Mili - Energy

7/31/2019 Mili - Energy

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Energy efficiency in green buildings- An

integrated approach to building design

Mili Majumdar

Fellow, The Energy and Resources Institute

Habitat Place, Lodhi Road, New Delhi -110003

Buildings as they are designed and used today, symbolise unrestrained

consumption of energy and other natural resources with its consequent negative

environmental impact. In India, the residential and commercial sector

consumes 25% of the total electricity usage of the country and a major portion of

this is utilised in buildings. Designing & developing new buildings based on

sound concepts of sustainability and applying suitable retrofit options to existing

buildings could substantially improve the energy use efficiency in the building

sector with an associated reduction in both local as well as global emissions. An"integrated approach" to building design involves judicious use and application

of-

w Efficient materials and construction practices

w Bio-climatic/solar passive architectural principles

w Efficient systems and equipments

w Renewable sources of energy

w Efficient waste and water management practices

Incorporating the above features in a holistic manner would result in buildings

that would impose a minimal impact on the environment while enhancing user

comfort and productivity.

Energy is used in various forms in a building e.g electrical energy is used to

power various appliances and equipment and thermal energy is used for

cooking. Typically electricity accounts for the major share in a building's energy

consumption. The primary end uses in a building, that use electricity are air

conditioning equipment, lights, fans, and office/household appliances or

machines. In a typical unconditioned building in India, lighting accounts for

maximum energy consumption, and in an air-conditioned building, 40-50% of

the total electricity consumption is accounted for by HVAC system, followed by

lighting system (20%). Other loads (pumps, equipment, etc.) contribute to

balance 2030%.


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An unconditioned green building would be designed to maximise thermal

comfort and avoid use of air-conditioners/air-coolers/heaters for maximum part

of the year. It would also have appropriate daylighting to reduce lighting energy

consumption. This is done through judicious use of passive solar principles

conducive to the climate in which the building is located e.g. in TERI-Bangaloreoffice building (fig 1) located in moderate climate of Bangalore, south facing

dark coloured solar chimneys create draft for exit of hot air, in turn drawing in

cool air from the open windows on north, ensuring adequate air flow at body

level to provide thermal comfort. Thermal performance of solar passive

buildings varies with changing outdoor conditions and in largely dependent on

weather conditions.

Fig 1: Cross section showing induced airflow pattern in Teri-Bangalore office

building

On the other hand, air-conditioned green buildings with maintaineduniform thermal conditions round the year are designed to minimise load on

conventional HVAC system. This is done by adopting appropriate passive solar

design strategies e.g. orientation, fenestration sizing and shading, landscaping,

day-lighting; and by using appropriate building materials and finishes, e.g.

thermal insulation, insulating glass units, heat reflecting paints, etc. A recently

concluded study by TERI has shown that for an institutional designed in a

composite climate, the cooling load could be reduced by 40% from the initial

estimated load (fig 2). The measures, which resulted in this load reduction,

were:w Use of over deck roof insulation using expanded polystyrene slabs/spray

applied polyurethane foam topped by reflective broken china mosaic

flooring.

w Use of double glazed windows with spectrally selective coating.

w Use of cavity wall construction with insulation infill.

w Use of energy efficient lighting.


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Fig 2:Reduction in cooling load for an institutional building by

incorporation of energy efficiency measures

72.1

120.0

99.087.1 82.0 81.5 78.4

3532322718

40

0102030405060708090

100110120130

Base load asper initial

estimate

Roof insulationusing EPS

Roof insulation+ efficient

windows

Wall and roofinsulation +

efficient

windows

Puf insulationon roof (rest

same as in run

4)

Efficientlighting with all

measures

EAT forprecooling

Energy saving opti ons

for building envelope

Tons of refrigeration (TR) % savings

w Use of underground earth air tunnel (EAT) to supply pre-cooled air to the air

handling units.

The excess investment incurred to incorporate the above mentioned measures

was estimated to payback in a year's time from savings in initial system costs

and reduced energy consumption. Thermal modelling and simulation tools

(Visdoe 3.1 and HAP 4.0) were used to accurately calculate the load reduction

and energy savings.

The thermal storage capacity of the earth being high, the daily and annual temperature

fluctuations keep decreasing with increasing depth of the earth. At a depth of about 4m belowthe ground the temperature remains constant round the year and is equal to the annual

average temperature of a place. For instance in Delhi this temperature is between 25-26 deg C.

The principle of the tunnel is to take advantage of constancy in temperature throughout the

year at a certain depth below ground. So if air is passed through such earth before funneling it

to a room, we can expect it to be cool in the summer and warm in the winter. An earth air

tunnel is a system in which air is forced through underground pipes or tunnels and then

circulated in the room. This system has been used to precool the fresh air input to the air

handling units thus reducing load on the AHUs.


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While building and system design interventions help downsize HVAC (Heating,

Ventilation and Air conditioning) systems, use of appropriate controls help to

reduce consumption of the optimised systems.

Lighting forms the major load centre in unconditioned buildings and next to

HVAC systems in conditioned buildings. Energy efficient lighting provides forright quality and quantity of light with minimal energy requirement. To

accomplish this step, the designer designs a lighting scheme for a specific

application based on illumination levels recommended by BIS or IESNA

standards. The aim of the designers should be to use efficient lamp and

luminaire combination to achieve the required illumination level (lux or foot-

candle level). This help the designer in ensuring that the lighting power density

(w/sq.ft) for a particular space is not exceeding the prescribed limits. The

ASHRAE 90.1-2001 standards specify the lighting power densities for different

space categories. Lighting simulation tools (e.g. lumen micro, etc) could be used

to design lighting schemes for a given power density.

Lighting schemes are normally designed for providing desired lux levels for

night-time conditions, i.e. without considering presence of daylight. Suitable

control strategies are then devised e.g. use of day-linked, occupancy sensors,

time switches, etc. to switch off or dim lights during daytime or when an area is

unoccupied. Design for day lighting further requires in-depth analysis of glare,

visible light transmission of glazing, sill level, window position and height,

orientation, outdoor obstruction, indoor reflectance etc: e.g. heat reflective

glasses used in buildings to reduce solar heat gain (to lower cooling load)

typically have low visible light transmission, thus reducing daylight into spaces.

In a predominantly hot climate like ours, glass with low shading co-efficient and

high visible transmittance should be selected to reduce solar gains and increase

visible light transmission.

Efficient design of building envelope and lighting is the foremost step in the

integrated design approach which helps to minimise space-conditioning loads.

The task of the designer is then to use efficient space conditioning equipment

and controls to further reduce energy consumption. In an air-conditionedbuilding use of efficient space-conditioning equipment and controls e.g. use of

efficient chillers, air handling units, pumps and cooling towers ; use of variable

speed drives at AHU fan motors, at cooling tower fan motors and secondary

chilled water pumps; use of low leakage dampers, enthalpy control, dry bulb

economiser are some of many energy conservation techniques possible for

HVAC systems.


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Use of natural cooling systems e.g wind towers, earth air tunnels etc. can be

integrated with conventional air conditioning systems to save energy.

Judicious building and system design can reduce energy consumption in a

building by 30-40% over conventionally designed buildings. After maximising

energy saving opportunities, in a building, a designer may consider use ofrenewable forms of energy to meet a part of the building's energy requirements

e.g. use of solar assisted water heating system, solar photovoltaic system can

reduce dependence on conventional/non-renewable forms of energy.

Conclusions:

With increasing energy prices, diminishing reserves of conventional

forms of energy, and increasing GHG emissions 'green buildings' are the

need of the hour. Globally speaking, in 1990, the residential, commercial,

and institutional building sector consumed 31% of global energy and

emitted 1900 mega tonnes of carbon and by 2050 its share would rise to

38% and 3800 mega tonnes respectively (IPCC,Nov.1996) .On the

brighter side, energy efficiency measures with paybacks in five years or

less can reduce global emissions by 40% by 2050. With increasing threat

on our planet earth caused by depleting resources and increasing

emissions it is absolutely pertinent that all our future buildings should be

designed to function as "green buildings".

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