24 degrees - confort productivity and energy consuption

8/7/2019 24 Degrees - Confort Productivity and energy Consuption

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24CStudy:Comfort, Productivityand Energy ConsumptionPublished by the British Council for Offices, January 2008Research conducted by Arup


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24C Study: Comfort, Productivity and Energy Consumption

Contents


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Foreword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Executive summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 COMFORT AND PRODUCTIVITY . . . . . . . . . . . . . . . . . . . . 9

2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 Existing guidance and standards . . . . . . . . . . . . . . . . . 10

2.3 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3 ENERGY CONSUMPTION. . . . . . . . . . . . . . . . . . . . . . . . 22

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4 ACKNOWLEDGMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . 29

5 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Contents


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ABOUT THE BRITISH COUNCIL FOR OFFICES

Established in 1990, the British Council for Offices mission is to research, develop and communicate best practice in

all aspects of the office sector. It delivers this by providing a forum for the discussion and debate of relevant issues

and works to promote co-operation and understanding between landlord and tenant, investor and developer, and owner

and occupier therefore encouraging efficiency and innovation in the office sector. The BCO has over 1,200 members,

who are organisations and individuals involved in creating, acquiring or occupying office space, both private and public

sector. The diverse nature of the BCO membership puts it in a unique position to advance the collective understanding

of its members, and the industry more generally, facilitating the creation of more effective office space.


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Currently the British Council for Offices recommends that thetemperature within an air-conditioned office should be

controlled to 22C and that the space should have an

acceptable level of humidity; that is, the relative humidity

should be within the range 40-60%. It is now widely

accepted that there is a need to reduce carbon emissions to

both reduce the rate of climate change and, equally

important, our dependence upon fossil fuels. Buildings are

responsible for a significant component of the United

Kingdoms carbon emissions and so any reduction will be a

useful contribution to the achievement of Government

targets in this area. One way to do this is to increase the

internal temperature in offices when cooling is used. The

British Council for Offices would like to increase the setpoint

by 2C, but have some concerns as to the effect on both

occupant comfort and productivity. They therefore

commissioned Arup to carry out a review of existing research

in these areas and also to assess the likely impact on energy

consumption The results of the study are summarised here.

COMFORT

The study was solely concerned with thermal comfort that

is the feeling of being hot or cold. Other aspects such as

noise and glare are unlikely to be affected by the change in

internal temperature. Furthermore, the study focussed on air-conditioned buildings rather than free-running buildings.

While the change of 2C might appear small, in practice

because of the way the space temperature controls function,

maximum space temperatures of 2C above the set value may

occur (and indeed are permitted in the BCO specification).

This means that 26C could occur during hot weather. Due to

sensors being mainly unable to measure operative

temperature, it is difficult to prescribe setpoint temperatures

based on operative temperatures alone, but it is suggested

that the air temperature setpoints are such that the

operative comfort range is ensured.

Researchers in the area of thermal comfort fall into twocamps; these are usually referred to as the Fanger approach

and the adaptive approach. The late P.O. Fanger carried out a

large number of comfort studies on people in controlled

environments from which he was able to develop a comfortequation relating the sensation of comfort (technically a

balance between heat generated and heat lost from the

body) to the following parameters:

Air temperature.

Temperature of the surroundings.

Air speed.

Humidity.

Type of clothing.

Type of activity.

This method is the basis for the current British and

International comfort standard.

The adaptive approach is based on surveys and looks at how

people respond in general. The most important result from

this work is that people will accept higher temperatures

inside buildings if, for example:

They can adjust the way they dress take jackets off

remove ties.

Alter air movement by desk fans for example.

Have control of blinds.

The external temperature has been high for a number of days.

Current guidance from the Chartered Institution of Building

Services Engineers includes both approaches for air-

conditioned and naturally-ventilated buildings. In practice,

the two are not really very different; the first allows an

examination of the detail, and the second takes real human

behaviour into account.

From an examination of these approaches it has been

concluded that a peak temperature of 26C will be

acceptable if a relaxed dress code can be adopted that is,

open necks and no jackets. At this temperature, humidity

becomes of greater importance than previously. However, the

current specification of a maximum of 60%RH should be

acceptable.

Executive summary


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The study recognises that it is unlikely that people in air-

conditioned buildings will accept temperatures much higher

than 26C and so designers will need to take care toaddress features such as solar shade. It may also be that a

more considered approach be taken in the design of the

heating, ventilation and air conditioning (HVAC) systems.

PRODUCTIVITY

There has been far less research in this area than in that of

comfort. This is probably because it is quite difficult to define

productivity. Furthermore, the effect that temperature has on

productivity is less clear and depends heavily on the type of

work being done. For example, typing speeds are shown to be

slower at higher temperatures, while memory improves with a

slow increase in temperature up to 26C. Along the same lines

as the placebo effect of thermal comfort, psychology also

affects productivity. According to the Hawthorne Effect, if

management shows an effort to improve conditions (regardless

of whether an improvement has actually been implemented),

occupants generally become more productive.

It is possible that people are most productive at work when

they are least aware of their surroundings. Apart from the

obvious implication of thermal, aural and visual comfort the

statement also implies there are no management issues that

could reduce motivation. Research has identified

management as a key issue in productivity. One researcherhas attempted to correlate productivity with space

temperature suggesting around a 3% drop in productivity at

26C from that achieved at 22C. However the researcher

has low confidence in the relationship. There is therefore

scant evidence to suggest that increasing the setpoint by

2C will have a noticeable effect upon productivity.

ENERGY CONSUMPTION AND CARBON DIOXIDE EMISSIONS

The effect of a change in internal temperature on the amount

of energy required to heat spaces is fairly obvious in that the

closer the internal temperature is to that outside the less heat

required. Things are not quite so simple when cooling. This isbecause the main sources of heat gain to the space, solar

radiation, office equipment and lighting are not affected by

the space temperature. The gains from people do reduce as

temperatures increase but only marginally. In addition current

levels of insulation mean that conduction of heat through the

building fabric is unlikely to be seriously affected. The load

imposed by air infiltration through the faade may under some

circumstances be affected but many designs attempt to

pressurise the building and so reduce this to an insignificant

load. The main influence of an increase in internal

temperature will be on the amount of energy required to cool

the fresh air supplied by the air-conditioning system to room

temperature. The study concentrated on this issue.

The design of the air-conditioning system will influence the

amount of energy saved by an increase in space temperature.

This is because some types only supply the minimum or near

minimum amount of fresh air required for a healthy

environment and make use of a secondary source of cooling(water coils) to remove the majority of the heat gain.

Examples are fan coil systems and active chilled beams.

Systems that provide all the cooling in the air supply

(variable air volume for example) handle much more air and

there is scope for free cooling if the external temperature is

not too high. Using a computer model written specifically

for this project the following systems were studied:

Fan coil units.

Variable air volume.

Active chilled beams.

Under floor air supply systems.

Performance was predicted for three locations within the

United Kingdom, London, Manchester and Edinburgh. In each

case the reduction in cooling energy and carbon dioxide

emissions due to the 2C change was calculated for a typical

office building that meets current building regulations.

Humidity was controlled to a maximum of 60%RH.

In addition to the savings made because air is not being

cooled as much, there is a more subtle potential saving

possibility. Reduced cooling of the supply air means that the

temperature of the water supply to the cooling coil could beincreased. This would result in an improvement in the

efficiency of the refrigeration plant, possibly by up to 7.5%.

The predictions suggested, with the exception of the under

floor supply system, savings of 6% (11%), 4%( 10%) and

3.5% (9.5%) in London, Manchester and Edinburgh

respectively. The figures in parenthesis indicate the effect of

an improvement in the efficiency of the refrigeration plant.

The reduction as the building moves North is simply due to

lower outside air temperatures. The predicted saving for the

under floor system are less at 1.1% (9%), 0.5% (7.5%) and

0.3% (7.5%) respectively. This is because air is supplied to

the space at a higher temperature (say 18C as opposed to12-14C). Because the fuel used is assumed to come from a

single source, grid supplied electricity, the percentage

reductions in energy and carbon emissions are identical.

Clearly the air-conditioning is only one source of carbon

dioxide emissions from the building. If all sources are taken

into consideration then, using typical building performance

figures, the maximum predicted reduction in electrical power

consumption is about 0.7%. The reduction in carbon

emissions could be slightly higher, but this will depend upon

the mix of fuels.

