The thermal performances of four heat pipe ETCs are measured. They are ETC 1, 2, 3 and 4. ETC 1 and 2 have 8 tubes of tube diameter 100 mm. ETC 3 and 4 have 20 tubes of tube diameter 70 mm. The difference between ETC 1, 3 and ETC 2, 4 is that ETC 1 and 3 have a curved/semi-cylindrical fin while ETC 2 and 4 have a flat fin. As shown in Fig.4, the performance ratio of ETC 3/ETC 4 is in the range of 0.73-0.94 meaning that the ETC with a flat fin performs better than the ETC with a curved fin. For a mean collector fluid temperature of 63°C, the ETC with a flat fin has a thermal performance 12% higher than the ETC with a curved fin. With an increase of the mean collector fluid temperature to 75°C, the thermal performance of the ETC with a flat fin is increased to be 15% higher than the thermal performance of the ETC with the curved fin. The comparison of a collector with a curved fin and a flat fin with a tube diameter of 100 mm is given by the ratio of ETC 1/ETC 2. The advantage of the ETC with a flat fin tends to weaken with an increase of the tube diameter. However, the ETC with a flat fin is better than the ETC with a curved fin for most of the test period.

Fig. 4. Performance ratio of the differently designed ETCs.

Thermal performance of the ETCs in phase 1 is summarized in Fig. 5. The measurements were carried out half a year from winter to summer with the aim to get a better estimation of the yearly collector performance. The result shows that the collectors with flat fins perform relatively better than the collectors with curved fins. For a collector with a tube diameter of 70 mm, types 3 and 4, there is an

increase of 13% of collector performance if a flat fin is used instead of a curved fin, while for a collector with a tube diameter of 100 mm, the extra thermal performance of the collector with a flat fin compared to the collector

curved fin absorbs 15.9% more solar energy annually than the flat fin. The explanation is the larger heat loss from the curved fin compared to that of the flat fin. The surface area of the curved fin is approx. 40% larger than the surface area of the flat fin, resulting in a higher heat loss from the curved fin and a lower thermal performance. It shall be noted that the location of the collector and the fact that He’s investigations only considered solar radiation from the front side might influence the conclusion as well. The measurement presented in this paper was carried out for a latitude of 56°, while in He’s investigations [3], a latitude of 40° was used.

The performance ratio between the ETC 2 and ETC 4 is less varying throughout the measuring period. This can be explained by the similarity of the fin design. The mean solar collector fluid temperature has a slight influence on the ratio of the thermal performance. There is an increase of the performance ratio with a decrease of mean solar collector fluid temperature, indicating that ETC 2 has a higher heat loss coefficient than that of ETC 4.

Up to now (August 2008) three mobile test facilities have been completed. The test facilities were delivery by SWT as complete, ready to use turn-key products. One of the facilities was sold to the South African Bureau of Standards (SABS) located in Pretoria, South Africa. This test facility is shown in Figure 9.

This test facility is the key component of a solar thermal test centre that was established, co­financed by the United Nations Development Program (UNDP), at the South African Bureau of Standards (SABS). The test facility shown in Figure 9 allows for testing of solar collectors and thermal solar systems according to ISO 9459-2 (CSTG-method).

Due to the installation of the above mentioned test facility the SABS is able to carry out thermal performance tests and durability tests of solar thermal systems and collectors according to the relevant standards. This is one important basis for the formal accreditation of the test laboratory according to ISO 17025.

Another test facility which allows testing of solar collectors according to EN 12975-2 (and solar thermal systems according to EN 12976 or ISO 9459-2 and ISO 9459-5 respectively is installed at the Research and Test Centre for Thermal Soar Systems (TZS) at the Institute for Thermodynamics and Thermal Engineering (ITW) at the University of Stuttgart.

