2.1 Research on the over-wintering technology of fish in the seawater solar pond

The area of the solar pond is 6000 m2 and the gradient is in 1.8. The interior of the pond is trapezium-structured, in which there are three contrast districts. The water is 4m deep in district □, 3.5m deep in district □ and 3m deep in district □. See fig 4

During the process of designing and constructing the pond, optimizing and simulating the ecological environment suitable for fish in seawater is attached importance to. In the temperature-preserving area, trapezium-shaped structure is adopted to provide fish with the convenience and freedom to choose suitable Living space. It is because that a large quantity of poisonous gas is accumulated continuously in the high-temperature zone during the operation of the solar pond.

Bulldozers, digging machines and slurry pumps are combined to work to construct the solar pond. The underground depth of the pond is 3.5m and upground depth is 0.5m. The upper slope of the pond is protected by cement board. The treatment of leakage-prevention is not adopted at the bottom of the pond, thus the underground brine is adopted to be poured into the solar pond and the salt concentration is regulated and controlled at about 25%o. The depth of water poured in pond for the fist time should be suitable for raising and managing young aquatic products, then go on pouring in water to 3/4 of the depth of the pond and should maintain the water level. When water temperature drops to 15°C,(It happens during the first-ten-day period of November) People should pour fresh water into the pond of about 20cm to 30cm, then pay attention to the change in weather. Fresh water of 20cm to 30cm should be poured into the pond before the cold current or at the time of the cold current. The depth of the fresh water should reach 60cm to 80cm when December arrives.

The crux of the safe working of the seawater solar pond is to maintain the existence of salt gradient and to upkeep good physico-chemical, biological factors of the heat-preserving areas. Fresh water should be added timely once the solution of the surface mixes. In winter, the ice surface

should be kept clean and accumulated snow is strictly forbidden in order not to affect photosynthesis and the effects of integrating and preserving heat.

In order to study and explore the law of change in physico-chemical factors in the seawater solar pond, water temperature, salt concentration, dissolved hydrogen, PH and so on are monitored regularly. See graph

Form the winter of 2001 to the spring of 2002, the working conditions of the seawater solar pond is fine and the salt gradient is controlled well. The temperature reaches 10.0”C, which is 4.0”C higher than the lowest working temperature. It meets the demand of the over-wintering of cultivation objects in the seawater of warm water kind in the Northern Part of China.

However, the physico-chemical target of the lower convective zone of solar pond exceeds limits, especially, the dissolved bydrogen in the water drops to zero. In the year of 2002, several measures were taken to improve the content of the dissolved hydrogen and better the water quality at the bottom.1) Develop and install the underwater hydrogen-increase devices at the interior of the solar pond.2) Design water circulating plastic pipeline at the bottom of solar ponds. The problems of water changing foul and low dissolved hydrogen are solved successfully through the two above-mentioned measures and the output of the over-wintering fish achieves 250kg per mu.

The set of coupled partial differential equations and the boundary conditions described in the previous section are converted to algebraic equations by means of finite-volume techniques using rectangular meshes on a staggered arrangement. Diffusive terms at the boundaries of the control volumes are evaluated by means of second-order central differences, while the convective terms are approximated by means of the high-order SMART scheme [13], with a theoretical order of accuracy between 1 and 3.

The domain where the computations are performed and a schematic of the mesh adopted is shown in Fig. 1b. The mesh is represented by the parameter n. According to Fig.1b, 1.5n*n* n control volumes are used (for example, when the problem is solved on 30 * 20 * 20 control volumes, it means n = 20). The numerical solutions have been calculated adopting a global /г-refinement criterion. That is, all the numerical parameters (numerical scheme, numerical boundary conditions, etc.) are fixed, and the mesh is refined to yield a set of numerical solutions. This set of numerical solutions have been post-processed by means of a tool based on the Richardson extrapolation theory and on the concept of the Grid Convergence Index (GCI) [4][5]. When the numerical solutions are free of programming errors, convergence errors and round-off errors, the computational error is only due to the discretization. The tool processes a set of three consecutive solutions in the /і-refinement. The main output is an estimate of the uncertainty of the numerical solutions due to discretization, the. Only solutions with order of accuracy between 1 and 3 GCI less than 1percent and Richardson nodes higher than 60 percent are considered. The mesh is intensified near the two plates using a tanh-like function with a concentration factor of 2 [10], in order to properly solve the boundary layer. This aspect is indicated in the Fig. 1b with solid triangles at the boundaries intensified.

The resulting algebraic equation system was solved using SIMPLE algorithm [2], and the iterative convergence procedure was truncated when relative increments of the computed Nusselt number in the last 50 iterations dropped below 0.0001%.

Investment

The following costs have to be considered, [11] : Ci=2673 Euros for air solar collectors, C2=1203 Euros for water storage tank, C3=1093 Euros for Trombe wall, C4=890 Euros for solar still, C5=6500 Euros PV system. The total cost of the equipment is C’=12539 Euros.

Installing cost is roughly C’’=C’/4=3100 Euros.

Total cost (equipment + installing) is Csun=15639 Euros.

Investment payback

Taking into account the electricity price in Romania, cl=0.06Euro/kWh, in 2003, the total useful energy supplied by the hybrid systems would cost yearly CL=694.7 Euros/year. If we do not take into account the rate of interest, the investment Csun would be paid back in a time T=Csun/cl=24.9 years. The life time of the installations is roughly t’’=40 years, that means the solar energy user would benefit from free energy during т1=15 years.

If the state supported VAT, the costs of the suggested solar system for a small or medium sized stock raising farm would be only C’sun=12668 Euros. The investment payback time would be reduced to only t2=20 years. In this case the solar energy user will benefit from free energy during 20 years.

Conclusions

The suggested solar system satisfies the requirements of the possible users by:

• Indoor air heating at the required thermal level.

• Water heating

• Water distillation

• Air ventilation

• Space illumination

Small and medium sized stock raising farms could be organised by modules. Each module would have lairs of one hundred young animals each. A part of the modules would be experimental and another part would be reference modules. The modules organised like reference modules would be energy supplied by means of conventional tools. The experimental modules would be energy supplied both by conventional and RES tools. The monitoring of energy consumption and expenses based on RES and conventional energy would allow the users to choose the most convenient way for their specific activity. The new RES legislation initiated by the Government could facilitate the implementation of RES (e. g. by exemption from taxes).

Rainer Becker, Dr.-Ing. Christof Wittwer

Fraunhofer Institute for Solar Energy Systems
Heidenhofstr. 2, 79110 Freiburg, Germany
Tel.: +49 (0) 761 / 45 88-54 09, Fax: +49 (0) 761 /45 88-94 09
rainer. becker@ise. fraunhofer. de, http://www. ise. fraunhofer. de

Within this paper the potentialities of using networked embedded systems will be discussed. New embedded hardware with network connectivity allows remote administration and software updates of solar thermal system controllers via internet. System self analysis helps to minimize breakdown times by sending email and SMS to request maintenance staff.

Basic Hardware and Software

Small computers with network capability (embedded systems) allow to get software updates via network, which is used to control and to monitor the behaviour of the components of a solar thermal system. The open source operating system Linux is a good choice for such purposes. Internet functionality is part of the system, it runs very stable and no license fees have to be paid. Today many consumer products are driven by Linux and so prices for ready-to-run embedded systems or processor boards decrease very fast. Ethernet is used for networking. It is standard and there is a non-proprietary protocol. It is cheap, reliable and there are gateways to many other network technologies. Network infrastructure needed for monitoring, visualisation and data acquisition (Fig. 1) is often already available or can be obtained cheaply.

Embcddcd-Linux-Compuier 1

Webportal. Service Provider. Data lagging

Embedded-!.inux-Computer 2

Monitoring Client

Internet

tome PC

Fig. 1: Network infrastructure for visualisation and monitoring purposes

Two embedded systems are running successfully in the Solarthermie2000 solar thermal system at the student hostel Vauban, Freiburg. The system has been updated with two compact units during the ConCheck project (Fig. 2, Fig 3, [1],[2],[3]), which are controlled by the embedded systems.

