A new design guideline for the heating and cooling curve in AHU of HVAC systems

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1 Document identification Type of documents: Glossary Cx process Project Tool Model Type of building: Type of commissioning: Initial-cx Re-cx Retro-cx Continuous-cx Glossary check: yes no MQC phase: Program Design Elaboration Realisation Operation Other remarks: A new design guideline for the heating and cooling curve in AHU of HVAC systems A method for the optimal adjustment of AHU in HVAC systems P.A. (Bert) Elkhuizen, H.C. (Henk) Peitsman TNO Building and Construction Research Delft The Netherlands KEYWORDS Control strategies, Building, Energy, Commissioning, HVAC ABSTRACT New Dutch office buildings are built under strict energy efficient legislation and are well euipped with insulation, condensing boilers, heat recovery systems, etc. Despite this, many buildings don t perform very energy efficiently and the comfort reuirements. This has led to a large number of complaints. The problem seems to be that common energy control strategies (heating/ cooling curves) are often used, most of the curves were based on buildings with low level of building shell heating resistance and no additional insulation and single glazing. When these control strategies are used in modern office buildings, too much energy is used due to a mismatch between heating and cooling demands. Energy savings of up to 35% can be realised without significant financial investments by devoting extra attention to the settings of the heating/ cooling curve in the central Air Handling Unit s (AHU s) of HVAC systems without loss of comfort. In most cases the number of complaints will also be reduced. The design method can be used in both new and existing buildings. The first part of this paper presents a brief description of the method used to realise this energy saving. The brief description follows with a practical example of this method. The intention is that the approach will generate a building-specific heating/cooling curve based on an energetic optimum between the demand for heating and cooling in the zone s. The input data used to generate the function cover the characteristics of the building, the organisation, and the HVAC euipment. A secondary effect for HVAC designers is that the way they design the type of installation can be changed. They will be able to see if their chosen HVAC system for the building design fits with the needs of the client. The new approach will be adapted for use in one of the most commonly used simulation programs for Dutch HVAC designers: VA 114. The program will generate the most energyefficient heating/cooling curve, but the designer will be able to change the function and will be given feedback about the effect of this on energy use. Figure 1 gives an example of a new heating/cooling strategy. Page: 1 of 26

2 BACKGROUND The majority of Dutch office buildings are euipped with HVAC systems that conditions the supply air in a centralised Air Handling Unit (AHU). A heating/cooling curve, as part of the control strategy, is used to condition the supply air to the zones. The common used settings to adjust the heating/cooling curve in most office buildings lead to problems relating to user comfort and excessive energy consumption. Even in the new office buildings, well build under the strict legislation, or in retrofit buildings the same problems occur and the predicted energy consumption or energy savings are far out of reach. Several studies on energy consumption confirm the existing gap between prediction and reality. Figure 1a presents visual the interaction between the central AHU and the cooling demand in the zones. Figure 1a: The control strategy determines the supply air temperature as a respond to the dynamic behaviour of the building The energetically optimised heating/cooling curve will help to decrease the existing gap between prediction and reality. The heating/cooling curves where for a long time no subject to further research and a forgotten subject. This project shows that it is a very interesting subject for research, since there is a strong relation between the actual heating/cooling curve and energy consumption and comfort. In the Dutch mild climate the need for space heating is reduced due insulation and airtightness, but on the other hand the need for room cooling is increased. During normal winter conditions or during summer conditions there seems no problem, there is a full load condition. But, with moderate temperatures there is a part load condition and a need for cooling and heating at the same time. Figure 1b shows a typical modern building heating and cooling demand profile. The demand for heating and cooling at the same time occur at 40% of the time. Figure 1b: Building related heating and cooling demand profile over a year period. (data points per hour) Page: 2 of 26

3 The control strategy determines how the HVAC installation responds to these conflicting energy demands. In the majority of the buildings there will be a contradicting heating and cooling supply at the same time, which will lead to excessive energy consumption. The development of control strategies stood still. The most common used control strategies are based upon knowledge from the past. These control strategies were developed during a time were the building had no insulation or double glazing, high internal heat loads, so the energy profile was clear. The standard control strategy fulfilled with no real problem, either you needed cooling or you needed heating. The uestion why the building should be controlled in that manner was never asked. The problem that nowadays occur with modern build office buildings or retrofit office building is that the control strategy doesn t fit the energy demands with the energy supply, too much energy is used due to a mismatch between demands and supply. Not only too much energy is used but there are also more comfort-related complaints than expected. In order to optimise the energy consumption and to reduce the complaints there should be much more attention to the control strategy and the heating/cooling curve. Despite many years of optimisation of single components, such as condensing boilers, there is no common knowledge about how to design your control strategy or how to design the heating/cooling curve. The fact is that there is no literature known about the relation between comfort, energy efficiency or energy consumption and heating/cooling curves. TNO Building and Construction Research developed the design method called Energetically optimised cooling/ heating curves, which allows designers and facility managers to design and check the optimal settings for their office buildings. The design method generates a buildingspecific heating/cooling curve for the centralised AHU system, which is optimally tuned to the building and the organisation occupying it. The optimal settings relate to the building's heating and cooling reuirements and the climate-control system's delivery of heating and cooling. Input data for the calculation method include building data, characteristics of the occupying organisation, and data relating to the climate-control system. Page: 3 of 26

