Economic and Exergy Analysis of Alternative Plants for a Zero Carbon Building Complex

Energy and Buildings 43 (2011) 787–795Contents lists available at ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild Economic and exergy analysis of alternative plants for a zero carbon building complex Tiziano Terlizzese, Enzo Zanchini ∗ Dipartimento di Ingegneria Energetica, Nucleare e del Controllo Ambientale, Università di Bologna, Viale Risorgimento 2, I-40136, Bologna, Italy a r t i c l e i n f o a b s t r a c t The feasibility of zero carbon emission plants for heating, air conditioning and domestic hot water (DHW) supply, is analyzed, with respect to conventional plants, for a new residential building complex to be constructed, in Northern Italy. Two zero carbon plants are considered: the first is composed of air-towater heat pumps for space heating and cooling, PV solar collectors, air dehumidifiers, thermal solar collectors and a wood pellet boiler for DHW supply; in the second, the air-to-water heat pumps are replaced by ground-coupled heat pumps. The conventional plant is composed of a condensing gas boiler, single-apartment air to air heat pumps, and thermal solar collectors. The economic analysis shows that both zero carbon plants are feasible, and that the air-to air heat pumps yield a shorter payback time. The exergy analysis confirms the feasibility of both plants, and shows that the ground coupled heat pumps yield a higher exergy saving. © 2010 Elsevier B.V. All rights reserved. Article history: Received 5 March 2010 Received in revised form 12 November 2010 Accepted 23 November 2010 Keywords: Zero carbon buildings Heat pumps Solar collectors Dynamic simulation Exergy analysis 1. Introduction Since a few decades, improving the energy efficiency of buildings, and possibly reach zero energy use for space heating and cooling and DHW production, is considered as an important technical target both in industrialized and in developing countries; thus, much research activity in this field has been performed worldwide. Peippo et al. [1] proposed a procedure for the optimum design trade-off strategy for solar low energy buildings, and reported some qualitative results of the procedure for a single family residential house and a large electricity intensive office building, with reference to three different climatic zones in Europe. Balaras [2] audited 8 apartment buildings, located in three climatic zones of Greece, and showed that a considerable energy saving in heating, air conditioning, DHW production and lighting can be obtained by proper retrofit actions. Iqbal [3] studied the feasibility of a zero energy one family home in Newfoundland, Canada, in which a grid connected 10 kW wind turbine provides the electric energy for space and water heating, cooking, lighting and appliances; he found that the total cost of the wind energy system is about 30% of the cost of the house. Rijksen et al. [4] studied, both experimentally and through dynamic simulation, the reduction of peak cooling requirement for an office building obtainable by means of thermally activated building systems (TABS); TABS have pipes embedded in the concrete floor, to carry water for heating and ∗ Corresponding author. Tel.: +39 051 2093295; fax: +39 051 2093296. E-mail address: [email protected] (E. Zanchini). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.11.019 cooling. Zhao et al. [5] designed and studied numerically a novel dew point air conditioning systems, and Zhao et al. [6] investigated the feasibility of this system in several China regions. Chan et al. [7] pointed out advantages and limitations of passive solar heating and cooling technologies and suggested research guidelines to improve the economic feasibility of these techniques. Wang et al. [8], discussed possible solutions for zero energy building design in UK. They showed that zero energy buildings, in which energy for heating, air conditioning, DHW, lighting and home appliances is provided by PV and thermal solar collectors and wind turbines, are theoretically possible in UK. They also provided optimization criteria for the building insulation and orientation, but did not perform an economic or exergy feasibility analysis. The aim of the present paper is to analyze the economic and exergy feasibility of zero carbon emission plants for heating, cooling and DHW supply, for a residential building complex planned for construction in a village close to Bologna, in Northern Italy. The transmittance of walls and windows is assumed as fixed, and two alternative zero carbon plants are designed and studied by dynamic simulations, performed through TRNSYS 16, and life cycle analysis. The first plant is based on air-to-air heat pumps and PV collectors, the second on ground coupled heat pumps and PV collectors. The economic and exergy payback time of these plants is determined with respect to a traditional plant, composed of a condensing gas boiler and single-apartment heat pumps for air conditioning. This kind of plant is still the most commonly employed for residential buildings in Northern Italy, where winter loads are important and air conditioning is usually not provided by the building constructor, but installed by single apartment owners. For all the plants a living room. The wood beams.938 MJ/(m3 K) [10]. two bedrooms. and their thermal properties are listed in Table 1. 