Long-term performance potential of concentrated photovoltaic (CPV) systems
Introduction
The use of renewable energy as primary energy supply is hailed to be the probable solution for future sustainable energy supply and environment development. High dependency on fossil fuels has increased the greenhouse gas emissions to an alarming level that leads to higher increase in the ambient temperature if such fuel consumption is unabated [1], [2], [3], [4], [5]. Among all renewable energy resources, solar energy has the highest energy potential and is feasible to fulfill the global energy needs [6], [7]. Moreover, the direct conversion of solar energy into electricity through photovoltaic systems provides a direct and elegant utilization of solar energy [8], [9], [10].
Ranging from the stationary and non-concentrating silicon-based PVs to two-axes tracked concentrated photovoltaic (CPV) systems, the diverse photovoltaic market offers many system configurations and technologies, with different performance levels at assorted working conditions. Concentrated photovoltaic (CPV) system provides highly efficient photovoltaic technology [11], utilizing multi-junction solar cells and concentrators. Solar energy hitting the earth surface is in the form of both, beam and diffuse radiations and their share in total received solar energy depends upon the local climate, weather and sky conditions i.e. the amount of pollutants present in the air and the clouds. As concentrators only concentrate and respond to beam radiations of solar energy [12], thereby require solar tracking. The commercial CPV systems are designed as gigantic units, based upon their targeted installation in the open desert regions, than the other climate regions, due to high availability and share of beam radiations in the desert region. On the other hand, the conventional stationary PV system can respond to beam and diffuse, both parts of the solar energy. Therefore, despite lower efficiency, conventional stationary PV can still produce power in cloudy conditions, as compared to the CPV.
The overwhelming catalogues of PV and CPV manufacturers, provide only the rated maximum efficiency of the system, based upon standard testing conditions, STC (IEC 60904-3) or nominal operating cell temperature, NOCT (IEC 61215 and IEC 61646) conditions tested in laboratory, as per IEC standard [13]. During actual operation in the field, as shown in Fig. 1, the output rating is far from the above-mentioned rated conditions. The field conditions differ significantly [14], [15] from the laboratory due to three factors. Firstly, the solar irradiance, as well as the share of beam and diffuse radiations, are constantly changing due to solar geometry and local cloud cover. Secondly, the temperature of photovoltaic modules is dependent largely on the ambient temperature which is dictated by seasonal variations. Thirdly, the frequent dust storms or dust particles [16] in the air can affect photovoltaic modules performances as it reduces the solar irradiance reaching the cells. From plant designer and customer point of view, the total energy output is of the main interest from any power plant. These changing field conditions affect the performance and the overall energy output of the system, which are far different than the rated performance [17], [18]. That is why, despite highest rated efficiency, CPV systems are targeted to be installed in open desert regions due to favorable working conditions of high beam radiations. It depicts that the feasibility of any photovoltaic system should not be based upon its rated performance and working conditions, like targeted installation of CPV system only in desert region. Total energy output, over a specific period, of a system is the true performance indicating parameter, which is of main interest for plant designers and customers.
The objective of this paper is to report the actual field performance of the CPV system in the tropical weather of Singapore. In addition, the feasibility and potential of CPV system is examined for the tropical weather conditions as opposed to the clear sky in conditions of semi-arid desert regions as well as comparing the CPV versus the stationary flat-plate PV panels. These PV performances are rated and compared by using a yearly electrical rating of output to input; it gives the total annual electrical energy uploaded to the grid from the PV array in the field operation versus the annual insolation input of the locality, both parameters expressed in kW h/m2/year. A complete description of the photovoltaic systems considered, namely a double reflection Cassegrain CPV (parabola dish and hyperbola mirror with maximum efficiency of 17–18%) and the Fresnel lens of 12 × 12 cm2 (with efficiency of 27–28%) and three pre-installed conventional stationary PV systems at CITI (BCA) Singapore, can be found in the published literature [19]. For the CPV systems, the experiments were conducted over a period of 12 months, from September 2014 to August 2015, and the monitored data included the electrical power output and the received direct normal irradiance (DNI). This is the first ever CPV long-term performance testing in the tropical region.
Section snippets
Development of CPV system
Fig. 2(a) and (b) shows the schematic and the concentrating assembly arrangement of the two developed CPV systems, namely the double-reflection mini dish CPV of Cassegrain arrangement and the Fresnel lens CPV unit. In these CPVs, solar beam radiation are concentrated to its focal point at inlet of glass homogenizer which has two functions: (i) it guides and distributes the solar beam radiation uniformly onto the multi-junction solar cells (MJC), glued to the outlet aperture of homogenizer
Testing methodology
To analyze the actual field performances of the CPVs as well as the conventional stationary PV, total long-term energy output of the systems is recorded and presented in the form of electrical rating i.e., expressed in kW h/m2/year as ratio of output to input energy. Such long term data, not the intermittency of instantaneous output, is of the key interest for the PV system designers and owners because it includes the effect of all of the performance affecting parameters during actual field
Results and discussion
For DNI, the field measurements were taken with uncertainty of ±0.75% (95% confidence level) and for GHI, the uncertainty in the measurement was recorded as ±91% (95% confidence level). In additions, the measured data was found to be independent of the temperature effect. However, for current and voltage measurements, from MPPT, uncertainty of ±0.66% and ±0.58% were recorded, respectively with 95% confidence level. Therefore, based upon the recorded uncertainties in the measured quantities, the
Conclusion
The long-term (twelve months) electrical rating of two types of CPV system, namely the mini dish and the Fresnel lens CPVs, has been successfully conducted under the outdoor tropical weather of Singapore. Such an outdoor field test is meaningful and accurate to the planners for estimating the field output of renewable energy systems that includes all losses due to solar irradiance intermittency, surface cleanliness of glazing, cell temperature effects and DC to AC converter. Under the same
Acknowledgement
This research was supported by the International Research Scholarship of Mechanical Engineering Department, National University of Singapore and collaborated with King Abdullah University of Science and Technology.
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