Energy performance of a residential building

International Journal of Low-Carbon Technologies Advance Access published November 5, 2013

Energy performance of a residential building-integrated micro-cogeneration system upon varying thermal load and control logic†

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Abstract This work investigated the energy performance of a 6.0 kWel micro-cogeneration device integrated with a multi-family house by using a dynamic simulation software. The analyses were performed upon varying the climatic conditions, the control logic of the cogeneration unit, the number of flats composing the building and the target temperature of hot water produced for heating purposes. The simulation data were used to compare the performance of the proposed system with those of a conventional system composed of a natural gas-fired boiler and a power plant mix connected to the electric grid from an energy point of view.

Keywords: MCHP; cogeneration; energy saving; control logic; TRNSYS *Corresponding author. [email protected]

Received 24 July 2013; revised 13 September 2013; accepted 14 September 2013

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1 INTRODUCTION The energy consumption of buildings significantly contributes to the overall European energy demand. Presently, this consumption is mainly due to the heating demand and it is expected to grow considerably within the next years [1]. Therefore, the achievement of sustainability in the building sector necessitates a tremendous effort to reduce both the energy demand and increase energy efficiency of the energy production systems. On the demand side, it is widely accepted that the improvement of the building envelope’s insulation takes top priority; from this point of view, the Italian government adopted a State Law [2] which enforces severe limits of thermal transmittances for walls and windows. Wide-ranging options exist on the supply side for the provision of electricity and heat; among these options, micro-cogeneration (defined as the combined production of electrical and thermal energy from a single fuel source, with electric output ,50 kW [3]) is a well-established technology

† A shortened version of this paper has been presented at “MICROGENIII - THE 3RD EDITION OF THE INTERNATIONAL CONFERENCE ON MICROGENERATION AND RELATED TECHNOLOGIES, NAPLES, April 14 –17, 2013”

and its deployment is considered by the European Community as one of the most effective measures to save primary energy and reduce greenhouse gas emissions [3– 5]. Micro-cogeneration devices are especially interesting for both the small and medium family house markets, as well as small buildings and small and medium scale enterprises: the MicroMap project reported that in Europe between 5 and 12.5 million dwellings could have micro-combined heat and power (MCHP) systems installed by 2020 [6]. There are several technologies being developed for micro-cogeneration including: (i) reciprocating internal combustion engine, (ii) micro-turbine, (iii) fuel cell and (iv) reciprocating external combustion Stirling engine. Among these options, reciprocating internal combustion engine-based cogeneration systems are usually the best choice for small scale cogeneration applications [5, 7– 9]: this is attributed to their well-proven technology, robust nature and reliability. The opportunity to use micro-cogeneration systems depends strongly on factors such as heat and power demand variations, control modes, capacity and efficiency of the residential cogeneration system, as well as electricity import/export conditions and modes. Therefore, the feasibility of a micro-cogeneration unit is a function of the design and size of the system as well as the building is intended for. Taking into considerations that

International Journal of Low-Carbon Technologies 2013, 0, 1– 14 # The Author 2013. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact [email protected]. doi:10.1093/ijlct/ctt075 1 of 14

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G. Ciampi, A. Rosato*, M. Scorpio and S. Sibilio Dipartimento di Architettura e Disegno Industriale, Seconda Universita` degli Studi di Napoli, via San Lorenzo, 81031 Aversa (CE), Italy

G. Ciampi et al.

† † † †

Palermo (latitude: 388 070 N; longitude: 138 200 E); Napoli (latitude: 408 500 N; longitude: 148 150 E); Roma (latitude: 418 530 N; longitude: 128 290 E); Milano (latitude: 458 280 N; longitude: 98 100 E).

The simulations were performed in a wide range of operating conditions; in particular, a comprehensive sensitivity analysis was performed in order to investigate the primary energy consumption upon varying: † the number of flats composing the building (both a three-dwellings multi-family house and a six-dwellings multifamily house were considered); † the set-point temperature for the production of the hot water for heating purposes (three different values were investigated: 50, 55 and 608C).

Figure 1. Scheme of the proposed system.

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International Journal of Low-Carbon Technologies 2013, 0, 1 –14

