Category Archives: EuroSun2008-5

The cases of not obligatory implementation of solar collectors

In buildings placed on historical zones is not obligatory to install solar collectors if the south- oriented roof area available is going to create a negative visual impact. As we have seen before, the energy produced by the solar collector is very important to meet de RCCTE requirements. Therefore, in building rehabilitations, not installing solar collectors means the Nac value is going to be only supported by conventional energies. To understand better the implications of this in a real case it were made some calculations using a building example situated on a historical zone and in a soft Portuguese climate zone. It is a typical Portuguese building of approximately hundred years old with a wood roof structure and large stone walls. From the calculation made we concluded that without solar collectors three mainly measures hat to be taken: increment strongly the envelope insulation, choose the more energy efficient equipments as possible and also select equipments based on gas or oil supply and not on electricity. The results showed that only selecting envelope insulations widths of minimum 8-10 mm the building could be straight with the regulations. Notice, this width is still uncommon in Portuguese projects and is going to occupy some useful internal space joining the fact that typical stone exterior walls have already great widths. Another problem detected is that usually the conventional energy to select must be electricity because usually there is not available natural gas supply on these zones. It seems by this example, that these regulations look a little restrictive for buildings in historical zones.

Other case is that collectors are not obliged to be implemented when the building south-oriented roof is or is going to be in the future shadowed during the solar period mentioned in chapter 3. One of the future obstacles on the efficient implementation of solar collectors is the fact that there is a lack of policies and regulations on urban planning to give the minimal guarantees to building sun exposure. In some cases, is difficult to know for sure what kind of constructions are going to be authorized in the future near the project site. It is an important problem related with governmental territory planning strategies.

Solar thermal collector yield — experimental validation of calculations. based on steady-state and quasi-dynamic test methodologies

P. Horta[13]* , M. J. Carvalho1 and S. Fischer[14]

1 INETI, Department of Renewable Energies, Campus do Lumiar do INETI, 1649-038 Lisbon, Portugal
2 Institute for Thermodynamics and Thermal Engineering (ITW) — University of Stuttgart,
Pfaffenwaldring 6, 70550 Stuttgart, Germany
Corresponding Author, pedro. horta@lneti. pt

Abstract

The characterization of collector efficiency is the fundamental tool for long term calculation of collector yield. It is, thus, one of the most important inputs in software tools aiming the design of solar thermal systems.

Presently two test methodologies are available for characterization of the efficiency of glazed collectors: i) steady state test and ii) quasi-dynamic test, methodologies based in different model approaches to a solar collector, providing different collector efficiency curve parameters and, consequently, imposing different power calculation algorithms.

Moreover, Horta et al (2008) demonstrated that the use of the collector efficiency curve derived from steady state test method is not enough for a thorough characterization of the long term performance of a collector.

The present work takes into account the introduction of the above referred test methodologies in the European Test Standard for Solar Thermal Collectors, and aims at clarifying how each test results should be used in long term thermal performance calculations.

The paper presents a synthesis of the different efficiency parameters provided by each test methodology and corresponding algorithms, applicable in the calculation of delivered power. Application of these algorithms to two days of measured data allows for a comparison of the results obtained with these different methodologies.

For validation purposes, results of tests performed on a CPC type collector with a concentration ratio C=1.72 are used. Measurement sequences are used to validate the calculation of power delivered by the collector using both algorithms based on steady-state methodology (with and without correction) and quasi-dynamic methodology.

Keywords: solar thermal energy; efficiency curve parameters; solar system simulation; long term performance assessment

Presently two test methodologies are available for characterization of the efficiency of glazed collectors: i) steady state test methodology according to EN 12975-2: section 6.1 and ii) quasi-dynamic test methodology according to EN 12975-2: section 6.3.

It should be stressed that these methodologies, based on different model approaches for a solar collector, provide different collector efficiency curve parameters and, consequently, impose different algorithms for calculation of the power (and energy) delivered by solar thermal collectors.

In recent studies, Horta et al. (2008) demonstrated that the use of the collector efficiency curve derived from steady state test method is not enough for a thorough characterization of the long term performance of a collector, especially if its optical characteristics differ from the simplest flat plate collector.

Considering that, at present, steady-state tests are more commonly used and the majority of available collectors are characterized by steady state based efficiency curve parameters, a methodology for correction of power/energy results obtained with those parameters was proposed by Horta et al. (2008).

Recently, in project NEGST (Carvalho et al., 2006) it was also highlighted that for a correct characterization of stationary collectors with special optical characteristics or for tracking collectors, the quasi dynamic test method is the most appropriate test methodology.

