KEYWORDS: concentrating solar power, parabolic trough collector, direct steam gen-
eration, two-phase flow, techno-economic, thermo-hydraulic, non-uniform solar flux,

metal foam inserts, thermal gradients.
Solar parabolic trough collector (PTC) is the most developed and deployed concentrat-
ing solar power technologies for generation of electricity. Majority of the present day

parabolic trough thermal power plants generate electricity by employing a Rankine cy-
cle. Synthetic oils and molten salts are majorly utilised as heat transfer fluids in the

solar field. Steam required to generate power is indirectly generated by employing heat

exchangers between solar field and the power block. Whereas, in direct steam gener-
ation water is used as the working fluid in both the solar field and the power block.

Main advantages of using water as heat transfer fluid are reduced investment cost, the
elimination of high-end equipment to pump the high temperature oils or molten salts,

elimination of the solar-to-steam heat exchangers, higher achievable steam tempera-
tures and higher plant efficiency. Direct steam generation in parabolic trough collec-
tors can operate in three modes: once-through, injection and recirculation mode. The

once-through mode is simple in configuration, involves low investment costs and lower
parasitic losses. Stability and controllability of the superheated steam at the outlet of
solar field are the key issues identified with this mode. Moreover, the non-uniform solar
flux and two-phase flow can lead to thermal gradients exceeding the tolerable limits.
The thermal gradients may lead to the bending and deflection of the receiver from the
focal line resulting in lower optical efficiencies. The thermal stress generated may even
result in the breakage of glass cover enclosing the metallic absorber tube.
Techno-economic comparative study of a large-scale concentrating solar plant with

thermic fluid and direct steam generation modes is carried out. The collector mod-
ules considered for techno-economic comparative study are based on a modified ver-
sion of an indigenous parabolic trough collector module designed and developed at Indian Institute of Technology Madras. The thermodynamic performance of the con-
centrating solar plants are evaluated at nominal and full-load conditions for the compar-
ative study. For the optimal size of solar field, the power plant efficiency of the direct

steam generating plant (18.19 %) has been found to be 7.11 % higher than the oil plant
(16.98 %). Moreover, the levelized cost of electricity for direct steam generating plant
(10.05 INR/ kW h) is 16.02 % lower in comparison to oil plant (11.97 INR/ kW h).
The comparative study concludes direct steam generation as a promising option for
enhancing the overall plant efficiency and reducing the cost of electricity generation.

Reliability and effectiveness of the automatic control systems for once-through di-
rect steam generation are based on the accuracy and computational speed of thermo-
hydraulic models. Therefore, a steady state one-dimensional thermo-hydraulic model

is developed to accurately model the direct steam process in a solar parabolic trough

collector. The two phase flow in the evaporating section is analysed using two em-
pirical correlations of heat transfer and pressure drop, and a flow map integrated heat

transfer and pressure drop model. The results of the thermo-hydraulic model using the
different two-phase heat transfer and pressure drop correlations were compared with
experimental data from literature. The simulation results using the different two-phase
models were found to be satisfactory and consistent within the experimental uncertainty.
However, the flow pattern map based model predicts the different flow regimes paving

a better understanding of the two-phase flow and helps in identifying the critical sec-
tions along the collector length. Moreover, the steady-state model can be modified and

extended to a dynamic model.
The flow pattern distributions inside the absorber tube are highly influenced by incident
direct normal irradiance, inlet temperature and inlet pressure conditions. Sensitivity
study can provide an insight into the influence of inlet conditions on the flow pattern

distribution along the direct steam generating collector loop. Sensitivity study for satu-
rated and superheated steam generation in PTC using the thermo-hydraulic model based

on the flow pattern map is carried out. Insignificant variations in the flow pattern map
are found for variation in the inlet temperature and incident direct normal irradiance
for both saturation and superheated steam generation. However, lower inlet pressures
are found favourable for increased annular flow regime along the superheated steam
generating collector loop. A drift flux model is developed to mimic the behaviour of direct steam generation pro-
cess in solar parabolic trough collector. The drift flux model presents the stability of a

single phase model and also reduces the computational cost involved. The numerical
results of fluid temperature, pressure drop and maximum circumferential temperature

difference along the loop length are compared with the experimental data from litera-
ture. The numerical results are found to be consistent and in good agreement with the

experimental data. Further, the drift flux model is utilised to study the influence of direct
normal irradiance and loop length on thermal gradients along the loop. Major results of
the study illustrate that the superheated section is prone to higher thermal gradients and
is at a potential risk of thermal damage.
Numerical investigations are carried to evaluate the performance of receiver tube with
metal foam inserts subjected to non-uniform solar flux. Configurations of absorber tube

with circumferential metal foam inserts in the lower side of the absorber tube with vary-
ing radial thickness are considered for the numerical analysis. Moreover, metal foam

inserts with a novel shape based on the non-uniform flux distribution around the lower
half of the absorber tube are also analysed. The maximum circumferential temperature

difference is found to reduce by 47 % to 72 % for the range of operating conditions con-
sidered in the study. Maximum enhancement of 5 and 10 times is found in the Nusselt

number and performance evaluation criteria respectively. Whereas, maximum gain of
3.71 % in the net energy efficiency and 2.32 % in exergy efficiency of the collector are
found for the metal foam inserted absorber tube configurations.