Exam (elaborations)

MTX 311 semester test 2 2017 memo

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Course
MTX 311 (MTX311)

Institution
University Of Pretoria (UP)

Book
Fundamentals of Thermodynamics

Thermodynamics semester test 2 MTX311 2016 worked out solutions. The memorandum has solutions are 100% correct including alternative working methods. The solutions are done in a step by step matter

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Uploaded on March 11, 2021
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Thermodynamics MTX 311

ST2 May 2017

Examiner

Dr AS Lexmond (University of Pretoria),

Dr WG Le Roux (University of Pretoria)

Instructions

1 Time: 90 min.
2 Answer all the questions
3 Full marks: 100
4 Neatness saves you marks, especially when only part of the question is correct.
5 Give the equations before you start filling in numbers; for the questions marked with an
*, also give a short but clear solution strategy
6 You are allowed to take any book and all notes with you
7 T0=25°C, P0=1 bar

fig. 1 power generation using ocean thermal energy conversion. Source:
http://en.wikipedia.org/wiki//Ocean_thermal_energy_conversion

The focus on renewable energy that we have seen over the last decade has generated lots of ideas
to produce work from free energy that is naturally available. Here is an idea to generate work from

, the temperature gradient between surface water and water at greater depths. At a few hundred
meters depth, ocean water is about 5 and 10°C warm. If we pump up this cold water, we can use it in
a power cycle to generate work.

1) What is the specific exergy of cold water of 15°C, 1 bar?
2) * Water at 5°C, 1 bar has a specific exergy of 2.95kJ/kg. What is the maximum amount of
work that we can generate from a process stream of 3.15kg/s of deep see water at 5°C , 1bar
in a reversible power cycle? (you can dump all waste heat at ambient conditions)

An example of a (not very smart) ocean thermal energy conversion process is given in fig. 1. The
inventor has forgotten a few important things. Most importantly, evaporation of the surface water
reduces the temperature in the vacuum chamber. To keep an acceptable temperature in the vacuum
chamber, most of the water has to be purged. A more complete process description is given in fig. 2.
Process flow properties are given in table 1. You can assume that the waste process steams (1C, 9,
11 and 12) are not used for anything else.

equipment
2 vacuum chamber
600W 11 3 turbine 950 W
7 condenser
10 10 vacuum pump 600 W

"forgotten" equipment
10 950 W 14 ambient water outlet pump 800W
15 condensate outlet pump 20W
16 cooling water pump 100W
3

1A 6A
9
1
2

7
1B

14 16
800W
8 13
1C 6 100W

15 20W

12

Fig. 2 more complete process flow diagram of ocean thermal energy conversion process

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