thermo-electrical solar hybrid system

8/3/2019 Thermo-Electrical Solar Hybrid System

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Thermo-electrical solar hybrid system

Katarzyna Znajdek, Technical University of Lodz

Abstract

This paper presents a conception of a new solarhybrid system which integrates photovoltaic (PV)module with solar thermal collector. Such thermo-electrical hybrid device provides a direct conversionof solar energy into electricity and heat energysimultaneously. A preliminary project of described

solution is proposed and a construction of theprototype is presented. Most significant operatingparameters of PV module, such as: efficiency,maximum power or fill factor, have been researchedincluding various performance of working mediumin thermal collector. Results have been comparedand analyzed. The optimum operation point hasbeen chosen and the efficiency of the whole systemhas been calculated.

Advantages of integration those two devices intoone hybrid system are presented and future plans forthe development and industrial application of this

solution are introduced.

1. Introduction

Sun is the greatest source of energy which isdelivered on Earth. One can take advantage of solarenergy in several ways. Among many methods ofphotoconversion (conversion of solar radiation), onecan distinguish photovoltaic and photothermalconversions. The fist one is a direct production ofelectrical energy through the generation andseparation of charges in the p-n junction ofsemiconductor. Second one is used to convert solar

radiation into heat, commonly used in solar thermalcollectors for preheating domestic water.

The aim of the project was to create a system thatconnects those two types of photoconversions, usesthem in one integrated device. The idea of suchsolution arose according to the observations ofdecreasing efficiency of photovoltaic modules underthe influence of high temperatures. Standardconditions for solar cell operation are defined by thetemperature of 25C. In practice, photovoltaicmodule under real atmospheric conditions (in centralEurope) can be heated up to 80C. The efficiency of

such PV module drops by about 0,4% to 0,9%absolute for each Celsius of increasing surfacetemperature. In order to minimize this effect,

applying of the copper pipes on the underside of thePV module is proposed. Working medium flowingthrough channels, located beneath the surface, coolsthe photovoltaic module and simultaneously,received thermal energy can be utilized for waterheating. Similar systems were mentioned by otherauthors before, however one cannot find anyexperimental measurements of real constructionperformance and it was not precisely investigated.Fig. 1 shows the cross section of the thermo-electrical solar hybrid device.

Fig.1. General idea of solar hybrid system.

2. Construction of the prototype

Proposed system prototype was constructedfallowing by the previously established project. Themodel device was designed for inside laboratory testsin undersized scale. It consists of nine single-crystalline silicon solar cells placed on previouslyisolated copper plate with copper coil pipeconnected to it. Fig. 2 presents the preliminary

project of constructed device.

Fig.2. System design: left front (top) side, right-

back (bottom) side.

Final construction was fabricated using available

techniques. Solar cells was connected in series-parallel way using metal tape and isolated from bothsides with high temperature spray plastic. Top side was covered with glass coat. Copper pipe was

XIII International PhD WorkshopOWD 2011, 2225 October 2011


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soldered to the bottom side of the plate in the wayshown in Fig. 2. Fig. 3 illustrates the realization ofthe project.

Fig.3. The realization of the solar hybrid systemprototype: left PV module, right- thermal collector.

The whole system was integrated, isolated andencapsulated in order to protect it from the influenceof the outer environment.

To conduct research the installation must beobviously equipped with light source (solar

simulator) and other indispensable peripheralapparatus (such as: meters, regulated load, pump andheat receiving system). Fig. 4 presents the systemduring measurements.

Fig.4. Thermo-electrical solar hybrid system under

illumination.

3. Obtained results and conclusions

This section present preliminary measurements ofsystem efficiency and in particular its dependence onthe fluid flux that flows through pipes.

3.1 Applied equations

To obtain photovoltaic module efficiency onefollows the formula:

ct

PVEA

P

=max

(1)

where: Pmax is electrical power in MPP (maximumpower point) [W], At represents surface area of themodule [m2 ] and Ec is solar radiation that hits thesurface [W/m2].

To find efficiency of solar thermal collector(equation 2), two basic parameters are needed:collector effective thermal power Qeff and collector

applied powerQapp.

app

eff

TQ

Q= (2)

Whereas Qeff is a product of working mediumfluid flux m [kg/s], its specific heat cp,f [J/kgK] andtemperature increment between input Tin and outputTout, andQappis represented by the value of absorbersurfaceAp[m2] multiplied by incident solar radiationper area unit Ec [W/m2 ], fallowing the formulas,respectively 3 and 4:

( )inoutfpeff TTcmQ = , (3)

cpapp EAQ = (4)

The efficiency of the whole hybrid system ismarked PV/T and determined as follows:

cp

eff

TPVEA

QP

+

=max

/ (5)

3.2 Measured parameters

As a first point of measurements, parameters ofphotovoltaic module without the support of solarcollector were obtained. The results show that afterabout 20 minutes, device temperature raised up to 50Celsius degrees. The consequence of increasedtemperature is efficiency drop. Its value at 50C was17% lower than the initial one (Fig. 5). This is onlythe confirmation of the literature reports of highnegative heat influence on PV module performance.In this particular case, deterioration of efficiency was

about 0.75% (absolute) for one Celsius oftemperature increase.

80

85

90

95

100

25 30 35 40 45 50 55

Temperature [oC]

Efficiencydecrese[%]

Fig.5. Absolute decrease of PV module under the

influence of increasing surface temperature.

Second and essential point of research was toinvestigate the operation of hybrid device describedabove. Basic parameters of the system were obtainedexperimentally and efficiencies were calculated usingformulas 1-5. Table 1 presents both measured andcomputed values. In this case, water was used asa fluid working medium. However for outdoorapplication water-glycol solution is more advisable

due to its lower freezing temperature allowingsystem to work in winter time as well.Parameters for thermo-electrical solar hybrid

system were measured in laboratory conditions


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under the illumination intensity of 363 W/m2. This isalmost three times lower value than the onerecommended for STC (standard test conditions:1kW/m2), which is probably the reason of relativelybad efficiency of PV module. However, herepreliminary research are presented and the majorobjective is to rather analyze the operation of

integrated appliances (photovoltaic module andthermal collector) or achieving the highest efficiency.

Tab.1.

Properties of the hybrid system at different fluid flux.

Ec

[W/m2]362.7

Ap [m2 ] 0.1764

At [m2 ] 0.1406

Qapp [W] 63.98

m [kg/s] 0.0225 0.0173 0.0150

Tin [C] 29.2 29.1 29.3

Tout [C] 29.6 29.7 29.9

T [C] 0.4 0.6 0.7

FF [-] 0.43 0.42 0.42

Pmax [W] 0.91 0.92 0.91

Qeff[W] 37.63 43.56 43.91

PV[%] 1.79 1.80 1.78T [%] 58.83 68.08 68.62

PV/T [%] 60.25 69.51 70.05

The efficiency of solar thermal collector, andconsequently the whole thermo-electrical hybridsystem, is dependent on the working medium fluidflux. One can observe that the efficiency value ofthermal collector reaches maximum at relatively lowfluid flux factor and decrease with faster flow. Thereason of such behavior is quite simple. Lower velocity of the liquid causes longer contact of theliquid with the inside surface of the pipe, whichallows it to receive more heat. As a result input andoutput collector temperature difference is higher.

Photovoltaic module efficiency is less sensitiveon the fluid flux value. One can see that it barelychanged in comparison to thermal efficiency. This isthe effect of relatively stable surface temperature ofphotovoltaic module, achieved thanks to theapplication of heat regaining system in the form ofliquid-based thermal collector.

Fig. 6 and 7 show thermographic pictures of thesystem surface, respectively: before and after turningon the flow of working medium.

Fig.6. Illuminated solar hybrid system working withoutliquid flow (solar thermal system is turned off)

Fig.7. Illuminated solar hybrid system several minutesafter turning on the liquid flow.

Temperature of the surface raised up from 27Cto 50C in about 20 minutes while cooling system was not running. Ten minutes after fluid fluxgeneration, surface temperature dropped to 36C.

Even such short experiment shows that thethermal-electrical hybrid system is working properly -efficiently cools photovoltaic plate and

simultaneously heats flowing water.

4. Future development

Designed system obviously requires furtherinvestigations and analysis of several omitted aspects.First of all measurements in standard test conditionsaccording to solar radiation are needed. Further stageshould include research in real atmosphericenvironment, as well as normalized mechanical,insulating and structural strength test. At the endindicated improvements should be made. However,after this preliminary analysis one can assume that

this integrated thermo-electrical solar hybrid systemis promising and future-oriented solution waiting forimplementation.

Bibliography

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[2] Smolec Wodzimierz: Fototermiczna konwersjaenergii sonecznej, Wydawnictwo Naukowe PWN,Warszawa 2000.

[3] Pluta Zbysaw: Soneczne instalacje energetyczne,Oficyna Wydawnicza PolitechnikiWarszawskiej, Warszawa 2007.


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[4] Chochowski Andrzej, Czekalski Dariusz:Soneczne instalacje grzewcze, Centralny OrodekInformacji Budownictwa, Warszawa 1999.

[5] Januszewski Jzef: Zasady projektowania urzdzesonecznych do celw ogrzewczych, WydawnictwoPolitechniki Wrocawskiej, Wrocaw 1986.

[6] Zawadzki Mirosaw: Kolektory soneczne, pompy

ciepa - na tak, Polska Ekologia, Warszawa 2003.[7] Wesoowski Maciej: Analiza funkcjonowania

cieczowej instalacji sonecznej, WydawnictwoUniwersytetu Warm.-Maz., Olsztyn 2006.

[8] Fotowoltaika - materiay szkoleniowe, PolitechnikaWarszawska, Warszawa 2011.

[9] Lewandowski Witold M.: Proekologiczneodnawialne rda energii, WydawnictwoNaukowo-Techniczne, Warszawa 2007.

Acknowledgements:

The author is a scholarship holder within theproject entitled "Bioenergy for the Region Integrated Programme for Ph.D. StudentsDevelopment" supported by European Social Fund.

Author:

MSc. Katarzyna ZnajdekTechnical University of Lodz211/215 Wolczanska90-924 Lodz, Polandtel. 48 42 631 26 81fax 48 42 636 80 24

email: [email protected]

thermo-electrical solar hybrid system

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