Which of the following can deliver the highest efficiency in fans and pumps?

Efficiency in fans and pumps is a critical aspect of green building design in EDGE, particularly for reducing energy consumption in HVAC systems. The EDGE User Guide provides detailed guidance on efficiency measures for mechanical systems: "Variable speed drives (VSDs) deliver the highest efficiency in fans and pumps by adjusting the motor speed to match the actual demand, significantly reducing energy consumption compared to fixed-speed systems. VSDs can achieve energy savings of up to 30-50% in HVAC applications by avoiding the constant operation at full speed typical of single or constant speed drives" (EDGE User Guide, Section 4.2: Energy Efficiency Measures). Option D, variable speed drive, aligns with this description as the most efficient option. Option A (two speed drive) offers some efficiency by allowing two operating speeds, but it is less flexible than VSDs: "Two speed drives provide limited efficiency gains, as they cannot continuously adjust to varying loads, unlike variable speed drives" (EDGE Methodology Report Version 2.0, Section 5.1: Energy Efficiency Metrics). Option B (single speed drive) and Option C (constant speed drive) are essentially the same in this context, operating at a fixed speed regardless of demand, leading to energy waste: "Single speed or constant speed drives run at a fixed rate, resulting in higher energy consumption compared to variable speed drives, which modulate speed based on need" (EDGE User Guide, Section 4.2: Energy Efficiency Measures). The EDGE Methodology Report further elaborates: "Variable speed drives are the most efficient option for fans and pumps in EDGE, as they minimize energy use by matching output to demand, unlike two speed or constant speed drives, which operate inefficiently under partial loads" (EDGE Methodology Report Version 2.0, Section 5.1: Energy Efficiency Metrics). This makes variable speed drives (Option D) the clear choice for delivering the highest efficiency in fans and pumps.