The Interaction of Standards and Innovation Hybrid

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The interaction of standards and innovation: Hybridphotovoltaicthermal collectors

Korbinian Kramer , Henning Helmers

Fraunhofer ISE, Heidenhofstr. 2, 79110 Freiburg, Germany

Received 8 January 2013; received in revised form 21 May 2013; accepted 3 August 2013Available online 7 November 2013

Communicated by: Associate Editor Bibek Bandyopadhyay

Abstract

Hybrid photovoltaicthermal solar collectors (PVT collectors) convert solar radiation into both electrical power and useable heat.The goal of combining these two forms of energy conversion in one product is to increase overall efficiency by accessing a higher technicalenergy potential. Combining the two types of energy transformation in a hybrid product is an innovative approach that is currentlyentering the market in the form of several products from several producers. As a result of boundary conditions (the early stage of marketdiffusion and the absence of standards, norms, and certifications), there is an enormous deficit of technical information for PVT collec-tors. This leads to restrained policy implementation from government entities, fewer incentives for producers and more wariness on thepart of the end consumer; the combination of these factors constitutes a strong market barrier. In addition, with respect to product qual-ity labels and product certification, PVT collectors must be discussed in a sophisticated way and, therefore, require an appropriate sci-entific description. In this paper, possible changes and steps with regards to standards, regulations and certification procedures are

suggested to provide solutions over the short, medium and long terms. In addition, an extended hybrid collector model (in analogyto the quasi-dynamic thermal performance model) is presented and proposed for implementation into the existing certification. 2013 Elsevier Ltd. All rights reserved.

Keywords: PVT collector; Hybrid collector; Product certification; Solar Keymark; Standardization; Combined heat and power

1. Introduction

Hybrid photovoltaicthermal solar collectors (PVT col-lectors) convert solar radiation into both electrical power

and useable heat. The aim of combining these two formsof energy conversion in one product is to increase overallefficiency by accessing a higher technical energy potential.Standard flat-plate photovoltaic modules based on crystal-line silicon convert approximately 15% of the incident solarradiation into electrical power (compare Fraunhofer ISE,2012); the remainderexcept for reflection lossis trans-formed into heat, as shown in Fig. 1.

From a technical point of view, many different varia-tions of collector configurations are possible. The simplestis to add a heat-removing hydraulic structure to the exist-ing standard PV module. If the collector design is not opti-

mized for good thermal bonding between the PV cell andthe hydraulic layout, the cell temperature is significantlyhigher than the temperature of the heat removing structure.Because the PV cells in such a configuration are not insu-lated from the environment well, thermal losses are rela-tively large and the temperature level is comparably low(40 C). Thus, such products are often sold in combina-tion with a heat pump that is used to raise the temperatureof the fluid in the heat storage tank to enable heating anddomestic hot water applications at temperatures from 60 to80 C (depending on the heating system), which is a normal

0038-092X/$ - see front matter 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.solener.2013.08.042

Corresponding author. Tel.: +49 761 4588 5139.E-mail address: [email protected](K. Kramer).

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range for domestic use. This type of product has beenlabeled an uncovered PVTcollector (seeFig. 2).

To increase overall efficiency, it is important not to nar-rowly focus only on the electrical energy segment, but toincrease the solar fraction of the solar thermal (ST) energy

part of the heating system throughout the year. This canonly be achieved by generating temperatures of approxi-mately 50 C and higher, particularly during winter times.To generate this type of heat, so-calledcovered PVTcollec-tors were developed. In such a product, the thermal absor-ber is constructed out of the PV cell, thermally coupledwith a heat removal structure and mounted within a collec-tor box; it also contains a transparent front cover andbackside insulation to reduce thermal loss. With theseproducts, significant solar gains (fsol> 10%) can be gener-ated even during the winter months in Central Europe.

To further reduce heat loss, solar radiation may beconcentrated by concentrator optics (typically reflecting

mirrors) onto a comparably smaller receiving area. Thistechnology is called concentrating PVT(CPVT). One mayfurther distinguish this technology into low- and high-con-centrating PVT (LCPVT and HCPVT, respectively). Theterm low typically denotes concentration ratios from 1 to

100, whereashighconcentration typically means concentra-tion ratios in the range of 3001000 and even higher. WithCPVT systems, heat can be generated at 120 C and abovereasonably efficiently. For high concentration ratios (con-centration ratio > 10), the effective solar resource isreduced to the direct beam radiation because of the concen-trating optics. Therefore, utilizing concentrator systemswith tracking is more appropriate in regions with a highproportion of direct normal irradiance (DNI). In theseregions, the heat generated can be used in process heatapplications for industry or in polygeneration approaches,such as thermally driven chillers or water desalination(compareHelmers, 2013a).