The simulation results were also used to compare the performance of the proposed system with those of a conventional system (CS) composed of a natural gas-fired boiler (for thermal energy production) and a power plant (PP) mix connected to the central grid (for electricity production) with the main aim to assess the suitability of the plant under investigation in comparison with the system based on separate energy production in terms of primary energy consumption. The analysis of the above-mentioned proposed system was already performed by the authors in Refs [13 – 15]. However, in Ref. [13], the simulations were performed only in the case of the building is located in Napoli, under both electric and thermal load-following logics by considering a three-dwellings multifamily house with 558C as set-point temperature for the production of hot water for heating purposes; the same residential building-integrated micro-cogeneration system investigated in Ref. [13] was analyzed in Ref. [14] by extending the simulations to four different climatic regions of Italy; in Ref. [15], the operation of the MCHP device under investigation in this paper was finally compared with those of two other cogeneration units in the case of a multi-family house composed of three-dwellings located in Napoli with two set-point temperatures (55 and 608C) for the production of hot water for heating purposes. In Section 2, the proposed MCHP-based system is described, whereas the reference system is depicted in Section 3. Section 4 shows the methodology used for evaluating and comparing the energy performance of the proposed and CSs. Finally, in Section 5, the simulation results are reported and discussed in detail, highlighting the influence of (i) the climatic conditions and MCHP control logic (Section 5.1), (ii) the hot water target temperature (Section 5.2) and (iii) the number of flats composing the building (Section 5.3).


experimental analyses are both expensive and time consuming, the simulation approach is usually preferred [9, 10]. In this paper, the performance of a hypothetical residential building-integrated micro-cogeneration system was simulated by using the dynamic software TRNSYS [11]. Simulations were limited to the heating period imposed by the Italian Law, by using a simulation time step equal to 1 min. The system under investigation basically consists of a 6.0 kWel internal combustion engine-based MCHP unit fuelled with natural gas [12], a natural gas-fired boiler devoted to the auxiliary thermal energy supply, and a combined storage tank storing heat for both heating purposes and domestic hot water production. The proposed system that is devoted to satisfy the requirements of a multi-family house located in the four following Italian cities representative of different climatic regions:

Energy performance of a building-integrated micro-cogeneration system

2 DESCRIPTION OF THE PROPOSED SYSTEM

2.1 Combined tank The combined tank was modelled by means of the Type60f included in TRNSYS library. This model allows to calculate thermal stratification in the component: in this study 10 temperature levels (nodes) were assumed in the tank; a uniform tank loss coefficient per unit area equal to 3.0 kJ/hm2K was assumed [17]. A vertical cylindrical hot water storage unit with one flow inlet and one flow outlet was considered: the cold water coming from the fan coils installed within the building enters the tank through the flow inlet, while the hot water going towards the fan coils installed within the building exits the tank through the flow outlet. The tank was equipped with three internal heat exchangers: the hot water coming from the MCHP unit flows through the internal heat exchanger located in lower

2.2 MCHP unit A 6.0 kWel reciprocating internal combustion engine-based micro-cogeneration unit [12] was investigated in this study. The main characteristics of this device are reported in Table 2. Thermal power during unit operation is recovered from the exhaust gases and the engine jacket of the micro-cogenerator by means of a water– glycol mixture and transferred to the water within the storage tank by using water as coolant. The MCHP device under investigation can operate under both electric and thermal load-following control strategies. Both operation logics were simulated in this study: Table 1. Main characteristics of the hot water storage [18]. Tank volume (m3) Tank height (m) Height of flow inlet (m) Height of flow outlet (m) Temperature levels used in the tank Tank loss coefficient (kJ/hm2K) Destratification conductivity (kJ/hmK) Number of internal heat exchangers Height of thermostat controlling the MCHP (m) Height of thermostat controlling the boiler (m) The fraction of glycol contained in the IHE1 by volume (%) IHE1 inside diameter (m) IHE1 outside diameter (m) IHE1 total surface area (m2) IHE1 length (m) IHE1 wall conductivity (W/mK) IHE1 material conductivity (W/mK) Height of IHE1 inlet (m) Height of IHE1 outlet (m) The fraction of glycol contained in the IHE2 by volume (%) IHE2 inside diameter (m) IHE2 outside diameter (m) IHE2 total surface area (m2) IHE2 length (m) IHE2 wall conductivity (W/mK) IHE2 material conductivity (W/mK) Height of IHE2 inlet (m) Height of IHE2 outlet (m) The fraction of glycol contained in the IHE3 by volume (%) IHE3 inside diameter (m) IHE3 outside diameter (m) IHE3 total surface area (m2) IHE3 length (m) IHE3 wall conductivity (W/mK) IHE3 material conductivity (W/mK) Height of IHE3 inlet (m) Height of IHE3 outlet (m)

0.855 2.040 0.125 1.691 10 3.0 0 3 0.816 1.836 0 0.027 0.029 3.1 37.0 45 45 0.813 0.144 0 0.026 0.028 2.5 30.5 45 45 1.521 1.048 0 0.023 0.025 7.8 106.5 16 16 0.108 1.703

