Provoked by recent advancements in additive manufacturing, 3D printable concrete has been explored as an alternative method to construct concrete structures [1][2][3]. This new construction approach has the potential to address several major challenges in current concrete industry: it eliminates the use of temporary formwork and vibrations that are typically necessary for consolidating wet concrete. This can in turn significantly reduce the material and labor costs involved in the formwork construction; the highly automated construction process can reduce resource demands, greenhouse gas (GHG) emissions, material waste, cut down construction time, and reduce human error [4,5]. In addition, it allows a new approach for concrete design -the material and component can be parametrically designed tuning to the optimal structural performance and functionalities. While numerous research efforts are still underway in various areas including properties of 3D printable cementitious materials [6] -mechanical and rheological properties [7][8][9], material design optimization [10,11], as well as behaviors of the 3D printed elements [6], including reinforcement strategies for 3D printed concrete structures [12]. Concrete 3D printing technologies including extrusion-based and powderbedding (e.g. D-shape) printing processes [13], robotics [14][15][16], and 3D printing of cementitious material under special environment such as underwater [17] and space constructions [18] are also investigated. Besides the research in laboratories, over the last decade, some large-scale applications and showcase examples have become available -e.g. office building in Dubai by Winsun, the interior of a hotel suite by Total Kustom [19] and residential house by Apis Cor [13].
One of the most striking features of 3D printing of concrete (or any other building materials) is the ability to produce architectural components with complex 3D geometry for lower cost in comparison with traditional casting processes [5], hence pushing the boundaries of the design space available for architects and engineers. With large-scale 3D printing technology, unprecedented level of structural efficiencies and/or multifunctionality through geometry can now be achieved [20] -i.e., additional functions can be embedded in the structural parts, the architectural components are not only constructed for mechanical properties but also serve other functions such as soundproofing and thermal insulation. One of the most loud-speaking examples is the Additive Manufacturing Integrated Energy (AMIE) project initiated by Oak Ridge National Laboratory (ORNL) and Skidmore, Owings & Merrill LLP (SOM) [21] -i.e., the 3D printed AMIE C-shape modular exterior wall explores the potential to condense multiple functions into an integrated shell. AMIE's organic geometries are optimized to reduce localized stress and mitigate turbulent exterior airflow; while the panels' interior ribs are designed to host ultra-high-efficiency atmospherically insulated panels which forms an efficient energy-conserving enclosure. Another example is the building retrofit project developed by Gosselin et al. [16] where the building retrofitting element served as exterior supporting walls for an existing building -the 3D printed elements were designed as absorptive formworks to be filled either with ultra-high performance fiber-reinforced concrete for structural parts or with an insulating material for thermal insulation. Multiobjective optimization was conducted to minimize thermal bridging whilst maintaining its structural performance.
While building energy-saving features have been integrated into the design of 3D printed architectural elements [14,20,[22][23][24], there has not been a systematic study on the energy saving potential of 3D printed concrete buildings. One of the most convenient and easy-to-implement strategies for 3D printed concrete buildings are the integration of passive energy-saving features as such green roof [25][26][27] and vertical greenery systems (VGS) [28][29][30][31]. VGS, often referred to as green façades and living wall systems, are considered as one of the most viable greenery options for buildings since they do not occupy additional space and they can be easily integrated into most new constructions as well as existing building projects [32,33]. Recent studies have indicated that in addition to their aesthetic benefits, VGS also help to improve air quality, reduce noise level, enhance building thermal performance, and minimize heat island effects [34][35][36][37]. When compared with green roofs, green walls have larger potential surface area for greening. Thus, VGS has significant potential to contribute to the insertion of vegetation in the urban context without occupying extra space at street level. At the building scale, green walls can reduce the energy demands of buildings by the combined effects of additional insulation provided by the greenery together with the shading for building surfaces and the cooling effects on the surrounding surfaces and canopy air [38].
