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How to reduce a buildings energy consumption by - Vabi Software

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How to reduce a building’s energy consumption by up
to 25%, without incurring hardware costs
Energy optimised heating/cooling curves for air-conditioned
office buildings
By Wim Plokker MSc, Vabi Software, Delft, the Netherlands
Bert Elkhuizen BSc, Cofely GDF Suez, the Netherlands
Summary
The top priority for the UK Government’s Construction Strategy is to reduce cost inefficiencies by up to
20% by the end of Parliament; closely followed by their green building goals. The current economic
climate is a both a catalyst and inhibitor for these requirements, with the costs of retrofitting balanced
against the business benefits of greener buildings.
Green building is set to boom on a global level. Retrofit projects are forecast as the principal
1
construction activity for the UK, and the number two globally after commercial new builds.
As well as the long term environmental benefits, green buildings are increasingly regarded as a
business imperative enabling direct cost savings, higher rent, and longer economic lives.
This paper focuses on commercial air-conditioned office buildings, and explains how, both within and
outside of key retrofit cycles it is possible to significantly reduce a building’s energy consumption by up
to 25%, without incurring significant hardware costs. This is achieved through the optimisation of the
heating and ventilation systems (HVAC).
Case studies have also shown the additional benefit of enhanced occupant comfort.
Overview
The Challenge
Typical modern office buildings, including those which are compliant with the Energy Performance of
Buildings Directive (EPBD), have costly and inefficient heating and air conditioning systems.
Thermal comfort within the modern office is determined at two levels – centrally, for the whole building
and locally, at room level by the occupants. Centralised heating and air conditioning systems are
typically optimised for the extremes of winter and summer. Outside of these periods, thermal comfort
is achieved by adjusting the temperature at room level, thus it is commonplace for buildings to be
heated at a central level whilst being cooled by occupants at a room level. The reverse is also true.
This results in unnecessary and wasteful energy consumption.
The Solution
The solution is to match the central supply to the minimum demand by reducing the supply
temperature in winter and raising the supply temperature in summer.
The correct supply temperature can be determined with Vabi Elements software used in conjunction
with a five step methodology to optimise air supply temperatures in order to prevent concurrent
heating and cooling. This results in 5 to 25% less energy consumption for heating and cooling and
enhanced thermal comfort. The method does not require additional investment in HVAC equipment
and can be applied to both new and existing buildings.
Page 1 of 9
Several case studies have shown an energy saving potential of up to 25%. Heating and cooling
demand as a function of the environmental temperature is central to the methodology. For new
buildings, this can be obtained from dynamic building simulation calculations using Vabi Elements
software. For existing buildings, this information comes from building energy management systems.
The methodology determines at which outside temperatures the air supply temperatures can be
lowered in order to prevent simultaneous heating and cooling. The methodology is applicable to most
HVAC systems.
Introduction
HVAC systems are usually designed for performance under full load conditions. Partial load conditions
in the intermediate season (the period outside deep winter and high summer) get less attention. The
design of air handling units is typically undertaken independently of the local heating and cooling
equipment. In many cases standard air supply temperatures are chosen for partial load conditions.
Figure 1.
Optimised vs. traditional
heating/cooling curve.
The blue curve is the standard curve used in many air conditioned buildings. Below 10В°C outside
temperature the supply air temperature is 20В°C or even higher, above 18 В°C outside the air supply
temperatures are 16В°C or lower. The green line indicates where air supply temperatures are equal to
the outside temperatures. The transition range denotes where the building turns over from heating to
cooling. This is described as the turnover temperature.
For well insulated buildings the traditional air supply temperatures will result in excessive energy
consumption during the transition between the cooling and heating period (40% of the year). In this
period there is a demand for heating (centralised air handling unit) and cooling (localised fan coil,
induction or chilled beam) at the same time because of the high air supply temperatures.
For the commonly used standard heating/cooling curve (blue curve in figure 1), the energy supply
does not match with the energy demand. The red curve in figure 1 is an example of an energy
optimised supply curve. Figure 2 shows the energy (heating and cooling) demand as a function of the
outside temperature for a typical office building.
Page 2 of 9
Figure 2.
Heating and cooling demand
as a function of the ambient
temperature
The red and blue dots represent the heating and cooling demand respectively. On the horizontal axis
the outside air temperature is given. On the vertical axis the hourly energy demand is given. During
the transition period (ranging from 5 В°C to 15 В°C) a large number of hours occur for which there a
mismatch in centrally supplied heat, and heat/cooling is required at a local level. The purple dots
indicate concurrent heating and cooling.
Figure 3.
Indoor air temperatures
as a function of the
ambient temperature
Figure 3 shows the resulting indoor temperatures as a function of the outside temperature. The red
area indicates excessively high indoor temperatures (in three classes), the blue area indicates indoor
temperatures which are too low (in three classes). Temperatures of 25 ВєC are not unusual for the
intermediate season. In a number of hours this leads to temperatures well above the heating set point
or even above the cooling set point, which requires cooling energy.