3. the effective thermal conductivity is reported in Table 1. evaluated according to EN ISO 6946:2008. Terlizzese. the internal air temperature is set at 28 ◦ C during the day. and a conventional plant. provided with a humidity control and heat recovery system. For air layers. 1. provide an additional thermal resistance. and 0. In the dynamic simulation. A view of a house with 4 apartments and a map of the first floor are illustrated in Fig. therefore. The internal heat loads have been evaluated. Plant A is composed of air-to-water heat pumps (AWHPs). The heat exchange between building and ground has been evaluated by considering the real. is 1. determined by means of TRNSYS Type 501. The first floor is composed of a kitchen with dining room. 1. Plant C is composed of a central condensing gas boiler for heating and single apartment heat pumps for air conditioning. The width of the shading devices placed above the windows (see Fig.3 h−1 and the employment of a heat recovery system with efficiency 0. the frame area is 20% of the total window area. Description of the building complex and of the plants The building complex is composed of seven four-apartment houses and five two-apartment houses. which covers about 22% of the total roof area. named Plant C are considered. which provide DHW. is the same in all cases: it provides 70% of DHW energy use. 3. The window transmittance. between the timber pillars. these data are available in the default TRNSYS 16 climatic data packages. evaluated according to EN ISO 6946:2008. the cooling system is turned off during the night.170 W/(m2 K) in correspondence of the wood fiber insulation (layers listed in Table 1) and 0. so that the total heated floor area of the complex (38 apartments) is about 4234 m2 . and the glazed surface solar factor is g = 0.4 W/(m2 K). The ground is composed of heavy clay with 15% water content. ( cp )gd = 2. 2) has been designed in order to shelter completely the direct solar radiation from April 15th to September 15th for windows facing South. Zanchini / Energy and Buildings 43 (2011) 787–795 els are employed and fresh air is supplied by a forced ventilation circuit. except for bathrooms. with reference to the typical meteorological year (TMY) determined by Remund and Kunz [11]. the monthly averaged air temperatures are reported in Table 2.41 m2 . is 0. starting from outside. and are insulated with wood-derived insulating materials. Two alternative zero carbon plants. Each apartment has two floors.788 T. In each case. The transmittance of the external wall. The heat capacity of internal walls has been taken into account. The weather data for Bologna have been considered. for each hour. cooling and DHW supply The component materials of the external wall. thermal solar collectors and a wood pellet boiler. where it is kept 2 ◦ C higher. Oriented Strand Board (OSB) is manufactured from waterproof wood strands. time-dependent. The roof transmittance. 2. is 0. Plant B is similar to Plant A. including frame. while the relative humidity of the internal air is kept at 50% both night and day. Layout of the building complex. The economic analysis shows that both zero carbon plants are feasible. The following values of the ground thermal conductivity kgd and heat capacity per unit volume ( cp )gd have been considered: kgd = 1. during each month. The heating and cooling heat loads for the .15 W/(m2 K) in correspondence of the timber frame. During summer. On the other hand.6. 2. a layout of the complex is reported in Fig. the latter covers about 10% of the total wall area. The roof has a composition similar to that of the external vertical wall. the thermal resistance of the external surface has been evaluated as a function of the wind velocity and of the external surface temperature. which are placed under the roof. named Plant A and Plant B.70 W/(m K). DHW is supplied by thermal solar collectors and by the gas boiler. The heat loss due to ventilation has been determined by assuming an air change rate of 0. The thermal solar plant for DWH. designed by the f-chart method [9] as illustrated in Section 3.589. evaluated as = s R (1) Fig.197 W/(m2 K). the internal air temperature is set at 20 ◦ C during the day and at 18 ◦ C during the night.21 W/(m2 K) elsewhere. and that the air-to air heat pumps yield a shorter payback time. Double glazed windows with 4 mm thick panes separated by a 16 mm thick argon layer have been considered. but AWHPs are replaced by ground-coupled heat pumps (GCHPs). Shadowing effects have been considered to evaluate solar energy gains. Energy demand for heating. so that the average transmittance of the external wall is about 0. Each apartment has a heated floor area of 111. During winter. temperature distribution in the soil. while the values of both the beam and the diffuse solar radiation incident on a horizontal surface. the ground coupled heat pumps appear as preferable from the exergy analysis viewpoint. which