Building HVAC Load Profiling Using EnergyPlus - Nanyang ...

Building HVAC Load Profiling Using EnergyPlus Dong Wang

Abhisek Ukil, Senior Member, IEEE

Ujjal Manandhar

School of EEE Nanyang Technological University Singapore Email: [email protected]

School of EEE Nanyang Technological University Singapore Email: [email protected]

School of EEE Nanyang Technological University Singapore Email: [email protected]

Abstract—Building load profiling plays an important part in the development of optimal energy management systems. Heating, Ventilation, Air Conditioning (HVAC) systems consume around 40% to 50% of the total energy consumed by buildings. Using this load data, optimal operation scheduling can be determined to reduce the peak energy requirement and minimize the overall energy consumption. In this paper, EnergyPlus software is used to simulate the load profiles for HVAC loads, for a given building design. Different types of buildings, namely residential and commercial are considered. The main difference between the different types of buildings is in the operation scheduling. The scheduling and parameter like temperature could be altered to derive energy savings. The energy savings would significantly contribute to reduction in energy peak, minimizing the overall energy consumption. Keywords— Building load, HVAC, Energy efficiency, EnergyPlus, load profiling, smart building, smart grid.

I. I NTRODUCTION Buildings around the world account for a significant portion of the total energy consumption, roughly about 30-40% [1]. Singapore buildings consumed more than 30% of Singapore total power in 2012, which amounted to 42.6 billion kWh [2]. In 2010, the Business Monitor International (BMI) predicted that between 2010 and 2014, Singapores total energy consumption will grow by 12% due to the population increase [2]. Heating, Ventilation and Air Conditioning (HVAC) systems consume around 40% to 50% of the total energy consumed by buildings [1]. Therefore, energy efficiency in the buildings is a major research focus in Singapore [1-4], as well as worldwide. In this paper, a detailed modeling of the HVAC load for different types of buildings is presented. Residential buildings refer to the public and the private housings. Commercial or office buildings are those utilized for purposes like banking, administrative, legal, architectural services, shopping malls, etc. [1]. Industrial buildings refer to factories, production units, etc. [1]. EnergyPlus [5] software is used for simulation and analysis in the different scenario. The remainder of the paper is organized as follows. In section II, the construction of building model is described. Section III discusses about the HVAC system in buildings, and its three main loops. Section IV presents the detailed results This work was supported in part by the grant (M4061309.040) by NEC Laboratories Inc. America.

for the residential and the commercial buildings, followed by conclusions in section V. II. B ULDING M ODEL An example building model, the general type that is utilized in this work, is shown in the Fig. 1. The materials of the building are the same as normally used in the construction, such as concrete, gypsum, glass, roof insulation, and so on. The properties of the building materials such as specific heat, density and conductivity will be set to normal factors. For the whole building, there are two floors, each floor having 4 small rooms, 2 rooms being in front, and the other 2 being at back, with stairs in between. Three surface sides of each room will be decorated with window. So, for this building model, surfaces will be exposed under sunlight and wind, or neither. All these natural weather conditions are taken into consideration in the simulation. The whole building is split into two zones. 8 rooms are grouped into zone-1, and zone-2 includes the stairs. The net building area is 251m2 , total floor area of zone-1 being 225m2 and 26m2 for zone-2. Different scheduling will be applied to the respective zones [6].

Fig. 1. Building model construction in EnergyPlus

III. H EATING , V ENTILATION , A IR -C ONDITIONING S YSTEM Heating, Ventilation and Air Conditioning, or HVAC system is widely used in large buildings, especially in the commercial and industrial buildings. Due to the special weather in

