Analysis of innovative micro-CHP systems to meet household energy demands

Enrico Saverio Barbieri, Pier Ruggero Spina, Mauro Venturini

⇑

Dipartimento di Ingegneria, Università degli Studi di Ferrara, Via G. Saragat, 1, 44122 Ferrara, Italy

a r t i c l e

i n f o

Article history:
Available online 28 December 2011

Keywords:
Combined heat and power
Building heating
Energy demand
Primary energy saving

a b s t r a c t

This paper presents an analysis aimed to evaluate the feasibility of micro-CHP systems to meet the house-
hold energy demands of single family users. The considered CHP systems, with an electric power output
of a few kW, are based on technologies (internal combustion engines, micro gas turbines, micro Rankine
cycles, Stirling engines and thermophotovoltaic generators) which are already available on the market or
will be available in the near future. Each CHP system is composed of (i) a prime mover, (ii) a thermal
energy storage unit and (iii) an auxiliary boiler used to cover peak thermal demands.

The analyses performed in this paper aim to evaluate (i) the energy performance of the CHP systems to

meet the energy demand of two single-family dwellings, with different floor areas and shape factors, and
(ii) the maximum cost allowed for each CHP system. The results presented in this paper allow the most
suitable technology, prime mover and thermal energy storage unit size to be identified.

From an energy point of view, the considered CHP systems usually satisfy most of the thermal and elec-

tric energy demand, with a primary energy saving index that is always higher than 20%. Moreover, the
correct sizing of the thermal energy storage unit capacity proves crucial. As regards economic feasibility,
it is shown that a reasonable target for the marginal cost of a CHP system for household heating is
approximately 3000 €/kW

e

, even though the absence of incentives does not make any of the technologies

very attractive at present. It is also shown that the highest profitability can be obtained by using the
prime mover (i) with the lowest electric power output closest to that of the peak electric demand and
(ii) with an electric-to-thermal ratio which fits the electric-to-thermal ratio of the users.

Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Combined heat and power (CHP) technology has been wide-

spread for many years in all activities characterized by high electric
and thermal demands which are fairly constant and predictable,
such as in the industrial sector. Since the most feasible applications
(in terms of both primary energy saving and economic profit) for
CHP in the industry scenario have already been exploited, as testi-
fied by

[1–3]

for medium and large users, today the main potential

for CHP diffusion seems to be in the residential sector. Until the
present, even though (i) the interest in domestic micro-CHP began
in the late 1980s, as shown in a pioneering study of the application
of small-scale CHP systems to building services

[4]

and (ii) it was

analytically demonstrated that distributed CHP systems increase
the site fuel energy consumption

[5]

, by reducing the primary en-

ergy consumption, several technical

[6–8]

, environmental

[9]

, eco-

nomic

[10]

and legislative

[11]

problems have curbed the spread of

CHP technology in this sector, especially for electric power sizes of
a few kW. One of the main difficulties is that the currently avail-
able CHP technologies in this range of electric power output, which

are basically internal combustion engines, do not have the charac-
teristics of high efficiency, low cost, silent operation, low pollutant
emissions and reduced maintenance, to render the CHP a real op-
tion as a substitute or as an integration of the traditional household
boiler

[6]

.

The aim of this paper is to evaluate the capability of innovative

micro-CHP systems as an alternative to traditional household boil-
ers. The considered micro-CHP systems are the ones based on mi-
cro gas turbine (MGT), micro Rankine cycle (mRC), Stirling and
thermophotovoltaic (TPV) technologies, which are already avail-
able but not yet widespread and industrialized, or available in
the near future. These technologies are also compared to the most
well-established technology for small size CHP, i.e. the internal
combustion engine (ICE). Fuel cell technology is instead not con-
sidered in this paper, due to the fact that this technology is not
likely to be available in the near future. Useful energy and opera-
tional analyses on PEM fuel cells are presented in

[12,13]

.

A literature review was preliminarily carried out in this paper to

evaluate residential building energy demands (electric, thermal
and cooling). Subsequently, for two single-family dwellings, char-
acterized by different floor areas and shape factors, a specific en-
ergy performance analysis was carried out, in order to investigate
the effects of different yearly energy demands.

0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:

10.1016/j.apenergy.2011.11.081

⇑

Corresponding author. Tel.: [phone]; fax: [phone].
E-mail address:

[e-mail]

(M. Venturini).

Applied Energy 97 (2012) 723–733

Contents lists available at

SciVerse ScienceDirect

Applied Energy

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p e n e r g y

Five CHP systems are considered in detail. Each of them is com-

posed as follows:

A prime mover (PM), based on one of the five considered tech-

nologies (ICE, MGT, mRC, Stirling, TPV). The five PMs were cho-
sen according to the fact that (i) they are already available on
the market or available in the near future, and (ii) they have
an electric power output which suits a single family user, i.e.
the electric power is up to 3 kW.

A thermal energy storage (TES) unit, which either allows the

peak thermal demands to be covered or the thermal energy to
be stored, when the produced thermal power is higher than
the actual thermal demand.

An auxiliary boiler (AB), to cover peak thermal demands, which

cannot be met by means of the PM in CHP configuration and TES
units.

The energy performance, in terms of primary energy saving, and

the maximum cost allowed for each CHP system are evaluated. The
latter analysis is required since all these technologies are not wide-
spread on the market (some of them are not currently industrial-
ized or are likely to be available in the near future) and therefore
their market price is not well-established. The maximum cost al-
lowed only considers the marginal cost of the CHP system with re-
spect to a traditional boiler, since the considered CHP systems are
intended as an alternative to traditional household boilers. Finally,
the determination of the most suitable technology, PM and TES size
is carried out. For this purpose, the influence of specific tariff con-
ditions in the present market scenario are evaluated thoroughly.

