Energy efficiency terms explained
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Not diabatic, occurring without gain or loss of heat.
An=not iso=equal tropic=directed.
The chi-value applies to a point thermal bridge. This can be a bolt or fastener - anything with not much of an extension in two of the three dimensions. Greek symbol χ. Unit: W/K. A single point thermal bridges can be neglected for the energy balance, but recurring point thermal bridges, as they occur e.g. in curtain wall facades, need to be accounted for.
Energy supplied to consumers, where it can be converted into useful energy, e.g. electricity into light or warmth, using respective devices.
Final energy is derived from primary energy, usually taking in transformation and distribution loss in the process.
The g-value gives the total solar energy transmittance, i.e. the fraction of solar energy (direct and indirect) that enters the building through a transparent element. The direct gains equal the total short wave transmissivity of the element. This fraction is zero with opaque elements. Indirect gains are obtained via absorption of solar energy, which is then radiated as heat. This fraction is > zero with both, opaque and transparent building elements. In steady state calculations, the g-value is usually only used with transparent elements, nevertheless. As input value for PHPP, the g-value needs to be assessed using EN 410.
Glazing for Passive Houses
Physical properties for window glazing of interest for planning Passive Houses are total solar energy transmittance and U-value, predominantly.
The formal definition of density is mass per unit volume. In some contexts the density is expressed in grams per mL or cc. In the building sector, kg/m3 is more commonly used. Mathematically a "per" statement is translated as a division. Density = Mass/Volume, in the building sector most commonly expressed in kg/m3.
Heat is the least "noble" energy form. According to the second law of thermodynamics, heat cannot be transformed into higher forms of energy - like mechanical or electrical energy - without a loss, whereas the reciprocal process can happen loss free. While heat is a scalar and thus undirected, heat transfer always occurs from warm to cold, and never the other way around, unless work is added. As heat is at the end of a chain of energy transformations, it can be the tip of an iceberg of energy usage. Using "nobler" forms of energy to generate heat, e.g. using electricity to generate space heating, should be minimised, and waste heat from mechanical or electrical processes should be utilised to preserve energy sources.
Applied to a building element, however, heat flow turns into a heat flow rate in W/m². It looses is undirectedness, becoming a vector. The heat flow rate is proportional to the temperature difference at both ends of the system, for example at building element surfaces.