4 1. INTRODUCTION Energy savings of up to 35% can be realised without significant financial investments by devoting extra attention to the settings of the central AHU in HVAC systems. The first part of this paper presents a brief description of the method used to realise this energy saving. The brief description follows with a practical example of this method. The second part describes in detail the methodology. 1.1 How is the calculation method used? The calculation method can be applied to office buildings yet to be designed and to existing office buildings. The calculation method can be used to: - Determine the energetically optimised heating/ cooling curve for individual buildings; - Estimate the possible energy saving; - Execute a uick scan for one building or large-scale building stocks; - Optimise the energy savings for other measures. 1.2 Who can use the calculation method? Customers for whom the calculation method is developed, are: (1) designers, (2) installers, (3) building managers, (4) TAB (Testing, Adjusting and Balancing) companies. Turnover temperature 25 Supply air temperature ['C] Standard Energetically Optimised heating AHU5 cooling AHU Transition range Outside air temperature ['C] Figure 1c: The new heating curve (T-outside = f(t-supply) of an AHU)) 1.3 How does the calculation method work? The calculation method is based on the hourly heating and cooling demand within the building, which can be determined using a building simulation program such the Dutch computer program VA114. TNO Building and Construction Research developed a simplified method, which was published in the Dutch ISSO publication 68. Input data for the calculation method include (1) building data, (2) characteristics of the occupying organisation, and (3) data relating to the climate-control system. 1.4 Turnover- or free temperature In general, when the outside temperature is eual to the Turnover temperature (figure 1c), no heating or cooling demand exists within the building. This is also called free temperature of the building. That means, no heating or cooling is supplied to the mechanical ventilation air. 1.5 Transition range The transition range (figure 1c) can be defined as the temperature interval for the outside temperature in which either a heating or cooling demand may exist. No heating or cooling is therefore added to the mechanical ventilation air. Page: 4 of 26

5 1.6 Standard heating/cooling curve A standard heating/ cooling curve such as often applied in practice does not take the abovementioned concepts (i.e. turnover temperature and transition range) into account. The mismatch between building and climate-control system is visualised clearly in figure Turn over temperature 00 Heating or cooling demand Outside air temperature Heating/cooling supply by standard curve Transition range Figure 2: Mismatch building and climate-control system. 2. An example from practice By paying additional attention to the settings of the HVAC systems, energy savings of up to 35% can be realised without large financial investments. This second part of the paper, using a example calculation, explains how this energy saving is realised in combination with an improvement in thermal comfort by means of a method developed by TNO Building and Construction Research. 2.1 Sample building This example relates to a recently completed office building (year of construction: 1998). A representative ground plan and the façade view are shown in figure 3. 22,8 m 14,4 m 68,4 m Figure 3: The ground plan and the façade of the selected building. Building - Gross Floor Area (GFA) : 6,895 m 2 - RC value building shell : 2.5 [m 2 K/W] - Window percentage exterior (North and South wall) : 40 % - Window system (HR+ glass) : U value glass = 1.5 [W/(m 2 K)] : ZTA value glass = 0.60 Page: 5 of 26

6 Organisation - Average internal heat production workspaces : 35 [W/m2 North East] (including freuency of use) HVAC system - Heated/cooled ventilation air, heating and cooling by means of 4-pipe induction system - Set point : winter = 22 C; summer = 24 C - Quantity of fresh air per person : 50 m 3 /h per person 2.2 Application of the method 'Energetically optimised heating and cooling curves' The energetically optimised heating and cooling curve is determined on the basis of the relation between the hourly heating and cooling reuirements and the outside temperature. The form of the energetically optimised heating and cooling curve diverges considerably from the standard heating and cooling curve that is generally used in actual practice (see figure 2). 2.3 Energy saving Energy saving 50% 40% % % 10% 0% Heating Cooling Heating 2.4 Improved comfort The comfort improvement is visualised in figure 4 by comparing the occurring room temperatures in the case building to the temperatures specified for the standard heating and cooling curve (in red colour) and the energetically optimised heating and cooling curve (in blue colour). In the situation with the standard heating and cooling curve, room temperatures of 24 C (set point cooling) occur at an outside temperature of 4 C. In the case of the energetically optimised heating and cooling curve, these room temperatures do not occur until an outside temperature of 10 C is reached. High room temperatures in a winter situation (4 C) will lead to complaints regarding the thermal comfort. Local cooling will be active to reach the comfort level. The number of comfort-related complaints is reduced considerably when the Cooling 35 W/m2 50 W/m2 Internal heat load/ production The energy saving that can be realised by adjusting the heating and cooling curve from the standard curve to the energetically optimised heating and cooling curve for the case building is as follows: - For heating 13 %, and - For cooling 10 %. If an organisation with an average internal thermal load eual to the thermal load as specified in the operational reuirements (= 50 W/m 2 N.E.) is occupying the same sample building, the energy saving is even higher (36 % for heating and 19 % for cooling). energetically optimised heating and cooling curve is applied. Inside Air temperature ['C] By Local cooling ['C] Setpoint Cooling Setpoint Heating Outside Air temperature ['C] Figure 4 Comfort improvement. Page: 6 of 26