The main contribution provided by this paper is a general guide-

line to lawmakers and investors, for the identification of the

maximum allowable marginal cost for each technology, the required
target energy performance and PM optimal size. The influence anal-
ysis on costs is performed by using a parametric approach, so that
the results can be extended to different economic scenarios. In this
manner, strategies could arise from the government through legisla-
tion and incentives and from utility companies through structure
rates and selling policies. Moreover, this paper covers all micro-
CHP technologies available on the market at present or in the near
future, while most of the papers only focus on internal combustion
engines

[14]

. Finally, this paper addresses the problem of the feasi-

bility of micro-CHP systems tailored for single-family users, while
state-of-the-art literature usually covers multiple users, such as
apartment blocks, hotels or sport centers

[14,15]

.

2. Energy requirements for residential buildings

2.1. Thermal energy demand

The primary energy demand for residential building heating is

discussed in the Energy Performance of Buildings Directive (EPBD)
2002/91/EC

[16]

, which sets the basic principles and requirements

of the energy performance of buildings; the Concerted Action EPBD

[11]

provides an overview of the implementation of this EU legis-

lation and reports the required energy performance limits EP

H,W

for

EU countries. These limits are related to the thermal energy de-
mand through the overall efficiency of heat production and distri-
bution, which is the product of the efficiency of the heat generator
and the heat distribution system. The range of EP

H,W

is consider-

ably large, ranging from approximately 10 to 300 kWh/(m

2

yr),

since the actual thermal energy demand is very case-sensitive.

Nomenclature

Symbols
C

electric to thermal power (or energy) ratio

c

specific cost (€/kWh) or (€/Sm

3

), TES fluid specific heat

(kJ/(kg K))

d

discount rate (%)

E

energy (kWh)

EER

energy efficiency ratio

EP

energy performance (kWh/(m

2

yr))

F

cash flow (€/yr)

h

yearly hours of operation (h/yr)

I

investment (€)

m

NG

yearly mass of natural gas (Sm

3

/yr)

N

number of years for NPV calculation (yr)

NPV

net present value (€)

p

correction factor which accounts for grid losses and
transformation losses

P

power (kW)

PBP

payback period (yr)

PES

primary energy saving index

r

revenue (€/kWh)

S

surface (m

2

)

t

taxes on natural gas (excise) (€/Sm

3

)

V

volume (m

3

)

VAT

Value Added Tax (%)

D

T

temperature difference (K)

g

efficiency

q

TES fluid density (kg/m

3

)

Acronyms
AB

auxiliary boiler

CHP

combined heat and power

ICE

internal combustion engine

mRC

micro Rankine cycle

PM

prime mover

TES

thermal energy storage

TPV

thermophotovoltaic

Subscripts and superscripts
AB

auxiliary boiler

a&l

appliances and lighting

C

air conditioning

CHP

combined heat and power

dem

demand

e

electric

Ee

electric energy

f

fuel

h

household

H

heating

max

maximum

NG

natural gas

prod

produced

purc

purchase

re

reduction of excise

s

separate

sale

sale to grid

sc

self-consumption

stor

storage

t

thermal

trad

traditional scenario

user

user

W

domestic hot water

724

E.S. Barbieri et al. / Applied Energy 97 (2012) 723–733

2.2. Electric energy demand

The electric demands of European residential users for appli-

ances and lighting can be estimated by means of the results from
various studies

[17–19]

. In particular, the ODYSEE Project

[17]

highlighted how the electric demand per dwelling did not change
significantly in the decade 1996–2005: even though household
appliance consumption reduced, electric appliance penetration
and use increased. The same report

[17]

shows that the range of

specific electric consumption for lighting and appliances in various
EU countries is considerably large: with reference to 2004, it
ranges from 14 kWh/(m

2

yr) in Romania to 48 kWh/(m

2

yr) in Fin-

land. The mean value is 29 kWh/(m

2

yr), which is slightly higher

than 25 kWh/(m

2

yr) in Italy.

2.3. Cooling energy demands

The cooling energy demands for residential users are extremely

case specific, and, in fact, it was not possible to find data of general
validity for Europe. However, since the cooling energy demand for
residential building conditioning during summer is usually met by
means of electric air conditioners, the electric demand for cooling
can be considered included in the overall electric demands for
appliances and lighting discussed in the previous section.

3. Prime mover technologies

The considered micro-CHP systems are based on ICE, MGT, mRC,

Stirling and TPV technologies. The latter three technologies are
external combustion systems (thus, pollutant emission levels are
comparable to those of boilers), characterized by high reliability,
low noise, and potentially high values of the overall CHP efficiency.
Similarly, Monteiro et al.

[15]

analyzed three small size CHP sys-

tems (based on internal combustion engines, micro gas turbines
and Stirling engines), but they were applied to a gym with an in-
door swimming pool, which clearly requires higher thermal energy
than single-family users.

3.1. Internal combustion engines

Internal combustion engines are the most well-established

technology for small- and micro-CHP applications. For applications
that suit domestic installations, the electric efficiency ranges from
20% to 26%, with a potential CHP efficiency up to 90%

[6,20]

.

3.2. Micro gas turbines

Gas turbines are a well-established technology for micro-CHP

applications with electric power outputs higher than approxi-
mately 30 kW

[6,20]

. The major technical factors that challenge

the development of micro turbines of a few kW are related to the
small-scale effects (e.g. large fluid dynamics, heat and mechanical
percentage losses) and costs

[21,22]

. To overcome these limita-

tions, Moss et al. present in

[23]

a recuperated gas turbine, with

an electrical and thermal power of 5 kW and 8 kW respectively,
where the centrifugal compressor and turbine are replaced with
reciprocating engines. From the simulated results, an electrical
generation efficiency of 33% appeared possible. At present, a recu-
perated micro turbine prototype has been documented in

[22]

.

Experimental tests demonstrated an electric power output of
2.7 kW at 12.3% electrical efficiency; nevertheless the authors of
paper

[22]

are reasonably confident that a demonstrator rated at

3 kW with a target electric efficiency of 16% will be available soon.
Studies to increase the MGT electric efficiency focus on ceramic

materials

[24]

and hybrid power plants consisting of an MGT inte-

grated with a solid oxide fuel cell

[25,26]

.