These savings are small but are a contribution to thereduction of carbon emissions at virtually no cost and with a

reduction in the fuel bill. It may also be possible to use a

slightly smaller capacity plant and so reduce the capital cost.


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Currently the British Council for Offices recommends that the

temperature within an air-conditioned office should becontrolled to 22C and that the space should have an

acceptable level of humidity; that is, the relative humidity

(RH) should be within the range 40-60%. It is now widely

accepted that there is a need to reduce carbon emissions to

both reduce the rate of climate change and, equally

important, our dependence upon fossil fuels. Buildings are

responsible for a significant component of the United

Kingdoms carbon emissions and so any reduction will be a

useful contribution to the achievement of Government

targets in this area. One way to do this is to increase the

internal temperature in offices when cooling is used. The

British Council for Offices would like to recommend an

increase in the setpoint by 2C, but have some concerns as

to the effect on both occupant comfort and productivity.

They therefore commissioned Arup to carry out a review of

existing research in these areas and also to assess the likely

impact on energy consumption. This is in two main sections:

Comfort and productivity.

Energy consumption.

The first contains a review of current guidelines and research

while the second contains an analysis of the effect of

temperature on the energy consumption and hence carbon

emissions for four typical air conditioning systems.

1. Introduction


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The purpose of this study is as follows:

To summarise existing guidance and standards related to

comfort in the built environment.

To conduct a state-of-the-art literature review on comfort

in air-conditioned offices.

To ascertain the effects of increasing the office control

setpoint temperature from 22C (2C) to 24C (2C)

during summer conditions.

To conduct a state-of-the-art literature review on

productivity and the anticipated effects of increasing the

indoor design temperature by 2K on productivity.

The study of comfort is inevitably one that combines both

objective (quantitative) and subjective (qualitative) findings.

Both types of findings will be reported here. As the main

reason for carrying out this literature review is to understand

the ramifications of increasing the indoor design temperature

on an office environment, the focus of the report is on

thermal comfort rather than other forms of comfort such as

visual, aural, and olfactory comfort. This is not to say that

the other parameters are unimportant, as studies have found

them to have impacts on workplace performance; however,

these studies fall outside the scope of this review.

Similarly, the review of productivity is limited to studies

conducted in office environments and to studies related toindoor temperatures rather than other factors.

2.1 DEFINITIONS

When designing for mechanical heating and/or cooling, it is

important to distinguish what the reason is for tempering

the environment. That is, is it a question of health and

safety, or is it a question of comfort? If it is a question of

health and safety, a wider range of indoor temperatures is

allowed as long as temperatures do not reach levels that

induce heat or cold stress in the upper and lower extremes,

respectively, of the temperature range. If, however, it is a

question of comfort, a much narrower range is consideredtolerable by occupants of the indoor space. Clearly, there are

temperatures outside a comfortable range at which an

occupant is not at risk from a health and safety point of

view. This study focuses on the comfort aspects of air-

conditioned office spaces. The definition of air-conditionedis the control of air temperature and relative humidity.

2.1.1 Thermal Comfort

Thermal comfort is defined as that condition of mind which

expresses satisfaction with the thermal environment and is

assessed by subjective evaluation (ASHRAE). The following

is a list of environmental factors affecting thermal comfort:

Air temperature.

Radiant temperature.

Air speed.

Humidity.

Additionally, there are personal factors that affect thermal

comfort, and these are as follows:

Clothing insulation measured in clo (1 clo = 0.155m2 C/W).

Activity level measured in met (1 met = 58.2 W/m2).

A typical business suit has an insulation level of 1 clo, and

normal office work corresponds to an activity level of 1 met.

As the aim of this study is to look at increasing the control

setpoint temperature in offices specifically, the following

assumptions were made:

Offices are air-conditioned.

Occupants are seated; sedentary or near-sedentary physical

activity level.

Occupants are clothed between 0.5 and 1.0 clo (ASHRAE).

2.1.2 Productivity

Productivity, like comfort, is difficult to define objectively as

many subjective factors affect workplace performance.

However, it can be defined broadly as the ratio of output to

input (CIBSE, TM24:1999). What constitutes the output and

input, however, is something that is not clearly and

absolutely defined.

