Energy Department.

Faculty of Engineering, Mataria, Helwan University

Energy Conservation and Management, ENE405
‫كلية الهندسة ـ المطرية‬

Co-generation
Up to 65% of the energy potential is released
as waste heat.
 Using
the waste heat for industry, hotels, hospitals
commerce and home heating/cooling.
The term cogeneration describes the simultaneous generation of
electrical energy and usable heat from a single primary energy
source, often natural gas or biofuels. Also known as combined heat
and power (CHP),
Several cogeneration system definitions exist, but overall, the term
applies when a single fuel source produces two or more forms of
energy. Cogeneration is also sometimes called recycled energy
• When a power plant generates electricity, it produces heat. If the plant releases

that heat into the environment as exhaust, it represents a huge waste of energy.

• Most of that heat can be captured and used for other purposes. When that

repurposing of heat occurs, the power plant is working as a cogeneration system.

1- The cogeneration process can increase overall energy

efficiency, with typical systems ranging from 65 to 90 percent.

2- Electricity generated by the cogeneration plant is normally

used locally.

3- Businesses that use cogeneration can lower operational costs

and reducing greenhouse gas emissions and pollutants.

4- Transmission and distribution losses will be negligible.

5- Offers energy savings ranging between 15-40%.

6- Cogeneration engineering experience ranges from “micro”

cogeneration designs that can generate between 5-10 MW of

power to much larger cogeneration facilities.

• There are several benefits to using cogeneration. The main reasons
to use CHP are to save energy and costs by reducing fuel
consumption.

• For instance, in the U.K., existing users of CHP save 20 percent of

their energy costs.
• With CHP, when fuel energy is converted into mechanical or
electrical energy, the bulk of the heat that’s released isn’t wasted.

• Less fuel is needed to generate the same amount of useful work that
a conventional power plant would produce.
That reduced fuel use has several benefits, including:
 Lower fuel costs

 Reduced fuel storage and transportation needs

 Reduced emissions – CHP is one of the most cost-effective ways to

reduce carbon emissions

 Less wear on machinery due to reduced pollutant exposure.

Another benefit is security.

 Cogeneration is considered a secure power supply since it provides stand-
alone power that isn’t dependent on a municipal power grid.

 A business that uses cogeneration can operate off-grid or easily

supplement to meet a surge in power demands.
Industrial
1- Reduces energy cost
2- More reliable power supply
3- Improved power supply quality
National
1- Fewer electricity shortages
2- Primary fuel savings
3- Enhanced efficiency of electric utility
service
4- Improving (reduced burned fuel)
1- High initial costs

2- Operating and
maintenance costs
3- Careful design and operating optimization

4- Possibility of selling surplus power

Industrial

Cement Ceramics
Pharmaceuticals

etc.

Food processing
Paper and board
Buildings

Hospitals Swimming pools

Hotels

etc.

Airports College and schools

At the most basic level, a typical cogeneration plant has an electricity
generator (Topping cycle) and a heat-recovery system (Bottoming
cycle). Here are some basic elements of a CHP setup:

Prime movers: Converts fuel into heat and electrical energy that can be

used to generate mechanical energy. Examples of prime movers include

gas turbines and reciprocating engines

Electrical generator: Converts mechanical energy into electrical energy

Heat recovery system: Captures heat from the prime mover

Heat exchanger: Makes sure that the captured heat is put to use
 A prime mover

Steam turbine

Reciprocating engine
Gas turbine
An electricity generator
A heat recovery system
A control system
Topping cycle plants: A topping cycle system starts with electricity
generation.
Bottoming cycle plants: Generating heat is first — waste heat produces
steam that is then used to generate electricity.
Bottoming cycle plants are found in industries that use very high-
temperature furnaces. They’re less common than topping cycle plants in
part because it’s easier to sell excess electricity.

Fuel Topping cycle Power

Process heat
Process steam

Raw materials Industry Product

Waste heat

Bottoming cycle Hot water

Power Process steam
1- Topping cycles

Boiler

Combustion turbine

Reciprocating engine
Steam turbine
 2- Bottoming cycles
Organic fluid bottoming systems

Steam bottoming systems

1. Cogeneration unit(s) and associated plant.
2. Fuel supply, storage and handling.
3. Connection charges including local/national
electricity networks.
4. All associated mechanical and electrical services.
5. Any new buildings, modification to existing
buildings.
6. Operator training, first set of spare parts and any
special tools.
7. Engineering design.
• Fuel.
• Labour.
• Maintenance.
• Consumables,
e.g. lubricating oil, feed-water treatment.
• Chemicals, cooling tower dosing.
• Back-up electricity prices.
• Simple Payback
Simple Payback (SPB) is the number of years before
the savings “Payback” the investment.

SPB = INVESTMENT / NET ANNUAL SAVINGS

Example: A project has an investment of $ 100,000.

and Savings per year are $ 22,000 but added
maintenance because of the project is $ 2,000.What is
the SPB?

Net savings $ 22,000 - $ 2,000 = $ 20,000

SPB = ($ 100,000 / $ 20,000) = 5 YEARS

• Low investment cost.
• Low fuel price.
• High electricity price.
• Minimum cogeneration fuel price.
• High annual operating hours.
• High overall thermal efficiency.
• For conventional systems in which electrical and heat energy are
produced separately, total system efficiency is 55.33%; but in a
cogeneration plant with reciprocating engine, the total
efficiency is 89.92%. The electrical efficiency of the cogeneration
plant is 41.33% and heat efficiency of the plant is 48.59%
respectively

Operation scenario of the cogeneration plant is demonstrated below:

The plant is fuelled by natural gas. Gas engine provides three outputs
for heat generation as shown in Figure.
Operation of cogeneration plant
• Firstly, exhaust gas of engine at roughly 530°C enters into a hot
water boiler that is connected to the hospital boiler.

