4.1. Comparison of Peak Day Energy Consumption
In order to more clearly see the performance characteristics of each system in different climate regions, we selected the peak days of each city as objects for comparison. This part selects the peak day of each selected city and conducts a detailed comparative analysis through thermal conditions from TRNSYS18, such as the temperature (Figure 8
), HR (Figure 9
), and cooling load (Figure 10
). Comparisons of the temperature, HR, and cooling load of each selected city are shown in Figure 8
, Figure 9
and Figure 10
. Since the latent heat load values of the various cities are similar, they are not shown in the cooling load figure.
According to Figure 11
and Figure 12
, the peak day energy consumption is analyzed from two aspects: systematic difference and city characteristics. Since the peak day of the comparison is the hottest day, there is no heating load.
From the viewpoint of system differences, the following describe the aspects of Figure 12
in order from left to right, according to the data of the figure.
The DOAS used in this study is a system that targets ventilation and dehumidification. After the first dehumidification of the air through the EW in the cooling season, the air is dehumidified again to the target HR through the CC. If the temperature of the process air is lower than the SA set point, reheating is required. The reheat coil (RHC) heats the air to reach the target temperature; consequently, even in the cooling seasons, the system has energy consumption due its boiler and pump.
Since the DOAS is a ventilation system, the SA flow rate was maintained at the minimum required ventilation rate based on the ASHRAE standard 62.1-2010 [14
]. However, the HPLD-IDECOAS, as an air conditioner, needs to perform the function of removing the thermal load in the room, and as a fully external air system, 100% fresh air is used. Accordingly, the energy consumption of the fan for the air supply and exhaust is significantly greater than that of the DOAS.
The main energy consumption of the DOAS is the use of the dehumidification part of the CC and RHC, whereas the main energy consumption of the HPLD-IDECOAS is the heat-pump-integrated LD system.
The HPLD-IDECOAS uses more energy than the DOAS for dehumidification (scilicet, the energy consumption of the HPLD system is more than that of the chiller (CC), RHC, and pump (boiler) of the DOAS). However, owing to the use of the IEC and DEC for cooling in HPLD-IDECOAS, the overall energy consumption is lower than the DOAS using a chiller (parallel cooling).
The analysis of the different climate characteristics of each selected city is as follows. It seen from Figure 8
, Figure 9
and Figure 10
of the six selected cities that the thermal environments of the peak days of each city have distinct characteristics. Although Beijing, Kashi, and Lhasa belong to the same climate region, they show significant differences in Figure 11
and Figure 12
because of their special geographical location and geographical environment, but generally exhibit the three lowest HRs of the six selected cities.
As can be seen from Figure 11
, in the peak day of the cooling season, the daily energy consumption of the HPLD-IDECOAS operating in Guangzhou is the highest of the 12 cases, and the daily energy consumption of the HPLD-IDECOAS operating in Lhasa is the lowest. In terms of the peak day, the energy consumption of the HPLD-IDECOAS in Guangzhou is 40% higher than the DOAS, and is similar to the DOAS in Shanghai and Kashi, and in other cities, the daily energy consumption of the HPLD-IDECOAS is generally lower than that of the DOAS operating under the same conditions. The energy saving rates of the system operating the different cities are, from high to low, 52% in Lhasa, 30% in Beijing, and 21% in Kunming.
From Figure 12
, it can be seen that Guangzhou has the highest daily average temperature of the peak day and the highest daily average HR in the six cities. In order to make the temperature and HR of process air reach the set point after passing the IEC and DEC, the HPLD system requires a large degree of dehumidification. As a result, the daily energy consumption of the HPLD-IDECOAS is 40% higher than that of the DOAS, owing to the power consumption of its heat pump and the energy consumption of the pump therein. For the DOAS, the energy consumption for dehumidification is lower than that of the HPLD-IDECOAS during the peak day, because of the use of the EW.
In Lhasa, as shown in Figure 8
, Figure 9
and Figure 10
, the temperature and HR during the peak day is the lowest of all six cities, and the temperature difference between morning and evening is large. As it is the driest of the selected cities, the HPLD-part is not activated, and the process air reached the SA setting only by the IEC and DEC. However, in the operation mode of the DOAS, whether the volume flow rate of the SA is higher than the minimum ventilation is calculated according to the latent load that must be cleared from the room; therefore, the two functions of the chiller, dehumidification, and eliminating the sensible load in the room, are activated. As shown in Figure 11
, running the HPLD-IDECOAS in Lhasa on the peak day can save up to approximately 52% of the energy compared to the DOAS. In fact, it is known from the existing research that IDECOAS (without the LD system) is more suitable in such dry regions.
The analysis of other two of the three drier cities, Beijing and Kashi, is as follows.