3.3. Micro Rankine cycles

The micro-CHP systems based on Rankine cycles (which use

water or an organic fluid as the working fluid) with a power size
of up to 10 kW, which are mostly available on the market at a pro-
totype level only, have an electric power size ranging from 1 kW to
10 kW, with a corresponding thermal power size ranging from
8 kW to 44 kW. For this reason, they may represent a good alterna-
tive to household boilers. The electric efficiency ranges from 6% to
19%, with a potential overall CHP efficiency that is always higher
than 90%

[6,20,27]

.

3.4. Stirling engines

The micro-CHP Stirling systems available on the market with an

electric power size of up to 10 kW, which are mostly prototypes,
have an electric power size ranging from 1 kW to 9 kW and a cor-
responding thermal power size from 5 kW to 25 kW, which may
also represent a good alternative to household boilers. The electric
efficiency ranges from 13% to 28% with the CHP efficiency higher
than 80%, which may even go beyond 95%

[6,20,28]

.

3.5. Thermophotovoltaic (TPV) generators

TPV-based micro-CHP systems can be obtained from properly

designed boilers, which use a surface radiant burner, by placing
PV cells in front of the surface radiant burner inside the combus-
tion chamber

[6,20,29,30]

. All heat not converted into electric en-

ergy by the PV cells (such as heat removed from cells by the PV
cell cooling system) is usefully recovered. Thus, the extra-fuel sup-
plied for electric energy production with respect to the simple boi-
ler can be considered fully converted into electric energy. Although
the electric efficiency of TPV CHP systems is low (approximately 2–
5% for available prototypes and in any case less than 15%), the po-
tential CHP efficiency is always higher than 90%

[6,20]

.

Compared to other technologies, the TPV generators have no

moving parts, so that noise and vibrations are very low. However,
they still require remarkable technological development, both for
the design of the components and their integration.

3.6. Prime movers for single family users

The PMs available on the market (mostly at a prototype level

only) with electric power up to 5 kW and suitable for residential
single family users, are reported in

Table 1 [6,20,22,31–43]

.

It can be noted that the PMs are characterized by considerably

different values of electric and thermal efficiency, which reflect
on the electric to thermal power ratio C

CHP

. As regards the overall

CHP efficiency

g

CHP

, it has to be noted that all the PMs allow

g

CHP

values equal or higher than 84%.

The PMs considered for the analyses carried out in this paper,

one per technology, are highlighted in gray in

Table 1

. These sys-

tems were selected as the most suitable PMs for single family res-
idential users. In particular, the electric efficiency of the considered
MGT (16.0%) and TPV generator (12.3%) has to be considered as a
target value.

4. Methodology and hypotheses

The main hypotheses used for the analyses carried out in this

paper are resumed in the following:

E.S. Barbieri et al. / Applied Energy 97 (2012) 723–733

725

1. The PM is operated in an ON/OFF mode, without load modula-

tion and thermal energy dissipation, so that the PM has to meet
the thermal demand through a thermal load following operat-
ing mode, while the produced electricity is a secondary product,
depending on the specific electric-to-thermal power ratio C

CHP

.

2. Since the PM is operated without thermal energy dissipation, all

the recovered heat on a hourly basis is supplied to the utilities
or accumulated in the TES unit. If the produced thermal energy
is larger than the sum of the residual TES capacity and of the
thermal demand on a hourly basis, the PM is switched OFF.

3. The control logic of the PM is aimed to maximize the profitabil-

ity. For this reason, the PM is preferentially switched ON during
the so-called ‘‘peak hours’’, i.e. when the revenue for electric
energy production is the highest

[44–46]

.

4. An auxiliary boiler, which is part of the whole CHP system, is

used to meet peak thermal demands, when both the PM in
CHP configuration and the TES unit cannot meet the thermal
demand on a hourly basis.

5. The electricity distribution grid acts as a storage system for

electric energy: excess electricity produced by the PM, and
not self-consumed by the user, is sent to the grid, while the
electricity is taken from the grid when demand is greater than
consumption.

6. Summer cooling demand is met by electric air conditioners.

4.1. Energy analysis

The energy and environmental benefits, in terms of primary en-

ergy saving and reduction of CO

2

emissions, of the CHP technology

with respect to the separate production of electricity and heat can
be evaluated through the Primary Energy Saving (PES) index

[47]

defined as follows:

PES ¼ 1

E

f

E

e

g

es

þ

E

t

g

ts

ð1Þ

where E

e

and E

t

are the produced electric and thermal energy and E

f

is the energy of the fuel feeding the PM in CHP configuration, while

g

es

and

g

ts

are efficiency reference values for the separate produc-

tion of electricity and heat, respectively.

Since the CHP systems considered in this paper are used for

household heating, the reference scenario considers that (i) the
electric energy demand is met by means of the electric energy ta-
ken from the electricity distribution grid, (ii) conventional boilers
are used to cover thermal energy demand and (iii) electric air con-
ditioners are used for summer cooling. Therefore, the PES index can
be calculated by using the following values for efficiency reference
values

g

es

and

g

ts

:

g

es

= 0.46 p, i.e. the average electric efficiency of Italian thermo-

electric power plants

[48,49]

multiplied by the correction factor

p

[50]

, which accounts for grid losses and transformation losses

(p = 0.925 for electricity exported to the grid and p = 0.860 for
the electricity self-consumed on-site).

g

ts

= 0.7537 + 0.03 log

10

P

t

, which is the average overall effi-

ciency of heat production and distribution (equal to the product
of the efficiency of heat generator and of heat distribution sys-
tem) of a conventional heating system with a boiler of power P

t

expressed in [kW]

[49]

. For instance, for P

t

= 28 kW, which is the

thermal power of the boiler used in the reference scenario for
the considered case study,

g

ts

is equal to 0.7971.

