Category Archives: Particle Image Velocimetry (PIV)

LCA methodology

A Life Cycle Assessment study has been carried out at the Department of Mechanical and Aeronautical Engineering of the University of Rome "La Sapienza”, using SimaPro 5.1 software. Since each modification of the Pv system (glazed covering, heat recovery) on one side leads to a higher energy output, but, on the other side, it requires new components and materials with their energy content, the main aim of the LCA study is to investigate the effectiveness of these modifications. The energy and environmental impacts and savings of all the systems have been assessed by means of two aggregate indicators: Global Warming Potential at 100 years (GWP100) and consumption of Primary Energy Resources (PER). The characterization factors for both the indicators are taken from “Eco-indicator 95” method, implemented in the database of SimaPro 5.1 software. We focused our attention on these values because of their relevance and importance in environmental and energy saving strategies.

The comparison of the results and the effectiveness of the modification have been evaluated using two pay back time parameters: the Energy Pay Back Time (EPBT) and the CO2 Pay Back Time (CO2 PBT). As a matter of fact, by producing clean energy during their operation, PV and PVT systems avoid the Primary Energy Resources consumption and the CO2 emissions related to conventional energy sources. The PBT parameters are the outputs of an environmental cost — benefit analysis and they estimate the time period needed for the benefits obtained in the use phase to equal the impacts related to the whole life cycle of the analyzed systems. Only after those periods the real environmental benefit starts.

Table 3. Amounts of materials used in PV and PVT systems (3 kWp, 30 m2)

PV AND PV/T SYSTEM COMPONENT

Sub-component and material

Amount

(kg)

Multi-crystalline silicon photovoltaic module

PV cells (including cell contacts)

21

glazed covering (low iron glass)

225

lamination material (ethylen vinyl acetate)

39

aluminium frame

45

Electrical

Balance Of System

steel

10

copper

6

plastic (Poly Vinyl Chloride, PVC)

4

Reflectors

aluminium (diffuse reflector material)

90

galvanized iron (for reflector installation)

90

thermal insulation (polyurethane)

30

collector frame (aluminum profiles)

120

collector back cover (aluminium sheet)

30

Heat Recovery Unit

ONLY FOR PVT/TFMS SYSTEMS (aluminium sheet)

9

ONLY FOR GLAZED HRU: glazed covering (low iron glass)

375

ONLY FOR GLAZED HRU: additional collector frame for glazed covering (aluminium)

45

galvanized iron rods (support structure for horizontal roof)

120

Mechanical Balance Of System

galvanized iron rods (support structure for tilted roof)

90

aluminium (support structure for tilted roof)

30

pipes for air circulation (galvanized iron)

60

(support structure and air circuit)

copper

9

Fan for air circulation steel

12

Plastic (PVC)

3

heat exchanger (copper)

60

In the paper, the EPBT and CO2 PBT values for the analysed systems are given, considering the final use energy output as electricity for standard PV modules and as electricity and gas as heat for the suggested hybrid PVT systems. Life Cycle Assessment (LCA) methodology aims at assessing the potential environmental impacts of a product or a service during its whole life cycle, according to ISO 14040 international standards (1997). The performed study focuses on the life cycle of a 3 kWp PV (or PV/T) system, with an overall active surface of 30 m2. All the analyzed systems were modeled taking into account the following sub-parts: multi-crystalline silicon (mc-Si) PV modules; mechanical Balance Of System (BOS); electrical BOS (inverter and cables); PV module support structures for both horizontal and tilted roof installation; Heat Recovery Unit (HRU), with or without additional glazed covering, and, finally, the air circulation circuit. Consistently with the LCA approach, for all the listed components, the environmental indicators were calculated, from raw material extraction to end of life disposal. Table 3 summarizes, for each component, the type and the amount of material for the 3 kWp reference system.

The effect of collector slope on optical efficiency

The aim of this investigation was to observe the effect that collector slope has on the optical efficiency and corresponding efficiency curves for an evacuated tube collector.

