Tiêu chuẩn iso 15927 1 2003

Microsoft Word S029559e doc Reference number ISO 15927 1 2003(E) © ISO 2003 INTERNATIONAL STANDARD ISO 15927 1 First edition 2003 11 15 Hygrothermal performance of buildings — Calculation and presenta[.]

Terms and definitions

For the purposes of this European Standard, the following terms and definitions apply.

3.1.1 mixing ratio ratio of the mass of water vapour to the mass of dry air with which the water vapour is associated

3.1.2 water vapour pressure part of the total atmospheric pressure exerted by water vapour

3.1.3 saturated vapour pressure over water vapour pressure of moist air in equilibrium with a plane liquid water surface

3.1.4 relative humidity ratio of the vapour pressure of moist air to the vapour pressure it would have if it were saturated

3.1.5 reference wind speed wind speed measured at a height of 10 m above ground level in open country without nearby obstacles

3.1.6 gust speed greatest instantaneous wind speed observed during the period over which the mean is calculated

Copyright International Organization for Standardization

3.1.7 solar irradiance radiation power per area generated by the reception of solar radiation on a plane of any tilt and orientation

The following special quantities can be distinguished according to the conditions of reception:

3.1.7.1 global solar irradiance irradiance generated by reception of solar radiation from the full hemisphere

The reception of both direct and diffuse solar radiation on a horizontal plane is defined as global solar radiation For tilted surfaces, this definition also includes a portion of the ground-reflected global solar radiation.

3.1.7.2 direct solar irradiance irradiance generated by the reception of solar radiation from a conical angle which surrounds concentrically the apparent solar disk

NOTE 1 Also referred to as "beam solar radiation".

NOTE 2 The horizontal component of the direct solar irradiance is a part of the global solar irradiance.

NOTE 3 Any component of the direct solar irradiance is generated nearly exclusively from unscattered solar radiation.

The apparent solar disk has a diameter of approximately 0.5 degrees Due to technical limitations, radiometers measure direct solar irradiance from solid angles surrounding the solar disk, typically within field-of-view angles ranging from 3 to 6 degrees.

Diffuse solar irradiance refers to the solar radiation received from the entire sky hemisphere, excluding the portion used to measure direct solar irradiance.

For accurate practical measurements, a sun-following disk is essential, as it consistently shades the radiometer's receiver with a 'field of shade' angle that matches the field of view angle for direct solar irradiance measurement This setup enables the calculation of global irradiance by summing the diffuse solar irradiance and the horizontal component of direct solar irradiance.

NOTE 2 The use of a ring to shade the sun along its daily path instead of a disk requires an equation to correct for the corresponding losses of diffuse solar irradiance.

3.1.7.4 reflected solar irradiance irradiance generated by reception of the rising reflected global radiation on a downward looking plane

NOTE 1 The ratio of reflected solar and global solar irradiance is called albedo.

NOTE 2 Part of the reflected global solar radiation is received on any tilted plane.

3.1.8 solar irradiation radiant energy per area received from the sun on a plane of defined inclination and orientation during a given period of time

NOTE The same components as indicated in 3.1.7 for irradiance can be distinguished.

3.1.9 longwave (terrestrial) radiation radiation with wavelength greater than 3 m from surfaces at the ground and from the atmosphere

NOTE The exchange of longwave radiation occurs permanently between buildings, the ground and the atmosphere at temperatures between 240 K and 340 K.

3.1.10 thermometer screen white painted, wooden, plastic, or aluminium louvered enclosure, which allows a free flow of air over thermometers while shielding them from solar radiation, longwave radiation and precipitation

Symbols and units

D wind direction from North d m number of days in a month - d y number of days in a year -

G l,a longwave irradiancefrom the atmosphere on a horizontal plane W/m 2

G s,d solar irradiance direct (beam) solar irradiance diffuse solar irradiance

G s,r reflected global solar irradiance Wm 2

H effective height of topographic feature m

H s solar irradiation MJ/m 2 h m number of hours in a month -

L d actual length of downwind slope m

L e effective length of upwind slope m

L u actual length of upwind slope m

R rainfall total (or equivalent amount of melted solid precipitation) mm

P total atmospheric pressure hPa p water vapour pressure hPa p sat ( ) saturated vapour pressure over water at temperature hPa s scale factor for topography coefficient -

