energies
Article Experimental Evaluation of the Heat Balance of an Interactive Glass Wall in A Heating Season
Jerzy Szyszka Faculty of Civil and Environmental Engineering and Architecture, Rzeszow University of Technology, 35-959 Rzeszów, Poland; [email protected]
Received: 23 December 2019; Accepted: 31 January 2020; Published: 3 February 2020
Abstract: The paper presents an evaluation of the energy efficiency of an interactive glass wall (IGW) prototype. It is a design analogous to Trombe wall. It is capable of giving out the solar radiation heat gains after the sunset. It responds interactively to solar exposure and temperature conditions, regulating the thermal resistance adequately to the requirements. The evaluation of the efficiency of the IGW was based on the analysis of density of heat flux measured on the inner surface of the wall. The experiments were conducted in field conditions using a test chamber of regulated air temperature. The identified parameters of solar energy losses and efficiency enable the IGW heat balance in a heating season in selected climatic conditions to be predicted. In the present paper the IGW heat balance is calculated for the climate in Poland. The calculations proved that the gains of the heat absorbed from solar radiation wall overweigh the losses.
Keywords: interactive glass wall; evaluation of heat balance; transparent intelligent solar active wall; Trombe wall
1. Introduction The construction industry is responsible for the largest portion of the consumption of energy produced in the world. The predominant part of this amount is used for heating of buildings to ensure an adequate air temperature level. The activities aimed at the reduction of energy consumption are focused on protecting buildings against excessive heat losses. For this purpose, materials and technologies improving the thermal resistance of building exterior envelopes. These include advanced vacuum and electrochromic glazing [1,2], or new generation thermal insulation materials: VIP or nanotechnology based aerogel [3,4]. A considerable potential is demonstrated by the technologies based on the application of building management systems or the so-called intelligent systems that enable the building envelope characteristics to be adapted to changing weather conditions. The heat balance can also be improved by geothermal heat exchangers or recuperators recovering the heat from ventilation air or treated sewage [5]. A natural trend in limiting the energy demand for heat generation is to develop photothermal conversion of solar energy-based systems integrated with the building envelope. In view of ecology and economy, passive or semi-passive systems have a significant potential. Among these, walls that are capable of recovery of heat generated by photothermal conversion of solar energy are in increasing demand. The exterior glazing used for their construction enables solar radiation absorption thus restricting the heat flow transfer to the outdoor surroundings. These design strategies are called Trombe wall (TW) and have been described extensively in specialized literature [6–8]. The energy efficiency of Trombe walls is determined by, inter alia, the physical properties of the materials for their construction and the climatic conditions in which they function as well as the manner they are used [9]. In the classic design, the so-called ventilated wall (Figure1a) heat is distributed inside the building by means of a mass heating wall owing to heat transfer and as a result of air circulation through the vents in the wall’s bottom and top. This solution is highly efficient in solar radiation conversion into heat.
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wall’s bottom and top. This solution is highly efficient in solar radiation conversion into heat. However, its thermal resistance is low. The irregular solar radiation and low temperatures in heating However, its thermal resistance is low. The irregular solar radiation and low temperatures in heating season typical of the Central and Eastern Europe pose a risk of heat loss on days of insufficient solar season typical of the Central and Eastern Europe pose a risk of heat loss on days of insufficient solar exposure. This problem was noticed in the design proposed in [10,11]. The thermal resistance was exposure. This problem was noticed in the design proposed in [10,11]. The thermal resistance was improved by placing an insulating polystyrene wall behind the mass wall (Figure1b). The flow of the improved by placing an insulating polystyrene wall behind the mass wall (Figure 1b). The flow of heat generated by the solar irradiance was made possible by air circulation between the ventilated air the heat generated by the solar irradiance was made possible by air circulation between the ventilated cavity and the adjacent space owing to the vents in the top and bottom parts of the insulating wall. air cavity and the adjacent space owing to the vents in the top and bottom parts of the insulating wall. A similar mode of heat distribution with the circulating air was employed in the wall proposed in [12]. A similar mode of heat distribution with the circulating air was employed in the wall proposed in In this case, the thermal insulating layer directly behind the selective absorber (Figure1c). Based on [12]. In this case, the thermal insulating layer directly behind the selective absorber (Figure 1c). Based the experiments performed in the climate of the Qinghai province in China, the authors proved that on the experiments performed in the climate of the Qinghai province in China, the authors proved the efficiency of heat gains utilisation, calculated as the ratio between the heat entering a building that the efficiency of heat gains utilisation, calculated as the ratio between the heat entering a building and the sum of solar irradiance reaching the wall surface increased by 56% compared with the classic and the sum of solar irradiance reaching the wall surface increased by 56% compared with the classic Trombe wall operating in identical conditions. In [13] a hybrid heat collecting facade (HHCF) was Trombe wall operating in identical conditions. In [13] a hybrid heat collecting facade (HHCF) was proposed, which can reduce the heating demand by 40.2% and 21.5% compared with the conventional proposed, which can reduce the heating demand by 40.2% and 21.5% compared with the direct solar heat gain window and the Trombe Wall. conventional direct solar heat gain window and the Trombe Wall.
2 2 3 4
1 1 1
2 (a) (b) (c) Figure 1. Thermal operation of passive walls: ( (aa)) classic classic Trombe Trombe wall, ( b) composite composite Trombe wallwall withwith an insulating panel, (c) composite Trombe wallwall with a selectiv selectivee absorber. 1—glazing, 2—massive wall, 3—insulating polystyrenepolystyrene wall,wall, 4—insulating4—insulating layerlayer directlydirectly behindbehind thethe selectiveselective absorber.absorber.
Along with the studies on the improvementimprovement of TW thermal eefficiency,fficiency, modificationmodification proposalsproposals are formulatedformulated toto improveimprove theirtheir performance.performance. The The examples include a TWPV integratedintegrated withwith photovoltaicphotovoltaic cellscells [[14–16],14–16], a hybrid system composed of a phase change materials-ventilated Trombe wallwall (PCMs-VTW)(PCMs-VTW) [17[17]] or or A A thermal-catalytic thermal-catalytic (TC) (TC) Trombe Trombe wall wall with with a capacitya capacity for for the the reduction reduction of airof formaldehydesair formaldehydes [18]. [18]. Transparency Transparency may may be one be ofone the of desirable the desirable functions function of thes of thermal the thermal wall. Water wall. wallsWater have walls this have property this property while preserving while preserving the potential the potential for heat for absorption heat absorption and storage. and storage. In the design In the discusseddesign discussed in [19] a in mass [19] heating a mass wall heating was replacedwall was with replaced a glass with container a glass filled container with water filled safeguarded with water againstsafeguarded algae against growth. algae To intensify growth. solar To intensify absorption sola ther absorption water was the coloured water andwasspecial coloured light and di ffspecialusing insertslight diffusing were placed inserts in were the container placed in (Figure the container2). (Figure 2).
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1 1
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Figure 2. Water Trombe wall (transwall): 1—glass water container, 2—colored water, 3—diffusing-absorbing Figure 2. Water Trombe wall (transwall): 1—glass water container, 2—colored water, 3—diffusing- inserts.Figure 2. Water Trombe wall (transwall): 1—glass water container, 2—colored water, 3—diffusing- absorbing inserts. absorbing inserts. Another type of a transparent wall capable of interactive response to weather conditions is Another type of a transparent wall capable of interactive response to weather conditions is the the designAnother of thetype IGW of a presentedtransparent in wall the papercapable (Figure of interactive3). Because response of its capacityto weather for conditions solar radiation is the design of the IGW presented in the paper (Figure 3). Because of its capacity for solar radiation conversiondesign of the and IGW potential presented for solar in gainsthe paper generation, (Figure even 3). afterBecause the sunset,of its capacity it is a wall for analogous solar radiation to the conversion and potential for solar gains generation, even after the sunset, it is a wall analogous to the Trombeconversion wall. and The potential wall’s interactivityfor solar gains and generation, corresponding even after selective the sunset, thermal it is resistance a wall analogous result in to solar the Trombe wall. The wall’s interactivity and corresponding selective thermal resistance result in solar irradianceTrombe wall. intensification The wall’s andinteractivity heat loss and reduction correspondi in periodsng selective of insuffi thermalcient solar resistance irradiance. result in solar irradiance intensification and heat loss reduction in periods of insufficient solar irradiance. irradiance intensification and heat loss reduction in periods of insufficient solar irradiance.
FigureFigure 3. TheThe prototypeprototype of of the the tested tested interactive interactive glass glass wall (IGW):wall (IGW): 1—outer 1—outer glazing, glazing, 2—louvers 2—louvers headrail, Figure 3. The prototype of the tested interactive glass wall (IGW): 1—outer glazing, 2—louvers headrail,3—swivelling 3—swivelling louvers panels, louvers 4—moveable panels, 4—moveable absorber, 5—interiorabsorber, 5—interior glazing, 6—louvers glazing, 6—louvers panels swivelling panels actuator,headrail, 7—containers3—swivelling withlouvers heat panels, storing 4—moveable materials. absorber, 5—interior glazing, 6—louvers panels swivelling actuator, 7—containers with heat storing materials. swivelling actuator, 7—containers with heat storing materials. IGW was designed as an alternative to nonbearing panel walls. According to the requirements IGW was designed as an alternative to nonbearing panel walls. According to the requirements or anIGW architectonic was designed vision, as an it canalternative be used to to nonbearing fill in completely panel walls. or partially According the to space the requirements between the or an architectonic vision, it can be used to fill in completely or partially the space between structuralor an architectonic elements. Invision, the evaluation it can be of used the thermalto fill in effi completelyciency of passive or partially systems the numerical space between models the structural elements. In the evaluation of the thermal efficiency of passive systems numerical basedthe structural on the finiteelements. element In the method evaluation or the methodof the thermal of finite efficiency differences of arepassive often systems employed numerical [20–24]. models based on the finite element method or the method of finite differences are often employed Theymodels off baseder the on possibility the finite of element very accurate method mapping or the method of the model’sof finite thermaldifferences performance, are often employed which is [20–24]. They offer the possibility of very accurate mapping of the model’s thermal performance, confirmed[20–24]. They by experiment-basedoffer the possibility validation of very [25accu]. However,rate mapping they of require the model highly’s specializedthermal performance, knowledge which is confirmed by experiment-based validation [25]. However, they require highly specialized ofwhich numerical is confirmed modelling, by experiment-based building physics validation and thermodynamics. [25]. However, In they the evaluationrequire highly of the specialized thermal knowledge of numerical modelling, building physics and thermodynamics. In the evaluation of the eknowledgefficiency of of prototypes numerical the modelling, validation building of numerical physic modelss and isthermodynamics. strongly supported In the by theevaluation data obtained of the thermal efficiency of prototypes the validation of numerical models is strongly supported by the data fromthermal experimental efficiency of tests prototypes performed the onvalidation physical of models numerical in natural models [ 26is strongly] or laboratory supported conditions by the data [27]. obtained from experimental tests performed on physical models in natural [26] or laboratory Theobtained evaluation from ofexperimental the thermal e ffitestsciency performed of the IGW on presented physical inmodels the paper in wasnatural based [26] on theor analysislaboratory of conditions [27]. The evaluation of the thermal efficiency of the IGW presented in the paper was based theconditions data obtained [27]. The in evaluation the field tests of the on thethermal IGW effi usingciency a test of chamberthe IGW ofpresented controlled in the indoor paper temperature. was based on the analysis of the data obtained in the field tests on the IGW using a test chamber of controlled on the analysis of the data obtained in the field tests on the IGW using a test chamber of controlled indoor temperature. indoor temperature.
