Building envelope and heat capacity: re-discovering the thermal mass for winter energy saving

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1 346 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Building envelope and heat capacity: re-discovering the thermal mass for winter energy saving S. Ferrari Politecnico di Milano, Italy ABSTRACT In order to simplify the procedure for evaluating winter building energy performances, most of existing standard and rules make reference to the building energy balance under steady state conditions. In fact also the recent implementation of European Performance Building Directive (EPBD) in Italy is related to this simplified approach. Based on the above approach, the effect of thermal mass in building envelope is drastically reduced even if, as a matter of fact, the real heat transfer process in buildings construction hardly depends on material s thermal capacity. Moreover, it is common belief that the positive effect of thermal mass is associated only to building performance in summer season in warm climatic conditions. However, the well-established scientific literature (Olgyay 1963, Givoni, 1967, Szokolay 1980, ASHRAE 2001), as well as traditional architectural solutions, supports the importance of the thermal capacity also in winter conditions which is not considered in common building practices. In the frame of a research project on Massive Building Envelope Thermal Performances, developed in Politecnico di Milano, a set of building energy dynamic simulations has been carried out. The effects of light, medium and heavy envelope constructions, having the same U-values, and the effects of the positioning of the insulation layer in stratified walls, have been evaluated referring to different building typologies, orientations, heating and cooling systems operational modalities in Italian climatic context. As a main result, the thermal capacity reveals its decisive contribution for energy savings, both in winter and in summer conditions. The lower time response in heat transfer process of a building with high thermal inertia in comparison with a light structure, reveals both the decrement of the inside temperature oscillation trend in respect to the external one (more homogeneous levels of comfort conditions all over the 24 hours) and the decrement of the amount of heat flow through the walls (reduction of energy consumption for heating/cooling). Moreover, heat storage in building thermal mass delays the maximum HVAC system energy demand in the hours during which other heat sources could be less incisive (the heating or cooling needs become more homogeneous all over the 24 hours, and the designed plant power could be undersized). As well known, differently from the simplifications adopted in the steady-state building energy balance evaluations, these actual behaviors strongly depend on the variations of the boundary conditions (real environmental conditions) and their evaluation, linked with the heat transfer phenomena, needs to be deeply investigated by more complex methods (e.g. dynamic evaluations based on heat transfer functions method or finite difference method). That means the utilization of sophisticated software, based on at least hourly calculations. In first approximation, considering a regular periodic variation with 24 hours cycle of the main boundary conditions (e.g. external air temperature and solar radiation values), the heat transfer flow can be shown as in the Figure 1 (Szokolay, 2004), where the continuous line represents the heat flow through a massive wall, and the dashed one the heat flow through a hypothetical wall, with the same U-value of the other one, but without any heat capacity. 1. INTRODUCTION The thermal inertia means the heat storage performance of material or building structure and its potential of heat transmission delaying. For the typical European building technologies the thermal inertia, physically defined by the heat capacity value directly proportional to the specific heat and the density of the materials - is commonly associated to the heavy structures, generally also named massive. Figure 1. Hourly heat flow through walls with and without heat capacity However, if the same walls are considered with con- PALENC Vol 1.indd 346 3/9/2007 1:24:48 µµ

2 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and 347 stant boundary conditions (steady-state evaluation) the two heat flows result with identical pattern. As a matter of facts, this last reductive approach is mostly diffused in common building design practices. In fact detailed evaluation methods are still adopted rarely because of the higher know-how and the additional time required to manage appropriate tools. It leads to the risk of large gaps from the building design hypothesis and the actual thermal behavior of the realized building during its life, because of the omitted significant aspects such as the thermal inertia effect. For example, the Italian procedure for assessing heating energy performances in residential buildings (based on EN 832 European method) is limited in such a way. The method for the calculation of building heating energy demand consider an energy balance from thermal losses and thermal gains, evaluated with reference to the mean air temperature and solar radiation data (monthly or seasonal), and maintaining the internal set point temperature as a constant. The thermal inertia is considered only in relation to the building materials facing inside in terms of their effect on the useful internal heat gains reduction (utilization factor). Beyond the standards, several existing software are able to predict actual hourly thermal flows throughout the building components, utilizing annual hourly climatic data. These tools can evaluate internal hourly temperatures (free floating) or the heating/cooling energy demand for maintaining hourly scheduled indoor temperatures. The most sophisticated ones can also provide for the evaluation of the energy consumptions, by simulating the hourly performances of the plants: among them, one of the most utilized, since 70, is DOE-2 code (LBL, 1994), adopted for the evaluations reported in the present study through the interface VisualDOE-4.1 (AEC, 2005). Similar results could be obtained by other analogous simulation programs (Witte et al. 2001). 2. ASSESSING BUILDING MODELS FOR DYNA- MIC ENERGY SIMULATIONS The proposed main investigation on the effects of thermal mass started from the energy-performance comparison between a light building envelope (gypsum-plaster sandwich filled by mineral fiber) and a heavy one (plastered clay bricks) having the same U-value, solar absorption coefficient and infrared emissivity. In order to highlight the performance gap between the two cases, 95 cm of clay bricks has been assumed as the massive reference case (the maximum wall construction thickness allowed in DOE-2) corresponding, based on the same U-value, to 6 cm of mineral fiber for the light reference case one. Table 1: Vertical envelope constructions U-value W/m 2 K Heat Capacity kj/m 2 K Thickness excl. plaster Heavy cm Light cm 2.1 Common assumptions for the building simulation models The building floors, basements, roofs and internal partitions have been assumed with the same constructions for all the simulated cases, referring to the most common national practice adopted in residential buildings (masonry and hollowed clay blocks). Also windows have been assumed same for all the cases. The performances of both the cases have been evaluated in terms of annual energy demand for heating and cooling (energy demand from systems) by adopting a conventional system able to provide both heating and cooling needs (four-pipe fan-coil). Thermal zones have been described through their lighting, equipments, occupancy hourly scheduled loads (typical for residential buildings) and considering a constant air-change equal to 0.5 vol/hour. It has to be noted that the main focus of the study is to evaluate the comparative performances between the two kind of vertical envelopes, therefore the above common assumptions have rather insignificant effects on the evaluations. 2.2 Variable parameters for the building simulation models In order to cover several cases, common in actual built environment, following variables have been considered. Buildings characterizations Four main building typologies have been considered (Fig. 2) based on different envelope surface area/conditioned volume ratio (S/V). one storey building (cottage, 4 front facing, S/V = 0.83) two storeys small building (2 front facing, S/V = 0.64) four storeys building (4 front facing, S/V = 0.43); sixteen storeys tall building (4 front facing, S/V = 0.22) Buildings thermal zones have been designed considering the external interaction (different for each exposure), the central positions (without direct link to outdoor) and the temporary functions (unconditioned passageways). As a consequence, the extension of windows has been assigned in respect to the minimum national requirement for the natural air-luminance ratio (window surface 1/8 zone floor), prevailing one of the main building fronts for all building typologies except the tall one (two main opposite faces). PALENC Vol 1.indd 347 3/9/2007 1:24:48 µµ

3 348 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Figure 2: Building energy simulation models Systems operational modalities Three different system operational modalities have been considered: continuous, on-off, set back (in this case standard 20 C for heating and 26 C for cooling set point temperatures become respectively 16 C and 30 C in set back hourly schedule). Climatic locations A first sensitivity analysis of building performances corresponding to different national climatic conditions have been considered based on three different locations representative of north (Milan), center (Rome) and south (Palermo) using TRY-test reference year- data. Building orientation In order to check different effects of thermal capacity in relation to the sun exposure, four different azimuths for the buildings have been considered (prevailing window facing south, east, west and north). Figure 3: Base-case energy demand (comparison Light-Heavy envelope) Starting from the previous base-case, the consistency of the performance trend has been verified by simulating the following cases. Case 1: Base-case with different system operational modality. Variant A: modality On-Off. 20 C heating - 26 C cooling set point temperature scheduled during main day hours and thermostat switched off in the rest of the time (total activation 12 hours/day). The results are shown in the Figure ENERGY SIMULATIONS RESULTS By the comparison of the two solutions of opaque envelope (light vs. heavy) with the four alternative of building typologies, the three system operational modality, the three climatic locations and the four building azimuths, a matrix of 288 cases will result. For reducing the number of cases, with the aim of an easier management of the output data, three of the four variables have been fixed and, step by step, by varying the fourth parameter the consistency of performance trend has been tested. Based on this approach, the procedure started from a reference case: comparison