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Evaluation of four control strategies for building VAV air-conditioning systems
Energy and Buildings 43 (2011) 414–422
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Evaluation of four control strategies for building VAV air-conditioning systems Xue-Bin Yang, Xin-Qiao Jin ∗ , Zhi-Min Du, Bo Fan, Xiao-Feng Chai Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Min Hang, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 12 March 2010 Received in revised form 25 August 2010 Accepted 3 October 2010 Keywords: Control strategy Evaluation Energy consumption Indoor air quality Thermal comfort
a b s t r a c t It is necessary to adopt appropriate control strategies to save energy and improve the indoor air quality (IAQ). On the validated TRNSYS simulation platform, four different control strategies are investigated to examine the indoor air temperature, energy consumption, CO2 concentration and predicted mean vote (PMV) for the variable air volume (VAV) systems in an office building in Shanghai. As an original scheme, Strategy A using constant outdoor air intake fraction shows high energy consumption, low CO2 concentration and acceptable thermal comfort. By using minimum outdoor air ventilation based on dynamic occupancy detection, Strategy B can provide more than 15% energy saving, acceptable PMV value but high CO2 concentration in breathing zone. By using indoor air temperature reset, Strategy C presents the most energy savings beyond 20% reduction, low CO2 concentration but poor thermal comfort. In mild seasons, combining enthalpy-based outdoor airflow economizer cycle with supply air temperature reset, Strategy D can achieve 9.4% energy savings and the lowest CO2 concentration. Taken together, each strategy covers some strengths as well as some weaknesses. How to comprehensively assess a control strategy for all specific objectives should be considered in future studies. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the ventilation control at zone level is one of the key factors affecting energy usage and indoor air quality, VAV systems are used for many large buildings to deliver an appropriate amount of supply air. The energy consumption for VAV system is chiefly associated with the thermal loads (cooling or heating) and the power to deliver these loads. Of course, the waterside power energy determined by the airside thermal load will indirectly influence the chiller energy consumption. For saving energy and improving IAQ, some strategies are developed to achieve the minimum outdoor air ventilation, to increase the supply air temperature, or to raise the indoor air temperature set point. More than six categories of strategies have been proposed in the recent works. 1.1. Fixed outdoor air damper Fixed outdoor air damper with a minimum position is commonly used to meet the requirement of minimum outdoor air flow rate in VAV systems. With the decrease of supply air flow rate, however, outdoor air flow rate will drop directly because of a fixed outdoor air damper position. Effects of supply air flow rate on outdoor air flow rate are approximately linear [1]. Since the heating, ventilation and air-conditioning (HVAC) system in large buildings
∗ Corresponding author. Tel.: +86 21 34206774; fax: +86 21 34206774. E-mail addresses: [email protected], [email protected] (X.-Q. Jin). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.10.004
operates at part-load conditions frequently, the ventilation strategy of maintaining a constant outdoor air intake fraction always leads to far lower outdoor air flow rates than the minimum demand stipulated in ASHRAE standard. By using outdoor air intake based on the unfavorable zone requirement [2], a strategy was proposed to apply in VAV systems. This strategy is different from the constant outdoor air flow rate based on total outdoor air requirement and on constant air intake fraction. 1.2. Minimum outdoor air flow rates Cho and Liu [3] proposed an applicable terminal-box control algorithm using minimum airflow reset to avoid inappropriate operation control functions associated with occupant discomfort and energy wasting in conventional strategies. Also, the dedicated minimum ventilation damper with pressure control is a recommended method to dynamically control the minimum outdoor air flow rates in VAV systems [1]. 1.3. Demand controlled ventilation (DCV) methods Although CO2 -based DCV has been proposed and investigated in many literatures [4–6], there are several problems in practical application. As an energy-saving strategy, in some cases, CO2 -based DCV may sacrifice other core design objectives including occupant health, productivity and even the threats to building structure’s long-term integrity. Energy benefits can be realized by implementing a DCV strategy, if the number of occupants and the actual
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Nomenclature Symbols A C E h P Q R RE S t
v V W Z t
total occupied floor area (m2 ) CO2 concentration (ppm) energy consumption (kWh) specific enthalpy (kJ/kg) total number of occupants (person) cooling loads (kW) outdoor air flow rate (m3 /(s m2 )) relative energy consumption CO2 generation rate per person (10−6 m3 /(s person)) dry-bulb temperature (◦ C) air flow rate (m3 /s) total net occupied floor volume (m3 ) humidity ratio (kg water/kg dry air) zone outdoor air fraction sampling interval (s)
Greek letters air density (kg/m3 ) Subscripts a area i zone number min minimum o outdoor air oz outdoor air for a zone p primary pz primary air for a zone P person r return air s supply air set set point Superscripts i current sampling time i−1 previous sampling time Acronyms AHU air handling unit DCV demand controlled ventilation HVAC heating, ventilation and air-conditioning IAQ indoor air quality PMV predicted mean vote VAV variable air volume
415
strategy called split-signal damper control strategy [9], can provide minimum static pressure drop in economizer dampers and be extremely effective at reducing the energy consumption of return and supply fans. Compared with the existing three-couple and twocouple dampers, the improved economizer damper can save 5% annual energy at least. Another example is a method of the integrated damper and pressure reset [10]. For static pressure set point reset according to the zone demand, however, the control logic is difficult to implement because of the highly interactive relationship between duct static pressure and damper position [11]. Also, it was found that Trim and Respond logic is a more effective approach in comparison with PID logic [11]. 1.5. Supply air temperature reset The supply air temperature reset applications [12–15] showed some favorable benefits in reducing power energy, reheating of cold primary air, outdoor air flow rates and the consequent humidification. By collecting occupants’ requests for the air temperature from their own personal computers and by balancing occupants’ requirements and energy consumption, an interactive system using a variety of feedback to control air-conditioning system can save beyond 20% energy at a constant indoor air temperature set point of 26 ◦ C during the summer cooling period [12]. The risk of great increase in power requirement is relatively small if supply air temperature is lower than the optimum. On the contrary, higher supply air temperature may result in relatively higher power consumption. 1.6. Combining strategies Mossolly et al. [16] proposed two strategies. One was to adjust supply air temperature and outdoor air flow rate to ensure the acceptable IAQ. The other was to modulate supply air temperature, supply air flow rate and outdoor air intake fraction to provide an acceptable thermal comfort and IAQ in each zone. Compared with the conventional control strategy based on the fixed supply air temperature, the proposed strategies not only enhanced the thermal comfort and IAQ, but also reduced the operational cost by 11% for the first strategy and 30.4% for the second. Xu et al. [17] presented a strategy with two schemes. One dynamically corrected the total outdoor air flow rates by utilizing the unpolluted outdoor air from the over-ventilation zone. The other optimized the temperature set point for the critical zones to reduce the variation of the required outdoor air fractions among all zones. Two schemes were combined to reduce the total outdoor air volume for energy conservation. 1.7. Other strategies or methods
ventilation rate can be determined with a reasonable degree of accuracy; and if building pressure can be maintained simultaneously [7]. Furthermore, CO2 -DCV application should be limited to those spaces with high-density, unpredictably, variable or intermittent occupancy. In addition, CO2 -DCV strategy can be used only in the presence of a reliable method to maintain a continuous base ventilation rate while preserving a minimum pressurization flow. To overcome these difficulties when DCV control is combined with economizer control, Wang and Xu [8] developed a robust control strategy with “freezing”, gain scheduling, integral term reset and feedback transition control for different transition processes. 1.4. Minimum static pressure drop Discharge and recirculation air dampers always keep fully opening during the occupied times. For the economizer with outdoor, discharge and recirculation air dampers, an improved operating
There are three other strategies or methods. One is based on CO2 or other pollutants emission. The ventilation rates, which are mostly determined by pollution emission, have substantial effects on the annual building loads. Thus, it is very important to reduce the pollution emission from building material and other sources [18]. Chao and Hu [19] developed a dual-mode control ventilation strategy, namely CO2 sensor control and non-occupant-related indoor contaminants control, to decrease CO2 or other pollutants emission. The other strategy adopted free cooling such as natural ventilation. Outdoor air flow rates during the evening and early morning hours are higher than those of the occupied hours [20]. This free cooling strategy could increase the chilled water temperature and result in approximate 20% savings in energy consumption. The third strategy used high air velocity to improve the thermal comfort. From the thermal comfort concept, it can be seen that if higher air velocity is provided, less cooling power is required to maintain PMV values in the desired range [21].
