COPc decrease with decreasing the evaporator pressure. It is found that the evaporation of −50 to −80 °C can be obtained from the HGAX-Type B with the COPc of 0.58–0.3. 3. The highest desorption temperature decreases as low as 168 °C, and therefore the corrosion problem can be completely removed by adopting the HGAX-Type C. It is also found that the COPc in the HGAX-Type C increases as high as 1.19 by controlling the desorber pressure, which is about 19% higher than the COPc in the standard GAX cycle with the same thermal conditions. 4. In HGAX-Type D, the maximum hot eater temperature of 106 °C is obtained. Some of hot water at 47 °C (at the outlet of the absorber) can be applied for space heating, and the rest with the outlet temperature of 106 °C can be utilized for panel or floor heating application.