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Figure 1 shows the scheme of the proposed system. The whole plant basically consists of a micro-cogeneration device (MCHP), an auxiliary boiler, a combined storage tank with three internal heat exchangers (IHE1, IHE2, IHE3), a plate-fin heat exchanger (PHE), three pumps (P1, P2, P3), three thermostats (T1, T2, T3), two 3-way valves (V1, V2), two flow diverters (D1, D2) and a group of fan coils installed within the building. The system under investigation is devoted to satisfy the space heating sensible load, the domestic hot water requirements and the electric demand related to the heating season associated with two hypothetical buildings: a three-dwellings multi-family house and a six-dwellings multi-family house. The heating purposes and domestic hot water production are satisfied by heating up the water contained within the tank. The MCHP system and natural gas-fired boiler co-operate in order to guarantee a given water temperature level within the hot water storage: three different set-point temperatures were considered for both thermostats T1 and T2 (50, 55, 608C). The group of fan coils is supplied by the combined tank. According to the European Standard EN12831:2003 [16], 208C was assumed as set-point indoor air temperature whatever the city is. Domestic hot water is produced by means of the internal heat exchanger IHE3 located in the tank; in the case of the temperature of the water exiting IHE3 is ,458C, the required additional heat is provided by the auxiliary natural gas-fired boiler and transferred to the domestic hot water by means of the PHE. The electricity generated by the cogeneration device is used directly into the building for the lighting systems, domestic appliances and auxiliaries or is exported to the electric grid, that is also used to cover peak demand. Each component of the whole system was simulated by using the software TRNSYS [11], where each physical piece is modelled with a component (named ‘Type’) that is a FORTRAN source code model. In the following sections, the main features of the Types used in this study for simulating each plant component are reported. Additional details can be found in Refs. [13 – 15].

part of the tank (IHE1 in Figure 1); the hot water coming from the natural gas-fired boiler goes towards the internal heat exchanger located in upper part of the tank (IHE2 in Figure 1); the third internal heat exchanger (IHE3 in Figure 1) allows heat to be extracted for domestic hot water production. The main characteristics of the tank considered in this study are reported in Table 1 [18].

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Table 2. Main characteristics of the MCHP unit [12]. AISIN SEIKI unit Model Engine type

a

Ratio between electric output and primary power input. Ratio between heat supplied to the end user and primary power input.

b

(a) in the case of thermal load-following operation, the system operates according to the external temperature signal coming from the thermostat T1 (Figure 1) placed on the tank near the exit of the internal heat exchanger IHE1: when this temperature is lower than the set-point value Tset,T1, the unit provides its maximum electric and thermal outputs; when the water temperature in the storage exceeds the set-point value, the unit is turned off. The electricity generation is a by-product; (b) in the case of electric load-following operation, the cogeneration system produces electricity by modulating the power supplied through the inverter on the basis of the electric demand. Heat is also available during unit operation, even if its level depends on the system electric output; the electric grid is used when the electric demand becomes larger than the maximum MCHP electric output; in the case of electric load is lower than 300 W, the unit is turned off. The Italian Law does not limit the daily operation time of cogeneration systems: as a consequence, in this study, the MCHP unit was allowed to operate up to 24 h per day depending on thermal or electric requirement. The cogeneration unit was simulated by using the detailed dynamic model [19, 20] developed within the Annex 42 of the International Energy Agency. This model was calibrated and validated for the AISIN SEIKI unit [12] on the basis of several laboratory tests performed by the authors [21, 22] at the Built Environment Control Laboratory of the Seconda Universita` degli Studi di Napoli. During the experiments, the transient and steady-state operation of the cogeneration system was experimentally investigated upon varying the electric level, the coolant temperature and the coolant flow rate. The experimental tests showed that the values of thermal efficiency measured during transient operation are significantly lower than 4 of 14


2.3 Boiler A 20.0 kWth natural gas-fired boiler [23] was considered in this study. The boiler was modelled in TRNSYS by using the Type6 included in TRNSYS library. The auxiliary heater is activated only in the case of (i) the water temperature within the tank (sensed by the thermostat T2 in Figure 1) is lower than a given set-point value Tset,T2 or when (ii) the domestic hot water temperature at the outlet of the internal heat exchanger IHE3 (sensed by the thermostat T3 in Figure 1) is lower than 458C. The manufacturer [23] suggests 0.924 as boiler efficiency for a part load ratio equal to 30% and 0.927 as boiler efficiency in the case of operation at rated output. Therefore, in this paper, the boiler efficiency hSP B was evaluated by considering a linear interpolation of the manufacturer data: SP hSP B ¼ 0:924 þ 0:000214ðPth;B 6:0Þ

ð1Þ

The primary power consumption of the boiler was evaluated as follows: SP ¼ Pp;B

SP Pth;B

hSP B

ð2Þ

According to the Italian Law [24], Italy is divided into six different climatic zones (named A, B, C, D, E and F) based on the heating degree days (HDD) index [25]. A maximum daily operation time for the heating system depending on the climatic zone is specified in Ref. [24]; as a consequence, the boiler was allowed to operate only during the time intervals specified in Table 3 as a function of both the city and day (week day and weekend day are distinguished). Out of these periods, the boiler was turned off for heating purposes; the boiler was allowed to operate up to 24 h per day in the case of thermal energy is required for producing domestic hot water at 458C.

