In order to satisfy the requirements of the Reference (dynamic) Model, the features indicated below should be incorporated. This specification is not exhaustive but gives sufficient detail to provide a basis for assessing computer models.
5.A9.1 Analytical method
Calculations should be carried out for time increments not exceeding one hour using appropriate time sequences of climatic data, internal load patterns and required control set points. These may be hourly average values or, if the calculation requires a time increment of less than one hour, measured data corresponding to the time increment should be used or values may be interpolated from hourly data.
5.A9.2 Climatic data
Data for the following parameters are required at time increments not exceeding one hour:
— dry bulb temperature
— moisture content (or equivalent)
— solar irradiation, comprising direct, sky diffuse, ground reflected (taking account of site factors), sky temperature (or other parameter appropriate to the determination of longwave radiation from external surfaces)
— wind speed
— wind direction.
The effect on the convective heat transfer coefficient of wind speed and direction should be taken into account.
The solar component should include longwave radiation transfer to the sky and surroundings.
The conversion of solar irradiance data measured at a particular orientation and slope into values for other orientations and slopes should be achieved using the
methods described in CIBSE Guide J: Weather and solar data(A9.1).
Solar altitude and azimuth should be determined using the methods contained in CIBSE Guide J: Weather and solar data(A9.1).
The conversion of measured climatic data into the form required by the calculation procedure should be achieved using the relationships given in chapter 1 of CIBSE Guide C: Reference data(A9.2).
5.A9.3 Properties of opaque fabric
The following properties should be represented:
— thermal resistance
— thermal capacitance
— surface emissivity (at boundaries and internal cavities)
— surface absorption coefficient for shortwave radiation
— convective and radiant heat transfer characteristics within cavities.
The dynamic response of opaque components may be determined using finite difference techniques or by response factors; other methods may be used provided that it may be demonstrated that they can achieve equal precision(A9.3).
5.A9.4 Glazing
The following properties should be represented:
— thermal resistance
— solar absorption
— solar transmission
— surface emissivity
— convective and radiant heat transfer characteristics within internal cavities.
The performance of glazing systems should be based on the values of solar altitude and azimuth calculated at the solar time corresponding to the time for which the calculation is being performed. This may differ due to longitude and/or the effect of local adjustments for daylight saving.
The performance of glazing systems must take account of reflections between the elements comprising the system.
Separate calculations must be made for shaded and unshaded areas of glazed surfaces.
5.A9.5 Shading
Shading devices may consist of purpose built overhangs, side fins adjacent to or part of a window or moveable devices such as blinds, shutters or curtains.
The shading effect should be calculated for time incre- ments not exceeding one hour using values of solar altitude and azimuth at the appropriate solar time. Where shading devices may be adjusted or controlled the effect of such features should be represented.
The model should take account of the effect of shading on glazing performance, as follows:
— in the case of purpose built shades, the determi- nation of the amount and location of shade falling on the glazing; reflected radiation from the shades should also be considered
— for blinds and curtains, the absorbed and transmitted radiation to be calculated, if appro- priate, as a function of slat angle; the interaction between glazing elements and blinds due to reflection of radiation from blinds must be represented.
Other obstacles to radiation such as shading by adjacent buildings and other site features should also be included, as should self-shading by the building under analysis.
5.A9.6 Internal longwave radiation
Longwave radiant heat transfer between surfaces and convective heat transfer between room air and room surfaces should be modelled using the fundamental heat balance described in Appendix 5.A3.
Longwave interchange between sources of internal heat gain and room surfaces must be modelled. The location of heat emitters should be taken into account.
5.A9.7 Internal shortwave radiation (direct solar gain)
The distribution of shortwave energy should be determined by calculation of the amount of direct and diffuse transmitted solar radiation incident upon each room surface. If a surface transmits shortwave radiation the
quantity transmitted must be calculated using the same methods as for the transmission of solar radiation into the building. Reflections of shortwave radiation should be modelled.
The solar distribution must be calculated at the same frequency as that for the climatic data.
5.A9.8 Room air model
Convective heat gains may be assumed to enter directly into the air. The convective heat balance should include a representation of the thermal capacity of the room air.
Under some circumstances it may be appropriate to increase the heat storage capacity of the air artificially to take account of furnishings. However, there is little guidance available on when this is necessary.
