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Solar radiation data are available for many locations on an hourly basis. Simulation studies of solar energy systems have generally used these hourly values to estimate long-term annual performance, although solar radiation can exhibit wide variations during an hour. Variations in solar radiation during an hour, such as on a minute basis, could result in inaccurate performance estimates for systems that respond quickly and non-linearly to solar radiation. In addition, diffuse fraction regressions and cumulative frequency distribution curves have been developed using hourly data and the accuracy of these regressions when applied to short-term radiation has not been established. The purpose of this research is to investigate the inaccuracies caused by using hourly rather than short-term (i.e., minute and 3 rain) radiation data on the estimated performance of solar energy systems. The inaccuracies are determined by examination of the frequency distribution and diffuse fraction relationships for short-term solar radiation data as compared to existing regressions and by comparing calculated radiation on tilted surfaces and utilizability based on hourly and short-term radiation data. Keywords: Solar radiation; Short-term; Minute; Tilted; Diffuse

The clearness index for hourly and daily radiation is an important parameter in describing solar radiation. Liu and Jordan demonstrated that the monthly average daily clearness index could be used to predict the long-term distribution of daily solar radiation in a month. This paper reviews recent literature on the prediction of hourly and daily frequency distributions and cumulative frequency distributions of clearness indices. Ten years of measured weather data for six cities in the U.S. are used to investigate the nature of the hourly and daily frequency distributions. A second set of ten years of data for six cities is used to verify the predictions. A bi-exponential probability density function is proposed that fits the observed bimodal nature of the data better than existing models. A case is made for the function being universal. [DOI: 10.1115/1.1445443]

The National Institute of Standards and Technology Net-Zero Energy Residential Test Facility is a highly instrumented, highly configurable, single-family, net-zero energy house occupied by a virtual family of four. A detailed transient model of the Net-Zero Energy Residential Test Facility and the accompanying mechanical equipment was created using information available before construction; the model incorporated building geometric details and construction material properties, as well as manufacturers’ specifications for HVAC, water heating, solar photovoltaic and other equipment. This model represents the typical design paradigm, where actual building performance and detailed equipment operation are not known. This original model underpredicted the measured annual energy consumption by 13.8%. The measured data were used to understand and correct the sources of error at the component level; modifications to the heating, ventilation, and air-conditioning system, interior thermal capacitance, and domestic hot water system improved the energy consumption prediction to within 1.6% of measured data. The differences between the original and modified models are useful for understanding the sources, magnitudes, and possible corrections to errors in energy models for high-efficiency residences. The modified model will be used in future studies of alternative energy system configurations and control strategies, contributing to cost-effective and optimum design of net-zero energy houses in America. [ABSTRACT FROM AUTHOR]

The performance of an air breathing proton exchange membrane (PEM) fuel cell stack has been experimentally measured to investigate the steady-state and transient effects of temperature, humidity and air flowrate. The results show that hydrogen leaks to the cathode through the membrane causing internal heating of the fuel cell. The leakage rate is found to be linearly dependent on the pressure difference between the hydrogen side and air side which is at atmospheric pressure. Temperature was found to not have a significant effect on the PEM performance, except through its indirect effect on humidity. The humidity of the membrane is found to be the most significant variable in determining the fuel cell performance. The airflow also influences the performance of the fuel cell directly by supplying oxygen and indirectly by influencing the humidity of the membrane. Experiments show that an optimum air flowrate exists that is much larger than required for stoichiometric oxidation of the fuel. [DOI: 10; 1115/1.1385202]