provide heating and cooling. E. about 70% of the DHW energy use is supplied by thermal solar collectors. the average transmittance of the roof is about 0. For the TMY considered. are illustrated in Fig. that are arranged in cross-oriented layers.186 W/(m2 K). floor radiant pan- where s is the thickness and R is the thermal resistance per unit area of the layer. a bathroom and a garage (unheated). a bathroom and a small terrace. All houses have a timber frame and wooden walls. air dehumidifiers.326 W/(m2 K) in correspondence of the timber frame. with electric energy supplied by PV collectors. The ground floor is composed of an entrance hall. considered. according to ISO 13790:2008. as is Table 2 Monthly averaged temperatures of the TMY.000 0.071 0.638 0.1 4. Month January February March April May June July August September October November December Average temperature [◦ C] 1.2 12 2 0. Table 1 Materials of the external wall: s = thickness [cm]. in kW. Terlizzese.39 3. c = heat capacity per unit volume [MJ/(m3 K)].19 21.T.000 0.300. House with 4 apartments: view of the building and map of the first floor. E.756 1.701 0.85 MWh for dehumidifying.13 0.083 0.2 5 1.96 14.84 20.75 MWh for heating.083 0. Zanchini / Energy and Buildings 43 (2011) 787–795 789 Fig. 64.265 whole building complex.12 20.00 MWh for cooling. e = emissivity.034 0. = thermal conductivity (for air. 4.756 1. The domestic hot water demand has been determined by employing the national Technical Standard UNI TS 11.72 4.35 9.038 0.55 24. 2.43 13.840 0. 25. effective thermal conductivity) [W/(m K)].222 0. The annual energy need for the whole building complex is: 131.105 0.034 1.071 0.1 3.000 0.88 .44 8.701 0. Material 1: Plaster 2: Mineralized wood fiber 3: Air 4: Vapour barrier 5: Air 6: Low e layer 7: Mineralized wood fiber 8: OSB 9: Wood fiber 10: Air 11: Vapour barrier 12: OSB 13: Mineralized wood fiber 14: Cellulose–gypsum board s 0.5 1.034 0.5 5 4 0.13 0.9 0.5 0.1 1. are illustrated in Fig.32 c 1.149 0.45 24.111 0.083 0.077 0.3 0. dehumidifying. In Fig.70 heat pumps. Both for Plant A and for Plant B. during a typical meteorological year.97. the plot in black refers to the heat pump monitored by Marcic (supply water temperature 40 ◦ C). For Plants A and B.70 MWh.e. and DHW supply are summarized in Table 3. the average temperature of the ground from the soil surface to the BHE bottom (100 m). The GCHP system has two water tanks: WT1. has been assumed equal to 14 ◦ C. The water tank WT2 is present in all the plants considered. during the period 1988–1998. For a water temperature in WT2 equal to 35 ◦ C. the COP of the heat pumps considered in this paper is about 19% higher than that measured by Marcic. where red lines represent warmer water. while the heat pumps are not used for cooling (free cooling). blue lines cooler water. A recent report available in the literature [13] shows that.75 64. the coefficient of performance (COP) of the heat pumps has been evaluated for each hour.. so that the maximum max = 159 kW. before the beginning of the BHE field operation. the percent COP increase of air-to-water heat pumps from 1986 to 2004 is about 25%. reliable experimental data in working conditions similar to those employed in this paper are not available in the literature. A reliable experimental evaluation of the long term COP of air-to-water heat pumps operating in conditions similar to those considered in this paper has been provided by Marcic [12]. 6. Plant sizing and primary energy use A floor radiant panel heat distribution system is adopted. performed through TRNSYS.66 L per day. By assuming a temperature rise from 15 ◦ C to 40 ◦ C. with a supply water temperature of 35 ◦ C. the COP data provided by the manufacturer of the air-to-water heat pumps considered in this paper have been considered as reliable and employed in calculations. Ác = 0. one obtains a total energy need. 4. E. 4. between BHEs and Table 3 Annual energy needs for the whole building complex. The seasonal weighted mean values of the COP obtained are as follows: for Plant A. For Plant A. three plots of the COP of air-to-air heat pumps versus the external air temperature are reported: the plot in light gray refers to the heat pumps considered in this paper. For Plant B. the COP data provided by the manufacturer have been employed. grout thermal conductivity 1. Monthly values of beam and diffuse radiation on a horizontal surface for Bologna. for Plant B. Kind of service Heating Cooling Dehumidifying DHW Energy need (MWh) 131. The author presents the results of the monitoring. where TWT1 is the water temperature in WT1 expressed in degrees Celsius.60 during the cooling period. after a comparison with available experimental data. given by Endhw = 66. The distribution efficiency. =Q Ád Áe Ác (2) ˙ hp ˙ n is the net thermal power required by the building. the latter is sufficient to match the design heat load of 166.00 25. the emission efficiency and the control efficiency have been evaluated according to the national Technical Standard UNI/TS 11300. For heating power supplied by the heat pumps is Qhp each plant. Q where Q ˙ is the thermal power supplied by the heat pumps and Qaux