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Energy Efficiency E-modules - Guidance

Ventilation and Air Conditioning in Public Sector Buildings

Ventilation and Air Conditioning in Public Sector Buildings | 2

Contents 1 Introduction 4

2 Learning Objectives and Outcomes 4

2.1 Learning Objectives 4

2.2 Learning Outcomes 4

3 Overview and Principles of Ventilation and Air Conditioning Systems 5

3.1 Comfort 5

3.2 Comfort Conditions 6

3.3 Air Conditioning Systems 6

3.4 Air Management 6

3.5 Natural Ventilation 6

3.6 Mixed Mode Ventilation 8

3.7 Mechanical Ventilation 9

3.8 Fan types 10

3.8.1 Centrifugal fans 11

3.8.2 Axial fans 11

3.9 Specific Fan Power 11

Example 12

3.10 Decision Tree for Ventilation 12

3.11 Refrigeration 13

3.12 Refrigeration Cycle 13

3.13 Refrigeration Efficiency 14

3.14 Evaporative Cooling 15

3.15 Free Cooling 15

3.16 Variable Refrigerant Flow (VRF) 16

3.17 Good Practice in VAC 17

4 VAC for Data Centres 19

4.1 Benchmarking 19

4.2 Data Centre Auditing 20

4.3 Data Centre Savings 20

4.4 Aisle Containment 20

4.5 Blanking 20

4.6 Increase Temperature Setpoint by 3°C 21

4.7 Free Cooling 21

4.8 Server Virtualisation 21


5 Energy Performance Improvement for VAC 22

5.1 Fan Speed Control 22

5.2 Replace Oversized Motors 22

5.3 Upgrade Controls 23

5.4 Implement Heat Recovery 23

6 Good Housekeeping 25

7 Building the Business Case 26

7.1 Business Case – Case Study 26

8 Useful links and references 28


1 Introduction

This guidance follows the format of the e-module “Ventilation and Air Conditioning in Public

Sector Buildings” and provides further details on the subjects covered in the module.

Please note that module users working in a healthcare environment should always refer to

the relevant Scottish Health Technical Memorandum (SHTM) prior to considering installation

of the measures suggested in the module. The advice given in the SHTM may conflict with

the advice given in this module, as it has been developed for the wider public sector. The

relevant SHTM can be found on the Health Facilities Scotland website.

2 Learning Objectives and Outcomes

2.1 Learning Objectives

The learning objectives for this module are to:

Describe the different Ventilation and Air Conditioning technologies and their

applications; and

Understand the different measures which can be implemented to improve Ventilation

and Air Conditioning systems’ (VAC) efficiency.

2.2 Learning Outcomes

The learning outcomes for this module are to:

Understand the different ventilation and air conditioning technologies;

Describe when VAC systems can work as integrated systems and when they should

operate separately;

Identify where the opportunities for improving VAC systems exist in Scottish public

sector sites and buildings;

Be able to carry out audits of ventilation and air conditioning systems and identify

opportunities for making improvements;

Identify when VAC systems should be replaced and what technology could be applied;

Prioritise opportunities for improving VAC systems in public sector buildings; and

Understand the key aspects of VAC systems replacement when building a business

case.


3 Overview and Principles of Ventilation and Air Conditioning Systems

3.1 Comfort

Prior to introducing VAC systems, it is important to understand the application and purpose

of these systems. This is to neither heat nor to cool a building but rather to offer such

conditions to the building occupants that will make them feel comfortable to carry out their

tasks.

The American Society of Heating, Refrigeration and Air Conditioning define thermal comfort

as the condition of mind that expresses satisfaction with the thermal environment which is assessed by subjective evaluation. (ANSI/ASHRAE Standard 55).

With comfort conditions being subjective, it is not expected for them to be set with strict

numeric limits but rather on a range of conditions. They are also dependant on the activity

level and clothing of the building occupants. For example in Figure 3.1 the comfort

conditions as presented in ISO 7730 are shown. Defining a range of comfort conditions is

very important for sizing HVAC (heating, ventilation and air conditioning) systems as well as

designing their controls.

For example, the green coloured area in Figure 3.1 can be considered as probably the best

comfort zone for office tasks, but also as a limiting deadband for controls. The meaning and

importance of the definition of the deadband zone is given more consideration later in the

module. Temperatures edging into the blue may be more acceptable for some manual

workers.

Figure 3.1 - Comfort zone definition to ISO 7730


3.2 Comfort Conditions

Indicative comfort conditions for offices as presented in CIBSE Guide B and are shown in the

Table 3.1.

Table 3.1 – Indicative comfort conditions for offices

Building/room type Temperature (°C)

Offices 21-23

Educational buildings

- lecture halls 19-21

- seminar rooms 19-21

- teaching spaces 19-21

Other factors

Fresh air supply 8 l/s per person

Air movement 0.1 – 0.25 m/s

CO2 levels 800 – 1000 ppm

Humidity 65% at comfort conditions

3.3 Air Conditioning Systems

A great variety of VAC systems have been developed in the past. Some systems deal with

comfort factors better than others and in many applications this had been the main criterion

for selection. Energy efficiency is now critical in selection and is given equal importance to

each system’s ability to provide comfort conditions. The VAC systems can be broken down

to two main components:

Air management; and

Refrigeration.

