When dealing with solar irradiance forecasting and, at the same time, with the performance of photovoltaic systems based on a given solar energy resource, one must understand the weather characteristics of the location under investigation.
For Singapore, the country covered in this thesis, high average daily ambient temperatures and relative humidity are found year round. Located at 1 degree North of the Equator, and having a climate classified exclusively as tropical (FONG, 2012), Singapore has little or unnoticeable seasons, except for a clear differentiation between rainy and dry seasons, characterized by the presence of monsoons.
For a PV system, the local weather conditions can be translated into what circumstances an installation experiences on a daily basis. As an example, Figure 36 shows module temperatures in Germany and in Singapore, highlighting the warmer condition the tropical location system experiences in relation to the temperate location. At 1,000 W/m2, a PV panel in Singapore is ~10° C hotter than in Germany.
Figure 36: Module temperature increase versus irradiance on module plane as registered in Singapore and Germany (NOBRE et al., 2015 (under preparation)).
2.3.2 Singapore climate
Storms are common in tropical regions. For the case of Singapore, fast approaching rain clouds from Malaysia (from the North) and from Indonesia (from the East, West and South) can change weather conditions and irradiance levels quite rapidly. As an example, and as previously shown in Figure 11, a “Sumatra squall”, a commonly known weather phenomenon in the region, is characterized by a fast-moving and sudden rain-storm front originating from Indonesia, however, predominantly from the West (FONG, 2012).
There are two monsoon seasons in Singapore – a Northeastern, taking place between November and March, and a Southwestern, between April and June. The rainiest months of the year are usually November and December (FONG, 2012).
The local solar resource is abundant. A recorded TMY for annual global horizontal irradiance (GHI) and diffuse horizontal irradiance (DHI) is 1,631 and 926 kWh/m2 respectively for a ground meteorological station at Changi Airport (METEONORM, 2013). These annual averages represent a diffuse share of ~57% from the total global irradiance, characterizing common cloudy conditions for Singapore.
SERIS has commissioned in May 2010 a comprehensive meteorological station, with some of its sensors seen in Figure 37. The station was deployed primarily due to the lack of PV-focused irradiance measurements available in the country. The setup possesses 9 silicon sensors installed at several tilt angles (0, 10, 20, 30 and 40 degrees facing an arbitrary orientation, followed by 4 façade silicon sensors tilted at 90 degrees, facing North, South, East and West). Additionally, the station has two pyranometers for global (GHI) and one for diffuse irradiance (DHI) as previously shown in Figure 33, among other classical meteorological sensors such as air temperature & relative humidity, wind speed & direction, and atmospheric pressure.
Figure 37: SERIS meteorological station rack, with tilted silicon sensors ranging from 0 to 40 degrees (source: SERIS).
From Figure 38, which stems from data records of the SERIS meteorological station for year 2011, it can be seen that the little seasonality in Singapore is demonstrated by a small variation of the ambient temperature profile in the country, with a daily annual average around 27-29°C. Dips can be seen between the months of November and February, which are associated with the Northeastern monsoon period.
Figure 38: Ambient temperatures in Singapore for the year 2011. The daily averages are plotted in orange (SERIS, 2011).
The irradiance profile for the country also shows little seasonal variations. The SERIS meteorological station has recorded for 2011 the following results for global and diffuse irradiance components as shown in Figure 39. Important to note is the high diffuse content, registered at 55% for the year depicted (METEONORM, 2013).
Figure 39: Global (GHI) and diffuse horizontal irradiance (DHI) in Singapore for the year 2011 (SERIS, 2011).
The numbers shown in Table 4 highlight the high diffuse irradiation component in a tropical location, with shares of diffuse over global irradiation (diffuse fraction, kd) of above 50%. The typical meteorological year (TMY) averages for Singapore are also shown. It is extremely rare to have a total clear sky day condition in Singapore. One of such occurrences was the 5th of August 2011 and the plot of global horizontal irradiance (GHI) recorded by the SERIS meteorological station was seen in Figure 26. The cloudless day shown as a red line is an extremely rare condition, taking place once or twice throughout a year. The time series in blue, also plotted in the same figure, is more representative of a typical day in the island, with ups and downs in irradiance due to high cloud content and motion.
The clear sky example presented in Figure 26 also brings another piece of important information about irradiance conditions in the Singapore environment – the peak irradiance achieved at an approximate zenith time of 13:15 pm from that August day shows irradiances around 920-950 W/m2, not reaching the theoretical value of 1,000 W/m2,
discussed in section 2.2.1. That can be explained by the fact that Singapore has high values of relative humidity (see Figure 40), thus the water vapor content in the air presents extra blockage for the light to reach the Earth’s surface. Moreover, it could be inferred that the presence of aerosols in Singapore’s air has levels above thresholds which would normally be found in non-heavily urbanized locations of the world. Table 4: Typical meteorological year averages for Singapore and results for 2011 through 2014 for the SERIS meteorological station. TMY source: (METEOTEST, 2015). Other data source: SERIS.
TMY 2011 ΔTMY 2012 ΔTMY 2013 ΔTMY 2014 ΔTMY