Evaluation of numerical weather prediction for intra-day solar forecasting in the continental United States

Numerical weather prediction (NWP) is generally the most accurate tool for forecasting solar irradiation several hours in advance. This study validates the North American Model (NAM), Global Forecast System (GFS), and European Centre for Medium-Range Weather Forecasts (ECMWF) global horizontal irradiance (GHI) forecasts for the continental United States (CONUS) using SURFRAD ground measurement data. Persistence and clear sky forecasts are also evaluated. For measured clear conditions all NWP models are biased by less than 50 W m⁻². For measured cloudy conditions these biases can exceed 200 W m⁻² near solar noon. In general, the NWP models (especially GFS and NAM) are biased towards forecasting clear conditions resulting in large, positive biases.Mean bias errors (MBE) are obtained for each NWP model as a function of solar zenith angle and forecast clear sky index, kt^∗, to derive a bias correction function through model output statistics (MOS). For forecast clear sky conditions, the NAM and GFS are found to be positively biased by up to 150 W m⁻², while ECMWF MBE is small. The GFS and NAM forecasts were found to exceed clear sky irradiances by up to 40%, indicating an inaccurate clear sky model. For forecast cloudy conditions (kt^∗ < 0.4) the NAM and GFS models have a negative bias of up to −150 W m⁻². ECMWF forecasts are most biased for moderate cloudy conditions (0.4 < kt^∗ < 0.9) with an average over-prediction of 100 W m⁻².MOS-corrected NWP forecasts based on solar zenith angle and kt^∗ provide an important baseline accuracy to evaluate other forecasting techniques. MOS minimizes MBE for all NWP models. Root mean square errors for hourly-averaged daytime irradiances are also reduced by 50 W m⁻², especially for intermediate clear sky indices. The MOS-corrected GFS provides the best solar forecasts for the CONUS with an RMSE of about 85 W m⁻², followed by ECMWF and NAM. ECMWF is the most accurate forecast in cloudy conditions, while GFS has the best clear sky accuracy.     

Annual available radiation for fixed and tracking collectors

Single crystal silicon solar cells are potential elements of large scale solar energy conversion systems. Current costs of these cells are too high at least in part because current production methods require single crystal wafers obtained by slicing cylindrical single crystal ingots. This paper reviews a U.S. research program aimed at reducing the cost of silicon cells by developing new methods of growing silicon ribbons and sheets from which high efficiency solar cells can be fabricated. The paper also describes novel techniques for lower cost processes for ingot growth and wafer slicing which are included in this research and development program.     

A program for calculation of solar energy collection by fixed and tracking collectors

, a realistic and versatile program, has been developed to estimate net solar energy collected by a solar collector per unit collection area. This program was developed to study the properties of various solar collectors. It is made useful to a wide spectrum of users by allowing them to choose any or all of 15 possible solar collector types for calculation and comparison. Additional collectors can be included without undue labor. Either or both of two selective absorbers can be selected for energy collection calculations. allows input for a third selective absorber. is programmed to use solar radiation and surface meteorological data taken from The National Solar Radiation Data Base (NSRDB) for 239 stations over the USA. It can be adjusted to read other data sets. It takes 20 s on a Compaq Presario 2700 1.13 GHz computer to calculate net solar energy collection per unit area for one solar collector design using each of two selective absorbers at five fixed absorber temperatures for all the daylight hours of 1 year at one location. The program output includes sums of solar energy collection for each day, month and year along with averages and distributions. Averages and distributions for the solar radiation and surface meteorological data are also obtained so solar energy collection can be related to these data. can be downloaded from the website: www.sci.sdsu.edu/SOCOL/.     

Some recommendations for inclinations and orientations of building elements under solar radiation in Polish conditions

The paper considers solar energy availability on the different surfaces that constitute the building envelope. The main aim of the presented analysis is to give recommendations for architects to help them design standard and low energy buildings in a proper way, including integration of solar active and passive systems into building structure. This requires maximisation of solar energy gains during certain periods of time, and at other times shading.     

Characteristics of infrared sky radiation in the United States

Extensive measurements of the thermal IR radiance of the sky were obtained at 6 U.S. locations: Tucson, AZ; San Antonio, TX; Gaithersburg, MD; St. Louis, MO; West Palm Beach, FL; and Boulder City, NV. Fifty thousand observations were obtained at half hour intervals during 1979 and 1980. Each observation consists of measurements in 7 spectral bands, wavelengths (in microns) of 8.1–13.7, 8.3–9.1, 9.4–9.9, 10.0–11.4, 14.0–15.8, 16.6–21.6 and 6–17 at zenith angles of 0, 20, 40, 60 and 80°. The data have been expressed and presented as apparent sky emissivities. It is shown that the measured spectral and angular sky emissivities can be reliably estimated from a knowledge of the total (global) sky emissivity, using an empirical “sky emissivity” equation. The results are of particular relevance to the performance of radiative cooling systems designed to make use of spectral and/or angular selectivity.     

