Solar PV saw its lowest levelized cost of energy (LCOE) — 1.78 US cents/kWh — in a bidding contest in October 2017 to Saudi Arabia’s first 300 MWp utility-scale solar PV Plant in Sakakah. Compared to the lowest LCOE bids of 3 US cents/kWh submitted to DEWA in May 2016; the cost has come down almost 40 percent for every kWh sold to the Grid.
Optimization of solar PV plants’ output plays an essential role in not only making “solar cheap” but also ensuring its long and durable life — increasing dependability on the intermittent sunlight.
The output of a solar PV module varies constantly throughout the day, as the sun completes its path in the sky. It is because of the dependence of the semiconductor material technology involved in making a solar cell. This results in complex and variable power generation of a solar PV plant making it hectic for the Designers and operators to accommodate the load profiles.
While increase in solar irradiance has an increasing effect on power generation by the solar cell, increase in temperature results in reduction in power production. The solar cell is also dependent on the incident spectrum of the sunlight, represented by airmass number, to produce more or less power in different parts of the day. The standard spectrum, airmass 1.5, is shown in Figure 1.
Figure 1 — The standard ASTM G173 Solar Spectrum depicting Airmass 1.5. Source: https://qph.ec.quoracdn.net/main-qimg-09fabce3df7ba786e38dc91a182a7247
The position of the sun in the sky changes 24 hours a day, 365 days a year. During the day the solar spectrum, incident on the earth’s surface, changes continuously. Depending upon the position of the sun in the sky, the sun light has to travel more or less distance, through the atmosphere, to reach the earth’s surface. The shifts in spectral changes throughout the day are shown in Figure 2.
Figure 2 — Variation in incident Solar Spectrum during time of Day. Source: Solar spectral irradiance measurements relevant to photovoltaic Applications – Cristina CORNARO, Angelo ANDREOTTI1, Department of Enterprise Engineering, CHOSE, University of Rome Tor Vergata, Via del Politecnico, 1 00133, Rome, Italy
The variation in the incident spectrum results in different amounts of power production that change throughout the day. Each wavelength from the spectrum contributes in different amount of current generation. The current produced through every wavelength in the spectrum is known as the spectral response of the semiconductor. The spectral response determines the semiconductor technology capability to produce, either more or less power, depending on the incident spectrum of sunlight on the PV module. Quantifying gains and losses due to spectral variations, can have significant effects in design optimization, energy forecast analysis, and in calculations of net present value (NPV), LCOE and similar financial indicators deciding plant feasibility.
The different commercialized semiconductor technologies; Crystalline Silicon, Copper Indium Gallium Selenide (CIGS), Cadmium Telluride (CdTe), Amorphous Silicon and more, are either more or less responsive to different parts of the solar irradiance spectrum, in different parts of the day. The results are displayed in Figure 3, showing the spectral response curves for different semiconductor technologies.
Figure 3 — Spectral Response Curves for different semiconductor technologies for PV. Source: European Commission, Joint Research Centre, Via Fermi 2749, Ispra 21027, Italy
The diversity in the semiconductor technologies allows PV systems designers to provide optimized designs of plants as per the environment of the location and translate the excess production into dollars, in turn decreasing the cost. As the location and its climate changes, the design of the PV plant requires a more optimized approach.
The terrain of Pakistan from North to South provides an excellent example to depict the optimization technique and establish a baseline through calculations and simulations.
This article studies three major areas of Pakistan from the North, Central and South regions — Islamabad, Bahawalpur, and Karachi — for a 100-MW solar PV plant in every city. The three cities show very different climate patterns and a great deal of weather variation throughout the year, encompassing all the four seasons. The annual average values of environmental data for the three cities are displayed in Table 1.
