UNIVERSITY OF NOTTINGHAM
Building Analysis 2 (K13BA2) Group coursework (Group G) Dainius Alonderis Valentas Rutkauskas Kyriakos Orfanos Xiaofang Xu Jong Hee Paik
[4122017] [4119827] [4114038] [4158955] [4158998]
20/03/2013
Abstract This report focuses on the design of a working HVAC system for the ZEB building in Singapore. Various systems are considered, including VAV and VRF. A variable refrigerant flow system (VRF) was chosen to provide cooling while a separate demand controlled variable air-volume-ventilation system is utilized to provide fresh air. The feasibility of the use a heat recovery system is also investigated giving a significant reduction in ventilation heat gains. The components of the systems are specified and drawings of the system layouts are provided, showing the accommodation of the systems in the building. Control systems are also designed for the optimum and most efficient operation of the HVAC. Finally, an energy consumption evaluation is performed, comparing the results with the energy generated by renewables. Zero energy building status has been achieved for the office building in Singapore. 0
Table of Contents List of Figures ......................................................................................................................... 3 List of Tables .......................................................................................................................... 4 1. System Selection ................................................................................................................ 5 1.1 Key selection criteria .................................................................................................... 5 1.2 Air-Conditioning systems comparison, ranking and final selection............................. 5 1.2.1 VAV ........................................................................................................................ 6 1.3 Feasibility of heat recovery system ........................................................................... 13 1.3.1 Heat recovery system .......................................................................................... 13 1.3.2 Selection of heat recovery system ...................................................................... 13 1.3.3 Possible energy savings ....................................................................................... 14 1.3.4 Energy required to operate heat recovery system ............................................. 14 1.3.5 Final thoughts ...................................................................................................... 15 2. Components Specification ............................................................................................... 16 2.1 VRF System Specification ........................................................................................... 16 2.1.1 The working principle of VRF system .................................................................. 16 2.1.2 VRF System Configuration ................................................................................... 17 2.1.3 Conclusion of VRF system specification .............................................................. 20 2.2 Ventilation System ..................................................................................................... 20 2.2.1 Ventilation System Requirements....................................................................... 20 2.2.2 Component Specification .................................................................................... 22 2.3 Selection of Heat Recovery System ........................................................................... 23 2.4 Psychrometry Analysis ............................................................................................... 24 2.4.1 Sensible Heat Gains ............................................................................................. 24 2.4.2 Latent Heat Gains ................................................................................................ 24 2.4.3 Supply Air ............................................................................................................ 24 2.4.4 Supply and Room Air Moisture Content ............................................................. 25 2.4.5 Psychrometric Charts .......................................................................................... 25 2.4.6 Heat Removal by Cooling Coil ............................................................................. 26 3. Accommodation of the Systems ...................................................................................... 27 1
3.1 VRF System Accommodation ..................................................................................... 27 3.1.1 Selection Criterion ............................................................................................... 27 3.1.2 Selecting Outdoor VRF Units ............................................................................... 27 3.1.2 Selecting Indoor VRF Units .................................................................................. 29 3.1.3 Verification of VRF Systems Accommodation ..................................................... 31 3.2 VAV System Accommodation .................................................................................... 32 3.2.1Ventilation System Accommodation and Layout................................................. 32 3.2.2 Duct Sizing for Ventilation System ...................................................................... 33 3.2.2 Fan Sizing................................................................................................................. 36 4. Controllability .................................................................................................................. 38 4.1 Range of Operating Conditions .................................................................................. 38 4.2 The Parameters of the Control System ...................................................................... 39 4.2.1 VRF Multi-Split System ........................................................................................ 40 4.2.2 VAV Air Supply Control System ........................................................................... 45 5. Annual Energy Consumption ........................................................................................... 48 5.1 VRF Air Conditioning Electricity Consumption ........................................................... 48 5.2 Equipment and Lighting Electricity Consumption ..................................................... 48 5.3 Total Electricity Consumption .................................................................................... 49 5.4 Potential of Renewable Energy Utilisation ................................................................ 49 References ........................................................................................................................... 51
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List of Figures Figure 1 Maximum acceptable dimensions for a VRF system ............................................. 10 Figure 2 Office building dimensions for the maximum potential VRF system pipework .... 11 Figure 3 Moisture recovery efficiency ................................................................................. 13 Figure 4 Example of refrigeration cycle ............................................................................... 16 Figure 5 VRF system with multiple indoor evaporator units............................................... 17 Figure 6 Example of a VRF system and its components ...................................................... 18 Figure 7 Typical layout of a VRF system .............................................................................. 18 Figure 8 VRF separation tubes and headers ........................................................................ 19 Figure 9 Example of simple extract-and-supply ventilation system with heat recovery .... 21 Figure 10 Basic layout of a VAV system with heat recovery ............................................... 22 Figure 11 VAV supply diffuser ............................................................................................. 23 Figure 12 Schematic representation of ventilation processes ............................................ 26 Figure 13 Specifications of outside condenser units provided by Fujitsu ........................... 28 Figure 14 Maximum VRF system dimension provided by Fujitsu ....................................... 29 Figure 15 VRF indoor unit - 4-way ceiling cassette ............................................................. 30 Figure 16 Specifications of VRF 4-way ceiling cassettes provided by Fujitsu ...................... 30 Figure 17 Ventilation system index runs on the ground floor ............................................ 33 Figure 18 Schematics of index runs ..................................................................................... 34 Figure 19 Elements of control system ................................................................................. 40 Figure 20 Electronic expansion valve .................................................................................. 42 Figure 21 Control schematic diagram of VRF system 1 ....................................................... 44 Figure 22 Control schematic diagram of VRF system 2 ....................................................... 45 Figure 23 Control schematic diagram for VAV system ........................................................ 47 Figure 24 Dimensions of Sunpower PV panel ..................................................................... 50
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List of Tables Table 1 Weight and rank method for comparing air-conditioning systems ......................... 6 Table 2 Energy and electricity savings by heat recovery system ........................................ 14 Table 3 Energy consumption due to pressure drop in heat recovery system .................... 15 Table 4 Heat gains taking into account heat recovered ...................................................... 24 Table 5 Latent heat gains..................................................................................................... 24 Table 6 Required volume flow rate for ventilation and proportion of fresh air ................. 25 Table 7 Supply and room moisture contents ...................................................................... 25 Table 8 Heat removal by cooling coils ................................................................................. 26 Table 9 Measurements of available building spaces for systems accommodation ............ 31 Table 10 Pressure drop in supply index run ........................................................................ 35 Table 11 Pressure drop in extract index run ....................................................................... 36 Table 12 Design requirements for comfort ......................................................................... 38 Table 13 Maximum cooling loads on hottest day in Singapore .......................................... 38 Table 14 Temperature sensors ............................................................................................ 41 Table 15 Humidity sensors .................................................................................................. 42 Table 16 Pressure sensors ................................................................................................... 43 Table 17 CO2 sensor ............................................................................................................. 46 Table 18 Electricity requirements for cooling ..................................................................... 48 Table 19 Equipment electricity consumption...................................................................... 49
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1. System Selection 1.1 Key selection criteria In order to choose an air-conditioning (A/C) system, the main requirements needed to be set first. For this particular building, a good control of indoor comfort conditions is required as there are different zones with varying occupancies. The office building has been divided into separate zones each represented by actual floors, and a precise control of the temperature and humidity within them is important. Moreover, there is a high humidity of the air in Singapore. Therefore, an air conditioning system incorporating efficient dehumidification function is essential and will be considered. Most importantly, the A/C systems should also provide a suitable indoor comfort as energy efficiently as possible. There is no need for winter heating thus an A/C which is designed and/or work properly with cooling loads would be considered first. Finally, the spaces available for installation of building services will be evaluated in order to choose the right system in terms of dimensions and applicability. There are ceiling voids and plant rooms in each floor which can be used for ducting, chiller and air handling. Thus, no great limitations in the spaces available are present for the installation of central plant and duct work.