3.4 Air Management

The reason for supplying and extracting air to and from a conditioned space is that it is a

very convenient means for providing almost all of the comfort requirements. Air can be

supplied treated or untreated, by natural force or introduced mechanically and this covers a

significant aspect of buildings services ventilation. In general, four different ways for

applying ventilation to a building can be identified:

Natural ventilation;

Infiltration (unintentional air supply);

Mixed mode ventilation; and

Mechanical ventilation.

3.5 Natural Ventilation

The driving force of air movement is pressure difference. The basic principle is to induce a

pressure difference between the building inside and the outside environment in order to


force air in to the building. There are two ways to achieve this. One way is through the

pressure difference due to wind applying air pressure on the sides of the building (see

Figure 3.2) and the other is by a high internal temperature (see Figure 3.3).

When air is heated, it expands and the buoyancy (or stack) effect drives the hot volume

upwards. The hot air stream leaves the building and a vacuum is created. This vacuum will

be filled with cooler air from outside. This is called the make-up air.

Figure 3.2 - Wind driven ventilation

Figure 3.3 - Buoyancy driven ventilation

This principle or approach gives a great opportunity for cost savings as the outdoor

conditions are taken advantage of. However, implementing natural ventilation in its simple

form may be more challenging than it sounds. There are particular limitations depending on

the building usage. For example in the Health Sector there is usually a limitation on the

number of windows that can be opened for safety reasons or, in other cases opening

windows is avoided if the building is located on a busy street to reduce noise.

For buildings with natural ventilation, it has to be accepted that during peak summer

conditions the internal temperatures may exceed 25° C for some periods of time. However,

high mass buildings can take advantage of their thermal storage capacity. They do this by

changing the time where the building has higher internal temperature so as not to coincide

with the time of highest outdoor temperature. Some buildings may require mechanical

cooling if this is not practical.


These buildings can benefit from night cooling. Here, the ventilation remains open at night

in order to cool down the building elements by discharging the stored heat thus allowing

them to store heat again the next day.

A disadvantage of natural ventilation is the limited opportunities for heat recovery during

the heating season. The high temperature differences will also induce higher air flow rates

creating cold draughts. Excessive ventilation during the heating season would also

drastically increase energy consumption. To achieve high controllability an additional

automated system would have to be applied, usually comprising sensors to monitor

occupancy, air quality and temperature linked to actuators to open or close windows and

louvers.

The advantages and disadvantaged of natural ventilation are listed in Table 3.2.

Table 3.2 – Natural ventilation summary

Advantages Disadvantages

No capital and operational costs for fans Limited opportunities for heat recovery during the

heating season

No refrigeration equipment needed for the cooling season

High capital cost for increased controllability

No background noise from fans Has to be incorporated in the design

Additional loads from ventilation are handled by

increasing the heating systems

To assess the effectiveness of using natural ventilation in the building the following rules of

thumb can be applied:

Natural ventilation can cope with heating loads of up to 40 W/m² (CIBSE AM10); and

Effectiveness of ventilation from a single window opening is up to 6 meters plan depth.

Some older guides indicate that the maximum effective depth is 12 metres.

When a building’s heating load (or gains) is more than 40 W/m² it can be difficult to

successfully apply natural ventilation. It is advisable to consider ways to reduce heat gains

in these cases. For example, consider shades to decrease solar gains or replacing (or

reducing) lighting or upgrading to more efficient electrical equipment.

Comprehensive information on Natural Ventilation can be found in CIBSE Guide AM10 -

“Natural Ventilation”.

3.6 Mixed Mode Ventilation

Mixed mode ventilation can be incorporated in the design of the building or can be applied

later. There can be spaces with different comfort requirements in a building or there may be

changes in the floor plans during the building’s life that may affect the effective operation of

natural ventilation.

In simple terms, mixed mode adds a key mechanical aspect to a system: a fan. Fans will

assist the movement of air and can help to overcome some of the controllability and

security issues associated with natural ventilation. These include the need to have external


openings for night cooling and achieving minimum ventilation without the need for input

from the occupants.

There are variations in the operation of mixed mode systems. These depend on the building

layout and controls. In general, the operation of such a system should be driven by the

effectiveness of natural ventilation. Air quality controls or time-clocks can be used to control

fan operation.

The advantages and disadvantaged of mixed mode ventilation are listed in Table 3.3.

Table 3.3 – Mixed mode ventilation summary


More effective control of comfort conditions

especially during the mid-season when the driving

forces for natural ventilation are milder

Increased operational costs associated with

fan operation

Less capital cost required compared to an

automated natural ventilation building

Comprehensive information on Natural Ventilation can be found in CIBSE Guide AM13 –

“Mixed Mode Ventilation”.

3.7 Mechanical Ventilation

Mechanical ventilation can be implemented with different arrangements and strategies. A

mechanical ventilation system normally comprises two streams of air; a supply and an

extract. This is known as a balanced system. However there are applications where only one

is used.

The operations associated with the exhaust stream typically include the removal of

pollutants and local flow control for example through the use of extraction hoods. Those

associated with the supply stream are external inlets for replacement (or ‘make-up’) air, air

recirculation, air conditioning (temperature and humidity control) and dilution of pollutants.

Recirculation systems are not generally favoured in new systems, with heat recovery being

preferred (refer to the heat recovery module for a detailed presentation to heat recovery

options in HVAC). Recirculation was used in the past in an attempt to decrease energy use

by decreasing the amount of outdoor air that had to be conditioned to a given humidity and

temperature. However, these systems present a significant challenge in terms of control. As

a result they can easily be found running using 100% fresh air, leading to the highest

possible heating and cooling loads.

