The Concrete Centre. Dynamic Thermal Properties Calculator

Transcription

1 The Concrete Centre Dynamc Thermal Propertes Calculator The Concrete Centre Dynamc Thermal Propertes Calculator

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3 The Concrete Centre Dynamc Thermal Propertes Calculator Contents Page 1 Introducton 1 2 User gude Software requrements 3 3 Theory Admttance parameters (ISO 13786) Internal heat capactes, and 30 (ISO 13790) 5 4 References and Further Informaton 6

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5 The Concrete Centre Dynamc Thermal Propertes Calculator 1 Introducton Ths gudance note descrbes The Concrete Centre s Dynamc Thermal Property Calculator tool (developed for TCC by Arup). The tool s an Excel spreadsheet that calculates parameters relatng to the dynamc thermal storage of constructon elements. Two groups of parameters are calculated: 1) thermal admttance, Y, and related quanttes, as specfed n EN ISO 13786:2007 and 2) surface heat capactes, (or Cm ), as specfed n EN ISO 13790:2004, whch are used n the SAP(2009 draft) and SBEM calculaton tools supportng Part L of the Buldng Regulatons. The specfc parameters calculated are: Thermal admttance parameters: Surface heat capactes - Thermal admttance, Y [W/m²K]. Ths s the amount of energy leavng the nternal surface of the element nto the room per unt degree of temperature swng. Ths s under theoretcal condtons where the nternal envronmental temperature undergoes perodc oscllaton and the external envronmental temperature s constant. - Tme lead for thermal admttance [hours]. Ths s the tme dfference between the tmng of the peak heat flow at the nternal surface and tmng of the peak nternal temperature. - Decrement factor [dmensonless]. Ths s the rato of the peak heat flow out of the external surface of the element per unt degree of external temperature swng to the steady state heat flow through the element per unt degree of temperature dfference between the nternal and external envronmental temperatures. - Decrement delay [n hours]. Ths s the tme lag between the tmng of the nternal temperature peak and the peak heat flow out of the external surface. - Surface factor [dmensonless]. Ths s the rato of the swng n heat flow from the nternal surface of the element to the swng n heat flow receved at the nternal surface of the element (such as solar gan from a sun patch). - Tme lag for the surface factor [n hours]. Ths s the tme lag between the tmng of the peak heat flow enterng the surface and peak heat flow leavng the surface nto the room. - Areal heat capacty [kj/m²k]. Ths s the amount of energy stored n the element over the frst half perod of the heat flow swng per unt area of element per unt degree of temperature swng. The same amount of heat s released n the followng half perod. - U-value [W/m²K]. Ths s the steady state heat flow through the element per unt degree of temperature dfference between the nternal and external envronmental temperatures per unt area. Note that ths quantty may dffer from other U-values quoted as t does not take account of thermal brdgng and s senstve to the values of surface thermal resstances used. - [kj/m²k]. Ths s the volumetrc heat capacty of the thermally actve part of the constructon up to a maxmum thckness of 100mm [kj/m²k]. Ths s the equvalent of but for a maxmum thckness of 30mm 2 User gude The spreadsheet contans three sheets: Input, Full Results and Reference Data. Error! Reference source not found. shows a screen shot of the Input sheet. On ths sheet the user specfes the constructon buld up and can alter (optonal) the tme perod, the nteror and exteror heat transfer coeffcents and the locaton of the element (external or nternal). The user can also enter project detals. Further detals on data entry are gven below. Page 1

