storage of food in developing countries

7/29/2019 Storage Of Food In Developing Countries

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Storage Of Food In Developing Countries; Engineers Without Borders 1

Summary:

The proposed cold store for agricultural produce in India, uses a novel combination ofa solar chimney and ground source cooling. A theoretical solar chimney model isdeveloped and predicts 8.5 air room changes per hour for a glazed design in India.The maximum pressure drop for the design is 0.317 Pascal. An experimental scalemodel does not conclusively verify the theoretical model.

The ground source cooling pipe is modelled in ANSYS and predicts 3.2K of coolingfor 25 metres of pipe, when combined with the solar chimney. The solar chimney isdeemed unsatisfactory for creating the necessary pressure drop for effective groundsource cooling. Further studies must be conducted in the field to ascertain groundsource cooling potential.


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CONTENTS:

Pg

SUMMARY 1

CONTENTS 2

1. INTRODUCTION 31.1 Context 4

1.2 Problem Description 4

1.2.1 Existing Cold Storage Room

1.3 Project Overview 5

1.4 Aims and Objectives 6

2. ANALYSIS OF EXISTING DESIGN 7

2.1 Simplified Schematic of Existing Design

2.2 Assumptions

2.3 Parameters 8

2.4 Building Heat Transfer Theory:

2.4.1 Thermal Transmission

2.4.2 Solar Load through Walls

2.5 Cooling Load Calculation 9

2.6 Discussion of Existing Design 10

3. REVIEW OF POTENTIAL COOLING METHODS 103.1 Mechanical Vapour Compression Refrigeration 11

3.1.1 Grid Electricity

3.1.2 Diesel Generation

3.1.3 Solar Photovoltaic Systems

3.2 Absorption Refrigeration 12

3.3 Natural Cooling Techniques

3.3.1 Evaporative Cooling Swamp Cooler

3.3.2 Roof Ponds

3.3.3 Ground Source Cooling:

3.3.4 Solar Chimneys

3.5 Passive Cooling Techniques 15

4. SELECTION OF COOLING TECHNIQUES & FINAL DESIGN CONCEPT

4.1 Final Design Concept 16


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5. DERIVATION OF CLIMATIC DATA FOR PABAL 17

5.1 Solar Data derivation & Ambient Air Temperature

5.2 Solar Geometry 18

6. SOLAR CHIMNEYS 20

6.1 Theoretical Volumetric Flow Rate Derivation

6.2 Selection of Parameters 23

6.3 Solar Chimney Volumetric Airflow 24

6.4 Variation of Solar Chimney Height and Diameter 26

6.5 Solar Chimney Pressure Drop and Comparison with Empirical Correlation

6.6 Design Recommendations 27

7. EXPERIMENTAL WORK 27

7.1 Introduction

7.2 Experimental Design 28

7.3 Experimental Procedure 29

7.4 Experimental Results

7.5 Comparison of Theoretical and Experimental Results 30

7.5.1 Calculation of Theoretical Average Air Velocity

7.5.2: Calculation of Average Air Velocity Using Experimental Data

7.6 Discussion 31

7.7 Conclusion 33

8. GROUND SOURCE COOLING

8.1 Determining Pipe Depth for Ground Source Cooling 33

8.2 ANSYS Ground Source Cooling Model 34

8.2.1 Aims

8.2.2 Modelling and Assumptions

8.2.3 Parameters

8.3 Results and Discussion 35

8.4 Design Recommendations 36

9. CONCLUSIONS AND FUTURE WORK 37


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Figure 1.1: Existing Cold Storage Room

1. INTRODUCTION

1.1 Context

The storage of food in developing countries is vital for sustaining life in remoteagricultural villages located in the Indian and African subcontinent. A very highambient air temperature is encountered in these regions; there is often no reliableelectricity connection and the local population earns a minimal disposable income.Consequently, this project focuses on Short Term Food Preservation as analternative means to conventional food refrigeration. Short Term Food Preservation isa cold storage method, enabling freshly harvested agricultural produce to be kept forup to a week without significant deterioration, prior to its final consumption orwholesale.

This project is carried out in partnership with the engineering charity Engineerswithout borders UK (1). It is based in the village of Pabal, located in the state ofMaharashtra, India at latitude 1804926 North and longitude 7400392 East (2).Currently a third of the produce rots before it can be sold in India (3); this leads tosignificant food wastage and economic loss. Short Term Food preservation could

therefore benefit many people in a nation, where almost 750 million people live inrural areas and the majority earn their livelihood as subsistence farmers.

1.2 Problem Description: (refer to(4) for a full project description)

During the dry season agricultural produce becomes less saleable, due to itsdeterioration in a climate, where external temperatures approach 400C. A novel ShortTerm Preservation method is required in a village that faces 8 hours of power cuts aday and a scarcity of water. The proposed solution should enable crops to bepreserved at an acceptable quality for a longer period of time; this will allow farmersto demand higher prices and limit food wastage.

1.2.1 Existing Cold Storage Room

The existing cold store is constructed out of bricks in a rectangular formationwith a 9 wall thickness. This design is advantageous, since there is a cavitylocated at the centre of each of the offset rectangular formations. These cavitiesare filled with sand. During cold store operation, water is intermittently pumpedto the top of the wall cavity and allowed to run through the sand.


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The corrugated metal roof is enclosed in a polythene mesh. This reduces thesolar induced thermal flux and therefore its influence on internal roomtemperature.

The system appears to dissipate heat by direct evaporative cooling from theporous brick walls and achieves a temperature gradient of 6-80C. However, thisis not sustainable in the drought prone region of Pabal, where there is a scarcityof water.

1.3 Project Overview:

Section 2 is an analysis of the existing design. A schematic is proposed along withrelevant parameters and heat transfer theory, enabling a quantitative calculation ofbuilding cooling load. This concludes with an evaluation of the cold store design andthe use of passive cooling methods is recommended.

Section 3 is a review of potential cooling methods for Pabal. Mechanical Vapourcompression systems and heat driven absorption systems are examined, but aredismissed due to expense. Natural and passive cooling methods are then examinedand deemed more appropriate for Pabal. Section 4 deals with the selection andexplanation of the novel solar chimney and ground source cooling method.

Section 5 is about the derivation of solar and ambient air temperature data for Pabalfrom the SoDa database. Solar geometry is then introduced, with aim of computingsolar irradiation occurring on a vertically inclined surface as in a solar chimney.

Section 6 is focussed on the theoretical derivation of volumetric flow rate in a solarchimney. Suitable parameters are then introduced, for calculation of flow rate

performance throughout the day in simple and glazed solar chimneys. The effect ofvarying stack height and diameter is then discussed, along with a calculation ofmaximum pressure drop, which shows good agreement with an empirical formula.Suitable design recommendations are suggested.

Section 7 details the experimental work, which compares predictions of thetheoretical model, to solar chimney scale model data. A full size scale model isdiscounted due to climatic considerations. The experimental design, procedure andresults, are followed by a theoretical and experimental calculation of average airvelocity and final comparison. It is not possible to conclusively verify the theoreticalmodel, due to experimental limitations.

Section 8 focuses on ground source cooling; the determination of ground source pipedepth is discussed. The parameters and assumptions are introduced for a model inANSYS. These results are then discussed, along with variation in pipe length andrecommendations. It is found that the pressure drop provided by the solar tower, isnot sufficient to guarantee effective cooling.


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InternalConditions

To achieve 20-25 degrees Celsius cooling with an internaltemperature of no more than 20oC.

To maintain internal relative humidity between 85-95% to suitmultiple crops (leafy vegetables and hardy produce).

