Material properties included in the model have been previously described [Incropera & Dewitt, 1996]. Nevertheless, these values can vary from the real ones (i. e. the cooper on the absorber, or the crystal of the covering) depending on the manufacturing processes. In this sense after eq. (8) and (11) solving, results are not satisfactory enough. In fact, convection parameters associated to heat transmission to water vary drastically in each case. For this reason, parameter optimization technique is used to determine those real values. In this case, the parameters that need to be optimized can be divided into two categories:

— Material properties, such as crystal extinction coefficient ke (m-1), crystal refraction index n2 (dimensionless), directional absorbance of cooper black plate ae (dimensionless), plate emmitance £p (dimensionless), crystal emmitance £p (dimensionless), conduction coefficient of the isolator (aluminium sandwich model) KiT (W/mK), and conduction coefficient of cooper kp (W/mK)

-Heat transfer properties, such as convection coefficient pipe-fluid h (W/m2 K) (where Kh is optimized), and convection factor through working fluid Uo (w/m3 K) (where KuO is optimized).

The optimization process consists of minimizing an objective function V, obtained from the sum of squares of the differences between the model values and the empirical values [Gill et al., 1981]. The system supplies two values of the temperature: the output temperature of the fluid, Tfo, and the plate temperature at the measurement point, Tp, near the top of the absorber, by means of a bulb thermometer. The optimization process makes possible to find the minimum value of the objective function.

^ = lTfo — Tfo f+lTp — TP ) (12)

At any step of the optimization process, the previous equations set (eq. (8) and (11)) is evaluated and the temperatures distribution is obtained. Two of those values are used to determine V next to the other two real ones. The optimization algorithm changes the parameter values, and repeats the process with the new ones, until the objective function reaches a minimum. This process is based on a Taylor series expansion of V, where the parameters are modified with the gradient descendent method [Gill et al., 1981]. At this point, modelled and real values are almost the same, and the temperature distribution is considered to be correct, and close to reality.

M. J. Marcos1, C. A. Perez-Rabago2 ,C. A. Estrada1’2, M. Romero1

1CIEMAT-PSA, Avda. Complutense 22, 28040-Madrid, Spain. e-mail: mj. marcos@ciemat. es

2CIE-UNAM, AP 34, 62580 Temixco, Morelos, Mexico. e-mail: cestrada@cie. unam. mx

Abstract

This paper presents the theoretical study of the heat transfer process that takes place in a special calorimeter of conical cavity named CAVICAL1. This instrument is used to measure the thermal power of a point focus solar concentration system named DEFRAC and developed at the Center of Energy Research of the National University of Mexico. The DEFRAC concentrator has 1.3 kWt and it has a very fine optical system. The opening of the calorimeter cavity is 14 cm2. The detailed heat transfer study is done using the FLUENT code. The heat transfer mechanisms that are taking into account in the analysis are the radiative energy absorbed by the inner wall cavity, the energy transfer from the wall cavity to the air by natural convection, the energy transferred by conduction through the metallic wall of the calorimeter and by forced convection through the fluid in the cooling system. The calorimetric information allows determining the thermal power that the concentrator is able to capture. Temperature and velocity fields are determined for each of the thermal fluids considered inside of the calorimeter. The analysis gives the thermal losses and the efficiency of the calorimeter. The information generated is useful to optimize the design of the calorimeter.

Key words: Concentrated solar radiation, heat transfer analysis. Cavity calorimeter.

1. Introduction

In the near and middle future, the point focus solar concentration systems will play and important roll in different industrial applications, like the generation of solar thermal power electricity, the production of solar fuels or the destruction of hazardous materials. The Center for Energy Research of the National University of Mexico built a solar concentration system named DEFRAC (Devise for the study of high radiative concentrated flux, acronyms in Spanish) to support basic and applied research on those systems. DEFRAC is a point focus solar concentrator with an equatorial solar tracking system. It consists of two frames: one is used as the main structural support and the other of hexagonal shape, it holds 18 first surface paraboloidal mirrors, 30 centimeters in diameter, made of aluminized glass with 0.95 reflectivity. The equivalent focal distance of the whole set of mirrors is 2 meters. The hexagonal frame axis is supported by two lateral journal bearings, attached to the main frame. The main frame has an electric motor, sensors and a control mechanism to follow the sun with high accuracy [1]. Figure 1 shows a schematic view of DEFRAC.