T temperature K v wind speed m/s vˆ gust wind speed m/s

The article discusses various parameters related to wind speed and atmospheric conditions, including the mean wind speed (\$v_r\$) measured in meters per second (m/s) and the site-specific mean wind speed (\$v_s\$) It also defines the mixing ratio (\$x\$) in grams per kilogram (g/kg) and the saturated mixing ratio (\$x_{sat}\$) concerning liquid water at a given temperature Additionally, it mentions the horizontal distance (\$y\$) from the crest of a topographic feature in meters (m) and the height above ground (\$z\$), including the minimum height (\$z_{min}\$) and roughness height (\$z_0\$) Lastly, it refers to the ratio of the gas constant of dry air to that of water vapor.

- air temperature C upwind slope of topographic feature - relative humidity -

The article defines key terms related to atmospheric measurements: "a" refers to atmosphere, "dm" indicates the mean over a day, "dx" signifies the maximum over a day, and "dn" denotes the minimum over a day Additionally, "h" represents values that are either instantaneous measurements or the average of multiple readings within an hour, while "ic" describes the inclination of a surface Lastly, "mm" stands for the mean over a month.

N values indicate the number of hours (e.g., 3 h, 6 h, or 12 h, but less than 24 h) and can represent either instantaneous measurements or the average of multiple readings during that period The longwave pq value is exceeded for q % of the time, while s denotes the solar standard deviation.

4 Periods over which parameters are calculated

The techniques outlined in clauses 5 to 9 are applicable for calculating monthly averages or totals, whether for a specific month, such as January of a given year, or for all corresponding months across multiple years, like all Januarys within a 30-year dataset.

Calculations of the standard deviation of daily means or totals about the monthly or annual means or totals (see 5.3 and 5.4) shall refer to a specified month or year.

The specified year or the multi-year period over which all parameters are calculated shall be quoted with the values of the parameters.

Sources of data

The dry-bulb air temperature data used to calculate monthly means shall come from observations from a thermometer screen fitted with louvers to allow a free flow of air.

Calculation of the monthly mean

The hourly temperature may be either: a) the mean of continuous measurements recorded during that hour or b) measurements recorded at a particular moment within the hour (e.g on the hour).

The monthly means shall be calculated as: m h h h m h m

(1) where h is the hourly temperature, in C; h m is the number of hours in the month under consideration.

5.2.2 From data measured at intervals of 3 h or 6 h

When dry-bulb outdoor air temperature data is collected at 3-hour or 6-hour intervals, the monthly mean can be determined using either the average of continuous measurements or instantaneous readings taken during those intervals.

(2) where n m = 8 d m for data at three-hour intervals;

= 4 d m for data at six-hour intervals; d m is the number of days in the month under consideration.

5.2.3 From daily maximum and minimum data

To calculate the daily mean outdoor air temperature when only the daily maximum and minimum temperatures are available, use the formula: \[\text{Daily Mean} = \frac{\text{Daily Maximum} + \text{Daily Minimum}}{2}\]This method provides an accurate representation of the average temperature for each day of the month.

6 and the monthly means obtained as: m m d d m d d

(4) where d m is the number of days in the month under consideration.

Daily temperature means derived from maximum and minimum values typically differ from those calculated using hourly data, with 95% of differences falling within 1.0 °C, and a maximum range of up to 2.0 °C Similarly, monthly means based on daily maximum and minimum temperatures also show discrepancies when compared to hourly calculations, where 95% of differences are between 0.2 °C and the maximum difference reaches 0.25 °C.

5.2.4 From instantaneous data at 07:30, 14:30 and 21:30 or at other similar times

When outdoor air temperature data is only accessible at specific times such as 07:30, 14:30, and 21:30, the daily mean temperature for each day can be determined using Equation (5) or a corresponding equation suitable for those time intervals.

30 : 14 30 : m 07 d (5) and the monthly mean obtained from Equation (4).

Calculation of the standard deviation of daily means about the monthly mean

If not defined in 5.2.2, 5.2.3 or 5.2.4, the daily mean temperatures for each day in the month are calculated from data measured at one, three or six hourly intervals using: d

(6) where n d = 24 for data at one-hour intervals;

= 8 for data at three-hour intervals;

= 4 for data at six-hour intervals; or from daily maximum and minimum data using Equation (3) or at 07:30, 14:30 and 21:30 or similar times using Equation (5).