To sum up, the key A/C system selection criteria are as follows:
Zone control
Dehumidification
Reliability and energy efficiency
High and efficiency cooling capacity
Space availability
1.2 Air-Conditioning systems comparison, ranking and final selection Based on the main requirements, three A/C systems were considered: variable air volume (VAV), variable refrigerant flow (VRF) and fan coil units (FCU). Chilled ceiling and chilled beams systems were not taken into the account as they do not provide humidity control and may cause problems such as insufficient cooling and water condensation. Evaporative cooling was also not considered as it is relatively unsuitable for humid climates. 5
A rank and weight method was used to identify the most suitable system taking into account various criteria and their level of importance. The most important criterion is the energy efficiency. As you can see from the table 1, VRF system got the highest score but VAV falls just slightly behind. Thus, a further appropriate analysis of both systems, defining the key operational characteristics and applicability for the office building in Singapore, is required. This would identify the key strengths and weaknesses of each system and therefore a final reasoned selection of an AC system could be reached. Table 1 Weight and rank method for comparing air-conditioning systems Criteria
Rank
VAV
Small plant space
6
5
30
7
42
6
36
Small ducting space
6
5
30
8
48
7
42
Stable control
7
6
42
8
56
7
49
Responsive control
7
6
42
9
63
5
35
Low capital cost
7
7
49
9
63
6
42
Efficiency
10
7
70
10
100
7
70
Low complexity
8
6
48
8
64
8
64
Low noise
6
7
42
6
36
5
30
Incorporates provision of fresh air
8
10
80
0
0
5
40
Easy condensate removal
5
9
45
4
20
4
20
Total
VRF
478
FCU
492
428
1.2.1 VAV The fundamental principle of VAV system is that it can produce both cooling/heating and ventilation to the building through the ducts and properly designed air handling and damper units. It is highly suitable for all year round cooling loads and may provide a sufficient control of indoor building conditions. Moreover, the energy consumption of novel VAV systems can be rather small and thus low energy as well as good comfort solutions could be achieved.
However, there are disadvantages associated with VAV systems. First of all, the space requirements for the VAV system are rather large. This is mainly due to the dual function where both cooling and ventilation is provided by a particularly designed air volume flow 6
rate through the ducts. Thus, as the supply air rate can be extensive due to the requirements of air conditioning in the building, the ducts may accommodate large space area. Moreover, the central or split VAV air handling units as well as chiller would occupy substantial area of the available room. Secondly, the VAV system energy losses are noticeable. The energy could be lost due to the large surface area of the ducting which carries substantially lower temperature medium. In addition, the higher the supply air requirements for desirable comfort conditions, the larger pressure drop is likely to occur in the system. This is mainly due to the pressure drop which increases exponentially with rise in air velocities. Thus, there are limitations associated with VAV system accommodation and running efficiency in the building where the available room space and system energy losses are factors to be considered.
However, one of the key possible problems with VAV systems is that ventilation rate might not be sufficient if cooling load reduces significantly. Because of that separate ventilation system might be needed, but that would increase complexity and cost of the system. Therefore, a further analysis was carried out in order to check if the VAV system and predetermined office building conditions would not cause any problems of insufficient ventilation. The procedure of the evaluation and results are presented in the following section of the report. 1.2.1.1 Fresh air supply in VAV system
It was investigated if VAV system would provide enough fresh air for the office building even when the cooling loads reduces to the minimum. It is inevitable to have cooling load when people are present in the building; and people also require fresh air. When heat gains from each person are known, volume flow rate for cooling can be found and compared to the fresh air requirements.
It is usually assumed that each person emits 100W of heat, hence it is the amount of cooling required to offset heat gains from people. The calculations of volume flow rate are shown below.
Supply air conditions (15
and 95% RH)
Air conditions were found using psyhrometric properties calculator 7
(http://www.sugartech.co.za/psychro/index.php).
Qs = M x s x (Tr-Ts) Qs- sensible heat gains, kW M - mass flow rate, kg/s s - specific heat of air, kJ/ kg K Tr - room air temperature, °C Ts - supply air temperature, °C Hence mass flow rate can be found: M = Qs/(s x (Tr - Ts)=0.1/1.01(27-15) = 0.008251 kg/s Volume flow rate required: V = M x v = 0.008251 x 0.828745 = 0.00684 m3/s = 6.8 l/s V - volume flow rate m3/s v - specific volume m3/kg
Above calculations take into account only heat gains from people. However, other internal heat gains from equipment and lighting are associated with people, hence it needs to be linked with occupants. The lowest equipment and lighting heat gains are in the first floor (library), with a value of 140W. Thus, it would result in another 9.52 l/s of air required to cool the space. In total 16.32 l/s of air is required to remove the heat from people and associated equipment and lighting.
To sum up, in order to remove the heat from one person and associated equipment 16.32 l/s of air at decided supply conditions is needed, in the zone with lowest internal heat gains. That is more than sufficient to allow required ventilation of 10 l/s per person. Thus, it can be concluded that VAV system would provide enough fresh air even if the total cooling load reduced considerably.
1.2.1.2 VAV conclusion The key advantage of the VAV system is that it can provide a good control of the internal comfort conditions for the various building zones. It is also competently suitable for 8
cooling and incorporates both ventilation and air conditioning. Moreover, the system is thought to be complex and a proper design of the operation and accommodation could provide efficient energy solution. Moreover, the problem of insufficient supply of fresh air was evaluated for the VAV system in the office building. It has been concluded that this would not cause any issues because the heat gains are highly linked to the people. Thus, although the VAV system may require large space for installation and there are likely system energy losses associated, it could provide a reasonable AC solution for the building. 1.2.2 VRF
Variable refrigerant flow system is essentially a multi-split system of an outdoor condenser unit connected to a number of evaporators - cooling units located in the building. The working principle is basically based on a refrigeration cycle where the cooling/heating output can be controlled by variable expansion valve and compression work. In addition, the system can produce both cooling and/or heating by reversing the refrigeration cycle. However, the VRF system is different from VAV in a way that it does not provide both cooling/heating and ventilation. It has a capability only to provide either heating or cooling for the building. Thus, a supplementary ventilation system would be required in order to supply a sufficient and if needed, variable amount of fresh air.
The key advantages of the VRF systems are relatively low space requirements, high and precise responsiveness and controllability of indoor comfort conditions, and high cooling efficiency. Moreover, they produce a good dehumidification and can be properly designed for various zone conditions.
However, there are also disadvantages associated with VRF systems. As it has been mentioned before, the VRF is basically an air conditioning system which only produces a required, precise cooling or heating. Thus, another system would be required to be installed separately providing conditioned or unaltered fresh air to the building which in turn increases the energy demand and costs. Moreover, the output of VRF system is highly dependent on the refrigerant pipe lengths. An excessive pipe length can cause too high pressure drop and the capacity of the farthest evaporator unit would be dramatically reduced causing insufficient indoor environment control. Therefore, it was necessary to check the building dimensions and find a potentially longest pipe path from the condenser 9
to the evaporator units. Thus, the evaluation has been performed and is presented in the following section together with compliances and guidance for maximum allowed distances via VRF systems. 1.2.2.1 Verification of the VRF system suitability (dimensions)
Although the maximum allowed refrigerant pipe lengths for the VRF system varies with different manufacturers, the guidance for the maximum measurements between the various units was used provided by ASHRAE. Figure 1 below shows the suggestive pipe lengths for diverse system layout.
Figure 1 Maximum acceptable dimensions for a VRF system
Thus, according to ASHRAE, the maximum pipe distances are as follows:
The maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is 164 ft - 50m;
The maximum permissible vertical distance between two individual indoor units is 49 feet - 15m;
The maximum overall refrigerant piping lengths between outdoor and the farthest indoor unit is up to 541 ft - 165m.
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However, the likely maximum refrigerant pipe length between the condenser and furthest evaporator units could be roughly represented by the dimensions of the office building as shown below in figure.
Figure 2 Office building dimensions for the maximum potential VRF system pipework
According to the measurements, the maximum pipe length encountered would be approximately 6 + 18 + 54.5 = 78.5m. Thus, the size of the building where VRF may be installed is sufficiently small and any possible system inadequacy may be avoided. However, before selecting an exact VRF system the compliance with manufacturer data and the office building would be checked in order to make sure the longest path of refrigerant pipe is sufficiently short.
In conclusion, the VRF disadvantage in term of limited pipe lengths should not be a problem for the AC system design in the building located in Singapore. 1.2.2.2 VRF Conclusion The key strengths and weaknesses were identified for the VRF systems. Those are the minimum space requirement, sensible control of building zones and good energy efficiency. However, the key disadvantages are that the system only provides cooling/heating and separate ventilation system would be required. Thus, an air conditioning provided by VRF 11
would be satisfactory for the building in terms of low running costs, system accommodation and its fast precise response for cooling. 1.2.3 Final selection between VAV and VRF for full air conditioning system As both VAV and VRF systems were found suitable for the air conditioning in Singapore, the key selection is to be based on the total energy efficiency, comfort and weight and rank results. According to the various literature sources, the total energy demand for air conditioning and ventilation is lower for the VRF system compared to VAV. The total energy reduction of up to 50% could be achieved, though the initial cost for installing VRF may reach 20% in excess to that of VAV. Moreover, the controllability and reliability of VRF systems are primary compared to VAV air conditioning systems. VRF systems use variable compressors which can maintain a precise temperature control within 0.6 degree C range. Therefore, based on the key assumptions and operation characteristics, the VRF system was chosen for the office building in preference to VAV.
However, the ventilation system was needed to be decided for use in conjunction with VRF. The fresh air requirements are directly linked to the people, thus a precise control of fresh air supply rate would be instrumental for minimising the total energy demand. The VAV system is suitable for such situation and therefore was selected. It is going to be installed separately from VRF providing the necessary amount of fresh, conditioned air for the particular building zones. Moreover, the heat recovery system can be included in the air handling unit and will be considered in the later sections of the report.
Thus, the two systems have been selected to provide HVAC for the office building. The cooling will be provided by VRF and ventilation with VAV systems separately. The option of using two systems was highly dependent on the overall energy efficiency and comfort. Thus, the lower energy demand and better comfort levels inside the building zones would be achieved due to less amount of energy being used for cooling and/or ventilation.
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1.3 Feasibility of heat recovery system 1.3.1 Heat recovery system Heat recovery system for HVAC is a system that transfers heat from the exhaust ventilation air to supply air. In cool weather exhaust air preheats supply air while in hot weather exhaust air pre-cools supply air. Heat recovery systems reduce the energy requirements for treating supply air as it doesn’t need to be cooled or heated that much. The technology is extremely beneficial in temperate climates, where temperature difference between inside and outside usually remains quite high. However, in Singapore the temperature difference between inside and outside is relatively small and mainly humidity needs to be reduced from incoming air. Thus, it was decided to investigate how efficient heat recovery system could be in Singapore. 1.3.2 Selection of heat recovery system According to Hoval “Rotary Heat Exchangers for Heat Recovery in Ventilation Systems” heat recovery systems with sorption wheel can have moisture heat recovery of up to 83% even when condensation potential is negative. Condensation potential - moisture content difference between saturated colder air and warmer air. For the building in Singapore condensation potential is remains around -4 g/kg throughout a year. Hence sorption wheel is the best suited option that could provide significant moisture recovery rate.
Figure 3 Moisture recovery efficiency
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1.3.3 Possible energy savings Possible energy saving from heat recovery system can assumed to be equal to all the heat gains from ventilation throughout year (found in BA1) times efficiency. Electricity saving could be approximated to be around three times smaller compared to energy savings (because coefficient of performance of cooling system is likely to be around 3). Calculations for annual savings are shown in Table 2 below.
Energy savings = (Ventilation heat gains) x (Heat recovery system efficiency) Electricity savings = Energy savings / COP Table 2 Energy and electricity savings by heat recovery system
Total
Ventilation
Ventilation
Possible savings
Possible
cooling
percentage, %
heat gains,
with heat recovery
electricity
MWh
system efficiency
savings, with
of 75%
COP of 3, MWh
load, MWh GF 277.3
37.4
103.7
77.8
25.9
FF
127.3
19.4
24.7
18.5
6.2
SF
175.3
14.2
24.9
18.7
6.2
1.3.4 Energy required to operate heat recovery system
Even though heat recovery reduces heat load, it comes with an expense of increased pressure drop along the system, hence bigger load for a fan and more electricity use. Ventilation flow rate remains the same throughout a day as occupation is the same, hence energy requirement for ventilation rate remains the same and energy required can be simply found from pressure increase and known volume flow rate. According to ‘Rotary Heat Exchanger for Heat Recovery in Ventilation Systems’ by Hoval pressure drop because of rotary heat exchanger varies between 80-130, value of 100 Pa was used for further calculations. Typical efficiencies for fan power - 70%, electrical motor power - 90% as were provided in Building Services Design 3. Assumed occupation is throughout a year 11 hours a day from 7am to 6pm. Thus, increased electricity consumption due to bigger pressure drop can be found using the equations below and are represented in Table 3.
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Air power = (Pressure drop) x (Volume flow rate) Fan power = Air power / Efficiency Electrical power = Fan power/ Efficiency Energy consumption throughout a year = (Electrical power) x (Operation time) Table 3 Energy consumption due to pressure drop in heat recovery system
Air flow
Pressure
Air
Fan
Electric
Electricity
rate (m3/s)
drop, Pa
power, W
power, W
power, W
consumption per year, MWh
GF 3.37
100
337
481
535
2.15
FF
1.01
100
101
144
160
0.64
SF
1.01
100
101
144
160
0.64
1.3.5 Final thoughts
It can be seen that if electricity consumption by fan is compared to energy saving by heat recovery system it only accounts for around 10%. However, few things need to be noted:
Bigger saving would be possible with bypassing the air, when outside air is cooler/less humid than the inside air.
The same assumptions for occupation were used for both energy savings and energy requirement to operate heat recovery system, hence the proportion between increased electricity consumption and energy savings is likely to remain very similar with varying occupation.
It is assumed that our building is not sensitive for cross-contamination because thermal wheel does not provide total separation between the supply and extract air.