Table 1 — Weather Data for three locations chosen for analysis
The output of the 100-MW PV installations has been analyzed against the weather patterns and for two different semiconductor technologies for the PV cells — Poly Crystalline Silicon, and Cadmium Telluride (same efficiency PV modules). The output has been simulated using PV simulation software — PV*SOL from Valentin Software and Plant Predict from First Solar.
The PV thermal losses have been quantified and compared with in the range of selected, different semiconductor technologies. The spectral responses for different semiconductor technologies’ PV modules have been quantified using the 2-param model that incorporates the airmass, humidity and the ambient temperature and gives a more precise and accurate analysis of the losses and gains in power production for the different semiconductor technologies. The spectral responses were obtained from the Plant Predict Software that provides a comprehensive analysis of the energy production. The results for energy produced by the 100-MW PV plants is then translated into financial parameters from an earnings perspective.
Islamabad displays quite a diversity in its climate. From the cold and dry winters, it takes little time to move to hot and humid summers. The temperature and humidity variance of Islamabad are shown in Graph 1 and Graph 2, respectively.
Graph 1 — Temperature Variance — Islamabad. Source: www.weatherspark.com
Graph 2 — Humidity Variance — Islamabad. Source: www.weatherspark.com
An average tariff of 6 US cents per kWh, the current tariff rate in Pakistan, has been assumed to predict the financial returns of the plants of two different technologies. With an average temperature of 22 degrees Celsius and an annual sum of 1722 kWh/m2 global horizontal irradiance (GHI) serves Islamabad a very promising location for solar PV installations, both large-scale plants and small-scale, distributed systems.
The monthly averaged thermal losses, as per the variance of temperature, for the two semi-conductor technologies have been explained in Graph 3.
Graph 3 – Thermal Losses as a percentage for Monthly Power Production – Islamabad
The different spectral variations, varying vastly as per the seasons, incur different spectral response shifts for the two semiconductor technologies. The results are displayed in Graph 4.
Combined with better high temperature performance, 5 percent, for the month of July, the added production due to better spectral performance; 4 percent, the CdTe technology adds more kWhs to production by a combined 9 percent. Adding the advantage with other months and compounding the annual added advantage, the CdTe technology, over the 25-year operation of the plant, counting the degradation rates and other variables, adds an extra almost US$13 Million over poly-crystalline silicon technology, at an average tariff of 6 US cents/kWh. The results and some basic information are displayed in Table 2.
Graph 4 – Variation in Performance due to changes in Incident Spectrum – Islamabad
Table 2 — Comparison of Results for Islamabad
The Bahawalpur city lies in Central Pakistan and typically consists of hot temperatures rising above 45-47 degrees Celsius in summers — thus a huge rise in module temperature is expected. The city is rich in solar irradiance and has abundant sunny days in a year for a solar PV plant to function. The area also suffers high humidity in summers. Graph 5 depicts the changes in temperature over the years while Graph 6 depicts the annual changes in humidity levels.
Graph 5 — Temperature Variance — Bahawalpur. Source: www.weatherspark.com
Graph 6 — Humidity Variance — Bahawalpur. Source: www.weatherspark.com
In the city’s vicinity lies the 100-MW Quaid-e-Azam Solar Power Plant, which had been the first IPP solar PV installation for Pakistan commissioned in 2015. The same PV installation has been subjected to a comparison with the thin film — CdTe — technology and simulations have been run for analysis. The financial calculations are based on the actual feasibility report provided for the 100-MW installation in 2014 and all the parameters have been assumed for that time, including the U.S. dollar rates, discount rates and tariffs. The 100-MW array is of silicon poly-crystalline technology and an equally efficient CdTe module has been used for comparison.
Graph 7 — Thermal Losses as a percentage of Monthly Power Production — Bahawalpur
Graph 7 displays the thermal losses due to high temperature of the location for 100-MW arrays for both technologies, while Graph 8 displays the gains and losses in production due to variation in the incident spectrum. The climate and environment of the Quaid-e-Azam Solar PV Plant is completely different from that of the 100-MW site in Islamabad. There is sufficiently more GHI, but it has increasingly high temperatures as well. The humidity is much higher in summers, where maximum production is required due to peak load demand in the country. For the month of August, due to better high temperature performance, the CdTe technology has an advantage of about 5.5 percent, with a better spectral response production of about 3.5 percent; adding to a cumulative 9 percent more production.
Graph 8 — Monthly Variation in Performance due changes in incident Spectrum — Bahawalpur
The results for the two different arrays at the location are displayed in Table 3.
Table 3 — Comparison of Results for 100 MW Quaid-e-Azam Solar PV Plant at Bahawalpur
The CdTe technology, 25-year production for the 100-MW plant, earns an excess of around US$31 million at the two different tariffs mentioned in Table 3. This excess revenue is a result of better high temperature performance and better spectral responses of the technology as per the climate of the location, incurring no extra costs. The LCOE sees a decline of 1 US cent per kWh for the 100-MW plant. While there is an excess generation of 0.5 billion kWhs over a span of 25 years.
Karachi is one of the biggest cities in Pakistan. It lies next to the Arabian Sea and, because of the coast, it has high humidity and mild temperature throughout the year. The same model as Islamabad has been applied for Karachi. 100-MW installations for poly-crystalline along with the CdTe technology have been simulated and the results have been assessed. An average tariff of 6 US cents has been kept making financial calculations. Graph 9 shows the annual temperature variance throughout the year while Graph 10 shows the annual humidity variance.
Graph 9 — Temperature Variance — Karachi. Source: www.weatherspark.com
Graph 10 — Humidity Variance — Karachi. Source: www.weatherspark.com
While the average monthly temperatures stay low in Karachi, the two semiconductor technologies, poly-crystalline silicon and CdTe, suffer thermal losses that affect performance. The thermal losses for each technology is shown in Graph 11.
Graph 11 — Thermal Losses as a percentage for Monthly Power Production — Karachi
The spectral responses, depicting losses and gains are shown in Graph 12. During the summer season, in the month of June, the humidity is at its maximum while the peak load also increases in the grid. The CdTe technology, clearly, surpasses the crystalline silicon technologies, reaching a 4 percent increase in production due to better spectral performance, while an advantage of 5 percent due to better high temperature performance, leading to an accumulative advantage of 9 percent. Compounding the gains and losses; CdTe earns an extra US$8 million over the span of 25 years. The comparison of results of the two semiconductor technologies are stated in Table 4.
Graph 12 – Monthly Variation in Performance due changes in incident Spectrum — Karachi
Table 4 — Comparison of Results — 100 MW PV Plant Karachi
The study of the three major cities lying in the North, Central, and South of Pakistan depicts that even at lower tariff rates, the CdTe technology helps earn millions of US dollars as extra revenue due to better high temperature performance and better spectral responses. It adapts well to high humidity and has a better low-light performance, during sunset and sunrise, when compared to conventional poly-crystalline silicon, adding more kWhs per day and hence more revenue. The right technology selection helps in optimizing the design and helps in bringing the LCOE to a minimum, without bearing any extra costs, as evidenced from the 100-MW Quaid-e-Azam Solar PV plant in Bahawalpur.
Pakistan is a country rich in solar irradiance, and with the solar PV favoring policies of the government, it is an excellent choice for solar PV investment. However, the varying topography incurs different climates and different challenges. From the methods and approach used in this article, the right technology selection can not only develop more profit but also aid in making solar PV cheap and durable.
The list of assumptions, simulation reports, calculation sheets, feasibility reports, datasheets of products used, and financial calculations are available and proof for excess kWh generation can be provided. Special thanks to Mavetech Pvt Ltd whose resources I used to perform calculations and simulations for the PV Plant sites and the Plant Predict Software team whose aid in calculating Spectral Responses for different technologies was essential.
Lead image credit: CC0 Creative Commons | Pixabay
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