Summer Design Weather Data

Location tab on model data at Site level

Cooling design and simulation autosizing simulations use simplified summer design weather based on worst-case design days.

The weather data used in cooling design calculations is as follows:

Maximum outside dry-bulb temperature - the maximum dry bulb air temperature over the day
Minimum outside dry-bulb temperature - the minimum dry bulb air temperature (night time)
Wet-bulb temperature at the time of the maximum dry-bulb temperature.

In addition for zone cooling load simulation autosizing this additional data is used:

Dry bulb temperature range modifier type to help define the daily profile of the outside air temperature
Wind speed and direction for use in Calculated natural ventilation autosizing calculations.

The degree of 'confidence' of the design data can be selected, i.e. the probability that the design data will occur in practice based on conditions encountered over recent years. This risk can be specified based on dry or wet-bulb temperatures, for 99.6, 99 and 98% confidence (i.e. 0.4, 1 or 2% chance of more extreme weather occurring).

See also the Site level data and weather files and Cooling Design Calculations tutorials

Sky

Solar model indicator

Choose the method to be used for solar modelling in the calculation of the solar radiation in the summer design day(s):

1-ASHRAE Clear Sky, the default model described in ASHRAE HOF 2005 Chapter 31, can be used to estimate hourly clear-day solar radiation for any month of the year in U.S. or similar temperate climates in the northern hemisphere. EnergyPlus calculations extend the clear sky application to both northern and southern hemispheres. Calculations use a Clearness value as part of their calculations. The Clear Sky model usually over estimates the amount of solar radiation available to the building.
2-ASHRAE Tau, aka the ASHRAE Revised Clear Sky Model is introduced in the ASHRAE 2009 HOF as a revised clear sky model based on location-specific optical depths for direct and diffuse radiation. These values are tabulated by month for all 5564 locations in the ASHRAE design data that accompanies the 2009 HOF. This data is included with EnergyPlus installations in C:\EnergyPlusV8-9-0\PreProcess\WeatherConverter\ASHRAE_2013_Monthly_DesignConditions.csv. The Taub and Taud design values for each month of the year are provided in the first 24 data columns. Studies done as part of ASHRAE research projects show that the revised tau model produces more physically plausible irradiance values than does the traditional clear sky model. In particular, diffuse irradiance values are more realistic. Taub and Taud values can be entered as described below.
3-Zhang Huang, developed for initial use in modelling typical meteorological years for China weather sites, this model seems to be good for other locations as well. Using total cloud cover, dry-bulb temperature, relative humidity, and wind speed as the independent variables, the total (global horizontal) solar radiation is estimated through a simple equation. This is the same model used to estimate the global solar radiation value when it is absent from the source data in the Weather Converter. Calculations use a Clearness value as part of their calculations.
4-Schedule, allows you to enter custom Beam solar day and Diffuse solar day scheduled values for the day’s profile (below).

Beam solar day schedule

Select your own custom day profile of beam solar values (Wh/m2) when the Solar model indicator is selected as 4-Schedule. These values will replace the calculated values during design day processing. Only Day schedules can be selected here.

Diffuse solar day schedule

Select your own custom day profile of diffuse solar values (Wh/m2) when the Solar model indicator is selected as 4-Schedule. These values will replace the calculated values during design day processing. Only Day schedules can be selected here.

ASHRAE clear sky optical depth for beam irradiance (Taub)

The Optical depth for beam radiation must be entered when the Solar model indicator is selected as 2-ASHRAE Tau. Taub and Taud values are tabulated by month for 5564 locations worldwide on the CD that accompanies the ASHRAE 2009 HOF and in ASHRAE_2013_Monthly_DesignConditions.csv that is installed with EnergyPlus.

ASHRAE clear sky optical depth for diffuse irradiance (Taud)

The Optical depth for diffuse radiation must be entered when the Solar model indicator is selected as 2-ASHRAE Tau. Taub and Taud values are tabulated by month for 5564 locations worldwide on the CD that accompanies the ASHRAE 2009 HOF and in ASHRAE_2013_Monthly_DesignConditions.csv that is installed with EnergyPlus.

