Comparative study on the overall energy performance between photovoltaic and Low-E insulated glass units

Highlights

• A comparative study between photovoltaic and low-e insulated glass units were conducted experimentally.

• The net energy saving potential of the BIPV IGU was identified based on the power, thermal and daylighting performance.

• BIPV IGU is better than Low-E IGU in reducing discomfort glare.

• BIPV IGC can reduce the cooling energy consumption significantly.

• About 16.8% electricity can be saved by replacing Low-E IGU with BIPV IGU.

Abstract

A novel semi-transparent building integrated photovoltaic (BIPV) laminate was developed and introduced in this paper. It was produced by cutting standard mono-crystalline silicon solar cells into small strips and then making electrical connections between each strip before laminating the cells between two layers of glass. The overall energy performance and energy saving potential of the BIPV insulated glass unit (IGU) under real world conditions were identified through a side by side comparative study. Compared to the reference IGU, the BIPV IGU had lower solar heat gain coefficient (SHGC) but much higher U-factor. The average HVAC electricity saving of the BIPV IGU was about 10% relative to the reference IGU. Daylighting measurement and analysis were carried out to evaluate the trade-offs associated with the BIPV IGU between daylight, glare, and lighting energy use. The results indicated that the BIPV IGU is better than the reference IGU in reducing discomfort glare. However, if the most conservative viewpoint near the window is used for the assessment, a lower transmittance BIPV IGU is required to bring the overall discomfort levels below the perceptible level. Lastly, the net energy saving potential associated with the novel BIPV IGU was identified based on the power, thermal and daylighting performance. On average, the BIPV IGU saved 16.8% of the total electricity use of the room. Further studies and improvement on the energy conversion efficiency of solar cells, the optimal transmittance as well as the thermal properties would make this technology more energy-efficient and affordable.

Introduction

In 2010, 41% of total energy consumption, equivalent about 40 quadrillion British thermal units (Btu), was consumed in residential and commercial buildings in the U.S. Among the total energy consumed in buildings, space heating, cooling and lighting accounted for >50% (Buildings Sector Energy Consumption. Energy Efficiency & Renewable Energy, 2012). Windows, as the main envelope element connecting outdoor and indoor environments, present a significant effect on the energy consumption of space heating, cooling and lighting, especially for modern high-rise buildings with large window to wall ratio (WWR). It was estimated by the Department of Energy (DOE) that 30% of the energy used to heat and cool all buildings in the U.S. was lost through inefficient windows, corresponding to a waste of 4.1 Quads¹ energy per year (Arasteh et al., 2006). Also, it is well known that windows providing sufficient daylighting illuminance and appropriate glare control could also significantly reduce the artificial lighting energy use. Thus, there is no doubt that high efficient window technologies offer huge energy saving potentials for buildings.

According to the functions of windows, there are mainly three performance criteria to evaluate the thermal and daylighting performance of window systems. The first assessment criterion is solar heat gain coefficient (SHGC), which evaluates the amount of solar energy through the window. SHGC is a dimensionless number from zero to one that represents the fraction of solar energy incident on the exterior of a window and frame that is transmitted to the interior. Usually, the higher the SHGC, the larger the cooling energy use in summer. The second assessment criterion is the U-factor, which is associated with the space heating energy consumption in winter. The term of U-factor is defined as the rate of heat loss through a window assembly. The lower the U-factor, the greater the thermal insulating performance. The last assessment criterion is the visible light transmission (T_vis), which is related to evaluate the daylighting performance of windows. High visible light transmission could increase daylighting illuminance level and in consequence reduce artificial lighting energy use and improve the quality of lighting, while too much direct light transmission may cause discomfort glare. Thus, a trade-off between harvesting daylighting illuminance and controlling glare should be considered.

Guided by the above three criteria, many high-performance window technologies, such as insulated glass windows, inert gas filled windows, Low-E coating windows, chromogenic windows, and vacuum glazing windows, have been developed and utilized to improve the energy efficient and occupant comfort in recent years (Ismail et al., 2008, Collins and Simko, 1998, Chow et al., 2010, Aydın, 2006, Kaklauskas et al., 2006, Papaefthimiou et al., 2006). However, all these technologies can only reduce power consumption in a passive way such as reflecting solar irradiation or preventing heat gain and daylighting penetration. They cannot work in an active way via absorbing and converting solar energy into electricity just like building integrated photovoltaic windows (BIPV windows) do.