During this study the irradiance from the solar simulator was always normal to the absorber surface as illustrated in Figure 5a. This was purely an attempt to monitor the effect of collector slope and not the relative characteristic dealing with incidence angle of irradiance for a fixed collector angle. The irradiance power density was 800 ± 50 Wm-2, the mass flow-rate was held at 0.07 kgs-1 throughout the experiments, the slope p was varied between 0° and 60° with respect to the horizontal plane.

The observations showed that the optical efficiency remained constant with collector slope as summarised in the table below Figure 5b, the average value of the optical efficiency was 0.83 ± 0.02 over the range. The a1 value of linearly dependent collector losses were found to decrease by ~9% whereas the less influential square dependences terms a2 where found increase by 750%, with increasing slope. The raw data efficiency curves were corrected to an irradiance of 800 Wm-2 and presented according to standard EN12975-2 as seen in Figures 5 c and d. The overall result showed that increasing the slope was found to have a beneficial effect on collector performance. This improvement was assumed to be due to the fact that an increased slope will promote thermosyphonic mechanisms within the collector tube.

Further investigation at a collector slope of 90° was not carried out due to a technical problem with the solar simulator, this data set will be completed at a later date.

Figure 5 [a] Schematic of the experimental set-up, [b] summary table of the optical efficiencies and corresponding loss coefficients for different slopes, [c] plot of the raw data efficiency curves at 800Wm-2 and [d] plot of the standardised efficiency curves at 800Wm-2

The technical concept of AndaSol

The basic principle of a solar thermal plant is to convert primary solar energy into electricity for homes, business and industry by means of a collector field, steam turbine and electric generator. The solar field consists of parallel loops of parabolic trough collectors. The new parabolic SKAL-ET collectors, developed and qualified by the Solar Millennium group and its partners for the AndaSol projects, track the sun from East to West in order to reflect and concentrate the direct solar radiation about eighty times onto the absorber tubes installed in the focal line of the reflecting surface. An absorber tube consists of a stainless steel tube with a selective coating that is covered by a glass envelope tube to reduce thermal losses. The annular space between absorber tube and glass tube is evacuated. Through the absorber tubes circulates a heat transfer fluid (HTF), normally synthetic oil, which is heated by the concentrated solar radiation up to a temperature of almost 400 °C.

In the direct operation mode, the HTF is circulated through the solar field (see Figure 2) where it is heated and supplied through a main header to the heat exchangers located in the power block, where superheated steam is produced at a temperature of 370 °C and a pressure of 100 bar. After passing through the HTF side of the heat exchangers, the cooled HTF is then re-circulated through the solar field to repeat the process. In this way, the HTF fluid acts as the heat transfer medium between the solar field and the power block of the steam cycle, heating up in the solar collectors and cooling down while producing steam for the steam generator. The superheated steam is then fed to the high-pressure
casing of a conventional steam reheat turbine. The steam is reheated before being fed to the low-pressure casing. The exiting steam from the turbine is condensed in a conventional steam condenser and returned to the heat exchangers via condensate and feed-water pumps to be transformed back into steam. With this process, the collected and concentrated solar radiation from the solar field is converted into electricity and afterwards fed to the general power supply.

To extend the operation of the AndaSol solar power plant beyond sunshine hours a thermal energy storage will be integrated into the plant design. Solar energy collected by the solar field during the day will be stored in the storage system and can be dispatched after sunset. The medium, which stores the thermal energy, is a molten salt of similar type like used as agricultural fertilizers. To charge the storage the salt will be heated up to about 385°C. To discharge the system the salt is cooled down again to about 295°C. At both temperatures the salt is still liquid. Cold and hot salt are stored in separate tanks.

