Multi-skill project scheduling problem under time-of-use electricity tariffs and shift differential payments

In real-world environments, project activities may be performed using multi-skill resources. When both manpower and equipment are needed to perform project activities, wage and energy costs may form the main components of total project cost. Most energy-consuming activities do not need to be performed at specific times, and regarding time-of-use electricity tariffs may be performed during off-peak hours to reduce both energy cost and power peak demand. Performing such activities in off-peak hours obliges employers to pay shift differentials which increases the wage cost. Since energy and wage costs move in opposite directions, a model that optimally schedules activities, such that the total cost (sum of energy and wage costs) is minimized is of particular importance from the demand side viewpoint. The present paper presents a bi-objective optimization model for the multi-skill project scheduling problem considering shift differential payments and time-of-use electricity tariffs, aiming to minimize project total cost and completion time. The proposed model is solved using multi-objective decision making techniques. Results show that taking into account both time-of-use electricity tariffs and regular/overtime payments not only reduces energy cost and total project cost, but also reduces the power peak demand, thereby benefiting power distribution networks as well.