In this study, we have investigated a hybrid thin film PV vacuum glazing called: ‘PV VG-4L’. The glazing involves an integration between a thin film PV glazing with a double vacuum glazing (both manufactured independently), and an additional layer of self-cleaning coated glass which totalling four layers of glass. The mathematical model of the PV VG-4L designs were developed and numerically solved in MATLAB. To evaluate the performance of the PV VG-4L, the prototype was manufactured and investigated at lab-scale and also under real conditions. Lab-scale experiments were conducted at steady state conditions using a TEC driven calibrated hot box at the Sustainable Energy Research Lab, University of Nottingham, UK. Meanwhile, outdoors, the prototype was tested at a research house at the University of Nottingham, UK. Under the influence of solar irradiance, the electrical performance of the PV-VG and the temperature difference between the surfaces of the glazing were analysed. However, the measurement of U-value under real conditions is not reliable due to the influence of solar irradiance on the heat flux sensor and also due to the absorbed solar irradiance by the thin film PV layer. Nevertheless, during low to zero solar irradiance, the U-value of the prototype can be estimated. The developed model was then validated against the experimental results by direct comparison to the trend of the experimental and theoretical curves obtained, and also by conducting error analysis using root mean squared percentage deviation (RMSPD) method. Testing using the calibrated hot box, adhering closely to ISO 12567 standards, resulted in an average measured total U-value of 0.6 W/m^{2}K which is when compared to a single thin film PV glazing with a typical U-value of 5 W/m^{2}K; the U-value is higher by almost 90%. From the analysis, the computed RMSPD value for the glazing surface temperature and the U-value are 4.02% and 0.92% respectively. Meanwhile, field testing under real conditions with a 0.4 m × 0.4 m PV VG-4L prototype found that 14 W/m^{2} power can be generated by the PV VG-4L at average solar irradiance of ~600 W/m^{2}. RMSPD computed glazing surface temperatures, electrical power generated under real conditions and U-value are 2.90%, 8.70% and 2.89% respectively. The theoretical and experimental results are concluded to be in good agreement. This study has significant contributions to the knowledge of building integrated photovoltaic PV technology. The mathematical model that has been developed can be used for PV VG-4L design optimisation and also to simulate the performance of PV VG-4L under various conditions. At building efficiency level, the PV VG-4L not only can produce power, but it also has high insulating properties. The promising U-value implies its range of potential applications which can be improved depending on the energy needs and applications, such as for BIPV solar façade (PV curtain walling) in commercial buildings, greenhouses, skylight and conservatory.

In recent years, researchers have shown interest in the study of vacuum glazing technology. Innovative designs were developed and researched. Among them are; Bao, Liu (^{–2}K^{–1} to 0.22 Wm^{–2}K^{–1} for a 0.4 m by 0.4 m size triple vacuum glazing. Memon (^{–2}K^{–1} is found achievable. Memon and Eames (^{–2}, simulations show that the EC layer is opaque and the inside temperature of the glass pane is higher than the indoor temperature. Meanwhile, for solar irradiance of 1000 W m^{–2}, the outdoor glass pane temperature exceeds the indoor glass pane temperature, of which as a consequence heat is transferred from outdoors to indoors. Ghosh, Norton (

The design of PV VG-4L is illustrated in Figure

The thermal resistance network for the PV VG-4L is shown in Figure _{1}, the average internal glass temperature denoted by _{2}, the average temperature of the thin film PV glazing denoted by _{TF}_{4}. The lateral heat transfer ends at the point right at the edge sealing and the temperature was denoted as _{edge}_{1}. Due to high thermal conductivity of the edge sealing, it acts as a short circuit for the heat transfer at the edges of the glazing. Meanwhile, due to high thermal conductivity between _{edge}_{2}. For the uninsulated edges, there will be heat transferred from the internal ambient to the area around the edges.

The thermal resistance network implies that, at certain level of solar irradiance and ambient temperature, the internal surface temperature is in general lower than the external surface temperature. This can be explained as follows. When the PV panel absorbed solar irradiance, a fraction of the absorbed solar irradiance will be converted into electricity meanwhile a fraction will be wasted in the form of heat. This heat released by the PV panel is insulated from the building via the vacuum layer of which, the main heat transfer occurs in the gap are mainly due to radiation and conduction through the support pillars. In the summer, this can be considered as an advantage since the installation of PV glazing would normally cause additional heating during summer of which in return, increase the cooling load of the building. Based on the thermal nodal networks, the energy balance equations were developed. In order to simplify the analysis, the following assumptions have been made:

The heat transfer involved is assumed symmetrical. Therefore, we only consider a quarter of the PV VG-4L area in the heat transfer analysis.

In this study we have considered the lateral heat transfer through the glass slab in g1. This is due to the fact that, the vacuum glazing is highly thermally insulated which makes the lateral heat transfer to become prominent.

The edge boundaries of the PV VG-4L are well insulated and hence the edge losses are assumed negligible.

Due to high thermal conductivity of the edge sealing, the edge seal is assumed as a thermal short circuit.

Meanwhile, due to high thermal conductivity between

To simulate the performance of the PV VG-4L, the following energy balance equations were developed for each of the temperature nodes.

The heat transfer terms are defined as follows:

1: The rate of the solar energy received by the glass cover or surface facing indoors after transmission through different glass layers per unit area; 2: The rate of heat transfer from the indoor ambient to g1 per unit area; 3: The rate of heat transfer from g1 to g2 per unit area. The heat transfer includes heat conduction through the glass slabs of g1, which then followed by heat transfer in the vacuum gap which are due to the heat conduction of the gas particles, heat transfer via radiation and heat conduction through the support pillars, and then followed by heat conduction through the glass slab g4; 4: The rate of lateral heat conducted along the x- direction and y-direction of the PV VG-4L from the centre of the PV VG-4L.

The heat transfer terms are defined as follows:

5: The rate of heat transfer from the indoor ambient to the edge area per unit glazing area (if not insulated); 6: The rate of heat transfer from edge 1 to edge 2 through

The heat transfer terms are defined as follows:

7: The rate of the solar energy received by g2 after transmission through different glass layers per unit area; 8: The rate of heat transfer from g2 to TF through the EVA layer per unit area.

The heat transfer terms are defined as follows:

9: The rate of the solar energy received by TF after transmission through different glass layers per unit area; 10: The rate of heat transfer from

The heat transfer terms are defined as follows:

11: The rate of lateral heat conducted along the x-direction and y-direction of the PV VG-4L from the centre of the PV VG-4L.

The heat transfer terms are defined as follows:

12: The rate of the solar energy received by _{a}_{_}_{o}

The thin film PV VG-4L introduced in this study is new and has never been discussed in existing research. Therefore, in order to theoretically predict the glazing’s electrical performance, the equation that has been widely used in the study of PV/Thermal solar collector is utilized. The following correlation developed by Schott (

Where _{ref} is_{Tref}_{ref}_{Tref}

From the simulation, the values of the average temperature of the glass sheets (layers) were used to compute the heat transfer coefficients summarised in Table _{centre}_{edge}

The definition of the symbol used.