With this exception, mechanical ventilation is widely applied in public sector buildings

because of its high controllability. However, as is shown later and also in the controls

module, it is common for mechanical ventilation control systems to have erroneous

parameters, leading to excessive energy consumption.

Figure 3.4 is a screenshot from a BMS user interface showing the arrangement and elements of a typical mechanical ventilation system.


Figure 3.4 - Screenshot from BMS module controlling ventilation

Ductwork is used to distribute air around the building. Large buildings usually have complex

duct networks of combined hard and flexible ducts. Ductwork size is very important and

affects an important parameter of the system; pressure drop. This parameter defines the

type of the fan and the size and energy consumption of the motor. The pressure drop in the

ducts is associated with about 30-40% of the total air pressure drop. Other sources are the

pressure drops across heaters, noise attenuation devices, filtration units, louvers, dampers,

etc. The following should be considered with ductwork:

Its design will affect pressure losses;

Smaller ducts involve less capital cost; and

However smaller ducts have a greater operational cost from increased fan energy

consumption.

The driving force for air movement is pressure difference. In a mechanical system the

required pressure is supplied by the fan. There is a variety of fans and selection depends on

the application. This is dictated by variables such as the required flow rate and air pressure,

the operating environment (e.g. very high temperature exhausts, potential for dust

explosions, etc.) and the required availability. Fans are discussed in more detail in subsequent sections.

The air handling unit is a packaged system used for air-conditioning and includes the fan

and the other required elements such as filters, humidifiers and heating and cooling coils. Filters also account for a considerable proportion of the pressure drop in the system.

Diffusers efficiently and effectively deliver air to the ventilated air conditioned space. This

means that they have to be selected with minimal pressure drop while achieving the proper

air throw. Air throw is the distance the supplied air will travel until it slows down and starts

moving with natural buoyancy.

3.8 Fan types

Typically the following fan types are considered in HVAC applications: centrifugal fans and axial fans.


3.8.1 Centrifugal fans

In this design the airflow enters the fan from the side and is centrifuged, following a radial

path through the fan. They are characterised by the type of fan impeller used:

Forward curved (FC): These are usually operated at relatively low speed to give high

volume but low static pressure;

Backward Inclined (BI): This arrangement usually gives a more efficient fan in terms of

energy consumption compared to a FC fan, although it requires about twice the rotational

speed for a given amount of air volume. Higher static pressure is achieved with this type;

and

Airfoil. This has similar operational characteristics to BI fans; however it has more narrow

operational range meaning that a certain fan cannot be ramped down further than about

40% of the nominal airflow because it may cause instabilities to the fan leading to a

situation known as fan surge.

3.8.2 Axial fans

In this design the airflow passes straight through the fan in a parallel direction:

Propeller fans are the simplest form of axial fan and can produce high flows at low static

pressures; and

Axial fans have steeper vane angles and combined with the use of flow straighteners can

achieve high air volumes at higher static pressures. For high static pressure applications,

arrangements of axial fans in series are available.

3.9 Specific Fan Power

The energy performance of fans is measurable and is benchmarked in building standards.

CIBSE Guide F has published data from the Non-domestic Building Services Compliance

Guide, which sets the maximum limits for new and existing buildings. The specific fan power

is given in watts per litre per second and is calculated as shown below:

Where:

Psf: is the total fan power of the supply fans working for the system in question,

including all power losses.

Pef: is the total fan power of the extract fans working for the system in question,

including all power losses.

q: is the design air flow through the system, considering the greatest of either the

supply or extract air flow (l/s).

The calculation methodology is set out in detail in BS EN 13779:2007 Annex D.

In some cases it is easy to do an initial assessment of the SPF factor of a ventilation system

from the data available on an air handling unit plate or commissioning report. Otherwise, air

flow measurement and measurement of the power supplied to Variable Speed Drives

(VSD’s) or the fans directly are required.

Some SPF figures as set in the non-residential buildings guide are shown in Table 3.4.


Table 3.4 – Specific fan power for ventilation systems

Specific fan power (w/l/s)

Ventilation system type New buildings Existing buildings

Central mechanical ventilation including

heating and cooling system 1.8 2.2

Central mechanical ventilation system 1.4 1.8

This information is used when assessing the operation of an existing system or when

considering the procurement of a new system.

Example

The measured data collected from the inspection of an air handling unit with air mixing

supplying heating and cooling to a lecture theatre is shown in Table 3.5.

Table 3.5 –Example data for lecture theatre

Unit Airflow (m³/s) Fan load (kW)

Supply 2.63 4

Return 2.83 2.2

These measurements can then be applied to the equation as shown below:

This confirms that in this case the system is performing according to the standards expected

for an existing building.

3.10 Decision Tree for Ventilation

All ventilation is based on the movement of air. There are two options to achieve this

movement. The first is to take advantage of the natural buoyancy of air and how this

buoyancy changes with temperature. The second is to mechanically induce air movement

using fans.

Another key element of ventilation is the condition of the air. This usually relates to its

temperature, humidity and cleanliness. Air can be supplied to buildings at outdoor condition.

However, this can vary widely across the seasons and so this approach might only be

acceptable in certain applications and for the warmer months of the year. For this reason air

is usually conditioned using appropriate air conditioning (AC) equipment to control temperature, humidity and cleanliness.

