Building-integrated Photovoltaic Windows Research Articles

Evaluation of the colour properties of CdTe PV windows

Building-Integrated Photovoltaic Window can potentially improve building energy efficiency. However, with the integration of solar cells, the colour properties of daylight passing through PV windows may be affected. It is important to explore the colour properties of the transmitted daylight to ensure a comfort luminous environment for its occupants. In this study, the spectral transmittances of four types CdTe PV window have been measured, subsequently the colour properties of the tested windows are examined and evaluated by Correlated Colour Temperature (CCT) and Colour Rendering Index (CRI). It was found that CdTe PV windows can achieve recommended CCT (i.e. 3300-5000K) when the daylight source is in the range of 4000K and 6500K. The general CRI of all the CdTe PV windows can be maintained in a comfort range, which is higher than 90, under different daylight scenarios.

Film-based optical elements for passive solar concentration in a BIPV window application

In this work, film-based optical elements are used as a passive solar concentrator for Building Integrated Photovoltaic (BIPV) window applications. Micro-facets are used to induce total internal reflection allowing window glass to function as a wave guide, thus redirecting light onto the back side of bifacial photovoltaic (BFPV) cells. Seasonal variations in the incident angle at solar noon were considered for Chicago, IL. Three optical models were developed using commercial optical software, validating that the optical elements have a significant impact on the amount of light energy allowed to reach the back side of the bifacial cell. Further, an experimental validation was conducted using two commercially available films. Using a 1:1 film to PV-cell ratio, the first film demonstrated a maximum of 26.3% increase in power on the back side of the bifacial cell. The second film type allowed a maximum of 30.2% increase. Both films allowed solar energy to reach the back side of the cell over the range of incident angles that were evaluated, making it useful as a passive solar concentrator. Finally, both 2:1 and 3:1 film to PV-cell ratios were evaluated for each of the two film types. A maximum power increase of 35.1% was observed. The results indicate that film-based optical elements have potential as a passive solar concentrator for BIPV applications.

Heating and Cooling Performance of Office Buildings with a-Si BIPV Windows Considering Operating Conditions in Temperate Climates: The Case of Korea

This study analyzed the heating and cooling performance of an office building in Daegu, Korea, equipped with amorphous-Si (a-Si) building-integrated photovoltaic (BIPV) windows. EnergyPlus was used to simulate and compare the heating and cooling loads of models for clear glass double-layer, heat-absorbing glass double-layer, and low-emissivity (low-e) glass double-layer windows. In addition, the impact of changes in building operation time, temperature settings, air infiltration from the entrances, and internal load were also analyzed as these all have a large impact on heating and cooling loads. Finally, three types of heating and cooling equipment were tested, and their power and primary energy consumption analyzed, to determine the actual energy used. Under baseline conditions, there was an 18.2% reduction in heating and cooling loads when the BIPV model was used compared to when the clear glass double-layer window was used. In addition, increases in temperature settings and air infiltration from the entrances had a negative effect on the reduction of the heating and cooling loads demonstrating a need for intensive management of these features if a-Si BIPV windows are installed in a building.

Power Performance Loss Factor Analysis of the a-Si BIPV Window System Based on the Measured Data of the BIPV Test Facility

The application of a building-integrated photovoltaic (BIPV) module to an elevation means that the factors causing performance losses in a BIPV are relatively high compared to a photovoltaic (PV) that is installed at the optimal angle. Therefore, it is essential to evaluate the performance loss factors of BIPV and to examine the characteristics of each performance loss factor. Measured data were used to analyze the performance and loss factors (module temperature, dust and soiling, power conditioning system (PCS) standby mode, direct current–alternating current (DC-AC) conversion loss). A performance ratio of International Electrotechnical Commission (IEC) 61724 was used to power the generation performance analysis. The impact analysis of each loss factor is analyzed by using difference of the power generation, the module efficiency, irradiation, and the performance ratio according to the existence of a loss factor. The performance ratio analysis result of this BIPV system shows a range of 66.8–69.5%. The range of performance loss due to each loss factor was as follows; module temperature: 2.2–6.0%, dust and soiling: 2.2–23.1%, PCS standby loss: 4.9–15.7%, DC–AC conversion loss: 4.1–8.0%. Because the effects of the loss factors are different depending on the installation conditions, the performance loss of the system should be minimized by taking this into consideration in the design stage in the BIPV.

