EnergyPlus building simulation software:

Which Ground Temperature to use?

Azhaili Baharun, Siti Halipah, Mohamad Omar Abdullah, Ooi Koon Beng*

Universiti Malaysia Sarawak (UNIMAS)

Corresponding email: ooi_kb3@hotmail.com

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Abstract

Many low-energy residential buildings are single-storey, with the floor in contact with the ground.

There is less information about ground temperatures than weather data, thus normally, the former is

calculated from the latter. There are three 'Ground temperatures in the EnergyPlus building simulation software. The first, the Undisturbed 'Ground temperature in the Statistics and Design Day weather files and the second Ground temperature from the 'Slab' and 'Basement' preprocessors, are used in the same input object in the input data file (IDF). The third Ground temperature is given as an output variable when the ZoneEarthTube object is used in the IDF, and a CalcSoilSurfTemp

preprocessor is available to calculate for three input data viz. the average, amplitude and phase of the

temperature at the ground surface, for predefined conditions at the surface and below. The damping

depth is the depth below which the ground temperature is not affected by variations in surface

temperature. The damping depth due to daily variations in surface temperature is not significant and

thus temperatures below due to yearly surface temperature variations are considered. Descriptions of

each of the three 'ground temperatures is summarized in a table. The assumptions in the derivation of the three Ground temperatures differ in detail. While the outdoor temperature is used to calculate the Undisturbed ground temperatures, solar radiation, moisture evaporation at the surface and relative humidity of the outdoor air are also used by the CalcSoilSurfTemp preprocessor. The indoor condition

of the building also determines which set of ground temperatures to use. The Slab and Basement preprocessors are meant for buildings where the indoor is conditioned. The discussion includes

analysis of some derived ground temperatures. A chart is proposed to approximate ground temperatures using surface and deep ground temperatures.

Keywords: EnergyPlus, Chart, Ground temperatures, Surface and deep temperatures

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1.0 INTRODUCTION

A researcher using EnergyPlus to find the effect of the ground/earth/soil in the passive cooling

of buildings may find its three 'Ground temperatures confusing. The first Ground temperatures are the 'Undisturbed' ground temperatures at 0.5, 2 and 4 meter depths given in the Statistics (.stat) and Design

Day (.ddy) files that accompany the EnergyPlus weather (.epw) file, with typical meteorological year

(TMY) data that can be downloaded from the primary website www.energyplus.gov. The second ground temperatures are from the 'Slab' and 'Basement' preprocessors. These first and second set of monthly averaged temperatures, are entries into the Site:GroundTemperature:BuildingSurface object of the

EnergyPlus Input Data File (IDF). The third Ground temperatures are those output from simulations that use the ZoneEarthTube object.

The collection of weather data, mostly at international airports, over the last 20-30 years, has

enabled a set of hourly weather data for a typical meteorological year (TMY) at most World

Meteorological Organisation (WMO) or the U.S. World Bureau Army Navy (WBAN) stations. However,

the temperatures below the ground, where the floor or basement of a building may be in contact with, are

not so readily available. Thus, ground temperatures are normally calculated from weather data files.

2.0 LITERATURE REVIEW

Norfziger[1]

gave the formula for damping depth of D = (2/) where , the thermal diffusivity (thermal conductivity divided by specific heat) is assumed constant, and is the frequency of temperature variations at the surface.

While Anna Houston et al[2]

gives descriptions on how to experimentally determine thermal

conductivity, Givoni[3]

gives descriptions on how to experimentally determine the thermal diffusivity.

Givoni[3]

also states Soil temperatures, as a function of depth and time, is usually expressed as a sum of one or more exponentially damped sinusoidal temperature waves. The formulae for the

temperature waves are solutions to the heat equation in which the ground is considered as a semi-infinite

solid with a plane surface at a uniform temperature. If the ground surface temperature is known or can be

estimated, its temporal variation is the boundary condition. The three parameters in the temporal variations are the average temperature at the surface, the amplitude (or minimum) surface temperature

and the time the minimum temperature occurs. The thermal diffusivity of the soil is also used to find the

ground temperatures. Figure 1 shows the Undisturbed Ground Temperatures that use a standard thermal diffusivity of 0.002322576 m

2/day from the Energyplus statistical (.stat) file.

