Yiqiang Jiang

Improving the frosting and defrosting performance of air source heat pump units: review and outlook

ABSTRACT When air source heat pump (ASHP) units operate in heating mode at low temperatures in high humidity environments, frost forms and accumulates on the surface of its outdoor coils. This frost layer adversely degrades the operating efficiency of ASHP units rapidly, and can even result in sudden shutdown. Due to this operational problem, many researchers have conducted studies on frost-retardation measures. Since the frost that is present on the ASHP unit after shutdown has to be removed, defrosting becomes necessary. Among several reported defrosting methods, such as compressor shutdown defrosting, electric heating defrosting, hot water spray defrosting and hot gas bypass defrosting, the most popular method is reverse cycle defrosting. In addition, defrosting initiation and termination control play an important role in the frosting and defrosting cycle. Due to a scarcity of reviews of the literature on improving the frosting and defrosting performance of ASHP units, this paper provides a comprehensive review of experimental and modelling studies which explore this topic. This review can be used for guiding the future design and control strategy optimisation for ASHP units.

Predictive performance of a wastewater source heat pump using artificial neural networks

A pilot-scale wastewater source heat pump was operated for 30 days to recover heat from waste bathwater and to warm up fresh bathwater. The results indicated that the fresh water successfully warmed up to the designated 45℃, 50℃, and 55℃ with the coefficients of performance of 2.3–3.5. Artificial neural networks including back propagation, radial basis function, and nonlinear autoregressive model with exogenous input were used to simulate this process. The root-mean-square error and coefficient of variation of the simulated results, using the experimental data taken on the first 18, 21, 24, and 27 days, respectively, as a package of training data, showed that taking the data measured on more days as the training data improved simulation accuracy. The nonlinear autoregressive model with exogenous input needed at least 24 days’ training data to achieve acceptable simulation results, the back propagation needed 27 days, while the radial basis function did not achieve acceptable results. Predictions based on the nonlinear autoregressive model with exogenous input modeling showed that the performance of the wastewater source heat pump system could gradually be stabilized within 42 days. Practical application: This study showed that the wastewater source heat pump can recover heat from waste bathwater to warm up fresh bathwater, and also demonstrated that the artificial neural network, especially nonlinear autoregressive model with exogenous input, is appropriate for predicting heat pump performance. Using this system can reduce building energy consumption, while using the artificial neural network model can help operate and maintain the wastewater source heat pump system.

Simulation analysis of a solar-assisted heat pump system for space heating in severe cold areas

Simulation studies were conducted on a solar-assisted heat pump system for space heating. The system has been built and used for providing heating source for an office building in severe cold areas of Daqing, China, and the parameter setting for simulation was based on an actual project condition. In this study, the system principle and operation modes were introduced. The thermal heating performance during the whole heating season was investigated and compared. In addition, the parameter changing and its effects on the system performance were further discussed. Simulation results indicated that the solar fraction were 36% and 39% respectively in December and January and could reach up to 70% or even 100% in the other heating months, with an average value of 64%. The total operation time supplied by solar energy takes a ratio of 70%, while its heating capacity accounts for 50%. The results in this study can be used as a reference and would be helpful for guiding further design and application studies for solar heating projects in the northeast region of China.

Numerical Simulation of Condensation of Upward Flow in a Vertical Pipe

A transient three-dimensional volume of fluid (VOF) simulation on condensation of upward flow of wet steam inside a 12 mm i.d. vertical pipe is presented. The effect of gravity and surface tension are taken into account. A uniform wall heat flux have been fixed as boundary conditions. The mass flux is m=130∼6000 kg m−2 s−1 and the turbulence inside the vapor phase and liquid phase have been handled by Reynolds Stress (RS) model. The vapor quality of fluid x=0∼0.4. The numerical simulation results show that in all the simulated conditions only the bubbly flow, slug flow, churn flow and annular flow are observed, in addition the results of flow pattern are in good agreement with the regime map from Hewitt and Roberts. The typical velocity field characteristic of each flow pattern and the effect of velocity field on heat transfer of condensation are analyzed. It can be found that only slug flow has an obvious local eddy around the slug gas in all simulated flow patterns. The trend of heat transfer coefficients rises throughout with the increase of vapor quality for all simulated conditions, which is good agreement with the correlation from Boyko and Kruzhilin.Copyright

Research on the aquifer temperature field of Groundwater Heat Pump with Pumping & Recharging in the Same Well

Mathematical model of pumping and recharging well (PRW) for ground water heat pump(GWHP) was developed and two operating modes(one is operating only in winter and the other is continuous operating in winter and intermittent operating in summer) of the PRW in Shenyang for a office building was simulated and studied using the developed model. Simulation results showed that, when the GWHP operated only in winter, the largest temperature drop was about 8.3°C at the end of the first heating season. The temperature recovered only 1.2°C in the transition season when the heat pump stopped running and the groundwater will freeze in 2∼3 yeas if the system continuously running in operating only in winter mode. When the GWHP system runs in continuous operating in winter and intermittent operating in summer mode, the aquifer composed of temperature drop zone and the temperature raise zone, and the largest temperature drop and raise was about 4.5°C and 3.6°C respectively at the end of first running year. With the running of the ground water heat pump, the temperature drop zone enlarged gradually, while the temperature raise zone gradually shrunk. At the end of the fifth heating season, the temperature raise zone disappeared, the temperature drop of the aquifer was 7.5°C and the influence radius of PRW was about 58 m. It can be concluded that when the ground water heat pump operates in severe cold region, where the building heating and cold load was highly unbalanced, it was necessary to take varied seasonal thermal energy storage measures.