The same definition of PES was also adopted in

[51]

for a similar

analysis.

4.2. Economic analysis

The economic analysis consists of the determination of the per-

iod to pay back the marginal cost of the CHP system with respect to
a traditional boiler.

In the following, the situation in which no CHP system is in-

stalled is defined as ‘‘traditional scenario’’ (i.e. a boiler is used for
thermal energy production and all the electric energy is taken from
the national grid), while the situation in which a CHP system is
present is called ‘‘CHP scenario’’. The economic analysis developed
below makes use of differential cash flows between the CHP sce-
nario and the traditional scenario.

The payback period (PBP) was evaluated as the period which

makes the net present value (NPV), defined in Eq.

(2)

, equal to zero

NPV ¼ I þ

X

N

i¼1

F

i

ð1 þ dÞ

i

ð2Þ

where I is the marginal investment cost of the CHP system with re-
spect to the boiler, F

i

is the differential cash flow of the ith year and d

is the discount rate (assumed equal to 5%).

The differential cash flow F

i

can be expressed according to Eq.

(3)

:

F

i

¼ F

Ee
i

F

NG
i

F

Ee
i

¼ ðE

e

Þ

sale

½ðr

Ee

Þ

sale

þ ðr

Ee

Þ

sc

þ ðE

e

Þ

sc

ðc

Ee

Þ

purc

F

NG
i

¼ ðc

NG

þ t

h

Þ½ðm

NG

Þ

CHP

ð1 þ VAT

CHP

Þ ððm

NG

Þ

trad

ðm

NG

Þ

AB

Þð1 þ VAT

trad

Þ

ð3Þ

The cash flow for electric energy F

Ee
i

is composed of the following:

(r

Ee

)

sale

: revenue from the excess electric energy exported to the

grid and sold to the electric market.

(r

Ee

)

sc

: revenue from the electric energy sold to the electric mar-

ket and bought back and consumed on a yearly basis.

Table 1
Main characteristics of PMs suitable for single-family dwellings.

726

E.S. Barbieri et al. / Applied Energy 97 (2012) 723–733

(c

Ee

)

purc

: avoided cost of purchasing electric energy from the

grid for the fraction of electric energy produced by the CHP sys-
tem and self-consumed on-site.

It should be noted that both (r

Ee

)

sale

and (c

Ee

)

purc

account for a

reduction in the excise (r

Ee

)

re

on a fraction of the natural gas used

for electric energy production by means of the PM.

The cash flow for natural gas F

NG
i

is instead composed of the

following:

(m

NG

)

CHP

: yearly mass of natural gas used to feed the PM, for

which a reduced VAT is allowed (i.e. VAT

CHP

= 10% instead of

20%).

((m

NG

)

trad

(m

NG

)

AB

): difference between the yearly mass of

natural gas used in the traditional scenario and the yearly mass
of natural gas used to feed the AB. Both quantities are charged
at the full VAT (i.e. VAT

trad

= 20%).

Therefore, in Eq.

(3)

, c

NG

represents the natural gas specific cost

to which taxes for household users t

h

(‘‘excise’’) have to be applied.

It also has to be noted that the valorization of electricity drawn

from the grid (retail rate (c

Ee

)

purc

) and of excess electricity fed back

to the grid (wholesale rate (r

Ee

)

sale

) are considerably different and

depend on the time point of purchase/sale (i.e. peak or off-peak
hours). The ranges of these values are reported in the following,
while discussing economic assumptions.

The economic analysis also accounts for CHP system mainte-

nance, by means of a specific cost which depends on the consid-
ered technology, i.e. 0.014 €/kWh

e

for ICEs, 0.012 €/kWh

e

for

both MGTs and mRCs, 0.010 €/kWh

e

for Stirling engines and

0.006 €/kWh

e

for TPV generators

[6,20]

.

5. Case study

5.1. Buildings

Two single-family dwellings, which may be suitable for install-

ing a micro CHP system, are considered in this paper, to investigate
two different situations with different energy demands. The appli-
cation to single-family dwellings is quite innovative, since residen-
tial and service sector applications, such as apartment blocks, are
usually considered in literature.

The data reported in

Table 2

were obtained in agreement with

EPBD

[11]

for the two considered dwellings. Though specific to

the considered cases, the values reported in

Table 3

could be rep-

resentative of typical European single-family dwellings. In fact, as
regards the thermal energy demand, the values for both buildings
are close to the average European value of 120 kWh/(m

2

yr). As re-

gards the specific electric demand for appliances, lighting and cool-
ing (EP

a&l

+ EP

C

), the values reported in

Table 2

consider the Italian

mean value (i.e. 25 kWh/(m

2

yr)); the values of electric consump-

tion for cooling refer to electric conditioners with an EER equal
to 3. Cooling demands for summer conditioning are specific
to these two buildings, but they can be reasonably considered

representative of similar buildings located in regions with similar
yearly average ambient temperatures. Moreover, report

[17]

shows

that the average dwelling floor area in Europe is 96 m

2

, which is

very close to the value for building #1, while building #2 approx-
imately doubles this value. Finally, it has to be noted that both
buildings are located in the Italian climatic zone E. In this climatic
zone, the heating systems can be switched ON for 183 days (i.e.
from 15 October to 15 April). Therefore, in this paper, the PM is also
allowed to be switched ON during this 183-day period.

5.2. Tariff scenario

The tariff scenario can be very changeable over time and also

depends on the considered country. For this reason, the influence
of specific consumer tax-included price for household users of nat-
ural gas and electric energy in Europe was analyzed in

[53]

. The

specific values used in this paper to perform the economic analysis
(see Eq.

(3)

) are summarized in

Table 3

for the two considered

buildings, which have different energy demands, as reported in

Ta-

ble 2

. These values refer to the 2009 Italian scenario, but may also

be representative of other European country scenarios.