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Fig. 1. Outdoor air control strategy scheme of dynamic occupants detection.
Although many strategies have been focused on ventilation control for VAV system in many investigations, none of them can achieve the fundamental design objectives for HVAC system to save energy, to reduce maintenance costs, to improve IAQ and thermal comfort, to promote worker productivity, and even to diminish lifecycle cost. By weighing the benefits and risks prior to implementing a strategy, four control strategies are investigated in this study. Strategy A contains a constant outdoor air intake fraction, Strategy B involves in the dynamic-occupancy-detecting DCV, Strategy C is the indoor air temperature reset control, and Strategy D employs the enthalpy-based outdoor airflow economizer cycle combined with supply-air-temperature reset control. These four strategies are applied to examine indoor air temperature, energy consumption, indoor CO2 concentration and PMV values under three typical testing days in summer, mild and winter. 2. Description of four control strategies For energy savings, comfortable and healthy indoor environments, four control strategies used in the VAV systems are described in the following subsections. 2.1. Strategy A: constant outdoor air intake fraction If the total supply air ventilation is at a lower level, constant outdoor air intake fraction will induce the generic terminal-reheat VAV systems being starved of air ventilation during winter period. In spite of this weakness, Strategy A is still a most-often adopted ventilation strategy [7] and provides sufficient ventilation air if VAV system is operated as a rated constant air volume system. Assumed that the constant outdoor air intake fraction is a fixed value of 0.3 throughout the year, outdoor air flow rate will increase in proportion to supply air flow rate.
ber of occupants at time i, Ra is the outdoor airflow rate required per unit area, and A is the total occupied floor area. Providing an excellent supply air to dilute the occupant-related contaminants, the DCV strategy based on CO2 concentration has been widely used [19]. This strategy detects the dynamic occupants in breathing ventilation area based on the difference between indoor and outdoor CO2 concentration, and acquires the minimum outdoor air flow rate by Eq. (1). As shown in Fig. 1, the indoor and outdoor CO2 concentrations as well as outdoor air flow rate measured by sensors are saved in the storage model. Subsequently, the values in previous and current time period are the inputs of the dynamic occupant supervisory model and thus the current–time population Pi can be calculated by Eq. (2) [23]. Pi =
i i (vio + vi−1 V × (Cri − Cri−1 ) o ) × (Cr − Co ) + 2S St
(2)
where vo denotes the total outdoor air volume. Cr and Co indicate the CO2 concentration of return air and outdoor air, respectively. The superscripts i and i−1 represent the current and previous sampling time, respectively. S is the CO2 emission rate per person. V is the total occupied floor volume, and t means the time step. In certain workdays, the comparison between actual and dynamically simulated population for an office room is demonstrated in Fig. 2. Note that a clear reduction can be seen at the lunch break time from 12:00 to 14:00. The indoor occupancy peak is up to 17 persons during the working period. To sum up, the simulation results are very close to the actual ones although the maximum simulation error is near 8.7%. This suggests that the DCV strategy based on CO2 concentration levels could obtain accurate prediction
2.2. Strategy B: dynamic-occupancy-detecting DCV The outdoor air is used to dilute the concentration of indoor air pollutants. Obviously, the minimum outdoor air volume is used to ensure the well IAQ. Considering the impacts of CO2 concentration and nonhuman air contaminants, DCV is introduced to achieve acceptable IAQ and low energy consumption. Outdoor air volume required at breathing zone is determined by people-related and area-related contaminant concentrations. The equation stipulated by ASHRAE Standard [22] can be depicted as
vio,min = RP × P i + Ra × A
(1)
where vio,min is the total minimum outdoor air flow rate at time i, RP is the outdoor airflow rate required per person, Pi is the total num-
Fig. 2. Comparison of population between actual and dynamically detecting occupants for an office room.
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Table 1 Control schemes for enthalpy-based outdoor airflow economizer cycle. Cases
Control schemes
ho > hr > hs
Outdoor air precooled by partial return air, and minimum zone outdoor air intake fraction Maximum outdoor airflow rate with return air damper shutoff Outdoor air blended with partial return air
hr > ho > hs hr > hs > ho
Fig. 3. Control strategies of indoor air temperature reset.
for indoor dynamic occupancy to determine the minimum outdoor air flow rate. 2.3. Strategy C: indoor air temperature reset control As to the comfort air-conditioning system used in an office building, it is unnecessary to maintain indoor air temperature and relative humidity at a fixed value but is significant to regulate them within a given range. The control strategy of indoor air temperature reset targets to establish the logical relationship between indoor air temperature and regulated variables such as occupants, energy consumption, CO2 concentration, PMV values and so on. The changing range of indoor air temperature can thus be controlled within people’s comfort zones as far as possible. Although the humidity also affects the health and comfort, this effect is evidently less than that of indoor air temperature. To simplify the control process, the value of relative humidity in occupied space should be limited to no more than 70% in summer and no less than 30% in winter. As shown in Fig. 3, the indoor air temperature is set at 28 ◦ C in summer and 20 ◦ C in winter, respectively. This is different from the varying set points in mild seasons. If the indoor air temperature is lower than 20 ◦ C, the supply air damper stays at a minimum opening. If the terminal reheat operates, the damper opening will become larger. Also, the damper opening gradually enlarges along with the increasing cooling load, and reaches the maximum if the indoor air temperature is higher than 28 ◦ C. 2.4. Strategy D: enthalpy-based outdoor airflow economizer cycle combined with supply air temperature reset control Most of traditional air-conditioning systems are usually designed based on typical meteorological conditions and minimum outdoor air flow rate in summer/winter design day. With the increase of modern office equipments and subsequent thermal loads in existing commercial buildings, chilled air should be supplied to ventilation space to transfer these loads even in mild seasons. Thus, it is difficult for the traditional design to fully consider both annual operational control and thermal comfort. In this case, if the outdoor air enthalpy is lower than the indoor air enthalpy, outdoor air ventilation, as free cooling, can compensate for a part of inner cooling load. Consequently, this will save the energy of air handling and shorten the running time of chillers. Note that outdoor air ventilation should meet the sanitation requirement of minimum volume rather than the annual fixed intake fraction. The VAV systems with fixed outdoor air fraction often have substantial energy conservation in typical summer/winter workdays, but may result in energy wasting in mild seasons or under non-typical
conditions. For these reasons, the strategy combining DCV with enthalpy-based outdoor airflow economizer cycle is developed to achieve the acceptable IAQ and the lower coil energy consumption. As listed in Table 1, ho , hr and hs denote the specific enthalpy of outdoor air, return air and supply air, respectively. Because the supply air enthalpy hs is lower than the outdoor air enthalpy ho in summer workdays in Shanghai, the fixed outdoor air fraction is set at 0.3, which is the same with Strategy A. In addition, the outdoor air enthalpy ho is usually considerably lower than the supply air enthalpy hs in winter workdays, so the fixed outdoor air fraction of 0.3 can save the energy consumption of system. The strategy with the identical control to Strategy A in summer/winter workdays is therefore especially suitable for the mild seasons. To avoid the instability phenomena such as alternation and oscillation of outdoor air damper control whose signals are determined from both traditional supply air temperature strategy and DCV strategy [24], the strategy of supply air temperature reset is specially employed to provide the control signals for supply air damper. Supply air temperature ts can be determined by Eq. (3) [25], and hs can be derived from Eq. (4). hs = 1.01ts + W (2501 + 1.86ts )
(3)
where hs is the specific enthalpy set point, ts is the dry-bulb temperature of supply air, and W is the humidity ratio. Qi = vpz,i (hset − hs )
(4)
where Qi is the cooling loads for zone i, is the air density, vpz,i indicates the primary airflow for zone i, and hset denotes the specific enthalpy set point.