Then the standard deviation of the daily means from the monthly mean is given by:

Calculation of the annual mean and standard deviation

The annual mean temperature shall be calculated from the daily means using: y d d d y d y

The standard deviation of the daily means from the annual mean shall be calculated by:

Expression of results

Monthly average dry-bulb outdoor temperatures should be reported to the nearest 0.1°C, and it is essential to specify the type of data (such as hourly or daily) utilized in the calculation of these monthly means.

Each month, the following parameters will be reported: the measurement dates used for calculations, the monthly average of the dry-bulb outdoor temperature, and the standard deviation of the daily mean dry-bulb temperature relative to the monthly average Additionally, when available, the report will include the maximum and minimum hourly dry-bulb outdoor temperatures, as well as the hourly dry-bulb outdoor temperature values at the 1%, 5%, 10%, 90%, 95%, and 99% percentiles.

If four or more reasonably spaced dry-bulb outdoor temperature values are available each day, hourly values can be estimated through linear or other interpolation methods This allows for the calculation of the specified statistical values.

These parameters summarised in b) to f) shall be presented in tabular form similar to the example shown in Table 1.

Table 1 — Sample table of monthly and annual mean temperatures JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC YEAR mm 5,7 5,4 6,2 8,1 10,4 13,1 14,1 14,7 13,0 10,9 8,1 6,2 9,7

Sources of data

Humidity data for calculating monthly averages must be sourced from one of the following: mechanically ventilated wet and dry bulb thermometers, chilled mirror dewpoint meters, capacitance hygrometers, or hair hygrometers.

Data obtained from wet and dry bulb temperatures measured in non-ventilated thermometer screens or from hair hygrometers are often too inaccurate for reliable monthly mean calculations.

Relationships between temperature and humidity parameters

6.2.1 Saturated vapour pressure and temperature

The saturated vapour pressure is given as a function of temperature in Table 2.

Table 2 — Saturated vapour pressure in hPa as a function of temperature + T

Intermediate values may be found by linear interpolation.

NOTE Saturated vapour pressure can also be calculated using the empirical equations:

6.2.2 Mixing ratio and vapour pressure

Vapour pressure is calculated from mixing ratio using x p xP

(12) where P is the total atmospheric pressure measured at the site, in hPa.

Relative humidity is calculated from dry-bulb temperature and vapour pressure using p sat p (13) where the saturated vapour pressure is derived from temperature as specified in 6.2.1.

NOTE 1 Relative humidity is commonly expressed as a percentage.

NOTE 2 Relative humidity can also be measured directly.

Calculation of monthly mean

To calculate the monthly mean temperature, hourly values of temperature and vapour pressure (or mixing ratio) must be available The monthly mean temperature is determined as outlined in section 5.2.1, while the monthly mean vapour pressure is calculated using the specified formula.

1 m (14) and the mean mixing ratio from: m h h h m h x x m

1 m (15) where h m is the number of hours in the month under consideration.

The hourly saturated vapour pressure shall be obtained from Table 2 or calculated from the hourly temperatures using Equation (10) or (11) as appropriate.

The monthly mean relative humidity is determined by calculating the monthly mean vapor pressure and saturated vapor pressure, as outlined in Equation (13) If needed, Equation (12) can be utilized to convert the monthly mean mixing ratio into the monthly mean vapor pressure.

To calculate monthly means, first determine the hourly vapor pressure using the available hourly temperature and relative humidity data This is done by applying the formula for saturated vapor pressure.

Calculating monthly mean vapour pressure using mean relative humidity and temperature can result in substantial inaccuracies, particularly in warm climates, due to the non-linear relationship between saturated vapour pressure and temperature To achieve more accurate results, it is essential to derive monthly mean vapour pressures from the highest frequency vapour pressure data available.

6.3.2 Data measured at intervals of three, four or six hours

When temperature and vapor pressure data are collected at three, four, or six-hour intervals, they should be averaged monthly to calculate the mean relative humidity, following the method outlined in section 6.3.1.

6.3.3 From instantaneous data measured at 7:30, 14:30 and 21:30 or at other similar times

To calculate the daily mean relative humidity when data is only available at specific times such as 7:30, 14:30, and 21:30, use Equation (17) or its equivalent for the relevant time intervals.