Design

Attributes

The cold store is to be 3048 x 3048 x 3048 mm (A communityrefrigeration scheme for approximately 5 farmers).

Easy to build and simple maintenance.

Pollution free (Sustainable). To maximise the usage of local labour, materials and tools.

EnergyDemand

Minimal reliance on mains electricity and water.

Cost To satisfy a total capital cost of Indian Rs.10000 116. Therunning cost for the cold storage unit should be minimal.

Table 1.1: Cold Storage Solution Objectives

1.4 Aims and Objectives:

Aims: To design a cold storage unit for the short-term preservation of agriculturalproduce in the village of Pabal, India.

Objectives:

Table 1.1 lists objectives such as cold store internal requirements and costs. Aweighted objectives tree can be used to rank their relative importance in the design inFigure 1.2. The project aim is split into two further levels, each specifying theobjectives in increasing detail.

The detailed objectives in Level 2 are each assigned an absolute score in the bottom

right hand box. This represents their fraction of importance in the overall design andwill be used to select the final cooling method in section 4.

0.4 0.12

Operating Costs0.3 0.3

Cost

Figure 1.2: Weighted Objectives Tree

0.6 0.24

Temperature

0.4 0.16

Relative Humidity

0.6 0.18

Construction Cost

0.3 0.3

Design Attributes

0.4 0.4

Internal Conditions

1 1

Cold Storage Concept

0.1 0.03

Local Labour

0.1 0.03

Pollution Free

0.1 0.03

Ease of Construction

Low Electricty/water usage

0.7 0.21

Level 1Level 2


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Tint 3048mm

229m 229m 2590m

Ghorz

Brick

Moist Sand

Corrugated Steel Roof

Figure 2.1: Simplified Cross Sectional Schematic of an Existing Design

Note:All exteriorsurfaces immersedin ambient fluid of

temperatureTamb

Gvert Solar Irradition

Solar Irradition

2. ANALYSIS OF EXISTING DESIGN:

The cooling load for the existing design will be calculated; this is the amount of heatthat must be removed in order to maintain a desired internal temperature. Aschematic of the existing design is proposed, followed by the analysis assumptionsand relevant parameters. Finally the appropriate heat transfer theory is introduced,along with a cooling load calculation and an evaluation of the existing design.

2.1 Simplified Schematic of Existing Design

The schematic below illustrates a cross-section of the existing design, which featuresa simplification of the brick sand cavity and a single layer corrugated steel roof. It isexposed to respective solar irradiation values of Ghorz and Gvert.

2.2 Assumptions:

Building heat transfer is modelled as a one-dimensional steady heat flow, with aninternal temperature of Tint and an ambient temperature of Tamb. Heat storage isassumed to be negligible within the brick wall, since its thickness is small.

Parameters occurring at midday will be used to compute the peak cooling load, since

this coincides with maximum solar irradiation. The roof and south facing wall areexposed to solar irradiation Ghorz and Gvert respectively, whereas the other walls areonly subject to convective heat transfer from the ambient air.

Heat Transfer by radiation and infiltration are assumed to be negligible. Heat transfercoefficients are applied assuming wall airspeed is less than 0.1ms-1 (5). 67


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2.3 Parameters:

The roof is modelled as single laminar surface. The thermal material parameters inand heat transfer coefficients are for typical building applications, obtained fromCIBSE Guide A (5). The thermal conductivity of sand varies between 1.5-2.5 Wm-1k-1depending on its degree of saturation; a median value has been used. Climatic datais derived from section 5.1.

2.4 Building Heat Transfer Theory:

2.4.1 Thermal Transmission:

The overall thermal transmission TQ

into a building with N individual

components (eg: a north facing composite wall and roof) is given by (8):

=

=

=Ni

i

AmbiiT TTAUQ1

int )( [1]

Uican be defined as the overall heat transfer coefficient of the ithcomponent (a

function of thermal conductivity k, thickness x and film heat transfer coefficient)and Ai the surface area of each component. In our model the overall heattransfer coefficient for each of the composite walls can be defined as:

sand

sand

brick

brick

wallwall k

x

k

x

hhU

+

++= 2

111

int

[2]

The overall heat transfer coefficient Uroof for the roof can be defined as:

steel

roof

roofroofk

x

hhU

++=

int

111[3]

Layer Material LayerThicknessx/(m)

ThermalConductivityk/(Wm-1k-1)

Thermal Absorptivity

Brick 0.102 0.65 (5) 0.49 (5)Sand (saturated) 0.025 2 (6)Roof: Mild Steel (0.5%C) 0.025 54 (7) 0.20 (5)

Table 2.1 Material Parameters

Solar Irradiation G(N/m2)

External HeatTransferCoefficient hext(Wm

-2K

-1)

Gvert Ghorz

Tamb/(k) Tint (K)

hroof hwall

Internal Heat Transfer

Coefficient hint (Wm-2K-1)

389 935 308 300 0.7 (5) 2.5 (5) 2.5 (5)

Table 2.2: Climatic Data and Film Heat Transfer Coefficients(Climatic data derived later in section)


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Convection

Reflection

SUN

TambTint

solq

Twall

Figure 2.2: Solar Load on Wall

2.4.2 Solar Load through Walls

When a wall is exposed to solarirradiation a proportion of this energy isimmediately lost by reflection. Theabsorbed energy is either conducted

through the wall with a heat flux of

solq or lost the surroundings by convection orradiation. Heat lost by radiation isneglected. The situation can be reducedto the following heat resistance diagramin Figure 6 (5).

2 heat flow equations can be writtenfrom Figure 6 by using equation [1]and summing them in accordancewith Kirchoffs law. Algebraicmanipulation results in an expressionfor overall heat flux through the wallas quoted in (5):

( )

+=

ext

vertambwallsol

h

GTTUq

int [4]

The total building-cooling load

CQ can be calculated by the summing of heat

flow by thermal transmission in [1] and total solar load heat flow

SolQ for allcomponents derived from [4]:

( )

++=+=

=

=

=

=

ext

ii

amb

Ni

i

ii

Ni

i

AmbiiSolTCh

GTTAUTTAUQQQ

int

11

int )( [5]

2.5 Cooling Load Calculation:

The surface area and the overall heat transfer coefficient for have been computedusing [2] for the walls and [3] for the roof. This has enabled the calculation of overall

thermal transmission using [1]. Similarly the individual solar loads have beencalculated for i=2 and i=5 and summed. Finally, the total cooling load has beenobtained using [5].

Tamb Tint

Twall

Solq

exth

1Gvert

iU

1

Gqsol

Figure 2.3: Thermal Resistance

Twall


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2.6 Discussion of Existing Design

Significance of cooling Load calculation

Under steady conditions the estimated peak cooling load is 2262W; this energyis supplied for the specific heat and latent heat of vaporisation of water.

This equates to a significant consumption of water by indirect evaporativecooling (greater if radiative heat transfer and heat infiltration was considered)and is extremely wasteful in a drought region (9). The existing temperaturegradient is not satisfactory for effective short-term food preservation.

Heat gain by thermal transmission only accounts for 11% of the total coolingload; the effects of solar load are far more significant, as indicated by Stoeckerand Jones (5). Furthermore, 62% of the heat flow enters through the roof; this isconsistent with studies by Nahar et al (10) who stipulate that almost half the heat

gain enters through the roof.

Design Improvements

This model suggests that solar irradiation accounts for the majority of coolingload; passive cooling techniques such as the use of external shading andvegetation could be very beneficial in the future design (see section 3.5).

Methods to reduce heat flow through the roof could involve increased roofthickness, the introduction of insulation or even roof shading (11). Low costinsulation within the building walls and roof are currently absent; this wouldreduce heat by thermal transmission. Implementing these passive methods inPabal will reduce cooling load.

