BUILDING SIMULATION - CASE STUDY: ERGOHOME DWELLING

Chapter 4: CASE STUDY: ERGOHOME DWELLING

4.3 BUILDING SIMULATION

The house was simulated in its current construction. The simulation software which is used to simulate the house was tested and calibrated against the field readings as was discussed earlier in this chapter. Several assumptions were made to provide a full representation of the house. Firstly, analytical tests were performed throughout a range of comparison of some selected thermal parameters (i.e. U-value, Y-value, g-value) and simplified calculation methods including Steady State Heat Loss (e.g. using CIBSE Simple Model) and CIBSE admittance method (using CIBSE cyclic model). These provide a quick check of the model accuracy by comparing the simulation results with a known solution. Model assumptions and detailed calculation methodologies are included Appendix B.1. <VE> simulation suite application has ApacheCal tool performs heat loss and heat gain calculations. Heat Loss utility applies CIBSE Simple Model to perform steady-state heat losses calculation and Heat Gain program employs CIBSE Cyclic Simple Model to predict peak temperature in hot days.

The CIBSE steady state methods in use for analytical verification include Simple Model and Cyclic Model which are simplifications to facilitate manual calculation. The calculated results using these two models were compared with IES<VE> results driven from ApacheCal tool which employs the same assumption and calculation methods. The Simple Model calculates the maximum heating load at a fix design condition including the given external design air temperature and internal air or operative temperature. The Cyclic Model utilises idealised (sinusoidal) weather and thermal response factor (e.g.

admittance value, decrement factor and time lag) that are based on 24 hours frequency.

It is used to predict the thermal capacity of the building envelope whilst determining the peak internal temperature during summer period and the associated maximum cooling load. However, the methods only provide one hour of winter and/or summer design condition while lack of integrated design solutions such as use of natural ventilation, mix mode operation (i.e. combined use of mechanical and natural ventilation), real climatic conditions and encountering other impacts like duration of high/low temperature, period of occupancy (i.e. use in weekend is different to use in weekdays).

4.3.1 Analytical verification using CIBSE Simple Model

CIBSE Simple Model is developed for building designer to size emitters in order to achieve a specified operative temperature. The design scenario was that the dwelling

was heated to an operative temperature of 20ºC and the external design temperature as in winter was selected at -5ºC. For simplification in manual calculation using the climatic data in CIBSE Guide J (CIBSE, 2002), the long sided wall with higher glazing area faces due south while in reality the building orientation onsite is 33º due South.

With the given building geometry, layout and constructional information, the simulation model of the test building was conducted using CIBSE Simple Model allowing numerical verification of the model as can be seen in Table 4-4. Inside air temperature and mean surface temperature are then calculated in correspondence with these assumptions and analysis. Model assumptions and detailed calculation methodologies are included in Appendix B Section B1.1 and B.1.3. IES<VE> simulation suite application has ApacheCal tool performs heat loss and heat gain calculations. Heat Loss utility applies CIBSE Simple Model to perform steady-state heat losses calculation and Heat Gain program employs CIBSE Cyclic Simple Model to predict peak temperature in hot days. Table 4-4 shows comparison of calculation data and IES<VE> outputs for Steady State Heat Loss in five thermal zones. Details calculation can be found in Appendix B Section B.1.3.1.

Table 4-4: Comparison of heat loss results utilising CIBSE Simple Model Thermal zone Thermal analyses Calculation

results

<VE>

outputs

Percentage of difference (%) Living space Heat loss (kW) 1.125 1.094 -2.4

Air temperature (°C) 19.57 19.44 -0.4

Bedroom 1 Heat loss (kW) 0.459 0.441 -2.73

Air temperature (°C) 19.44 19.57 -0.5

Office Heat loss (kW) 0.227 0.231 1.8

Air temperature (°C) 19.30 19.45 1.8

Bathroom Heat loss (kW) 0.221 0.225 1.8

Air temperature (°C) 19.30 19.45 1.8 Hall entrance Heat loss (kW) 0.27 0.243 -2.4

Air temperature (°C) 19.45 19.75 0.5 EH unit Total heat loss (kW) 2.502 2.454 1.8

4.3.2 Analytical verification using CIBSE Cyclic model

The cyclic model in which analysis is carried out uses a sequence of identical days for which, the external conditions vary on a chosen 24 hour basis (CIBSE, 2006a). The calculation sequence starts with an assessment of whether a building needs to be cooled down, by calculation of summertime peak temperature (e.g. CIBSE admittance method).

Short-wave solar radiation reaches the building envelope surface and is absorbed at the external surface. After a delay due to thermal storage capacity (known as thermal mass), the external temperature of external surface receiving short-wave radiation increases.

Different temperatures on both sides of a building element results in heat flow (or long wave radiation) through this element and into the room by convective heat transfer (CIBSE, 2006a). Thus the summertime peak temperature is determined at the time when the peak solar radiation occurs together with thermal response of building envelope to short-wave radiation.

The results of analytical verification using CIBSE Cyclic Model are shown in Table 4-5, predicting cooling load and peak in room air temperature in a hot summer day with the peak in solar radiation. Detailed calculations for calculated results can be found in Appendix B Section B.1.3.2 with the thermodynamic values of building envelope previously calculated in Appendix B. Section B.1.2.2.

Table 4-5: Comparison of cooling results using CIBSE Cyclic Model

Thermal zone Thermal analyses Time peak Calculation results <VE> output Percentage of error (%)

Living space

Peak air temperature (°C)