A further all-in-one test facility was delivered to the Hydrometeorological Service located in Skopje, Republic of Macedonia where if forms the basis for a solar thermal test and competence centre (see Figure 10)

The results of table 9 show that, with a proper selection of energy supply equipments and vectors, it is possible to fulfil the regulation and to achieve class A in all locations and even class A+ in some locations. It is however important to clarify if class A+ can be achieved in all the studied locations. In order to assess this, a new round of improvements was implemented for the locations where class A+ had not yet been achieved. The improvements consisted on the adoption of a better solar collector and on several improvements to the building envelope. Table 10 shows a detailed list of the improvements made at each of the locations where the A+ had not yet been achieved.

Table 11 and Table 12 show the results after this new round of improvements. The results confirm that class A+ is reachable in all locations both for the apartment and for the dwelling. The level of constructive and technological sophistication differs according to the building and location. For the dwelling in Guarda and in Penhas Douradas, reaching class A+ implied adopting mechanical ventilation with heat recovery, a solution not common in Portugal so far.

4. Conclusions

This study analysed the compliance of different sets of constructive and technological solutions of two residential buildings with the thermal regulations in different places in Portugal, as well as the corresponding energy class.

The first major conclusion form the results is that it is possible to comply with the regulation and to achieve the best energy classes (A and A+). Since the studied locations included extreme cold and extreme hot locations of Portugal, it is possible to infer that this holds true for the whole country.

The second major conclusion is that the requirements for the envelope are determined essentially by the need to comply with the heating requirements. It was found that the thermal quality level of the envelope needed to comply with the regulations is substantially different between the mildest and the coldest regions of the country. While 3 to 4 cm of insulation seem to be enough in the locations close to the coast, the interior mountains regions it may require about 6 to 8 cm. Nevertheless, compliance seems to be possible without interfering significantly the architectural appearance of the buildings (glazed area, solar orientation).

Regarding the energy class, it was found that is determined essentially by the equipment choices for heating, cooling and domestic hot water, with a decisive influence of this later one, due to the weighting factors 0.1 / 0.1 / 1.0 used in equation 1. In this aspect it was confirmed that a solution of domestic hot water heated exclusively with an electrical resistance seems to be incompatible with the regulation. Nevertheless, optimization of the envelope seems to be also needed to make difference for class A+ in the coldest regions, and in some cases even mechanical ventilation with heat recovery may be required to reach that energy class.

Regarding the importance of solar collectors, apart of the issues of its obligation and of the reasons that allow to drop that obligation, it is clearly demonstrated by the fact that they were part of all the sets of solutions that achieved class A+ and of most of those that achieved class A. Naturally, since Ntc is evaluated in primary energy, solutions of solar collectors complemented with gas boilers lead to better results than those complemented with electric resistances. In the coldest regions where class A+ is more difficult to achieve, a high quality solar collector (or a collector area higher that strictly required by the regulation) is one of the most effective elements in finding solutions towards class A+.

5. References

[1] Regulamento das Caracteristicas de Comportamento Termico dos Edificios — RCCTE, Decree-Law 80/2006 in Portuguese).

[2] Sistema Nacional de Certificagao Energetica e da Qualidade do Ar interior dos Edificios, Decree-law 78/2006,

[3] CAMELO, S. Camelo, P dos Santos et al.: Regulamento das Caracteristicas de Comportamento Termico dos Edificios (RCCTE). Manual de apoio a aplicagao do RCCTE (in Portuguese).

Prior to the start of the 1998 Sacramento demonstration, individual fourteen tube modules were tested on Sandia National Laboratory’s two-axis tracking (AZTRAK) platform. See Winston et al [3]. These tests measured losses from the new ICPC collector operating at about 150C on a 35C summer day of only 120w/m2 of collector aperture.

1.2 Sacramento Demonstration

A 100 m2 336 Novel ICPC evacuated tube solar collector array has been in continuous operation at a demonstration project in Sacramento California since 1998. The evacuated collector tubes are based on a novel ICPC design that was developed by researchers at the University of Chicago and Colorado State University in 1993. The evacuated collector tubes were hand-fabricated from NEG Sun Tube components by a Chicago area manufacturer of glass vacuum products.

