Building A Truly Wireless Data Communications Solution Part 2


Shared on October 02, 2012 written by USAT Corp

Part 2: Solar Irradiance Across the USA

We are sometimes asked, how many milliamps will the Raven or GX-440 modem draw so I can size the panel. That is but one piece of the model required for building a solar powered system. USAT Corp. often speaks to the 'nuances' of a solar system design. A 140 watt panel is not a 140watt fixed "generator", it is a 140 watt rating that then must be derated and modified based on numerous environmental characteristics.

Environmental characteristics are difficult to simply assign a percent 'derate' or deduction to a panel's output. A great example is looking at the variance in solar irradiance across the country—how much bright sun you can count on per day in various locations, and at various time of year. For an off-grid solar setup, we are most concerned with the least productive solar day and least productive month, thereby identifying the period of slowest recovery of the battery levels. Identifying the least productive month can be contingent on panel angle and other variables. For this illustration we drew up at USAT Corp., let's look at the month of December. For this we will assume a constant 'typical' setting for a number of variables such as array azimuth (where it is facing), using the latitude to set the fixed array tilt, fixing a derate amount that is identical with each setup, and so forth.

NREL, or National Renewable Energy Labs, provides location based research that is vital to developing a truly wireless installation. Taking a few example cities, we can see the impact of location on solar radiation. This measurement provides a vital insight into the variability of productivity which we need to take into account when designing systems.

Las Vegas produces almost double the output from the same panel in December.

This illustration bespeaks the nuances of solar system design. Solar radiance levels don't follow an easy latitudinal threshold as one might hope. For example Houston, Texas at 29.98° N latitude to Las Vegas's 36.08° N latitude, for reference, averages 3.34 kWh per day (kWh/m 2/day) of solar radiance to Las Vegas's 5.05 kWh per day. Despite its relative longitudinal proximity, the city closer to the equator produces less solar energy in December.

Below is an illustration showing solar resource across the country during December as compiled by NREL's statistical analysis.

Solar Resources Across The Country

USAT does the research and modeling when considering system panel sizing and battery storage sizing to ensure that the cellular wireless modems attached work every day they are deployed as expected.

Keywords: USAT Tech Talk, USAT M2M Solutions, Wireless Data Communications

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