is the auxiliary thermal power for heating supplied by the wood pellet boiler.790 400 2 MJ/m 350 300 250 200 150 100 50 0 January March May July T. hence borehole thermal resistance 0. between heat pumps and radiant panels. with reference to the same supply water temperature. arrows in lines denote the water flow direction. have been selected. prescribed by the Regional Law 156/2008. For Plant B. 5. COP = 3. two heat pumps. i. double U tube borehole heat exchangers (BHEs) with the following features have been considered: high density polyethylene tubes SDR 11 with external diameter 32 mm. with a constant value of the water temperature in WT2 (35 ◦ C). .095 m K/W. WT2. and large arrows at the sides of the heat pumps denote the energy flow direction. borehole diameter 156 mm. per each apartment. Therefore. kW 160 heating 120 cooling 80 40 0 0 1460 2920 4380 5840 7300 8760 heating hours Fig.1 W/(m K). with a heating power of 79.5 kW each. Heating (dark gray) and cooling (gray) heat load for the whole building complex during a typical meteorological year. the power supplied to the building is ˙s = Q ˙n Q ˙ hp + Q ˙ aux . For this tank.85 66. a maximum water temperature equal to 35 ◦ C has been assumed.32 during the heating period. in each plant for heating. the COP as a function of the water temperature in WT1 is given by 0. which uses GCHPs. Zanchini / Energy and Buildings 43 (2011) 787–795 Beam Diffuse September November Fig. by considering the external air temperature (Plant A) or the water temperature in WT1 (Plant B). on account of technological improvement. of an air-to-water heat pump installed in 1988 which supplies water at a mean temperature of 40 ◦ C. The figure shows that. in Plants A and B also for cooling. 3. is 165. for the whole building complex.99.12 TWT1 + 4. for the DHW demand. the plot in dark gray refers to the heat pumps considered in this paper.4.99. their values are. Terlizzese. Áe = 0. The annual energy needs for heating. cooling. Therefore. with a supply water temperature of 40 ◦ C. The total length of the BHEs has been designed by iterative simulations. A scheme of Plant B during winter operation is reported in Fig. The result. COP = 5. The undisturbed ground temperature. respectively: Ád = 0.81 during the heating period and COP = 3.9 kW (external temperature – 5 ◦ C). the COP of the air-to-water heat pumps as a function of the external air temperature and of the supply water temperature provided by the manufacturer has been employed. in order to obtain a minimum temperature of WT1 not lower than 4 ◦ C. For the piping system between WT2 and the radiant panels. Zanchini / Energy and Buildings 43 (2011) 787–795 791 thermal solar collectors PV solar collectors building complex DHW tank boiler WT2 WT1 heat pumps BHEs Fig. which corresponds to 40 BHEs 100 m deep. E. Finally. for a period of 2 years. for each room. nevertheless. the internal set point temperature (28 ◦ C) is reached in summer by free cooling. A total length of 4000 m has been obtained. Ehp = 26. COP of air-to-air heat pumps versus external air temperature. the electric energy required by the heat pump system has been determined.T. For Plants A and B. for Plants A and B. and the maximum cooling power per unit floor area during summer is equal to 28. per unit floor area. The thermal power subtracted from the building by the radiant panels. is 28. Terlizzese. it has been verified that. the thermal energy 5 COP 4 Present paper.72 MWh for cooling)..34 MWh per year for Plants A and B. 35 °C Present paper. during summer nights the water flow in radiant panels is stopped. For Plant B. Ehp . The electric energy use for water circulation.9 kPa and 8. Scheme of Plant B. 6. Simulations of the apartments have been performed by TRNSYS under the following constraints: the maximum heating power during winter.10 MWh for Plant A (36. 7.38 MWh for heating and 18.59 MWh per year for Plant C (where radiant 20 °C 18 16 14 12 Marcic 3 10 8 6 2 1 4 2 0 0 2 4 6 8 10 12 °C 14 0 2920 5840 8760 11680 14600 17520 hours Fig. The total length of the BHEs has been determined by iterations. The figure shows that the temperature of WT1 during summer exceeds 18 ◦ C only exceptionally. i. The estimated electric energy consumption is 5.e. is reported in Fig. by employing the data obtained with Eq. 7. for Plant B. the internal air temperature is usually lower than 29 ◦ C and exceeds this value only exceptionally. 5. dehumidification (Plant A and Plant B) and single apartment air conditioning (Plant C) has been evaluated as follows. sending water directly from WT1 to the radiant panels. A plot of the temperature of WT1 versus time. Temperature of WT1 versus time.9 W/m2 . (3).03 L/s. The electric energy consumed by the heat pumps per year. Fig. is equal to the design heating power. the total head loss and flow rate are respectively 69. during one year of Q The results are: Ehp = 55. 40 °C supplied for heating by the wood pellet boiler during one year has been evaluated.9 W/m2 . for heating (free cooling is adopted). for a period of two years. . the power extracted from the ground to meet the winter heat load is given by ˙ gd = Q ˙ hp 1 − Q 1 COP (3) Simulations of the BHEs have been performed through TRNSYS Type 557. with a water inlet temperature of 18 ◦ C and an internal air temperature of 28 ◦ C. has been determined as the integral. ˙ hp /COP. These temperature conditions and 50% relative humidity have been considered as satisfying. Moreover. 3.05 MWh for Plant B. By means of these simulations. A wood pellet boiler with 200 kW power and an efficiency equal to 0. Thermal solar collectors have been sized to yield f = 0.05.5 0. single glazed flat plane thermal solar collectors with a selective absorbing surface have been chosen.89.824. Since electric energy for heat pumps. while only the relative humidity is controlled during the night. This consumption corresponds to 5. Eaux is the auxiliary thermal energy for heating per year supplied by the boiler.79 MWh of electric energy for Plant A. since Endhw = 66. the total thermal energy supplied by the wood pellet boiler during one year is given by Ewpb = Esdhw (1 − f ) + Eaux .67 Plant B (MWh) 26.4 panels are used only for heating). which states . combined PV system losses 25.67 MWh of electric energy. the storage and the supply efficiency for the DHW system is (Ád Ást Ás )dhw = 0. azimuth angle −21◦ .30 5.05 – 32. In analogy with Plants A and B. 8. the design software available in Ref. The product of the distribution. For Plant C. where FR is the heat removal factor and ( ˛)0 is the effective transmittance–absorptance product at normal incidence. so that Ewpb = 24.35 3.56 MWh is the total primary energy use of the building complex for heating. the total plant efficiency is 1. The following PV system features have been considered: tilt angle 14◦ .70. [14].5 m2 . the wood pellet boiler and the heat pumps for Plants A and B. as a function of monthly thermal loads. azimuth angle 0◦ . Thermal collectors are placed on the roof of a detached plant room. Thus. about 60 m2 for Plant A and about 43 m2 for Plant B. each with 5 BHEs piped in parallel. Clearly.05 – 72. also the energy use for the BHE loop pumping must be considered.6 m2 for plant B.6 0. E. The total PV collector area is 569.68 MWh for cooling and 8.10.30 5. and Áwpb is the boiler efficiency.05 8. In Fig.5%.51 52. dehumidifying and DHW supply.70 MWh. the emission and the control efficiency for the heating system is Ád Áe Ác = 0. for each BHE.05 – 12.1. and thus accounts for the effects of clouds. in each house. which have been evaluated according to EN 15316-3-1:2007. and the weighted mean value of the COP is 3. and the estimated energy consumption is 8. For Plants A and B. (Ád Ást Ás )dhw (4) 40 60 80 100 2 120 140 Collector area [m ] Fig. Plant A (MWh) Heating Cooling Dehumidifying Radiant panel pumping DHW loop pumping BHE loop pumping Total 36. the auxiliary thermal energy for heating per year.00 for heating and 0. the condensing gas boiler for Plant C. storage and supply efficiencies for the domestic hot water system. where UL is the overall heat transfer coefficient.2 kWp (peak power) for Plant A. for Plants A. for cooling.7 0. in a typical meteorological year. must be added. for Plant A and Plant B.34 0. Ást and Ás are the distribution. Their product is 0. for a house with four apartments. supplied by the wood pellet boiler.68 8.792 Table 4 Annual electric energy use for Plants A. to which the use of 32. is 93. the thermal loads have been evaluated by assuming that both temperature and relative humidity are controlled during the day. and the energy use to provide space heating and 30% of the DHW energy need is 153.30 MWh per year. For Plant C. Zanchini / Energy and Buildings 43 (2011) 787–795 0. Áwpb (5) where Ád .94 MWh.6 m2 for plant A and 409. the fraction f of the annual thermal energy use for DHW provided by solar collectors is plotted versus the transparent collector area. For all the plants considered.56 MWh. FR ( ˛)0 = 0. Terlizzese.11 MWh. climatic data. are summarized in Table 4. To meet a part of the thermal load Esdhw .2 kWp for Plant B. collector performance parameters. the result is 12. cooling.25 0. dehumidifying and pumping.80 kWh/(m2 year). The desired energy supply is obtained by a PV system with 71. The plant has been sized by the f-chart method [9]. thus. dehumidifiers and water circulation is provided by PV collectors. for Plants A and B Ewpb = 24. and WT1 for Plant B.95.79 T. namely 72..03 MWh per year (20. and the product of the distribution. The total head loss.35 MWh for dehumidifying).04 MWh.38 18. which contains WT2 and the DHW tank. an analysis of the technical data provided by constructors has shown that the efficiency of wood pellet boilers produced nowadays ranges from 0. the electric energy consumed by the singleapartment heat pumps for cooling and dehumidifying has been determined by considering the hourly thermal loads evaluated by TRNSYS and the COP data provided by the constructor. FR UL = 3. is Eaux = 0. storage volume 75 kg/m2 .66 W/(m2 K). The BHE piping system is composed of 8 parallel loops.9 to 0. This consumption corresponds to 36. The PV collectors are roof-integrated.8 0. Plot of the fraction f of the annual thermal energy use for DHW provided by solar collectors as a function of the transparent