3.1. Comparison of Simulation and Experiment
Control simulations were performed under experimental conditions defined section 2.4 to verify the reliability of the simulation results. The virtual load patterns for the experiments were selected using the loads of days that showed 60%, 80%, and 100% of the daily design heating load. Heating load patterns and outdoor air temperatures for three days are derived from TRNSYS using an office building model in Seoul. A chiller is used to provide heating load according to given heating load. To obtain the performance variation of the heat pump by the outdoor temperature change, heat pump is installed in a temperature controlled environmental chamber. Electricity power meter is installed on the heat pump to measure the power consumption. The measurement and control program for the system is developed using LabVIEW (Laboratory virtual instrument engineering workbench). The symbols in
Figure 10,
Figure 11 and
Figure 12 represent the comparison of hourly average heat capacity of the thermal storage tank and heat pump of the control methods with results from the simulation model. Experimental data appear as a symbol at about 0.6 or more under the three selected load patterns. Heating capacity of the thermal storage tank and heat pump of the three control methods were found to fit well with previous experimental results.
Table 8 indicates the comparison between the experiments and simulations in terms of heating capacity of the heat pump and storage tank and power consumption of the heat pump during daytime and nighttime. To compare the results with experimental outcomes, the results from the thermal storage priority method were normalized when the load was 100% of the design heating load. Nighttime power consumption is equal to heat pump’s power consumption required for recharging the amount discharged from tank during daytime.
When load is 100% of design load, experimental results and simulations for all control methods show a heat pump capacity of 0.60–0.65 and thermal storage capacity of 0.35 to 0.40, with differences within 5%. The reason for this similarity is the comparable modes of operation for all control methods, since the heat pump and thermal storage tank operate almost under design conditions, which are 0.6 and 0.4, respectively, as the load approaches 100% of design load. Therefore, in terms of power consumption, the experimental and simulation results show similar values of 0.56–0.60 during daytime and 0.39–0.43 during nighttime.
When the load is 80% of design heating load, each control method generates variations in heating capacity and power consumption due to the different operational characteristics. In both experimental and simulation results, heating capacity of the heat pump is largest in the heat pump priority method, followed by the region control method and thermal storage priority method, whereas that of the thermal storage tank shows the reverse. The heating capacity values obtained experimentally and via simulations showed high consistency, with differences within 0.02. It is expected that daytime and nighttime power consumption are proportional to heating capacity of the heat pump and thermal storage tank, but COP of the heat pump affects practical power consumption. Indeed, total power consumption is lowest in the heat pump priority method, which is expected to operate the highest part load ratio of the heat pump. However, nighttime power consumption, which experiences relatively inexpensive rates, was the largest in the thermal storage priority method, while the region control method lay in the middle of the other methods in terms of power consumption. The simulation results coincide well with the experimental results, with excellent pattern prediction.
When the load is 60% of design heating load, the heat pump priority method controls the heat pump to meet the load entirely on its own. In the thermal storage priority method, heating capacity of the thermal storage tank is the highest at 0.36, while that of the heat pump decreases as the load falls. Overall power consumption of this method is larger than those of the other two methods, as PLR of the heat pump falls. When the load is 80% and 60% of design load, the heat pump operates at full load under the heat pump priority method, as opposed to at its optimum heating capacity under the region control method; however, under the thermal storage priority method, the heat pump operates at lower PLR, as the load decreases to meet heating load.
3.2. Winter Simulation Results
Control performance simulations are performed during winter using the heat pump priority method, thermal storage priority method, region control method, and dynamic programming method.
Figure 14a shows the frequency ratio of heating load against design heating load in winter, along with daytime and nighttime daily average outdoor temperatures at a specific load. Heating load during winter shows a higher frequency at 20–80% of design load, and the number of days with low load (0–20%) or high load (>80%) are relatively few. Heating load tends to rise as outdoor temperature decreases. Given that daytime load is the same, the nighttime outdoor temperature is 3.8 °C lower than its daytime counterpart.
