UC San Diego

Clear-sky modeling is critical for the accurate determination of Direct Normal Irradiance (DNI), which is the relevant component of solar irradiance for concentrated solar energy applications. Accurate clear-sky modeling of DNI is typically best achieved through the separate consideration of water vapor and aerosol concentrations in the atmosphere. Highly resolved temporal measurements of such quantities are generally unavailable unless a meteorological station is close. When this type of data is not available, attenuating effects on the direct beam are modeled by Linke turbidity-equivalent factors, which can be obtained from broadband observations of DNI under cloudless skies. We present a novel algorithm that allows for a time-resolved estimation of the average daily Linke turbidity factor from ground-based DNI observations under cloudless skies. This requires a method of identifying clear-sky periods in the observational time series (to avoid cloud contamination) and a broadband turbidity-based clear-sky model for implicit turbidity calculations. While the method can be applied to the correction of historical clear-sky models for a given site, the true value lies in DNI forecasting under cloudless skies through the assumption of persistence of average daily turbidity. This technique is applied at seven stations spread across California, Washington, and Hawaii while using several years of data from 2010 to 2014. Performance of the forecast is evaluated by way of the relative Root Mean Square Error (rRMSE) and relative Mean Bias Error (rMBE), both as a function of solar zenith angle, and benchmarked against monthly climatologies of turbidity information. Results suggest that rRMSE and rMBE of the method are typically smaller than 5\% for both historical and forecasted CSMs, which compare favorably against the 10–20\% range that is typical for monthly climatologies. Clouds significantly attenuate ground-level solar irradiance causing a substantial reduction in photovoltaic power output capacity. However, partly cloudy skies may temporarily enhance local Global Horizontal Irradiance (GHI) above the clear-sky ceiling and, at times, the extraterrestrial irradiance. Such enhancements are referred to here as Cloud Enhancement Events (CEEs). In this work, we study these CEEs and quantitatively assess the occurrence of resulting coherent Ramp Rates (RRs). We analyze a full year of ground irradiance data recorded at the University of California, Merced, as well as nearly five months of irradiance data recorded at the University of California, San Diego, and Ewa Beach, Hawaii. Our analysis shows that approximately 4\% of the data points qualify as potential CEEs, which corresponds to nearly 3.5 full days of such events per year if considered sequentially. The surplus irradiance enhancements range from 18 W m$^{-2}$ day$^{-1}$ to 73 W m$^{-2}$ day$^{-1}$. The maximum recorded GHI of 1,400 W m$^{-2}$ occurred in San Diego on May 25, 2012, nearly 43\% higher than the modeled clear-sky ceiling. Wavelet decomposition coupled with fluctuation power index analysis shed light on the time scales on cloud-induced variability and CEEs. Results suggest that while cloud fields tend to generate variability most strongly at the 30 min time scale, they have the potential to cause CEEs that influence variability on time scales of several minutes. This analysis demonstrates that CEEs are indicators for periods of high variability and therefore provide helpful information for solar forecasting and integration.

Finally, we report on experimental results for natural convection evaporation from a free surface of water into air at low Rayleigh numbers. Experiments were performed for air maintained between 285 K and 310 K, water surface temperatures ranging from 284-308 K, and relative humidity (RH) values ranging from 15-85\%. During an experiment, no external heat was added to the liquid requiring the ambient to provide the energy required for evaporation. A geometry-independent length scale is employed, and we compare results to various other geometries and conditions. The combination of parameters (length scale, temperature, and relative humidity) results in Rayleigh numbers near zero (both positive and negative). Rayleigh numbers near zero have historically been challenging to measure because the driving potentials are relatively small, and several transfer mechanisms are of comparable magnitudes resulting in transfer rates that are generally unstable. Empirical results suggest that two distinct flow regimes exist, which we attribute to the presence or absence of a dominant recirculation zone over the evaporative pool. These distinct flow regimes can be accurately described by the following simple correlation. When a recirculation zone exists the correlation is \begin{equation*} \ShL = 0.179\left(\RaL + 52.5 \right) ^ {1/2}, \end{equation*} and in the absence of a recirculation zone, the correlation is \begin{equation*} \ShL = 0.206\left(\RaL + 55.3 \right) ^ {1/2}, \end{equation*} where $\mathscr{L} = A/P$. A discussion on the effect of flow recirculation zones that lead to the bifurcation of the Sherwood number correlations above is also presented.