Mechanical ventilation and air conditioning can also be used to overcome other problems.

Examples are where outside noise pollution is an issue, or where the building occupants

require high quality air, such in certain medical situations.


The schematic shown in Figure 3.5 is a simple guide that can be used when considering any upgrade in ventilation systems.

Figure 3.5 - Decision Tree for Ventilation

3.11 Refrigeration

Heat moves naturally in one direction only; from hot to cold. To move heat the other way

work needs to be applied such as via a heat pump. A heat pump needs a heat sink to take

heat from or move heat to. This might comprise ground, air or water. Using a heat pump,

even a volume of air considered to be cold might have enough heat energy available to be

extracted by mechanical means.

As heating systems are described in the module 2, Steam and High Temperature Hot Water

Boilers, the options available for cooling only will be outlined.

Before describing the main AC systems available, the cooling methods available should be

explored. However, many of the AC systems can work on reversible cycles, so can provide

both heating and cooling.

The main cooling methods are:

Evaporative cooling;

Free cooling;

Gas compression refrigeration; and

Gas absorption refrigeration.

The most extensively used method of refrigeration is the gas compression cycle.

3.12 Refrigeration Cycle

Figure 3.6 shows how a refrigeration cycle works and shows how heat can be moved to

service a heating or cooling load.


Figure 3.6 - Refrigeration Cycle

Refrigeration is a 4-stage cyclic process:

Compression. The refrigerant enters the compressor at low pressure in the gaseous

phase. The compressors are sensitive and cannot process liquids or they will be

damaged. Thus the aim is to bring the gas into the compressor at a “superheated”

condition (i.e. with no liquid), where small variations in temperature or pressure will not

affect the gas condition and will not cause the liquid to condense. The gas exits the

compressor as a high pressure and high temperature gas.

Condensation. The high pressure hot gas is taken through the condenser which is a

form of heat exchanger used to remove as much heat as required to condense the gas,

but no more. The refrigerant normally exchanges heat with, the external environment.

For example in a high efficiency air source heat pump, fans with speed control are used

to control the volume of air blowing through the condenser and achieve high control of

the condensing process.

Expansion. The refrigerant, still at high temperature and high pressure, moves to the

expansion valve which throttles the refrigerant causing it to lose pressure. The result of

this process is the expansion of the refrigerant which causes it to cool.

Evaporation. The cooled mixture of low pressure gas and liquid goes through the

evaporator where it absorbs heat from the environment and evaporates completely until

it becomes a superheated gas and the cooling cycle starts again.

3.13 Refrigeration Efficiency

The efficiency of refrigeration plants is characterised by the ratio of the energy output

(heating or cooling) to energy input (electricity).

For heating applications the COP (Coefficient of Performance) is used. This is the ratio of the

heat released at the condenser to the energy input to the compressor.

For refrigeration the EER (Energy Efficiency Ratio) index is used. This is the ratio of the

heat absorbed at the evaporator over the input to the compressor.


The efficiency of refrigeration equipment is not constant throughout the year. This is

because it is affected by the environmental conditions of the evaporator and condenser, in

particular the ambient temperature.

In general COP increases with increasing evaporation temperatures and drops with

increasing condensing temperatures. For this reason the European Directive for Eco-design

of Energy Related Products requires the manufacturers to publish the seasonal efficiency

ratios for air conditioning equipment with an output less than 12kW.

3.14 Evaporative Cooling

Evaporative cooling is an energy efficient method for cooling and can be applied in warm

conditions as well. The basic concept is that when a liquid evaporates (as described earlier

with the refrigerants) it absorbs heat from its surroundings. In evaporative cooling water is

used for heat absorption. Two methods are available:

Direct evaporative cooling; and

Indirect evaporative cooling.

The difference between the two methods is that in the direct method, water is added

directly to the air stream as an atomised stream thus increasing its moisture content

(humidity). Pads are used to block any water droplets formed and carried by the air stream.

This method is used where there are not strict humidity control requirements. In the

indirect method, a secondary stream of air is cooled with the atomised water before going

through an air-to-air heat exchanger where it absorbs heat from the primary stream.

Evaporative cooling is also commonly used to cool a water stream. This is the process used

in cooling towers for various applications from small to medium size cooling loads to process

plants and power stations. Cooling towers in the range of building applications can operate

very effectively when the ambient temperature is up to 15°C.

Figure 3.7 - Cooling Towers

3.15 Free Cooling

Free cooling is a method where low external temperature conditions are used to chill the

cooling medium. This makes it very energy and cost efficient in applications where the air-

conditioning system has to handle high internal cooling loads throughout the year such as in

data centres. The concept is simple, and can be applied to many different cooling system

arrangements.

The main equipment required is a so called dry cooler. This is a heat exchanger where the

cooling medium (refrigerant or water) is cooled by an air stream induced by a fan.


This technology can be applied on Direct Expansion systems as well but the capacity of the

system is significantly reduced (to around 22%). This is because the refrigeration process

relies upon the phase change of refrigerant at a standard pressure.

To economically justify the capital cost of such a system a steady cooling load throughout

the year is required. In the public sector these systems can offer an opportunity for savings

in applications like data centres.

Figure 3.8 - Dry Cooler

3.16 Variable Refrigerant Flow (VRF)

It is common when sizing air conditioning equipment to select it to match the peak demand.

