Abstract:
Direct steam generation coupled is a promising solar-energy technology, which can
reduce the growing dependency on fossil fuels. It has the potential to impact the
power-generation sector as well as industrial sectors where significant quantities of
process steam are required. Compared to conventional concentrated solar power
systems, which use synthetic oils or molten salts as the heat transfer fluid, direct
steam generation offers an opportunity to achieve higher steam temperatures in
the Rankine power cycle and to reduce parasitic losses, thereby enabling improved
thermal efficiencies. However, its practical implementation is associated with non-trivial
challenges, which need to be addressed before such systems can become more
economically competitive. Specifically, important thermal-energy processes take place
during flow boiling, flow condensation and thermal-energy storage, which are highly
complex, multi-scale and multi-physics in nature, and which involve phase-change,
unsteady and turbulent multiphase flows in the presence of conjugate heat transfer.
This paper reviews our current understanding and ability to predict these processes,
and the knowledge that has been gained from experimental and computational
efforts in the literature. In addition to conventional steam-Rankine cycles, the
possibility of implementing organic Rankine cycle power blocks, which are relevant
to lower operating temperature conditions, are also considered. This expands the
focus beyond water as the working fluid, to include refrigerants also. In general,
significant progress has been achieved in this space, yet there remain challenges
in our capability to design and to operate high-performance and low-cost systems
effectively and with confidence. Of interest are the flow regimes, heat transfer
coefficients and pressure drops that are experienced during the thermal processes
present in direct steam generation systems, including those occurring in the solar collectors, evaporators, condensers and relevant energy storage schemes during
thermal charging and discharging. A brief overview of some energy storage options are
also presented to motivate the inclusion of thermal energy storage into direct steam
generation systems.