Control

Conventional controller hardware cannot easily be provided with software updates. In contrast to this controllers, software of embedded systems can be updated via Internet. Using simulation software (e. g. ColSim [4]) control algorithms are improved at Fraunhofer ISE in an iterative process. The embedded system can communicate with other components of the heating system (e. g. gas burner, apartment operating unit, …) by using its network connectivity. For example the pump for loading the storage of the solar thermal system at the student hostel Vauban, Freiburg, is controlled by Mbus communication. Control strategies can be optimised by using solar irradiation prognoses and current data from the Internet. The data can also be used for energy gain calculations to check whether the system works properly or not.

In [1] a simulation of yearly performance of the ASD was presented. The modifications made to the model, would yield a different estimate. Below we attempt a new prediction of performance on a yearly basis, for present state ASD, even though we do not have the results from sunnier and warmer days. This estimate can be seen in Fig. 6, also showing what would be the production in a conventional open pond, with the same size.

These results show that the yearly performance of the prototype, as it is now, should be at least on the order of 2 times (evaporation enhancement factor) the performance of a
conventional salt works. The modifications discussed above should at least bring us to a performance, at least 6 times better than the conventional salt works solution, according to the result of a simulation for yearly operation of a lowered evaporation channel ASD (0.3 m channel height).

4. Conclusions

Preliminary performance measurements of our ASD prototype showed that there is no sufficient air flow at the brine/air boundary, hindering the results. Two major modifications of the design were identified and proposed, and will be implemented as soon as possible. These are a reduction of the height of the evaporation channel and the adoption of CPC type non-imaging optics for full illumination of the solar chimney, increasing airflow velocities over the brine surface, with consequent improved convection conditions, and increase of the airflow driving force.

It was shown that a description of ASD operation can be well modelled by introducing in it internal airflow dynamics into the numerical model, namely after a more suitable calculation of head loss and airflow velocity profile around the interface region brine/air. Monitoring is on allowing for assessment of ASD operation under spring and summer conditions, permitting an even better tuning of head loss and velocity profile calculations over a more representative set of data.

Conclusion of ASD prototype testing will lead us to a final design for optimized (passive) brine evaporation, applicable to MED plant effluent treatment and salt production, as well as a software tool for ASD proper modelling.

These results will certainly be the staring point for the application of the concept and in other applications, such as wood or food drying or even in (direct) water distillation.

Novel ICPC

After a year and a half of operation there were two distinct glass crack development patterns. One was related to the production sequence and the other to the end of the tube where the crack occurred.

Tube End: The end caps shown in figure 4 were used on both ends of the evacuated tubes. They consisted of a dish shaped piece of glass and a metal cup bonded to the glass using a housekeeper seal.

12mm metal pipes were brazed to the metal cups to provide flow of heated fluid at the top end and as a means to evacuate and tip off the tube at the bottom end. Thus, only the top end was subject to both thermal stress (the 155C fluid) and mechanical stress (partial
support of the fin and heat transport tube). Therefore, one might expect failure rates due to cracking of the glass to be higher at the top end of the tube than at the bottom. In fact the opposite occurred. Out of 19 cracked tubes after one year, 7 were cracked at their tops and 12 at their bottoms. Statistically, if one assumes that the true proportion of cracks at the top to be 60 percent, then there is only a 0.1 percent chance that one would observe seven or fewer cracks located at the top out of a total of 19. Thus, the evidence supports that there is no difference in quality due to absorber orientation and that the higher operating temperatures do not cause higher rates glass cracking.

Production Sequence: One and a half years after installation, 1.2 percent of evacuated tubes in the first half of the production run versus 9.8 percent of the tubes in the second half had developed cracks. This strongly suggests distinct differences in quality between the first half of the production run and the second half. Statistically, assuming that the entire production run is characterized by the overall fraction of cracked tubes of 0.059, the likelihood that the first half of the production run coming from such a process is less than 0.3 percent. Moreover, after six years of operation only 3.6 percent of the original vertically finned tubes had developed cracks, whereas the horizontally finned tubes continued to develop cracks at a much higher rate. Since fabrication methods typically trend toward higher quality (lower failures during operation), the first part of the production run seems to be a better baseline for expected manufactured tube characteristics.

Solar energy has been used widely in many industrial areas. One of the widely used solar energy applications is water heating. Energy storage is much more important where the energy source is intermittent, such as solar energy. While the water exit from the tank to usage, the temperature of the hot water in the tank stars to decrease because of the mixing of the cold water from the main lines and remaining hot water in the tank. In this work, using of the some obstacles has been suggested to supply higher thermal stratification in the tank during the water usage from the tank.