Symbol | Definition | Symbol | Definition |
---|---|---|---|

Solar absorptance | Solar transmittance | ||

_{edge} |
Correction factor due to the total area of edge sealing to the centre area | _{rad} |
Total global solar irradiance incident upon the glazing structure. |

_{edge} |
The edge sealing thickness | _{edge} |
Conductive heat transfer coefficient due to the edge sealing |

_{P} |
Heat transfer coefficient through the support pillars | _{iedge} |
Total heat transfer coefficient from the indoor ambient air to the uninsulated edge sealing area |

_{k_edge} |
The lateral heat transfer to the glass edge | _{g1_g2} |
The total heat transfer coefficient from g1 to g2 of vacuum glazing |

_{a_g1} |
Total heat transfer coefficient from the internal ambient to g1 | _{g2_TF} |
The total coefficient of heat transfer by conduction from |

_{TF_g4} |
The total heat transfer coefficient by conduction from the thin film PV glazing layer |
_{edge} |
Thermal conductivity through the edge sealing |

_{o} |
Total heat transfer coefficient from the external glass |
_{a_i} |
Indoor ambient temperature |

_{a_o} |
Outdoor ambient temperature |

Hence, the centre U-value (_{centre}_{total}

In this study, to solve the energy balance equations, we have used the inverse matrix method. MATLAB is used to carry out the iteration process. Newton-Raphson iteration technique is used to estimate the temperature and hence the temperature-dependant heat transfer coefficients of the variables.

A PV VG-4L prototype using an amorphous silicon (

The PV VG-4L prototype.

The calibrated hot box with the installed sample.

Comparison between the Theoretical and Experimental Results for the glazing surface temperature and U-value using the calibrated hot box under different sets of controlled ambient temperature conditions.

Cond. | Text (°C) | Tint (°C) | Tint (Exp) (°C) | Text (Exp) (°C) | Tint (Theo) (°C) | Text (Theo) (°C) | U-value total (Exp) | U-value total (Theo) |
---|---|---|---|---|---|---|---|---|

1 | 12.7 | 32.70 | 30.34 | 13.50 | 29.90 | 13.37 | 0.56 | 0.57 |

2 | 17.5 | 27.5 | 27.40 | 18.65 | 26.42 | 17.84 | 0.57 | 0.57 |

3 | 7.6 | 27.83 | 25.19 | 9.13 | 25.67 | 8.29 | 0.56 | 0.56 |

The mathematical model validation method is performed by comparing the results obtained experimentally and theoretically based on the trends shown on the related graphs. In this study, the mathematical model has been validated against the indoor experimental data with the input parameters recorded in the experiment were used in the computer simulation for all the three different conditions. In addition to the direct comparison between the simulation and theoretical curves, the validation of the mathematical model is further justified using root mean square percentage deviation (RMSPD). As shown in Figure

Comparison between the theoretical and experimental results for the glazing surface temperature and U-value using the calibrated hot box.

Measured parameters and uncertainties.

Parameters | Sensor | Uncertainties |
---|---|---|

Temperatures | K-type Thermocouples | ±1.5°C |

Heat flux | Hukesflux Thermal Sensors | ±1.9 × 10^{–6} V/(W/m^{2}) |

Solar irradiance | Pyranometer | ±5% |

Maximum power produced by the PV VG-4L | RO4 with Keysight 34972A | Electric current (I) (±1.5 μA) |

The developed mathematical model has been validated against indoor experimental analysis. Nonetheless, the true performance of the PV VG-4L under real sky conditions still needs to be investigated in order to further justify the validity of the mathematical model especially that the electrical performance of the thin film PV glazing could not be evaluated indoors. That said, this section first discusses the performance of the PV VG-4L under real conditions. To carry out the testing, the prototype was installed at E.ON 2016 research house at the University of Nottingham, United Kingdom with latitude of 52.9438°N, and longitude of 1.1934°W. It is worth emphasising that the outdoor monitoring of the PV VG-4L has been conducted under two conditions; during the day and during the night (at zero solar irradiance). The thermal and electrical characteristics of the PV VG-4L under real conditions were monitored using the sensors as summarised in Table