3. REVIEW OF POTENTIAL COOLING METHODS:

Potential cooling methods for cold storage are reviewed and compared with theproject objectives in section 1.4. Mechanically driven vapour compression systemsand their various power sources are evaluated, followed by a review of heat drivenabsorption refrigeration. The review then focuses on natural and passive coolingtechniques, which are simple low cost cooling solutions, involving minimal reliance onmechanical systems.

i Buildingcomponent

Ai(m ) Ui(Wm- k-1)

TQ

(W)

SolQ (W)

i=1 North facing wall 7.89 0.89 56.2i=2 South facing wall A1 U1 56.2 590i=3 West facing wall A1 U1 56.2

i=4 East facing wall A1 U1 56.2i=5 Roof 9.29 0.55 40.9 1406N=5 =266 =1996

cQ

= 2262W

Table 2.3: Cooling Load Heat Balance Calculation


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3.1 Mechanical Vapour Compression Refrigeration

A liquid refrigerant is evaporated at a low pressure and temperature due to heattransfer from the area to be cooled. Mechanical Compression (for different energysources see 3.1.1, 3.1.2 and 3.1.3) results in an increase in entropy, temperature andpressure. The fluid is passed through the condenser at a constant pressure, losingheat to the surroundings and is finally expanded back to its original pressure.

This method requires a complex refrigerant and can only be purchased from amanufacturer. Although vapour compression cooling effectively satisfies the requiredinternal condition, purchasing costs are high and range from 1900 - 2800 (12) for acold store of similar dimensions.

3.1.1 Grid Electricity

Vapour compression systems are usually powered by grid electricity; thisrequires a reliable continuous power supply, which does not exist in Pabal. ColdStore refrigeration in India entails a running cost of 31 per cubic metre per year

(13), which would equate to an unsustainable expense of 837 a year for theproposed cold store. There is also the cost of maintenance, requiring skilledpersonnel and replacement parts.

3.1.2 Diesel Generation

Skilled personnel are permanently required on site, to maintain this form ofenergy supply. Apart from the purchase cost, there is also the associated cost offuel and spare parts, which is comparable to the high cost of grid generation.

3.1.3 Solar Photovoltaic Systems

The abundant source of solar irradiation in Pabal could be converted intoelectricity by photovoltaic cells and stored in batteries; however the currentcoefficient of performance is only 0.3 (14). These systems are used for criticalapplications such as vaccine preservation, since the capital cost amounts to3000-5500 (15) for only 60-80W of cooling. The cooling load required for theexisting design is much greater (see section 2.5).

Figure 3.1: Simple Vapour Compression Cycle


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3.2 Absorption Refrigeration:

Absorption refrigeration is similar to the vapour compression cycle, except it is a heatdriven cycle (5). The low pressure vapour in the evaporator is absorbed by a liquidsolution in the absorber, which rejects the heat to the surroundings as in Figure 9(16). The pressure is then elevated by a pump (minimal work), which delivers thefluid to the generator. This receives heat from a high temperature source, whichdrives off the high pressure vapour to the condenser and the cycle then continues asin the vapour compression cycle.

Absorption refrigeration does not depend upon moving parts, simplifyingmaintenance and reducing running costs. They have been simple to manufactureusing local labour in developing countries (15). However the cost of purchasing andrunning a kerosene or gas power unit would cost 2000 over 10 years and wouldonly provide 60 -100W of cooling (15).

Solar powered absorption units are also available on the market; they are relativelynew and feature an improved net coefficient of performance of 0.5 (solar tocooling)(14). Florides (17) absorption air-cooling system in a Cypriot house incurred

a capital cost of 1300 including solar panels, which is uneconomical in Pabal.

3.3 Natural Cooling Techniques:

Natural cooling involves the use of natural heat sinks for internal heat dissipation(18); evaporative cooling, ground source cooling and solar chimney ventilation will beexamined. The methods attempt to use renewable energy and minimal reliance onmechanical systems.

3.3.1 Evaporative Cooling Swamp Cooler

Evaporative cooling involves the liquid phase change of water to water vapour(10). The fan in the swamp cooler drives external air over water soaked pads.The air provides the latent heat of vaporisation for the water and is consequentlycooled. The performance is based on the saturation efficiency formula (18) andthe air cannot be cooled below the dew point, which falls between the dry bulband wet bulb temperatures.

Figure 3.2: The Absorption Cycle


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These devices are cost effective starting at 71 (19) and would functionsuccessfully in the low humidity environment of Pabal. However, the systemconsumes an unsustainable 0.45 litres of water per hour and requires mainselectricity for operation.

3.3.2 Roof Ponds

Roof ponds involve the low cost construction of water ponds over non-insulatedflat roofs (18). The water surface is shaded during the day to avoid excessiveheating and opened during the night. A shaded roof pond cools by evaporation;the roof below by heat conduction to the roof pond. Tang (9) demonstrates that6-90C cooling is possible by shaded roof ponds in a single storey building.

Roof ponds are effective natural heat sinks, although Givoni (18) stipulates thatthe wet bulb temperature should be less than 200C for effective application,whereas in Pabal at midday this is 290C. This system consumes a similaramount of water to the swamp cooler and requires a construction that is capableof bearing high structural loads.

3.3.3 Ground Source Cooling:

During the summer period, ground temperatures remain significantly lower thanambient air temperature, with much smaller daily variation (18). In an open loop

system, the ambient air is passed through PVC pipes buried underground, intothe cold store. The ground acts as a heat sink and the cooling of ambient air isachieved. The temperature the air can be cooled by, is dependent on inlet airtemperature, ground temperature at pipe depth, air velocity, pipe length and soilthermal properties (18).

Figure 3.4: Roof Pond

Fi ure 3.3: Swam Cooler


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Incident Solar

Irradiation

Figure 3.6: Solar Tower

In a study by Sharan (20) of a very similar open loop system in Gujrat, India, 50metres of pipe was buried 3 m below the surface. During testing ambient air of400C was cooled almost 140C, with a coefficient of performance of 3.3. Thesolution is very simple; underground pipes could easily be integrated with thebuilding foundation utilising local labour. However, energy is required to power afan or other device to create a pressure drop across the buried pipes.Furthermore, it is vital that vermin and rainwater are prevented from entering thepipe system.

3.3.4 Solar Chimneys:

A solar chimney consists of a vertical brick or concrete stack, which is exposedto solar irradiation. A temperature difference is created between the warmed airadjacent to the hot interior wall and the quiescent air. This induces buoyancyforces, resulting in a vertical free convection flow, referred to as the stack effect.When the flow is fully developed there is also a temperature difference betweenthe top and bottom of the chimney.

Solar Chimneys often feature a glazed faade in order to increase the stackeffect. An experimental study in rooms with a floor area of 6m2 by Afonso (21),

demonstrated a ventilation rate amounting to 20 air changes per hour with solarchimneys. The highest ventilation rate occurs at maximum solar irradiation,which coincides with the greatest cooling demand. However, solar chimneys areineffective during periods of low solar irradiation, such as cloudy days.

Ambient Air Tin

Figure 3.5: Open Loop Ground Cooling


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3.5 Passive Cooling Techniques:

Passive cooling aims to reduce cooling load in buildings, through wall and roofinsulation, shading devices and surrounding vegetation. Possible wall improvementscould include low conductivity cork insulation and using an air cavity of 25mmoptimum thickness (5). Positioning a low emissivity material such as aluminium foil inthe air gap, acts as radiant barrier by reflecting internal radiation (18). These are lowcost methods of reducing building cooling load and are very simple to incorporateinto any wall construction as in Figure 3.7.