In the AndaSol configuration, HTF from the solar field is diverted through a heat exchanger that is used to charge the thermal storage system. For charging, cold salt is pumped from the cold tank to the hot tank passing the salt-to-oil heat exchanger. In the heat exchanger the salt is heated up to the hot tank temperature. During night, the storage is discharged through the same oil-to-salt heat exchanger by changing the flow direction of the salt. The salt is pumped from the hot to the cold tank and the stored heat is transferred to the oil via the heat exchanger. The cooled salt is returned to the cold storage tank. The heated oil is fed to the steam generator to generate high-pressure steam. The storage system of the first two AndaSol projects is designed to store approximately 880 MWht each, of thermal energy. This is enough to operate the plants for 6 hours at full load during non-sunshine hours.

The principle of the AndaSol solar field layout is illustrated in Figure 3: Two SKAL-ET150 collectors with a length of 150 m are connected ign series to form a row running in a north — south direction. Two adjacent rows are connected via a cross over pipe to a loop, consisting of four SKAL-ET150 collectors. The cold heat transfer fluid is going from the power block to the cold header pipe and then entering the parallel loops. The fluid is heated up in the loops and then going back to the power block through the hot header. Each AndaSol plant has a reflecting surface of approximately 500.000 m2 made up by 624 SKAL-ET 150 collectors in 156 loops. Altogether, the solar field of one AndaSol project covers approximately 200 hectars of flat land; 1500 m in east-west direction and 1300 m in north-south direction. All SKAL-ET collectors of the AndaSol-1 & 2 solar fields are controlled from the control room by a central computer. It starts up the solar field in the morning according to weather and availability conditions and stows it for the night and during strong winds.

A Small PV-Module for 3.6 kW(thermal) Stand-alone. Solar Oven Tracking System

J. Antonio Urbano Castelan*#, Yasuhiro Matsumoto * y Rene Asomoza P.* # #

Alexei Martinez Munoz#, German Escoto Mora, Alfonso Sotelo Trujillo#

Francisco J. Aceves Hernandez y Antonio Jacome Rodriguez#
*Section of the Solid State Electronics, Electric Engineering Department CINVESTAV-IPN
P O Box. 14 -740 Mexico D. F. C. P.07360 Tel (52) 55 5061-3783, Fax (52) 55 5747-7114

e-mail: jurbano@mail. cinvestav. mx

# SEPI-ESIME and ICE-ESIME-IPN Professional Unit "Adolfo Lopez Mateos"
Zacatenco, Lindavista C. P.07738 Mexico D. F.

ABSTRACT

This article shows how 3.6 kWTH solar oven concentrating system is operated by using only 5 watt-peak PV-module for its tracking system. The solar oven is autonomous and designed for Marfas Islands, Mexican rural area for food cooking, sea water distillation and medical instrumental sterilization.

The PV-module charges continuously energy in a capacitive storage to move two 12 DCV motors of 36W each. These electric motors adjusts azimuth and altitude depending of solar position determined by an electronic optical sensor for an optimum concentration position. There have been made two versions for this purpose, the first one consists on digital states (on/off) for light and dark conditions over a “Greek cross”, and the second one provides redundancy because it has two detection elements besides it is possible to have coarse, medium and fine detection.

The solar oven system of 360 mirrors (10X10cm2 each) achieves temperature of about 300°C at its oil-container. This oil-container transmits the heat directly to the commercial kettle, maintaining 120°C at the pressure of »1.05kg/cm2. Maria s Islands has an average of 340 sunny days in a year.

We expect a contribution for forest conservation avoiding firewood consumption in the near future. Foregoing Mexican rural area has great potential in solar resources for their inhabitant needs. Food cooking, water distillations are most important daily-life activities. In Mexico, solar cooking oven has been introduced since 1955, however, this cooking technique was not assimilated due to the local people’s social and cultural aspects.

This development is intended to be applied in those rural areas in Mexican Republic because there are 28 million people that still use firewood leading to diverse problems such as health pulmonary emphysema, body burnings, deforestation, and CO2 emissions. It has been estimated that one individual consumes about 1.2 tons of firewood [5]; an additional reason to promote this use is due to high solar radiation over semi-desertic and desertic areas, which represent about 3/4 of the Mexico’s territory.