The ranges reported in

Table 3

for the two buildings are due to

the different CHP systems which lead to different amounts of elec-
tric and thermal energy produced on a yearly basis and to different
amounts of electric energy exported to the grid and self-consumed
on-site during peak and off-peak hours.

The following comments can be made about the values reported

in

Table 3

:

(r

Ee

)

sale

: the values were obtained by considering the amount of

electric energy exported to the grid during peak and off-peak
hours, with reference to northern Italy. The order of magnitude
is approximately 12 c€/kWh.

(r

Ee

)

sc

: the revenue from the electric energy sold to the electric

market and bought back and consumed on a yearly basis
depends on the total amount of the electric energy taken from
the grid on a yearly basis and on the amount of electric energy

Table 2
Main characteristics of the two considered dwellings.

Building #1

Building #2

Floor area (m

2

)

98

223

Volume (m

3

)

265

602

Shape factor, S/V (m

2

/m

3

)

0.86

1.24

Yearly average ambient temperature (°C)

[52]

14.2

13.1

Thermal energy demand (kWh/yr)

11,823

23,162

EP

H

(kWh/(m

2

yr))

85

104

Hot water production (kWh/yr)

1811

3496

EP

W

(kWh/(m

2

yr))

18

16

Appliances and lighting (kWh/yr)

1764

4014

EP

a&l

(kWh/(m

2

yr))

18

18

Electric consumption for cooling (kWh/yr)

684

1555

Specific electric consumption for cooling,

EP

C

(kWh/(m

2

yr))

7

7

Electric to thermal energy ratio,

C

user

= (EP

a&l

+ EP

C

)/(EP

H

+ EP

W

)

0.18

0.21

Table 3
Assumptions for economic analysis.

Building #1

Building #2

Specific revenue from the electric energy sent to the grid

(r

Ee

)

sale

(c€/kWh)

10.6–13.0

11.1–12.9

Specific revenue from the electric energy sold to the grid and bought back and consumed on a yearly basis

(r

Ee

)

sc

(c€/kWh)

0.1–1.4

1.0–10.1

Specific cost of electric energy purchased from the grid

(c

Ee

)

purc

(c€/kWh)

16.8–32.8

30.2–41.9

Natural gas specific cost without taxes

c

NG

(c€/Sm

3

)

27.0 or 54.0

Taxes on natural gas for household users (excise)

t

h

(c€/Sm

3

)

20.1–21.7

Natural gas specific cost for household users (including taxes and VAT for household users)

(c€/Sm

3

)

56.5–88.9

58.4–90.8

E.S. Barbieri et al. / Applied Energy 97 (2012) 723–733

727

sold to the electric market and bought back and consumed dur-
ing peak and off-peak hours. Therefore, the range of variation of
(r

Ee

)

sc

values is very large.

(c

Ee

)

purc

: the values depend on the amount of the electric energy

taken from the grid during peak and off-peak hours on a yearly
basis (which depends on the electric energy demand and on the
electric energy produced from the CHP system and self-con-
sumed on-site) and, therefore, it changes by passing from build-
ing #1 to building #2.

c

NG

: the highest value (i.e. 54.0 c€/Sm

3

) applies in Italy to band

D2 (annual consumption of natural gas between 20 GJ and
200 GJ, i.e. between 525 Sm

3

and 5254 Sm

3

). A halved value of

c

NG

, i.e. 27.0 c€/Sm

3

, is also considered in this paper, to repre-

sent another possible scenario where the cost of natural gas is
considerably lower.

5.3. Electric, thermal and cooling demand time profile

A key aspect to be accounted for is the trend over a given time

frame (e.g. one day) of electric, thermal and cooling demands. In
general, the demand time profile depends on the season (e.g. sum-
mer or winter), on the day typology (working or non-working day),
on the considered building (e.g. single-family dwelling or apart-
ment blocks) and on the type of occupants (e.g. single occupant
or family).

In this paper, for both the considered single-family dwellings,

only one time profile has been assumed representative of the aver-
age trend over time of each month, i.e. each month has been as-
sumed to be composed of days with the same time profiles of
electric, thermal and cooling demands. The trends of the assumed
daily electric, thermal and cooling demands are reported in

Fig. 1

.

It should be considered that (i) the total thermal demand is the
sum of the demand for space heating and hot water production

and (ii) the heating period starts on 15th October and ends on
15th April, while the cooling period starts on 1st June and ends
on 31st August. The values chosen for making all the trends non-
dimensional (P

dem_max

) are reported in

Table 4

for the two consid-

ered buildings.

Table 4

also reports the ratio between the peak va-

lue of the month and the absolute peak value of the year

[51]

. This

means that all the values in

Fig. 1

have to be scaled by using the

scaling factors reported in

Table 4

. As expected, it can be noted that

the highest peak of thermal demand occurs in January, while cool-
ing is required only in June, July and August. Electric demand for
appliances and lighting and thermal demand for hot water produc-
tion are instead constant over the year.

5.4. Thermal energy storage unit

The optimal size of the TES unit, in terms of thermal energy that

can be stored, depends on (i) overall thermal energy demand, (ii)
daily distribution of the thermal demand and (iii) CHP size, i.e. ra-
tio between PM thermal power size and user yearly thermal de-
mand peak

[51]

.

The TES volume depends on the difference

D

T between the tem-

peratures of the fluid at the TES inlet and outlet section. In this pa-
per,

D

T was assumed constant and equal to 30 °C and the fluid was

assumed to be water. Eq.

(4)

relates the TES volume to TES equiv-

alent hours h

stor

.

V ¼

ðP

tH;W

Þ

max

h

stor

q

c

D

T

ð4Þ

6. Results and discussion

6.1. Energy analysis

The energy analysis was carried out for both buildings (B#1 and

B#2) and for the five different CHP systems.

