The InfoWater Pro energy management simulator calculates pump energy charges based on user-specified energy charge rates, one-time demand charges for maximum power consumption and pump efficiencies for any given hydraulic conditions within the network. Such calculations can be used to establish energy-efficient and cost-effective pump operation.
The energy charge rate is defined as the charging rate per unit of energy usage ($/kWh). Demand (or capacity) charge is defined as the cost associated with the maximum power consumed within the charging period ($/max. kW). This is a one-time charge due to peak power used during each billing period. Both charging rates are represented as a time-varying pattern (constant or variable rate schedule) for the simulation period. The pump efficiency data describe the relationship between the overall pump efficiency as a function of flow and are expressed as a percentage. The overall pump efficiency is the wire-to-water (total) efficiency, which is the product of the pump efficiency and driver efficiency. This overall efficiency is used in calculating the energy cost of pumping water.
The following figure illustrates an example relationship between average energy cost and energy usage during a 24-hour extended period simulation:
Two options are available for specifying pump efficiency data. A constant efficiency or multiple efficiency-flow operating data points can be specified for any pump. In the latter case, when only three data points are provided, InfoWater Pro fits a quadratic curve to this data of the form:
e = a + bq + cq2
Where: the coefficients a, b, and c are determined by the program. When more than three points of efficiency-flow data are provided, InfoWater Pro uses linear interpolation to calculate pump efficiency.
If the pump is operating outside the flow range specified, that is above the largest flow data point or below the smallest flow data point, and then a constant efficiency is used for all cost calculations in this flow region. This efficiency is set equal to the efficiency associated with the closest operating flow. This gives a realistic description of the pump operation in the extended flow range. The figure below illustrates this situation.
Since pumps can operate at different operating points during the course of a simulation period, InfoWater Pro calculates the power used at each operating point (simulation time step) to determine power cost. At any operating point, the useful power, pu (hp), can be computed from:
Where: q is the flow rate (cfs), h is the pump head (ft). The required power consumption, pc (kW), is determined from:
Where: e is the overall pump efficiency for the operating point in fractional form (<1.0). For each pump, the cost of operation can then be computed for each hydraulic time step of the simulation period.