INTRODUCTION

In 1955 the Wisconsin University donated 30 solar ovens to the northern states of Coahuila and Nuevo Leon in Mexico, they were parasol inverted type with aluminum painting and parabolic bend, this is shown in the Fig 1, [1]. One week later it was thought that it would be the most important contribution from an American University to the developing countries; one month later the ovens were abandoned. The Rockefeller

Foundation, worried by this result, sent Technicians and Sociologists to find out the possible causes of the failure. They were sent after identical ovens were applied parallely in American Indian communities. They had the following conclusions and detected disadvantages:

1. The social — cultural aspect and feeding habits, were the decisive facts, since the ancestral tradition of cooking inside the house beside the bonfire was broken. Cooking outdoors and under the sunbeams broke the millennial culture of their ancestors. On the other hand, in the indigenous areas it is used to prepare meals at a very early hour of the day so people return by the afternoon or night to have dinner, both conditions are when the solar resource is no longer present.

2. The housewife was forced to become and Astronomer since she had to distract her activities to locate the oven every certain time since the Sun evolves 15° per hour on the sky: she had to check the angles of altitude and azimuth of the Sun’s path.

3. The unwanted gleams of the oven due light beams outside of they focal condition hurted the housewife’s eyes leading an obvious discouragement to continue using the oven.

4. In those countries located within the tropical area (between the tropics of Cancer and Capricorn), the users are forced to use the oven in an horizontal position during certain times of the year, it caused that the bottom of the parasol got dirty by drippings produced while the product was being cooked.

5. The parabolic type solar oven, hinders its own use when the Sun is in the zenith and the diameter of it surpasses 1.5 meters.

6. The fragility of the oven structure (parasol parabolic type) let the wind sweep it easily, so the oven were found in the neighbor’s house damaged by the air.

7. The absence of the Sun, affects directly the cooking process, since this concentration system operates with the direct component of the Sun’s light.

OBJECTIVE

To design, to build and to test a small PV-Module tracking system for a 3.6kWTH stand­alone solar oven. The target objective was to build a simple, economical and robust solar oven for Mexican rural area Here, we will focus attention to its mechanical, thermal efficiency and tracking system aspects. The outcome is shown in FIG 2 and FIG. 3.

Economic Analysis

For the economic analysis an emerging market for solar tower plants is assumed with concentrator costs of 132 €/m2 for a 120 m2 glass-metal heliostat. The receiver costs are 16 k€/m2, 33 k€/m2 and 37.5 k€/m2 for the low, medium and high temperature receiver.

The investment costs for the conventional part (power block incl. BOP) are 560 €/kW for the Mercury system and 510 €/kW for the PGT10 Combined Cycle system.

For the detailed cost calculations a 2nd generation plant is considered. The engineering cost for gas turbine solarisation are shared between 10 similar plants, and the plant is operated remotely in a ‘virtual park’ of 4 similar plants with a high degree of automation. For the calculation of levelized electricity costs (LEC) a debt rate of 75% at 4.2% interest rate is assumed. Other financial parameters are: common equity rate of 25% at 14% rate of return, 2.5% inflation rate, 12 years debt payback time, 20 years plant lifetime.

The results of the LEC calculations for the stand-alone option can be found in [9]. The total LEC range from about 6.3 €cent/kWh to 20 €cent/kWh depending on power level, capacity factor and solar share. The solar incremental LEC are calculated as the additional costs compared to the generation at a market price of 4 €cent/kWh divided by the incremental solar generated electricity. They range from about 12.8 €cent/kWh to 41.4 €cent/kWh for the 2nd generation plants in daytime operation (Table 2).. For the largest system a total LEC of 8.6 €cent/kWh at a solar share above 50% is calculated.

power system

Mercury-50 800°C

PGT10 800°C

PGT10 1000°C

ISO rating

[MW]

4.2

16.1

16.1

annual DNI

[kWh/m2]

2791

2791

2791

receiver therm. power at DP

[MW]

3.5

18.5

28.59

solar fraction at DP

38.3%

57.8%

87.8%

total refl. area

[m2]