vpz,i =
voz,i Zp
(5)
where vpz,i denotes the primary air flow rate for zone i, voz,i is the outdoor air flow rate for zone i, and Zp is the zone primary outdoor air fraction. 3. Building models and simulations The VAV systems are installed throughout a certain floor of an office building located in Shanghai and the schematic diagram can be found in the published literature [25,26]. The office building is occupied only from 8:00 to 20:00. The floor has a total floor area of 1166 square meters and is divided into eight thermal zones. The simulation program developed on TRNSYS [26,27] is applied as the platform to validate four control strategies. The program includes some models involving the VAV systems and their control loops. With low initial cost and energy consumption, the employed single-duct VAV system comprises three basic parts, an air handling unit (AHU), some air ducts and VAV terminal boxes. A pressureindependent VAV terminal with electric reheat is installed in each thermal zone. The electric reheat and pressure-independent VAV controller are employed to control the air temperature in each zone. By regulating the VAV damper signalled from a PID temperature controller, the zone supply air flow rate can be varied to provide a desired space temperature. To maintain a supply air static pressure near its set point, a PID controller regulates supply fan speed
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Fig. 4. Outdoor air conditions in three testing days (a) dry-bulb temperature and (b) humidity ratio
to provide sufficient air volume. Also, an ex-filtration flow controller adjusts return fan speed to change the difference between the supply and the return air flow rates at a certain set point and to maintain positive pressure. Supply air temperature is provided by adjusting water valve and thus modulating the chilled water flow rate into the AHU coil. In practice, a single controller is used to provide certain outdoor air intake fraction by positioning outdoor, recycle and exhaust air dampers.
Fig. 5. Dynamic change of indoor air temperature for four different strategies (a) summer, (b) mild, and (c) winter.
4. Results and discussion Four performance indices, including indoor air temperature, energy consumption, CO2 concentration and PMV, are examined in three typical days, namely a summer day, a mild day and a winter day. Four performance indices are examined and compared under identical working conditions. The outdoor air dry-bulb temperature and humidity ratio in three typical days in Shanghai are shown in Fig. 4. 4.1. Indoor air temperature Fig. 5 demonstrates the variation of indoor air temperature for four control strategies in three typical testing days. In summer, indoor air temperature can reach the set point without drastic fluctuations and meet the system requirements. The indoor air temperature set point for Strategy A and B are both 24 ◦ C while that for Strategy C is 28 ◦ C. It can be seen in Fig. 5(b) that the relatively low outdoor air temperature leads to small indoor cooling load in the morning and evening in mild seasons. During the hours of 8:00 to 10:00 and 17:00 to 20:00, the indoor air temperatures for Strategy A, B and D maintain at their set point (24 ◦ C) even though the supply air damper reaches the minimum position. Although a terminal reheat equipment is employed to maintain the indoor air temperature at
its set point, this will inevitably lead to partial cooling load offset by reheating and subsequent energy wasting. In practice, the traditional design and commission only cover the summer/winter operation modes for air-conditioning system, so it is very difficult to maintain the system operating reliability under the cooling mode in mild seasons. Fortunately, Strategy C can set low temperature point under low load conditions in the morning or evening due to enlarging the fluctuation scope of indoor air temperature. In this case, it is within an hour for indoor air temperature to reach the allowable lower limit of 20 ◦ C, so the reheating time is shortened and the cooling load offset by reheating is significantly reduced. Furthermore, the indoor air temperature set point increases with the increase of outdoor air temperature, and thus thermal conduction load decreases. As shown in Fig. 5(c), the indoor air temperatures in winter for Strategy A and B are near their set point of 24 ◦ C and that for Strategy C is 20 ◦ C. Also, it takes about 3 h for the indoor air temperatures of Strategy A and B to reach their set points. It can be seen that the system dynamic response is insufficient because the excessively low indoor air temperature in winter evening may result in a peak load before chiller start-up. Hence, it is necessary to keep a minimum indoor air temperature in winter evening or to operate chillers before the building is occupied.
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Fig. 6. Energy consumption for four different strategies.
4.2. Energy consumption The energy consumptions for four control strategies in three testing days are shown in Fig. 6. As an original scheme, Strategy A is the comparing benchmark to the other three strategies. Compared with Strategy A, in summer, Strategy B and C can reduce energy consumption by 23.6% and 32.4% respectively. Strategy B can save energy by detecting the dynamic occupants and then controlling the outdoor air volume. Less outdoor air volume results in lower outdoor air cooling load and smaller supply air volume. For Strategy C, higher indoor air temperature set point leads to lower thermal conduction load and smaller cooling load. Due to increasing the indoor air temperature set point, Strategy C can provide significant energy savings. In mild, compared with Strategy A, the energy consumption of Strategy B is increased by 5.6%, while that of Strategy C and D is reduced by 24.9% and 9.4%, respectively. For Strategy B, less outdoor air is supplied into the building because the minimum outdoor ventilation is varied by detecting the dynamic indoor occupants. Outdoor air flow rate is declined and the subsequent fans’ energy may be reduced, but less mild outdoor air as free cooling is supplied into ventilation space. The total energy consumption, therefore, is not reduced but increased. Strategy C reduces outdoor air ventilation, but raises the supply air temperature under the peak system load. Thus, the cooling load reduced by raising supply air temperature is much greater than that increased by lower outdoor air ventilation. The total cooling load, therefore, is reduced. Because indoor cooling load is partially reduced by mild outdoor air, the energy of Strategy D is also reduced. Generally, Strategy C can distinctly reduce the energy consumption while Strategy B may increase the energy usage. If the outdoor air climatic conditions are favorable in mild season, much more outdoor air ventilation will be beneficial to save power energy responding to such free cooling opportunities. In winter, compared with Strategy A, the energy consumption of Strategy B and C is decreased by 15.2% and 21.9%, respectively. This is mainly because that the smaller outdoor air flow rate results in lower thermal load. As a whole, Strategy C exhibits the most effective energy usage beyond 20% reduction compared with Strategy A. The energy consumption of Strategy B can be reduced by more than 15% either in summer or in winter, but increased by 5.6% in mild. Dissimilar to Strategy A merely in mild, Strategy D Exhibits 9.4% decrement in energy consumption.
Fig. 7. CO2 level in breathing zone for four different strategies (a) summer, (b) mild, and (c) winter.
4.3. CO2 concentration Better IAQ is directly involved in human mood, healthy state, working efficiency and so on, and thus favorable CO2 concentration should be as low as possible if no energy is wasted. Obviously, the indoor CO2 concentration holds a strong function with occupancy number since occupants are the predominant CO2 generation source inside the office building. Under the identical occupied conditions, more outdoor air flow rates will conduce to lower CO2 concentration because people-generated CO2 volume is diluted by outdoor air. 100% outdoor air ventilation, therefore, is prior to be adopted to achieve low CO2 concentration. Fig. 7 demonstrates the indoor CO2 profile in three testing days. In the morning before fully occupied, the noon for lunch and the evening before unoccupied, indoor CO2 concentration is lower indicating a few occupants. Nevertheless, CO2 concentration becomes higher with the increase of occupants in other period. As depicted in Fig. 7(a), Strategy B always shows relatively higher CO2 concentration during the entire working period, but the CO2 level is still in the allowable range. Because the system load and outdoor air ventilation are both reduced by high indoor air temperature set point, Strategy C exhibits higher hourly CO2 concentration than the other strategies during the whole working period. Fig. 7(b) shows that the CO2 level varies with different indoor occupants. As shown in Fig. 7(c), indoor CO2 concentration of Strategy B shows the maximum value due to the minimum outdoor air ventilation, but is
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Fig. 8. PMV value for four different strategies (a) summer, (b) mild, and (c) winter.