The paper presents a synthesis of the different efficiency parameters provided by each test methodology and corresponding algorithms, applicable in the calculation of delivered power (see section 2). A validation of the methodology proposed by Horta et al. (2008), for the correction of long term performance calculations based on steady-state parameters, is also presented, after the results of tests performed on a CPC type collector with a concentration ratio C = 1.72.

Characteristic heat balance equations

In a solar thermal collector the heat balance can be written according to the following equation:

C — d t

 

(1)

 

image175

image176

T represents the collector temperature and C its effective thermal capacity. qabs is the power absorved by the collector from incident solar radiation, qioss, is the power lost to the environment. The power delivered by the collector for use is given by:

quse = m cp (te — ti) (2)

With m being the flow rate, te and ti , the outlet and inlet collector temperature respectively and Cp the fluid heat capacity. In the SS test method the term on the right, in equation (1), is considered equal to zero and it is possible to define a steady-state efficiency:

Л= Л0 — a1(tm — ta)/G — a2(tm — ta)2/G (3)

where tm is the mean fluid temperature and ta the ambient temperature. Parameters q0, ai and a2 are determined based on test data that respects the conditions listed in Table 1 and using a least square fit method. Since one of the conditions is that the incidence angle on collector aperture is lower then 20°, dependence of optical efficiency, q0, on incidence angle has to be determined in a separate test. In this test, the Incidence Angle Modifier (IAM) is determined based on global irradiance incident on collector aperture.

Also the thermal capacity of the collector is only determined if and optional test is performed (see section 6.1.6 of EN 12975-2:2006)

In the case of QD test method, equation (1) can be written, for the case of glazed collectors, as:

Q/A = F(xa)enКдь (0)Gb + F(ia)enKMGd — c1(tm — tj-c2(tm — tj — c5dtm /dt

where A is the collector reference area, Gb and Gd are beam and diffuse irradiance incident on the collector, respectively.

In QD test method, the parameters that characterize the collector are:

F(xa)en;Keb (0);K9d ;c1;c2;c5 (5)

i. e., optical efficiency (equivalent to ^0 in SS), IAM for beam radiation, IAM for diffuse radiation, thermal loss coefficients (equivalent to a1 and a2 in SS) and effective thermal capacity. The fact that the IAM is decoupled in its component for beam radiation, K0b(0), and the component for diffuse radiation, K0d, is an additional advantage of QD test method, specially in the characterisation of collectors with more complex optical characteristics than flat plate collectors [3,4], e. g, evacuated tubular collectors or CPC type collectors.

Transmittance-absorptance product

The transmittance-absorptance product describes the properties of the glass and the absorber. The products are for all four solar collectors varied in the interval from 0.77 to 0.90. As expected the best performance is seen with a high transmittance-absorptance product. Fig 5 shows the performance as a function of the transmittance-absorptance product for the solar collectors located in Nuussuaq. The

higher the product, the higher the thermal performance. The variations of thermal performance in the investigated interval are slightly higher in Nuussuaq than in Sisimiut and Copenhagen. An increase in the transmittance-absorptance product from 0.83 to 0.9 will result in a increased performance of about 12 % for all the collectors. A change in the transmittance — absorptance product from 0.83 to 0.9 results in an increased optimum tilt of the collectors of 6° for the collectors located

Подпись: Fig 5. The thermal performance as a function of the transmittance- absorptance product for the collectors in Nuussuaq. in Nuussuaq. In Sisimiut and Copenhagen the transmittance-absorptance product will only slightly influence the optimum collector tilt. The optimum orientation by changing the transmittance- absorptance product from 0.83 to 0.90 is changed in such a way that the collectors in Nuussuaq should be turned even more towards east to about 60° from south towards east. In Sisimiut the best azimuth for the collectors with curved absorbers is 2° from south towards west. The azimuth of the collectors with flat absorbers is 35 ° towards west from south. In Copenhagen a change in transmittance — absorptance product will result in the best azimuth to be 4° from south towards west for all four collectors. In improvement of the transmittance-absorptance product will result in great improvement of the thermal performance of all the collectors. With a mean collector fluid temperature of 60 °C, the improvement rages from 11 % to 14 %, with the highest improvement in Nuussuaq. The thermal performance increases with high mean collector fluid temperature for all collectors at all locations.

System tests according to ISO 9459-2 and ISO 9459-5

In part two and five of the standard series ISO 9459, two possibilities for performance testing of domestic solar thermal hot water systems are described.

