The PV modules have been replaced by a wooden target that has identical dimensions.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.

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

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.

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

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

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.

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.

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

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.

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.

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

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.

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

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

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:

NuDh = 5.811( Г)-°’237

3.4. Thermal resistance

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

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

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.