In order to avert this disparity between centrally supplied energy and local energy demand, the
centrally supplied energy should be reduced (lowering the supply temperatures), thus avoiding
excessive energy supply from this source. The optimised heating cooling (red) curve from figure 1
fulfils this requirement.
Page 3 of 9
Figure 4.
Heating and cooling
demand after adjustment
Figure 5.
Indoor temperatures
after adjustment
Figures 4 and 5 show that the extent of concurrent heating and cooling has significantly reduced; the
overheating is also reduced to a minimum.
The energy-optimised heating/cooling curve can be characterised by a turnover temperature (the
outside temperature at which the building is in thermal equilibrium with its environment), and the
transition range. The turnover temperature is dependent on both the building/system characteristics
and the organisational loads (internal heat gains). The size of the transition range is determined by the
variation in internal heat gains (both solar radiation and gains due to people, equipment and lighting).
When the variation increases the transition range also increases. This means the supply temperatures
in the winter will be lower and local heating equipment will buffer the variation in the heating demand.
During the summer supply temperatures will be higher and local equipment will buffer the variation in
heating demand.
The Methodology
Despite many years of optimisation of single components, such as condensing boilers, there is no
common knowledge about how to design a building’s control strategy or how to design the
heating/cooling curve. This was the starting point for the development of this methodology.
The design method is based on the hourly heating and cooling demand within the building, which can
be determined using Vabi Elements software. This program is a dynamic building energy simulation
tool, which is used by almost all leading consultancy firms in the Netherlands. The method is
[ii].
published in the Dutch ISSO publication 68 The methodology has also been applied on existing
[iii],
buildings and is currently integral part of several ISSO publications concerning sustainable facility
[iv]
management
Page 4 of 9
The method is cut into 5 major steps. Figure 6 shows these 5 steps.
Figure 6. Schematic overview of the methodology
Who can use the calculation method?
Customers for whom the design method is suitable for: (1) designers, (2) installers, (3) building
managers, (4) TAB (Testing, Adjusting and Balancing) companies.
Step 1: Collect all necessary data
Step 1 is based on the collection of the necessary �project’ data, which are relevant for the
determination of the Energy Optimised Heating/Cooling Curve. The �project’ parameters/data are
related to (1) the building properties, (2) the type of HVAC system, (3) the building user.
Step 2: Determine the heating and cooling demand as a function of the outside temperature.
Step 2 is the determination of the heating/cooling demand of the building as function of the outside air
temperature. The verification of the heating/cooling demand of the building is a complex analysis and
is based on the use of Vabi Elements software.
Step 3: Determine the basic shape of the new curve
Step 3 is a very important step. In a new design, the designer has to choose the type of HVAC system
and how the required capacity is split over the air and water supply. The air supply should cover the
minimum energy demand for cooling or heating, in the transition range it should follow the outside
temperature plus a temperature rise for fan dissipation. The local water supply should cover the
variation in heating or cooling demand. The basic shape of the energy optimised heating/cooling curve
is determined which provides a description of the heating and cooling production of the HVAC system.
This will be based on the outcome of the calculated heating and cooling demand as outlined in step 2.
Page 5 of 9
The results of step 3 are:
п‚·
The calculated value of the transition temperature (free temperature) of the building. The
transition temperature is the temperature where no heating or cooling is required.
п‚·
The transition range.
п‚·
The sensitivity of the heating and cooling demand in relation to the outside air temperature.
Step 4: Discount other aspects of the HVAC system in the heating/ cooling curve
Discount the energy contribution of the different HVAC components (AHU, fans, heat recovery, etc.)
from the basic shape (step 3) of the heating/cooling curve. In the transition range the concept is that
no heating or cooling is applied to the supply air. Without fan dissipation this would result in outside air
temperatures as desired air supply temperatures.
When the fan dissipation is 1В°C, the air supply set point should be 1В°C above the outside air
temperatures. A number of aspects of the air-conditioning system that affect the heating and cooling
supply have not yet been included. These include heat recovery, which affects the heating and cooling
supply in the air-handling unit.
The use of a heat recovery system in the HVAC system should not conflict with the optimum
heating/cooling curve. It is important to make sure that when a heat recovery system is operated, the
optimum energy heating/cooling curve is not exceeded.
Step 5: Account for other preconditions in the heating/cooling curve
The effect of the physical boundary conditions of the HVAC system should be taken into account. It
gives the limits within which the heating/cooling curve could be defined without causing complaints
from the users of the building.
If the boundary conditions as calculated in this step hinder the right choice of the heating/cooling
curve, it may be necessary that the design conditions of the HVAC system are adjusted. In this case
step 4 must be revisited.
Preconditions to be taken into account are: comfort aspects, condensing risks, characteristics of air
supply grills, humidifying and dehumidifying and draught risks.
Case study
Since the introduction of this method a number of consulting companies have successfully applied the
methodology on existing buildings. For large building complexes the yearly savings for energy
reduction can go up to 100,000 Euro per year. For a subset of these buildings a pre and post
occupant survey was performed.
