Wildor Maldonado Carbajal
Manuel Collares Pereira

Ao Sol Energias Renovaveis S. A., Parque Industrial de Porto Alto, Sesmaria Limpa, Samora Correia,

Portugal, +351263 65 13 05/06,

wildormaldonado@aosol. pt; mpicp@aosol. pt
Abstract

The integration of solar collectors on building facades is becoming a much more common option, and a true alternative to the usual placement of collectors on terraces or on roofs, seeking an orientation towards azimuth south (on the northern hemisphere) and a tilt equal or higher than latitude.

For a stationary collector the choice of azimuth south and a tilt around the value of latitude maximizes yearly energy collection, or Winter energy collection, when a tilt value higher than that of local latitude is chosen.

However when collectors have to go on a vertical faqade the results obtained are much different, specially on latitudes typical of southern Europe, i. e. from 35° to 55°; from winter to summer, due-south may no longer be the azimuth of choice. In fact, in the summer time, the sun is “very high” in the sky and is basically reflected off the collector glass cover, greatly reducing the collected radiation.

In this paper the authors wish to make a collector performance evaluation of three different collectors, operating on vertical surfaces. The three collector types are a flat plate collector and two CPC collectors, one with absorber fins running horizontally (HF) and another with the absorber fins running vertically (VF). The comparison will be made at constant inlet temperature to the collectors. The flat plate collector is a good selective surface one and the CPCs chosen are: (i) a standard CPC manufactured by AO SOL for the VF case and (ii) a new proposed design for the HF case.

This HF case is an attempt at having an optics developed to somehow compensate the grazing incidence angles of the sun in the summertime, when the sun is highest in the sky and the azimuth of the vertical wall approaches zero (due-south).

For faqade azimuth varying between 0° to 90° we calculate the solar energy collected (direct and diffuse radiation) for apparent solar annual motion, and use recent refinements proposed to standard calculation methods [1,2]. A comparison will also be made with the same collectors operating according to the usual “due south-tilt equal latitude” procedure.

The analysis is done for two locations Lisbon (latitude 38.75°) and Freiburg (latitude 48°) as two representative cases.

The results show that the behaviour of the collectors on vertical walls is more or less the same on an yearly basis for several different azimuths, (a result which was not intuitive to the authors) although they are quite different on a seasonal basis.

As for the comparison between the three collectors considered it is seen that the CPC HF is better than the other two, in particular for azimuths closer to zero, both because of its optical behaviour and because of its better thermal performance.

1. Introduction

The integration of solar collectors on building facades is becoming a much more common option, and a true alternative to the usual placement of collectors on terraces or on roofs, with an orientation towards azimuth south (on the northern hemisphere) and a tilt equal or higher than latitude.

In this paper the authors wish to make a collector performance evaluation of three different collectors, operating on vertical surfaces, with different azimuths. The three collector types are a flat plate collector and two CPC collectors, one with absorber fins running horizontally (HF) and another with the absorber fins running vertically (VF). The comparison will be made at constant inlet temperature to the collectors. The flat plate collector is a good selective surface one and the CPCs chosen are: (i) a standard CPC manufactured by AO SOL for the VF case and (ii) a new proposed design for the HF case.

This HF case is an attempt at having an optics developed to somehow compensate the grazing incidence angles of the sun in the summertime, when the sun is highest in the sky and the azimuth of the vertical wall approaches zero (due-south).

The relevant characteristics of all collectors are presented in chapter 2. and their performance is calculated (at constant inlet temperature) in chapter 4 for the following situations:

1) Yearly performance for the 5 azimuths 0, 22.5, 45, 67.5 and 90 deg.

2) Comparison of the values obtained for azimuth zero, with what would be obtained if the collectors were operated at tilt-equal latitude.

3) Monthly performances at Tin = 40°C, for the same azimuths as in 1).

The paper ends (chapter 4) with a summary of the main conclusions.

2. Collector description

Table 1 summarizes the optical and thermal characteristics of the collectors chosen for this study.

Table 1. Involved Collectors. Collector F’n 0 F’UL (W/m2/°C) Flat plate selective 0.80 4.7 CPC 3E+ NS 0.74 4.2 New CPC EW 0.80 3.4

All collectors are characterized in the so called linear approximation, with a coefficient describing its optical efficiency (F’q0) measured at normal incidence and another (F’UL) its heat loss.

The flat plate is taken as a standard good selective coated one. The two CPCs used in this study are:

1) VF CPC — standard 3E+ CPC manufactured by AO SOL Energias Renovaveis S. A. [3], as in Fig. 1, with an effective concentration of 1.15X, after truncation.

2) HF CPC — a collector with the cross section of Fig. 2 is considered. Its final concentration is 1.5X, a CPC with a full acceptance angle of 110° deg. Notice the asymmetric nature of this collector, expressly conceived so that the fin has a tilt with respect to the entrance aperture (the vertical glass cover) of 20° deg. This tilt is not the yet the result of an optimization, but it is a first attempt at reducing on average, incidence angles on the absorber.

(a)

Fig. 2. HF CPC: (a) cross section, (b) on a vertical wall.

This collector has not yet been built and its performance is extrapolated from the one obtained at AO SOL with a symmetric collector with a similar concentration value [8].

Calculation of collector thermal performance at different inlet temperatures is made with a standard utilizability [4] based method. The optical performance is calculated taking into account the corrections and the methodology described in [1, 2], corrections arising from the fact that standard efficiency tests were not designed for CPC collectors.

The proper incidence angle modifiers in each case (longitudinal and transversal), evaluated through detailed ray tracing is used, as well as the behaviour with respect to diffuse radiation.

All calculations are done for two cities: Lisbon with a latitude of 39° deg and Freiburg with a latitude of 48° deg.

3. Results

3.1- yearly performance evaluation at five different azimuths

The three collectors are placed vertically and their performance is evaluated for the following azimuths: 0°, 22.5°, 45°, 67.5° and 90°. These are taken from South (0°) to West (90°) assuming that towards the East the behaviour is identical (symmetrical).

Fig. 3 to 7 show the yearly performance for each collector type, as a function of inlet temperature, at two different latitudes, that of Lisbon and that of Freiburg, and at azimuth zero (vertical due south).

The VF CPC is more optically penalized by the vertical wall than the flat plate, but the HF CPC has a better behaviour resulting for an acceptance angle more favourably defined with respect to collector orientation vs. apparent motion of the sun.

A striking observation — see Fig. 3 to 7- is that there is not much difference on yearly collected energy, even for walls with very large deviations from zero azimuth!.

This non intuitive result will be explained in the coming sections, once the monthly behaviour is presented.

3.2- Penalty introduced by the vertical wall: a comparison with the usual tilt equal latitude at azimuth zero recommended orientation.

Fig. 8, 9 are a reproductions of Fig. 3, with the addition of the results [2] of the calculation of energy delivered at tilt equal latitude and the same azimuth, to give an idea of the penalty introduced by the unfavourable vertical wall (tilt equal 90° deg).

It can be seen that the difference between the VF CPC and the flat plate is attenuated for tilt equal latitude (a tilt for which the CPC in question was designed).

The reverse is true for the HF CPC, now with not so good a behaviour in the tilt equal latitude mode, a logical consequence of the fact that it was designed for the vertical wall.

It can also be seen that in general there is a penalty on the order of a factor of two for the operating temperature range considered, for going on the vertical wall as opposed to working within the more favourable orientation.

The HF CPC was not meant to operate at tilt equal latitude and that shows in the poorer result obtained when that tilt is used in this case. This difference is still pronounced but less significant at the higher latitude (Freiburg).

Fig. 8. Comparison of yearly collected energy on the vertical wall and at tilt equal latitude in Lisbon, Fig. 9.