External shading such as horizontal overhangs and balconies are often effective lowcost methods of protecting the building exterior from direct solar irradiation and canbe analysed using shading coefficients.

Surrounding vegetation is advantageous; a fully-grown tree evaporates water on asunny day, which is equivalent to almost 870MJ cooling capacity (18). As well asproviding shade, vegetation can reduce surrounding temperature by about 2-30C.

4. SELECTION OF COOLING TECHNIQUES & FINAL DESIGN CONCEPT

Each of the cooling methods evaluated in section 3 have been scored against theobjectives featured in section 1.4. The total scores in Table 4.1 will be used to selectthe most appropriate cooling methods for Pabal.

The vapour compression and absorption refrigeration systems receive low scoresdue to their unfeasible expenditure and will be discounted. The swamp cooler androof ponds are both low cost and effective, but their water consumption isundesirable. Consequently, solar chimneys received the highest score of 8.56 andwill be selected as the most cost effective and sustainable method. Similarly, groundsource cooling achieves the best performance by natural cooling. These methods willbe combined with passive cooling techniques, to maintain a desirable internal

temperature.

AluminiumFoilCork Insulation

Air Gap

Figure 3.7: Thermal Insulation


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4.1 Final Design Concept:

The final concept is novel and will integrate a solar chimney and ground sourcecooling. The idea is inspired by a simple schematic in Figure 16, which has beenproposed by Vail (22):

As discussed in section 3.4.5 a chimney stack is exposed to solar irradiation, which

creates a vertical free convection current of heated air. This induces a pressure dropbetween the top and bottom of the chimney and drags ambient air through an openloop ground source cooling pipes. The ambient air is cooled to a certain temperature,through heat exchange with the surrounding thermal mass of the soil, before enteringthe building. This technique will be used to maintain a desirable internal temperature.

Figure 16: Concept Proposed By Vail (22)

Score out of 10Temperature

RelativeHumidity

ConstructionCost

OperatingCost

LowElec/WaterUsage

EaseofConstruction

PollutionFree

LocalLabour

TotalScore

Vapour Compression Refrigeration

(I) Grid Electricity 10 7 0 2 0 0 6 0 3.94

(ii) Diesel Generation 10 7 0 2 7 0 6 0 5.41

(iii) Solar Photovoltaic Systems 5 7 0 7 10 0 10 0 5.56

Absorption Refrigeration

(I) Kerosene/Gas 5 7 0 2 7 5 6 6 4.54

(ii) Solar Irradiation 7 7 0 7 10 5 10 5 6.34

Swamp Cooler 6 10 7 5 0 5 7 5 5.41

Roof Ponds 5 7 9 7 3 10 10 9 6.28

Ground Source Cooling 7 7 8 8 8 10 10 10 7.78

Solar Chimneys 6 7 10 10 10 10 10 10 8.56

Passive cooling Techniques 5 7 8 10 10 10 10 10 7.96

Table 4.1: Design Objective Scores & Overall Absolute Scores for Cooling Methods


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5. DERIVATION OF CLIMATIC DATA FOR PABAL

This section deals with the derivation of average hourly solar irradiation values for ahorizontal surface and ambient air temperature in Pabal. These parameters influencethe air mass flow rate in the Solar chimney and cooling potential of the ground sourcesystem. Relevant solar theory is then introduced, with the aim of calculating theincident solar irradiation on a vertically inclined surface as is encountered in a solarchimney.

5.1 Solar Data derivation & Ambient Air Temperature

Since it is not possible to obtain climatic data measured directly at the site in Pabal,the required data has been acquired from a solar interpolation and database servicecalled SoDa (23).

The resource can calculate hourly values for horizontal clear sky radiation, modellingdirect and diffuse radiation components. Irradiation values are calculated accordingto Aguiar and Collares-Pereira (23). The hourly ambient air temperature isinterpolated from the temperature distribution of the nearest measurement station

and hourly global irradiation.

The incident angle , between the normal of the surface and the suns rays, dictatesthe magnitude of solar irradiation. It is influenced by latitude, date and solar time (7).Consequently, the Latitude-longitude coordinates and solar time settings are requiredfor the database.

Hourly horizontal solar irradiation and ambient air temperature values have beencalculated for each day in the month of April. This month corresponds to the peaksummer dry season in Pabal. The values have been averaged for each solar hour;they are converted to vertical surface solar irradiation values, using theory in section5.2. The variation in vertical solar irradiation and ambient air temperature is illustrated

in Figures 5.2 and 5.3.

SUN Normal

Figure 5.1: Incident Angle


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100

150

200

250

300

350

400

6 7 8 9 10 11 12 13 14 15 16 17 18

Solar Hour/(hour)

So

larIrradiation/(Wm-2)

29.0

30.0

31.0

32.0

33.0

34.0

35.0

36.0

6 7 8 9 10 11 12 13 14 15 16 17 18

Solar Hour/(hour)

AmbientAirTemp.(

0C)

The database provides an acceptable basis for deriving climatic data, since it is of asimilar magnitude to climatic data from nearby Pune city. However, the derived solarirradiation values only offer an estimate of real solar values, due to the error in

interpolative calculation methods, uncertainty in the value of diffuse radiation andclimate variations such as cloudy skies. Reflected radiation, which is influenced bythe immediate surroundings, is also neglected.

5.2 Solar Geometry(7)

Relevant equations will be introduced to convert horizontal surface solar irradiation tovertical solar irradiation, as occurs in the Solar Chimney.

Solar Irradiation is composed of three radiation components; direct radiation from thesun ID, diffuse radiation from the sky Id and reflected radiation Ir (depending uponsurface properties known as albedo)illustrated by Figure 5.4.

Consequently total Solar Irradiation G is a function of the three radiation componentsand incident angle (7):

rdD IIIG ++= cos [6]

Assuming that Ir is a negligible component of the total solar irradiation and thatdiffused radiation Id accounts for approximately 30% of total incident irradiation, directradiation on a horizontal surface with angle of incidence horz (24):

Figure 5.4 Composition of Solar Irradiation

Figure 5.2: Variation in Vertical SurfaceSolar Irradiaton in Pabal during the day(April)

Figure 5.3: Variation in Ambient AirTemperature during day (April)


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horz

horz

D

GI

cos7.0= [7]

Determining the angle of angle of incidencehorzon a horizontal surface

The calculation of horz is necessary, to compute the value of direct radiation ID from

[7] at each hour of the day. The angle of incidence for a horizontal surface horz isgiven by:

solarhorz =090 [8]

where solar is the solar altitude angle defined as the angle from the horizontal to thesun as depicted by Figure 20.

solaris a function of latitude L, hour angle H and the suns declination (7):

sinsincoscoscossin LHLsolar += [9]

The hour angle varies with time and is a function of the solar hour:

)12(15

0

= SolarHourH [10]

The suns declination is a function of n, which is the day of the year numbered fromJanuary 1st. A value of n corresponding to April 15th is used.

365

)284(360sin47.23

n+= [11]

Determining the angle of incidence on a vertical surfacevert

The angle of incidence on a vertical surface vert is given by (7):

coscoscossolarvert

= [12]

Where is the solar surface azimuth, which is the angle between solar azimuth

and the surface azimuth given by:

= [13]

Figure 5.5: Solar altitude angle


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The solar azimuth is defined as the angle between horizontal projection of the sun

(Figure 5.5) and the south given by:

cos

sincossin

H= [14]

The surface azimuth is an attribute of wall orientation. It is defined as the angle thenormal of the vertical surface makes with the South. Therefore = 00 for our south

facing solar chimney.

Determining total vertical solar irradiation:

If reflected radiation Ir is neglected and equations [6] and [7] are combined, it ispossible to compute vertical solar irradiation, using calculated values of horz andvert:

horzvert

horz

horz

vert GG

G 3.0coscos

7.0 +=

[15]

6. SOLAR CHIMNEYS

This section aims to derive a theoretical formula for the air mass flow rate in a SolarChimney. Parameters are then introduced in order to calculate volumetric flow rate. The effect of altering solar chimney height and diameter are also examined.

Two types of solar chimney are considered (Figure 6.1). The simple solar chimneyconsists of a cylindrical stack made of either concrete or brick and with an inletlocated near its base. The glazed solar chimney is very similar, except a portion of

the South facing faade is replaced with glazing. These structures can beincorporated on a rooftop or free standing.

6.1 Theoretical Volumetric Flow Rate Derivation

Since the solar chimney wall is usually of minimal thickness (


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rate. Although this effect is significant, Afonso (21) suggests it can be neglected fordesign purposes, due to the unpredictability of wind velocity. The model will solelyconsider solar irradiation.

The piezometric head lost to friction for a fully developed flow in a pipe is given byDarcys Formula (25):

2

2

avf

umflp = [16]

where l is the length of pipe, f the friction factor and m the hydraulic mean depthwhich for a circular cross section diameter d can be defined as (25):

4

4

ncecircumfere

areasectional-cross

2

d

d

d

m ===

[17]

Substituting [17] into [16]and summing with minor friction losses the total frictionalhead becomes where uav is average air velocity:

+= k

d

flup

avf

4

2

1 2 [18]

The solar chimney is modelled as cylindrical stack of height H (where H = H1+H2),exposed to vertical solar irradiance Gvert. The stack is attached to a hypotheticalfrictionless thermosiphon; air entering from the cold store at a temperature Tint anddensity 1. The air is uniformly heated in the stack resulting in a density change from2 to 3, exiting with temperature Tout (Figure 6.2).

Figure 6.2: Hypothetical Thermosiphon model

2 Gver

ToutS=0

Length S along perimeter(dashed perimeter)

Z

1

H1

H2

1=

z

s1=

z

s

S=LTint

3


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Considering a small section of pipe of length s the overall pressure loss combinedwith the hydrostatic component is:

( )

+

+

=

s

zg

L

sk

d

sfu

s

pav

4

2

1 2 [19]

Integrating [19] between S=0 and S=L with respect to S calculate the total pressure:

[20a]

04

2

1 21 =

+ gHk

d

fHuav [20b]

The fluid close to the hot right hand wall is less dense than the remainder of the fluid.The temperature difference creates a density gradient, appearing in the hydrostaticterm gH in equation [20b]. Consequently, buoyancy forces induce a free

convection boundary layer in which the heated fluid rises vertically.

If density variations exist only due to temperature variations the volumetric thermalExpansion Coefficient is (constant pressure):

pT

=

1 [21]

It can be expressed as the Boussinesq approximation, where T is the absolute fluid

temperature and T is the free stream temperature (26):

TTT =

11 )( = TT [22]

Substituting equation [22] into [20b]:

+= k

d

fHugHTT

4

2

1)( 211 [23]

Equation [23] equates buoyancy pressure to friction pressure losses. The absolutetemperature T can be approximated as an average of Tint and Tout. [23] can berewritten as:

+=

+k

d

fHugHT

TT out 4

2

1

2

2

intint [24]

The air mass flow rate within the stack is driven the vertical temperature differenceand depends on specific heat of air Cp. This is equated to heat flow into the chimney;

a function of vertical solar irradiation Gvert, stack diameter d and collector efficiency :

23121

2

10

4

2

10 gHgHgHk

d

fHupp avL ++

+==


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2)( int

dHGTTCm vertoutp

=

[25]

Substituting mass flow rate into [25] for flow using an average air density air:

2)(

4int

2dGH

TTuCd

outpair

= [26]

Combining equations [24] [25] [26] and rearranging for average air velocity :

3

1

2

4

2

+

=

kd

fHdc

gGHu

pair

av

[27]

Volumetric Flow rate within the Solar Chimney is therefore given by:

4

2

avud

v

=

[28]

6.2 Selection of Parameters

Equations [27] and [28] can be used to predict solar tower mass flow rates in Pabal.This will be investigated after selecting suitable fixed parameters.

Thermal Parameters:2728

If the air is assumed to be an ideal gas = p/RT the volumetric thermalcoefficient can be rewritten as:

TRT

RT

RT

p

T p

11122===

=

[29]

T is the absolute temperature can be approximated by an average of Tint andTout. A typical temperature gradient of 7K will be used as achievedexperimentally by Afonso (21) in solar chimneys of 2m high, allowing calculation

of . Although this is an approximation, increasing the temperature gradientvalue to 12K would only introduce a 0.33% error in [27]; therefore is near its

Internal

ColdstoreTemperature

Tint/(k)

Volumetric

ThermalCoefficient

/(K-1)

Average

AirDensity

air/(kgm-

3)

Average Air

SpecificHeat

Cp/(Jkg-1K-1)

Friction

Factorf

Total Minor

lossCoefficients

k

Unglazed

CollectorEfficiency

293 0.00337 1.190 (27) 1004.8 (27) 0.01 (21) 2 (28) 0.11 (7)

Table 6.1: Thermal and Fluid Parameters for Model


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true value. Average air density and specific heat values have been selected atabsolute temperature T.

Fluid Friction Losses:

The magnitude of the friction factor f has been found to be less than 0.01 insolar chimneys (21). The total minor loss coefficient k was calculated by the sumof values for contraction on entrance, a 900 bend and expansion on exit.

Collector Efficiency:

The collector efficiency is the ratio of heat flux transferred to the fluid to incidentvertical solar irradiation (7):

vertG

q

= [30]

For the simple solar chimney stack, collector efficiency is a function of heat fluxdue to solar load from [4] in section 2.4.2 and vertical solar irradiation. Theefficiency of the glazed solar chimney is a function of the optical and thermal

properties of the glazing and solar chimney wall; vertical solar irradiation, fluidentry temperature Tin and ambient air temperature Tambare also influential (7).

Figure 6.3: Efficiency of a Single Glazed Flat Plate Collector

00.10.20.30.40.50.60.70.80.9

0 0.02 0.04 0.06 0.08 0.1

(t in-tamb)/Gvert , (Km2W-1)

CollectorEfficiency

However it is more appropriate to obtain efficiencies for a glazed solar chimneyfrom Figure 5.8 (7). Values will be extrapolated for each solar hour of day, sinceinput values are a function of ambient air temperature and solar irradiation andefficiency varies from 0.19-0.49. However, this graph is an approximation since itis for flat plate collectors. Due to smaller daily efficiency variations for the simplesolar chimney, an average unglazed collector efficiency value in Table 5.1 willbe used.

6.3 Solar Chimney Volumetric Air Flow

The parameters in section 6.2 and climatic data in 5.1 are used to model the simpleand glazed solar chimneys, each height 2 metres and diameter 0.5 metres.


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Figure 6.4: Volumetric Flow Rate During Day (April) For

Glazed and Simple Solar Chimneys

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

6 8 10 12 14 16 18

Solar Hour/(hour)

VolumetricFlowRate/(m3/s)

GlazedSolarChimney

Simple

SolarChimney

The air flow rates are very similar for both chimneys early and late in the day, withonly a 6.0% difference (Figure 6.4). This is due to the low solar irradiation occurringin this period. Similarly, the peak air flow rate in both chimneys correspond to

maximum solar irradiation occurring at midday.