7 W arm te- en koelbehoefte tijdens kantooruren Tbuiten [C] A40-E-M4-NL-TNO-1 3. Introduction Detailed description of the Energetically Optimised Heating/Cooling Curves in HVAC systems 3.1 Description of the method The method is build up in 5 steps. The separate steps are presented in figure 5. Step_1 Collect all necessary data (building properties and design data) for the project Step_2 Determine of the heat- and cooling demand of the building as a function of the outside air temperature Energiebehoefte [0,1 x kwh] Step_3 Determine the basic shape/ form of the new heating-/ cooling curve Step_4 Discount other aspects of the HVAC system (e.g. recirculation heat recovery) in the heating-/ cooling curve Step_5 Discount other preconditions/ limitations (draught, moisture, condensation) in the heating-/ cooling curve Figure 5: The steps to be executed for an optimised Heating/ Cooling curve. In this paragraph a brief description is given of the steps to be executed to find an optimised Heating/ Cooling curve of HVAC systems. The following paragraph provides a detailed explanation of the mentioned steps. Step 1 is based on the collection of the necessary Project data which are relevant for the determination of the Energetically 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 is the determination of the heating/cooling demand of the building as function of the outside air temperature. The determination of the heating/cooling demand of the building is a complex analysis and is based on the use of building simulation programmes. Because not everybody is able to work with sophisticated building simulation programmes, a simplified method has been developed to solve this problem. In Step 3 the basic shape of the Energetically Optimised Heating/Cooling Curve is determined and is thus a description of the heating and cooling production of the HVAC system. This will be done on the outcome of the calculated heating and cooling demand as calculated in step 2. The outcome of step 3 is: (1) the calculated value of the turn-over temperature (free temperature) of the building. The turn-over Page: 7 of 26

8 temperature is the temperature where no heating and cooling is reuired. (2) the transition range and (3) the sensitivity of the heating and cooling demand in relation to the outside air temperature. In step 4, the energetic contribution of the different HVAC components (AHU, Fans, Heat recovery, etc.) to the basic shape (step 3) of the Energetically Optimised Heating/Cooling Curve will be determined. In step 5 account is taken of the physical boundary conditions of the HVAC system. It gives the limit within which the Energetically Optimised Heating/Cooling Curve could be defined without complaints being received 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 must be adjusted. In this case we will have to go back to step Description of the method Energetically Optimised Heating/Cooling Curves in HVAC systems The description of the method focuses on the following HVAC types: Central heating and cooling of the mechanical ventilation air by AHU; Decentralised heating and cooling in the zones by e.g. induction units (local end -units/ terminal units) The method contains the following steps: Step 1 : Collection of the necessary Project / boundary conditions The following project data must be collected for each building part or for each zone: Building properties Gross Floor Area (GFA); Functional effective Floor Area (FFA); Area of facades, roofs, first floor; Orientation of facades; Window area of each facades; Percentages of window area in the facades. Building design physics Rc- value of the building shell Window system/ type U- value glass g- value glass U- value window frame Organisation/ Building user Maximum number of persons Internal thermal heat production zones/ workspaces Average Maximum Terms of references (Programme of reuirements) Internal thermal load corridors and other spaces; Number of persons per space Page: 8 of 26

9 HVAC system Type/ concept of central HVAC system Type decentralised (zone) HVAC end units Setpoint winter period Setpoint summer period Amount of fresh air per person total Humidity Amount of recirculation of return air Exhaust of air by lighting units Fan properties: Fan pressure of supply fan or electrical power capacity Fan pressure of exhaust fan or electrical power capacity Only in the case of existing buildings Capacity heating coils AHU Capacity cooling coil AHU split up in latent cooling and perceivable cooling. Current heating/cooling curve Step 2 : Determination of the heating/cooling demand of the building as function of the outside air temperature. A. Determine the turn-over point temperature (see figure 1 and 2) The value of the turn-over point temperature will usually be between 5 C and 15 C. All heat sources and heat flows are included to determine the turn-over point. (See appendix system boundary for the heating and cooling reuirement). The turn-over point temperature is determined by taking the average of the setpoint temperature for heating and the setpoint temperature for cooling, minus a ratio between heat gains (internal thermal heat production and solar energy) and the overall heat losses of the building to the outside air (transmission and ventilation). This is expressed in formula [1]: { φ + * A * g }* η INTERN; AVG SUN; AVG GLASS GLASS HEAT DISSIPATION TO = I; AVG [1] a* U * ALOSSES + VENT ; OUTSIDE* ρ * CP TO = the turn-over point(or free) temperature [ C] = the average AHU setpoint temperature for summer and winter period [ C] I ; AVG φ INTERN ; AVG = the average internal heat production [W] ; = the average daily solar radiation [W/m2] SUN AVG A GLASS = total glass surface area for the building section [m2] g = g value of the glazing (not including switching sun blind) [-] GLASS η HEAT = 0,64 = utilisation factor for the heat gains [-] φ = the heat dissipation of HVAC components [W] DISSIPATION + φ Page: 9 of 26