In this research, the versatile geometry design enabled by concrete 3D printing was leveraged to seamlessly integrate vertical living wall system into a building's exterior enclosure to create a 3D printed concrete green wall system (i.e., 3D-VtGW), see Fig. 1. The 3D-VtGW building was assembled with prefabricated (3D printed) multifunctional wall modules, which serve as the backbone for green wall system to improve building's energy efficiency.
To quantify energy-saving potential of the 3D-VtGW system, a thermal network model was developed to simulate energy behavior and thermal comfort of buildings with 3D-VtGW system. Whole-building energy simulations were carried out using Chinese Standard Weather Data (CSWD) of Nanjing, China. This study represents an example demonstrating the extended design space for multifunctional building envelopes enabled by digital construction technique, which allows integration of multiple architectural functions (e.g., energy-saving, acoustic).
The prototype building is located at Jiangbei Kechuang Stadium, Nanjing, China. It has a steel frame structure enclosed with the 3D Wall layer printed vertical green wall system (3D-VtGW). 3D-VtGW is a living wall system assembled with 3D printed modular elements that can serve as the backbone for growing vertical greenery (Fig. 1). As aforementioned, one of the most attractive features of concrete 3D printing of concrete is its ability to produce building components with highly complex 3D geometry and less cost/time as compared to traditional casting process. 3D printing offers technical advantage of reconciliating non-standard shapes to achieve additional architectural functions (other than load-bearing and space separation) such as aesthetics and building energy-saving features. In this study, each 3D-VtGW module is comprised of double-layer supporting wythes as well as a wave-shaped surface wythe to grow greenery, as shown in Fig. 1. The cavity between the two supporting wythes can either be filled with reinforcements (e.g., rebar) and high performance concrete (e.g., UHPC) for load bearing, or thermal insulation foams to improve the building's energy performance. In addition, sinusoidal-shaped exterior surface wythe is integrally printed with the wall module such that the space enclosed by sinusoidalshaped wythe and the supporting wythes can be filled with soil to grow greenery. The moist soil and vegetation serve as heat sink and shield against solar radiation. In combination with evapotranspiration within the 3D-VtGW module, the integrated living wall will significantly reduce through-wall heat flux and save building energy use during summer months. It is also worth noting that, the designed 3D-VtGW system mostly harvests rain water and building's greywater for irrigation, aiming to achieve easy-to-maintain system with low maintenance cost.
As illustrated in Fig. 2 (a), the 3D-VtGW prototype building was constructed through on-site assembly of prefabricated (3D printed) concrete modules. For the 3D printing process of 3D-VtGW module, a low-critical shear stress mortar premix with only fine aggregates was prepared with a rheological behavior appropriate for pumping and extrusion-based printing. The mortar ink was kept in a shearing mixer to avoid setting before it was conveyed towards a screw-driving extrusion printing head using a peristaltic pump. Then, the preprinted and cured 3D-VtGW modular elements were assembled on-site around a load-bearing steel frame. The joints were grouted with a non-shrinkage cementitious-based sealant to prevent water intrusion and minimize air infiltration. After erecting the roof and placing fenestration components such as windows and door frames, the envelope was painted with the water-proofing surface paint. Lastly, soil with growing vegetation was placed into each green wall module after the installation of irrigation system. Fig. 2 (b) shows pictures of the completed 3D-VtGW buildings at Jiangbei Kechuang Stadium before the vertical greenery was placed. Through a combination of concrete 3D printing technology and modular assembling, the prototype building was complete in less than 30 days, from component production (3D printing) to construction completion.
The plant in 3D-VtGW is a living component of the building envelope that responds to the environment in a very complicated way [39]. The simulation of heterogenous vertical green wall response is generally not included in commercially available softwares for transient thermal simulation of buildings, making the simulation and design difficult for architects and building designers. In order to study the energy saving potential and thermal comfort performance of buildings with the 3D-VtGW envelope, a hydro-thermal network model was established to perform energy analysis of the heterogeneous building envelope.
