A Combined Solar Photovoltaic Distributed Energy Source Appliance

doi:10.4236/nr.2011.22010

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Natural Resources, 2011, 2, 75-86

doi:10.4236/nr.2011.22010 Published Online June 2011 (http://www.scirp.org/journal/nr)

A Combined Solar Photovoltaic Distributed

Energy Source Appliance

Himanshu Dehra

1-140 Avenue Windsor, Lachine, Québec , Canada.

E-mail: [email protected]

Received November 29th, 2010; revised January 18th, 2011; accepted January 25th, 2011.

ABSTRACT

The paper has analysed the state-of-art technology for a solar photovoltaic distributed energy source appliance. The

success of implementation of photovoltaic (PV) power project is increased when PV module system is integrated with

building design process and is used as multi purpose appliance for use with building elements. The improvement in

overall system efficiency of building integrated PV modules embedded in building façade is achieved by minimizing and

capturing energy losses. A novel solar energy utilisation technology for generation of electric and thermal power is

presented by integration of ventilation and solar photovoltaic device with the heating, ventilating and air conditioning

(HVAC) system. The testing appliance named as photovoltaic duct wall was a wooden frame assembly of double wall

with air ventilation: two adjacent glass coated PV modules, air column, plywood board filled with polystyrene and

dampers. The measurement data is collected from various sensors to read measurements of solar intensity, ambient air

temperature, room air temperature, electric power, surface temperatures of PV modules and plywood board, air veloci-

ties and air temperatures in the air column. The enhancement in the air velocity of the air column is fulfilled with an

exhaust fan fixed in an outdoor room. The simulation model is used to perform the two dimensional energy analyses

with applied one dimensional solution of steady state heat conduction equations. The bases of simulation model are

conjugating energy travel paths with network boundary conditions of convection, radiation exchange, heat storage ca-

pacity, thermal storage capacity and heat transport.

Keywords: HVAC, Energy Source, Energy Appliance, Energy Distribution, Photovoltaic Duct

1. Introduction

With increase in cost of fuel and electricity and increase

in greenhouse gas productions, there is a rise in activity

trend towards use of solar energy utilisation technologies

for energy and environment conservation. The traditional

techniques of passive solar heating are applied in build-

ings since historical times by generation of solar heat and

transmission of heat flow inside the building with use of

thermo siphon principle to move warm air [1-4]. The

active solar systems make use of mechanical and electri-

cal equipment such as fans and pumps to circulate the

heat of fluid effectively [5-7].

The examples of active solar systems are roof top col-

lectors for heating of water and air for inside use in the

building. In active space heating systems, either air or

liquid or phase changing material is used to collect solar

heat and to supply to the occupied space either directly

or its heating load is combined with building heating

utility system. The examples are solar air heaters, solar

chimney, ventilated façades and earth-coupled heat

structures. Solar energy is also used to operate cooling

equipment by supplying solar heat to any of several types

of heat generation cycles.

The first usable solar cell was invented at Bell labora-

tories in 1954. A photovoltaic (PV) or solar cell is a solid

state device that converts sunlight and electromagnetic

radiation into electricity. The energy losses in a solar cell

are to the tune of 85%, which result from several factors

[1]. The intensity of solar radiation is usually lower than

standard test value of 1000 Wm−2, as atmospheric ab-

sorption of the solar radiation spectrum reduces the

available solar radiation below this value [1]. The elec-

trical efficiencies are also reduced because incident solar

radiation is often not perpendicular to the plane of the

photovoltaic modules. Moreover, solar cell temperatures

are often higher than the nominal operating cell tem-

perature (NOCT) of 25˚C, so that the diode current that

opposes the light generating current is higher than in test

76 A Combined Solar Photovoltaic Distributed Energy Source Appliance

conditions, reducing the output current. The electric

conversion efficiency decreases approximately by 1%

with every 25˚C increase in surface temperature of crys-

talline silicon based solar cells [1]. The result of these

lower efficiencies in PV modules is that their payback

period becomes longer and they become less financially

attractive for power plant owners and designers [1].

1.1. Sustainable Energy Source

The main advantages of PV modules in power generation

are [1]: i) PV modules have long life of about 20 years

and advantage of providing direct room temperature

conversion to high-grade energy; ii) they do not have any

moving parts, thus having low maintenance costs and

have ability to function unattended for long periods; iii)

they have modular nature, high reliability and instance

response with nearly zero time constant; and iv) they are

non-polluting, no sound generation and promote distrib-

uted generation, de-centralized, micro-power or green

power generation concept. The three most prominent

reasons for not gaining acceptance in consumers are [1,3]:

i) their high manufacturing cost; ii) low energy conver-

sion efficiency to electricity; and iii) intermittent nature

and requires electrical storage, which has its own techni-

cal problems. Unlike a conventional building, the cost of

structure of a prefabricated outdoor or portable room per

unit area is independent of location and end use. The

capital cost and operational cost of the prefabricated

outdoor rooms is often included in its design process

depending on the total time of operation of the outdoor

room at the site of work. The energy requirements for

prefabricated outdoor rooms are fulfilled by a portable

generator set and a utility supply. The capital and opera-

tional cost of energy supply per kW is much higher in

comparison to the bulk utility supply to a building.

Therefore, any small amount of solar energy supply re-

placing conventional energy supply does not have a ma-

jor impact on cost of prefabricated rooms. The schematic

of a prefabricated outdoor room used for conducting en-

ergy trials on an appliance is illustrated in Figure 1. A

PV power generating system of size 80 WP covering 1 sq.

m of façade would approximately generate on an average

97.2 kWh per year, displacing electricity with an emis-

sion factor of 0.7 kilograms of CO2 per kWh during peak

electricity use [3]. Table 1 has presented calculation

sheet for reduction of GHG emissions with integration of

80 WP of photovoltaic and solar ventilation technologies

into a prefabricated outdoor room [4].

