Determination of Adequate Type of Stirling Engine for Cogeneration

in Industrial Sector

Kaoutar Laazaar, Noureddine Boutammachte

and Nadia Rassai

Department of Energy, ENSAM, Moulay Ismail University, Marjane 2 B.P. 15290 Al-Mansour, Meknès, Morocco

Keywords: Stirling Engine, Cogeneration, waste heat, cement plant, CHP.

Abstract: In the present paper, a comparison study of three types of Stirling Engine (Alpha, Beta and Gamma) was

realized for cogeneration purpose in industrial sector (cement plant). The different configurations of Stirling

engine are simulated by PROSA software. Several parameters are analysed such as working gas pressure,

engine speed, hot source temperature, cold source temperature and working fluid type. The results show that

the Alpha Stirling engine is the best type for integration in cogeneration system purpose due to its high overall

efficiency and output power, high compression ratio and low thermal losses.

1 INTRODUCTION

The depletion of fossil resources and their impact

on the environment impose an energy resolution,

which necessarily translates into a wide spread

application of energy efficiency and massive use of

renewable energies. Thus for the same comfort, we

can pay less energy bill thanks to concepts optimizing

the energy consumed and thus integrating the

principle of energy efficiency. It is in this context that

cogeneration (Combined Heat and Power, CHP)

systems are integrated to improve the efficiency of

industrial production processes. The Stirling Engine

(SE) is a promising technology for cogeneration since

it is an external combustion hot air engine, i.e. the

heat required for its operation can be from multiple

sources: solar energy, biomass, geothermal energy or

even industrial heat waste.

Existing works in the literature have treated

cogeneration with the SE only in residential building,

unfortunately no work has addressed the possibility

of integration SE in industrial sector for combined

production of heat and electricity (CHP).

The SE is classified in three types: Alpha, Beta

and Gamma, and each type is characterized by its

advantages and limitations. The choice of a particular

type for cogeneration requires preliminary study to be

able to integrate it into the industry in an efficient

way.

The aim of this work is the comparison between

Stirling engine types for determination of the best

configuration that deliver high output power and

overall efficiency and so improve the performance of

cogeneration based SE system.

2 COGENERATION SYSTEM

The World Alliance for Decentralized Energy

(WADE) defines cogeneration as: «The process of

producing electricity and the useful thermal energy

(heat or cold) at high efficiency and close of the point

of use». The idea of cogeneration is based on the fact

that electricity generation releases a large amount of

heat usually dissipated in the environment. For this

reason, CHP techniques consist on recovering as

much as possible this residual and available thermal

energy.

The interest of cogeneration is the increase of

production system efficiency corresponding to a more

efficient use of primary energy resources.

Cogeneration systems can minimize energy losses,

reduce emissions and the investment price if the

system is well designed. Fig.1 shows the process of

cogeneration. CHP systems are distinguished from

traditional systems by their high overall efficiency,

Eq. (1) describes the efficiency of a traditional system

and Eq. (2) presents the overall efficiency of

cogeneration unit.

Laazaar, K., Boutammachte, N. and Rassai, N.

Determination of Adequate Type of Stirling Engine for Cogeneration in Industrial Sector.

DOI: 10.5220/0009773603690374

In Proceedings of the 1st International Conference of Computer Science and Renewable Energies (ICCSRE 2018), pages 369-374

ISBN: 978-989-758-431-2

369

output

input





(1)

output

overall

input





(2)

Where

output

is generated electrical power in

output

is generated electrical and thermal

powers in

and

input

is the fuel introduced into

the system input in

Page Setup

The paper size must be set to A4 (210x297 mm). The

document margins must be the following:

 Top: 3,3 cm;

 Bottom: 4,2 cm;

 Left: 2,6 cm;

 Right: 2,6 cm.

3 ANALYSIS OF THE STIRLING

ENGINE

3.1 Description of the Stirling Cycle

The Stirling engine (SE) is an external

combustion hot air engine, it was invented by the

Scottish clergyman and engineer Robert Stirling in

1816. The Stirling machine is a device that operates

in a closed cycle according to a thermodynamic cycle,

which in theory is described as a group of

thermodynamic processes comprising two isotherms

and two isochores. Fig.2 depicts the Pressure-Volume

(P-V) diagram of the Stirling cycle.

The area under P-V diagram presents the work

obtained from the operation of SE. As the Fig.2

shows, the cycle is divided into for processes:

- Process 1-2: Isothermal compression of

working fluid and release of heat to the

external source.

- Process 2-3: Isochoric heating given by the

regenerator to the system.

- Process 3-4: Isothermal expansion of

working fluid by introduction of the heat to

the engine from external source.

- Process 4-1: Isochoric cooling of the engine

and absorption of heat from the regenerator.

Theoretically, the efficiency of Stirling cycle is

the same as Carnot efficiency and is worth:

output

stirling

input h







(3)

Where

output

is the output work in

input

the input heat in W and

are the hot and cold

sources temperature respectively.

The output power of SE can be calculated

approximately from the Beale formula:

0.015

pfV

(4)

Where

P is the output power of SE in

p is

the mean pressure in

bar ,

f is the frequency of the

cycle in

Hz and

is the displacement of the piston

cm .

The pressure of the SE is an important parameter

that is used for calculating the engine work, it is as

follows:

exp

reg comp

Ereg C

VVV

TT T





(5)

Where

is the pressure of Stirling engine,

exp

reg

and

comp

are expansion, regenerator and

compression space volumes respectively in

m ,

T ,

reg

and

T are expansion, regenerator and

compression spaces temperature in K.

Figure 1: Cogeneration process.

Figure 2: Stirling cycle diagram.

ICCSRE 2018 - International Conference of Computer Science and Renewable Energies

370

The SE differs from traditional engines by several

advantages such:

- Multi-fuel capability.

- High efficiency.

- Low operating noise.

- Low emissions.

3.2 Classification of the Stirling Engine

As mentioned before, there are three types of SE:

Alpha, Beta and Gamma, that are different by the

geometric configuration of each type as illustrated in

Fig.3.

The Alpha configuration has two pistons in hot

and cold cylinders respectively, it is a V-shape

formed by two pistons that are joined at the same

point on a crankshaft. The working fluid is moved

between the two pistons. A heater, regenerator and

cooler are grouped in series. The Alpha type can be

joined with a configuration of multiple cylinders,

hence a high power output can be reached, which is

adaptable with motorized machines.

The Beta configuration possesses a single

cylinder containing both a power piston and a

displacer that moves the working gas between the

cold and hot ends. This configuration is used with a

rhombic drive in general in order to keep the phase

angle difference between the power and displacer

pistons, but they may be joined on a crankshaft. Beta

type has less technical problems than the other types

because the power piston is away from the hot fluid.

The Gamma configuration has two separated

cylinders like Alpha configuration, but it possesses a

piston and displacer as the same as the Beta type, and

the pistons are joined in parallel on a crankshaft.

This configuration produces lower compression ratio

as the compression space is split up between the two

cylinders, but it is mechanically simpler than the other

configurations.

3.3 Application of the Stirling Engine

for Cogeneration in Cement Plants

Cement plants are a major source of heat ejection

that is lost in the air without any exploitation. In this

study, we are interested in the recovery of thermal

losses ejected by the process of clinker (cement

component) cooling leaving the kiln of cement plants.

The hot air generated from the cooling will be

recovered using a heat exchanger to be the heat source

of the SE which will simultaneously produce

electricity and heat for cogeneration purpose.

The specifications and operating conditions of

the SE types used in this study are summarized in

Table 1.

4 RESULTS AND DISCUSSION

4.1 Effect of Pressure

The pressure parameter has an important

influence on the operation of SE since it contributes

on the displacement of the working gas.

Figure 3: Diagram of Stirling engine configurations.

Determination of Adequate Type of Stirling Engine for Cogeneration in Industrial Sector

371

Table 1: Technical specifications of Stirling engine.

Parameters Values

Pressure (bar) 350

Hot source temperature (˚C) 1110

Cold source temperature (˚C) 61

Engine speed (rpm) 1500

Matrix outer diameter (mm) 104.24

Length of the cooler pipes

(mm)

220

Number of the cooler pipes (-

)

161

Length of the heater pipes

(mm)

246.3

Number of the heater pipes (-) 80

Fig.4 shows the pressure impact on the overall

efficiency, i.e. thermal and electrical efficiencies, for

the three types of SE. It is seen that the overall

efficiency increase as the pressure increase and reach

the values of 71.27 %, 72.46 % and 72.61 % for Beta,

Gamma and Alpha types respectively for 500 bar

value. The increase of overall efficiency for all the

types is due to the fact that the area under the P-V

diagram becomes larger at higher pressure which is

equal to power output. Hence the SE will produce

more power and it efficiency will increase. The Alpha

exceeds the other types in terms of overall efficiency

because it has a high compression ratio and its

pressure drop are low.

