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In this section, we first evaluate the performance of the REST2 model, then the accuracy of the estimated GHI and DNI, and finally, the forecast accuracies for time horizons between 30–180 min. All evaluations are detailed under the four sky conditions.
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Figure 4 shows the scatter statistics of the clear-sky GHI and DNI estimates based on the REST2 model. The GHI estimates were very close to the ground-based measurements, with nRMSE and nMBE of 4.93% and 1.68%, respectively. The REST2 model tended to slightly overestimate GHI on clear-sky days, and it tended to overestimate DNI by a larger magnitude compared to GHI, with an nMBE value of 8.67%. Large overestimations of DNI were observed in low and moderate DNI conditions. One explanation is that a lower DNI is associated with a high aerosol loading, which is generally underestimated by the MERRA-2 reanalysis (Huang et al., 2021). Notably, the model estimates were very close to the surface measurements when DNI was high. The high output power of solar PV plants depends on high DNI values. Compared to the low and moderate DNI values, the fluctuations of high DNI values will induce heavier impact on the safe operation, control, and management of solar PV plants. Therefore, the accurate calculation of high DNI values is crucial for the utilization of solar PV and is the focus of DNI calculations. Overall, the REST2 model performed well in supporting the GHI and DNI estimates and nowcasting evaluations at the Xianghe station under different conditions.
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Figure 5 presents the density plot of FY-4A GHI and DNI under all-sky conditions against surface observations at Xianghe. The estimated GHIs show good correlation with the surface measurements, with nRMSE and nMBE of 22.4% and 5.9%, respectively. The percentage of the estimated GHI values exceeding the measurements by 100 W m–2 was approximately 15.9%. These overestimated GHI values were concentrated at 200–500 W m–2, comprising the dominant contribution to GHI overestimation. Accurate calculation of high GHI values is crucial for solar energy estimation and forecasting. The good performance of the FY-4A GHI estimation model when the GHI is high indicates that the model can support FY-4A GHI estimation and nowcasting under all-sky conditions. Compared with the estimated GHI, the performance of the estimated DNI is worse, and the model needs to be improved in this aspect. In general, the FY-4A DNI was overestimated with an nMBE value of 24.1% (Fig. 5b). The empirical-cloudy model performs well with a high DNI condition and tends to largely overestimate DNI when DNI values are lower than 700 W m–2. One explanation is that if the ground station is covered by the shadows of clouds or surrounding features (namely, the 3D effect), its real-time measured DNI will be lower than that of other locations within the matching pixel in satellite images, thus the measured DNI values will be lower than satellite-derived values (Huang et al., 2019; Jiang et al., 2019). The deviation of clear-sky DNI background and CI is also a contributor to the overestimation.
Figure 5. Density plot of the SSI estimation model GHI (a) and DNI (b) versus SSI measurements under all-sky conditions in 2018. The black line denotes a 1:1 line. The red lines indicate liner regression results.
Table 1 shows the statistical indices of the estimated GHI and DNI in this study compared with those in previous studies based on empirical methods. The nRMSE and nMBE of the GHI estimates in Shanghai obtained by Chen et al. (2020) were 34.7% and 11.4%, respectively, which were higher than those obtained in our work by 12.3% and 5.5%. The nRMSE values of the estimated GHI and DNI in our study were lower by 3.5% and 3.9%, respectively, compared to those obtained by Jia et al. (2021) in their study of northern China. The improvement of our estimation model eliminates the bias related to SZA variation by introducing a parameterization of the FY-4A clear-sky reflectance to SZA.
Table 1. Statistical indexes of instantaneous FY-4A SSI in the present study and previous studies.
The evaluation of the estimated GHI and DNI for different sky conditions is shown in Fig. 6. The accuracies of GHI and DNI estimations declined with an increase in cloud occurrence (from clear-sky C1 to overcast C4). GHI estimates feature less deviation under clear, moderately clear, and cloudy conditions. The MBE values of GHI varied from –22.4 W m–2 to 23.5 W m–2, and the RMSE values increased from 41.1 W m–2 to 90.5 W m–2 as the sky conditions varied from C1 to C3. The estimation accuracy declined sharply under C4 conditions, with the MBE and RMSE reaching 62.6 W m–2 and 144.5 W m–2, respectively. The performance of the estimated DNI was significantly worse than that of the GHI estimation, featuring larger deviations under all four conditions. The MBE values varied from –9.1 W m–2 to 173.1 W m–2, while the RMSE values increased from 94.2 W m–2 to 253.7 W m–2. The poor performance under the C4 condition had a limited impact on the PV plants because the grid-connected power allocation can be adjusted to suit C4 conditions based on regional numerical models. Therefore, the performance of the GHI and DNI estimates under other sky conditions, especially C1 and C2, is the focus of GHI and DNI estimation and nowcasting and is crucial in the utilization of solar PV.
