2012_ J.cho Thermal Modelling

8/17/2019 2012_ J.cho Thermal Modelling

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CIBSE ASHRAE Technical Symposium, Imperial College, London UK – 18th and 19

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Thermal modelling and parametric study of transpired solar collector

YJ Cho*a, A Shuklaa, D N Nkwettab! P Jonesa Welsh School of Architecture, Cardiff Universitya

Sustainable Building Envelope Centreb

*[email protected]

AbstractBeing a cost effective solar application, Transpired Solar Collector (TSC) system hasbeen widely used in North America and Europe. In order to estimate the energyperformance of TSC system, software programs such as RETScreen and SWift arecurrently available. These softwar e programs’ empirical models are specificallydesigned for the analysis of TSC. However, TSC estimation tools have rarely beencritically analysed by other researchers.

This paper introduces a new estimation tool for modelling the energy performance ofbuilding integrated TSC. The program is called SBET (Sustainable BuildingEstimation Tool). This paper focuses on three parts of calculation algorithms, i.e. thecapture of solar energy delivered, together with destratification and insulationsavings. The calculation algorithms for these savings are presented here togetherwith an explanation of their derivation. Lastly, the TSC thermal performance results ofSBET estimation tool is compared with those of currently available softwareprograms, using a consistent benchmark design parameters and weather data.

Keywords Solar air heating, transpired solar collector, heating

1.0 IntroductionMany designs for solar air heating have been reported and discussed in the literature[i ,ii ,iii ,iv ,v ,vi ] e.g. bare plate, back-pass, glazed, unglazed, covered, uncovered,perforated, un-perforated, single pass, double pass, triples pass etc. Transpired solarcollector is a modified form of these solar air heaters in a cost effective way.

Transpired Solar Collector (commonly known as Solarwall) has widely been used inCanada, USA and Europe over the past decade because it provides considerablesaving in energy and installation cost[vii ]. As can been seen in Figure 1, thisunglazed solar collector with a perforated absorber layer is integrated to the building

façade. The collector absorbs solar radiation using the absorber surface, made of ametallic absorber sheet. The ambient air in the vicinity of absorber surface is heatedand transferred through the multiple small perforations into the cavity between theskin and façade. Heated air in the cavity is drawn into the building in order to providespace heating.

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Figure 1 - Schematic of Transpired Solar Collector (TSC) system

With regards to the influential parameters on Transpired solar collectors (TSC), the

parameters such as perforation configurations, solar radiation, air flow rate, approach

velocity and absorptivity have been studied both theoretically and experimentally

[v,vi ,viii ,ix ,x ,xi ] .

In order to estimate the energy performance of this solar air heating system, software

programs such as RETScreen[xii ] and SWift [xiii ] are currently available. These

software programs use empirical models based on IEA report [vii ] and Car penter’s

model [xiv ]. However, despite the market dominance of these estimation tools, they

have rarely been the subject of academic review. Therefore, the characteristics of

these tools are briefly explained in Table 1.

Parameters RETScreen SWift

Use of weather data NASA monthly data are

applicable for worldwide.

Hourly data for North America but

monthly data for the other

regions.

Calculation algorithms i.e.

manual, of software are

provided?

Yes No, however the calculation

algorithms are assumed to be

similar to RETScreen.

Applicable for various types

of buildings?

Yes Yes

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Parameters RETScreen SWift

Azimuth and slope

adjustment of collector, in

relation to solar radiation.

Yes, various angles are

adjustable by the user.

Limited angles are provided.

Selection of absorber’s

colours relevant to

absorptivity

Yes, various colours are

selectable and user definable

absorptivity coefficient.

Limited colours are available.

Canopy type and colour No Limited

Exhaust flow rate and

location

No Limited

Roof area for

destratification calculation

Floor area applied for simplicity. The area is not specified by the

user. However, the floor area

seems to be derived using a

certain occupancy density and

the designed flow rate.

The effect of terrain type

and building height on wind

speed

For a simplistic way, the wind

speed at collector is obtained

using a correction factor of 0.35.

Unknown.

