The degree day method is a simple approach which can be used to quickly calculate the heating or cooling energy consumption of a building and is recommended in the ASHRAE Standard Methods of Measuring, Expressing and Comparing Building Energy Performance to compare energy performance of buildings [8]. The HDD are calculated by multiplying the difference between the ambient air temperature and a certain base temperature by the time spent at this temperature. The heating degree days were calculated from the Sydney TMY2 weather data used for the simulations. This showed that Sydney had 760 HDD [K. d/a] per year assuming a base temperature of 18 °С. Hart and Dear [9] examined the mean daily temperature associated with the minimum heating and cooling energy consumption for Sydney and found that a degree-day base temperature of 18 °С is appropriate to use for the calculation of both heating and cooling degree-days. The number of heating degree days for Sydney as determined from the TMY2 weather data are shown in Table 2. Table 2. Monthly breakdown of heating degree days for Sydney

The following equation was used to calculate the monthly heat transmission energy requirement, Qreq = (UA)eff X HDD X 24 (1)

where Qreq is the simple transmission heat energy required and (UA)eJf is the U-value multiplied by the external area of the building. The following equation was used to calculate the monthly ventilation heat loss energy requirement,

Qvent = ACH X Vair X Cp air X HDD x 24 (2)

where Qvent is the heating demand energy loss due to ventilation and infiltration, ACH is the air change rate for the building, Vair is the air volume, and Cp air is the specific heat capacity for air.

Internal gains of 3W/m2 were assumed and the solar gains through the windows, the monthly utilisation factor and the resulting heating demand were calculated using the following equations,

where nutilisation is a simple approximation for the utilisation factor for the whole heating period, Qgains is the sum of the internal and solar gains, Qlosses is the sum of the heating transmission and ventilation losses and Qdemand is the heating requirement of the house.

From the simulations conducted in TRNSYS and the PVT performance equations the PVT system was found to have a thermal efficiency of up to 27%, however the average efficiency of 22% was used to calculate the supplied PVT heating. The results of the simple HDD calculations are shown in Figure 1 below. This graph shows that heat removed from a 16m2 (1.6 kWp) PVT array will meet the HDD heating requirement of a well insulated Sydney house in winter but in the month of July the TRNSYS heating demand is greater than the heating supplied by the PVT system. The heating demand as calculated in the IDA ICE and TRNSYS simulations included in this graph show good agreement with the simple HDD calculations. More detailed analysis is required to determine the day to day heating effect of the PVT system.