Figure 6. Density plot of the SSI estimation model GHI (a–d) and DNI (e–h) versus observed SSI under different sky conditions. The black line denotes a 1:1 line. The red lines indicate linear regression results.
To characterize seasonal variations in model accuracy, the monthly mean estimated GHI and DNI and surface measurements were compared in Fig. 7. The monthly mean estimations of GHI and DNI were larger than the surface measurements. The difference between model and observation presented a seasonal pattern with a large discrepancy in spring–summer and a small difference in autumn–winter. The seasonal pattern of the GHI and DNI model discrepancy is related to the seasonal variation in cloud occurrence. In spring and summer, the proportion of days with the C4 condition was higher than 40%, especially in July, with the proportion reaching 90% (as shown in Fig. 2). High cloud occurrence reduces the accuracy of the estimated GHI and DNI. With the decrease in cloud occurrence, the discrepancy between estimated GHI and DNI and their respective measurements was reduced in autumn and winter.
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A case of forecasted and observed CI, GHI, and DNI is presented in Fig. 8, with a 60-min forecast horizon at 0140 UTC 1 December 2018. The CI at 0240 UTC was forecasted from the CI observations and CMV information at 0140 UTC. The estimated GHI and DNI and the CI forecast were used to obtain the GHI and DNI nowcasts at 0240 UTC. The FY-4A-based GHI and DNI nowcasts and the observations had similar spatial distribution patterns. An apparent divergence between forecasts and observations occurred in the red rectangular area with broken clouds (Figs. 8a and 8d) because the CMV derivation using the block-matching method does not consider the formation and dissipation of clouds. Therefore, the CMV forecast method has relatively low predictability under the broken-cloud situation.
Figure 8. An example of FY-4A-based (a–c) nowcasting and (d–f) observation for CI, GHI, and DNI at 0240 UTC on 1 December 2018.
The quality of GHI and DNI nowcasting within 0–3 h ahead, under all-sky conditions, is summarized in Table 2. The statistical indices for 0 min ahead were taken as deviations between the GHI and DNI estimations and the surface measurements. For 0–3-h forecast horizons, the nRMSE values of the GHI and DNI nowcasts increased slowly to 30.8% and 53.4%, respectively, surpassing those at 0 min ahead only by 8.4% and 8.0%, respectively. The nMBE values of GHI and DNI decreased with increasing forecast horizon from 5.9% and 24.1% at 0 min ahead to 0.8% and 18.6% at 3 h ahead, respectively. The GHI and DNI nowcasting methods offset the overestimation of the hybrid estimation method.
SSI Metric 0 min 30 min 60 min 90 min 120 min 150 min 180 min GHI mean 475.9 469.4 479.9 510.9 500.9 494.8 485.6 RMSE 106.6 117.8 125.5 138.5 144.4 143.2 149.5 nRMSE 22.4 25.1 26.2 27.1 28.8 28.9 30.8 MBE 28.0 34.5 28.3 27.2 14.9 18.9 3.7 nMBE 5.9 7.3 5.9 5.3 3.0 3.8 0.8 DNI mean 444.7 430.3 438.0 452.7 447.2 441.7 419.4 RMSE 201.9 207.3 208.3 220.2 222.0 228.3 232.6 nRMSE 45.4 48.1 47.6 48.6 49.6 51.7 53.4 MBE 107.2 105.0 98.5 99.2 87.8 90.8 81.1 nMBE 24.1 24.4 22.5 21.9 19.6 20.6 18.6 Table 2. Statistical indexes (the mean value, RMSE, nRMSE, MBE, and nMBE) under all-sky conditions with forecast horizons from 0–180 min (units: W m–2).
In the FY-4A satellite-based study conducted for Chengde by Yang et al. (2020), the nRMSE values of the GHI and DNI forecasts at 3 h ahead were 36.8% and 58.8%, respectively. The nRMSE of the FY-4A GHI forecasts for Shanghai calculated by Chen et al. (2020) was nearly 60% with a forecast horizon of 3 h. Both of these studies presented higher nRMSE values than the present study, suggesting that the GHI and DNI nowcasting system in this study performed better for forecast horizons up to 3 h. The CMV derivation algorithm used in the reference and our work has a similar performance (Cros et al., 2014). In this study, the correction (the parameterization of FY-4A clear-sky reflectance to SZA before CI calculation) decreases the deviations of the estimations and further improves the performance of the GHI and DNI nowcasting systems.