The air flow rate for the glazed solar chimney is on average 59.7% greater than thesimple chimney, due to it greater collector efficiency. The glazing admits a highproportion of solar irradiation through transmittance and reduces convection andreradiation losses from the back solar wall. However, the difference in air flow rate isexaggerated, since an average collector efficiency value is used for the simplechimney.

Figure 6.5: Air Changes Per Hour During Day

(April) For Glazed and Simple Solar Chimneys

0

2

4

6

8

10

6 7 8 9 10 11 12 13 14 15 16 17 18

Solar Hour/ (hour)

Glazed

Solar

Chimney

Simple

Solar

Chimney

The volumetric air flow rates are of the same order of magnitude as experimentaldata obtained by Afonso (21), for solar chimneys of the same dimensions. Thissuggests that the theoretical model provides a reasonable estimate.

In Figure 6.5 the air changes per hour reaches a maximum of 8.5 in the glazed stack;this suggests that glazed solar chimneys are more effective at incrementing naturalventilation.

This model provides a conservative estimate of volumetric airflow rates. Theintermittent action of wind forces will raise airflow. The external ambient temperatureis always greater than the average chimney temperature in this model; thistemperature difference will increment buoyancy forces in the chimney and increaseair velocity.


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6.4 Variation of Solar Chimney Height and Diameter

Figures 6.6 and 6.7 illustrate the number of room air changes per hour (ACH)achievable at maximum solar irradiation, for a range of Solar Chimney heights anddiameters. The diameter was kept constant at 0.5 metres for Figure 6.6 and height at2 metres for Figure 6.7.

Figure 6.6: Effect o f Stack H eight On Number of

Roo m A ir changes Per hour

0

2

4

6

8

10

1214

0 0.5 1 1.5 2 2.5 3 3.5 4Stack Height H /(m)

NumberofAir

ChangesPerHour Glazed

SolarChimney

Simple

SolarChimney

Figure 6.7: Effect o f Stack D iameter On Number

of Ro om Changes Per Hour

0

20

40

60

80

100120

140

160

0 0.5 1 1.5 2 2.5 3Stack Diameter d /(m )

Glazed

SolarChimney

Simple

Solar

Chimney

Figure 6.6 shows that as stack height is increased, there is a rise in ACH for bothchimneys and also a divergence between both values. This is due to a greater

collector area, and cause major difference in the glazed case. However, the increasein ACH diminishes as stack height rises; the same result has been obtained inexperimental work (21).

Figure 6.7 shows that there is a rise in ACH for both chimneys, as stack diameter isincreased and a divergence between both values. Using larger stack diametersraises ACH more significantly compared to stack height; increasing stack diameter by0.5 metres leads to an initial rise of 8.49 ACH, but only 2.06 ACH for height.Volumetric air flow rates are most effectively maximised by using a larger stackdiameter.

6.5 Solar Chimney Pressure Drop and Comparison with Empirical Correlation

Maximum pressure drop p can be predicted for a glazed solar chimney(dimensional parameters in section 6.6) by combining [20a] and [22]:

+=

2

4

2

1 int2 TTgHkd

fHup outairavair [31]


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where average air velocity uav is calculated from [25] using parameters in Table 6.1at midday. An empirical correlation for the stack effect in buildings is obtained from(5):

=

int

113455

TTHp

out

[32]

Table 6.2 shows a good agreement between the predicted theoretical and empiricalpressure drop values for the solar chimney, since the results are the same order ofmagnitude. However the empirical correlation is approximate, since it does notconsider cross sectional area.

6.6 Design Recommendations

A glazed solar chimney is recommended since it increments volumetric air flowrate most effectively.

A stack height of 3 metres is structurally attainable and any increase in heightwill only provide a minimal flow rate gain.

Although increasing stack diameter raises air flow rate most effectively, it alsoresults in a larger stack outlet cross sectional area. Therefore air could enter byinfiltration and increase cooling load. A stack diameter of 0.7 metres isrecommended as a suitable compromise.

7 EXPERIMENTAL WORK

7.1 Introduction

It is difficult to assess the performance of the cold store experimentally in the UK, dueto the differences in climate, soil types and soil thermal properties. Therefore,experimentation using a 1:1 scale model is inappropriate. Ground Source cooling canonly be effectively assessed by experiments in the field, due to the difficulty ofemulating soil behaviour. Consequently, this section compares experimental datafrom a model solar chimney with the theoretical model in section 6.

Barrozi et al (29) have used a 1:12 experimental scale model of a solar chimney andbuilding, together with a solar simulator and controlled temperature environment, tomonitor performance. However in order to ensure dimensional similarity of the scalemodel, higher temperature differences and working fluids other than air are required.

Therefore, a solar chimney model has been used, but not with the intention of directlyverifying predictions made for Pabal in section 6.3. Instead, the experimental air flowrate data will be compared with the theoretical model, specifically for this case.

Theoretical Pressure Drop p/(Pa) Empirical Pressure Drop p/(Pa)-0.317 -0.825

Table 6.2: Theoretical and Empirical Pressure Drop Values


28/39


7.2 Experimental Design

A 1:3 scale model has been constructed, combining an elementary solar chimneywith a cold store enclosure. This consists of a box made of 6 square medium densityfibreboard sheets and each side is 762 millimetres in length (the sheets arebracketed together in the interior).

Heat transfer to the box is not investigated; its purpose is to form a rigid enclosureand only admit air through two small holes drilled on one side (Figure 7.1).The Solar Chimney consists of a 900 elbow pipe, attached to a vertical down pipe,fitted near the top of the box. The stack is fabricated out of PVC and is 0.110 metresin diameter and 0.582 metres in height. Although this is not a typical solar chimneymaterial, the geometry is more significant in this instance and friction losses can becompensated accordingly.

This scale model is not exposed to solar irradiation, due to the complexity ofrecreating it. Instead a rectangular aluminium plate is placed within the vertical downpipe and held by an internal steel bracket (Figure 7.2). It is the same height as the

stack .

The aluminium plate represents the inner wall of the solar chimney and is heated.The area behind the plate is insulated, ensuring convection from the front of the plateonly. Aluminium possesses a similar specific heat capacity to concrete and thereforeits rate of cooling will be alike. The plate is located 35mm tangentially from the pipeedge; plate temperature is monitored using a thermocouple attached to a Squirrel

Figure 7.2: Overhead view of Chimney stack

Insulation

Bracket

PVC pipe

Aluminium Plate

Figure 7.1: Solar Chimney and Cold Store Scale Model

PVC Stack

MDF Enclosure

Air Inlet Hole


29/39


data logger. Air velocity and outlet air temperature are measured using a VelociCalcAir flow meter at the top of the stack.

7.3 Experimental Procedure

The aluminium plate was heated in the oven until it reached a steady uniformtemperature of 750C and was placed in the bracket. Plate temperature wasmonitored every thirty seconds, using the data logger. 2 minutes was allowed for thefree convection flow to develop in the stack.

The radial variation in air velocity across the stack was measured for a range of platetemperatures; starting 2 millimetres from the plate edge and at increments of 5millimetres.

Exit temperature Tout was measured every thirty seconds using the flow meter probe.Temperature profiles were measured radially in increments of 2 millimetres acrossthe stack for a range of plate temperatures.

7.4 Experimental Results:

An internal temperature Tint=20.50C and ambient temperature of Tamb=22.3

0C wererecorded during the experiment. Figures 7.3 and 7.4 depict velocity and temperatureprofiles across the stack for a range of average plate temperatures and are theaverage of two experimental runs.