There is some previous analysis about these subjects as; ALIZADEH, has investigated the thermal behavior of a horizontal cylindrical storage tank. He used one dimensional the Turbulent Mixing Model and Displacement Mixing Model in numerical calculations. He has used some models to prevent unsteady behavior of the vertical temperature distributions [1].

AL-NIMR has solved and presented some mathematical models to determine the effect of the different design parameters on the thermal stratification within the tank and the time required by tank to supply water within a specified outlet temperature [2].

MISRA has analyzed the thermal stratification both theoretically and experimentally in hot water storage tank for the thermosyphon effect in solar water heating systems. He has given the analytical expressions to obtain temperature distributions in the tank. He has also given the diagrams depends on the time to present conductive heat transfer between the layers in the storage tank [3].

HELVA ad MOBARAK have investigated the effect of the amount of the hot water using into the temperature distribution in solar water heating storage tank [4].

HARIHARAN and BADRINARAYANA have analyzed the thermal stratifications numerically and experimentally in the hot water storage tank. They have studied the effect of surrounding and operating conditions into the thermal stratifications. They have observed that stratification improves with increasing AT and water flow rates [5].

HAHNE and CHEN have studied numerically about the flow and heat transfer characteristics in a cylindrical hot water store. They have used the storage efficiency to obtain thermal stratification. They have found that the increase of the Richardson and Peclect number has an effect that increases the storage efficiency [6].

Mo and MIYATAKE have carried out the transient numerical analysis for the thermal stratifications in the storage tanks. They have used turbulence model (k-s model). They have presented the effect of exchange cold water with hot water into the thermal stratifications [7].

EAMES and NORTON have investigated the effect of the tank geometry into the thermal stratification for sensible heat storage for low Reynolds number. They have presented the effect of inlet and outlet port locations on store performance [8].

In this study, the effect of the using some different obstacles for obtaining higher thermal stratifications has been analyzed numerically. The different kinds of obstacles are placed in the cylindrical tank to get the best performance for thermal stratifications inside the tank between all investigated cases. The water has been used as fluid.

We calculate the collector coefficients and their uncertainties with the least square and the weighed least square methods for a quasi-dynamic test [ 1 ].

The 95% confidence limits for each collector coefficient are calculated as well as the respective limits for the collector efficiencies resulting from the identified model. Using the test data, the confidence limits for the efficiencies can be validated, which proves that the uncertainties of the “collector coefficients» as well as the “efficiency curve" of the collector can be determined in a reliable way. However, a review of these uncertainty statements still requires a more comprehensive analysis of several complete tests.

The weighted least square method (WLS) gets slightly different coefficients with the same collector as the least square method (LS). It should be the more accurate one, as mentioned earlier. This statement has yet to be supported by the use of a more extended database.

Like both the quasi-dynamic and the steady-state tests are performed under outdoor conditions, from the measurement conditions Point of view it is impossible to repeat them, because weather conditions (combination of solar radiation and ambient temperature) always vary. Thus, to get a quantitative statement on the test reproducibility, the need for an extended database is again underlined. It should be remarked that this reproducibility is strictly related to the defined standard test conditions (or data selection conditions) as given by EN 12975 [ 1 ].

The easy implementation in EXCEL™ spreadsheet makes it possible to apply the WLS method regularly for collector test evaluations.

Acknowledgements: The test data was gained using the equipment of ITW at the University of Stuttgart, for which we express our gratitude.

4. REFERENCES

[ 1 ] CEN (2000) Standart 12975-2: Solar thermal systems and components — Solar collectors — Part 2: Test methods, European Committee for Standardisation.

[ 2 ] ISO (1994) Standard 9806-1: Test Methods for Solar Collectors. Part 1: Thermal Performance of Liquid Heating Collectors Including Pressure Drop. ISO, Switzerland.

[ 3 ] Press W., Teukolsky S. A., Vetterling W. T., Flannery B. P. (1996). Numerical Recipes, 2nd ed. Cambridge University Press, Oxford.

[ 4 ] NBR 10184 (1988) Coletores solares planos para liquida — Determinagao do rendimento termico, ABNT, Brazil.