light-envelope vs. heavy-envelope of a four storeys building typology (S/V=0.43) with south facing prevailing, located at Rome (center Italy), with continuous system operational modality (20 C heating and 26 C cooling temperature set point for 24 hours/day all over the year). As we can see in the Figure 3, from the comparison of the two alternatives of simulated buildings the heavy envelope results a sensible reduction on heating and cooling energy demand. Figure 4: Case 1 variant A energy demand (comparison Light- Heavy envelope with modality system on/off) Variant B: modality set-back. 20 C heating - 26 C cooling set point temperature scheduled during main day hours, 16 C and 30 C respectively in the rest of the time (total full activation 12 hours/ day). The results are shown in the next graph (Fig. 5). Figure 5: Case 1 Variant B energy demand (comparison Light- Heavy envelope with modality system set-back) PALENC Vol 1.indd 348 3/9/2007 1:24:49 µµ

4 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and 349 In both the previous cases, massive envelope results better in energy performances, in particular concerning the heating energy demand. With the set-back modality, the difference between the two building envelope performances is slightly higher with respect to the switched-off case (on the annual basis, the reduction of 31% for heating energy demand and 6% for cooling energy demand become 29% and 3% respectively in the case on/off). This is due to the larger period in which thermal mass could perform its role of heating storage: during set-back period the system is always running, while during the same hours of the other case the system is switched off. Case 2. Base-case with different building typologies The goal of the second case is the comparison between the light envelop performance and the heavy one by varying the S/V ratio of the buildings. The results in terms of annual energy demand for each square meter of floor are shown in the next graph (Fig. 6). climatic conditions. The results are shown in the next graphs (Fig. 7-8). Figure 7: Case 3 energy demand (comparison Light-Heavy envelope at Milan) Figure 6: Case 2 annual energy demand (comparison Light-Heavy envelope in several typologies) Based on the results, all building typologies with massive envelope have better performances, also in this case in particular concerning heating energy demand. It can be observed that heating energy demand decreases by lowering S/V ratio. This is because of the lower winter specific transmission losses through the surface envelope (very incisive on energy balance of the building in heating season). However, the variation of S/V ratio is not so effective on cooling energy demand, since major amount of heat to be extracted is related to the internal heat gains (same values for each square meters), and to the solar gains through the windows (proportionally dimensioned to the floor areas). It is interesting to note that the two storey small building results the lower specific cooling demand: even if it has the same window/floor ratio, it doesn t have opening facing east and west, where solar radiation in summer is higher. Case 3. Base-case with different climatic conditions. Through the adoption of the Milan and Palermo TRY weather files, the light vs. heavy base-case models have been simulated considering typical north and south Italy Figure 8: Case 3 energy demand (comparison Light-Heavy envelope at Palermo) Along with the results of the reference base-case (location Rome), the consistency of the better energy performance trend for the massive solution is also confirmed for other national climatic conditions, obviously with the mix of heating and cooling annual energy demand that change its proportion moving from north to south Italy. On average, the national heating and cooling energy demand reduction due to the thermal mass correspond to 30% and 15% respectively. It has to be also underlined that, in some cases, during the mid seasons, the monthly energy demand appears only for the light envelope solution: that correspond to a high potential of free floating period for buildings. Case 4. Base-case with different azimuths. The base-case (4 storeys building, located in Rome, with continuous system operational modality) has been simulated by varying the orientation. The energy demand reduction due to the heavy envelope is reported in percentage in the Figure 9. PALENC Vol 1.indd 349 3/9/2007 1:24:49 µµ

5 350 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and 3.1 Further investigations In addition to the matrix consistency validations, the following two investigations have been carried out. Case 5. Dimensioning the heat capacity by calibrating heating energy demand The goal of this investigation is to find out the U-value of the envelopes with significant heat capacity that results the same heating energy demand of the light envelope considered in this study. Referring to the base-case light envelope (U-value 0.66 W/m 2 K) heating energy demand, a set of simulations have been done in order to obtain the same seasonal values both with a heavy envelope (clay bricks walls, as considered before, density 1800 kg/m 3 ) and with a medium one (45% hollowed clay bricks walls, density 800 kg/m 3 ). As shown in the Figure 10 the heating energy demand calibrations result 55 cm of thickness for the heavy wall and 27 cm thickness for the medium one, with corresponding to U-value of 1.05 W/m 2 K and 0.9 W/m 2 K respectively. Figure 9: Case 4 annual energy demand reduction for several exposures (% due to the heavy envelope) The variation of azimuth doesn t highlight