“ENERGY”
Energy is the ability/capacity to do work. It is the quantitative property that must be transferred to an object or system in order to perform work on, to heat and to operate the object or system. The amount of energy required by the HVAC system to accomplish a defined task (heating-cooling-ventilation) is of prime importance for evaluating the building performance. “Power” is the instantaneous snapshot of work (without a time dimension), “Energy” is the integration of this instantaneous power over time (per hour, per day, per year).
“EFFECTIVENESS”
Effectiveness is a qualitative descriptor that describes the success in meeting a pre-defined objective for a system. Any HVAC system that delivers the Owner’s Project Requirements (OPR) would be properly termed as “effective”.
“EFFICIENY”
Efficiency is defined as a ratio of output to input which is a quantitative assessment of the ability of a system to provide some effect relative to the resource of doing so. An HVAC system that provides a lot of useful heat while requiring just slightly more heat input would be termed efficient. A system can be efficient while ineffective and vice versa. A high-performance building design seeks systems that are simultaneously “effective” and “efficient”.
“ENTHALPY”
Enthalpy is the sum of sensible and latent heat content of the unit mass of moist air (air-moisture mixture). It is also defined as the “total” energy content of a sample of air. Different methods of conditioning air (heating, cooling, humidifying, dehumidifying) changes its “enthalpy” and the load on HVAC systems can be inferred from this change of energy content.
“ENTROPY”
Entropy is a degree or measure of disorder of a system. High entropy indicates a diminished capacity of a system to generate work out of natural heat conversion processes. Most building systems lose order over time thus increase entropy.
“EXERGY”
Exergy is simply a measure of a utility of an energy exchange. The exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with the heat reservoir, reaching maximum entropy. While energy is converted from some form to another during the process, the exergy is always destroyed when a process is irreversible (e.g., loss of heat as an unusable by product to the environment). Exergy is also synonymous with available energy, exergic energy, utilizable energy, available useful work, reversible work or ideal work.
Adapted from ”Mechanical and Electrical Equipment for Buildings” 13th Edition by W.T. Grondzik and A. G. Kwok. Page 535.
Omer T. Karaguzel, PhD