The same for Freiburg.

3.3- The seasonal behaviour of the collectors for the different azimuths

In this analysis only the performance at Tin = 40°C is presented, for the sake of space. Monthly energy delivered is presented for each azimuth and collector type.

It can be seen that the differences between winter and summer months are much larger for the wall facing south than for the others. In fact the intermediate azimuths show a more constant monthly performance. When totals are calculated this yields the results referred in 3.1.

4. Conclusions

Three collector types — a flat plate collector and two CPC type collectors — were placed on vertical walls and their performance evaluated at different temperatures, in two locations, Lisbon and Freiburg, respectively a lower and a middle latitude in Europe.

It was shown that this collector mounting yields significantly less energy than the usual tilt equal latitude, due south, collector orientation usually recommended. These differences are larger for Lisbon (almost a factor of two) than for Freiburg, at a higher latitude, thus with the sun on average more perpendicular to the vertical wall, all year round.

The results for annual energy yield on vertical walls for the three collector types, show that the behaviour (average energy delivered at constant operating temperature) of each of them is more or less insensitive to the direction the wall faces up to very large deviations (67.5° deg) from azimuth zero (a wall facing due south). This is a result which was not intuitive to the authors.

However on a monthly basis the differences are quite apparent, with the wall due south showing a larger winter time contribution and a larger difference between winter and summer months.

As for the relative yield of each collector type, the comparison shows that the CPC HF (a first and very simple attempt at designing a CPC to operate on vertical mounting) is better than the other two, in particular for azimuths closer to zero, both because of its optical behaviour and because of its better thermal performance, underlining the fact that it was really meant to operate on the vertical as the other two were not.

In the future, the wider use of solar energy to heat and cool buildings as well as to provide DHW, will require a much wider use of building facades to place solar collectors, quite beyond the usual placement on the roof. The kind of study presented here will certainly be made for new collectors,

meant to take the best advantage of the new 90° deg tilt, trying to reduce the penalties it imposes,

when compared to more usual tilt and orientations.

References

[1]Pedro Ribeiro Horta, MSc. Eng.; Maria J Carvalho, PhD; Manuel Collares-Pereira, PhD; Wildor Carbajal, Eng. — “Long term performance calculations based on steady state efficiency test results :analysis of optical effects affecting beam diffuse and reflected radiation” — submitted to Solar Energy Journal (2007).

[2]Manuel Collares-Pereira, PhD; Wildor Maldonado, Eng. — “Efficiency testing of solar collectors and long term performance simulation tools: application to flat plate and CPC type collectors”. ESTEC 2007.

[3]AOSOL, Energias Renovaveis S. A. is a company owned by a Portuguese investing holding group ENERPURA. It designs and supply thermal solar collectors of the CPC type. www. aosol. pt.

[4]Manuel Collares Pereira, Maria Joao Carvalho — “Dimensionamento de sistemas solares; sistemas de aquecimento de agua com aramazenamento acoplado”- Laboratorio Nacional de Engenharia e Tecnologia Industrial Industrial, Departamento de Energias Renovaveis (1990).

[5]Carvalho, M. J. et al 1987. Economic Optimization of Stationary Non evacuated CPC Solar Collectors. Journal of Solar Engineering vol. 109 pp.40-45.

[6]William R. Mclntire. 1982.”Factored approximations for biaxial incident angle modifiers”. Solar Energy, Vol. 29, N° 4, pp. 315-322, 1982. Pergamon Press Ltd.

[7]Ari Rabl. 1985. “Active Solar Collectors and Their Applications”. New York: Oxford University Press, Inc.

[8]M. Collares Pereira, M. J. Carvalho, J. Correia de Oliveira. "A new low concentration CPC type collector with convection controlled by a honeycomb TIM material: a compromise with stagnation temperature control and survival of cheap fabrication materials" ISES Solar World Congress, Goteborg, June, 2003.