Grantee Research Project Results
Final Report: Solar Window for Water Collection and Purification
EPA Grant Number: SU840165Title: Solar Window for Water Collection and Purification
Investigators: Dyson, Anna H , Kim, Jaehong
Institution: Yale University
EPA Project Officer:
Phase: I
Project Period: December 1, 2020 through November 30, 2021 (Extended to November 30, 2022)
Project Amount: $24,851
RFA: P3 Awards: A National Student Design Competition Focusing on People, Prosperity and the Planet (2020) RFA Text | Recipients Lists
Research Category: P3 Awards , P3 Challenge Area - Safe and Sustainable Water Resources
Objective:
Multifunctional Building-Integrated Systems for Solar Water Disinfection
This project addresses strategic water and energy management techniques for the built environment through investigating the integration of novel enhanced Solar Water Disinfection (SODIS) techniques into a building envelope assembly for on-site renewable energy-based water recuperation (Figure 1). This project incorporates ongoing multifunctional building envelope research to investigate the potential for the integration of combined solar energy collection and water disinfection into curtain wall systems and structural facades that provide architectural requirements, such as transparency to outside views, reduction of solar glare, modulated daylighting, and reduction in lighting loads (Dyson et al., 2012). For this project, we investigated the integration of a novel photochemical enhancement of solar-driven water disinfection (enhanced-SODIS), within an advanced facade typology for solar water disinfection, daylighting, and heat remediation (Dyson et al., 2012; Novelli et al., 2021). We explored how a solar concentrating optical geometry could enhance the synergistic effectiveness of photochemical technologies in SODIS and solar pasteurization (SOPAS). We hypothesize that the integration of optical components into a building envelope module could enhance the effectiveness of renewable point-of-use water treatment by strategically increasing solar capture with available building surface area, and by maximizing the potential for solar-driven processes through increasing flux using solar concentration.
Solar concentrating collectors and the incorporation of plant-based photosensitizers have several advantages with building integration:
1.) The approach increases energy onto a system process and could increase the water production rate, especially significant to overcoming seasonal periods with high cloud cover or shorter and lower intensity daily solar hours,
2.) Building integration is a point-of-use application that directly benefits households by ensuring Water Safety,
3.) It provides strategic management of how and where incident solar energy transmits through or is functionalized by the building envelope,
4.) the system demonstrates a sustainable approach to using available resources in each climate,
5.) Leverages building-scale means of providing household water storage, and thereby
6.) Through integration, they maximize solar energy for household energy, daylighting, water heating, and thermal comfort needs. Furthermore,
7.) It harnesses renewable, non-toxic, and plant-based photosensitizers at little to no consumer cost to households
8.) The photosensitizing dyes can be harvested from plants grown by households, and
9.) The photosensitizing dyes visually indicate when the water is safe to drink, assuring water security.
Figure 1 Building-Integrated Solar Disinfection: Incorporating renewable photochemical water treatment technologies into the Solar Enclosures for Reuse (SEWR) Framework (Dyson et al., 2012; Dyson et al., 2020; Ryberg et al., 2018; Ryberg et al., 2021).
Conclusions:
The design development of a low-cost, manufacturable, and building-integrated solar concentrating collector for distributed use by households requires a multifunctional trade-off between system costs, deployability and performance. The experimental work presented in this report, covers a range of test scales that were used to produce multifunctional proof-of-concept evaluations. For this stage of work, we have focus on evaluating whether the concept achieves viable levels of solar concentration by using thermal, photochemical and water disinfection indicators measured in non-concentrating and concentrating test conditions. We compared simulated with measured performance data to continually evaluated performance potential, viability, and further iterate the system design towards a more optimal mediation of design criteria.
Simulated Optical Performance of the Solar Concentrating Collector in Ideal, Realistic, and Building-Integrated Conditions
We expected that incorporating a CPC-type reflective profile into a building system concept would produce expected losses to the ideal optical performance by introducing material properties, realistic thicknesses, and the system's demands as an insulating envelope. We conducted optical simulations to estimate the impact of integration on optical efficiency.