As shown in Figure 8
, Figure 9
and Figure 10
, Beijing’s daily average temperature is in third place and its daily average HR is in fourth place of the six selected cities on the peak day. Although the temperature is high, the temperature difference between day and night is large, and it is dry. Thus, as can be seen from Figure 12
, the DOAS system running on Beijing’s peak day not only needs to deal with the latent load of process air but also with the sensible load when using the chiller to dehumidify. Thus, in terms of dehumidification, it consumes more energy than the HPLD part that only needs to deal with the latent load of the process air. Using the HPLD-IDECOAS as an air conditioner, the dehumidified air can satisfy the air supply condition without using the chiller, thereby saving more energy. As shown by the broken line diagram in Figure 11
, the overall energy savings are approximately 30%.
Kashi’s daily average temperature is the fourth highest and its daily average HR is the second lowest on the peak day; thus, it is concluded that the drying condition is similar to that in Beijing, but the temperature is slightly lower than that in Beijing. However, because the elevation is much higher than that of Beijing, the sensible load in the room is higher than that in Beijing. Therefore, a more sensible load needs to be eliminated in the operation of the HPLD-IDECOAS. Consequently, its energy-saving potential is not as good as that in Beijing. As a result, the energy consumption of the HPLD-IDECOAS is similar to that of the DOAS.
In Kunming, the annual solar projection angle is large, and the hot-weather conditions are not extreme. The average daily temperature is far lower than that of Beijing, and the HR shows that this is a mild place, but the HR is higher than that in Beijing, at third place. Thus, the latent load in the room that needs to be eliminated in Kunming is higher than that in Beijing when running the HPLD-IDECOASD. The energy-saving potential is not as good as that of Beijing, but it is higher than the DOAS in the same region, which is approximately 21%.
In Shanghai, the average daily temperature and the sensible load treated in the room are the second highest, and the daily average HR is the highest. Due to its high humidity, the HPLD-IDECOAS has the lowest energy-saving potential among the selected cities. However, owing to its mild climate, Figure 8
, Figure 9
and Figure 10
show that the daily temperature difference is small, but the sensible load in the room is higher than in Beijing. As mentioned earlier, the HPLD-IDECOAS needs to remove more latent load owing to the efficiency effects of the IEC and DEC (set to 70% in this study), such that the process air can reach the SA set condition after passing through the IEC and DEC, therefore, there is no significant different in energy consumption between the two system on the peak day.
In summary the results of the peak day energy consumption analysis are as follows.
The energy-saving potential of the HPLD-IDECOAS is most affected by the sensible load that needs to be eliminated from the room. For example, the energy-saving potential of Lhasa is greater than those of Beijing, Kashi, Shanghai, and Guangzhou in turn.
When the difference in the HR is large, the energy-saving potential of the HPLD-IDECOAS is more affected by the HR, such as is Beijing and Kunming.
The energy performance of the DOAS in each representative city is opposite to that of the HPLD-IDECOAS. For example, when there is too much sensible load eliminated from the room, the energy-saving effect is significantly higher than the HPLD-IDECOAS, and at other times, the energy consumption is much higher than the HPLD-IDECOAS.
4.3. Comparison of Seasonal Energy Consumption (Cooling Season and Heating Season)
In Beijing, as shown in Figure 19
, during the cooling season, the two systems heat the SA differently. In the LD-IDECOAS, using the HC to heat the RA first and using the IEC as the sensible heat exchanger, the OA reaches the SA temperature by heat transfer between the process air and the RA on the IEC. In the DOAS, the OA is preheated by the EW and then directly heated to the SA set temperature by the HC. Therefore, it is seen from the different heating modes of the two systems during the cooling season that the energy consumption of the HC of the LD-IDECOAS is higher than that of the DOAS. It is concluded that the greater the difference, the lower the local air temperature, which shows that Beijing’s winter is cold.
In Shanghai, the difference in energy consumption between the dehumidifying parts of the two systems seems to be larger than in Beijing (i.e., the energy consumption of the heat pump in the HPLD-IDECOAS and that in the CC plus the RHC in the DOAS seem to be larger than in Beijing) Similar to Beijing, the energy consumption of the RHC in the HPLD-IDECOAS seems to be larger than that in DOAS. The difference in the energy consumption of the two systems is divided into two. One appears in the dehumidification section and one has the ability to save energy compared to the DOAS, because the HPLD-IDECOAS does not require parallel cooling and, instead, uses the IEC and DEC.
The difference in energy consumption is mainly in two aspects. First, the energy consumption of the chiller and RHC for dehumidification is the highest of the six selected cities. Owing to the large difference, the proportion of the total energy consumption is also large. The relates to parallel cooling, and accounts for a larger proportion. From the perspective of energy saving, the HPLD-IDECOAS using the IEC and DEC instead of the chiller, in regions that have a higher temperature and need more cooling, has a higher energy-saving potential.
The annual solar projection angle of Kunming is large. The humidity is not high and, thus, the energy consumption of the chiller and RHC for dehumidification is the lowest of the six selected regions. The main difference is in the removal of the sensible load and latent load from the room, although the difference is small. However, the total energy consumption is low and, thus, the proportion is significant. In such climatic conditions, the HPLD-IDECOAS has more energy-saving potential in terms of sensible load and latent load in the room compared with the DOAS.
The temperature of Kashi in winter is low but not extreme, there is a large change in the annual temperature and daily temperature, and the temperature difference between day and night is large. Therefore, it seen that in Kashi, heating consumes much energy compared to cooling and dehumidification. Since both systems use parallel heating, the difference between the two systems is not large in this region.
Lhasa has a large daily temperature difference, such that one can feel the temperature of four seasons in a single day. However, it is on the whole a cold and dry city, with the energy consumption mainly accounted for by heating. Therefore, the energy consumption of the two systems was essentially similar.
In summary, it can be seen that during the cooling season, similar to the peak day, owing to the use of the heat pump in the HPLD-IDECOAS to reduce the regenerative heat, and then through the IEC and DEC for cooling, the energy-saving effect in each comparison city is better than the DOAS, which uses a chiller. In the heating season, because the HPLD-IDECOAS is designed to supply the same temperature as the DOAS (i.e., 20 °C), parallel heating is required. The overall heating energy consumption of the HPLD-IDECOAS is slightly lower than that of the DOAS, as the DOAS uses the EW to recycle the EA and, thus, the heating energy consumption is slightly lower than that of the HPLD-IDECOAS.
4.4. Comparison of Annual Energy Consumption
Beijing has four distinct seasons; summer is hot and rainy, the rainy season is concentrated in summer, which often includes heavy raining, and winter is cold and dry. The RHC of the LD-IDECOAS is mainly used in winter, and the RHC of the DOAS is mainly used in summer. The difference in energy consumption between the HPLD-IDECOAS and DOAS is mainly reflected in the dehumidification part; although the difference is large, the overall proportion is not large. The overall energy difference is 39%, shown in Figure 25
Shanghai has four distinct seasons, with a mild and humid climate. It exhibits a short spring and autumn, and a long winter and summer. There is a moderate rainy season throughout the year, with high humidity in winter. The total energy consumption of the HPLD-IDECOAS saves 14.8% compared to the DOAS.
Guangzhou has a long summer and a short winter. It has a high temperature and high humidity. In this case, the savings compared to the DOAS is 36%.
The summer in Kunming is not particularly hot, the winter is not particularly cold, and the four seasons are as warm as spring. Therefore, the total energy consumption is lower in the six comparison cities, only higher than the driest Lhasa. The annual energy consumption of the HPLD-IDECOAS saves a total of 51% compared to the DOAS.
Although the summer of Kashi is long and hot, the low-temperature period is long. The total energy consumption of the two systems is similar to Beijing at 30%.
Lhasa is a relatively dry place, and does not experience particularly extreme heat in summer; it has the lowest temperature in summer compared to other cities, and the temperature of winter is not too low. Because of the dryness, the LD system is not activated. In order to deal with the sensible heat load in the room, the HPLD-IDECOAS exhibits higher energy-saving potential at 35% due to the use of IEC and the DEC than the DOAS using a chiller.
In the previous study, Kim et al. [3
] compared the annual energy consumption of the LD-IDECOAS with the traditional VAV system, and found that the energy-saving potential of the LD-IDECOAS system is 68%. On this basis, Li et al. compared the energy-saving performance of the conventional VAV system and the LD-IDECOAS in different climate regions of China, and the results show the LD-IDECOAS can save considerable energy in all regions. Therefore, in this study, we obtained the difference in energy consumption between the DOAS and the HPLD-IDECOAS in various climate regions.
Similar to Li et al. [5
], the simulation results also showed the HPLD-IDECOAS has higher energy-saving potential in hot and humid areas, even in warm regions such as Kunming. Moreover, the system which is used the CC to remove the sensible heating loads of the server room or dehumidification will consume more energy compared to the system that is not used the CC. For Li et al. the LD-IDECOAS, which does not used the CC, showed more energy-saving potential compared to the VAV system that used the CC. In this study, the HPLD-IDECOAS, which does not used the CC, showed higher energy-saving performance compared the DOAS with a parallel system that used the CC. The reason is as follows, to demand the sensible heating loads of the server room, the HPLD-IDECOAS used the direct and indirect evaporation cooling, what means that there is no energy consumption in this part, however, the DOAS with a parallel system used a chiller (the energy consumption of the chiller is expressed in Figure 25
as “parallel cooling”) to demand the sensible heating loads, which consumes much energy. To remove the latent heating loads of the server room, the HPLD-IDECOAS used an HPLD unit and the energy consumption of this part is expressed as “heat pump” in Figure 25
, and the DOAS with a parallel system used an EW for free cooling and then the remaining latent heating loads will be removed by the chiller, which is expressed as “CC(chiller)” in Figure 25
. Although the DOAS with a parallel system had already used an EW to recycle the waste heat from the RA to reduce the energy consumption, however, it can be easily seen in Figure 25
that the sum of the energy consumption of the chiller part (the sum of the “parallel cooling” and the “CC (chiller)” in Figure 25
) is much higher than that of the “heat pump”.
Compared to the Li et al. [5
], this study uses an HPLD unit instead of the LD system which is used in Li et al. [5
] to make control manageable, and existing results show the HPLD system is more energy efficient than the LD system at the energy level [12