HKIE Professional Assessment - Energy Discipline cheat sheet

Energy is conserved (First Law); exergy is destroyed by irreversibility, so exergy analysis locates true inefficiency where energy balances show none.

Primary energy to delivered energy: conversion chain losses (generation, transmission, distribution) mean site electricity typically carries ~2.5-3x the primary energy of a direct fuel use.

SI units: 1 kWh = 3.6 MJ; 1 tonne oil equivalent (toe) = 41.868 GJ; 1 tonne coal equivalent (tce) = 29.308 GJ.

Power vs energy: kW is instantaneous rate, kWh is energy over time; demand charges in tariffs are set by maximum kW, consumption charges by kWh.

Capacity factor = actual annual energy output / (rated capacity x 8760 h). Typical: nuclear ~90%, offshore wind 40-50%, solar PV 12-20% in Hong Kong latitude.

Load factor = average load / peak load over a period; a low load factor signals peaky demand and poor asset utilisation, raising unit cost.

Energy return on investment (EROI) = energy delivered / energy invested over lifecycle; EROI below ~3 is generally judged unable to support modern society.

Sankey diagrams obey conservation at every node; the width of each flow is proportional to energy rate, so branch widths must sum to the input.

First Law for a steady-flow open system: Q - W = m[(h2 - h1) + (V2^2 - V1^2)/2 + g(z2 - z1)]; neglect kinetic and potential terms for most plant calculations.

Carnot efficiency = 1 - Tc/Th with absolute temperatures (K). It is an upper bound for any heat engine operating between those reservoirs; no real cycle reaches it.

Second Law: entropy of an isolated system never decreases; for a control volume, entropy generation S_gen = sum(m_out s_out) - sum(m_in s_in) - sum(Q/T_boundary) >= 0.

Rankine cycle efficiency is improved by raising boiler pressure/temperature, lowering condenser pressure, and adding feedwater heating (regenerative) or reheat.

Conduction: Fourier's law q = -kA dT/dx; plane wall resistance R = L/(kA); cylindrical wall R = ln(r2/r1)/(2 pi k L).

Convection: Newton's law q = hA(Ts - Tf); Nusselt number Nu = hL/k links convection to conduction; Nu = C Re^m Pr^n for forced convection correlations.

Radiation: q = epsilon sigma A (Ts^4 - Tsurr^4), sigma = 5.67e-8 W/m2K4. Radiation dominates at high temperature and is independent of air movement.

Overall heat transfer coefficient: 1/UA = 1/(h1 A1) + R_wall + 1/(h2 A2); the largest resistance controls the design and the fouling allowance.

Heat exchanger effectiveness NTU method: effectiveness = actual heat transfer / maximum possible; for counterflow, higher NTU and higher Cr ratio give higher effectiveness.

Rankine cycle: pump work is small relative to turbine work; cycle efficiency = (W_turbine - W_pump)/Q_in. Superheat and reheat raise both efficiency and exhaust dryness fraction.

Gas turbine Brayton cycle: efficiency depends mainly on pressure ratio and turbine inlet temperature; simple-cycle efficiency ~35-40%, combined cycle (CCGT) 55-62%.

Combined cycle: exhaust gas from the gas turbine generates steam in an HRSG for a bottoming steam cycle; overall efficiency is roughly the sum of the two cycles minus losses.

Diesel engine indicated thermal efficiency = brake thermal efficiency / mechanical efficiency; specific fuel consumption (g/kWh) = 3.6e6 / (LHV in MJ/kg x efficiency).

Boiler efficiency by indirect (loss) method: efficiency = 100 - (dry flue gas loss + moisture loss + unburnt carbon + radiation/convection loss). Flue gas loss dominates; each 20 C reduction in stack temperature saves about 1% fuel.

Steam turbine heat rate (kJ/kWh) = 3600 / cycle efficiency; a lower heat rate means better plant performance and is the standard performance-test metric.

Cooling: condenser vacuum is set by cooling water temperature plus terminal temperature difference; a 1 C rise in condenser pressure degrades output by roughly 0.5-1% for a steam turbine.

Combustion stoichiometry: for a hydrocarbon CxHy, theoretical air moles O2 = x + y/4 per mole fuel; excess air ~3-10% for gas, 10-20% for oil, 20-40% for coal in practice.

Plant auxiliaries (ID/FD fans, pumps, mills) consume 5-10% of gross output; net output = gross output - auxiliary load, and net efficiency is the figure used in performance guarantees.