The building-integrated solar concentrating collector adapts an optical profile with a reported optical efficiency of 85% and a half-acceptance angle of 60 degrees in ideal conditions (Winston et al., 2005). The addition of building-integration parameters such as optimal tilt angle (OPTA) and building orientation decreased the simulated optical concentration on the receiver to 81.6% at direct normal incidence. From a geometric concentration of 3.2x (relative irradiance on a plane focused on the tube), the simulated losses reduce the solar flux concentration to an estimated 2.6x concentration ratio within the proposed building configuration. The estimated flux that would transmit through the receiver into the water volume within a transparent tube condition with upwards of 90% probable transmission and a refractive index of 1.46 suggest that 61.9% of direct normal solar incidence on the system would reach the water target. At the latter optical efficiency, the solar concentrating reflector produces a 2.0x concentration of solar energy on the water target.
In suspension-based water treatment technologies such as photosensitizers, the flux per unit water volume is more significant than the conventional flux/surface area metric used with solar thermal systems. Estimates factoring average optical efficiency suggest that the receiver surface would intercept a minimum of 2.1 times more flux than in a non-concentrating condition (measured in W/ mL). At the simulated concentration of solar flux and based on photosensitization parameters regarding flux dosage, the system could double the rate of broad-spectrum photosensitization water production compared to non-concentrating conditions, with similar effects on the rate of solar pasteurization, albeit dependent upon ambient conditions and expected rates of heat loss.
Solar Concentration Potential: Comparative evaluation of simulated and measured indicators of the optical concentration of solar flux on the target receiver
We estimated realistic levels of solar concentration using several indicators of flux conversion in the system. Further to assessing the viability of the solar concentrating approach, we simultaneously evaluated the impact of concentrating solar on the water treatment techniques.
Solar Concentration with Photosensitization
In the incorporation of broad-spectrum photosensitization, we used the linear photo-decay rate of the photosensitizing dye as an indirect indication of solar concentration. Compared across concentrating and non-concentrating conditions, the solar concentrating treatment increased the dye degradation rate by 26% and suggests that a probable 1.26x solar concentration ratio is produced in the visible spectrum. In contrast to the 2.1x concentration ratio predicted through simulations, the differences between measured and simulated performance data suggests that optical losses of up to 40% resulted due to realistic environmental, geometric, and material effects. In application, the solar concentrating collector could improve the photosensitization- based water treatment rate by reducing the residence time up to ~80% to that required under non- concentrating conditions. More significantly, solar concentration could increase the functional solar flux threshold to include solar inputs as low as 600 w/m2 to achieve 99.99% inactivation through minimal effective solar dosages. This suggests that incorporating solar concentration with photosensitizing dyes could ensure effective SODIS function on low-sun or partly cloudy days with variable sky conditions, extending the viability for water treatment seasonally. In the point-of- use context, variable solar conditions would reduce the viable period for safe water treatment. Therefore, meeting the minimum solar dosage in an expedient manner would offer significant security in resource-scarce informal settings dependent on renewable point-of-use treatment.
Solar Concentration and Solar Water Heating for Pasteurization
The thermal absorption of incident solar energy by the fluid volume was measured across various configurations with indoor and outdoor testing. With lab-scale testing in indoor high bay labs (21°C 70%RH), the rate of temperature increased more than doubled relative to the control condition. In contrast to indoor test data, the thermal data collected from experiments conducted under full sun showed a threefold increase in the water heating rate with solar concentration compared to results for the non-concentrating control condition. The thermal behavior in the system corroborates what was simulated in terms of flux per water volume (W/mL) based on a high absorption factor on the receiver surface, likely due to several heat transfer mechanisms in addition to direct solar concentration.
Compared with non-concentrating solar collectors, this suggests an increased rate of hourly water production dependent upon targeted effluent water temperatures for solar pasteurization and domestic water heating. At the highest degree of water safety at an effluent temperature of >80°C, the solar collector could produce on average 5-8L/hr/m2 under peak solar conditions or at least 2.5L/ hr/m2 under the variable low sun conditions (300-800 W/m2). At a lower target temperature for SOPAS at an effluent temperature of >65°C, the collector output could be upwards of 12L/hr/m2 or 100L/ m2/day. The solar concentration of incident solar energy also increases the use of intermittent periods of direct solar on days with incremental cloud cover in the mornings or evenings, maximizing how much water can be treated within non-ideal solar conditions. At a three- fold flux concentration, we observed water heating at low irradiance levels of 500 W/m2.