Fig.5 illustrates the influence of pressure on the

thermal power. It is shown that Gamma configuration

can produce more thermal power compared to the

other types. This is because Gamma type has more

heat losses so the cogeneration based SE system

recovers these thermal losses to generate thermal

power. However, the Alpha type produce more

electrical power so its overall efficiency is higher than

Beta and Gamma types as it is depicted in Fig.4.

4.2 Effect of Hot Temperature

The electrical efficiency for all SE types can be

improved when the temperature of hot source rises, as

displayed in Fig.6, and reaches maximum values of

50.88 %, 41.79 % and 40.32 for Alpha, Beta and

Gamma respectively at 1500 ˚C. It is due to the

Carnot efficiency equation which reveals that the

increase of hot temperature will improve the

efficiency, hence the output power will also increase.

Figure 4: Effect of pressure on overall efficiency.

The Alpha type remains the most powerful

configuration for CHP than the others, as the volumes

of hot and cold sources are separated, therefore it does

not release a lot of thermal losses and the input heat

is used properly.

Figure 5: Effect of pressure on thermal power.

4.3 Effect of Cold Temperature

As it is depicted in Fig.7, the performance of SE

decreases with the increase of the cold source unlike

that of the hot source. It can be seen that the efficiency

increases almost linearly for the three configurations

as long as the cooling temperature decreases and

reaches the values of: 46.07 %, 37.27 % and 34.45 %

for Alpha, Beta, Gamma respectively at 0 ˚C.

ICCSRE 2018 - International Conference of Computer Science and Renewable Energies

372

Figure 6: Effect of hot temperature on electrical efficiency.

This result is theoretically explained by the fact

that it is necessary to have a large temperature

difference between the hot and cold sources of the

Stirling engine to optimize its performance.

The efficiency of the Alpha engine exceeds that

of the other types because it is characterized by its

capability to operate with a large temperature

difference, which is the case of this study.

4.4 Effect of Engine Speed

Fig.8 compare the electrical power of SE with two

working fluid (Helium and Air) at different engine

speed values. The use of Helium as working gas

improve the output power because of its low heat

capacity and high thermal conductivity unlike Air.

Figure 7: Effect of cold temperature on electrical efficiency.

Figure 8: Effect of engine speed on electrical power.

The increase of electrical power at higher speed is

due to the repetition of the P-V cycle. However, the

rise of speed decreases the efficiency because of the

increase of the engine friction, so the heat exchange

between working gas and heat source does not happen

properly.

In high engine speed values (≥ 1500 rpm), the

Alpha SE provides high electrical power thanks to its

low dead volume and low friction in working space.

4.5 Effect of Thermal Losses on SE

Performance

In Fig.9, the thermal losses of three types of SE are

given for different pressure. It is noticed that the

thermal losses increase as the pressure increase. This

is due to leakage of piston rings, heat leak in the

conduct between the working gas and the hot source

and heat losses in regenerator of the SE.

The Gamma type has great thermal losses which

is owing to the no capability of this type to operate

with large temperature difference. Therefore, a lot of

heat input will be lost.

The Alpha SE remains the type that has the

minimum of thermal losses thanks to the fact that no

mixture between hot and cold working fluid is

performed.

The numerical results of this work simulation are

given in Table 2.

Determination of Adequate Type of Stirling Engine for Cogeneration in Industrial Sector

373

Figure 9: Thermal losses depending on the engine pressure.

Table 2: Numerical results of the comparison study

Electrical power (KW) Electrical efficiency (%)

Alpha Beta Gamma Alpha Beta Gamma

45.29 43.57 44.61 40.01 33.68 30.53

5 CONCLUSIONS

In the present work, a comparison study between

three types of Stirling engine for cogeneration

purpose in industrial sector (cement plant) is carried

out.

First, the influence of engine pressure on overall

efficiency and thermal power respectively was

analysed. The results show that Alpha type provides

the best overall efficiency thanks to its large

compression ratio. However, it does not provide the

best thermal power.

It is also noticed that the increase of hot source

temperature positively varies the efficiency of all

types, as it is necessary to apply a large temperature

difference between the hot and cold sources to

improve the engine performances. The results show

that Alpha type remains the best configuration for

industrial cogeneration due to its low thermal losses,

so the input heat is well used by the system.

The SE is optimized when Helium is utilized as

working gas because it has a high heat transfer

coefficient. It was found that the Alpha type can

provide high efficiency at high speeds due to its low

dead volume, hence the Stirling cycle can be repeated

several times without lot of mechanical friction.

The Alpha Stirling engine can generate a great

power output with high overall efficiency for

industrial cogeneration (cement plant in this study),

because of several advantages including high

compression ratio, low dead volume, separation of

hot and cold working gas spaces and capability of

operation in a high temperature difference.

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