by Ismail Zain, ST – Studio Gentra (Green Building Simulation Studio)
The building performance simulations and calculations are very important things for the future performance prediction of the building we have designed. The simulation process has been supported by various tools such as Ecotect, Energyplus, etc, that each software has some specific abilities for some specific simulations. As example for energy simulation, Ecocect is useful for early stage of the design process, but as the design nears completion it needs Energyplus for more accurate simulation. Any building performance that can’t be simulated yet had to be calculated manually such Overall Thermal Transfer Value (OTTV) but some parts of the formula can be supported by Ecotect to calculate U-value and shading percentage (G).
In the Greenship rating system of Green Building Council Indonesia (GBCI), the chance for green building simulation has been opened especially on daylighting, air movement and energy simulation. For early stage of design process, it is very important to uses a software that enable us to integrate intuitively architect’s ways with an easy understanding simulation result for architects. Ecotect can simulate some building performances with this criteria with a wide range of simulation such as sun-shading design, daylighting, artificial lighting, cfd simulation, thermal comfort, cooling load and electricity consumption. On the contrary, Energyplus is not user-friendly software for architects because it is intended for mechanical engineers.
Nevertheless, Ecotect has some limititations that we have to consider because they can influence the final result of certain simulations. One of the limitations is some data like Time lag and Solar Heat Gain Coefficient (SHGC) have to be calculated outside Ecotect. Recently there is a tool that has abilities to solve the problems, called Ecotect Supporting Program (ESP) released by Studio Gentra, Bandung. The basis for Time lag and SHGC calculation have been adapted from a book titled House, Climate and Comfort by Martin Evans.
Another limitation that has not been fully supported by Ecotect is weather files for any city in Indonesia. There are some steps to convert the local weather data to be a compatible one with Ecotect. Some data such as temperature, relative humidity and wind can be obtained from local wheater stations . But direct radiaton and diffuse radiation have to be calculated graphically through use of sun-radiation diagram overlaid on sun-path diagram according to a city where the site located.
One of the greenship item that can’t be simulated yet is OTTV calculation. The formula is very specific and has not been supported yet by any simulation tools. It needs extra efforts for us to calculate the OTTV formula because there are some variables such as shading coefficient and U-value have to be calculated with caution for the complexity of the formulas. Ecotect can help simplify the process just for the calculation of U-value and Shading percentage (G).
The Data needed for The Green Building Simulations and Calculations
The green building simulations need various data for the simulation process to be run well. The data types divided into two categories, external data which is weather data and internal data which is inherent to the building we have designed.
For Sun-shading design, the external data needed is the latitude of the site. For daylighting simulation, the external data needed is design sky illuminance for the city where the site is located and the internal data needed are glass transmittance, interior and exterior surface reflectance. For thermal comfort and energy simulation, the external data needed is temperature and sun radiation while the internal data needed are thermal property of the material (thermal conductivity, density, specific heat, solar reflectance, emissivity, time lag, shgc and light transmittance), type of clothes’s users, room function, room capacity, hours of operation, type and amount of lamps and appliances.
Meanwhile, for OTTV calculation, external data needed is solar factor that can refer to Standar Nasional Indonesia (SNI) No. 03-6389-2000 about Energy Conservation on the Building Envelope while the data are aimed only for the city of Jakarta but it can be treated as a baseline for a fair comparison for another cities in Indonesia. The internal data needed are wall’s absorptances, u-value for walls and windows, density and width of walls, shading coefficient (SC) of glasses and of shading devices and finally, window to wall ratio (WWR).
The sun-shading design has to refer to sun-angle that have to be considered when the sun penetration is allowed and when is not. But the sun-shading device is usually intended to block direct radiation which have more radiation intensity than diffuse radiation that sun-shading device cannot deal with them (unless we intend to design for it but it will create a trade-off, decreasing daylighting level) . For anticipate the diffuse radiation, we have to choose some glasses with a best performance on reducing heat affected by sol-air (come from combination of solar radiation and air temperature) . To avoid the glare, the combination of sun-shading device and glass performance can be carried out.
The simple guideline for the range of time we can simulate for sun-shading device simulation on tropical zone are from 10.00 AM to 04.00 PM which the sol-air are high. We can activate the sunpath diagram on 3d environment in Ecotect that shows us any sun-position according to time and date on the whole one year. Ecotect has a tool for showing which the areas that still have sun-penetration on this range of time and which is not. Thus we can modify our sun-shading device to cover the problems. But the evaluation is still needed wether our sun-shading device will decrease lighting level or will increase SC for shading device significantly.
For Daylighting simulation, Ecotect uses The Building Research Establishment (BRE) Split-Flux method that is a slightly more complex but widely recognised technique that is used in many building regulations around the world.