Several producers today offer hybrid PVT products onthe market. To encourage transparency in this emergingmarket and support its development, the information typ-ically required in both relevant market sectors should beprovided to consumers.

This article first describes the market status of theseproducts at present. Second, the existing normative andquality-assuring processes of the relevant types of products(PV and ST) are analyzed with respect to PVT collectorsand existing gaps and ambiguities are identified. Third, dis-tinct proposals are made to bridge these gaps over theshort, middle and long term. In the final chapter, we gather

the results of our analysis to find a solution.

2. The market status of PVT

At present, the market share of PVT collectors on theglobal solar thermal product market is small (0.02% ofnewly installed collector areas in 2010, see Wei, 2012).However, heating systems that combine heat pump tech-nology with PVT were introduced in a growing numberof markets at various exhibitions over the past two yearsin Germany and other Central European countries. PVTproducts will benefit by the development of the PV branch

and existing market distribution systems of the ST branch.

Nomenclature

Aa aperture area of the PVT collector [m2]

a0 a6 collector coefficients [various]b0 b3 collector coefficients [various]

Geff effective irradiance [W/m

2

]Pel electrical power output [W]Pth thermal power output [W]Pout total power output, Pel+ Pth [W]

Ta ambient temperature [C]Tm mean fluid temperature [C]Tsky sky temperature [C]

t time [s] wind speed [m/s]DT reduced temperature, Tm Ta [K]

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

300 700

a

b

1100 1500 1900 2300

S

pectralIrradiance[W/m2/nm]

Wavelength [nm]

Solar Spect rum AM 1.5

a + b : Part of the spectrum

absorbed i n the PV module

b : PV conversion

a : Heat production

Fig. 1. The graph shows the spectral irradiance distribution of the solarradiation (according to AM1.5g). Part of this solar irradiance is lostthrough reflection. In a PV module, the part of the irradiance that isshown as areas a+ b can be received. The part of the irradiance that isconverted into electricity in a typical silicon solar cell is shown as area b.The majority of the radiation power, however, is absorbed and convertedinto heat, which is shown as area a. In a PVT collector, this large fractionis utilized as additional power output. (Dupeyrat et al., 2012) (Reprintedfrom Energy and Buildings, P. Dupeyrat, C. Menezo, S. Fortuin, Study ofthe thermal and electrical performances of PVT solar hot water system,

Copyright 2012, with permission from Elsevier.)

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PV products are cheaper than they ever have been and have

established widespread market penetration. Thus, innova-tive technologies are required to justify higher prices andtrigger new market sector development Because of its earlystage of market penetration and the absence of standardsand quality certification mechanisms, PVT collectors mustcontain trustworthy technical information. The absence ofstandards and proper certification mechanism leads to gov-ernmental reluctance to implement PVT products intoappropriate policy schemes. As a consequence, producersare not able to make substantial investment decisions infavor of PVT collector production. Conversely, end con-sumers and installers cannot compare different productson an informed basis. As a consequence, the informationdeficit is a strong market barrier. Thus, a common stan-dard and certification scheme must be crafted and imple-mented to increase the transparency of these products.

3. Status of relevant product standards

3.1. Solar thermal

Solar thermal collectors are currently tested accordingto EN 12975-1,2:2006-A1:2011, and both standards haverecently been mandated for revision. The technical partof the revision process will be finalized in the spring of2013, and the new standards should be published by Janu-ary 2014 at the latest. These standards define productrequirements (part 1) and establish and explain the appro-priate testing procedures that allow validation of the prod-uct requirements (part 2). The applicability of therespective standards to particular products is defined inthe respective scope. PVT collectors are not explicitlyexcluded from EN 12975-1,2:2006-A1:2011. Thus, from aformal point of view the existing collector test standardalready applies to PVT collectors. From a purely adminis-trative point of view, it appears to be possible to certify aPVT collector with a Solar Keymark (compare Nielsen,

2012) at first glance. However, the CCB (CEN Certification

Board) prevented this possibility in March 2012 (compare

Druck, 2012). Since that date, it has not been possible tolicense Solar Keymark labels to PVT collectors.1 Recentlythe Solar Keymark Network could establish a workinggroup to define minimum requirements for PVT collectorcertification on the basis of PV safety test to bridge thegap for the time being. Due to this work, certification ofPVT collectors according to Solar Keymark scheme rulesis now possible again. This quality label is the most estab-lished guarantee for solar thermal products in Europe andis therefore decisive in being able to sell a solar thermalproduct. For deeper understanding, it should be recalledthat EN 12975-2:2006-A1:2011 covers two topics, i.e.,information on both efficiency and functionality of arespective product.

