The impact of increased cooling setpoint temperature during demand response events on occupant thermal comfort in commercial buildings: A review

Energy & Buildings 173 (2018) 19–27

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The impact of increased cooling setpoint temperature during demand response events on occupant thermal comfort in commercial buildings: A review Sama Aghniaey, Thomas M. Lawrence∗ College of Engineering, University of Georgia, Athens, GA, USA

a r t i c l e

i n f o

Article history: Received 7 December 2017 Revised 16 March 2018 Accepted 30 April 2018

Keywords: Thermal comfort Demand response Energy consumption

a b s t r a c t This paper provides a review of the literature concerning the impact of temporarily increased cooling setpoint temperature on occupant thermal comfort during demand response (DR) events in commercial, airconditioned buildings. We address concerns regarding thermal comfort as it relates to zone temperature modifications that may be implemented as a part of DR measures. Increased zone setpoint temperatures during the cooling season can adversely affect building occupants physically and psychologically, and impair their perceived indoor air quality and self-estimated performance. In some cases, however, improved occupant thermal comfort due to warmer zone setpoint temperatures during DR events has also been reported. In order to reduce the negative impacts on building occupants, DR must be implemented with careful control and monitoring. There are significant differences in assumptions made about the way people would respond to a thermal environment between the static heat balance model and the adaptive approach, with the adaptive approach offering a wider range of acceptable indoor temperatures. Therefore, application of the adaptive approach could be one option to improve building energy performance by taking advantage of occupant adaptive behaviors during DR events. Depending on the building services and systems available, expectations of the occupants, and the control options they are given to adjust to their thermal environment, occupants could potentially adapt to temperatures higher than what are currently being practiced in buildings. However, an upper limit threshold for temperature modifications, even if temporary ones, must be recognized to minimize adverse impacts on building occupants prior to DR implementation. Therefore, the ideal goal would be to develop menthods that would identify an optimum balance between energy consumption and the building occupant thermal comfort before applying DR strategies. Prior review papers relevant to DR have generally concentrated on the energy saving potential alone, or where energy savings have been prioritized over occupant thermal comfort. This paper reviews implementation of DR from the perspective of occupant thermal comfort and presents a summary of the most relevant experimental and field studies regarding occupant thermal comfort during DR events in commercial, air-conditioned buildings. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Mechanically conditioned buildings are significant contributors to overall global energy consumption and the corresponding impact on electricity demand and greenhouse gas emissions. Building heating, ventilating, and air conditioning (HVAC) systems are responsible for roughly 20% of total energy consumption in developed countries and many developing economies as well ([22], [46]). Providing occupant thermal comfort alongside a supply of

∗

Corresponding author. E-mail address: [email protected] (T.M. Lawrence).

https://doi.org/10.1016/j.enbuild.2018.04.068 0378-7788/© 2018 Elsevier B.V. All rights reserved.

fresh air, maintaining their health and well-being, is the main purpose of HVAC system operation in buildings but also results in the consumption of a considerable amount of energy and money expenditures. Therefore, in an effort to more effectively manage energy consumption and peak energy demand, it is important to appropriately define thermal comfort and its relation with indoor temperature and other influential factors, such as relative humidity, air speed, radiant temperature, time of exposure at a given temperature, etc. Ultimately, we would like to achieve an optimized balance between thermal comfort and energy consumption. Electrical energy production differs from other types of energy (such as liquid fuels) in that the production and consumption occur essentially simultaneously. This feature critically affects power

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Nomenclature AC AMV APD CLO DR HVAC IEQ MM NV PAQ PMV PPD SSP

Air-Conditioned Actual Mean Vote Actual Percentage Dissatisfied Clothing Insulation Value; 1 CLO equals 0.155 m2 K/W (0.88 ft2 °Fhr/Btu) Demand Response Heating, Ventilation, and Air Conditioning Indoor Environmental Quality Mixed Mode Naturally Ventilated Perceived Air Quality Predicted Mean Vote Predicted Percentage Dissatisfied Summer Set Point

