Environmental Engineering Reference
In-Depth Information
There is an alternative to producing mechanical power by the above methods. The electro-
chemical cell, or fuel cell, supplied with fuel and oxidant, can convert part of the fuel energy
directly to electrical power. Operating much like a battery, electrochemical reactions at positive
and negative electrodes generate an electric current within the cell that flows from the negative to
positive terminal, as in an electric generator. These reactions are sustained by the oxidation of the
fuel, which occurs at moderate temperatures well below those encountered in mechanical engines.
Fuel cell efficiencies are about 45%.
The science of thermodynamics provides a guide for improvements in the performance of
combustion systems, but cannot substitute for the invention and ingenuity needed to bring them
about.
Problem 3.1
It takes 2.2 million tons of coal per year to fuel a 1000-MW steam electric plant that operates at a
capacity factor of 70%. If the heating value of coal is 12,000 Btu/lb, calculate the plant's thermal
efficiency and heat rate. (The latter is defined as the ratio of fuel heat in Btu to electric output
in kWh.)
Problem 3.2
Given a pressure ratio
p
2
/
35 of
the working gas fluid, calculate the ideal thermal efficiency of a Brayton cycle gas turbine plant.
Explain why real gas turbine plants achieve only 25% to 35% thermal efficiency.
p
1
=
12 across a gas turbine and a specific heat ratio
c
p
/
c
v
=
1
.
Problem 3.3
A Rankine cycle ocean thermal power plant is supplied with heat from a flow of warm ocean
surface water, which is cooled by 15
◦
C as it passes through the boiler. If the plant electrical output
is 10 MW and the thermal efficiency is 5%, calculate the volume flow rate of warm water, in m
3
/s,
supplied to the boiler.
Problem 3.4
A synthetic fuel consists of 50% by weight CO and 50% by weight H
2
. Calculate its fuel heating
value, MJ/kg fuel and MJ/kg product, and stoichiometric air-fuel ratio using the data of Table 3.1.
Problem 3.5
A combined cycle power plant has a gas turbine cycle thermodynamic efficiency of 30% and a
steam cycle efficiency of 30%. Calculate the combined cycle thermodynamic efficiency, the ratio