From 1998 through 2002 demonstration project ICPC solar collectors supplied heated pressurized 150C water to a double effect (2E) absorption chiller. The ICPC collector design operates as efficiently at 2E chiller temperatures (150C) as do more conventional collectors at much lower temperatures. This new collector made it possible to produce cooling with a 2E chiller using a collector field that is about half the size of that required for a single effect (1E) absorption chiller with the same cooling output. Data collection and analysis has continued to the present [5, 6, 7]

Fig. 1: 1998 Daily Collection Performance for Fig. 2: 1998 Daily Collection Performance for

Operation at 90 to 110C Collector to Ambient Operation at 110 to 130C Collector to Ambient

Temperature Differences. Temperature Differences.

As can be seen in Fig. 1 and 2, the non­tracking ICPC evacuated solar collector array provided daily solar collection efficiencies (based on the total solar energy falling on the collector) approaching fifty percent and instantaneous collection efficiencies of

about 60 percent at the 140C to 160C collector operating temperature range. Daily chiller COPs of about 1.1 were achieved. The ICPC array has recently been operating at the lower temperatures to drive a single effect absorption chiller. The ICPC array has provided daily solar collection efficiencies approaching

fifty-five percent at the 80C to 100C collector operating temperature range.

Maria Isabel Abreu1* and Rui Oliveira2

1 Polytechnic Institute of Bragan^a, Campus de Sta Apolonia, Apartado 1134, 5301-875 Bragan^a, Portugal
* Corresponding Author, isabreu@ipb. pt

Abstract

The use of solar energy constitutes a great concern of national and international bodies, as a result of a strategic policy towards green energy consumption. The Portuguese regulations on building thermal behaviour and energy efficiency, recently enacted by the Portuguese Government, in line with the European Union Directive 2002/91/CE, have introduced the obligatory use of solar collector technology for hot water production applied to new building projects and to some important retrofit works. To cope with the prescriptions of these regulations, the solar technology market is been growing and the Portuguese project designers and construction professionals are founding new challenges. The purpose of this study is to identify and analyse the major problems and initial impacts of the obligatory implementation of solar collector technology in buildings and provide contributions to improve solar energy use in the future. The results show that, as almost all new technology implementation, some obstacles have always to be faced initially.

Keywords: Solar Collector, Solar DWH, Building Thermal Regulations, Portugal.

1. Introduction

In residential buildings the energy for hot water production (DHW) contributes strongly to the final building consumptions of useful energy [1,2]. In the beginning of this decade, the Portuguese Program E4 — Energy Efficiency and Endogenous Energies have proposed an ambitious goal of 1 million m2 of solar collector area in Portugal until 2010. To cope with this, it was implemented in 2001 by Portuguese Government the National Program Solar Hot Water for Portugal (IP-AQSpP) [3]. A more recent government effort is The National Plan for de Energy Efficiency (PNAEE), approved some months ago, that include important measures for buildings, ambitioning an improvement of 1% per year on energy efficiency [4]. The Portuguese new Thermal Regulations (RCCTE) opens to all constructions partners a new opportunity for implementing more strongly renewable energy technologies in buildings. Using solar hot water systems it can represent a saving of 20% on the total energy consume of a family in electricity and gas [3].

2. Methodology

The methodology used on this study consisted of a literature review on government and institutions publications and information, statistics and also interviews to professionals. Secondly, to complement the analysis, it was made simulations with the regulations methodology and with the official software for solar collectors, Solterm, with the consequent discussion of results.

The criteria are used to measure the performance of the different failure detection approaches. The selection is very important, since the result will be different, if not all relevant criteria are included.

As most of the methods are still in development, a qualitative evaluation is applied. The criteria are as follows:

• Automatic Failure detection included?

• Accuracy/effectiveness of failure detection

• Automatic Failure identification included?

• Accuracy/effectiveness of failure identification

• Costs (operational and hardware)

3. Overview of Failure Detection Methods

The methods for failure detection will be described in the next sections. Table 3.1 lists some characteristics of the different methods, like what time scale and for what type of systems they are applied to.

key figures were studied. These are compared to the values determined in the planning phase, which are based on the irradiation and temperature profile of a typical reference year [2].

The optimisation phase was very effective regarding failure detection, however, it is time-consuming and therefore costly. The routine supervision phase is not that time-consuming, but does not deliver a quick feedback if the system is working properly and it does not locate failures.

The idea is to mount a tracker, the thermostat and the complete data acquisition equipment on a carriage on tracks, which allows the collectors on the testing surface to be tested outdoors under natural conditions as well as indoors under the solar simulator. The tracker’s testing surface is 2.5 x 5 m: space enough for four standard solar collectors.

The hydraulic system and the data acquisition equipment are such that all four collectors can be tested simultaneously outdoors. However, as the indoor testing surface is limited to 2 x 2.2 m, collectors under the solar simulator can only be tested consecutively.

The thermostat for parallel testing of up to four collectors, on a movable frame, was specially developed by TUV Immissionsschutz und Energiesysteme GmbH and Lauda GmbH for this application. In this unique design, the cold production, tempering and data acquisition are combined in one unit, which means that only the electrical connection and one network cable need to be disconnected for a change of position. To function smoothly under the Libyan climate conditions, both the control cabinet with the data acquisition equipment and controls and the tempering unit cabinet are air-conditioned.

The moveable frame is a modification of a trolley such as is used in the cement industry to move heavy loads. measX used the measurement system Almemo® by Ahlborn for the data acquisition. This system controls the thermostat as well as the data acquisition and can, for example, set the temperature test values.

To prevent the collector tests from being distorted by the heat radiation between the collector and the solar simulator’s hot metal halide lamps, a cold artificial sky is used. For the first time, this artificial sky, which consists of two parallel non-reflective, low-iron panes of glass, between which chilled air is circulated, was mounted directly on the lamp field.

The CSES test facilities conform to all current international testing standards in the field of solar thermal and PV modules. For solar thermal, this includes the ISO 9806 set of norms for collectors as well as the ISO 9459 set of norms for systems. There is also a testing facility for solar storage according to the European Technical Specification EN TS 12977-3 (there is currently no corresponding international norm).

In addition, the design includes all necessary test facilities to fulfill the requirements of ISO 9806-2 for serviceability tests of solar thermal liquid collectors. Besides smaller testing equipment, this includes a stand for testing performance in rain, shock and exposure conditions.

For long-term management of the operation with local personnel, BEG makes sure that the necessary know-how is communicated and offers training workshops in Germany and Libya. Other project partners are also involved in these training sessions.

The complete facility was installed in Libya at the end of 2007.

As for determination of daily system performance, the system shall be allowed to operate for 12 h, from 6 h before solar noon until 6 h after solar noon. At 6 h after solar noon the collector shall be shielded, and water drawn off from the store at a constant flow rate of 600 lit/hr. The cold make-up water shall be at the temperature tmain defined during the preconditioning of the system. The temperature of the water being drawn off (td) shall be measured at least every 15 s and an average value recorded at least every time a tenth of the tank volume is drawn off. A volume of water equal to three times the tank volume shall be drawn off. If the temperature difference between the water drawn off and the cold water entering the store is greater than 1 K after three tank volumes, then the draw-off shall be continued until the temperature difference is less than 1K. And especially during the draw-off,

the temperature of the cold water entering the storage tank shall not fluctuate by more than ±0.25 K

and shall not drift by more than 0.2 K during the draw-off period. The flow rate during the draw-off of hot water from the store is very important, and can greatly influence the draw-off temperature profile. The flow controller must therefore maintain a constant flow rate through the storage vessel at 600lit/hr

±50lit/hr.

Figs. 5-7 present the main experimental results gained by the automatic system. Daily draw-off performance, inlet temperature and flow rate profiles can be seen in Fig. 5. And the Fig. 6 shows that the temperature difference between outlet temperature and inlet temperature of system is suitable to the requirements of the standard. From Fig. 7 the fluctuation and drift of the temperature of the entering the storage tank during the preconditioning period and drawn-off period separately can be seen.

The aim was to develop a procedure with which it is possible to extrapolate the performance test results obtained by the test of only one system to other systems of the same product line. For the validation of the tool, different thermosiphon DHW systems of one product line have been tested at three different test laboratories (INETI — Instituto Nacional de Tecnologia Industrial, Lissabon, Portugal; CSTB — Centre Scientifique et Technique du Batiment, Sophia Antipolis, France; ITW, University of Stuttgart, Germany). The thermal performance data obtained by the tests of the systems and determined by the calculation tool have been compared.

2.1. Setting up a mathematical model

The mathematical model establishes a relationship between the solar fraction fsol of a solar domestic hot water system, the collector aperture area (Ac) and the store volume (Vsto),

fsol = f(A, vsto) (1)

To identify the influence of the collector size and the storage volume on the solar fraction, a product line of thermosiphon systems with the store on the roof and without auxiliary heating was modelled with the simulation software TRNSYS. The collector aperture area of the systems varies between 2 m2 and 8 m2 in steps of 1 m2 and the storage tank volumes varies between 0.2 m3 and 0.6 m3 in steps of 0.1 m3.

In figure 1 the values of the solar fraction are plotted depending on the collector aperture area and the storage tank volume. It can be seen that the values describe a surface in the three­dimensional space.

The surface fSol = f (Ac, Vsto ) may be described with a second order polynomial,

Fig 1. Calculated solar fraction fsol in dependency of the collector aperture area Ac and the storage tank volume Vsto

fsol(Ac, VSto) = a1 + a2 ■ Ac + a3 ■ Ac + a4 • VSto + a5 • Ac ‘VS

+ a6 ■ Ac 2 ■ Fsto + a7 ■ Ac ■ VsJ + a8 ■ Vsto

where the values of a1…a8 can be determined by regression analysis. With equation (2) it is possible to calculate the solar fraction of systems with arbitrary sizes of storage tank and collector aperture area for one product line.

For each specific product line, a new function fsol = f (Ac, Vsto) has to be determined. As the

simulations necessary to derive the function are quite complex and time-consuming it is not reasonable to perform this calculation for every product line of hot water systems on the market. Hence it was necessary to find a different approach where the effects of collector aperture size and storage tank volume on the solar fraction can be calculated or at least estimated more easily.

In addition to collector aperture area and storage tank volume, there are further parameters which have an impact on the solar fraction of a thermosiphon system. The main influence parameters are:

• Geographic position (Solar radiation, mean ambient temperature)

• Performance of the thermal collector (e. g. collector efficiency)

• Performance of the storage tank (e. g. heat losses)

In order to take these different effects into account for a variety of thermosiphon systems, the main influence parameters will be classified. An example of the classification is given in Table 1.

In Figure 2, three surface plots

corresponding to equation (2) offsoi are depicted for different collector types or efficiencies, respectively. The storage volume has been kept constant in this case.

/

Wolfgang Striewe*, Jan Steinmetz, Thomas Biel, Daniel Volker,

Korbinian Kramer, Stefan Mehnert, Matthias Rommel

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

Introduction

Factory made and pre-assembled solar domestic hot water (SDHW) systems may provide a range of advantages in comparison with individually constructed systems. It is observed that the market share of these systems increases.

In the course of this trend the quality assurance of the products becomes more important. It is only profitable for the customer to install a SDHW system if it provides high solar gains over a long service and life-time.

Testing of these systems is done according to the testing standard EN 12976 at Fraunhofer ISE to ensure the quality of SDHW systems. It is the European testing standard for factory made thermal solar systems. Testing according to this standard is also part of the requirements to receive the Solar Keymark quality label.

It includes an outdoor test for the experimental determination of system parameters. These parameters are used to make a performance prediction by means of a computer simulation. Furthermore, several quality tests are carried out. These include the testing of the stagnation behavior of the SDHW system and other tests.