Henrik Davidsson*, Bengt Perers, Bjorn Karlsson

Energy and BuildingDesign, Lund University, B. O Box 118, SE 221 00 Lund, Sweden
Corresponding Author, henrik. davidsson@ebd. lth. se
Abstract

A building-integrated multifunctional PV/T collector have been developed and evaluated. The PV/T solar window is constructed of PV cells laminated on solar absorbers and is placed in a window behind the glazing. To reduce the costs of the solar electricity, reflectors have been introduced in the construction to focus radiation onto the solar cells. The tiltable reflectors render a possibility to control the amount of radiation transmitted into the building. The insulated reflectors also reduce the thermal losses through the window. A model for simulation of the electric and hot water production was developed. The model can perform yearly energy simulations where different effects such as shading of the cells or effects of the glazing can be included or excluded. The simulation can be run with the reflectors in an active, up right, position or with the reflectors in a passive, horizontal, position. The simulation program was calibrated against measurements on a prototype solar window placed in Lund in the south of Sweden and against a solar window built into a single family house, Solgarden, in Alvkarleo in the middle of Sweden. The results from the simulation shows that the solar window produces about 56% more electric energy per unit cell area compared to a vertical flat PV module.

Keywords: solar window, PV/T

1. Introduction

A diversity of technical solutions needs to be applied and developed if solar electricity is to become cheap enough to compete with grid electricity. One technique for reducing the price of solar electricity is to use the reflector to focus radiation onto the PV cells, thus allowing expensive PV cells to be replaced by considerably cheaper reflector material. Active water cooling on the back side of the cell gives both relatively cold, high efficient cells, and hot water for domestic use. Further price reduction is possible if the solar modules can be integrated into the building construction. Integration makes it possible to use existing frames and glazing for the solar modules or, alternatively, to replace roofing materials and windows by solar modules. Wall integrated solar collectors using reflectors have been shown to increase the electrical output substantially [1] compared to flat vertical PV modules. All these technologies have been combined in the PV/T hybrid technology presented in this work.

A building integrated multifunctional solar window was proposed and developed by Fieber [2]. The solar window, se figure 1, is constructed of absorbers on which the PV cells have been laminated. The solar window is building-integrated into the inside of a standard window, thus saving frames and glazing and lowering the total price of the construction. In order to minimize the PV cell area, reflectors have been placed behind the absorber. When tilting the foldable reflectors to a vertical position the solar radiation is focused onto the absorbers. When the reflectors are tilted to a horizontal position the solar radiation is let into the building to allow for passive heating. This means that the

reflectors in a closed position increase the radiation on the cells, reduce the thermal losses through the window and also work as a sun shade. The glazing of the window in front of the absorbers is anti reflection treated to maximize the transmittance.

Fig 1. Left; the solar window with water cooled solar cells, insulated and tiltable reflectors and anti reflection treated glazing. Right; illustration of the parabolic reflector and the absorber

2.1. Geometry

The geometry of the solar window is shown in figure 1 above. The optical axis, v, of the parabolic reflector is directed 15° above the horizon with focus on the front edge of the absorber. This means that all radiation from 15° and higher solar altitudes will hit on the absorber between the focal point, F, and the reflector. The focal length is denoted p, the height of the glazing h and a is the absorber width. The angle w is the angle between the glazing and the absorber plane and qNS is the incident angle of the solar radiation projected in the north-south vertical plane. The absorbers are 1.11 m long and 8 cm wide, and the PV cells are 12.5 cm * 6.25 cm. The solar window in Solgarden is constructed of 8 absorbers per window unit, and the prototype solar window is constructed of 5 absorbers, see figure 2. The Solgarden solar window has 64 PV cells in series and the prototype solar window has 8 PV cells in series. The total window area is 16 m2 in Solgarden and about 1.2 m2 for the prototype solar window.

The reflector parabola is described in Eq. (1). r is a vector from F to a point on the parabola at angle ф.

г(ф) = p/cos2^) (1)

Both h and a can be expressed by r for the two angles w=105° and u+v=35°, respectively for the solar window. The ratio between h and a, which is defined as the geometrical concentration factor, can be calculated to be 2.45 for the construction.