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2. IGW Prototype Design and Test Method This section is devoted to the details of the IGW prototype design and the test stand. The adopted method, based on experimental tests, of the dete determinationrmination of the specific specific parameters of the IGW prototype heat balance componentscomponents isis discussed.discussed.
2.1. Description of the Tested Prototype 2.1. Description of the Tested Prototype The prototype of a wall element IGW (Figure4) was made from three triple glazed units each of The prototype of a wall element IGW (Figure 4) was made from three triple glazed units each of 2 overall heat transfer coefficient of Ug = 0.5 W/m K (gain factor g = 0.55, emissivity e = 0.92). To facilitate overall heat transfer coefficient of Ug = 0.5 W/m2K (gain factor g = 0.55, emissivity e = 0.92). To facilitate the solar radiation access inside the components and intensify heat transfer to the building, the central the solar radiation access inside the components and intensify heat transfer to the building, the central unit was made in thethe louverlouver windowwindow technology.technology. The absorbers made of perforated black sheet (absorption coefficient α = 0.95) were attached to the swivelling louver panels so that, after swivelling (absorption coefficient α = 0.95) were attached to the swivelling louver panels so that, after swivelling the panels, the absorber was exposed as vertically to the sunlight rays as possible. The operation of the the panels, the absorber was exposed as vertically to the sunlight rays as possible. The operation of glass panels swivelling was made automatic by connecting the actuator that regulates the swivelling the glass panels swivelling was made automatic by connecting the actuator that regulates the with a controller responding to air temperature and solar irradiance Ig measured in the space between swivelling with a controller responding to air temperature and solar irradiance Ig measured in the the glazed units. The threshold values were adopted at 20 ◦C for the temperature T1 of the air in 1 space between the glazed units. The threshold valu2 es were adopted at 20 °C for the temperature T the space between the glazed units and 50 W/m for the solar irradiance Ig measured behind the of the air in the space between the glazed units and 50 W/m2 for the solar irradiance Ig measured outer glazing. behind the outer glazing.
High thermal resistance Solar profit mode mode (louver closed) (louver open) 4 5 8 2 3 Inside Outside. Outside. Inside 1 7 T2 3 9 Ti qi T1 Ig Te Ie
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Figure 4. The prototype of thethe testedtested wall:wall: 1—exterior1—exterior glazing,glazing, 2—louvers2—louvers headrail,headrail, 3—swivelling3—swivelling louvers panels, panels, 4—moveable 4—moveable absorber, absorber, 5—interior 5—interior glazing, glazing, 6—louvers 6—louvers panels panels swivelling swivelling actuator, actuator, 7— 7—containerscontainers with withheat storing heat storingmaterials materials containers, containers, 8—aerogel mat 8—aerogel seal, 9—relay mat rods, seal, 10—temperature 9—relay rods, 10—temperaturesensors, 11—density sensors, heat 11—densityflux sensor, 12—pyranometers. heat flux sensor, 12—pyranometers.
The storage of of the the heat heat absorbed absorbed inside inside an an IGW IGW was was made made possible possible by bythe the containers containers placed placed on onthe thesides, sides, filled filled with with phase phase change change materials materials of of the the total total weight weight of of 5.55 kg.kg. Rubitherm GmbH manufactured commercial phase change material RT 28, with the value of enthalpy of fusion and solidification interval of ∆Hfus = 220–225 J/g, m.p. = 28 ◦C and solidification temperature = 21 ◦C solidification interval of ΔHfus = 220–225 J/g, m.p. = 28 °C and solidification temperature = 21 °C was wasused. used.
2.2. Description of Test Stand
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2.2. Description of Test Stand Energies 2020, 13, x FOR PEER REVIEW 5 of 16 The energy efficiency of IGW was evaluated on the basis of the results obtained in a test chamber withThe regulated energy indoor efficiency air temperature of IGW was Tevaluatedi in field conditions.on the basis The of the prototype results obtained of the IGW in a of test dimensions chamber with890 regulated885 mm wasindoor installed air temperature in a facing Ti southin field wall conditions. of the test The chamber prototype (Figure of the5 ).IGW It was of dimensions thermally × 890separated × 885 mm from was the wallinstalled in which in a itfacing was fixed south by wall means of the of extruded test chamber polystyrene (Figure layer 5). It 0.08 was m thermally thick and separatedpolyurethane from foam. the wall in which it was fixed by means of extruded polystyrene layer 0.08 m thick and polyurethane foam.
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Figure 5. TheThe chamber chamber for for field field tests: tests: 1—IGW 1—IGW module, module, 2—pyranometer 2—pyranometer (I g)) placed placed behind behind the the exterior exterior
glazing, 3—pyranometer (Ie)) placed placed outside outside the the chamber. chamber.