2. Comfort and productivity


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2.2 EXISTING GUIDANCE AND STANDARDS

2.2.1 BRITISH COUNCIL FOR OFFICES GUIDE 2005

The British Council for Offices Guide 2005: Best practice in

the specification for offices currently specifies the following

design conditions for air-conditioned offices:

Summer:

Design indoor temperature: 22C 2C

Note: The industry standard for summertime relative

humidity limits is 50% 10%.

Winter: Design indoor temperature: 20C 2C

A minimum recommendation of 35-40% RH has been set

by the BCO if the fresh air rate equals or exceeds 2.0

l/s/m2

For buildings without mechanical cooling, the BCO guide

states that predicted thermal comfort is based on the

percentage of occupied hours for which particular internal

temperatures are expected to exceed 25C. The thermal

comfort is measured using the percent people dissatisfied

(PPD) method from the International Standard, ISO 7730,

aiming for a maximum PPD of 10%.

The BCO guide also points out a 1999 field study by de Dear

et al. in which the preferred temperature was determined to

be 23.5C.

2.2.2 CIBSE Guide A: Environmental Design

The Chartered Institution of Building Services Engineers

(CIBSE) published the 2006 version ofCIBSE Guide A:

Environmental Design, and Chapter 1 addresses

Environmental criteria for design. Guide A is a best practice

guide which is based on the ISO Standard 7730. Guide A

uses an operative temperature to convey the combined

effects of air temperature and mean radiant temperature. Theoperative temperature is defined as follows:

Tc = HTai + (1H)Tr (Equation 3.2.1)

Where:

Tc = operative temperature (C)

H = hr/ (hc+ hr)

hr = surface heat transfer coefficient by radiation

hc = surface heat transfer coefficient by convection

Tai = indoor air temperature (C)

Tr = mean radiant temperature (C)

A comfort zone is defined by -0.5 < PMV < 0.5, or 80%occupant acceptability, which is the same as ASHRAE

Standard 55-2004 (see Section 3.3). This 80% allows for

10% dissatisfaction with general thermal comfort and an

additional 10% dissatisfaction due to local thermal

discomfort. Guide A points out that the predicted mean

vote (PMV) combines the influence of air temperature, air

movement and humidity with that of clothing and activity

level into one value on a thermal sensation scale. However,

the PPD assumption of uniform clothing may actually

overestimate discomfort since, in reality, personal choice of

clothing is at play.

2.2.2.1 Temperature

Guide A states that temperature is usually the most

important environmental variable affecting thermal comfort,

and a change of 3K correlates to 1 scale unit difference on a

thermal sensation scale for sedentary subjects.

For offices in the summertime, Guide A, Table 1.5

recommends a temperature range between 22C and 24C for

offices in which the assumed activity level is 1.2 met and

clothing insulation level is 0.7 clo.

Temperature differences in the vertical should be limited to

a maximum of 3K between the ankles and head, but

temperature differences in the horizontal may be desirable as

this allows occupants to choose to move from a less

comfortable to more comfortable location.

2.2.2.2 Air Speed and Movement

The recommended upper limit for air speeds in a

mechanically-ventilated building is 0.3 m/s, according to

Guide A. Air speeds greater than this would be considered

unacceptable due to draught dissatisfaction, which is a

function of air speed, local air temperature, and fluctuations

in air speed. The draught rating (DR) is given by the

following equation:

DR = (34Tai)(v0.05)0.62(0.37vTu + 3.14) (Equation 3.2.2.1)

Where:

v = local mean air speed (m/s)

Tu = local turbulence intensity (%)

CIBSE Guide A points out that acceptable operative

temperatures can rise with increased air speeds. This

phenomenon will be discussed further in a later section.

Also, as activity levels increase, so do the relative air speeds

over the body surface.


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2.2.2.3 Gender and Age DifferencesCIBSE Guide A explains differences between genders and age

groups. In general, the clothing insulation level for women

is lower than that for men, so women tend to vote lower on

a thermal sensation scale. Regarding age, there is not much

variation as the lower metabolic rate in older people is

compensated by a lower evaporative loss.

