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A Design Strategy to Reach nZEB Standards Integrating Energy Efficiency Measures and Passive Energy Use
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 111 (2017) 205 – 214
8th International Conference on Sustainability in Energy and Buildings, SEB-16, 11-13 September 2016, Turin, ITALY
A design strategy to reach nZEB standards integrating energy efficiency measures and passive energy use a
, Fausto Barbolinia, Paolo Cappellaccib, Luca Guardiglia* a
University of Bologna, viale Risorgimento, 2, Bologna, 40136, Italy b GIA, The Whitehouse, Belvedere Road, London, SE1 8, UK
Abstract In this paper a design strategy is presented to integrate energy efficiency measures and passive energy use, in order to meet near Zero Energy Buildings requirements in European temperate climates. In particular, a hybrid system for the integration of natural and mechanical ventilation at different times of the year is proposed. ZEBs and nZEBs usually employ mechanical ventilation systems to provide air changes and energy saving. This common solution is in contrast with the principles of natural ventilation and adaptive comfort. It is well known that natural ventilation can significantly reduce the energy demand during the summer. The case study is represented by a social housing complex located in the periphery of Modena (Italy), dating back to 1980. The project consists in the deep energy renovation of some buildings of this complex to accomplish nZEB standards. The proposal envisages two different modes of operation for the buildings, one for the cold season and one the warm season. For the cold season, a mechanical ventilation system with earth tubes and heat recovery has been designed, together with airtightness, solar greenhouses and high thermal mass and insulation. For the warm season the design allows a free-running use: open trickle ventilators applied to windows which provide background ventilation, mass and insulation mitigate the heat loads, vertical ventilation shafts support natural ventilation and free night cooling. The ventilation shafts are designed with aerodynamic principles to provide each apartment with additional (and maximised) differences of pressure due to the stack effect. The ventilation shafts have an important role for the free cooling. The renovated buildings are also equipped with active systems to compensate the remaining energy demand: a Combined Heating Power System (CHP), PV panels and solar thermal collectors. The indoor comfort conditions in the warm season are evaluated according to the ASHRAE 55 adaptive model for free-running buildings. The internal comfort in the warm season is verified with a multizone dynamic simulation and a CFD analysis. The results of the study confirm that in the warm season acceptable indoor comfort conditions can be achieved in a free running nZEB. © 2017 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 KES International. Keywords: Near Zero Energy Buildings; integrated design; hybrid ventilation systems, ventilation shafts.
* Corresponding author. Tel.: +39-051-2093179; fax: +39-051-2093156. E-mail address: [email protected]
1876-6102 © 2017 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 KES International. doi:10.1016/j.egypro.2017.03.022
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Fausto Barbolini et al. / Energy Procedia 111 (2017) 205 – 214
1. Introduction In general terms a Zero Energy Building (ZEB), or Net-Zero Energy Building (NZEB), uses all cost-effective measures to reduce energy usage through energy efficiency and includes renewable energy systems that produce enough energy to meet remaining energy needs during [1]. The wording “Net” emphasizes the energy exchange between the grid connected building and the energy infrastructure [2]. In fact, most zero net energy buildings get part of their energy from the grid and return the same amount during the year, when their production of renewable energy is higher than the demand. Conversely, according to the EU Directive of 2010 (EPBD), a near Zero Energy Building (nZEB) still requires a small amount of energy on a yearly basis [3]. In the last years many definitions of NZEBs were given, depending on how and where this renewable energy is produced [4]. The same acronym is often used for the similar concept of zero emissions buildings, which is slightly different. Regardless these definitions, the efficiency required to achieve zero or near-zero energy goals leads essentially to two different actions: reducing energy demand by means of energy efficiency measures and passive energy use, and generating energy from renewable sources. There are many design strategies to increase energy efficiency, including airtightness to avoid infiltration and mechanical ventilation systems with heat recovery to provide air conditioning and indoor air quality (IAQ). The designed energy balance of a ZEB can however be invalidated with an improper use of technologies by occupants, such as opening windows, changing the operative temperature or not providing the right maintenance of systems [5]. Natural ventilation systems generally contrast with the principle of mechanical control of indoor environment [6, 7]. The mechanical control of thermal comfort, much emphasized in ZEBs, aims to reduce the interaction of users with the outdoor environment and this contrasts with the principles of psychological comfort. Besides, monitoring of ultra-low energy buildings in Italy has revealed that occupants rarely use mechanical systems properly and the energy consumption often exceeds the expected results. Thermal comfort is defined as that condition of mind which expresses satisfaction with the thermal environment [8] and depends on physiological and psychological aspects. The former have been widely investigated by Fanger and other scholars, the latter seem to be neglected, at least in the current design. Psychological aspects of comfort involve the interaction of occupants with the environment and vary with latitude, cultural and social factors. Adaptive thermal models, essentially valid for free running buildings, are based on the assumption by Humphreys and Nicol: ‘if a change occurs such as to produce discomfort, people react in ways which tend to restore their comfort’. These models are entirely focused on psychological aspects and allow wider tolerances of indoor thermal comfort conditions than the physiological-only ones [9]. The goal of this paper is to propose a design strategy to integrate natural and mechanical ventilation systems in a nZEB at different times of the year, overcoming airtightness related problems and thermal comfort ones. Energy balances are estimated according to current Italian standards; the concept of near Zero Energy Building is then referred to the global Energy Performance Index (Ep gl) [Kwh/m2y], as defined by the recent Decreti Interministeriali (June 2015). The Index takes into account the annual renewable demand of primary energy for space heating, water heating, cooling, lighting and equipment, in relation to the usable surface of the building, and should be near zero. By the beginning of 2021 all new Italian buildings must be nZEBs. 2. Design strategies The design strategies are applied to the deep renovation of a social housing complex in the city of Modena, dating from 1980 (Figure 1a). The costs of this operation are not discussed in this paper. However, there are different ways of accomplishing the renovation, in relation to the amount of original structure that is going to be kept in place. The new buildings have approximately the same shape and volume of the old ones, except for new sunspaces and tilted roofs to host Photovoltaic panels. The existing buildings are located in a typical temperate climate: winter is not very cold, with temperatures rarely under 0°C; summer can be slightly hot, with the average of daily temperatures in the warmest months around 30°C. Wind speed is generally low at all times of the year, with values around 1,5 m/s, except for some gust at 5-8 m/s. The site of the building has 2258 Degree Days. The project (Figure 1b) seeks to follow a cost control strategy, taking into account three main objectives: x Energy Saving through Building Design
Fausto Barbolini et al. / Energy Procedia 111 (2017) 205 – 214
x Energy Efficiency of Mechanics and HVAC x Energy Production from Renewable Sources. The integrated design concerns all these objectives at the same time, with choices that have influences on the whole system according to an holistic approach. 2.1. Building Design The Energy Saving goal is achieved through Building Design (Figure 2), which concerns: x Optimizing the shape of building, with a new tilt angle of roof and shadow analysis; x Maintaining the exposed surfaces of facades, which are relatively limited compared to their inner volume, with a ratio of 0,3; x Optimizing solar gains through greenhouses; x Passive cooling [10, 11]: cross ventilation for all apartments and construction of new internal shafts for additional ventilation in the warm season [12]; x Increasing green surfaces for climate mitigation [13] x Air tightness windows with additional trickle ventilators (to be opened in the warm season) [14]. x Choice of insulating materials as reported in table 1.
Fig. 1. (a) existing building; (b) new proposal.
Fig. 2. Passive Building Design strategies.
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The expected solar gain of the newly designed greenhouses is 15% of total winter heating demand for adjacent rooms. Photovoltaics are placed on the roof, in order to reach a production of 15 Kwh/m2 y (electricity). The heat contribution from greenhouses is given by our previous studies on similar constructions [15], and corresponds to literature data [16]. Table 1. U-Values of building after rehabilitation. Element
Materials
U-Value (W/m2 K)
External wall
10 mm internal plaster 250 mm perforated bricks 320 mm rockwool panels 20 mm reinforced external plaster
0,107
Outer floor
30 mm finishing flooring 50 mm sand-cement screed 240 mm concrete floor 320 mm rockwool panels 20 mm reinforced external plaster
0,109
Roof
70 mm terracotta shingles 320 mm rigid rockwool panels 240 mm concrete floor 10 mm internal plaster Triple glazed 3x4mm,
0,110
Windows (general)
14 mm air gap Double glazed 3x4mm,
1,00
Windows (on greenhouses)
14 mm air gap
1,30
The focus of the project is then the use of hybrid ventilation [17]: mechanical in the cold season, natural in the warm one. In extremely hot conditions in summer natural ventilation can be assisted by HVAC systems. A new ventilation shaft is placed at the centre of the building, near the staircase. The ventilation shaft is designed to maximise the indoor-outdoor difference of pressure in all the apartments, to provide enough ventilation in the warm season, applying the stack effect equation:
'ps
§
·
© Ti
¹
U Ui g H NPL H U ¨¨ Ti Te ¸¸ g H NPL H
(1)
The height of HNPL has been estimated according to Ashrae guidelines, with values typically of 0,7 of total height of the shaft. To optimise its effect, the shaft has been divided in two separate portions, the former for the lower stories, the latter for the two upper ones, as shown in Figure 3. The difference of temperature from the inlet to the outlet of the shaft has been initially settled at 25 °C, considering the presence of a solar chimney at the top. The pressure of wind has been also calculated at the various stories:
'p w
Cp U
U 2 § Ti Te · ¨ ¸ g H NPL H 2 ¨© Ti ¸¹
(2)
Cp coefficient has been estimated through both empirical tables and CpCalc+ software. As shown in Figure 4, in the climate of the building site, the effects of wind pressure have little relevance compared to the stack effect ones [18]. Once ∆p has been known, the airflows are calculated by the equation: § 2'p · Q Cd ¨¨ ¸¸ © U ¹
0,5
(3)
Fausto Barbolini et al. / Energy Procedia 111 (2017) 205 – 214
Cd is a discharge coefficient for the sharp-edged orifice, taken as 0,61. The areas of the inlet openings in the shaft are set as 0,06 m2 (30cm x 20 cm). By applying the equation (3), a first empirical verification shows that the airflow through the shaft at each story allows a air change rate of at least 1 volume per hour. The same results are given for a reduced difference of temperature between inlets and the outlet of the shaft of 10°C. The results suggest that a free running use of the building guarantees a good IAQ.
Fig. 3. Differences of pressure due to wind and stack effect. Shaft optimisation.
