Case Study Buildings

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nergyEfficiency

Case Studies

Roeland De MeulenaereLaborelec

January 2008

Case study: Buildings


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Energy Efficiencywww.leonardo-energy.org

Energy Efficiency

IntroductionWith all the environmental problems caused by CO2emissions, energy saving

is a hot topic at the moment. But how much do energy savings cost? How long

does it take until you recover your investment? Where can you save energy?

And how much CO2emissions are actually avoided?

This document provides answers to questions such as these, based on

commonly used methods of saving energy. Various possibilities for saving

energy are discussed in detail.

The five case studies presented in this document are:

1. Frequency regulator and oxygen sensor for superheated boilers

2. Energy management system

3. Thermal bridges

4. Ventilation

5. Lighting

The following energy prices are used in the case studies:

Electricity: 75/MWhGas: 25 /MWh

Fuel oil: 0.60 /l


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Case Study: Buildingswww.leonardo-energy.org

Case Study: Buildings

Frequency regulator and oxygen sensor for

superheated boilers

Introduction

Many buildings are heated with natural gas. In many cases the boilers in which

the gas is converted into heat do not operate efficiently, due to faulty regulation

or poor adjustment. In this case we look at the real-life example of a hospital.

Present situation

The boilers in the hospital work with simple on/off regulation and an excess

supply of air in order to guarantee full combustion of the natural gas. The air is

brought to the gas by means of a fan.

In such a system the excess air which does not actually contribute towards the

combustion also has to be heated up by the flame. The hot air disappears up

the flue along with the combustion gases. This can be avoided by controlling

the supply of air more accurately, resulting in lower gas consumption because

less air has to be heated up.

In this case four superheated boilers are used, one of 8 MW and three of 11.7

MW.

Proposal

We examined whether one of the 11.7 MW boilers could be fitted with a

frequency regulator on the air supply fan, and/or an oxygen sensor in the flue

pipe with feedback to the regulator.

A frequency regulator yields electricity savings compared to the present

regulation with a damper valve (depending on the load on the boiler), while an

oxygen sensor yields fuel savings (depending on the burner setting).

Savings and investment

The air supply fan was fitted with a frequency regulator. The power

consumption is proportional to the cube of the speed (rpm). In other words, a

small reduction in speed yields a large saving in power.

The oxygen sensor makes it possible to raise the efficiency of the boiler by

reducing the amount of excess oxygen, so that less fuel is used to generate the


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same amount of heat.

Theoretical calculations were carried out using simulation software with the

frequency curve based on a standard normal distribution of the fan power. The

savings yielded by the O2 sensor were calculated on the basis of the

combustion coefficient at different O2levels and the thermal consumption of the

system.

These calculations yielded the following savings, assuming 6000 hours of

operation per year and a flue temperature of 190C:

Table 1: Calculated values and the savings achieved using an oxygen sensorand a frequency regulator

Various installers were invited to submit offers. One of these offers (in Belgium)

represented a capital cost of 39.883 (incl. VAT), which in turn represented a

payback period of 1.6 years.

Energy management system

Introduction

An energy management system (EMS) is a central controller for utilities, with a

single computer controlling everything throughout the building: temperatures,

lighting, time switches etc. The computer also logs all the data. Such a system

can yield considerable energy savings if properly used. For example,

ventilation/airco/heating can be switched off at night or during weekends. Note

that an EMS by itself does not yield any energy savings: what counts is making

the right use of it!

Present situation New situation Saving

Electrical power of fan 22 kW 22 kW

Electricity consumption 132 MWh/year 64 MWh/year 68 MWh/year

Excess oxygen 5% 2%

Combustion efficiency 90.1% 91.4%

Thermal consumption 54756 MWh/year 53977 MWh/year 779 MWh/year

24575 /year


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Present situation

In a hotel the rooms are kept at a temperature of at least 20C. If there are

guests in the room they can adjust the temperature as they wish. The

ventilation system is already fitted with a heat recovery system.

Proposal

By using an energy management system the temperature in the rooms can be

controlled separately.

If a room is unoccupied the temperature can be set lower than the present 20

C. In the winter a minimum of 16C is enough, and by having a minimum of 18

C in summer no cooling is necessary, only heating. There is a difference in

temperature between winter and summer because the comfort temperature is

different.

Note that the temperatures chosen depend on the thermal inertia of the

building. If the building has high inertia it is better to set the temperature higher,

because people will set the heating very high when they come in and not turn it

down again, as the walls take a long time to warm up. Conversely, if the inertia

is low then the temperature can also be set lower.


The simulation software used for this calculation determines the amount of heat

required in terms of degree-hours. A degree-hour represents the difference

between the actual temperature and the reference temperature, summed per

hour over a certain period. For instance, if the reference temperature is 15C,

the software calculates how many hours the average outside temperature was

less than 15C during the time intervals concerned. If the average outside

temperature is 13C for one hour and 14C for another hour, then there are 3

degree-hours during this two-hour period. This information can be used to

calculate the amount of heating required.