10 a = weighting factor [-] a = 1 for constructions that separate the building (section) from the outside air or water a = 1/(1+U) for constructions that separate the building (section) from the ground or from a crawling space a = 0 for constructions that separate the building (section) from a heated area a = 0.5 for constructions that separate the building (section) from an unheated area U = average thermal transmittance coefficient of the building shell (incl. glass) [W/m2.K] A = total loss surface of the building (section) [m2] Losses VENT ; OUTSIDE = fresh air uantity of mechanical ventilation air [m3/s] ρ = 1,2 = specific mass of the air [kg/m3] C P = 1000 = specific heat of air [J/kg.K] The expression: a * U * ALOSSES in euation [1] is eual to the specific heat loss through transmission as defined as the variable H TR in paragraph 6.4 of the Dutch standard NEN 2916 (Energy Performance of NON Residential buildings). This standard is available in English (1 pages). NB: Particularly for the management of the HVAC installation, it is advisable to keep the derived building data so that they are available to interested parties. These are the building properties relating to Energy : 1) the heat transmission ( a * U * ALOSSES ), 2) the total solar energy transmission of the glass ( A GLASS * ZTAGLASS ) and 3) the total necessary energy for the fresh air contribution for mechanical ventilation ( VENT ; OUTSIDE * ρ * CP ). It is often difficult to re-determine these data (no longer available) or extremely labour-intensive. If the details are directly available the heating/cooling curve can be adjusted more uickly if necessary. Taken on average, the figure value for the entrance of solar radiation is expressed in formula 2: 80* Z SUN ; AVG = ORIENTATION [2] Z ORIENTATION = orientation factor of the solar radiation. The following can be applied for a building or building section with opposite orientations: Z ORIENTATION =1.0. If the building or building section comprises several orientations, a glass surface area average orientation factor can be determined. For each orientation a value for Z ORIENTATION can be applied as shown in table 1. SUN ORIENTATION ORIENTATION FACTOR N 0.6 NE 0.7 E 0.9 SE 1.1 S 1.3 SW 1.3 W 1.2 NW 0.9 Horizontal 1.6 Page: 10 of 26

11 Table 1 : Values for the orientation factor Z ORIENTATION The formula for the turn-over point temperature can be worked out in more detail and all terms divided by the Gross Floor Area (GFA) yield in formula [3] in which as well as all properties of the building and the organisation the building form is also clearly presented as a variable. TO φintern ; AVG AGLASS φdissipation { + SUN; AVG * * gglas}* ηheat + = GFA GFA GFA I ; AVG [3] A LOSSES VENT ; OUTSIDE a* U * + * ρ * CP GFA GFA GFA = Gross Floor Area of the building (section) [m 2 ] φ /GFA = internal thermal heat production per m 2 gross floor area. [W/m 2 ] INTERN;AVG ; /GFA = the fresh air contribution (outside air) to the mechanical [m 3 /s.m 2 ] VENT OUTSIDE ventilation per m 2 floor area (comparable with the reuirements of the Dutch Building Decree for air circulation). Note: The terms A GLASS /GFA and A LOSSES /GFA depend importantly on the building shape. The term A LOSSES /GFA is designated as the shape factor. Possible values for the shape factor that are achieved in practice are given in figure 6. 6 Building shell losses / GFA Gross Floor Area (GFA) Figure 6 : Practical values for the shape factor It can be deduced from figure 6 that, especially for small buildings (GrossFloorArea < 2500 m 2 ), the effect of the shape factor is significant and that the shape factor partly determines the turn-over point temperature. For buildings with a GrossFloorArea exceeding 5000 m 2 the shape factor will be 0.5 to 1.0. The term φ DISSIPATION (see formula [4]) represents the heat dissipation of the HVAC components. This could include, for instance, heat dissipation through ventilators and heat that gets into the HVAC system due to armature extraction taking place in the lighting. Page: 11 of 26

12 φ = φ + φ * R + R [4] DISSIPATIO N DISSFAN1 DISSFAN 2 φdisslight * φ DISSIPATION = the heat dissipation of HVAC components [W] φ = the heat dissipation of ventilators in supply channel or area [W] DISSFAN1 φ = the heat dissipation of ventilators in the exhaust channel [W] DISSFAN 2 φ DISSLIGHT = the heat extracted in the lighting armatures [W] R = the recirculation factor (see step 4) [-] Ventilators Ventilators are normally used in the HVAC system to transport the centrally conditioned air in the AHU to the rooms. Ventilators are also used in some types of local systems (e.g. fan coil units). These ventilators use electrical energy that is partly fed back into the building as heat. The thermal energy emitted by the ventilators no longer necessarily has to be provided by the central AHU in the heating season. In the cooling season the same amount of thermal energy does however have to be extracted additionally in the central AHU. Q Q = Σ 0,65* P ) = Σ( * dp ) [5] DISSFAN1 ( FAN ; EPOWER;1 VENT ; TOTAL SUPPLY = Σ 0,65* P ) = Σ( * dp ) [6] DISSFAN 2 ( FAN; EPOWER;2 VENT ; TOTAL SUPPLY P FAN;EPOWER;1 = electrical power (capacity) of all ventilators in room or supply channel [W] P FAN;EPOWER;2 = electrical power (capacity) of all ventilators in the exhaust channel [W] 0,65 = the efficiency of the ventilator [-] ; = total air uantity of mechanical ventilation air [m 3 /s] VENT TOTAL dp SUPPLY = ventilator pressure of the supply ventilator(s) [Pa] dp = ventilator pressure of the exhaust ventilator(s) [Pa] EXHAUST Lighting armature extraction Part of the convection heat yielded by the switched-on lighting emitted to the ventilation discharge air (circa 50%) is emitted via the armature extraction. The heat emitted by convection is a fraction (average approx. 68%) of the lighting energy. The temperature of the ventilation discharge air increases as a result of the extracted heating light. There are a number of circumstances in which this emitted lighting output is supplied to the air supply circuit and thus affects the heating/cooling curve. This is the case in the event of recirculation and with the application of heat recovery. The expression of the light contribution is given in formula [7]. φ = Σ 0,34* ) [7] DISSLIGHT ( P LIGHT ;EPOWER φ DISSLIGHT = the heat extracted from the lighting armatures [W] P LIGHT ; EPOWER = average switched on lighting capacity [W] 0.34 = is the convection factor of the lighting (0.68) multiplied [-] by the extraction output from the armature (0.5) Page: 12 of 26