Figure ^{rd} to 24^{th} of May 2019, the internal heat source was switched off during the day meanwhile, the average heating temperature was set at 30°C during the night (i.e. from 8 p.m. to 4 a.m. the next day) to heat the ambient room at 24 to 25°C. On the day of testing, the sky was in a clear sky condition and hence clear pattern of solar irradiance curve with the time of the day was obtained. Another set of experiment was conducted to evaluate the performance of the PV VG-4L on the 20^{th} to 21^{st} of June 2019. However, as can be seen on Figure ^{2} to 60 W/m^{2} and the temperature profiles clearly follows an exponential decay lagging behind the step change. Additionally, by carefully examining Figure

Variation in the external glazing surface temperature (Texternal), internal glazing surface temperature (Tinternal), Outdoor ambient temperature (To_ambient), indoor ambient temperature (Ti_ambient) and solar irradiance with time of the day (23^{rd} of May (7:00 a.m) to 24^{th} of May 2019 (4:00 a.m.)).

Variation in the external glazing surface temperature (Texternal), internal glazing surface temperature (Tinternal), Outdoor ambient temperature (To_ambient), indoor ambient temperature (Ti_ambient) and solar irradiance with time of the day (20^{th} of June from 9:15 a.m. to 21^{st} of June 2019 9:15 a.m.).

It is worth noting that, in both graphs, at low–zero solar irradiance, the indoor surface temperature of the PV VG-4L is in general higher than its external surface temperature. However, as the incident solar irradiance increases, the external surface temperature increases. The trend of the graph is explained as follows; when the PV component of the PV VG-4L absorbed the incident solar irradiance, a fraction was converted into electrical energy meanwhile the rest was wasted in the form of heat. However, the vacuum layer behind the PV component of the PV VG-4L act as the insulation layer or barrier to the wasted heat from being transferred indoors. As a result, the external surface temperature of the PV VG-4L became higher compared to its internal surface temperature. In the summer, this will be an advantage in comparison to the typical installation of BIPV in double glazed configuration. For the data analysis, the temperatures, solar irradiance and heat flux were recorded for every 1 s meanwhile, the electrical parameters were recorded for every 10 s.

The outdoor experimental results obtained from the 23^{rd} to 24^{th} of May, as discussed previously, were compared with the theoretical results using the developed mathematical model. Due to the varying condition of the ambient climate, the electrical performance and thermal characteristics of PV VG-4L in a steady state condition are analysed as per time constant of the PV VG-4L. From our analysis, it is concluded that the computed time constant for the PV VG-4L is approximately 30 minutes. The experimental results were compared based on the PV VG-4L surface temperature difference and electrical performance at quasi-steady state during the day and thermal transmittance or U-value during the night. The comparisons between the outdoor experimental and theoretical results are represented in Figures ^{2} is approximately 14W/m^{2} for 0.4 m × 0.4 m PV VG-4L made of amorphous silicon solar cells. It is worth emphasizing that, a different power produced is expected when a different type of thin film PV is used as the prototype. For example, using the validated mathematical model, it is predicted that, if amorphous/microcrystalline silicon solar cells at 20% transparency is used as the thin film PV layer, the power output at the same average solar irradiance can achieve as high as 32 W/m^{2}. In order to further justify the validity of the mathematical model, error analysis using RMSPD analysis was performed. The average RMSPD for the glazing surface temperatures, the power produced and U-value are 2.90%, 8.7% and 2.89% respectively. It is worth noting that the derived absolute errors for the power produced are too small to be included in the plotted curves.

Top: Outdoor Experimental and Theoretical Curves for the Glazing surface temperatures (Tint for the internal surface and Text for the external surface) and bottom: Power produced per m^{2} for 0.4 m × 0.4 m prototype as per time constant of the PV VG-4L with time of the day as (data was taken on the 23^{rd} of May 2019 during the day).

Top: Outdoor Experimental and Theoretical Curves for the Glazing surface temperature and bottom: U-value for 0.4 m × 0.4 m prototype as per time constant of the PV VG-4L with time of the day as (data was taken on the 23^{rd} of May 2019 to 24^{th} of May 2019 at low-zero solar irradiance).

