آب گرم کن پمپ حرارتی میکرو کانال دی اکسید کربن دوگانه: قسمت دوم - شبیه سازی و بهینه سازی سیستم

This paper presents the development of a transcritical CO2 heat pump water heating system model incorporating analytical heat exchanger models and an empirical compressor model. This study investigated the effects of a suction line heat exchanger (SLHX) and once-through versus recirculating water heating schemes. The once-through systems outperformed the recirculating systems by 10% for the system without an SLHX and 15% with an SLHX. However, a gas cooler twice as large is required. The SLHX was shown to benefit system performance at higher evaporator temperatures with improvements of 16.5% for the once-through and 4% for the recirculating systems. This study can be used to improve the design of microchannel based transcritical CO2 heat pumps; evaluate the impact of varying water inlet temperature, desired outlet temperature and evaporation temperature on system performance; and quantify the effect of differential diurnal electricity rates on system operating costs for these different operation schemes.

Heat pump water heaters have been shown to be one of the most promising applications for using low global warming potential (GWP) CO2 as a working fluid (Kim et al., 2004 and Neksa et al., 1998). The high temperature lifts required in water heating match well with the temperature glide exhibited by the supercritical CO2 during heat rejection. The glide allows water delivery temperatures of up to 90 °C without significant degradation in system efficiency (Kim et al., 2004). Heating water to this temperature with a conventional system (e. g., R134a) can only be done by raising the compressor discharge pressure substantially to avoid temperature pinches. In conventional systems therefore, as the condenser saturation pressure increases, the available enthalpy difference across the vapor-liquid dome decreases, and the compressor pressure ratio increases, drastically reducing system efficiency. Neksa et al. (1998) developed and validated a system model assuming a tube-in-tube gas cooler in a 50 kW system, heating water from 8 to 60 °C with a COP that varied from 3.0 to 4.3. Neksa et al. (1998) demonstrated a decreasing COP with increasing water temperature at a fixed evaporator pressure. Cecchinato et al. (2005) and Rigola et al. (2005) each presented comparisons between R134a and CO2 based heat pump water heaters. Again, each researcher assumed a tube-in-tube gas cooler. Cecchinato et al. (2005) showed CO2 offers improved performance at many operating conditions. However Rigola et al. (2005) showed operating at high gas cooler water inlet temperatures can severely degrade CO2 heat pump performance. Thus, Cecchinato et al. (2005) also show that CO2 systems benefit from stratified storage tanks when used in a closed water heating loop, as the water temperature entering the heat pump is kept at a minimum. Similar observations on the impact of water inlet temperature were made in models developed by White et al., 2002 and Stene, 2005, and Sarkar et al., 2006 and Sarkar et al., 2009. Kim et al. (2005) discussed the impact of a suction line heat exchanger (SLHX) on system level performance of a CO2 heat pump for water heating. One impact of the SLHX is the increase in superheating of the refrigerant at the compressor suction port. The additional heating and pressure drop through the SLHX resulted in reduced specific volumes at the suction port, and therefore lowered the system mass flow rates. This reduction in mass flow rate would typically lead to reduced component capacities, but the relatively constant compressor work input increases the discharge temperature and enthalpy. The increased discharge temperature leads to an increase in the driving temperature difference through the gas cooler and effectively offsets the penalty due to the reduced mass flow rate. Many of the experimental facilities and simulations that have been used to explore the suitability of CO2 for water heating applications have been based upon concentric tube-in-tube heat exchangers. While these heat exchangers have a high effectiveness at long lengths, they are generally far from compact at the capacities needed for domestic water heating. Little work has been performed on the use of microchannel-based heat exchangers in these systems, especially for use in hydronically-coupled systems. Microchannel heat exchangers have been shown to reduce heat exchanger size and refrigerant charge of heat pump systems and could aid in the continued successful commercialization of CO2 water heaters. One additional means of reducing heat exchanger size that has not previously been widely investigated is through repeated recirculation of tank water at a high water flow rate through the gas cooler to meet the desired high water temperature needed for domestic purposes. Much of the prior research used an SLHX in the experimental facilities for cycle testing. Little has been reported on the direct impact of these components, although their use is common. Further research is required to analyze the potential benefit of these components and determine which system operating conditions will benefit the most from their use. Therefore, this work focuses on extending the previous work on CO2 heat pump water heaters to include compact microchannel heat exchangers. Two water heating schemes are investigated to analyze the impact on system heat exchanger size and the associated performance tradeoffs. Finally, the performance impacts of an SLHX on the heat pump water heater system are analyzed.

Analytical models of microchannel based heat exchangers and an empirical model of a reciprocating compressor developed and validated with experiments in Part I of this study were used to design a residential water heating system and develop a detailed system level model. Investigations on two system configurations, with and without an SLHX, and two water heating schemes, once-through and recirculating, were conducted. Results from a simplified thermodynamic state point model were used to define the design conditions for each heat exchanger, which were then sized using the previously developed component models and incorporated into a detailed system model. The gas cooler for the once-through system had 20 water plates; while for the recirculating system, the gas cooler had 10 plates. The evaporator of the no-SLHX system had 11 plates, while the evaporator of the SLHX system had 10 plates. The SLHX length was determined to be 0.23 m to meet the required 60% effectiveness. The detailed system model calculated gas cooler COP and water outlet temperature at a single instant under varying conditions for each water heating scheme and system configuration. The effect of the SLHX on system COP was analyzed and shown to be detrimental to performance at the −5 °C evaporator temperature, with penalties of nearly 5%. At an evaporator temperature of 25 °C, the SLHX benefited the once-through system by nearly 25% for the 35 °C gas cooler water inlet case. Addition of the SLHX to the once-through system resulted in a 23% increase in total system heat transfer area. The recirculating system also indicated an improvement with the use of the SLHX, with the 25 °C evaporator and 45 °C gas cooler inlet temperature case showing a 14% improvement in COP. The addition of the SLHX to the recirculating system resulted in an increase of 16% in total heat transfer area. Finally, an approach was developed to simulate heating a tank of water from 15 to 60 °C using the instantaneous results of the detailed model. The recirculating system required 2 to 4 water recirculations to provide the desired level of heating. The once-through, no-SLHX system outperformed the recirculating, no-SLHX system by an average of 10.2% when a 15 °C tank of water was raised to 60 °C. For the SLHX systems, the once-through system outperformed the recirculating by 15.2%. To obtain these higher performance values, a gas cooler that was twice the size of the gas cooler for the recirculating system was needed. While this study has provided insight into the benefits and tradeoffs of using an SLHX and two heating schemes, additional work can be performed to further improve the accuracy of the model or investigate different options to increase performance. The detailed model utilizes a fixed superheat and optimal pressure based on each operating condition. This implies either an adequately sized low pressure receiver, or absent that, a method of varying system volume to control superheat and high-side pressure. Developing a model that predicts how superheat and high-side pressure change in a system without a properly sized receiver at over and undercharged scenarios would yield a useful understanding of the sensitivity of the performance of the two systems and water heating strategies to refrigerant leaks. Additionally, considering advanced components such as scroll compressors or ejectors may be warranted to investigate potential system performance increases.