7038

37615

62733

receiver aperture

[m2]

12

55

82

optical tower height

[m]

50

100

130

capacity factor

54.0%

54.0%

54.0%

net. electric energy

[MWh]

17170

66294

62738

solar incremental electricity

[MWh]

3060

20511

32914

incremental solar share

17.8%

30.9%

52.5%

incremental CO2 avoidance

[t/a]

1680

9537

15304

incr. solar to electric efficiency

15.6%

19.5%

18.8%

total investment costs

[T€]

6302

20421

26023

thereof solar equipment

33.8%

37.2%

47.1%

spec. investment costs

[€/kWe]

1801

1438

1859

fixed O&M costs

[T€/a]

725

1572

1887

thereof personal expenses

33.5%

47.2%

49.7%

fuel costs

[€/MWh LHV]

13.43

13.43

13.43

levelised electricity cost (LEC)

[€/kWh]

0.1066

0.0764

0.0860

CO2 avoidance cost

[€/kg]

0.6804

0.2531

0.1884

solar incremental LEC

[€/kWh]

0.4135

0.1577

0.1276

Table 2: Summary of performance calculation and cost analysis for daytime operation

Relating the solar LEC to the annual amount of CO2 that can be avoided3 when operating the hybrid plant instead of the pure fossil reference plant, the CO2-avoidance costs can be calculated for each individual plant. For the higher power level a value well below 200 €/ton CO2 can be reached.

a CO2 production from natural gas of 200 kg/MWh LHV is assumed.

Development and Experimental Results of. Thermal Energy Storage Technologies for. Parabolic Trough Power Plants

Doerte Laing, Wolf-Dieter Steinmann, Rainer Tamme, Christoph Richter DLR — German Aerospace Centre — Institute of Technical Thermodynamics

Introduction

Economic thermal storage is a technological key issue for the future success of solar thermal power plant technologies. Storage technology becomes an urgent issue for the successful implementation of commercial solar thermal plants in the context of an accelerated promotion of such plants in those EU member countries, which are relying on solar power to reach the community goal of doubling the share of renewable energy in the EU energy balance from 6 % today to 12 % in 2010.

Without storage, a solar plant in Southern Europe will operate only 800 hours in real full load and over 2400 hours in part load, yielding about 2000 equivalent full load hours or a solar fraction of about 25% in a base load solar steam plant. With 5-6 hours of storage capacity, a solar plant in Southern Europe will operate about 4000 hours in real full load and reduce its part load operation to 400 hours. Due to improved part load efficiency and doubled utilisation of the power block, solar thermal production costs can be reduced with storage by up to 20%. Larger storage capacities may lead to further reduced costs.

At DLR different storage concepts are developed for application in commercial parabolic trough power plants:

> solid media sensible heat storage for parabolic trough power plants using thermal oil as heat transfer medium in the solar collectors

> phase change energy storage systems for direct steam generation in parabolic troughs

> steam accumulators as buffer storage system for compensation of fast transients in parabolic trough power plants

The temperature range for these storage concepts is between 150°C and 450°C.

Solid media sensible heat storage for thermal oil heat transfer media

Solid media sensible heat storage units have been developed in the project WESPE [1], funded by the German Government from December 2001 until December 2003. The focus of this project lay on the development of an efficient and cheap sensible storage material, on the optimization of the tube register heat exchanger and on the demonstration of this technology with a 350 kWh test unit.

Results and discussions

2.1 Effect of temperature

Fig.3 shows the effect of temperature at 20C, 30C, 45C and 60C when the airflow velocity is 1.35m/s.

From Fig3a, it is seen from four curves that evaporation process is quickened rapidly with the increase of the temperature. Fig3b gives the result that the evaporation velocities fluctuate mainly between 0.40×10 kg sm and 1.27×10 kg sm at 20C, and reaching

0. 80-7.00×10-4kg s-1m-2 at 60C, which is approximately 7 times that at 20C. Since solar collector can heighten the temperature of seawater because of the greenhouse effects, and the results show that temperature is a very important factor to evaporation process, using solar chimney for drying is beneficial to the evaporation process.