Solar PV: module rated at STC (1000 W/m2, 25 C, AM1.5). Power temperature coefficient is typically -0.3 to -0.45 %/K, so cell temperature above 25 C reduces output.

PV yield estimate: annual kWh = installed kWp x peak sun hours x performance ratio (PR). PR of 0.75-0.85 accounts for temperature, soiling, inverter and wiring losses.

Wind power in the swept area: P = 0.5 rho A V^3 Cp. Betz limit Cp = 16/27 = 0.593; modern turbines achieve Cp 0.35-0.45 and cut-in/cut-out typically 3-4 m/s and 25 m/s.

Wind energy is proportional to the cube of wind speed, so a 10% speed error gives about a 33% energy error; site wind measurement campaigns of at least 12 months are standard.

Solar thermal: flat plate collectors suit low temperature (<80 C); evacuated tube and concentrating collectors reach higher temperatures; efficiency falls as the collector-to-ambient temperature difference rises.

Hydropower: P = rho g Q H eta; small hydro needs head and flow data over a full hydrological year, and design flow is usually set near the flow exceeded 30-40% of the time.

Biomass and waste-to-energy: net calorific value on a wet basis is lower than dry basis; moisture content directly reduces combustion temperature and boiler efficiency.

Grid integration: variable renewables require firm capacity, flexibility and curtailment rules; Hong Kong's CLP and HK Electric schemes set feed-in tariff rates by technology and capacity band.

Hydrogen: electrolysis efficiency 60-80% (LHV); round-trip efficiency via fuel cell is typically 30-40%, so hydrogen suits long-duration or hard-to-abate uses rather than routine storage.

Energy audit hierarchy: preliminary (walk-through) identifies saving opportunities; detailed audit measures and verifies them with sub-metering, trend logs and regression against production or weather.

Energy baseline: use regression of consumption against drivers (degree days, tonnes produced, occupancy). Adjust baseline for changes in conditions before claiming savings (IPMVP Option C whole-facility or Option B retrofit isolation).

Simple payback = capital cost / annual saving. Life-cycle cost and NPV must be used where equipment life exceeds payback by a wide margin; include maintenance and energy escalation.

Load profile analysis: coincident peak and load duration curve determine whether a measure reduces demand charges, energy charges, or both. Shifting load does not save energy by itself.

Demand-side management: peak clipping, valley filling, load shifting and strategic conservation. Interval metering at 15-30 minute resolution is the basic data source.

Motor and drive savings: affinity laws give flow proportional to speed, head to speed squared, power to speed cubed. A VSD at 80% speed uses about 51% of full-speed power.

Lighting: efficacy in lm/W; LED 100-150 lm/W versus fluorescent 60-90 lm/W. Lighting power density (W/m2) is the code metric; controls add 20-40% savings in daylit or intermittently occupied spaces.

Compressed air is among the most expensive utilities; leaks typically waste 20-30% of output. Measure with a no-load run-down test or ultrasonic leak detection.

Measurement and verification requires an uncertainty statement; savings smaller than twice the metering uncertainty should not be claimed as verified.

Cooling load components: conduction through envelope, solar gain through glazing, internal gains (people, lighting, equipment), ventilation and infiltration latent and sensible loads. Peak load sets plant size; annual energy sets operating cost.

Chiller efficiency: COP = cooling output / electrical input. kW/RT = 3.517 / COP; a chiller at 0.6 kW/RT has COP about 5.9. Water-cooled chillers typically 0.55-0.65 kW/RT, air-cooled 0.9-1.1 kW/RT.

Chilled water supply/return temperature difference: a low delta-T syndrome raises pumping energy and forces more chillers on. Design delta-T is commonly 5-7 C for standard systems.

Fan and pump laws: power varies with the cube of speed; system curve intersection determines the actual operating point, so throttling wastes energy where VSD control is possible.

Hong Kong Building Energy Code (BEC) under the Buildings Energy Efficiency Ordinance sets prescriptive requirements for HVAC, lighting, and power; designers may instead meet the performance-based route via whole-building energy simulation.

Fresh air: ASHRAE 62.1 or local code sets minimum outdoor air per person and per floor area. Demand-controlled ventilation using CO2 sensors saves energy where occupancy varies.

