LECTURE N 2. - Meteorological Quantities and Climate Parameters - IDES-EDU

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1 Lecture contributions Coordinator of the lecture: Contributor : LECTURE N 2 - Meteorological Quantities and Climate Parameters - Prof. Marco Perino, DENERG Politecnico di Torino, C.so Duca degli Abruzzi 24, Torino, marco.perino@polito.it, Dr. Valerio LoVerso Politecnico di Torino, C.so Duca degli Abruzzi 24, Torino, valerio.loverso@polito.it, Prof. Marco Perino, DENERG Politecnico di Torino, C.so Duca degli Abruzzi 24, Torino, marco.perino@polito.it, 2 1

2 INTRODUCTION Meteorological measurements for weather forecasting and climatology have been carried out on a regular basis for centuries. However, the data acquired can only be evaluated and interpreted after having statistically recorded medium-term and longterm atmospheric conditions. ES -E D All meteorological parameters are influenced by solar radiation, directly or indirectly, and this results in typical daily or yearly trends. All meteorological parameters are also subject to short-term variations, normally caused by turbulences within the atmosphere. U The main meteorological parameters significant for energy demand in buildings and for renewable energy industry are: Wind speed and direction Air temperature Air pressure Air humidity Solar and terrestrial radiation Precipitation THE AIR TEMPERATURE - 1 Air is heated by the sun indirectly: through a high wavelength radiation exchange with the ground (albedo) ID through a convection exchange with the ground through water vapor condensation and subsequent freezing processes, yielding evaporation and fusion heat being exchanged with air As a result: the air temperature shows a fluctuating trend with a double period: daily and Temperatur a dell aria yearly. 1 the daily maximum outdoor temperature, due to the atmospheric and solar energy Irraggiament o del Sole exchanges, typically occurs 2 3 hours 0 0 Irraggiament after the solar radiation reaches its o del suolo maximum value the minimum daily temperature value 0 6 happens at sun rise Typical daily variation of air temperature, in normal conditions, the air temperature decreases with the height from the sea Sun radiation, ground radiation level (around 0.65 C every 100 m) Radiation W/m2 Temperature C 2 0 2

3 THE AIR TEMPERATURE - 2 The connection between solar radiation throughout the year: and air temperature can be observed the lowest temperature values are recorded about one month after the winter solstice (i.e. lowest solar radiation), while the highest temperature values about one month after the summer solstice (i.e. highest solar radiation). THE AIR TEMPERATURE - 3 C Temperatura giornaliera [ C] (2) (1) Qualitative seasonal variation of air temperature (1) and solar radiation (2) DAILY VARIATION OF AIR TEMPERATURE AT DIFFERENT LATITUDES MARCH DECEMBER C ORA Irraggiamento [kwh/(m 2 giorno)] JUNE C 13 8 TEMPERATURA Turin TORINO TEMPERATURA Messina MESSINA TEMPERATURA Rome ROMA ORA 3

4 THE AIR TEMPERATURE - 4 Besides the daily cyclic variation, the air temperature shows stochastic components due to weather phenomena. These effects may be so high to completely change the predictable behavior due to the solar and atmospheric exchanges (so that a day may be characterized by air temperature values far different from those typical of the season). Nevertheless, if the homologous values (corresponding to the same hour of the same days for different years) are averaged over many years, the resulting daily temperature profiles show a recognizable, typical, periodic trend (stochastic influences are phased-out). Rarely the month average air temperatures differ from one year to other more than 2-3 C. -E D U Air temperature is also influenced by local features and phenomena (effect at meso and micro scale) and varies with the height from the ground. ES THE AIR TEMPERATURE - 5 Influencing factors at a meso-climatic and micro-climatic scale: TOPOGRAPHY ID water reservoirs depressions and bottomlands valleys TERRAIN SURFACE thermal inertia type of coverage and color LOCALITATION rural or urban area 4