Fig. 2. Air loop in HVAC system

Singapore, heating function of HVAC is rarely used. The airconditioning (AC) is mainly used. There are three main loops in this system: Air, Chilled Water and Condenser [7]-[8]. A. Air-Loop The main function of the Air Loop is to distribute air into desired zone which needs air supply. The schematic diagram is shown in Fig. 2. The whole loop splits into two parts: supply and demand side. In Fig. 2, the four nodes 3, 8, 9 and 10 would consist of an outdoor air system. Node 3 is the inlet node of the demand side. The return air from the supplied zone will pass through this node. Node 9 is the stream to relief the return air to the outdoor environment. Node 8 is the stream for the system to absorb outdoor air into the system. Node 10 will mix the outdoor air with return air to continue supplying air. The air will pass through water cooling coil and be cooled to desired temperature. In this paper, the temperature of the supplied air is set to 12.8 ◦ C. Fan delivers the air through the duct. The cooled air will pass through a splitter to be distributed to different zones. The return air from each zone will pass through a mixer, and sent to node 3. This continuously forms an air loop. Each zone will be installed with a Variable Air Volume (VAV) controller. It monitors the zone temperature. User can adjust the temperature through the thermostat, sending the feedback to the VAV controller. It automatically calculates and adjusts the air flow volume. B. Chilled Water Loop The chilled water flows in the cooling coil. The schematic diagram of the chilled water loop is shown in Fig. 3. The chilled water loop is also divided into two sides: supply and

Fig. 3. Chilled water loop in HVAC system

demand. Supply side consists of an electric chiller, and demand side has the cooling coil. A bypass branch exists in each side. This will bypass the operating equipment and ensure the condition that the equipment are not required. The working fluid can flow in the bypass pipe, instead of the equipment. Due to the presence of multiple branches, splitter and mixer are required at each side. C. Conderser Loop The condenser loop supplies cooling water in the cooling tower to water-cooled electric chiller in the chilled water loop. The schematic diagram of the loop is shown in Fig. 4. In condenser loop, supply side consists of a cooling tower and demand side consists of an electric chiller. The chiller scheme will be same as in the chilled water loop. The temperature set-point of the cooling tower is set at outlet node. The operation of cooling tower is regulated by the outdoor air wet bulb temperature, which is monitored. The outdoor air condition is achieved from the weather file. D. Equipment Settings 1) Fan: Fan is used to deliver the air to the desired zone. The specification of fan used in this paper is shown in Table I [9]. The fan total efficiency and motor efficiency are set to 0.6045 and 0.93 respectively. The schedule value is set to 1, which means that the fan is on, and value 0 means the fan is off. In this work, the fan value has been set at 1, all the time. 2) Pump: In chilled water loop, the pump is used to pump the water to the chiller. In condenser loop, the pump is used to pump the water to the cooling tower. The specification for the pump is shown in Table II [9]. The inlet and the outlet nodes will be set according to the schematic diagrams of the chilled water loop Fig. 3 and

TABLE II P UMP SPECIFICATION

ture will be set to 22 ◦ C and 25 ◦ C in certain time period for comparison. Fig. 5 shows the energy consumption of the main electric components for one week period. For the lights, energy consumption in both zone 1 and 2 will be the same through all days due to the unchanged scheduling. For day 1 and day 7, which are weekends, the energy consumption is different from the weekdays. For the chiller, different internal heat and weather condition cause the chiller energy to be different. Fig. 6 is obtained by changing the temperature set-point, according to the real situation. During the weekdays and weekend daytime, the temperature will be set to 25 ◦ C. The most significant change to notice is the chiller energy consumption, dropping rapidly in daily data. As shown in Table IV, the energy consumption of cooling reduces from 2.06 to 1.46 GJ. Fan energy drops from 0.09 to 0.06 GJ, 0.33 GJ of energy being saved by the pump. Due to reduction of power in the electrical components, the corresponding heat generation will decrease. The heat rejection reduces by 0.22 GJ. The total site energy consumption drops from 4.43 to 3.24 GJ. The formulation for calculating the percentage of HVAC system energy consumption in total energy is as follows. X F acility = Building+HV AC +P lant+Exterior (2) X Building = Zone (3)

Fig. 4. Condenser loop in HVAC system TABLE I FAN SPECIFICATION

condenser loop Fig. 4. The rated pump head is 179352 Pa. The fraction of full load power is determined by the equation: F raction F ull Load P ower = C1 + C2 P LR + C3 P LR2 + C4 P LR3

(1)

where, C1 , C2 , C3 and C4 are coefficients and P LR is Part Load Power. The coefficients of a typical pump, that are used in this paper, are shown in Table II [9]. As the coefficients C3 and C4 are set to 0, Eq. (1) becomes a linear one. IV. S IMULATION R ESULT A. Residential Building The scheduling is set according to the real situation. The detail setting is shown in the Table III. The cooling tempera-

EnergyP ercentage = (HV AC + P lant)/F acility

(4)