In the chamber the air temperature was stabilized at the level of 20 ◦°C.C. The tested parameters were recorded by heat flowflow density sensors ALMEMOALMEMO FQ A020 C and temperature sensors PT1000 2 as well as pyranometers Delta OHM LPPYRA03AC (sensitivity 10 mVmV/(kW/m/(kW/m2)) interacting with 16 Channel Data Acquisition Monitoring System CometComet MS6D. The arrangement of the sensors is shown in Figures 4 4 and and6 6.. The The data data were were recorded recorded with with the the frequency frequency of of every every five five minutes. minutes. The The louver louver was was controlled byby MP018 MP018 Relays Relays Module Module Output Output interacting interacting with thewith Comet the recorder.Comet recorder. List of the List employed of the employedapparatus apparatus and sensors and together sensors with together their measurementwith their measurement accuracy has accuracy been listed has been in Table listed1. in Table 1. Table 1. Measurement accuracy of employed sensors and Data Acquisition Monitoring System.
TableKind 1. Measurement of Sensor accuracy of employed Type sensors of Sensor and Data Acquisition Monitoring Accuracy System. A class TemperatureKind sensor of Sensor Type of PT1000Sensor Accuracy (<0.2 ◦C) DeltaOhm LP Pyra12 <1 % (first class) IrradiatonTemperature sensor A class DeltaOhmPT1000 LPPyra03 < 0.2% (second class) ± Heat flow densitysensor ALMEMO FQ A020 C <0.2 °C)<6% of measured value IrradiatonComet MS6DDeltaOhm 16 Channel Data LP AcquisitionPyra12 Monitoring<1 System% (first class) DC 4 to 20 mA 0.1% ( 0.02 mA) ± ± DC sensor DeltaOhm10 LPPyra03 V to +10 V <±0.2% (second 0.1%class) ( 10 mV) − ± ± TemperatureHeat flow PT1000 <6% of measured0.2 ◦C( 200 ◦C to +100 ◦C) ALMEMO FQ A020 C ± − density value Comet MS6D 16 Channel Data Acquisition Monitoring System DC 4 to 20 mA ±0.1% (±0.02 mA) DC −10 V to +10 V ±0.1% (±10 mV) ±0.2 °C (−200 °C to Temperature PT1000 +100 °C)
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Figure 6. Measuring and control system diagram. Figure 6. Measuring and control system diagram. The tests were conducted in winter 2018/2019 in Rzeszów, Poland (50 02 01”N, 22 00 17”E). The tests were conducted in winter 2018/2019 in Rzeszów, Poland (50°02◦ 0′01″N, 22°00◦ 0′17″E). 2.3. Description of the Method for the Evaluation of IGW Thermal Efficiency 2.3. Description of the Method for the Evaluation of IGW Thermal Efficiency The parameter of IGW thermal efficiency evaluation that can be adopted as measurable is the wall’sThe heat parameter balance. Theof IGW energy thermal state ofeffi theciency wall evaluation separating that the indoorcan be environmentadopted as measurable from the outdoor is the wall’s heat balance. The energy state of the wall separating the indoor environment from the outdoor environment of a building can be measured by, inter alia, the density of heat flow rate qi taken at, environment of a building can be measured by, inter alia, the density of heat flow rate qi taken at, for for instance, its inner surface. It is a resultant of the flow connected with heat loss qH, and the flow instance, its inner surface. It is a resultant of the flow connected with heat loss qH, and the flow coming coming from solar radiation qS: from solar radiation qS: q = f (q , q ) (1) i = H S qi f (qH ,qS ) (1) The heat balance of the wall in time interval of
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irradiance, using Equations (2) and (3) the empirical coefficient of heat transfer UIGW_EMP can be determined: qi UIGW_EMP = (4) ∆T On the basis of the dependencies above also solar gains can be evaluated. The solar gains related heat flow qS is determined by solar irradiance and the properties of the wall in the aspect of heat transfer indoor. A significant amount of the absorbed heat is lost as an effect of additional losses resulting from the increase of the wall temperature. The absorption of sunlight after the time necessary for heat flow transfer to the inner surface results in a reduction of flow qi, from the value qH (Equation (3)) to a level proportional to the amount of the energy absorbed. Consequently, solar gains QS in the time interval of (t1:t2) can be calculated from the equation:
Zt2 Q = A (qH q )dt, (5) S · − i t1 where:
A—total pane area (m2).
Solar Heat Gain Utilisation (SHGU) efficiency ηSHGU referred to the sum of solar irradiance transferred through the outer glazing Sg in IGW balance, can be calculated from the equation:
Rt2 A (qH q )dt · − i t1 QS ηSHGU(Ig) = 100% = 100% (6) Rt2 · Sg · cg A Igdt · ·t1 where:
Cg—ratio of the outer glazing visible area (not covered by the glazing bead) to the total glazing inner area IGW (Cg = 0.89), 2 Sg—the sum of solar irradiance transferred through the outer glazing (Wh/m ).
3. Results and Discussion In this section, based on the experimental data, the specific parameters of IGW heat balance components are determined. The method was validated. Using the determined parameters and the climate data of a typical meteorological year the IGW heat balance was calculated for a heating season of a selected locality, which was followed by a discussion of the results. From the results recorded at the turn of November and December the period of low radiation and a period determined by heat gains from high irradiation period were selected (Figure7) On the basis of heat flux density distribution on IGW indoor surface at minor temperature variations at low solar irradiance an empirical value of heat transfer coefficient UIGW_EMP was determined. In the periods of high radiation during the day the temperature variations were large. During the day the temperature of air heated by the sun increased. At night-time, with no cloud cover, the radiation heat exchange with the sky increased the temperature drop. Energies 2020, 13, 632 8 of 16 Energies 2020, 13, x FOR PEER REVIEW 8 of 16
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Figure 7. Air temperature distribution vs. solar irradiance recorded outside the test chamber in Figure 7. Air temperature distribution vs. solar irradiance recorded outside the test chamber in analyzed period. analyzed period.