still within acceptable limits. Strategy A and C possess very similar trends. Their CO2 concentrations typically remain in the range from 350 to 650 ppm, and this is better than Strategy B. To sum up, different strategies have various CO2 concentration levels in three testing days. Strategy A possesses the lowest CO2 concentration in summer and better in mild and winter. Strategy B exhibits the maximum CO2 concentration during the whole year. Strategy C has the lowest one in winter and slightly higher than those of Strategy A in summer and mild. Strategy D possesses the lowest one in the mild testing day and close to those of Strategy A in summer and winter. Considering that CO2 level is an important index for IAQ, Strategy A and D provides the best IAQ in summer, Strategy D indicates the best in mild, and Strategy C provides the best in winter. 4.4. PMV According to the equations from ASHRAE Handbook 2005 [25], PMV value is applied to evaluate the thermal comfort. As a single comfortable index involving a range of thermal sensations, PMV contains the effects of air temperature, space relative humidity, radiant temperature, air velocity, clothing level and activity level. Taken these six variables into consideration, thermal sensation range scale of PMV can be listed in Table 2 [21]. The PMV values of four different control strategies are shown in Fig. 8. From Fig. 8(a), Strategy A and B exhibit desirable PMV values
and vary from −0.2 to 0.1 where is an ideal environmental region for human thermal comfort. Because the indoor air temperature set point is increased, Strategy C results in a certain degree of decline in thermal comfort and the corresponding PMV value mostly fluctuates in the vicinity of +1 during working time. According to Table 2, this region belongs to slightly warm feeling. Obviously, Strategy C shows the most unsatisfactory thermal comfort owing to increasing the indoor air temperature set point to 28 ◦ C. As depicted in Fig. 8(b), some PMV values are lower than −3 indicating the cold feeling for human thermal sensations. This is mainly because that the indoor air temperature is much lower at the beginning of chiller start-up (as seen in Fig. 5) when system operates under the summer cooling mode. Moreover, the cold feeling is slowly relieved subject to a gradual rise in indoor air temperature till 10:00. During the hours of 10:00 to 17:00, PMV value always exists at an acceptable limit of +0.5. Taken as a whole, PMV value for Strategy A, B and D approximates 0 and their regarding indoor air temperatures stabilize at the set points. Corresponding to the higher indoor air temperature set point, Strategy C exhibits high PMV value approaching to +0.5. After 17:00, PMV value is gradually reduced to −1 of slightly cool feeling with decreasing indoor air temperature set point. As shown in Fig. 8(c), PMV value of three strategies could reach a stable state in an hour following chillers start-up. The trends are similar to the indoor air temperature variations demonstrated in Fig. 5(c). Strategy C reveals the most unfavorable indoor thermal comfort, and its PMV value is close to −1.3 indicating slightly cool feeling. The other two strategies can make much desirable indoor thermal comfort and both PMV values exist in between −0.8 and −0.5. Taken the clothing condition into account, these values pertaining to slightly cool feeling might be acceptable in winter workdays. As a whole, Strategy C reveals relatively high PMV value meaning slightly warm feeling or slightly cool feeling, and the other strategies exhibit relatively low PMV value indicating the desirable indoor thermal comfort. In addition, the variation trend of PMV values is very close to that of indoor air temperature. For each strategy, it takes more than 1 h to reach a stable state in summer and winter, and more than 2 h in mild seasons. 5. Evaluation of four strategies Three performance indices including relative energy consumption, CO2 concentration interval and thermal comfort, are used to evaluate four control strategies. Relative energy consumption RE is defined in Eq. (6). CO2 concentration intervals are statistically from 10:00 to 18:00, and the thermal comfort is rated from the thermal sensation range scale of PMV. REStrategy j =
EStrategy j EStrategy A
j = A, B, C and D
(6)
where REStrategy j denotes the relative energy consumption of Strategy j, j = A, B, C and D. EStrategy j denotes the energy consumption of Strategy j, and EStrategy,A denotes the energy consumption of Strategy A. As shown in Table 3, different strategy possesses various strengths as well as weaknesses evaluated by three performance indices. Strategy A has the highest energy consumption, acceptable thermal comfort, and low CO2 concentration. Its CO2 concentration only in mild is slightly higher than that of Strategy D. Strategy B exhibits the best thermal comfort but the highest CO2 concentration. Its energy consumption is higher than Strategy C and lower than Strategy A and D. Strategy C possesses the lowest annual energy consumption, slightly higher CO2 concentration in summer and mild than Strategy A and D, and slightly warm thermal comfort in summer and slightly cool in winter. Strategy D shows the lowest
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Table 2 Thermal sensation range scale of different PMV value. PMV value Thermal comfort
+3 Hot
+2 Warm
+1 Slightly warm
0 Neutral
−1 Slightly cool
−2 Cool
−3 Cold
Table 3 Evaluation of four control strategies.
Relative energy consumption Summer Mild Winter CO2 concentration interval (ppm) Summer Mild Winter Thermal comfort Summer Mild Winter
Strategy A
Strategy B
Strategy C
Strategy D
1 1 1
0.76 1.06 0.85
0.68 0.75 0.78
1 0.91 1
[398, 593] [446, 927] [454, 705]
[839, 1020] [958, 1078] [824, 926]
[469, 736] [498, 1000] [438, 669]
[398, 593] [381, 662] [454, 705]
Neutral Neutral Neutral
Neutral Neutral Neutral
Slightly warm Neutral Slightly cool
Neutral Neutral Neutral
Table 4 Preferred strategies for different performance index. Performance index
Summer
Mild
Winter
Whole year
Lowest energy consumption Lowest CO2 concentration Best thermal comfort
Strategy C Strategy A or D Strategy B
Strategy C Strategy D Strategy B
Strategy C Strategy A or D Strategy A, B or D
Strategy C Strategy D Strategy B
CO2 concentration, acceptable thermal comfort, and lower energy consumption in mild than Strategy A. The preferred strategy for different performance index is listed in Table 4. Among these four strategies, Strategy A is the most unsatisfactory strategy in the whole year for any performance index. Strategy B provides the best thermal comfort, and Strategy C exhibits the lowest energy consumption. Identical with Strategy A in summer and winter, Strategy D shows the lowest CO2 concentration during the whole year. 6. Conclusion Strategy A is the benchmark to depict the increment or decrement in other strategies’ energy consumption. The evaluations of four strategies are as follows: (a) Strategy A possesses the lower CO2 concentration and the better PMV in summer and winter, but the largest annual energy consumption. (b) With more than 15% energy savings either in summer or in winter, Strategy B follows the similar PMV trends to Strategy A. The CO2 concentration level, however, is so high that it may result in more complaints from occupants. (c) Strategy C displays the lowest energy consumption beyond 20% savings and the lower CO2 concentration in both summer and winter. Because of the relatively high set point of 28 ◦ C in summer and relatively low one of 20 ◦ C in winter, this strategy results in slightly-warm thermal comfort in summer and slightly-cool in winter. (d) Dissimilar to Strategy A only in mild, Strategy D provides the lowest CO2 concentration, 9.4% energy savings, and similar PMV to Strategy A. Generally, Strategy C is a simple, effective and practical energysaving strategy. Since the conventional design of air-conditioning system only takes summer and winter operational modes into consideration, some measures should be taken to avoid the energy wasting resulting from partial cooling offset by reheating. Strategy
C uses indoor air temperature reset to decrease the temperature set point if the system load is low in the morning and evening in summer, and even in mild seasons. In winter, some measures such as maintaining an upper limit of the minimum indoor air temperature set point, or operating chillers before building is occupied, make sense to reduce the complaints about occupant comfort due to shortening the time to reach a set point. It is difficult to examine a control strategy whether perfect or not. To meet the requirements such as saving energy, desirable IAQ, acceptable thermal comfort and so on, the comprehensive assessment methods to weigh benefits and risks for all specific objectives should be further investigated. Acknowledgement The research work presented in this paper was financially supported by a grant from the National Natural Science Foundation of China under Grant No. 50976066. References [1] Advanced variable air volume system design guide, 2005. http://www. uccsuiouee.org/index.html. [2] T.A. Reddy, M. Liu, D.E. Claridge, A study of energy use and satisfactory zone ventilation of different outdoor air ventilation strategies for terminal reheat variable air volume systems, Energy and Buildings 29 (1) (1998) 65–75. [3] Y.H. Cho, M. Liu, Minimum airflow reset of single duct VAV terminal boxes, Building and Environment 44 (9) (2009) 1876–1885. [4] M. Krarti, M. Al-alaw, Analysis of the impact of CO2 -based demand-controlled ventilation strategies on energy consumption, ASHRAE Transactions 110 (1) (2004) 274–286. [5] T.M. Lawrence, J.E. Braun, Calibrated simulation for retrofit evaluation of demand-controlled ventilation in small commercial buildings, ASHRAE Transactions 113 (2) (2007) 227–240. [6] S.T. Taylor, Demand-controlled ventilation: CO2 -based DCV using 62.1-2004, ASHRAE Journal 48 (5) (2006) 67–75. [7] D.S. Dougan, L. Damlano, CO2 -based demand control ventilation, ASHRAE Journal 46 (10) (2004) 47–54. [8] S. Wang, X. Xu, Optimal and robust control of outdoor ventilation air flow rate for improving energy efficiency and IAQ, Building and Environment 39 (7) (2004) 763–773. [9] N. Nassif, S. Moujaes, A new operating strategy for economizer dampers of VAV system, Energy and Buildings 40 (3) (2008) 289–299.