With the CSTG (Complete System Testing Group) test method standardized in ISO 9495-2, only solar thermal systems without an integrated auxiliary heating element can be tested. The per­formance determination according to the CSTG method focuses only on sums of energy. For per­formance testing of solar hot water system according to the CSTG method, the solar irradiance dur­ing each test day is summed up. In a second step, the useful energy withdrawn from the system at the end of the day is calculated based on measurements of the fluid inlet and outlet temperatures and flow rate. Finally the withdrawn daily energy is divided by the daily solar irradiance. This test is performed for several days with different irradiance values. Based on the results obtained in this way the annual system performance can be calculated for specific reference conditions.

During the test of the system according to the DST-method (Dynamic System Test) standard­ised in ISO 9495-5 the system is operated for several days according to well specified test condi­tions. From the measured data recorded during this short-term test, specific system parameters are determined by means of parameter identification. Based on these parameters the thermal perform­ance of the system can be determined for specified reference conditions by means of annual system simulations.

CORRESPONDENCE BETWEEN TECHNOLOGICAL. SOLUTIONS AND THE ENERGY LABEL OF RESIDENTIAL. BUILDINGS IN PORTUGAL

Sebastiao Carvalho, Vftor Leal and E. Oliveira Fernandes

FEUP — Faculty of Engineering of the University of Porto
Rua Dr. Roberto Frias, s/n 4200-465 Porto PORTUGAL
* Corresponding Author: Vitor Leal, vleal@fe. up. pt

Abstract

Portugal has new building energy regulation since 2006, coupled with a building energy certification scheme in force since 2007. The regulation for residential buildings has several requirements that must be met, among which are limits to the nominal heating and cooling needs, to the nominal final energy for domestic hot water and to the nominal primary energy consumption. Because most of the requirements are performance goals rather than prescribed measures, there are always several options to meet the regulation requirements. Furthermore the certification/labelling scheme now in place establishes a differentiation between the buildings that comply, ranging from B — to A+ for new buildings.

This study presents an analysis of the correspondence between the constructive and technological solutions adopted and the compliance with the regulation as well as the resulting energy class. This is done using an apartment and a dwelling as case studies, for 7 different locations in mainland Portugal. The analysis of the results places a focus on the relevance of the domestic hot water solar collectors for compliance with the regulation and for the achievement of the best energy classes (A, A+).

Keywords: Buildings, Performance, Certification, EPBD, Solar collectors

1. Introduction

Buildings represent about 37% of the primary energy used in Portugal, slightly more than transportation or industry, and they are therefore a key-sector for achieving a low carbon society. One of the most important mechanisms designed to achieve this goal is the EPDB-related certification and labelling scheme, which in Portugal is called the SCE (decree-law 78/2006, [1]) and is supported on the energy regulations for residential buildings (decree-law 80/2006, [2]) and for energy-intensive buildings (decree-law 79/2006, [3]).

Solar collectors are a direct requirement of the regulation although it can be discarded in some cases (solar exposition not favourable, historic areas). Furthermore, the calculation method used in this labelling system recognizes important credits to solar energy, either that entering through the glazed envelope as direct solar gains or that captured by solar collectors for water heating.

In terms of energy requirements, the fulfilment of the regulation implies the simultaneous compliance with the following requirements:

i) Minimum requirements for the envelope elements (U-value and solar factor)

ii) Heating needs (useful energy, Nic) inferior to a maximum level allowed (Ni).

iii) Cooling needs (useful energy, Nvc) inferior to a maximum level allowed (Nv).

iv) Hot water needs (final energy, Nac) inferior to a maximum level allowed (Na)

v) Total primary energy (Ntc) inferior to a maximum allowed level (Nt).

The calculation of the total primary energy needs Ntc considers that the domestic hot water needs are satisfied at 100%, while the nominal heating and the cooling needs are only satisfied at 10% (due to traditional use patterns). It is computed as (eq.1) :

image152

(kgoe/m2.year)

 

Ntc

 

0.1

 

F +Nac■F

puv pua

 

(eq. l)

 

where hi and hv represent the conversion factors from final to useful energy, while Fpui, Fpuv and Fpua represent the conversion factors from final to primary energy.

If (and only if) all the previous criteria i) to v) are met simultaneously, then an energy class can be determined. The energy class is established through the quotient between the estimated primary energy use and its maximum allowed by regulation, with class transitions at each 25% improvement. The minimum allowed class for new or significantly retrofitted buildings is B — (table 1).

This study intends to perform an analysis of the correspondence between the constructive and technological solutions adopted and the compliance with the regulation as well as the resulting energy class. This is done using an apartment and a dwelling as case studies, for 7 different locations in mainland Portugal. The analysis of the results puts a focus on the relevance of the DHW solar collectors for compliance with the regulation and for the achievement of the best energy classes (A, A+).

Table 1: Energy labelling of new residential buildings as function of the relationship between the calculated primary energy use (Ntc) and the maximum allowed (Nt).

0.75< (Ntc / Nt) < 1

0.50 < (Ntc / Nt) < 0.75

0.25 < (Ntc / Nt) < 0.50

(Ntc / Nt) < 0.25

Building energy class

B-

B

A

A+

Factory made systems — EN 12976 Part 1 and 2

The standard series EN 12976 for factory made systems was changed e. g. with regard to the requirements concerning the suitability for drinking water according to EN 1717 (protection against pollution of portable water in drinking water installations and general requirements of devices to prevent pollution by backflow).

2. Custom built systems — CEN/TS 12977 Part 1, 2, 4 and 5 and EN 12977 Part 3

The most changes made during the revision were related to the standard series ENV 12977 for custom built systems since this standard series was fundamentally revised and extended by two additional parts. The new standard series CEN/TS[3] 12977 consisting of in total five parts (see Table 2) is expected to be published for formal vote during the second half of 2008.

The fundamental revision and the amendment of two additional parts were necessary due to the extension of the scope from solar domestic hot water systems to solar combisystems. Solar combisystems are systems that provide heat for domestic hot water preparation and for space heating.

Since the most significant differences between solar thermal domestic hot water systems and solar combisystems are related to the heat store and to the controller specific parts related to these components were added.

The performance testing of stores for solar domestic water heating systems is described in EN 12977-3. Since the core procedures in this standard were already included in the previous version ENV 12977-3 /3/ it was decided to establish this standard as a real EN standard. The formal vote by the CEN member countries related to this part was carried out during the summer of 2008 with a positive outcome. Therefore the official publication of this standard can be expected by the end of 2008.

The first new part of the standard series CEN/TS 12977 is part 4 dedicated to performance testing of combistores. The amendment of the standard series for custom built systems by this part is a logical consequence of the extension of the whole standard series to combisystems.

Since the controllers used in solar combisystems are typically much more complex and advanced than the ones used in solar domestic hot water systems it was also necessary to account for this aspect by additional procedures. Therefore CEN/TS 12977-5 named “Performance test methods for control equipment” was added as the second new part. Beside quality assurance and improvement of the reliability of all control equipment, the parameters of the controllers determined by the test procedures of part 5 are required in order to determine the performance of thermal solar systems by means of computer simulation /4/.

At present (summer 2008) the formal vote within the CEN member countries for CEN/TS 12977 Part 1, 2, 4 and 5 is under preparation. In case of a positive voting result the publication of the final version of these standards can be expected for early 2009.

The revised version of the standard series EN 12975, EN 12976 and CEN/TS 12977 will form an excellent basis for the further development of the European solar thermal market and for Solar Keymark certification. However, in order to consider future technological developments such as tracking concentrating solar collectors or solar cooling systems periodical revisions of the standards will be required

Quality Assessment for Concentrating Collectors

The solar field within a solar power plant has to be optimized in several aspects. First optical and thermal design is necessary to develop technical solutions with large efficiency. Secondly the materials used shall comply with the necessary optical and thermal performance specifications, and have to withstand the environmental and operation stresses for sufficiently long time. Thirdly the assembly of individual components has to be controlled and qualified. And last but not least, the costs of the collector have to be minimized. Having costs in mind, optical and thermal performance as wells as longevity become the key issues in collector optimization.

When overall performance does not meet the expectations, it is often difficult to trace back the problems to subsystems. Therefore a chain of quality measures should be used parallel to the assembly process of a collector. The measures generally consist of a testing procedure plus technical specifications. The testing can be either on-site in an outdoor environment or indoors in a laboratory or production place. We were working on the latter approach for several reasons:

• — for measurements in the laboratory external perturbations can be minimized and therefore the level of precision in general is higher

• — rejections within the production can be noticed in an early stage

• — testing methods developed can be modified to support quality control in a production line

Optical and thermal qualification will help manufacturers and construction companies to control collector performance as stipulated in the specifications. Some properties of the key components, i. e., thermal receiver, secondary and primary mirrors, have to be determined.

General difficulty

The thermal performance of concentrating collectors depends strongly on the concentration ratio C. The concentration ratio for collectors having tubular absorbers is calculated by the ratio between

image111 image112 Подпись: dfu Подпись: (1)

aperture area and unrolled absorber area. Simplified it can be assumed that the diffuse irradiance available for the collector is reduced by the quotient 1/C. Using this assumption the irradiance Gnet which can be used by the collector can be calculated using equation 1.