5 THE AIR TEMPERATURE - 6 To characterize the air temperature of a location, the following values are usually adopted: average daily temperature (daily mean), average monthly temperature (monthly mean), average yearly temperature (annual mean), thermal excursion (the difference between the minimum and the maximum value of the daily outdoor air temperature) extreme (summer/winter) temperatures and time at which these peak values occur THE AIR TEMPERATURE - 7 Calculation of the average values: Daily mean If all the values at each hour of the day are known (measured), the daily mean can be assessed as the usual arithmetic average of all the values, lf the only available values are the daily maximum and minimum for each day of the month, the daily mean, θ dm, can be calculated as: θ dm θ = dmax + θ 2 dmin lf the only available values are those at certain times (usually: 07:30, 14:30 and 21:30), the daily mean, θ dm, can be calculated as: θ = + θ 3 + θ d7:30 d14:30 d21: 30 θdm 5

6 THE AIR TEMPERATURE - 8 Monthly mean If all the values at each hour of the month are known (measured), the monthly mean can be assessed as the usual arithmetic average of all the values, lf the only available values are the daily mean temperatures, θ dm, then θ mm can be calculated as: n θ mm = d= 1 Estimating the daily temperature profile (hourly) for the COOLING DESIGN DAY The daily air temperature profile can be approximated through the following function: Where: θ(τ) = θ θ n dm f( τ) dmax θ d,max θ dmax = maximum daily temperature, θ Max = maximum daily thermal excursion, f = reduction factor (given in the following table) THE AIR TEMPERATURE - 9 Estimating the daily temperature profile (hourly) for the COOLING DESIGN DAY The hourly reduction factor, f, is given hour by our for the design day: (From: Fracastoro, 1982) 6

7 THE AIR TEMPERATURE - 10 Estimating the daily temperature profile (hourly) for a GENERIC DAY The hourly profile of the air temperature during a generic day (24 hour) can be assessed through the following relation: Where: ( ) θ τ = θ + π sin 12 ( τ ϕ) π ( τ ϕ) + K K1 2 sin θ= daily mean temperature ϕ = phase angle = (τ Max -τ min )/2 K 1, K 2 = constants Constant K 1 and K 2 are obtained solving the following system: b θ ( ) K1 sin + K2 sin b = 2 2 b K1 cos + 2 K2 cos( b) = 0 2 Max Being: THE GROUND TEMPERATURE 6 b = π ϕ τ Max = time at which the maximum temperature occurs τ min = time at which the minimum temperature occurs The ground temperature is subject to the same periodic variations during the time as the air temperature, however its swing amplitude is far lower, thanks to the huge thermal mass of the soil. At few centimeters below the ground level the daily oscillation of the air temperature is practically undetectable. The ground temperature varies in time with a quite predictable law. This allows to obtain the time profile through a mathematical function, as proposed by Hadvig: Where: θ(h, τ) = θ y + A e π -h α Ys 2π cos Ys ( τ τ ) max - h θ(h,τ) = ground temperature at depth h and at time τ ( C) θ y = annual mean outdoor air temperature ( C) A = yearly thermal excursion ( C) Y s = year duration in seconds (that is: ) (s) τ max = instant of the year (in seconds) when the maximum outdoor air temperature is reached (s) α = thermal diffusivity of the soil (m 2 /s) π α Y s 7

8 THE RELATIVE HUMIDITY - 1 A small amount (grams per kilograms of dry air) of water vapor is present in the outdoor air. This water vapor derives from the evotraspiration phenomena (vegetation), evaporation from oceans, water reservoirs and the soil. Influencing factors at a meso-climatic and micro-climatic scale: TOPOGRAPHY VEGETATION evaporation process due to the presence of water masses evaporation-transpiration of tree leaves, which absorb most of the incident heat During winter time, since the outdoor air temperature is low, the specific humidity is very low (water vapor content per unit mass of dry air). Nevertheless, the RH is usually very high (75 % and above) for many hours of the season. In summer, instead, the water vapor content per unit mass of dry air may be 2 to 3 times higher than in winter, but RH is lower due to the high air temperature. THE RELATIVE HUMIDITY - 2 During the day the vapor pressure shows a rather constant trend with time (black curve in figure); as a result, relative humidity tends to increase when air temperature decreases and vice versa. DAILY VARIATION of RH 8

