Energy Management and Emissions

Energy audit and conservation measures

An energy audit is a systematic study of how a facility uses energy, carried out to find and evaluate savings. The standard procedure runs in stages. The first stage is the walkthrough, or preliminary, survey: a site visit in which operations are observed and obvious opportunities are identified. Next, past energy consumption and cost data are collected and analysed to establish a baseline against which later savings are measured. Each candidate energy conservation measure (ECM) is then evaluated for technical and economic feasibility, for example by its payback period. A historic profit and loss statement is a financial accounting record. It is not a stage of the energy audit itself.

Boiler blowdown is the deliberate discharge of concentrated boiler water to control dissolved solids. It cannot be eliminated completely, but part of its energy can be recovered. The usual route is flashing. High-pressure blowdown is released into a flash vessel held at a lower pressure, where part of it vaporises instantly into low-pressure flash steam that can be used in the plant. Returning blowdown to the steam drum only reinjects dissolved solids and recovers no useful heat. Mixing it with feedwater gives only modest preheating and is not the standard steam-recovery route.

• Blowdown — controlled discharge of boiler water to limit dissolved solids. e.g. A boiler with high TDS must blow down regularly, so the goal is recovery, not avoidance.
• Flash steam — vapour formed when pressurised hot water drops to a lower pressure. e.g. Hot blowdown flashed at atmospheric pressure yields steam at that lower pressure.

Luminous efficacy of lighting

Luminous efficacy is the ratio of the luminous flux emitted by a light source to the electrical power it consumes, \eta_v = \Phi_v / P. Luminous flux is measured in lumen (lm) and electrical power in watt (W), so efficacy is expressed in lm W^{-1}. A light source that gives 800 lm while drawing 10 W therefore has an efficacy of 80 lm W^{-1}.

The term is easily confused with other lighting quantities. Illuminance is the luminous flux per unit area, measured in lux (lm m^{-2}). Luminous intensity is measured in candela (cd), which is lumen per steradian. A unit that adds a time factor, such as lm W^{-1} h^{-1}, does not represent efficacy. Efficacy compares light output with power input at a given instant.

• Luminous flux — total light power emitted in all directions, in lumen. e.g. Efficacy is the flux divided by watts, not the flux divided by area.
• Luminous intensity — light emitted per unit solid angle, in candela. e.g. A candela value describes a light beam's brightness in one direction, not its efficiency.