As the name implies, the energy can be estimated at different levels, and that depends on the application intended.
For example, Grid-connected PV systems are more flexible to solar energy intermittence since the utility grid is used as a backup source when more energy demand arises. With Stand-alone PV systems, since energy is delivered instantaneously to the load without utility grid backup, the load requirements are less flexible to energy supply. That is a consideration when sizing PV systems.
For all PV systems, the main sizing factor is energy consumption. That can be estimated or calculated depending on the energy data availability. The following scenarios summarize types of data provided to the PV designers before sizing the PV system:
- If the client can provide 12-month energy bills, the designer can then evaluate these bills to come up with the average monthly energy consumption and total annual energy consumption.
- If the client doesn’t have all 12 month bills but he/she can provide energy bills of some months, the designer should come up with a creative way to estimate the total annual energy based on the provided data.
- If the client doesn’t have any energy bill records at all, the designer should then run the estimation based on the client’s energy loads to come up with the total annual energy consumption. That is referred to as load analysis and it will be discussed in this topic.
As seen earlier, energy estimation can be as easy as reading an energy bill, or it can be as challenging as running a load analysis. How do we go about it in either case?
Analyzing Energy Bill:
The total energy consumed can be calculated by using all monthly energy bills (in kWh) for the entire year. This is considered the most accurate method to estimate the average monthly and total annual energy consumption based on real data provided by the client. However, this method doesn't provide consumption detail such as daily energy demand or hourly energy demand since the energy readings on the monthly bill are usually taken once a month.
In order for a designer to learn more about the daily or hourly energy demand of a property, more detailed calculations are required to achieve that task and that is usually done at the load level. The hourly and daily consumption can be measured for any property; however, this method requires using energy analyzing devices at the meter side for a significant period of time. This method will generate the most accurate energy consumption data. However, it requires more time and budget.
Since power demand is usually stated on the rating of each device/appliance individually, there should be an easier way to estimate daily energy demand. We learned in "Electricity Basics" in the Orientation that power is not the main sizing parameter since the same load can consume zero energy if it is turned-off or it can consume a lot of energy when it is turned-on for a long period. Energy consumption is based on the power demand over a period of time.
Since most loads don’t run continuously for the entire day, operating time is another parameter that should be considered when estimating the total energy consumption of a device.
When running load analysis calculations, it should be noticed that there are two types of loads:
- AC loads that requires AC power to run. (Note: this is what most of our loads are.)
- DC loads that can be run directly from the PV system.
Power generated from the PV array is DC power, and in case there are AC loads, power conditioning units are used as described in previous lessons. Since PCU is not 100% efficient, it will consume some energy. That should be added to the load analysis as an additional energy usage. The energy consumption of an inverter is estimated by its conversion efficiency.
As a result, the total required energy that should be provided by the PV array is calculated as follows:
Esdc is the required daily system DC (Wh/day)
Eac is the AC energy consumed by load (Wh/day)
Eff is the inverter efficiency
Edc is the DC energy consumed by loads (Wh/day)
To learn more about load analysis, please refer to the required reading of Chapter 9 in the text.