2. Literature Review

The double façade with integrated PV modules acts as

ventilated façade and source for generation of electric

A/C Unit

Exhaust Fan

Crossed Ventilated

Windows

Baseboard Heater

PV Module Test Section I

Data Storage

A.C. Power Supply

(120 V, 60 Hz , Single phase)

Intake Dampers

Test Section II

Exhaust Dampers

Water supply

Entrance door

Solar Collector

with PCM Board

2896 mm

3962 mm

2825 mm

2134 mm

937 mm

Solar Intensity Sensor

Figure 1. Pre-fabricated outdoor room.

Table 1. GHG emissions reduction.

Energy Production

Electricity generation per annum

(Basis: 200 clear sunny days, generating 0.486 units

per day)

97.2 kWh

Heating load reduction through waste heat recovery

per annum(Basis: Assuming heating load of 65kWh

per m2 of PV wall area per annum, 155 cold sunny

days meeting 10% of the load)

6.5 thermal kWh

(23.4 MJ)

CO2 reductions per year

Indirect: Electricity emissions 50.4 kg

Direct: Heating with fuel emissions 39 kg

Conversion factors

1 Kg CO2 emissions from crude oil

{Equivalent crude oil consumption: 1 × (12/44) /

(19.14×0.99) = 14.55 g (600 kJ)}

600 kJ

1 kWh (3413 Btu) 3.6 MJ

and thermal power [1-24]. The PV thermal collectors are

either constructed by pasting solar cells on the absorber

surface and creating air gap above it with transparent

cover or by fixing PV modules on absorber plate with air

plenum passage for air flowing in between absorber plate

and insulating back plate. The heat and fluid flow char-

acterization in ventilated channels are important criteria-

for maximizing the energy production in embedded sys-

tems. The ventilated channels are classified according to

flow conditions as naturally ventilated and mechanically

ventilated channels [1]. Forced convection in mechanic-

cally ventilated channels is classified on the basis of flow

regimes: entrance region, fully developed flow, laminar

and turbulent regime [1]. The most of the studies con-

ducted on thermal performance of building integrated

photovoltaic modules are one dimensional and are unable

to describe thermal characterisation in two dimensions.

The significance of radiation heat transfer and its effect

on reducing surface temperature of solar cells in PV

modules is not considered in most of the studies. There-

fore conjugate heat transfer analysis for a solar thermal

system is required for design of thermal and flow condi-

tions [4]. The thermal performance of building integrated

photovoltaic modules and as well as thermal analysis of

air flowing through ventilated façade is required under

variable environment conditions to ascertain the thermal

A Combined Solar Photovoltaic Distributed Energy Source Appliance 77

conditions of building integrated photovoltaic modules.

The thermal model ascertains electrical and thermal

power output from the power generation system. The

significance of long wave radiation exchange from

channel walls along with its effect on air convection heat

transfer from channel walls is also necessary to be per-

formed in the analysis of conjugate heat transfer in the

channel [5]. This ensures proper design of the heat-

ing/cooling equipment and assess indoor environment

based on the performance of thermal system integrated

with electric utility and HVAC system of the building.

The thermal model assesses the potential for energy

conservation in buildings. The heat dissipated from PV

modules has the option to transfer to air, water or phase

changing fluids and recovered heat can be used for dis-

tributed heating purposes in the building. In this regard

thermal losses should be kept at minimum from thermal

system. Table 2 gives various techniques for minimizing

thermal losses from a solar energy utilisation system.

2.1. Approaches and Issues

Air ventilation is used in building integrated PV module

for the purpose of achieving cooling of solar cells in PV

module, which also provides means for capturing ther-

mal power from solar radiation, which is otherwise not

converted into useful heat and is lost to surrounding en-

vironment. Despite insignificant loss in solar irradiation

on vertically inclined PV module, the main advantage of

PV modules embedded in building façade as compared

Table 2. Techniques for minimising thermal losses.

Technique Description

Selective

absorbers

They are applied as cover paint for increasing the asorbtan-

ce of the surface and absorb maximum solar energy by

minimizing the emission of thermal radiation. Usually base

plates (of copper, steel, aluminum or plastic) with flat

black paint on top acts as selective absorber. Other exam-

ples of selective absorbers are black chrome, black acrylic

paint etc.

Flow

Passage

With liquids and phase change materials, flow is usually

through tubes attached to or integrated with the absorbe

plate. Evaporation and condensation processes reduce

efficiency of system. With air, flow occurs above and/o

below the base plate. Heat transfer surface area is increased

y means of fins, slots and rough metals with minimum

increase in friction coefficient of the flow passage. Use o

forced convection controls the amount and rate of hea

flow at specified temperature.

Cover

plates

One, two or three transparent covers are used to reduce

convective and radiative heat losses to outside air. Tem-

pered glass and plastic materials are generally used.

Insulation

It is used to reduce thermal losses from back (rear) and

sides of the collector. Examples are low binder fibre glass,

isocyanurate polyurethane foam, polystyrene etc.

