On PV module temperatures in tropical regions
Highlights
► Influencing factors on PV module temperature in Singapore have been analyzed. ► Seven patterns of module temperature behaviors have been identified. ► Guidelines for PV system installations are derived for performance optimization.
Introduction
The efficiency of photovoltaic (PV) modules is strongly affected by their operating temperature. Typically for every 1 °C increase of module temperature, there is a ∼0.45% drop of module efficiency for crystalline silicon modules (Mau and Jahn, 2006, Skoplaki and Palyvos, 2009a). For thin-film modules, this efficiency loss is only about half of that of crystalline silicon technology.
There are several models that describe the correlation between module temperature and weather variables such as ambient temperature, irradiance and wind speed, with a good summary given by Skoplaki and Palyvos (2009b). The simplest and the most widely used model is given by:where Tmod and Tamb are module and ambient temperatures, respectively; the slope k is called the Ross coefficient which expresses the temperature rise above ambient with increasing irradiance (Ross, 1976). Earlier values of k were reported in the range of 0.02–0.04 °C m2/W (Buresch, 1983), with k = 0.025 °C m2/W being the most broadly accepted value (Sauer and Kaiser, 1994). This range was extended upwards to 0.02–0.055 °C m2/W by a more recent IEA study (Nordmann and Clavadetscher, 2003), categorizing the results qualitatively according to level of integration and size of air cushion (if any) behind the modules.
The above models with constant k hold for no wind and no electric load. Griffith et al. (1981) pointed out that with regard to the relevant weather variables, the k value is extremely sensitive to wind speed, less so to wind direction, and practically insensitive to the ambient temperature level. If wind effects cannot be neglected, an additional relation to wind speed or even a thermal model based on heat transfer and balance equations has to be applied to achieve a more accurate result. Two such models have been verified by Koehl et al. (2011) through experimental analysis to give relatively good predictions of module temperatures, if long-term time series of the following three climatic parameters are available: ambient temperature, irradiance and wind speed. Generally, there are two kinds of thermal models: the static and the dynamic. The static models assume the holding of a steady-state heat balance at any moment, from which many correlations between module temperature and weather variables are derived (Mattei et al., 2006, Skoplaki et al., 2008). Following the basic laws of thermodynamics, Kurnik et al. (2011) proposed a set of nonlinear equations to describe four different energy flows: solar energy that enters the PV module, electrical energy produced by the module, infrared radiation exchange and the energy flow due to conduction and convection between the module and the ambient. They build the framework to solve for the module temperature. The dynamic models also take into account the transition phase between two steady-state heat balances. With differential equations, it gives much more accurate results (40% improvement in RMSE) than static models (Amy de la Breteque, 2009), although it is not trivial to derive the necessary model parameters.
It should be noted that all studies mentioned before were conducted in higher latitudes. Trinuruk et al. (2009) investigated the suitability of some models under the tropical climate in Thailand, but their work is for building integrated photovoltaics (BIPV) only. Not much work has been reported for tropical regions especially with respect to the influencing factors on module temperatures. Such research becomes even more important as many countries in and near the tropics have started support schemes with hundreds of megawatts of PV installations expected over the next few years (e.g. India, Thailand, Malaysia, Philippines). So for tropical regions, it is therefore increasingly important to understand what will affect the module temperature and how to best control it. This paper gives the results of an analysis of module temperatures of 16 different PV systems in tropical Singapore (also comparing them to systems in Europe), and eventually suggests some guidelines for the installation of PV systems aiming for lower module temperatures.
The paper is organized as follows: Section 2 introduces the 16 PV systems across Singapore, which are under constant monitoring by the Solar Energy Research Institute of Singapore (SERIS). Based on these measurements, Section 3 analyzes the module temperatures in various PV systems in Singapore with a comparison with non-tropical systems given in Section 4. Section 5 discusses the reasons for the differences in module temperatures. Section 6 gives some guidelines for system design and installation in how to achieve reduced module temperatures in tropical climates for an optimized system performance. Conclusions are drawn in Section 7.