Table 1 – Characteristics of two popular estimation tools (SWift andRETScreen)

Through understating characteristics and calculation algorithms of two currentestimation tools, a new estimation tool of building integrated TSC is developed in theSustainable Building Envelops Centre (SBEC). The program is called SBET(Sustainable Building Estimation Tool). This paper focuses on calculation algorithmsfor three aspects of TSC's performance, i.e. the capture of solar energy delivered,plus destratification and insulation savings. These calculation algorithms arepresented here together with an explanation of their derivations. Lastly, TSC thermalperformance results from the developed SBET estimation tool are compared withthose of currently available software programs, using a consistent benchmark designparameters and the same weather data.

2.0 Thermal modellingThe SBET tool models solar energy delivered, solar air heating utilisation,destratification savings and insulation savings as follows.

2.1 Solar Energy DeliveredSBET includes for the use of both monthly and hourly weather data, so providing the

flexibility to model different location data. Tilt radiation is obtained using thehorizontal solar radiation based on the Liu and Jordan’s isotropic diffuse algorithm

[xv ]. The collector efficiency and the solar air heating utilisation, as described below,

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are used to obtain the total amount of usable solar energy, the average daily amount

of solar energy incident on the collector, and the collector efficiency.

2.2 Collector EfficiencyThe collector efficiency in SBET was derived in a similar way with the RETScreen

[xii, xiv ]. The SBET model considers influential parameters such as the designedflow rate, the wind speed, the absorptivity and radiation flux. The collector efficiency

is derived from a heat balance on the collector, incorporating the collector efficiency

curve in Figure 2.

pccolQ

f R

colQ

wind U

coll

1801 [1]

Where α: absorptivity, U wind : wind speed, colQ

: volumetric flux of collector flow

(m3/s.m2), f R : the regression coefficient; pc : heat capacity (1005 J/kg.K), : density

of air ( 1.213kg/m3).

The collector efficiency (col ) is analogous to that of Kutscher et al. [xvi ] and can be

explained as how efficiently the heat is delivered by transpired solar collector system.

The major parameters associated in the collector efficiency are absorptivity ( ), flow

rate, wind speed and regression coefficient.

2.2.1 Collector efficiency curveThe relationship between collector efficiency and flow rate in the SBET model are

shown in Figure 2.

Figure 2 - Collector efficiency curve

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Two SBET curves of different absorptivities and a regression coefficient of 7 are

compared with Carpenter’s model [xiv ].

2.2.2 Absorptivity

Absorptivity of is a major parameter that influences the collector efficiency [v , vii ].The increase in the absorptivity is strongly related to the increase in radiation heat

and can extend up to 1.

2.2.3 Designed flow rate The designed flow rate is determined by the required flow rate for occupants. An

increase incoll

Q

can give a higher collector efficiency [vii ,xvi ], however, any increase

incoll

Q

is limited by the damper control, in order to meet the ventilation requirement .

2.2.4 Wind speed The parameter of wind speed can reciprocally affect not only the collector’s flow rate

but also the collector’s efficiency [ vii , x ,xiv ]. The wind speed calculation contained

in the SBET model includes the effect of the terrain types and the building height.

Based on CIBSE AM10 [xvii ] , the wind speed is calculated as:

a

met wind zk U U [2]

Where U met is the wind speed at a height of 10m, provided from the Met office. The

reference wind speed of at height z, U met is affected by the building height and the

terrain types. The coefficients of k and a are determined by the terrain where thebuilding is situated, as shown in Table 2.

Terrain types K a

Open flat country 0.68 0.17

Country with scattered wind breaks 0.52 0.2

Urban 0.35 0.25

City 0.21 0.33

Table 2 - Terrain coefficients of k and a

2.2.4 Regression coefficient (Rf) When a collector area is designed, the increase in collector area results in more

absorbing solar energy and less air volume flux. Thereby, the temperature rise in

TSC is dependent upon the change in air volumetric flux and radiation flux [v,vii ].