Sky clearness

The sky “clearness” index for the summer design day and must be entered when the Solar model indicator is selected as 1-ASHRAE clear sky or 3-Zhang Huang. Along with the solar position as defined by the Location information and the date, this value helps define the solar radiation for each hour of the design day. Clearness may range from 0.0 to 1.2, where 1.0 represents a clear sky at sea level. Values greater than 1.0 may be used for high altitude locations.

Note: This “sky clearness” does not have the same meaning as the EnergyPlus output variable “Site Daylighting Model Sky Clearness”.

Weather Data Modifiers

The daily temperature profile used in the cooling design calculations is calculated from the maximum and minimum values using a modified sinusoidal curve and assuming that the maximum temperature occurs at 15:00 and the minimum at 5:00. You can change this behaviour by using the data described below.

Dry-bulb temperature range modifier type

DesignBuilder provides some options to give more control over the dry-bulb temperatures used for the design days:

1-Default multipliers - where EnergyPlus automatically generates the outside dry-bulb temperature profile based on the data provided in ASHRAE 2009 HOF.
2-Multiplier schedule - where you can define a day schedule that specifies a multiplier applied to the daily temperature range to create the proper dry-bulb temperature range profile for your design day. This works as described below.
3-Temperature profiles - allows you to use a Dry-bulb temperature profile schedule to define the dry-bulb temperature for the design day.

Important note: When using multiple summer design months these options are applied identically for each month. They are therefore generally of more use for single design months, i.e. when 1-Single design month is selected as the Design temperature period.

When using the 1-Default multipliers option, the multipliers are taken from the ASHRAE 2009 HOF. More specifically, EnergyPlus creates an air temperature for each timestep by using the entered maximum dry-bulb temperature in conjunction with the entered daily range and the above multiplier values. The actual equation used is shown below:

T_current = T_max - T_range x T_multiplier

where:

T_current= Air temperature of current hour of day

T_Max = User supplied Max Dry-bulb Temperature

T_range = The daily temperature range (T_max - T_min)

T_Multiplier = Range multiplier as shown on the above graph

The range multiplier values represent typical conditions of diurnal temperatures (i.e. the low temperature for the day occurring about 5:00 AM and the maximum temperature for the day occurring about 15:00. Note that EnergyPlus does not shift the profile based on the time of solar noon as is optionally allowed in ASHRAE procedures.

ASHRAE research indicates that dry-bulb and wet-bulb temperatures typically follow the same profile, so EnergyPlus can use the default profile to generate humidity conditions.

Dry-bulb temperature range modifier schedule

If you would like to override the default ASHRAE profile you can select the 2-Multiplier schedule option for the Dry-bulb temperature range modifier type setting. This shows allows a Day schedule to be selected to define the range multipliers for each time of the cooling design day.

Dry-bulb temperature profile schedule

Select the Day schedule to be used to represent the dry-bulb temperature for the design day.

Note: The temperature values in the schedule must be defined in °C even when SI units are selected.

Humidity condition type

This option allows the method to be used to define humidity to be defined. It is only made available when the Dry-bulb temperature range modifier type is set to 3-Temperature profiles. Select from:

1-Wet-bulb, the default option where a wet-bulb temperature at the time of maximum temperature is used. The as defined using the coincident wet-bulb temperature further down the data sheet.
2-Relative humidity, where the hourly variation of relative humidity is defined using the Humidity condition schedule.

Humidity condition schedule

Select the Day schedule which defines the relative humidity in %.

Wind Data

Wind speed

This is the wind speed to be used for calculating the exterior convective heat transfer coefficients and for use (only in Calculated natural ventilation autosizing simulations) for calculating the exterior wind pressure along with the surface wind press coefficients.

Wind direction

Used only for only in autosizing simulations of Calculated natural ventilation models for calculating the exterior wind pressure along with the surface wind press coefficients.

Sizing Period

When running simulations, some design features may benefit by using a longer design weather period for HVAC autosizing or loads calculations. Longer design periods may be set up by allowing the hourly weather file to be used in the sizing calculations.

Autosize method

Choose the design HVAC autosizing method to use for simulations from the following options:

1-Design day - the default option where data for one or more summer design days is used based on the settings described above.
2-Weather file days where you can select the period to be used from the weather file for sizing.
3-Weather file condition type where you can select from some standard per-defined types of period such as 1-Summer extreme.

When 2-Weather file days is selected, enter the From Start day and Month and the To Start day and Month as shown in the screenshot below.

Period selection

When the 3-Weather file condition type Autosize method is selected you must choose the Weather file condition type from the following list.

1-Summer extreme (default setting)
2-Summer typical
3-Winter extreme
4-Winter typical
5-Autumn typical
6-Spring typical
7-Wet season
8-Dry season
9-No wet season
10-No dry season
11-Tropical hot
12-Tropical cold

Note: Not all weather files have all of the valid choices so this option is not completely generic. For example there may be extreme cold periods in some weather files but extreme wet periods (tropical) in others.

Design Temperature Period

The design month for cooling design and autosizing simulations is not always clear from the outset and indeed is often different for different parts of the same building. You can therefore select up to 12 design months to be used in cooling design and autosizing simulations.

You can choose to enter design data for either 1 or multiple months:

1-Single design month where a single set of worst case design data is used for cooling design and autosizing simulations. In this case the design data is typically for the month of July in the Northern hemisphere and January in the Southern hemisphere.
2-Multiple design months where design data for more than one month is used for cooling design and autosizing simulations. In this case the range of months to be used are selected on either the Cooling design calculation options or Model options dialog and the design data to be used for each month is specified here on the Location tab.

Tip: The 2-Multiple design months option is the safest choice as the peak cooling month may vary across the building depending on the amount of glazing and shading on each façade. For example, for buildings in the Northern Hemisphere, large areas of glazing on south-facing façades may cause the peak design month to be delayed until August or even September when the sun is lower in the sky.

Note: The details of the design day(s): day of the month, the month range and day of week to be used for the Cooling design analysis can be defined on the Model options and Cooling design dialogs.

Yearly Design Temperatures

When the Design temperature period is selected as 1-Single design month the following data can be entered for a single month.

Dry-bulb and mean coincident wet-bulb data

Max dry-bulb temperature - the maximum dry bulb air temperature over the day.
Coincident wet-bulb temperature - the wet-bulb temperature at the time of the maximum dry-bulb temperature.
Min dry-bulb temperature - the minimum dry bulb air temperature (night time). The template values have been calculated by DesignBuilder by subtracting the 0.4% Monthly Design Dry Bulb Temperature with the Mean Daily Dry Bulb Temperature Range Coincident with 5% Design Dry Bulb Temperature.

Wet-bulb and mean coincident dry-bulb data

Max wet-bulb temperature - the maximum wet-bulb air temperature over the day.
Coincident dry-bulb temperature - the dry-bulb temperature at the time of the maximum wet-bulb temperature.
Min dry-bulb temperature - the minimum dry-bulb air temperature (night time). The template values have been calculated by DesignBuilder by subtracting the 0.4% Monthly Design Dry Bulb Temperature with the Mean Daily Dry Bulb Temperature Range Coincident with 5% Design Dry Bulb Temperature.

Monthly Design Temperatures

When the Design temperature period is selected as 2-Multiple design months then the following data can be entered for 12 months.

Dry-bulb and mean coincident wet-bulb data

Monthly Design Dry-Bulb Temperatures - the maximum dry bulb air temperatures over the design day for each month.
Monthly Coincident Wet-Bulb Temperatures - the wet-bulb temperatures at the time of the maximum dry-bulb temperature.
Monthly Minimum Dry-Bulb Temperatures - the minimum dry bulb air temperature (night time).

Wet-bulb and mean coincident dry-bulb data

Monthly Design Wet-Bulb Temperatures - the maximum wet-bulb air temperatures over the design day for each month.
Monthly Coincident Dry-Bulb Temperatures - the dry-bulb temperatures at the time of the maximum wet-bulb temperature.
Monthly Minimum Wet-Bulb Temperatures - the minimum wet-bulb air temperature (night time).