2.2 Effect of airflow velocity

Fig.4. shows the effect of airflow velocity when the temperature is 30C. The velocities were 0.52m/s, 1.39m/s,1.60m/s and 1.78m/s.

Clearly, evaporation process increases with the higher wind velocity (From 0.52m/s to 1.60m/s), but it declines when wind velocity reaches 1.78m/s. During the experiments, we find that there is a layer of salt covered the surface of seawater, which hinders the mass transfer from the main body of seawater to the air because of the strong evaporation enhancement. So the design of solar chimney dryer should limit the wind velocity to a proper range. The drive for evaporation is humidity gradients between the body of seawater and the air just above the surface of seawater. So we can enhance the humidity grads by using solar chimney.

Operation and Maintenance issues

Industrial turbines for saturated steam are not designed for daily start-up and shut­down, because this kind of operation would reduce its life time considerably and would require often revisions and costly maintenance. Manufacturers recommend to operate the turbine at low load conditions overnight by means of a fossil-fired back-up boiler or thermal energy storage. So, the use of saturated steam turbines in a solar plant demands the implementation of costly maintenance procedures to assure a good performance and durability. However, the extra maintenance cost required by a saturated steam turbine can not be quantified in general, because the service conditions imposed by every manufacturer are different. This can be done only on a case by case basis. What can be clearly stated is that the O&M cost of a saturated steam turbine is higher.

Concerning operation requirements, saturated steam turbines seems to be less flexible than superheated steam ones, because great changes in the steam parameters can have a more dramatic effect. Nevertheless, the water/steam separator at the interface between the solar field and steam turbine brings some benefits. If the volume of this separator is properly designed, it can also act as thermal energy storage if the turbine is operated with gliding pressure. The amount of saturated water and steam inside this vessel can feed the turbine with saturated steam at gliding pressure for a few minutes, thus overcoming short cloud transients. This is a very important advantage of the saturated steam option.

Summarizing, when O&M issues are considered, DSG solar power plants with saturated steam have advantages and disadvantages when compared to the superheated steam option. However, an economic assessment of these advantages and disadvantages is still unfeasible because of the lack of experience with commercial DSG power plants. Though a theoretical study could be performed, the wide range of uncertainties provoked by the lack of experience very much limit the accuracy of results obtained from such a study.

Conclusion

According to the investigation performed the saturated steam option has a 4 % higher net yearly electricity production of the power plant. On the other hand for the plant size analysed, the initial investment required by the saturated steam option is of about 5% higher than for the superheated steam option. Though its maintenance cost is higher, the saturated steam option requires a less complex solar field and the water/steam separator located at the power block inlet can act as a thermal energy storage, which can feed the steam turbine with saturated steam at gliding pressure for few minutes, thus overcoming short cloudy periods. Further advantages of the saturated steam option are the lower complexity of the solar field and the possibility to use simpler collector options that are able to operate with good efficiency at 260-300°C.

Due to the lack of experience with commercial DSG plants, a complete economic and technical assessment of both options including all the aspects (yearly performance, initial investment, operation and maintenance) is still unfeasible until the first DSG solar power plants are installed and deliver accurate information. Therefore the results presented have to be regarded as preliminary but nevertheless the saturated steam DSG plant seems to be an interesting option for near term application in the lower capacity range.

References

[1] Eck M., Zarza E., Eickhoff M., Rheinlander J., Valenzuela L.: Applied research concerning the direct steam generation in parabolic troughs’, Solar Energy, Vol. 74, No. 4 , April 2003 , pp. 341-351

Energy savings and Pay Back Time results

In spite of the material and energy requirement needed to produce and install PV and PVT systems (and to treat their components at the end of their technical life), during their operation, they produce clean electricity and heat, thereby displacing conventional energy. Therefore, environmental benefits due to avoided environmental impacts are associated to the system operation phase. We used the data from electricity and heat production by PV and PVT systems (Table 2) to achieve the following, assuming that PV systems electrical output displaces conventional grid electricity considering the European average electricity mix for the calculation. Basing on system energy output and on displaced conventional sources, the "environmental cost” of the systems, in a life cycle perspective, can be matched with the "environmental savings” obtained thanks to their clean operation phase. The values of the energy and CO2 Pay Back Times may be calculated, representing the time period needed for the benefits obtained in the use phase to equal the impacts related to the whole life cycle of the analyzed systems and are summarized in Table 4.