This example relates to an existing office building (year of construction: 1998). The building
characteristics of the selected building are as follows:
п‚·
U-value building shell: 0.4 W/m²K’Window
percentage exterior (North and South wall):
40 %
п‚·
Gross Floor Area (GFA): 6,895 m2
п‚·
Window system (HR+ glass) U value glass =
1.5 [W/(m2 • K)]: g-value glass = 0.60
Organisation
п‚·
Average internal heat production
workspaces: 35 [W/m2] (including frequency
of use)
HVAC system
Page 6 of 9
22,8 m
14,4 m
68,4 m
п‚·
Heated/cooled ventilation air, heating and cooling by means of chilled beam with integral
heating coil
п‚·
Set point: winter = 22 В°C; summer = 24 В°C
п‚·
Quantity of fresh air per person: 14 l/s per person
2500
2000
782
1500
895
Local
Plant
1000
500
1341
408
208
1004
[kWh]
[kWh]
[kWh]
[kWh]
Heating
Cooling
Heating
Cooling
0
Conventional
Figure 8.
Calculated energy
savings.
299
180
Optimised
Figure 8 shows the calculated energy savings for the reference case. The distributions over local and
central energy demands have changed.
Improved comfort
The comfort improvement is shown in figure 9 by comparing the occurring room temperatures in the
�case’ building to the temperatures specified for the standard heating and cooling curve (in red) and
the energy optimised heating and cooling curve (in blue). The situation with the standard
heating/cooling curve shows that the room temperatures of 24 В°C (set point cooling) occur at an
outside temperature of 4 В°C (figure 9). In the case of the energy optimised heating and cooling curve,
these room temperatures do not occur until an outside temperature of 10 В°C is reached.
High room temperatures in a winter situation will lead to complaints regarding thermal comfort. The
number of comfort-related complaints is reduced considerably when the energy optimised heating and
cooling curve is applied. Figure 10 shows an occupant survey before and after heating/cooling curve
adjustments.
Figure 9.
Improved comfort through
adjusting the heating / cooling
curve
Page 7 of 9
Figure 10.
Occupant survey before
and after adjustments
Integration with Vabi Elements
The method described above is integrated into Vabi Elements. This software automates the process of
transforming the heating and cooling demand profiles into an optimal heating/cooling curve. In the
automated way five runs are performed in batch mode. The first three are runs to generate a building
energy demand profile. In these three runs the internal loads are varied. The energy loads are
transformed into an air supply temperatures that hypothetically would exactly match the energy
demand. A linear regression is performed on this hypothetical air supply temperatures in order to
determine the parameters that describe the optimised heating curve.
The fourth run is completed with this optimised heating and cooling curve. The fifth and final run is
executed with the heating and cooling curve given by the user so that the improvements in energy
consumption and comfort can be determined.
Points for consideration
The use of (standard) heating/cooling curves may contribute to complaints of discomfort and to high
energy consumption. The heating cooling curve should be adjusted to the building and system
characteristics and the building loads. The proposed design method changes the way we look at the
design process of HVAC installations. The designer first determines the heating and cooling demands
and then the matches with the type of HVAC system. The annual profile of the heating and cooling
demand can be used as a strong communication mechanism to explain the principle choices that are
made in the design process.
The adjustment of the heating cooling curve to building characteristics and internal loads also means
that every time a building has a major change in internal heat loads, organisation, insulation, etc., the
heating/cooling curve should be redesigned.
This may also be the reason why after a major retrofit, the full benefit of the refit will not show unless
the heating/cooling curve is redesigned.
Conclusions
This is a proven method that will provide designers, installers, building managers, Testing, Adjusting
and Balancing engineers with guidance on how to adjust set points for air supply temperatures in
order to reduce energy consumption and improve thermal comfort.
Page 8 of 9
The methodology underlines the importance of the optimal tuning of heating/ cooling curves in HVAC
systems. The energy saving potential of optimal heating curves presents an expected saving on total
HVAC energy consumption of 5 – 25 % comparative to common heating curves. The method does not
require any additional hardware and enhances occupant comfort.
References
i.SMART MARKET REPORT - World Green Building Trends,Business benefits driving new and retrofit market opportunities in
over 60 countries, 2013- McGraw Hill Construction, produced in association with United Technologies, and in association with
the US Green Building Council and World Green Building Council
ii. Elkhuizen P.A, Plokker W. et Al, 2002, Energetisch optimale stook- en koellijnen, Rotterdam: stichting ISSO
iii Elkhuizen, P.A., Scholten J.E., Peitsman H.C. and Kooijman A., “The effect of optimal tuning of the Heating-/ cooling curve in
AHU of HVAC system in real practise, Proceedings, ICEBO, Paris, 2004.
iv. Elkhuizen P.A, Plokker W. et Al, 2002, Energetisch optimale stook- en koellijnen, Rotterdam: stichting ISSO
This paper is based on the presentation �Energy optimised heating/cooling curves for HVAC systems in air conditioned office
buildings’ presented by Wim Plokker at CIBSE Technical Symposium, Liverpool John Moores University, Liverpool, UK, 11-12
April 2013
Page 9 of 9
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