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HOMER Pro 3.10

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Deferrable Load

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Important: Deferrable Load is part of the Advanced Load module. See Adding Modules for more information on purchasing module.

A deferrable load is an electrical load that requires a certain amount of energy within a given time period, but the exact timing is not important; it can wait until power is available. Loads are normally classified as deferrable when they are associated with storage. Water pumping is a common example—there is some flexibility as to when the pump actually operates, provided the water tank does not run dry. Other examples include ice making and storage charging.

Click the Deferrable button to open the Deferrable Load page. The descriptive name, Deferrable Load, is a label used to identify the deferrable load in the schematic (instead of using the abbreviation).

shots_deferrable-load-label

Monthly Average Values

The baseline data is the set of 12 values representing the average deferrable load, in kWh/day, for each month of the year. The average deferrable load is the rate at which energy leaves the deferrable load storage tank. It is also the amount of power required to keep the level in the storage tank constant.

Enter the average deferrable load for each month of the year in the Enter Monthly Averages table on the left. HOMER assumes that the deferrable load is constant throughout each month. HOMER calculates the resulting annual average deferrable load and displays it below the table. The monthly average values are displayed in the deferrable load graph as you enter them.

Scaled Data for Simulation

HOMER scales the baseline deferrable load data for use in its calculations. To scale the baseline data, HOMER multiplies each of the 12 baseline values by a common factor that results in an annual average value equal to the value that you specify in Scaled Annual Average.

shots_annual_average

To determine the value of this factor, HOMER divides the scaled annual average by the baseline annual average. The scaled data retains the seasonal shape of the baseline data, but may differ in magnitude. The default value for the scaled annual average is the baseline annual average. When the two values are equal, the scaled data and baseline are identical. HOMER interprets a scaled annual average of zero to mean that there is no deferrable load.

You can use the scaled annual average to perform a sensitivity analysis on the size of the deferrable load.

Other Inputs

Variable

Description

Storage Capacity

The size of the storage tank, expressed in kWh of energy needed to fill the tank.

Peak Load

The maximum amount of power, in kW, that can serve the deferrable load. In a water pumping application, it is equal to the rated electrical consumption of the pump.

Minimum Load Ratio

The minimum amount of power that can serve the deferrable load, expressed as a percentage of the peak load. In a water pumping application, if the pump is rated at 0.75 kW and requires at least 0.5 kW to operate, the minimum load ratio is 67%.

Electrical Bus

Specifies whether the deferrable load must be served by alternating current (AC) or direct current (DC) power.

The deferrable load is second in priority after the primary load, but ahead of charging the batteries. Under the load following strategy, HOMER serves the deferrable load only when the system is producing excess electricity or when the storage tank becomes empty. Under the cycle charging strategy, HOMER also serves the deferrable load whenever a generator is operating and able to produce more electricity than is needed to serve the primary load.

Regardless of dispatch strategy, when the level of the storage tank drops to zero, the peak deferrable load is treated as a primary load. The dispatchable power sources (generator, grid or storage bank) then serve as much as possible of the peak deferrable load.

Example: Each day, 4.5 m3 of water is needed for irrigation. There is an 18 m3 water tank. At full power, the pump draws 400 W of electrical power and pumps 3 m3 per hour. To model this situation using HOMER:

The peak deferrable load is 0.4 kW, which is the rated power of the pump.

It would take the pump 6 hours at full power to fill the tank, so the storage capacity is 6 hours times 0.4 kW, which is 2.4 kWh.

It would take the pump 1.5 hours at full power to meet the daily requirement of water, so the average deferrable load is 1.5 hours per day times 0.4 kW, which is 0.6 kWh/day.

Note: To the right of each numerical input is a sensitivity button (shots_sensitivity-button) that allows you to do a sensitivity analysis on that variable. For more information, see Why Would I Do a Sensitivity Analysis?