Подпись: N Подпись: G„eL G Подпись: Gbeam + C Gdfu G Подпись: (2)

Dividing the useable irradiance Gnet by the hemispherical irradiance G yields the useful fraction N described by equation 2.

image119

Due to this dependence of the useful irradiance Gnet on the concentration ratio C more diligence has to be taken for the determination of the thermal performance of concentrating collectors compared to flat plate collectors.

10

Fig. 2. Fraction of useful irradiance N as a function of the concentration ratio C and the

diffuse fraction D

2. Test method

The European Standard EN 12975-2:2006 [1] allows for two test methods for the determination of the thermal performance: the test method under steady state conditions and the test method under quasi-dynamic conditions.

The more detailed test method under quasi-dynamic conditions differentiates between beam irradiance Gbeam and diffuse irradiance Gdfu. Together with the incidence angle modifier for beam irradiance Kbeam(0) and diffuse irradiance Kdfu it is therefore possible to model the influence of the beam irradiance under different incident angles as well as the dependency of the thermal performance on the diffuse fraction. In addition the implementation of the effective heat capacity ceff in the thermal performance model of the collector (see equation 3) allows the description of the dynamic behaviour of the collector under changing radiation levels. By means of the test method under quasi-dynamic conditions the thermal performance of stationary non-concentrating collectors can be determined as well as for tracking concentrating collectors. The relatively high level of detail of the model permits the use of test sequences with wide variation in the level of irradiance.

The simplified test method under steady state conditions abstains from the differentiation of the beam and diffuse irradiance and the metrological determination of the effective heat capacity (see equation 4). The major drawback of these simplifications is the fact that only data can be used for evaluation that was recorded under very constant (steady state) conditions. An additional uncertainty in the test results is created by the use of an incident angle modifier for the hemispherical irradiance K(0) which itself depends on the diffuse fraction predominant during the measurements. Due to these simplifications the test method under steady state conditions is strictly speaking only suitable for collectors with a thermal performance not depending significantly on the nature of the irradiance (beam or diffuse). Hence the test method is not suitable for concentrating collectors at all.

Подпись: (3) (4) ( d3 Л

Подпись: dtQ = A П0GbeamKbeam (в) + n0GdjuKdju — ai (f — 3a ) — «1 (f — 3a — Ceff

Q = A(no GK (в) — a, (V -*a ) — a (V -*a )[10] )

For CPC collectors available on the market today with a concentration ratio of 1 < C < 2 the fraction of useful irradiance is reduced up to 25% depending on the diffuse fraction. In the following the impact of this fact on the test results of CPC collectors is described.

Testing of solar air collectors

Christoph Thoma, Thorsten Weick, Jens Richter, Thorsten Siems, Matthias Rommel

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrahe 2, D-79110 Freiburg
Corresponding Author, christoph. thoma@ise. fraunhofer. de

Abstract

Solar air collectors do have only a very small market share. About 20 years back, in the beginning of solar collector developments in Europe, there was some development work carried out. But rather soon in those times it became clear that for the development of small solar thermal systems, designed only for domestic hot water in single-family-houses there were more advantages for collector systems using a liquid as heat transfer fluid. Although in most systems a water-glycol mixture is used as fluid, the simplifying term "water-collectors" will be used in this paper. The development aspects for water-collectors and air-collectors were pretty much seen in a direct competition. But solar thermal energy has made tremendous steps forward in the meantime. We think that it is necessary now to assess the possibilities for the development of solar air collectors again and in the light of the new technical possibilities that are available now. Fraunhofer ISE has therefore started to improve its existing testing facility for solar air collectors. Up to recently, it was directly installed in our solar indoor testing facility with solar simulator. That was convenient and helpful for the developments that we carried out in collaboration with the industry so far. But now we started to re-design the components of the testing facility. It is now possible to carry out test not only in the laboratory with the solar simulator, but also on our outdoor testing facility with tracker. And even testing of systems already installed in the field are made possible. The paper gives an overview on the achievements obtained so far.

Keywords: solar air collector, collector testing, air, European collector test standards, EN12975 [15]

In figure 1 some thermodynamic properties of air and water are compared to each other. The differences in the properties do have important consequences not only for the development of air — collectors, but also for testing them.

parameter

air

water

ratio

water / air

density p in kg

m3

1,185

998,200

~ 842

specific heat capacity c in kJ

kg ■ K

(mass-based)

1,004

4,179

~4

specific heat capacity c in kJ

m1 ■ K

(volume-based)

1,190

4171,478

~3505

thermal conductivity X in W/(m K)

0,026

0,599

~23

Table 1: Comparison of some thermodynamic properties of air and water: