Thermographic analysis of a building integrated photovoltaic system

Renewable Energy 26 (2002) 449–461 www.elsevier.com/locate/renene

Technical note

Thermographic analysis of a building integrated photovoltaic system Morgan D. Baziliana, *, Harry Kamalanathan b, D.K. Prasad a

a

National Solar Architecture Research Unit, Faculty of the Built Environment, UNSW, Sydney 2055, Australia b School of Mechanical and Manufacturing Engineering, UNSW, Sydney, NSW, 2052, Australia Received 4 June 2001; accepted 14 June 2001

Abstract A residential-scale building integrated photovoltaic (BiPV) cogeneration system has been thermographically investigated. The results are useful in calibrating the numerical models created to predict the system’s operational temperatures. The combined heat and power system is based on existing BiPV roofing technology with the addition of a modular heat recovery unit. The convection of the air behind the panels will serve both to cool the photovoltaic panels and provide a heat source for the residence. The analysis allows for the interpretation of the surface emissivities and operating temperatures, as well as qualitative graphic analysis of temperature gradients.  2002 Elsevier Science Ltd. All rights reserved. Keywords: IR Thermography; BiPV; Heat transfer; Photovoltaic temperature coefficients

1. Introduction A residential-scale building integrated photovoltaic (BiPV) cogeneration system has been designed and experimentally tested in Sydney, Australia. The photovoltaic (PV) cogen system is based on existing BiPV roofing technology with the addition of a modular heat recovery unit that can be used in new or renovation construction schemes. Utilizing waste heat from the BiPV array can help to increase the system’s overall efficiency (Fig. 1) and thus improve its economics. BiPV market penetration is in its infancy in Australia. Recently however, the political climate and govern* Corresponding author. Tel.: +61-2-9300-8191; fax: +61-2-9385-4507 E-mail address: [email protected] (M. D. Bazilian). 0960-1481/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 1 ) 0 0 1 4 2 - 2

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Nomenclature

⑀ k s Tsky Tpv lp F Rs Rsh c h Pm hr Voc qrad

Emissivity Boltzmann’s constant [Eq. (1)] Stefan–Boltzmann constant Sky temperature PV top surface temperature Peak wavelength Spectral irradiance Series resistance (Fig. 3) Shunt resistance (Fig. 3) Speed of light in a vacuum [Eq. (1)] Plank’s constant [Eq. (1)] Maximum power (electric) Radiative heat transfer co-efficient Open-circuit voltage, or voltage at maximum power Radiative heat loss

Fig. 1.

System processes estimated.

mental initiatives have allowed for a more positive outlook for market penetration. There is an existing BiPV roof top array (Fig. 2) installed on the northwest facing test-site roof. The PV array is acting as a roofing element as well as a power producer. A thermographic analysis of the system at the full building scale and at the component level was conducted. The quantitative results are in good agreement with experimental data of operating temperatures in various parts of the system. The qualitative results give good insight into the nature of BiPV installations in attic spaces and show an impetus for an interior cooling duct that will improve the electric output

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Fig. 2.

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Solar tile installation.

of the PV array, produce thermal energy, and act as an insulating layer for the building envelope. The graphic output of the infrared (IR) camera used to conduct the studies also gives good insights into the temperature gradients between the BiPV roofing and the metallic roofing. Likewise, the varied thermal qualities within the modules themselves can be witnessed clearly. The use of an IR camera is a fitting way to evaluate the temperature performance and parameters of an integrated solar energy system since it is a relatively quick procedure, and can be accomplished without the need for interrupting system operations. 1.1. Literature review Research on the topic of PV cogeneration and specifically heat transfer in the space behind PV arrays has been researched in some length by the EU PV-HYBRIDPAS Group. This includes the work of Bloem et al. [2], Moshfegh and Sandberg [15], Clarke et al. [5] and Wouters et al. [18] among others. Within this project, a brief thermographic study was conducted on an integrated PV roofing shingle product (Martin et al. [14]). The study observed no thermal difference between the asphalt shingle system and the BiPV shingles. Brinkworth et al. [3] has investigated a number of heat transfer equations and models for predicting this flow as has Garg and Adhikari [10]. Thermographic analysis of building envelopes is an emerging industry. It is discussed in Grinzato et al. [11], and Titman [16]. The use of IR data in calculating heat transfer coefficients is reviewed in Astarita et al. [1]. The relationship between PV cells, modules, and systems to increased heat is widely published. Specific studies with BiPV systems have been conducted by Fuentes and Roaf [9], and De Gheselle et al. [6]. Excellent accounts of thermal aspects of PV arrays in general can be found in Buecher [4] and Emery et al. [8].

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2. Light and heat The sun is often modeled as a blackbody (a perfect absorber and emitter of all radiation based on temperature) at a temperature of 6000K. The spectral irradiance of a blackbody is given by Eq. (1), which is known as Plank’s law (Honsberg and Bowden [12]). F(l) =

2phc2 l [exp(hc/klT)⫺1] 5

(1)

By differentiating the spectral irradiance and solving, by setting the derivative equal to 0 from the above equation, we are given the peak wavelength [Eq. (2)] that the irradiance is emitted at any temperature (Honsberg and Bowden [12]). lp(µm) =

2900 T

(2)

At ambient temperatures on earth this peak wavelength falls within the IR. It is invisible to the human eye and lies just above the visible spectrum of light. Photovoltaic modules can only use a small part of the incoming energy of light. This is limited by the band gap of the semi-conductor material used in the cell. The rest of the incoming light is either reflected or is lost to sensible heat. Efficiencies of PV cells today range from 6 to 18% commercially. The vast majority of BiPV systems are produced with either mono or polycrystalline silicon cells. The degradation in open-circuit voltage, efficiency, fill-factor, and maximum power output of these PV cell types due to increased temperature is well understood, and will play a role in the design of any PV cogen system. Monocrystalline and polycrystalline silicon cells have a negative temperature coefficient of ca 0.4–0.6%/°C (Wenham et al. [17]). Removing heat from the PV cells also has the benefit of reducing thermal cycles and stresses. It should be noted that amorphous silicon (a-Si) integrated roofing panels are available. In Australia, however, they are not readily available and their market penetration is almost non-existent. The majority of these roofing products are in the form of either shingle systems or mounted on metal-sheeted roofing. In both cases, it is believed that the thermal characteristics of these products are not grossly different than the roofing elements they serve to replace. The integration of PVs into buildings can affect the operating temperature and is discussed at some length in [4]. The open-circuit voltage loss is the primary factor affected by an increase in temperature as shown in Eq. (3) (Wenham et al. [17]). 1 dVoc = ⫺0.003/°C Voc dT

(3)

An equivalent cell model was created in MicroCap, which is a standard way of modeling various effects on semiconductor cells. It is a useful tool in analyzing electrical characteristics of PV cells. Here, it is useful in showing the relationship between increased temperature and lowered power output. The effects of the increase

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in the short-circuit current are minimal and are considered negligible for the purposes of this model. Also, the impacts of shading and the effects of bypass diodes will not be scrutinized as they are not essential in showing the basic relationship between heat and PV performance. MicroCap is a software tool designed for electronics engineers. It has graphical interfaces that allows the user to chose and combine the necessary components to create a wide array of circuitry. This graphic is shown in Fig. 3 and the results in power and open-circuit degradation are shown in Fig. 4.

Fig. 3.

Fig. 4.

Equivalent circuit diagram.

Resulting I–V and P–V graphs showing the effect of heat on PV cells.

3. Thermography Infrared thermography is the technology of converting IR radiation into visible light that can be then transferred to an electronic form for representation. If particles are moving than thermal energy is being created and is detectable. All objects give off thermal radiation if they are at a temperature greater than absolute zero. The image produced by the IR camera can then be used to depict and calculate the amount

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of energy being radiated by the object. By assigning a color scheme to various levels of output the image can be rendered useful for investigation. The use of IR thermography has grown considerably in the last decade. Handheld IR camera allow consultants and engineers to do on-site work in the built environment. The cameras, though expensive, are relatively robust. The IR detector is a transducer that reads the incoming radiative energy and transforms it into a voltage or a current so it can be digitally rendered (Astarita et al. [1]). Thermographic analysis is both non-destructive and non-evasive, which contribute to its benefits in investigating operating electric (PV) systems already installed as a roofing product (Titman [16]). This allows for a consultant to complete an analysis of a power system without having to interrupt its operation. In many cases the evaluative process can take place without ever having to enter a facility or structure. This is valuable in terms of time and safety. Thermography detects radiation in the long wave IR part of the spectrum in different windows, typically between either 3–5 or 8–14 µm (Grinzato et al. [11]). There are a number of variables that can affect the precision and quality of the IR images. These include: wind; the opacity of materials within the IR spectrum; the angle of incidence from the camera lens to the object; reflection; and water content of the surface or air. Wind can affect the outcome of a thermographic analysis because they cause surface temperature shear effects (Titman [16]). IR images allow for both qualitative and quantitative analysis. Qualitative analysis is sufficient to get a grasp of the thermal heat transfer that is witnessed in a BiPV array by simply comparing the color output of the image to a nearby or adjacent surface. Quantitative methods can be used to find surface temperatures and emissivities. The strength of thermography in this research lies in the graphic output rather than its ability to exactly determine surface temperatures or amounts of heat loss. Better methods for precise methods of temperature and emissivity measurement exist. Thermographic methods can serve to validate these transducers with no disturbance to the testing, however. If an area can be shown to be hotter relative to another surface, than in most cases, this should prove sufficient as a basis for evaluation. Normally, in the context of building envelope work, hot spots or leakages can be detected showing unwanted thermal bridging in an otherwise well insulated envelope. The IR interpretation of a building envelope can also be used to look at the integrity of older insulation in renovation projects and used as a post construction inspection check in both new an renovation scenarios. This is valuable at both a research level and in the case of a non-biased third-party evaluation.

4. Experimental site Sydney has a mild coastal climate at latitude of 33.76°S, longitude of 151.36 and an elevation of about 5 m above sea level. Fig. 5 shows daytime operating and ambient temperatures typical during the week of thermographic testing. The array being monitored is composed of 20 BiPV modules in plastic frames. The rest of the

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Fig. 5. Experimental test data on a typical sunny clear autumn day (irradiance, input temperature and PV cell temperature).

roof is covered in single overlapping layers of corrugated iron metallic roofing sheets colored dark green. The roof is uninsulated which allows for a good comparison between the PV modules and the standard roofing. It has a seamless acrylic, elastomeric finish on it that serves to act as an insulating layer. Each PV cell has an area of 10 cm2. Each tile has 24 monocrystalline BP cells. All cells are connected in series with one bypass diode. All of the modules are connected in series to obtain a high voltage for lower system losses. The system is rated at 740Wp DC peak in size. In a climate like Sydney’s the capacity factor will be in the range of 0.22, thus the system will produce in the neighborhood of 1.42 MWh/annum. Calibrated type T thermocouples were placed in a grid inside the ducting unit and on different areas of the PV array. The data was averaged over 15 s and logged every 5 min. Environmental data was also logged on-site and averaged every 10 s over the 5-min query period. A cooling/heating duct was designed and installed on the backside of the PV array. A high efficiency low voltage fan was used to create a flow in the duct. This heat recovery/cooling unit was insulated on the back and sides by 50 mm of rigid insulation.

5. Results IR thermographic testing was conducted during the week of 17–24th of April between 13:00 and 15:00 daily. The best images of the study came from sunny, windless days with low ambient moisture content. These times were chosen to witness high system operating temperatures. Historical experimental data showed that

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early afternoon is when peak temperatures occur due to the slightly west orientation of the BiPV array. The system was producing both thermal and electrical output throughout the examination. An IRCON model 100p was used for the testing. The IR camera is calibrated at ±2% or ±2°C, whichever is greater. It reads in the spectral band between 8 and 12 µm, and uses anti-reflection coated Germanium optics. The camera weighs ⬍2 kg, which allows for on-site use in a variety of conditions. Both qualitative and quantitative results will be presented. The most important results came in the form of: derived surface emissivities; temperature gradients across the PV modules between areas of cells and between the cells; and the temperature gradient between the metal roofing and the BiPV roofing elements. Emissivities were calculated by comparing the surface temperature on a known emissivity tape with the surface temperature on the solar array. Testing was conducted on both the interior and exterior of the building envelope. Current surface temperature data from thermocouples was available for both from the test site data acquisition system. Fig. 6 is an image taken from the inside of the attic looking at the BiPV array and the standard roofing. The coolest items in the image are the framing of the roof. Table 1 shows the calculated results. The image reveals the large interior temperature difference between the roof and the BiPV array. This was expected, but is significant in its impacts on the building envelope in both the heating and cooling seasons. The roofing is treated with the aforementioned special insulating surface, which produces a lower interior temperature. The BiPV array is up to 11°C hotter than the surrounding metal roofing. This in itself is an impetus to cool the PVs and confirms the usefulness for ventilation behind PV arrays in a graphical manner. This is unique image because the PV array is compared with an un-insulated roofing interior surface. Ideally the roof would be insulated and have a radiative foil barrier applied to the interior surface. If this more suitable system was in place then the BiPV array should

Fig. 6.

Interior view of BiPV array.