particular difference in terms of energy demand reduction due to the massive envelope. Anyway, reminding that building models in this study haven t any shading devices, it can be observed that the south facing main façade in massive envelope is the more effective one in terms of energy demand reduction for massive vertical envelope: for cooling, because of the larger amount of solar gains from the higher number of windows facing the sun path (internal storage effect) and, for heating, because of the prevailing heat losses through the main opaque surface facing north (better performance of the thermal mass in reducing heat flow). Moreover, in case of main facade facing east or west the thermal capacity advantages are quite similar (because of the sun path symmetry referring to the south) except for a more cooling benefit (internal storage effect) for the west facing one, due to the well known overheating effect in the afternoon for this exposure (direct solar radiation gains in addition to the heat gained during all over the daytime). Figure 10: Case 5 calibration on heating energy demand for lightmedium-heavy envelopes Case 6. The position of the mass into the stratified walls Because of the small thickness requirements in building walls, this investigation has been conducted in order to verify the effect of the thermal mass position into stratified walls with insulating layer. For this, the same basecase U-value (0.66 W/m 2 K corresponding to 95 cm of bricks for massive solution) has been obtained with 24 cm of clay bricks coupled with 4 cm of insulation material (mineral fibers). The evaluations have been considered moving its position, step by step, as internal layer, external layer or both (dividing it into two layers of 12 cm thickness each one and placing the insulating material in between). The simulations have been conduced for system modality both on/off and continuous. In the Figure 11 results are reported, together with the heavy and the light mono-layer ones of the base-case. Figure 11: Annual energy demand light, heavy, multi-layers (system modality continuous and on/off ) It can be observed that thermal mass is always effective and, coupled with insulating material, its position for the present cases is not crucial, even if the better one is facing inside: important result is that with 30 cm of multilayer wall thickness, it is possible to obtain a large amount of energy savings in respect to the light solution, having similar performance as the mono-layer heavy wall. PALENC Vol 1.indd 350 3/9/2007 1:24:50 µµ

6 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and CONCLUSIONS In present study two main opposite technological solutions for building envelope, one extremely light the other extremely heavy, have been evaluated for their energy performances. The two solutions could also remind to European historical building practices (the heavy one) and to the more recent and diffuse trend for new building constructions (the light one). As assessed in the old past, when no technological systems could remedy to the lack of building envelope performances, in Italy and more in general in Europe, the heating capacity (traditional architecture made by stones, bricks or wood) in building construction reveals better performances, both in cold and warm temperate climates. In fact, thermal capacity in this study have been tested from north to south of Italy, but certainly the consistency could also be validated moving more to north (Norén et al. 1999). Moreover, considering that cooling demand is growing globally because of our new comfort level needs, the passive cooling strategies like night free cooling are becoming rare and conditioning of buildings throughout the year is no more exceptional: when air conditioning systems operate both in winter and summer, as considered in this study, the percentage of energy demand reduction due to thermal mass results more significant for heating. European standards, building industries and designers should think beyond the on going trend that considers the insulation as a key issue in the building energy conservation. As shown in this study, the thermal capacity is certainly effective for energy savings, in reference to the actual dynamics of the physical heat transmission flows that determine the real energy balance of the building. Witte M.J., Henninger R.H., Glazer J., (2001) Testing and validation of a new building energy simulation program, Proceedings, 7 th International IBPSA Conference, Rio de Janeiro, Brazil, August AKNOWLEDGMENTS A. Campioli, Dept. BEST Politecnico di Milano, Italy G. Di Cesare ANDIL-Assolaterizi Rome, Italy REFERENCES AA.VV (2001), ASHRAE Handbook_Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta Architectural Energy Corporation (2005) Givoni B. (1967) Man, climate and architecture. 1st ed. London, Applied Science Publishers Ltd. Norén A. et al., (1999) The Effect of Thermal Inertia on Energy Requirement in a Swedish Building, International Journal of Low Energy and Sustainable Buildings, Vol. 1 Olgyay V. and A. (1963), Design with climate: bioclimatic approach to architectural regionalism Princeton, N.J. : Princeton University Press. Szokolay S. V. (1980), Environmental science handbook for Architects and Builders, vol. I and II, The construction press, Lancaster Szokolay S.V. (2004), Introduction to Architectural Science: The Basis of Sustainable Design, Elsevier - Architectural Press, Oxford. Simulation Research Group of Lawrence Berkeley Laboratory (1994) DOE-2 manuals, U.S. Department of Energy - University of California. PALENC Vol 1.indd 351 3/9/2007 1:24:50 µµ