c. Finally, the solar receiver was replaced by a two-tank moltensalt
storage system as the primary source of heat input (Fig. 3)
with HITEC molten-salt [21] as the storage medium. The high
temperature of the storage was fixed at 430 C (assumed to be
20 C below the turbine inlet temperature since the storage was
supposed to be charged by the vapour generated in the solar
receiver itself). Similarly, it was also assumed that the moltensalt
can only heat the working fluid up to 410 C (again 20 C
below the molten-salt hot temperature), and an additional
external heater (e.g. a fossil fuel or biomass fired burner) is
employed to heat the working fluid to 450 C, the turbine inlet
temperature, so that the power cycle always operates at the
design point. The cold temperature of the storage was then
calculated so as to maintain the pinch point condition for the
storage heat exchanger. It was assumed that the cold temperature
of the molten-salt (HITEC) cannot go below 180 C to avoid
freezing of the salt which occurs at 142 C [21]. The specific heat
capacity of HITEC was assumed to be equal to 0.373 cal/(g C)
[21] or 1567.1 J/(kg K). The molten-salt mass flow rate required
by the cycles was then evaluated for different turbine inlet
pressures and ammonia mass fractions. Only the discharge cycle
for the storage system was considered in the current study. This
part of the analysis was modelled using MATLAB (R2013a) as
Aspen Plus does not include HITEC in its component database.
Also, as MATLAB has a very good interface with REFPROP [22],
the thermophysical properties of the working fluid for this part
of the analysis were calculated using REFPROP (v9.0).
The following assumptions were made for the analysis:
a. Since very few central receiver STPPs with DSG are currently
operative [23], and very little operational data is available in
open literature regarding these plants, the solar receiver was
assumed to be similar to the one used in the PS10 STPP. Similar
to the PS10 plant, the receiver efficiency was assumed to be 90.2
% (an annual average value, including the heat losses to the
environment), and the peak design value of solar irradiance on
the receiver to be 0.65 MW/m2 [24]. However, the area of the
receiver was assumed as 42.65 m2 so that the heat input to the
working fluid became 25 MW for all the cases. It was also
assumed that the irradiance is evenly distributed over the
receiver area and that there were no pressure losses.
b. Pressure drops and heat losses were neglected in the other
components of both the cycles.
c. The recuperators in the KC had a PPTD of at least 5 C. The
condensers in both the cycles had a PPTD of at least 4 C. When
using the molten-salt storage system instead of the solar
receiver