The convective heat transfer coefficient at room surfaces should be calculated as a function of surface and air temperatures; suitable correlations are given by Alamdari(A9.4) and Hatton(A9.5). It is not considered practicable at present to include the influence of room air movement patterns.
5.A9.9 Infiltration and ventilation
The needs of design models and simulation models differ in that, for design purposes, it is usual to specify the value of infiltration whereas simulation techniques require this parameter to be calculated. Furthermore, ventilation to remove excess heat gain is an essential factor in the calculation of overheating risk. One way to determine ventilation rates is by means of a zonal airflow model. See chapter 4: Infiltration and natural ventilation for guidance on the calculation of natural ventilation rates. The program supplier should provide details of the method used and be able to justify the assumptions made in the model.
References for Appendix 5.A9
A9.1 Weather and solar data CIBSE Guide J (London: Chartered Institution of Building Services Engineers) (2002)
A9.2 Properties of humid air ch. 1 in Reference data CIBSE Guide C (London: Chartered Institution of Building Services Engineers) (2000)
A9.3 Building energy and environmental modelling CIBSE Applications Manual AM11 (London: Chartered Institution of Building Services Engineers) (1998)
A9.4 Almadari F and Hammond G P Improved data correlations for buoyancy driven convection in rooms Building Serv. Eng. Res.
Technol. 4(3) 106–112 (1980)
A9.5 Hatton A and Awbi H B Convective heat transfer in rooms Proc.
Building Simulation ’95, August 1995, Wisconsin, USA (1995)
6.1 Introduction
Internal heat gain is the sensible and latent heat emitted within an internal space from any source that is to be removed by air conditioning or ventilation, and/or results in an increase in the temperature and humidity within the space. It includes the following sources:
— bodies (human and animal)
— lighting
— computers and office equipment
— electric motors
— cooking appliances and other domestic equipment.
This chapter provides information on heat emission from various sources to enable designers to estimate internal heat gains. Designers can choose to estimate either the rate of internal heat gain, where sufficient is known about the use of the building, or base it on ‘benchmark’ values typical for the building and intended use and normally used by the industry. The choice will depend on the known or predicted use and likely change of use during the life of the building and building environmental services. If, for example, the building and services were to be designed speculatively in anticipation of a generic type of user, it could be appropriate to base the estimates of internal heat gains based on current practice or bench- mark values. However, benchmark values are only available for common buildings.
If the building use is known it may be more appropriate to estimate the level of internal heat gains by comparison with measurements from similar buildings or calculate a value using measured heat gains from individual heat emitting devices and first principles. Such estimates should allow for the probability that all devices do not emit heat concurrently and at a constant rate. This is particularly the case for office equipment manufactured to comply with the US ‘Energy Star’ program*. Diversity factors should be allowed for the values of heat gain for the design of centralised air conditioning systems. The diversity factor increases, i.e. the load reduces, as the sum of areas served by the cooling distribution system approaches to the central cooling source.
This chapter also provides information on the proportions of radiant and convective heat from various sources of heat gain and the time delay caused by thermal storage of heat gains in building fabric.
6.2 Benchmark values for internal heat gains
Benchmark values for internal heat gains are based on either surveys of measured internal heat gains from a number of buildings of particular types and usage, or empirical values found appropriate from experience and considered good practice in the industry.
6.2.1 Office buildings
Most of the published surveys of internal heat gains have been carried out in office buildings. The main sources of internal heat gains in offices are the occupants, artificial lighting and office equipment connected to the small power electrical distribution. Surveys have identified a relationship between internal heat gains from small power office equipment and the density of occupation.
Stanhope(1)commissioned surveys of a number of differ- ent office buildings with different densities of occupation in 1993 and 2000. There was no significant difference in the results of measured internal heat gains from small power office equipment between the two surveys. The results of the surveys showed that an allowance of 15 Wãm–2was adequate for practically all types of offices and that occupation densities of 12 m2/person and 16 m2/person were appropriate design occupation densities for city centre offices and business parks respectively. The only exception might be offices used for financial activities such as dealers’ rooms. The typical values were adopted by the British Council for Offices and recommended in the BCO’s Guide to best practice in the specification for offices(2). The results of these surveys were similar to surveys in the USA. Measurements on 44 office buildings were made in 1995 by Komor(3), who concluded that an allowance of not more than 13.4 Wãm–2for office equipment heat gains should not be exceeded unless the circumstances were exceptional.
