Designing for Value in Large-Scale PV Systems: Page 3 of 4
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Today, power plants operating under PPAs with TOD rate structures are designed with much higher dc-to-ac ratios, up to and exceeding 1.4. In cold, sunny weather, the dc system is capable of generating more power than both the inverters and ac system are designed to handle. Whenever this is the case, the inverter restricts dc power output by simply moving the array off its maximum power point. Since inverter power limiting results in a clipped, flat-topped power curve—as shown in Figure 1—this phenomenon is often referred to as clipping. While the same term also describes distortion in an audio waveform, there is no distortion in the voltage or current waveforms during power limiting, as an interactive inverter must always adhere to strict power-quality requirements. What suffers instead is the PV system’s production efficiency.
Why would developers spend money on extra PV modules, only to have the extra power output from those modules wasted? The first reason is the need to increase utilization of all fixed development costs and system structural costs. Developers have invested a lot of money in land, interconnection fees, lawyers and personnel to create the project opportunity. They have also built a substantial ac system infrastructure—one that includes inverters, transformers, switchgear and a substation—and they want to push as much energy as possible through that fixed investment over the life of the PPA, even if that means sacrificing production efficiency. The second reason for increased dc-to-ac ratios is that developers want to deliver the greatest possible quantity of highest-value energy, as defined in the PPA’s TOD rate structure. To capitalize on energy values that are two to three times the baseline rate, designers oversize the dc-to-ac ratio so that inverters run at full power when energy is the most valuable. The general idea is that you are willing to give away (via clipping) 2 MWh of energy at $100/MWh to get 1 MWh of energy at $250/MWh, because this nets you $50.
DC-to-ac optimization is based in part on the premise that increased temperature negatively affects PV module performance. As cells heat up during operation due to internal resistance and ambient weather conditions, operating voltage decreases, thereby dragging down performance. For example, a 300 W– rated module at 25°C and an irradiance of 1,000 W/m2 generates 300 W of power. If irradiance is constant but ambient temperature increases, causing cell temperatures to reach 50°C, the same module (assuming a -0.43%/°C temperature coefficient) now produces only 268 W of power (300 W x [1 + (50°C - 25°C) x -0.43%/°C] = 268 W).
Fortunately, module manufacturers can improve the module power and temperature relationship. For example, they can reduce the nominal operating cell temperature through the use of advanced materials designed to more quickly dissipate the module’s internal heat to the atmosphere, allowing it to run cooler. In addition, sophisticated cell technologies can improve the temperature coefficient of modules. Modules using standard crystalline PV cells have a temperature coefficient of about -0.45%/C, meaning that a 1°C increase in operating cell temperature decreases power output by 0.45%. Use of these advanced cells can reduce the coefficient to -0.43%/°C, or even -0.41%/°C.
In hot climates where modules are consistently operating at 50°C or higher, module temperature coefficient is a critical driver of system performance. As shown in Figure 2, a 0.04%/°C difference in module temperature coefficient can result in more than a 1% difference in annual energy yield. For an EPC to take advantage of a module’s improved temperature coefficient, it must collaborate closely with the module manufacturer. Most manufacturers continually update their own PAN files, which include temperature coefficients and are used in PVsyst to characterize a module’s performance parameters. However, it is important for EPCs to understand the assumptions behind the file inputs and to ensure that their module suppliers can support these inputs with real-world and statistically significant performance data.
Although smart module selection and testing can mitigate heat resistance effects, the power output of PV modules is always higher at low operating temperatures if irradiance is constant. On cold but sunny days—think crisp spring mornings—solar power plants are at peak dc output. These are the days when you are most likely to see power-curve clipping resulting from inverter power limiting. However, this often pays off later. Consider a hot afternoon in July when high temperatures reduce the dc power output. When the available power falls well below the ac system’s nameplate capacity, two consequences follow: First, you are not making good use of your investment in ac equipment; and, second, you are missing out on the most valuable revenue opportunity of the year, that summer afternoon high-TOD multiplier. To maximize economic performance, you need to increase the dc-to-ac ratio to capture more of this peak revenue opportunity, even though that will reduce system efficiency.
Tracker versus fixed-tilt mounting. When making the decision between installing a fixed-tilt racking system or a tracker, you must consider several different factors, including cost differences, land use, energy output and TOD rates. Many EPCs use an LCOE model for analysis, but this approach, without proper consideration of TOD rates, does not lead to the best design. In simplified terms, an LCOE model is the project’s all-in price divided by total energy generated over the life of the power plant. The all-in price includes up-front costs for land, construction and interconnection of the plant, plus annual O&M costs discounted back to the present day. Total energy output incorporates annual energy production estimates, which are also discounted back to the present. An LCOE model is great for finding the most cost-efficient form of energy generation, but it fails to consider the revenue side of the equation and TOD rate structures.