Land-use requirements and the per-capita solar footprint for photovoltaic generation in the United States

In this report, we estimate the state-by-state per-capita “solar electric footprint” for the United States, defined as the land area required to supply all end-use electricity from solar photovoltaics (PV). We find that the overall average solar electric footprint is about 181 m² per person in a base case scenario, with a state- and scenario-dependant range from about 50 to over 450 m² per person. Two key factors that influence the magnitude of the state-level solar electric footprint include how industrial energy is allocated (based on location of use vs. where goods are consumed) and the assumed distribution of PV configurations (flat rooftop vs. fixed tilt vs. tracking). We also compare the solar electric footprint to a number of other land uses. For example, we find that the base case solar electric footprint is equal to less than 2% of the land dedicated to cropland and grazing in the United States, and less than the current amount of land used for corn ethanol production.     

Vulnerability of wind power resources to climate change in the continental United States

Renewable energy resources will play a key role in meeting the world's energy demand over the coming decades. Unfortunately, these resources are all susceptible to variations in climate, and hence vulnerable to climate change. Recent findings in the atmospheric science literature suggest that the impacts of greenhouse gas induced warming are likely to significantly alter climate patterns in the future. In this paper we investigate the potential impacts of climate change on wind speeds and hence on wind power, across the continental US. General Circulation Model output from the Canadian Climate Center and the Hadley Center were used to provide a range of possible variations in seasonal mean wind magnitude. These projections were used to investigate the vulnerability of current and potential wind power generation regions. The models were generally consistent in predicting that the US will see reduced wind speeds of 1.0 to 3.2% in the next 50 years, and 1.4 to 4.5% over the next 100 years. In both cases the Canadian model predicted larger decreases in wind speeds. At regional scales the two models showed some similarities in early years of simulations (e.g. 2050), but diverged significantly in their predictions for 2100. Hence, there is still a great deal of uncertainty regarding how wind fields will change in the future. Nevertheless, the two models investigated here are used as possible scenarios for use in investigating regional wind power vulnerabilities, and point to the need to consider climate variability and long term climate change in citing wind power facilities.     

Thermal behavior of curved roof buildings exposed to solar radiation and wind flow for various orientations

In this study air flow, solar radiation and heat transfer from a two dimensional curved roof with north-south and east-west faced are determined and results are compared with flat roof for the same size and orientation. Comparison are performed for their corresponding roof surface temperature, and heat flow for several roof rim angles and also for various wind flow velocities, as well as for different wind directions. Turbulence is modeled by RNG k–ε method and solar radiation distribution over the roof is determined based on an appropriate model applicable to hot arid regions of Iran. Solar radiation is calculated based on the summation of beam and diffuse radiation and ground reflected radiation. For certain inside roof temperature, over all heat transfer to the building is determined with day time for various wind flows and arc shapes and results are compared with flat roof. It was found that various wind flow condition over the vaulted roof makes substantial difference on the convection heat transfer coefficient and finally on the rate of heat transfer to the building with respect to flat roof. Based on heat transfer simulation, roof temperature, heat transfer convection coefficient and heat flow though the vault for different roof arrangement and flat roofs have been determined and advantages of specific vault orientation and wind direction are specified.     

A novel prediction algorithm for solar angles using solar radiation and Differential Evolution for dual-axis sun tracking purposes

This paper deals with a dual-axis sun tracking system for a photovoltaic system. Its trajectories are determined by an optimization procedure. The optimization goal is the maximization of the electrical energy production within a photovoltaic system, by considering the tracking system consumption. The procedure used for determining the tilt angle and azimuth angle trajectories is described as a nonlinear and bounded optimization problem. Since an explicit form of the objective function is unavailable, a stochastic search algorithm called Differential Evolution is applied as the optimization tool. In order to evaluate the objective function, models for calculating the available solar radiation and tracking system consumption are applied together with the efficiencies of solar cells, a DC/DC converter and inverter. A new algorithm is introduced for the time dependent prediction of available solar radiation. It is based on the length of a sunbeam’s path through the atmosphere and the statistical data of a pyranometer measured total and diffuse solar radiation at a given location on the Earth. The optimization bounds are given in the form of angular speed, lower and upper bounds for both angles and angle quantization. The results presented in this paper show, that the optimal trajectories can help to increase the electrical energy production within photovoltaic systems by sun tracking.     