30 : 14 30 : m 07 d (17) and the monthly means are obtained using Equation (18):

6.3.4 Calculation of the annual means

The annual mean temperature shall be calculated using Equation (7) and the annual mean water vapour pressure calculated from: y d d d y d p p y

1 m m (19) and the mean mixing ratio from: y d d d y d x x y

1 m m (20) and the mean relative humidity calculated using Equation (13).

Mean values shall be expressed to the precision shown in Table 3.

Table 3 — Precision to which monthly and annual means shall be expressed

Each month and year, the following parameters must be reported: the measurement dates used for calculations, the mean temperature, water vapor pressure (or mixing ratio), and relative humidity Additionally, if hourly data is accessible, the report should include the 1%, 5%, 10%, 90%, 95%, and 99% percentiles, along with the absolute maximum and minimum values of water vapor pressure, mixing ratio, or relative humidity.

These parameters shall be presented in tabular form similar to the examples shown in Tables 4 and 5.

Table 4 — Sample table of monthly and annual means (with distribution of water vapour pressure)

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC YEAR mm 5,7 5,4 6,2 8,1 10,4 13,1 14,1 14,7 13,0 10,9 8,1 6,2 9,7

Table 5 — Sample table of monthly and annual means (with distribution of relative humidity) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC YEAR mm 5,7 5,4 6,2 8,1 10,4 13,1 14,1 14,7 13,0 10,9 8,1 6,2 9,7

Methods of measurement

Wind speed is measured with an anemometer and wind direction with a vane The reference mean wind speed v r is calculated over a time period ranging from 10 min to 1 h.

NOTE 1 The instantaneous speed is in fact a mean speed over approximately 2 s depending on the nature of the measuring instrument used.

NOTE 2 In reference conditions with a mean time period of 10 min the gust speed vˆ1,54v r , and for a mean time period of 1 hour vˆ1,65v r

NOTE 3 Wind speed data are sometimes expressed in knots (1 knot = 0,514 m/s).

Mean wind direction (D) is determined over the same timeframe as wind speed and is expressed in 10-degree sectors from north For instance, an east wind is represented by sector 90, noted as 09, while a west wind (270) is indicated as 27, and a north wind is denoted as 36 A calm wind is marked as 00, and if the wind direction varies, it is reported as 99.

Environmental influence on mean wind speed

Mean wind speed is influenced by local environmental factors such as topography, ground roughness, and nearby obstacles When comparing two sites at similar altitudes within a 10 km radius, significant differences in mean wind speeds are unlikely if their local environments are comparable To derive the reference regional wind speed (\$v_r\$) from observed site wind speed (\$v_s\$) or to estimate wind conditions at a site based on reference wind, it is essential to apply a correction to the mean wind speed, assuming the absence of nearby obstacles.

The roughness coefficient reflects the variability of mean wind speed at a location, influenced by both the height above ground and the terrain's roughness, which varies with the wind's direction.

The roughness coefficient at height z is given by:

C R (z) = K R ln(z min/z 0) for z < z min (22) where

K R is the terrain factor; z 0 is the roughness height; z min is the minimum height

These parameters depend on the terrain category as given in Table 6.

Table 6 — Terrain categories and related parameters

I Rough open sea; lake shore with at least

5 km fetch up wind and smooth flat country without obstacles

II Farm land with boundary hedges, occasional small farm structures, houses or trees

III Suburban or industrial areas and permanent forests

IV Urban areas in which at least 15 % of the surface is covered with buildings of average height exceeding 15 m

If there is a change of roughness upwind of a site within a kilometre, the smoothest terrain category in the upwind direction shall be used.

The topography coefficient is essential for understanding the increase in mean wind speed over isolated hills and escarpments, specifically in areas that are not undulating or mountainous This coefficient is directly related to the wind velocity approaching the hill and should be applied to locations situated more than halfway up the slope, within 1.5 times the height of the cliff from its base.

The equation \$C_T = 1 + 0.6s\$ applies for \$s > 0.3\$, where \$s\$ is a factor derived from Figures 2 and 3, scaled to the lengths of the upwind slope (\$L_u\$) and downwind slope (\$L_d\$) Additionally, the upwind slope is represented by \$H/L_e\$ in the direction of the wind.

L u is the actual length of the upwind slope in the wind direction;

L d is the actual length of the downwind slope;

L e is the effective length of the upwind slope defined in Table 7;

The effective height of the feature is denoted as $ H $, while $ y $ represents the horizontal distance from the top of the crest to the site Additionally, $ z $ indicates the vertical distance from the ground level at the site.