Figure 7.3: Radial Air Velocity Profile For Different

Plate Temperatures

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20

Distance from Plate/(mm)

AirV

elocity/

(ms-1)

44.3C

40.07C

37.7C

34.7C

33.3C

31.2C

In Figure 7.3 higher plate temperatures, results in greater air velocity; this is causedby a more rapid rate of cooling and a greater heat flow in the solar chimney. This

results in higher air velocities as predicted by [25] and is equivalent to a higher solarirradiation.

The best-fit line in Figure 7.3 is for the velocity profile at a plate temperature of31.20C. Due to the no slip condition, it would be expected that fluid velocityimmediately next to plate would be zero. However, fluid velocity increases to amaximum at 2 millimetres away from the plate and decreases with distance from theplate. Air velocities are only detected in a free convection boundary layer ofapproximately 22 millimetres in thickness. Beyond this point, the air in the pipe isstagnant.


30/39


Figure 7.4: Radial Temperature Variation For Different Plate

Temperatures

20

25

30

35

40

0 10 20 30 40Distance From Plate/mm

Temperature/0C

33.3C

36.3C

39C

48C

Figure 7.4 shows that the variation in temperature profile extends further away fromthe plate than for air velocity. High plate temperatures appear to result in higher valuetemperature profiles. However, this is not the case for a plate temperature of 48 0C,since the flow is still developing in the pipe and the air is increasing in temperature.

Temperature in the pipe remains constant after 30 35 millimetres. This radialtemperature difference in the pipe is responsible for inducing buoyancy forces.

7.5 Comparison of Theoretical and Experimental Results

In this section the theoretical expressions derived in section 6.1 will be adapted toenable predictions of average air velocity. This parameter is also computed forexperimental data, by using numerical approximation. The theoretical andexperimental results are then compared.

7.5.1 Calculation of Theoretical Average Air Velocity

Since plate temperature was measured with respect to time during the

experiment, plate heat flow in the pipe

plateQ is given by:

dt

dTmcQ

AlpPlate )(=

[33]

where m is the mass of the Aluminium plate and )(Alpc is its specific heat. The

rate of temperature change with respect to time is obtained from the gradient ofthe plate temperature time graph.

Substituting [33] for the GHd term in [27] and replacing the terms for circular

section area with experimental flow area Aexp[28]becomes:

31

exp

4

+

kd

fHCA

gHQu

pair

plate

av

[34]


31/39


Average air velocity can be calculated using parameters at 297K in Table 7.1and exit temperature Tout for each plate temperature. The friction coefficient f iscalculated assuming laminar flow (25), since the Reynolds number isapproximately Re=249 from the experimental results.

Re

16=f [35]

7.5.2: Calculation of Average Air Velocity Using Experimental Data

The results in Figure 7.3 provide radial profiles for air velocity across the stack.Average air velocity is approximated from this data using the trapezium rule; theboundary layer flow area will be divided into 5 strips of 2.5mm width, measuredfrom the plate edge (Figure 7.5). Each velocity measurement is located at thecentre of a trapezium.

The height of each strip is calculated using Pythagoras rule. The product ofeach strips air velocity and trapezoidal area is summed together to calculatevolumetric flow rate. Average air velocity is determined by dividing this by flowarea Aexp. This calculation was performed for each velocity profile.

7.6 Discussion

The experimental and theoretical average flow rates calculated in the previous

section are plotted against plate heat flow

plateQ in Figure 7.6.

Figure 7.5: Trapezoidal Strips across Boundary Layer for Numerical Approximation

PVC Pipe

Aluminium Plate

Air Density

air/(Kg/m3)

AluminiumSpecific HeatCp(al)/ (Jkg

-1K-1)

Air SpecificHeat Cp/(Jkg-1K-1)

Frictionfactor, f

FlowAreaAexp/(m

2)

AluminiumPlate Massm/(Kg)

Minor Losscoefficient

k

1.194 897 1004.8 0.064 0.00771 0.406 2

Table 7.1: Parameters for Theoretical Calculation


32/39


Figure 7.6: Experimental and The oret ical Average Air

Velocity Ver sus Plate Heat Flow

0.00

0.05

0.10

0.15

0.20

0.25

5 10 15 20 25Plate Heat Flow/(Watts)

AverageAirVelocity/(ms

-1) Experimental

Average Air

Velocity

Theoretical

Average Air

Velocity

For both results, average air velocity increases approximately at the same rate and isof a similar order of magnitude. However, the theoretical model predicts a higheraverage air velocity and differs by a factor of 5 from experimental values. This could

be attributed to the inadequacy of the scale model in simulating a true solar chimneyand error in experimental data and the method of analysis.

Scale model:

The heated plate in the scale model simulates the inner wall of the solar chimney.The plate cools by natural convection and is in a transient state. However, thetheoretical model proposed in section 6, assumes that the inner wall is at steadystate conditions, due to constant hourly solar irradiation.

Since plate heat flow

plateQ in the scale model changes with time, it is more difficult

for a fully developed heat flow to occur. This may have caused experimental air

velocity values to be less than theoretical predictions. A plate maintained at a steadystate have led to the higher velocities predicted by [34].

Experimental Error:

The airflow meter possesses a +/-0.015ms-1 error and therefore causes a 25%uncertainty in some of the low air velocity values. The readings are also at thethreshold of instruments measurement range of 0 30 ms-1, introducing additionalerror, questioning its suitability for low velocity applications.

The airflow meter also takes time to output a stable value when the probe is movedto a new location. This leads to inaccuracy, since the velocity profile could change inthis period. Measurement error also results from using a 5 millimetre probe to takereadings within a 22 millimetre boundary layer.

Analysis Error:

The analysis assumes that measured air velocities do not vary laterally across eachtrapezoidal strip. Using the trapezium rule to estimate volumetric flow rate in the pipealso leads to numerical error. However, these are small in comparison to thepreviously discussed errors.


33/39


Soil ThermalDiffusivity /(m2s-1)

Daily angularfrequency /(s-1)

Damping DepthD /(m)

5D / (m)

0.121 x 10-6 (18) 0.727 x 10-4 0.058 0.29

Figure 8.1: Calculation of Depth at which Temperature Variation is eliminated

7.7 Conclusion

It is difficult to verify the accuracy of the theoretical model presented in section 6using experimental data. This is due to the shortfalls of the experimental model inrepresenting a solar chimney; a more realistic model could recreate steady stateconditions by using a plate maintained at a constant heat flux. The high experimentalerror was due to the limitations of the measurement apparatus. This is overcome in asimilar study of free convection flow in a solar chimney, by using a low velocity gasanalyser with a tracer gas (29).

The reliability of the theoretical model can only be verified through furtherexperimentation. However it does provide predictions that are representative of freeconvection boundary flows; since experimental and theoretical results are a similarorder of magnitude. Therefore, it can be used to provide an approximate estimate ofsolar chimney volumetric air flow rate in Pabal.

8. GROUND SOURCE COOLING

8.1 Determining Pipe Depth for Ground Source Cooling

The soil depth at which the pipe is buried influences the magnitude of ground sourcecooling. Soil temperature is difficult to model at different depths since soil thermaldiffusivity varies with soil type, moisture and density. However, assuming ahomogenous medium, soil temperature Tsoilvaries sinusoidally with respect to time tand depth from soil surface Z (18):

+=

D

zteATztT D

z

ambsoilcos),( 0 [36]

where A0is the amplitude of variation and damping depth D dictates the extent oftemperature variation given by:

( ) 5.02

=D [37]

where is soil thermal diffusivity and is diurnal angular frequency. At a depth ofZ= 5D almost all daily temperature variation is eliminated (18).

Figure 8.1 shows that daily soil temperature variation only occurs up till a depth of0.290 metres. Therefore, ground source pipes should be ideally buried at a depth ofat least 1 metre; this will ensure a constant soil temperature required for effectivecooling. Tests in the field should be conducted to clarify this.