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Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Evaluation of four control strategies for building VAV air-conditioning systems Xue-Bin Yang, Xin-Qiao Jin ∗ , Zhi-Min Du, Bo Fan, Xiao-Feng Chai Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Min Hang, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 12 March 2010 Received in revised form 25 August 2010 Accepted 3 October 2010 Keywords: Control strategy Evaluation Energy consumption Indoor air quality Thermal comfort
a b s t r a c t It is necessary to adopt appropriate control strategies to save energy and improve the indoor air quality (IAQ). On the validated TRNSYS simulation platform, four different control strategies are investigated to examine the indoor air temperature, energy consumption, CO2 concentration and predicted mean vote (PMV) for the variable air volume (VAV) systems in an office building in Shanghai. As an original scheme, Strategy A using constant outdoor air intake fraction shows high energy consumption, low CO2 concentration and acceptable thermal comfort. By using minimum outdoor air ventilation based on dynamic occupancy detection, Strategy B can provide more than 15% energy saving, acceptable PMV value but high CO2 concentration in breathing zone. By using indoor air temperature reset, Strategy C presents the most energy savings beyond 20% reduction, low CO2 concentration but poor thermal comfort. In mild seasons, combining enthalpy-based outdoor airflow economizer cycle with supply air temperature reset, Strategy D can achieve 9.4% energy savings and the lowest CO2 concentration. Taken together, each strategy covers some strengths as well as some weaknesses. How to comprehensively assess a control strategy for all specific objectives should be considered in future studies. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the ventilation control at zone level is one of the key factors affecting energy usage and indoor air quality, VAV systems are used for many large buildings to deliver an appropriate amount of supply air. The energy consumption for VAV system is chiefly associated with the thermal loads (cooling or heating) and the power to deliver these loads. Of course, the waterside power energy determined by the airside thermal load will indirectly influence the chiller energy consumption. For saving energy and improving IAQ, some strategies are developed to achieve the minimum outdoor air ventilation, to increase the supply air temperature, or to raise the indoor air temperature set point. More than six categories of strategies have been proposed in the recent works. 1.1. Fixed outdoor air damper Fixed outdoor air damper with a minimum position is commonly used to meet the requirement of minimum outdoor air flow rate in VAV systems. With the decrease of supply air flow rate, however, outdoor air flow rate will drop directly because of a fixed outdoor air damper position. Effects of supply air flow rate on outdoor air flow rate are approximately linear [1]. Since the heating, ventilation and air-conditioning (HVAC) system in large buildings
∗ Corresponding author. Tel.: +86 21 34206774; fax: +86 21 34206774. E-mail addresses: [email protected], [email protected] (X.-Q. Jin). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.10.004
operates at part-load conditions frequently, the ventilation strategy of maintaining a constant outdoor air intake fraction always leads to far lower outdoor air flow rates than the minimum demand stipulated in ASHRAE standard. By using outdoor air intake based on the unfavorable zone requirement [2], a strategy was proposed to apply in VAV systems. This strategy is different from the constant outdoor air flow rate based on total outdoor air requirement and on constant air intake fraction. 1.2. Minimum outdoor air flow rates Cho and Liu [3] proposed an applicable terminal-box control algorithm using minimum airflow reset to avoid inappropriate operation control functions associated with occupant discomfort and energy wasting in conventional strategies. Also, the dedicated minimum ventilation damper with pressure control is a recommended method to dynamically control the minimum outdoor air flow rates in VAV systems [1]. 1.3. Demand controlled ventilation (DCV) methods Although CO2 -based DCV has been proposed and investigated in many literatures [4–6], there are several problems in practical application. As an energy-saving strategy, in some cases, CO2 -based DCV may sacrifice other core design objectives including occupant health, productivity and even the threats to building structure’s long-term integrity. Energy benefits can be realized by implementing a DCV strategy, if the number of occupants and the actual
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Nomenclature Symbols A C E h P Q R RE S t
v V W Z t
total occupied floor area (m2 ) CO2 concentration (ppm) energy consumption (kWh) specific enthalpy (kJ/kg) total number of occupants (person) cooling loads (kW) outdoor air flow rate (m3 /(s m2 )) relative energy consumption CO2 generation rate per person (10−6 m3 /(s person)) dry-bulb temperature (◦ C) air flow rate (m3 /s) total net occupied floor volume (m3 ) humidity ratio (kg water/kg dry air) zone outdoor air fraction sampling interval (s)
Greek letters air density (kg/m3 ) Subscripts a area i zone number min minimum o outdoor air oz outdoor air for a zone p primary pz primary air for a zone P person r return air s supply air set set point Superscripts i current sampling time i−1 previous sampling time Acronyms AHU air handling unit DCV demand controlled ventilation HVAC heating, ventilation and air-conditioning IAQ indoor air quality PMV predicted mean vote VAV variable air volume
415
strategy called split-signal damper control strategy [9], can provide minimum static pressure drop in economizer dampers and be extremely effective at reducing the energy consumption of return and supply fans. Compared with the existing three-couple and twocouple dampers, the improved economizer damper can save 5% annual energy at least. Another example is a method of the integrated damper and pressure reset [10]. For static pressure set point reset according to the zone demand, however, the control logic is difficult to implement because of the highly interactive relationship between duct static pressure and damper position [11]. Also, it was found that Trim and Respond logic is a more effective approach in comparison with PID logic [11]. 1.5. Supply air temperature reset The supply air temperature reset applications [12–15] showed some favorable benefits in reducing power energy, reheating of cold primary air, outdoor air flow rates and the consequent humidification. By collecting occupants’ requests for the air temperature from their own personal computers and by balancing occupants’ requirements and energy consumption, an interactive system using a variety of feedback to control air-conditioning system can save beyond 20% energy at a constant indoor air temperature set point of 26 ◦ C during the summer cooling period [12]. The risk of great increase in power requirement is relatively small if supply air temperature is lower than the optimum. On the contrary, higher supply air temperature may result in relatively higher power consumption. 1.6. Combining strategies Mossolly et al. [16] proposed two strategies. One was to adjust supply air temperature and outdoor air flow rate to ensure the acceptable IAQ. The other was to modulate supply air temperature, supply air flow rate and outdoor air intake fraction to provide an acceptable thermal comfort and IAQ in each zone. Compared with the conventional control strategy based on the fixed supply air temperature, the proposed strategies not only enhanced the thermal comfort and IAQ, but also reduced the operational cost by 11% for the first strategy and 30.4% for the second. Xu et al. [17] presented a strategy with two schemes. One dynamically corrected the total outdoor air flow rates by utilizing the unpolluted outdoor air from the over-ventilation zone. The other optimized the temperature set point for the critical zones to reduce the variation of the required outdoor air fractions among all zones. Two schemes were combined to reduce the total outdoor air volume for energy conservation. 1.7. Other strategies or methods
ventilation rate can be determined with a reasonable degree of accuracy; and if building pressure can be maintained simultaneously [7]. Furthermore, CO2 -DCV application should be limited to those spaces with high-density, unpredictably, variable or intermittent occupancy. In addition, CO2 -DCV strategy can be used only in the presence of a reliable method to maintain a continuous base ventilation rate while preserving a minimum pressurization flow. To overcome these difficulties when DCV control is combined with economizer control, Wang and Xu [8] developed a robust control strategy with “freezing”, gain scheduling, integral term reset and feedback transition control for different transition processes. 1.4. Minimum static pressure drop Discharge and recirculation air dampers always keep fully opening during the occupied times. For the economizer with outdoor, discharge and recirculation air dampers, an improved operating
There are three other strategies or methods. One is based on CO2 or other pollutants emission. The ventilation rates, which are mostly determined by pollution emission, have substantial effects on the annual building loads. Thus, it is very important to reduce the pollution emission from building material and other sources [18]. Chao and Hu [19] developed a dual-mode control ventilation strategy, namely CO2 sensor control and non-occupant-related indoor contaminants control, to decrease CO2 or other pollutants emission. The other strategy adopted free cooling such as natural ventilation. Outdoor air flow rates during the evening and early morning hours are higher than those of the occupied hours [20]. This free cooling strategy could increase the chilled water temperature and result in approximate 20% savings in energy consumption. The third strategy used high air velocity to improve the thermal comfort. From the thermal comfort concept, it can be seen that if higher air velocity is provided, less cooling power is required to maintain PMV values in the desired range [21].