Transparent bifacial a-Si:H solar cells employing silver oxide embedded transparent rear electrodes for improved transparency

We developed a transparent oxide–metal–oxide (OMO) structure using aluminum-doped zinc oxide and oxidized silver (AgOx) as a transparent electrode of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell for use in building-integrated photovoltaic (BIPV) windows. The oxygen (O2) addition (O2 flow rate) was optimized for a metal-to-dielectric intermediate-phase AgOx OMO to have high transparency and high conductivity, which were confirmed by finite-difference time-domain simulation. Using the AgOx OMO as a rear electrode, transparent a-Si:H solar cells were fabricated for BIPV window application. The performance of the fabricated cells showed highest bifacial efficiency (b-η) of 7.87% at AgOx OMO of 1 sccm, and highest average transmittance (T500–800, i.e., wavelength range: 500–800 nm) of 21.9% at AgOx OMO of 3 sccm, i.e., improvements from b-η = 7.42% and T500–800 = 18.8% at Ag OMO of 0 sccm. The cell with the optimized AgOx OMO (3 sccm) achieved b-η = 7.69% and the best figure of merit (product of b-η and T500–800) of 169%, i.e., 30% higher than the Ag OMO cell (139%). The developed AgOx OMO electrodes could be used in BIPV windows or in other optical devices requiring both high transparency and high conductivity.

Power performance analysis of a transparent DSSC BIPV window based on 2 year measurement data in a full-scale mock-up

Among transparent solar cells or modules, the DSSC (Dye-sensitized solar cell) has a relatively low production cost compared to silicon-based solar cells and provides a range of colors for building design. On the other hand, its commercialization has been limited by the low power generation efficiency and durability. Few studies have examined the power generation performance of a full scale DSSC module applied to actual buildings. In this study, a long-term performance test was conducted on vertical and inclined DSSC BIPV (Building Integrated Photovoltaic) windows applied to a full-scale mock-up facility to verify the power performance and power characteristics of transparent DSSC BIPV glazing under actual conditions over a 2 year period.The DSSC module was fabricated by the serial connection of nine 2.26Wp transparent DSSC sub-modules. The multi-layered DSSC BIPV glazing, which was composed of a “DSSC module (9mm)” + “air space (12 mm)” + “clear glass (5mm)” was joined to the curtain wall frame of the test cell facility.The unit size of the DSSC BIPV glazing was 0.975 m (W) × 0.965 m (H) and its maximum capacity was 20.34Wp. The size of the test cell facility was 3 m (W) × 3 m (H) × 9 m (D). The building is a container type insulated with 100 mm urethane foam. Four and two DSSC BIPV windows were installed on the vertical south wall and on the 30° sloped roof, respectively. Through the monitoring system, the power performance data of each DSSC BIPV windows and meteorological data were analyzed over a two year period (from 1st Jan. 2015 to 31st Dec. 2016).The 2 year data showed that the average daily power yield for each month of the vertical DSSC BIPV window ranged from 1.75 to 3.93 kWh/kWp·day (average 2.53 kWh/kWp·day), whereas that of the 30° sloped DSSC BIPV window ranged from 2.16 to 5.34 kWh/kWp·day (average 3.60 kWh/kWp·day).The operating power efficiency of the vertical DSSC BIPV window ranged from 2.65 to 4.14% with an average of 3.40%, whereas that of the sloped DSSC BIPV window was in the range of 2.64–3.63% with a mean of 3.01%. The power generation efficiency of the vertical plane BIPV window was higher than the sloped BIPV window. In contrast to the average daily power yield, the power efficiency of the vertical window was higher than that of the sloped window, which may be due to the higher efficiency of the DSSCs under overcast conditions. This is consistent with the power performance of the DSSC BIPV windows according to the sky conditions, which showed an increase in power efficiency with increasing cloud cover or decreasing solar intensity.

Bifacial color realization for a-Si:H solar cells using transparent multilayered electrodes

Transparent hydrogenated amorphous silicon (a-Si:H) thin-film solar cells, in which the colors of the front and rear faces can be adjusted individually, were developed for implementation in building-integrated photovoltaic (BIPV) windows. The proposed ultrathin transparent multilayered electrodes (TMEs) are highly conductive and transparent, consisting of a bottom layer of gallium-doped zinc oxide (GZO), a thin metal layer of Ag, and an optoelectronic controlling layer (OCL) of GZO. They can be used for both the front and rear electrodes of transparent a-Si:H solar cells, and the resulting solar cells show a 5.0% average power conversion efficiency and 18.3% average transmittance. The various dual colors on the front and rear of the cell can be individually adjusted by changing the thickness of the OCL on each face of the TME, without significantly changing the efficiency. The cell colors were quantitatively investigated using the color coordinates in a chromaticity diagram based on the International Commission on Illumination (CIE; 1931) standard to show that the simulated and observed colors match well under both LED and fluorescent light sources. We also show that the observable color of the BIPV windows is determined by indoor and outdoor light sources, and dynamically varies as a function of illuminance of the light source as the day gradually progresses into night. This study helps to elucidate the color behavior of the BIPV windows and to develop a-Si:H BIPV windows that show various colors with stable electrical power generation.

Design and analysis of a highly reliable large-area Z-type transparent module for dye-sensitized solar cells

Abstract The design methodologies for large-area transparent dye-sensitized solar cells (DSSCs) and modules for building-integrated photovoltaic (BIPV) windows are presented. Using electrochemical impedance spectroscopy and the optical/electrical measurements of a small-sized (∼9 × 5 mm) test cell, the basic circuitry of a DSSC is generated in a unit cell for building the 2-dimensional equivalent circuit model of a large-area Z-type transparent DSSC module (300 × 300 mm) on P-Spice. Based on solar irradiation at a window-side monitored for a year, we have proposed a DSSC module design that generates the maximal electrical energy for the best levelized cost of electricity. While simulations using the developed model estimated a module efficiency of ∼3.23% and an annual electrical energy generation of 448 kWh/m2, the fabricated module with a cell width of ∼18 mm exhibited an efficiency of ∼3.19%; the design is also conducive for practical manufacturing and thermal reliability. The developed model demonstrates the operational behaviors of DSSC modules under local shadowing conditions that match with the experiments. The feasibility of the DSSC as a reverse diode, working as a distributed by-pass diode eliminating the current mismatch loss is also demonstrated; this is beneficial for long-term operations. This work can be applied to other types of DSSCs and can be used for producing DSSC-based BIPV windows.

Energy benefits from semi-transparent BIPV window and daylight-dimming systems for IECC code-compliance residential buildings in hot and humid climates

Abstract Interest in semi-transparent solar cell window systems, also called building integrated photovoltaic (BIPV) windows, has increased worldwide because the windows generate electricity and also provide natural light transmission. This study evaluates the potential energy benefits of integrating semi-transparent solar cell windows and daylight-dimming systems for a 2009 International Energy Conservation Code (IECC) code-compliant residential building in a hot and humid climate. This study used whole-building energy simulation models, a DOE-2.1e BIPV window module created for this study, and an existing daylight-dimming system module. Based on the simulation results, peak demand, energy use, and electricity production from each orientation (east-, west-, south-, and north-facing) were all analyzed. The south-facing window showed the greatest potential to generate electric power and reduce cooling loads/use, whereas the east-facing window resulted in the largest annual lighting energy savings. As compared to the code-compliant base-case model, the BIPV windows showed great potential for energy savings: about 12–21% in annual energy use, and 14–26% in peak cooling demand. In addition, the parametric study varying the window-to-wall ratio (WWR) with the BIPV window resulted that the larger BIPV window provided more savings in annual total energy use.

Colored a-Si:H transparent solar cells employing ultrathin transparent multi-layered electrodes

We fabricated hydrogenated amorphous silicon (a-Si: H) transparent solar cells using ultrathin transparent multi-layered electrodes (TMEs) as rear-side transparent electrodes for building-integrated photovoltaic (BIPV) windows. Each TME included a bottom layer, thin Ag layer, and optoelectronic controlling layer (OCL). The TMEs were experimentally designed to have high transparency and conductance. The fabricated a-Si:H transparent solar cells showed optimal performance with a 6.36% power-conversion efficiency and 23.5% average transmittance (500–800nm) when TMEs with a thickness less than 160nm were incorporated. We demonstrated that the reflection color of the cell could be tuned without serious loss of cell efficiency by varying the thickness of the OCL. The backside colors were predicted to be the coordinates on a CIE 1931 chromaticity diagram using the reflection spectrum from the cell and emission spectrum of a 50W LED light source. The developed a-Si:H transparent solar cells exhibited high efficiency and show feasibility for incorporating various colors in the photovoltaic and aesthetic functionalities of BIPV windows.