Enclosure

A box or any enclosure to hold the components togethe

and to protect from weather. When solar energy system is

embedded in building façade, they also minimize distribu-

tion losses of transmitting heat flow into the building.

with the roof top system is that requirements of low en-

ergy heating are met easily according to the climate and

due to energy use near its point of generation. In roof top

systems, there is delay time, duct costs and energy dis-

tribution losses due to heat flow from ducts into indoor

environment. Thermal resistance of the building façade

increases with air ventilated building integrated PV

module system. This justifies protection from excessive

heating from solar radiation by passing and controlling

the amount of heat flow, which is not possible in case of

passive solar heated double façades, which are only

naturally ventilated. It also provides means for ventila-

tion in the form of pre-conditioning of fresh air into the

building and daylighting in the case of glazed section in

double façades. Examples of indirect solar gain system

are provided in Table 3. The approaches and issues for

investigating a thermal system with PV modules are pre-

sented in Table 4.

One of the important aspects of getting success in

photovoltaic based distributed power generation is to

consider PV modules in the integrated design process of

the building and using them either integrated with the

building façade or as roof top building integrated system.

The author of the present work has conducted various

experiments and models on the state-of-art overview and

design issues for recovering thermal power along with

Table 3. Examples of indirect solar gain system embedded in

building facade.

TechnologyDescription

Solar Air Col-

lectors

Solar air collectors are the cheapest mode of solar en-

ergy utilization. A flat plate solar air collector has a

sorber (selective or non-selective) and one or two glaz-

ing covers. Solar pre-heated air is used for warm ai

heating or ventilation in buildings. The solar air collec-

tor may be constructed with or without thermal energy

storage. Indirect solar gain system with thermal energ

storage also uses solar heat in non-peak hours of sola

energy availability.

Photovoltaic

Thermal

(PV/T) Col-

lectors

Photovoltaic module acts as absorber in the collecto

and efficiency of the PV/T collector is in between sola

-air collector and photovoltaic module. PV/T collectors

are used to increase the overall efficiency of the system,

plus they have the advantage of generating electricity.

Temperature of the recovered pre-heated fresh air is less

than in case of solar- air collector.

Solar Heated

Ventilated

façades

In ventilated façades, moderate temperatures of the pre

-conditioned air are possible. Mechanically ventilate

solar energy façades with either multiple building fa-

çades or ventilated ducts are used for controlled supply

of pre-heated or pre-cooled fresh air into the building.

These can be glazed or un-glazed.

Coupling of

solar-air, PV/T

collectors,

ventilated

facades and

passive build-

ing elements.

Coupling of different solar-thermal devices and passive

building components is done for meeting temperature

requirements of pre-conditioned fresh air as per climatic

conditions and replacing part of the heating/cooling an

electrical load of the building (e.g. Coupling of PV/T

and solar air collectors with ventilated façades).

78 A Combined Solar Photovoltaic Distributed Energy Source Appliance

Table 4. Approaches and issues for investigating building

integrated photovoltaic modules-thermal system.

Approaches Issues

Design of PV cooling

channels

Heat transfer and fluid flow passage model-

ling, Significance of radiation heat transfer in

different flow conditions, fluid flow charac-

terization (with air, water, liquid or phase

change fluids) for different flow regimes o

the channels and performance evaluation o

the optimised configurations.

Modelling energy gen-

eration and its applica-

tions

Thermodynamic modeling, energy storage,

entropy generation minimization and energy

evaluation of thermal system.

Combined performance

with building energy

supply

Dynamics of combined energy flow from

electric supplying utility, thermal system and

HVAC system into the building.

Modelling wind effects

and wind – induced flow

in solar energy collector

Modeling wind effects in external region a

the entrance of collector and its effect on flow

distribution in collector with respect to

low-rise and high-rise buildings.

Assessment of indoor

environment

Analysis of pre-conditioned fresh air entering

through HVAC system into the building in-

door environment as per thermal comfort,

health and environment standards.

improvement in the electrical power output from build-

ing integrated photovoltaic (BIPV) systems [8-10]. The

author has done thermal performance of building inte-

grated PV modules by considering energy rate balance

on a PV module [21]. Both steady state and transient

simulations are performed [11-13]. In a thermal model,

time constant of the PV module response is defined as

the time taken for the module temperature to reach 63%

of the total change in temperature resulting from a step

change in the irradiance level [10].

2.2. Considerations for Selecting Modeling

Techniques

For the case of building integrated PV modules embed-

ded in building façade, ventilated channel flow is appli-

cable for modeling [23]. Most work in the modeling is

done for simulation of performance of specific configu-

rations [21-24]. Both steady state and transient simula-

tions are performed by simulation of energy rate outputs

in the form of thermal energy and losses to surrounding

environment from PV ventilated façade system. Most of

the studies on thermal monitoring of PV ventilated fa-

cades are done near the outlet, where air is at exit of

channel. Work is done to simulate heat transfer by cou-

pling of convective and radiative heat transfer in PV

cooling ducts [9,10]. Research on heat transfer simula-

tion is conducted for limiting cases representing bound-

ary conditions. The major consequence of these studies is

that they are unable to design boundary surface for opti-

mum design of cooling channel or duct for maximizing

power generation from a thermal system.

Specific energy modeling issues are related to double

wall structures [14,15,19]. For example, the author has

simulated the photovoltaic solar wall with a thermal

network model [21]. The airflow is based on stack effect

with wind pressure through openings along the height of

the double façade. The airflow from ventilated façade

into the building indoor environment is not considered in

the model. Modeling techniques can be arranged in the

order of increasing complexity starting from single node

isothermal modeling. The basic types of modeling tech-

niques, which are in used for modeling embedded PV –

thermal system in building façade are described in Table

5. Numerical modeling using computational fluid dy-

namics is relatively new and considerable support work

is required in order to gain confidence in such models as

generally they are mainly illustrative without considering

three-dimensional and wind effects and without valida-

tion with experiments [1].

3. Prefabricated Outdoor Room

As is illustrated in Figure 1, the pre-fabricated outdoor

room was setup at Concordia University, Montreal, and

Québec, Canada for the purpose of conducting research

on various aspects of combined solar photovoltaic dis-

tributed energy source appliance [1]. The outdoor room

with section for testing photo vol-taic and solar ventila-

tion technologies was designed and built up for

Table 5. Basic types of modelling techniques.

Technique Description

Isothermal Mod-

eling

Steady state simulation for constant temperature

of surfaces and is used in the design of solar-air

collectors. Model assumes constant wall tem-

perature and steady heat transfer coefficient for

solving heat transfer problem.

Heat-balance

Modeling

Heat-balance is a method for the heat fluxes as a

non-steady variable of thermal gains. This

method is a function of space and non- steady

heat transfer coefficients. This method is used for

heat balances of the air mass and at the surfaces

of system element.

HVAC and air-

flow modeling

Non-steady state models are used for mass and

energy balances on common HVAC systems for

fans, coils, boilers and are represented by overall

efficiency values and calculated by curve fitting

technique from manufacturers’ data. This method

is used for estimating wind and buoyancy-driven

infiltration rates.

Network Model-

ing

Network airflow model represents thermal resis-

tances, capacitances and admittances for different

types of nodes (air and surface) and assume large

air volumes with uniform conditions, and predict

flow through discrete paths. A mass balance is

expressed for each node in the system.

Thermal and

airflow network

modeling

Airflow and thermal network models are joined

together to exchange data between solution do-

mains at each time step. Data exchange from

thermal model, at previous time step, is used to

establish temperature of surfaces by airflow

model in current time step.

A Combined Solar Photovoltaic Distributed Energy Source Appliance 79

conducting the practical investigations. As is illustrated

in Figure 1, the dimensions of the pre-fabricated outdoor

room were: interior length of 3962 mm and an interior

width of 2896 mm, with a total interior floor area of 8.61

m² and a height of 2825 mm. The prefabricated outdoor

room was inclined at 10˚ East of South on the horizontal

plane. The test section with photovoltaic modules was

assembled in components with two commercially avail-

able PV modules, air passage with air-gap width of 90

mm, plywood board filled with polystyrene as insulation

panel, side walls made up of Plexiglas and all parts con-

nected with wooden frames. The test section was con-

structed with two PV modules each of dimensions: (989

mm × 453 mm). The PV modules were having glass

coating of 3 mm attached on their exterior and interior

sides. Each PV module in the photovoltaic duct wall was

having 36 multi-crystalline solar cell units, with thin

transparent gaps in between them. The insulation panel

was assembled with 7 mm thick plywood board enclo-

sure filled with 26 mm polystyrene. The overall thick-

ness of the insulation panel with polystyrene was 40 mm.

The insulation panel was thermally insulated with ply-

wood board filled with polystyrene for minimizing any

heat transfer between the air passage and the room zone.

The overall dimensions of the test section with inlet

damper built in the test section were 1100 × 937 × 90 (L)

mm3. The side walls (PV module and insulation panel)

were of dimensions 989 (H) × 937 (W) mm2. The re-

mainder of the generated heat along with the heat gen-

eration from the upper portion of the test section was

transported into the building environment. The schematic

of test section showing PV modules with solar air venti-

lation is illustrated in Figure 2. The manually operated

inlet air damper (921 mm × 75 mm) was fixed at the

bottom section of PV modules to allow passage for the

inlet air. The total length of the air passage in test section

of PV modules with inlet damper was 1100 mm with a

volumetric capacity of 0.09 m3. The dampers were made

of wood covered with an aluminum sheet. The dimen-

sions of the dampers used for solar air ventilation are

provided in Figure 3. The electric circuit for the two

connected photovoltaic modules is illustrated in Figure 4

[1]. The heating, ventilating and air-conditioning (HVAC)

requirements were met in the outdoor room by a base-

board heater, an induced-draft type exhaust fan and a

split window air conditioner [1] heating was supple-

mented by conditioning from the fresh air entering from

the inlet damper through PV module test section. How-

ever, during the mild season of autumn for the duration

of conducting the practical trials in the test section, nei-

ther baseboard heater was used nor air-conditioning unit

was used for auxiliary heating or cooling inside the

pre-fabricated outdoor room.

Air Inlet

Air Outlet

Air Velocity

Sensor

Damper D1

(Open)

Damper D3

(Closed)

Damper D2 (Open)

PV Module 2

Insulation Panel

PV Module 1

Thermocouples

Figure 2. Schematic of the experimental setup showing

placement of sensors.

System Boundary

Back Panel

PCM Board

Interior Outlet Damper (D2)

(38 X 184)

Exterior Glazing

Solar-cell module

Outlet Damper (D3)

Inlet Damper (D1)

Wire Mesh (19 X 184)

Wood Frame (41 X 184)

Wood Frame (41 X 41)

Wire Mesh (38 X 184)

Wood Frame (41 X 184)

Wood Frame with

Strip (41 X 41)

Ambient ZoneBuilding Zone

Wood Frame (41 X 41)

Figure 3. Side elevation of photovoltaic duct wall (all di-

mensions are in mm: H = 989 mm and L = 90 mm).

Figure 4. Electric circuit for two PV modules.

4. Experimental Setup

A photovoltaic device was installed on south facing fa-

çade of prefabricated outdoor room. The outdoor room

was setup at Concordia University, Montréal, and Qué-

bec, Canada for conducting practical investigations [1].