Figure 1. Undisturbed Ground Temperatures from weather data and 0.002322576 m2/day thermal diffusivity. Source: Plotted from data in the statistics (.stat) file of typical meteorological year weather data.

3.0 METHODOLOGY

Based on Norfzigers formula, the damping depths due to day and year variations of surface temperatures are compared in Table 1 for the soil thermal diffusivities used in the calculations of the first

set of monthly averaged ground temperatures and the four predefined soil conditions below the surface

used in the CalcSoilSurfTemp preprocessor.

The damping depth due to daily variation is less than 16 cm and EnergyPlus calculations usually

use monthly averaged values in a years variation.

Table 1. Damping depths for the thermal diffusivities used in EnergyPlus.

Soil Condition (Type,

moisture content) below

surface

, Thermal Diffusivity (m

2/day)

Damping Depth for

day variations, Dd (m)

Damping Depth for year

variations Dy (m)

0.002322576 0.0278 0.5311

( for calculating Undisturbed Ground temperatures in .stat and .ddy files)

(Heavy soil, saturated) 0.0781 0.1575 3.01

(Heavy soil, damp solid

masonry)

0.055728 0.1332 2.545

(Heavy soil, dry) 0.04458 0.1189 2.2715

(Light soil, dry) 0.024192 0.0877 1.675 Source: (summarized from EnergyPlus v7 Documentation by ooi koon beng, 2012)

A summary of the features/data for the three ground temperatures in EnergyPlus, the method to derive the ground temperature or the average, ground surface temperature for the Earth Tube object, the software object where the ground data is used and the type of building where this feature is applicable, is given in Table 2. ODT is the outdoor dry bulb temperature. Descriptions of each of the three

Ground temperatures are given in the following subsections.

Table 2. Ground temperatures, method to derive the ground temperature, where the data is used or shown, and the building where this ground temperature is applicable.

# Feature/ground temperature data

Method to derive this

ground temperature Object/variable where

data is used/shown.

Building where

feature is applicable

1 Monthly Undisturbed

'ground temperatures at 3 depths given in .stat

(Statistics) and .ddy

(design day) files.

Based on Mean monthly

ODT, minimum ODT and

date of minimum ODT, of a

TMY and a constant soil

thermal diffusivity.

Site:GroundTemperat

ure:BuildingSurface

object of the input

data file (IDF).

Naturally ventilated

or free running Buildings.

2 The average ground temperatures at the outside faces of floors or

basement output by the

Slab or Basement

preprocessors

These 3-D heat transfer

preprocessors assume a

constant indoor air

temperature throughout the

year.

Site:GroundTemperat

ure:BuildingSurface

object of the input

data file (IDF). .

To find the energy

to condition the

indoor air to a

setpoint/comfort

temperature.

3 Ground temperatures at the outside face of the

earth tube, in contact with

the ground.

Uses the average, amplitude

and phase of the surface

temperature for 8 predefined

surfaces and 4 predefined

soil conditions below

Ground Interface temperature output with simulations that

use ZoneEarthTube

object.

When Earth Tube is

used to precool or

condition the air for

ventilating the

building.

Source: (original table by the author, ooi koon beng, March 2012)

2.3.1 UNDISTURBED GROUND TEMPERATURES

Statistics from the typical meteorological year (TMY) data is given in a .stat file. The monthly

averaged outdoor temperature (ODT), the minimum ODT and the time of the minimum ODT are used as

the temporal variation stated by Givonni. Using a soil thermal diffusivity of 0.002322576 m2/day, the

Undisturbed ground temperatures, Feature 1 in Table 2, were shown in Figure 1. The EnergyPlus team,

with purported references to T. Kusuda (1971[4]

, 1967[5]

) work gives the exponentially decaying

temperature wave that describes the Undisturbed 'ground temperatures at three depths. The amplitude of the sine wave is half of the difference between the maximum monthly mean and the minimum monthly

mean outdoor temperature.