34/39


8.2 ANSYS Ground Source Cooling Model

8.2.1 Aims:

ANSYS is a general purpose Finite Element Analysis package and is used tomodel heat transfer occurring in the ground source cooling pipe. A code wascreated for 4 distinct steps; creating a solid model, generating a finite elementmesh, applying boundary conditions and obtaining the final solution.

The aim is to predict the pipe outlet temperature Toutlet after ambient air has beendriven through the system for different pipe lengths (Figure 8.1). The air is drivenby pressure drop p created in the solar chimney (see section 6.5).

8.2.2 Modelling and Assumptions: (30)

Symmetry allows the horizontal section of the pipe length l, to be modelled 2dimensionally, along its radius R as in Figure 8.3. The vertical sections areneglected in Figure 8.2, as they comparatively small in length.

It is assumed that the flow is steady, fully developed and axisymmetric.

Velocity varies with radius only and shear force opposes fluid flow. The air

to be cooled is incompressible and fluid properties remain constantthroughout the pipe.

The pipe wall is subjected to a negative heat flux

q and is assumed to have

negligible thermal resistance. The pipe is subjected to pressure drop p.

8.2.3 Parameters:

Toutlet Tamb

p

Figure 8.2: Ground Source Cooling Pipe

w

q

CentrelineInlet Outlet

u(r) R

Pipe wall

Figure 8.3: ANSYS Geometric Model

PressureDropp/ Pa

Denisity/Kgm-3

AirViscosity/Km-1s-1

Air SpecificHeat Cp/Jkg-1K-1

PipeRadiusR /m

Ambient AirTemperatureT amb/K

HeatFlux

q /Wm-2

Air ThermalConductivityk/(Wm-1k-1)

0.317 1.190(27)

1.46x10-6

(27)1004.8(27)

0.05 310 -0.5 2.624x10-2

(27)

Obtained from project partners work Table 8.1: Parameters for ANSYS Model


35/39


The highest ambient temperature in Pabal has been selected, which occurs inconjunction with the maximum pressure drop. Air thermal properties have beeninterpolated at 305K, which is representative of mean pipe temperature. Thepipe radius is typically used in ground source systems. Pipe length has beenvaried in this exercise.

8.3 Results and Discussion:

Figure 8.3 shows the temperature profile along a 12 metre pipe; ambient air enters at310K and leaves the outlet at a minimum temperature of 308.9K. This pipe length istypically used in ground source applications, but seems to achieve very little cooling.The air closest to the pipe wall is cooled the most due to the stagnation of boundarylayer flows. The air travelling at the pipe centre only increases in temperature midwayalong the pipe, since there is low flow mixing. Figure 8.4 illustrates a 0.7K fall inradial outlet temperature.

The radial outlet velocity profile in Figure 8.5 shows a maximum velocity of 0.62ms -1

at the pipe centre line, which falls to zero at the pipe wall. The pipe velocityinfluences the axial temperature distribution in Figure 8.3, since a higher valuesresult in greater fluid mixing and increases air cooling. This is the reason why typicalground source cooling systems use much higher air velocities ranging from 4 to8 ms-1 (20).

Therefore, the maximum pressure drop provided by the solar tower, is insufficient toachieve desired ground source cooling. Furthermore, the pressure drop will be lessat other hours of the day due to lower solar irradiation. The maximum pressure drop

Figure 8.3 Axial Temperature Profile Along Ground Source Pipe (12 Metres)

Figure 8.4: Outlet Radial Temperature Profile Figure 8.5: Outlet Radial Velocity Profile


36/39


value is also based on an internal cold store temperature of 293K; this is notachieved with this pipe length.

Increasing ground source pipe length raises heat exchange area, allowing greatercooling as in Figure 8.6. However, only 3.25K of cooling is obtained with 25 metres ofpipe metres. Beyond this length, the ANSYS model seems to predict air coolingincreasing proportionally with pipe length.

Figure 8.6: Ground Source Cooling Achieved w ithVariation in Pipe Length

0

2

4

6

8

10

12

0 10 20 30 40 50Pipe Length/m

AchievedAirCooling/K

However, the ANSYS model does not account for the influence of the friction factor inhigh pipe lengths, or the pressure losses encountered in the vertical pipe sections. Aconstant heat flux is assumed; this value is dependent on soil depth and the variationin soil thermal properties. This model only provides a crude model of daily cooling ina ground source pipe, since parameters such as soil temperature and type can onlybe obtained from the site. There is also an associated error of using a finite elementanalysis method.

Nonetheless, the ANSYS model indicates that the solar chimney pressure drop is toolow, to create the required pipe air velocity for effective ground source cooling.

8.4 Design Recommendations

Ground Source Cooling is an effective form of natural cooling, but a solarchimney does not provide the necessary pressure drop for cooling. Instead, a lowcost battery powered fan or blower mechanism, could be installed at one end ofthe system, to ensure air velocities of 4 to 8 ms-1.

The best thermal contact should be sought with the pipe and surrounding soil;improved thermal conductivity could be achieved by covering the pipe with a 0.05m of sand, when it is buried.

The horizontal pipe section should be positioned at a 1% gradient slope from inletto outlet, to ensure that water does not accumulate in the system and is expelledfrom a small hole drilled in the elbow. Furthermore, the pipe inlet should begauzed to prevent vermin entering the system and sheltered from the rain.

Pipes should be buried at a soil depth over 1 metre; detailed tests must beconducted to find the optimum depth for lowest soil temperature. Pipe length andpossible cooling can then be predicted from the ANSYS model using a suitablepressure drop.


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9. CONCLUSION AND FUTURE WORK

The existing cold store analysis, has highlighted that passive cooling methods mustbe used in any future design; this will significantly reduce building cooling load. Acombined solar chimney and ground source cooling system has been proposed,since it best satisfies the cost and energy requirements for Pabal. Consequently, thisproject has investigated the combined cooling performance of this system.

The theoretical model predicts that the glazed solar chimney is the most effective atincrementing ventilation rate, rising to a maximum of 8.5 room changes per hour.However, the theoretical model underestimates ventilation rate, since it does notinclude wind effects or buoyancy forces induced by ambient air. The most criticalassumption is that internal temperature is maintained at 293K; this has not beenachieved using the ground source cooling system. Furthermore, solar collectorefficiency graph used in the model, is more appropriate for flat plate collectors andtrue solar chimney efficiency is unknown.

The model has enabled solar chimney height and diameter to be specified; theestimated maximum pressure drop for Pabal (0.317Pa) shows good agreement with

an empirical correlation, which suggests that it is a reliable estimate of the true value.However, it is not possible to corroborate this model with experimental data, due tothe inadequacy of solar chimney representation and apparatus limitations.

The ANSYS model demonstrates that the solar chimney is not satisfactory inproviding the required pressure drop for ground source cooling. However, it shouldbe retained as an effective natural ventilation method. Instead, the pressure dropshould be augmented using a fan or blower device for ground source cooling. Themodel is a crude approximation of ground source cooling, due to the uncertainty ofsoil thermal properties in Pabal and error in the finite element method.

There is much future research that could be conducted in solar chimneys and groundsource cooling. The study of free convection flows within a solar chimney model in a

steady state situation, would allow true verification of the theoretical model. Similarly,more research is required into solar chimney collector efficiency, as this dictatesperformance and varies with design. Testing the performance of a full solar chimneyscale model in Pabal would be desirable. A true assessment of ground sourcecooling potential in Pabal, can only be achieved by performing further tests in thefield, due to the complexity of soil thermal behaviour.


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