Knight and Dunn(4)calculated the internal heat gains of 30 air conditioned office buildings in the UK, based on surveys. The occupant densities in the surveys ranged from 4 to 24 m2 per person. The results show that total internal heat gains are proportional to occupant density.
The relationship can be seen in Figure 6.1.
The internal heat gains include lighting and occupant gains in addition to equipment. Benchmark values for lighting heat gain are given in Energy Consumption Guide ECG019: Energy use in offices(5). The maximum value for offices is 12 Wãm–2. The results of the above surveys of benchmark values for lighting energy use are combined in Table 6.1 to give benchmark values for total internal heat gains for typical offices at various occupant densities.
6 Internal heat gains
* www.energystar.gov
6.2.2 Other building types
There are few published surveys of measured internal heat gains for other types of buildings. Table 6.2 (opposite) provides typical internal heat gains for some common buildings and uses.
6.3 Occupants
All active animal bodies including humans lose heat to their surroundings. This section deals with human beings but a method for estimating the heat gains from animals is included as Appendix 6.A1.
The emission of heat from a human body in relation to the surrounding indoor climate is discussed in chapter 1 section 1.3.1.5. Table 6.3 provides representative heat emissions (sensible and latent) from an average adult male in different states of activity. The figures for a mixture of males and females assume typical percentages of men, women and children for the stated building type.
The latent heat gain from a human body results in an instantaneous addition to the moisture content of the air, whereas the part of the sensible heat gain is absorbed by the surrounding surfaces and stored in the material.
Between 20 and 60%(6)of the sensible heat emission, can be radiant depending on type of clothing, activity, mean radiant temperature and air velocity. Indicative values for high and low rates of air movement are shown in two columns on the right hand side of the table.
Heat gains / Wãm–2 totalfloor area
24
6 8 10 12 14 16 18 20 22
4
Occupant density / m2 total floor area per person 120
110 100 90 80 70 60 50 40 30 20 10
Figure 6.1 Variation of calculated total heat gains with occupation density(4)
Table 6.1 Benchmark values for internal heat gains for offices (at 24 °C, 50% RH)
Building type Use Density of Sensible heat gain / Wãm–2 Latent heat gain / Wãm–2 occupation
People Lighting Equip’t People Other / personãm–2
Office General 4 20 12 25 15 —
8 10 12 20 7.5 —
12 6.7 12 15 5 —
16 5 12 12 4 —
20 4 12 10 3 —
Table 6.3 Typical rates at which heat is given off by human beings in different states of activity.
Degree of activity Typical building Total rate of Rate of heat emission for mixture Percentage of sensible heat emission of males and females / W heat that is radiant heat for
for adult male stated air movement / %
/ W Total Sensible Latent High Low
Seated at theatre Theatre, cinema (matinee) 115 95 65 30 — —
Seated at theatre, night Theatre, cinema (night) 115 105 70 35 60 27
Seated, very light work Offices, hotels, apartments 130 115 70 45 — —
Moderate office work Offices, hotels, apartments 140 130 75 55 — —
Standing, light work; Department store, retail 160 130 75 55 58 38
walking store
Walking; standing Bank 160 145 75 70 — —
Sedentary work Restaurant 145 160 80 80 — —
Light bench work Factory 235 220 80 140 — —
Moderate dancing Dance hall 265 250 90 160 49 35
Walking; light machine Factory 295 295 110 185 — —
work
Bowling Bowling alley 440 425 170 255 — —
Heavy work Factory 440 425 170 255 54 19
Heavy machine work; Factory 470 470 185 285 — —
lifting
Athletics Gymnasium 585 525 210 315 — —
Source: ASHRAE Handbook: Fundamentals (2001)(6)
6.4 Lighting
6.4.1 General
All the electrical energy used by a lamp is ultimately released as heat. The energy is emitted by means of conduction, convection or radiation. When the light is switched on the luminaire itself absorbs some of the heat emitted by the lamp. Some of this heat may then be
transmitted to the building structure, depending on the manner in which the luminaire is mounted. The radiant energy emitted (both visible and invisible) from a lamp will result in a heat gain to the space only after it has been absorbed by the room surfaces. This storage effect results in a time lag before the heat appears as a part of the cooling load.
In determining the internal heat gains due to artificial lighting the following must be known:
Table 6.2Benchmark allowances for internal heat gains in typical buildings
Building type Use Density of Sensible heat gain / Wãm–2 Latent heat gain / Wãm–2 occupation
People Lighting* Equip’t† People Other / personãm–2
Offices General 12 6.7 8–12 15 5 —
16 5 8–12 12 4 —
City centre 6 13.5 8–12 25 10 —
10 8 8–12 18 6 —
Trading/dealing 5 16 12–15 40+ 12 —
Call centre floor 5 16 8–12 60 12 —
Meeting/conference 3 27 10–20 5 20 —
ITrack rooms 0 0 8–12 200 0 —
Airports/stations‡ Airport concourse 0.83 75 12 5 4 —
Check-in 0.83 75 12 5 50 —
Gate lounge 0.83 75 15 5 50 —
Customs 0.83 75 12 5 50 —
/immigration
Circulation spaces 10 9 12 5 6 —
Retail Shopping malls 2–5 16–40 6 0 12–30 —
Retail stores 5 16 25 5 12 —
Food court 3 27 10 † 20 §
Supermarkets 5 16 12 † 12 §
Department stores:
— jewellery 10 8 55 5 6 —
— fashion 10 8 25 5 6 —
— lighting 10 8 200 5 6 —
— china/glass 10 8 32 5 6 —
— perfumery 10 8 45 5 6 —
— other 10 8 22 5 6 —
Education Lecture theatres 1.2 67 12 2 50 —
Teaching spaces 1.5 53 12 10 40 —
Seminar rooms 3 27 12 5 20 —
Hospitals Wards 14 57 9 3 4.3 —
Treatment rooms 10 8 15 3 6 —
Operating theatres 5 16 25 60 12 —
Leisure Hotel reception 4 20 10–20 5 15 —
Banquet/conference 1.2 67 10–20 3 50 —
Restaurant/dining 3 27 10–20 5 20 —
Bars/lounges 3 27 10–20 5 20 —
* The internal heat gain allowance should allow for diversity of use of electric lighting coincident with peak heat gain and maximum temperatures. Lighting should be switched off in perimeter/window areas (up to say 4.5 m) and no allowance account for any dimming or other controls.
† Equipment gains do not allow for large duty local equipment such as heavy-duty photocopiers and vending machines.
‡ The exact density will depend upon airport and airplane capacity, the type of gate configuration (open or closed) and passenger throughput. Absolute passenger numbers if available would be a more appropriate design basis. Appropriate building scale diversities need to be derived based on airport passenger throughput.
§ Latent gains are likely but there are no benchmark allowances and heat gains need to be calculated from the sources, e.g.
for meals, 15 W per meal(6)served, of which 75% is sensible and 25% latent heat; see also Appendix 6.A2
— total electrical input power
— fraction of heat emitted which enters the space
— radiant, convective and conductive components.
Both the total electrical input power and the distribution of the heat output will vary with manufacturer. In particular, the optical properties of the luminaire can affect greatly the radiant/convective proportion emitted by the lamp. All figures quoted in the following section are typical. Manufacturers’ data should be used where possible.
6.4.2 Total electrical power input
The total electrical power input to the lighting installation must be known. For lamps with associated control gear, it
is important to add the power dissipated by the control gear to that dissipated by the lamp. The control gear power loss is likely to be about 10% of the lamp rating for electronic ballast and about 20% for conventional ballast(7).
Case studies carried out on a number of offices built or refurbished between 1977 and 1983 found that the lighting loads were between 10 and 32 Wãm–2for a maintained illuminance levels of 150–800 lux(8). Surveys carried out on newer buildings found that the lighting loads were in the range 8–18 Wãm–2for a maintained illuminance levels of 350–500 lux(9).
Where the actual installed power is not known reference should be made to Table 6.4(10), which provides target installed power densities for various task illuminances.
Table 6.4 Lighting energy targets
Application Lamp type Task illuminance Average installed
/ lux power density / Wãm–2
Commercial and Fluorescent-triphosphor 300 7
similar applications 500 11
(e.g. offices, shops*, 750 17
schools)
Compact fluorescent 300 8
500 14
750 21
Metal halide 300 11
500 18
750 27
Industrial and Fluorescent-triphosphor 300 6
manufacturing 500 10
750 14
1000 19
Metal halide 300 7
500 12
750 17
1000 23
High pressure sodium 300 6
500 11
750 16
1000 21
* Excluding display lighting Source: Code for Lighting (2002)(10) Table 6.5 Measured energy distribution for fluorescent fittings having four 70 W lamps(7)
Type of fitting Energy distribution / %
Mounting Schematic Description Upwards Downwards
Recessed Open 38 62
Louvre 45 55
Prismatic or opal diffuser 53 47
Surface Open 12 88
Enclosed prismatic or opal 22 78
Enclosed prismatic on metal 6 94 spine
6.4.3 Fraction of emitted heat entering the space
The proportion of heat entering the space depends upon the type and location of the light fittings.
Where the lamp or luminaire is suspended from the ceiling or wall-mounted or where uplighters or desk lamps are used, all the heat input will appear as an internal heat gain.
Where recessed or surface-mounted luminaires are installed below a false ceiling, some of the total input power will result in a heat gain to the ceiling void. An accurate assessment of the distribution of energy from particular types of luminaire should be obtained from the manufacturer. In the absence of manufacturer’s data, Table 6.5 provides an indication of the energy distribution for various arrangements of fluorescent lamp luminaires, based on laboratory measurements(7).
For air handling luminaires, up to 80% of the total input power can be removed by the air stream, leaving only 20%
to enter the space as heat gain. The specific manufacturer should be contacted for actual test data. Heat taken away from a luminaire through a ceiling plenum, or directly from the luminaire itself, will not form part of the room sensible heat gain but may still constitute part of the total refrigeration load.
6.4.4 Radiant, convective and conductive components
Little information exists on the proportions of radiant, convective and conducted heat gain from lighting. Lamps radiate in both the visible and invisible wavebands and there will be a net gain of infrared radiation from the lamp and luminaire due to their radiant temperature being above the room mean radiant temperature. Table 6.6 provides approximate data for different lamp types and shows that a substantial proportion of the energy dissi- pated by all sources is emitted as radiant heat. Radiant heat can cause discomfort to the occupants. It is mainly detected by the occupants on the forehead and the backs of the hands as these parts are more sensitive to radiant heat than other parts of the body. The optics and body design of the luminaire can reduce substantially the radiant component and, for the purposes of determining room cooling load, it may be sufficient to assume that the heat is purely convective.
6.5 Personal computers and office equipment
6.5.1 General
Personal computers (PCs) and associated office equipment result in heat gains to the room equal to the total power input. The internal heat gains for this equipment is normally allocated as an allowance in watts per square metre (Wãm–2) of net usable floor area. Typical values are given in section 6.2 above.
The internal heat gains can be estimated from basic data but care must be taken to allow for diversity of use, idle operation and the effects of energy saving features of the equipment
6.5.2 Individual machine loads
It is well documented that nameplate power overstates the actual power and consequent heat gain. Hosni et al.(11) found with nameplate consumption of less than 1000 W the ratio of heat gain to nameplate power ranged from 25%
to 50% and concluded the most accurate ratio for determining heat gain was 25%.
The heat gain from PCs will fall significantly when they are equipped with the Energy Star(12)feature. The Energy Star features apply to all office equipment including PCs,
CRT and flat screen monitors, printers, fax machines, photocopiers and scanners. The Energy Star qualification started in the US and has been adopted by the European Union(13). To qualify as Energy Star compliant equipment has three levels of power consumption: normal, standby and sleep. The specifications set out the maximum levels of power consumption in the sleep mode and the default time for the equipment to enter sleep mode. Wilkins and Hosni(14)measured the power consumption of various PCs and other office equipment. Tables 6.7 and 6.8 show their results as typical heat gains from PCs and monitors in the continuous and energy saver modes. (Similar data for flat screen monitors could not be located.)
Table 6.6 Energy dissipation in lamps(10)
Lamp type Heat output / %
Radiant Conducted/ Total
convected*
Fluorescent 30 70 100
Filament (tungsten) 85 15 100
High pressure mercury/ 50 50 100
sodium, metal halide
Low pressure sodium 43 57 100
*The power loss of ballasts should be added to the conducted/convected heat.
Table 6.7 Typical heat gains from PCs(14)
Nature of value Value for stated mode / W
Continuous Energy saving
Average 55 20
Conservative 65 25
Highly conservative 75 30
Table 6.8 Typical heat gains from PCmonitors(14)
Monitor size Value for stated mode / W
Continuous Energy saving
Small (13–15 inch) 55 0
Medium (16–18 inch) 70 0
Large (19–20 inch) 80 0