Regarding PBT values, it should be noted that they are, in any case, considerably lower than the expected lifespan of the systems. From these results we observe that the highest PBT values are about 3 years and 3 months, while PV systems lifespan could be assumed to be nearly one order of magnitude higher. As a matter of fact, the most conservative assessments (Kato et al,1998) indicate expected life periods of 15П25 years, while other sources (Travaglini et al, 2000), thanks to aging tests conducted on operating plants, suggest a lifespan of more than 30 years. Besides, LCA results underline that the proposed improved configurations for PV systems (heat recovery by air cooling and TFMS modification) enable the energy output to be significantly increased. The higher energy production from improved PV systems and the consequent energy savings, overcome the increased impacts due to the additional system components (HRU). Thus, the proposed configurations show lower values for the PBTs. Additionally, the heat production compensates the impacts due to the HRU. When the HRU of the PVT system is equipped with a glazed covering, though, the increase in thermal energy production allows a considerable lowering of the PBT values.

Concluding, the use of a glazed covering lowers the electrical output because of the reflection and absorption from the glazing but, on the other side, thanks to the greenhouse effect inside the collector, the amount of heat recovered is widely increased and the result of this two opposite effects is positive, thereby achieving lower PBTs. It is noticeable that the better performance of the studied systems is achieved the more the thermal energy

demand is constant during the year, even though, as underlined in the previous parts of the paper, the "12 months air scenario” is somewhat ideal, since referred to strictly particular industrial cases. The most interesting scenario for domestic applications (in spite of the increased material requirement for the heat exchanger) is the combination of air and water heat recovering systems, that leads to lower the environmental PBT in all the analysed configurations.

Conclusions

Hybrid Photovoltaic/Thermal solar systems with air heat extraction were developed by University of Patras, aiming to the increase of the total efficiency of photovoltaics by providing simultaneously electrical and thermal output. We calculated the energy output for operation and the Energy Pay Back Time (EPBT) and CO2 Pay Back Time (CO2 PBT) of all studied systems, considering the corresponding materials of the horizontal and tilted building roof installation of systems. Estimating all together the extracted results we notice that the system that combines the higher total energy output with the lower values of EPBT and CO2PBT are the PVT/GL and the PVT/TFMS both considered in the configuration with reflectors. These systems can be used on horizontal or tilted building roofs, with better performance for the horizontal roofs. The mounting of the thin flat metallic sheet inside the air channel (TFMS modification) gives higher electrical and thermal output compared to the similar unglazed type of PVT/AIR systems. The addition of the booster diffuse reflectors is positive in all cases although the reduction of EPBT and CO2PBT is small. Concluding, the heat extraction from the PV modules results to cost effective solar devices, that are of positive performance regarding their environmental impact, compared to standard PV modules. The advantages of the hybrid PV/T solar systems makes them attractive for a wider application of photovoltaics, providing heat apart of electricity and increasing therefore the total efficiency of the converted solar radiation into useful energy.

