Energy efficiency terms explained
Special | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | ALL
A |
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AdiabaticNot diabatic, occurring without gain or loss of heat. | |
AnisotropicAn=not iso=equal tropic=directed. | |
C |
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Chi-valueThe 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. | |
E |
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F |
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Final EnergyEnergy 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. | |
G |
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g-ValueThe 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 HousesPhysical properties for window glazing of interest for planning Passive Houses are total solar energy transmittance and U-value, predominantly. | |
Gross densityThe 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. | |
H |
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HeatHeat 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. | |
Heat flowHeat flow describes heat per unit of time in J/s or W; it is also a measure of power. Heat flow is a scalar, therefore not directed and not affected by changes of the co-ordinate system. 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. | |
Heating Demand | |
Heating LoadHeating load describes the required size of a heating system for providing comfortable indoor temperatures even on the coldest design day of the year. As this is sometimes confused with heating demand, one way of differentiating the two with an analogy might be thinking of the heating load as the power of a motor in horse power or kW, and heating demand as the energy you need to feed this motor for it to do what you want it to do, e.g. taking you from A to B. In PHPP the heating load is assessed for two weather variations: a cold, clear day and a moderate, overcast day. On a cold, clear winter day you can expect temperatures to be in the lowest range, as nothing (no cloud cover) stops the earth's surface from radiating back heat to the sky. On the plus side, solar radiation will be higher than on an overcast day. Whatever scenario turns out to result in the highest heating load will be applied for further considerations. For a Passive House, the heating load should not exceed 10W/m2 treated floor area. Example: with a treated floor area of 100 m2, the heating load should be no more than 1,000 W or 1 kW. How is the maximum heating load explained? By multiplying the specific fresh air requirement (30W/person) with the specific heat capacity of air at 20℃ and the difference of maximum supply air temperature (at about 52℃) and the minimum temperature at which air leaves the heat exchanger (16.5℃). The approximate result of this calculation is 350W/person. Divided by the average space requirement per person (assumed as 35m²) a maximum heating load of 10W/m² results: | |
K |
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k-ValueRefer to thermal conductivity. | |
L |
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Lambda valueRefer to thermal conductivity. | |
Low-e glazingLow-e stands for low emissivity. Microscopically thin, nearly invisible, metal or metallic oxide layers are coated to the outside of the interior pane (thus located in the interstitial cavity) of a multi-pane Insulating Glass Unit (IGU) to reduce the heat conductance by minimising radiative heat flow. A low-e coating is usually transparent to the solar spectrum (visible light and short-wave infra-red radiation) and reflective of long-wave infra-red radiation. In triple glazed IGUs there sometimes are two low-e layers. | |
P |
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Passive HouseA very comfortable and healthy house that needs very little energy. Quantitative performance benchmarks include:
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PHPPPHPP is the Passive House Projecting Package, a spreadsheet based tool for planning and optimising highly energy efficient buildings. It can be obtained from the Passivhaus Institut in Germany and various national bodies. | |
Primary EnergyPrimary energy is energy contained in raw fuels or other naturally occurring forms of energy before it undergoes transformation or is put to a use. It is a measure of the energy potential of a source, what could be used if conditions were ideal - which they are however not, for the largest part. Electricity derived from fossil fuels e.g. loses around 2/3 of its energy potential in the transformation process. In other words: it needs roughly 3 parts fossil fuel to generate one part of electric energy. | |
Psi-valueCoupled 2D thermal bridge coefficient, used to gauge the numeric impact of thermal bridges. Ψ, upper case psi (English: sigh), is the most commonly used symbol. The unit for psi is W/(m K). It applies to a length, i.e. the length of the thermal bridge. It denotes the thermal conductivity of an assembly of materials. The higher the value, the higher the additional heat loss through that joint. | |
R |
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R-valueIs the measure of thermal resistance, used in the building and construction industry. The higher the value, the better the insulation effectiveness of a material layer. The R-value is always the property of a material layer in m2 K/W, derived from dividing the material layers thickness (m) by the material's thermal conductivity. The R-value is therefore not intelligible without the material layer thickness. Several indices indicate more detailed information about what sort of R-value is considered. RT for example indicates the total R-value of the sum of all material layers in a building element. If calculated in accordance with international standard ISO 6946, it also includes repeating thermal bridges. | |
S |
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SHGCRefer to g-value. | |
T |
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Thermal ConductivityThermal conductivity is a material property. It indicates the heat flow (W or J/s) occurring in 1m of material length at a temperature differential of 1 Kelvin on both ends of this length. | |
U |
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U-valueU-values gauge how well a building element allows heat to pass through. The lower the U-value, the greater a building element's resistance to heat flow and the better its insulating value. | |
Λ |
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λRefer to thermal conductivity. | |
Χ |
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χRefer to chi-value. | |
Ψ |
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ΨRefer to psi-value. | |