2.4 Building characteristics and loads The geometrical layout of the hypothetical building investigated in this report is basically a multiplication of a single family


GECC60A2 (NR-P) Reciprocating internal combustion engine, water cooled, four cycles, three cylinders Displacement (cm3) 952 Speed revolution (rpm) 1600 4 1800 Fuel Natural gas, LPG Generator type Permanent—magnet type, synchronous generator 16 poles Rated electric output (kW) 6 Rated thermal output (kW) 11.7 Operating sound at 1.0 m distance 54 and 1.5 m height (dB) Electric efficiencya at maximum 28.8 load (%) Thermal efficiencyb at maximum 56.2 load (%)

those measured during steady-state operation. The analysis of the tests also indicated that both electrical and thermal efficiencies are strongly affected by the electric output of the unit: in particular, the experiments showed a thermal efficiency increasing at decreasing the electric output, and an electric efficiency increasing with increasing the electrical power. The measurements were compared with the simulation data obtained by using the ANNEX 42 model in order to empirically validate the model itself; the data measured during a 24 h dynamic test performed by applying a realistic daily load profile representing the Italian domestic non-heating ventilation and air conditioning (HVAC) demand profile for a multi-family house of five dwellings were used: the predicted fuel use, the electric output and heat recovery over the whole duration of the data set differed from measured values by 26.1, 20.2 and 21.0%, respectively.


Table 3. Time intervals during which the boiler is allowed to operate for heating purposes. Week day

Weekend day

Palermo (climatic zone B, HDD ¼ 751)

Napoli (climatic zone C, HDD ¼ 1034)

Roma (climatic zone D, HDD ¼ 1415)

Milano (climatic zone E, HDD ¼ 2404)

Palermo (climatic zone B, HDD ¼ 751)

Napoli (climatic zone C, HDD ¼ 1034)

Roma (climatic zone D, HDD ¼ 1415)

Milano (climatic zone E, HDD ¼ 2404)

7:30–10:00 16:00– 21:30

7:30–11:00 15:00–21:30

6:30– 11:00 15:00– 22:30

6:30–12:00 14:30–23:00

8:30– 10:00 16:00– 22:30

8:30–11:00 15:00–22:30

8:30–12:00 14:00–22:30

8:30–13:30 14:30–23:30

house type building geometry. The main geometrical characteristics of each single flat as well as the building orientation are shown in Figure 2. All floors have the same useable floor area (96.0 m2), while the net height of each single flat is 3.0 m. Five windows were considered for each single floor. Starting from 1 January 2010, the Italian Law [2] specifies the threshold values of thermal transmittance for both walls and windows of renovated buildings depending on (i) the climatic zone where the building is located and (ii) the wall type (external wall, ground and roof ). These threshold values of thermal transmittance Ulim for property renovations located in Palermo, Napoli and Roma e Milano are specified for walls and windows, respectively, in Tables 4 and 5. In Table 4, the characteristics of building walls are also specified: in particular, the internal (hi) and external (he) convective and radiative heat transfer coefficients of walls, wall thermal transmittance (Uwall ), layers material, thickness (s), density (r), specific heat (c) and thermal conductivity (l ) are reported for each wall type and for each city. As it can be derived from Table 4, the hypothetical building was built by equating the thermal transmittance values of each wall to the given threshold

† † † †

Palermo: from December 1 to March 31; Napoli: from November 15 to March 31; Roma: from November 1 to April 15; Milano: from October 15 to April 15.

As above-mentioned a maximum daily operation time for the heating system depending on the climatic zone is specified in Ref. [24]. As a consequence, the indoor air temperature was controlled only during the time intervals specified in Table 3: out of these time intervals the indoor air temperature was not controlled. Heat coming from occupants, personal computers and lighting systems was assumed to contribute to the internal gains of the building. Figure 3 shows the number of occupants of each single floor and the occupants-related sensible heat gain of each single floor as a function of the time during a week day International Journal of Low-Carbon Technologies 2013, 0, 1– 14 5 of 14


Figure 2. Main geometrical characteristics of a single flat of the whole multi-family house.

value: this result was obtained by adjusting the building insulation upon varying the climatic zone. The values of hi and he were derived by the European Standard EN ISO 6946 [26], while the values of r, c and l were defined based on Italian Standard UNI 10351:1994/EC [27]. The main characteristics of building windows are reported in Table 5: in particular, the internal (hi) and external (he) convective and radiative heat transfer coefficients of windows, frame thermal transmittance Uframe, glazing thermal transmittance Uglazing, window type, glazing width, spacing width, spacing gas, ratio between frame area Aframe and window (glazing þ frame) area Awindow are specified for each city. As it can be derived from Table 5, the hypothetical building was built by equating the thermal transmittance values of each window to the given threshold value; values of Uframe and Uwindow were calculated based on the European Standard EN ISO 10077-1 [28]; all windows are provided with shadings, whose zone and wall reflection coefficients were considered equal to 0.5. According to the Italian Law [29], the area of each window was defined as the 12.5% of the ground area of the room where the window is located. According to the European Standard EN12831:2003 [16], the indoor air set-point temperature was established at 208C whatever the city under consideration is. The Italian Law [24] specifies the duration of the heating season depending on the Italian region where the plant is located; according to the limits specified in Ref. [24], in this study, the following periods of simulations were considered:

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Table 4. Characteristics of building walls. Wall type

Ulim (W/m2K)

hi (W/m2K) he (W/m2K) Uwall (W/m2K)

Layer materials

r (kg/m3) c (kJ/kgK) l (W/mK)

s (m)

Gypsum plaster Bricks

External wall Palermo ¼ 0.48 7.7 Napoli ¼ 0.40 Roma ¼ 0.36 Milano ¼ 0.34

25.0

Roof

Palermo ¼ 0.38 10.0 Napoli ¼ 0.38 Roma ¼ 0.32 Milano ¼ 0.30

Ground

25.0

Palermo ¼ 0.49 5.9 Napoli ¼ 0.42 Roma ¼ 0.36 Milano ¼ 0.33

25.0

900 1800 20

1.00 0.84 1.20

0.250 0.720 0.041

900 1800 1000 1900 1200 20

1.00 1.00 0.84 0.84 1.00 1.20

0.250 0.700 0.300 1.060 0.170 0.041

1700 1500 2300 2300 1500 1700 20

0.84 1.00 0.84 0.84 1.00 0.84 1.20

1.400 0.720 0.720 0.720 0.720 1.400 0.041

1200 1900 1000 1800

1.00 0.84 0.84 1.00

0.170 1.060 0.300 0.700

Table 5. Characteristics of building windows.

Palermo Napoli Roma Milano

Ulim (W/m2K)

hi (W/m2K)

he (W/m2K)

Uframe (W/m2K)

Uglazing (W/m2K)

Window type

Glazing width (mm)

Spacing width (mm)

Spacing gas

Aframe/Awindow (%)

3.0 2.6 2.4 2.2

7.7

25.0

3.96 2.94 7.90 6.56

2.83 2.54 1.43 1.53

Double glazing

4 6 4

16

Argon

15

(Figure 3a) and a weekend day (Figure 3b). Sensible heat coming from each occupant was assumed equal to 75.0 W: this values is suggested by the Standard ISO 7730:2005 [30] in the case of light work/typing as degree of activity. Figure 4 shows the sensible heat flux related to the lighting appliances installed within each single flat; light sources with an installed total electric capacity of 294.0 W were considered; thermal power coming from each lighting system was assumed equal to the 75% of its nominal electric capacity. Sensible heat produced by lighting systems was considered as transferred to the indoor air by both radiation (70%) and convection (30%) [31]. Concerning the infiltration, the European Standard EN 12831:2003 [16] was used to calculate the air exchange rate, i.e.

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the number of interior volume air changes that occur per hour, induced by wind and stack effect on the building envelope: based on this European Standard, the air exchange rate was assumed equal to 0.28 h21. The internal gains coming from occupants, personal computers and lighting appliances associated with the whole building were generated by multiplying the load profile presented for one dwelling by the number of dwellings; no difference among the four cities was considered in terms of internal gains. A group of fan coils, supplied by the hot water storage, was installed into the building in order to balance the space heating sensible load. A temperature difference of 58C was assumed for the hot water crossing the fan coils, with a variable-flow pump controlling the fan-coils outlet temperature.


0.02 0.33 Palermo ¼ 0.48 Palermo ¼ 0.053 Napoli ¼ 0.40 Expanded polyester Napoli ¼ 0.070 Roma ¼ 0.36 Roma ¼ 0.082 Milano ¼ 0.34 Milano ¼ 0.088 Gypsum plaster 0.02 Gypsum plaster 0.015 Cement and bricks 0.300 Concrete 0.084 Bitumen 0.010 Palermo ¼ 0.38 Palermo ¼ 0.052 Napoli ¼ 0.38 Expanded polyester Napoli ¼ 0.052 Roma ¼ 0.32 Roma ¼ 0.072 Milano ¼ 0.30 Milano ¼ 0.080 Sand 0.050 Lime and cement adhesive 0.010 Tiles 0.010 Tiles 0.010 Lime and cement adhesive 0.010 Sand 0.050 Palermo ¼ 0.49 Palermo ¼ 0.026 Napoli ¼ 0.42 Expanded polyester Napoli ¼ 0.040 Roma ¼ 0.36 Roma ¼ 0.057 Milano ¼ 0.33 Milano ¼ 0.067 Bitumen 0.005 Concrete 0.077 Bricks 0.300 Gypsum plaster 0.015


The Type56a (included in TRNSYS library) was used for modelling the building envelope, indoor air set-point temperature, infiltration and internal gains. Type15-6 (included in TRNSYS library) was used for reading the external EnergyPlus weather data files [32] of each city.

