A comparative energy analysis of three electrochromic glazing technologies in commercial and residential buildings
Introduction
U.S. buildings currently consume approximately 14.4 quads (15.2 EJ) annually, including 73% of national electricity consumption [1]. Much of this energy is consumed by lighting, heating, ventilation, and air-conditioning (HVAC), which are each end-uses affected by the thermal and optical properties of a building’s windows. Investigations on this topic have consistently shown the significant impact of window thermal and optical properties on building energy consumption [2], [3], [4], [5], [6], particular in locales with hot climates [7]. Dynamic window technologies, whose properties can be controlled actively or changed passively due to environmental conditions, are a promising category of advanced building technologies that can contribute to reducing energy use in buildings [8], [9], particularly if they are capable and quick and responsive switching [10]. Prominent dynamic window technologies include electrochromic glazings, whose optical properties undergo reversible changes under and applied voltage [11], [12]; thermochromic glazings, whose optical properties undergo reversible changes under changes in temperature [13], [14]; and photovoltachromic glazings, which incorporate solar energy collection to self-power switching of optical properties [15], [16]. Among dynamic window technologies, electrochromic (EC) glazings have emerged as a viable advancement over static technologies, with several manufactures bringing EC window products to the U.S. market [8], [17]. In a building window application, electrochromic materials can be actively modulated in order to control the transmittance of visible and near-infrared solar energy [18]. In so doing, EC glazings can selectively block or transmit solar heat and light to optimize building energy use. While many lighting and HVAC functions in buildings are still manually controlled, EC control strategies are likely to be driven by data from thermal and lighting sensors in response to lighting and HVAC set points to ensure effective operation.
EC glazings of various types have been in development for decades, and have been the focus of many studies related to their underlying material properties, as well as their suitability for deployment in building window applications [12], [19]. For established EC technologies, dynamic glazing properties and their resulting building energy impacts are reasonably well understood. Recent advances in material science has created the potential for new EC technologies that exhibit dramatically different dynamic optical properties and switching ranges [19]. For these new, novel glazings, the potential energy benefit versus either static or other dynamic glazings is not currently known. An enhanced understanding of these new glazings’ energy performance and quantification of the relative value of EC switching across the visible and near infrared (NIR) spectrums would allow researchers to target material properties of EC glazings that maximize their effectiveness in building applications. The objective of this paper is to compare the energy performance of two novel EC glazings (near-infrared EC and dual-band EC) to a well-established EC glazing (conventional EC) and several high performance static windows.
Electrochromic glazings based on tungsten oxide films have been in development for decades and are currently available as commercial products from a number of window and glass manufacturers [8]. These glazings, denoted as “conventional” electrochromic (CEC) glazings in this paper, exhibit broadband switching, meaning their transmission of near-infrared (NIR) and visible light are reduced in unison when switching states [11], [12]. Current commercially available coatings reduce transmission by absorption although prototypes of reflective devices have been demonstrated [20]. This functionality gives CEC glazings the potential to be highly effective at blocking solar heat and mitigating glare [4]. Typically, the CEC blocks some portion of near-infrared heat in both states. When transitioning to a “dark” state, the CEC reduces transmission of both NIR and visible light, resulting in a visually perceptible darkening.
The energy and occupant comfort benefits of CEC have been investigated extensively and are the focus of a number of simulation and experimental studies. Findings suggest that when deployed with appropriate controls, CEC glazings can be highly effective at reducing peak cooling loads, and in modulating daylight so as to capture electric lighting savings while still reducing glare. A simulation of U.S. commercial building stock shows primary energy savings potentials of 10–20% in perimeter zones from CEC deployment [21]. Simulations of CEC glazings in Mediterranean climates demonstrate savings as high as 37 kWh/m2 of glass for east and west facing facades [22], and annual energy savings as high as 54% [23]. Another simulation study found dynamic glazings applied to a split-pane window produced lighting savings of 37–48% versus static window with occupant controlled blinds [24]. Dynamic glazings have been simulated or experimentally evaluated in a number of additional studies [25], [26], [27], [28], [29], [30], [31], [32], many of which suggest variable but positive energy impacts of CEC deployment. Several papers reviewing the current state of advanced window technologies, including CEC and other dynamic glazings have also been published recently [8], [17], [33]. Research on tungsten oxide based EC glazings is ongoing [34], and their manufacturing cost [35], durability [36], and other properties [37] continue to improve incrementally.
In the past several years a transparent electrochromic film capable of modulating NIR transmission without affecting visible light transmission has been developed [38], [39]. This feature may give the transparent NIR-switching electrochromic (NEC) glazing an aesthetic advantage over conventional electrochromic glazings in a variety of building applications and climates. Current NEC glazings are based on a plasmonic electrochromic effect that dynamically modulates the localized surface plasmon resonance of doped semiconducting nanocrystals [40]. Additionally, because of the novel coating design, NEC glazings have the potential to be manufactured using lower cost methods, including spray or slot die coating of a nanocrystal based ink, followed by an annealing process to fix the film to the substrate [41], as opposed to conventional dynamic coatings, which today use more complex and expensive vacuum-based sputter coating processes to manufacture [18]. Although recent research has uncovered lower-cost manufacturing techniques for CEC glazings [35], these are not currently used for commercially available products. Consequently, the potential for manufacturing cost savings could increase the market competitiveness of NEC glazings. Furthermore, the reduced energy intensity of manufacturing means that NEC glazings can achieve life-cycle energy impacts 15–21% lower than comparable CEC glazings [42].
A previous study simulated a broad range of performance characteristics in south-facing building zones. It found that high performing NEC glazings could produce energy savings ranging from 6 to 11% for commercial buildings and 8 to 15% for residential buildings, primarily in middle and northern U.S. regions, relative to high performing static glazings [43]. That analysis found NEC savings in hot, cooling-dominated regions to be less significant, as the NEC transmitted more insolation than optimal during portions of the year. Additional simulations of multiple commercial building types found that a hypothetical NEC glazing outperformed commercially available CEC glazings, but relied on improved thermal properties to do so [44].
More recently, research efforts have been focused on developing a composite glazing that embodies the properties and switching ranges of both CEC and NEC glazings. The first demonstrated so-called “dual-band” electrochromic (DBEC) relies on a synergistic interaction between indium tin oxide (ITO) nanocrystals and niobium oxide (NbOx) glass [45], [46]. Both of these materials exhibit electrochromic properties, however in different regions of the spectrum. This combination allows for an ITO-NbOx composite glazing to independently modulate transmission in both the NIR (ITO) and visible (NbOx) regions. A DBEC material with improved modulation range is under development using tungsten oxide (WO3-x) nanocrystals instead of ITO in combination again with NbOx glass [47]. As with the NEC, these nanocrystal coatings can be applied to glass through a lower capital cost process and less energy intensive application and annealing process [45], meaning the manufacturing cost of the coated glazing may be lower relative to sputtered glazings.
Dynamic switching of the DBEC glazing can be understood by referring to Fig. 1, Fig. 2. Fig. 1 provides a simplified illustration of the functionality of each EC state. The DBEC modeled in this investigation can be understood as being composed of three EC states. Each of these states is defined such that it embodies properties archetypal to a specific operational mode. I.e. the “bright” EC state has been defined to be highly transparent in the visible and NIR spectrums; the “cool” state transparent in the visible and blocking in the NIR; while the “dark” state is blocking in both spectrums. Fig. 2, shows the optical properties of and possible transitions between each state. Solar heat gain coefficient (SHGC) represents the fraction of total insolation that is transmitted through the window unit; while Tvis is the fraction of visible light transmitted. In these figures, CEC can be understood as consisting of the “cool” state and “dark” state, as well as the intermediate states between these two, while the NEC can be understood as consisting of the “cool” and “bright” states, which modulate NIR transmission without impacting visible light transmission.
Recent advances in material science mean that new dynamic window technologies, like DBEC glazings, are close to becoming commercially available, at scale for deployment in buildings throughout the world. Despite the proximity to market availability, the performance benefits of the DBEC over competing technologies have not been quantified beyond preliminary scoping studies in building test cells [48]. Furthermore, there are no published findings of how various external factors (e.g. climate, building geometry, and occupancy and use patterns) will affect the potential benefits of dual-band switching. Given the potential of DBEC glazings to reduce building energy consumption, an analysis quantifying the energy performance is a valuable contribution to the body of knowledge related to building technologies. As the properties and performance of dynamic windows grow more complex, such analyses have proven valuable for decision-making related to development and deployment [49].
This paper presents an analysis of the energy benefits of a dual-band electrochromic glazing deployed in two commercial building types and one residential, in each of the 16 climate zones that compose the United States. This investigation draws on well-vetted methods using building energy modeling software Energy Plus, and utilizes type-representative building reference models developed by the U.S. Department of Energy. This analysis expands on previous simulation work, which assessed the energy performance of NIR electrochromic relative to conventional electrochromic and high performance static glazings.
In this paper, the performance of each of these technologies will be compared to that of a novel, dual-band electrochromic glazing. The results are used to assess the added benefit of selective NIR and visible spectrum switching enabled by DBEC adoption in each region and building type. Furthermore, both energy and energy costs savings are explored to identify market segments within the U.S. where the DBEC glazings achieve the highest savings, which helps inform decision-making related to the ongoing development and future deployment of this technology.
Section snippets
Dynamic window properties
A hypothetical DBEC can be defined by the properties of its three archetypal states. In this investigation, the optical properties have been based on projected values for the fully developed DBEC product, although these have not necessarily been achieved yet in prototypes. The intention of this investigation is to illustrate the technical potential of a hypothetical, high performance DBEC glazing, as opposed to the research glazing as it currently exists. In the simulation each window is
Total building energy
The effect and magnitude of varying window thermal and optical properties can first be explored by examining total building energy use. Fig. 7 presents the total building energy consumption by relevant end-uses for large office buildings and select climate zones: 2A, 4A, and 5A. The first two columns in these graphics show the local ASHRAE-compliant windows, with properties that vary by region. The first of the ASHRAE cases does not include daylighting control, while the second and all
Conclusions
This paper has presented a comparative analysis of three advanced dynamic, electrochromic window technologies: conventional EC, Near-infrared EC, and dual-band EC by simulating their energy performance in three building types and 16 climate zones representing the U.S. The results of these simulations are used to assess the relative value of each of the EC technologies’ switching ranges, as well as their overall value relative to available static window alternatives. Several simple control
Acknowledgements
Lawrence Berkeley National Laboratory is operated for U.S. Department of Energy (DOE) under Contract Grant No. DE-AC02-05CH11231. This project was supported by a DOE ARPA-E grant to D.J.M.
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