Wednesday May 22, 2024

The facts of going solar

By Omar S Cheema
April 30, 2017

Part - II

There is no single international rate for the payment of solar photovoltaics (PV) in terms of the lifecycle cost (the levelised cost of electricity). There wasn’t one at the time when the Quaid-e-Azam Solar Park was set up near Bahawalpur and there isn’t now.

It’s disingenuous to say the cost of PV was globally replicable at four cents per kWh back then or 2.3 cents per kWh presently. Solar power was being commissioned elsewhere at higher prices – and that’s the case even now. Soon after the Bahawalpur project kicked off, I helped plan a utility-scale PV plant in Jordan at a sovereign-backed purchasing price (also known as feed-in-tariff) of 16.9 US cents/kWh for 20 years. The buying price in Jordan has dropped this year to 6 US cents/kWh – which is still above the 2.3 and 4 cents per kWh numbers.

Even with the exact same physical conditions at two separate sites, identical insolation and temperature, the cost per kWh can differ. The cost of project financing – ie, the cost of borrowing (or equity investment of) the required capital – varies according to the credit risk of the off-taker and the profit expectation of the financier: a matter of risk vs expected returns. Country risk is a major factor. All other things being equal, the cost of a solar project in a country with a weaker sovereign credit rating is going to be higher. It is, therefore, wishful thinking to expect the cost of a state-sponsored solar power plant to be the same for both Pakistan and the UAE.

Let’s address the accusation made in the article, ‘The perks of going solar’ – published in these pages on March 15 – that somehow the Chinese turnkey system installer at the solar park in Bahawalpur short-changed a contractual commitment of 100MW by only delivering 18 MW “because of the poor quality of materials supplied” – as if poor materials could be responsible for an 82MW capacity gap. This notion exposes a lack of understanding of PV project planning and overlooks the fact that the earth rotates and results in the alternation between day and night.

The first solar PV installation at the Bahawalpur park would have had a nameplate DC power rating of 100MW. The full nomenclature is 100MW-peak-DC, ie it has a peak capacity of 100MW-DC power output at the standard testing conditions (STC). The AC capacity ratings are less than the DC rating because of conversion losses. The same holds true for the UAE PV plant near Abu Dhabi and 1,000MW is only the nameplate peak power capacity rating at the STC. The STC is an arbitrary reference to compare one PV product to another: a solar flux of 1,000 watts per metre squared at 1.5 global (G) air mass (AM).

To achieve the peak rating – a capacity factor of 100 percent – three conditions must apply. First, the PV array must receive 1,000 watts per square metre x 24 hours/day x 60 minutes/hr x 60 seconds/min x 365.25 days/year divided by a 3,600,000 joules-to-kWh conversion factor = 8766 kWh per square metre per year solar irradiation. Second, the array efficiency must be the same in the actual operating conditions as at the STC. Third, there must be no other energy losses due to ohmic resistance of the cables or from the array’s DC output to the AC injection into the grid.

There is no place on the planet that 24 x 7 receives 1,000 watts per square metre, the required 8,766 kWh per square metre per year for a 100 percent capacity factor – since obviously at night the solar array will receive 0W per square metre . The actual power output of the Bahawalpur (or the Abu Dhabi) plant will be a fraction of 100MW-peak (or 1,000MW-peak)– a capacity factor of much less than 100 percent, primarily because neither receive anything close to the STC solar flux nonstop due to the rotation of the earth and deviations from the two other conditions: a lower array efficiency in actual operating conditions and the energy losses in the system (unless you have a perfect inverter and superconducting cables). While the actual vs STC efficiency and system energy losses depend on the technology design and its implementation, the solar irradiation factor is beyond human control, it’s down to Mother Nature.

What is the total annual solar irradiation that can be expected for a fixed-tilt array at Bahawalpur or Abu Dhabi? For a first approximation, you can consult the public PVGIS database. At Bahawalpur and Abu Dhabi, the optimum fixed-tiltplane-of- array (POA) irradiation is 2,279 kWh/per metre squared per year and 2,484 kWh per metre squared per year respectively. The POA irradiation will increase for a tracker-mounted array. Now assume an ideal PV system with no drop from the STC array efficiency and no energy losses. The ideal fixed-tilt capacity factors at Bahawalpur and Abu Dhabi would come to 26 percent and 28 percent (the ratios of 2,279 and 2,484 to 8,766). As a result, the maximum power outputs will be 26MW and 280MW, respectively, for the 10MW and 1,000MW installed in Bahawalpur and Abu Dhabi. The difference between the 100MW peak rating and the reported 18MW rating at Bahawalpur is not mainly because of “poor quality of materials” but a result of how the STC are defined and the actual POA irradiation that can be received in comparison.

If the Bahawalpur PV plant is indeed operating at 18MW and the incident solar irradiation matches that predicted by the PVGIS, it means there is a gap of about 8MW to the idealised fixed-tilt maximum of 26MW. There are valid grounds to scrutinise this performance, the scope for improvement and determine how the gap between the actual 18 percent (18MW) and utopian 26 percent (26MW) capacity factors can be narrowed for further installations at the park. However, it is ridiculous to blame “poor quality of materials” for an 82MW difference from the 100MW peak capacity nameplate rating – at least 74MW of that has nothing to do with the Chinese suppliers and installers.

How can we judge whether the Bahawalpur PV project is working according to satisfactory, internationally competitive technical standards? The industry criterion for gauging PV systems is the performance ratio (PR) metric. The PR formula makes an adjustment for the actual solar irradiation received, so you can compare the design and performance of PV plants at different sites. A reasonable PR benchmark for the first 100MW at Bahawalpur is 0.8 (80 percent). Typically, an independent engineer verifies the PR. To determine whether the installation is suffering from “poor quality of materials”, a simple objective step is to check the independent engineer’s PR record.

Solar PV and wind energy have, in recent years, been globally dominating new electric power. China and India are cutting back on coal, despite having far greater reserves than Pakistan and doubling down on solar. It’s a no-brainer for Pakistan to prioritise and develop a solar-heavy energy portfolio. Instead of disseminating misleading information about solar projects, spreading confusion about the technology and its economics, the emphasis should be on constructive criticism and positive competition, outperforming and overtaking previous projects and delivering cheaper and more plentiful electricity to the country.



The writer is based in London and provides advisory services for renewable energy and energy efficiency ventures and projects worldwide. Email: