Category Archives: EuroSun2008-3

The experiment

The PV modules have been replaced by a wooden target that has identical dimensions. The target was painted white for better contrast in the pictures. On that target a set of parallel lines in different colours are drawn accurately with a 2 cm difference between each of them. These lines are used as reference edges in the target from which the irregularities of the reflected pattern (Figure 2) can be observed. A set of pictures were taken with a digital camera placed at a distance perpendicular to the concentrator’s axis which is oriented towards it. A movable platform was used to insure the correct location of the camera along the normal to the concentrator. The principle of the two-point perspective is used to detect, with the aid of numerical software, the edges of the target.

A geometrical algorithm is then used to calculate the normal vector to reference line. An error map

of the normal vectors for each mirror is further constructed and the RMSE calculated. The above procedures were made for a vertical and tilted concentrator positions.

image059

Fig. 2. Reflection of the reference coloured lines of the wooden target 4. The geometric algorithm

Figure 3 shows a camera placed at a distance S normal to the plane of the concentrator. The target (absorber) is located at the focal plane at a distance f. A mirror X meters from the centre of the concentrator and inclined an angle ad degrees, shows an image i of the edge or the reference lines of the target. The normal vector n of any point on the mirror’s surface is theoretically inclined with respect to the concentrator plane with an angle nth-

nth = 90 — ad (1)

From simple geometrical relations it can be proved that the values of angles a and a are related to real tilt angle qexp which is computed as per equation (4). Irregularities in the mirror’s surface affect the tilt angle of the normal vector n. Errors are computed from the discrepancies between the theoretical and experimental values of q.

a’ = arctan(Xm)

(2)

. X + Ax — A d. a = arctan( ) f — Ay

(3)

nexp = 90 — (a+2a)

(4)

X

 

Дх

 

ik

 

S

 

Fig. 3. The Absorber Reflection Method algorithm

 

image060

The algorithm requires the knowledge of the image coordinates of the reference line i or more precisely the length Xm in space dimensions. The image coordinates of points along the reference lines are first located in pixels and then transformed into actual distances as shown in the following section. The straight reference lines will appear deformed if the surface underneath is not totally flat.

An Experimental Study of Air Flow and Heat Transfer in an inclined Rectangular Channel with Wood Strips on the Bottom Plate

D. C. Diarra1*, L. Candanedo2, S. J.Harrison1, and A. Athienitis2

department of Mechanical and Materials Engineering, Queen’s University, K7L 3N6, Kingston, Canada Telephone: 1 6135332591, Fax: 1 613 5336489, email: diarra@me. queensu. ca 2 Department of Building, Civil and Environmental Engineering, 1455 de Maisonneuve Blvd. West, Montreal, QC, H3G 1M8, Concordia University, Montreal, Quebec *Corresponding Author: diarra@queensu. ca

Abstract

Designs and configurations of building-integrated photovoltaic thermal (BIPV/T) air systems are based on the type of PV modules, the location, and the geometry of the framing on which the modules are to be mounted. Moreover, the unpredictable behavior of airflow in Building Integrated Photovoltaic ducts under natural convection requires high accuracy velocity measurement techniques to successfully predict the airflow rate and its pattern. Using an asymmetrically heated channel from the top to represent a BIPV configuration under field conditions, the air flow pattern and the temperature distribution of the system components were investigated. A Particle Image Velocity (PIV) system was used to study the air flow pattern in the system.

The results obtained gave a better understanding of the air flow pattern and the heat transfer mechanisms in practical roof-integrated BIPV and BIPVT system configurations and the formulation of some design guidelines for building integrated Photovoltaic systems in natural convection.

Keywords: heat transfer, wood strips.

1. Introduction

The introduction of BIPV market, the diversification of the designs and the various efforts to improve the systems, led to practical application of air-cooled PV/T and BIPVT systems across the world [1]. Consequently, building integrated photovoltaic systems are increasingly popular. There are many previous studies of natural and forced convection over vertical and horizontal plates, and through inclined channel formed by smooth parallel plates [2, 3] outlined some design procedures for smooth PVT ducts under natural convection, while [4], proposed a correlation for the calculation of local and average Nusselt number for asymmetrically heated channels with inclination angles ranging between 18° and 30°. Liao et al. [5] also presented a computational fluid dynamics (CFD) study of heat transfer for a BIPV/T facade. Brinkworth and Sandberg [6], reported the effects of ribs on the buoyant flow induced in a duct. It is found that the additional hydraulic resistance due to the presence of the ribs does not affect the flow-rate greatly, since the flow varies roughly as the cube root of the total resistance and secondly, an additional resistance will have a noticeable effect only if it is greater than the fixed values already present, arising from the flow losses at the inlet and along the duct walls.

Usually natural ventilation by air has low flow rates especially in residential areas. Heat and energy transfer processes in a practical BIPVT system (under field conditions) is a complex scenario. Currently limited information and data are available on BIPV/T energy systems analysis taking into account the real configurations and framing of the air duct. The current experimental test apparatus was designed to represent a practical model, configuration, and setup of BIPVT systems operating under field conditions. The thermal components of the PV module were simulated by an aluminium plate heated by uniform heat fluxes, and a particle image velocity system was used to study the air flow pattern in the channel.

2. Experimental Model

2.1 Components of the Channel

image122
image123

Figure 2 shows a schematic of the experimental PVT air duct system.

The plate was 2.4 m long and 0.34 m wide, and made of 0.002m thick aluminium sheet with an emissivity of 0.27. The bottom plate was also 2.4 m long, 0.34 m wide and made of Plexiglas with 0.006 m of thickness. Both side plates were also in plexiglass with 0.003 m thickness, and 0.14m height. The surface emissivity of the plexiglass was 0.97. The space (H) between the two plates could be varied by moving the bottom plate up or down. To ensure a two-dimensional flow, the channel height used in the experiment could be adjusted in the range of 0.015 m to 0.050 m. The channel was mounted on a steel frame with an adjustable tilt angle of 20° to 44°.

2.2 Measurements

The temperature distribution along the top surface was measured with copper/constantan thermocouples starting at 0.15 m and spaced at 0.3 m intervals. Three rows of eight thermocouples each were placed along the centre left-hand and right-hand sides of the top plate, giving an average temperature (Tt ) representing the top plate temperature. On the bottom plate of the channel, eight thermocouples were placed along the centre line opposite those as the top wall to give the average temperature (Tb ) of the

bottom plate. Temperatures of the insulation TftM(top plate insulation), Tbins (bottom plate insulation),

T (side insulation) were also measured at different points along the channel. Two Vaisala humidicap

sensors (Vaisala HMT 333) were used to measure the relative humidity and temperature of the air at both the inlet and outlet of the duct. Figure 3 illustrates the temperature sensors’ location for the air between and under the wood strips along the channel.

0.34m

Figure 3: Wood strips and thermocouple location
inside the duct

Experience: the EURAC study case

In Bolzano, the capital of the most northern Italian Province, three buildings are equipped with solar collectors assisted by one Combined Heat and Power generator. For one of them, which is the seat of EURAC, a large amount of information could be collected thanks to a monitoring system which has worked since 2005.

image069

The main features of the EURAC energy facility are reported in Table 1. Fig. 1. and Fig. 2 show the plant layout respectively for winter and summer operation mode. A more detailed description of the system can be found in [3].

Table 1.Main features of the SHC-CHP installation in EURAC, Bolzano

Heat Production Facility

Solar Collectors — Gross Area

615 m2

1 Cogeneration Unit

180 kWe/ 330 kWth

2 Condensing Boilers

350 KWth each

Cold Production Facility

1Absorption chiller

300 kWc

2 Compression chillers

315 kWc each

Storage tanks

2 Solar tanks

5,000 l each

1 Cold tank

5,000 l

Fig. 1.Layout of the SHC-CHP installation at EURAC, Bolzano : winter operation mode.

One critical aspect within this plant is the presence of a hydraulic junction where all the hot and cold streams are mixed, in particular the ones of the cogenerator and the solar loop which often have different temperatures, especially in winter. Besides increasing entropy generation, mixing flows at different temperatures can decrease both collectors’ and cogenerator’s efficiency, in winter and in summer as well. In fact, in winter solar fraction usually has a temperature lower than the one of
cogenerated heat. Hence, solar fraction can be stored and used for SDHW supply or mixed. If it is mixed, collectors’ efficiency can be negatively affected by a too high mean temperature in the hydraulic junction which is due, on one hand to the high temperature provided by the cogeneration, on the other hand to the high temperature returning from the distribution system (high temperature radiators are included). On the contrary, in summer, high temperatures are delivered by the solar loop, increasing the main temperature in the hydraulic junction. In this case, when the absorption chiller works at partial loads (i. e. the “V Abs” in Fig. 2. reduces the mass flow entering the generator of the absorption chiller, thus the mass flow between the hydraulic junction and the valve is recirculated), the engine cooling stream temperature risks to be too high. The cogenerator is put in alarm and it switches on/off continuously, first because it is controlled by the electricity demand, secondly because it has no heat storage. Furthermore, the nominal hot mass flow entering the generator of the absorption chiller is higher than the sum of the nominal flows of the cogenerator and the solar loop, thus the boilers have to be used if the absorption chiller has to be run at nominal conditions. Whenever not all of them work and the cooling peak load is reached at the same time, colder flows from the bottom of the hydraulic junction enter the absorption machine and the inlet temperature is decreased.

image070

Fig. 2. Layout of the SHC-CHP installation at EURAC, Bolzano: summer operation mode.

Solar8 vs. traditional PV module based on cells area

One of the most common arguments in favour of PVT concentrating systems is the higher electrical efficiency per cells area when compared with a regular PV module with the same cells area. In this situation and based only on the PV cells point of view, Solar8 has a considerable higher efficiency per cell area when compared with the PV module (Table 9). This result can be explained by the higher irradiation the cells receive due to the reflector concentration factor and the tracking system. The thermal output can be seen just as an additional output one can get by cooling down the cells.

Table 9. Solar8 and traditional PV module electric output comparison based on cells area. PV module inclination is 40° in Stockholm, 30° in Lisbon and 20° in Lusaka. Aceels=0.33m2.

Electric annual output per cells area (kWh/m2)

Stockholm (lat=59.2°N)

Lisbon (lat=38.7°N)

Lusaka (lat= 15.4°S)

Solar8 tracking N-S (50°C)

661.8

1204.7

1467.6

Traditional static PV module (25°C)

192.4

309.7

360.6

Ratio Solar8/PV module

3.4

3.9

4.1

For this simulation it was considered that the PV module has16% efficiency at 25°C, the same cells area as Solar8 and that they cover 90% of its glazed area.

It is important to notice that the thermal and electric outputs shown previously don’t take into account system distribution losses, array shading effects and load distribution.

4. Conclusions

With this study several conclusions can be taken not only for Solar8 but also perhaps to the general photovoltaic/thermal concentrating hybrids being developed:

1. Solar8 can be replaced by a traditional side-by-side system using less space and producing the same electric and thermal output.

2. Local diodes installed in each cell can be able to bypass the current over the poorest cells and help reducing the problem with uneven radiation.

3. One axis tracking around North-South direction is considerably better than tracking around an axis placed on East-West direction.

4. The global irradiation on a static surface is higher when compared with the beam irradiation towards a tracking concentrating surface.

5. The ratio between electric and thermal output decreases when Solar8 is moved to the equator where the beam irradiation values are higher.

6. This PV/T combination still present lower outputs when compared with the traditional side-by­side system for the same glazed area. It is possible to say that there is chain efficiency around the most important components in Solar8. If every part of this chain works accurately and perfectly integrated in the system, higher efficiencies can be achieved in future models.

3. References

[1] Measurement report: Test of PVT module “PVtwin”. IEA task 35. Danish Technological Institute.

[2] Duffie, J. A., & Beckman, W. A. (1980). Solar Engineering of Thermal Process. Wiley Interscience, New York.

4. Acknowledgement

This study was supported by SolNet — Advanced Solar Heating and Cooling for Buildings — the first coordinated international PhD education program on Solar Thermal Engineering.

Modelling the Energy Contributions of a PVT System to a Low Energy

House in Sydney

S. Bambrook* and A. Sproul

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052.

NSW, Australia

* Corresponding Author, s. bambrook@student. unsw. edu. au

Abstract

A hybrid photovoltaic/thermal (PVT) air system integrated into a low energy residential house in Sydney is modelled. The thermal and electrical energy contribution of the PVT system to the house is examined to investigate the suitability of these systems in appropriately designed houses to eliminate the need for space heating systems, reducing household energy consumption and greenhouse gas emissions, and decreasing the peak load on the electricity grid. Simple heating degree day calculations showed that the PVT heating energy was slightly in excess of the monthly winter heating demand of the house and more detailed simulation results showed that PVT systems can provide an acceptable indoor temperature in winter in well insulated Sydney houses.

Keywords: Photovoltaic/thermal, PVT, modelling, building simulation.

1. Introduction

Sydney has a temperate climate with warm, humid summers and mild winters. Surprisingly,

Sydney is predominately a heating climate with significantly more heating degree days than cooling degree days. In Australia, 42% of household energy consumption is attributed to space heating and 2% is attributed to space cooling [1]. While only a small proportion of the space heating uses electricity as the fuel source, there are still significant greenhouse gas emissions resulting from the use of other fuels such as natural gas and liquid petroleum gas. A corresponding increase in the peak electricity demand requires significant expenditure on upgrading the electricity distribution network to cope with this peak demand.

To address these problems a holistic approach to energy efficient residential house design and implementation of a PVT air system is proposed. A well insulated house with passive solar design would have a much lower heating and cooling demand than the average Australian house. The silicon photovoltaic (PV) system is mechanically ventilated to lower the cell operating temperature in order to improve the efficiency and provide a higher power output. The waste heat energy can be used to heat the house in winter. This work aims to investigate the practical feasibility of a PVT system for meeting the winter heating requirement of the house.

The majority of research work to date on PVT air systems has focused on analysing the PVT system itself and examining performance improvements. Charalambous et al [2] provided a comprehensive review of various PVT collector types with particular emphasis on the parameters affecting PVT performance.

This work has a focus on the overall system, examining the effect of integrating a PVT system into a building. The Swiss AERNI factory building features one of the early PVT air system

installations. In this system the photovoltaic modules are ventilated and thermal energy is used directly in the building in winter and stored in earth storage during summer [3]. Lloret et al [4] conducted a case study of a PVT system integrated into the Mataro Library in Spain. The hybrid PVT air modules forming the facade wall and roof mounted array provide heated air which is used as an input to the gas heating system in winter and, in summer, the warm air is ventilated to the outside of the building. Cartmell et al [5] reported on the simulation and initial monitoring results from a combined ventilated PV and solar thermal air system installed at the Brockshill Environment Centre in the United Kingdom. Simulations used ESP-r to generate the hourly load requirements for the various zones within the building and this output was then used in the TRNSYS simulation to determine the contribution of the ventilated PV and solar air systems towards meeting the heating demand of the building. TRNSYS simulations were used by Bakker et al [6] to show that a PVT water system coupled with a ground source heat pump could meet the heating and most of the electricity demand of a single family house in the Netherlands.

This work represents the first steps in analysing the energy contribution of a PVT system integrated into a Sydney house in order to eliminate the need for space heating. The analysis focused on using building energy simulation programs to determine the energy requirement of the house and the expected thermal and electrical output of the PVT system. For initial calculation and comparison the heating degree day (HDD) method was also used.

Distribution of temperatures

Fig. 5 shows the averaged temperatures for the channel walls and the bulk fluid:

image091

Z (m)

Fig. 5. Average temperature for different walls and bilk fluid, a = 2.43 and Re = 250.Example caption for

figures.

A linear increase in bulk mean temperature along the tube length can be appreciated. This is a natural result of energy balance under uniform heat flux. The temperature difference between the channel walls and the fluid attains its minimum at the channel entrance region, and gradually reaches a constant value. This is in agreement with the variation of Nusselt number as will be discussed in a following section. In figure 5, the variation of the average T of each wall and the mean bulk T, obtained with the CFD simulations is shown. It can be noticed that bulk T has a linear variation and Twalls an exponential as it was expected.

3.3. Nusselt number

The average Nu number for each wall is obtained through the local Nu as:

1

Nuz, ave = — j NUzdl (Eq. 5)

Lc 0

where Lc is the width of each wall.

image092 Подпись: (WNUave,bottom + WNUave,top + 2HNUave,side ) 2(W + H) Подпись: (Eq. 6)

The overall average Nu for each aspect ratio is obtained as the proportional addition of the average Nu of each wall [1].

In the figure 6, the variation of the Nu with the Re, for each aspect ratio is shown

Подпись: 0 250 500 750 1000 1250 1500 1750 2000 ReDh Fig.6. Average Nusselt numbers. 14

13.5 13

12.5 12

11.5 11

10.5 10

9.5 9

8.5 8

7.5 7

6.5 6

5.5 5

image096 image097

The thermal behaviour of the system can be understood as a function of the aspect ratio (a) and the Reynolds number. However, in order to obtain replicable results for tubes of any length, diameter, etc. A dimensionless parameter is defined as the equivalent distance of the fully developed flow from the entrance of the tube (L+, dimensionless length). This parameter has a similar definition to X+ [6], but is dependent of the total length of the tube. This parameter is known as the Graetz variable, and is defined as:

In the figure 7 the variation of Nu with the L+ is shown. Making a minimum quadratic residual analysis a correlation can be obtained. Its mathematical expression is:

Подпись:NuDh = 5.811( Г)-°’237

3.4. Thermal resistance

image099 Подпись: (Eq. 9)

The operation of the heat sink is usually evaluated by the value of the thermal resistance, defined as:

Подпись: Fig. 8. Comparision of thermal resistance: numerical vs experimental (Aspect ratio = 2.43).

Where Tw out is the fluid outlet temperature and Tin the fluid inlet temperature.

In literature we can find some correlations or values of the thermal resistance for linear concentrating systems:

Table 3. Inverse heat transfer coefficients for a=2.43.

Re

Mass flow (kg/m2s)

1/hc (m2K/W)

125

0.16

1.71×10-3

250

0.31

9.44×10-4

500

0.62

5.14×10-4

750

0.91

3.81×10-4

1000

1.22

3.01×10-4

1500

1.81

2.16×10-4

2000

2.41

1.69×10-4

Подпись: Table 2. Thermal resistances Authors 1/hc (m2K/W) Chenlo y Cid 8.7x10-4 (Re = 5000) Coventry. 1.3x10-3 (Mass flow = 0.348 kg/m2s)

To compare with these values we should calculate the thermal resistance per unit area, obtaining the table 3.

The values of thermal resistance to the proposed sink are lower compared to those presented by Chenlo and Cid and Coventry. It should be mentioned that the values that can be compared to a greater degree

[5] , were acquired with the prototype working under real conditions. The values shown of the sink

design are made at the laboratory, so when comparing it is necessary to be critical quantifying differences.

5. Conclusion

The heat exchange properties of the aluminium improve increasing the aspect ratio of its cross section, in addition the pressure drop or in consequence the pumping power is higher when the hydraulic diameter (which is directly related with the cross section) is lower.

Nevertheless, a big aspect ratio implies a much more difficult mechanical procedures, suck as hydraulic connections, isolation. Moreover, is necessary to mention that the main aluminium factories don’t manufacture pipes of one centimetre width with aspect ratios higher than 2.43.

Attending to this explanations, the pipe selected to be include in the PVT systems under concentration is with a cross section of 20x10cm2 (a = 2.43).

Acknowledgments

This work was supported by the MCYT (Spain) (ENE2007-65410).

References

[1] P. Lee, S. V. Garimella, D. Liu. Investigation of heat transfer in rectangular microchannels. International Journal of Heat and Mass Transfer 48 (2005) 1688-1704.

[2] J. I. Rosell, X. Vallverdu, M. A. Lechon, M. Ibanez. Design and simulation of a low concentrating photovoltaic/thermal system. Energy Conversion and Management 46 (2005), 3034-3046.

[3] F. Chenlo, M. Cid, A linear concentrator photovoltaic module: analysis of non-uniform illumination and temperature effects on efficiency, Sol. Cells 20 (1987) 27-39.

[4] L. W. Florschuetz, C. R. Truman, D. E. Metzger, Streamwise flow and heat transfer distributions for jet array impingement with crossflow, J. Heat Transfer 103 (1981) 337-342.

[5] J. S. Coventry, Performance of the CHAPS collectors, Conference record, Destination Renewables — ANZSES 2003, Melbourne, Australia, 2003, pp. 144-153.

[6] F. P. Incropera, D. P. DeWitt, Fundamentals of Heat andMass Transfer, fourth ed, Wiley, New York, 1996.

[7] R. F. Russell, Uniform temperature heat pipe and method of using the same, Patent US4320246, 1982, USA.

[8] M. W. Edenburn, Active and passive cooling for concentrating photovoltaic arrays, Conference record, 14th IEEE PVSC, 1980, pp. 776-776.

[9] M. J. O’Leary, L. D. Clements, Thermal-electric performance analysis for actively cooled, concentrating photovoltaic systems, Sol. Energy 25 (1980) 401-406.

Experimental setup

Silicon is deposited by CVD on a layer of silicon powder. The deposited layer is easily separated from the quartz substrate owing to the presence of the powder layer and constitutes a self supported pre­ribbon. By ZMR processing this pre-ribbon is converted into a multicrystalline ribbon that can be used as substrate for solar cells.

 

6

 

3

 

5

 

4

 

1

 

2

 

Fig. 2. Experimental setup for the CVD deposition step.

 

image167image168

The CVD deposition step was tested so far in a specially designed batch mode furnace (see Fig.2). The substrate (1) is a single plate of quartz covered by a layer of silicon powder (2). It remains stationary in the central stage below the quartz window (6). The powder layer is heated by the radiation from halogen lamps (7); silane gas flows (4) through the furnace and thermally decomposes depositing silicon on top of the powder layer. Conditions in the furnace are set to ensure an approximately uniform growth of the deposited section of silicon (3) ribbon on top of the flattened powder layer (2).

2. Results

Several runs were performed to optimize CVD deposition conditions. It was observed that film porosities below ~50% allowed reproducible ZMR [3].

The SDS pre-ribbons produced during the CVD step are intrinsic. To be used as base material for solar cells they must be doped. We used a spray doping technique [4] to do the p-doping and ZMR in one step.

The recrystallized ribbons were 300 pm thick, with a crystalline structure consisting of grains of a few centimetres long and a few millimetres wide, p-doped, with resistivities of 1.5 Qcm.

image169

Fig.3. Top view of a SDS ribbon after the final ZMR step. The ribbon is 3 cm wide and the thickness is 300 pm.

We prepared very simple (no passivation nor anti reflection coatings) solar cells on this new material to assess the overall potentiality of the whole process.

Подпись: Fig. 4 - I-V and Spectral Response results for a control cell (bold line) and for the best SDS cell in this study.
image171

The lower values of the short-circuit current in SDS samples when compared to the control samples are in agreement with the lower quantum efficiency values in the infrared region of the spectrum in the Spectral Response measurements.

Nevertheless, the results on these first SDS solar cells, namely the obtained value for the diffusion length in the best SDS (70 pm) clearly demonstrate the ability to achieve high efficiencies.

4. Conclusion

The SDS-Silicon Dust Sheet method for the formation of silicon ribbons based on a gaseous feedstock was described, and the feasibility demonstrated by the preparation of full working photovoltaic cells on a SDS ribbon silicon. The advantages of the SDS process are (i) no substrate is required, thus reducing sources of impurity and cost; (ii) low energy budget because CVD is performed under atmospheric pressures and at low temperatures; (iii) high quality, free standing, crystalline silicon sheets are made possible by the non-contact float-zone crystallisation with in-situ doping.

5. References

[1] Wald F., Crystals: Growth, properties and applications 5, Springer: New York, 1981, 147-198

[2] Ciszek T., Silicon Processing for Photovoltaics I, North-Holland, 1987; 131-166

[3] Pinto C. R., Serra J. M., Brito M. C., Gamboa R., Maia Alves J., Vallera A. M., Proceedings of 21stEuropean Photovoltaic Solar Energy Conference and Exhibition, Dresden, 2006, 1099-1101

[4] Silva J. A., Brito M. C., Costa I., Maia Alves J., Serra J. M. and Vallera A. M., Solar Energy Materials and Solar Cells 2007; doi:10.1016/j. solmat.2007.08 .002

Method to find Xm

The parallel lines in 3D space appear in the image to meet at a point when their plane is not parallel to the image plane (Figure 4). This point of convergence is called the vanishing point and can be used to determine distances in the real world [2]. The vanishing point of lines is sufficient to analyze and detect the edges without the need for completely calibrating the camera used. In the case of the BiFres system, the structure frame of the concentrator is a rectangle with four metal sides. The four corners are easy features to locate from the pictures and their coordinates are used to form the equations of lines. The equations of lines are solved simultaneously to locate their intersection; the vanishing point. With the knowledge of the actual dimensions of the concentrator and their corresponding image pixel counts, a scaling factor is obtained which converts the image pixels to space dimensions. Using triangle similarities, the coordinates of any image point on the reference lines are then directly related to the actual distance Xm. This information is further fed into the geometrical algorithm for the calculation of the real surface slope at the specified point on the mirror. It is important to mention that the above method is an approximation because the mirrors do not lie in the same plane of the concentrator frame; each mirror has a different tilt angle.

However, the effect of this tilt is less apparent because the camera is located at a large distance from the concentrator between 18 to 30 meters away.

image061

6. Results and analysis

The following graphs show the error map of the surface of each mirror as well as the root mean square error for the whole mirror with the concentrator at vertical and inclined positions. Presented here are only the 15 mirrors of the left part of the concentrator.

image062

Fig. 5. Surface error map of mirrors 1-15 (from top) at inclined (left) and vertical (right) positions.

Colour coded scales are in mrad

image063

The two graphs show that the mirrors at the inclined position posses surface errors within the range of 3 mrad compared to the vertical position that shows errors up to 4.5 mrad. In the vertical position, RMS errors are between 0.54 and 2.63 mrad while in the inclined position they are between 0.23 and 1.46 mrad. These values are within the range of RMSE and comply with surface errors quoted for other concentrator systems [1, 4, 10]. The results contradict the expectations that at inclined positions the RMSE should be higher than that at the vertical position due to the mirrors bending under their own weight. The reason for this is that during tracking, the motor exerts a lot of power to retain the concentrator to its horizontal position. The motor moves slower than usual and this affects the tracking precision. Therefore a heavy counter balance is attached to the concentrator frame to help the motor during its movement. The balance is made of heavy iron bars connected to centre body of the concentrator and balanced with cable wires to the corners of the structure. In the vertical position, the bars are perpendicular to the frame and parallel to the ground. At this position the cable wires are at their highest tension and exert stresses on the structure and consequently the mirrors holders. On the other hand, when the concentrator is at an angle, the bars exert less torque on the structure and the overall effect on the mirrors is less pronounced.

7. Conclusion

The absorber reflection method has been used to assess the surface slope errors of a linear Fresnel PV/T concentrator. The absorber reflection method has proved to be an effective and simple tool to obtain slope error map of reflecting concentrating systems. Results show that mechanical stresses on the structure, where mirrors are attached, play an important role in the errors. Structure manufacturing should be precise because mirror misalignments can cause the reflected rays to be blocked by the preceding mirror thus increasing the non-uniform illumination and shading effects on the modules. The counter balance should be redesigned to avoid its negative implications. The experiment has shown that the accuracy in locating the camera is a crucial point for the reliability and accuracy of the measurements.

8. Acknowledgments

This work has been supported by the 6th European Union Research Programme’s Marie-Curie early

stage research training network “Advanced solar heating and cooling for buildings — SOLNET”.

References

[1] J. Coventry (2004), A solar concentrating photovoltaic / thermal collector. PhD thesis. Australian National University

[2] H. Doehler, B. Korn, Robust position estimation using images from an uncalibrated camera. Digital

Avionics Systems Conference, 2003. DASC ’03. The 22nd Volume 2, 12-16 Oct. 2003. Pages:9.D.2-9.1-7

[3] E. T Franklin, J. S. Coventry, Effects of highly non-uniform illumination distribution on electrical performance of solar cells. Proceedings of Solar 2002 — Australian and New Zealand Solar Energy Society

[4] S. A. Jones, J. K. Gruetzner, R. M. Houser, R. M. Edgar, T. J. Wendelin, VSHOT: a tool for characterizing large, imprecise reflectors. Annual international symposium on optical science, engineering, and instrum­entation, Denver, Colorado USA, November 1996. Department of Energy

[5] S. Kammel, F. P. Leon, Deflectometric measurement of specular surfaces. Instrumentation and Measurement (2008), IEEE Transactions 57, pp. 763-769.

[6] M. C. Knauer, J. Kaminski, G. Hausler, Phase measuring deflectometry: a new approach to measure specular free-form surfaces. Optical Metrology in Production Engineering. Proceedings of the SPIE (2004), volume 5457, pp. 366-376

[7] K. Pottler, E. Lupfert, G. Johnston, M. Shortis, Photogrammetry: A powerful tool for geometric analysis of solar concentrators and their components. Journal of Solar Energy Engineering (2005). 127, pp. 94-101.

[8] M. R. Shortis, G. Johnston, Photogrammetry: An available surface characterization tool for solar concentrators, Part II Assessment of surfaces. Journal of Solar Energy Engineering (1997). 119, pp. 286­291

[9] M. R. Shortis, G. Johnston, Photogrammetry: an available surface characterisation tool for solar concentrators — Part I: measurement of surfaces. Journal of Solar Engineering (1995). 118 (3), pp. 146-150.

[10] S. Ulmer, B. Heinz, K. Pottler, E. Lupfert, Slope error measurements of parabolic troughs using the reflected image of the absorber tube. 13th International Symposium on concentrating solar power and chemical energy technology: SolarPaces. Seville, Spain. June 20-23, 2006.

[11] T. Wendelin, K. May, R. Gee, Video scanning Hartmann optical testing of state-of-the-art parabolic trough concentrator. Solar 2006 Conference (ISEC’06).July 8-13, 2006.Denver, Colorado USA

[12] T. Wendelin, Parabolic trough optical characterization at the National Renewable Energy Laboratory. 2004 DOE Solar Energy Technologies Program Review Meeting October 25-28, 2004 Denver,

Colorado

[13] T. Wendelin, G. J. Jorgensen, R. L. Wood, SHOT: A method for characterizing the surface figure and optical performance of point focus solar concentrators. In: Solar Engineering. American Society of Mechanical Engineers, New York, (1991) pp. 555-560.

Experimental layout of the system

Подпись: Figure 4. Experimental rig Подпись: Figure 5. PIV system

The experiment was conducted in the solar calorimetry laboratory. The tests were carried out in an air — conditioned room having an average temperature of 23°C. Figure 4 shows the pictures of the experimental test rig, while figure 5 shows the PIV system.

PV Thermal Systems — Capturing the Untapped Energy J. Hollick

Conserval Engineering Inc., 200 Wildcat Road, Toronto Ontario Canada M3J 2N5
Corresponding Author, ihollick@solarwall. com

Abstract

Canada’s National Solar Test Facility and the Danish Technological Institute have completed testing of PV Thermal modules as part of the International Energy Agency Task 35 Project. The data shows that it is possible to capture two to three times more thermal energy than electricity from a PV array. Panels from various manufacturers were tested under NOCT conditions, and the results showed that when PV modules were mounted on top of SolarWall® transpired collector panels, the total solar efficiency was in the 25% to 50% range depending on the PV module tested, compared to the typical 6 to 12% for PV modules alone.

By removing the excess heat generated by the PV modules, the electrical output is increased. Modules can commonly operate at temperatures over 50 degrees above ambient temperature resulting in a performance reduction of more than 25%. By removing the heat from the module and lowering the operating temperature, significant gains can be made in system performance and the heat can be utilized for practical heating purposes. The economics of a PV system that incorporates a thermal component can also be improved on buildings where the PV heat can be used to displace space heating energy.

Keywords: PV thermal, transpired collector, BIPV

1. Introduction

The trend with photovoltaic (PV) installations is towards building integrated systems, and while this is advantageous in many regards, there are problems associated with conventional methods of integrating PV directly into a building.

The main problem with building integrated photovoltaic (BIPV) systems is heat retention under the PV modules. The heat produced can be as much as 50°K (90°F) over ambient temperature resulting in two concerns. The first is the possible structural damage from heat if panels are not vented or if heat is not recovered. The second is the lower efficiency of most PV modules with increasing temperature. Crystalline cells are affected by temperature and the performance drops as cell temperature rises. It has been shown that for each °C increase in temperature, the power production drops by ~0.5%. This means that a BIPV 100 W crystalline module at 65°C is only delivering 80 W of power compared with the 25°C name plate rating.

A PV module operating at its stagnation temperature of 50°K above ambient, when the heat is not removed and if ambient temperature is 30°C, will experience a module temperature of 80°C, or even higher, on some tiled roofs.

Another issue facing installers and customers is competition for roof space and deciding on which solar technology should have priority. Covering a roof with PV modules only uses 10% to 15% of available solar energy and eliminates the possibility for future solar thermal systems with much

higher solar conversion efficiencies. A client may not be able to install solar panels to heat water, a pool or the building when the roof is already covered with a solar electric technology.

Grid tied PV systems have a high initial cost and are generally sold only with generous incentive programs. A possible solution to the long payback situation is to see whether the "waste" solar heat can be recovered and used to lower heating costs.