3.1. Empirical Heat Transfer Coefficient UIGW
3.1.IGW Empirical can operate Heat Transfer in two Coefficient modes. U InIGW one, the “high thermal resistance” the closed inner louver ensures maximumIGW can operate thermal in resistance.two modes. In one, the otherthe “high one, thermal the “solar resistance” profit mode” the closed opening inner thelouver louvers enablesensures more maximum efficient thermal heating resistance of IGW interior. In the other as a resultone, the of “solar reduced profit thermal mode”resistance. opening the To louvers determine the empiricalenables more values efficient of the heating heat of transfer IGW interior coeffi cientsas a result in bothof reduced modes, thermal from resistance. the recorded To determine data the days werethe selected empirical on values which of low the solarheat tr irradianceansfer coefficients was accompanied in both modes, by from minor the fluctuationsrecorded data of the outdoor days air temperature.were selected In theon which period low between solar irradiance November was 2018accompanied and mid-February by minor fluctuations 2019 the of wall outdoor operated air in temperature. In the period between November 2018 and mid-February 2019 the wall operated in the the interactive mode. Using the data from this period the empirical heat transfer coefficient U(Rmax) interactive mode. Using the data from this period the empirical heat transfer coefficient U(Rmax) corresponding to the maximum thermal resistance of IGW was identified (Figure8, Table1). From the corresponding to the maximum thermal resistance of IGW was identified (Figure 8, Table 1). From turnthe ofFebruary turn of February and March and March 2019 tests2019 tests were were conducted conducted with with the the blocked blocked open open innerinner louver. From From this periodthis data period that data were that used were for used the for identification the identification of heat of heat transfer transfer coe coefficientfficient U U(R(Rminmin) corresponding to a reducedto a reduced value of value thermal of thermal resistance resistance of IGW of IGW were were selected selected (Table (Table2). 2).
Table 2. Averaged daily values of heat flux density, outdoor temperature and empirically determined heat transfer coefficient.
q [ W ] ◦ ( )[ W ] Number of Day i m2 ∆T [ K] UEMP Rmax m2K 1 3.409 20.70 0.165 2 3.685 20.67 0.178 3 3.425 20.12 0.170 4 3.375 20.16 0.167 5 3.242 20.02 0.162 Mean value x 0.168 r P 2 (x x) − 0.006 Standard deviation (n 1) −
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FigureFigure 8. Distribution 8. Distribution of of outdoor outdoor air air temperature temperature ( Te),), heat heat flux flux density density meas measuredured on the on inner the inner surface surface of theof IGW the IGW and and solar solar irradiance irradiance measured measured behind behind thethe outer glazing glazing in in the the ve veryry low low radiation radiation period. period.
Finally, on the basis of the results (Tables2 and3) the IGW heat transfer coe fficient was described Table 2. Averaged daily values of heat flux density, outdoor temperature and empirically determined with Equation (7): heat transfer coefficient. ( ) T ◦ I W 0.255 if 1 > 20 C and g > 50 m2 UIGW_EMP = , (7) W 0.168 ino other cases W Number of Day q [ ] ΔT [ K] U EMP (R ) [ ] i m2 max m2K With the known heat transfer coefficient, it is possible to predict the heat flux component q 1 3.409 20.70 0.165 H(U) T connected with the heat2 loss proportional3.685 to air temperature20.67 difference ∆ 0.178on both sides of the wall, which can be expressed3 with a formula3.425 (Equation (8)).20.12 0.170 4 3.375( 20.16 0.167) T ◦ I W 5 3.242 0.255 if 120.02> 20 C and q > 50 0.162m2 qH(U) = UEMP ∆T = ∆T, (8) Mean· value x 0.168 in other cases 0.168 · Σ(x − x)2 Standard deviation 0.006 Table 3. Averaged daily values of heat flux density,(n −1) outdoor air temperature and empirically determined heat transfer coefficient in “solar profit” mode. Finally, on the basis of the results (Tables 2 and 3) the IGW heat transfer coefficient was described with Equation (7): q [ W ] ◦ ( )[ W ] Number of Day i m2 ∆T [ K] UEMP Rmin m2K W 1 5.080.255 if T > 20o C and 20.14I > 50 0.252 U = 1 g 2 , (7) 2IGW _ EMP 5.05 20.45m 0.247 0.168 in other cases 3 5.38 20.23 0.266 With the known heat transferMean coefficient, value x it is possible to predict the heat flux component0.255 qH(U) r P 2 connected with the heat loss proportional to air(x temperaturex) difference ΔT on both sides of the wall, Standard deviation − 0.01 which can be expressed with a formula (Equation(n 1) (8)). − > o > W 0.255 if T1 20 C and Iq 50 q = U EMP ⋅ ΔT = 2 ⋅ ΔT, 3.2. Determination of theH Solar(U ) Heat Gain Utilisation Efficiency (ηSHGU) m (8) 0.168 in other cases The impact of solar irradiance on IGW results in the reduction of the heat flux density. Depending on the irradiance, the resultant heat flux may reach a negative value, which indicates heat transfer to the building interior. The comparison of the real flux qi with flux qu calculated after Equation (7) for air temperature difference ∆T on both sides of the wall in a given time interval was a basis for the Energies 2020, 13, x FOR PEER REVIEW 10 of 16
Table 3. Averaged daily values of heat flux density, outdoor air temperature and empirically determined heat transfer coefficient in “solar profit” mode.
W o W Number of Day q [ ] ΔT [ K] U EMP (R ) [ ] i m2 min m2K 1 5.08 20.14 0.252 2 5.05 20.45 0.247 3 5.38 20.23 0.266 Mean value x 0.255 Σ(x − x)2 Standard deviation 0.01 (n −1)
3.2. Determination of the Solar Heat Gain Utilisation Efficiency (ηSHGU) Energies 2020, 13, 632 10 of 16 The impact of solar irradiance on IGW results in the reduction of the heat flux density. Depending on the irradiance, the resultant heat flux may reach a negative value, which indicates heat calculationtransfer to ofthe the building actual solarinterior. gains TheQS comparison. The knowledge of the of real solar flux gains qi with referred flux toqu the calculated sum of solarafter irradianceEquation (7) that for generated air temperature these gains difference enables Δ theT on estimation both sides of of the the solar wall heat in a gain given utilisation time interval efficiency was ηaSHGU basisin for IGW the (Equationcalculation (6)). of Thethe actual efficiency solar referred gains Q toS the. The sum knowledge of solar irradiance of solar gains transferred referred through to the thesum outer of solar glazing irradianceSg, is expressed that generated by the equation: these gains enables the estimation of the solar heat gain utilisation efficiency ηSHGU in IGW (Equation (6)). The efficiency referred to the sum of solar irradiance t2 transferred through the outer glazing Sg, is expressedR by the equation: A (q ( ) qi)dt · t 2 H U − QS t1 ηSHGU(Ig) = 100% = A⋅ (q − q )dt 100%, (9) S · H (Ut)2 i · η =g QS ⋅ = t1 R ⋅ , SHGU (I g ) 100% cg A t 2Igdt 100% (9) Sg · · c ⋅ A t⋅1 I dt g g t1 where: where: I [ WW]—solar irradiation transferred through the outer glazing in time interval (t1:t2). gI m[2 ]—solar irradiation transferred through the outer glazing in time interval (t1:t2). g m2 To determine efficiency ηSHGU using the dependencies described by Equations (8) and (9) the To determine efficiency ηSHGU using the dependencies described by Equations (8) and (9) the period of high sums of daily solar irradiation was selected (Figure9). period of high sums of daily solar irradiation was selected (Figure 9).
Figure 9. Distribution of air temperature outside the chamber Te, heat flux density measured on the IGW inner surface and solar irradiation measured behind the glazing in the period of considerable solar radiation.
Solar heat gain utilisation efficiency ηSHGU in IGW calculated from Equation (9) and its components are tabulated in Table4. Energies 2020, 13, x FOR PEER REVIEW 11 of 16
Figure 9. Distribution of air temperature outside the chamber Te, heat flux density measured on the IGW inner surface and solar irradiation measured behind the glazing in the period of considerable solar radiation.
Solar heat gain utilisation efficiency ηSHGU in IGW calculated from Equation (9) and its Energiescomponents2020, 13 are, 632 tabulated in Table 4. 11 of 16
Table 4. Daily sums of solar gains Qs, solar irradiance measured behind the outer glazing of IGW and Table 4. Daily sums of solar gains Qs, solar irradiance measured behind the outer glazing of IGW and calculated efficiency ηSHGU. calculated efficiency ηSHGU.
WhWh WhWh Number of Day [n] Qs [ 2 ] Sg [ 2 ] ηSHGU Number of Day [n] Qs [m 2 ] Sg [m 2 ] ηSHGU 10 131.61m 608.1m 21.64 10 131.61 608.1 21.64 11 276.60 1440.8 19.20 11 276.60 1440.8 19.20 12 289.75 1398.1 20.73 12 289.75 1398.1 20.73 1313 249.51 1190.6 1190.6 20.96 20.96 1414 236.32 1143.3 1143.3 20.67 20.67 1515 104.56 438.6 438.6 23.84 23.84 MeanMean value value xx 21.1721.17 r P 2 Σ (x−x) 2 Standard deviation (x − x) 1.53 Standard deviation (n 1) 1.53 (n−−1) 3.3. Validation of IGW Heat Balance Calculation Mode 3.3. Validation of IGW Heat Balance Calculation Mode For the validation of the possibility of the IGW heat balance prediction the results recorded in the periodFor the of validation 1–31 January of the 2019 possibility (Figure 10of) the were IGW chosen. heat balance The wall’s prediction heat balance the results calculated recorded on the in the period of 1–31 January 2019 (Figure 10) were chosen. The wall’s heat balance calculated on the basis of the recorded heat flux density on the inner surface IGW Q(qi) was compared with the balance basis of the recorded heat flux density on the inner surface IGW Q(qi) was compared with the balance calculated on the basis of the determined heat transfer coefficient UIGH and the efficiency of solar calculated on the basis of the determined heat transfer coefficient UIGH and the efficiency of solar energy utilisation ηSHGU. energy utilisation ηSHGU.
900
10 800 ] ]
2 700 2 0 C]
o 600 [ [W/m [W/m e i g -10 500
400 -20 300 Temperature T Temperature Solar Irradiation I Heat q flux density 200 -30 100
-40 0 0 1 2 3 4 5 5 6 7 8 9 1011121314151616171819202122232425262627282930
Time [days]
Figure 10. Distribution of air temperature Te, heat fluxflux densitydensity measured on the inner surface of the IGW and solar irradiationirradiation measured measured behind behind the outerthe outer glazing glazing in January in January 2019. 2019. The validation study was based on the comparison of IGW heat balance calculation based on the The validation study was based on the comparison of IGW heat balance calculation based on the heat flux density recorded in the period of 1–31 January 2019 on the inner surface Qi (Equation (2)) heat flux density recorded in the period of 1–31 January 2019 on the inner surface Qi (Equation (2)) with the balance QH calculation based on heat transfer coefficient UIGW (Equations (7) and (8)), and the with the balance QH calculation based on heat transfer coefficient UIGW (Equations (7) and (8)), and solar gain QS calculation based on solar heat gain utilisation efficiency ηSHGU in IGW (Equation (9)) the solar gain QS calculation based on solar heat gain utilisation efficiency ηSHGU in IGW (Equation referred to the sum of solar radiation recorded behind the glazing. The calculation results are tabulated in Table5. Energies 2020, 13, 632 12 of 16
Table 5. Heat balance components calculated for January 2019. R R R Qi (QH QS) Qi=A qidt QH=A qH(U)dt QS=ηSHGU cg A Igdt QH QS ∆Q= − − 100% · · · · · − Qi · 575.89 [Wh] 3084.79 [Wh] 2419.46 [Wh] 665.33 [Wh] 15.53%
From the tabulated results it can be concluded that the difference ∆Q between the balance determined from the heat flux density measured on the IGW inner surface and the balance calculated on the basis of heat transfer coefficient UIGW and solar heat gain utilisation efficiency ηSHGU is 15.53%. This difference indicates that either the solar gains component is underestimated or the heat loss component overestimated. In the context of the calculations of the potential solar irradiation derived heat gains the difference is on the margin of calculations safety.
3.4. Prediction of IGW Heat Balance in a Heating Season Based on Climate Database To predict IGW balance the database of 30-year observation based typical meteorological years, available on website of the Ministry of Digitisation RP was used data [28]. It had been developed for the necessary calculations of energy consumption/generation in the construction industry. It can be used for the calculation of energy characteristics of buildings and issuing energy certificates for buildings, in energy auditing as well as the design and energy simulation of buildings. Balance calculations (Equation (10)) were performed in a calculation spreadsheet with hourly temperatures of outdoor air and hourly sums of solar radiation striking the surface vertical to the meteorological station proper for the city of Rzeszów. The calculations were done for South (S), South–East (S–E), South–West (S–W, East (E) and West (W) orientations of the IGW in the months of October to April. X X Q = A U (T Te) A g η Cg Se , (10) · IGW_EMP · i − − · · SHGU · · h where:
A—surface of IGW (1 m2),
UIGW—heat transfer coefficient (after Equation (7)), Ti—indoor air temperature (Ti = 20 ◦C)], Seh—hourly sum of solar irradiation recorded in front of the glazing, g—gain factor (g = 0.55 according to the manufacturer’s specifications of the glazing used).
Based on the g value, sum of solar irradiation behind the outer glazing Sgh can be determined:
Seg = S g, (11) eh · The results of the calculations of heat balance per the area of 1 m2 in each month of the heating season are tabulated in Table6 and Figure 11. The Negative values indicate the overbalance of heat gains over losses. Energies 2020, 13, 632 13 of 16
Table 6. Heat balance calculated for the months of the heating season depending on the orientation with respect to cardinal points.
Heat Balance [Wh/m2] Month E SE S SW W October 2442.03 3526.65 4226.97 3592.03 2492.84 − − − − − November 42.10 993.54 1691.16 1240.25 132.42 − − − − December 835.30 237.27 841.21 390.31 727.05 − − − January 764.06 571.02 1160.47 421.21 870.04 − − − February 884.79 2124.54 2690.94 1839.41 692.03 − − − − − March 3260.19 4298.28 4679.78 3861.34 2921.9 − − − − − Energies 2020, 13April, x FOR PEER REVIEW6536.78 7243.21 7272.98 6953.11 6259.6 13 of 16 − − − − −
January 1000 Wh 0
-1000
December -2000 February -3000
-4000 -5000
-6000
-7000
-8000
November March
E SE S SW W
October April
Figure 11. Heat balance calculated for the months of th thee heating season depending on the orientation with respect to cardinal points chart. with respect to cardinal points chart. What is noticeable in the presented results is the overbalance of the irradiation derived gains What is noticeable in the presented results is the overbalance of the irradiation derived gains over losses in each month of the heating season in the wall oriented towards south-east, south and over losses in each month of the heating season in the wall oriented towards south-east, south and south-west. The solar irradiation of the eastern and western orientation did not compensate for the south-west. The solar irradiation of the eastern and western orientation did not compensate for the heat loss in the period November–January. Considering the fact that the proposed method of balance heat loss in the period November–January. Considering the fact that the proposed method of balance estimation is biased with ca. 15.5% margin of heat gains underestimation, the results can be regarded as estimation is biased with ca. 15.5% margin of heat gains underestimation, the results can be regarded satisfactory with respect of calculations safety. The target of the IGW design optimization was not the as satisfactory with respect of calculations safety. The target of the IGW design optimization was not maximization of gains but instead its functionality related to its transparency properties. Unlike typical the maximization of gains but instead its functionality related to its transparency properties. Unlike windows, the IGW heat thermal capacitance enables the irradiation derived gains to be transferred typical windows, the IGW heat thermal capacitance enables the irradiation derived gains to be also to the time after the sunset. This property is illustrated in the diagram in Figure 12. transferred also to the time after the sunset. This property is illustrated in the diagram in Figure 12.
20 600
]
]
2
2
m 0 500
m
/
/
W
W
[
[
i
-20 400 g
q
I
y
t
n
i
o
s
-40 300 i
t
n Solar gain delay
a
e
i
d
d
a
x
-60 200 r
u
r
l
i
f
r
t
a
a
l
e 100
o
H
S 0
0 1 2 3 4 5 6 7 8 9 0 1 4 6 7 9 1 2
2 5 8 0 3
1 1 1 1 1 1 1 1 1 2 2 2 2 Time [h]
Figure 12. An example of heat flux density distribution on the IGW inner surface during a day.
Depending on the daily sum of solar radiation the IGW inner surface generates a flux oriented towards the building interior even eight hours after the sunset. It should also be noted that the heat flux given up by the wall surface during the day did not exceed 40 W/m2. It is a value ten times lower
Energies 2020, 13, x FOR PEER REVIEW 13 of 16
January 1000 Wh 0
-1000
December -2000 February -3000
-4000 -5000
-6000
-7000
-8000
November March
E SE S SW W
October April
Figure 11. Heat balance calculated for the months of the heating season depending on the orientation with respect to cardinal points chart.
What is noticeable in the presented results is the overbalance of the irradiation derived gains over losses in each month of the heating season in the wall oriented towards south-east, south and south-west. The solar irradiation of the eastern and western orientation did not compensate for the heat loss in the period November–January. Considering the fact that the proposed method of balance estimation is biased with ca. 15.5% margin of heat gains underestimation, the results can be regarded as satisfactory with respect of calculations safety. The target of the IGW design optimization was not the maximization of gains but instead its functionality related to its transparency properties. Unlike Energiestypical2020 windows,, 13, 632 the IGW heat thermal capacitance enables the irradiation derived gains 14to of be 16 transferred also to the time after the sunset. This property is illustrated in the diagram in Figure 12.
20 600
]
]
2
2
m 0 500
m
/
/
W
W
[
[
i
-20 400 g
q
I
y
t
n
i
o
s
-40 300 i
t
n Solar gain delay
a
e
i
d
d
a
x
-60 200 r
u
r
l
i
f
r
t
a
a
l
e 100
o
H
S 0
0 1 2 3 4 5 6 7 8 9 0 1 4 6 7 9 1 2
2 5 8 0 3
1 1 1 1 1 1 1 1 1 2 2 2 2 Time [h]
Figure 12. An example of heat flux density distribution on the IGW inner surface during a day. Figure 12. An example of heat flux density distribution on the IGW inner surface during a day. Depending on the daily sum of solar radiation the IGW inner surface generates a flux oriented towardsDepending the building on the interior daily sum even of eight solar hours radiation after th thee IGW sunset. inner It should surface also generates be noted a flux that oriented the heat fluxtowards given the up building by the wall interior surface even during eightthe hours day after did notthe exceedsunset. 40 It Wshould/m2. Italso is a be value noted ten that times the lower heat 2 thanflux given the solar up by radiation the wall transferredsurface during through the day the did outer not glazing.exceed 40 This W/m property. It is a value is significant ten times for lower the occupants’ comfort.
4. Conclusions The present paper is devoted to the evaluation of the energy efficiency of an interactive wall responding to changing conditions of solar radiation and air temperature proposed by the author. Following the analysis of the results of field experimental tests conducted using a physical model the parameters characterising the heat balance components were determined. They were employed in the simplified method of IGW heat balance prediction on the basis of weather data of a typical meteorological year. A number of final conclusions were formulated.
(1) The prediction of heat balance calculated for a heating season for a selected locality indicated the overbalance of heat gains over losses in all the months of the season when the IGW was oriented to the south, south-east and south-west. The western and eastern orientations in November and December resulted in the predominance of heat losses. (2) The tests indicated that, unlike conventional windows, apart from transparency, owing to the use of phase-change materials (PCM) in the IGW structure, it has the capacity of giving up heat gains even eight hours after the sunset. (3) The test results confirm the potential of the interactive designs, which apart from transparency, have the capacity to reduce the conventional energy demand and exert a favourable impact on the functionality of a building and occupants’ comfort. The use of cutting-edge technologies and their increasing availability for the shaping of the outer envelope of a building opens up new possibilities of construction engineering. 4) The validity of the method discussed in the paper indicated a difference in the heat balance calculated on the basis of the recorded heat flux density at the level of 15.53% of heat gains underestimation or its losses overestimation, which can be considered a result satisfactory with respect of calculations safety.
The heat characteristics if IGW, the validation of heat balance prediction method based on experimentally identified UEMP and ηUSRH presented in the paper should be regarded as one of the stages of the research. Further research is planned on a greater scale, in which special attention will be focused on the improvement of the thermal capacitance and optimization of glazing selection in order to increase heat gains and the capacity of their storage. Energies 2020, 13, 632 15 of 16
5. Patents Szyszka, J., Rzeszow University of Technology, A collector-accumulative transparent wall for construction engineering; Patent application 12.04.2018 r. nr P.425203 PL.
Funding: This research received no external funding. Acknowledgments: The author would like to thank Prof. Lech Lichołai making available the infrastructure and equipment necessary to conduct the tests. Conflicts of Interest: The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
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