Thus, the regression coefficient considers the additional geometrical term relating

the effect of the temperature rise due to this radiation flux factor for collector

efficiency is defined using the ratio of iriseT , to iref T , and written as:

51,

,

iref

irise

f T

T R [3]

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where the reference temperature rise in a month i ( iref T , ) is obtained based on the

solar radiation flux of 1000W/m2 and a certain amount of designed flow rate, see the

following regression graph in Figure 3. The designed temperature rise ( iriseT , ) is

derived from the user specified tilt radiation flux and the designed flow rate.

Figure 3 - Temperature rise according to the flow rate and the tilt solar

radiation flux

2.3. Solar Air Heating UtilisationIn transpired solar collector heating systems, useable energy capable of reducing the

heating load demand is imperative and any non-useable energy should be avoided

by using bypass dampers. To simulate this system, the factors such as the damper

control, the usable delivery temperature, the supply temperature and so on should be

known. These factors are described below.

2.3.1 Damper controlModulating dampers are most often used in solar air heating systems to utilize the

useable solar energy of TSC. Dampers modulate to maintain some preset

temperatures for an optimum air heater system efficiency. The failed modulating can

prevent the air handling unit from operating properly for maximum energy efficiency.

When the preset temperatures are specified by a user, they should consider the

actual temperature rise, the design flow rate, the collector flow rate due to the

weather conditions.

In SBET model, this modulating process is simplified with two preset temperatures,

i.e. minimum supply temperature and maximum supply temperature. The user

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specified temperatures are associated not only with damper control but also

distinguishing the seasons.

Figure 4 - Schematic of Temperature and Flow control

The minimum supply temperature and the maximum supply temperature are used to

classify cold days and hot days respectively. Also, these supply control temperatures

are associated with controlling the collector flow rate on warm/cold days. On cold

days the solar collector alone may not achieve the space set temperature. In case,

the air heater system would utilize the recirculation air from the space. The warmer

recirculation air induced would be mixed with the TSC air, and the mixed air is

heated by a gas burner to achieve the space set temperature. Therefore, TSC

controller controls the collector flow rate and recirculation flow rate using the

motorised dampers. For simplicity, the flow and temperature controls for solar air

heating system can be controlled as can be seen in

Figure 4. A simple heat flow balance is written as:

sdesignrecirccoldesignact col T QT QQT Q

)( [4]

where T s : the supply temperature, normally 8 ~10oC higher than space set

temperature, T recirc : the recirculation temperature =(Tspace +Tstrat)/2, T tsc : the

collector temperature,coll

Q

: the collector flow rate,designQ

: the design flow rate.

Using the eq.[4], the collector flow rate is obtained as:

[5]

#$%&'%()*+&,-

./0

123

sdesign T Q

/4*%$

recirccoldesign T QQ

)(

act col T Q

indesign T Q

design

recirctsc

recircs

col QT T

T T Q

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Figure 5 - Calculation process for solar energy delivered

The ratio of collector flow rate and recirculation flow rate would dynamically vary

corresponding to the actual TSC delivery temperature and preset supply temperature

of AHU. Consequently, the modulating damper affects the air heater system

efficiency and the useable solar energy collected.

Since it is difficult to calculate the constant quantities, the iteration algorithm is

applied in a similar way with RETScreen methodology. Thus, three iterations areused in the program as can be seen in the calculation process of Figure 5.

2.3.2 Usable delivery temperature rise(ΔT usa )The usable delivery temperature rise can be defined as the maximum increase in

airflow temperature inside collector and calculated using the efficiency and the airflow

rate of the collector and is given as:

isunlight pcol

itilt col

iusa

hC Q

GT

.

.

.

[6]

where itilt G . is the solar radiation incident on the collector at a month i , derived from

the radiation model based on Liu and Jordan’s isotropic diffuse algorithm. isunlight h . is a

sunlight hour obtained using Cooper’s equation [15].

Using the Eq[6] , from external temperature and offset temperature, the usable

temperature is derived as:

)( .... ioffset iext iusaiusa T T T T [7]

Calculate initial collector flow rateusing Eq.[10], initial collector efficiencyusing [1] and initial actual deliverytemperature using Eqs.[6]-[8].

Calculate the variation in actualdelivery temperature withconsideration of damper control, usingflow rate of Eq [4].

Calculate solar energy delivered (Q sol ).