The above Undisturbed ground temperatures are based on ground without the building over it. If

there is a building over the ground, the 'Ground temperature could be different because the building covers the ground and the temperature on the indoor face of the floor is that of the indoor air and not that

of the outdoor air. These Undisturbed ground temperatures may be used as the Site:GroundTemperature:BuildingSurface for naturally ventilated or free running buildings.

2.3.2 Ground Temperature from Slab and Basement Preprocessors

The Slab and Basement preprocessors will calculate the monthly ground temperatures on the outside face of floors or basements, in contact with the ground. These are also used in the

Site:GroundTemperature:BuildingSurface object. The purpose of these 3-D heat transfer preprocessors is

to enable a more accurate calculation of the energy required to condition the indoor air to a setpoint,

normally the comfort temperature. By default, EnergyPlus computes for the dynamic heat transfers through the planar building surfaces using 1-D heat flow algorithms.

For large floors, EnergyPlus can also give separate ground temperatures at the Core and

Perimeter of the slab. EnergyPlus documentation (Auxiliary Programs) also reports that a 4C range in the daily indoor air temperature variation may not have any significant effect on the heat fluxes through

floors. Iterative use of these preprocessors would give better results of temperatures at the inside and

outside faces, and thus more accurate energy for conditioning the indoors.

Since EnergyPlus version 6, users may use the Detailed Ground Heat Transfer group of objects

instead of these two preprocessors. This means that the 12 monthly averaged (mean) ground temperatures

output from the preprocessor are automatically transferred to and entered into the 12 fields of the

Site:GroundTemperature:BuildingSurface object within each simulation run. The fields in the

GroundHeatTransfer:Control object enable the user to control whether the Slab or Basement preprocessor

is to be executed during a simulation run.

2.3.3 Ground Temperature on the outside surface of EarthTube, output from a simulation.

The following fields of the ZoneEarthTube object in the Input Data File (IDF) require mandatory

data:

Soil Condition, around the earth tube. Select from 4 predefined soil conditions, given in Table 1.

Average Soil Surface Temperature {C}

Amplitude of Soil Surface Temperature {C}

Phase Constant of Soil Surface Temperature {days}

The average, the amplitude and the phase of the approximated sinusoidal annual temperature

variation at the ground/earth/soil surface can be found by the CalcSoilSurfTemp preprocessor, for eight

predefined (four each for Bare and Covered) surfaces, and four predefined soil conditions under the

surface (around the earth tube), for any location defined by the EnergyPlus weather (.epw) file.

The CalcSoilSurfTemp preprocessor considers the solar radiations during the day, longwave

radiations at night, and relative humidity in the outdoor air and the rate of evaporation of the moisture at

the surface. EnergyPlus Engineering Reference[7]

gives the indicated fraction of the evaporation rate, f,

which depends mainly on the soil cover and the soil moisture level.

Figure 2 shows the average, amplitude and phase of the soil surface temperatures from the

CalcSoilSurfTemp preprocessor, for the 8 predefined soil conditions at the surface, for Heavy and

Saturated Soil condition below. When the surfaces are Covered, the annual average temperature of the

surface is higher than those when the surfaces are Bare and there is no difference between annual average

temperatures for Covered and Bare surfaces when the surface is dry. What is not shown is that the

average surface temperature is not affected by the soil condition below.

Figure 2. Average, amplitude and phase of Soil Surface Temperatures with CalcSoilSurfTemp utility. Source: Plotted from data output by the CalcSoilSurfTemp preprocessor using Kuching weather data.

Figure 3 shows the Amplitude of the Soil Surface Temperature for 4 soil conditions (Type of soil

and moisture) around the earth tube.

Figure 3. Amplitude of Soil Surface Temperature for 4 soil conditions (type of soil and moisture) around

Earth Tube. Source: Plotted from data output by the CalcSoilSurfTemp preprocessor using Kuching weather data.

3.0 DISCUSSIONS

1) Figure 4 shows that there are insignificant differences of the ground temperatures at 0.5 meter depth for the thermal diffusivities used for the Undisturbed ground temperatures and the four predefined soil conditions around the earth tube.