The BRE Split-Flux method is a widely recognised and very useful technique for calculating daylight factors. This method is based on the assumption that, ignoring direct sunlight, there are three separate components of the natural light that reaches any point inside a building:
Sky Component (SC) – Directly from the sky, through an opening such as a window.
Externally Reflected Component (ERC) – Reflected off the ground, trees or other buildings.
Internally Reflected Component (IRC) – The inter-reflection of 1 and 2 off surfaces within the room.
Separate consideration of these three components is justified by the fact that each is affected by different elements within the design. The daylight factor is thus given as a percentage and is simply the sum of each of these three components.
DF = SC + ERC + IRC
However, the original BRE Estimating Daylight in Buildings paper makes a number of assumptions and simplifications which were necessary for quick hand calculations. For more accurate calculation, Ecotect has added the increased accuracy mode which brings some variables to be included such as refractive index of each impacted window and external reflectance value of each surface in the model.
Once the Daylight factor has been calculated, we can convert them into illumination level for each room by multiplied with design sky illuminance (Ecocet will calculate it automatically). Thus the result can be read from the analysis grid which represents various illumination levels at any point in the room. Design sky illuminance based on the worst-case scenario that is uses the overcast day light level . Design sky illuminace for cities in Indonesia refer to 10.000 lux based on SNI 03-2396-2001 about Code on Daylighting System Planning for Buildings.
Greenship states that 30 % of the floor area have to achieve the illumination level requirement. Ecotect can integrate daylighting simulation with artificial lighting simulation to increase illumination level at the points that can’t achieve the illumination level requirement for each room. The Point Method is used for the artificial lighting simulation in Ecotect. This method is aimed to determine direct illumination level of artificial lighting by use of photometric data. After that, we can measure the percentage of working year lighting off for lighting sensor use.
For physically accurate and comprehensive lighting analysis, Ecotect can output files data for direct input – vice versa – into the RADIANCE Lighting Simulation Software developed by Greg Ward at Lawrence Berkeley Laboratories. But it should be carried out when the design process nears completion.
Two steps have been made that show the balance between the creation of shading device in one side and achievement for the illumination level requirement (30% of the floor area) in the other side.
Thermal Comfort Simulation
Thermal comfort has been a main issue for building performance all the time. One of the criterion of thermal comfort is Mean Radiant Temperature (MRT) which means, the result of surface temperature from sorrounding objects. MRT is a heat sensation comes from the radiation of materials and must be differentiate with air temperatur that is a represent of the temperatur of air molecules.The MRT that can be convert to another criterion for easy reading such as Predicted Mean Vote (PMV) that scales betwen -4 for coldest one and +4 for hottest one.
In Ecotect , MRT simulation is applied to a grid analysis thus we can quickly understand which point in a room that received hotter surface temperature or colder ones. So we can modify the thermal property of the material to achieve better ones. In Tropical zone , we have to reduce the amount of MRT in order to give a low contribution to internal temperatur of a room. The Internal temperatur is known as a dry resultant temperature.
The dry resultant temperature is the temperature registered by a thermometer at the centre of an externally blackened sphere 150 mm diameter, being a function of air temperature, mean radiant temperatures and wind velocity. It is used as an index temperature for comfort where the air velocities are low (CIBSE Guide A, 1999).
This statement indicate a method called admittance method or steady state method. The method uses idealised (sinusoidal) weather and thermal response factors (admittance, decrement factor and surface factor) that are based on a 24-hour frequency.
Cooling Load Simulation
For air-conditioning buliding, space load (heating/cooling load) calculation is very important thing to estimate the capacity of air-conditioning plant. In the tropical zone, only cooling load calculation that is needed. The admittance method is also being used to calculate cooling load.
The admittance method was originally intended to calculate peak internal temperatures in buildings that is affect the peak cooling load to ensure that it would not become uncomfortably hot during sunny periods. But This method frequently over exceed on its prediction. So, it is useful only in early stage of design process for comparing various design alternatives.
The validation is needed to have more accurate calculation as recently a new method called a dynamic simulation has been implemented in order to solve the problems. Energyplus have adopted this method on its simulation engine.
In Ecotect, the results of cooling load simulation are shown as a graphic that represent monthly cooling load for a year.
For a comprehensive energy modeling, Ecotect have a tool that enable us to display the monthly electricity consumption in one year that comes from use of various applliances and lamps. we just insert 3d applliances and lamps into the model and Ecotect will calculate automatically for the result.
The electricity consumption for one year can be easily converted to the amount of emmission of CO2 that correspond with. But for the effective of the design process, just the lamps and certain applliances that should be incorporated into the model because it is just aimed for comparing various design alternatives. For more accurate estimation of electricity consumption, Energyplus is needed to deal with.