collector area. The figure shows that solar collectors provide 70% of Esdhw with a transparent area of about 87. The electric energy use for pumping domestic hot water is about 0. For Plants A and B. storage volume and collector area. with 51. dehumidifying and pumping.9 kPa. the efficiency of the condensing gas boiler has been considered as equal to 1. The result is 29. with zero carbon emission. B and C.05 MWh per year. cooling. i.92 has been chosen. B and C. the PV collectors have been sized in order to supply exactly the total use of electric energy reported in Table 4. For Plants A and B. The values of the electric energy used for heating. the same climatic data employed for the building simulation [11] have been used. with the following plant features: tilt angle 45◦ . [15] has been employed.34 0. one obtains Esdhw = 74.59 0.72 12. For Plant B. The water flow rate is 20 L per minute.89. The use of primary energy which corresponds to this use of electricity has been determined according to the Resolution EEN 3/08 of the Italian Agency for Electric Energy and Gas (AEEG). evaluated as suggested in Ref.9 f Plant C (MWh) – 20. 8. the fraction f of the annual thermal energy use for DHW provided by solar collectors. has been evaluated by assuming the COP of air dehumidifiers equal to 2.95. Indeed. The electric energy use for dehumidification.94 for DHW supply.51 MWh per year. and 52. The latter employs hourly data of both direct and diffuse radiation on a horizontal surface.25 MWh of electric energy for Plant B. the thermal energy supplied per year to the DHW system is Esdhw = Endhw .15 kWh/(m2 year). which allows to determine where f is the fraction of Esdhw supplied by the thermal solar collectors.e. The total radiation per unit area incident on the collector surface has been evaluated by TRNSYS Type 16. and show that Plant A remains the most convenient.700 D 3300 D 26. water distribution system to radiant panels. has been assumed. C.70 D /m3 for natural gas.000 D 114. The operating costs have been evaluated as follows.000 D 20. PV systems require periodical maintenance activities such as module cleaning. present in all plants. Clearly. for summer cooling and dehumidifying.500 D 245. 0. B and C has been performed.400 D 11. These components are: radiant panels.500 D 341.500 D 40. 9.23 D /kg for wood pellet. B. per unit mass.400 D 793 Income that 1 kW h of electric energy corresponds to 2. have not been considered. B. excluding the common components. for a period of 20 years. because also Plant C has heat pumps. Indeed. by assuming that the additive maintenance cost is due only to the PV system. the costs of the common components. The same thermal solar collector system has been considered. As usual. Therefore. An annual maintenance cost equal to 46 D /kWp. the result is 78.200 D 1200 D 22. For heat pumps. for roof-integrated PV Panels. the embodied energy of the common components of Plants A. a capital cost of 4800 D /kWp has been considered. together with the mass fractions of the constituent materials given in Ref.000 D 525.27.25 D /kW h for electricity. for Plants A. tank WT2. are reported in Table 5. The table shows that Plant B is the most expensive and that the ratio between the capital cost of Plant B and that of Plant A is about 1. In analogy with the economic analysis. Plant A Heat pumps Dehumidifiers PV solar collectors Pellet boiler Total Plant B BHE. C. with respect to Plant C. with respect to Plant C. B and C are reported in Table 6. for each plant. all the PV electricity produced is paid by the State at the rate 0. [17]. we will call embodied energy of a plant component the exergy loss due to its construction and installation. it may be interesting to perform a comparative economic analysis of Plants A. B and C has been performed.175 kWh of primary energy. an additional maintenance cost has been considered. The current rates of fuels and electricity in Bologna have been considered: 0. 10.000 D 414.300 D Plant A Wood pellet PV electricity Maintenance Annual income Plant B Wood pellet PV electricity Maintenance Annual income Plant C Methane Electricity Annual cost Table 6 Annual cost (income) for energy use (production). the real mass has been considered. . E. for all plants. 9 are strongly influenced by the presence of PV systems with different areas and by the State incentives to PV electricity production. Therefore. Since a comparative economic analysis of Plants A. B and C. The capital costs of Plants A. while that of Plant B is about 11 years. Economic analysis The economic feasibility of Plants A and B has been analyzed by comparison with a conventional heating and cooling plant. visual checking of the electrical wiring system. and C has not been considered.422 D /kW h. is about 6 years. For each plant. For the high density polyethylene tubes of BHEs. For the PV systems.800 D 12.200 D 8200 D 19.800 D 12. the additive maintenance cost has been evaluated as equal to 3300 D /year for Plant A. a cost of 50 D /m has been considered for the BHEs (length 4000 m and total cost 200. for a time interval of 20 years. For Plant A and Plant B. loop and pump Cold tank Heat pumps Dehumidifiers PV solar collectors Pellet boiler Total Plant C Gas boiler Air to air heat pumps Total 11.200 D 2000 D 40. we have performed our economic analysis by assuming zero cost of money and zero annual increase of fuels and electricity costs. boiler.000 D 125. the results illustrated in Fig. 0.000 D 20. For the BHE system of Plant B. 6.000 D 205. Capital plus operating cost versus time. 5. while the value of the embodied energy of each material. has been taken from Ref.67 MWh. [16]. Zanchini / Energy and Buildings 43 (2011) 787–795 Table 5 Plant costs. B. plus a cost of 2600 D for pipes and pumps and a cost of 2600 D for labour and machinery use. The State financial support given for PV electricity production in Italy has been taken into account: only the annual difference between the electric energy consumed by the plant and the electric energy produced by the PV system is paid by the user (zero in this case). The figure shows that Plant A is the most convenient. moreover it has a total cost always lower than that of Plant B. Exergy analysis A comparative exergy analysis of Plants A. the real mass has been con- Fig.T.63 MWh of primary energy. The total capital plus operating cost versus time is plotted in Fig. and to 2400 D /year for Plant B. thermal solar collector system. called Plant C. The annual operating costs/incomes for Plants A. which corresponds to the average service cost of local maintenance companies.000 D ). The results of this analysis are reported in Fig. Terlizzese. the total use of primary energy per year for plant C is 231. the embodied energy of each non-common component has been evaluated as follows. dehumidifiers and tanks. Hence. On account of the uncertainty in the previsions of the cost of money and on the annual increase of the unit costs of fuels and electricity. Cost 1200 D 30. 9. Its payback time. and checking of module watertight seals.000 D 2400 D 18. DHW distribution circuit. in the absence of PV systems. have been studied by means of the simulation code TRNSYS and compared with a conventional plant. Zanchini / Energy and Buildings 43 (2011) 787–795 4000 MWh Plant A Plant B Plant C 3500 3000 2500 2000 1500 1000 Plant A Plant B Plant C € 500000 400000 300000 200000 100000 500 0 0 5 10 15 0 0 5 10 15 years 20 years 20 Fig. cooling. the exergy analysis has been repeated by excluding the embodied energy and the annual exergy production of the PV system. Capital plus operating cost versus time. The exergy loss due to borehole drilling has been evaluated by considering a diesel fuel consumption of 1 L per each meter of borehole (typical consumption for the soil considered).5 MWh/kWp. Similarly.02 kW h/L [19.175 = 138. B.0 MWh 46. namely 10. for Plants A.6 MWh 3.20].56 + 52. With this approximation. Table 7 Values of the embodied energy for the non common components of Plants A. Therefore. the data for Plant C do not change.8 MWh 905.43. B and C.1 MWh 392.2 MWh 6. by considering the feedstock energy as no longer available. while the annual exergy use for Plant C is 231. for a new building complex in Northern Italy.32 MWh. 11. the total is 182.88 MWh. The figure shows that the lowest exergy use after 20 years is obtained by Plant A.79 × 2. C. Plots of the total exergy loss due to the plant construction and operation versus time. B. as is shown in Section 3.20 MWh. B.0 MWh MWh years 20 Fig. 12. Plant A Heat pumps Dehumidifiers PV solar collectors Pellet boiler Total Plant B Heat pumps Cold tank Boreholes BHE pipes Dehumidifiers PV solar collectors Pellet boiler Total Plant C Gas boiler Air to air heat pumps Total 8. for Plants A. Fig. for plant B one obtains a total exergy use per year equal to 24.56 MWh. For each plant. by approximating the fuel (methane or wood pellet) exergy with its lower heating value. humidity control and domestic hot water supply. in the absence of PV collectors. B.7 MWh 3.25 × 2. due to the consumption of wood pellet. 12. in this scenario. [18].2 MWh 18. in the absence of PV collectors. 7. C. Both plants employ heat pumps which receive electricity by PV panels and thermal solar collectors for DHW supply.2 MWh 6.175 = 158. namely 72. E. in the absence of the PV system. Plant A employs air- . for Plants A.4 MWh 435. The values of the embodied energy for the non common components of Plants A. Total (construction + operation) exergy use versus time.8 MWh 634.6 MWh 8. The embodied energy of PV collectors has been evaluated by assuming an embodied energy per unit peak power equal to 8. and by approximating the diesel fuel exergy with its lower heating value. the annual exergy use for Plants A and B is 24. as in the case considered here. 11 for Plants A. are illustrated in Fig. Conclusions Two alternative zero carbon plants for heating.5 MWh 18. To obtain a direct comparison. the exergy loss due to the plant operation during a typical meteorological year has been evaluated. Clearly. 