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Fig. 1. Outdoor air control strategy scheme of dynamic occupants detection.
Although many strategies have been focused on ventilation control for VAV system in many investigations, none of them can achieve the fundamental design objectives for HVAC system to save energy, to reduce maintenance costs, to improve IAQ and thermal comfort, to promote worker productivity, and even to diminish lifecycle cost. By weighing the benefits and risks prior to implementing a strategy, four control strategies are investigated in this study. Strategy A contains a constant outdoor air intake fraction, Strategy B involves in the dynamic-occupancy-detecting DCV, Strategy C is the indoor air temperature reset control, and Strategy D employs the enthalpy-based outdoor airflow economizer cycle combined with supply-air-temperature reset control. These four strategies are applied to examine indoor air temperature, energy consumption, indoor CO2 concentration and PMV values under three typical testing days in summer, mild and winter. 2. Description of four control strategies For energy savings, comfortable and healthy indoor environments, four control strategies used in the VAV systems are described in the following subsections. 2.1. Strategy A: constant outdoor air intake fraction If the total supply air ventilation is at a lower level, constant outdoor air intake fraction will induce the generic terminal-reheat VAV systems being starved of air ventilation during winter period. In spite of this weakness, Strategy A is still a most-often adopted ventilation strategy [7] and provides sufficient ventilation air if VAV system is operated as a rated constant air volume system. Assumed that the constant outdoor air intake fraction is a fixed value of 0.3 throughout the year, outdoor air flow rate will increase in proportion to supply air flow rate.
ber of occupants at time i, Ra is the outdoor airflow rate required per unit area, and A is the total occupied floor area. Providing an excellent supply air to dilute the occupant-related contaminants, the DCV strategy based on CO2 concentration has been widely used [19]. This strategy detects the dynamic occupants in breathing ventilation area based on the difference between indoor and outdoor CO2 concentration, and acquires the minimum outdoor air flow rate by Eq. (1). As shown in Fig. 1, the indoor and outdoor CO2 concentrations as well as outdoor air flow rate measured by sensors are saved in the storage model. Subsequently, the values in previous and current time period are the inputs of the dynamic occupant supervisory model and thus the current–time population Pi can be calculated by Eq. (2) [23]. Pi =
i i (vio + vi−1 V × (Cri − Cri−1 ) o ) × (Cr − Co ) + 2S St
(2)
where vo denotes the total outdoor air volume. Cr and Co indicate the CO2 concentration of return air and outdoor air, respectively. The superscripts i and i−1 represent the current and previous sampling time, respectively. S is the CO2 emission rate per person. V is the total occupied floor volume, and t means the time step. In certain workdays, the comparison between actual and dynamically simulated population for an office room is demonstrated in Fig. 2. Note that a clear reduction can be seen at the lunch break time from 12:00 to 14:00. The indoor occupancy peak is up to 17 persons during the working period. To sum up, the simulation results are very close to the actual ones although the maximum simulation error is near 8.7%. This suggests that the DCV strategy based on CO2 concentration levels could obtain accurate prediction
2.2. Strategy B: dynamic-occupancy-detecting DCV The outdoor air is used to dilute the concentration of indoor air pollutants. Obviously, the minimum outdoor air volume is used to ensure the well IAQ. Considering the impacts of CO2 concentration and nonhuman air contaminants, DCV is introduced to achieve acceptable IAQ and low energy consumption. Outdoor air volume required at breathing zone is determined by people-related and area-related contaminant concentrations. The equation stipulated by ASHRAE Standard [22] can be depicted as
vio,min = RP × P i + Ra × A
(1)
where vio,min is the total minimum outdoor air flow rate at time i, RP is the outdoor airflow rate required per person, Pi is the total num-
Fig. 2. Comparison of population between actual and dynamically detecting occupants for an office room.
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Table 1 Control schemes for enthalpy-based outdoor airflow economizer cycle. Cases
Control schemes
ho > hr > hs
Outdoor air precooled by partial return air, and minimum zone outdoor air intake fraction Maximum outdoor airflow rate with return air damper shutoff Outdoor air blended with partial return air
hr > ho > hs hr > hs > ho
Fig. 3. Control strategies of indoor air temperature reset.
for indoor dynamic occupancy to determine the minimum outdoor air flow rate. 2.3. Strategy C: indoor air temperature reset control As to the comfort air-conditioning system used in an office building, it is unnecessary to maintain indoor air temperature and relative humidity at a fixed value but is significant to regulate them within a given range. The control strategy of indoor air temperature reset targets to establish the logical relationship between indoor air temperature and regulated variables such as occupants, energy consumption, CO2 concentration, PMV values and so on. The changing range of indoor air temperature can thus be controlled within people’s comfort zones as far as possible. Although the humidity also affects the health and comfort, this effect is evidently less than that of indoor air temperature. To simplify the control process, the value of relative humidity in occupied space should be limited to no more than 70% in summer and no less than 30% in winter. As shown in Fig. 3, the indoor air temperature is set at 28 ◦ C in summer and 20 ◦ C in winter, respectively. This is different from the varying set points in mild seasons. If the indoor air temperature is lower than 20 ◦ C, the supply air damper stays at a minimum opening. If the terminal reheat operates, the damper opening will become larger. Also, the damper opening gradually enlarges along with the increasing cooling load, and reaches the maximum if the indoor air temperature is higher than 28 ◦ C. 2.4. Strategy D: enthalpy-based outdoor airflow economizer cycle combined with supply air temperature reset control Most of traditional air-conditioning systems are usually designed based on typical meteorological conditions and minimum outdoor air flow rate in summer/winter design day. With the increase of modern office equipments and subsequent thermal loads in existing commercial buildings, chilled air should be supplied to ventilation space to transfer these loads even in mild seasons. Thus, it is difficult for the traditional design to fully consider both annual operational control and thermal comfort. In this case, if the outdoor air enthalpy is lower than the indoor air enthalpy, outdoor air ventilation, as free cooling, can compensate for a part of inner cooling load. Consequently, this will save the energy of air handling and shorten the running time of chillers. Note that outdoor air ventilation should meet the sanitation requirement of minimum volume rather than the annual fixed intake fraction. The VAV systems with fixed outdoor air fraction often have substantial energy conservation in typical summer/winter workdays, but may result in energy wasting in mild seasons or under non-typical
conditions. For these reasons, the strategy combining DCV with enthalpy-based outdoor airflow economizer cycle is developed to achieve the acceptable IAQ and the lower coil energy consumption. As listed in Table 1, ho , hr and hs denote the specific enthalpy of outdoor air, return air and supply air, respectively. Because the supply air enthalpy hs is lower than the outdoor air enthalpy ho in summer workdays in Shanghai, the fixed outdoor air fraction is set at 0.3, which is the same with Strategy A. In addition, the outdoor air enthalpy ho is usually considerably lower than the supply air enthalpy hs in winter workdays, so the fixed outdoor air fraction of 0.3 can save the energy consumption of system. The strategy with the identical control to Strategy A in summer/winter workdays is therefore especially suitable for the mild seasons. To avoid the instability phenomena such as alternation and oscillation of outdoor air damper control whose signals are determined from both traditional supply air temperature strategy and DCV strategy [24], the strategy of supply air temperature reset is specially employed to provide the control signals for supply air damper. Supply air temperature ts can be determined by Eq. (3) [25], and hs can be derived from Eq. (4). hs = 1.01ts + W (2501 + 1.86ts )
(3)
where hs is the specific enthalpy set point, ts is the dry-bulb temperature of supply air, and W is the humidity ratio. Qi = vpz,i (hset − hs )
(4)
where Qi is the cooling loads for zone i, is the air density, vpz,i indicates the primary airflow for zone i, and hset denotes the specific enthalpy set point.
