Abstract
This paper presents an analytical investigation of heat-transfer limits of a novel solar loop-heat pipe developed for space heating and domestic hot water use. In the loop-heat pipe, the condensate liquid returns to the evaporator via small specially designed holes, using a mini-channel evaporator. The study considered the commonly known heat-transfer limits of loop-heat pipes, namely, the viscous, sonic, entrainment, boiling and heat-transfer limits due to the two-phase pressure drop in the loop. The analysis considered the main factors that affect the limits in the mini-channel evaporator: the operating temperature, mini-channel aspect ratio, evaporator length, evaporator inclination angle, evaporator-to-condenser height difference and the dimension of the holes. It was found that the entrainment is the main governing limit of the system operation. With the specified loop design and operational conditions, the solar loop-heat pipe can achieve a heat-transport capacity of 725 W. The analytical model presented in this study can be used to optimise the heat-transfer capacity of the novel solar loop-heat pipe.
Highlights
Loop-heat pipes (LHPs) are two-phase heat-transfer devices that are able to transfer large amounts of heat over long distances due to a capillary or gravitational structure [1,2]
The major advantages of LHPs compared to heat pipes (HPs) are an ability to operate against gravity and a higher maximum heat-transport capacity [1]
Due to the above advantages, LHPs are ideally suitable for use in solar collection systems for hot water and space heating use, which allow heat to be collected by an evaporator outside a building, and transfer it to water flowing across the heat exchanger
Summary
Loop-heat pipes (LHPs) are two-phase heat-transfer (evaporation/condensation) devices that are able to transfer large amounts of heat over long distances due to a capillary or gravitational structure [1,2]. The major advantages of LHPs compared to heat pipes (HPs) are an ability to operate against gravity and a higher maximum heat-transport capacity [1]. The minimum value of these limits represents the maximum heat-transport capacity of the LHP system, which is the maximum amount of heat the LHP system could transfer. This maximum heat-transfer capacity represents the most significant performance characteristic of an LHP [3]. Existing solar LHPs are not technically mature; there are still opportunities to enhance their maximum heat-transport capacity and thermal performance [3]. An efficient way to enhance the thermal performance of solar LHPs could be the use of mini-channel heat pipes (MCHPs) in the evaporator
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