9 THE ATMOSPHERIC RADIATION - 1 The solar radiation, once has crossed the atmosphere, reaches the ground and is partly absorbed and partly reflected back towards the space. The absorbed quota heats up the soil causing a re-emission of radiation (in the range of IR between about µm, with a peak around 10 µm). This radiation is named terrestrial radiation. The terrestrial radiation is then mostly absorbed and reflected by the atmosphere, that, in turn, re-emits a radiative heat flux. The sum of the reflected terrestrial radiation and of the quota re-emitted by the atmosphere is named atmospheric radiation. The atmospheric radiation depends on: the quantity of water vapor in the atmosphere, the presence of clouds and the air temperature of the lower layers of the atmosphere. (From: Terry s Lab Website) THE ATMOSPHERIC RADIATION - 2 There are numerous relations to assess the atmospheric radiation. A simplified formula to evaluate the atmospheric radiation on an horizontal surface, G a [W/m 2 ], is the one proposed by Cole: G a = θ a + 65 c θ a c Where: θ a = air temperature [ C] c = cloud cover factor (fraction of the sky covered by clouds) [-] 2 [ W/m ] For inclined surface, G a can be corrected by means of the following relation: G a 2 ( Σ) = G K + b K σ θ [ W/m ] a Where: Σ = is the tilt angle with respect to the horizontal 1 2 a 9

10 THE ATMOSPHERIC RADIATION - 3 Coefficients K 1 and k 2 are given in the following table: (From: Fracastoro, 1982) b can be assessed through the following relation: b = c θ SOLAR RADIATION AND WIND Due to the ample extent of information needed in relation to solar radiation and wind, for designing buildings and energy systems based on RES, these quantities will be thoroughly dealt with in the following lectures. In particular : Lectures 3 will treat the topics: SOLAR ENERGY AND SOLAR RADIATION, Lectures 4 will treat the topic: AVAILABLE SOLAR RADIATION, Lectures 6 will treat the topics: WIND AND WIND ENERGY. a c 10

11 CLIMATE PARAMETERS THE SOL-AIR TEMPERATURE - 1 Sol-air temperature, θ sol-air, is the fictitious outdoor air temperature that, in the absence of all radiation exchanges on the outer surface of the building envelope, gives rise to the same heat flux through the surface as would the combination of incident solar radiation, radiant energy exchange with the sky and other outdoor surroundings and convective heat exchange with outdoor air. G T θ equivalent θ sol-air 11

12 THE SOL-AIR TEMPERATURE - 2 Sol-air temperature, θ sol-air, can be assessed through the following relation: θ α G = θ + ε R T sol air (1) h o ho α = absorptance of surface for solar radiation [-] G T = total solar radiation incident on the surface [W/(m 2 )] h o = surface heat transfer coefficient by long-wave radiation and convection at outer surface [W/(m 2 K)] θ = outdoor air temperature, [ C] ε = hemispherical emittance of surface [-] R = difference between long-wave radiation incident on surface from sky and surroundings and radiation emitted by blackbody at outdoor air emperature [W/m 2 ] For many practical cases θ sol-air can be assessed, with a sufficient approximation, with a simplified relation obtained assuming R = 0. DESIGN CONDITIONS - 1 Outdoor design conditions must represent the worst, most severe, scenarios that the building and the HVAC system are asked to cope with. It has to be underlined that these conditions are not the worst-of-all weather conditions experienced over the years in a location, but they must represent the statistically relevant worst conditions (those which typically occur from time to time). ASHARE suggests to adopt the following criteria to select the design conditions: Design conditions for warm-season: outdoor air temperature and humidity are based on annual percentiles of 0.4 %, 1.0 %, and 2.0 %. Design conditions for cold-season: are based on annual percentiles of 99.6 % and 99.0 % of the outdoor air temperature. The use of annual percentiles to define design conditions ensures that they represent the same probability of occurrence in any climate, regardless of the seasonal distribution of extreme temperature and humidity. In many countries outdoor design conditions are defined in technical standards and fixed by law. 12

