Nowadays, based on the rapid industrialization and the improvements in quality of life, especially in OECD countriesm the share of electricity… [603714]

INTRODUCTION
1. Solar energy
Nowadays, based on the rapid industrialization and the improvements in quality of life,
especially in OECD countriesm the share of electricity produced by Renewable Energy Sources
(RES) is rapidly growing. [10]
With the growing e nvironmental concerns and the depletion of fossil fuel, the world is
paying more attention to the use of renewable energy. One source of renewable energy is solar
energy . [12]
Solar energy is , among others, a energy resource that is ecofriendly, renewable and freely
available on earth. But, of all renewable energy sources, solar energy seems to be more promising,
sustainable energy resources. Thus, the solar energy based systems can meet energy demands to
some extent and keep the balance in the ecosystem . [4] Solar energy is one of the most widely
adopted renewable energy source that can be utilized in various applications such as thermal
management or electricity. [2] Solar radiations can be converted into either thermal energy , using
thermal collectors or electrical energy , through special optical solar cells, also known as
Photovoltaic(PV) cells or both. [4]
Also, in recent year has grown the interest in sustainable development and growth,
motivating the development of environmental benign energy tec hnologies. Research on
applications of solar energy technologies have as a consequence expanded rapidly, exploiting the
abundant, free and environmentally benign characteristics of solar energy. However, widespread
acceptance of solar energy technology dep ends on its competitiveness, considering factors such as
efficiency, cost -effectiveness, reliability and availability. Some solar thermal systems, such as
solar water heaters, air heaters, cookers, dryers and distillation devices, have advanced notably in
decades in terms of efficiency and reliability. Efficiencies of these devices typically range from
about 40% to 60% for low – and medium -temperature applications [6]

1.1. Advantages of solar energy
Renewable Energy Source
Among all the benefits of solar panel s, the most important thing is that solar energy is a
truly renewable energy source. It can be harnessed in all areas of the world and is available every
day. We cannot run out of solar energy, unlike some of the other sources of energy. Solar energy
will be accessible as long as we have the sun, therefore sunlight will be available to us for at least
5 billion years when according to scientists the sun is going to die.

Reduces Electricity Bills
Since you will be meeting some of your energy needs with the e lectricity your solar
system has generated, your energy bills will drop. How much you save on your bill will be
dependent on the size of the solar system and your electricity or heat usage. Moreover, not only
will you be saving on the electricity bill, but if you generate more electricity than you use, the
surplus will be exported back to the grid and you will receive bonus payments for that amount
(considering that your solar panel system is connected to the grid). Savings can further grow if you
sell excess electricity at high rates during the day and then buy electricity from the grid during the
evening when the rates are lower.
Diverse Applications
Solar energy can be used f or diverse purposes. You can generate electricity (photovoltaics)
or heat (solar thermal). Solar energy can be used to produce electricity in areas without access to
the energy grid, to distill water in regions with limited clean water supplies and to powe r satellites
in space. Solar energy can also be integrated into the materials used for buildings. Not long ago
Sharp introduced transparent solar energy windows.
Low Maintenance Costs
Solar energy systems generally don’t require a lot of maintenance. You o nly need to keep
them relatively clean, so cleaning them a couple of times per year will do the job. Most reliable
solar panel manufacturers give 20 – 25 years warranty. Also, as there are no moving parts, there
is no wear and tear. The inverter is usually the only part that needs to changed after 5 – 10 years
because it is continuously working to convert solar energy into electricity (solar PV) and heat
(solar thermal). Apart from the inverter, the cables also need maintenance to ensure your solar
power sy stem runs at maximum efficiency. So, after covering the initial cost of the solar system,
you can expect very little spending on maintenance and repair work.
Technology Development
Technology in the solar power industry is constantly advancing and improvem ents will
intensify in the future. Innovations in quantum physics and nanotechnology can potentially
increase the effectiveness of solar panels and double, or even triple, the electrical input of the solar
power systems.