Highly transparent amorphous silicon solar cells fabricated using thin absorber and high-bandgap-energy n/i-interface layers

We used very thin (i.e., ~115-nm-thick) absorbers to fabricate full-penetration (FP) semitransparent a-Si:H solar cells for building-integrated photovoltaic (BIPV) windows and introduced high-bandgap (HB) n/i-interface layers to decrease shunt loss and increase carrier collection. The optoelectronic properties of the fabricated cells showed that the HB layers improved the fill factor by decreasing shunt loss and increased the short-circuit current density by enhancing generated-carrier collection. We used scattering matrix analysis to simulate the transmittance characteristics of the semitransparent solar cells fabricated using the HB layers and showed that the HB layers shifted the transmission peak to longer wavelengths. We used a figure of merit (FOM), the product of the efficiency and the average transmittance in the range 400–800nm, to represent the performance of the semitransparent solar cells and compared the FOMs of the cells fabricated for application to BIPV windows. The cell fabricated using triple HB layers showed the highest FOM, more than 20% higher efficiency than the reference cell (fabricated without using any HB layers), and its transmittance was similar to that of the reference cell. Fabricating FP semitransparent a-Si:H solar cells with triple HB layers and evaluating their optoelectronic performances will be very useful for implementing high-performance BIPV windows.

Building energy performance evaluation of building integrated photovoltaic (BIPV) window with semi-transparent solar cells

This study evaluates the effects of electrical and optical parameters of building integrated photovoltaic (BIPV) windows with a semi-transparent solar cell on the overall energy performance of a typical mid-sized commercial building in various climate conditions. For precise evaluation of thermo-optical performance of the BIPV windows, we have fabricated semi-transparent amorphous silicon (a-Si:H) solar cells in various conditions and evaluated optical properties of the cells in addition to the energy conversion efficiency. More importantly, we have taken optical parameters of the cells in different sun wavelength spectra on the front and back side of the cell because optical response at each wavelength affects differently on the energy conversion efficiency of the cell and thermal properties of BIPV windows. By using three different semi-transparent solar cell types, the annual whole building energy simulation was performed to evaluate the BIPV window performance in six different climate conditions. A significant change in the overall building energy consumption was observed by varying the thermo-optical characteristics of BIPV windows. Our results suggest it is required to customize the BIPV window characteristics with real optical data through sun wavelength spectra which is sensitively varying by the fabrication conditions of solar cells, in order to maximize building energy performance for different climate conditions at the building location.

Lifetime performance of semi-transparent building-integrated photovoltaic (BIPV) glazing systems in the tropics

Abstract The use of semi-transparent BIPV as a form of on-site renewable energy generation in energy efficient sustainable buildings is increasing. Its popularity is due to its contribution towards zero-(or even plus) energy buildings. In urban cities where the buildings have limited rooftop area but large facade areas, adopting semi-transparent BIPV windows is an alternative for conventional windows. The semi-transparent BIPV windows not only solely generate electricity but also affect the buildings' electricity usage through daylighting and heat gain/loss. In tropical climates where it is hot and humid all year round, the importance of window facade elements is even more pertinent due to high cooling load from the solar heat gains. This paper examines the life cycle environmental and economic performance of commercially available semi-transparent BIPV modules for window application under the tropical conditions of Singapore. Energy simulations, previously performed, were adopted to conduct a life cycle assessment to determine the long term performance in terms of energy and carbon emissions, as well as cost considerations. The EPBT and EROEI for the modules ranged from 0.68 to 1.98 and 11.72 to 34.49 respectively. After considering government subsidies, some modules cost lower than conventional windows, while half of the remaining modules, achieved payback periods of 1.1–13.1 years. These performance indicators were used to form a decision-making tool to assist architects and building designers in the selection of BIPV modules for windows during the early design stages.

Energy analysis of semi-transparent BIPV in Singapore buildings

Buildings consume large amounts of energy and semi-transparent building-integrated photovoltaic (BIPV) has the potential to increase their energy efficiency. BIPV windows affect building energy consumption through solar heat gain, daylighting and electricity production. This study examines six commercially available semi-transparent BIPV windows; four single-glazed and two double-glazed. A new index was formulated to evaluate the overall energy performance of semi-transparent BIPV in terms of increase/decrease of cooling energy, artificial lighting energy and generation of electricity. Parametric simulation studies based on different window-to-wall ratios and orientations to obtain annual energy consumption were performed for the case of Singapore. Selected results of semi-transparent BIPVs were also compared against commonly used window types such as single-glazing, doubled-glazing and low-emissivity windows. The results showed the potential to adopt semi-transparent BIPV across all orientations in tropical countries such as Singapore. A variety of design strategies optimizing the window-wall-ratio for various orientations will be necessary to achieve the highest electricity benefit with different modules.