13 DESIGN CONDITIONS - 2 Design data based on dry-bulb temperature represent peak occurrences of the sensible component of ambient outdoor conditions. Design values based on wet-bulb temperature are related to the enthalpy of the outdoor air. Conditions based on dew point relate to the peaks of the humidity ratio. The designer, engineer, or other user must decide which set(s) of conditions and probability of occurrence apply to the design situation under consideration; as a general rule: The 99.6 and 99.0% design conditions are often used in sizing heating equipment. The 0.4, 1.0, and 2.0% dry-bulb temperatures and mean coincident wet-bulb temperatures often represent conditions on hot, mostly sunny days. These are often used in sizing cooling equipment such as chillers or air-conditioning units. Design conditions based on wet-bulb temperature represent extremes of the total sensible plus latent heat of outdoor air. This information is useful for cooling towers, evaporative coolers, and fresh air ventilation system design. DESIGN CONDITIONS DIFFERENT APPROACH BETWEEN HEATING AND COOLING SYSTEMS Design conditions for heating systems 10 usually refer to a fixed condition, 8 corresponding to the statistically worst 6 Heating 4 hour of the year as far as heating demand 2 0 is concerned. This is due to the fact that design procedures for heating system -6 (specially in case of simpler systems) is based on a steady-state calculation -12 Days (therefore a single point condition is sufficient). In case of cooling systems design, it is necessary to rely on calculation methods based on non steady-state regime, able to quantify the energy storage/release of the building structures and furniture's (thermal mass) and the thermal dynamics. This implies that a time profile of the design Cooling conditions is required. Typically a design day (e.g. an infinite sequence of 24 hourly values) has to be defined. Outdoor air Temp. [ C] 13

14 ANNUAL HEATING AND HUMIDIFICATION DESIGN CONDITIONS ASHRAE suggests to adopt the following meteorological parameters to establish the heating design conditions: Choose the coldest month (i.e., month with lowest average dry-bulb temperature); Assess the dry-bulb temperature corresponding to 99.6 and 99.0% annual cumulative frequency of occurrence (statistically relevant coldest conditions); Assess the dew-point temperature corresponding to 99.6 and 99.0% annual cumulative frequency of occurrence and the corresponding specific humidity, calculated at standard atmospheric pressure at elevation of station (grams of moisture per kg of dry air) and mean coincident dry-bulb temperature; Assess the wind speed corresponding to 0.4 and 1.0% cumulative frequency of occurrence for coldest month; Assess the mean wind speed coincident with 99.6% dry-bulb temperature and the corresponding most frequent wind direction. ANNUAL COOLING AND DEHUMIDIFICATION DESIGN CONDITIONS - 1 ASHRAE suggests to adopt the following meteorological parameters to establish the cooling design conditions: Choose the hottest month (i.e., month with highest average dry-bulb temperature); Assess the daily temperature range for hottest month (defined as the mean of the difference between daily maximum and daily minimum dry-bulb temperatures for hottest month); Assess the dry-bulb temperature corresponding to 0.4, 1.0, and 2.0% annual cumulative frequency of occurrence (statistically relevant warmest conditions), Assess the wet-bulb temperature corresponding to 0.4, 1.0, and 2.0% annual cumulative frequency of occurrence and the mean coincident dry-bulb temperature and the corresponding mean coincident dry-bulb temperature; Assess the mean wind speed coincident with 0.4% dry-bulb temperature and the corresponding most frequent wind direction (degrees from north); Cont d 14

15 ANNUAL COOLING AND DEHUMIDIFICATION DESIGN CONDITIONS - 2 Summer design day U Assess the dew-point temperature corresponding to 0.4, 1.0, and 2.0% annual cumulative frequency of occurrence and the corresponding humidity ratio, calculated at the standard atmospheric pressure at elevation of station (g of moisture per kg of dry air) and mean coincident dry-bulb temperature; Assess the enthalpy corresponding to 0.4, 1.0, and 2.0% annual cumulative frequency of occurrence and the mean coincident dry-bulb temperature. Day from any calendar month with a specified return period for extreme values of the significant meteorological parameters (usually dry bulb temperature). ES -E D It is a series of 24 hourly values of: temperature, temperature swing, dew point (or RH), solar irradiation and wind speed EXTREME DESIGN CONDITIONS In case of critical applications design, where even an occasional short-duration capacity shortfall is not acceptable, ASHRAE suggests to adopt the following extreme annual design conditions: ID Wind speed corresponding to 1.0, 2.5, and 5.0% annual cumulative frequency of occurrence, m/s. Extreme maximum wet-bulb temperature, C , 10-, 20-, and 50-year return period values for maximum and minimum extreme dry-bulb temperature, C. 15