The architectural implication such as light distribution has been investigated [2]. Following this, long term measurements were performed regarding energy production, both electrical and hot water. This was carried out on a prototype solar window placed in Lund in the south of Sweden as well as from a solar window built into a residential building in Alvkarleo about 100 km north of Stockholm, Sweden.

In this paper, we describe a model developed to simulate the yearly energy production of the hybrid window system from climatic data. The model uses both experimentally measured parameters and theoretically derived values and functions in the calculations. It takes into account shading caused by the window frames and also includes the transmittance through the glazing and the angular dependence of the PV cells. The model also allows for analyzing different limiting effects such as shading or transmittance through the glazing. This makes it possible to study the potential of development for the solar window.

Method

Measurements of the performance of the multifunctional PV/T hybrid solar window were carried out during 2006 on a prototype solar window placed in Lund, Sweden (55.44N, 13.12E). A full scale system combining 4 of these solar windows, another 4 is planed, was installed in a single family home called Solgarden in Alvkarleo, Sweden (60,57N, 17,45E) and evaluated during 2006-2008. The window was directed 23° towards east. The solar windows can be seen in figure 2. The measurements of the generated current and voltage produced by the prototype solar window were carried out using a Campbell CR1000. The radiation, temperatures and water flow through the absorbers was measured using a Campbell CR10 logger. The temperature measurements were carried out using PT100 sensors. All measurements made in Solgarden used a Campbell CR10. Measurements were monitored both with the reflectors in a horizontal and in a vertical position. The prototype solar window was supplied with water of constant inlet temperatures and the measurements were carried out during both day and night. Night time data were used for determining the thermal losses of the window.

Fig 2. Left figure; the prototype solar window with five absorbers. Right figure; the solar window in Solgarden with closed reflectors.

A simulation model was developed to evaluate the solar window. The model uses the direct and diffuse radiation together with the inlet water temperature, the ambient temperature and the time, and thus the solar angles, as inputs. The outputs are thermal and electrical delivered power. In order to simplify the calculations the power delivered by the solar window was divided into three components, Pdirect,

Preflector, and Pdiffuse. The first is Pdirect, power caused by the direct radiation that hits the absorber directly, the second component is Preflector, power caused by the direct radiation that goes via the reflector. The third component, Pdiffuse, is the power contribution caused by the diffuse radiation. Figure

3 graphically explains the three different components of radiation. Ptotal is the total power delivered by the window.

Pdirect=Ib*Tglass(©1)* Opv(©2)*fshading(©3)*ACell*npv *^(©2) (2)

Preflector Ib *Tglass(©1)* apv(©4)*freflector(©5)*Areflector*npv*Rreflector*cos(©5) (3)

Pdiffuse= Idiffuse *C1,2 (4)

Ptotal= Pdirect+Preflector+ Pdiffuse (5)

Ib and Idiffuse are the beam radiation and the diffuse radiation against the window. Tglass describes the angular dependent transmittance through the glazing; apv describes the angular dependence of the absorptance of the PV cells, and fshading describes the shading of the PV cells caused by the window frame. freflector is a correction factor for the shadow effects for the radiation which is reflected. This function includes the shading of the reflector. The angles ©1 to ©5 are the different incidence angles for the beam towards the components of the solar window. Acell and Areflector are the areas of the PV cell and the reflector, respectively. npv and Rrefiector are the efficiency of the solar cells and the reflectance of the reflector. C12 is a response function for the diffuse radiation obtained from measurements during cloudy days, when the beam radiation has negligible influence on the performance. Measurements during cloudy days were performed with the reflector in both horizontal and in vertical positions, allowing both C1, horizontal reflector and C2, vertical reflector, to be determined. The transmittance, Tglass, through the window was calculated using Fresnel’s equations and Snell’s law. The shading factors fshading and freflector were calculated theoretically from the PV/T window geometry. A measurement was performed to determine apv, the angular dependence of the PV cells.

In order to calculate the thermal output a fourth term has to be added to describe the thermal losses in the absorber. The thermal losses, Pthermal loss prototype for the prototype solar window and the thermal

losses Pthermal loss Solgarden is shown below.