13 B. Determining the derivative of the heat/cooling demand as function of the outside temperature (see figure 2 and 8). The average slope of the derivative of the heat demand as function of the outside temperature can be expressed in: Slope H:. The average slope of the derivative of the cooling demand as function of the outside temperature can be expressed in Slope C:. These have been placed on par with each other for the simplified calculation method. The derivative of the heating and cooling demand of the building as a function of the outside temperature is determined using the formula [8]. Slope = Slope = a* U * A * ρ * C )*1.25 [8] h; φ C; φ ( LOSSES + VENT ; OUTSIDE P Slope H :φ = average slope of the heating demand as function of the outside temperature [W/ C] Slope C:φ = average slope of the cooling demand as function of the outside temperature [W/ C] a = weighting factor [-] a = 1 for constructions that separate the building (section) from the outside air or water a = 1/(1+U) for constructions that separate the building (section) from the ground or from a crawling space a = 0 for constructions that separate the building (section) from a heated area a = 0.5 for constructions that separate the building (section) from an unheated area U = average heat transmission coefficient of the building shell [W/m 2.K] A LOSSES = total loss surface of the building (section) [m 2 ] VENT ; OUTSIDE = fresh air uantity from mechanical ventilation air [m 3 /s] ρ = 1.2 = specific mass of the air [kg/m 3 ] C P = 1000 = specific heat of the air [J/kg.K] 1.25 = is the correction factor for static to dynamic heat balance and accounting for the energy losses as a result of air infiltration. The heat flows that are linked to the outside temperature are the heat losses resulting from transmission and ventilation of outside air. This dependence of the heating/cooling demand on the outside temperature must be translated into an increase in the temperature of the ventilation flow of fresh air (the effect of recirculation is accounted for in step 4). The slope is therefore a ratio between the sum of the overall heat losses as a result of transmission and ventilation of outside air divided by the heat losses as a result of ventilation (see [9]): Slope H ; T ( a* U * ALOSSES + VENT ; OUTSIDE * ρ * CP ) = SlopeC; T = *1.25 [9] * ρ * C VENT ; OUTSIDE P Slope : = average slope of temperature rise to the outside temperature [ C/ C] H T Slope : = average slope of the temperature reduction to the outside temperature [ C/ C] C T C. Determining the size of the transition area The size of the transition area for the building section is determined by variations in the thermal load resulting from the entrance of solar radiation and the difference in set points for heating and cooling. The determination of the heating curve as described here is for a building (section) that consists of a number of separate areas. Up until now the average internal heat production of all areas within the building Page: 13 of 26

14 (section) has been assumed. There can however be substantial mutual differences between the rooms in the building (section). (See figure 7). 1 person absent 2 persons AHU 1 person Average presence = 50% absent 2 persons = room with minimal heating demand = room with minimal cooling demand Figure 7 : The mutual differences between rooms The reuired level of comfort has to be brought about at all times for each room in the building (section). A worst-case scenario must be assumed to compensate for this dependence on rooms. This can be achieved by basing the calculation of the heating curve in the heating season on a minimum heating reuirement (with eual rooms this is the space with maximum internal thermal heat production) and the calculation of the cooling curve in the cooling season on areas with a minimum cooling reuirement (with eual rooms this is the room with minimum internal thermal heat production). This is processed into this methodology by assuming the maximum internal thermal heat production for the winter season and the minimum internal heat production for the summer season. The result of this is that the size of the transition area increases. This increase is related to the degree of mutual differences in internal thermal heat production per room (expressed in the variable F VAR ). The total size of the overall transition area can be determined as follows [10]: d TRANSITION {2.0* SUN ; AVG * AGLASS * gglass + φintern ; MAX * FVAR}* ηheat = + ( SET ; SUMMER SET ; WINTER ) [10] a* U * A + * ρ * C LOSSES VENT ; OUTSIDE P d TRANSITION = temperature interval size of the transition area [ C] F VAR = factor that indicates the spread between thermal internal heat production [-] load per room mutually = setpoint temperature in the heating season [ C] SET ;WINTER SET ;SUMMER = setpoint temperature in the cooling season [ C] Practical values for the size of the transition area vary (depending on the specific project conditions) between 5 C and 15 C. The average presence of people in a building provides information about the average internal thermal heat production. The maximum number of people in a room provides information about the concurrence of people (and therefore the thermal heat production) in the room. For a building with one person rooms, the spread of the internal thermal heat production will be large in each room since this depends on whether or not that person is present in the room. Page: 14 of 26

15 The spread of the internal heat production will reduce in keeping with the increase in people in the room. The biggest spread is found between a fully-occupied room (Ф INTERN;MAX ) and an empty room (Ф INTERN = 0). The spread of the internal heat production per room is expressed as the variable F VAR [11]. F VAR max min = [11] max F VAR = factor that indicates the spread between thermal heat production per room mutually [-] MAX = the maximum internal thermal heat production found in a certain room per m 2 area [W/m 2 ] = the minimum internal thermal heat production in a certain room per m 2 floor area [W/m 2 ] MIN For a single-person room this will be : F VAR = 0.5 to 1.0 Note: This is based on an eual distribution of the rooms with/without presence through the building. Because of the mutual heat exchange between the rooms (via open doors)the value of F will not be eual to 1.0 but will be lower. For rooms designated for more than one person the value of F VAR will be between 0.0 and 0.5. For large rooms with many work stations (e.g. open plan offices) the value of F VAR will be 0.0. VAR The figure values of F VAR given here are no more than indicative. The value of F VAR will have to be determined with the given formula for each individual situation. The limit values for the outside temperature to which the transition area applies are given by the temperatures L and H. The accompanying values L and H are determined by [12] and [13]: dtransition L = TO 2 [12] dtransition H = TO + 2 [13] L = low transition temperature [ C] H = high transition temperature [ C] KP = the turn-over point temperature [ C] d = temperature interval size of the transition area [ C] TRANSITION Step 3 : Determining the basic shape of the optimum heating/cooling curve The basic shape of the heating/cooling curve is derived from the variables that are determined in step 2. These variables are shown in figure 8. The red/blue line shows the basic shape of the heating and cooling of the ventilation air (outside air part). This basic shape indicates the extent to which the fresh outside air has to be conditioned to insure that the supply temperature is altered in such a way that the uantity of heat or cold added to the air is eual to the minimum reuirement for heat and cooling in the building (section). Page: 15 of 26

16 d = warming up of supply air AHU (K) Slope H; Outside air temperature E (ºC) L TO H Slope C; Figure 8: The basic shape of the desired heating and cooling of the ventilation air. In view of the fact that the heating/cooling curve is not expressed in temperature differences but in actual temperature levels, the desired heating and cooling must be added to the outside temperature. This is shown in figure 9. The dotted line is the Y=X line, which means that the base is eual to the outside temperature. To add the desired heating or cooling the basic form (see figure 8) is added to the Y=X line. 40 Supply air temperature AHU ( ºC) 10 Slope H; Y=X Slope C; E (ºC) Outside air temperature L TO H Figure 9 : The basic shape of the heating/cooling curve Page: 16 of 26

17 3.2.4 Step 4 : Discounting aspects of the air-conditioning system The steps given below relate to the central AHU of the HVAC system, which means that all zones belonging to an AHU have to be taken together. For each AHU a choice has to be made (in view of the underlying zones and the accompanying limits for heating/cooling curves) regarding which heating/ cooling curve is to be continued in the analysis. If one AHU is present in each zone, the analysis for each zone can be continued. A number of aspects of the HVAC system that affect the heating and cooling supply have not yet been included. These aspects are as follows: heat recovery (affects the heating and cooling supply in the AHU). recirculation (affects the temperature level of the heating/cooling curve). Heat recovery Heat recovery is an alternative for the AHU to apply the desired conditions to the ventilation air with the advantage that no primary energy is reuired for the recovered heat or cold. When heat recovery is applied, the fresh outside air is pre-heated or pre-cooled by heat recovery from the ventilation exhaust air. The uestion of whether or not to operate heat recovery is generally determined by the temperature difference between the outside air and the ventilation exhaust air. Reasoning from the perspective of the preceding description of the heating and cooling demand, it can be stated that this is not the correct regulation criterion. It is also the case with heat recovery that no more heat or cold must be fed to the mechanism that the heating or cooling reuirement prevailing at that moment in time. It is therefore important to make sure that when the heat recovery system is activated the optimum energetic heating/ cooling curve is not exceeded and/or that the cooling curve is not fallen short of. The energetic effect of heat recovery and the areas in which this should be activated is shown in figure 10. The heat recovery system may only be activated if water humidification (adiabatic) is applied so that the fall in temperature in the air humidifier can be compensated with the extra recovered heat. The example given in figure 10 is based on a situation in which the optimum energetic heating/cooling curve can be achieved without having to make adjustments under the influence of the filters from STEP 5. Heat recovery can achieve savings because the recovered heat or cold that is added to the fresh air no longer has to be fed into the AHU. d = warming up of supply air AHU by use of heat recovery unit (K) Heat recovery (HR) 0 heat recovery ON maximum L heat recovery ON, between 0-100% TO η H HR heat recovery OFF *( Exhaust E ) Outside air temperature E (ºC) Exhaust heat recovery ON maximum Page: 17 of 26