Figure 9 shows the nRMSE and nMBE values of GHI and DNI nowcasting under four sky conditions with forecast horizons of 0–3 h. The nRMSE values of GHI nowcasting are between 8% and 22% at 30–180 min ahead, and the increase with growing forecast horizon is relatively small under C1, C2, and C3 conditions. The nRMSE values under C4 increased rapidly as the forecast horizon increased—from 44.3% at 30 min ahead to 52.6% at 180 min. The nRMSE of DNI nowcasting exhibits similar trends, but the values are larger. The nRMSE is about 12%–13% under C1 conditions, with the 150-min forecast horizon having the smallest value. For C2 and C3 conditions, the nRMSE values increased with increasing forecast horizons—from 25.4% and 53.1% at the 30-min horizon to 33.1% and 57.9% at the 180-min horizon, respectively. Under the C4 condition, the nRMSE values of DNI show big changes over the forecast horizons of 30–180 min, ranging from 189.5% to 201.5%.
Figure 9. The nRMSE (%) and nMBE (%) of SSI under different sky conditions with forecast horizons from 30–180 min.
The nowcasting system tends to overestimate DNI. The most severe overestimations were under C4, followed by under the C3, C2, and C1 conditions. The nMBE values of DNI showed small changes under all sky conditions, except under C4. The highest nMBE value of DNI was 31.6% at 30 min ahead under cloudy conditions. Note that the nRMSE and nMBE values of DNI nowcasting are significantly higher under C4 conditions than under other sky conditions due to the low DNI.
Fengyun-4 Geostationary Satellite-Based Solar Energy Nowcasting System and Its Application in North China
- Manuscript received: 2021-12-20
- Manuscript revised: 2022-03-04
- Manuscript accepted: 2022-03-17
Abstract: Surface solar irradiance (SSI) nowcasting (0–3 h) is an effective way to overcome the intermittency of solar energy and to ensure the safe operation of grid-connected solar power plants. In this study, an SSI estimate and nowcasting system was established using the near-infrared channel of Fengyun-4A (FY-4A) geostationary satellite. The system is composed of two key components: The first is a hybrid SSI estimation method combining a physical clear-sky model and an empirical cloudy-sky model. The second component is the SSI nowcasting model, the core of which is the derivation of the cloud motion vector (CMV) using the block-matching method. The goal of simultaneous estimation and nowcasting of global horizontal irradiance (GHI) and direct normal irradiance (DNI) is fulfilled. The system was evaluated under different sky conditions using SSI measurements at Xianghe, a radiation station in the North China Plain. The results show that the accuracy of GHI estimation is higher than that of DNI estimation, with a normalized root-mean-square error (nRMSE) of 22.4% relative to 45.4%. The nRMSE of forecasting GHI and DNI at 30–180 min ahead varied within 25.1%–30.8% and 48.1%–53.4%, respectively. The discrepancy of SSI estimation depends on cloud occurrence frequency and shows a seasonal pattern, being lower in spring–summer and higher in autumn–winter. The FY-4A has great potential in supporting SSI nowcasting, which promotes the development of photovoltaic energy and the reduction of carbon emissions in China. The system can be improved further if calibration of the empirical method is improved.
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Keywords:
- Fengyun-4A,
- surface solar irradiance,
- estimates and forecasting,
- cloud motion,
- block-matching
摘要: 碳中和目标背景下,中国未来将显著增加光伏等新能源在能源结构中的占比。光伏太阳能的间接性和不稳定性是太阳能并网利用中的重大挑战之一。除了发展新能源储能等技术之外,发展太阳能短临预报技术是提高太阳能利用率的经济有效途径。我国新一代静止气象卫星风云四号(FY-4A)的发射给太阳能短临预报(<3小时)提供了新的观测手段。本文利用FY-4A多通道反射率数据建立了地表太阳辐照度估算和短临预报系统。该系统由两个关键部分组成,第一部分为基于物理晴天模型和经验云天模型构建的地表太阳辐照度混合估算方法,第二部分为地表太阳辐照度短临预报模型,其核心是通过块状匹配法推导出云运动矢量,进而预报未来3小时内地表太阳辐照度场。该系统目前能够同时实现水平面总辐射和法向直接辐射的估算和短临预报。验证结果表明:水平面总辐射估算值的准确性高于法向直接辐射,二者归一化均方根误差分别为22.4%和45.4%。30-180分钟预测范围内水平面总辐射和法向直接辐射预测值的归一化均方根误差分别在25.1%-30.8%和48.1%-53.4%之间。地表太阳辐照度估算结果的准确性取决于云的出现频率,即春夏较低,秋冬较高。本研究工作表明新一代静止气象卫星在地表太阳辐射短时临近预报中的广阔应用前景,将显著促进我国光伏太阳能能源发展和利用。该系统在华北地区具备良好的性能,未来进一步改进将侧重于对地表太阳辐照度估算模型的校准并推广其在整个中国地区的适用性。