3. Mechanics and HVAC Energy Efficiency is achieved by the choice of high performance systems together with the entire design of the building. Designing a ZEB (or nZEB) necessarily requires to choose renewable energy technologies, and the choice of HVAC systems is strictly related to this condition. A first overview of the project highlights questions to be solved: x The surfaces for PV and solar thermal collectors of roofs are relatively small related to the numbers of apartments, and the energy production does not supply all the needs of the building; x An additional and renewable system of energy production has thus to be provided to achieve the Zero Energy goal; x The heating system of the cold season should be able to operate in reverse even in the warm season, so that a single mechanical system serves the building; x HVAC should be shut down in the warm season in behalf of natural ventilation; x IAQ has to be guaranteed at all times of the year, despites air tightness of windows. The choice adopted for the building consists of a mechanical ventilation system equipped with earth tubes, heat recovery and a heating unit. During the cold season the system provides heating and ventilation in each apartment (the ventilation shaft is closed). During the warm season HVAC system should be off in behalf of natural ventilation: the background ventilation is then provided by the trickle ventilators in windows and the open shaft. In case of extreme climate conditions the HVAC system can be turned on and the ground-coupled heat exchanger provides cool and de-humidified fresh air. Hot water for domestic use and heating and electricity are then provided by a Combined Heating Power System (CHP), as long as PV and solar thermal collectors are not sufficient.The Figure 4 shows the operational scheme of the system in the cold season. Figure 5 shows the ventilation system scheme. Figure 8 represents a relevant section of the building. To optimise the sun-exposed surfaces of the roofs, PV is designed to be settled for 604 m2, while photovoltaic thermal hybrid solar collectors are designed for the remaining 250 m2. The total energy production is about 155.100
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kWh per year, and the hot water production supplies all the needs of the building for the most of the year.
Fig. 4. Operational scheme in the cold season.
Fig. 5. Ventilation system scheme.
The electric needs of the building are estimated in 171.000 kWh per year through the SVI index (StromVerbrauchsIndex) [19] related to the number of occupants of every single apartment according to the
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equation 4: N. of occupants x 500 kWh + 500 kWh = goal value per apartment in kWh
(4)
A CHP system supplies the electric deficit of PV and also provides hot water for heating and domestic use in the cold season. A power of 80 kW is sufficient to furnish hot water and the residual electric energy needed. The building energy rating is evaluated according to EN ISO 13790:2008 in 7,7 kWh/m 2 per year [20]; almost all the energy demand is balanced by the production from renewable sources; this production could easily turn into a slight surplus, with a few more devices. As the global Energy Performance Index is less than 25% of the Italian standard (Ep gl < 0,25 Epgl lim), the building belongs to the top class A4, which corresponds to a near Zero Energy Building if more than 50% of the energy demand is covered by renewables. There is no actual need to push the performance to an energy plus standard [21], once the nZEB goal is achieved. 4. Thermal comfort analysis Thermodynamic and a CFD analyses have been carried out to evaluate the effectiveness of the adopted solutions. The main purpose of the analyses was to estimate the indoor thermal comfort conditions, especially in the warm season when a free running use of the building is desired. The results have been checked through the Ashrae 55 standard for buildings with no HVAC systems working [22, 23]. 4.1 Multizone thermodynamic analysis A relevant portion of the building has been modelled and analysed in the typical design summer week. The internal environments have been modelled in different ways to evaluate the influence of the shaft on performances. The settings of the simulation included air infiltration for 0,5 volumes per hour in each apartment, a night cooling program of 5% windows openings between 8pm and 7am and an advanced mathematical pattern for ventilation calculation. The output results of the simulation are copious and detailed. Hence the main and interesting outputs are presented. A global overview of the internal temperatures, presented in Figure 6, shows that the average values range between 25°C and 28°C, despites the daily temperature fluctuations.
Fig. 6. Internal and external temperatures in the typical design summer week.
A detailed analysis of the various zones shows that local temperatures vary at each story. If the lower stories are the cooler ones, with temperatures of 1°C below the building average, the upper floors reach temperatures of 29°C in the hottest hours of the day. The entire results for each story and zone of the model have been evaluated according to Ashrae 55 standard and are reported in Figure 7. As it is clearly visible, thermal comfort conditions are satisfied within a 80% of acceptability limit in the hottest days of the year. A more accurate analysis has been lead for the rooms served by the shaft to check the internal conditions of comfort. A simplified model of seven rooms, one for each story, and the annexed ventilation shaft has been analysed. The boundary conditions have been set adopting the output of the thermodynamic simulation in a typical situation of night cooling, at 4am of 7th August, when the outdoor air temperature is about 20°C and the average of the whole building is 26,5°C. The grid used for the model has steps of 0,1m, with 0,05m subdivisions close to the openings, the shaft and the surfaces of walls and floors.
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The results of the simulation show the temperature gradient across the height of the building, as expected, and the air speed, never exceeding 0,25m/s in the rooms (Figure 9b). The air speed and the temperature in the shaft are instead much higher and this causes the cooling effect in the rooms. A further detailed look at the results in every single room of the model shows the convective movements of air just adjacent the floor and the walls, with the air speed in the middle of the rooms quite little (Figure 9c, figure 10).
Fig. 7. Internal comfort conditions in Ashrae 55 adaptive model chart. In blue the upper floor, in yellow the lower floor, and in red the average of the whole building.
Fig. 8. Relevant section of the ventilation shaft and the HVAC system shaft.
Fausto Barbolini et al. / Energy Procedia 111 (2017) 205 – 214
Fig. 9. Model (a) and some results of the CFD analysis: vertical section with the distribution of the temperature in the stories (b) and temperature and air speed distribution in a first floor room (c).
Fig. 10. Temperature (a) and air speed distribution (b) in a first floor room.
