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Solar power |
Solar cells represent the fundamental power conversion unit of a photovoltaic system. For practical operation, solar cells are usually assembled into modules.
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Many different solar cells are now available on the market, and yet more are under development. The range of solar cells spans different materials and different structures in the quest to extract maximum power from the device while keeping the cost to a minimum. Devices with efficiency exceeding 30% have been demonstrated in the laboratory. The efficiency of commercial devices, however, is usually less than half this value. |
Crystalline silicon cells hold the largest part of the market. To reduce the cost, these cells are now often made from multicrystalline material, rather than from the more expensive single crystals. Crystalline silicon cell technology is well established. The modules have a long lifetime (20 years or more), and their best production efficiency is approaching 18%.
Cheaper (but also less efficient) types of silicon cells, made in the form of amorphous thin films, are used to power a variety of consumer products like solar-powered watches and calculators, but larger amorphous silicon solar modules are also available.
A variety of compound semiconductors can also be used to manufacture thin-film cells (for example, cadmium telluride or copper indium diselenide). These modules are now beginning to appear on the market and hold the promise of combining low cost with acceptable conversion efficiencies.
A particular class of high-efficiency solar cells from single silicon or compound semiconductors (for example, gallium arsenide or indium phosphide) are used in specialized applications, such as to power satellites or in systems which operate under high-intensity concentrated sunlight.
PV modules can be mounted in a variety of methods. They can be mounted on a frame at ground level set in concrete, or they can be mounted on a pole or a roof of a building. In the case of applications within the build environment, it would be more suitable to either retrofit the modules to the façade or the roof or to mount them on the roof as free-standing.
However, there are several aspects to remember for the mounting arrangement. In general, the modules should be rigid and secure in order to withstand strong winds. They should be upwind from chimney fumes and dusty roads to minimise build up on the glass of ashes and other dirt carried by the wind, and they should be safely accessible for regular cleaning to maintain the best performance. The mounting should also be positioned where the wind can help to cool the modules. No part of the modules |
More importantly, the modules should be tilted to face south in the northern hemisphere and to face north in the southern hemisphere (to within 20°) with a tilt angle equal to the local latitude.
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This stationary mount with a fixed tilt angle would maximize the annual energy output, but at some installations, it might be cost-effective to adjust the tilt seasonally. A tilt angle equal to the local latitude would be suitable for spring and autumn, but a flatter angle in the summer and a steeper angle in the winter would optimise the obtained power in these seasons in particular, and an adjustment of 10 or 15° would be recommended. However, the tilt angle should not be less than |
Appreciating that simulation would be a powerful tool for estimating supply profiles from renewable resources, we tried to simulate the performance of PV’s in the build environment in order to estimate their output power. We made a simple model of a single free-standing roof-mounted PV module using the ESP-r. In our model, we assumed the module type used to be the RMS95 from R&S Renewable Energy Systems which was a polycrystalline PV module of efficiency about 10%.
The total module area was 0.874m2 with cell area 0.734m2 (84% of the total module area). It consisted (from top to bottom) of low-iron glass, EVA and white Tedlar with the PV cells embedded within the Tedlar, and then an aluminium backsheet.
Since our case study was based in Glasgow (Strathclyde university buildings), we made our module face south with a fixed tilt angle equal to the local latitude which was about 56° for Glasgow. We ran the simulation on 1 hour time steps for a whole year of the Glaswegian climate, and we got a results file that contained the hourly insolation absorbed by the module, the temperature of the module and the power output from the module. From this data, we were able to calculate the power output during a typical summer day, a winter week, a spring month, etc.... We also found that the total output power from a single module of the above mentioned size and characteristics was estimated to be approx. 80kWh per year, i.e., the output was 91.6kWh per m2 of PV module area per year.
We decided to also study how effective seasonal module tilt adjustments would be in our case. Therefore, we ran a 2nd simulation after changing the tilt of the module for the summer months to be 46° (a flatter angle) and for the winter months to be 66° (a steeper angle), and we obtained another results file. The results obtained showed that, in our case, the overall increase in the module annual output power by changing the tilt angle was very modest (approx. 3% from 91.6kWh to 94.3kWh per m2 of module area per year), and so seasonal modification of the module tilt would not be very advantageous in our case. It is worth mentioning, however, that we observed that the summer months benefited from the modified tilt more than the winter months did. We assumed that this was maybe due to the modest insolation in the Glaswegian winter in the first place so there wasn’t much insolation to capture no matter how we changed the module tilt.
Therefore, we have adopted the scenario of stationary PV modules with a local latitude tilt angle for our case study, and we used the corresponding supply profiles in our decision support tool.
We used simulation for the assessment of photovoltaics of the above mentioned type and characteristics as an example, but different PV types of different efficiencies and orientations could be used. Also, similar simulations could be done for other renewable resources like wind or CHP.