Section snippets
Measurement setup
The PV systems under SERIS’ monitoring are all mounted on rooftops. A broad variety of locations around the island (see Fig. 1) with different system sizes and module technologies has been selected, with the aim of making sample as representative as possible. Further information about these systems can be found in Table 1, with the “roof distance index” explained in Table 2. There are two types of roof – PV array relationships for the monitored systems: (1) inclined roof with PV array mounted
Results
The monthly averaged module temperatures for each site since commissioning until February 2011 are shown in Fig. 2.
A stable and concurrent data stream for all systems is available from September 2010 to November 2010 (data recording at system A was interrupted from December 2010 until February 2011). For the analysis in this paper, data for the month of November 2010 have been chosen to avoid any possible distortion from the relatively high air pollution levels (locally referred to as “haze”)
Comparison with PV systems in non-tropical regions
To put the observations in Section 3 into perspective, the temperature graphs were compared to module temperatures of PV systems in non-tropical regions. As in the case of the systems in Singapore, ΔT = Tmod − Tamb is used instead of Tmod to remove the effect of ambient temperatures at different locations. Comparative data are from the same month (November 2010).
The PV systems selected for the comparison are from southern Germany, northern Germany and southern Spain. The German systems are mounted
Discussion
There are several factors that affect the rise of module temperatures over ambient temperatures in PV systems, in particular: rooftop materials, ventilation, module framing and other environmental conditions. To segregate these factors, all systems were categorized (see Fig. 7) according to their k-value, and the combination of rooftop material and roof distance index (as introduced in Table 1, Table 2), where “M” and “C” stands for “metal” and “concrete”, respectively.
Guidelines
Based on the analysis in the previous section, some guidelines for the installation of PV modules can be drawn to achieve a lower module temperature in PV systems in tropical regions such as Singapore (in decreasing order of influence):
- •
Ventilation. Beneficial temperature effect: up to 50%
- –
make roof-to-PV module distance as large as technically and economically possible and permissible from an architectural and safety point-of-view;
- –
remove/avoid any unnecessary obstructions to airflow;
- –
choose well
- –
Conclusion
In order to optimize the performance of PV systems in constantly hot climates such as the tropics, it is paramount to keep PV module temperatures as low as possible. Through proper site selection and leveraging the guidelines presented in this paper, module temperature rise over ambient temperature can be substantially reduced. The monitoring results show a factor of three between the lowest and the highest temperature rise. With a 0.45%-drop for every 1 °C higher temperature of silicon
Acknowledgement
The project “High Performance Photovoltaic Systems for Tropical Regions” is funded by National Research Foundation (NRF) of Singapore under the Clean Energy Research Programme (CERP).
References (15)
Thermal aspects of c-Si photovoltaic module energy rating
Solar Energy
(2009)- et al.
Modeling of the nominal operating cell temperature based on outdoor weathering
Solar Energy Materials and Solar Cells
(2011) - et al.
Outdoor testing of PV module temperature and performance under different mounting and operational conditions
Solar Energy Materials and Solar Cells
(2011) - et al.
Calculation of the polycrystalline PV module temperature using a simple method of energy balance
Renewable Energy
(2006) - et al.
On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations
Solar Energy
(2009) - et al.
Operating temperature of photovoltaic modules: a survey of pertinent correlations
Renewable Energy
(2009) - et al.
A simple correlation for the operating temperature of photovoltaic modules of arbitrary mounting
Solar Energy Materials and Solar Cells
(2008)
Cited by (60)
Environmental benefits of co-located photovoltaic and greenery systems: A review on the operational performance and assessment framework across climate zones
2023, Sustainable Energy Technologies and AssessmentsOptimal heat treatment furnace based on new robust point approximation strategy
2020, Journal of Cleaner Production