Experimental analysis of micro-cogeneration units based on

reciprocating internal combustion engine

R. Possidente b, C. Roselli a, M. Sasso a,*, S. Sibilio b

a Dipartimento di Ingegneria, Universita degli Studi del Sannio, Piazza Roma, 82100 Benevento, Italyb DISPAMA Seconda Universita di Napoli Borgo San Lorenzo, 81031 Aversa (CE), Italy

Received 19 July 2005; received in revised form 8 January 2006; accepted 13 March 2006

Abstract

The cogeneration, or the combined production of electric and/or mechanical and thermal energy, is a well-established technology now, which

has important environmental benefits and has been noted by the European Community as one of the first elements to save primary energy, to avoid

network losses and to reduce the greenhouse gas emissions. In particular, our interest will be focused on the micro-cogeneration, MCHP (electric

power�15 kW), which represents a valid and interesting application of this technology which refers, above all, to residential and light commercial

users [M. Dentice d’Accadia, M. Sasso, S. Sibilio, Cogeneration for energy saving in household applications, in: P. Bertoldi, A. Ricci, A. de

Almeida (Eds.), Energy Efficiency in Household Appliances and Lighting, Springer, Berlin, 2001, pp. 210–221; Directive 2004/8/EC of the

European Parliament and of the Council of the 11 February 2004 on the promotion of cogeneration based on the useful heat demand in the internal

energy market and amending Directive 92/42/EEC, Official Journal of the European Union (2004)]. In particular, our work group started a R&D

programme on micro-cogeneration in 1995: a laboratory, equipped with the most common appliances (washing-machine, dishwasher, storage

water heater, . . .), has been built and some MCHP prototypes have been tested too. In this article, the results of an intense experimental activity on

three different micro-cogenerators, one of them made in Japan and in a pre-selling phase, are reported. In a previous paper a detailed analysis of the

test facility, with the description of the equipments and the data acquisition systems, can be found [M. Dentice d’Accadia, M. Sasso, S. Sibilio, R.

Vanoli, Micro-combined heat and power in residential and light commercial applications, Applied Thermal Engineering 23 (2003) 1247–1259]. A

typical 3-E (Energetic, Economic and Environmental) approach has been performed to compare the proposed energy system, MCHP, to the

conventional one based on separate ‘‘production’’. In the energetic analysis the amount of primary energy savings provided by micro-cogeneration

unit has been evaluated for different types of MCHP units and at various working conditions. Furthermore the evaluation of the equivalent CO2

emissions of the compared systems, MCHP and conventional systems, allows to calculate the MCHP potentials to reduce greenhouse gas

emissions. Finally the Simple Pay Back approach has been considered to define the economic feasibility of cogeneration in small size applications

with the varying of some economic variables (first cost, gas price, operating hours per year . . .).# 2006 Elsevier B.V. All rights reserved.

Keywords: Micro-combined heat and power (MCHP); Energetic analysis; Experimental analysis

www.elsevier.com/locate/enbuild

Energy and Buildings 38 (2006) 1417–1422

1. Introduction

The technological evolution and the changes of national and

international energy market are affecting the development and

the commercial penetration of new energy systems such as the

cogeneration, the combined ‘‘production’’ of power and heat by

a single energy source [1–3]. Particularly, our attention is

focused on micro-cogeneration (MCHP), with a supplied

electric power less than 15 kW, a typical small-scale load, such

as the domestic one. Various micro-cogeneration technologies

* Corresponding author. Tel.: +39 0824305509; fax: +39 0824325246.

E-mail address: sasso@unisannio.it (M. Sasso).

0378-7788/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.enbuild.2006.03.022

have been developed, based on different prime movers such as

reciprocating Joule-cycle engines, reciprocating internal

combustion engines, fuel cells, stirling engines [4], gas and

Rankine-cycle micro-turbines. Although some micro-cogen-

erators are already available there are some factors that limit

their application in the European market [5]. The restraints to

the diffusion of MCHP are not only of technical and economical

type, as everyone would think, but are due, at least in Italy, to an

objective difficulty in distributed electric energy ‘‘production’’:

the breaking up process of electric monopoly, though the

approval of some laws, is again too far away to be realized in

practice. The most important restraints of the European market

for micro-CHP systems are:

R. Possidente et al. / Energy and Buildings 38 (2006) 1417–14221418

Nomenclature

E power (kW)

PER primary energy ratio (%)

h efficiency (%)

PES primary energy savings (%)

DCO2 avoided equivalent CO2 emissions (%)

Subscript

p primary

el electric

th thermal

CS conventional system

AS alternative system

US end-user

PP power plant

B boiler

MCHP micro combined heat and power

� h

igh manufacturing costs,

� c

heap and efficient condensing boiler,

� u

ncertainty about future gas/electricity price ratio,

� r

eluctance of utilities,

� r

eluctance of installers,

� o

vercapacity of centralized plant,

� la

ck of public incentives,

� w

arm weather.