Figure 5 reports the whole building sensible thermal loadduration diagram obtained from simulations, with the heatdemand values sorted in descending order for each city; in particular, Figure 5a is related to the building with three flats, whereas Figure 5b shows the values associated to the



Figure 3. Occupants-related sensible heat gains for a single flat during week day (a) and weekend day (b), whatever the city is.

G. Ciampi et al.

Figure 5. Thermal load-duration diagram for the three-dwellings building (a) and the six-dwellings building (b) as a function of the city.

six-dwellings multi-family house. The thermal load-duration diagrams associated to the whole building were developed by multiplying the thermal load profile of the single dwelling by the number of dwellings. As it can be derived from Figure 5, space heating demand has a quite different duration depending on the city under investigation: it is equal to 940 h for Palermo, 1370 h for Napoli, 1860 h for Roma and 2550 h for Milano. In addition, it can be noticed that the maximum building sensible

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thermal load is always 31.7 kW: this value was set in TRNSYS simulations as the sum of the thermal output of the boiler of the proposed system (20.0 kW) and the maximum thermal output of the MCHP unit (11.7 kW). Figure 6 shows the simulation values of thermal energy required for heating purposes of the whole building per unit area as a function of both the city and the simulation period; in particular, Figure 6a is related to the building with three flats,


Figure 4. Daily heat flux profile associated to the lighting appliances of a single flat, whatever the city is.


Figure 7. Single flat daily domestic hot water demand profile during three typical days whatever the city is (a) and single flat domestic hot water duration curve as a function of the city (b).

while Figure 6b shows the values associated to the six-dwellings multi-family house. According to this figure, it can be noticed that, whatever the period under investigation and the number of flats are, the maximum and the minimum amounts of thermal energy are required in the case of the building is located in Milano and Palermo, respectively; the thermal energy required by the building located in Napoli is always larger than that one used by the building located in Roma. In this study, the domestic hot water demand profile with an average basic load of 200 l/day in the time scale of 1 min specified within IEA-SHC Task 26 [33, 34] was used. Figure 7a shows the daily domestic hot water demand of each single flat for three typical days (December 31, January 31 and March 15) whatever the city is; Figure 7b reports the hot water demand-duration

diagram of each single flat as a function of the city, with the domestic hot water flow rate values sorted in descending order. In this paper, the domestic hot water demand profile of the entire building was generated by multiplying the domestic hot water demand profile of the single dwelling by the number of dwellings. Figure 8a highlights the daily electric-demand profile resulting from the operation of both lighting systems and other domestic appliances (such as vacuum cleaner, dishwasher, washing machine, PC, TVs, fridge) installed within a single dwelling. The electric consumptions of each appliance were derived from Ref. [35]. The daily electric-demand profile of the whole building was defined by multiplying the single flat profile reported in Figure 8a by the number of dwellings. The electric requirements



Figure 6. Thermal energy per unit area required for heating purposes for the three-dwellings building (a) and the six-dwellings building (b).

G. Ciampi et al.

considered in this study correspond to an electric consumption for the whole building equal to 109.7 Wh/m2 per day. Figure 8b shows the electric load-duration diagram (with the electric-demand values sorted in descending order) of each single flat upon varying the city. The electric consumption of the pumps was also taken into account during the simulations, even if not included in Figure 8.

3 DESCRIPTION OF THE REFERENCE SYSTEM

ð3Þ

The primary power consumption of the boiler considered for 10 of 14 International Journal of Low-Carbon Technologies 2013, 0, 1– 14

CS Pp;B ¼

CS Pth;B

hCS B

ð4Þ

Concerning the efficiency of the PP connected to the national electric grid hPP, a figure of 0.461 was assumed: this value represents the PP average efficiency in Italy, including transmission losses [36].

4 METHODOLOGY

The main focus of this study is comparing the energy performance of the proposed plant based on a micro-cogeneration device with those of a CS based on separate energy production. In the following, a CS composed of a natural gas-fired boiler (for thermal energy production) and a PP mix connected to the electric grid (for electricity production) is considered. The comparison was performed by assuming for the reference system the same electric and thermal energy output obtained for the proposed system. A 32.0 kWth natural gas-fired boiler [23] was selected for the thermal energy production within the reference system. The boiler manufacturer [23] suggests a value of 0.911 as boiler efficiency for a part load ratio equal to 30% and a value of 0.937 as boiler efficiency in the case of operation at rated output; in this study, the boiler efficiency of the CS hCS B was evaluated by considering a linear interpolation of the manufacturer data: CS hCS B ¼ 0:911 þ 0:001067ðPth;B 10:44Þ

the CS was evaluated as follows:

The proposed system was compared with the reference system in terms of primary energy consumption by using the so-called primary energy saving (PES) indicator: PES ¼

CS SP ðEp;TOT Ep;TOT Þ CS Ep;TOT

ð5Þ

SP is the primary energy consumed by the proposed where Ep;TOT CS system and Ep;TOT is the primary energy consumed by the CS for supplying the same energy output of the proposed system. A positive value of PES indicates that the MCHP system allows for a primary energy saving (PES) in comparison with the CS. SP CS and Ep;TOT were computed as reported The values of Ep;TOT below: SP Eel;buy þ SP hPP hB SP SP SP Eel;buy Eth;B Eel;MCHP þ þ hPP hPP hCS B

SP SP Ep;TOT ¼ Ep;MCHP þ CS ¼ Ep;TOT

SP Eth;MCHP

hCS B

þ

SP Eth;B

ð6Þ ð7Þ

where the primary energy consumed by the MHCP unit SP Eth;B ; the thermal energy provided by the boiler of the proposed


Figure 8. Daily electric-demand profile for a single flat whatever the city is (a) and single flat electric load-duration diagrams as a function of the city (b).


SP system Eth;B ; the electric energy purchased by the proposed SP , the thermal energy supplied system from the electric grid Eel;buy SP , and by the MCHP unit to the water within the tank Eth;MCHP SP the electric energy produced by the MCHP unit Eel;MCHP were derived from simulations.

5 RESULTS AND DISCUSSION

(1) the MCHP control strategy strongly affects the energy performance: (a) the values of PES are always positive under thermal load-following operation: this means that the proposed system allows for an energy saving in comparison to the reference system based on separate energy production; (b) the CS is more convenient than the proposed system from the energy point of view under electric loadfollowing operation (values of PES are always negative). (2) the influence of climatic conditions on the energy performance is relevant (mainly in the case of thermal loadfollowing logic): (a) in the case of thermal load-following operation: the maximum value of PES (6.5%) is obtained when the building is located in Napoli or Milano, while the minimum value of PES (2.3%) is reached when the city under consideration is Roma; (b) in the case of electric load-following operation: the maximum value of PES (223.0%) is obtained when the building is located in Napoli, while the minimum value of PES (225.3%) is reached when the city under consideration is Roma.

5.2 Influence of hot water set-point temperature This section shows how changing the values of both Tset,T1 and Tset,T2 can affect the energy performance of the proposed system.

5.1 Influence of climatic conditions and MCHP control logic The effects of both climatic conditions and MCHP control strategy are described in this section. The values of primary energy saving as a function of the city (Palermo, Napoli, Roma and Milano) and the control logic of

Figure 9. PES as a function of both city and MCHP control logic in the case of three-dwellings building with Tset,T1 ¼ Tset,T2 ¼558C.

International Journal of Low-Carbon Technologies 2013, 0, 1 –14 11 of 14


In the following sections, the influence of climatic conditions, MCHP control logic, set-point hot water temperature and number of flats on the energy performance of the proposed system are described and discussed in detail. The influence of both climate and operational logic on the performance of combined heat and power (CHP) and combined cooling heat and power (CCHP) systems were also analyzed in Refs [37 – 44]. In particular, Cardona and Piacentino [37] proposed a general and innovative criterion on plant management and suggested some correlations that allow to size the main components of the plant based on several case studies; Cardona and Piacentino [38] also compared from an energetic and economic viewpoints an existing medium size CCHP pilot plant that covers the electrical, thermal and cooling loads of two office buildings situated in Sicily (Italy) with other plant configurations upon varying both for machine sizes and management criterion; Mancarella and Chicco [39] studied the local and global emission impact of distributed cogeneration systems operating under general and realistic loading conditions with respect to average and state-of-the-art conventional technologies; Chicco and Mancarella [40] also presented and discussed a novel approach to assess the emission reduction of CO2 and other greenhouse gases (GHGs) from CHP and CCHP systems with respect to the separate production; Fumo et al. [41] compared a building-CHP system and a building baseline in which the HVAC system is composed of a vapor compression system for cooling and a furnace for heating by using the Primary energy operational strategy to maximize energy savings for the CHP system. Mago et al. [42] contrasted the performance of CCHP and CHP systems operating following the electric load or the thermal load with a CS based on separate energy production in terms of primary energy consumption, operation cost and carbon dioxide emissions for different climate conditions; Fumo and Chamra [43] derived some mathematical relations to define conditions a CCHP system should operate in order to guarantee PESs; Rong et al. [44] presented an improved unit decommitment algorithm for the unit commitment problem in combined heat and power production planning.

the cogeneration unit (electric load-following logic and thermal load-following logic) are reported in Figure 9. The values reported in Figure 9 were obtained by considering the multifamily house composed of three flats, with Tset,T1 ¼ Tset,T2 ¼ 558C. According to this figure, it is worth noting that:

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In particular, Figure 10 shows the primary energy saving as a function of the city (Palermo, Napoli, Roma and Milano) and the set-point temperature for the production of hot water for heating purposes; with respect to the values of Tset,T1 and Tset,T2, three different cases were studied: (a) Tset,T1 ¼ Tset,T2 ¼ 508C; (b) Tset,T1 ¼ Tset,T2 ¼ 558C; (c) Tset,T1 ¼ Tset,T2 ¼ 608C.