In this research, the vertical greenery is represented by three types of node: a node to represent the foliage layer shown in Fig. 3, which captures the sensible heat exchanges, i.e. the short and long-wave radiation balance with the environment and the sky, and convection between the greenery and the surrounding air, the latent heat expelled by the plant transpiration; a second node for the canopy air, where the convective heat balance with outdoor air, foliage, and with the surrounding environment is computed; and another node representing the exterior surface layer of the 3D printed concrete wall which exchanges heat with the two previous nodes through long-wave radiation and convection and also receives a portion of the incoming radiation, and transmits heat to the internal wall node [37]. Similarly, the horizontal greenery is also represented by three types of nodes representing foliage layer, canopy air and soil, where the latent heat exchange brought by evapotranspiration is incorporated in the model. Similar to other existing models for VGS [37], the hydro-thermal network model developed herein is based on a finite difference approach, considering only one-dimensional heat transfer without internal transverse fluxes. The details on the calculation of the evapotranspiration, heat balance, moisture balance and thermal comfort are discussed in the subsequent section.
The indoor space heat exchange considers energy balance of indoor air and interior wall surfaces, including the convective heat transfer on interior wall surfaces, long-wave and short-wave radiations from internal sources including human activity, lighting and equipment, as well as the thermal flow from air conditioner and infiltration. The short-wave and long-wave radiations transmitted through glazing, long-wave radiant interaction amongst interior wall surfaces and the glazing are also considered, see Fig. 4 (a) and (b).
The heat diffusion equation for any point P with temperature T (x, y, z) in a 3D-VtGW module can be expressed as:
where t is time;c p , q, and k are the material's specific heat, density, and thermal conductivity, respectively; and Q 000 V x; y; z ð Þis the rate of heat transferred from energy sources (e.g., solar radiant energy, latent heat).
For a differential control volume, Equation ( 2) is expressed as:
where V is volume and E V is the rate of heat transferred from energy sources in this differential control volume.
As aforementioned, each 3D-VtGW module is discretized into a number of finite volumes, as shown in Fig. 3 (a), where the energy balance method [40] is applied. Thus, Equation (2) can be rewritten as a general form for node i in a thermal network as:
where subscript i represents the node number; H iÀ1 i is the heat transfer coefficient representing conduction between node i-1 and node i; Q i is the rate of heat transfer from energy sources in the finite volume of node i.
The heat transfer process of plant foliage includes convective heat exchange with ambient air, short-wave radiation (Q i,sw,f ), long-wave radiation (Q i,lw,f ) [41] and latent heat from evapotranspiration (Q i,lat,f ). Thus, equation (3) for the foliage node can be written as:
where H af i;f is the heat transfer coefficient between ambient air and node i in foliage layer; T af is near-canopy air temperature for foliage layer; F is view factor, in which superscript is emitting surface and subscript is receiving surface; b i;f is foliage coverage ratio of surface for node i; Q ik, lat is latent heat generated from evapotranspiration for node i and k; other notations in Equation ( 4) and following equations can be found in the nomenclature.
Similar to the heat balance of foliage, the heat transfer process within the soil can be expressed as:
where T as is canopy air temperature near the soil layer; a i;s is solar absorptivity.
The heat transfer within the concrete wyeth in a 3D-VtGW module shown in Fig. 3 can be expressed as one of the three node types:
(1) Exterior surface node
where H j i;w is heat transfer coefficient between air node j and wall node i; F k i is view factor for node i with exterior emitting surface k -i.e., wall or window and foliage surfaces.
(2) Internal node
(
ð8-cÞ 3.1.5. Heat and moisture balance of canopy air Heat balance of canopy air node j can be expressed as the following.
where H i;f j is heat transfer coefficient representing convection between canopy air node j and foliage node i.
The moisture balance of canopy air node j is expressed as:
where W j is water mass in the canopy air node j;w f j ; w s j and w air ex j are moisture transfer rate from foliage, soil and outdoor air, respectively.
The heat transfer process for indoor air includes convective heat exchange with interior wall and window surfaces, air conditioning, infiltration and absorption of heat from internal gains Q j;int generated by occupant activity, Q people j;conv lighting Q light j;conv , and equipment Q equip j;conv . The heat balance of indoor air node j is expressed as:
-aÞ
where _ m sys j and _ m inf j are mass flow rate into indoor zone for air node j from air conditioner and outdoor air, respectively; T sup;j is the supply air temperature of air conditioner for indoor air node j.