vpz,i =
voz,i Zp
(5)
where vpz,i denotes the primary air flow rate for zone i, voz,i is the outdoor air flow rate for zone i, and Zp is the zone primary outdoor air fraction. 3. Building models and simulations The VAV systems are installed throughout a certain floor of an office building located in Shanghai and the schematic diagram can be found in the published literature [25,26]. The office building is occupied only from 8:00 to 20:00. The floor has a total floor area of 1166 square meters and is divided into eight thermal zones. The simulation program developed on TRNSYS [26,27] is applied as the platform to validate four control strategies. The program includes some models involving the VAV systems and their control loops. With low initial cost and energy consumption, the employed single-duct VAV system comprises three basic parts, an air handling unit (AHU), some air ducts and VAV terminal boxes. A pressureindependent VAV terminal with electric reheat is installed in each thermal zone. The electric reheat and pressure-independent VAV controller are employed to control the air temperature in each zone. By regulating the VAV damper signalled from a PID temperature controller, the zone supply air flow rate can be varied to provide a desired space temperature. To maintain a supply air static pressure near its set point, a PID controller regulates supply fan speed
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Fig. 4. Outdoor air conditions in three testing days (a) dry-bulb temperature and (b) humidity ratio
to provide sufficient air volume. Also, an ex-filtration flow controller adjusts return fan speed to change the difference between the supply and the return air flow rates at a certain set point and to maintain positive pressure. Supply air temperature is provided by adjusting water valve and thus modulating the chilled water flow rate into the AHU coil. In practice, a single controller is used to provide certain outdoor air intake fraction by positioning outdoor, recycle and exhaust air dampers.
Fig. 5. Dynamic change of indoor air temperature for four different strategies (a) summer, (b) mild, and (c) winter.
4. Results and discussion Four performance indices, including indoor air temperature, energy consumption, CO2 concentration and PMV, are examined in three typical days, namely a summer day, a mild day and a winter day. Four performance indices are examined and compared under identical working conditions. The outdoor air dry-bulb temperature and humidity ratio in three typical days in Shanghai are shown in Fig. 4. 4.1. Indoor air temperature Fig. 5 demonstrates the variation of indoor air temperature for four control strategies in three typical testing days. In summer, indoor air temperature can reach the set point without drastic fluctuations and meet the system requirements. The indoor air temperature set point for Strategy A and B are both 24 ◦ C while that for Strategy C is 28 ◦ C. It can be seen in Fig. 5(b) that the relatively low outdoor air temperature leads to small indoor cooling load in the morning and evening in mild seasons. During the hours of 8:00 to 10:00 and 17:00 to 20:00, the indoor air temperatures for Strategy A, B and D maintain at their set point (24 ◦ C) even though the supply air damper reaches the minimum position. Although a terminal reheat equipment is employed to maintain the indoor air temperature at
its set point, this will inevitably lead to partial cooling load offset by reheating and subsequent energy wasting. In practice, the traditional design and commission only cover the summer/winter operation modes for air-conditioning system, so it is very difficult to maintain the system operating reliability under the cooling mode in mild seasons. Fortunately, Strategy C can set low temperature point under low load conditions in the morning or evening due to enlarging the fluctuation scope of indoor air temperature. In this case, it is within an hour for indoor air temperature to reach the allowable lower limit of 20 ◦ C, so the reheating time is shortened and the cooling load offset by reheating is significantly reduced. Furthermore, the indoor air temperature set point increases with the increase of outdoor air temperature, and thus thermal conduction load decreases. As shown in Fig. 5(c), the indoor air temperatures in winter for Strategy A and B are near their set point of 24 ◦ C and that for Strategy C is 20 ◦ C. Also, it takes about 3 h for the indoor air temperatures of Strategy A and B to reach their set points. It can be seen that the system dynamic response is insufficient because the excessively low indoor air temperature in winter evening may result in a peak load before chiller start-up. Hence, it is necessary to keep a minimum indoor air temperature in winter evening or to operate chillers before the building is occupied.
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Fig. 6. Energy consumption for four different strategies.
4.2. Energy consumption The energy consumptions for four control strategies in three testing days are shown in Fig. 6. As an original scheme, Strategy A is the comparing benchmark to the other three strategies. Compared with Strategy A, in summer, Strategy B and C can reduce energy consumption by 23.6% and 32.4% respectively. Strategy B can save energy by detecting the dynamic occupants and then controlling the outdoor air volume. Less outdoor air volume results in lower outdoor air cooling load and smaller supply air volume. For Strategy C, higher indoor air temperature set point leads to lower thermal conduction load and smaller cooling load. Due to increasing the indoor air temperature set point, Strategy C can provide significant energy savings. In mild, compared with Strategy A, the energy consumption of Strategy B is increased by 5.6%, while that of Strategy C and D is reduced by 24.9% and 9.4%, respectively. For Strategy B, less outdoor air is supplied into the building because the minimum outdoor ventilation is varied by detecting the dynamic indoor occupants. Outdoor air flow rate is declined and the subsequent fans’ energy may be reduced, but less mild outdoor air as free cooling is supplied into ventilation space. The total energy consumption, therefore, is not reduced but increased. Strategy C reduces outdoor air ventilation, but raises the supply air temperature under the peak system load. Thus, the cooling load reduced by raising supply air temperature is much greater than that increased by lower outdoor air ventilation. The total cooling load, therefore, is reduced. Because indoor cooling load is partially reduced by mild outdoor air, the energy of Strategy D is also reduced. Generally, Strategy C can distinctly reduce the energy consumption while Strategy B may increase the energy usage. If the outdoor air climatic conditions are favorable in mild season, much more outdoor air ventilation will be beneficial to save power energy responding to such free cooling opportunities. In winter, compared with Strategy A, the energy consumption of Strategy B and C is decreased by 15.2% and 21.9%, respectively. This is mainly because that the smaller outdoor air flow rate results in lower thermal load. As a whole, Strategy C exhibits the most effective energy usage beyond 20% reduction compared with Strategy A. The energy consumption of Strategy B can be reduced by more than 15% either in summer or in winter, but increased by 5.6% in mild. Dissimilar to Strategy A merely in mild, Strategy D Exhibits 9.4% decrement in energy consumption.
Fig. 7. CO2 level in breathing zone for four different strategies (a) summer, (b) mild, and (c) winter.