1.2. Solar Energy Disadvantages
Cost
Nowadays, the initial cost of purchasing a solar system is fairly high, but, as we know,
solar technologies are constantly developing, so it is safe to assume that prices will go down in the
future.
Weather Dependent
Although solar energy can still be collected during cloudy and rainy days, the efficiency of
the solar system drops. Solar panels are dependent on sunlight to effectively gather solar energy.
Therefore, a few cloudy, rainy days can have a noticeable effect on the energy system. You should
also take into account that solar energy cannot be collected during the night. On the other hand, if
you also require your water heating soluti on to work at night or during wintertime, thermodynamic
panels are an alternative to consider.
Solar Energy Storage Is Expensive
Solar energy has to be u sed right away, or it can be stored in large batteries. These batteries,
used in off -the-grid solar systems, can be charged during the day so that the energy is used at night.
This is a good solution for using solar energy all day long but it is also quite expensive. In most
cases, it is smarter to just use solar energy during the day and take energy from the grid during the
night (you can only do this if your system is connected to the grid). Luckily our energy demand is
usually higher during the day so we can meet most of it with solar energy.
Uses a Lot of Space
The more electricity you want to produce, the more solar panels you will need because you
want to collect as much sunlight as possible. Solar panels require a lot of space and some roofs are
not b ig enough to fit the number of solar panels that you would like to have. An alternative is to
install some of the panels in your yard but they need to have access to sunlight. Anyways, if you
don’t have the space for all the panels that you wanted, you can just get a fewer and they will still
be satisfying some of your energy needs.
Associated with Pollution
Although pollution related to solar energy systems is far less compared to other sources of
energy, solar energy can be associated with pollution. Tran sportation and installation of solar
systems have been associated with the emission of greenhouse gases. There are also some toxic
materials and hazardous products used during the manufacturing process of solar photovoltaics,
which can indirectly affect th e environment. Nevertheless, solar energy pollutes far less than the
other alternative energy sources.

2. Photovoltaics cells

The photovoltaic phenomenon has been recognized since 1839, when French physicist
Edmond Becquerel was able to generate electricity by illuminating a metal electrode in a weak
electrolyte solution. The photovoltaic effect in solids was first studied in 1876 by Adam and Day,
who made a solar cell from selenium that had an efficiency of 1 –2%. The photovoltaic effect was
explained by Alb ert Einstein in 1904 via his photon theory. A significant breakthrough related to
modern electronics was the discovery of a process to produce pure crystalline silicon by Polish
scientist Jan Czochralski in 1916. [6]
Photovoltaic (PV) cell or solar cell di rectly converts solar energy into electricity by the
photovoltaic effect . The PV cell circuit is the fundamental building block of PV systems and it is
sealed in an environmentally protective laminate on the PV module. On the other hand, PV array
is the c omplete power generating unit, consisting of number of series or parallel (or combination
of both) connected PV modules and panels (one or more PV modules assembled as a pre -wired,
field-installable unit. [14]
The efficiency of first generation silicon cel ls was about 6%. It was first practical p -n
junction type solar cell and was demonstrated at the Bell Telephone Laboratories in 1954. This
efficiency is considerable lower than that of contemporary solar cells, that is about 14 – 20%. [6]
PV is currently a technically and commercially mature technology able to generate and
supply short/mid -term electricity using solar energy. Although the current PV installations are still
small and provide only 0.1% of world total electricity generation.
A market review indicated that the annual growth since 2000 of global PV installations
has exceeded 40% and the total installed capacity worldwide has reached about 22 GW. [15]
Photovoltaic systems can be further distinguished based on the solar cell technology
presente d in Figure 1. Silicon (Si) based technologies can be categorized as a crystalline silicon
and amorphous silicon or thin film, and are considered the most mature. Crystalline silicon cells
can have different crystalline structures: single crystalline silic on, multi -crystalline silicon and
ribbon cast multi -crystalline silicon . [6]

Figure 1. Various PV technologies1

About 90% of total PV market is seen to be based on crystalline silicon (c -Si), with the
shares of mono c -Si and poly c -Si comprising abou t 42% and 45%, respectively. In recent years,
significant growth has been reported for applications of amorphous or thin film Si technology,
likely due to its advantages over c -Si and poly c -Si cells, which include lower cost per unit collector
and little variation of cell efficiency with temperature. However, the lower cell efficiency, rapid
performance degradation and shorter life of thin film cells relative to c -Si are likely to hinder
growth in utilization. Other PV technologies such as CdTe, CIS are in the early phase of
development, but are considered promising due to their use of multi -junction cells. [6]
With continuous technical advance, increased installation volume, reduced price and
encouraging legal policies, PV will certainly continue on the fa st-growing pace and eventually
become an important energysupplier in the world. It is predicted by IEA at its recent Technology
Roadmap – Solar Photovoltaic Energy that PV will deliver about 5% of global power need by 2030
and 11% by 2050. The accelerated use of PV will result in more than 100 giga -tonnes (Gt) of CO2
emission reduction during the period of between 2008 and 2050. [15]

1 Kumar et al., 2011

3. Solar thermal collectors
A solar collector converts solar radiant energy into useful thermal energy, which is
transferred t o a fluid (water, air, glycol, oil, etc.) flowing through the collector.
Solar thermal is one of the most cost effective renewable energy technologies and has huge
market potential globally. It representing more than 90% of the world -installed solar capac ity. The
collected energy is utilized for various purposes, such as steam generation, water or space heating,
solar assisted cooling and industrial process heating or it can be stored in a thermal storage for
later use. [15]
A common classification of sola r thermal collector divides them in 2 categories:
– non-concentrating (e.g., flat plate)
– concentrating (e.g., compound parabolic concentrators)
In the non -concentrating type, the collector area (the area that intercepts the solar radiation)
is the same as t he absorber area (the area that absorbs the radiation) [3]. Flat plate collectors (FPC)
and evacuated tube collectors (ETC) are non -concentrating type collectors. These collectors are
mainly designed for solar hot water and industrial process heat applicat ions which require energy
delivery at temperatures in the range of 60 -250
C. These collectors use both diffuse and beam
solar radiation and do not require tracking of the sun. They are mechanically simpler than
concentrating collectors and require less mai ntenance. In Figure 2 are shown the different types of
non-concentrating and concentrating solar collectors. [18]

Figure 2. Types of solar collectors2

2 Review of sputter deposited mid – to high – temperature solar selective coatings for Flat Plate/Evacuated tube
collectors and solar thermal power generation applications

Flat plate solar collectors
The flat plate solar collector is most common solar thermal device and has been used for
many decades. The schematic diagram of a typical flat -plate solar collector is shown in Figure 3.
A flat -plate collector consists of: an absorber, a transparent cover, a heat -transport fluid (water,
air, etc.), housing or casing and insulatio n. Usually a low -iron tempered glass is used as a
transparent cover. The absorber surface is the heart of the flat plate solar collector and is typically
made of copper or aluminum and painted black or selectively coated. When used, insulation is
located a t the back and sides of the collector to limit heat losses. The most common insulations
material for flat plate collectors are polyurethane and mineral wool. A frame of tubes is attached
to the absorber surface to allow for the flow of a heat transfer flui d. The thermal efficiency of this
collector type normally ranges from 40% to 60% for the low to medium temperature applications,
and decreases rapidly as temperatures exceed 60 °C. These collectors are generally used for
temperatures less than 100 °C and a preferred choice for combined photovoltaic – thermal
modules. [6]

Figure 3. Schematic diagram of a flat -plate collector3

In the concentrating type solar collector, various types of mirrors, reflectors or
concentrators are used to concentrate the solar energy and they provide higher temperatures (i.e.,
400–1000 °C) than non -concentrating type collectors.
Compound parabolic concentrator (CPC), central receiver or solar tower, parabolic trough
collector and parabolic dish collectors are concentrating type collectors and are known as
concentrated solar power (CSP) systems. The concentration ratio (i.e., ratio of the area of aperture
to the area of the receiver) varies from less than unity to high values of the order of 105. The
concentrating type collectors are classified into three types:
– non-imaging ;

3 Review of sputter deposited mid – to high – temperature solar selective coatings for Flat Plate/Evacuated tube
collectors and solar thermal power generation applications

– line focusing ;
– point focusing collectors.

Non-imaging collectors have a low concentration ratio; line focusing collectors have
intermediate concentration ratio and point focusing collectors have high concen tration ratio. The
CPC collector is a non -imaging concentrating collector. Whereas, parabolic trough and central
receiver are line focusing and point focusing collectors, respectively. These CSP systems are
mainly used for solar thermal power generation. [18]

Another categorization of solar collectors , based on heat transfer fluid separates them also
into 2 categories:
– liquid heating
– air heating types.