However, throughout the year the demand can vary widely and in some cases peak loads

only occur for a few hours. For systems with traditional controls this would mean that when

the demand is low they would operate at small on-off cycles. Due to the high capacity of the

equipment against the low load, this will cause the system to overshoot the desired room

conditions thus leading to energy waste.

VRF was developed in an attempt to overcome the limitations of a cyclic single compressor

system. This technology is mainly characterised by the use of a power inverter that controls

the speed of the compressor and the mass flow of the refrigerant.

An advantage of the VRF systems against conventional, is the option to install the system in

a 3-pipe configuration. This enables the provision of both heating and cooling in the air

conditioned building at the same time. It can also use the variable heat demand for

advanced heat recovery applications. Another advantage is the use of multiple outdoor units

on the same system which enables a high degree of resilience.

The disadvantages come with the size of the system. In many cases VRF systems are

designed to cover large areas of the building. This requires long pipe runs, a large number

of joints, substantial refrigerant and lubricating oil charges and multiple electronics. These

are all factors that increase the probability of failure.

The advantages and disadvantages of VRF systems are shown in Table 3.6.


Table 3.6 – Summary of VRF systems


Can adjust the load on the compressor to

meet space heating/cooling requirements

Centralised VRF systems in big buildings have long

pipe runs, number of joints, substantial refrigerant and

lubricating oil charge and multiple electronics all being

factors that increase the probability of failure

Multiple outdoor units can be combined to

increase resilience

3-pipe systems applications allow

concurrent heating, cooling and optimised

heat recovery across multiple room units

by using a single heat pump system

3.17 Good Practice in VAC

Before starting to investigate a new VAC system you should consider how you can reduce

heating or cooling loads. For example:

Have you considered upgrading the lighting system with more efficient lamps/fittings

that will reduce heat gains from lighting?

Is your building fully insulated?

Have you considered upgrading the building shell?

Have you considered upgrading the glazing system and applying reflective membranes?

Have you considered shading systems?

When this is complete, consider if air-conditioning is required. Use the decision tree (Figure

3.5) to identify system requirements.

Complex control systems are difficult to commission correctly and can be difficult to

operate. It is also important to maintain flexibility within your control strategy to allow for

future changes to the building and its use. As a result you consider if some or all of the

following controls are required:

Temperature;

Occupancy; and

Air quality.

Effective control strategies are discussed in Module 5, Overview of HVAC System Controls,

but the first step is always to consider the purpose of the space being serviced, what

comfort factors are important and if the space is continuously or intermittently occupied.

Demand control ventilation (DCV) is considered state-of-the-art in VAC control systems.

DCV is suitable where there is a high variation in occupation during the day (e.g. auditoria),

but less efficient where general dilution ventilation is required (e.g. hospitals).

You should also consider zoning. If there are areas with different comfort conditions

required, a system that can meet such requirements must be selected (e.g. 3-pipe VRF)

along with suitable zone control.


Whether it is an upgrade or a new plant, the design of the distribution system is important.

For example:

Identify best plant location. Locating the plant as close to the demand areas as possible

minimises distribution losses;

Strictly define the occupancy levels. This will avoid under or oversizing of the system;

Specify high efficiency fans. Defining the static pressure of the system will allow high

efficiency BI centrifugal or axial fans to be selected; and

Apply heat recovery.

The need for humidity control should also be considered. Providing humidification is

expensive in terms of capital and operational costs. In most applications humidity is

acceptable in the range 40-70%. Due to the weather in the UK, humidity in buildings is

normally within this range without additional humidity control.

Noise can also be an issue with some systems and the requirement for noise attenuation

must be assessed relative to building use.

Table 3.7 is an example of the noise levels that must be met after the commissioning of a

ventilation system at a health facility according to SHTM 03-01, Ventilation for Healthcare

Premises.

Table 3.7 – Noise levels for ventilation system

Room Overall noise level -

NR

Ventilation plant

commissioning - NR

Ventilation plant

design - NR

Ward areas 33 30 30

Circulation areas 50 45 40


4 VAC for Data Centres

4.1 Benchmarking

The power demand of data centres can be up to 50 times the demand of an office space,

with much of this coming from the requirement for cooling. Data centre design is a

relatively new field and new technologies are emerging to reduce the energy consumption of

both computing and cooling equipment.

In order to reduce energy consumption, data must be collected on energy usage which can

be benchmarked and assessed for efficiency. Due to the increase in data centres there has

been a great interest in benchmarking energy use. A popular benchmarking tool is PUE

which stands for Power Usage Effectiveness.

First, comprehensive data regarding the energy consumption of the data centre must be

collected. Best practice is to use sub-meters to monitor the power delivered to the data

centre.

IT Equipment Power includes everything that supports IT equipment load such as:

Computer, storage, network equipment, KVM switches, monitors; and

Workstations/laptops used in monitoring, controlling the data centre.

Total Facility Power includes:

Power delivered to the equipment above including the power losses. Usually data

centres include an UPS, so the power delivered to the UPS must be included;

Cooling system components such as chillers, computer room air conditioning units,

pumps, cooling towers; and

Other miscellaneous loads such as data centre lighting.

PUE performance ranges are noted in Table 4.1.