In this paper, the results of an intense experimental analysis

conducted in a wide range of operating conditions performed on

three different MCHP prototypes have been shown. The most

important energetic, economic and environmental impact

performance indices have been evaluated and compared to that

ones of conventional system, characterized by an electric and

thermal separate ‘‘production’’ to supply the demands of the user.

2. Comparison between the micro-cogenerators and the

reference system

Among the different MCHP technologies at the moment,

that one based on reciprocating engines seems to be the most

Fig. 1. Energy flows of alternative (MCHP) and c

interesting both referring to R&D projects and to available

system [6–11]. According to a typical 3-E (Energetic,

Economic and Environmental) approach the performances of

the alternative system, in this case a MCHP, are compared to

that ones of the conventional energy system, in this case the

electric grid and a boiler. In Fig. 1 the energy flows are shown:

alternative and conventional systems can satisfy both the

electric power, Eel;US, and the thermal power, Eth;US, require-

ments for heating and/or domestic hot water production. In the

same figure the conversion parameters and primary energy

flows are also shown.

2.1. Alternative system

The three MCHPs tested [12] are based on gas fuelled

internal combustion engine. In Table 1 the main parameters of

the alternative systems are shown. In Table 1 it is evident that

the analyzed models cover a large electric power range (1.7–

6.0 kW), typical of a lot of users: from domestic to light

commercial. The three considered models are very different, as

visible in Table 1. Prototypes #1 and #2, designed during

MCHP research and that the authors are developing, use large

diffusion and low cost components, not optimized for

cogenerative use, in order to minimize MCHP first costs.

The engines, derived by a stand-by unit, are air-cooled and

modified to use natural gas. Thermal energy is recovered from

exhaust gas and, for MCHP #1, also from air that cools the

engine and the generator. MCHP #3 is, instead, in a selling-

phase in Japan, and it uses high technological level

components, designed to cogenerative purpose with a water-

cooled engine and an exhaust gas heat exchanger. It’s equipped

with all the setting and control systems that optimize its use in a

lot of working conditions. The three cogenerators have been

subjected to intense experimentation in an appropriate facility

test. The most important thermodynamic properties have been

measured to evaluate the energetic and mass flow rates. The

MCHPs have been tested in a wide range of working conditions

varying both electric, Eel;US, and thermal, Eth;US, energy

supplied to the end-user, as shown in Fig. 2 for each MCHP.

MCHPs #1 and #2 can cover a range of Eel typical for domestic

application (<3 kW). MCHP #1 delivers a large thermal power

onventional (power plant and boiler) systems.

R. Possidente et al. / Energy and Buildings 38 (2006) 1417–1422 1419

Fig. 3. Ratio of electric power to thermal power vs. the supplied electric power.

Table 1

Most important parameters of the alternative systems

MCHP #1 MCHP #2 MCHP #3

Displacement (cm3) 359 226 952

No. of cylinders (–) 1 1 3

Eel (kW) 3.00 1.67 6.00

Eth (kW) 10.5 4.00 13.5

Ep (kW) 18.3 10.2 22.7

hel (%) 16.4 16.3 26.5

hth (%) 57.4 38.8 59.5

PER (%) 73.8 55.1 86.0

Fig. 4. Electric efficiency vs. the supplied electric power.

due to its more efficient heat recovery system. The cogenerator

#3 works in a higher range of electric output (2–6 kW) typical

of light commercial user. As regards the thermal output it is

important to underline that, in order to simulate a typical

domestic application, the MCHP #3 has been tested not at

maximum thermal output. For instance for a Eel ¼ 6 kW the

maximum thermal output in the experimental analysis is equal

to 12.5 kW (due to different LHVof natural gas used and water

flow rate), while the nominal thermal output available by the

manufacturer data is equal to 13.5 kW. For this reason in Fig. 2

and in the following figures a dotted line is used to show the

MCHP #3 nominal performance referred to maximum thermal

output. In Fig. 3 is shown the ratio Eel=Eth as a function of the

Eel: this ratio is usually adopted to match the user loads

especially if is not possible to sell electric energy surplus. This

parameter increases with the electric output due to the hel,

increase. For MCHP #3 the measured values are in good

agreement to those (0.2–0.9) of large size cogenerator based on

the same engine (Eel > 0:1 MW). It is very important to evaluate

the hel, of the three micro-cogenerators in different working

conditions, Fig. 4. The hel, according to the larger sized CHPs,

increases with the Eel remarking the need to use for large

number of hours the MCHPs, at maximum load. In the better

conditions, MCHP #3, Eel ¼ 6 kW, hel, is equal to 27%, very

Fig. 2. Thermal power at different supplied electric power.

close to the performances of larger size cogenerators

(30 � hel � 40%). In order to evaluate the potential of primary

energy savings, it is interesting to evaluate the primary energy

ratio (PER), defined as the ratio of useful (thermal + electric)

energy to primary input energy, related to the fuel consumption.

