Co-ordination Action for Autonomous Desalination Units ... - ADU-RES

feed water circulating through the distillation plant. The collectors must be able to
heat the thermal fluid up to medium temperatures so that after appropriate heat
transfer, the water fed to the evaporator reaches temperatures between 70 °C and
120 °C.
The most known solar thermal distillation combination is solar Multistage Flash
(MSF) and solar Multi-Effect (ME) evaporation. From the energy point of view, the
main supply to the desalination plant is a large thermal input. Like all thermal
processes, distillation demands a high-energy input (due to the energy required for
change of phase). Besides, some auxiliary electricity is also required for pumping
(electricity could be produced via photovoltaics).
On the other hand, the solar thermal systems are so much dependent on the
radiation (day/night) that some heat storage is always required. The accumulator
may thus become the main subsystem of the plant and the adopted control
strategies become of particular importance.
For MSF evaporators, the performance ratio increases with temperature, so that high
temperatures (up to 120°C) are preferred. This in turn increases the risks of
suffering from scaling and corrosion. MED evaporators operate nowadays at lower
temperatures (around 70 °C) and those hazards are reduced.
Finally, the control of such evaporators must be very accurate, and particularly the
flash equilibration in MSF. The system is unstable in small sizes. This leads to the
use of medium and large size evaporators (thousands of m 3 /day capacity) which do
no quite fit with the sizes and capacities usually applied with renewable energies,
unless a huge solar field could be built, which in turn implies large ground surfaces.
Therefore, the combination solar thermal - distillation seems best suited for medium
and high production capacities. However research has be done also in small
capacities. An example is the MSF plant installed in 1987 in El Paso, Texas. The
combination was somewhat unusual involving a 3,355 m 2 solar pond and a
cogeneration system, producing electricity in a Rankine cycle and water in a 24
stage MSF evaporator capable to produce 19 m 3 /d.
More reference cases can be found at S. Luis de la Paz (Mexico) where a double
solar field (194 m 2 flat collectors plus 160 m 2 concentrating) provides heat for a 10
m 3 /d MSF unit, with 10 stages. The plant was commissioned in 1980. One more
WP2 ADU-RES 9