Energy Efficiency, Auditing and Demand-Side Management
Domain 4 tests whether you can find, quantify and verify savings without double counting. The examiner looks for a clear audit boundary, a defensible baseline, correct use of part-load laws and awareness of Hong Kong’s statutory audit regime.
Core rules
- Audits range from a walk-through survey to a detailed audit and an investment-grade study. Define the boundary (meter, plant, building, portfolio), collect at least 12 months of data, and normalise for weather, occupancy and production.
- A baseline must be adjusted before savings are claimed. Use regression of energy against a driver such as cooling degree-days or output; report uncertainty and the measurement and verification option (IPMVP Options A–D).
- For fans and pumps, the affinity laws give flow ∝ speed, head ∝ speed² and power ∝ speed³, but only where system curve and static head allow. A 20% speed reduction can cut power by about 49%, not 20%.
- Demand-side management includes peak clipping, load shifting, valley filling and strategic conservation. Power-factor correction reduces current and losses but not the real kWh consumed.
- Under the Energy Efficiency Ordinance (Cap. 610), prescribed commercial buildings must undergo an energy audit every 10 years and implement outlined energy-management opportunities; the Building Energy Code sets design requirements for new and altered installations.
Worked example
A 75 kW fan runs 4,000 hours/year at full speed. Throttling reduces flow to 80%. With a VSD and a system dominated by friction, power at 80% speed is 75 × 0.8³ = 38.4 kW. Saving = 75 − 38.4 = 36.6 kW; annual energy saving = 36.6 × 4,000 = 146,400 kWh. At HK$1.20/kWh, saving = HK$175,680/year. A HK$300,000 retrofit has a simple payback of 1.7 years. If static head is significant, the cube law overstates savings, so check the system curve.
Traps
Claiming speed savings without checking static head; comparing post-retrofit consumption with an unadjusted baseline; counting the same saving in two measures; ignoring interactive effects; and treating power-factor correction as an energy-saving measure.