The moisture balance for indoor air node j is:
where W j is water mass of indoor air node j, w int j ; w sys j and w inf j are moisture transfer rate for indoor air node j from internal latent heat gains, air conditioning system supply air and infiltration.
Evapotranspiration is the combination of two different processes that occur simultaneously, evaporation from the soil surface or the wet vegetation and transpiration from the vegetation [42,43]. Over the past years, a number of empirical models have been developed to simulate the latent heat brought by the evapotranspiration process within the greenery system [38,42,44,45]. Among them, the Penman-Monteith equation originally developed for crops [42] has been widely adopted for the simulation of the evapotranspiration process for green walls and green roofs [37,41,43,46,47]. Studies by Ouldboukhitine et al. [48], Davis and Hirmer [49] pointed out that the FAO-56 Penman-Monteith equation tends to under predict the evapotranspiration in building greenery systems, whereas a number of experimental studies [37,46,47] have indicated that the generic format of Penman-Monteith equation is suitable for green walls/roofs after minor modifications based on plant species and irrigation condition etc. [43,49,50]. In this study, the soil is assumed to be constantly irrigated for the simplicity of analysis and illustration.
The general form of Penman-Monteith equation can be written as [42]:
where D is the slope of the saturation vapor pressure-temperature curve; G is heat absorbed by soil; q a is air density; c p,a is the specific heat of air; c is psychrometric constant; e s is saturation vapor pressure of air, e is the actual vapor pressure; r s is (bulk) surface resistance [42]:
where LAI active is the active (sunlit) leaf area index [43]; and r l is bulk stomatal resistance. R n is net radiation at foliage surface [43]:
and r a is aerodynamic resistance adopted from [43,50]:
where d is the leaf characteristic length and u is the air velocity [43].
Thermal comfort indices -i.e., the Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) based on Frager's model [51] were used to evaluate thermal comfort-time performance of building with 3D-VtGW. PMV predicts the mean value of the votes of a large group of people considering influence from indoor environment (humidity, air temperature and velocity, mean radiant temperature) and human metabolism, while PPD gives a quantitative prediction for the percentage of thermally dissatisfied occupants shown as the following function of PMV:
The Long-term Percentage of Dissatisfied (LPD) reflects occupants' long-term thermal comfort [52] defined by the following equation:
where p r;n is the occupation rate at n th time step for r th zone; PPD r;n is the percentage of dissatisfied; Dt n is time step duration. This normalized index gives an overall impression of the building thermal comfort performance for all the occupants over the whole time. Frager's model is used to calculate percentage of dissatisfied for LPD.
The whole building simulation model is the network through assembling of all the layer nodes and air nodes as shown in Fig. 4 (b), based on heat and moisture balance equations. The state-space representation [53] of the thermal network can then be written as:
The building's thermal behavior is solved by the discrete state space model through discretization of time: In the heat balance equation, long-wave radiation is linearized using the same method performed in reference [54]. Simple glazing system model [55] was used for windows in this building model, in which window layer is no-mass layer. Infrared heat transfer through window and back out short-wave radiation from indoor space were also considered. Model in reference [55] was used for calculating internal gains from occupants, lighting, and equipment. For each time step, heat balance and moisture balance were computed separately; model variables (e.g., air relative humidity) and air conditioning control were updated based on information from last step.