10.70 W/(m K)).67 MWh. 4000 3500 3000 2500 2000 1500 1000 500 0 0 5 10 15 Plant A Plant B Plant C sidered and the value of the embodied energy per unit mass has been taken from Ref. The total embodied energy becomes 28.4 MWh 605.794 600000 T. B. C. and C are summarized in Table 7.5 MWh 38. Total (construction + operation) exergy use versus time. for a period of 20 years. and of the primary energy equivalent of the electric energy use per year. the exergy analysis reveals that ground-coupled heat pump systems yield the lowest consumption of primary energy sources. The figure shows that. 10 does not yield a direct comparison between the exergy use of an air-to-water heat pump system and that of a ground-coupled heat pump system. Plots of the total exergy loss due to the plant construction and operation versus time. because Plant A and Plant B have different PV collector areas. even in a ground with a rather low thermal conductivity (kgd = 1. The table shows that the total embodied energy for Plant B is greater than that for Plant A. as reported in Ref.8 MWh for Plant A and 470 MWh Plant B. and that (excluding the components common to all plants) the ratio between the embodied energy of Plant B and that of Plant A is about 1. [17].4 MWh 40. are illustrated in Fig. The exergy analysis illustrated in Fig. The annual exergy use for Plant A is given by the sum of 24. the lowest exergy use after 20 years is obtained by Plant B. for a period of 20 years.56 MWh. Terlizzese. and C. Automotive fuels – Diesel – Requirements and test methods. Jones. Kavanaugh. http://ec. [20] EN 590.ec. J. D.N.A.T.O. Vartiainen. K. Report of the Swedish Heat pump Association and of the European Heat pump Association.jrc. [13] M. Lund. [11] J. Feasibility study of a novel dew point air conditioning system for China building application. meteotest. [16] G. Renewable and Sustainable Energy Reviews 14 (2010) 781–789. E. S. Rafferty. Z. Iqbal.B. Energy and Buildings 36 (2004) 185–193. Flue gases analysis and measurement on site of combustion efficiency. .K.php. L. J. Review of passive solar heating and cooling technologies. Riffat. [18] J. [3] M. However.P. Zafirakis. [8] L.A. Energy and Buildings 29 (1999) 189–205. A design procedure for solar heating systems. Jones.eu/environment/ ecolabel/about ecolabel/reports/hp tech env impact aug2005. METEOTEST. [15] Joint Research Centre.pdf. Kunz.B. Kondili. S. Reducing peak requirements for cooling by using thermally activated building systems. (Naples. Marletta.B. C.I.J. 1. J. Applied Thermal Engineering 28 (2008) 1942–1951. Multivariate optimization of design trade-offs for solar low energy buildings. [19] UNI 10389-1. http://re. Zhao. but are economically less feasible. S. Energy 34 (2009) 1187–1198. [10] ASHRAE Handbook – HVAC Applications. K. September 2009. The exergy analysis has shown that Plant A yields also a lower total exergy consumption after 20 years of operation. [12] M. Renewable Energy 29 (2004) 277–289. Kaldellis. Hammond. Proceedings of Institution of Civil Engineers. Energy and Buildings 42 (2010) 298–304. Potential for energy conservation in apartment buildings. E. October 1st 2009.D. C. primary energy use (wood pellet) and different PV collector areas. The economic analysis has shown that both Plant A and Plant B are feasible. [4] D. A feasibility study of a zero energy home in Newfoundland. 32. they have the same.eu/ pvgis/apps3/pvest. Solar Energy 18 (1976) 113–127. Argiriou. Zhu. [5] X. http://www. Building and Environment 44 (2009) 1990–1999. W. Chan. E. Asimakopoulos. Riffat. Zhao. [2] C. P. Institute for Energy. Proceedings of CLIMA 2000/Napoli 2001 World Congress.M. Energy 161 (2) (2008) 87–98. Rijksen.A. then the lowest exergy consumption after 20 years is obtained by Plant B. ASHRAE (1997). [7] H. Energy and Buildings 31 (2000) 143–154. Forsén. Yang. Duan. Riffat. 2001). A. this result is due to the higher PV collector area employed in Plant A. Embodied energy versus energy efficiency in building heating systems. Case study of zero energy house design in UK. [6] X. Cammarata.M van Schijndel. Zanchini / Energy and Buildings 43 (2011) 787–795 795 to-water heat pumps. Long-term performance of central heat pumps in Slovenian homes. P. S. Peippo. whereas Plant B employs ground-coupled heat pumps. D. The results point out that ground-coupled heat pumps ensure a lower environmental impact than air-to-water heat pumps. Wisse. A. Embodied energy and carbon in construction materials.T. Heat pumps technology and environmental impact. nearly vanishing. Terlizzese. Lee. [14] S. Marcic. Balaras.W. Optimum autonomous stand-alone photovoltaic system design on the basis of energy pay-back analysis. Heat generators. Meteonorm Version 5. Ch. [17] G. Ground-source heat pumps: design of geothermal systems for commercial and institutional buildings.A. Wang. [9] S. Droutsa.P. If the exergy analysis is repeated without considering the PV panels. 15–18. Numerical study of a novel counter-flow heat and mass exchanger for dew point evaporative cooling. 2005. Remund. Duffie. Gwilliam. Therefore. References [1] K.europa.com. a specific financial support for the installation of ground-coupled heat pumps should be given by public administrations. J. at least in a ground with a low or medium thermal conductivity. Energy and Buildings 41 (2009) 1215–1222.Y.A Beckman. and that Plant A has a lower financial payback time (6 years) than Plant B (11 years). S.europa. Klein.