Figure 4. 0.5m deep Undisturbed Ground Temperatures for five thermal diffusivities. Source data using the Fortran formula that calculates the Undisturbed ground temperatures in the .stat file.

2) Figure 5 shows the maximum, average and minimum monthly mean Ground temperature, after a simulation run using a small (0.05m radius) object Earth Tube, with zero air flow, surrounded by Light

and Dry Soil, under Bare and Wet Surface, for Kuching, at depths down to 10m. The ground surface

temperatures are from the CalcSoilSurfTemp preprocessor. Givonni (1994) has also shown that at 10 m

depth the temperatures do not vary and this depth can be considered deep.

Figure 5. Maximum, Medium and Minimum Ground temperatures at various depths. Source: Charted from Outputs of simulation runs using the ZoneEarthTube object.

Measurements in Kuching in September 2011 (Ooi, 2011) [8]

show that the peak temperature of

turfed surfaces can be up to 7C lower than the outdoor temperature. Measurements in Singapore in November 2001 by Wong (Chan, 2008) show that the peak daytime temperature of turfed surfaces are

also lowered by up to 7C if covered by shrubs, and the temperatures of shrub covered turfed surfaces

during the day are about 2C higher than those at night. The maximum monthly mean temperature being 1.6C and 2.2C higher than the minimum monthly mean temperature for Kuching and Singapore respectively, show that the measured ground surface temperatures are close to twice the 2.5C amplitude of the monthly mean temperatures for bare and moist surfaces calculated by the CalcSoilSurfTemp pre-

processor shown in Figure 3. Verification of the calculations requires long term measured data.

4.0 CONCLUSIONS

When data on surface temperatures with thermal diffusivity of the soil is available for a year, the

maximum, average and minimum temperatures at various depths can be obtained from a chart. Hopefully,

this paper would help to save the time of new users in their research into the use of the ground

temperatures for passive buildings.

ACKNOWLEDGEMENT

Linda Lawrie of DHL Consulting LLC, member of EnergyPlus development team, for providing the

Fortran formula in the Weather preprocessor that calculates the undisturbed ground temperatures in the

.stat file.

Michael J Witte of GARD Analytics for clarifications on the use of Slab and Basement preprocessors and

other EnergyPlus developers/users for their contributions in the yahoo forum.

REFERENCES:

[1] D.L. Nofziger, Soil Temperature Changes with Time and Depth: Theory,

http://soilphysics.okstate.edu/software/SoilTemperature/document.pdf

[2] Anna Houston, Grant Tranter, and Ian Miller, Temperature Waves,

http://www.usyd.edu.au/agric/web04/Temperature%20Waves_final.htm viewed 15th February 2012.

[3] Givoni B. (1994) Passive and Low Energy cooling of buildings Van Nostrand Reinhold [4] T. Kusuda, Earth Temperatures Beneath Five Different Surfaces. Institute for Applied Technology, NBS Report 10-373, 1971, National Bureau of Standards (NBS), Washington DC 20234

[5] T. Kusuda, Least Squares Technique for the Analysis of Periodic Temperature of the Earths Surface Region, NBS Journal of Research, Vol 71C, Jan-Mar, 1967, pp 43-50.

[6] ASHRAE Handbook of HVAC Applications (Table 4, pp 11.4)

[7] EngineeringReference, EnergyPlus Documentation v7. www.energyplus.gov

[8] Ooi K.B 2011. The use of Dynamic Thermal Modelling and simulation in designing a comfortable

passive Malaysian Building. PhD Thesis, Civil Engineering, Universiti Malaysia Sarawak.

[9] Chan, S. A. (2008). Green Building Index - MS1525.Applying MS1525:2007 Code of

Practice on Energy Efficiency and Use of Renewable Energy for Non-Residential

BuildingsKuala Lumpur: Pertubuhan Arkitek Malaysia.CPD Seminar 14th

February 2009.

http://www.greenbuildingindex.org/Resources/20090214viewed 22nd

August 2011