16 THE DEGREE DAY CONCEPT - 1 The degree days refer to the concept of accumulated temperature differences. Degree days are a relatively simple form of climatic data, useful as an index of climate severity. Energy consumption for space heating and cooling is a function of the degree days. Calculation or estimation of degree days needs to introduce the concept of a base temperature, θ b. In theory, the base temperature should reflect the point at which buildings begin to need heating or cooling to maintain the required internal temperatures. However, the base temperatures are freqeuntly established and fixed conventionally, at national level, through technical standards and/or decree and laws. A degree day is computed as the integral of a function of time that generally varies with temperature. The function is truncated to upper and lower limits that vary by organism, or to limits that are appropriate for climate control. The assessment of the degree days is different between heating period (HDD = Heating Degree Days) and the cooling period (CDD = Cooling Degree Days) THE DEGREE DAY CONCEPT - 2 The degree day is, therefore, a measure of heating or cooling requirements. Degree days may be used for the following purposes: a) monitoring the amount of energy used by heating/cooling plant, and thus its efficiency (the energy management use); b) providing an index of climatic severity as it affects energy use for space heating and cooling (the comparison use); c) comparing the actual energy consumption for heating/cooling in a specific period with the consumption in a standardized period, in order to determine the operational rating (the energy modeling use); d) predicting, on a preliminary level, the economic consequences of different interventions for energy efficiency (e.g. thermal insulation) (the energy policy use). 16

17 THE DEGREE DAY CONCEPT - 3 Energy management (purpose a) requires the assessment of new degree days data at regular intervals. These degree days are calculated from actual meteorological quantities measured at weather stations. They reflect the real weather conditions, vary year by year and are calculated, on the basis of standard base temperatures. They are, usually, published for each month of the heating/cooling season, as soon as these can be computed from verified meteorological observations. Comparison, energy modeling and energy policy purposes (b, c and d), require degree days representative of the typical climate of a region (and not of actual weather data related to a certain month or season). For this reason they must be assessed on the basis of data collected over many years (possibly giving extremes as well as mean values), to typify the severity of the climate of a locality, area or region. THE HEATING DEGREE DAY (HDD) - 1 Heating degree day (HDD) provides a simple metric for quantifying the amount of heating that buildings in a particular location need over a certain period (e.g. a particular month or year). Heating degree days are a measure of the severity and duration of cold weather. The colder the weather in a given month, the larger the degree-day value for that month. The heating requirements for a given building at a specific location can be roughly considered to be directly proportional to the number of HDD at that location (In conjunction with the average U- value for a building and the average ventilation loss coefficient they, they provide a means of roughly estimating the amount of energy required to heat the building over that period). 17

18 THE HEATING DEGREE DAY (HDD) - 2 One HDD means that the temperature conditions outside the building were equivalent to being below a defined threshold comfort temperature inside the building by one degree for one day. Thus heat has to be provided inside the building to maintain thermal comfort. HDD can be added over periods of time. Heating degree days are defined relative to a base temperature, θ b, and a threshold temperature, θ th. The most appropriate base temperature for any particular building depends on indoor building conditions. The threshold temperature is the outside temperature above which a building needs no heating. Therefore, θ th, for any particular building depends on indoor building conditions (set point temperature) and the nature of the building (including the heatgenerating occupants and equipment within it). For calculations relating to any particular building, HDD should be selected with the most appropriate base and threshold temperatures for that building. However, for practical reasons HDD are often made available with conventional base temperatures and heating periods. THE HEATING DEGREE DAY (HDD) - 3 HDD are usually calculated using simple approximation methods that adopt daily temperature readings instead of more detailed temperature records, such as hourly readings. One popular approximation method is to take the average temperature on any given day, and subtract it from the base temperature. If the value is less than or equal to zero, that day has zero HDD. If the value is positive, that number represents the number of HDD on that day: Where: HDD = τ τ N 0 + ( θ θ ) dτ = ( θ θ ) b dm N d= 1 θ b = base temperature, θ dm = daily mean temperature, τ = time N = number of days in the heating (cooling) season b + dm d [ C day] Superscript + means that only positive differences have to be considered 18