Solar thermal air collectors

The working principle of a solar thermal air collector is similar to th at for a flat plate solar
collector, the main differences being the heat transfer fluid (air) and the air flow passage
configuration. Solar thermal air collectors have some advantages over conventional flat plate
collectors:
– The use of air as the heat tran sfer fluid avoids the need for special heat transfer fluids (oil
or glycol) able to withstand freezing conditions.
– Corrosion is less of a concern.
– Leakage through joints and ducts is less of a concern.
– High -pressure protection is not required.
– The device i s more compact and lightweight, less complicated and easy to install.

Nonetheless, the solar thermal air heater has some shortcomings relative to flat plate liquid
collectors. In particular, the heat transfer rate is relatively slow due to lower thermal c onductivity
of air, and a greater volume of air per unit collector area is required to store the thermal energy
due to the lower specific heat capacity of air. [6]

The global solar thermal market has been continuously growing since the beginning of the
1990 and in Europe, solar thermal market was tripled from 2002 to 2006 and still in booming. A
vision plan issued by European Solar Thermal Technology Platform (ESTTP) indicated that by
2030 up to 50% of the low and medium temperature heat will be delivered through solar thermal.
The European Solar Thermal Industry Federation (ESTIF) has predicted that by 2020, the EU will
reach a total operational solar thermal capacity of between 91 and 320 GW, thus leading to saving
of equivalent to at least 5,600 tonnes crude oil. By 2050, the EU will eventually achieve 1,200 GW
of solar thermal capacity. [15]

Chapter 1. Photovoltaic -thermal (PVT) systems

Solar energy conversion systems as thermal collectors and PVs are devices that absorb
solar radiation and conver t it to useful energy as thermal and electrical, respectively.
The conversion rate of solar radiation into electricity by PVs depends on cell type and is
between 5% and 20%. Thus, the greater part of the absorbed solar radiation by PVs is converted
into h eat (at about 60 –70%), increasing the temperature of cells. This effect results in the reduction
of their electrical efficiency and there is an essential difference between solar thermal collectors
and PVs regarding the required conditions for their effect ive operation. The solar thermal
collectors aim to achieve higher absorber temperature in order to provide heat removal fluid (HRF)
efficiently and at higher temperature, while the PV cells operate at lower temperatures in order to
achieve higher efficienc y in their electrical output. [1]
The heat that is a by -product of electricity generation by PV cells can be utilized in hybrid
system designs instead of simply dissipating it to the environment. Hybrid photovoltaic –thermal
(PVT) systems offer a practical solution to increase the electrical power production from PV panels
in addition to the recovery of heat extracted from the panels. Waste heat recovery permits the
utilization of waste heat to supply space or water heating in a way allowing improved overa ll
system efficiencies to as high as 70 %. [2]
A PV –thermal (PVT) collector is a module in which the PV is not only producing
electricity but also serves as a thermal absorber. In this way both heat and power are produced
simultaneously. [3] Waste heat recovery permits the utilization of waste heat to supply space or
water heating in a way allowing improved overall system efficiencies to as high as 70%.
The solar energy conversion into electricity and heat with a single device called hybrid
photovoltaic thermal collector (PVT) as illustrated in Figure 4. In this way, heat and power are
produced simultaneously and it seems a logical idea to develop a device that can comply with both
demands.

Figure 4 . Schematic of various solar technologies.4
The purpos e of PV systems is to produce electricity, so it may be more desirable to use the
PV by -product heat to generate supplemental electricity.
The attractive features of the PVT system are:
– It is dual -purpose: the same system can be used to produce electricity and heat output.
– It is efficient and flexible: the combined efficiency is always higher than using two
independent systems and is especially attractive in building integrated PV(BIPV) when
roof-panel spacing is limited.
– It has a wide application: the hea t output can be used both for heating and cooling
(desiccant cooling) applications depending on the season and practically being suitable
for domestic applications.
– It is cheap and practical: it can be easily retrofitted/integrated to building without any
major modification and replacing the roofing material with the PVT system can reduce
the payback period. [4]

