Cogeneration or combined heat and power (CHP)

Background
·
Cogeneration or combined heat and
power (CHP) is the use of a power station to
generate electricity and useful heat at
the same time from the combustion of a fuel or an alternate thermal energy
source.
·
In separate production of electricity, some
energy must be discarded as waste heat, but in cogeneration
this thermal energy is put to use. All thermal power plants
emit heat during electricity generation, which can
be released into the surroundings through cooling
towers, flue gas, or by other means. In
contrast, CHP captures some or all of the by-product for heating,
either very close to the plant, or—especially in cold climates —as hot water
for district heating with temperatures ranging from
approximately 80 to 130 C. This is also called combined heat and power
district heating (CHPDH). Small CHP plants are an example of decentralized
energy. The process heat at moderate temperatures (100–180 C) can
also be used in absorption refrigerators for
cooling.[ref: Wikipedia].
·
With respect to desalination, cogeneration
is the process of using excess heat from electricity generation, or useful process
heat, for another task: in this case the production of potable water
from seawater or brackish groundwater in an integrated, or
“dual-purpose”, facility where a power plant provides the energy for
desalination.
·
Cogeneration takes various forms, and
theoretically any form of energy production could be used. However, the
majority of current cogeneration desalination plants use fossil
fuels andnuclear power as their source of
energy. Most desalination plants located in the Middle East or North
Africause their petroleum resources to offset limited water resources.
The advantage of dual-purpose facilities is they can be more efficient in
energy utilization, thus making desalination a more viable option for drinking
water. [ref: Wikipedia].
Project Problem Statement
·
This mini-project is to be completed in partial
fulfillment of the course “Desalination & Power, EMC-4923”; Students will
work in groups of 3 – 4, and submit a report documenting their solutions, and a
presentation of the results.
·
In this mini-project, you will consider
the study of a hypothetical cogeneration-desalination problem in order to gain
some insight into the process, the calculations involved, and the behavior of
the various outputs. This should bring
together the different fragments of the process covered in the [Desalination
and Power, EMC-4923] course at ADMC.
·
The purpose of the report is to document the
solution to the given cogeneration-desalination problem using what you learned
in class. A discussion of the results is
to be documented and presented.
·
In the project problem description statement
given below, the process heat generated is used as the heating load to a single-effect
evaporator (SEE) to generate a distillate product. The SEE process hardware is described by
figure 1a. Figure 1b shows a general
profile of the temperature variation across the hardware. Detailed description of the SEE process and
the mathematical model can be found in the class notes [EMC-4923] for learning
outcome 4.
·
Help for the treatment of a cogeneration problem
can be found in the class notes for learning outcome 5.

a.
SEE Schematic Diagram [EMC 4923]

b. Apparent temperature variation

Figure 1.
Single Stage Evaporation Process [EMC 4923]

The above SEE desalination configuration is common to all
the projects. The following set of
specifications is to be used in the solution of the SEE process:
·
The seawater temperature, Tcw, varies
over a range of 5°C to 30°C.
·
The feed water temperature, Tf, is
less than the brine boiling temperature by 4 to 15 °C.
·
The steam temperature, Ts , is higher
than the brine boiling temperature by 4 to 15°C.
·
The seawater salinity, Xf, range is
32,000 to 42,000 ppm.
·
The salinity of the rejected brine, Xb,
is 70000 ppm.
·
The boiling temperature, Tb, varies
over a range of 55 to 100 °C.
·
The heat capacity of seawater, distillate, and
reject brine are assumed constant and equal to 4.2 kJ/kg °C.
·
You must first solve the cogeneration problem,
and use the results to continue the solution of the SEE process. The connection between the two processes
(power generation and desalination) is through the process heater block (left to
students), where you will use your understanding of the individual processes
that make up the overall problem.
·
While the cogeneration process is well defined
and fixed, the desalination of seawater via SEE can be analyzed by varying some
key operating parameters (the cooling water temperature, seawater and brine
salinity, and feedwater temperature) in order to investigate the behavior of
the SEE process based on the mathematical model formulated in class. This means that you will rely on the same
formula sheets provided for SEE (LO4) and CHP (LO5) in order to develop your
solution. The analysis should shed light
at the behavior of key output and control parameters (such as the cooling water
mass flow rate, the performance ratio, and the evaporator and condenser heat
transfer areas.)
·
You may use MS Excel, Matlab, EES, or similar tools to
help you do the calculations… But I
would encourage everyone to first do the work on scratch paper, and then type
your report when done. If you need help, do not hesitate to see me immediately…
·
You will need to use the steam tables A4 – A7
(Y. Cengel’ s Thermodynamics textbook)
·
Your report should include the material
presented above to begin with. Add
sections under the headings: Calculations (for parts a, b, c, d, e, f, g, h,
i, j, k, l);Discussions (for parts a, b, c, d, e, f, g, h, i, j, k, l);
·
Marking:
10 marks each for parts a through j.
20 marks for part k. 10 marks for
part l.

P1.
Cogeneration Plant & SEE
Consider an ideal cogeneration steam plant that is to
generate electrical power and process heat to be used for desalination in a
single effect evaporation process. A
schematic of the hardware and T-s diagrams is shown in figure 2 for the
cogeneration plant. Steam enters the turbine of a cogeneration plant at 7
MPa and 500°C. One-fourth of the steam is extracted from the turbine at 600-kPa
pressure for process heating. The remaining steam continues to expand to 10
kPa. The extracted steam is then condensed and mixed with feedwater at constant
pressure and the mixture is pumped to the boiler pressure of 7 MPa. The mass
flow rate of steam through the boiler is 30 kg/s. Disregarding any pressure
drops and heat losses in the piping, and assuming the turbine and the pump to
be isentropic, determine
a. the mass flow rate and temperature of the process heater steam;
c. the process heat rate produced;
c. the net power produced by the plant;
d. the utilization efficiency of the cogeneration plant.

a. Block diagram

b. T-s diagram

Figure 2. Cogeneration plant

Based on the results of the cogeneration system, choose
proper SEE operating parametersaccording to general specifications given
earlier, and determine
e. the distillate product mass flow rate.
f. The desalination performance ratio.
g. the heat transfer area

of
the evaporator.
h. the heat transfer areas

of
the condenser.
i. the cooling seawater mass flow rate,

.
j. Discuss your results.
k. Produce plots of the above output and control parameters
against variations of the key input parameters in the proper range suggested at
the beginning of this problem statement.

l. Discuss your results…
Possibly recommend a setting for optimal operation.

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