6 On the Input sheet some key output results are also shown for convenence. These are admttance, decrement factor, decrement delay and value (note that decrement factor and decrement delay are not calculated for nternal elements). A graph showng the ampltude and phase of the swngs n nternal and external heat flows and nternal temperature s also dsplayed. The Full results sheet contans the full set of calculated parameters, lsted n Secton 1 together wth a summary of the constructon buld up and other assumptons on whch the calculaton s based. The Reference Data sheet contans a lst of thermal propertes of common constructon materals, taken from CISBE Gude A. The followng nstructons explan how to use the spreadsheet. 1) Fll n the calculaton settngs. a. Enter the perod of oscllaton of the nternal envronmental temperature. b. Enter the nternal surface resstance for the element. (Refer to Table 1 for typcal values). c. Enter the external surface resstance. d. Select whether the element s an external element (.e. separatng a condtoned space from the outsde) or an nternal element (.e. separatng two condtoned spaces). 2) Fll n the Element constructon. a. Select the layer type for each layer before enterng any other detals for the layer. Select Sold Layer f the layer s a layer of buldng materal. Select Cavty Unlned f the layer s a cavty wthout a fol lnng. Select Cavty Fol-lned f the layer s a cavty wth a low emssvty fol lnng. If these automatcally calculated cavty resstances are not approprate, select the Cavty User defned thermal resstance. b. Once the layer types have been selected, the parameters requred for each layer wll be hghlghted. All of the hghlghted felds must receve entres for the calculaton to be carred out. c. Enter a layer name to dentfy each layer. d. Enter the thckness of each layer. e. Enter the densty, specfc heat capacty and thermal conductvty of each sold layer. Values of these propertes for common materals are provded n the Reference data sheet. These may be coped and pasted nto the nput felds. f. Enter the thermal resstance of each cavty wth a user defned thermal resstance. The layers do not need to be n consecutve rows n the Element Constructon table. Empty rows wll be gnored by the calculaton. Once all the requred parameters for a layer have been entered, the layer may be coped and pasted nto other locatons wthn the Element Constructon table. To delete a layer, delete the layer type for the layer. 3) Clck the calculate button to calculate the result for the element. The results wll be presented n three formats. a. Key. Ths shows the most commonly used results. b. Heat flow plot. The plot shows a unt perodc swng n the nternal envronmental temperature. The plot also shows the surface heat flows nto the nternal surface and out of the external surface resultng from the plotted swng n nternal temperature and a constant external temperature. Ths s a graphcal nterpretaton of the admttance and the decrement factor, respectvely. Page 2

7 The Concrete Centre Dynamc Thermal Propertes Calculator c. Full Results sheet. Vew ths by clckng on the tab at the bottom of the wndow. Ths shows all the nput parameters that have been used n the calculaton and the thermal resstance used n the calculaton for each cavty layer. The Full Results sheet also gves a full lst of the dynamc thermal propertes calculated. If any error messages are produced whlst usng the calculator, follow the nstructons to check the nput parameters are vald. The programme checks that the user nputs are adequate to perform the calculaton. There are no restrctons, however, on whether the nput values are realstc. If the error message perssts when there seems to be no problem wth the entered data, open an empty verson of the calculator and re-enter the data. 2.1 Software requrements The tool was developed on the 2003 Servce Pack 3 verson of Excel and has also been tested on Excel Theory 3.1 Admttance parameters (ISO 13786) Thermal admttance s a framework to quantfy how a constructon element responds to perodc (snusodal) cycles n temperature and heat gan. Ths framework s descrbed fully n EN ISO 13786:2007. The admttance framework also provdes the bass for the CIBSE Smple Dynamc Model for calculatng coolng loads and summertme space temperatures (CIBSE (2005) Gude A: Envronmental desgn). The admttance framework calculates parameters descrbng the dynamc thermal propertes of constructon buld ups composed of homogeneous, planar elements, from the thermal propertes (thermal conductvty, specfc heat capacty and densty) of the materal layers and cavtes that make up the constructon. The two prncpal admttance parameters are admttance and decrement factor. These descrbe the response of the constructon element to a perodc (snusodal) thermal forcng consstng of a heat flow nto the surface from the room (by radatve and conductve heat transfer processes). The admttance descrbes the rate of heat flow nto the nteror surface of the element and decrement descrbes the rate of heat flow from the exteror surface of the element. In both calculatons t s assumed that the room ar temperature s varyng and the outsde ar temperature s constant (smlar concepts can be defned reversng these assumptons). Each parameter descrbes the ampltude of the response and also has an assocated tme lead or lag between the tmng of the response and of the forcng. The tme perod for the thermal forcng s usually taken to be 24-hours, but any tme perod can be used and the tool enables the tme perod to be used to be specfed by the user. Although admttance s sometmes taken to be a measure of thermal storage, ths s not strctly true, snce Y only descrbes the heat flow nto the surface, whereas t s the dfference between the heat flow nto the nteror surface of the element and that leavng the exteror surface that relates to the heat storage acheved. To calculate the effectve thermal storage, therefore, requres both Y and decrement factor to be used and the assocated tme delays. Ths s done n the calculaton of the areal heat capacty whch descrbes the effectve heat storage acheved over one half cycle. The thrd parameter calculated s the surface factor. Ths s not ncluded n ISO but s used n the CIBSE smple dynamc model. It relates to the ablty of a surface element to absorb ncdent radatve heat gan, e.g. from a sun patch on the surface. Ths parameter also has an ampltude and a tme lag. The theoretcal scenaros formng the bass of these three parameters are shown schematcally n Fgure 1. Page 3