Table 4.1 – PUE performance ranges

PUE Efficiency & Opportunities

>2.0 Energy Inefficient. Improvement opportunities are likely to give attractive paybacks

1.6-2.0 Good efficiency. The closer the PUE to 1.6 the longer the payback time on investment in energy efficiency

<1.6 High efficiency, purpose built data centres. Energy efficiency improvement opportunities will be fewer with high capital cost and long payback periods

The current trend in the public sector is an increase in computer power and thus electrical

demand. In many cases data centres are housed in non-purpose built rooms, which means

they are characterised by inefficient cooling systems, inefficient lighting and low room

temperature setpoints.


4.2 Data Centre Auditing

Inefficient Data Centres usually have:

Computer room air conditioning (CRAC) units blowing air towards the server racks with

a free airflow in the room;

Hot air from the server racks blended with cold air from the units. This is typical when

no containment is applied;

Air flows through the server racks and not just through the server;

Low temperature setpoints. In many data centres temperature is set down to 16 °C.

Trying to keep the room temperature at this setpoint when there is hot and cold air

blending decreases the efficiency of the refrigeration equipment; and

Refrigeration running all of the time.

4.3 Data Centre Savings

Table 4.2 outlines a list of the technologies that can be applied to reduce energy

consumption in data centres.

Table 4.2 – Opportunities for energy saving

Opportunity Savings Cost/Payback

Aisle Containment Up to 30% Medium/Reasonable

Blanking panel (with containment implemented) Up to 10% Medium/Reasonable

Set point temperature increase by 3°C Up to 7% Low/Quick

Free cooling Up to 50% High/Reasonable

Virtualisation Up to 50% Medium/Reasonable

4.4 Aisle Containment

Greater performance of the cooling system is expected with the complete isolation of the

hot and cold aisles within a structure. There are many different options including either

standard or non-standard containment structures depending on the consistency of sizes and

manufacturers of the racks.

Aisle containment has become an industry standard for data centres. Overall it can reduce

the total air supply required because of the prevention of air blending and it can ensure an

increased return air temperature which in turn increases the efficiency of the cooling

system.

However, in the case of a power failure, contained aisles cannot take advantage of the

building thermal capacity, so the temperature in the racks will increase faster. A

containment system that will be compatible with the fire detection and firefighting system of

the facility also has to be selected. Clearly an assessment has to be performed to of the

balance of benefit and risk.

4.5 Blanking

Even when the server racks are organised in a hot-cold aisle arrangement, there is air

blending occurring before the air enters the rack. This is caused from recirculation of hot air


from the hot aisle and through the racks which mixes with the cold air in the cold aisle. This

effect is more apparent in the higher units of the rack.

4.6 Increase Temperature Setpoint by 3°C

While many newer computing systems have the capacity to run at a higher temperature,

this may not be the case with older systems. This makes this a difficult measure to be

implemented and the IT personnel have to be included in all decisions, along with the

cooling system commissioning company. However, increasing the cooling setpoint will

decrease the cooling load significantly and thus reduce energy consumption.

4.7 Free Cooling

Free cooling has become a data centre industry standard that can offer substantial savings.

Free cooling methods can be implemented in a few different ways and can be very efficient

in colder climates.

Free cooling systems take advantage of the ambient temperature being lower than the

CRAC supply temperature by using the temperature of the surrounding environment to cool

the data centre. By using free cooling, the refrigeration compressor can be by-passed,

reducing the electrical power of the system. The thermal fluids usually used in free cooling

applications are air and water.

Usually free cooling is fully available as long as the ambient temperature is at least 1°C

below the room setpoint temperature.

4.8 Server Virtualisation

Virtualisation is a software implementation involving setting up multiple servers within a

server. This increases the utilisation of each server and decreases the number of physical

servers that have to run simultaneously. This is one of the few opportunities to decrease IT

load.


5 Energy Performance Improvement for VAC

After exploring VAC systems and measures of efficiency, there are several common

practices to consider:

Reduce fan speed;

Upgrade controls and review strategies;

Implement Heat Recovery;

Replace refrigeration equipment with high efficiency units; and

Use multiple chillers and sequencing.

5.1 Fan Speed Control

Affinity laws describe the relationship between various characteristics of fans at different

ratios like rotating speed, air flow, static pressure, density and fan power. Understanding

the relationship between the air flow and the fan power enables a ready assessment of

saving opportunities from using variable speed control. VSD reduce power consumption

according to the affinity laws. VSD alter the electricity frequency and reduce the rotating

speed of the motor.

It is common that fans and pumps are oversized. This is because building utilities are

designed to meet peak loads. However, only rarely do buildings operate at these peak load

demands. Adjusting flow by mechanical means, for example by using dampers in ducts, will

reduce the airflow giving savings in heating or cooling loads, but it does not reduce the

electricity consumption of the fans. Another option is to apply VSD’s instead of using

mechanical means.

5.2 Replace Oversized Motors

As noted, building services systems are usually oversized. The same applies to motors and

drives. Figure 5.1 is indicative of the course of decisions that lead to an oversized motor.

Figure 5.1 - Motor oversizing

Although applying VSD control can be used to match system requirements, this does not

mean that the best possible performance will be achieved. There are two reasons for this,

the standard losses of a VSD because of the power conversions required to control

frequency and the efficiency of the motor.

A motor sized to match to the actual load requirements would operate at its maximum

efficiency point. So, when having to replace a motor you should consider a higher efficiency

one.


There are various standards relating to motors efficiency. For example IEC3 motors have an

efficiency of up to 97%.