5. Discussion The case study presented shows the possibility to achieve good levels of comfort in near Zero Energy Buildings in temperate climates through passive strategies of design, in the perspective of minimum impact on the environment and resources. The results are the sum of various systems that work together, from building design to mechanics and HVAC systems. The ventilation shaft, which is an element that characterise the project, is a part of this integrated process of design. There are a few examples of these solutions in the literature review, but none of them seems to have been applied to nZEBs [24]. Indeed adopting passive strategies of cooling is not a requirement to design a ZEB, or nZEB, as the zero energy goal can be achieved in many other ways. If in winter heating is required, acceptable indoor thermal comfort conditions are still achievable even without the use of HVAC systems in the warm season. Adaptive comfort models indeed guarantee high psychological and physical satisfaction by occupants, moreover admitting wider ranges of temperature. The outputs of thermal simulations show acceptable conditions and are still preventive: indeed, a simple change of a parameter can adulterate the results. This is the reason why settings have been precautionary, e.g. the percentage of opening of the
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windows have just been set at 5% and could be more, or the timing of windows opening has not intentionally set in the best desirable conditions. A consideration should be also done on the possibility of using the HVAC system in the hottest days: the position adopted for the project is not absolute, and in extreme climate conditions mechanics can be turned on at any time. Therefore, in extremely hot conditions natural ventilation can be assisted by HVAC systems without any relevant impact in the annual energy balance. Moreover, the ventilation shaft represents an option among the various possible, which needs to be checked with the layout of the building and the construction constraints. In the site of the case study its contribution to the final result is significant, as it increase the airspeed in the room and accelerates the cooling, but it is part of a complex. For higher heights and windier locations its contribution for free cooling could be decisive. 6. Conclusions The case study presents a strategy of integrated energy efficiency measures and passive energy use in a high-density building. In particular, the use of hybrid ventilation in buildings is suggested: mechanical ventilation in the cold season and natural ventilation in the warm one. This strategy leads to an adaptive thermal comfort during the summer. This is in contrast with the current tendency to use HVAC systems during the summer in ZEBs. The paper analyses only one case study; more likely, the results can be extended to other similar projects of deep renovation, where the ventilation shafts can be successfully adopted with some adjustments. References [1] US Department of Energy. A Common Definition for Zero Energy Building. September 2015. p. 1. [2] Pless S, Torcellini P. Net-Zero Energy Buildings: A Classification System Based on Renewable Energy Supply Options. NREL TP-55044586; 2010. [3] Directive 2010/31/EU of the European Parliament and the Council of 19 May 2010 on the energy performance of buildings (EPBD); 2010. [4] Torcellini P, Pless S, Deru M. Zero Energy Buildings: a critical look at the definition, Pacific Grove, California; 2006. [5] Liddament M.V. Occupant Impact on Ventilation. AIVC Technical Report 53. INIVE EEIG, Sint-Stevens-Woluwe (Bruxelles); 2001. [6] Allard F. Natural ventilation in buildings – A Design Handbook. London: James & James Ltd; 1998. [7] ASHRAE, 2016 ASHRAE Handbook – HVAC Systems and Equipments. Atlanta; 2016. [8] ANSI/ASHRAE Standard 55-2013. Thermal Environmental Conditions for Human Occupancy. [9] Humphrey MA, Nicol JF. Understanding the Adaptive Approach to Thermal Comfort. ASHRAE Transactions 104 (1); 1998. p. 991-1004. [10] Short CA, Lomas KJ, Wood A. Design strategy for low-energy ventilation and cooling within an urban heat island. Build Res Inf. 2004;32(3):187-206. [11] Santamouris M, Asimankopoulos D. Passive cooling of buildings. London: James & James Ltd; 1996. [12] Axley JW. Residential Passive Ventilation Systems: Evaluation and Design. AIVC Technical Report 54. INIVE EEIG, Sint-StevensWoluwe (Bruxelles); 2001. [13] Fioretti R, Palla A, Lanza LG, Principi P. Green roof energy and water related performance in the Mediterranean climate. Build Environ. 2010;45(8):1890-1904. [14] Karava P, Stathopoulos T, Athienitis AK. Investigation of the performance of trickle ventilators. Build Environ. 2003;38(8);981-993. [15] Bigelli E, Guardigli L, Venturi D. Solar Greenhouse and natural ventilation. In: 31st Air Infiltration and Ventilation Centre Conference 2010: Low Energy and Sustainable Ventilation Technologies for Green Buildings. 1. New York: Curran Associates Inc; 2012. p. 504-523. [16] Asdrubali F, Cotana F, Messineo A. On the Evaluation of Solar Greenhouse Efficiency in Building Simulation during the Heating Period, Energies 5; 2012. p. 1864-1880. [17] Guardigli L, Cappellacci P, Barbolini F. Passive cooling Through Ventilation Shafts in High-Density Zero Energy Buildings: A Design Strategy to Integrate Natural and Mechanical Ventilation in Temperate Climates. In: 35th AIVC Conference: Ventilation and airtightness in transforming the building stock to high performance; 2014 Sept 24-25; Poznan (Poland); 2014. [18] Grosso M, Marino D, Parisi E, A wind pressure distribution calculation program for multi zone air flow models. In. Building Simulation ’95; 1995 Aug 14-16; Madison (USA); 1995. p. 105-118. [19] Verbraucherzentrale SüdTirol, SVI Index; 2008. [20] EN ISO 13790:2008 – Energy performance of buildings – calculation of Energy use for space, heating and cooling; 2008. [21] Rodriguez-Ubinas E, Montero C, Porteros M, Vega S, Navarro I, Castillo Cacigal M, Matallanas E, Gutierrez A. Passive design strategies and performance of Net Energy Plus Houses. Energ Buildings. 2014;83:10-22. [22] Brager GS, De Dear RJ. Thermal comfort in naturally ventilate buildings: revisions to ASHRAE Standard 55; 2002. [23] AIVC Centre – AIVC, Ventilation Information Paper 12 – Adaptive Thermal Comfort and Ventilation, INIVE EEIG, Sint-Stevens-Woluwe (Bruxelles); 2006. [24] Prajongsan P, Sharples S. The ventilation shaft: An alternative passive cooling strategy for high-rise residential buildings in hot-humid climates. In: 28 th International PLEA Conference: Opportunities, Limits & Needs Towards an environmentally responsible architecture; 2012 Nov 7-9; Lima (Perù); 2012.
ScienceDirect Energy Procedia 111 (2017) 205 – 214
8th International Conference on Sustainability in Energy and Buildings, SEB-16, 11-13 September 2016, Turin, ITALY
A design strategy to reach nZEB standards integrating energy efficiency measures and passive energy use a
, Fausto Barbolinia, Paolo Cappellaccib, Luca Guardiglia* a
University of Bologna, viale Risorgimento, 2, Bologna, 40136, Italy b GIA, The Whitehouse, Belvedere Road, London, SE1 8, UK
Abstract In this paper a design strategy is presented to integrate energy efficiency measures and passive energy use, in order to meet near Zero Energy Buildings requirements in European temperate climates. In particular, a hybrid system for the integration of natural and mechanical ventilation at different times of the year is proposed. ZEBs and nZEBs usually employ mechanical ventilation systems to provide air changes and energy saving. This common solution is in contrast with the principles of natural ventilation and adaptive comfort. It is well known that natural ventilation can significantly reduce the energy demand during the summer. The case study is represented by a social housing complex located in the periphery of Modena (Italy), dating back to 1980. The project consists in the deep energy renovation of some buildings of this complex to accomplish nZEB standards. The proposal envisages two different modes of operation for the buildings, one for the cold season and one the warm season. For the cold season, a mechanical ventilation system with earth tubes and heat recovery has been designed, together with airtightness, solar greenhouses and high thermal mass and insulation. For the warm season the design allows a free-running use: open trickle ventilators applied to windows which provide background ventilation, mass and insulation mitigate the heat loads, vertical ventilation shafts support natural ventilation and free night cooling. The ventilation shafts are designed with aerodynamic principles to provide each apartment with additional (and maximised) differences of pressure due to the stack effect. The ventilation shafts have an important role for the free cooling. The renovated buildings are also equipped with active systems to compensate the remaining energy demand: a Combined Heating Power System (CHP), PV panels and solar thermal collectors. The indoor comfort conditions in the warm season are evaluated according to the ASHRAE 55 adaptive model for free-running buildings. The internal comfort in the warm season is verified with a multizone dynamic simulation and a CFD analysis. The results of the study confirm that in the warm season acceptable indoor comfort conditions can be achieved in a free running nZEB. © 2017 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 KES International. Keywords: Near Zero Energy Buildings; integrated design; hybrid ventilation systems, ventilation shafts.
* Corresponding author. Tel.: +39-051-2093179; fax: +39-051-2093156. E-mail address: [email protected]
1876-6102 © 2017 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 KES International. doi:10.1016/j.egypro.2017.03.022
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Fausto Barbolini et al. / Energy Procedia 111 (2017) 205 – 214
1. Introduction In general terms a Zero Energy Building (ZEB), or Net-Zero Energy Building (NZEB), uses all cost-effective measures to reduce energy usage through energy efficiency and includes renewable energy systems that produce enough energy to meet remaining energy needs during [1]. The wording “Net” emphasizes the energy exchange between the grid connected building and the energy infrastructure [2]. In fact, most zero net energy buildings get part of their energy from the grid and return the same amount during the year, when their production of renewable energy is higher than the demand. Conversely, according to the EU Directive of 2010 (EPBD), a near Zero Energy Building (nZEB) still requires a small amount of energy on a yearly basis [3]. In the last years many definitions of NZEBs were given, depending on how and where this renewable energy is produced [4]. The same acronym is often used for the similar concept of zero emissions buildings, which is slightly different. Regardless these definitions, the efficiency required to achieve zero or near-zero energy goals leads essentially to two different actions: reducing energy demand by means of energy efficiency measures and passive energy use, and generating energy from renewable sources. There are many design strategies to increase energy efficiency, including airtightness to avoid infiltration and mechanical ventilation systems with heat recovery to provide air conditioning and indoor air quality (IAQ). The designed energy balance of a ZEB can however be invalidated with an improper use of technologies by occupants, such as opening windows, changing the operative temperature or not providing the right maintenance of systems [5]. Natural ventilation systems generally contrast with the principle of mechanical control of indoor environment [6, 7]. The mechanical control of thermal comfort, much emphasized in ZEBs, aims to reduce the interaction of users with the outdoor environment and this contrasts with the principles of psychological comfort. Besides, monitoring of ultra-low energy buildings in Italy has revealed that occupants rarely use mechanical systems properly and the energy consumption often exceeds the expected results. Thermal comfort is defined as that condition of mind which expresses satisfaction with the thermal environment [8] and depends on physiological and psychological aspects. The former have been widely investigated by Fanger and other scholars, the latter seem to be neglected, at least in the current design. Psychological aspects of comfort involve the interaction of occupants with the environment and vary with latitude, cultural and social factors. Adaptive thermal models, essentially valid for free running buildings, are based on the assumption by Humphreys and Nicol: ‘if a change occurs such as to produce discomfort, people react in ways which tend to restore their comfort’. These models are entirely focused on psychological aspects and allow wider tolerances of indoor thermal comfort conditions than the physiological-only ones [9]. The goal of this paper is to propose a design strategy to integrate natural and mechanical ventilation systems in a nZEB at different times of the year, overcoming airtightness related problems and thermal comfort ones. Energy balances are estimated according to current Italian standards; the concept of near Zero Energy Building is then referred to the global Energy Performance Index (Ep gl) [Kwh/m2y], as defined by the recent Decreti Interministeriali (June 2015). The Index takes into account the annual renewable demand of primary energy for space heating, water heating, cooling, lighting and equipment, in relation to the usable surface of the building, and should be near zero. By the beginning of 2021 all new Italian buildings must be nZEBs. 2. Design strategies The design strategies are applied to the deep renovation of a social housing complex in the city of Modena, dating from 1980 (Figure 1a). The costs of this operation are not discussed in this paper. However, there are different ways of accomplishing the renovation, in relation to the amount of original structure that is going to be kept in place. The new buildings have approximately the same shape and volume of the old ones, except for new sunspaces and tilted roofs to host Photovoltaic panels. The existing buildings are located in a typical temperate climate: winter is not very cold, with temperatures rarely under 0°C; summer can be slightly hot, with the average of daily temperatures in the warmest months around 30°C. Wind speed is generally low at all times of the year, with values around 1,5 m/s, except for some gust at 5-8 m/s. The site of the building has 2258 Degree Days. The project (Figure 1b) seeks to follow a cost control strategy, taking into account three main objectives: x Energy Saving through Building Design
Fausto Barbolini et al. / Energy Procedia 111 (2017) 205 – 214
x Energy Efficiency of Mechanics and HVAC x Energy Production from Renewable Sources. The integrated design concerns all these objectives at the same time, with choices that have influences on the whole system according to an holistic approach. 2.1. Building Design The Energy Saving goal is achieved through Building Design (Figure 2), which concerns: x Optimizing the shape of building, with a new tilt angle of roof and shadow analysis; x Maintaining the exposed surfaces of facades, which are relatively limited compared to their inner volume, with a ratio of 0,3; x Optimizing solar gains through greenhouses; x Passive cooling [10, 11]: cross ventilation for all apartments and construction of new internal shafts for additional ventilation in the warm season [12]; x Increasing green surfaces for climate mitigation [13] x Air tightness windows with additional trickle ventilators (to be opened in the warm season) [14]. x Choice of insulating materials as reported in table 1.
Fig. 1. (a) existing building; (b) new proposal.
Fig. 2. Passive Building Design strategies.
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The expected solar gain of the newly designed greenhouses is 15% of total winter heating demand for adjacent rooms. Photovoltaics are placed on the roof, in order to reach a production of 15 Kwh/m2 y (electricity). The heat contribution from greenhouses is given by our previous studies on similar constructions [15], and corresponds to literature data [16]. Table 1. U-Values of building after rehabilitation. Element
Materials
U-Value (W/m2 K)
External wall
10 mm internal plaster 250 mm perforated bricks 320 mm rockwool panels 20 mm reinforced external plaster
0,107
Outer floor
30 mm finishing flooring 50 mm sand-cement screed 240 mm concrete floor 320 mm rockwool panels 20 mm reinforced external plaster
0,109
Roof
70 mm terracotta shingles 320 mm rigid rockwool panels 240 mm concrete floor 10 mm internal plaster Triple glazed 3x4mm,
0,110
Windows (general)
14 mm air gap Double glazed 3x4mm,
1,00
Windows (on greenhouses)
14 mm air gap
1,30
The focus of the project is then the use of hybrid ventilation [17]: mechanical in the cold season, natural in the warm one. In extremely hot conditions in summer natural ventilation can be assisted by HVAC systems. A new ventilation shaft is placed at the centre of the building, near the staircase. The ventilation shaft is designed to maximise the indoor-outdoor difference of pressure in all the apartments, to provide enough ventilation in the warm season, applying the stack effect equation:
'ps
§
·
© Ti
¹
U Ui g H NPL H U ¨¨ Ti Te ¸¸ g H NPL H
(1)
The height of HNPL has been estimated according to Ashrae guidelines, with values typically of 0,7 of total height of the shaft. To optimise its effect, the shaft has been divided in two separate portions, the former for the lower stories, the latter for the two upper ones, as shown in Figure 3. The difference of temperature from the inlet to the outlet of the shaft has been initially settled at 25 °C, considering the presence of a solar chimney at the top. The pressure of wind has been also calculated at the various stories:
'p w
Cp U
U 2 § Ti Te · ¨ ¸ g H NPL H 2 ¨© Ti ¸¹
(2)
Cp coefficient has been estimated through both empirical tables and CpCalc+ software. As shown in Figure 4, in the climate of the building site, the effects of wind pressure have little relevance compared to the stack effect ones [18]. Once ∆p has been known, the airflows are calculated by the equation: § 2'p · Q Cd ¨¨ ¸¸ © U ¹
0,5
(3)
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Cd is a discharge coefficient for the sharp-edged orifice, taken as 0,61. The areas of the inlet openings in the shaft are set as 0,06 m2 (30cm x 20 cm). By applying the equation (3), a first empirical verification shows that the airflow through the shaft at each story allows a air change rate of at least 1 volume per hour. The same results are given for a reduced difference of temperature between inlets and the outlet of the shaft of 10°C. The results suggest that a free running use of the building guarantees a good IAQ.
Fig. 3. Differences of pressure due to wind and stack effect. Shaft optimisation.
3. Mechanics and HVAC Energy Efficiency is achieved by the choice of high performance systems together with the entire design of the building. Designing a ZEB (or nZEB) necessarily requires to choose renewable energy technologies, and the choice of HVAC systems is strictly related to this condition. A first overview of the project highlights questions to be solved: x The surfaces for PV and solar thermal collectors of roofs are relatively small related to the numbers of apartments, and the energy production does not supply all the needs of the building; x An additional and renewable system of energy production has thus to be provided to achieve the Zero Energy goal; x The heating system of the cold season should be able to operate in reverse even in the warm season, so that a single mechanical system serves the building; x HVAC should be shut down in the warm season in behalf of natural ventilation; x IAQ has to be guaranteed at all times of the year, despites air tightness of windows. The choice adopted for the building consists of a mechanical ventilation system equipped with earth tubes, heat recovery and a heating unit. During the cold season the system provides heating and ventilation in each apartment (the ventilation shaft is closed). During the warm season HVAC system should be off in behalf of natural ventilation: the background ventilation is then provided by the trickle ventilators in windows and the open shaft. In case of extreme climate conditions the HVAC system can be turned on and the ground-coupled heat exchanger provides cool and de-humidified fresh air. Hot water for domestic use and heating and electricity are then provided by a Combined Heating Power System (CHP), as long as PV and solar thermal collectors are not sufficient.The Figure 4 shows the operational scheme of the system in the cold season. Figure 5 shows the ventilation system scheme. Figure 8 represents a relevant section of the building. To optimise the sun-exposed surfaces of the roofs, PV is designed to be settled for 604 m2, while photovoltaic thermal hybrid solar collectors are designed for the remaining 250 m2. The total energy production is about 155.100
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kWh per year, and the hot water production supplies all the needs of the building for the most of the year.
Fig. 4. Operational scheme in the cold season.
Fig. 5. Ventilation system scheme.
The electric needs of the building are estimated in 171.000 kWh per year through the SVI index (StromVerbrauchsIndex) [19] related to the number of occupants of every single apartment according to the
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equation 4: N. of occupants x 500 kWh + 500 kWh = goal value per apartment in kWh
(4)
A CHP system supplies the electric deficit of PV and also provides hot water for heating and domestic use in the cold season. A power of 80 kW is sufficient to furnish hot water and the residual electric energy needed. The building energy rating is evaluated according to EN ISO 13790:2008 in 7,7 kWh/m 2 per year [20]; almost all the energy demand is balanced by the production from renewable sources; this production could easily turn into a slight surplus, with a few more devices. As the global Energy Performance Index is less than 25% of the Italian standard (Ep gl < 0,25 Epgl lim), the building belongs to the top class A4, which corresponds to a near Zero Energy Building if more than 50% of the energy demand is covered by renewables. There is no actual need to push the performance to an energy plus standard [21], once the nZEB goal is achieved. 4. Thermal comfort analysis Thermodynamic and a CFD analyses have been carried out to evaluate the effectiveness of the adopted solutions. The main purpose of the analyses was to estimate the indoor thermal comfort conditions, especially in the warm season when a free running use of the building is desired. The results have been checked through the Ashrae 55 standard for buildings with no HVAC systems working [22, 23]. 4.1 Multizone thermodynamic analysis A relevant portion of the building has been modelled and analysed in the typical design summer week. The internal environments have been modelled in different ways to evaluate the influence of the shaft on performances. The settings of the simulation included air infiltration for 0,5 volumes per hour in each apartment, a night cooling program of 5% windows openings between 8pm and 7am and an advanced mathematical pattern for ventilation calculation. The output results of the simulation are copious and detailed. Hence the main and interesting outputs are presented. A global overview of the internal temperatures, presented in Figure 6, shows that the average values range between 25°C and 28°C, despites the daily temperature fluctuations.
Fig. 6. Internal and external temperatures in the typical design summer week.
A detailed analysis of the various zones shows that local temperatures vary at each story. If the lower stories are the cooler ones, with temperatures of 1°C below the building average, the upper floors reach temperatures of 29°C in the hottest hours of the day. The entire results for each story and zone of the model have been evaluated according to Ashrae 55 standard and are reported in Figure 7. As it is clearly visible, thermal comfort conditions are satisfied within a 80% of acceptability limit in the hottest days of the year. A more accurate analysis has been lead for the rooms served by the shaft to check the internal conditions of comfort. A simplified model of seven rooms, one for each story, and the annexed ventilation shaft has been analysed. The boundary conditions have been set adopting the output of the thermodynamic simulation in a typical situation of night cooling, at 4am of 7th August, when the outdoor air temperature is about 20°C and the average of the whole building is 26,5°C. The grid used for the model has steps of 0,1m, with 0,05m subdivisions close to the openings, the shaft and the surfaces of walls and floors.
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The results of the simulation show the temperature gradient across the height of the building, as expected, and the air speed, never exceeding 0,25m/s in the rooms (Figure 9b). The air speed and the temperature in the shaft are instead much higher and this causes the cooling effect in the rooms. A further detailed look at the results in every single room of the model shows the convective movements of air just adjacent the floor and the walls, with the air speed in the middle of the rooms quite little (Figure 9c, figure 10).
Fig. 7. Internal comfort conditions in Ashrae 55 adaptive model chart. In blue the upper floor, in yellow the lower floor, and in red the average of the whole building.
Fig. 8. Relevant section of the ventilation shaft and the HVAC system shaft.
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Fig. 9. Model (a) and some results of the CFD analysis: vertical section with the distribution of the temperature in the stories (b) and temperature and air speed distribution in a first floor room (c).
Fig. 10. Temperature (a) and air speed distribution (b) in a first floor room.