Actual scale of the heat recovery system for the building would be slightly smaller, because extract air from kitchen and possibly cafe could not be used due to higher degree of contamination in the air.
Thus, it was decided to use heat recovery system as it would provide significant energy savings.
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2. Components Specification Specification of the main VAV and VRF system components and parameters has been carried out in order to better understand the layout and operation of HVAC scheme applied for the building. This is an essential part of the report and any assumptions made as well as findings will be used for the systems accommodation and evaluation. Thus, an in-depth investigation has been performed for the both systems separately while explaining the key operational principles.
2.1 VRF System Specification 2.1.1 The working principle of VRF system The operation of VRF system is based on the refrigeration cycle (Figure 4). It has a condenser, evaporator, compressor and expansion valve, all connected with specially designed pipes for a particular refrigerant flow.
Figure 4 Example of refrigeration cycle
However, VRF system is considered as a multi-split system where there are multiple evaporator units provided for the one refrigeration cycle. It also comprises of expansion valves and compressor(s) in order to provide controllability of the system.
Thus, VRF system is basically a very sophisticated system operating on the principles of refrigeration cycle and there are different components and specifications to be defined.
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Therefore, the following section of the report focuses on the VRF systems and their key properties and parameters.
2.1.2 VRF System Configuration Figure 5 below shows a typical layout of VRF system designed for the purpose of cooling the building.
Figure 5 VRF system with multiple indoor evaporator units
As it can be seen, the main system components are:
An outdoor condenser unit which is commonly placed on the roof or mounted on the exterior wall where good outside air cooling is available.
Indoor evaporator units which are installed in the building zones according to the cooling requirements.
An electronic expansion valve (EEV) or pulse modulating valve (PMV) that provide output control of the indoor evaporator unit.
Refrigerant flow pipes. Those are mainly two types: gas pipe and liquid pipe. The liquid is provided for evaporator unit and after the cooling takes place the vapour (gas) is returned to condenser unit through compressor.
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Thus, the VRF system comprises of an outdoor condenser unit which is connected to a number of indoor evaporator units via specially designed refrigerant flow pipes. The cooling capacity is controlled by either the compressor or the EEV/PMV. VRF systems also incorporate separation tubes and/or headers in order to split the system for a number of indoor units (Figure 6). One of the advantages using such splitting components is that each of the evaporator could be controlled separately. Moreover, VRF system extensions could be applied if there are additional header outlets provided. This is beneficial for the building which is likely to be modified or the building zone comfort requirements change.
Figure 6 Example of a VRF system and its components
In addition, the VRF systems can be designed to have a number of condenser units. This is likely to depend on the total cooling demand because a limited number of indoor units can be provided by a condenser. Figure 7 below shows a typical layout of the VRF system designed to use three connected condenser units.
Figure 7 Typical layout of a VRF system 18
Moreover, it can be seen from the above figure that the system operation is not restricted if one of the evaporator units fails. It is due to the property of multi-split system obtained through the separation tubes or headers. Thus, the VRF system could provide a reliable operation; and the cooling output is dependent on the properties of condenser and evaporator units.
Figure 8 represents a configuration of both separation tubes and headers.
Figure 8 VRF separation tubes and headers
The control of VRF system is achieved through the EEV/PMV and compressor. The expansion valve as well as the compression work defines the refrigeration cycle efficiency and capacity. Thus, varying these parameters could alter the cooling capacity and the efficiency of the system. It is complex control system and an appropriate design is necessary to provide sensible operation. Therefore, it is important to make sure that these components are properly selected together with VRF system from various manufacturers’ catalogues.
A key parameter which defines the VRF system capacity is the coefficient of performance (COP) and energy efficiency ratio (EER) which depends on the properties and design of system’s components and operating temperatures. As the design of air conditioning cooling for the office building only concerns the output capacities, the key parameters of 19
cooling and dehumidification will be taken into the account. That is, the VRF system will be selected according to the requirements to maintain indoor comfort levels high. Thus, the manufacturer catalogues will be investigated and the properties of system such as cooling, dehumidification and applicability will be appropriately considered. 2.1.3 Conclusion of VRF system specification VRF system is a sophisticated split system working on the principles of refrigeration cycle. The key components are:
condenser,
evaporators,
complex expansion valves,
compressor(s),
gas and liquid refrigerant pipes and
separation tubes and/or headers
An efficient cooling is achieved by the indoor evaporator units. The air conditioning system is highly reliable and controllable due to the separation tubes/headers and the sophisticated expansion valves and/or compressors. Moreover, the efficiency of the VRF system is based on the COP and EER, and the capacity of evaporator and condenser is to be primarily decided to provide sufficient cooling for the building. Thus, the VRF system is a complex system and specifications of various units and components are important for the reasoned selection. 2.2 Ventilation System 2.2.1 Ventilation System Requirements The air conditioning in our building is provided by a VRF system; hence a separate ventilation system is required to provide fresh air, as the VRF system does not incorporate ventilation. Mechanical is the obvious choice between mechanical or natural ventilation because Singapore’s humid climate makes the latter choice less feasible. The basic types of ventilation systems are extract only, supply only and extract-and-supply. An extract-andsupply system with heat recovery was decided. This choice was made because this system type allows for better treatment of the supply air as well as increases the efficiency of the 20
system. It also allows us to control the supply and extracting points. The heat recovery is achieved through a heat exchanger which cools and/or dehumidified the warm fresh air with the cool outgoing air. Figure 9 below shows a basic layout of a simple extract-andsupply system with heat recovery.
Figure 9 Example of simple extract-and-supply ventilation system with heat recovery
The fresh air also needs to be filtered in order to avoid pollutants being transferred in the building, compromising the supply air quality. In addition to the heat recovery system, a control system is considered, which monitors the CO2 levels inside the building and regulates the flow of fresh air accordingly. This control system could save energy when certain areas of the building are less occupied or not being used.
Hence, the requirements for the ventilation system are:
Extract and supply system
Heat recovery
Filtering
Control system adjusting air flow rate according to CO2 levels
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2.2.2 Component Specification The diagram below shows the basic layout of the ventilation system (figure 10). Fresh air is driven through the supply duct by a fan. Passing through the heat exchanger, it releases some of its heat and moisture and is then filtered. The fresh air is then supplied to the rooms through supply diffusers. In the final stage, the air is extracted through the extract duct with the help of a fan and after passing through the heat exchanger is released outside. CO2 sensors inside the rooms give feedback to the supply diffusers which regulate the amount of fresh air supplied, according to the CO2 levels. Pressure sensors placed in the supply and extract ducts give feedback to the variable speed drives which regulate the speed of the fans according to the required supply and extract volume flow rate.
Figure 10 Basic layout of a VAV system with heat recovery
The main components of the ventilation system are:
Heat exchanger (heat recovery system)
Supply and extract ducts
Filter
Supply and extract fans
Variable speed drivers
CO2 sensors
Variable-geometry air supply diffusers
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The use of variable-geometry supply diffusers in the terminal units ensures improved control of air velocity for good air flow into the space. Figure 11 shows an example of such supply diffuser. The dumper regulates the area through which air can flow in response to the requirement for ventilation.
Figure 11 VAV supply diffuser
The size of the components in the ventilation system is specified by one parameter and that is air volume flow rate. These include the duct size, fans and air supply diffusers.
2.3 Selection of Heat Recovery System Hoval CARS (Computer Aided Rotary Heat Exchanger Selection) software was used to find the most suitable rotary heat exchanger. Selected models and design conditions can be found in Appendix 1: Rotary Heat Exchangers.
Notes:
Bigger rotary wheels provide smaller pressure drop and higher recovery efficiency. Thus, the option of using biggest rotary wheels was chosen.
Efficiency of both heat and moisture recovery is around 75%.
The same volume flow rate was assumed for both extract and supply. However, there might be differences due to separate air extraction in a kitchen and air density variation due to temperature.
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2.4 Psychrometry Analysis 2.4.1 Sensible Heat Gains
First of all, sensible heat gains were taken into account for each floor. Knowing that heat recovery system could recover up to 80% of heat gains through ventilation (including moisture), it was decided to assume efficiency of 70% to allow additional heat gains from fans and other inefficiencies. Table 4 Heat gains taking into account heat recovered
Ground Floor 1st Floor 2nd Floor
Heat gains, kW 110 39 54
Ventilation gains, kW 60 18 18
Gains taking into account heat recovery, kW 68 26.4 41.4
2.4.2 Latent Heat Gains
Afterwards, latent heat gains were found. It was assumed that only latent heat gains from people occur, while other latent heat gains can be neglected. According to the Engineering Toolbox [http://www.engineeringtoolbox.com/persons-heat-gaind_242.html] latent heat gains from one person in an office is around 55W. Thus, latent heat gains were simply found by known number of people and heat gains from each person. Table 5 Latent heat gains
Floor Ground Floor 1st Floor 2nd Floor
Number of people 337 101 101
Latent heat gains per person, W 55 55 55
Latent heat gains, kW 18.54 5.56 5.56
2.4.3 Supply Air
Amount of air supplied through air conditioning system on each floor were found (method shown in VAV air conditioning system consideration) along with amount of fresh air supplied, total amount supplied to a floor and proportion of fresh air.
24
Table 6 Required volume flow rate for ventilation and proportion of fresh air
Ground Floor 1st Floor 2nd Floor
Fresh air volume flow rate, m3/s 3.37
Heat gains, kW
Volume flow rate from AC, m3/s 4.65
Total volume flow rate, m3/s 8.02
Proportion of fresh air, %
68
Mass flow rate of air from AC, kg/s 5.61
1.01 1.01
26 41
2.18 3.42
1.81 2.83
2.82 3.84
36 26
42
2.4.4 Supply and Room Air Moisture Content
Mass flow rate is the total supplied air to a floor (through air conditioning and
ventilation systems). Latent heat = (mass flow rate) x (Latent heat of vaporisation of water) x (return minus supply air moisture content) Return minus supply air moisture content difference was found from known latent heat gains, mass flow rate and latent heat of vaporisation. Afterwards, room moisture content was found as supply air moisture content is also known and was found from psychrometric chart from set air supply conditions (15 and 95% RH)
Table 7 Supply and room moisture contents
Floor
Ground Floor 1st Floor 2nd Floor
Latent heat gains, kW 18.54
Mass flow rate, kg/s 8.54
Latent heat, [kJ/kg] 2450
Return minus supply air moisture content [kg/kg] 0.00089
Return moisture content [kg/kg] 0.01099
Room relative humidity, % 52
5.56 5.56
3.06 4.29
2450 2450
0.00074 0.00053
0.01085 0.01064
49 48
2.4.5 Psychrometric Charts
Psychrometric charts can be found in appendixes. Schematic representation of air mixing and points are shown in Figure 12 below.
25
Figure 12 Schematic representation of ventilation processes
Where: O - Outside air HR - Fresh air supplied to a room after heat recovery M - All air that is supplied to a room (from AC and ventilation systems) and assumed to be mixed R - Room air S - Supply air
2.4.6 Heat Removal by Cooling Coil
Mass flow rate was found in the previous parts while supply air and mixed air enthalpies were taken from psychrometric charts. From the known enthalpy difference and mass flow rate required heat removal by cooling coils was found.
Table 8 Heat removal by cooling coils
Floor
Mass flow rate, kg/s
Supply air enthalpy, kJ/kg
Mixed air enthalpy, kJ/kg
Enthalpy difference, kJ/kg
Removal by cooling coils, kW
Ground Floor
5.61
41
57.5
16.5
93
1 Floor
2.18
41
57
16
35
2 Floor
3.42
41
56.5
15.5
53
26
3. Accommodation of the Systems System accommodation stage of the report was carried out in order to evaluate the requirements of both the VAV and VRF systems so that a compatible and well-reasoned installation is achieved for maximum comfort and energy efficiency. The systems were selected from the manufacturer’s catalogues according to the energy and ventilation demand of each comfort zone in the building. This included the real likely physical sizes and key operation properties of the components. Knowing the system requirements and parameters to cope with the worst case scenario, the components were evaluated and selected. Moreover, the further assumptions were made for the heat recovery and ventilation systems. 3.1 VRF System Accommodation 3.1.1 Selection Criterion The cooling demand of each comfort zone was evaluated for the worst case scenario in order to select an appropriate VRF system and its components. According to the results from the psychrometric and building energy analysis, the maximum cooling demand is approximately 181 kWh. However, the building zones/floors were investigated separately and the maximum cooling loads of each were defined as follows:
Ground Floor Zone - 93 kWh.
First Floor Zone - 35 kWh.
Second Floor Zone - 53 kWh.
3.1.2 Selecting Outdoor VRF Units
The various manufacturers and suppliers were investigated in order to choose the most efficient and suitable VRF system. According to the findings, Fujitsu provides a top-class VRF system configuration for the maximum efficiency and reliability. However, the maximum cooling output provided by a single VRF system loop was found to be limited at around 125 kWh. Therefore, it was necessary to consider at least two separate VRF systems for the air conditioning of the building.
27
Due to the nature of subdivision of the cooling loads in the building, it was decided to provide two VRF systems for the ground floor and first and second floors respectively. Thus, the selection criterion of two separate systems has been based on the two cooling demands: 93 kWh and 88 kWh (35 kWh + 53 kWh).
The Fujitsu Airstage V-2 outdoor condenser units were considered suitable as they can deliver first-class energy efficiency and reliability. Figure 13 shows the technical data of two condenser units provided by Fujitsu.
Figure 13 Specifications of outside condenser units provided by Fujitsu
28
Thus, the two groups of condenser units providing 95.9 kWh and 89.4 kWh of cooling have been chosen for the VRF systems of the ground floor and first and second floors respectively. Those systems can also supplement up to 48 indoor units and have relatively high efficiencies - EER at approximately 3.6. Moreover, the dimensions are relatively moderate, thus the condenser units could be placed on the roof without any substantial restrictions.
However, before the final decision on the condenser units, it was necessary to check the system’s compliance with our building dimensions. Figure 14 below shows the maximum refrigerant piping lengths that the Fujitsu Airstage V-2 can sustain.
Figure 14 Maximum VRF system dimension provided by Fujitsu
Thus, as our building dimensions are fairly lower (see Section 2 of the report), the Fujitsu VRF outdoor units are suitable and any insufficiencies of the piping system could be avoided. 3.1.2 Selecting Indoor VRF Units
There is a number of different VRF indoor units available on the market that could be selected in preference to comfort and sufficiency required. However, the Fujitsu four-way ceiling cassette was chosen to be most suitable for the purpose of cooling the office 29
building (figure 15).
Figure 15 VRF indoor unit - 4-way ceiling cassette
It can provide a uniform air flow and temperature distribution inside the conditioned building zones. Moreover, the space requirements are moderate and a range of cooling capacities are available for selection. The figure below represents the key specifications of the various four-way ceiling cassettes provided by Fujitsu.