3.2. Photovoltaic

Non-concentrating terrestrial photovoltaic modules aretested world-wide under IEC 61215:2005 (crystalline PV)or IEC 61646:2008 (thin film PV). The only exceptionsare the USA and Canada with their own additional regula-tions. These tests are performed for design homologationand type certification. Moreover, the relevant safety testsare mandatory according to IEC 61730-1, 2:2004. Specify-ing the electrical power output is detailed in IEC 60904.IEC 61215 and IEC 61730 are currently under revision.The expected date of availability (DAV) is 2013/2014.IEC 61215 and IEC 61646 focus on the electrical and basicfunctionality of the modules and ensure that the electricalperformance is kept stable under typical operating condi-tions. IEC 61730-1 defines requirements for the use of thePV module and also specifies safety features. Part 2 ofIEC 61730 finally gives explanations about how to performthese safety tests in a laboratory. If one would look on a

Fig. 2. From left to right: non-covered PVT air-heating collector (Reprinted with permission from Grammer Solar) covered flat-plate water-heating PVTcollector (Dupeyrat et al., 2012) (Reprinted from Energy and Buildings, P. Dupeyrat, C. Menezo, S. Fortuin, Study of the thermal and electricalperformances of PVT solar hot water system, Copyright 2012, with permission from Elsevier.); low-concentrating CPVT collector ( C= 37;Coventry,2005) (Reprinted from Solar Energy, 78(2), J.S. Coventry, Performance of a concentrating photovoltaic/thermal solar collector, pp. 211222, Copyright2005, with permission from Elsevier.); high-concentrating PVT collector (C= 522;Helmers et al., 2013b).

1 It should be noted that the situation was not defined in detail beforeMarch 2012. Certain products had somehow previously obtained the Solar

Keymark label; for these products, the status quo was fixed.

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PVT collector as a pure PV-module, it is of course coveredwithin the scope of the standards discussed above. There-fore, it would be formally acceptable to test a PVT accord-ing to the above-discussed IEC standards. Consequently, itis possible to certify a PVT collector according to IEC61215/61646 with safety class 2, which defines the relevant

certification for the market.In particular, tests on functionality are not completelyadequate or even applicable for PVT collectors, as shownbyDupeyrat et al. (2011). This is true because PVT collec-tors, particularly covered ones, are built with an openframe, as opposed to standard PV modules. Therefore,the functionality test on the dielectric strength from PVtesting would destroy the PVT product completely or havean enormous influence on the thermal performance. Con-versely, in the case of a covered PVT collector, the mechan-ical load test of IEC is not necessary because the coveringbox interacts with the ambient conditions. These examplesshow the need to adapt and change the current testing

instructions and methods related to PVT collectors indetail.

4. Certification and CE-marking

Similar to testing for standardizing product-qualitylabels, product certification for PVT collectors must be dis-cussed and must require appropriate scientific informationdisclosure. To sell a PV module on the European market,the product must be CE marked because it is an electricaldevice (compareSchonau and Na, 2012). The declarationof conformity must be in compliance with the EU Low

Voltage Directive, on the basis of the harmonized EN61730-1, 2 standard that is defined in the formal publica-tion of the EU parliament (seeAmtsblatt der EuropaischenUnion, 2009). If the product fulfills the requirements of thestandard, the manufacturer is allowed to affix the CE markon it and offer the product to the EU market. In addition,there are voluntary quality labels available for the PV mar-ket (compare Schonau and Na, 2012). At present, themost relevant label appears to be from the IECEE (World-wide System for Conformity Testing and Certification ofElectro Technical Equipment and Components) (comparealsoIECEE, 2012).

From the solar thermal perspective, a CE labeling of theproduct is only mandatory if the collector (a) is to be inte-grated in the facade/roof of a building, (b) contains a largevolume of fluid under pressure or (c) consists otherwise ofparts addressed in any EU directive. In the case of a fac-ade/roof integration, the EU directive on constructionproducts applies and the product must be double-checkedfor conformity with the directives requirements. The har-monization process of the relevant product standard thatis necessary to base such conformity declaration on itsresults is under development. In the near future, it will bestandard that the conformity can be declared on the basisof EN 12975-1, which means that the test procedures of

EN ISO 9806 will have been passed and all requirements

of EN 12975-1 will have been fulfilled, including allrequirements of the building product directive.

The most important voluntary quality label in the mar-ket for solar thermal products is the Solar Keymark label.The label is product related and granted if the product ful-fills all requirements of the Solar Keymark scheme rules,

which includes compliance with the relevant Europeanproduct standard (EN 12975).The consequence for PVT collectors at the moment is

the following:A conformity declaration according to the Low Voltage

Directive is mandatory because PVT includes PV. For theST side, even more declarations may be required, depend-ing on the context. This may be required with building inte-gration (EU Building products directive), installation of alarger fluid volume under pressure (EU Pressure directive)or when a electro-mechanical tracking device is being inte-grated (EU Machinery directive).

At present, the only voluntary quality label that can be

granted is based on IECEE. The Solar Keymark label iscurrently not applicable for PVT collectors (status,November 15, 2012). For accelerated market penetration,it is important to reduce the hurdles for market entranceby enabling the application of Solar Keymark for PVTproducts, which would generate confidence in the end con-sumer (compareTrommsdorff and Steinhoff, 2007). In thefollowing chapter, possible changes and implementationalsteps are suggested to solve the situation over the short,medium and long term.

5. Approach

To make the Solar Keymark label applicable for alltypes of PVT collectors, the following steps are suggested:

1. Development of consistent test procedures for PVT col-lectors and discussion of the results/experiences in thetesting community (e.g., Solar Keymark Network).

2. Integration of PVT collectors into the scope of an inter-national and/or European standard, i.e., includingdeveloped and/or accepted procedures with detailed testprocedures and references on existing standards withproducts where possible.

3. Including PVT collectors into the Solar Keymark sys-tem, which means adopting the power output calculator(see chapter 5.1) for PVT, integrating PVT products inthe scope of the scheme rules and requiring mandatorycertificates for safety testing.

4. Integration of all defined test procedures at accredited(according to EN ISO 17025) test laboratories withintheir scope of accreditation (which must cover all rele-vant test standards).

The basic characteristics of neutrality on technical vari-ants and significant regarding, accuracy repeatability, and

fairness must be implemented. The test procedures for

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PVT (similar to ST and PV) must be categorized into per-formance, functionality and accelerated aging tests.

5.1. Performance (power output)

The Solar Keymark label demands an annual energyoutput calculation for a solar thermal collector with theSCEnO-Calc2 tool. The calculation is based on either thesteady state or the quasi dynamic performance model ofEN 12975-2. There is no analogous standardized perfor-mance model for both electrical and thermal energy outputfor PVT collectors. To overcome this deficiency, we presenta model that may be used in a way similar to the existingpurely thermal model. A detailed derivation of the modeland an exemplary application have been presented by Hel-mers and Kramer (2013c). It should be noted that themodel applies for both non-concentrating and concentrat-ing collectors.

Similar to the quasi dynamic method for solar thermalcollectors, as described by Perers (1997), the proposedmodel comprehensively describes a PVT collector by a setof empiric coefficients. The coefficients are obtainable byperforming multi-linear regressions on the usual standardmeasurement data, i.e., the typical ambient and thermalmeasurement data plus the electrical power output Pel.Because the principle loss mechanisms to the environmentremain unchanged whether solar cells are present, the totalpower output Pout= Pth+ Pel of a PVT collector isassumed to be well described by the existing quasi-dynamicmodel (see nomenclature for the definitions of thesymbols):

Pth PelAa

aDG a1DT a2DT2 a3DTm a4DTsky

a5dTm

dt a6Gm 1

In addition, the following expression for electrical powerPelwas derived (seeHelmers and Kramer, 2013c):

Pel

Aa bDG b1GTm b2GTa b3G

2 2

The derivation of Eq. (2) is based on the assumption of

the linear dependence of the PV efficiency on temperatureand general energy balance considerations of a PVT collec-tor. Furthermore, reasonable simplifications are under-taken, as described inHelmers and Kramer (2013c).

Subtracting Eq. (2) from Eq. (1) yields the expression forthe thermal power Pth:

Pth

Aa aD bDG b1GTm b2GTa b3G

2 a1DT

a2DT2 a3DTm a4DTsky a5

dTm

dt a6Gm 3

Thus, a PVT collector can be comprehensively parame-terized by the sets of coefficientsaDto a6and bDto b3. Eq.(3) represents the interaction of the electrical power outputexpressed in the coefficientsbiwith thermal operation con-ditions and vice versa. Note that the set of coefficientsdescribing the PVT collector in maximum electrical powermode (mp) of the PV differs from the set of coefficients foroperation in, for example, electrical open circuit mode (oc);it holds ampi a

oci and b

mpi b

oci . Finally, implementing Eqs.

(2) and (3) into the existing framework of SCEnO-Calcenables annual energy output calculations for PVTcollectors.

5.2. Functionality tests and accelerated aging

To transfer and modify functionality tests from PV andST to PVT collectors, it is reasonable to distinguish short-,mid- and long-term solutions. In the following, solutionsare proposed:

5.1.1. Short-term

At present, a Solar Keymark certification of PVT collec-tors regarding functionality is not permitted. Thus, a ruleof exemptions for PVT collectors is required that allowsfor an alternative presentation of the power output resultsand requires the safety relevant tests from PV only. It must

be proven that the required tests were performed on thecomplete PVT collector at an accredited (according toEN/ISO 17025) test laboratory (for testing according toEN 12975-1, ISO 9806, IEC 61730-1, 2 and IEC 61215).Regarding the test procedure in the test lab, the proceduredescribed by Hofmann et al. (2010) should be followed.Such an exemption should be made until more elaborateprocedures are prepared.

5.1.2. Mid-term

The functionality tests should be processed into a Tech-nical Regulation (TR).3 Such a regulation must combine

the relevant functionality tests from EN 12975 and ENISO 9806, in addition to IEC 61730-1, 2 and IEC 61215.Further, guidelines must be implemented that offer moredecisive instructions on how to handle different types ofproducts. The implementation of an exemplary test reportdefault and suggestions for how to present the results in aharmonized way are also required. Nevertheless, thescheme rules will cover only the requirement for the proofof the safety test according to IEC 61730-1, 2; however, in

2 SCEnO-Calc (compare SCEnO-Calc, 2011) is an Excel-based grosscollector output calculator, which is part of the Solar Keymark scheme,rules V.19. It sums hour-by-hour yield calculations based on climate dataof a given site. It is mandatory to publish the power output calculated withthis tool together with other descriptive technical data on the product

described onwww.solarkeymark.org.

3 Technical Regulations (TRs) are not standards; they are descriptiveonly and give guidance about technical processes. After several iterationsand discussions within the relevant expert rounds, TRs often become

standards.

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addition to this, the national certifiers will request testreports according to this TR.

5.1.3. Long-term

The product test norm for collectors should integrate allthe experiences generated by running tests on PVT accord-

ing to IEC, EN and the TR and should cover PVT collec-tors in its scope. In addition, new test sequencessuch ascyclic internal shock tests, definitions of reference areasfor PVT collectors or the electric isolation between cellsand metallic parts of the hydraulicsshould be integratedin the collector test standard and the relevant test methods.Relevant accelerated aging tests performed according toIEC 61730-1, 2 and IEC 61215 should be implemented orreferred to and applied to the complete PVT collector.

The CE declaration of conformity can then be issued onthe basis of the harmonized collector test standard.

6. Summary and outlook

Currently, the market for PVT collectors lacks transpar-ency. This leads to uncertainty and confusion for all theparticipants in the market, from producers and sellers toconsumers and even the public authorities. To remove thismarket barrier of uncertainty, clear rules for this marketsegment must be developed.

Possible steps for developing short, mid and long termquality assuring processes were offered in the paper. Theoverall goal is to harmonize the situation for PVT collec-tors with the situation for current solar thermal collectors,particularly with respect to the regulation and quality sys-

tem; necessary actions to accomplish this goal wereidentified.

The implementation of a hybrid PVT collector perfor-mance model plays a central role in the integration of thecertification process. Therefore, the suitability of therecently proposed model by Helmers and Kramer (2013c)was discussed.

Finally, drafting and publishing a Technical Regulation(TR) as an intermittent step toward full normative cover-age of PVT collectors is recommended.

Intensive scientific treatment is required for comparabil-ity and accurate testing. A discussion among market partic-ipants should be sought to target the development ofregulations and standards that move toward a market rel-evant direction, which might possibly occur within the con-text of a follow-up task to Task 35 in the framework of theSolar Heating and Cooling Program of the IEA (Interna-tional Energy Agency).

Acknowledgements

This paper is part of the Ph.D. thesis of K. Kramer,counseled by Prof. G. Oesten, Prof. V. Wittwer (Universityof Freiburg, Germany), Prof. M. Rommel (Hochschule furTechnik Rapperswil, Switzerland) and Prof. E. Meidinger

(Buffalo University of New York, USA). H. Helmers grate-fully acknowledges the scholarship support from the Ger-man Federal Environmental Foundation (DBU).

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