The photovoltaic appliance was built as a parallel plate

duct wall with the plywood board, and was vertically

inclined at 10° East of South on the horizontal plane. The

test section was assembled in components with two

V s

PV module

PV module #2

80 A Combined Solar Photovoltaic Distributed Energy Source Appliance

commercially available PV modules, air passage with

air-gap width of 90 mm, plywood board filled with poly-

styrene as insulation panel, side walls made up of Plexi-

glas and all parts connected with wooden frames. The

combined solar photovoltaic appliance section was con-

structed with two glass coated PV modules each of di-

mensions: (989 mm × 453 mm). The PV modules were

having glass coating of 3 mm attached on their exterior

and interior sides. The plywood board was assembled

with 7 mm thick plywood board enclosure filled with 26

mm poly-styrene. The overall thickness of plywood

board with polystyrene was 40 mm. The exterior damp-

ers were made of wood covered with an aluminum sheet.

The heating, ventilating and air-conditioning (HVAC) re-

quirements were met in the outdoor room by a baseboard

heater, an induced-draft type exhaust fan and a split

window air conditioner [1]. The heating was supple-

mented by conditioning from the fresh air entering from

the inlet damper through photovoltaic duct wall. How-

ever, during the mild season of autumn for the duration

of conducting experimental runs, neither baseboard

heater was used nor air-conditioning unit was used for

auxiliary heating or cooling inside the pre-fabricated

outdoor room.

The pair of PV modules used for conducting experi-

mental investigations was connected in series for genera-

tion of electric power with a rheostat of maximum vary-

ing resistance up to 50Ω. T-type thermocouples were

used for obtaining thermal measurements from the test

section of photovoltaic module. As is illustrated in Fig-

ure 1, three thermocouple sensors were placed at the top,

middle and bottom locations in the PV module,

air-passage and insulation panel of plywood board filled

with polystyrene were used to measure local tempera-

tures. Two thermocouples were used to measure the in-

side test room air temperature and ambient air tempera-

ture. The hybrid air ventilation created for the PV mod-

ule test section was by natural wind, or through buoy-

ancy effect in the absence of wind [1]. The fan pressure

was used to achieve higher air velocities by operation of

the exhaust fan fixed on opposite wall with respect to

wall of the test Section [1]. The slight negative pressure

was induced for drawing low air velocities in absence of

wind-induced pressure from the inlet damper into the test

section through the test room [1]. Air velocity sensor

was placed perpendicular to the walls of the PV module

test section to record axial air velocities near its outlet.

The thermocouple outputs, currents, voltages, solar irra-

diation and air velocity signals were connected to a data

logger and a computer for data storage. The measure-

ments collected from the sensors were recorded as a

function of air velocities or mass flow rate from the test

section with use of fan pressure. The experimental data

from the data acquisition system was collected and

stored every two minutes in the computer [1].

5. Simulation Model

5.1. Assumptions

The assumptions applicable to the simulation model are

[1]: i) quasi steady state heat transfer analysis has been

performed for a photovoltaic duct wall assuming parallel

plate configuration; ii) uniform average air velocity dis-

tribution; iii) temperature variation only in y-direction

(vertical), being taken as lumped in other directions

(x-axis and z-axis); iv) air properties are evaluated at

film temperature of 300 K; v) negligible heat transfer

from side walls/plywood board and room air zone; vi)

conduction (diffusion) equation for performing energy

balance on air nodes is not taken into consideration; vii)

no infiltration or air leakage sources from the test section;

and viii) ambient air and room air temperatures are

specified.

5.2. Governing Equations

For photovoltaic module of the duct with a steady solar

heat flux generation, Poisson’s heat equation with

boundary conditions is written as:

22 0







 in 0

t 0

H (1)





ap a

ShT T







 at (2) 0x

 

pa pipi

hTThr TT



 









(3)

For plywood board of duct with insulation, Laplace’s

heat equation with boundary conditions is written as:

0 in

ppi

Ltx Ltty H









 

(4)





ss i

hT T





 at

Lt t (5)

() (

ii apiip

hT Thr T T



 





)

Lt (6)

5.3. Two Dimensional Energy and Mass Balance

Equations

The energy balance between convective flux and en-

thalpy flux of air with its boundary condition is written

as:

A Combined Solar Photovoltaic Distributed Energy Source Appliance 81







 





 

in 0

H (7)

() ()

Ty Tk at 1

yk n









(8) 0( 1)kn 

vLc





 (9)

The conjugate heat exchange analysis is performed for

a section of photovoltaic duct wall with one PV module

and corresponding areas of air passage and insulating

plywood board. The finite difference approximations are

used to develop algebraic nodal equations for each node

in the computational grid. The nodal equations are de-

veloped by performing an energy balance on surface and

air nodes in the control volume i.e. control volume finite

difference method (CVFDM) is used for representing

algebraic nodal Equation [1]. The general energy balance

equation for ‘p’ surface nodes of PV module is written as

[1]:

,1,1

1,1, ,

()( )

()() ()

oapapyp ppp

pp pppapapipi

TTUqATT U

TTU TTUUTT





 



0



,1

(10)

Similarly, the energy balance equation for ‘i’ surface

nodes of insulating plywood board is written as:

,1,11

()() ()

() ()0

is isiiiiiiii

iaiaipi p

TTU TTUTTU

TTUUTT

 



 

 (11)

The energy balance equation for ‘a’ air nodes is writ-

ten as:

,11 ,

()()()

aaaaaiaiaap

UTT UTTUTT



  (12)

The solution of nodal equations of thermal network

comprises formulation of conductance’s matrix and heat

source matrix [1]. Matrix equation has heat source vector

terms for Q’s and U’s are the conductance terms at vari-

ous nodes. The general matrix form of nodal equation is

written as [1]:

,, ,

()

mn mnmn

UT Q



 



(13)

6. Results and Discussions

6.1 Temperature Solutions

The sample measurement results obtained from section

of photovoltaic duct wall are presented in Table 6. The

location of sensors as per Figure 1 is presented in Table

7. The measurements that were obtained from experi-

mental setup of photovoltaic duct wall are for cases of

buoyancy-induced and fan-induced hybrid ventilation.