The values reported in this figure were obtained by considering the multi-family house composed of three flats; taking into consideration that the electric load-control strategy gives negative values of PES (Figure 9), only the thermal load-following operation was considered in this section. Figure 10 highlights that: † whatever the set-point temperature is, the system based on separate energy production is always less convenient from an energy point of view with respect to MCHP unit-based system; † whatever the city is, the values of PES increase when the setpoint temperatures are reduced; with respect to the case with Tset,T1 ¼ Tset,T2 ¼ 558C, lowering the values of both Tset,T1 and Tset,T2 allows to obtain an increment of PES ranging from 0.1% (Roma and Palermo) up to 0.4% (Napoli); † whatever the city is, the values of PES decrease when the setpoint temperatures becomes larger; with respect to the case with Tset,T1 ¼ Tset,T2 ¼ 558C, increasing the values of both Tset,T1 and Tset,T2 provides a PES reduction ranging from 1.1% (Roma) up to 1.4% (Napoli); † by comparing the case of Tset,T1 ¼ Tset,T2 ¼ 608C with the case of Tset,T1 ¼ Tset,T2 ¼ 508C, it results a difference in terms of PES ranging from 1.2% (Roma) up to 1.8% (Napoli). 12 of 14 International Journal of Low-Carbon Technologies 2013, 0, 1– 14

Figure 11. PES as a function of both city and number of flats in the case of Tset,T1 ¼ Tset,T2 ¼ 558C, with MCHP unit operating under thermal load-following logic.

5.3 Influence of number of flats In this section, the influence of number of flats composing the building is investigated. In particular, the values of PES as a function of the city (Palermo, Napoli, Roma and Milano) and the number of flats composing the multi-family house (three-dwellings building and six-dwellings building) are reported in Figure 11. The values depicted in Figure 11 were obtained by considering Tset,T1 ¼ Tset,T2 ¼ 558C; taking into consideration that the electric load-control strategy gives negative values of PES (Figure 9), only the thermal load-following operation was analyzed in this section. The results showed in Figure 11 allow to draw the following conclusions: † the proposed system allows for an energy saving in comparison with the reference system based on separate energy production also in the case of the six-dwellings multi-family house is considered; † whatever the city is, the values of PES become larger when the number of flats increases; with respect to the three-dwellings building, the six-dwellings multi-family house is characterized by higher values of PES, with a difference ranging from 0.2% (Napoli) up to 5.1% (Roma).

6 CONCLUSIONS The performance of a hypothetical residential buildingintegrated micro-cogeneration system was dynamically simulated upon varying the climatic conditions, the control logic of cogeneration unit, the number of flats composing the building and the target temperature of hot water produced for heating purposes. The simulation results were used to assess the potential energy saving achievable by using the proposed system in comparison with a CS based on separate energy production. The main strength of the work is especially related to the fact that the transient nature of building and occupant driven loads,


Figure 10. PES as a function of both city and set-point temperatures in the case of three-dwellings building, with MCHP unit operating under thermal load-following logic.


[10]

[11] [12] [13]

[14]

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Acknowledgements The work described in this paper was undertaken as part of IEA/ ECBCS Annex 54 (www.iea-annex54.org). The Annex was an international collaborative research effort and the authors gratefully acknowledge the indirect or direct contributions of the other Annex participants.

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the interaction between the loads and the system output and the part-load characteristics of the cogeneration unit were taken into account in great detail. On the other side, a simplified model of the multi-family house was built by setting-up the thermal transmittance values for both walls and windows suggested by Italian Law; in addition, a streamlined approach was adopted in defining the electric-demand profile, the domestic hot water requirement, as well as the internal gains of the whole building. The performed analyses showed that the energy performance are mainly affected by the MCHP control logic: whatever the city, set-point temperature of hot water for heating purposes and number of flats are, only the heat-led control logic reduced the primary energy consumption (up to 6.5%) in comparison to the reference system, while the electric load-following logic was not suitable from an energy point of view (negative values of PES). The effect of climatic conditions was also relevant: a difference in terms of PES equal to 4.2% was observed between the best and the worst cases under MCHP thermal load-control logic. The influence of set-point temperatures appeared significant only in the case of a variation of 108C was proposed, while the number of flats composing the building substantially affected the values of PES mainly when the thermal load was low.

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