Three successiveiterations

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where ioffset T . is the offset temperature and can be defined as the difference between

the max temperature (T max.i ) and the average temperature (T avg.i ) as the daytime

ambient temperature is normally higher than the average daily temperature.

The usable delivery temperature ( iusaT , ) in Eq.[7] is compared with the supply

temperature ( sT ) in order to determine the actual delivery temperature (T act.i ) and the

equation is written as:

),min( ,. iusasiact T T T [8]

The usable delivery temperature in this equation [8] is controlled by the preset supply

temperature. This controlled delivery temperature is called the actual delivery

temperature. This actual delivery temperature is used to justify how actually the

usable heat is delivered from TSC to the space.

2.3.3 Calculation process with three successive iterations

In order to determine the actual usable heat from TSC, the actual delivery rise is

needed and obtained as :

)( .... ioffset iext iact iact T T T T

[9]

In order to apply the three iteration method, initially the collector flow rate is obtained

from the relationship between the collector heat flow and the design heat flow rateand calculated as:

designinscolext s QT T QT T

)()( )0( [10]

As the supply air temperature for heating is is assumed to be normally 8 oC higher

than the space temperature (T in ), the difference between T s and T in is regarded as T s

- T in = 8 for simplicity, the equation [9] is rewritten as:

designext s

col

QT T

Q

))(

8,1min(

)0(

[11]

When the supply temperature is less than the external ambient temperature, the

system mode is considered as the cooling mode, thus the negative results are not

allowed.

The initial flow rate of

)0(

colQ is used for calculating the initial collector efficiency ( )0(

col )

from the Eq. [1] and these quantities are applied into the Eqs [6]-[8] to obtain the

actual delivery temperature)0(

act T .

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This)0(

act T is applied to the Eq. [4] to get

)1(

colQ . Then, three iteration process is

continued until the last actual delivery temperature )3(act T and the last collector

efficiency )3(col are attained.

To determine the energy delivered by the collector over a whole a year ( solQ ), themonthly contributions are summed-up using Eq. [12]:

12

1

...

)3(

.

i

iopicolitilt sol f AGQ icol

[12]

where Qsol :solar energy delivered and f op.i

: the operation function which is

obtained from the Eq. [13] :

iopisys

isunlight

idaytimeop

iop h f

h

h f ..

.

..

.

[13]

2.4Destratification savingThe stratification effect occurs in the space with a high ceiling. Typically the

industrial building has a strong stratification because the high ceiling allows very

warm air to rise and settle near ceiling. A higher ceiling temperature due to this

stratification attributes to a significant amount of additional heat loss through the roof.

Thus, a TSC system provides a ventilation air with through a ceiling duct, and creates

destratification. Airflow from the duct mixes with a much warmer air in the vicinity ofthe ceiling. This mixing process induces a lower temperature of the ceiling.

The SBET calculation model considers the destratification saving which is comprised

of the infiltration heat saving and the conduction heat saving. The equation of

destratification saving is written as:

12

1

..inf .. )(

i

isavisavcondestrat QQQ [14]

Practically, the factors such as the infiltration rate and the ceiling temperature are

critically associated with the heat loss through the roof, and provide a distinguishable

difference between the base building and the proposed building. The equations

below demonstrate how the conduction saving and infiltration saving are determined:

roof roof f strat bstrat iopisavcon AU T T hQ )( ......

[15]

pext strat ext strat iopisav C T T QT T QhQ

)()('inf

'

inf ....inf

[16]

Whereisavcon

Q.. Saving in conduction heat loss (kWh),

isavQ

..inf : Saving in infiltration

heat loss (kWh), inf

Q : Infiltration rate in the proposed building (m3/s), inf '

Q : Infiltration

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rate in the base building (m3/s), bstrat T . : Stratification ceiling temperature in the base

building(oC), f strat T . : Stratification ceiling temperature in the proposed building(oC),

ext T : external air temperature(oC), roof U : U -value of roof (W/m

2.K ), roof A : Roof area

(m2).

