Low Inefficiency in High Temperature

BIPV systems provide an alternative source of energy for the building and the local grid. It is widely established that the energy-generating efficiency of the photovoltaic modules depends on multiple factors including incidence angle, irradiance, ambient temperature as well as humidity. The variability of these factors often leads to a sub-optimal efficiency. When photovoltaic modules are integrated into a building, other design considerations compete with maximising energy production.

BIPV systems often produce a greater proportion of energy at low irradiance than conventional photovoltaic systems due to architectural constraints (i.e. lacking dark coloured panels to absorb agents of sunlight) in the design of BIPV arrays. In the light of this, the low irradiance level and the high ambient temperature often become the predominant factors that limit the efficiency of the BIPV systems. In such case, high radiation input is not enough as the efficiency of the panel determine the energy output. Thus, the high temperature of the surrounding ambience is a reason for low efficiency – it limits the power output and wastes the input of high radiation.

Effect of Temperature on Efficiency

To quantify the impact of radiant temperature to the efficiency of PV modules, Skoplaki & Palyvos (2009) have formulated a more theoretical approach. This is modelled by the equation:

where is the module’s electrical efficiency at the reference temperature, and at solar radiation flux of 1,000W/m². This model suggests that an increase in air temperature will lead to a decline in efficiency of the PV module and this further illustrated in Figure 1.

Figure 1. Increase in Temperature leads to Lower Efficiency (Photo Credit: Skoplaki & Palyvos, 2009)

As illustrated in Figure 1, the negative gradient indicates an inversely proportional relationship between the operating temperature and the efficiency – higher temperature leads to lower efficiency. The five lines indicate the different materials used for the PV modules. However, in this case, all five materials have a decrease in efficiency against higher temperature levels – only difference is the tolerance of each material. Thus, given with the constant input, this clear shows that as the temperature increased, the power output for BIPV modules decreases.

Effect of Radiation on Power Output

Figure 2. Increase in Light Intensity leads to Higher Efficiency

As illustrated in Figure 2, the positive gradient indicated a proportional relationship between the light intensity (radiation divided by area) and the power output. The power output increases as the light intensity increases. Similarly, like the effect on temperature, different materials have a different threshold.

Significance of Temperature VS Radiation to Power Output

According to the study done by Yoo & Lee (2002) which compares the efficiency of the BIPV system throughout the different seasons, the PV modules are the most efficient during the winter season where the solar radiation and temperature was moderately low. This is further illustrated in Figure 3.

Figure 3. Efficiency Metrics throughout the Year (Photo Credit: Yoo & Lee, 2002)

Thus, this concludes that PV modules works best with low radiant temperature. The results of these studies suggest that BIPV modules are most suitable for regions of strong irradiance level and low temperature, country such as Tibet, where the high altitude caused the ambient temperature to be consistently low where light incidence is strong.

Table 1: Comparison of Efficiency and Output of PV Panels

In Table 1, the radiation level in hotter seasons gives 2.5 times more advantage over colder seasons. However, despite the colder weather, winter seasons are able to produce 1.2 times higher power than in summer. As efficiency is calculated by output over input, the resultant effect on efficiency has a whopping 3.3 times difference between the two season. Thus, it is concluded that high temperature significantly undermines the efficiency of the panels as well as outweighing the positive effect of higher radiation levels – thus, giving a lower output.This also reveals a major problem where

This also reveals a major problem where high-temperature levels during summer period produce a substantially low amount of energy. This serves a potential danger for BIPV implementation in Singapore, who has hot and humid weather all year round.