See Figure 1 for clarification of these quantities.

C crest of hill w wind direction

Figure 1 — Definition of factors determining topography coefficient

Figure 2 — Factor s for cliffs and escarpments

Figure 3 — Factor s for hills and ridges

Statistical elements

The monthly or annual mean wind speed may be calculated from data measured either continuously or at intervals not exceeding 3 h.

The monthly mean wind speed shall be calculated by: m m N

1 m (24) where n m is the number of wind speed readings in the month in question.

The annual mean wind speed shall be calculated by y y N

1 m (25) where n y is the number of wind speed readings in the year.

7.3.4 Cumulative frequency distribution of wind speeds

The cumulative frequency distribution of wind speeds indicates the proportion of time, either monthly or annually, that the mean wind speed remains below a specified threshold This can be expressed through a cumulative distribution function, where the probability of exceeding a wind speed $ v $ is represented as $ 1 - P $, with $ P $ being the probability that the wind speed is below that value The overall wind speed distribution, denoted as $ P(v) $, is effectively modeled by the Weibull distribution.

P v 1e ( / ) (26) where c is a scale parameter; k is a shape parameter.

NOTE 1 The scale parameter, c, and the shape parameter, k, are determined from the wind data.

NOTE 2 It is also possible to define a Weibull distribution for each direction.

The article presents statistical tables detailing the distribution of mean wind frequencies categorized by direction and speed, organized by month and annually, as illustrated in Table 8 It is essential to document any local obstructions that may affect the accuracy of wind direction distribution.

Table 8 — Sample table of monthly or annual frequency of wind by speed and direction

A "wind rose" graph visually represents wind statistics, where the length of symbols in various directions corresponds to the cumulative frequencies of wind speeds below specific thresholds.

Each month or year, the following parameters must be reported: the measurement dates used for calculations, the anemometer's location and any local features affecting its readings, the frequency of the original data for mean and distribution calculations, the monthly or annual mean wind speed, a cumulative frequency distribution of wind speed, or the parameters of the corresponding Weibull distribution Additionally, if necessary, a table detailing the frequency distribution of wind speed by direction for the specified period should be included.

Sources of data

Rainfall measurement must adhere to WMO Guidelines No 8 (1996) using a properly designed rain gauge Newly fallen snow and solid precipitation should be measured to the nearest centimetre on level ground near the gauge Solid precipitation collected in the gauge must be melted using a method that minimizes evaporation, and the resulting liquid volume should be recorded Additionally, it is important to note the days when the ground is covered with snow.

Calculation of monthly total

The hourly values of precipitation and melted solid precipitation shall be totalled to give the monthly precipitation.

The daily totals of precipitation and melted solid precipitation shall be totalled to give the monthly precipitation: d m d d m R

The report must include the measurement dates for parameter calculations, the frequency of original precipitation data recording, and the specific time of day for daily measurements It should detail the monthly total precipitation rounded to the nearest 1 mm, the count of days with precipitation of 0.1 mm or more, and the maximum daily precipitation for the month, also rounded to the nearest 1 mm For hourly measurements, the report should specify the maximum hourly precipitation to the nearest 0.1 mm and the total hours in the month where precipitation exceeded 0.1 mm Additionally, it should include the total depth of newly fallen snow or solid precipitation and the number of days with snow cover on the ground.

Sources of data

Solar irradiance and solar irradiation are quantified following the WMO Guidelines No 8 from 1996, with a focus on measurements taken on a horizontal plane and the four vertical planes of orientation.

Global horizontal irradiance is often the primary measurement, while other solar irradiation components can be estimated using auxiliary data Key parameters to consider include hourly, daily, monthly, and annual total solar irradiation.

Calculation of monthly total solar irradiation

Usually daily, monthly and annual data are expressed in energy rather than in mean power.

9.2.2 From irradiance or hourly irradiation measurements on any plane

The monthly total global, direct, and diffuse irradiation can be calculated from the mean solar irradiances, $ G_{s, ic, i} $, measured on a surface with a specific inclination, $ ic $, over a time interval $ t $ (typically 1 hour or 3 hours) at a given time $ i $.

N m is the number of measurements in the month; and the subscripts denote g global radiation; d diffuse radiation; b direct radiation; ic a surface of any inclination.

For accurate mean calculations, it is crucial that the measurement planes are uniformly oriented Additionally, the time intervals must remain constant and encompass all hours of the month without any overlaps.

