Thermodynamics MTX311 2019 worked out solutions. The memorandum has solutions are 100% correct including alternative working methods. The solutions are done in a step by step matter
The Kathu solar power plant is a 4.5km², R12 billion, trough-based concentrated solar power plant
that has just been built in the Northern Cape. At full capacity, it can produce about 100 MW in
electricity. A process flow diagram of a 100MW trough-based solar power plant is given in fig. 1. The
process consists of 2 cycles; a molten salt cycle that adsorbs sunlight and a steam cycle to generate
electricity. Using parabolic mirrors, sunlight is concentrated and adsorbed by pipes through which
the molten salt flows. A big advantage of this design is that it is relatively cheap; a big disadvantage
is that relatively large amounts of heat escape from the pipes. Another advantage of this process is
that the molten salt can be stored and used to generate steam (and thus electricity) at night or on
cloudy days. Typical process flow conditions are given in table 1.
table 1 Process flow conditions of a solar thermal power plant
nr fluid description T (°C) P (kPa) m (kg/s) H (MW) Ψ (MW)
1 molten salt solar field inlet 150 400 1100 210.48 34.85
2 molten salt solar field outlet 400 325 1100 631.27 222.69
3 molten salt steam generator inlet 375 300 1100 589.18 199.59
4 molten salt steam generator outlet 150 250 1100 210.49 34.81
5 liquid water steam generator inlet 50 1600 121 24.51 -0.46
6 steam turbine inlet 350 1600 121 380.59 126.22
7 wet steam turbine outlet 55 16 121 257.34 -1.62
8 liquid water condensed steam 48 16 121 24.31 0.29
The concentrated solar power plant has 4 main components (see fig. 1); the solar field, the steam
generator, the steam turbine and the condenser.
The solar field is effectively a huge heat exchanger that adsorbs 525MW in sunlight (so Q=525MW)
to heat up the molten salt. Note that sunlight is just heat that is being radiated from the surface of
the sun (which is 5500°C). A lot of heat is lost from the pipes in the solar field. The average surface
temperature of the pipes is 400°C.
, The steam generator is a heat exchanger that transfers heat from the molten salt cycle into the
steam cycle.
The generator produces electricity and the condenser cools down and condenses the steam coming
out of the turbine. Heat from the condenser is not being used.
4A (5) What are the irreversibility and second law efficiency of this solar thermal power plant?
4B (12) Fill in table 2: What are the irreversibility and second law efficiency of the 4 main
components?
Table 2
I (MW) η2nd
solar field
steam generator
turbine
condenser
4C (18) There are 4 causes of irreversibility in the solar field: 1) fluid friction, 2) heat loss,
3) irreversibility due to heat transfer from the sun to the wall and 4) irreversibility due to heat
transfer from the wall into the pipes. Make an estimate of these irreversibilities. To help you, an
overview of the energy flows in the solar field is given in fig. 2
sunlight in
heat loss
wall (400°C)
molten salt in molten salt out
fluid in the pipes
fig. 2 schematic of energy streams in the solar field
5 (15) A mixture of 20g/s, consisting of 20% by mass of ammonia and 80% by mass of oxygen is
cooled down from 120°C, 50 bar at constant pressure until the ammonia just starts to condense.
How much heat has to be removed in this process? Assume C p is constant.
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