Lecture 5: Efficiency and Heat Rate in cogenerative power system

1. Types
Efficiency & Heat Rate

2. Efficiencies and Heat Rate
Turbine work Gross work
Net work

3. Heat rate (HR) reflects fuel economy & is conversion efficiency of Power Plants.
It is amount of heat added (in Btu) to produce a unit amount of work (in kWh).
HR is inversely proportional to efficiency.
There are various HRs corresponding to the work used in the denominator. e.g.
Efficiencies and Heat Rate
Net cycle HR = Rate of heat added to cycle (Btu/h) = Heat added to cycle (Btu)
Net cycle power (kW) Net cycle work (kWh)
Gross cycle HR = Rate of heat added to cycle (Btu/h)
Turbine power output (kW)
Net station HR = Rate of heat added to steam generator (Btu/h)
Net station power (kW)
Gross station HR = Rate of heat added to steam generator (Btu/h)
Gross turbine-generator power (kW)
HR is related to thermal efficiency by:
th
HR

3412

Note: 1 kWh = 3412.12 Btu
In default, HR and  refer to net station values.

4. Combined Heat and Power
Cogeneration

5. Cogeneration (or CHP)
Cogeneration uses both electricity & heat and therefore
can achieve an efficiency of up to 90%, giving energy
savings between 15-40% when compared with the
separate production of electricity from conventional
power stations and of heat from boilers. Usually
efficiency ranges from 50-70%. It is one of the most
efficient ways to use fuel and, thus, save energy costs.

6. Cogeneration (or CHP)
KEY POINTS:
•Helps reduce CO2 emissions
significantly.
• A number of different fuels &
reliable technologies can be used.
• A concurrent need for heat and
electricity indicates a suitable site.
Roughly, probably suitable where
there is a fairly constant demand for
heat for at least 4,500 hrs /yr.
• Payback period & profitability
depends crucially on the difference
between the fuel price and the sales
price for electricity.
• Global environmental concerns &
projected energy demand growth in
developing countries are likely to
improve market conditions for
cogeneration in the near future. A Simple
cogeneration plant.

7. Cogeneration plant efficiency (also called Utilization factor) is:
Cogeneration (Plant efficiency)
For separate generation of electricity and steam, the heat added per unit total energy
output is
A
s
co
Q
H
E 



h
e
e
e


)
1
( 

]
/
)
1
[(
)
/
(
1
h
e
c
e
e 





The combined efficiency for separate generation is given by:
c

………………………(1)
.………………………(2)
Cogeneration is beneficial if the efficiency of the cogeneration plant exceeds that of
separate generation.
where e = electrical fraction of total energy output =
s
H
E
E


and E = electric energy generated ; Hs = heat energy (in process steam)

8. Two types: (i) Topping cycle (ii) Bottoming cycle
Cogeneration (Types & Classification)
(i) Depending on requirements, steam is either (a) extracted (from turbine) at
intermediate stage (pass-out turbine), or (b) taken at turbine exhaust (back-
pressure turbine).
Electricity is generated in the usual manner.
(ii) Heat at the higher temp is used directly for process requirements. The low-grade
heat is then used to generate electricity. It has a combined efficiency < Eq. (1).
Cogeneration is classified according to prime mover:
• Steam turbines;
• Reciprocating engines;
• Gas turbines;
• Combined cycle.
New developments are bringing new technologies towards the market like:
• Fuel cells;
• Stirling engine;
• Micro-turbines.

9. A cogeneration plant with
Pass-out Turbine.
Cogeneration (Types)
Cogeneration plant with
Back-pressure turbine.

10. Cogeneration (Types: Another Example)
Assignment # 4
Draw the T-s diagram for this system. Clearly write any assumption
that you might make.

11. The reliability of a prime mover is a measure of its susceptibility to
unscheduled shutdown but its availability takes into account all outages.
The time period used is typically a year(8,760 hrs).
The formulae below illustrate one method of making the calculations:
% Reliability = T - (S+U) x 100
T – S
% Availability = T - (S+U) x 100
T
where S = scheduled maintenance shutdown, U = unscheduled shutdown and
T = time period when plant is required to be in service or available for
service, all in hrs/yr (usually 8760 hrs).
Cogeneration (Reliability & Availability)

12. Steam enters the turbine at 7 MPa and 500 °C. Some steam is extracted from the
turbine at 500 kPa for process heating. The remaining steam continues to expand to 5
kPa where it is condensed and then pumped to boiler pressure. At times of high
demand for process heat, some steam leaving the boiler is throttled to 500 kPa and is
routed to the process heater. The extraction fractions are adjusted so that steam leaves
the process heater as a saturated liquid at 500 kPa. The mass flow rate of steam
through the boiler is 15 kg/s. Disregarding any pressure/heat losses in the piping and
assuming an isentropic turbine and pumps, determine the
Cogeneration (Example)
(a) maximum rate at which process heat can be supplied, (b) power produced and the
utilization factor when no heat is supplied, and (c) rate of process heat supply when
10% of the steam is extracted before it enters the turbine & 70% of the steam is
extracted from the turbine at 500 kPa for process heating.