Figure 14b indicates the simulation results for control methods based on the heating load throughout winter. When the heat pump priority method is used, the heat pump handles entire heating load when it is 60% or less. When it exceeds 60%, the thermal storage tank increases heating capacity to handle the load, as the heat pump cannot increase its capacity any further. In the thermal storage priority method, heating capacity of thermal storage is fixed at 0.4 when the load is 40% or more; otherwise, entire heating load is handled by the thermal storage tank. The region control method shows a pattern similar to that of the thermal storage priority method when the heating load is 40% or less. When the load exceeds 40%, the heat pump does operate, but with a capacity lower than that of the heat pump priority method where the heat pump operates at full capacity. As indicated in
Figure 12, the heat pump is operated at optimal PLR for heating load of 0.48–0.88, distinguishing this method from the heat pump priority method with a full-load heat pump. In the dynamic programming method, the thermal storage tank handles most of the load when it is less than 40%. For load over 40%, heating capacity of the heat pump becomes larger than that of the thermal storage tank. Unlike the region control method, the dynamic programming method does not divide the control area with respect to load size, but follows the minimum cost path. Nevertheless,
Figure 13 shows similar patterns arising from the two methods.
Power consumption to generate heating capacity of the heat pump with control methods is closely related to COP of the heat pump. The heat pump’s COP against its heating capacity is shown in
Figure 15a. The heating capacity is divided by design load for normalization. COP increases with heating capacity, as displayed in
Figure 7b, and reaches a peak when heating load is 0.48, which is equivalent to PLR of 0.8. COP varies depending on the control method, even when heating capacity is identical, because the heat pump has different operating conditions for each control method.
In the thermal storage priority method, the thermal storage tank handles the heating load first, and the rest is handled by the heat pump as the load rises. Therefore, the heat pump operates when the load is relatively larger compared to the other methods. Large heating loads mean low outdoor temperatures, which leads to low COP of the heat pump with this method, even when the heat pump capacity is identical. In the heat pump priority method, the heat pump operates when the outdoor temperature is high and the load is small, so COP is higher than in the thermal storage priority method and full-load operation is frequent. The heat pump in the region control method operates only in the regions with large heating capacity due to its control characteristics. The dynamic programming method is expected to minimize power consumption and shows the highest COP of the heat pump under the same heating capacity conditions.
Figure 15b shows COP of the heat pump when different control methods are selected to respond to different levels of heating load. During nighttime, the heat pump charges an amount of heat equal to that discharged from the storage tank during daytime. COP distribution of the heat pump at night is also shown. COP of the heat pump depends on outdoor temperature, heating capacity, and outlet water temperature, and the performance varies in accordance with operational characteristics of the heat pump, which is governed by a control method to meet the heating load.
When the load is low, at a level of 0.4 or less, the heat pump priority method uses the heat pump solely. COP is low because PLR is small. When the load is higher than 0.4, heating capacity of the heat pump increases with high COP in all methods except the thermal storage priority method. However, in terms of daytime COP, the region control method with optimal PLR and the dynamic programming method with minimum cost path are found to be superior to the heat pump priority method at full load.
COP is lowest in the thermal storage priority method, because the heat pump begins operating from the heating load of 0.4 and continues operating with small PLR despite large heating load. When the load increases to 0.88 or more, the operational conditions are close to design load; therefore, the control methods show high PLRs and thus similar COPs.
At night, the heat pump operates at full load to charge an amount of heat equal to that discharged from the thermal storage during daytime. Therefore, COP is higher than during daytime with low PLR. Nonetheless, as presented in
Figure 14a, outdoor temperatures are lower at night than during the day, and hence, nighttime COP is lower than daytime COP with a high PLR.
Large heating load indicates that the outdoor temperature is low, during which time nighttime COP of the heat pump decreases. In contrast, when the heating load is 0.4 or less, nighttime COP does not vary significantly depending on the control method, except in the heat pump priority method, as only the thermal storage tank handles heating load. When heating load is between 0.4 and 0.88, nighttime COP is lowest in the thermal storage priority method; this is because nighttime charging operation takes longer as the discharged amount is larger in the thermal storage tank. In winter, the outdoor temperature declines as the night progresses, decreasing performance of the heat pump operating at night. For the same reason, nighttime COP is higher in the heat pump priority method, which uses the thermal energy tank in thermal storage less and thus operates the heat pump for a shorter period at night. Nighttime COP for the region control and dynamic programming methods are lower and higher than those for the heat pump priority and thermal storage priority methods, respectively. When nighttime heating load exceeds 0.88, nighttime COPs of all control methods converge to design specifications.
Figure 16a,b shows daily average power consumption and electricity cost according to heating load during winter, respectively. These values are demonstrated non-dimensionally based on the thermal storage priority method when the heating load is 80% or more. The upper portion of the bars indicate power consumption and electricity cost of the heat pump during daytime, while the bottom portion refers to those during nighttime.