The air temperatures in photovoltaic duct wall are de-

veloped as a function of magnitude of the ambient air

temperature. The maximum convection heat transfer was

observed between the ambient cold air entering into the

inlet damper and the air passage of the test section. The

dissipation of heat at the inlet opening has resulted in

considerable increase in temperature of the air. For the

general case fan-induced hybrid ventilation, the tem-

perature difference between top and bottom node for PV

module was around 2˚C, for insulating plywood board

between 8˚C-10˚C and temperature difference between

ambient air and air leaving the outlet of photovoltaic duct

wall varies between 5-9˚C [1]. The insulating plywood

board was heated by long wave radiation heat exchange

from PV module and its temperature at top node was ~

15˚C higher than ambient air and surface temperature

difference between top and bottom nodes of the insula-

tion panel was ~ 10˚C [1]. The temperature differences

were higher for the case of buoyancy-induced hybrid

ventilation. Thermal power is easily calculated for vari-

ous components of photovoltaic duct wall from the tem-

perature differences.

The results obtained from two dimensional numerical

solutions for Run no. 1 of Table 6 are illustrated from

Figure 5(a) to Figure 5(c). The temperature profiles of

PV module, air and insulation panel are obtained with

respect to the height of photovoltaic duct wall. The initial

boundary values in the model are based on the measured

values of quasi steady-state solar irradiation, ambient air

temperature, and room air temperature and air velocities.

The measured boundary values were then used to obtain

two dimensional numerical solutions for one dimensional

heat equations of the photovoltaic duct wall. The nu-

merical results are obtained by assuming constant air and

surface properties. The mean air velocity was assumed

be constant in the test section for representing nodes of

air with same thermal capacity conductance terms, mcp

also termed as thermal capacity rate

C through out its

height [1]. The amount of dissipated heat from the PV

module taken up by air is dependent on mass flow rate

passing over it, thermal capacity of air and temperature

difference between air-column in the test Section [1].

The dissipated heat from the face area (W × H) of PV

module is ventilation heat rate is convection heat loss

from the walls of PV module test section due to differ-

ence in temperature between the walls of test section and

air flowing across cross-sectional area of test section

across one PV module [1].

6.2. Sensible Heat Storage Capacity

In Table 8, heat storage capacity has been calculated

both for temperature differences in y and x-directions.

Thermo-physical properties of various components of

photovoltaic duct wall were evaluated at 300 K.

Thermo-physical properties of plywood board with insu-

A Combined Solar Photovoltaic Distributed Energy Source Appliance

lation were obtained from tests conducted with heat flow

meter and related specifications from the manufacturer.

The temperature differences along y-direction are ob-

tained from the two dimensional numerical model. The

temperature differences along x-direction are obtained by

assuming same temperature difference per unit thickness

of material along x and y-directions. The photovoltaic

duct wall with wooden frame was composed of non-

homogeneous materials having different densities, spe-

cific heats and thicknesses. The pair of PV module was

having three layers of material viz., a flat sheet of solar

cells, with glass face sheets on its exterior and interior

sides. The surface temperature of PV module was as-

sumed to be uniformly distributed in the three layers.

The heat capacity of the wooden frame and sealing mate-

rial was having negligible effect on the temperature of

PV module, air or insulation panel because wood was

Used as construction material and moreover the magni-

Table 6. Sample experimental results.

Run No.

(Wm−2)

(W)

(°C)

(ms−1)

Tp(b)

(°C)

Tp(m)

(°C)

Tp(t)

(°C)

Tb(b)

(°C)

Tb(m)

(°C)

Tb(t)

(°C)

Ta(b)

(°C)

Ta(m)

(°C)

Ta(t)

(°C)

Fan-induced hybrid ventilation

1 725.4 31.1 14.5 22.6 0.425 34.2 32.9 36.3 19.9 24.0 29.4 18.8 19.2 22.4

Buoyancy-induced hybrid ventilation

2 697.5 28.8 13.3 24.9 0.17 39.9 45.0 46.8 28.4 35.0 38.3 21.7 28.3 29.8

Table 7. Location of sensors in the experimental setup.

Distance of

sensors (Figure

Tp(b)

(°C)

Tp(m)

(°C)

Tp(t)

(°C)

Tb(b)

(°C)

Tb(m)

(°C)

Tb(t)

(°C)

Ta(b)

(°C)

Ta(m)

(°C)

Ta(t)

(°C)

Air velocity

sensor

y (cm) 15 55 94 15 55 94 15 55 94 99

z (cm) 60 60 60 60 60 60 60 60 60 60

x (mm) 6.2 6.2 6.2 96.2 96.2 96.2 51.2 51.2 51.2 51.2

Note: x is horizontal; y is vertical; z is adjacent 3rd axis of x-y plane of Figure 2

Table 7. Location of sensors in the experimental setup.