Fig. 2

shows that PM yearly operating hours increase, by

increasing TES size (expressed as TES equivalent hours)

[51]

. As

an example, in the case of ICE and building #2, the number of
yearly operating hours increases from about 2700 h/yr to approxi-
mately 4000 h/yr, which is close to 4392 h/yr, which is the maxi-
mum number of operating hours in 183 days of possible PM
operation.

It should be considered that, according to

[51]

, the curves in

Fig. 2

tend to an asymptotic value, which corresponds to the opti-

mal TES size, beyond which the PM yearly operating hours do not
increase any further. The optimal TES size depends on the consi-
dered PM technology (i.e. PM thermal power size), on PM control
logic and on the considered building (i.e. thermal energy demand).
The optimal TES volumes are reported in

Table 5

for the considered

0.00

0.20

0.40

0.60

0.80

1.00

0

6

12

18

24

Daily hour

P

/P

max

Space heating

Hot water

Electric

Electric for
cooling

Fig. 1. Energy demand over time

[18,54]

.

Table 4
Absolute peak power demands (kW) of the year and ratio between peak value of the month and absolute peak value of the year.

Power

Space heating

Hot water

Electricity

Electricity for cooling

Building

#1

#2

#1

#2

#1

#2

#1

#2

P

dem_max

(kW)

8.369

14.978

1.067

2.060

0.326

0.742

0.968

2.202

January

1.00

1.00

1

1

1

1

0

0

February

0.75

0.83

1

1

1

1

0

0

March

0.38

0.51

1

1

1

1

0

0

April (first half)

0.08

0.16

1

1

1

1

0

0

April (second half), May

0

0

1

1

1

1

0

0

June, July, August

0

0

1

1

1

1

1

1

September, October(first half)

0

0

1

1

1

1

0

0

October (second half)

0.04

0.09

1

1

1

1

0

0

November

0.50

0.53

1

1

1

1

0

0

December

0.86

0.90

1

1

1

1

0

0

728

E.S. Barbieri et al. / Applied Energy 97 (2012) 723–733

PM technologies, control logic and buildings, together with maxi-
mum PM yearly operating hours.

Figs. 3

a and b, respectively show the fraction of the electric and

thermal energy demand met by the PM in CHP configuration, as a
function of the PM technology and operating hours (which depend
on TES size, according to

Fig. 2

), for both buildings (B#1 and B#2).

It can be observed that:

Only the electric energy demand for building #1 can be com-

pletely met by the ICE and almost totally by the MGT, while
the ratio between the produced and the required electric energy
for all the other technologies and buildings is usually lower
than about 85%.

All CHP systems (with the exception of ICE for building #2)

meet at least 80% of the demand of thermal energy.

The primary energy saving of the considered PM in the CHP con-

figuration has been evaluated in terms of both relative energy sav-
ing, through the PES index defined in Eq.

(1)

, and absolute energy

saving, expressed as equivalent tonnes of oil per year, in

Figs. 4

and 5

, respectively.

Fig. 4

highlights the effect of the PM technology (characterized

by the electric-to-thermal power ratio C

CHP

and overall CHP effi-

ciency

g

CHP

reported in

Table 1

) on PES index. The CHP system

based on the MGT seems preferable, followed by the CHP system
based on the Stirling engine.

Fig. 5

reports the absolute primary energy saving obtainable for

building #1 and #2 by installing the considered CHP systems, both
with and without the TES unit. In particular, when a TES unit is
present, an optimal TES size was considered (see

Table 5

).

To conclude,

Figs. 2 and 3

highlight that the presence of a TES

unit of proper size proves crucial: this reflects on the increased
absolute primary energy saving shown in

Fig. 5

.

6.2. Economic analysis

The influence of (i) cooling energy demand, (ii) incentives on

produced electric energy and (iii) natural gas purchase cost is ana-
lyzed for the two buildings and five CHP systems, in the case of a
TES unit of optimal size (see

Fig. 2

and

Table 5

).

The base case identified for comparison purposes is character-

ized by:

– Presence of cooling demand.
– Presence of the incentive (r

Ee

)

sc

on produced electric energy,

according to Eq.

(3)

, and of the incentive due to the reduction

in the excise (r

Ee

)

re

on a fraction of the natural gas supplied to

the PM, which is already included in (r

Ee

)

sale

and (c

Ee

)

purc

values

in

Table 3

.

– Natural gas cost equal to 54.0 c€/Sm

3

.

The results are reported in

Fig. 6

in terms of the marginal cost of

the CHP system (with respect to a traditional system) that can be
paid back over 10 years, for both building #1 and #2, and over
5 years, for building #2 only. In fact, due to the low thermal energy

0

1000

2000

3000

4000

5000

0.0

1.0

2.0

3.0

4.0

TES equivalent hours [(kWh

t

)

stor

/(kW

t

)

dem_max

]

PM yearly operating hours [hr/yr]

ICE (B#1)

ICE (B#2)

MGT (B#1)

MGT (B#2)

mRC (B#1)

mRC (B#2)

Stirling (B#1)

Stirling (B#2)

TPV (B#1)

TPV (B#2)

Fig. 2. TES equivalent hours vs. PM yearly operating hours.

Table 5
Optimal TES size and maximum PM yearly operating hours.

Building #1

Building #2

V (m

3

)

PM yearly oper. hours (h)

ðP

t

Þ

PM

ðP

t

Þ

dem max

V (m

3

)

PM yearly oper. hours (h)

ðP

t

Þ

PM

ðP

t

Þ

dem max

ICE (HONDA, Ecowill)

0.68

3340

0.34

0.73

4053

0.19

MGT (MTT)

1.08

788

1.59

1.96

1589

0.88

mRC (OTAG, Lion)

1.08

727

1.70

1.96

1499

0.94

Stirling (MICROGEN)

0.68

2073

0.64

1.22

3510

0.35

TPV (JX Crystal)

1.08

1285

0.99

1.22

2591

0.55

0

20

40

60

80

100

0

1000

2000

3000

4000

PM yearly operating hours [hr/yr]

(

E

e

)

CHP

/(

E

e

)

us

e

r

[%]

ICE (B#1)

ICE (B#2)

MGT (B#1)

MGT (B#2)

mRC (B#1)

mRC (B#2)

Stirling (B#1)

Stirling (B#2)

TPV (B#1)

TPV (B#2)

0

20

40

60

80

100

0

1000

2000

3000

4000

PM yearly operating hours [hr/yr]

(

E

t

)

CHP

/(

E

t

)

use

r

[%]

ICE (B#1)

ICE (B#2)

MGT (B#1)

MGT (B#2)

mRC (B#1)

mRC (B#2)

Stirling (B#1)

Stirling (B#2)

TPV (B#1)

TPV (B#2)

(b)

(a)

Fig. 3. Ratio between electric (a) and thermal (b) energy produced by CHP system and electric and thermal energy demand vs. PM yearly operating hours.

