Heat Pipe and Loop Heat Pipe Technologies and Their Applications in Solar Systems

Solar energy is considered as the renewable and carbon-neutral energy source of enough scale to replace fossil fuels. The direct utilisation of solar energy can be categorised into two types, i.e. photovoltaic (PV) and solar thermal. However, some disadvantages, e.g. high thermal losses, low conversion rate, still limit the widespread of the solar systems. The solar systems using the heat pipe (HP) and loop heat pipe (LHP) technologies have been developed to tackle the existing problems of the solar system. In this chapter, the working principle and classification of HPs and LHPs for use in the solar system would be comprehensively introduced. The mathematical methods related to the heat transfers limits (i.e. capillary limit, entrainment limit, viscous limit, boiling limit, sonic limit and filled liquid mass limit) and thermal balance (i.e. heat input into the evaporator, heat transportation from the evaporator to the condenser via evaporation and condensation of the heat transfer fluid in the heat pipes, and heat output from the condenser) will be presented. The research works relating the solar systems using heat pipes and loop heat pipes will also be reviewed and analysed from the aspects of characteristic performance, on-site testing, and economic and social assessment. This chapter potentially revealed the further development of HP and/or LHP for use in the solar system.