2.4.1 Stratification temperature

The stratification temperature of f strat T . is obtained from the regression model based

on the empirical graph in Figure 6 and the models are written as:

C T of I Case forT T o

bstrat bstrat f strat 8.2007.0785.0 ... [17]

1.148.2537.1246.0 ... bstrat o

bstrat f strat T C of II Case forT T [18]

C T of III Case forT T obstrat bstrat f strat 1.145 ... [19]

Figure 6 - Stratification temperature (reproduced from [xii])

When the stratification temperature in the base building ( bstrat T . ) is provided by a user,the stratification temperature of the proposed building ( f strat T . ) is calculated using the

regression model. Therefore users can easily specify the stratification temperature of

their own existing buildings.

2.4.2 Comparison with RETScreen model

The model for the destratification saving is compared with RETScreen model in

Table 3.

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Terms RETScreen SBET

Flow rate designQ

The flow rate for thedestratification saving is used asa ventilation flow rate.

inf '

Q and inf

Q Two infiltration rates arerespectively used for thebase building without TSC

and the proposed buildingwith TSC.

Area floo r A

For a simplicity purpose, thefloor area is used instead of roofarea.

roof A

The area applied in theequation is a roof area.

Specification ofstratificationtemperature

The ceiling temperature of the

building with TSC is an input,

specified by users in an arbitrary

way.

As the ceiling temperature of

base building is an input, the

proposed building’s ceiling

temperature is predicted

based on a regressionmodel.

Table 3 - Comparison of destratification saving calculation

2.5 Insulation savingWhen a TSC is installed on building, there is an additional benefit to save a building

energy. The air cavity inside TSC can keep the heat provided from the absorber plate

and the conductive heat from the space to the TSC wall.

The metal plenum of TSC provides the protection from wind and the provision of

solar energy, the external surface resistance is greater than its normal value of 0.04

m2K / W. The thermal resistance of the TSC cavity varies 0.10 ~ 0.29 m2K / W ,

according to the degree of emissivity and the heat flow pattern in Table 4.

Ventilated air space types Thermal resistance

High-emissivity surface, heat flow horizontal (wallapplications)

Rse = 0.13 m2K/W

Low-emissivity surface, heat flow horizontal (wallapplications)

Rse = 0.29 m2K/W

High-emissivity surface, heat flow upwards (roof applications)Rse = 0.10 m

2K/W

Low-emissivity surface, heat flow upwards (roof applications)Rse = 0.17 m

2K/W

Table 4 - Thermal resistances in ventilated air space types [xviii ]

The model estimates insulation savings under three different patterns based on

sunlight hours: TSC daytime operation, TSC night time operation and no TSC

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operation during daytime . The equations below demonstrate how the insulation

saving is determined:

12

1

......... )(i

idaynopinsinight opinsidayopinsins QQQQ

[20]

where idayopinsQ ... is the daytime insulation saving while the Air Handling Unit (AHU) is

operating to collect the solar energy from TSC for the month i .inight opins

Q ... is the

night time insulation saving, while the AHU is operating in the night time when the

sunlight hour is longer than daytime in a day.idaynopins

Q ... is the insulation saving while

the AHU is not operating in the daytime for the month i .

)}()({ ........inf iext ieff colcwiext incolwidayopidayop T T AU T T AU hQ [21]

)}()({ ........inf iext ieff colcwiext incolwinight opinight op T T AU T T AU hQ

[22]

)}()({ ........inf iext ieff colcwiext incolwidaynopidaynop T T AU T T AU hQ [23]

where h op.day.i : operation time during daytime for a month i, h nop.day.i : non-operation

time during daytime for a month i, U wall : The u value of the wall without TSC, U cw :

The u value of the wall with TSC, T in : Space set temperature, T eff,i : The effective

temperature, T eff,i = 2/3T col,i +1/3 T eff,i consists of one third of external temperature

(T ext,i ) and two third of collector temperature (T col,i ).

There are the difference in space set temperature (T in ) for idayopinsQ ... and idaynopinsQ ... The

T in of non operation time in the daytime is assumed to be 15degC.

Compared to RETScreen, the SBET model does not take into account the insulation

saving from the non operation mode in the night time because the sunlight hour is

normally not longer than the daytime in the winter season. Thus, the insulation saving

in the TSC non operation daytime is only achievable when sunlight is available during

a day. When the space is not in the operating mode, the space temperature could

not be constant and vary with the outdoor temperature. Therefore, the spacetemperature is assumed to be 15degC.