Power factor: kVA = kW / pf. Power factor correction to 0.95+ avoids utility penalties and reduces cable and transformer losses; capacitors are sized in kVAr = kW (tan phi1 - tan phi2).

Electrical distribution losses: I^2 R losses fall with voltage; transformer no-load loss is continuous, so lightly loaded transformers should be de-energised or replaced with high-efficiency units.

Harmonics from VSDs and LED drivers cause neutral overheating and transformer derating; total harmonic distortion limits are typically 5% voltage at the point of common coupling.

Levelised cost of energy (LCOE) = sum of discounted costs / sum of discounted energy output. It ignores system integration cost, so it is not a full comparison of dispatchable versus variable generation.

NPV and IRR: accept projects with positive NPV at the hurdle rate; IRR is the discount rate giving zero NPV, and it can mislead when comparing projects of different scale or timing.

Carbon intensity: grid emission factor in kg CO2e/kWh. Hong Kong's CLP and HK Electric factors are published annually and are used in BEAM Plus and government carbon audits.

Scope 1 (direct combustion and fugitive), Scope 2 (purchased electricity, heat, steam) and Scope 3 (value chain) follow the GHG Protocol; Scope 2 may be reported location-based or market-based.

Carbon offsets and renewable energy certificates must be additional, verifiable and retired; buying certificates does not remove the need to measure and reduce direct emissions first.

Hong Kong policy instruments: feed-in tariff, renewable energy certificates, BEAM Plus certification, the Buildings Energy Efficiency Ordinance and the Energy Efficiency Ordinance registration scheme for prescribed products.

Marginal abatement cost curve: rank measures by cost per tonne CO2e abated; negative-cost measures are those with net financial benefit over life and should be pursued first.

Electricity tariff structure: CLP and HK Electric tariffs combine a basic charge (energy) and a fuel clause adjustment (fuel cost pass-through); demand charges apply to larger customers.

Discount rate choice matters: a 3% versus 8% rate can reverse the ranking of long-life renewables against short-life efficiency measures.

Project lifecycle: feasibility, design, tender, construction, commissioning, handover, operation. Energy performance guarantees should be tied to defined test conditions and measurement methods.

Commissioning: functional performance testing verifies that systems meet design intent; retro-commissioning of existing buildings typically yields 5-15% energy savings.

Hazardous energy control (lockout/tagout) is mandatory before work on rotating plant, electrical panels or pressurised steam systems; isolation must be verified by test, not assumption.

Confined space entry into boilers, tanks and ducts requires atmosphere testing (O2 19.5-23.5%), permit-to-work, standby person and rescue plan.

Hong Kong statutory framework: Factories and Industrial Undertakings Ordinance and its subsidiary regulations, the Electricity Ordinance, the Boilers and Pressure Vessels Ordinance, and the Air Pollution Control Ordinance.

Environmental compliance: Air Pollution Control Ordinance caps emissions from specified processes; noise from fixed plant is controlled under the Noise Control Ordinance with permitted levels at noise sensitive receivers.

Environmental Impact Assessment Ordinance requires designated projects to pass the EIA process and obtain an environmental permit before construction.

Risk register with probability and consequence scoring; safety integrity for high-hazard controls follows a defined safety lifecycle with documented proof testing.

Contract forms: HKIA/HKIS standard forms, NEC4 for collaborative delivery; liquidated damages, retention and performance bonds allocate schedule and performance risk.

HKIE members are bound by the Code of Conduct: honesty and integrity, competence within one's field, public safety and health first, and disclosure of conflicts of interest.

Signing and sealing work certifies personal technical responsibility. A member should only sign work they have personally supervised or reviewed, and should record the basis of the judgement.

Whistleblowing duty: where a member becomes aware of a practice endangering public safety, the obligation is to report internally first and to the authority if not resolved.

Conflict of interest: declare in writing, withdraw from decision-making, and never accept benefits that could influence judgement; gifts beyond token value should be declined or declared.

Continuing professional development is a condition of registration; HKIE requires a documented CPD record, typically a minimum number of hours per year across technical and non-technical categories.

Engineering judgement under uncertainty: state assumptions, quantify uncertainty where possible, apply the precautionary principle to safety, and document the decision trail for later review.

Sustainable development duty: engineers should minimise whole-life resource use and emissions, and should not accept a brief that is technically unsafe or environmentally non-compliant without recorded objection.

Expert evidence and reports must distinguish fact from opinion, cite verified data, and disclose the limits of the analysis performed.