grids reliability. To increase grid reliability and to address power capacity shortages, buildings could be integrated with a smart grid to participate in demand-side management to reduce peak demand, total energy consumption, and utility cost [36] and participation in DR programs or temporary HVAC systems adjustment is one tool to achieve this [47]. DR is defined as “a set of timedependent program activities and tariffs that seek to reduce electricity use or shift usage to another time period. DR provides control systems that encourage load shedding or load shifting during times when the electric grid is near its capacity or electricity prices are high” [39]. Smart buildings and their associated controls and equipment are capable of responding to DR requests from the utility or system operator to manage peak demand. The benefits to building operations are a minimized demand charge or minimizing total utility cost based on the real-time price of electricity, or taking advantage of other financial incentives that might be offered. Some response measures may involve adjustments in HVAC operational setpoints (such as zone temperatures or supply airflow temperatures and flow rates) or lighting, thus opening up potential occupant comfort perception problems. When done right, a smart building can also provide a preferred or at least adequate indoor environmental quality (IEQ) for the building occupants. In this context, the important areas of IEQ affected include temperature, humidity, ventilation rates, and lighting levels [36]. Demand Response programs are becoming increasingly important globally as a powerful tool to manage peak demand and increase electrical power network reliability and sustainability. However, the potential for zone temperature changes used in DR to adversely impact occupants, even if they are temporary, points to a need for research to study and identify the impact of these measures on building occupants. The focus of these studies should be on the occupant thermal comfort, health, and overall productivity. It is essential to identify an optimum trade-off between energy consumption and the building occupant thermal comfort before extensively applying DR strategies in buildings [39]. Based on a large body of evidence in the literature, it is apparent that in many cases the current cooling temperature setpoints in buildings are lower than absolutely necessary, and thus there is a energy saving potential in even minimally increasing the cooling setpoint temperature [15,47]. The measure of the overall performance of a building depends heavily on the occupant thermal perception. Considering the subjective nature of thermal comfort and the complexity of interactions between occupants, buildings, and environment, it is important to accurately understand, define, and predict thermal comfort [17]. There are various definitions of thermal comfort used by different researchers, but these are all generally associated with a

thermal balance of the body with respect to the local ambient conditions [22]. One well-known definition states this as “the condition of mind which expresses satisfaction with the thermal environment” [5]. Thermal comfort from this viewpoint is very subjective and it is hard to deal with in practical terms. Occupant health and performance that is affected by their thermal comfort is of a paramount importance in commercial buildings, particularly in building use types such as offices or classrooms where people have less control options and freedom to adjust the thermal environment. Thus, it is particularly important to maintain occupant thermal comfort and wellbeing with maximum care in commercial buildings. Prior review papers relevant to DR have generally concentrated on the energy saving potential alone, or where energy savings have been prioritized over occupant thermal comfort. The main goal of this article is to review prior work that describe the impact of zone temperature adjustments on building occupants in public spaces, such as might be experienced during a DR event. This is important since a better understanding of these correlations could result in fulfillment of the maximum potential for DR to save peak demand on the grid and money for the building owners while maintaining sufficient health, comfort, and wellbeing for the occupants. To set the stage for this review, we first provide a background on the various approaches to achieve thermal comfort from the perspective of the two most well known thermal comfort models; (1) the heat balance model or rational approach and (2) the adaptive model. Further background is given with a brief outline of the energy saving potential that DR can provide. This paper is not intended to review all topics associated with thermal comfort perception in buildings, as they have been the subject of various other prior reviews and publications. Rather, we are focusing here on our primary objective to provide a review of the impact of DR on occupant thermal comfort in mechanically conditioned, commercial buildings based on results reported in previous experimental studies. 2. An overview of the most well-known thermal comfort models This following two sections provides a brief background introduction for the larger objectives of this review study, with this section covered thermal comfort models and the next the energy savings potential for DR events. This particular section is not intended to be a full review of all relevant literature associated with these thermal comfort models, but rather just as an introduction to the concepts associated with thermal comfort and DR. 2.1. Heat balance model In the 1960 s, Fanger proposed a heat balance model to enable HVAC engineers to predict the overall acceptability of a given thermal environment for a large group of occupants. The input data for this model was generally developed from college-age students exposed to steady-state situation in climate-controlled chambers. The comfort equation with this heat balance model is based on conditions for achieving thermal neutrality. Fanger proposed the Predicted Mean Vote (PMV) index to be a representative of a “mean thermal sensation vote for a large group of building occupants for any given combination of thermal environmental variables, activity and clothing levels” [53]. This index incorporates the seven points scale which ranges from −3 for cold to +3 for hot. The PMV equation is a function of air temperature, mean radiant temperature, air velocity, relative air humidity or water vapor pressure of air at the local conditions, metabolism or type of activity, and clothing level [45,53].

