The load following strategy is a dispatch strategy whereby whenever a generator operates, it produces only enough power to meet the primary load. Lower-priority objectives such as charging the storage bank or serving the deferrable load are left to the renewable power sources. The generator may still ramp up and sell power to the grid if it is economically advantageous.
Under the load following strategy, HOMER dispatches the system's controllable power sources (generators, grid, storage bank) so as to serve the primary load and the thermal load at the least total cost each time step, while satisfying the operating reserve requirement. The total cost includes the cost of fuel, operation and maintenance, and replacement. To accomplish this, HOMER calculates the fixed and marginal cost of each dispatchable power source:
•A generator's fixed cost is equal to its hourly operation and maintenance cost plus its hourly replacement cost plus the cost of its no-load fuel consumption. Its marginal cost is equal to its fuel curve slope times the fuel price. If waste heat can be recovered from the generator and the waste heat is needed to serve the thermal load, the generator's marginal cost is reduced by the value of the thermal energy it produces (which is equal to the marginal cost of thermal energy from the boiler). If a cost is assigned to carbon emissions, the generator's marginal cost is increased accordingly.
•The storage bank's fixed cost is zero and its marginal cost is equal to the storage wear cost.
•The grid's fixed cost is zero and its marginal cost is equal to the grid power price. If a cost is assigned to carbon emissions, the grid's marginal cost is increased accordingly.
Once it characterizes each dispatchable source in this way, HOMER searches for the combination of generation sources that satisfies the primary load, required operating reserve, and thermal load at least cost.