Since the solar window in Solgarden experiences thermal losses to two different temperatures, the ambient temperature and the indoor temperature, two different U-values where used. The Usolgarden out is the thermal loss to the outside and the Usolgarden in is the thermal loss to the inside. Awindow is the total window area. DeltaT out is the temperature difference between the ambient temperature and average water temperature and DeltaT in is the temperature difference between the indoor temperature and the average water temperature. Uprototype is the U-value for the prototype solar window and DeltaT is the temperature difference between the ambient temperature and the average water temperature.

Result

Two different types of graphs were used to validate the model. The first type is shown in figure 4, where results from measurements and simulations are compared. The short circuit current Isc in the right figure is from a cell placed in the solar window. The days were chosen to illustrate different weather conditions, such as different ambient temperatures and cloudy weather with sunny intervals. The days where also chosen to show different seasons and thus different solar angles.

Fig 4. Measured and simulated thermal and electrical output for the window in Solgarden (upper) and in the prototype window (lower). Blue is the simulated output and purple is the measured output. On the x-axis is the time of the day.

During the measurements on the prototype solar window two different, not perfectly synchronized, loggers for monitoring the electrical output and the radiation were used. This means that synchronization problems could arise during partly cloudy days. If the electrical output was measured during a cloudless time and the irradiance was measured during a cloudy time the result from the simulation, using the irradiance as input, differs from the measurement. To solve this problem the simulated and the measured output was integrated daily. Then this irregularity will disappear. The result from this analysis is shown below in figure 5 where the integrated daily measured output on the y-axis is plotted against the integrated daily simulated output on the x-axis. A perfect agreement between simulation and measurement would put all the points on the line, x=y. This analysis was performed both for the thermal output, left figure, and the electrical output, right figures. Validation from the prototype solar window is in blue and the validation from Solgarden is in purple. All values have been normalized to the highest output in each series. The correlation is high for all four validations.

0,2 0,4 0,6 0,8 1 Normalized simulated output
0	1,2
► Solgarden	Prototype

Fig 5. The thermal energy production (left) and the electrical energy production (right). The dots in the graphs
are the integrated daily energy production, the simulated value on the x-axis and the measured value on the y-
axis. The blue dots are from Solgarden and the pink dots are from the prototype window.

Yearly simulations where made for the solar window and for two flat PV-modules. The PV-modules has the same efficiencies and areas as the string module in the solar window but without shading effects and reflectors. The PV-modules are installed on a wall alternatively tilted 20° on a roof. The wall mounted PV module is not shaded like the solar window but still benefits less from the diffuse radiation due to less favourable angles between the cells and the sky. This is also the case for the direct radiation, as can be seen in figure 6. When the PV module is located on a roof at a low tilt it receives more diffuse radiation than a wall mounted PV module since the module can see a larger part of the diffuse sky. This is clearly visible in figure 6. The increase of the electrical output from the direct radiation on the module is due to less loss in the glazing and the possibility for the roof module to utilize the radiation which comes from directions behind the wall. Note that the increase of the diffuse radiation on the roof mounted module almost compensates the reflector contribution on the cells in the solar window. The diffuse irradiation is treated as isotrop.

Fig 6. The annual electrical output from the prototype solar window and from two flat PV-modules on a wall at
90° tilt and on a roof at 20° tilt. In the figure the blue part is electricity produced by the direct radiation that hits
the absorber directly. The red part is the electricity caused by direct radiation that goes via the reflector. The
yellow part is the diffuse radiation that goes directly on the absorber and the light blue is the electricity caused by
the diffuse radiation that goes via the reflector. All results have been normalized to the total annual output from

the solar window.

The same analysis, in this case using TRNSYS, was performed to investigate the thermal properties. A TRNSYS-deck including the solar window or flat solar collectors, pumps, a storage tank, etc and a heating load was constructed. In the simulation all parameters but the areas of the wall collector and the roof collector was kept constant. Figure 7 shows a graph of the area of the flat collector required to

produce the same annual amount of thermal energy as the solar window in Solgarden, turned 23° from south towards east. The roof collector was placed at 20° tilt and the wall collector is placed at 90° tilt to the horizontal. The roof mounted collector can see a larger part of the diffuse sky and has more preferable incidence solar angles and thus gain and produce more energy compared to the wall mounted collector. The absorber area in the solar window is 5.06 m2 and the total window area is 16m2.