Using the validated mathematical model, the U-value of the PV VG-4L was evaluated at different pairs of width and length as summarised in Table

The parameters and the computed U-values.

U-value (W/m^{2}K) |
||
---|---|---|

0.3 | 0.3 | 0.66 |

0.5 | 0.5 | 0.55 |

1.0 | 1.0 | 0.51 |

1.5 | 1.5 | 0.50 |

At 1.0 m × 1.0 m in size, the influence of solar irradiance _{rad} to the increase in PV temperature and hence the PV performance of the thin film PV glass was evaluated. The thin film PV performance was simulated with the variation in solar irradiance at fixed outdoor temperature of 15°C. The size of the window was fixed at 1 m by 1 m. The aim is to keep the indoor temperature at 23°C. Figure ^{2}. On average, the drop in the electrical efficiency due to the presence of vacuum level is only 0.1%.

The variation in, the electrical efficiency

The variation in, temperature of the PV cells, with the increase in solar irradiance at fixed indoor and ambient temperature for PV VG-4L at high and low vacuum level.

Additionally, the temperature of the PV VG was also predicted with the change in the outdoor ambient temperature ranging from –10°C to the extreme of 40°C at fixed solar irradiance and indoor ambient condition of 700 W/m^{2} and 23°C respectively Figure

The variation in the electrical efficiency with the change in outdoor ambient temperature at fixed solar irradiance and indoor temperature.

An innovative Thin Film Photovoltaic Glazing with Vacuum Insulated Layer (PV VG-4L) is presented in this paper. A mathematical model was developed by taking into account all the parameters related to the individual component of a typical vacuum glazing unit which is the dominant in the design and also the fraction the solar irradiance absorbed by the different layers of PV VG-4L. To validate the mathematical model, a lab-scale prototype was manufactured and tested indoors using a calibrated hot box, and outdoors by installing the sample at a research house. Under controlled conditions, the overall U-value of the PV VG-4L was measured to be as low as 0.6 W/m^{2}K. When investigated under real conditions, an obvious trend in glazing surface temperature variation with solar irradiance was obtained. During low to zero solar irradiance, the internal glazing surface temperature is on average higher in comparison to the external glazing surface temperature. However, as the solar irradiance increases, the by-product of the absorbed heat by the thin film PV glazing layer, has led to an increase in the external glazing surface temperature as the heat is hindered from being transferred indoors by the vacuum layer. At average solar irradiance of 600 W/m^{2} the PV VG-4L can produced in total of 14W of power per m^{2} of panel. Meanwhile, at low to zero solar irradiance (i.e. during the night), at outdoor ambient temperature of 14°C, the average U-value of the typical PV VG-4L was found to be as low as 0.6 W/m^{2}K while maintaining the indoor ambient temperature at 30°C. Please note that high internal ambient temperature was obtained due to the use of heater at its maximum setting. For a conventional thin film PV glazing, to improve the thermal performance of the thin film PV glazing, a combination with a double-glazing unit is possible with the estimated U-value of 2.5–2.8 W/m^{2}K depending on the type of gas used to provide the insulation in the air gap. Clearly, the vacuum layer introduced in the PV VG-4L design presented in this paper is better in performance with the slim configuration of the glazing unit as an additional benefit. The results also show that the PV VG-4L not only can produce power but also has high insulation properties when compared to a single thin film PV glazing with a typical U-value of 5 W/m^{2}K; the U-value is higher by almost 90%. The promising U-value implies its range of potential applications can be improved depending on the energy needs and applications, such as for BIPV solar façade (PV curtain walling) in commercial buildings, greenhouses, skylight and conservatory.

The authors gratefully acknowledge Innovate UK’s financial support through Newton Fund (Project reference no: 102882) and International Science & Technology Cooperation Program of China (No. 2016YFE0124300).

Saffa Riffat is the Editor in Chief of the journal and was removed from all editorial processing for this paper.