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The heat transfer performance of a high-density polyethylene helical coil heat exchanger (HCHE) which is adopted by a seawater-source heat pump system (SWHP) is experimentally and numerically studied. The effects of seawater flow rate on heat transfer performance of the HCHE are investigated and the correlation between Nusselt and Reynolds number is fitted by the experimental results of cooling condition. The fitted results are compared with other similar studies of helical heat exchangers. Mathematical models in cooling condition, non-icy condition and icy condition are developed with the calculated external convective heat transfer coefficient. The simulation results are verified by the experimental data of icy condition and they show good agreement with each other. The effects of different key parameters on the heat transfer performance of the HCHE are investigated, such as inlet temperature, velocity of intermediate medium, pipe length and diameter, temperature of seawater and icing outside the pipes. The results indicates that the consideration of seawater flow rate in the study of subsea heat exchangers is much closer to the reality and the development of the mathematical models is very helpful in the designing and optimizing of the subsea heat exchangers used for the SWHP system. [ABSTRACT FROM AUTHOR]

• An unsteady state model of seawater heat exchanger with wave theory is established. • Thermal performance of the heat exchanger under icing condition is investigated. • Heating performance of SWHP system at various seawater temperature is analyzed. • A macro spatial analysis of SWHP in numerous coastal cities is performed. Seawater-source heat pump (SWHP) system exhibits merits of building energy conservation and reducing carbon emissions, while enhancing the heat transfer of seawater heat exchangers is the key to improve the system performance. Here, a model of SWHP system with staggered tube-bundle heat exchanger is established and its performance in winter is numerically studied and verified by experiments. The wave theory is introduced into the mathematical model to describe the flow field of actual ocean and improve the accuracy. The heat transfer performance of staggered tube-bundle exchanger is investigated under icing conditions, and the results indicate that the ice layer thermal resistance outside the tube accounts for more than 28% of the total thermal resistance. The impacts of seawater temperature on the SWHP system performance are discussed and the average increment of heating capacity and COP is 0.12 kW and 0.046 respectively with the increase of seawater temperature by 1 °C. Furthermore, a spatial analysis of SWHP system in 29 coastal cities of China are analyzed and compared with air-source heat pump (ASHP) system. The results show that SWHP system has greater advantages than ASHP system, especially in the southern cities. This research comprehensively analyzes the technology, economy and geography of SWHP system from a more macro perspective, and it can promote the application of ocean thermal energy. [ABSTRACT FROM AUTHOR]

In this paper, the performance of a humidification-dehumidification (HDH) desalination system with heat pump unit is experimentally investigated. The newly developed HDH system is comprised of humidifier, heat pump unit, 1st stage dehumidifier, plate heat exchanger and 2nd stage dehumidifier. The proposed system takes advantages of heat pump unit, which transfers heat to the seawater through the plate heat exchanger and absorbs heat from air through the 2nd stage dehumidifier in the meantime. Fresh water is accumulated from both the 1st stage dehumidifier and 2nd stage dehumidifier and the productivity of the proposed system is acceptable compared with other reported HDH systems and unaffected by solar radiation. The effects of seawater temperature, air temperature and air flow rate on system productivity are investigated. Seawater temperature has a positive influence on the productivity, while the effect of air temperature is insignificant. As the air flow rate increases, the productivity increases to the maximum value at a certain air flow rate and then decreases as the air flow rate still increases. The maximum productivity of the proposed system and the estimated cost of the fresh water are 22.26 kg/h and 0.051 USD/kg respectively, which are superior compared with other HDH systems. [ABSTRACT FROM AUTHOR]

• A novel heat exchanger with unequally-pitch arrangement is proposed. • The thermal, hydraulic and thermal-hydraulic performance are analyzed. • The novel heat exchanger is superior to that with equally-pitch arrangement. A novel staggered tube-bundle seawater heat exchanger with unequally-pitch arrangement, which the transversal and longitudinal tube pitches are inhomogeneous, is firstly proposed and applied in the closed-loop seawater source heat pump system. An experimental system that can induce the regular wave to simulate the seawater flow is established to investigate the thermal resistance ratio, overall heat transfer coefficient, thermal efficiency of heat exchanger under various wave parameters. The thermal performance, hydraulic performance, and comprehensive thermal-hydraulic performance of heat exchangers with unequally-pitch arrangement and equally-pitch arrangement are compared. The results indicate that the unequally-pitch arrangement of tube-bundle heat exchanger can significantly enhance the overall heat transfer coefficient and thermal efficiency, meanwhile decrease the pressure drop, and consequently improve the comprehensive performance. The present study is conducive to improve the performance potential of staggered tube-bundle seawater heat exchanger and promote the exploitation of ocean thermal energy. [ABSTRACT FROM AUTHOR]

• An unsteady mathematical model was developed based on wave theory. • The model was validated by the tested results. • Effects of key parameters on the thermal performance were studied. Seawater heat pump system exhibits merits of reducing building energy consumption, while enhancing the heat transfer of seawater heat exchangers is the key to improve the system performance. The effect of seawater flow behavior on the thermal performance of seawater heat exchanger immersed in oscillating flow is significant but often neglected. An unsteady mathematical model was firstly developed to describe heat transfer of oscillating flow around the staggered tube-bundle heat exchanger and verified by experimental results. Effects of the structural and operational parameters, such as the tube length, heat transfer fluid velocity, wave parameters and sinking depth on the thermal performance were analyzed. It was shown that the thermal efficiency of seawater heat exchanger ascended with the increase of wave amplitude and decrease of the heat exchanger sinking depth. Additionally, the tube length and heat transfer fluid velocity could be reasonably designed to achieve the optimum value between cost and performance. The heat transfer mechanism in seawater side was predominated by forced convection to natural convection with the increasing sinking depth under various wave parameters. The proposed model is favorable to delicately characterize the heat transfer of seawater heat exchanger with the consideration of seawater flow. The results in this paper is helpful to guide the optimization on designing the staggered tube-bundle heat exchanger in oscillating flow and to promote the application of ocean thermal energy. [ABSTRACT FROM AUTHOR]

• The effects of the oscillating flow on the thermal performance were studied. • An empirical correlation with acceptable accuracy was proposed. • Comparisons of various correlations and experimental results were conducted. Owing to the growing demands to develop sustainable energy, ocean energy is attracting increasing attention for the large amount of renewable, available, and potential energy resources stored in the ocean. An experimental system was constructed to study the heat transfer characteristics of staggered tube bundle heat exchanger immersed in oscillating flow. The results revealed that the instantaneous horizontal and vertical velocities of the oscillating flow exhibited a periodic fluctuation trend. Moreover, the cycle-averaged velocity and the external heat transfer coefficients were clearly affected by the wave height, whereas the wave period had little effect. To quantitatively describe the heat convection of staggered tube bundle heat exchanger in oscillating flow, a correlation of the Nusselt number that accounted for the combined effects of the wave parameters (including the wave height and wave period), water depth, and location depth of the submersed tube bundles was proposed. Comparisons with various correlations revealed that previous correlations could not describe the external heat convection in oscillating flow accurately. The newly proposed correlation was proven to predict the external Nusselt number with an acceptable accuracy under various testing conditions, which benefitted the design of the seawater heat exchanger. [ABSTRACT FROM AUTHOR]