8 Tˆ Admttance, Y Tˆ Tˆ e Decrement factor, f Tˆ U e absorb Surface factor, F absorb Fgure 1: Schematc of the scenaros under whch the dynamc thermal characterstcs are calculated. s the swng n heat leavng the nternal surface of the element nto the room, Tˆ s the swng n nternal envronmental temperature, Tˆ e s the swng n external envronmental temperature, U s the U-value of the element and absorb s the heat absorbed by the surface (heat flows are shown n orange and temperatures n green). To calculate the admttance parameters, the tool follows the methodology set out n ISO 13786:2007 Thermal performance of buldng components Dynamc thermal characterstcs Calculaton methods. Two mportant features of the calculaton requrng clarfcaton are the values used for the surface heat transfer coeffcents and the thermal resstance of any nternal cavtes n the constructon Surface heat transfer coeffcents The surface heat transfer coeffcents used n the default calculaton are those for horzontal heat transfer. Ths s obvously approprate for a vertcal wall. For a horzontal slab, typcally the drecton of heat transfer alternates between upwards and downwards over the durnal cycle. It s therefore approprate to use these ntermedate values for the surface heat transfer coeffcent. In cases where the heat transfer s ether always upwards or always downwards the user may alter the surface heat transfer coeffcents to the conventonal surface resstances gven n BS EN ISO 6946:2007 Buldng components and buldng elements Thermal resstance and thermal transmttance Calculaton method and shown n Table 1. Page 4

9 The Concrete Centre Dynamc Thermal Propertes Calculator Surface Resstance m 2 K/W Drecton of Heat Flow Upwards Horzontal Downwards Rs Rse Table 1: Conventonal heat transfer coeffcents gven n ISO Cavty thermal resstance In stuatons where the constructon buld up has cavtes the thermal resstance of these cavtes needs to specfed or calculated. The tool allows the user to ether nput a cavty thermal resstance based on an offlne calculaton or can calculate the thermal resstance from a specfcaton of cavty thckness. The calculated values based on thckness are taken from the tabulated values gven n the CIBSE Gude A (2005). Two types of cavty can be specfed: unlned or fol-lned (low-e). To mprove the nterpolaton of ntermedate values between these tabulate values the methodology set out n BS EN ISO 6946 was used. To use ths methodology to reproduce the values gven n the CIBSE Gude A requred a number of assumptons to be made, n lne wth the standard. For a fol lned cavty, the convectve heat transfer coeffcent could not be less than 1.39 W/m 2 K as the cavty got wder; the black-body radatve coeffcent was 5.7 W/m 2 K; the emssvty of the fol-lned surface was 0.16; and the emssvty of the unlned surface was 0.9. The drecton of heat transfer through the cavty s assumed to be horzontal for the same reasons as the heat transfer from the nternal and external surfaces s assumed to be horzontal. If ths s not approprate the user may set the thermal resstance of the cavty by selectng the Cavty - user defned thermal resstance layer type. 3.2 Internal heat capactes, and 30 (ISO 13790) The tool calculates two surface heat capactes, and 30. These quanttes are not drectly related to the admttance framework but to a smpler framework for calculatng thedynamc thermal response of buldng elements used n ISO 13790:2004 Thermal performance of buldngs Calculaton of energy use for space heatng. 1 These quanttes relate smply to the volumetrc heat capacty of dfferent parts of the constructon element. The way n whch they are calculated s descrbed below. s used n SAP (2009, draft) and the SBEM calculaton tools supportng Part L of the Buldng Regulatons. It s calculated (n unts of kj/k per m² of surface area) from: 10 6 c d where: s the densty of layer n the constructon (kg/m³) c s the specfc heat capacty of layer (J/kg K) d s the thckness of layer (mm). 1 Note that ISO uses the notaton rather than and n SBEM was formerly referred to as Cm. Page 5