To quantify thermal performance and energy saving potential of buildings built with the 3D-VtGW, energy analyses were performed on two commercial buildings with different envelopes: one with the 3D-VtGW and another with conventional aerated concrete block wall. The buildings selected for comparative study have the same roof, floor and fenestration (windows and door) materials. The window-to-wall ratio is 25% for the exterior envelopes. The building with 3D-VtGW was implemented with greenery on all four exterior walls (opaque portion of the vertical envelope); the baseline exterior wall was made of aerated concrete with the same overall R-value (1 KÁm 2 /W) as the 3D-VtGW. Both buildings were assumed to be used as small commercial buildings (i.e., cafeteria) with the dimension of 8.7 m (L) Â 5.8 m (W) Â 3.74 m (H). Automatic irrigation system was assumed to be installed on the top of each wall module, in conjunction with rain water harvest, to ensure proper water content in the soil. Other main simulation parameters include occupant activities, lighting, equipment and infiltration, which are enlisted in Fig. 5. It was also assumed that the long-wave absorptivity and long-wave emissivity are equal for both the soil and wall surfaces. CSWD of Nanjing, China were used for the simulation, Fig. 6 presents typical weather data during three representative seasons. Indoor temperature was controlled by air conditioner within 20.5-23.9℃ in summer and 21.1-24.9℃ in winter.
Whole building energy simulation and thermal comfort analysis were carried out using the hydro-thermal network model developed in Section 3 over a typical meteorological year. The simulation results indicated that 3D-VtGW system has substantial potential to improve both energy conservation and thermal comfort performance of buildings.
As is shown in Figs. 7 and8, an evident difference can be observed for the thermal behaviors between the 3D-VtGW wall and the aerated concrete block wall baseline. Similar to the results from previous studies on green walls and green roofs [39], the exterior wall vertical surface temperature (weight average by area) of 3D-VtGW was notably lower in comparison with that of the baseline aerated concrete block wall. In summer, the maximum surface temperature of the aerated concrete wall baseline (west) reached over 60℃, whereas it remained only up to 35℃ for the 3D-VtGW during summer daytime as shown in Fig. 7. The westfacing wall is exposed to sunlight during afternoon hours and thus the temperature rises rapidly and reaches its peak value. The exterior surface temperature of 3D-VtGW also increases during day hours, but the peak temperature is significantly lower than that of the conventional wall -i.e., the rise of greenery system surface temperature is much more gradual. The vegetation together with wet soil protect building envelope from rapid temperature rise through evapotranspiration effects and solar shading similar to other VGS systems [39].
Apart from the benefits mentioned, 3D-VtGW also provides additional insulation (from plants and soil) to help maintaining indoor thermal environment. Under different weather conditions, VGS often shows notably different performances -i.e., generally living walls show a better thermal performance than common walls in summer than that during winter and transitional seasons [39]. Because of the combined effects of evaporative cooling, foliage shading, and heat storage of substrate soil, average surface temperature of the concrete support layer within 3D-VtGW showed delay and significant attenuation compared with that of the baseline aerated concrete wall, see Fig. 7. It was observed that even during the nights of hot summer days, the surface temperature of exterior concrete support layer of the 3D-VtGW is still below the surface temperature of aerated concrete block wall due to the evaporation promoted by hot exterior air temperature. During transitional seasons, the 3D-VtGW only shows cooling effect during daytime. In winter, because the average indoor temperature is higher than average outdoor temperature, 3D-VtGW serves as heat sink in the daytime and resistance at night. Fig. 8 shows the heat flux through each exterior wall. The results indicate that the 3D-VtGW system effectively reduces the peak heat flux through exterior wall, especially during summer daytime through high solar-induced evapotranspiration. Since orientation has a significant influence on the solar radiation acting on the building envelope, the heat flux through the west-facing wall is notably higher than the other walls. This orientation-dependent heat flux difference is particularly evident with the baseline aerated concrete block wall due to rapid exterior temperature rise as previously discussed. Whereas for 3D-VtGW, there is relatively small difference in through-wall heat flux of different orientations in both summer and winter, see Fig. 8. Relative heat flux reduction for 3D-VtGW compared with the baseline indicates that west wall brings largest thermal benefits in summer; and south 3D-VtGW wall exhibits most beneficial behavior as compared to baseline.
