# 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.