Figure 16 Specifications of VRF 4-way ceiling cassettes provided by Fujitsu
The capacities of ceiling cassettes have been selected according to the cooling requirements in each building floor. The selection criterion has also been linked to the layouts of the VRF systems which were provided so that a good temperature distribution is achieved. Thus, the layout of the VRF system and its components was produced and is shown in HVAC drawings provided at the end of report, also identifying the indoor evaporator capacities.
30
3.1.3 Verification of VRF Systems Accommodation Since the key components of VRF system have been chosen, it was necessary to check and validate that they can be properly accommodated in the building spaces available. The building has ceiling voids, roof space and plant rooms which can be used for installation of building systems. Therefore, the dimensions were evaluated and compared to the space requirements of each VRF system component. Table 9 below shows the key building measurements considered for a validation. Table 9 Measurements of available building spaces for systems accommodation
Building Space
Measurement
Ceiling Void Height
75 cm
Plant Room Area (Ground Floor)
52.7 m2
Plant Room Area (First Floor)
24.2 m2
Plant Room Area (Second Floor)
52.2 m2
Roof Space (Loft) Height
2.5 m
According to the results, the VRF systems can be properly accommodated:
The condenser units are to be placed on the roof in order to provide air cooling and sound insulation. As the height of condenser units is 1.69 m they can be suitably installed.
The indoor 4-way ceiling cassettes will be installed in the ceiling voids. As the maximum height of indoor unit would reach only 28 cm, it is well below limits. Thus, ceiling cassettes could be sufficiently accommodated.
Moreover, the refrigeration pipes will be provided from the roof condenser units down to the vertical loops of indoor evaporators. Thus, it was necessary to provide vertical shaft-like structure for carrying pipes. This has been provided by the plant rooms which are relatively on the top of each other. Thus, the openings could be drilled to provide a path for refrigeration pipework. In addition, as the pipes of VRF system are low in diameter, they will easily fit the ceiling voids.
The layout has been provided to show the relative position of the outdoor and indoor VRF units and refrigeration pipework in the office building (see HVAC drawings).
31
Thus, a VRF system for the ground and first and second floors has been properly accommodated. The layouts and key dimensions of the VRF system components and building spaces were investigated and evaluated. It has been proven that the VRF air conditioning systems could be installed using the spaces provided by building roof, ceiling voids and plant rooms. Moreover, appropriate structural considerations were taken into account for accommodation of refrigeration pipework.
3.2 VAV System Accommodation 3.2.1Ventilation System Accommodation and Layout
The availability of an AHU room on each floor allowed for the use of a separate air handling unit for each floor. This gives a simpler design which consumes less energy, as it avoids the vertical transfer of air between floors and gives the shortest duct length possible. The air inlet was placed on the external wall of the AHU room. This provides the shortest route but it should be assumed that the inlet is placed as high on the wall as possible, in order not to affect the people passing through the corridor. The air outlet was placed on the south wall. It is placed several meters away from the east and west side to prevent the contact of the extracted air with occupants. It also provides the shortest route possible. Both the supply and extract units and ducts were placed on the ceiling, as there is thought to be enough space in the void to accommodate the system. Supply diffusers are placed along the east and west side of the ceiling close to the windows, where the building has the most heat gains. Three-way diffusers were used which insure even distribution towards the central area. The extract points were placed along the centre of the ceiling. The distance between the supply and extract points for most of the building area is about 7m which is long enough to insure no short circuiting takes place. The ducting was designed so as not to interfere with the structure of the building. The layout of the ventilation system is provided in the HVAC drawings section of the report. Openings on the internal walls were designed in order to insure good air circulation in smaller areas where the space is not adequate for both supply and extract points. Having only supply air and no extract, the pressure is slightly higher than the rest of the building area, forcing the air to escape the small rooms through the wall openings, thus achieving air circulation. A separate air extraction fan was placed on the south wall of the kitchen on 32
the second floor in order to circulate the air more efficiently and simplify the layout of the system. Having no air supply, the pressure is lower than the main area, which forces fresher air close to supply points to flow through the opening on the internal wall, circulating the air in the kitchen. 3.2.2 Duct Sizing for Ventilation System In order to get an idea about duct sizes and pressure drop, index runs (the path from air handling unit to the furthest extract/supply point) were found. Calculated duct sizes can suggest if ceiling voids are big enough, while pressure drop and volume flow rate define requirements for a fan. 1. Index runs, both supply and extract on the ground floor were chosen as represented
on building layout as seen in Figure 17, while the schematic representations for extract index run and supply index run can be seen in Figure 18.
Figure 17 Ventilation system index runs on the ground floor
33
Figure 18 Schematics of index runs
2. Fresh air supply rate for the ground floor was known. It was split between zones
according to the area and it was assumed that each supply/extract point in a zone provides the same amount of airflow. 3. Velocities in ducts were chosen: 7 m/s for main ducts and 5 m/s for branches.
CIBSE Guide B, 2005, Table 3.2 suggest using 6.0 m/s for branches and 7.5 m/s for mains ducts in general offices. However, lower velocity reduces noise and reduces energy consumption (by both reducing fan power required and heat gains from the fan). 4. Diameter of the ducts and pressure drop per unit length were found using duct
sizing chart. 5. Pressure drop due to friction in ducts was found by multiplying known length of a
duct and pressure drop per unit length. 6. Total friction pressure drop was found by adding up all the pressure drops in the
index run.
Afterwards pressure drop due to the components was found. 1. Velocity pressure was found using formula: 0.5ρv2. ρ - density of air, v - known velocity in the duct. 2. Component loss coefficients were determined. 0.15 for branch fittings, 1.0 for exits, 1.7 for 90* branches and 0.67 for mitre bends. 3. Pressure drop due to components was found by multiplying velocity pressure and component loss coefficient. 34
4. Total pressure drop due to component was found by adding up all the pressure drops in components along index run.
Total pressure drop in the index run was found by summing up the total friction and total component pressure drops. All the above steps are represented in Table 10 and 11.