BIPV windows refer to the use of semi-transparent PV (STPV) laminates to substitute for conventional glazing to constitute window systems (Peng et al., 2015). Compared to other advanced window technologies, the most significant advantage of BIPV windows lies in that they can actively and appropriately utilize the incident solar irradiation for power generation through photovoltaic effect, and at the same time regulate solar heat gain and control daylight glare by adjusting the transmittance of PV laminates. In another word, an optimally designed BIPV window can not only reduce additional solar heat gain and unwanted daylighting glare but also actively convert the part of undesirable and excessive incident solar energy into electricity rather than passively reflect or prevent it. To some extent, BIPV windows are characterized by both functions of building energy efficiency and distributed renewable power generation. Thus, they provide pretty good choices for high-rise office buildings which are characterized by large window area, high solar heat gain as well as big peak load. With the further improvement of energy conversion efficiency and reduction of costs, semi-transparent BIPV windows with customized sizes, patterns and colors would achieve a much better overall energy performance and economic returns in future.

Owing to the above-mentioned advantages, the energy performance of semi-transparent BIPV windows/facades has been extensively investigated, including the heat transfer mechanism and thermal performance (Fung and Yang, 2008, Han et al., 2009, Han et al., 2010, Pal et al., 2016, Chow et al., 2009, He et al., 2011), air conditioning load reduction and solar heat gain (Yoon et al., 2013, Chen et al., 2012, Lu and Law, 2013, Miyazaki et al., 2005, Wong et al., 2008, Peng et al., 2013, Cuce et al., 2015, Motuziene and Bielskus, 2014), thermal comfort (Bizzarri et al., 2011, Polo Lopez and Sangiorgi, 2013), daylighting performance (Ng and Mithraratne, 2014, Lynn et al., 2012, Li et al., 2009, Chow et al., 2007, Leite Didoné and Wagner, 2013, Chae et al., 2014, Kapsis and Athienitis, 2015), annual thermal and electrical simulation (Park et al., 2010, Li et al., 2009, Xu et al., 2014), energy saving potential (Olivieri et al., 2014, Ng et al., 2013, Peng et al., 2016) and outdoor performance tests (Olivieria et al., 2014, Yoon et al., 2011, Peng et al., 2013, Peng et al., 2015). Through literature review, it was found that the transparency of STPV laminates used in BIPV windows was normally achieved by one of three design approaches. For the first thin-film PV approach, the deposited solar cell layer can be so thin that it supported some visible light transmission, but the transmittance was usually as low as 5%, which can’t meet the desired daylighting requirement. The energy conversion efficiency of this kind of BIPV window is limited by the thin film technology and a typical value was less than 8%. Laser cut technology was introduced in the second thin-film PV approach, in which thin-film solar cell layers was cut away and patterned by a laser cut process to increase the transparency. Theoretically, any visible transmittance can be achieved with this approach by adjusting the cut area of solar cells. However, the energy conversion efficiency would be lower than 10% if the high transmittance is desired. For the third approach, transparency was achieved by capturing a patterned array of opaque crystalline silicon (c-Si) solar cells between the layers of a laminate with the desired interval of unobstructed space between the cells, such that there was light transmission between the cells. The energy conversion efficiency of this kind of BIPV window depends on the transmittance. Taking the BIPV window with 30% transmittance as an example, its energy conversion efficiency can be as high as 15%. However, patterned STPV laminates with large opaque crystalline solar cells (156x156 mm²) will likely disrupt the view of building occupants and result in visual discomfort.

In summary, both the existing semi-transparent thin-film PV laminates and the crystalline silicon PV laminates have advantages and disadvantages regarding energy conversion efficiency, appearance aesthetics and/or visual comfort. In this context, a novel STPV laminate was developed and introduced in this paper. This STPV laminate was produced by cutting standard crystalline silicon solar cells into narrow strips and then automatically welding and connecting the strips into strings for laminating. As this STPV laminate combined the advantages of both the laser groove thin-film PV laminates and the conventional c-Si STPV laminates, it not only possesses beautiful appearance and pretty good visual effect but also characterized by relatively high energy conversion efficiency. In addition, although much research related to the energy performance of STPV windows have been reported in recent years, the comparative experimental study of the overall energy efficiency between a semi-transparent BIPV window and a commonly used window was not found. In this paper, to evaluate the overall energy performance of the novel semi-transparent BIPV insulated glass unit (IGU) relative to a typical Low-E coated reference IGU, a side by side outdoor comparative test was conducted on FLEXLAB (Facility for Low Energy Experiment in Buildings) at Lawrence Berkeley National Laboratory (LBNL). Various energy consumption related parameters were measured during this test, including daylighting illuminance, lighting electricity use, air-conditioning cooling/heating water flow rate, as well as power generation from the BIPV IGU. The thermal properties of both the BIPV IGU and the Low-E reference IGU were measured and analyzed. Also, HVAC electricity uses of the both test cells equipped with the BIPV IGU and Low-E IGU were calculated respectively based on the measured cooling/heating water flow rate and the corresponding temperature difference. Lastly, the overall energy performance of the BIPV IGU was determined and its energy saving potential compared to the Low-E coating reference IGU was identified.