Taking a product off-grid and knowing that it will continue to work year after year at the highest level is no simple task.
Understanding the power budget of your system is crucial to creating a successful solar-powered product.
A power budget consists of the following pieces: daily device power consumption, storage element, self-discharge, power converter efficiencies, available illumination, charger efficiency, and tolerance to downtime/factor of safety.
Daily Device Power Consumption
As we all know, the sun rises and sets each day. This provides a daily cadence for studying device power consumption. We will look at device power consumption based on Watt-Hours per day.
To determine the number of Watt-Hours of power required to operate the device using the following equation:
Storage Element Self-Discharge
In addition to the power consumed by your device, there will also be some parasitic discharge from the storage element. This discharge is based on the type of storage element and the capacity of the storage element.
Most self-discharge is a percentage of the capacity per day. Discharge rates are also strongly dependent on ambient temperature. Discharge rates will increase with temperature.
Below is a table of discharge rates for common types of storage elements. Estimates of self-discharge may or may not be important depending on the application.
Self-discharge is important for low-power draw applications or applications where a device may be dormant for extended periods of time.
Power Converter Efficiencies
When a device requires an input voltage that must be significantly different than the storage element’s voltage or stable over the entire range of a storage element, a power converter must be placed between the storage element and the device.
For example, if a device operates on 5V, but is connected to a 12V battery, then a 12V to 5V buck converter will need to be placed between the battery and the device. Converter efficiency can be measured by operating the system and measuring the voltage and current at the input and output of the converter.
The storage element and/or the solar panel supply the input power. The output is the power consumed by the device to be powered. To calculate the converter efficiency, use the following equation:
Now that the converter efficiency is known, it needs to be accounted for in the daily device power consumption calculation.
This can be done in one of two ways. The first way is using the original watt-hour equation and substituting the input voltage and current values.
The second way is to use the following equation:
Available Illumination
Unfortunately, outdoor light is rarely consistent.
It varies widely based on cloud cover, upper atmosphere particulates (due to things like forest fires), reflection from ground cover (such as snow), facing south vs. north, and location of the sun in the sky (time of day/year).
This makes for a very dynamic system to design within. All of these factors will shift both the solar spectrum and the intensity.
Regional average solar illumination data can be found using NREL’s PV Watt calculator.
This calculator is a great tool to estimate illumination in the area you plan to use your solar product if your product is deployed outdoors in an area that is not shaded. When sizing a system for continuous year-round operation, use the information for the worst-case month of the year.
If the product is deployed indoors or in a shaded area, a more in-depth look at hours of illumination and intensity will need to be performed.
Charger Efficiency
In many cases, a charge controller will be necessary for prolonged battery life and safety. There are special cases where a charge controller is not necessary.
When charging a lead acid battery, if the charge current in amps is less than 1/100th the amp hour capacity, a charge controller will not be needed.
Adding a charge controller to the system will introduce another parasitic loss. Charge controller efficiencies range from 60-95%.
The panel size will need to be increased to account for this efficiency loss. Calculating the size of a panel will be discussed in the next section.
Tolerance to Down Time/Factor of Safety
The available illumination, especially in outdoor applications, assumes the panel will see the average solar radiation daily to maintain operation with no disruption.
Realistically there will be days overcast and sunny days, so a battery storage system will need to be large enough to provide power during extended periods of low solar radiation.
This means you will need to use your calculated average daily power consumption and multiply it by the desired number of run days between charges.
Then choose a storage element with a total watt-hour capacity close to that. Next, the panel must be large enough to recharge the storage system during good solar radiation.
To effectively charge the storage element, the solar panel must produce enough power to overcome the battery self-discharge, the daily power consumption, and an additional amount based on the amount of time required to charge a fully depleted system.
This calculation is shown below:
As you can see, developing and successfully executing a power budget requires attention to detail and extensive planning and execution.
Now that you know how to determine your power budget, the panel size, and the storage element to meet that budget, you are well on your way to a successful solar product. In our next post, we will apply all the elements we have discussed to an example application.
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