18 Figure 10: The effect by use of a heat recovery system Recirculation Recirculation can be used to make sure that the temperature difference between the supplied air and the room/ zone air does not become too big. Recirculation of air does not affect the building's heating and cooling reuirement, and therefore has no influence on the heating and cooling supply uantity as given in step 2 (which is the building's pure heat balance without taking account of the comfortable supply temperatures). The energy consumption for heating and cooling will therefore not be affected by recirculation (assuming that the amount of fresh outside air remains unchanged). It is possible that the ventilators will reuire a higher capacity for recirculation, which will result in an increase in the energy used for the ventilators. If recirculation is applied, it will be necessary to increase the mixing of the fresh outside air and the recirculation air. The final supply temperature will therefore consist of a mixture of the temperature given in figure 9 and the temperature of the recirculation air. The value of the exhaust temperature is eual to: EXHAUST φ + φ DISSFAN 2 DISSLIGHT = I + [14] VENT ; TOTAL * ρ *CP EXHAUST = temperature of the mechanical discharge air [ C] I = the room/ zone temperature [ C] (make a distinction between heating and cooling season) φ = the heat dissipation of ventilators in exhaust channel [W] DISSFAN 2 φ DISSLIGHT = the heat extracted at the lighting armatures [W] ρ = 1,2 = specific mass of the air [kg/m 3 ] C P = 1000 = specific heat of the air [J/kg.K] The level of recirculation is expressed in the recirculation fraction R: R VENT ; TOTAL VENT ; OUTSIDE = [15] VENT ; TOTAL R = the recirculation factor [-] ; = total air uantity of mechanical ventilation air [m 3 /s] VENT TOTAL ; = fresh air uantity of mechanical ventilation air [m 3 /s] VENT OUTSIDE The heating and cooling curve, including the recirculation air, can be calculated as indicated in figure 11. Page: 18 of 26

19 Supply air temperature AHU (ºC) 40 Basic shape Heat Exhaust (1-R) (R) Outside air temperature L TO E (ºC) H Exhaust Figure 11: Calculating the heating/cooling curve, including recirculation air The example given in figure 11 is based on a uantity of recirculation air that is eual to the amount of outside air. In this case, the recirculation factor R is 0.5. The effect of the recirculation factor R is shown in the figure. The resulting air temperature (= green dotted line) is determined on the basis of the course of the temperature of the fresh outside after treating for heat or cooling reuirement (= red/blue line) and the course of the exhaust temperature (= purple dotted line) using the recirculation factor R. The heat dissipation of the supply ventilator must be added to calculate the final heating/cooling curve. The level of the heating of the ventilation air can be calculated as follows: d SUPPLY, FAN = dpsupply /10 [16] = heating up of the supply air resulting from the supply ventilator [K] d SUPPLY, FAN dp SUPPLY = ventilator pressure of the supply ventilator [Pa] 10 = 10 = heat capacity of the air [J(/m3.K)] Use of free cooling In the greater part of the range of the outside temperature, it is advantage from a minimum energy consumption viewpoint to ventilate a minimum necessary uantity of fresh air. If reuired, the total air flow can be increased with a share of recirculation air. For a specific area in the temperature range of the outside temperature, it is worthwhile to have the amount of fresh outside air increased. This makes it possible to make use of what is known as 'free cooling', which makes optimum use of the cooling capacity of the outside air. There are two conditions that 'free cooling' has to meet: there is a cooling demand in the building (section) the outside temperature is lower than the exhaust temperature. The temperature area that meets both conditions and in which free cooling can be applied is the temperature area of the outside temperature between the high temperature of the transition area H and the exhaust temperature EXHAUST. Page: 19 of 26

20 The desired settings of the heating/cooling curve are not affected by the application of free cooling (the total air flow remains constant) Step 5 : Discounting all preconditions and limitations In order to finally calculate the heating/cooling curve, it is desirable to indicate which areas are still free to select the temperature level without this being accompanied by extra energy consumption (see steps 2 to 4) of that the heating/cooling curve leads to comfort complaints or other problems in the building. Use is made of filters in step 5 to prevent this from happening. These filters will indicate which temperature areas are undesirable for certain reasons. Filter 1 Filter relates to the minimum heating and cooling demand aspect of the building (see steps 2 to 4). The analysis of the building's heating and cooling reuirement makes it possible to define a temperature area of the central supply air in which the heating/cooling curve must be NOT located from the viewpoint of minimum energy consumption. This is shown in more detail in figure 12. The areas shaded blue and red are 'undesirable' (forbidden) because these areas no longer meet the condition of minimum heating or cooling demand. The green line in figure 11 forms the basis for the content of filter 1 (see step 4). The example given in figure 12 is based on mechanical ventilation without recirculation (R=0). Supply air temperature AHU (ºC) 40 Forbidden zone heating demand Slope C; 10 Slope H; Forbidden zone cooling demand Outside air temperature E (ºC) L TO H Figure 12: Filter 1, which is determined by the building's heating and cooling reuirement pattern. Page: of 26

