Technical factors to consider when implementing solar photovoltaic systems
By Cheng Zhi Wei
Solar energy systems are often hailed as one of the more viable renewable energy sources for sunny Singapore. Given that solar module prices are rapidly falling, Singapore will surely reach grid parity in the near future. Grid parity refers to the point where the cost of solar power is equivalent to the cost of electricity generated from conventional fossil fuel power plants. Grid parity could result in an explosion in solar system implementation as consumers and businesses begin to see it as a cost-effective alternative to traditional sources of electricity.
At the Clean Energy Expo Asia 2011 Conference, one of the sessions covered Solar Power Technology with a focus on certain technical considerations that must be factored in when implementing solar photovoltaic (PV) systems. The session was moderated by Mr Christophe Inglin, Managing Director, Phoenix Solar Pte Ltd, and included speakers, Mr Horst Kruse, Director Sales Photovoltaics Asia, SCHOTT Solar AG, and Dr Jiang Fan, Manger of Technology Centre of Energy Conservation, Singapore Polytechnic. In particular, Mr Kruse focused on the tropicalization of PV modules and Dr Jiang focused on the effect of partial shading.
Traditionally, solar PV modules have been developed and used in temperate countries, namely the European and North American countries. Therefore, the modules and testing standards have been designed for operation in such climates. These tests include Thermal Cycles and the Damp Heat test amongst others. The former exposes the module to continuous cycles of temperature change and the latter exposes the module to high heat and humidity conditions for prolonged hours.
These tests are meant to simulate actual operation conditions but are recognized to fall short in replicating the effects of equipment aging. This is particularly important as the solar modules are reported to have life expectancies of between 20-30 years. Furthermore, many systems have a payback period of between 8 to 16 years, depending on operational conditions. Thus, the lifetime of these solar modules is key to whether they represent a worthwhile investment.
However, standard industry tests might not be adequate as they do not consider tropical environmental conditions. Some manufacturers such as SCHOTT Solar AG are compensating for this by putting their modules under more stringent tests including doubling the standard number of Thermal Cycles, doubling the duration of the Damp Heat test and even field testing.
Another implementation consideration is the effect of partial shading. Partial shading refers to the phenomenon where solar module output drops drastically when even a small portion of it is obscured by shade. It may initially seem counter-intuitive but this phenomenon is due to fail-safes that have been included to protect the solar modules. When a solar cell is obscured, it is no longer able to generate current and becomes a resistor in the circuit. It will start to to heat up and would be permanently damaged if the shade is not removed.
The solution for this is to build a bypass diode into the circuit so the entire cell is bypassed if it is obscured. Currently, bypass diodes are implemented along strings of several cells each. So, if any cell along the string is obscured, the bypass diode would deactivate the entire string, resulting in a drastic loss of output. Therefore, small patches of shade can have an amplified effect on the loss of power output especially if it affects multiple strings.
There are several solutions to counter the effect of partial shading. Several manufacturers are looking to decrease the cell-to-diode ratio to make their solar modules more partial-shade resistant. However, this would result in more expensive modules so the need for such shade-resistant features needs to be thoroughly considered.
Another solution is to conduct a sun-path study prior to the implementation of the PV system, which involves studying the shadows cast by the sun across the proposed PV system over a year. This gives a rough idea what sort of shadows and partial shading would affect the proposed system. It is also important to consider the layout of the substrings within a solar module. For example, the partial shade would have less affect on power output if it obscured two cells along the same string versus two cells along two different strings.
Another factor that must be considered is the possibility of future urban developments around the PV system. For example, a newly constructed high-rise building next to the PV system could severely limit the power output of the PV system and render it worthless. Thus, it is important to consider nearby urban developments in the 15-20-year period ahead when choosing a site for a PV system.
Finally, it is important to take note of the warranty coverage offered by the solar module manufacturer. Many companies offer a 25-year warranty but this may be affected by operational conditions. For example, some companies may not offer the full 25-year warranty if the module is being used near the sea. Also, attention must be paid to the conditions of the warranty. For example, some companies may offer additional solar modules should existing modules suffer from excessive power output loss. This may not be helpful if the PV system site has limited space to accommodate additional modules.