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Table 1 Derived surface temperatures on interior of array and roofing (to go with Fig. 6) Label

Average

SD

L1 L2 L3 L4 L5

46.5 53.4 55.9 53.0 56.0

0.51 0.66 0.33 0.40 0.38

be dealt with to isolate it from losing or gaining inordinate amounts of heat through the year. The emissivity of the backside white tedlar was calculated to be 0.85. This is only slightly (0.02) lower than those values found for a typical white tedlar in the literature. As mentioned earlier the other gradient of interest is that across the PV module. A distinct difference in transmitted thermal energy is witnessed from the areas between the cells and the cells themselves. This is shown in Fig. 7. This image shows the cooler spaces between the cells from the inside. The lighter part at the bottom of the image is the roof framing. The monocrystalline cells are black and are conducting their thermal energy to the inside of the attic more than the areas that are transparent. This will have a small, but important impact on the thermal model being created to predict the overall U values and possible output temperatures of a heat recovery unit. A view of the outside of the roof (Fig. 8) shows that the BiPV array is slightly cooler than the surrounding roofing material, but is not being conducted through the roofing as well because of the insulating and reflective nature of the applied covering on the metal as we have seen. A quantitative analysis of the outside of the PV modules was undertaken to evalu-

Fig. 7.

PV cell.

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Fig. 8.

Exterior view of BiPV array.

ate the emissivity of the top surface. Fig. 9 shows a close-up image of the solar tile with a piece of foil of known emissivity. The emissivity of the BiPV array top cover is found to be 0.81 (Table 2). This figure can be used in calculating the top surface radiative heat loss. This heat loss makes up a large part of the overall heat loss of the system. On day with little wind the radiative component is often higher than the convective heat loss. The loss is found by using Eq. (4) (Krieth and Kreider [13]) qrad = ePVs(Tpv⫺T4sky)

(4)

The relative impacts of the radiative and convective heat loss from the PV array to

Fig. 9.

Emissivity and temperature testing of exterior of PV.

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Table 2 Emissivity and temperature from Fig. 9 Label

Emissivity

Average

SD

A1 A2 L1 L2 L3

0.81 0.81 0.93 0.93 0.81

52.1 52.6 55.7 55.5 55.6

0.31 0.47 0.20 0.27 0.27

Fig. 10. Convective versus radiative heat transfer losses from the PV array.

the ambient are shown in Fig. 10. (This graph would have the convective losses dominant on a windy day.) A small part of the interior cooling of the array being accomplished through forced convection is captured by the thermographic analysis (Fig. 11). The lighter shades

Fig. 11.

View of convective cooling.

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at the lower extremities of the BiPV modules reveals the convective cooling taking place on the inside. This is another case where the thermographic technique shows its non-evasive strength. The fan can be operating while the analysis is taking place, in this way all of the system components can be evaluated under normal working conditions. The internal heat transfer coefficient can be found using the derived emissivity value for the backside of the PV modules using Eq. (5) (Duffie and Beckman [7]). hr =

sT¯ 3 (1/e1) + (1/e2)⫺1

(5)

(The emissivity of the back of the heat recovery unit is of a known emissivity.) These heat transfer losses can then be used in a numerical model to predict system surface temperatures and output efficiencies.

6. Conclusion An IR thermographic analysis was conducted on a BiPV roofing element to graphically and numerical investigate the heat transfer properties of the system. The system is being used to test the feasibility of a heat recovery unit that will utilize the waste heat from the back of the array while cooling the PV cells. The interior of the BiPV array was found to be distinctly hotter than the surrounding roofing on a clear sunny day. This will have impacts both for the building envelope and the PV cell thermal performance. It will also provide a graphical way to view the nature of BiPV systems that use typical PV module construction or architectural glass–glass construction. The thermographic images help to calibrate the numerical models being written for the cogeneration system by deriving surface emissivities. Data from on-site temperature transducers were also validated. An impetus for removal of heat off of the back of BiPV modules was shown. Thermographic investigation is a useful tool for investigating various aspects of components while systems under normal operation.

Acknowledgements The principal author would like to acknowledge the support of the Faculty of the Built Environment at the UNSW in Sydney and an ACRE postgraduate research scholarship. The work in this paper was supported by the Australian Cooperative Research Centre for Renewable Energy (ACRE) among others. ACREs activities are funded by the Australian Commonwealth’s Cooperative Research Centre’s Program. Professor Graham Morrison from the School of Mechanical Engineering at UNSW was also indispensable in this work.

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