4 Kumar et al., 2015

1.1. Classification of PVT hybrid systems
PVT systems are classified into different categories depending on the structure or
functionality of the de signs. According to their operating temperature PVT solar energy systems
can be divided into three systems:
– Low temperature – up to 50 °C;
– Medium temperature – 50 to 80 °C;
– High temperature – over 80 °C.
The hybrid PVT systems that are referred to appli cations of very low temperatures (30 –
40 °C) are associated with air or water preheating and are considered the most promising PVT
category. The PVT systems that use typical PV modules and provide heat above 80 °C have
lamination problems due to the high operating temperatures and need further development. [1]
In terms of heat extraction employed, PVT modules could be classified as: [2]
– Air – based types;
– Liquid – based types;
– Heat pipe – based types;
– Phase change materials (PCM) – based types;

1.1.1. Air base d PVT collectors
PVT air systems are utilized in many practical applications due to low construction
(minimal use of material) and operating cost among others. Air -based PVT collectors are
formulated by incorporating air channels often present at the rear of a PV laminate allowing
naturally or forced ventilated air to flow and extract accumulated heat through convective heat
transfer. [2]
A PVT air collector is simply a flat plate solar air heater with photovoltaic cells pasted on
the black absorber plate. It has the advantage of generating both thermal (low grade) and electrical
(high grade) energies from the same unit, and hence, it is less costly than two separate units.
Flat plate air collectors exist in many designs, but the most common models are sho wn
below, in Figure 5. In these single glazing collectors, air may flow over the absorber (Model I) or

below it (Model II), and even on both sides of the absorber in a single pass (Model III) or in a
double pass (Model IV) fashion.
Although the Model I co llector has the simplest design, Model II, of conventional design,
is the PVT collector which has been widely studied. [5]

Figure 5. Schematics of the various PVT models along with heat transfer coefficients. 5

5 Hegazy, 2000

Numerical solutions of the energy balance s indicate that the electrical and thermal outputs
for models II –IV are similar and superior to that for model I, and that the pumping power needed
is lowest for model III and second lowest for model IV.
Tonui and Tripanagnostopoulos improved PVT air coll ectors by enhancing heat extraction.
They addressed some inherent shortcomings of PVT air collectors, such as the low density,
volumetric heat capacity and thermal conductivity of air, by using a thin suspended flat metallic
sheet (TMS) between the absorbe r surface and back plate and/or by using fins on the back plate of
the air duct (FIN). [6]
Below, in Figure 6 it is illustrated a cross -sectional view of the air channels for the 3 types
of systems that was studied: typical (reffered as REF) and modified ( TMS and FIN).

Figure 6. Cross -sectional view of PVT/AIR collector models. Flow direction is perpendicular to
the page.6

The REF system represents the typical single -pass air channel attached behind the PV
module and is used as a datum system. The sheet in TMS creates a kind of double -pass
configuration and doubles the heat extraction surface. The FIN system, on the other hand, consists
of fins with rectangular profiles (Π) attached to the back wall of the air duct, for practical reasons
and oriented para llel to the flow direction. Attaching them on the PV rear surface results in better
PV cooling and increased heat production due to high PV temperature. In this case it requires
incorporating the fins in the production phase of the PV module for good back thermal contact but
makes it (PV) cumbersome, complex and heavy adding to the already high PV panel cost and

6 Tonui et Tripanagnostopoulos, 2007

transportation cost as well. In the suggested method, the locally fabricated fins may be fitted easily
to the building fac -ade or tilted roof makin g them cost effective.
Both methods increase heat transfer from the channel walls to the circulating air but
increase pressure drop (in the system).
The energy efficiency that can be achieved by the REF, TMS and FIN systems from
experiments are 25%, 28% an d 30%, respectively.

1.1.2. Liquid based PVT collectors
At high operating temperature conditions, air cooling fails to accommodate the
temperature rise at the surface of PV cells causing critical drop in their conversion efficiency.
Liquid cooling offers a be tter alternative to air cooling utilizing coolant as heat extraction medium
to maintain desired operating temperature of PV cells and a more efficient utilization of thermal
energy captured. [2]
Liquid -based PVT collectors are superior to air -based ones due to higher specific heat
capacity of coolants employed leading to further improved overall performance. In addition,
liquid -based PVT collectors offer less temperature fluctuations compared to air -based PVT making
them more favourable. When liquid is us e with the PV panels is made through a heat exchanger,
while in case of using air, the contact with PV panels is direct. [1]
The most common working fluid in liquid based PVT collectors are water, water/air and
most recently refrigerant. The water type PVT collectors are the most widely system studied.
Classification scheme for PVT liquid collectors is given below, in Figure 7.

Figure 7. Liquid based collectors classification7

1.1.2.1. PVT water collectors
Several water -based PV/T collector designs having differe nt flow patterns have been
introduced and investigated both theoretically and experimentally to achieve efficient cooling of
PV cells as we can see in Figure 8.

7 Daghigh et al., 2011

Figure 8. Different configuration of water -based PV/T collectors8

However, the most common collector design studied consists of a PV module attached to
an absorbing collector with serpentine of a series or parallel tubes at the rear surface of PV
modules (sheet – and – tube), Fig ure 9 .

Figure 9. Sheet and tube PV/T collector9

8 Makki et al., 2015
9 Makki et al., 2015

The PVT wat er collector operates such that water is forced to flow across tubes extracting
the excessive heat from the PV cells, hence reducing the operating temperature of PV cells and
transferring heat to be utilized for water and space heating applications. Rese archers have
identified several environmental and design parameters affecting the performance of such type of
collectors including: mass flow rate, water inlet temperature, number of covers, absorber to fluid
thermal conductance, PV cells packing factor, collector length, duct depth absorber plate design
parameters. [2]
Zondag et al. make a comparaison of the efficiency of seven different design type of PV/T
collectors to demonstrate the effects of design choices on the electrical and thermal efficiency.
There are studied 7 designed types of PV/T collectors which can be classified in four
groups, and presented in Figure 10:
A. Sheet -and-tube PV/T collectors;
B. Channel PV/T collectors;
C. Free flow PV/T collectors;
D. Two absorber PV/T collectors.

Figure 10 . Variou s collector concepts: (A) sheet -and-tube PV/T, (B) channel PV/T, (C) free flow
PV/T, (D) two -absorber PV/T (insulated type)10

10 Zondag et al., 2003

The simplest way to construct a PV/T collector is to rely entirely on well -known available
technology by taking standard photovolt aic panel and integrate it into a thermal collector without
any modification. Sheet -and-tube PV/T collectors are examined with zero, one and two covers.
In case of the channel PV/T collector with the channel on top of the PV, the configuration
impose const raints on the choice of the collector fluid. In this case water is used, which has a small
overlap in absorption with the PV which leads to a 4% relative decrease in the electrical
performance. A variation of the channel design is obtained by letting the w ater flow underneath
the PV panel. In this article, two cases are studied: a channel below a conventional opaque PV
panel and a channel below a transparent PV panel with a separate black thermal absorber
underneath the channel.
In a free flow panel unrest rained fluid flows over the absorber. This design, in comparasion
with the channel case, eliminates one glass layer, but, as in the case of the channel PV/T collector,
the fluid flowing over the PV panel needs to be transparent for the solar spectrum. It l ooks like
water is the natural choice but the evaporation will create problems at higher temperatures because
the water evaporation pressure is not very low. Reflections and material costs are reduced while
the mechanical problem of breaking the glass cov er is avoided. A disadvantage of this design is
the increased heat loss due to evaporation.
The design with two -absorber panel uses a transparent PV laminate as a primary absorber
and a black metal plate as a secondary absorber. The panel contains tow wate r channels on top of
each other. The water flows in through the upper channel and is returned through the lower
channel.

1.1.2.2. Refrigerant based PVT collectors

Refrigerant fluids are normally used in systems combining PVT collectors and solar –
assisted heat pu mps (SAHP), in which the PVT collector serves as an evaporator where the
refrigerant absorbs thermal energy available at the PV cells. Low evaporation temperature (0 –20
°C) of refrigerant allows efficient cooling of PV cells to be achieved leading to sign ificant increase
in the performance.
The performance of PVT solar -assisted heat pump cogeneration system was investigated
theoretically and experimentally by Ji et al.. The system comprised of nine (3X3) mono -crystalline

PVT collectors serving as direct -expansion evaporators for the heat pump in which R22 refrigerant
inside copper coil vaporizes at low temperature transferring absorbed heat to the condenser
section, Fig ure 11 .
Dynamic distributed model was developed to predict various parameters includ ing the
pressure distribution of the PV evaporator and along the copper coil attached, temperatures of PV
cells and base panel thermal collector, thermal and electrical efficiencies of the system, in addition
to vapour quality and enthalpy. Validation of the simulation results exhibited good agreement
with measured data except for the pressure drop of the PV evaporator which was found much
higher than the simulation prediction attributed to underestimation of the saturated temperature
gradient and highe r temperature gradient along the copper coil observed during the experiment.
[2]

Figure 11. Schematic diagram of PV solar -assisted heat pump11

11 Makki et al., 2015

1.1.3. Heat pipe based PVT collectors
Heat pipes are considered efficient heat transfer devices that combine the principles of
both thermal conductivity and phase transition. A typical heat pipe consists of three sections
namely, evaporator, adiabatic, and condenser sections, Fig ure 12 .
Heat pipes provide an ideal solution for heat removal and transmission, with o ne end
serving as a thermal energy collector and the other end as a thermal energy dissipator. Heat pipes
have been considered for thermal management applications of PV technology due to the
advantages such technology provide over other cooling means suc h as aiding uniform temperature
distribution of PV cells, elimination of freezing that thermosyphon tube can suffer from in higher
latitudes, in addition to resistance to corrosion. However, the design of efficient heat pipe involves
careful selection of a suitable combination of the heat pipe container material, working fluid, and
wick structure . [2]

Figure 12. Schematic diagram of heat pipe12

George F. Russell developed a cooling approach for concentrated PV (CPV) systems
utilizing heat pipes, to ena ble operation at elevated temperature and utilization of extracted heat
for beneficial use. The systems comprised of a row of PV cells mounted on the outer surface of a
heat pipe, where heat pipes are arranged next to each other to form a panel, and Fres enl lenses
were used to provide high solar energy intensity, as the scheme presented in Figure 13 . The heat

12 Makki et al., 2015

pipe used had internal tubes for circulating a fluid coolant through the heat pipe vapour field to
promote heat extraction. Details of the performan ce parameters were not revealed in this study.
The pipe was made out of extruded aluminium surface and the evaporative working fluid was
benzene. Under ambient temperature of 40 °C and concentrated solar radiation of 19.2 kW/m2, a
minimum wind speed of 1 m /s was required to keep the evaporator temperature below 140 °C
which limits the cooling capability of such system.

Figure 13. Heat pipe cooling of PV cells13

1.1.4. PCM based PVT collectors
Phase changes Materials (PCMs) are substances that are able to abso rb and release large
amount of energy as latent heat through a reversible isothermal process at a particular phase
transition temperature .
As far as PV systems are concerned, conventional passive cooling techniques are unable
to provide the required cooli ng during peak solar radiation periods leading to a deteriorated
performance of PVs. Furthermore, inhomogeneous temperature profiles which affect the
generation capacity of PV systems stand as a limitation in other passive cooling methods .

13 Makki et al., 2015

Various PCM -based PV concept designs have been reported in literatures. The most
common system studied considers incor poration of PCMs in Building Integrated Photovoltaic
(BIPV), Fig ure 1 4. [2]

Figure 14. Schematic drawing of a PV/PCM system14

Stand -alone PV colle ctors with PCM thermal storage have also been reported i n few
literatures, Fig ure 15 . These designs have the same basic components in common from which the
system is constructed. However, diversity in the heat transfer mechanisms from the PV module
was noted. The unutilized part of solar radiation striking the surface o f PCM -based PV systems is

14 Makki et al., 2015

conducted as heat to the PCM through the PCM container material causing increased temperature
of PCM. At a certain phase transition temperature the PCM starts melting and due to continuous
temperature rise the melt extends into the PCM. During latent heat transfer process the PCM
effectively acts as a heat sink maintaining a regulated temperature of the PV modules close enough
to its melting/freezing point. Once the melting process is complete, any further heat stored will
manifest as a temperature rise. Such process is reversible where insolidification cycle takes place
as the temperature drops below the melting point. [2]

Figura 15. V -trough stand -alone PV/PCM system15

15 Makki et al., 2015

Chapter 2 . PVT air -based