4.3. CO2 concentration Better IAQ is directly involved in human mood, healthy state, working efficiency and so on, and thus favorable CO2 concentration should be as low as possible if no energy is wasted. Obviously, the indoor CO2 concentration holds a strong function with occupancy number since occupants are the predominant CO2 generation source inside the office building. Under the identical occupied conditions, more outdoor air flow rates will conduce to lower CO2 concentration because people-generated CO2 volume is diluted by outdoor air. 100% outdoor air ventilation, therefore, is prior to be adopted to achieve low CO2 concentration. Fig. 7 demonstrates the indoor CO2 profile in three testing days. In the morning before fully occupied, the noon for lunch and the evening before unoccupied, indoor CO2 concentration is lower indicating a few occupants. Nevertheless, CO2 concentration becomes higher with the increase of occupants in other period. As depicted in Fig. 7(a), Strategy B always shows relatively higher CO2 concentration during the entire working period, but the CO2 level is still in the allowable range. Because the system load and outdoor air ventilation are both reduced by high indoor air temperature set point, Strategy C exhibits higher hourly CO2 concentration than the other strategies during the whole working period. Fig. 7(b) shows that the CO2 level varies with different indoor occupants. As shown in Fig. 7(c), indoor CO2 concentration of Strategy B shows the maximum value due to the minimum outdoor air ventilation, but is
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Fig. 8. PMV value for four different strategies (a) summer, (b) mild, and (c) winter.
still within acceptable limits. Strategy A and C possess very similar trends. Their CO2 concentrations typically remain in the range from 350 to 650 ppm, and this is better than Strategy B. To sum up, different strategies have various CO2 concentration levels in three testing days. Strategy A possesses the lowest CO2 concentration in summer and better in mild and winter. Strategy B exhibits the maximum CO2 concentration during the whole year. Strategy C has the lowest one in winter and slightly higher than those of Strategy A in summer and mild. Strategy D possesses the lowest one in the mild testing day and close to those of Strategy A in summer and winter. Considering that CO2 level is an important index for IAQ, Strategy A and D provides the best IAQ in summer, Strategy D indicates the best in mild, and Strategy C provides the best in winter. 4.4. PMV According to the equations from ASHRAE Handbook 2005 [25], PMV value is applied to evaluate the thermal comfort. As a single comfortable index involving a range of thermal sensations, PMV contains the effects of air temperature, space relative humidity, radiant temperature, air velocity, clothing level and activity level. Taken these six variables into consideration, thermal sensation range scale of PMV can be listed in Table 2 [21]. The PMV values of four different control strategies are shown in Fig. 8. From Fig. 8(a), Strategy A and B exhibit desirable PMV values
and vary from −0.2 to 0.1 where is an ideal environmental region for human thermal comfort. Because the indoor air temperature set point is increased, Strategy C results in a certain degree of decline in thermal comfort and the corresponding PMV value mostly fluctuates in the vicinity of +1 during working time. According to Table 2, this region belongs to slightly warm feeling. Obviously, Strategy C shows the most unsatisfactory thermal comfort owing to increasing the indoor air temperature set point to 28 ◦ C. As depicted in Fig. 8(b), some PMV values are lower than −3 indicating the cold feeling for human thermal sensations. This is mainly because that the indoor air temperature is much lower at the beginning of chiller start-up (as seen in Fig. 5) when system operates under the summer cooling mode. Moreover, the cold feeling is slowly relieved subject to a gradual rise in indoor air temperature till 10:00. During the hours of 10:00 to 17:00, PMV value always exists at an acceptable limit of +0.5. Taken as a whole, PMV value for Strategy A, B and D approximates 0 and their regarding indoor air temperatures stabilize at the set points. Corresponding to the higher indoor air temperature set point, Strategy C exhibits high PMV value approaching to +0.5. After 17:00, PMV value is gradually reduced to −1 of slightly cool feeling with decreasing indoor air temperature set point. As shown in Fig. 8(c), PMV value of three strategies could reach a stable state in an hour following chillers start-up. The trends are similar to the indoor air temperature variations demonstrated in Fig. 5(c). Strategy C reveals the most unfavorable indoor thermal comfort, and its PMV value is close to −1.3 indicating slightly cool feeling. The other two strategies can make much desirable indoor thermal comfort and both PMV values exist in between −0.8 and −0.5. Taken the clothing condition into account, these values pertaining to slightly cool feeling might be acceptable in winter workdays. As a whole, Strategy C reveals relatively high PMV value meaning slightly warm feeling or slightly cool feeling, and the other strategies exhibit relatively low PMV value indicating the desirable indoor thermal comfort. In addition, the variation trend of PMV values is very close to that of indoor air temperature. For each strategy, it takes more than 1 h to reach a stable state in summer and winter, and more than 2 h in mild seasons. 5. Evaluation of four strategies Three performance indices including relative energy consumption, CO2 concentration interval and thermal comfort, are used to evaluate four control strategies. Relative energy consumption RE is defined in Eq. (6). CO2 concentration intervals are statistically from 10:00 to 18:00, and the thermal comfort is rated from the thermal sensation range scale of PMV. REStrategy j =
EStrategy j EStrategy A
j = A, B, C and D
(6)
where REStrategy j denotes the relative energy consumption of Strategy j, j = A, B, C and D. EStrategy j denotes the energy consumption of Strategy j, and EStrategy,A denotes the energy consumption of Strategy A. As shown in Table 3, different strategy possesses various strengths as well as weaknesses evaluated by three performance indices. Strategy A has the highest energy consumption, acceptable thermal comfort, and low CO2 concentration. Its CO2 concentration only in mild is slightly higher than that of Strategy D. Strategy B exhibits the best thermal comfort but the highest CO2 concentration. Its energy consumption is higher than Strategy C and lower than Strategy A and D. Strategy C possesses the lowest annual energy consumption, slightly higher CO2 concentration in summer and mild than Strategy A and D, and slightly warm thermal comfort in summer and slightly cool in winter. Strategy D shows the lowest
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Table 2 Thermal sensation range scale of different PMV value. PMV value Thermal comfort
+3 Hot
+2 Warm
+1 Slightly warm
0 Neutral
−1 Slightly cool
−2 Cool
−3 Cold
Table 3 Evaluation of four control strategies.
Relative energy consumption Summer Mild Winter CO2 concentration interval (ppm) Summer Mild Winter Thermal comfort Summer Mild Winter
Strategy A
Strategy B
Strategy C
Strategy D
1 1 1
0.76 1.06 0.85
0.68 0.75 0.78
1 0.91 1
[398, 593] [446, 927] [454, 705]
[839, 1020] [958, 1078] [824, 926]
[469, 736] [498, 1000] [438, 669]
[398, 593] [381, 662] [454, 705]
Neutral Neutral Neutral
Neutral Neutral Neutral
Slightly warm Neutral Slightly cool
Neutral Neutral Neutral
Table 4 Preferred strategies for different performance index. Performance index
Summer
Mild
Winter
Whole year
Lowest energy consumption Lowest CO2 concentration Best thermal comfort
Strategy C Strategy A or D Strategy B
Strategy C Strategy D Strategy B
Strategy C Strategy A or D Strategy A, B or D
Strategy C Strategy D Strategy B
CO2 concentration, acceptable thermal comfort, and lower energy consumption in mild than Strategy A. The preferred strategy for different performance index is listed in Table 4. Among these four strategies, Strategy A is the most unsatisfactory strategy in the whole year for any performance index. Strategy B provides the best thermal comfort, and Strategy C exhibits the lowest energy consumption. Identical with Strategy A in summer and winter, Strategy D shows the lowest CO2 concentration during the whole year. 6. Conclusion Strategy A is the benchmark to depict the increment or decrement in other strategies’ energy consumption. The evaluations of four strategies are as follows: (a) Strategy A possesses the lower CO2 concentration and the better PMV in summer and winter, but the largest annual energy consumption. (b) With more than 15% energy savings either in summer or in winter, Strategy B follows the similar PMV trends to Strategy A. The CO2 concentration level, however, is so high that it may result in more complaints from occupants. (c) Strategy C displays the lowest energy consumption beyond 20% savings and the lower CO2 concentration in both summer and winter. Because of the relatively high set point of 28 ◦ C in summer and relatively low one of 20 ◦ C in winter, this strategy results in slightly-warm thermal comfort in summer and slightly-cool in winter. (d) Dissimilar to Strategy A only in mild, Strategy D provides the lowest CO2 concentration, 9.4% energy savings, and similar PMV to Strategy A. Generally, Strategy C is a simple, effective and practical energysaving strategy. Since the conventional design of air-conditioning system only takes summer and winter operational modes into consideration, some measures should be taken to avoid the energy wasting resulting from partial cooling offset by reheating. Strategy
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