In Fig. 7, the respective percentages of electrical loads in residential buildings for operation at 22 ◦ C, for 7 days are: 90.17%, 86.43%, 86.92%, 85.42%, 86.91%, 86.66%, 90.76%. In Fig. 8, the respective percentages for 7 days are: 86.26%, 81.84%, 82.34%, 82.14%, 82.00%, 83.06%, 87.16%, for operation at 25 ◦ C. After comparison, it can be noted that the percentage drops about 3-4% in corresponding day. B. Commercial Building The scheduling of the office building is shown in Table V. The scheduling is slightly different from the residential building, because during the daytime of office building, people will work there, and less will stay at home. For the weekends, we assume no people to work in the office.

TABLE IV R ESIDENTIAL BUILDING: TOTAL END-USER ENERGY AT 22 ◦ C AND 25 ◦ C

Fig. 5. Weekly energy consumption in residential building at 22 ◦ C.

Fig. 6. Weekly energy consumption in residential building at 25 ◦ C.

Fig. 7. Residential building electrical loads at 22 ◦ C.

Figs. 9 and 10 show the energy consumption in the commercial building, for operation at 22 ◦ C and 25 ◦ C, respectively. Scheduling for lightings remains unchanged. The significant change can be noticed in the plant equipment, namely, the fan and the chiller. From the Table VI, the energy consumption for cooling reduces from 1.70 to 0.94 GJ. Fan energy increases from 0.11 to 0.14 GJ, 0.45 GJ of energy is saved by pump. Due to reduction in power of the electric components, the corresponding heat generation will decrease. The heat rejection reduces to 0.3 GJ. The total site energy consumption drops from 4.13 to 2.65 GJ. Using Eq. (2) and (4), the percentage of HVAC system energy consumption in total energy in commercial building is calculated. As shown in Fig. 11, the respective percentages

Fig. 8. Residential building electrical loads at 25 ◦ C.

of electrical loads in commercial buildings for operation at 22 ◦ C, for 7 days are: 97.06%, 76.45%, 77.43%, 77.87%, 77.19%, 76.79%, 97.30%. As shown in Fig. 12, the respective percentages of electrical loads in commercial buildings for operation at 25 ◦ C, for 7 days are: 93.78%, 66.78%, 67.56%, 67.00%, 66.54%, 66.84%, 94.4%, for operation at 25 ◦ C. In commercial building, due to the daytime weather condition and heat from the sunlight, the fan power will be higher than other time period in one day. If the temperature changes from 22 ◦ C to 25 ◦ C, in order to keep the desired temperature in the space and reduce the chiller power, the fan electric power needs to be increased. This is reflected in Figs. 11 and 12. However, higher energy savings can be achieved by reducing the chiller power.

TABLE III S CHEDULING OF RESIDENTIAL BUILDING

Fig. 9. Weekly energy consumption in commercial building at 22 ◦ C.

Fig. 11. Commercial building electrical loads at 22 ◦ C.

Fig. 10. Weekly energy consumption in commercial building at 25 ◦ C.

Fig. 12. Commercial building electrical loads at 25 ◦ C.

TABLE V S CHEDULING OF COMMERCIAL BUILDING

TABLE VI COMMERCIAL BUILDING: TOTAL END-USER ENERGY AT 22 ◦ C AND 25 ◦ C

cause the percentage of HVAC system energy consumption to be lower than the residential building. Different experiments are performed, in terms changing the operating temperature, and changing the schedules. The difference in energy utilization are noted. Between residential and commercial building, the latter shows impact of weekends on energy consumption. Accordingly, different scheduling for weekdays and weekends could be implemented in the commercial buildings to derive more energy savings. R EFERENCES

Computational fluid dynamics-based thermal modeling of HVAC [10] could further optimize the building energy management. Multisensor monitoring of the optimal temperature [11] could be utilized to control the temperature of the HVAC properly, avoiding over- or under-cooling. V. C ONCLUSION In this paper, models for HVAC load profiling of different types of buildings, namely residential and commercial are developed and analyzed. EnergyPlus [5] software is used for simulation and analysis. From the detailed analysis, in the residential building, electric equipment will not be used for long time in daily life, but the HVAC system runs for long time period. So, the weightage in total facility energy will be higher. The commercial building will be slightly different, due to many electric types of equipment used during the office hour. The energy consumption will be high, and consequently it will

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