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Fig. 1. Predicted percentage of dissatisfaction (PPD) as a function of predicted mean vote (PMV), [5].

There is an empirical relationship between the PMV index and the Predicted Percentage Dissatisfied (PPD), as shown in Fig. 1. The PPD predicts the percentage of building occupants that would be dissatisfied with the existing thermal environment. The subjective nature of thermal comfort causes the PPD index never to reach zero; even a PMV of zero results in a PPD value of 5%. Many global HVAC standards for the built environment are based on this PMV-PPD relationship, for example in Europe with EN ISO 7730 (ISO 1984) [45] and EN 15251 (British Standards [11]), or with ANSI/ASHRAE Standard 55 [5] in the US. The ASHRAE Standard defines the thermal comfort zone to fall between a PMV of −0.5 and +0.5 with the corresponding PPD value of 10% or less. Thus, it is believed that at least 90% of occupants would be satisfied with a thermal environment that fell within that PMV range. Since this model was developed primarily based on the steadystate controlled laboratory experiments, some researchers believe that it does not always represent real situations in buildings as it does not incorporate some important parameters that affect occupant thermal comfort in the real world.

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Fig. 2. Percent energy savings for widened air temperature setpoints in office buildings from different U.S. climate zones [22].

indoor temperatures that could result in potential energy savings in both the cooling and heating seasons for mechanically conditioned and naturally ventilated buildings [31,57]. Rather than the six environmental and human factors utilized by the heat balance model, there are a number of other environmental, psychological, cultural, and socio-economic factors involved with a person’s thermal comfort perception, as well as the adaptive options and the level of personal control over a thermal environment [44]. To highlight the need to consider additional factors, one study showed that occupants with pro-environmental policy attitude tend to be more tolerant of non-optimal thermal conditions in buildings that have been defined as a ‘green building’ [57]. The topic of thermal comfort has been studied for decades now, and this paper is not attempting to review and summarize prior work. This has already been done in papers such as [9,12,13,17,19,28,57], and [48]. Unfortunately, none of these review papers studied thermal comfort in light of new challenges, such as the effect of increased cooling setpoint temperatures from implementation of demand response measures on building occupants and their thermal comfort perception; this is the primary topic of this review paper.

2.2. Adaptive model 3. Discussion on the potential energy savings with DR Acceptable thermal comfort temperatures can be determined based on field studies without using the heat balance theory but rather based on the adaptive principle of thermal comfort [23]. The adaptive model relates comfortable temperatures indoors to the current outdoor temperature [51] and was originally proposed by Dr. Thomas Bedford [6,9]. This model was partially the result of field studies in which occupants in their everyday environment were questioned about their thermal sensation, thus, more closely relevant to people’s ordinary living conditions [44]. The adaptive principal is expressed as: “…if a change occurs such as to produce discomfort, people react in ways which tend to restore their comfort” [43]. This adaptation is the basis for an adaptive thermal comfort model. This definition emphasizes that people are not passive recipients of their thermal environment, but rather can and will adapt [9,43]. Although Standards based on the heat balance model partially reflect some degree of behavioral adaptation through clothing level, they fail to account for the potential of psychological adaptation. Psychological adaptation refers to changes in the occupant’s expectations and satisfaction as a result of their recent thermal experience or their interactions with the environment [10]. There are some differences in assumptions about the way people would respond to a thermal environment between the static heat balance model and the adaptive approach. The adaptive comfort model allows for a wider range of

This section provides additional background for the study of thermal comfort and DR programs by giving a brief summary from several studies of the energy savings potential with DR measures. One study of a typical office building in Zurich shows that a 2–4 °C increase in zone setpoint temperatures would result in a reduced annual cooling energy consumption by a factor of three (due to the relatively low need for cooling in that location). Another study predicted energy reductions by a factor of two to three if thermostat setpoint increases from 23 to 27 °C were implemented for nighttime in Hong Kong apartment bedrooms [47]. Admittedly, these two situations are on the optimistic side for percentage savings since there is generally a limited need for cooling in Zurich and with a lower nighttime cooling demand compared to daytime cooling in Hong Kong. Fig. 2 depicts the energy saving potential of widening cooling and heating thermostat setpoint temperatures for four different cities in the United States based on building energy simulation modeling conducted by Hoyt et al. [22]. They investigated the potential for energy savings of an expanded thermostat setpoint temperature in office buildings. They concluded that when increasing the cooling setpoint temperature from 24 to 25 °C, cooling energy saving of 7–15% could be achieved depending on building operational features and climate zone. The energy saving potential in

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that study was also reported to reach 35–45% if the setpoint temperature was extended from 24 to 28 °C, however the effect on occupant thermal comfort at these temperatures was not reported. Increased zone temperature setpoints with DR measures may increase energy used for reheat if not done in coordination with other control system changes. If the supply air temperature were also correspondingly increased, there is a potential for an increased humidity level. Higher moisture levels could result in mold and mildew problems and odors. Increased humidity associated with higher temperatures may also affect indoor air quality (IAQ) through release of sorbed pollutants in building materials. While this is a valid concern, particularly in humid climates, with careful monitoring and control this issue can be avoided while still implementing HVAC system adjustments associated with DR measures [1]. A higher indoor temperature can also result in an increased relative humidity and impaired indoor air quality through release of sorbed pollutants in building materials. These negative impacts can be controlled through careful HVAC adjustments, use of personal conditioning systems, and cautious control and monitoring. Additional studies exist that similarly investigate the energy saving potential of DR without considering its impact on building occupants. However, a DR program could not be successfully implemented in the real-world conditions if there is no balance between energy saving and occupant thermal comfort. The consideration of the adverse impact on occupant thermal comfort should be balanced with the amount of energy consumption that is avoided to keep indoor temperatures uniform within a narrow range.

ergy saving potential from DR. This section reviews studies done on increased cooling setpoint temperatures as well as the rate of change of those temperatures. 4.1. Experimental studies of increased cooling setpoint temperature impact on building occupants A number of studies have shown that occupants are able to accept a much wider temperature range in real-world practices compared to a controlled laboratory setting. It has been also reported that a uniform or neutral temperature does not necessarily translate to comfort. Table 1 summarizes some of the more relevant experimental studies investigating the impact of increased cooling setpoint temperature on occupants in air-conditioned building for commercial (non-residential) buildings. The results of one study by De Dear and White [14] concerning thermal comfort and DR is worth further discussion. In this study, the authors analyzed the effect of DR on residential building occupant’s thermal comfort associated with Critical Peak Pricing and Direct Load Control strategies. They emphasized that, based on the adaptive thermal comfort model, “there is no absolute reference point or threshold on comfort continuum”; a repeated exposure of occupants to an increased indoor temperature would eventually increase their tolerance of those temperatures, as they get accustomed to the wider temperature swings that existed for prior generations before mechanical cooling systems. 4.2. The importance of the rate of temperature change

4. Potential impacts of zone temperature changes with DR on the building occupants Although DR measures can reduce peak demand, overall energy consumption, and utility costs, they can also affect the building occupants physically and psychologically. The overall potential impacts on occupants include thermal discomfort, decreased productivity, mood swings, and even their overall health. A decreased thermal comfort can adversely affect people’s self-estimated performance and their perceived air quality [58]. Those authors also discussed that the perceived air quality and self-estimated performance are strongly correlated with thermal comfort perception rather than indoor air temperature. Increasing the zone setpoint temperature during a DR event does not necessarily impair air quality perception or productivity of occupants if they can maintain their thermal comfort elsewise, such as through personalized conditioning systems, ceiling fans, etc. In some cases, HVAC system changes can actually result in an improved thermal comfort vote, and thus these could be considered for a long-term energy efficiency measure [39]. An example of this is for overcooled buildings that end up with an improved thermal comfort for many occupants during DR events. Building overcooling during the summer is a common problem in many buildings across the United States ([1,16,38], among others), and it is often the result of oversized HVAC systems with poor latent control or the operations staff wanting to avoid any hot temperature complaints. Although there are many published studies investigating the energy saving potential for DR, the number of studies targeting the impact of DR on building occupants is much lower. In this section, we review how thermal comfort considerations can, or should be, involved in DR measures in order to minimize the adverse impacts on building occupants. The adaptive principle for thermal comfort assumes that adaptive characteristics can make it possible for occupants to adjust to the resulting changing indoor thermal environment if DR measures were implemented. However, it is important to identify the acceptable or allowable ranges for reduced HVAC services in order to minimize adverse impacts on occupants and maximize the en-

How building occupants accept DR and load shedding via HVAC system operation adjustments significantly depends on the rate of the load shedding, overall magnitude of the changes, and their frequency [8,21]. However, in most DR practices stable ambient temperatures are usually reached in no more than one hour after changing the zone control set point temperature. Table 2 reviews the impact of rate of changes in increased cooling setpoint temperature (drift or temperature ramp) on building occupants in commercial, air-conditioned buildings. Temperature drift is defined as monotonic, steady changes in temperature with time that is characterized by a starting value, amplitude, and the rate of changes [21]. The findings from Sprague and McNall [50] agreed with the ASHRAE accepted temperature threshold for the rate of temperature changes, although the ASHRAE Standard took a more conservative approach, since it allowed for a lower maximum temperature amplitude. Fig. 3 depicts the limit of temperature variation rate listed in ASHRAE Standard 55 compared to the findings from Sprague and McNall. As that Figure suggests, a temperature fluctuation up to 5 °F (2.8 °C) is feasible; i.e. for T equal to 5 °F, the maximum cycles per hour (CPH) must be approximately 1.5. The results clearly suggest that a higher temperature fluctuation is possible compared to the ASHRAE limit that was in place at the time when this study was published (1970). Later, in 2013, ASHRAE released new guidance about the limits on temperature drifts and ramps that was in a general agreement with findings from the Sprague and McNall study, allowing even for a higher temperature amplitude over a longer period of time [4]. This newer guidance is summarized in Table 3, and these values have been retained in the latest version of this Standard that was released in 2017. 5. Summary This paper reviewed the impact of using HVAC setpoint changes that could be included as part of a building’s demand response measures on the thermal comfort of the occupants. The heat balance model is a good predictor of occupant’s thermal comfort in

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Table 1 Summary of studies on the impact of increased cooling setpoint temperature in air-conditioned, commercial buildings on occupant thermal comfort. Author

Study/ building type

Location/ climate

[18]

Climate controlled chamber

ConnecticutUSa moderate climate

[27]

Field study MM office buildings

Helsinki, Finland, Humid Continental

[24]

Field studies

Various locations

[29]

Field study AC/NV classrooms

Hawaii USA Tropical

[30]

Field study AC/NV classrooms

Japan subtropical climate

[22,3,26]

Theoretical and field studies

Various locations

[25]

Field study AC/NV campus classrooms

Taiwan, subtropical climate

[44]

Field study AC/NV office buildings

Europe France, Greece, Portugal, Sweden and the UK

Cool Biz Campaign [52],

Field study AC office buildings

Japan, temperate climate

[41]

Field study AC office buildings

UK, Temperate Maritime Climate

Zhang et al., 2008 [59]

Climate controlled chamber

Beijing, China, temperate and continental monsoon

[53]

Review paper

Various locations

Cool Biz Campaign [20]

Field study AC office buildings

Japan, temperate climate

Results Thermal discomfort rapidly increases when increasing indoor temperature up to 28 °C compared to temperatures above 28 °C. However, thermal sensation vote evenly increases around both sides of neutral thermal conditions when changing a warmer or cooler thermal environment to neutral. A hysteresis effect is more striking when changing the thermal environment from cold to neutral than changing it from warm or hot to neutral. Body temperature that stimulates physiological and behavioral thermoregulatory follows occupant thermal sensation and discomfort. Behavioral adaptation provides a long-term thermoregulation while physiological responses provide a short-term solution. The sick building syndrome (SBS) and sensation of dryness are linearly correlated to room temperature as the most important indoor air parameter, adversely affecting occupants for temperatures above 22 °C. There was an excess of SBS symptoms, both when the thermal environment perceived as too cold and too warm. A higher percentage of workers were thermally satisfied for temperature below 22 °C. Showed a correlation between neutral or comfort temperature and the mean indoor temperature that occupants had been recently exposed to. The occupant’s preferred comfort temperature closely followed the prevailing mean indoor temperature, suggesting that occupants could feel comfortable in a wider range of thermal conditions, especially when there are sufficient options to adapt to that environment. Rather than occupants voting for ± 1 and 0, a significant number of subjects voting for ± 2 and ± 3 based on ASHRAE seven-point scale for thermal sensation, still found the thermal environment as acceptable. The neutral effective temperature for occupants in AC buildings was 27.4 °C while the preferred temperature was 4 °C lower than that. The comfort temperature for naturally ventilated buildings was reported to be 3 °C higher than that for air-conditioned buildings during the cooling season. These results imply how human adaptive nature could help them widen their thermal acceptability range. Students felt slightly cool when temperatures were held as specified in the ASHRAE comfort zone, and the temperature for an occupant’s neutral thermal sensation differed from their preferred thermal state as observed in conditioned classrooms. A uniform or neutral temperature and/or keeping operative temperature indoors in a narrow range does not necessarily translate to thermal comfort. Occupants are able to accept wider temperature ranges than what is normally practiced in buildings. Studied thermal acceptability, neutrality, and thermal preferences in classrooms in hot and humid climate: Although 86% of physical measurements fell outside the ASHRAE comfort zone, more than 80% of students felt thermal satisfaction in their thermal environment. In Taiwan, occupants are acclimatized to hot and humid climate and could accept higher indoor temperatures. Neutral temperature differs from ideal comfort temperature. Tried to determine the maximum comfortable temperature with and without mechanical heating and cooling: The level of thermal discomfort is a function of the normal indoor temperature in that space and the occupant’s current preferred temperature. They realized that for temperature changes of ± 2 °C from the optimal comfort temperature, thermal discomfort would increase considerably. Office building temperatures were set at 28 °C and employees were encouraged to dress lightly; short sleeves with no tie and jacket. It is believed that removing the tie and jacket would decrease body temperature up to 2 °C, enabling occupants to adjust to 28 °C The campaign resulted in some public behavioral changes towards reducing energy consumption, provoking the private sector to incorporate similar procedures. The energy saving potential of this practice was emphasized over its adverse effects on the occupants and their work performance. For indoor temperatures between 24 and 26 °C the indoor environment was considered acceptable. The PPD for air velocity of 0.1 ms−1 , metabolic rate of 1, and CLO of 1 was more than 20%, however, for CLO levels equal or less than 0.5, the PPD was reported to be less than 20%. Thermal sensation, acceptability, and comfort were closely correlated under uniform thermal conditions. Under non-uniform thermal environment, thermal acceptability and comfort were correlated. The range of thermal acceptability ran between PMV values of 0 and 1.5 (between slightly warm and warm) and subject were either comfortable or slightly uncomfortable in this range. While the overall thermal sensation remained the same, the thermal unacceptability and vote for uncomfortable environment was higher under non-uniform thermal conditions. Because of differences in occupants’ thermal preferences, thermal neutrality does not necessarily reflect the optimum comfort conditions while very high or very low PMV values do not necessarily cause thermal discomfort. During the Cool Biz campaign, sometimes zone temperature exceeded 30 °C and more than 70% of occupants were dissatisfied with their thermal environment. Variability within the indoor environment from the designated ‘setpoint temperature’ was reported. This new thermal environment resulted in more fatigue among the occupants and adversely affected their self-estimated performance. (continued on next page)

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Table 1 (continued) Author

Study/ building type

Location/ climate

Results

[55]

Field study office building AC

The British Council for Offices [40]

Theoretical studies AC office buildings

CA, San Bernardino, Hot/Dry Summers UK, Temperate Maritime Climate

[58]

Field study/ controlled chamber AC/NV/MM

Different locations (Australia, Canada)

[49]

Field study AC office buildings

Australia different cities, humid subtropical

[56]

Field study AC open office buildings

[33,35]

