Design Day Weather Overview

Advanced HVAC allows for detailed HVAC sizing simulations using EnergyPlus. These simulations utilise 'Design Day' weather input to generate a full day of weather data. You can specify multiple design days, and input parameters are mostly self-explanatory. Additional details are provided in fields like Daily Dry-bulb temperature range, Solar Model Indicator, and Sky Clearness.

Advanced HVAC includes one Annual Summer (0.4% cooling) Design Day and one Annual Winter (99.6% heating) Design Day. These design day conditions are generated using the latest available .ddy file from climate.onebuilding.org for the closest weather file location to your project site.

Daily Dry-bulb Temperature Range

The daily dry-bulb temperature range spans from the maximum to minimum temperatures of this Design Day. EnergyPlus allocates this range across the 24 hours of the day, illustrated below.

The default multipliers are based on ASHRAE 2009 HOF. Specifically, EnergyPlus computes the air temperature for each timestep using the provided maximum dry-bulb temperature along with the entered daily range and the given multiplier values. The precise equation is displayed below:

Tcurrent=TmaxTrangeTmultiplerT_{current} = T_{max} \cdot T_{range} \cdot T_{multipler}

Tcurrent = Air temperature of current hour of day

TMax - User-supplied max dry-bulb temp

Trange = User-supplied daily temperature range

Tmuliplier = Range multiplier as shown on the above graph

The range multiplier values depict diurnal temperature patterns, where the lowest temperature occurs around 5:00 AM and the highest around 3:00 PM. It's important to note that EnergyPlus doesn't adjust the profile based on solar noon, as permitted in ASHRAE procedures.

According to ASHRAE research, dry-bulb and wet-bulb temperatures generally follow a similar pattern. Therefore, EnergyPlus can employ the default profile to determine humidity conditions.

Solar Model Indicator

The Solar Model Indicator offers a choice between ASHRAEClearSky, ASHRAETau, ASHRAETau2017, and ZhangHuang for solar modelling when calculating solar radiation on the design day.

ASHRAEClearSky and ZhangHuang incorporate the Clearness value in their computations. ASHRAETau applies the updated model detailed in Chapter 14 of the ASHRAE 2009 Handbook of Fundamentals, utilising Taub and Taud values.

Similarly, ASHRAETau2017 employs the revised model specified in Chapter 14 of the ASHRAE 2017 Handbook of Fundamentals, utilising Taub and Taud values (outlined below). For comprehensive technical information on these models, consult the Engineering Reference.

Sky Clearness

This numerical value should be provided if ASHRAEClearSky or ZhangHuang is selected in the Solar Model Indicator field. It represents the "clearness" value for the day. Alongside the solar position determined by the Location information and the specified date for the design day, this value helps determine the solar radiation levels for each hour of the day. Clearness can range from 0.0 to 1.2, with 1.0 indicating a clear sea-level sky. Values exceeding 1.0 may be used for high-altitude locations. Typically, 0.0 clearness is used for Winter Design Days. Please note that this "sky clearness" differs from the meaning of the output variable "Site Daylighting Model Sky Clearness".

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