If hourly irradiation data is available for each day of the month, the monthly solar irradiation can be easily calculated by summing the values using the appropriate formulas.

From the daily total irradiation H s,j of the day d, on a surface of inclination ic, the monthly total global, direct and diffuse irradiation are given in turn by: dm d d ic ic H

The same remarks on the inclination of the plane apply as in 9.2.2.

Each month, the following parameters must be reported for each radiation component: the measurement period for recorded parameters, the interval of radiation recording, and the total solar irradiation for the month.

When the monthly totals are based on hourly values, the following additional statistics shall be reported: the minimum, 10 percentile, the median, the 90 percentile and the maximum irradiance.

Estimating irradiances that are not measured

The techniques outlined in sections 9.2.2 and 9.2.3 are applicable only when a series of measurements for the required quantity is accessible However, this situation is rarely encountered when dealing with irradiation on a vertical plane, as well as diffuse or direct irradiation.

To accurately reconstruct diffuse, global, and direct irradiance for any inclination and orientation, it is essential to have a series of measurements that include global irradiance on a horizontal plane, air temperature, humidity, solar constant, albedo, solar time, and cloud characteristics While established methods for data computation exist, it is always preferable to use measured data over computed data when available.

NOTE Annex A gives recommended methods for the computation of non measured radiation parameters.

General

Longwave radiation, characterized by wavelengths exceeding 3 micrometers, is released by the atmosphere, ground, and buildings at temperatures ranging from 240 K to 340 K The impact of sky radiation can be quantified using an equivalent sky temperature, which is essential for thermal calculations.

Sources of data

Longwave, or terrestrial, radiation is quantified following the WMO Guidelines No 8 from 1996 This measurement can either represent the average longwave irradiance or the total irradiation accumulated over a specific duration, typically an hour or a day.

NOTE Measured data are obtainable from climate archives of national weather services cooperating with WMO, or from the World Radiation Data Center (WRCD) of WMO in St Petersburg.

Calculation of monthly total longwave irradiation from the atmosphere (sky)

From the mean longwave irradiance G l, a, i measured during a time interval t (usually 1 h or 3 h) at time i, the monthly total longwave irradiation is given by:

10.3.2 From daily total irradiation measurements

From the daily total irradiation H l,j of the day d, the monthly total longwave irradiation is given by: dm d d m H

Each month, the following parameters must be reported: the recording period, the interval of radiation measurements, the total monthly longwave irradiation, and if the monthly totals are derived from hourly values, the minimum statistics should also be included.

10 percentile, the median, the 90 percentile and the maximum irradiance.

Methods for splitting global solar irradiance into the direct and diffuse parts

Various models have been developed through statistical analysis of measured data to differentiate between direct and diffuse global solar irradiance, particularly effective in mid-latitude climates, while other models may be better suited for tropical regions The European Solar Radiation Atlas (3rd edition) provides comprehensive methods for solar data analysis, and the 4th edition (2000) includes a CD-ROM with a database and ten algorithmic chains for deriving modified quantities.

Global irradiance on a horizontal plane can be divided into direct and diffuse components by using the diffuse fraction derived from Erbs' statistical results The direct irradiance is determined by subtracting the diffuse irradiance from the total global irradiance The formula for calculating the diffuse fraction is given by $ k T_{0.22}: \frac{G_{s,d}}{G_{s,g}} = 1.0 - 0.09 k T $.

4 (A.1) k T 0,80: G s,d /G s,g = 0,165 where k T is the clearness index of the atmosphere related to extraterrestrial global irradiance;

G s,d is the diffuse fraction of global irradiance, in W/m 2 ;

G s,g is the global irradiance on a horizontal plane, in W/m 2

The clearness index of the atmosphere, k T, is the ratio of the extraterrestrial global irradiance on the ground to the measured global irradiance l) terrestria (extra

The extraterrestrial irradiance can be calculated from astronomical data (distance to sun, incidence angle, etc.) using the methods specified in, for example, [5, 13].

When a plane is tilted, it can receive radiation reflected from the ground, and since it does not have a full view of the sky hemisphere, the diffuse irradiance must be calculated in a more complex manner The angle-dependent components of diffuse irradiance, including the circumsolar fraction, horizon ribbon, and isotropic fraction, require separate calculations For detailed methodologies on deriving these components, refer to the papers listed in the Bibliography [5, 8, 9, 10, 11, 14].