19 THE HEATING DEGREE DAY (HDD) - 4 In theory the value of N is determined choosing the threshold outdoor air temperature, θ th (daily mean). This is the outside daily mean temperature above which a building needs no heating (typically 12 C, but its value can vary according to the building type, location and internal conditions). Nevertheless, N is frequently fixed at national level, by means of technical standard and/or laws, decrees. θ θ b θ th θ dm τ 0 N HDD THE COOLING DEGREE DAY (CDD) - 1 τ N Day cooling degree day (CDD) provides a simple metric for quantifying the amount of cooling that buildings in a particular location need over a certain period (e.g. a particular month or year). While method to assess the HDD is well established, the method for calculating the cooling degree days is not unique. According to the most often adopted definition, cooling degree-days (CDD) are calculated as: τn N + + CDD = ( θdm θb ) dτ = ( θdm θb ) [ C day] d τ d= 1 Where: 0 θ b = base temperature, θ dm = daily mean temperature, τ = time N = number of days in the heating (cooling) season The plus sign (+) of the equation indicates that only positive values are to be counted, meaning that if θ dm < θ b then CDD = 0. 19

20 THE COOLING DEGREE DAY (CDD) - 2 Daily values of CDD are summed to calculate the total number of cooling degreedays over a period in question. Other methods for assessing CDD can be found in the literature. Calculation of CDD can be achieved by using monthly-average daily temperatures as well as monthly-average solar radiation and ambient temperature data in combination (sol-air temperature degree days). DEGREE DAYS AND BUILDINGS DYNAMIC ENERGY SIMULATION Degree days are suited to roughly assess the energy performance of relatively small buildings with simple heating systems and controls, using quasi steady-state thermal analysis. A more reliable calculation of the energy performance of a building, or the modeling of the performance of larger and more complex buildings, cannot be done with the degree days concept. In such cases more extensive climatological data sets are needed, such as full or short reference weather year. Software are available to simulate the annual energy performance of buildings requiring a 1-year data set (8760 records, one for each hour of the year) of weather conditions (that is a reference weather year ). 20

21 BUILDINGS DYNAMIC ENERGY SIMULATION AND METEO DATA - 1 A reference weather year (variously known as Test Reference Year - TRY, Typical Meteorological Year - TMY, International Weather Years for Energy Calculations IWECs, ) is a single year of hourly data (8760 hours), selected to represent the range of weather patterns that would typically be found in a multi-year dataset. Therefore, it is a sort of average year or typical year for a given location and time frame. It is intended to allow more economical simulation than multi-year datasets, and to form an equitable basis for comparing the predicted typical energy consumptions of different building designs and, in some cases, the typical performance of solar collectors. ES -E D U Definition of a Reference Year depends on satisfying a set of statistical tests relating it to the multi-year parent dataset, from which it is drawn. Some sources have preferred to identify a continuous, 12-month period, as typical; whereas others have applied the criteria to individual months, subsequently assembled into a composite 12-month year. BUILDINGS DYNAMIC ENERGY SIMULATION AND METEO DATA - 2 ID Many data sets in different record formats have been developed to meet these requirements. The reference weather data, typically and traditionally used in building and solar energy simulations, are: International Weather for Energy Calculations (IWYEC) and Typical Meteorological Year (TMY, Type 2 and 3) in the United States and Canada, the Test Reference Year (TRY) in Europe. These data represent a typical year with respect to weather-induced energy loads on a building. The main difference between the various type of reference weather years (TRY, TMY, IWEC) lies in the way data are collected, analyzed and assembled. LIMITATIONS: because reference weather years represent typical and average (over long periods) rather than extreme conditions: they do not meet the worst-case conditions occurring at a location, therefore they are NOT SUITED FOR DESIGNING SYSTEMS. no indication of the full range of possible conditions is given, therefore there are several issues when considering, peak heating loads, human comfort or overheating levels. 21