The European Union has announced minimum mandatory minimum efficiency requirements

for AC induction motors. From the 1st January 2015, all motors from 7.5kW to 375kW must

meet IEC2 or IEC3 efficiency levels and be driven by a VSD.

5.3 Upgrade Controls

As building load demands vary through the day, so as well as trying to meet a revised peak

load, the demand trends should also be followed.

In order to reduce running costs, a ventilation demand control strategy can be implemented

where humidity, temperature and air quality (CO2) are monitored and controlled to meet

standard comfort conditions.

By monitoring the concentration of CO2 in a room a direct indication of its occupancy can be

gathered and thus the amount of fresh air supplied to the room can be directly controlled.

Implementing air quality monitoring and control can produce great savings in spaces where

there is a varying level of occupancy.

Implementing Demand Controlled Ventilation is relatively easy and effective and can

achieve savings of more than 50%. In the case of all air systems however, demand

controlled air conditioning requires a sophisticated set of control settings. A system with

sophisticated sensors but improper settings might not produce the savings envisaged, as it

may be running at full capacity to maintain a high set-back temperature of an unoccupied

space for example.

Another opportunity is to control air conditioning load demand of an unoccupied area. To

reduce wasting energy in unoccupied areas, PIR sensors can be used to detect presence in a

room. If the room remains unoccupied for some time, then a set-back temperature setpoint

can be implemented which is lower than that needed for occupied spaces. The reduced

setpoint should be estimated in such a way that when the room becomes occupied the

comfort conditions can be achieved in a reasonable time.

5.4 Implement Heat Recovery

Heat recovery has the potential to deliver high energy savings. A run around coil, which

recovers heat from the ventilation exhaust used to preheat the fresh air supply is typically

used in all air systems. More information is available in the Heat Recovery e-module.

To estimate the amount of heat that can be recovered from an extract stream, the following

calculation can be used:

Where:

QE is the airstream load in kW

Qv is the airstream flow rate in m³/s

Tin is the indoor temperature in °C

Tout is the outdoor temperature in °C


If the airflow and the indoor and outdoor temperatures is known then the wasted load is

estimated in kW. Multiplying this figure by the hours of operation gives the total energy

wasted to the environment.


6 Good Housekeeping

Good housekeeping is very important in order to achieve high performance of VAC

equipment. VAC applications involve a variety of systems. A planned preventive

maintenance regime should be created for all this equipment. A full and updated list of the

VAC equipment must be available (i.e. an asset register).

With a simple walk around inspection of your facilities you will be able to identify if these

practices are followed.

HVAC equipment must be insulated. Insulation tends to deteriorate, especially in

outdoors systems, because of exposure to the elements (sun, rain, frost). Please refer

to the Building Fabric Measures e-module to obtain a good understanding of the savings

that can be achieved. This includes piping (water and refrigerants) and supply and

return ducts.

Regularly inspect refrigeration equipment for leaks. Installations with more than 3kg of

refrigerants are required to keep records of the type of F-gas refrigerants and quantities

in each system. The regulations also require regular inspections for leaks. The main

focus of the F-Gas registers is to protect the environment from the emission of HFC

which are Greenhouse Gases (GHG). Gas leaks apart from being harmful to the

environment also significantly reduce the efficiency of refrigeration equipment.

Keep the heat transfer equipment clean. Blocked condensers operate at higher

temperature. An increase of 1°C in condenser’s temperature can increase the required

compressor work by up to 4%.

Assess condenser size. A relatively larger condenser can actually increase the system

efficiency.

Allow unrestricted air movement around the condensing units and cooling towers. Lower

condensing temperatures will be achieved, increasing efficiency. Consider relocating the

condensers if this is not the case.

Check motor drives on fans and pumps. Bearings and couplings with insufficient

lubrication, as well as loose belts, increase the motor transmission losses and reduce

the system efficiency.

Check air filters and replace them. Blocked filters increase the pressure drop across the

system, increasing motor drive consumption and reducing air delivery.

Locate and replace crushed or leaking ducts. Replacing leaks, increases the system

transmission efficiency.

Check BMS settings. Adjust heating and cooling settings to have at least a 3°C

deadband.

Define air flow requirements and adjust air flow.

Interlock heating and cooling controls. In improperly controlled systems sometimes

heating and cooling both operate simultaneously to adjust to the right temperature.

Care should be taken if dehumidification is required.

Audit for oversized fans if the building has been recently refurbished and distribution

ducts have changed.


7 Building the Business Case

Things to consider prior to deciding to upgrade a ventilation and air-conditioning system

are:

Is the system well controlled?

Is the air handling unit beyond its economic lifetime?

Is there heat recovery within the system?

Is the air conditioning system efficient for partial loads?

These questions are similar to the ones you should ask when considering a design brief.

Other information you will need in order to build an informed business case includes:

The activity carried out in the building;

The expected occupancy;

The current air flow rates;

The current specific fan power;

The current controls;

The current scheduling, and

The age of the equipment (AHU, refrigeration, heating).

7.1 Business Case – Case Study

Here is an example of building a business case for the installation of VSD on an air handling

unit. First the installation was audited to give the following information:

Supply - 2.5 m³/s;

Controls - local temperature control for heating battery;

Theatre capacity - 90 persons;

Supply fan rating - 5.5 kW;

Extract fan rating - 3 kW;

Heat recovery existing; and

VSD at 100%.

Energy consumption of ventilation systems and associated opportunities are difficult to

calculate. A number of assumptions based on experience have to be made. However, some

basic calculations from first principles can be undertaken to give an estimate of

consumption.

A calculation for sufficient air supply is required. Assuming 8 l/s/p for sufficient ventilation

then, airflow = 80 x 9 = 720 l/s = 0.72 m³/s. That is an excess of 1.4 m³/s air supply

Applying the affinity laws formula identifies that the drive power for the supply fan can be

reduced to 0.7 kW. For the return fan this revised figure is closer to 0.4 kW. This is a

reduction of almost 88%.

Assuming the unit is scheduled to run for 1,470 hours a year, electricity consumption will be

E = (5.5 + 3) x 1,470 = 12,495 kWh.


Assuming the reduced power demand the electricity consumption would be E = (0.7 + 0.4)

x 1,470 = 1,617 kWh.

Assuming electricity at 10.5/kWh total savings would be (12,495 - 1,617) x 0.105 =

£1,142.20 per year. By comparing this with the cost of upgrade to VSD the simple payback

can be estimated.

The survey also showed that the room is booked for only 300 hours per year. By installing

occupancy sensors up to 970 hours of operation can be saved (a morning start-up has been

included as standard load). The electricity consumption savings are E = (0.7 + 0.4) x 500 =

550 kWh, giving a total savings of (12,495 - 550) x 0.105 = £1,254 per year.

Based on a typical cost to implement BMS and controls upgrade of £7,000 this gives a

simple payback of 5.6 years.

For simplicity the gas heating savings were not included.


8 Useful links and references

Title Source Link

Zero Waste Scotland - www.zerowastescotland.org.uk

Maintenance Engineering and

Management CIBSE

www.bsria.co.uk/information-

membership/bookshop/publication/cibse-guide-m-maintenance-

engineering-and-management

Scottish Health Technical Memorandum

03-01 Part A

Health Facilities

Scotland

www.hfs.scot.nhs.uk/publications/1360855592-SHTM%2003-

01%20Part%20A%20Feb%202013.pdf

AM10: Natural Ventilation CIBSE www.cibse.org/knowledge/cibse-am/am10-natural-ventilation-in-non-

domestic-buildings

A Natural Choice CIBSE www.carbontrust.com/media/81365/ctg048-a-natural-choice-natural-

ventilation.pdf

AM13: Mixed Mode Ventilation CIBSE www.cibse.org/knowledge/cibse-am/am13-mixed-mode-ventilation

Part F: Ventilation Planning portal www.planningportal.gov.uk/buildingregulations/approveddocuments/p

artf

How to implement Air Quality Sensors The Carbon Trust www.carbontrust.com/media/147151/j7972_ctl059_implement_air_qu

ality_aw__interactive.pdf

How to implement heat recovery in

heating, air conditioning and ventilation

systems

The Carbon Trust www.carbontrust.com/media/147119/j7948_ctl030_how_to_impleme

nt_hvac_heat_recovery_aw.pdf

Heating, ventilation and air conditioning The Carbon Trust www.carbontrust.com/media/7403/ctv046_heating_ventilation_and_ai

r_conditioning.pdf

PUE:A Comprehensive Examination of the

Metric The Green Grid

www.thegreengrid.org/en/Global/Content/Books/Book1-

PUEAComprehensiveExaminationoftheMetric

http://www.zerowastescotland.org.uk/

http://www.bsria.co.uk/information-membership/bookshop/publication/cibse-guide-m-maintenance-engineering-and-management/



http://www.hfs.scot.nhs.uk/publications/1360855592-SHTM%2003-01%20Part%20A%20Feb%202013.pdf

http://www.hfs.scot.nhs.uk/publications/1360855592-SHTM%2003-01%20Part%20A%20Feb%202013.pdf

http://www.cibse.org/knowledge/cibse-am/am10-natural-ventilation-in-non-domestic-buildings

http://www.cibse.org/knowledge/cibse-am/am10-natural-ventilation-in-non-domestic-buildings

http://www.carbontrust.com/media/81365/ctg048-a-natural-choice-natural-ventilation.pdf

http://www.carbontrust.com/media/81365/ctg048-a-natural-choice-natural-ventilation.pdf

http://www.cibse.org/knowledge/cibse-am/am13-mixed-mode-ventilation

http://www.planningportal.gov.uk/buildingregulations/approveddocuments/partf/

http://www.planningportal.gov.uk/buildingregulations/approveddocuments/partf/

http://www.carbontrust.com/media/147151/j7972_ctl059_implement_air_quality_aw__interactive.pdf

http://www.carbontrust.com/media/147151/j7972_ctl059_implement_air_quality_aw__interactive.pdf

https://www.carbontrust.com/media/147119/j7948_ctl030_how_to_implement_hvac_heat_recovery_aw.pdf

https://www.carbontrust.com/media/147119/j7948_ctl030_how_to_implement_hvac_heat_recovery_aw.pdf

http://www.carbontrust.com/media/7403/ctv046_heating_ventilation_and_air_conditioning.pdf

http://www.carbontrust.com/media/7403/ctv046_heating_ventilation_and_air_conditioning.pdf

http://www.thegreengrid.org/en/Global/Content/Books/Book1-PUEAComprehensiveExaminationoftheMetric

http://www.thegreengrid.org/en/Global/Content/Books/Book1-PUEAComprehensiveExaminationoftheMetric

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