Table 10 Pressure drop in supply index run Section
Volume Flow rate, m3/s
Velocity, m/s
diameter, m
Pressure drop per unit length , Pa/m
Length, m
Pressure drop due to friction, Pa
AB Branch B BC Branch C CD Branch D DE Branch E EF Branch F FG Branch G GH Branch H HI Branch I IJ Branch J JK Outlet K Total
3.36
7
0.8
0.58
8
4.64
1.33
1.33
1.33
1.12
0.98
0.82
0.66
0.44
0.22
5
5
5
5
5
5
5
5
5
0.6
0.6
0.6
0.52
0.5
0.46
0.43
0.35
0.25
0.49
0.49
0.49
0.5
0.58
0.65
0.7
0.9
1.5
4.4
10.6
2
7.5
2.8
6.1
2.8
7.3
8.2
35
Velocity pressure, Pa
Pressure drop per fitting, Pa
1.7
29.4
49.98
0.67
15
10.05
0.67
15
10.05
0.15
15
2.25
0.15
15
2.25
0.15
15
2.25
0.15
15
2.25
0.15
15
2.25
0.15
15
2.25
1
15
15
2.16
5.19
0.98
3.75
1.62
3.97
1.96
6.57
12.3
43.139
Total pressure drop in supply index run 141.6 Pa
Comp. loss coefficient
98.58
Table 11 Pressure drop in extract index run Section
Volume flow rate m3/s
Velocity, m/s
diameter, m
AB Branch B BC Branch C CD Branch D DE Branch E EF Branch F FG Branch G GH Branch H HI I outlet Total
3.36
7
0.8
2.66
2.31
1.96
1.64
1.32
0.88
0.44
5
5
5
5
5
5
5
0.8
0.75
0.72
0.68
0.58
0.5
0.33
Pressure drop per unit length Pa/m 0.58
0.32
Length, m
Pressure drop due to friction
4
2.32
5.8
0.34
6.4
0.38
3
0.45
6.1
0.47
3.2
0.58
6
0.9
8.9
Comp. loss coefficient
Velocity pressure, Pa
Drop per fitting, Pa
1.7
29.4
49.98
0.15
15
2.25
0.15
15
2.25
0.15
15
2.25
0.15
15
2.25
0.15
15
2.25
0.15
15
2.25
1
15
15 78.48
1.856
2.176
1.14
2.745
1.504
3.48
8.01 23.231
Total pressure drop in extract index run 101.7 Pa
Conclusion for duct accommodation Even though the biggest diameter of a duct is 0.8m, the height of ceiling void available is approximately 0.75m. It can be easily solved by either increasing velocity in the duct (8 m/s velocity would require duct of only 0.65m) or splitting branches in the air handling unit room. Moreover, first and second floor have more than three times smaller volume flow rates, thus accommodation of ducts should not be a problem. 3.2.2 Fan Sizing Air power required to run a fan is equal to pressure difference across the fan multiplied by volume flow rate. However, the electrical power required to actually run the fan is higher, as fan efficiency could be approximately 80% and efficiency of a motor could have efficiency around 95%. Thus, electricity requirement for a fan is equal to the required
36
pressure increase multiplied by volume flow rate and divided by efficiency. Moreover, pressure drop in the heat recovery unit (around 100 Pa) is incorporated into the calculations.
Supply: Electrical power required = [3.36 x (101.7+100)]/(0.8*0.95) = 891.7 W
Extract: Electrical power required = [3.36 x (141.639+100)]/(0.8*0.95) = 1,068.3 W
37
4. Controllability The use of control systems not only keeps the building in comfortable conditions, but also operates systems efficiently and maintain safe environment in various unforeseen circumstances. A good control system can reduce the use of energy, as well as the operating and life cycle costs of the building systems. Thus, it can have great effect on the building performance: HVAC and lighting systems. 4.1 Range of Operating Conditions
The temperature and ventilation rate requirements for the building are listed in Table 12. The comfort temperatures range from 21-25 ° C and a minimum of 10 l /s of fresh air is required per person. In the worst case scenario, a maximum of 16.32 l/s of fresh air will be supplied. Table 12 Design requirements for comfort
Zone
Temperature (°C)
Ventilation Rate
Office
22-24
10 l/s/p
Exhibition Hall
21-23
10 l/s/p
Café (bars/lounge)
22-24
10 l/s/p
Reading Room
24-25
10 l/s/p
Book Storage Space (library lending desk)
21-23
10 l/s/p
Toilets
21-23
> 5ach
Table 13 shows the total cooling loads and ventilation heat gains for each floor on the worst case scenario, which give a total of 135.8kW with heat recovery. Table 13 Maximum cooling loads on hottest day in Singapore
April 22nd
Cooling load (kW) Ventilation heat gains (kW) Total load with heat recovery (kW)
Ground floor 110
60
68
First floor
39
18
26.4
Second floor
54
18
41.4
96
135.8
Total 203
38
4.2 The Parameters of the Control System Building Management System (BMS) is a stand-alone computer system that monitors, controls and optimises the operation of the facilities and systems in the building. This allows the building to have good control of the internal conditions, increased comfort levels for occupants, effective monitoring of energy consumption, and saves time and money during overall maintenance.
A BMS normally consists of many systems, such as lighting, HVAC, security system, fire alarm. Variable Refrigerant Flow Air (VRF) is used for cooling and Variable Air Volume system (VAV) is for ventilation purpose.
In a control system, there are three basic elements: a sensor, controller, and a controlled device. Figure 19 shows an example of how control system works and the components involved. First, the sensor measures a variable such as the temperature in a room, and the values are transmitted to the controller through the signal conditioner and a transmitter. Then the controller takes the values into account and alters the output signal to the controlled device through the actuator. An actuator provides the mechanical action to operate the final control device, which is typically a valve or damper. Finally, the device will change the output of the load based on the signal received from the controller. Depending on what variables are required to be controlled, the components, such as sensors and the controlled devices, will differ in each control system.
39
Figure 19 Elements of control system
PI controller has been chosen for the both control systems. It is the most widely used controller in HVAC controls. It is a combination of proportional and integral (P+I) control. Proportional control produces a control output proportional to the deviation of the controlled variable from the desired set point. Integral control produces a control signal proportional to the time integral of the deviation from the set point (CIBESE Guide H, 2006). By combining these two, it allows to eliminate the load error over time. On the other hand, sensors and controlled devices depend on the controlled variable in each system. They will be discussed in the following sections 4.2.1 VRF Multi-Split System VRF system provides the cooling for the building. It is energy efficient due to its highly responsive cooling and includes outdoor units and indoor units as shown in Figure 21. It is especially important to control this system well, since the comfort of the occupants majorly depends on the indoor temperature. To control the temperature and humidity in the building, the following elements have been chosen for the system.
40
4.2.1.1 Temperature and Humidity Sensors
Temperature is the most widely measured variable in HVAC. Temperature sensors give guidelines to the AC system to provide the supply air with the desired temperature. Different types of sensors are listed below in Table 14. Due to its high accuracy, stability and reliability, platinum resistance thermometers (PRT) have been chosen to measure the temperature in the building. An accuracy of 0.6K achieved over a 15 to 25
range is
suitable for zone air temperature measurement, while an accuracy of 0.25K is needed for control of chilled water temperature. Table 14 Temperature sensors
Another important variable that needs to be taken into account is the humidity. Excessive humidity levels can cause discomfort, health issues and affect productivity. There is also a risk of condensation. Thus, appropriate humidity control is essential, especially in Singapore.
Table 15 shows two types of humidity sensors that are most widely used in HVAC control. The capacitive polymer film sensor provides a direct measure of relative humidity. However, drift occurs when exposed to high RH. The dew point sensor, on the other hand, has a very high accuracy control. Thus, the chilled mirror dew point sensor has been selected to measure the dew point directly.
41
Table 15 Humidity sensors
After the temperature and humidity have been monitored, the signals are transmitted to the controller. The PI controller will then alter the signals depending on the indoor temperature to expansion valves via actuators. 4.2.1.2 Expansion Valves In the VRF system, the expansion valves act as the controlled devices which alter the output of the system. There are electronic or pulse modulating expansion valves located near each indoor evaporator unit, which will control the flow properties of the refrigerant entering the evaporator. They are able to follow the radical changes in capacity required, enabling to maintain the optimized working conditions with precise temperature control and quick cooling. This adds efficiency to the system and reduces carbon footprint. Figure 20 illustrates typical electronic expansion valve.