Field study AC office buildings

Korea Seoul, subtropical climate UK, Temperate Maritime Climate

Any increase in cooling temperature setpoint above 77 °F (25 °C) would result in a more than 20% dissatisfaction rate. The normal setpoint temperature for that office is 74° F (23.3 °C). Suggested that all air-conditioned offices raise their setpoint temperature (typically at 22 ± 2 °C) by 2 °C. This decision was the result of theoretical studies based on the adaptive model, which suggested that PPD value for an office building operating at 26 °C with air speed equal to 0.1 m/s and a CLO equal to 0.5 or less would be less than 10%. There were no supportive field studies to verify these results. Investigated the acceptable temperature threshold in AC buildings: By increasing the indoor air temperature, the perceived indoor air quality and self-estimated rating of occupant performance do not necessarily decrease if occupant thermal comfort is maintained through ceiling fans for air movement, radiant cooling or heating system, or personal conditioning systems. Thermal acceptability decreases to less than 80% for operative temperatures more than 26 °C. Studied occupant comfort and energy use for two strategies; static control strategy or increasing setpoint temperature 1 °C higher than normal and, adjusting indoor temperature according to the ambient temperature. Due to the increased complaint rate, occupants were adversely affected but possibly due to lower ventilation rate. Daily HVAC energy consumption was predicted to be reduced by 6% for the static and 6.3% percent for the dynamic control strategy. Occupants feel comfortable even at 28 °C depending on the prevailing mean temperature outdoor. The normal operating temperature in this building is 26 °C.

[34]

Field study AC office buildings

UK, Temperate Maritime Climate

[37]

Field study AC/NV/ MM, AC in summer office buildings

India, cities in different climate zones

[2]

Field study AC campus classrooms

Athens, GA, USA, Subtropical

Investigated the potential for increasing minimum cooling setpoint temperature: Although increasing the setpoint temperature from 22–23 to 24 °C made occupants warmer, it did not significantly affect their self-reported thermal comfort. Increasing indoor temperature in open-plan office area to 24 °C did not substantially increase thermal discomfort. The actual percentage dissatisfied (APD) level for this temperature was near maximum, implying that 24 °C might be the maximum cooling setpoint temperature for this office environment. The APD level was higher than the theoretical PPD from Fanger’s heat balance model, suggesting that local discomfort or other factors should be incorporated when choosing a cooling setpoint temperature for DR measures or as a “new normal”. Studied the potential for introducing a minimum summer setpoint (SSP) temperature by UK government in office buildings: Summer setpoint (SSP) temperature was lower than 24 °C for more than 60% of buildings. Occupants in public organization generally preferred a mandatory SSP ≥ 24 °C while those in private organizations preferred a SSP ≤ 24 °C. Recommended SSP was generally preferred over mandatory SSP. The adaptive model was valid for both NV and AC mode in mixed mode buildings.

Fanger’s heat balance model was over-predicting thermal sensation in the warmer side of the seven-point sensation scale (cold, cool, slightly cool, neutral, slightly warm, warm, hot), represented by actual thermal comfort vote scores of −3 through 3, respectively. There was a unit change in thermal comfort sensation vote for every 4 °C change in indoor temperature in AC buildings. By temporarily increasing the classrooms cooling setpoint temperature from the typical 21–22 °C (70–71 °F) up to 25 °C (77 °F), the occupant’s AMV values did tend to increase but for most cases the class averages remained in an acceptable thermal comfort range (−0.5 < AMV < + 0.5). None of the respondents who perceived their thermal environment as slightly cool, neutral, and slightly warm considered the environment as being thermally unacceptable while 75% of occupants voting for cool and 62% of those voting for warm still considered their thermal environment as acceptable.

Note: In this Table, AC = air conditioned; NV = naturally ventilated; MM = mixed-mode ventilated and conditioned.

air conditioned buildings, and is the basis for most of the thermal comfort standards in existence today. However, since it is mostly based on experiments conducted in laboratory settings under steady-state environmental conditions, it may not always be fully applicable to the operation of actual buildings, or in conditions where the indoor temperatures were gradually increased for a DR response measure. An alternative method for predicting occupant thermal comfort is based on the adaptive approach. This concept was derived using information gathered from field studies and allows for wider indoor temperature ranges that more closely follows the outside air condition. This model encourages designers of HVAC systems to consider individuals as active agents that can adapt to the local thermal environment, with these including behavioral, psychological, and physiological adaptations. Operating buildings using this approach has the potential to lower global en-

ergy consumption and corresponding greenhouse gas emissions, thus improving the overall sustainability of the built environment [9]. A major focus of this review paper was on the impact of increased cooling setpoint temperature on building occupants in airconditioned buildings during DR events. There is a great potential for energy savings (both peak demand and total consumption) through increasing buildings setpoint temperatures (even temporarily) in cooling seasons, however, the potential for a negative impact on building occupants from increased temperatures has been reported in some cases, especially for temperatures higher than 28 °C, although for temperatures less than this the study results did not indicate significant negative impacts. It is believed that there is not any one specific point where occupants start to feel thermal discomfort when increasing the cooling setpoint