22 TYPICAL METEOROLOGICAL YEAR - TMY A typical meteorological year (TMY) is a data set of hourly values of solar radiation and meteorological quantities for a 1-year periods, for a specific location, generated from a data set much longer than a year in duration. The data set is created through selecting, by statistical methods, one Typical Meteorological Month (TMM) for each of the 12 calendar months from a period of years (preferably 30) of data and concatenating the 12 months to form a TMY. The final TMY file consists of hourly data for an annual period, but each month is from a different year. There exist different formats for TMY (TMY2, TMY3). U TMY represents the range of weather phenomena for the location in question, while still giving annual averages that are consistent with the long-term averages for the location. ES -E D Their intended use is for computer simulations of solar energy conversion systems and building systems to facilitate performance comparisons of different system types, configurations, and locations. For example, EnergyPlus, TRNSYS and PVSOL support simulations using TMY. TEST REFERNCE YEAR - TRY ID A test Reference Year (TRY) is a data set with a structure similar to that of the TMY. It is a 1-year sequences of 8760 hourly values of: dry bulb temperature, vapor pressure (or other humidity parameter), global solar radiation or both direct and diffuse radiation on a horizontal surface and wind speed at a height of 10 meters, together with the date and time stamps. The details (location and altitude) of the station, the period of the original data set and the years from which the individual months were taken shall be reported. The main difference between the TMY and the TRY is the procedure and the statistical methods used to assemble the data from databases constituted by 10, 20 or 30 years of measured data. Care has to be taken since different assembling methods are used in different countries to create TRY. One great limitation involved in the creation of a TRY weather file is that, frequently, it does not contain any measured solar radiation values. Reported radiation data are often estimated and obtained through calculation of solar radiation based on the cloud cover factors and cloud type. 22

23 INTERNATIONAL WEATHER FOR ENERGY CALCULATIONS (IWEC WEATHER FILES) - 1 The selection criteria for the ASHRAE International Weather Years for Energy Calculations (IWECs) files is similar to the TMY selection process, but instead uses nine weighted weather parameters. IWECs data sets contain "typical" weather data in ASCII format (a header and 8760 hourly records). ES -E D U The International Weather for Energy Calculation (IWEC) files are derived from up to 18 years of hourly weather data. The weather data is supplemented by solar radiations estimated on an hourly basis from earth-sun geometry and hourly weather elements (cloud cover, atmospheric features). INTERNATIONAL WEATHER FOR ENERGY CALCULATIONS (IWEC WEATHER FILES) - 2 The IWEC files are well suited to the following uses: ID Input to building energy hourly simulation software (such as DOE-2, BLAST or EnergyPlus) to estimate the typical annual energy consumption of buildings. The IWEC files are not suited to the following uses: simulation of wind energy and/or solar energy systems (the weights used for the selection of typical months are heavily biased toward dry bulb temperature and solar radiation is estimated). Sizing of systems. The IWEC files are 'typical years' that normally stay away from extreme conditions. 23

24 ARTIFICAILLY GENERATED TMY Weather data are not available for any site worldwide. Moreover, for many sites the data are available in an aggregated form (e.g. daily or monthly average values). Furthermore, measured data when existing are typically referred to a weather station that can be further apart from the building location. In these cases, software are needed to calculate hourly values from the monthly values using stochastic models and to interpolated between weather stations. Generally, a statistical approach should be considered as a last resort. Weather files generated from statistics will not exactly match the normal hour-to-hour and day-today variability seen in measured data. ES -E D U An example of software suited for these purposes is Meteonorm (meteonorm.com). The program's calculation algorithms provide the basis for generating hourly values for global radiation, temperature and other meteorological parameters. The resulting time series correspond to "typical years". A sophisticated interpolation models allows a calculation of at any site in the world. CARE IN USING METEOROLOGICAL QUANTITIES AND CLIMATE PARAMETERS ID Most of the observed data for which design/typical conditions were assessed are collected from airport observing sites, the majority of which are flat, grassy, open areas, away from buildings and trees or other local influences. Temperatures recorded in these areas may be significantly different (3 to 5 C lower) compared to areas where the design conditions are being applied. Significant variations can also occur with changes in local elevation, across large metropolitan areas, or in the vicinity of large bodies of water. Judgment must always be exercised in assessing the representativeness of the design conditions. Wind speed and direction are very sensitive to local exposure features, such as terrain and surface cover. The original wind data used to calculate the wind speed and direction design conditions are often representative of a flat, open exposure, such as at airports. Wind engineering methods can be used to account for exposure differences between airport and building sites. These topics will be dealt with in lectures 7 and 8. 24

25 References and relevant bibliography 2005 ASHRAE Handbook of Fundamentals - Chapter 28 Climatic Design Information, ASHRAE, Atlanta, USA, Elementi di Climatologia Edilizia, G. Fracastoro, Ed. Celid ISBN , Torino, Italy, 1982 Standard EN-ISO parts 1, 2, 4 and