Figure 20 Electronic expansion valve
42
4.2.1.3 Pressure Sensor In addition to the temperature and humidity sensors, pressure sensors have been located near the outdoor unit to control the speed of the compressor. Working together with the electronic expansion valve, the overall refrigerant flow is controlled by the variable-speed compressor that is linked to the pressure sensor. Table 16 shows a number of pressure sensors. Table 16 Pressure sensors
43
4.2.1.4 The Control Schematic Diagrams for VRF System Schematic diagrams of the VRF control system are shown in Figure 21 and Figure 22. Figure 21 is a wiring diagram that shows how the sensors, controllers and controlled devices are connected. Moreover, the PI controller is wireless and controls the cooling in each zone. The components are then linked with network converters and the control centre, which is further connected to other building systems such as the ventilation system. Figure 22 indicates the locations of the electronic expansion valves near the indoor units. For simplicity, the outdoor units were combined in the drawing. The fan speed will be held constant across the whole system.
Figure 21 Control schematic diagram of VRF system 1
44
Figure 22 Control schematic diagram of VRF system 2
4.2.2 VAV Air Supply Control System The VAV system is responsible for supplying fresh air into the building. The amount of the fresh air supplied will be controlled depending on the occupancy. The more the occupants are in the building, the muggier they will feel due to the increase of heat and moisture emitted. In other words, the occupants might feel discomfort as the level of occupancy increase and will require more fresh air. Thus, to control the amount of fresh air, following elements have been considered in the system. 4.2.2.1 CO2 Sensor CO2 level is a parameter for occupancy in the space, and it is becoming the most generally accepted measure of indoor air quality. Other than CO2 sensor, there is also indirect method such as predicting the occupancy based on the work schedules. However, it will be hard to predict the occupancy for libraries and exhibition halls, since there are no set schedules. Table 17 shows the details of a CO2 sensor. After the level of CO2 has been measured, it will be transmitted to the PI controller and further to the controlled device – VAV damper unit. 45
Table 17 CO2 sensor
4.2.2.2 Dampers Ventilation dampers are used to control the flow of fresh air in buildings. Usually in a VAV system, the dampers will be connected to thermostats to control the supply air temperature as part of an air conditioning system. In our building, however, the VAV system is only used for ventilation, thus the dampers are linked with CO2 sensors to control the indoor air quality. The PI controller will alter the output signals, and then the damper will be adjusted via actuator to change the output.
4.2.2.3 Pressure Sensor Similar to the VRF system, pressure sensors are also a part of the control system. These pressure sensors are located in ducts, connected to variable speed devices (VSDs). The VSD controls the fan to alter the velocity of supply and extract air. This is to maintain the constant static pressure of the airflow in both the supply and extract ducts.
4.2.2.4 The Control Schematic Diagram for VAV System
The schematic diagram of the VAV ventilation system is shown in Figure 24. The figure shows the wiring of the CO2 sensors to the control centre, and to the dampers. In addition, there are pressure valves located near the fans, connected via variable speed devices to control the fan speed to maintain the constant airflow.
46
Figure 23 Control schematic diagram for VAV system
47
5. Annual Energy Consumption 5.1 VRF Air Conditioning Electricity Consumption The heat gains for the worst case scenario were calculated in BA1, while the load for the cooling coil was found in psychrometry section (see 2.4.5). The ratios between the heat gains and the loads for the cooling coils were found in order to approximate the total annual cooling load. Electricity consumption for cooling was found by dividing annual cooling load by COP of the VRF system (taken as 3.6). Ecotect calculations assumed that the building is fully occupied from 8:00 to 17:00 every day (including weekends). For manual energy demand calculations, it has been assumed that the exhibition centre and library (ground and first floors) are occupied 50% on average and 80% for the offices (second floor). Table 18 derives the total electricity requirement for cooling. Table 18 Electricity requirements for cooling
Zone
Heat
Cooling Ratio Annual
Annual
Electricity
Total
gains,
heat
cooling
requirements,
electricity
kWh
gains,
load,
kWh
requirement
kWh
kWh
for cooling, MWh
Ground
68
93
1.37
186,537
255,117
35,433
1 floor
26
35
1.33
106,398
141,058
19,591
2 floor
41
53
1.29
155,048
199,940
44,431
floor 99
5.2 Equipment and Lighting Electricity Consumption Assumptions made:
The number of VRF indoor units and electricity consumption were taken from the VRF system components specification.
Electricity requirement for fans on the ground floor is nearly 2kW. The required fan power for first and second floor combined should be slightly lower than for the GF, thus it can be approximated that total fan power required is 4kW to allow additional losses 48
Equipment and lighting electricity consumption were found in BA1
Taking into account that the electrical equipment is not operating the whole time, the total electricity consumption was multiplied by a factor of 0.8.
Table 19 Equipment electricity consumption Number
Electricity
Hours of
Electricity
Electricity
Total
of units
used, kW
operation
consumption,
consumption,
electricity
per year
kWh
MWh
consumption, MWh
VRF units
11
0.059
2,920
1,895
21
0.039
2,920
2,391
-
4
2,920
11,680
11.68
Equipment
-
38
2,920
110,960
110.96
Lighting
-
14.6
2,920
42632
42.63
in the building Ventilation
4.29
135.65
fans
5.3 Total Electricity Consumption
Total electricity consumption is equal to the sum of electricity used for AC system, equipment and lighting.
99.5+135.7 =235.2 MWh
5.4 Potential of Renewable Energy Utilisation In BA1, the amount of maximum energy that can be generated from the PV panels installed on the roof was calculated. Up to 350 MWh of electricity could be generated with the one of the best PV panels available on the market. However, in reality the efficiency is lower due to various factors. Moreover, in BA1, the available roof area was assumed to be 1450m². However, the number of PV panels that could be installed on the roof depends on the dimensions of the panels. Thus, the dimensions of the actual PV panel from the Sunpower manufacturer’s catalogue were found and are shown in Figure 24. 49
Figure 24 Dimensions of Sunpower PV panel
Thus, a total number of 644 PV panels can be installed on the roof. This gives the total 341MWh of energy that can be generated.
As the total building electricity consumption is lower than the electricity that could be generated by PV solar cells, the energy consumed by the building could be completely supplied by the renewable energy produced:
235.2 MWh < 341 MWh
The Zero Energy Building (ZEB) was achieved!
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References CIBSE Guide H. (2009) Building Control Systems, Norfolk; Great Britain CIBSE Guide A Yeboah, S. (2010) Air Conditioning Systems, Lecture, University of Nottingham, Ningbo China, unpublished Kokogiannakis, G. (2010) Control Systems 1, Lecture, University of Nottingham, Ningbo China, unpublished Kokogiannakis, G. (2010) Control Systems 2, Lecture, University of Nottingham, Ningbo China, unpublished Johnson Controls, Commercial & Industrial HVAC,York Carbon Trust. Heat Recovery Carbon Trust. Heating, Ventilation and Air Conditioning Product Data Sheet of Sunpower PV Panels <http://us.sunpowercorp.com/cs/BlobServer?blobkey=id&blobwhere=1300271295172 &blobheadername2=Content-Disposition&blobheadername1=ContentType&blobheadervalue2=inline%3B+filename%3D11_318_sp_e20_435_ds_en_w_ltr. pdf&blobheadervalue1=application%2Fpdf&blobcol=urldata&blobtable=MungoBlobs> Psychrometric Calculations ,http://www.sugartech.co.za/psychro/index.php Hoval, Rotary Heat Exchangers for Heat Recovery in Ventilation Systems BHATIA, A. HVAC Variable Refrigerant Flow Systems. ASHRAE Guide, American Society of Heating, Refrigerating and Air-conditioning Engineers Fujitsu V-2 VRF System Catalogue
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