5. Discussion The case study presented shows the possibility to achieve good levels of comfort in near Zero Energy Buildings in temperate climates through passive strategies of design, in the perspective of minimum impact on the environment and resources. The results are the sum of various systems that work together, from building design to mechanics and HVAC systems. The ventilation shaft, which is an element that characterise the project, is a part of this integrated process of design. There are a few examples of these solutions in the literature review, but none of them seems to have been applied to nZEBs [24]. Indeed adopting passive strategies of cooling is not a requirement to design a ZEB, or nZEB, as the zero energy goal can be achieved in many other ways. If in winter heating is required, acceptable indoor thermal comfort conditions are still achievable even without the use of HVAC systems in the warm season. Adaptive comfort models indeed guarantee high psychological and physical satisfaction by occupants, moreover admitting wider ranges of temperature. The outputs of thermal simulations show acceptable conditions and are still preventive: indeed, a simple change of a parameter can adulterate the results. This is the reason why settings have been precautionary, e.g. the percentage of opening of the
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windows have just been set at 5% and could be more, or the timing of windows opening has not intentionally set in the best desirable conditions. A consideration should be also done on the possibility of using the HVAC system in the hottest days: the position adopted for the project is not absolute, and in extreme climate conditions mechanics can be turned on at any time. Therefore, in extremely hot conditions natural ventilation can be assisted by HVAC systems without any relevant impact in the annual energy balance. Moreover, the ventilation shaft represents an option among the various possible, which needs to be checked with the layout of the building and the construction constraints. In the site of the case study its contribution to the final result is significant, as it increase the airspeed in the room and accelerates the cooling, but it is part of a complex. For higher heights and windier locations its contribution for free cooling could be decisive. 6. Conclusions The case study presents a strategy of integrated energy efficiency measures and passive energy use in a high-density building. In particular, the use of hybrid ventilation in buildings is suggested: mechanical ventilation in the cold season and natural ventilation in the warm one. This strategy leads to an adaptive thermal comfort during the summer. This is in contrast with the current tendency to use HVAC systems during the summer in ZEBs. The paper analyses only one case study; more likely, the results can be extended to other similar projects of deep renovation, where the ventilation shafts can be successfully adopted with some adjustments. References [1] US Department of Energy. A Common Definition for Zero Energy Building. September 2015. p. 1. [2] Pless S, Torcellini P. Net-Zero Energy Buildings: A Classification System Based on Renewable Energy Supply Options. NREL TP-55044586; 2010. [3] Directive 2010/31/EU of the European Parliament and the Council of 19 May 2010 on the energy performance of buildings (EPBD); 2010. [4] Torcellini P, Pless S, Deru M. Zero Energy Buildings: a critical look at the definition, Pacific Grove, California; 2006. [5] Liddament M.V. Occupant Impact on Ventilation. AIVC Technical Report 53. INIVE EEIG, Sint-Stevens-Woluwe (Bruxelles); 2001. [6] Allard F. Natural ventilation in buildings – A Design Handbook. London: James & James Ltd; 1998. [7] ASHRAE, 2016 ASHRAE Handbook – HVAC Systems and Equipments. Atlanta; 2016. [8] ANSI/ASHRAE Standard 55-2013. Thermal Environmental Conditions for Human Occupancy. [9] Humphrey MA, Nicol JF. Understanding the Adaptive Approach to Thermal Comfort. ASHRAE Transactions 104 (1); 1998. p. 991-1004. [10] Short CA, Lomas KJ, Wood A. Design strategy for low-energy ventilation and cooling within an urban heat island. Build Res Inf. 2004;32(3):187-206. [11] Santamouris M, Asimankopoulos D. Passive cooling of buildings. London: James & James Ltd; 1996. [12] Axley JW. Residential Passive Ventilation Systems: Evaluation and Design. AIVC Technical Report 54. INIVE EEIG, Sint-StevensWoluwe (Bruxelles); 2001. [13] Fioretti R, Palla A, Lanza LG, Principi P. Green roof energy and water related performance in the Mediterranean climate. Build Environ. 2010;45(8):1890-1904. [14] Karava P, Stathopoulos T, Athienitis AK. Investigation of the performance of trickle ventilators. Build Environ. 2003;38(8);981-993. [15] Bigelli E, Guardigli L, Venturi D. Solar Greenhouse and natural ventilation. In: 31st Air Infiltration and Ventilation Centre Conference 2010: Low Energy and Sustainable Ventilation Technologies for Green Buildings. 1. New York: Curran Associates Inc; 2012. p. 504-523. [16] Asdrubali F, Cotana F, Messineo A. On the Evaluation of Solar Greenhouse Efficiency in Building Simulation during the Heating Period, Energies 5; 2012. p. 1864-1880. [17] Guardigli L, Cappellacci P, Barbolini F. Passive cooling Through Ventilation Shafts in High-Density Zero Energy Buildings: A Design Strategy to Integrate Natural and Mechanical Ventilation in Temperate Climates. In: 35th AIVC Conference: Ventilation and airtightness in transforming the building stock to high performance; 2014 Sept 24-25; Poznan (Poland); 2014. [18] Grosso M, Marino D, Parisi E, A wind pressure distribution calculation program for multi zone air flow models. In. Building Simulation ’95; 1995 Aug 14-16; Madison (USA); 1995. p. 105-118. [19] Verbraucherzentrale SüdTirol, SVI Index; 2008. [20] EN ISO 13790:2008 – Energy performance of buildings – calculation of Energy use for space, heating and cooling; 2008. [21] Rodriguez-Ubinas E, Montero C, Porteros M, Vega S, Navarro I, Castillo Cacigal M, Matallanas E, Gutierrez A. Passive design strategies and performance of Net Energy Plus Houses. Energ Buildings. 2014;83:10-22. [22] Brager GS, De Dear RJ. Thermal comfort in naturally ventilate buildings: revisions to ASHRAE Standard 55; 2002. [23] AIVC Centre – AIVC, Ventilation Information Paper 12 – Adaptive Thermal Comfort and Ventilation, INIVE EEIG, Sint-Stevens-Woluwe (Bruxelles); 2006. [24] Prajongsan P, Sharples S. The ventilation shaft: An alternative passive cooling strategy for high-rise residential buildings in hot-humid climates. In: 28 th International PLEA Conference: Opportunities, Limits & Needs Towards an environmentally responsible architecture; 2012 Nov 7-9; Lima (Perù); 2012.