Distance of

sensors (Figure

Tp(b)

(°C)

Tp(m)

(°C)

Tp(t)

(°C)

Tb(b)

(°C)

Tb(m)

(°C)

Tb(t)

(°C)

Ta(b)

(°C)

Ta(m)

(°C)

Ta(t)

(°C)

Air velocity

sensor

y (cm) 15 55 94 15 55 94 15 55 94 99

z (cm) 60 60 60 60 60 60 60 60 60 60

x (mm) 6.2 6.2 6.2 96.2 96.2 96.2 51.2 51.2 51.2 51.2

Note: x is horizontal; y is vertical; z is adjacent 3rd axis of x-y plane of Figure 2

Air

y = 14.866e

0.0366x

= 0.9944

4.9514.8524.75 34.65 44.55 54.45 64.35 74.25 84.15 94.05

Height (cm)

Temperature (

Insulating plywood Board

y = 2.4418Ln(x) + 19.448

= 0.9942

4.9514.8524.7534.6544.5554.45 64.3574.2584.1594.05

Height (cm)

Temperature (

PV Module

y = 32.197e

0.0097x

= 0.9743

32.5

33.5

34.5

35.5

4.9514.85 24.75 34.65 44.55 54.45 64.3574.25 84.15 94.05

Height (cm)

Temperature (

(a) (b) (c)

Figure 5. Temperature plots with height of photovoltaic duct wall: (a) PV module; (b) Air; (c) Insulating plywood board.

A Combined Solar Photovoltaic Distributed Energy Source Appliance 83

tude of the heat capacity of wood framing material was

not proportional to the face area of glass coated PV

modules.

6.3. Thermal Storage Capacity

Thermal storage capacity of photovoltaic duct wall has

been obtained from individual thermal conductivities of

glass coating, solar cells, air, plywood board and poly-

styrene. Time constant (/

dpdd d

TCdh





) for each

component of duct is calculated from individual heat

capacities and film coefficients for various component of

photovoltaic duct wall. Equivalent thermal conductivity

of glass coated PV module is calculated to be 0.91 W

m−1·K−1. Temperature differences are obtained along y-

direction i.e. along height of PV module test section

(0.993 m) are obtained from Table 6 for Run No. 2 in

the critical case of buoyancy-induced hybrid ventilation.

Temperature differences along x-direction i.e. along

thicknesses of each component of PV module test section

are obtained proportionate to temperature differences

along y-direction. Thermal storage capacity of PV mod-

ule test section along its height is 15.9 KJ. Thermal stor-

age capacity in x-direction is negligible in comparison

with the thermal storage capacity in y-direction. There-

fore temperature measurements were also felt necessary

along the height of photovoltaic duct wall to consider

pattern of heat flow and heat transport. The procedure for

obtaining thermal storage capacities of components of

photovoltaic duct wall is described in Table 9.

Traditional one dimensional (1-D) temperature solu-

tions obtained from Equations (1) to (7) does not take

into account heat flow conductance across all thicknesses

towards x, y and z axes. The error induced by not con-

sidering conduction heat flow analysis along height of

photovoltaic duct wall (y-direction) is found to be sig

nificant. In addition to this, traditional 1-D solutions do

not take into effect integrated radiation heat exchange in

between the composite nodes. The small increase or de-

crease in surface temperature causes exponential increase

or decrease in temperature of air. From Equations (10) to

(13), conjugate heat exchange paths are bi-directional in

U-matrix to allow adjustment of any imposed errors due

to calculation of constitutive relations (or boundary con-

ditions) with use of temperatures obtained from the tradi-

tional 1-D temperature solutions.

Heat storage capacity of photovoltaic duct wall is con-

sidered as nil by assuming lumped temperature distribu-

tion. The error induced by neglecting heat storage capac-

ity is small because of discretisation procedure. As the

induced error due to non consideration of heat capacity is

Table 8. Sensible heat storage capacities.

Component ρn

(kg·m−3)

(J·Kg−1·K −1)

(m·10−3)

dnρnCn

(J·m−2·K−1)

Hpv-T

(J·K−1)

Glass coating 3000 500 3 4500 4171.5

PV module 2330 677 0.2 315.48 292.45

Glass coating 3000 500 3 4500 4171.5

Sub-total - - - - 8635.5

Air 1.1174 1000 90 100.56 93.22

Plywood 550 1750 7 6737.5 6245.66

Polystyrene 1050 1200 26 32760 30368.5

Plywood 550 1750 7 6737.5 6245.66

Sub-total - - - - 42953.0

Total - - - - 51588.5

Note: Heat capacities are calculated for face area of PV module test section of 0.927 m2.

Table 9. Thermal storage capacities.

Component kd

(Wm−1·K−1)

dnρnCn

(J·m−2K−1)

(W·m−2·K−1)

(sec)

ΔTV

(K)

ΔTH

(K)

(KJ)

(J)

PV module 0.91 9315.48 10 932 6.9 0.04 5.8 0.2

Air 0.02624 100.56 10.0 10 8.1 0.75 0.0 0.0

Plywood 0.0835 6737.5 10.0 674 9.9 0.40 0.55 0.16

Polystyrene 0.02821 32760 1.0 32760 9.9 0.40 9.0 9.6

Plywood 0.0835 6737.5 10.0 674 9.9 0.40 0.55 0.16

Total - - - - - - 15.9 10.12

84 A Combined Solar Photovoltaic Distributed Energy Source Appliance

dependent on grid size, therefore with increased number

of nodes in the grid, the induced error is negligible by

not takeinto effect integrated radiation heat exchange in

between the composite nodes. The small increase or de-

crease in surface temperature causes exponential increase

or decrease in temperature of air. From Equations (10) to

(13), conjugate heat exchange paths are bi-directional in

U-matrix to allow adjustment of any imposed errors due

to calculation of constitutive relations (or boundary con-

ditions) with use of temperatures obtained from the tradi-

tional 1-D temperature solutions.

Heat storage capacity of photovoltaic duct wall is con-

sidered as nil by assuming lumped temperature distribu-

tion. The error induced by neglecting heat storage capac-

ity is small because of discretisation procedure. As the

induced error due to non consideration of heat capacity is

dependent on grid size, therefore with increased number

of nodes in the grid, the induced error is negligible by

not considering heat storage capacity.

The constitutive relations for conductance terms in

Equations (10)-(13) are calculated over discretised con-

trol areas in y-z plane for conductive heat flow, radiation

and convective heat exchange factors. Heat transport

conductance terms are calculated from mass flow rate

crossing the control volume in x-z plane i.e. same

throughout photovoltaic duct wall assuming no leakage

or infiltration sources. The two dimensional numerical

solutions have considered the effect of thermal storage

by incorporating conduction heat flow factors in y-di-

rection. The error induced by not considering heat stor-

age capacity is negligible in comparison with the error

induced by not considering thermal storage in the walls

of photovoltaic duct. The glass coated photovoltaic mod-

ules have low specific heat in comparison with air and

insulated plywood board filled with polystyrene. This

results in small value of heat capacity for the

semi-conductor material of photovoltaic modules in

comparison with plywood board with polystyrene insula-

tion. The plywood board and polystyrene have high spe-

cific heats and are widely used for building insulation.

The low thermal conductivities of plywood board and

polystyrene give poor thermal storage in comparison to

glass coated photovoltaic modules. The counter-effects

of good heat storage capacity and poor thermal storage in

plywood board and poor heat storage capacity and good

thermal storage in photovoltaic modules are balanced by

considering the conduction heat flow along y-direction

both for outer and inner wall of photovoltaic duct wall in

obtaining two dimensional numerical solutions. There-

fore, it is definitely not essential to obtain solutions of

two dimensional steady heat conduction equations

(Equations 1 and 4) in comparison with the necessity of

performing transient analysis for the case where inner

wall of the duct is massive and has high heat storage ca-

pacity. Therefore, only effective boundary conditions

(Equations 3 and 4) are utilized for obtaining two dimen-

sional solutions of one dimensional heat conduction and

heat transport equations.

7. Conclusions

A combined solar photovoltaic distributed energy source

appliance is presented. A pair of photovoltaic modules is

combined with a plywood board and fixed in the wall of

an outdoor room. The advantages of a photovoltaic de-

vice as a sustainable energy source for reducing green

house gas emissions are presented.

A combined photovoltaic appliance was a duct wall

assembly in an outdoor room, which was built with a pair

of glass coated PV modules installed on a wooden frame

of a duct wall, leaving air passage with the wall section

of plywood board filled with polystyrene. A study is

conducted to analyze the state-of-art for an energy ap-

pliance.

A literature review elaborating various details of an

appliance, approaches and issues are enumerated in tabu-

lar format. Some sketches of drawings of the energy ap-

pliance are illustrated for giving an idea of the effiiency

of the appliance along with HVAC system.

A simulation model based on measured data of air ve-

locity, solar intensity, outdoor room and ambient tem-

peratures is presented. The model equations comprising

of heat conduction equations along with the boundary

conditions of integrated radiation exchange, convection

heat transfer and heat transport equations are specified.

The steady state solution of one dimensional heat con-

duction and heat transport equations were obtained by

performing two dimensional energy and mass balances

on section of PV duct wall. The effect of heat storage

capacity and thermal storage capacity for an appliance is

included in the simulation model.

The heat and thermal storage capacities of PV duct

wall were obtained for comparing the errors and validat-

ing the assumptions made in the proposed model. The

simulation model has predicted fairly well the tempera-

tures along the height of PV device in comparison to the

measurements obtained from the test section of the ap-

pliance.

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A Combined Solar Photovoltaic Distributed Energy Source Appliance 85

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86 A Combined Solar Photovoltaic Distributed Energy Source Appliance

Nomenclature

∆Tm,n Temperature difference at node m,n (K)

A Face area of PV module (m2)

cp Specific heat of air at constant pressure (J·kg−1·K−1)

Ep Electric power generated

H Height of photovoltaic duct wall (m)

ha Ambient heat transfer coefficient (W·m−2·K−1)

hi Convective heat transfer coefficient for insulating

plywood board (W·m−2·K−1)

hc Average convective heat transfer coefficient

(W·m−2·K−1)

hp Convective heat transfer coefficient for PV module

(W·m−2·K−1)

hr Linearised radiation heat transfer coefficient

(W·m−2·K−1)

hs Film coefficient for the insulating plywood board

facing room air zone (W·m−2·K−1)

L Air gap width of duct (m)

m Steady mass flow rate (kg·s−1)

n Number of discretised elements in y-ordinate

N Total number of discretised elements in grid

Qm,n Heat source term at the node (m,n)

S Incident solar intensity (W·m−2)

Up,i Conductance between nodes p and i (direction p to i)

(W·K−1)

Um,n Conductance Term at node (m,n),

Sp Absorbed solar radiation by PV module (Watts)

Tm,n Temperature matrix for discretised nodes (m·X·n)

Ta Temperature variable for the air (K)

Ti Temperature variable for the insulating board (K)

ti Thickness of Back panel (m)

To Ambient air temperature (K)

Tp Temperature variable for the PV module (K)

tp Thickness of PV module (m)

Ts Room air temperature (K)

v Air velocity (m·s−1)

W Width (m)

Subscripts

p PV

module

i Insulating plywood board

a Air

m Position of row in U-matrix

n Position of column in U-matrix

o Ambient air

s Room