Design of an MHHC through thermodynamic simulations

This section depicts the energy impact of the PCM floor tiles on the airport terminal space, and is divided into the performance of the Ebb® tiles and the Energain® tiles. A time-step of 360s is employed for both the CFD and TRNSYS models, with an average of 40 iterations per time-step for the CFD model. The simulation time for each season was 14 hours with a 3GHz i7 processor. The contour and vector plots for the indoor air-movement are similar to Figs. 8.2 and 8.3.

8.2.1 ‘Ebb’ Tiles

The Ebb tiles are placed on the floor of the airport terminal space as described in section 7.1, and the enhanced phase change model developed in this study is used to simulate phase change. The temperature and load trends of the zone, with the Ebb floor tiles, are shown in Fig. 8.5.

Fig. 8.5(a). Ambient and zone temperature (Tf) profiles with and without night ventilation for the Ebb-Tiles’ case

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Fig. 8.5(b). Heating (+) and cooling (-) load profiles for the Ebb-Tiles’ case, with/ without night ventilation, for 2D geometry

It can be observed from Fig 8.5(a) that the general temperature trends for the Ebb tiles’

for all seasons are similar to the ‘DC-only’ case, whereby: the indoor space is maintained in the comfort range only for the intermediate season; whilst the space overheats for part of the summer. The temperature impact of the PCM tiles in the summer can be quantified by the fact that the indoor space overheats (i.e. Tf > 23°C) for 22% and 21.5% of the time for the non ventilated ‘DC + Ebb tiles’ and the ‘DC + Ebb tiles Vent’ cases, respectively, compared to 23% for the ‘DC-only’ case. Furthermore, the zone temperature during the winter season is below the comfort requirements, similar to the ‘DC-only’ case.

The load profiles for both the ‘DC-only’ and the non-ventilated ‘DC + Ebb tiles’ cases are very similar, as shown in Fig. 8.5(b). During the intermediate season, heating is required during the early morning hours, while cooling dominates during the day.

During summer, heating may be required, but cooling is most prominent, whilst in winter, the airport terminal requires only heating. For the ‘DC + Ebb tiles Vent’ case, a noticeable change in the load profile can be observed. The time of maximum cooling load for both the summer and intermediate seasons is found to shift by an average of 0.8 hrs, whilst the heating load for these two seasons is found to increase in the early

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morning hours compared to the ‘DC-only’ and ‘DC + Ebb tiles’ cases. During winter, the heating load trends for all configurations are similar.

Thus, the addition of the Ebb tiles without ventilation is found not to have a major impact on both the temperature and energy load profiles of the airport terminal, whilst recharging the Ebb tiles through night-ventilation shifts the time of maximum cooling load during the day by 0.8 hours during the intermediate and summer seasons.

However, night-ventilating the indoor space also increases the heating load required by the space during the early morning hours in the intermediate and summer seasons.

Hence, although the percentage overheating time in the summer is similar for the non-ventilated and non-ventilated cases, the overheating of the space occurs 0.8 hrs later when ventilating the indoor space at night. There is no apparent impact of the Ebb tiles during the winter season.

Fig. 8.6. Heating (+) and cooling (-) demands with and without night ventilation, with Ebb floor tiles

Fig. 8.6 shows that employing night ventilation in the space reduces slightly the cooling demand during the summer and intermediate seasons, but at the same time, increases the heating demand. Thus, night ventilation is effectively shifting the load from cooling to heating, and therefore the annual energy savings will depend on the balance between the two opposing loads. Furthermore, compared to the ‘DC-only’ case from Fig. 8.4, it can be observed that the cooling loads are lower when employing the Ebb floor tiles, whilst

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heating load increases when using night ventilation with the Ebb floor tiles. During winter, the heating loads for the Ebb floor tiles are slightly lower compared to the ‘DC-only’ case.

8.2.2 ‘Energain’ Tiles

The thermophysical properties of the Energain tiles are given in section 7.1. This tile is used to show the impact of floor tiles with higher latent enthalpy than the Ebb tile, on the energy performance of the indoor space. The temperature and load trends of the zone for the Energain floor tiles are shown in Fig. 8.7.

Fig. 8.7(a). Ambient and zone temperature (Tf) profiles with and without night ventilation for the Energain-Tiles’ case

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Fig. 8.7(b). Heating (+) and cooling (-) load profiles for the Energain-Tiles’ case, with and without night ventilation, for 2D geometry

The general observations from the trends in Fig. 8.7 are similar to the Ebb tiles’ case, that is: the comfort conditions are satisfied for the intermediate season; there is overheating in the summer; and the indoor temperatures are below the comfort requirements during winter. However, the swing in the indoor temperature is more noticeable when employing the Energain floor tiles. For instance, the summer indoor temperatures are found to increase at a slower rate compared to the ‘DC-only’ case, whilst the reduction in the winter indoor temperatures during the night is lower when the Energain tiles are used, compared to the ‘DC-only’ case.

Consequently, the temperature impact of the PCM tiles in the summer can be quantified by the fact that the indoor space overheats (i.e. Tf > 23°C) for 19.7% and 19.0% of the time for the non ventilated ‘DC + Energain tiles’ and the ‘DC + Energain tiles Vent’

cases, respectively, compared to 23% for the ‘DC-only’ case. This also shows an improvement in indoor thermal comfort, compared to using the Ebb floor tiles.

Similar to the Ebb tiles case, the heating energy load is found to increase (when night ventilating the space) in the early morning due to the higher thermal mass of the building that needs heating, while the cooling energy decreases during the day, compared to the ‘DC-only’ case. However, when using the Energain tiles, the reduction in cooling energy during the day is more prominent for both the non-ventilated and

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ventilated cases, compared to both the ‘DC-only’ and Ebb-tile cases. Additionally, the time of maximum cooling load is found to shift by an average of 1.3 hrs for both the non-ventilated and ventilated scenarios, compared to the ‘DC-only’ case. During winter, there are no apparent changes in the energy demand for the space.

Fig. 8.8. Heating (+) and cooling (-) demands with and without night ventilation, with Energain floor tiles

Fig. 8.8 shows that similar to the Ebb tiles, using the Energain tiles reduces the cooling load during the day for the intermediate and summer seasons, compared to the ‘DC-only’ case. However, the reduction in cooling load is more pronounced when using the Energain tiles, due to the higher latent heat capacity of the PCM. The heating demand for the summer and intermediate seasons is reduced when employing the Energain tiles in the non-ventilated mode, whilst ventilation significantly increases the heating load, compared to the ‘DC-only’ case. It can also be observed that the winter heating load is reduced when employing the Energain tiles, compared to both the Ebb-tiles and the

‘DC-only’ cases.

Generally, it was found that increasing the latent heat capacity of the floor tiles reduces the cooling load during the summer and intermediate seasons, but ventilating the space at night increases the heating loads for these two seasons. In winter, the heating load also decreases with the use of higher latent heat capacity PCM in the floor tiles, and ventilating the space at night has a very low impact on the heating energy demand of the space.

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