By comparing the old and new situations and comparing them with one

another, it is possible to see how much heat was being wasted in the old

situation.

If these data are multiplied by the boiler efficiency and the efficiency of the heat

recovery system, we obtain the ultimate saving. The price of the energy

management system is not taken into account in the calculation, since prices

vary greatly according to the number of measuring points, the regulation

functions, read-outs etc.


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Table 2: Calculated values and the savings achieved using an energy

management system

Other utilities connected to the energy management system:

outside lighting

corridor lighting (set to 50% at night)

ventilation system in conference rooms and restaurant

lobby lighting

etc.

N of rooms 146

Occupancy 61%

Efficiency of heat recovery system 50%

Present situation:

Zero occupancy 20CConsumption 1714 kWh

Proposal:

Zero occupancy

Winter (November to March) 16 C

Summer (April to October) 18 C

Consumption

Winter 763 kWh

Summer 451 kWh

Total consumption 1213 kWh

Saving:

1 room empty all year round 501 kWh

All rooms, actual occupancy 29091 kWh

727 /yearr


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Thermal bridges

Introduction

A cold bridge is formed where a material used in part of an outside wall has better heat conduction than

the wall itself, if there is no additional insulation around it.

Thermal bridges are a frequent problem in buildings, allowing heat to escape in winter and to enter in

summer, so that more energy has to be used for heating and cooling respectively.

The best way to deal with thermal bridges is avoid them during construction of the building in the first place.

In existing buildings they can be eliminated by fitting additional insulation. However, the insulation must be

properly fitted and must include vapour-proof flashing.

Thermal bridges can be detected using thermal imaging.

In the case study considered here, the thermal bridges are load-bearing beams that are not insulated.

Present situation

In the walls of the building there are 35 metal beams that are not insulated, forming thermal bridges with all

the attendant energy losses. They were detected using thermal imaging.

Fig. 1: A cold bridge


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Fig. 2: A cold bridge made visible by IR thermal imaging

Proposal

Thermal bridges can best be eliminated at the design stage of the building, as

prevention is better than cure.

However, if they are detected at a later stage then they should be insulated as

best possible.

In order to fulfil the requirements of vapour proof flashing, the metal beams

were insulated with rolls of polyurethane (PU) covered with a thin layer of

aluminium on the outside.


By definition, a cold bridge has a very high U value (heat transfer coefficient):

the higher the U value, the greater the heat losses. The U value depends on

the thickness of the part and the material of which is made. The aim of fitting

extra insulation is to make the U value as low as possible. In this case the U

value of the beams was reduced from 5.87 to 0.6 W/m.K.

The difference in the U value is multiplied by the difference between the

average inside and outside temperature and the operating hours of the heating

system. By taking into account the efficiency of the heating system and the

cross-sectional area of the beam, the annual saving can be calculated.


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Table 3: Calculation of the savings achieved by eliminating thermal bridges

Heat transfer coefficient (U) of the beam with-out insulation 5.87 [W/(mxK)]

Heat transfer coefficient (U) of the beam withinsulation 0.6 [W/(mxK)]

Inside temperature during the day 18C

Efficiency of the heating system 58%

Operating hours 5800hours

Annual energy saving (per m of insulatedbeam) 316kWh/year

Area to be insulated 43.89m(0.33m x 3.8m x35 pieces)

Annual energy saving 13.9MWh/year

Fuel: fuel oil

Energy price: 0.6/litre

Annual saving in energy costs (per m ofinsulated beam) 18.96

Annual financial saving 832.15

Cost of the insulation work 5.08/m(36.58/(12m*0.6m))

Payback period 0.3years


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Ventilation

Introduction

New buildings nowadays are designed to be as airtight and well-insulated as

possible, but it is still important for the building to be properly ventilated. The

ventilation can be controlled by making the building airtight and installing a

good ventilation system. This makes it possible to ensure not too much or too

little ventilation, and avoids wasting energy by having to cool the air too much.

Older ventilation systems are frequently overdimensioned, and so it can help to

retrofit them with a frequency regulator combined with CO2 sensors. The

sensors make it possible to maintain good air quality without supplying too

much fresh air. However, such retrofitting is not always possible and in some

cases can also be very expensive. One alternative is to recirculate the excess

amount of air extracted, as the extraction air does not have to be heated up so

much with respect to the fresh outside air drawn in, or even does not have to be

heated at all. The aim in this case is to recirculate as much air as possible.

Present situation

The ventilation system in this case is a ventilation group with a mixer section,

set to a fixed minimum amount of fresh air (20%). This applies to all operating

times, despite being set to the minimum.

Fig. 3: The ventilation group

Proposal

The aim was to limit consumption outside office hours. This was achieved by

reducing the amount of fresh air drawn in (so that less cold air has to be


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warmed up), and by lowering the temperature at which air is blown in.

It was not technically feasible to fit frequency regulation without replacing a

large part of the ventilation group. It was therefore decided to adjust the mixing

register more accurately, so that more air was recirculated and thus less air had

to be heated up.

The proportion of fresh air was reduced to 10% during the hours when there is

nobody in the offices, and the temperature was lowered to 18C.


Table 4: Calculation of the savings achieved by readjusting the ventilation

system

In this case also the calculation is based on the degree-hour method.

For a ventilation group with a flow rate of 10,000 m/h this can yield a gas

saving of 24.6 MWh if the minimum air settings are reduced to 10% and 18C

during office hours. This represents an annual saving of 615. The onlyinvestment is the labour cost necessary to adjust the settings.

Heating

Energy consumption ofair processing Former situation New situation

Operating times 0000-2400 0600-1900 1900-0600

Air flow rate 10000 m/h 10000 m/h 10000 m/h

Proportion of fresh air 20% 20% 10%

Fresh air flow rate 2000 m/h 2000 m/h 1000 m/h

Room injection tempera-ture 21 C 21 C 18 C

Degree-hours at desiredinjection temperature 90504 C*h 46921 C*h 32248 C*h

N of operating hours ofventilation system 8331 h 4439 h 3629 h

Amount of energy required 81.08 MWh/year 42.03MWh/year 14.44 MWh/year

Saving 24.60 MWh/year


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Lighting

Introduction

Lighting is an item that normally has little attention paid to it, as relighting is

seldom economically feasible (due to a large number of separate users, low

consumption per user and low number of operating hours). Relighting is usually

done as part of a larger project, when it is necessary to draw up a complete

lighting plan.

Present situation

The office rooms (10 x 30 m on five floors) in this case study have 150 lighting

units (30 per floor), each with two fluorescent tubes of 36 W. The units are fitted

with an electromagnetic ballast and have white reflectors, which however have

become dulled over the course of the years. The units are attached to the

system ceiling in three rows.

Fig. 4: Schematic representation of one floor

30m

10m

One floor

WINDO

WOS

Lighting units


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Proposal

Initial energy savings were obtained by fitting new lighting units (at the same

points) with an electronic ballast instead of an electromagnetic one. Further

savings were achieved by the new units having more efficient mirror optics, so

reflecting the light better. This enabled a lower lighting power to be used.

Yet more energy savings were obtained by regulating the lighting level

according to the amount of natural light. At times when there is a lot of daylight

the lights nearest to the window are switched off entirely, while the ones in the

middle operate at a lower level.


Data:

The lights burn from Monday to Friday, 0600 to 1900, 52 weeks per year. The

number of operating hours is therefore 3380 hours per year.

There are 150 lighting units, each with two fluorescent tubes.

Savings possibility 1:

In the current situation the energy consumption per lighting unit is 86.4 W (sum

of the two tubes plus the starter losses), giving a consumption of 43.8 MWh per

year.

The energy saving in the new situation is obtained by using electronic ballasts

whose energy loss is practically nil (1 or 2% of the total consumption). In

addition, more efficient mirror optics enable the consumption to be lowered to

32.3 MWh per year.

Savings possibility 2:

The electronic ballasts afford the possibility of dimming the lights, and with a

simple extension they can be regulated according to the amount of natural light,

so that the lighting units do not have to operate constantly at full power.

It is assumed that the lights nearest the windows will operate at 100% for 3

hours per day. The ones in the middle row will operate at 50% for 10 hours per

day and at 100% for 3 hours, while the ones farthest from the window will

operate constantly at 100%.

The installed capacity of the new lighting units (and ballasts) is 9577 W.

1/3 operating constantly: 10.8 MWh

1/3 operating for 10 hours at 50% and 3 hours at 100%: 6.6 MWh

1/3 operating only 3 hours per day: 2.5 MWh


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Total consumption: 19.9 MWh

The additional saving is 12.5 MWh.

Investments:

Savings possibility 1 costs 75 per lighting unit (total: 11,250), while

possibility 2 costs 20 per unit ( 3,000).

Summary:

Table 5: Calculation of the savings achieved by lighting modifications

Here it should be noted that possibility 2 is not independent of possibility 1,

since only electronic ballasts can be dimmed.

Consump-tion Saving

Cumulativesaving

Invest-ment

Cumulativeinvestment

Paybacktime

Cumulativepayback

Present 44 MWh

Saving 1 32 MWh 26% 26% 11250 11250 13.2 13.2 years

Saving 2 20 MWh 39% 55% 3000 14250 3.2 7.9 years

Case Study Buildings

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