E.S. Barbieri et al. / Applied Energy 97 (2012) 723–733

729

0.18

0.20

0.22

0.24

0.26

0.28

PES

ICE

MGT

mRC

Stirling

TPV

Fig. 4. PES index.

0.0

0.2

0.4

0.6

0.8

1.0

Primary energy saving [toe/yr]

Optimal energy storage capacity

Null energy storage capacity

ICE

MGT mRC Stirling TPV

ICE

MGT

mRC Stirling TPV

Building #1

Building #2

Fig. 5. Primary energy saving.

0

2000

4000

6000

8000

ICE

MGT

mRC

Stirling TPV

Marginal cost allowed [€/kW]

Building #1
PBP = 10 yr
c

NG

= 54 c€/Sm

3

0

2000

4000

6000

8000

ICE

MGT

mRC

Stirling TPV

Marginal cost allowed [€/kW]

Building #1
PBP = 10 yr
c

NG

= 27 c€/Sm

3

0

2000

4000

6000

8000

ICE

MGT

mRC

Stirling TPV

Marginal cost allowed [€/kW]

Building #2
PBP = 10 yr
c

NG

= 54 c€/Sm

3

0

2000

4000

6000

8000

ICE

MGT

mRC

Stirling TPV

Marginal cost allowed [€/kW]

Building #2
PBP = 10 yr
c

NG

= 27 c€/Sm

3

0

2000

4000

6000

8000

ICE

MGT

mRC

Stirling TPV

Marginal cost allowed [€/kW]

Building #2
PBP = 5 yr
c

NG

= 54 c€/Sm

3

0

2000

4000

6000

8000

ICE

MGT

mRC

Stirling TPV

Marginal cost allowed [€/kW]

Building #2
PBP = 5 yr
c

NG

= 27 c€/Sm

3

Base case

Null cooling energy demand

Null (

r

Ee

)

sc

Null incentives on el. en. (null (

r

Ee

)

sc

& (

r

Ee

)

re

)

Fig. 6. Marginal cost allowed for each technology, to reach a payback period of 10 years (B#1 and B#2) or 5 years (B#2), by using a TES of optimal size, in the case of
c

NG

= 54.0 c€/Sm

3

or c

NG

= 27.0 c€/Sm

3

.

730

E.S. Barbieri et al. / Applied Energy 97 (2012) 723–733

demand of building #1, the PBP for building #1 is always higher
than 5 years. The marginal cost is reported per unit of installed
electric power of the CHP system.

Fig. 6

highlights that:

– As expected, the thermal energy demand strongly influences

CHP system profitability; in fact, the thermal energy demand
of building #2 is almost twice the thermal energy demand of
building #1 and this allows a more than double marginal cost
of the CHP system.

– The influence of the presence of electric air conditioners for

cooling is relevant, since it increases the annual electric energy
consumption and, as a consequence, the differential cash flow
for electric energy F

Ee
i

in Eq.

(3)

.

– The incentive (r

Ee

)

sc

is not much relevant in many cases.

– The absence of the incentive, due to the reduction in the excise

(r

Ee

)

re

on a fraction of the natural gas supplied to the PM, is

instead very relevant, since it reduces (r

Ee

)

sale

and (c

Ee

)

purc

val-

ues in

Table 3

by approximately 5 c€/kWh.

– The influence of the natural gas cost, decreased in this analysis

from 54.0–27.0 c€/Sm

3

, is very low, since the differential cash

flow for natural gas F

NG
i

in Eq.

(3)

is very small.

The key information provided by

Fig. 6

is the quantitative eval-

uation of the maximum marginal cost allowed for each technology.
In fact, by considering the base case and the natural gas cost of
54.0 c€/Sm

3

, it can be observed that the marginal cost per unit of

installed electric power is in the range of:

– 600–1800 €/kW

e

, for B#1 and PBP = 10 years.

– 2200–6600 €/kW

e

, for B#2 and PBP = 10 years.

– 1200–3700 €/kW

e

, for B#2 and PBP = 5 years.

where the lower limit always occurs for MGTs, while the upper

limit refers to Stirling engines. It can be observed that, for a user
with a thermal energy demand similar to that of building #1, the
application of these micro-CHP systems is unlikely to be feasible
at present technology costs. Otherwise, in the case of a single-fam-
ily user similar to that of building #2, it is possible to establish a
reasonable target marginal cost for the feasibility of micro-CHP
systems for household heating at about 3000 €/kW

e

.

As a final comment, it can be stated that the absence of incentives

and, in particular, of excise reduction on a fraction of the natural gas
supplied to the PM, does not make any of the technologies particu-
larly attractive at present, since the marginal cost allowed is always
too low. Moreover, the most feasible PMs are the ones with the low-
est electric power output (i.e. ICE and Stirling), since these two PMs
allow the highest fraction of self-consumed electric energy.

6.3. Optimal prime mover sizing

A concluding analysis is carried out in this section to identify

the optimal sizing of the PM. The analysis is carried out for the base
case, with reference to:

– Building #2, characterized by C

user

= 0.21, peak thermal demand

equal to 17.038 kW (sum of peak thermal demands for both
space heating and hot water production, as reported in

Table 4

),

peak electric demand equal to 0.742 kW (

Table 4

) and TES unit

size equal to 1.96 m

3

(

Table 5

).