To investigate the cooling mechanism of the 3D-VtGW, an energy balance analysis was carried out using the simulation results. As shown in Fig. 9, the radiative, conductive, latent, convective energy flow and energy balance are obtained from the simulation results. It was found that on the foliage surface net solar radiation is the major heat gain in the daytime for both summer and winter months. Evapotranspiration dissipates most of heat gain on the foliage surface in summer. In winter, long-wave radiation and convection also contributes to the heat dissipation from the foliage. This observation is consistent with the findings from other studies on living walls [37] showing the influence of the outdoor environment via both the convective energy flow from canopy airflow and long-wave radiation exchanged with the outdoor environment. Fig. 9 (b) and (d) present the heat balance at the soil node for a typical summer and winter day, respectively. In summer, water evaporation from the soil layer would dissipate a large portion of the in-coming heat, further reducing the through-wall heat flux and fostering the passive cooling effect from greenery, see Fig. 9 (e). It is worth noting that this configuration with complex geometric features was enabled by concrete 3D printing technique with much lower construction cost as compared to conventional construction technique. Under the combined effect of evapotranspiration, vegetation shading and the thermal storage, the average heat flux transferred into the room through 3D-VtGW is 3.94 W/m 2 less than that through baseline aerated concrete block wall during summer months. During transitional seasons, the plant transpiration still provides effective cooling during the daytime, where the average through wall heat flux was also largely reduced. Stomatal resistance of plant increases in winter responding to lower outdoor temperature and solar radiation compared with that in summer [38], evaporation cooling effect from the greenery system is relatively small, whereas the greenery serves as heat sink and reduces unbeneficial heat flux through exterior envelope by 2.88 W/m 2 compared with baseline.
The monthly air conditioning (AC) load was obtained for the case study building with the 3D-VtGW. The AC load reduction is obtained by comparing to a reference building (same configuration) with the common aerated concrete block wall. Fig. 10 (a) presents comparison of the daily AC loads compared between the 3D-VtGW building and baseline building during two typical summer days in July. The AC load reduction ranges from 15% to 20% over typical summer days during the occupied hours with some fluctuations during the day. It is noted that the AC is scheduled off 1:00 am -5:00 am every day when the building is unoccupied. As a result, during warm summer nights the indoor air temperature of reference building may continuously rise during these hours, whereas the greenery system kept the indoor air temperature of 3D-VtGW building relatively cooler when AC is off. The AC load reduction is higher during afternoon hours due to the high evapotranspiration from the 3D-VtGW, which is consistent with the temperature and heat fluxes presented in the previous sections. Fig. 10 (b) presents the monthly AC loads comparison (and reduction) for the case study building with 3D-VtGW and the aerated concrete block wall baseline. The simulation results show that the energy saving potential for the 3D-VtGW envelope is higher during summer months (May-August) than in transitional season and winter month, with maximum reduction occurs in June (12.00% cooling load reduction). It should be noted that the seemed higher percentage saving in winter months (November-February) than that in transitional seasons is mostly due to the relatively lower monthly energy use. The percentage of saving greatly depends upon external factors such as building type, building dimension, and environmental factors. When considering the monthly variation of energy saving it is consistent with the previous experimental study [38], which showed that when solar radiation is higher, living wall systems can provide higher energy saving -when a vertical greenery system receives higher solar radiation, the thermal benefits increases. Since the building studied herein is a cafeteria with high internal heat gains from occupants, lighting and equipment, thus, the winter month heating energy demand is lower than the cooling load demand in the summer.
In order to evaluate the potential of 3D-VtGW to improve building's indoor thermal comfort, the Frager's model is adopted [51]. The predicted percentage of dissatisfied (PPD) index is obtained from the simulation results based on the formula given in Section 3. It is evident that the 3D-VtGW system reduces the predicted percentage of discomfort for all seasons. Fig. 11 (a) presents the PPD values for both buildings with 3D-VtGW and aerated concrete block wall during two typical summer days, where the PPD for the baseline building hovers round 25% throughout the operation hours. The higher PPD value occurs when the outside temperature or the occupation rate is high (during afternoon and evening hours). The building with 3D-VtGW shows similar trend, whereas the PPD values are lower than that of the baseline building with PPD remains under 20% for most of the operation hours. Fig. 11 (b) presents the monthly long-term percentage of dissatisfied (LPD) of the 3D-VtGW building as compared to the baseline building. Similar to the AC load reduction as shown in Fig. 10 (b), the thermal comfort improvement shows highest potential during summer months (May through September), with the annual LPD decreased by 10.20% (i.e., the Yearly LPD reduced from 12.26% for the baseline to 11.00% for the building with 3D-VtGW).
