Dynamic Simulation on Energy Performance of a School 1876 6102 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY NC ND license (http //creativecommons org/lic[.]
Available online at www.sciencedirect.com ScienceDirect Energy Procedia 101 (2016) 1026 – 1033 71st Conference of the Italian Thermal Machines Engineering Association, ATI2016, 14-16 September 2016, Turin, Italy Dynamic simulation on energy performance of a school Loreti L.a, Valdiserri P.a, Garai M.a * a Department of Industrial Engineering (DIN), Viale Risorgimento 2, 40136 Bologna, Italy Abstract Building retrofitting is the most feasible and cost-effective method to improve building energy efficiency The paper presents a comparative analysis of two different strategies to enhance the energy performance of an existing building, trying to enhance indoor comfort conditions too On one hand, the strategy aimed at reducing the heat transfer by transmission, on the other one at decreasing the heat transfer by ventilation The study has been applied to a school, located in the surroundings of Bologna, Italy Potential energy savings were calculated by dynamic simulation using Trnsys© software © Published by Elsevier Ltd This © 2016 2016The TheAuthors Authors Published by Elsevier Ltd is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the Scientific Committee of ATI 2016 Peer-review under responsibility of the Scientific Committee of ATI 2016 Keywords: educational building; retrofit; mechanical ventilation; TRNSYS simulation Introduction Energy consumption for civil use in Italy has been estimated around 36% of the total primary energy used, with a 43 Mtep use according to [1] Moreover, if the analysis is restricted to the Public Administration building stock, the electric and thermal consumptions cover the 8% and 10% of national demand, respectively In order to reduce its consumption, the European Community issued Directive 2002/91 on Building Energy Certification [2] which came into effect in Italy through the Legislative Decree no 192/2005 [3] and no 311/2006 [4] With the Directive * Corresponding author Tel.: +39-051-2093281; fax: +39-051-2093296 E-mail address: lisa.loreti2@unibo.it 1876-6102 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the Scientific Committee of ATI 2016 doi:10.1016/j.egypro.2016.11.130 L Loreti et al / Energy Procedia 101 (2016) 1026 – 1033 1027 2006/32 [5] the European Community highlighted the crucial role of public administrations that may be a prime example in terms of energy efficiency interventions; it came into effect in Italy through the Legislative Decree no 115/2008 [6] The 2010/31/EC [7] updated the European framework for building energy consumption and defined NZEB, whose Italian adoption produced the Legislative Decree no 63/2013 [8], further modified with the Law 90/2013 [9] The latter integrated the Legislative Decree no 192/2005 in order to harmonize its contents to latest European directives The importance of improving the existing public building stock is demonstrated by Legislative Decree no 91/2014 [10] It includes the improvement of energy standards in schools allocating grants for more than 300 million Euros with a 0.25% cut rate for the works that improve energy efficiency of buildings by at least two energy classes in three years A further evidence of national energetic policy is the Green Public Procurement that has been recently approved [11] Moreover the relevance of these aspects is documented by the number of research projects that have been conducted in the last years The most relevant aspects are related to energy saving strategies that involve retrofit measures [12][13], but also operating and maintenance measures to reduce consumptions [14] In the present study, transient simulations using TRNSYS© software [15] were applied for a retrofit intervention in a public educational building The model consisted of the whole building and energy saving measures The tuning procedure of the model is based on heating energy consumption data Annual and monthly energy savings related to the different measures are synthetically represented in clusters: administration offices, classrooms, hall and gym Since building retrofits are often related to consistent initial investments, the economic issue was considered in order to get a framework on the feasibility of the investment Description of the building The case study is a secondary school building located in the surroundings of Bologna, in the North-East area of Italy The school is located between a county road and hills, in a residential district that is densely built; nevertheless the distance between adjacent buildings allows to take advantage of the solar radiation during the whole year The school was built in the early ‘70s; therefore it was not designed in terms of energy consumption optimization The whole structure has a gross surface of 4500 m2 and gross volume of 16000 m3 and it houses about 375 pupils The school has an articulated plan and spreads over three levels: the main block hosts both the administration area and some classrooms at the ground floor, and the remaining at the first and second floor; the gym block consists of two halls and of the dressing rooms The building plans are shown in Fig The building is characterized by a concrete in sight structure and ribbon windows shaded by projecting concrete elements Internal walls are made of a double layer of bricks without insulation, both floors and roof are made of mixed concrete and masonry without insulation as well Windows cover the 32% of exterior vertical surfaces, are double glazed with iron frame with poor thermal performances and allow high air permeability with relevant air infiltration rates U-values of the envelope components are evaluated by inspection and typical construction types of the time: main concrete walls 0.943 W/m2K; stair case walls 1.73 W/m2K; internal brick walls 0.855 W/m2K; roof 1.1 W/m2K; attic floor 0.98 W/m2K; ground floor 1.12 W/m2K; windows 2.8 W/m2K The heating system consists of 732 kW gas condensing boiler installed in the last decade, operating with a climatic control (outdoor temperature) The emission system consists of radiators placed under the windows in the main building and heat convectors in the gym No room or zone temperature controls are installed The existing heating system consists of four hydraulic loops: atrium, administration offices, classrooms and gym block There is no mechanical ventilation thus the air quality is supplied only by manual windows opening and the amount of fresh air cannot be adequately controlled A previous study [16] focused on the overall indoor quality highlighted that students complained for cold temperatures and air drafts, moreover the measured CO2 concentration inside classrooms was found considerably high There is no cooling system since the school is practically unused from early June until mid September Thus, summer operating conditions are not taken into account for the present study 1028 L Loreti et al / Energy Procedia 101 (2016) 1026 – 1033 a b c Fig School plans: (a) Ground floor; (b) First floor; (c) Second floor 2.1 Heating energy consumptions It was possible to gather energy consumptions during the heating period for three years: heating consumptions are shown in MWh (since they refer to energy counter that are placed on the delivery of each water loop) and they are plotted against mean external temperatures in Fig 2(a) and cumulate percentage degree-days in Fig (b) The data are aggregated according to the four hydraulic loops before mentioned: atrium, administration offices, classrooms and gym The plots show aggregated data; analyzing the energy consumptions it is possible to assess: x consumptions are proportional to the mean external temperature, indicating that the climatic control works; x school energy consumptions are consistent (74 kWh/m2year) because of poor performance of building elements but they were reduced in the last two years thanks to energy management measures; x atrium accounts for 10% of consumptions, administration for 8%, classrooms for 40% and gym for the remaining 42% Consistent gym consumptions are imputable to its exploitation both for student gym classes during mornings and for different sport activities during afternoons and evenings [17] a b Fig Heating energy consumptions as a function of: (a) mean external temperatures (Text,m in °C); (b) degree days a b Fig Tuning of the model VS real consumptions Heating energy consumptions as a function of: (a) clusters; (b) cumulated degree days L Loreti et al / Energy Procedia 101 (2016) 1026 – 1033 1029 2.2 TRNSYS model The present study focused on the evaluation of building retrofit strategies to enhance the energy performance of an existing building, trying to improve indoor comfort conditions too Dynamic simulations were exploited in order to get reliable results: using TRNSYS© simulation program and Meteonorm climatic data file A numerical model was created on the basis of design drawings and inspections It consists of the whole building; each room was modeled as a single thermal zone in order both to meet software requirements and to get detailed results Building construction characteristic together with shading elements, boundary conditions, internal gains, heating set points and schedules are assigned according to real operating conditions Unknown variables (i.e infiltration and ventilation rates) are determined on the basis of heating energy consumption data during the tuning process Given the proportional relation between heating consumptions and external temperatures, a target consumptions profile was created proportionally to the degree-days of the climatic data file The model baseline (#0) was considered tuned when the variance between simulations and target consumptions was found below 5%, see Fig Existing plant system elements were not modeled in order to focus on envelope elements Moreover available consumption data refer to energy counters, thus heating system efficiency are already extracted and target consumption data can be easily compared with simulated results Design strategies The analysis of the actual situation in terms of consumptions and needs were the starting point to assess the energy measures to optimize the space heating service Measures were addressed to improve both the building envelope insulation and air tightness but also indoor air conditions of classrooms and offices The following measures are applied to the model: x #1: installation of new windows, natural ventilation The measure implies a reduction of actual infiltration rates with unchanged actual natural ventilation profiles; x #1IAQ: installation of new windows (Ug= 0.59 W/m2K, Uf=1.2 W/m2K, g-value=0.584, luminous transmittance 74%), improved natural ventilation The measure implies a reduction of actual infiltration rates but scheduled natural ventilation profiles are applied in order to guarantee better ventilation rates to classrooms and administration offices, respectively [vol/h] (5 minutes each hour) and [vol/h] (5 minutes each couple of hours); x #2IAQ: installation of heat recoveries (0.7 sensible effectiveness), improved ventilation rates The measure implies the installation of a mechanically controlled ventilation system, which guarantees [vol/h] ventilation rates to classrooms and [vol/h] to administration offices The system is modelled taking into account zoning and school occupation profile: administration in the basement (installed power 300 W), two classrooms clusters in the basement (450 W and 600 W), two classrooms clusters at the first floor (900 W both) and one classrooms cluster at the second floor (900 W) x #3: internal insulation of the attic floor and of the roof, natural ventilation The measure lefts infiltration rates and natural ventilation profiles unchanged; x #3IAQ: internal insulation of the attic floor and of the roof, improved natural ventilation The measure considers scheduled natural ventilation profiles in order to guarantee better ventilation rates to classrooms and administration offices, respectively (1/h) and (1/h); x Combinations of the previous measures: #1+2IAQ, #1+3IAQ, #2IAQ+3 Solutions that consider the installation of a mechanical ventilation system imply higher costs but were considered in order to give reference results Moreover the scheduled natural ventilation profiles that are considered would imply an active and conscious role of school users that cannot be guaranteed Nevertheless their commitment may be improved trying to increase their awareness towards behaviors aimed at saving energy and improving indoor comfort conditions too [18] 1030 L Loreti et al / Energy Procedia 101 (2016) 1026 – 1033 3.1 Simulation results In Fig results are presented in comparison to the target simulation (#0) which follows consumption profile data (see Fig 3), and can be considered the school baseline Only the heating season data are plotted and exploited as terms of comparison since the school has no cooling system It is worth mentioning that, when comparing building envelope measures #1, #1IAQ and #3, #3IAQ the savings reduction in the administration and classroom clusters are related to the augmented ventilation rates That is why #*IAQ measures are compared to the analogue #0IAQ that is the model baseline with improved natural ventilation, in order to get more comparable results From the plots emerges that the new windows installation entails large energy savings, -28% or -26% on the whole (#1 and #1IAQ, Fig (a) and (b) respectively): that is because windows account for a 32% of the school building faỗade This intervention maybe advisable in order to meet faỗade passive acoustic requirements as well since it has been proved to be essential [16], as it usually happens in national educational buildings [19] The insulation of the attic floor and of the roof is a consistent measure as well: it implies respectively -18% or 17% of whole energy saving (#3 and #3IAQ, Fig (e) and (f) respectively) The soften impact on consumption is due to the fact that, even if the involved surface is larger than the glazed one, actual thermal performances are less demanding, albeit not insulated The administration area only partially takes advantages form this measure since it is located at the ground floor The installation of heat recoveries (#2IAQ, Fig (c)) will improve both indoor air conditions and comfort since they prevent external cold air to cause air flows during windows opening Nevertheless the energetic cost is consistent, +12% on the whole The energetic outlook improves if the installation of the mechanical ventilation system is joined to one of the envelope measure: #1+2IAQ implies a whole saving of -13% and #2IAQ +3 a reduction of -4% on the whole system consumptions (see Fig (d) and (h) respectively) The combined application of both measure and (#1+3IAQ), involving the renovation of about 58% of the external building surfaces, entails a whole -44% reduction of heating consumptions (see Fig (g)) Economic issue The retrofitting solutions presented in this study aimed at decreasing energy demand and improving indoor air quality of classrooms and offices Nevertheless, the application of energy saving measures usually needs to be evaluated in relation to economic assessments Therefore, the economic aspects were taken into account for configurations analyzed in Section In the evaluation, we considered a maintenance cost of 0.2% for new windows and 4% for the heat recovery system The yearly cost for heating system are reported below together with CO2 savings Carbon dioxide emission savings accounted for natural gas used for the heating system, with an emission of 0.20 kg/kWh primary energy Given an 80 % efficiency of the system, Table shows a comparison among different scenarios Electricity consumptions of heat recoveries are taken in account as well Simple Payback Time (SPBT) and Net Present Value (NPV) were exploited as financial parameters for evaluating the economic feasibility of the different scenarios: the first to get an immediate overview on the investment feasibility since it refers to the period of time required to reach the break-even point, the latter to evaluate the competing long-term measures The economic analysis was carried out for the retrofitting scenarios and was based on the technical-economic situation in Italy in the last ten years The inflation cost was set to 1%, the cost of capital 0.25% for the first 20 years [10] then to 0.5%, and the cost of natural gas was 0.12 €/kWh and the cost of electricity was 0.18 €/kWh The cost of gas includes a plant efficiency of about 80% In Table the initial investment, the energy saving obtained during the first year, and the SPBT are reported NPV was calculated for each scenario considering different increments of the cost of energy (3% or 5%) According to the Italian regulation on energy saving, it is not possible to claim a tax refund for public buildings, while a private may claim a tax refund of 65% of the cost of the investment in 10 years NPV was reported either without tax refund claim (reference case: r=0.5% for the whole period and discount interest rate case:r=0.25% for the first 20 years, then r=0.5%) or considering a 65% tax refund in 10 years (r=0.5%) in order to get a comparison between public and private investments scenario that may occur when dealing with educational buildings Accordingly, the private investment scenario is evaluated considering a cost of capital equal to 0.5% NPVs calculated for the different measures are reported in Table L Loreti et al / Energy Procedia 101 (2016) 1026 – 1033 -28% a IAQ -13% IAQ -17% IAQ -4% IAQ b +12% IAQ c d -18% e f -44% g -26% 1031 IAQ h Fig Different energy measures: comparisson of heating seaso on consumptions too the reference ones (#0 and #0IAQ basselines) Heating ennergy consumptions on the lefft axis and variationns (%) on the right aaxis 1032 L Loreti et al / Energy Procedia 101 (2016) 1026 – 1033 Table Yearly cost for heating and CO2 savings for the different measures and the baselines (#0 and #0IAQ) Yearly cost for heating (€) CO2 savings (kg) #0 #0IAQ #1 #1IAQ #2IAQ #3 #3IAQ #1+2IAQ #1+3IAQ #2IAQ+3 €39,237 €42,230 €28,317 €31,203 €47,809 €32,319 €35,220 €37,161 €23,515 €40,966 - 22,750 22,973 -12,120 14,414 14,605 10,064 38,989 2,136 - Table Initial investment, energy saving during the first year and Simple Payback Time for the different scenario #1 #1IAQ #2IAQ #3 #3IAQ #1+2IAQ #1+3IAQ #2IAQ+3 Investment (€) €370,100 €370,100 €35,472 €228,101 €228,101 €405,572 €598,201 €263,573 Energy saving (€) €10,920 €11,027 -€5,579 €6,918 €7,010 €5,069 €18,715 €1,264 SPBT (years) 33.9 33.6 - 33.0 32.5 80.0 32.0 208.5 Table Net Present Value calculated for all measures at different increment of the cost of energy (ie) considering or not the tax refund of 65% (+TR) that is the private investment scenario A lifespan of 25 years is considered Reference investment case: Public investment Discount interest rate case: Public investment Private investment Measures ie = 3% ie = 5% ie = 3% ie = 5% ie =3% + TR (65%) #1 -€7,987 €117,241 -€ 1,085 €126,339 €226,093 #1IAQ -€4,251 €122,204 €2,723 €131,394 €229,829 #2IAQ -€267,947 -€331,924 -€272,386 -€337,485 -€245,511 #3 €13,708 €93,048 €18,320 €99,050 €157,977 #3IAQ €16,913 €97,304 €21,586 €103,386 €161,182 #1+2IAQ -€285,433 -€227,299 -€ 283,151 -€223,998 -€28,918 #1+3AQ €36,343 €250,957 €48,440 €266,817 €414,691 #2IAQ+3 -€256,876 -€242,379 -€256,754 -€ 242,003 -€90,172 Discussion The analysis highlights that retrofitting an existing school building may produce both significant energy and CO2 emission savings Nevertheless, consistent installation costs reflect in SPBT greater than 30 years Fig shows the variation of Net Present Value during the 25 years for the different measures analyzed with or without tax refund Here, NPV was calculated using an increment of the cost of energy of 3% per year Fig (a) shows that without tax refund (public investment scenario) the NPV is around 20,000 € only for scenario #3 and #3IAQ and around 50,000 € for measure #1+3IAQ It is below 5,000 € for new windows installation, with pay back investment time of about 33 years a b Fig NPV of the different energy measures: (a) without tax refund (r=0.25% or the first 20 years, then r=0.5%), (b) with tax refund (r=0.5%) L Loreti et al / Energy Procedia 101 (2016) 1026 – 1033 1033 Contrarily, claiming a 65% tax refund in 10 years (private investment scenario), Fig (b) results in pay back investment times consistently shorter: between 10 and 12 years for all measures excluding those that include the installation of the mechanical ventilation system That is because it implies on one hand relevant operating costs, on the other it requires more expensive annual maintenance costs The change of the slope at year 10 is due to the end of the period of tax refund in this case After 25 years, the retrofitting solution with the highest NPV is the one provided by #1+3IAQ, nevertheless this combined measure requires the major investment costs Conclusion In the present work, a dynamic simulation was performed on an existing educational building located in Bologna – Italy A retrofit strategy is proposed to improve heating energy consumptions and indoor comfort as well Several measures are presented taking into account two design strategies: reducing the heat transfer by transmission, and decreasing the heat transfer by ventilation Energy savings are calculated by transient simulation using TRNSYS© The evaluated energy measures consisted of windows replacement, internal insulation of attic floor and roof and installation of a heat recovery system at the service of administration offices and classrooms The improvement of actual ventilation rates are considered as well Baseline conditions both for actual ventilation rates and improved ones are presented in order to get more comparable results Finally the evaluation of CO2 emission savings and the economic issue are presented Simple Payback Time (SPBT) and Net Present Value (NPV) were exploited as financial parameters for evaluating the economic feasibility of the different measures, and results are reported either without tax refund claim or considering a 65% tax refund in 10 years, in order to get a comparison between public and private investments that may occur when dealing with educational buildings Advantageous interventions resulted to be those involving the building envelope retrofit, due to large yearly energy savings, albeit they imply consistent investment costs Clearly, if the cost of energy arises energy saving measures appear more feasible Nevertheless, given the actual cost of energy, the costs of investment for building retrofit we believe that the key strategy to make it 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better ventilation rates to classrooms and administration offices,... performances are less demanding, albeit not insulated The administration area only partially takes advantages form this measure since it is located at the ground floor The installation of heat recoveries