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Table 2 The impact of temperature drift or ramp rate on building occupants. Author

Study type

Location

Results

[50]

Climate controlled chamber

Manhattan, KS, USA

[54] (cited in [42])

Climate controlled chamber

Norway

[7]

Climate controlled chamber Climate controlled chamber

New Haven, CT, USA

Investigated the effect of a fluctuating dry bulb temperature for sedentary subjects: Fluctuations in standard temperature would not result in thermal comfort complaints if the peak-to-peak amount of temperature change were kept below that shown in Equation: T2 (CPH) < 15 Where CPH = cycles per hour or cycle frequency and T is the zone dry bulb temperature°F. If T = 2° F (1.1 °C) and CPH = 3.75 cycles/hours or less, the fluctuation would be acceptable. Examined quick cyclic temperature ramp, starting from 25 °C at the rate of 0.5 °C/min. Occupants were able to change the ramp direction when they felt “Too Hot” or “Too Cold”. Large and rapid temperature ramps were undetectable by occupants since skin temperature would lag behind rapid ramps. Larger ramps were more detectable to resting subject than working occupants. A slow ramp of 0.5 °C/h around thermal neutral temperature is not detectable by occupants and even a 2 °C deviation from the neutral temperature would not cause more than 20% thermal dissatisfaction among occupants if done slow enough. Investigated the effect of temperature drift on buildings occupant’s thermal comfort perceptions; studied an 8½ hour ramp for 24 sedentary subjects (half men and half women) with CLO around 0.5: A temperature ramp of 0.6 °C/h from 23 to 27 °C (73–81 °F) would be acceptable for at least 80% of the occupants, and increasing humidity would not cause a significant difference as long as the dew point temperature remained below 20 °C (68 °F). Studied the acceptability of periodic variations and ramp in indoor temperature. If the operative temperature changes as slow as 0.6 K/h during the daytime and in an upward direction, it would be acceptable provided that temperature drift does not extend beyond the comfort zone by more than 0.6 K and for longer than one hour. Studied the effect of simultaneous light dimming and cooling load shedding on office workers. A steady increase in temperature up to 1.5 °C from the normal temperature over a 3-h period (0.5 °C/h) is not detectable for office workers, or if detectable would be considered acceptable in the circumstances studied. Longer exposure to temperature drift (even moderate ramps such as ± 0.6 K/h for 3–4 h) will eventually result in sick building syndromes and decreased task performance among occupants. There was no significant difference in AMV values between occupants with and without dress code freedom.

[8]

New Haven, CT, USA

[21]

Review paper

Various locations

[42]

Climate controlled chamber

Ottawa, Canada

[32]

Climate controlled chamber

Lyngby, Denmark

Fig. 3. Suggested maximum temperature amplitude for thermal comfort versus cycle time for triangular wave temperature variations, 1970 [50].

temperature, since there is not a given temperature and humidity range within which all people feel thermally comfortable. People could feel thermally comfortable in any number of temperature, humidity, airspeed, etc. combinations depending on building context, local culture or societal expectations, the person’s recent thermal exposure history, and so forth. Negative impacts of potential thermal discomfort from increased cooling setpoint temperature might include fatigue and sleepiness, as well as decreased productivity and self-estimated performance. Some studies revealed an improved occupant thermal comfort during implementation of DR measures, due mostly to the tendency to overcool buildings in many areas.

The energy saving potential of DR implementation has to date unfortunately been emphasized more than the impact on occupant thermal comfort. It is important to also consider the impact of DR on building occupants through sufficient field studies in different climates, building types, applications, and services. From this review article, we conclude that adjusting HVAC system operation can be a viable option for peak electrical demand reduction without adversely impacting the occupant thermal comfort and their perceptions of the indoor environment, when done with reasonable care and operational controls. Additional supporting research will contribute toward an evolution in thermal comfort standards

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S. Aghniaey, T.M. Lawrence / Energy & Buildings 173 (2018) 19–27 Table 3 Limits on temperature drifts and ramps as listed in ASHRAE Standard 55–2017. Limit of temperature drifts and ramps Time period, h

0.25

0.5

1

2

4

Maximum operative temperature change allowed, °C (°F)

1.1 (2.0)

1.7 (3.0)

2.2 (4.0)

2.8 (5.0)

3.3 (6.0)

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