Optimizing the Solar Photovoltaic Energy Capture On Sunny and Cloudy Days Using a Solar Tracking System

Solar energy is a clean, renewable way to provide electrical power, as well as to power the future hydrogen economy. We envision using solar-powered photovoltaic (PV) devices to drive the electrolysis of water to make hydrogen in both centralized and home fueling systems for fuel-cell electric vehicles (FCEVs). In this way the solar energy is stored in the hydrogen. At the GM R&D Center, we have investigated ways to optimize the solar to hydrogen process by photoelectrochemical and photovoltaic-electrolyzer (PV-electrolyzer) systems. By improving the efficiency of the solar-water-splitting process, we can help to bring a solar-powered hydrogen home fueler for closer to reality. Using renewably-generated hydrogen to power future FCEVs can eliminate air emissions from vehicles and contribute to the diversity of the possible sources of hydrogen. PV-electrolyzer systems have only been developed for the purposes of demonstration and proof of concept due to their high system costs, system complexity, and, in some cases, low system efficiencies for generating the high-pressure hydrogen needed for hydrogen storage. Although it is feasible to optimize the efficiency of PV-electrolyzer systems for making hydrogen, there is also a need to improve the individual solar and electrolysis systems. We have determined a method in which a 2-axis solar tracking system is optimized to capture the maximum amount of solar energy on both sunny and cloudy days. Previously, solar tracking systems have only been optimized for sunny days, when direct sunshine that accounts for 85-90% of the total solar radiation, predominates. However, improving the harvesting of solar energy on cloudy days is important to using solar energy on a daily basis for fueling FCEVs because it improves the hydrogen yield on the days with the lowest hydrogen generation, which in turn reduces the system size and cost.

In order to design an optimal solar tracking system for capturing solar energy we have analyzed an extensive set of measurements of the solar irradiance obtained using four identical solar arrays and associated solar sensors (collectively called solar collectors) with different tilt angles relative to the earth's surface. The measurements included a variety of ambient conditions including different seasons and both cloudy and cloud-free conditions. One solar array (and its associated solar sensor), for a period around solar noon, was always approximately pointed directly toward the sun (DTS); it captured the direct component of the solar radiation that predominates on sunny days. On sunny days, solar arrays and sensors with a DTS configuration captured up to twice as much solar energy as solar collectors with a horizontal (H) orientation in which the flat surface of the array was tilted toward the zenith. This finding was in agreement with the cosine response law for flat-surfaced solar collectors. Another set of solar collectors always had an H orientation, and this best captured the diffuse component of the solar radiation that predominates on cloudy days. During cloudy periods, we found that an H configuration increased the solar energy capture by nearly 50% compared to a DTS configuration during the same period. A simple model for diffuse radiation, referred to as the Isotropic Diffuse Model, agrees qualitatively with the angular dependence that we measure for the horizontal/2-axis tracking irradiance ratio on cloudy days. However, it predicts a lesser dependence for the magnitude of this ratio than we measured. We propose three embodiments for an optimized 2-axis tracking system that uses simple algorithms and sensors to determine when it is cloudy, and then orients the solar modules to an H configuration for maximum solar system performance the during periods of cloud cover.

The importance of improving the solar energy output on cloudy days is illustrated in the following scenario for a home hydrogen fueling system. Assume that a solar-powered hydrogen fueling system that is designed to produce, on average, sufficient solar energy to generate 0.5 kg of hydrogen per day via water electrolysis. This amount of hydrogen would be sufficient to for an FCEV on an average daily commute of 30 miles. Typically, solar systems are sized to produce the desired amount of energy on an average solar day for the location of interest. The size (area) of the solar system depends many factors, including the location, the season, and the weather on a given day. The output of solar systems is highly variable; on short, cloudy winter days the system will make much less that the average amount, while even on the longest sunniest summer day it will make no more than twice the average amount. Ideally, the hydrogen storage system must be able to hold enough hydrogen to supply the vehicle for a few bad days in a row ? this is like the autonomy built into an off-grid solar system that uses batteries to store solar energy. Of course the vehicle can occasionally use the centralized hydrogen dispensing system, but this is to be avoided as much as possible to maximize the use (convenience and cost) of the solar home fueling system. Therefore, improving the solar energy capture on the worst days is important to minimize the solar and storage system size and maximize the overall use of the home fueling system.