To study the limiting factors in the solar window a simulation was carried out where the factors fglass(©1), fpv(©2) and fshading(03) in Eq. (2,3 and 4) was set to 1, see figure 8. Since the angular dependence of the PV cells is large only for high angles the impact of setting fpv(02) to 1 will be small, the shading is already deteriorating the performance for high solar angles. If the glazing is omitted the yearly electrical output would increase by about 15% and if the shading effects can be removed completely the increase would be as much as 21%. If the shading effect is very large it is better to have one cell less, since large shading is caused by the window frame on the outer cells.

Limiting effects

1,4

1,2

c.

Fig 8. Different limiting factors affecting the solar window. The first bar is the complete simulation. In the
second bar the angular dependence of the PV cells have been removed. In the third bar all shading effects have
been removed and in the last bar the effects from the glazing have been removed. In blue are simulations
performed without the influence of the reflector and in red are simulations including the reflector contribution.

Discussion

The focus of the work in this article is to reduce the total costs of a building including a solar energy system. One solution is to use building integrated PV/T hybrid collectors using reflectors to focus the
radiation onto the absorbers. Different collectors have been proposed [1]. Using such technique fa? ade elements can be saved to reduce the costs. To further develop the building integration technique a multifunctional PV/T hybrid solar window was proposed by Fieber [2]. Integrating the proposed collector into a window saves both frames and glazing. The total price of the construction is reduced further since the concentrating reflectors are tiltable and thus provide flexible solar shading for the building.

The results from the simulation program developed to evaluate the window closely match the measured data. The simulated annual electrical energy production clearly shows the importance of utilizing the diffuse radiation. About 40% of the electrical energy produced in the window is due to diffuse radiation. The comparison performed in Figure 6 shows that the solar window produces about 56% more electrical energy per unit area of PV cells compared to a flat PV module placed on a wall at a 90° tilt. However the roof mounted PV module performs about 2% better per unit area than the solar window. The roof mounted PV module receives more diffuse radiation than the wall mounted system, and thus produces more electrical energy.

The simulation presented in figure 7 shows that the solar window produces less thermal energy per absorber area compared to a flat vertical solar collector or a roof collector installed at 20° tilt to the horizontal. Due to the complex design of the solar window the U-value of the collector is relatively high. The thermal losses from the solar window collector is approximately 50% larger compared to a normal plat solar collector. However, a large part of the thermal losses will heat the building passively. This positive effect is not included in the values in figure 7. A full investigation including the passive effects, such as passive heating of the building due to thermal losses from the collector and taking into account the decrease of passive heating through the windows due to solar radiation utilized in the collector instead of the passive heating, will be presented in future papers.

The results presented in Figure 8 clearly show the importance of choosing the best available glazing for the window. The importance of avoiding shading caused by the frames of the window is also clear. If the shading is extensive it is better to have one less PV cell per absorber. Heavy shading can occur if the cells on the outer edges are placed too close to the window frame. The angular dependence of PV cells is only apparent for large incident angles, and large incident angles are already heavily shaded by the frames and heavily suppressed by low transmission through the glazing. The annual performance can be increased by up to 30% if the impact of shading and angular effects is minimized.

As can be seen in figure 6 it is possible to run simulations with the reflectors in both active, vertical, or passive, horizontal, positions. This keeps the simulation realistic by allowing control mechanisms, based on human behaviour, to decide whether or not to have closed reflectors. For instance there is a possibility to cool the building at night by simply opening the reflectors and thus increasing the U — value of the window. This is not a possibility for a standard window with low U-value.

References

[1] H Gajbert et al, Solar Energy Materials & Solar Cells 91 (2007) 1788-1799

[2] A Fieber, Building Integration of Solar Energy, Lic. Thesis (2005) Report EBD-T—05/3