– Specific marginal cost for the micro-CHP system, assumed equal

to 3000 €/kW

e

, according to the results highlighted in the previ-

ous section.

In order to compare different PMs, characterized by different

electric and thermal power output and overall CHP efficiency, both

the net present value NPV, calculated after 15 years, and the
payback period PBP are used. The CHP parameters varied for this
analysis are (i) the CHP efficiency

g

CHP

(in the range 0.85–1.00),

(ii) the electric-to-thermal power ratio C

CHP

(in the range 0.15–

0.30) and (iii) the electric power (equal to 1.0 kW or 1.2 kW). The
electric and thermal power values, together with the ratio with
respect to the respective peak values, are reported in

Table 6

. It

can be noted that, in case of C

CHP

= 0.15 and P

e

= 1.2 kW, the

thermal power is equal to 8.25 kW. This thermal power fully meets
the thermal demand during the coldest month (January), i.e. the AB
is never switched on during this month and 2984 h of operation are
allowed during the year.

Fig. 7

shows that the highest NPV can be obtained by using the

PM with the lower electric power output (i.e. 1.0 kW) and an
electric-to-thermal ratio C

CHP

that best fits the electric-to-thermal

ratio of the users C

user

(i.e. 0.21). In fact, the NPV for C

CHP

= 0.20 is

the highest for

g

CHP

values higher than approximately 0.93, while

the optimal C

CHP

value is 0.25 for

g

CHP

values lower than

Table 6
Electric and thermal power values for optimal prime mover sizing.

C

CHP

(P

e

)

PM

= 1.0 kW

(P

e

)

PM

= 1.2 kW

(P

e

)

PM

/(P

e

)

dem

_

max

= 1.35

(P

e

)

PM

/(P

e

)

dem

_

max

= 1.62

(P

t

)

PM

(kW)

(P

t

)

PM

/(P

t

)

dem

_

max

(P

t

)

PM

(kW)

(P

t

)

PM

/(P

t

)

dem

_

max

0.15

6.67

0.39

8.25

0.48

0.20

5.00

0.29

6.00

0.35

0.25

4.00

0.23

4.79

0.28

0.30

3.33

0.19

4.00

0.23

2000

4000

6000

8000

0.85

0.90

0.95

1.00

η

CHP

NPV @ 15 years [

]

C = 0.15; P

e

= 1.0 kW

C = 0.15; P

e

= 1.2 kW

C = 0.20; P

e

= 1.0 kW

C = 0.20; P

e

= 1.2 kW

C = 0.25; P

e

= 1.0 kW

C = 0.25; P

e

= 1.2 kW

C = 0.30; P

e

= 1.0 kW

C = 0.30; P

e

= 1.2 kW

Fig. 7. Net present value after 15 years vs. overall CHP efficiency

g

CHP

– influence of

electric to thermal power ratio C and electric power P

e

.

3

5

7

9

0.85

0.90

0.95

1.00

η

CHP

PBP [yr]

C = 0.20; P

e

= 1.0 kW

C = 0.20; P

e

= 1.2 kW

Fig. 8. Payback period vs. overall CHP efficiency

g

CHP

– influence of electric power

P

e

for C = 0.20.

E.S. Barbieri et al. / Applied Energy 97 (2012) 723–733

731

approximately 0.93. Therefore, as a general conclusion, it can be
stated that the optimal electric power size is the lowest value, clos-
est to that of the peak electric demand, while the optimal electric-
to-thermal ratio C

CHP

has to fit the electric-to-thermal ratio of the

users.

Fig. 8

reports the PBP values for C

CHP

= 0.20 and P

e

equal to

1.0 kW or 1.2 kW. The PBP values for C

CHP

= 0.25 are not reported

since they are the same as those ones obtainable in case of
C

CHP

= 0.20, owing to the fact that the differential cash flows are

evaluated on a yearly basis. It can be easily seen that the decrease
of the electric power size from 1.2 kW to 1.0 kW allows a decrease
of PBP of approximately 1 year. In any case, the values range from 4
to 7 years, with a reasonable target PBP of 5–6 years.

7. Conclusions

This paper discussed the feasibility of micro-CHP systems,

based on internal combustion engines, micro gas turbines, micro
Rankine cycles, Stirling engines and thermophotovoltaic genera-
tors, to meet household energy demands. Two single-family dwell-
ings were considered and fully characterized regarding their
energy performance and daily profiles of electric, thermal and cool-
ing loads. In addition to the PM, the CHP systems are composed of a
thermal energy storage unit and an auxiliary boiler used to cover
peak thermal demands.

The analysis of energy performance showed that the CHP units

usually satisfy at least 80% of the thermal energy demand, while
the ratio between the produced and the required electric energy
usually remains lower than approximately 85%. The primary en-
ergy saving index (in the range of about 20–28%) only depends
on the prime mover technology, while the correct sizing of the
thermal energy storage unit capacity proved crucial to maximize
the absolute value of the saved primary energy.

The economic analyses highlighted that a reasonable target for

the marginal cost of a CHP system for household heating is approx-
imately 3000 €/kW

e

. The CHP system based on Stirling engine

proved to be the best solution in almost all scenarios. Moreover,
the most influencing incentive proved to be the reduction of the
excise on a fraction of the natural gas supplied to the PM. In any
case, the absence of incentives and, in particular, of the excise
reduction does not make any of the technologies very attractive
at present, since the marginal cost allowed is always too low.

Finally, the analysis aimed to identify the optimal sizing of the

prime mover showed that the highest profitability can be obtained
by using the prime mover with the lowest electric power output
closest to that of the peak electric demand. Moreover, the
electric-to-thermal ratio of the CHP system should fit the
electric-to-thermal ratio of the users.

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