• Secondly, jacket water of the engine transfers its heat by the high
temperature exchanger to the hospital. 90°C HT exchanger input
transfers its energy to hospital and cools down the engine by
completing the circuit with a temperature of 78°C.

• Finally, after-cooler of the engine transfers its heat by the low

temperature exchanger to the hospital. 44°C LT exchanger input
transfers its energy to hospital and cools down the engine by
completing the circuit with a temperature of 40°C.
 Payback period calculation of the cogeneration plant
• A payback period calculation contains savings and expenses of the
cogeneration plant.

• Electrical power Savings. Generating electricity by cogeneration plant

reduces electrical energy consumption from the grid, thus not buying
expensive electricity from the grid is a significant saving.
• Electrical savings calculation is indicated below:

• ES is electrical savings for one year,

• OH is operating hour of cogeneration plant during a year,
• P is electrical output power produced for one hour,
• PU is the unit price of grid electricity,
• UF is utilization factor
• If we consider that cogeneration plant will operate 24 hours per day

(considering 8400 hours per year excluding failures and maintenances

(360hr, 15 days)), produce 849kW electrical output power in an hour,

utilization factor is 73.5% (average for a typical day is 624 kW) and the

unit price of grid electricity is 0.121 $/kWh, then electrical saving of the

cogeneration plant is calculated as 634,249 $/year.

• Heat savings are hot water savings, jacket water savings and
after-cooler savings. Producing hot water and warm water in the
cogeneration plant reduces heat costs of the hospital by less use
of natural gas. Heat saving calculation is illustrated below:

• HS is heat saving for one year,

• HWS is hot water boiler saving,

• JWS is jacket water saving,

• AS is after-cooler saving respectively.

 Hot water boiler saving is calculated as

• BOP is boiler output power during one hour,

• OH is operating hour of cogeneration plant during a year
• PNG is the unit price of natural gas,
• UF is utilization factor,
• LHV is lower heating value,
• η is boiler or exchanger efficiency.

If we consider that boiler heat output power is 446kW per hour, efficiency is
95.9% for boiler, utilization factor is 76% and unit price of natural gas is 0.393
$/m3, then hot water boiler saving of the cogeneration plant is calculated as
116,340 $/year.
Jacket water exchanger saving is calculated as

• JWOP is jacket water exchanger output power during one hour,

• OH is operating hour during a year

• PNG is the unit price of natural gas

• UF is utilization factor

• LHV is lower heating value

• η is exchanger efficiency.

If we consider that jacket water exchanger heat output power is 483 kW per
hour, efficiency is 90% for exchanger, utilization factor is 75.35% and unit
price of natural gas is 0.393 $/m3, then jacket water exchanger saving of the
cogeneration plant is calculated as 133,103 $/year.
After-cooler exchanger saving is calculated as

AOP is after-cooler exchanger output power during one hour.

If we consider that after-cooler exchanger heat output power is 50 kW
per hour, efficiency is 90% for exchanger, utilization factor is 79.21%
and unit price of natural gas is 0.393 $/m3, then after-cooler exchanger
saving of the cogeneration plant is calculated as 14,485 $/year.

So Heat saving is calculated from the third equation as 263,928

$/year.
Savings of the cogeneration plant is 898,177 $/year.
 Expenses.
1- Total fuel consumption of the gas engine can be calculated as given
below:

where CTF is total fuel consumption.

For 8400 operation hours, fuel consumption CF of 204.8 m3/ year and
utilization factor is 73.5%, and unit price of natural gas is 0.393 $/m3
total fuel consumption is calculated as 496,923 $/year.
2- internal electrical consumption (auxiliary equipment power
consumption)
Calculation of internal electrical consumption for auxiliary equipment
of the cogeneration plant is indicated below:

where CIE is internal electrical consumption for one year,

PAUX is electrical output power of auxiliary equipment for one hour,
and utilization factor is 73.5%.

If we consider that auxiliary equipment consume 30 kW electrical

power in an hour, and the unit price of grid electricity is 0.121 $/kWh,
then internal electrical consumption expense of the cogeneration plant
is calculated as 22,412 $/year
3- service and spare part expenses
• For cogeneration plants, service and spare part expenses are
considered as a single fee per operating hour.
• If the cogeneration plant will operate 8400 hours in a year and if
service and spare part expense is 10 $/h for one engine-generator
set, total expense is calculated as 84,000 $/year.
• The cogeneration plant operates continuously in three shifts by
three different operators and gross salary for each operator is
1,000 $/month. For one year, total operator expense is calculated
as 36,000 $/year.

• Expenses of the cogeneration plant is 639,335 $/year.

Payback period.
• Total revenue (TR) is the difference between savings and expenses
and that is 258,842 $/year.

• Investment cost for the cogeneration plant is 600,000 $. (Capital

Cost)
• Finally, payback period (Tp) can be found illustrated below:

 Due to the total revenue founded above and investment cost for the
cogeneration plant, the payback period is calculated as 2.32 year.
Example:
 A waste heat recovery system is to be used with 0.5 MW diesel

generator to produce drinking water from sea water. Estimate the

produced drinking water (L/hr) if the system configuration having

the following data: the generator having an efficiency of 80%,

intake water temperature is 27C, saturated temperature is 147C,

steam latent heat is 2000kJ/kg, waste heat boiler efficiency is 80%.