Estimated Water Disinfection Capacity
We assessed the bacterial inactivation capacity as a function of solar pasteurization through outdoor testing in field settings. The system was challenged-tested with Escherichia coli (E. coli) as an indicator for enteric bacterial pathogen disinfection efficiency to evaluate the system’s bacterial log reduction value (LRV) under variable solar conditions. Under full sun, E. coli spiked feed solutions (106 CFU/ ml) were inactivated entirely within 60 minutes, demonstrating an up to 6-log reduction value under peak solar (>1000 W/m2). When evaluated in partly cloudy conditions with an average 570 W/m2 solar input, the system showed a reduced LRV of 5-Log within 2hrs due to reduced incident solar energy.
Preliminary estimates based on these results provide a reasonable range of viable water treatment annually across changing seasons. During peak solar periods over the dry season over a 6-hr solar period, estimates suggest a daily production rate of 30-50L per square meter of collector area, depending on the target temperature range for pasteurization. Furthermore, in non- ideal solar collector conditions with periods of cloud cover – the more realistic baseline scenario for household water treatment - the system could still produce a minimum of 15L per m2 collector per day, providing at least 7.5L of safe drinking water per person per day as a 2m2 roof light system or 100L per household as a 7-8m2 building-integrated roof enclosure (UNHCR Minimum Water Allocation per person daily) year-round. In locations with long dry seasons and a direct solar climate, such as Phoenix, Arizona, the system could provide >145L treated water and all domestic hot water needs with a 3m2 collector area that could be integrated as a roofing daylight system in numerous architectural design instances.
Ensuring year-round household water safety using renewable energy requires the treatment approach to be resilient against changing solar and source water quality conditions. At the minimum, a multi-barrier water treatment system for rain, spring, and municipally supplied water sources should include prefiltration and disinfection treatment stages to ensure adequate inactivation of variable pathogen loads. For instance, prefiltration has a 1-log removal capacity for viruses, and although solar pasteurization has a 6-log removal capacity for most waterborne pathogens, the approach depends on daily climate conditions and could result in periods of insufficient water pasteurization. Therefore, combining several treatment mechanisms and a building-integrated installation harnessing prefiltration and water storage, we target a minimum year-round 4-log water disinfection capacity based on the EPA-SWTR 99.99% (4-log) virus inactivation standard. With future work, we will evaluate the probable system performance across the range of environmental conditions of the case study sites.
Estimated Multifunctional Performance with Building Integration: Combining the need for solar concentration, mediated daylight transmittance and mitigated solar heat gain
To achieve greater sustainability in urban energy and water systems, we must maximize the distributed and functional use of environmental resources incident to buildings. The proposed system, therefore, challenges the status-quo discretization of solar energy applications by developing means of modulating light transmittance to simultaneously concentrate solar while providing sufficient indoor daylighting by selectively transmitting light in the visible spectrum.
Incorporating the system into the building façade design would support passive daylighting design, solar heat gain mitigation, and carefully considering views through transparent apertures without the harmful risks of visual glare. Effective daylighting design can produce energy savings of up to 75% of the energy used for electric lighting in a building (Torcellini et al., 2004), reducing cost, energy, and material burdens on homeowners, utilities, and the environment. Electric lighting accounts for 25% of the total electricity consumed in buildings in the United States (US-DOE 2006), and buildings account for over 75% of the total electricity demand nationwide (US-EIA 2008). An effective daylighting design using high-performance glazing systems can provide quality daylight simultaneous to mitigating solar heat gain. Daylighting-associated solar gains have been shown to be half the heat gains of an artificial lighting system required to produce the same interior lighting levels. Furthermore, greater use of natural daylighting could reduce waste heat produced by artificial lighting, thereby reducing building cooling loads. Therefore, effective daylighting designs can reduce building cooling loads by 10-20% by reducing electric lighting (Anders., 2011; LANL, 2003). Incident solar energy is a critical resource supporting human health, safety, and productivity as natural daylighting (Wirz-Justice et al., 2021). The physiological benefit of natural daylight depends on exposure to broad-spectrum power distribution of sunlight which artificial lighting such as LEDs, fluorescents, or incandescent sources cannot provide. Natural daylighting exposure has been shown to improve moods (Beute & Kort., 2014), support better sleep (Boubekri et al., 2014), is directly responsible for essential circadian rhythm entrainment of our biological clocks (Nagare et al., 2021), and counters a 1.4x more significant risk for depression associated with inadequate lighting (Brown & Jacobs., 2011). Cumulatively, sufficient daylight exposure is critical to reducing disease burden and resulting impacts on productivity.
Therefore, towards more sustainable use of environmental resources in buildings, we have simultaneously investigated the building-integrated concept in terms of water disinfection capability and in building performance as a translucent rooflight for daylight, thermal regulation, and view finding. In the ongoing investigation, we will compare the proposed systems' building envelope performance criteria with minimum energy-efficient daylighting standards. For instance, in Phoenix, Arizona's direct solar desert climate, a system must meet a U-value of <0.40 and a <0.25 solar heat gain coefficient (SHGC) to ensure building energy efficiency and mitigate additional cooling loads. Furthermore, the management of solar transmittance to meet the SHGC criteria will impact the daylighting design of the system, shaping the system's surface area extent and visible transmittance (Vt) rating. On average, double-glazed systems meeting respective SHGC ratings have a visible transmittance of 0.5-0.6 Vt. However, as a translucent roofing system with larger surface area extents, we targeted a Vt range of 0.35-0.5 in a skylight configuration or 0.1-0.2 Vt when the proposed system is employed as roofing (O'Connor et al., 1997).
We compared optical material treatments that could be functionalized for the dual function of solar concentration and used as glazing materials for daylighting purposes. The degree of translucency for daylighting must be carefully considered as it is a critical factor impacting effective solar concentration, solar heat gain to the building interior, views through the system, and daylighting contribution. The introduction of translucency would produce losses to the ideal concentrating profile by reallocating this energy to daylighting. Initial optical simulations, modeled according to surface treatment’s reported reflectivity properties, showed that introducing translucency would reduce the collector’s optical efficiency to 70%, with an effective 2.3x solar concentration under direct normal irradiance. Therefore, in contrast to the opaque condition with high reflectivity, the translucent condition would be 84% as effective in concentrating solar flux. At both physical test scales (device and control), the semi-reflective treatment performed at an average 87% efficiency relative to the fully reflective treatment when tested indoors under solar simulation. Similarly, the translucent condition produced a comparable reduction in the thermal behaviors as with the optical behaviors. The relative performance compared with the introduction of translucency to the reflective conditions was corroborated with outdoor test data, with an 84% relative performance in water heating.
We evaluated each system configuration’s light transmission as a measure of the ratio between solar irradiance above and below the surface. The non-reflecting control condition produced a 30% transmission of the incident solar energy, demonstrating the effects of reflectance and absorbance through multiple transparent surfaces on the resulting daylight transmittance. In the testing of the semi-translucent condition, the results corroborated the manufacturer’s reported transmittance of 6-7% total solar transmittance. Through photographs and observation, the experiments suggest that an appropriate amount of visual clarity is maintained regardless of the low transmittance value. Glare from the overhead solar source was appreciably reduced, and a muted tint was visible on the glazing surface.
Supplemental Keywords:
Solar concentration, solar collectors, photoreactor design, building facade design, Environmental justice, sustainable infrastructure design, environmentally benign substitute, resource recovery, design for the environment, holistic design, model for sustainability, environmental education, renewable energy, monitoring resource consumption, green building, architectural design, sustainable construction materials, green chemistry, biotechnology, biopolymers, waste to value, conservation, urban water planning, water purification technologies, drinking water treatment, disinfection, pathogen detection and removal, photocatalyst.Relevant Websites:
Yale Center for Ecosystems in Architecture Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.