Methods for estimating the longwave atmospheric irradiances

(longwave sky irradiances)and the sky temperature

The irradiance generated on a horizontal plan by the reception of longwave radiation from the atmosphere can be described with the Stefan-Boltzmann law using a special atmosphere emittance a < 1

G l,a is the atmospheric longwave irradiance, in W/m 2 ; a is the atmospheric emittance; is the hemispherical Stefan-Bolzmann constant = 5,6697 10 -8 , in W/(m 2 K 4 );

T is the air temperature measured at 2 m height in a screen, in K.

Reference [13,14] give information on the calculation of sky emissivity for cloudy and cloudless skies.

If measured dewpoint temperatures, dp, and observations of the cloud cover, c, are available, Equations (B.2) and (B.3) should be used (from Unsworth [12]). c D c

0 is the emissivity for clear sky conditions; c is the cloud cover fraction (0 c 1); dp is the dew point temperature measured at 2 m height in a screen, in °C;

A, D are fitted parameters from measurements;

B is a fitted parameter from measurements, in °C -1

If no measured data are available, the following values can be used:

If the air temperature, T, is measured and the cloud cover observations for low n L, middle n M or high cloud n H, Equations (B.4) and (B.5) should be used [13].

According to [13, 14] the following values should be used:

Assuming that the sky behaves as an ideal black body radiator (emissivity equals 1), a corresponding sky temperature T a can be calculated:

[1] SMITHSONIAN INSTITUTION, Smithsonian Meteorological Tables Washington, 1958.

[2] CIBSE, Guide Volume C, Reference Data Chartered Institute of Building Services Engineers, London, 1986.

[3] LINKE and BAUR (1970), Meteorlogisches Taschenbuch, Neue Ausgabe, Band II, zweite Auflage Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G, 1970.

[4] DEUTSCHER WETTERDIENST, Aspirations-Psychrometer-tafeln Vierte, erweiterte Auflage Verlag Friedrich Vieweg & Sohn, Braunschweig, 1963.

[5] EC: GREIF, J and SCHARMER, K, The European Solar Radiation Atlas 2000 (ESRA 4) Distributed by: Ecole des Mines de Paris, 2000; 60, Boulevard Saint-Michel, 75272 Paris Cedex 06.

[6] PALZ, W and GREIF, J., European Solar Radiation Atlas: Solar Radiation on Horizontal and Inclined Surfaces 3 rd Edition: Springer, 1996.

[7] ERBS, D.G., Estimation of the diffuse radiation fraction for hourly, daily and monthly average global irradiation Solar Energy Vol 28/4, 1982.

[8] PEREZ, R et al., An anisotropic hourly diffuse radiation model for sloping surfaces – Description, performance validation, site dependency evaluation Solar Energy 36, p 6, 1986.

The study by Perez et al (1987) explores how variations in luminous efficacy of global radiation and zenith illuminance are influenced by weather conditions It also describes a potential method for generating essential daylight availability data using existing solar radiation databases.

[10] PEREZ, R et al, A new simplified version of the Perez diffuse irradiance model for tilted surfaces Solar Energy 39, 3, pp 221-231, 1987.

[11] PEREZ, R et al, Modelling daylight availability and irradiance components from direct and solar irradiance Solar Energy 44,5, pp 271-289, 1990.

[12] UNSWORTH, M.H et al., Longwave radiance at the ground Quarterly Journal of the Royal Meteorological Society, 1975.

[13] CZEPLAK, G., and KASTEN, F., Parameterisierung der atmosphọrischen Wọrmestrahlung bei bewửlktem Himmel Meteorologische Rundschau 6, l987.

VDI 3789, Part 2, focuses on environmental meteorology, specifically examining the interactions between the atmosphere and surfaces It provides methodologies for calculating both shortwave and longwave radiation, contributing to a deeper understanding of atmospheric processes This document, published by the Association of German Engineers in October 1994, serves as a crucial resource for professionals in the field.

Tiêu đề	Hygrothermal Performance of Buildings — Calculation and Presentation of Climatic Data — Part 1: Monthly Means of Single Meteorological Elements
Trường học	International Organization for Standardization
Chuyên ngành	Hygrothermal Performance of Buildings
Thể loại	international standard
Năm xuất bản	2003
Thành phố	Geneva

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