In Fig. 5 the PER of the three MCHPs is reported as a function

of the electric output. In particular PER increases with the Eel.

Also for low cost equipments, MCHP #1 and #2, the

optimization of the thermal energy recovery system allows

to reach satisfactory PER (MCHP #1, 66 � PER � 73%).

MCHP #3 in the test conditions reaches PER values of 82%; in

the nominal conditions maximum PER is about 86%,1 in good

agreement to the performances of larger size CHP. Finally, in

order to remark the optimal operating conditions, Eth and Eel, in

Fig. 6 are reported the iso-PER curves for MCHP #3. This graph

is fundamental to define the domain for an efficient use of the

MCHP from an energetic point of view.

1 Nominal working condition: Tin = 333 K, Tout = 338 K, water flow =

0.645 kg/s.

R. Possidente et al. / Energy and Buildings 38 (2006) 1417–14221420

Fig. 5. PER vs. the supplied electric power.

Fig. 6. Iso-PER curves of MCHP #3 vs. the electric and thermal power.

2.2. Conventional system

The conventional system that supplies the electric and

thermal demands of the users, is based on a separate energy

‘‘production’’ of a power plant (PP) connected to the user by

the electrical grid, and of a gas fuelled boiler (B). The

efficiency reference values depend on the technology used to

produce electricity and heat. Furthermore with reference to

the analysis of the potential diffusion of cogeneration in a

geographic area the mix of energy conversion systems

adopted in that area must be considered. Therefore, as regards

the Italian situation and legislation restraints the efficiencies

of 0.39 and 0.85 are used as reference values for separate

production of electricity and heat respectively. These values

have been adopted also as for other EU countries [8] and as for

Japan [7,10]. Using these efficiency values, considered as

constant in all of the tested operative conditions

(0 � Eel � 6 kW and 2 � Eth � 12:5 kW) the PER of the

conventional system is in the range 61–69%. In order to

compare the MCHP units with the best available and

economically and justifiable technology for separate produc-

tion, a reference value of 0.55 typical of gas fuelled combined

cycle power plant has been considered too.

2.3. Energetic analysis

To compare the alternative energy system able to satisfy the

same user, it’s important to evaluate the primary energy savings

(PES) defined as (1)

PES ¼ Ep;CS � Ep;AS

Ep;CS

(1)

From Fig. 7 it is possible to remark that there is an energy

Fig. 7. PES vs. the supplied electric power.

saving (PES > 0) using MCHP #1 for electric power values

higher than 2 kW, and its maximum value is less than 10%. Is

necessary to remark that these low performances are due to the

use of cheap technologies not optimized for cogenerative use.

MCHP #3 in the test conditions performs a saving of primary

energy as regards the conventional system for an electric power

higher than 3 kW. In the best conditions PES is about 25%.

Using the MCHP #3 cogenerator at its maximum thermal

capacity (nominal) the PES is always greater than 0. If the Eel is

equal to 6 kW the energy savings of MCHP #3 is reduced to

14% if compared to the best available reference system and

economically justifiable technology for separate production of

electricity based on gas fuelled combined cycle with electrical

efficiency of 55%.

2.4. Environmental impact analysis

The environmental impact is really important by choosing a

technology and a simplified approach is based on the evaluation

of the emissions of equivalent CO2 of the compared energy

systems. In the following, CO2 emissions in the power plant for

an electrical unit converted are assumed equal to 0.7 kg/kW h

and the equivalent CO2 emissions due to the fuel, natural gas,

burnt into the boiler and MCHP are equal to 0.2 kg/kW h. Also

in this case the equivalent CO2 emissions due to electricity

conversion are typical to a mix of technologies adopted in the

R. Possidente et al. / Energy and Buildings 38 (2006) 1417–1422 1421

Fig. 8. DCO2 trend vs. the supplied electric power.

Fig. 9. SPB vs. operating hours per year of the MCHP.