4.0 Comparison with other software programsIn this section, the predictions of SBET model are compared with the results from two

other simulation programs.

The SBET model uses the system design parameters along with hourly/monthly

weather data to determine annual energy savings. The concept is similar to that used

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in RETScreen and SWift. Four different design configurations in Table 5 are tested

and compared on an annual basis to the RETScreen and SWift programs.

Table 5 - Design configurations

The weather data for Cardiff in the United Kingdom was chosen for the comparisons.

The model to model test conditions reflect a typical warehouse building and are given

below.

4.1 Test results and discussions

Three estimation tools are used to estimate the energy savings for the four different

cases using identical weather data. Four different cases are described as follow.

Parameters Case A1 Case A2 CaseB1 Case B2

Flow rate ( m3

/h) 5400 7200 5400 7200Stratification temperature (oC) 22

Indoor set temperature (oC) 19

Max. supply temperature (oC) 28 35

Min. supply temperature (oC) 15 15

Collector size ( m ) 200 100

Weather data: Cardiff

Floor area: 3000m2

Roof area: 3000m2

Building size: 100mx 30mx 15m (L x W x H)

Collector depth: 0.1m

Azimuth : South

Tilt: 90degree wall.

Operation schedule: 7/24

)(a )(b

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Figure 7 - The saving results from three different software programs

Stratification temperature

As can be seen in Figure 7a, three different programs provide different total annual

saving results. The SBET model produces a total saving output that lies at a midpointbetween SWift and RETScreen tools. It is worth noting that the greater difference ofannual saving predictions between RETScreen and SWift was 15% when a validationtest of RETScreen was performed by comparison with SWift [xii ]. However, thisresult shows that the annual saving prediction by RETScreen provides 30~37% lowerthan that produced by SWift, even though there is no difference in weather inputsource.Whereas, the SBET program provides the 4 ~17% decreased values, compared toSWift. The discrepancy is mainly due to different data applications and differentcalculation algorithms.

The destratification savings in Figure 7b shows that the savings from RETScreen andSBET are higher than the SWift. The SWift does not include the parameter ofbuilding geometry but the programs of SBET and RETScreen use the actual buildinggeometry for the destratification saving calculation. Thus the SWift canunderestimate the destratification saving.

Figure 7c shows that the heating delivered of the SWift software program is muchhigher than the other software programs. The overestimation of the SWift is due toabnormally large amount of savings in the summer periods, which has lower HeatingDegree Days (HDD).

The insulation saving results are shown in Figure 7d. There are in generally goodaccordance.

Figure 8 shows how the SBET and SWift programs estimate the annual profile of thesolar energy delivered to the building. The solar energy delivered for two softwareprograms are compared based on the Heating Degree Days (HDD). The difference inheating degree days between SBET and SWift is due to different base temperature.The base temperature of the SWift’s HDD (HDD-SWift) is 18degC but that of SBET’s HDD is 15.5degC.

)(c )(d

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Figure 8 - Solar energy delivered using SBET and SWift

As a result, the system profiles of SBET model automatically responds according tothe heating degree days and reduce the solar heating delivered by itself. However,the SWift program doesn’t react automatically against low heating degree days in

summer periods.

5.0 Conclusions A newly developed model of SBET is introduced in order to estimate the performance

solar air heating system. The calculation algorithms for solar energy delivered,

destratification saving and insulation saving are described in this paper.

The SBET model prediction is compared with two current software programs, based

on the same weather data and the design conditions. The comparison results show

that the saving results lie in between the RETScreen and the SWift models. For afuture work, the monitoring results relevant to TSC parameters would be included for

the comparison with other software programs. The monitored data of TSC

parameters such as TSC supply flow rate and temperature, recirculation flow rate

and temperature, AHU supply temperature, ambient temperature, solar radiation

(beam and diffuse), wind velocity would be useful to examine the model's predicted

data.

AcknowledgementsThis project is supported by the European Regional Development Fund through theWelsh Government.

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