10 The summaton s done over all layers n the element, startng at the nsde surface and stoppng at whchever of these condtons occurs frst (whch may mean part way through the layer) half way through the constructon an nsulatng layer a maxmum thckness of 100mm An nsulatng layer s defned here as an element havng a thermal conductvty of less than 0.08 W/mK. Ths threshold s taken from the User Gude to SBEM verson 3.3.b (Note: ISO and SAP(2009) do not specfcally defne a crtera for an nsulatng layer). In the case of cavtes, an effectve thermal conductvty for the layer s calculated by dvdng the cavty thckness by the cavty thermal resstance. The surface heat capacty, 30, s calculated n a smlar way to but the maxmum thckness for the summaton s 30mm. Ths quantty s used n ISO to model thermal storage effects where heatng s ntermttent. It s not used n SAP or SBEM. Note: s a relatvely smplstc way of charactersng the thermal mass of a constructon element as, n addton to the assumptons made regardng the thermally actve layer thckness, t does not take account of the effect of thermal conductvty n determnng the tmescale for heat to enter and leave the element. The areal heat capacty defned n ISO provdes a more accurate measure of the effectve heat capacty of a constructon element. 4 References and Further Informaton Envronmental Desgn CIBSE Gude A Chartered Insttute of Buldng Servces Engneers 2006 EN ISO 13786:2007 Thermal performance of buldng components Dynamc thermal characterstcs Calculaton methods Internatonal Organzaton for Standardzaton 2007 EN ISO 13790:2004 Thermal performance of buldngs Calculaton of energy use for space heatng Internatonal Organzaton for Standardzaton 2004 EN ISO 6946:2007 Buldng components and buldng elements Thermal resstance and thermal transmttance Calculaton method Internatonal Organzaton for Standardzaton 2007 Harrngton-Lynn, J. The Admttance Procedure: Intermttent Plant Operaton Buldng Servce Engneerng Volume 42 pg Mlbank, N. O. Harrngton-Lynn, J. Thermal Response and the Admttance Procedure Buldng Servce Engneerng Volume 42 pg Loudon, A.G. Summertme Temperature n Buldngs Wthout Ar-condtonng IHVE/BRS Symposum 1968 Daves, M.G. Hourly Estmaton of Temperature Usng Wall Transfer Coeffcents School of Archtecture and Buldng Engneerng, Unversty of Lverpool 1995 Thermal Mass Explaned - The Concrete Centre 2009 Publshed by MPA The Concrete Centre Gllngham House, Gllngham Street, London SW1V 1HU Tel: +44 (0) All advce or nformaton from MPA The Concrete Centre s ntended only for use n the UK by those who wll evaluate the sgnfcance and lmtatons of ts contents and take responsblty for ts use and applcaton. No lablty (ncludng that for neglgence) for any loss resultng from such advce or nformaton s accepted by MPA The Concrete Centre or ts subcontractors, supplers or advsors. Readers should note that publcatons from MPA The Concrete Centre are subject to revson from tme to tme and they should therefore ensure that they are n possesson of the latest verson. Page 6