Italian geographic area. Comparable values are adopted in

similar analysis performed in other countries [8,9,11]. Also in

this case, in order to compare the cogeneration units with the

best available reference system, a value of 0.4 kg CO2/kW h

has been considered. The parameter that will be used is the CO2

avoided given by (2)

DCO2 ¼CO2;CS � CO2;AS

CO2;CS

(2)

From Fig. 8 it is possible to remark that the CO2 emissions

Fig. 10. SPB vs. operating hours per year of the MCHP with economic

supports.

are lower than the conventional system (DCO2 > 0) for an

electric power higher than 1 kW. In the best condition,

Eel ¼ 3 kW, MCHP #1 allows a reduction of greenhouse gas

of about 20%. MCHP #3 has always a lower environmental

impact than the conventional system, reaching also 40% of the

avoided emissions. At nominal operation mode the MCHP #3

avoided emissions are always greater than 20%. The DCO2 is

equal to 17% if the MCHP #3 is compared at Eel equal to 6 kW

to a benchmark situation of CO2 emissions of 0.4 kg/kW h. The

analysis of these trends is important to support this technology

which could give, in a brief period of time, benefits in terms of

polluter emissions. In fact, a massive use of this technology

could contribute in a very strong way to the emissions

reduction, according to national and international restraints of

greenhouse effect.

2.5. Economic analysis

Finally, to complete the analysis of small-scale cogenera-

tion, it is necessary the evaluation of economic performance

indices. In fact, aiming at a large diffusion of CHP technology,

characterized by energetic and environmental advantages, it is

necessary a first cost able to allow a short pay back period.

However, the number of the parameters to consider does not

permit to obtain homogeneous results depending, above all, by

the different conditions in the various countries. Therefore

there are a great number of subjects involved in the definition of

the economic variables including the institutional sectors and

the private sectors (gas utilities, manufacturers, . . .). For

example, the possibility to obtain funds as well as to sell the

electric surplus to the grid at good price, could strongly

contribute to CHP market penetration. However, in the

following, in order to give general indications, the simple

pay back (SPB) of MCHP #1 and #3 will be evaluated. MCHP

#1, derived from mass produced equipments, is very cheap, so

its first cost can be estimated of about lower than 1000 s/kWel.

Whereas the model #3, not yet sold in Europe, the first cost is of

about 2500 s/kWel, evidently too high, compared to the market

cogeneration standards. According to the Italian market, an

electric energy price of 0.15 s/kW h and a natural gas price of

0.50 s/S m3 have been assumed. In Fig. 9, the SPB at different

number of working hours per year is shown. It is evident that

only very peculiar conditions, characterized by an intensive

use of CHP, allows acceptable SPB. Then, the SPB has been

estimated in presence of economic action to support this

R. Possidente et al. / Energy and Buildings 38 (2006) 1417–14221422

technology. So, it has been considered a contribution equal to

30% on the first cost (by Institutions, manufacturers, gas

utilities, . . .) and a reduction of the natural gas price for

cogenerative use (0.35 s/S m3). The results (Fig. 10) seems to

be very promising for MCHP #1 while the SPB of the MCHP #3

is too high also in these conditions. It is evident that the greatest

obstacle to the diffusion of small size cogeneration is the first

cost, too high, in comparison to similar technologies developed

for higher sizes.

3. Conclusions

In this paper it has been analyzed the possibility of a small-

scale cogeneration development valuing the Energetic,

Economic and Environmental impact implications (3-E

approach). Attention has been paid to the use of this well-

established technology that has been noted by European

community as one of the first elements to save primary energy

and to reduce the greenhouse gas emissions, also for

residential and light commercial users. In the paper the

results of an intense experimental analysis in a wide range of

operative conditions performed on three MCHP units, both

different for engineering designing and for first cost, have

been reported. The most important energetic, economic and

environmental impact performance indices have been

evaluated and compared to the same parameters of the

conventional system for separate production of heat and

electricity. The results obtained are encouraging, in fact the

micro-cogeneration already allows to obtain primary energy

savings up to 25% and a polluter emissions reduction up to

40%. As regards the economic analysis it is evident that the

greatest obstacle to diffusion of small-scale cogeneration is

its first cost in comparison to similar technologies developed

for higher sizes. It can be finally stated that the cogeneration

represents a mature technology also in small size applica-

tions: MCHP units based on reciprocating internal combus-

tion engine are at the moment available on the market and

they allow to reduce primary energy consumption and

polluter emissions in comparison to conventional systems. In

EU countries (UK and Germany) and in Japan economic

actions to support this technology are allowing an encoura-

ging diffusion of MCHP units.

Acknowledgements

This work was supported by a grant from Regione Campania

by means of Legge 41/1994; many thanks to Napoletanagas

Clienti Spa, Tecnocasa srl and Bruno srl.

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