Technical and economic performance of residential solar water heating in the United States

This paper examines the regional, technical, and economic performance of residential rooftop solar water heating (SWH) technology in the U.S. It focuses on the application of SWH to consumers in the U.S. currently using electricity for water heating, which currently uses over 120 billion kWh per year. The variation in electrical energy savings due to water heating use, inlet water temperature and solar resource is estimated and applied to determine the regional “break-even” cost of SWH where the life-cycle cost of SWH is equal the life-cycle energy savings. For a typical residential consumer, a SWH system will reduce water heating energy demand by 50–85%, or a savings of 1600–2600 kWh per year. For the largest 1000 electric utilities serving residential customers in the United States as of 2008, this corresponds to an annual electric bill savings range of about $100 to over $300, reflecting the large range in residential electricity prices. This range in electricity prices, along with a variety of incentives programs corresponds to a break-even cost of SWH in the United States varying by more than a factor of five (from less than $2250/system to over $10,000/system excluding Hawaii and Alaska), despite a much smaller variation in the amount of energy saved by the systems (a factor of approximately one and a half). We also consider the relationships between collector area and technical performance, SWH price and solar fraction (percent of daily energy requirements supplied by the SWH system) and examine the key drivers behind break-even costs.     

A comparison of daily solar radiation estimates for the Northeastern United States using the Northeast Regional Climate Center and National Renewable Energy Laboratory models

Observed solar radiation data at three sites in the northeastern United States are compared with values estimated for nearby airport locations using the National Renewable Energy Laboratory (NREL) and Northeast Regional Climate Center (NRCC) models. A tendency toward considerable overestimation of relatively low values of observed solar radiation is evident in the NREL model. This bias is apparently regardless of season. A similar bias is not detected in the NRCC model. For moderate to high values of solar radiation both models produce estimates with similar accuracy for most practical applications. However, these models both tend to underestimate observed solar radiation on days when near maximum possible radiation levels are received. The tendency for the NREL model to overestimate low solar radiation values appears to be linked to the use of total sky cover, rather than the combination of cloud coverage and cloud base height information. Although total sky coverage data may be superior for estimates of moderate to high daily solar radiation values, it appears that information regarding the height of low overcast layers and the presence of obstructions to visibility, such as fog or haze, is required to accurately estimate low daily solar radiation totals.     

Effect of manual tilt adjustments on incident irradiance on fixed and tracking solar panels

Hourly typical meteorological year (TMY3) data was utilized with the Perez radiation model to simulate solar radiation on fixed, azimuth tracking and two axis tracking surfaces at 217 geographically diverse temperate latitude sites across the contiguous United States of America. The optimum tilt angle for maximizing annual irradiation on a fixed south-facing panel varied from being equal to the latitude at low-latitude, high clearness sites, to up to 14° less than the latitude at a north-western coastal site with very low clearness index. Across the United States, the optimum tilt angle for an azimuth tracking panel was found to be on average 19° closer to vertical than the optimum tilt angle for a fixed, south-facing panel at the same site. Azimuth tracking increased annual solar irradiation incident on a surface by an average of 29% relative to a fixed south-facing surface at optimum tilt angle. Two axis tracking resulted in an average irradiation increase of 34% relative to the fixed surface. Introduction of manual surface tilt changes during the year produced a greater impact for non-tracking surfaces than it did for azimuth tracking surfaces. Even monthly tilt changes only resulted in an average annual irradiation increase of 5% for fixed panels and 1% for azimuth tracked surfaces, relative to using a single optimized tilt angle in each case. In practice, the decision whether to manually tilt panels requires balancing the added cost in labor and the panel support versus the extra energy generation and the cost value of that energy. A Supplementary spreadsheet file is available that gives individual results for each of the 217 simulated sites.     

Solar geometry for fixed and tracking surfaces

A general expression for the solar radiation incidence angle in terms of the slope and surface azimuth is derived for both fixed and tracking surfaces. Using this expression, relationships for the slope and azimuth of optimally tracked one- and two-axis tracking surfaces are developed. This information is necessary in determining incident solar radiation based upon horizontal measurements.     

Experimental and theoretical investigation of global and diffuse solar radiation in the United Arab Emirates

In the present study, the global, direct and diffuse components of solar radiation as well as temperature, relative humidity and wind speed have been continuously monitored and analysed on an hourly, daily and monthly basis. The monthly average daily total solar radiation varies from 2700 W h/m² in December to 8000 W h/m² in June with an average clearness index of 0.65. Experimental data are compared to the predictions of different theoretical models as functions of declination and hour angles. Correlations are obtained describing the variation of hourly, daily and monthly averages of total and diffuse solar radiation using polynomial expressions. Empirical correlations describing the dependence of the daily average diffuse to total radiation ratio on the clearness index are also obtained. Data for the daily diffuse to total radiation ratio are compared to correlations obtained by other investigators. The comparison shows a reasonable agreement with some scatter due to the seasonal dependence of the correlation. Comparison of calculations with experimental measurements under clear sky conditions show excellent agreement with a maximum error of 8%.     

Supporting solar power in renewables portfolio standards: Experience from the United States

Renewables portfolio standards (RPS) have become an increasingly popular option for encouraging the deployment of renewable electricity. It is a relatively new policy mechanism, however, and experience with its use is only beginning to emerge. One key concern is whether RPS policies offer adequate support to a wide range of renewable energy technologies and applications or whether, alternatively, they will favor a small number of the currently least-cost forms of renewable energy. This article documents the design of and early experience with state-level RPS programs in the United States that have been specifically tailored to encourage a wider diversity of renewable energy technologies, and solar energy in particular. As shown here, state-level RPS programs specifically designed to support solar have already proven to be an important driver for solar energy deployment, and those impacts are projected to build in the coming years. State experience in supporting solar energy with RPS programs is mixed, however, and full compliance with existing requirements has not been achieved. The comparative experiences described herein highlight the opportunities and challenges of applying an RPS to specifically support solar energy, as well as the importance of policy design details to ensuring that program goals are achieved.     

Exploring the impact of permitting and local regulatory processes on residential solar prices in the United States

This article statistically isolates the impacts of city-level permitting and other local regulatory processes on residential PV prices in the United States. We combine data from two “scoring” mechanisms that independently capture local regulatory process efficiency with the largest dataset of installed PV prices in the United States. We find that variations in local permitting procedures can lead to differences in average residential PV prices of approximately $0.18/W between the jurisdictions with the least-favorable and most-favorable permitting procedures. Between jurisdictions with scores across the middle 90% of the range (i.e., 5th percentile to 95th percentile), the difference is $0.14/W, equivalent to a $700 (2.2%) difference in system costs for a typical 5-kW residential PV installation. When considering variations not only in permitting practices, but also in other local regulatory procedures, price differences grow to $0.64–$0.93/W between the least-favorable and most-favorable jurisdictions. Between jurisdictions with scores across the middle 90% of the range, the difference is equivalent to a price impact of at least $2500 (8%) for a typical 5-kW residential PV installation. These results highlight the magnitude of cost reduction that might be expected from streamlining local regulatory regimes.     

A preliminary assessment of avian mortality at utility-scale solar energy facilities in the United States

Despite the benefits of reduced toxic and carbon emissions and a perpetual energy resource, there is potential for negative environmental impacts resulting from utility-scale solar energy (USSE) development. Although USSE development may represent an avian mortality source, there is little knowledge regarding the magnitude of these impacts in the context of other avian mortality sources. In this study we present a first assessment of avian mortality at USSE facilities through a synthesis of available avian monitoring and mortality information at existing USSE facilities. Using this information, we contextualize USSE avian mortality relative to other forms of avian mortality at 2 spatial scales: a regional scale (confined to southern California) and a national scale. Systematic avian mortality information was available for three USSE facilities in the southern California region. We estimated annual USSE-related avian mortality to be between 16,200 and 59,400 birds in the southern California region, which was extrapolated to between 37,800 and 138,600 birds for all USSE facilities across the United States that are either installed or under construction. We also discuss issues related to avian–solar interactions that should be addressed in future research and monitoring programs.     

Nuclear energy for the United States

The historical trend toward increasing electrification is expected to continue, requiring substantial increases in U.S. electric power generating capacity. Nuclear power and coal are expected to be the only alternatives capable of making a major contribution to meeting this demand for the next several decades. This paper examines what nuclear could do to assure the United States of adequate supplies of energy at reasonable prices through the turn of the century and beyond. The approach used was to determine how rapidly nuclear generating capacity could be expanded if a national commitment was made to solve the licensing, regulatory and political problems which are currently discouraging utilities from making further nuclear commitments. It was concluded that a total of 550 GWe of nuclear capacity could be in operation by the year 2000. Achieving this desirable goal would require a strong commitment by government and industry to work together to replace the current adversary relationship with one of mutual cooperation.     

The costs and benefits of large-scale solar photovoltaic power production in Abu Dhabi,United Arab Emirates

The potential for a 10 MW photovoltaic power plant in Abu Dhabi is examined in this paper using RETScreen modeling software to predict energy production, financial feasibility and GHG emissions reductions. Initial results show high energy production potential, generating 24 GWh and saving over 10,000 tons of GHG emissions annually, but poor financial prospects yielding a net present value (NPV) of ?$51 million. Benefits of reducing GHG and air pollution emissions by replacing natural gas with PV generation are calculated to have a net present value of $47 million, with a large range of possible values. Results show that the high initial costs and low expected price for electricity generated are driving reasons why photovoltaic systems are not being implemented in Abu Dhabi. A feed-in tariff rate of $0.16/kWh is recommended to make large-scale PV systems profitable.