2.2.2.4 Adaptive Approach

Humphreys and Nicol proposed using an adaptive approach

to designing for the indoor environment. Their approach is

based on extensive field studies of subjects in their everyday

work environment rather than in a climate chamber. Thus,

the effects of the subjects adapting to the thermal

environment by adjusting say, their clothing insulation level

or behaviour, would be accounted for in the thermal comfort

predictions. Humphreys and Nicol argue that occupant

adjustments are a result of recent past experience. For

example, if yesterday was too cold, occupants would be more

likely to increase clothing insulation levels the next day.

Based on this logic, historical daily mean external air

temperatures are weighted according to their proximity to

the current day. The following equation gives the running

mean external air temperature for day n:

Trm(n) = (1rm)Te(d1) + rmTrm(n1) (Equation3.2.4.1)

Where:

Trm(n) = running mean temperature (C)

rm = a constant between 0 and 1 that defines

the rate at which the running mean

temperature responds to external temperature

Te(d1) = daily mean external temperature for

the day before the previous day (C)

Trm(n1) = running mean temperature for day (n-1) (C)

For heated and/or cooled buildings, the upper and lower

comfort bands are given by the following equations,

respectively:

a) Tcom = 0.09Trm + 24.6 (Equation 3.2.4.2)

b) Tcom = 0.09Trm + 20.6 (Equation 3.2.4.3)

Where Tcom = comfort temperature (C)

Figure 1 shows the comfort bands for buildings withheated/cooled operation.

Figure 1 80% comfort zones (2K) in offices related to the

running mean of the outdoor temperature

The adaptive approach allows operative temperature to drift

so long as it occurs slowly over the period of a few days.

Note: A similar approach is proposed for a revised Dutch

comfort standard (Raue et al.).

2.2.2.5 Additional Recommendations

When the operative temperature is high, CIBSE Guide A

recommends the following measures:

Relaxing the office dress code.

Allowing individual control (e.g., adjusting blinds, moving

away from sunny areas).

Permitting flexible working hours.

Increasing air movement.

Providing hot or cold drinks (Note: Hot drinks trigger a

sweating response.)

0

5

10

15

20

25

30

Comfort temperature upper limit (C)

Comfort temperature lower limit (C)

302520151050

Outdoor running mean temperature (C)

Indoortemperaturelimits(C)


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2.2.3 ASHRAE STANDARD 55-2004The American Society of Heating, Refrigeration and Air-

Conditioning Engineers (ASHRAE) last updated their thermal

comfort standard in 2004, following ISO 7730:1995. ASHRAE

Standard 55-2004: Thermal Environmental Conditions for

Human Occupancy uses the operative temperature to

determine a comfort zone. An operative temperature for

given values of humidity, air speed, metabolic rate, and

clothing insulation, combines the dry-bulb air temperature

with the mean radiant temperature into a single value.

2.2.3.1 Assumptions

For office settings, it is assumed that the metabolic rates

would be between 1.0 and 1.3 met, and the clothing

insulation would be between 0.5 and 1.0 clo. It is also

assumed that the conditions are at steady state, and the

occupants are healthy adults at atmospheric pressure up to

an altitude of 3000m. They must also be in the office space

for at least 15 minutes.

2.2.3.2 PMV-PPD Index

The standard also illustrates the Predicted Mean Vote (PMV)

and Percent People Dissatisfied (PPD) method of determining

the comfort zone, as developed by Fanger. His studies were

carried out in climate chambers, which are carefully

controlled environments that are not entirely representative

of everyday office work environments. However, they provide

the advantage of being able to adjust certain variables while

keeping others constant.

The PMV-PPD index uses a heat balance model to arrive at

predictions of thermal comfort, and the heat balance model

accounts for clothing insulation, metabolic rate, and

environmental parameters. A deep-body temperature of 37C

is sought as the point for the human thermo-regulatory

system.

The 7-point ASHRAE thermal sensation scale is defined as

follows:

+3 hot

+2 warm

+1 slightly warm

0 neutral

-1 slightly cool

-2 cool

-3 cold

PPD is related to PMV by the following equation:

PPD = 100 95e

(-0.03353PMV4 0.2179PMV2)

(Equation 3.3.2.1)

Figure 2 shows the relationship between PPD and PMV.

Figure 2 Percentage Persons dissatisfied (PPD) as a function

of predicted mean vote (PMV)

2.2.3.3 Acceptability Criteria

In this ASHRAE standard, a comfort zone is defined by -0.5

24 degrees - confort productivity and energy consuption

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