Apart from some other parameters impacting the behavior of greenery systems, such as LAI, surface reflectivity [37], exterior environment (solar radiation, relative humidity) [38], the thermal insulation of supporting layers adjacent to soil and foliage has a significant influence on the effectiveness of the 3D-VtGWs (i.e., AC load reduction and thermal comfort improvement are shown in Fig. 12 (b)). Fig. 12 (a) shows that under the internal gains level of a commercial building (high internal gain), 3D-VtGWs with low thermal insulation, R int , as shown in Fig. 12 (c) brought higher AC load reduction in summer and transitional season as compared to 3D-VtGWs with higher thermal insulation levels. Even during winter months, lower insulation level also shows beneficial effect in terms of energy saving -i.e., the yearly AC load reduction ratio of 3D-VtGW with low thermal insulation R int was 22.95%, which was much higher than 9.12% AC load reduction for 3D-VtGW with high thermal insulation R int . This is mainly due to that lower level of insulation improves the utilization of greenery cooling effect by promoting heat conduction from the greenery layer to the hot indoor air. Similarly, improvements in thermal comfort was also observed for building with lower R int during spring, summer and fall. Based on this observation, it is projected that the performance of living wall systems is dependent on both the environmental conditions, infiltration level, the level of internal gains (building type, occupancy conditions etc.), as well as the envelope insulation level. Therefore, it is speculated that living wall systems including 3D-VtGWs may show higher benefits with dynamic insulation [56,57] that can modulate insulation levels based on the demand.
Large-scale 3D printing has opened enormous opportunities for multifunctional building envelope design. In this research, a modular building envelope with integrated vertical greenery system, namely 3D printed Vertical Green Wall (3D-VtGW), was developed with the aid of concrete 3D printing technology. The 3D-VtGW was designed and produced using an integrated design-build (print) platform, enabling workflow for rapid customized design optimization, and prototyping of high-performance building envelops to satisfy multiple design requirements. The energy saving potential of a small commercial building assembled from 3D printed modular living wall system was demonstrated. A thermal nodal network model was formulated to calculate the thermal performance of buildings with 3D-printed heterogenous building envelope with vertical greenery system, with the consideration of both heat and moisture balance of the living wall components and air. Heat transfer process through the heterogenous living wall was captured by discretized state space equation.
Energy analyses of two archetype buildings were carried out as case studies: one with the 3D-VtGW and another baseline building with conventional aerated concrete block wall with similar thermal insulation level. The simulation results indicate that 3D-VtGW system exhibited excellent energy saving potential and improved thermal comfort. For the case-study archetype buildings (i.e., small commercial building) demonstration herein, the 3D printed green wall system largely reduces exterior wall surface temperature in summer as well as the heat flux through exterior walls. 3D-VtGW system brought 11.20% AC load reduction in summer and yearly 9.12% AC load reduction compared with baseline. The building with 3D-VtGW provided improved yearly thermal comfort, especially during summer and transitional season reducing LPD of baseline by 10.20%.
For buildings with high internal gain like the ones demonstrated in this study, lower insulation of exterior building envelope brought beneficial thermal behavior due to the promoted heat dissipation through the living wall. The observation leads to the belief that variable/dynamic insulation may enhance the performance of green wall systems, although the topic falls outside the scope of the current study. This research serves as a demonstration of the application of large-scale 3D printing to enable the construction of otherwise hard-to-produce or costly building components with complex geometry. The advancements in additive and subtractive manufacturing techniques and computational tools over the decade have opened pathways for producing building structures and components that were previously impossible or time-consuming to create. This largely expands the design space for highperformance building materials, components, and assemblies that have integrated functionalities such as structural, architectural (e.g., acoustic, thermal insulation, self-shading), and environmental functions (air quality melioration). Future research and developments are encouraged to explore these expanded spaces.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.