[ 5 ] Mathioulakis E., Voropoulos K., Belessiotis V. (1999) Assessment of uncertainty in solar collector modeling and testing, Solar Energy Vol. 66 No. 5, 337-347.

[ 6 ] Muller-Scholl Ch., Frei U. (2000) Uncertainty analyses in solar collector measurements, Proc. of the Eurosun 2000.

[ 7 ] Fischer S., Heidemann W., Muller-Steinhagen H., Peters B., Berquist A. (2001) Collector test method under quasi-dynamic conditions according to the European Standard En 12975-2,

Proc. ISES Solar World Congress.

[ 8 ] Sabatelli V., Marano D., Braccio G., Sharma V. K. (2002) Efficiency test of solar collectors: uncertainty in the estimation of regression parameters and sensitivity analyses, Energy Conversion & Management, Vol. 42.

[ 9 ] Kratzenberg M., Beyer H. G., Colle S. (2002) Setup of a test facility for the characterization of thermal collectors according to the Euronorm at the “Universidade Federal de Santa Catarina”, Proc. “Sun at the end of the world” International solar energy congress and exhibition, Universidad Tecnica Federico Santa Marfa.

[ 10 ] Kratzenberg M., Beyer H. G., Colle S., Albertazzi Gongalves A. (2003) Test facility for quasi­dynamic collector tests for the characterization of thermal solar collectors in accordance with the international norms, metrologia-2003 — Metrologia para a Vida, Sociedade Brasileira de Metrologia (SBM).

[ 11 ] Kratzenberg M., Beyer H. G., Colle S., Petzoldt D. (2004) Bestimmung der Kollektorparameter und ihrer Unsicherheiten uber die Methode der gewichteten Fehlerquadrate fur den statischen und den quasi-dynamischen Kollektortest, Proc. Otti-Kolleg Thermische Solarenergie.

F. Cruz*, M. P. Dorado, J. M. Palomar, V. Montoro Dep. of Mechanics and Mining Engineering University of Jaen (Spain)

Corresponding author

Temperature distribution analysis over the absorber surface and the working fluid requires the application of an experimental study based on heat transfer basic principles, i. e. bi-dimensional conduction. In this sense, several researchers have decided to measure the temperatures over the plate [Chuawittayawuth et al., 2002], while others have preferred to simulate the whole process, sometimes based on commercial software with high computational capacities [Kalogirou & Papamarcou, 2000]. In our work, modelling and optimization were made by previously analyzing and programming the process. According to the general theory of heat transfer, we have proposed to develop a two-dimensional model of conduction and working fluid heat transfer [Incropera & Dewitt, 1996], together with the conventional theory of solar collectors [Duffie & Beckman, 1991], for both steady state and transient regime. Analysis were made using the following considerations [Norton et al., 2001], although in a preliminary stationary analysis some of them could be neglected: a) thermal capacity of absorber plate is considered; b) density, specific heat, viscosity, conductivity and Prandtl number of refrigerant are approximated by polynomic functions depending on temperature; c) heat transfer coefficients depend on the medium temperatures of the plate and the environment on each time; d) time variations of irradiation, ambient temperature, and inlet and outlet flow temperatures are assumed; e) radiation transmission through transparent surface varies with solar time and day of the year. Later, to validate the described models, a parameter identification procedure was formulated. This procedure was based on the gradient descendent method, related with the Taylor series expansion of an objective function. This function was defined using the comparison between real and modelled parameters [Gill et al., 1981].

The analysis of the dynamic processes in section 5 shows that there is no unique value of an effective capacity that well describes all the relevant processes. Furthermore, none of the existing procedures for the determination of the capacity is suitable for all the processes. The capacity resulting from the calculation or the J.2 procedure only poorly describes fast fluctuations of irradiance. On the other hand, dynamic effects that are caused by fluctuations of the fluid inlet temperature are strongly overestimated by the J.3 procedure.

So which determination procedure is best suited for describing realistic collector operation?

As long as one-node models are used, a compromise is needed. For this, it is important to find out for which of the mentioned dynamic processes the collector gain depends on the effective capacity. A determination procedure is only well suited if it well describes those processes where the capacity strongly influences the daily or yearly collector gains. The correct modelling of other processes is less important.2