In greenship, energy modelling is encouraged with the potentiality of high scores that can achieve maximum scores about 20 points. Based on Energy Efficient Index (EEI) standar for a certain building type as a baseline, then our building model is being calculated for its electricity consumption. Finally we have to calculate the percentage of the reduction of the electricity consumption based on the baseline. The high scores will be granted if the percentage of the reduction achieve 60 %.
Air Movement Simulation
Introducing the outdoor air is important for occupants to conncet with the nature for healthy reason. One of the good criteria for the indoor air movement is to have inlet and outlet openings to achieve a cross ventilation. The simulation for this can be carried out in Ecotect alongside with Winair as the cfd engine.
Initial setting for this simulation is conducted by including the prevailing local wind – their directions and speeds. Then we can run the simulation in Winair and displayed it back into Ecotect. The results show which points in a room that have a good ventilation or not.
In Greenship, OTTV is one of the prerequisite item in Energy Efficient and Conservation category. OTTV or Overall Thermal Transfer Value, is one of the parameters to measures the efficiency of the building energy which a building envelope performance have to achieve ≤ 45 W/m2.
The calculation refer to SNI 03-6389-2000 about Energy Conservation on the Building Envelope. The formula is :
OTTV = α [(Uwx (1- WWR)] x TDeq + (SC x WWR x SF) + (Uf x WWR x ΔT)
OTTV = overall thermal transfer value of the external wall (W/m2)
α = wall absorptance (dimensionless)
Uw = U-value of opaque wall (W/m2.K)
TDeq = equivalent temperature difference (K)
Uf = U-value of fenestration (W/m2.K)
DT = temperature difference between interior and exterior (K)
SC = shading coefficient of fenestration (dimensionless) = SCwin x SSF
SCwin = shading coefficient of window glass (dimensionless)
SSF = solar shade factor of external shading devices (dimensionless)
SF = solar factor of fenestration (W/m2)
WWR = window-to-wall ratio (gross wall area)
To obtain the data that will be included to this formula is comes from any sources and many techniques. Solar Factor and DT can be obtained directly from The SNI. To have a TDeq value, we have to insert the density and the width of a material. SCwin can refer to technical data from any glass manufactures. The WWR is found out directly from the building drawing.
Another data such as Uw, Uf and SSF need further calculation. To find out the Uw or Uf, many steps have to be conducted. First we have to find out the resistance value by divided the thickness of the material with its thermal conductivity. Then we sum all of the resistance values of the material and the external and internal surface resistances. The U-value is the reciprocal of the sum of the resistances.
SSF has a number of complex calculations, unfortunately the formulas are very specific according to the shape of the given shading device type. In SNI , there are no calculation examples related to the formula and no additional information about ID or Direct radiation and Id or diffused radiation. For comparison of how the formula is excercised, we have to refer the guideline of Envelope Thermal Transfer Value (ETTV) calculation realesed by Building and Construction Authority (BCA) , Singapore.
The formulas are given below ,
Q = Ae x ID +A x Id
Q = solar heat gain (W/m2)
A = total area of window or Ae + As (m2)
Ae = exposed area of window (m2)
As = shaded area of window (m2)
ID = direct radiation (W/m2)
Id = diffused radiation (W/m2)
to find SC fenestration,
SC = G x ID + Id
SC = shading coeficient fenestration (dimensionless)
G = the fraction of area exposed to direct solar radiation or Ae / A (dimensionless)
ID = direct radiation (W/m2)
Id = diffused radiation (W/m2)
To determine the effective SC of a shading device, theoretically, the computation has to be carried out for 12 months of the year. However, as the computation involved is rather tedious and the degree of accuracy required is not a critical factor, it is deemed sufficient to base the SC computation on 4 representative months of the year, March, June, September and December. The representative days of these 4 months are March 21, June 22, September 23 and December 22. Further, since the solar data for March 21 and September 23 are almost identical, it suffices to compute the solar heat gain for March and double it to take account of the heat gain for September. Mathematically, the effective SC of a shading device is given by:
Effective SC = ∑M(G x ID x Id) + ∑J(G x ID x Id) + ∑S(G x ID x Id) + ∑D(G x ID x Id)
∑M IT + ∑J IT + ∑S IT + ∑D IT
M denotes March
J denotes June
S denotes September
D denotes December
To determine G value there are a number of different formulas depend on the type of shading. But there are limitations for calculate SC of more complex types. In Ecotect, there is a feature to simulate G-value, the fraction of area exposed to direct solar radiation. The feature that enable to simulate G-value present the hourly percentage of shading. By this feature we can simulate any type of shading device easily.
Use of a computer simulation, especially that suitable for an architect’s workflow in early stage of design process, can help architect to judge his work effectively without manually excercise a number of the formulas that too complex to dealing with. The other hand, a combination with any simulation tool that encourages a more accurate result is very important in a final stage of the design process. But there are a number building performance calculations that have not been supported by computer simulation and hopely one by one can be adapted to be a computer program, especially an architect-friendly one.
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