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Heat loss mechanisms for an evacuated tube collector

Developments in absorber technology have generated selective coatings with high values of absorbance over the solar spectrum in parallel with low levels of emissivity in the infrared region. Using the method described by Bhowmik6 it was possible to show that absorber plate using black chrome coatings exhibit selectivity’s of ~10 whereas the newer so called ‘blue’ coatings exhibit improved selectivities of ~25 due to lower emissivity values. As a result radiation losses have been minimised for evacuated solar tubes in comparison to earlier absorber plate technologies. Also Chow et al7 showed that
conduction and convention loss mechanisms shutdown within evacuated tubes at pressures less than 1×10-3 mbar. The dominant mechanisms of heat loss for the evacuated collector systems are therefore due to convection and conduction in the area of the manifold and corresponding tube connections. The use of Infrared-imaging techniques can reveal this to be the case; Figure 6 shows a thermal map for a vertically mounted direct-flow collector system installed outdoors. The glass temperature of the evacuated tubes was observed to be 12.5 ± 2.5 °C whereas the temperature of the manifold cover indicated by the dashed box was observed to be 32.5 ± 2.5 °C. Modern insulations with low k-values in the order of 2×10-2 Wm-2K-1 can minimise these heat losses from the manifold, which is especially important in low flow systems.

2 Conclusion

It was reported that the optical efficiency of an evacuated tube collector of direct-flow design was stable with incident irradiance power densities. The effect of mass flow-rate on the optical efficiency was found to be significant. The low flow-rate penalty was found to decrease efficiency by up to 25% of the quasi-stable value at higher mass flows. Using a simple empirical technique thermal losses from the collector were calculated under these conditions where Tc was held at 3 K above the ambient temperature. These losses were shown to depend heavily on mass flow and incident irradiance. The influence of increasing collector slope on collector performance was found to be beneficial. Linear losses within the collector were found to decrease by 9% over the range of 0° to 60°, however square dependency losses were found to increase by 750%. However, square dependency losses have a minimal effect on the collector performance and therefore the overall result on increasing the slope was found to be favourable. Losses from the collector system were found to be concentrated around the manifold and the connections to the solar tubes. Losses within the evacuated tube were found to be minimal, radiation losses were dominant for pressures less than 1×10-3 mbar.

3 Further work

Plans to repeat this work for comparison with an evacuated heat-pipe collector system are in the pipeline. Also mathematical modelling of the collector and solar simulator using TRNSYS based set-up may be employed at a later stage for theoretical comparison with experimental results.

Acknowledgements

I would like to thank my colleagues at Thermomax R&D, David McClenaghan, Gareth McWha and Richard Pelan who helped me with the construction of Thermomax’s first solar simulator, thanks guys I couldn’t have done it without you.

Nomenclature

Symbol

Name

Units

a1

Linear dependency of heat loss coefficient

Wm-2K-1

a2

Square dependency of heat loss coefficient

Wm-2K-2

Ac

Collector Absorber Area

m-2

Cp

Fr

Specific Heat Heat Removal Factor

Jkg-1K-1

Gt

Total incident Irradiance

Wm-2

m

Mass flow-rate

kgs-1

Qu

Useful Gain

W

01

Ta

Ambient Temperature

°C

Tc

Collector Temperature

°C

Ti

Collector Inlet Temperature

°C

Ul

Coefficient of Heat Loss

Wm-2K-1

P

Collector slope

degs

ДТ

Temperature difference across the manifold

K

ATexp

Л0

Measured Temperature difference Optical efficiency

Transmittance-absorptance product

K

References

[1] Solar Energy — The State of the Art: Edited Jeffrey Gordon — Chapter 5 Solar Water Heating by G. L. Morrison: James and James (Science Publishers) Ltd, London, 2001

[2] EN 12975-2:2001 — Thermal solar systems and components — Solar collectors

[3] SRCC STANDARD 100 — Test methods and minimum standards for certifying solar collectors, 1995

[4] C. Muller-Scholl, S. Brunold, U. Frei / Proceedings EUROSUN 2002 Conference in Bologna Italy

[5] K. A.R Ismail, M. M. Abogderah: ASME Journal of Solar Energy Engineer Engineering, 120, 51-58, 1998

[6] N. C. Bhowmik, J. Rahman, M. A. Alam Khan, Z. H. Mazumder: Renewable Energy, 24, 663, 2001

[7] S. P. Chow, D. R. Mills and G. L. Harding: Solar Energy 31 (4), 433, 1983