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ASHRAE Research Project 1309: Developing Solar Radiation Models For Tropical Locations

ASHRAE Research Project 1309

Developing Solar Radiation Models For Tropical Locations

From ASHRAE Journal Newsletter, October 13, 2020

The ability to estimate solar radiation in a certain location is important to solar energy management.

A previously published report for ASHRAE Research Project 1309, Development of Solar Radiation Models For Tropical Locations, provides an overview of existing solar radiation models as well as testing of predictions from selected models against measured data. The report presents the recommended solar model that is suitable for predicting hourly solar radiation in international locations, including tropical climates.

Moncef Krarti, Ph.D., P.E., LEED AP, from University of Colorado Boulder, discusses the research.

1. What is the significance of this research?

The main contribution for this research is providing ASHRAE engineers a simplified calculation method to estimate hourly solar radiation specific to tropical locations. The method is based on weather parameters that are readily available from standard weather reports, including cloud cover, change in dry-bulb temperature over the past three hours, wind speed and relative humidity. Since solar radiation data are not recorded by weather stations, the method developed as part of this research project makes it possible to convert what had been unusable raw weather data into usable weather files suitable for detailed building energy calculations.

2. How does this research further the industry's knowledge on this topic?

The industry has had access to similar solar models developed for North American or European climates. However, when such models were applied to tropical locations, the results were mixed and unreliable. This research was proposed and conducted to address this deficiency by developing a solar model specifically for use in tropical climates.

3. What lessons, facts, and/or guidance can an engineer working in the field take away from this research

Much of the project effort involved evaluating the model against available field measurements and subsequent calibration of the model coefficients. After calibration, the model was found to correlate well with the measured data on an average basis, although instantaneous hourly results can still show significant scatter as documented in the final report as well as publications in the ASHRAE Transactions.

4. Were there any surprises or unforeseen challenges for you when preparing this research?

There were several surprises and unforeseen challenges that were not recognized until subsequent ASHRAE Research Project 1477, Development of Over 2,500 Weather Files for International Locations, used the solar model extensively to create 3,012 ASHRAE IWEC2 weather files for international locations. The main surprise was that a set of coefficients would produce similar characteristic hourly profiles for direct normal solar radiation in different locations, and the unforeseen challenge was the difficulty in producing a direct normal hourly profile on clear days that matched the profile from a dedicated clear sky solar model.

5. What are the next steps to further this research?

While the developed model from Research Project 1309 remains valid as a cloud-based solar model for tropical locations, no further steps are planned for this research. Indeed, cloud-based solar models have been mostly replaced by satellite-derived solar data that have much more extensive coverage and have been shown to be more reliable when compared to detailed ground measurements.