21 Filter 2 Filter 2 is based on preconditions that are often set for technical reasons. These preconditions vary between projects. For this reason, the preconditions must be set individually for each project. The explanation given below is based on a model situation. Draught risk Comfort complaints resulting from draughts can be caused by the temperature difference between the supply temperature and the ambient temperature being too great. The risk of draughts depends significantly on the type of end device used. comfort criteria If the effect of turbulence and the temperature gradients is neglected, it will generally be the case that the following criteria must be operated for the air temperature and the air velocity: summer: Operative < 26 C and v < 0. m/s; (PMV = +0.5) winter: Operative > C and v < 0. m/s. (PMV = -0.5) Cooling and heating in the rooms can be achieved by mixing cold air or warm air with the ambient air. The end devices/ terminal units (air supply grids, fan coil units, induction units, etc.) are determinative for the air velocities and the distribute of the temperature in the room. This is of course within the restrictions of the room geometry, the positioning options and the necessary capacities (and/or temperature differences). Air supply grid The limits that can be set for the maximum and minimum temperatures of the supply air depend significantly on the specific situation (size of room, position of air supply grid, etc.) For this reason, the example is based on an average situation. Assuming the customary offices and the current air supply grid types, the design values given below are applicable to the maximum and minimum temperatures of the supply air. TYPE OF GRID TEMPERATURE DIFFERENCE BETWEEN THE SUPPLY AIR AND THE AMBIENT AIR [ C] Minimum (cooling) maximum (heating)* Wall grids Wall grid Ceiling grids Line grid Induction grid Vortex grid * The maximum temperature of the supply air is not critical. As things stand, when heating, a temperature difference higher than +10 C will not be selected. When selecting the air supply grid, account must be taken of matters such as the circulation multiple to be achieved. The maximum circulation multiple for a line grid is approximately 15. For a vortex grid, however, the circulation multiple can be a maximum of 50 to guarantee comfort. other end devices/ terminal unit The other end devices such as radiators, fan coil units, induction units and climate ceilings do of course have a considerable effect on the comfort aspect. For the fan coil units and induction units installed in the ceiling, the centrally conditioned ventilation air has already been mixed with ambient air that has been extracted from the room. The resulting temperature from both of these units will from the viewpoint of the draught risk resulting from an excessively large temperature difference between the supply Page: 21 of 26

22 temperature and the ambient temperature be lower than the units fitted to the wall in which the air is supplied to the habitation zone. Supply air temperature AHU (ºC) terminal unit Maximum over (surplus) temperature Maximum under temperature terminal unit I Outside air temperature E (ºC) TO Figure 13 : Filter 2 relating to the draught risk section Humidification in the winter period When humidifying, the air will have to be humidified to a moisture content of 5 to 6 g/kg. With an average moisture production level in the building of 1.5 g/kg ( =7 g/kg), at a room/ zone temperature of 22 C the relative humidity (RH) will be eual to approx. 40%. Following the humidification battery in the AHU there will be an RH of approx. 90%. To achieve 5.5 g/kg with RH=90%, the air temperature (assuming steam humidification) after the humidifying battery will have to be at least 7 C. Dehumidifying in the summer situation During the summer period, cooling to a minimum of 16 C (11.5 g /kg) must be applied in connection with dehumidifying the outside air. At an average humidity production of 1.5 g/kg ( =13 g/kg) and a room/ zone temperature of 24 C, the relative humidity will be 70%. If the ventilator dissipation is discounted, the supply temperature is eual to C. 40 Supply air temperature AHU (ºC) forbidden zone de-humidify (summer) forbidden zone humidify (winter) Outside air temperature E (ºC) Page: 22 of 26

23 Figure 14 : Filter 2, relating to the section humidifying and dehumidifying If filter 1 and filter 2 are combined, this results in a new filter that is shown in figure 15. Condensation risk Pipes For the end devices/ terminal units that are supplied with preheated or cooled water, temperature range under which they are mainly applied can be defined. The following temperature ranges are the most current ones: LTC MTC HTC LTH HTH radiators X X fan coil units X X X induction units X X X climate ceilings X X X X Cooling LTC: low temperature cooling (6-12 C); MTC: medium temperature cooling (12-16 C); HTC: high temperature cooling (16 - C); Heating VLTH: very low temperature heating (< 35 C) LTH: low temperature heating (35-45 C); MTH: medium temperature heating (45 55 C) HTH: high temperature heating (< 55 C). Air ducts The condensation risk in air ducts is partly determined by the surface temperature of the air ducts. This surface temperature is determined by the air temperature of the mechnical ventilation air, which is created by the central AHU. This air temperature is the result of the analysis described in this chapter. The condensation risk on pipes and on ducts can be estimated by making an assessment of the dewpoint temperature (which is an indicator of the absolute humidity) in the room and comparing it with the surface temperatures of pipes and ducts. If the surface temperature falls beneath the dewpoint temperature of the room/ zone air, condensation will occur. The air humidity in the room is determined by the air humidity of the mechanical ventilation air, the air humidity of the outside air (natural ventilation and infiltration), the moisture production and the moisture accumulation in the walls. Since the condensation risk depends greatly on the project-specific conditions (especially the air humidity achieved in the room), no further details of this section are given. It is however advisable always to make a risk assessment for condensation. Figure 15 shows the result of filter 1 and filter 2. Page: 23 of 26