When heating load is small (0–40%), the thermal storage tank handles the entire load in the thermal storage priority and region control methods. Therefore, same amount of power is consumed at night in both cases. In the heat pump priority method, the heat pump alone carries the heating load with low PLR. As indicated in
Figure 15b, COP is lower during daytime than nighttime when load is small, so power consumption is higher in the heat pump priority method. In the dynamic programming method, the thermal storage tank handles most of the load, so power consumption is lowest, albeit by a narrow margin.
In medium-load range (40–80%), the thermal storage tank and heat pump carry the heating load simultaneously in the thermal storage priority method, and the heat pump capacity increases with load size. This method shows the greatest power consumption due to the lowest COPs both during the night and day. The heat pump priority method has lower daytime COP due to full-load operation than the region control and dynamic programming methods, but has the lowest power consumption because the charging thermal energy is small during nighttime, when COP is low. In the region control and dynamic programming methods, the heat pump charges less thermal energy at night than in the thermal storage priority method, but more than in the heat pump priority method. These advanced methods operate the heat pump more than the heat pump priority method in the medium-load range, but their high daytime COP tends to reduce total power consumption. Further, the dynamic programming method has better daytime COP than the region control method, which explains its lowest power consumption level.
When heating load is high (>80%), the thermal storage priority method shows a slightly higher level of power consumption than the heat pump priority method, since COP of the heat pump is lower at night than during the day. The region control and dynamic programming methods show slightly lower power consumption than the heat pump priority method. The heat pump priority method operates the heat pump at full load, but, as
Figure 15b indicates, the region control and dynamic programming methods operate the heat pump with better COP during daytime than that of the heat pump priority method. As heating load increases, operational conditions of the heat pump and the thermal storage tank become similar to design conditions, which eliminates operational differences among the control methods.
It was confirmed that power consumption in the region control and dynamic programming methods was lesser than that in the conventional methods, and it decreased further in the medium-load range, which occurs most frequently. This is because conventional methods operate the heat pump either at full load or low PLRs, while the advanced methods operate it in the highly efficient medium-load range, resulting in reduced power consumption.
Based on information presented in
Table 6, the nighttime rates are cheaper than the daytime rates. Therefore, if the total power consumption is the same, the electricity cost is lower when nighttime heating load is higher. In the low-load region, a vast majority of the electricity used in each control method is consumed during either daytime or nighttime. The heat pump priority method results in high power consumption and the electricity costs are dramatically higher than those in other methods because it utilizes expensive daytime electricity. The dynamic programming method has the lowest power consumption, but, since it depends partially on daytime electricity, the electricity cost is only slightly less than that in the conventional methods, which primarily use nighttime electricity.
In the medium- and high-load regions, the thermal storage priority method shows the highest power consumption. However, as it relies largely on nighttime electricity for heating load over 0.6, the electricity cost is lower than the heat pump priority method. In contrast, the heat pump priority method shows low overall power consumption, yet results in high electricity costs due to its dependence on daytime operation. The region control method shows higher overall power consumption than the heat pump priority method, but tends to be more economical because it largely utilizes nighttime electricity and has less total power consumption than the thermal storage priority method. The dynamic programming method’s response is similar to that of the region control method, however the electricity costs are the lowest owing to excellent daytime performance and low daytime power consumption.
In the high-load region, the electricity costs are 1.01 for the heat pump priority method, 0.97 for the region control method, and 0.97 for the dynamic programming method. The characteristics of each method become similar as the load increases, resulting in less variation when compared to the low-load range. Accordingly, the region control and dynamic programming methods result in cheaper electricity costs than the heat pump priority method, because they utilize less-expensive nighttime electricity. Compared to the thermal storage priority method, they also have cheaper electricity costs due to the amount saved from high COP of the heat pump during the day and night, and this trend is most remarkable in the medium-load region.
Table 9 shows simulation results throughout winter, including daytime and nighttime power consumption and electricity costs by control methods. It is confirmed that the region control and dynamic programming methods are more economical than the conventional methods in terms of electricity costs. The storage utilization and heat pump efficiency have been improved through the region control method, which utilizes optimal PLR of the heat pump, and the dynamic programming method, which explores the minimum cost path. Cheap nighttime rates render these methods highly economical compared to the conventional methods. However, to implement the dynamic programming method in a heat pump–thermal storage system practically, it is necessary to know the heating load pattern in advance, which can be challenging. Therefore, the region control method has an edge over other methods as it is equally economical and can be easily implemented practically.