In the past, a flat plate calorimeter has been used to measure the solar concentration power of DEFRAC. It had the function to capture concentrated solar radiation and transfer it to a thermal fluid. The calorimeter was made of stainless steel, with cylindrical geometry and it had two circular flat plates (11 cm in diameter with 1 cm of gap) where circulates the cooling thermal fluid (usually water). The calorimeter acts as a receiver of DEFRAC and in its external surface the sun spot was formed [2]. Figure 2 shows a picture of the flat

temperature to the ambient. This method has Fig. 1. Schematic view of DEFRAC resulted effective,

because it had allow us to

determine the concentration power of DEFRAC. However, it has been demonstrated that the temperature of the external surface was not close to ambient; 470 oC, it has been calculated, based on measurements, at the center of the plate, with concentrated solar energy coming from only 6 mirrors, even though the mass flow rates were relatively high (1.11kg/min) and the increments of temperature between outlet and inlet calorimeter water flow were low (~ 5 °C). Knowing the plate temperature distribution, the convective and radiative thermal loses have been calculated, and they have resulted to be less that 2.5%. [2]. On the other hand, the determination of the concentrated solar power by DEFRAC was calculated by the addition of the powers obtained by each group of 6 mirrors of the DEFRAC. But, with this flat plate calorimeter, it has not been possible to measure directly the solar concentration power of the 18 mirrors, due to the fact that the external surface was degraded with the high solar flux (~ 4000 suns) and thus the high temperature.

In order to improve the calorimetric measurements of

the DEFRAC concentrated solar power, a cavity calorimeter was designed, built and tested. This calorimeter was named CAVICAL1 (cavity calorimeter No. 1). It allows us to handle high temperatures, to have a better control on the thermal emittance and also it allows us to calculate the concentrated solar power of DEFRAC using all 18 mirrors of the system. This paper deals with the preliminary results of the heat transfer study using a computer fluid dynamics

Fig. 2. Sun spot picture of the flat plate calorimeter code (CFD) to determine during one test. temperature distributions of

different device’s components and heat losses from the calorimeter to the surroundings. The validation of the computer simulation was carried out by comparison with experimental results.

So far, very little attention has been focused on forced circulation solar stills. In this study, the heat and mass transfer relationships in the forced circulation solar still with enhanced water recovery will be developed. Following this the simulation model for this still
will be incorporated into the SOLSTILL program. Finally, the SOLSTILL program will be validated by comparing its results with those from the experimental model.

The forced circulation solar still has been chosen in this study for several reasons. Compared with other types of solar powered distillation systems such as the solar multistage flash distillation, solar vapour compression, solar powered reverse osmosis, solar powered electrodyalisis, and solar membrane distillation systems, solar stills represent simple, yet mature technology. This is suitable for developing countries like Vietnam.

The low efficiencies of a conventional solar still may be overcome by changing the principle of operation as follows:

• Using air as an intermediate medium and substituting forced convection for natural convection to increase the heat coefficients in the still, resulting in increased evaporation of water.

• Replacing saturated air in the standard still by “drier” air to increase the potential for mass transfer in the still, leading to higher outputs.

• Circulating the air-vapour mixture from the standard still to external water cooled condensers to gain efficiency from a lower condensing temperature. The cooler the cooling water available, the more effective this condensing process will be.

• Recovering some of the heat extracted in the condensing process and using it to preheat the air-vapour mixture entering the still.

• Substituting the condensing area of the flat sheet covers in the standard still by the external condenser with much larger heat exchange areas to increase condensation efficiencies.

The issues was put forward in 2001 officially and the technical research on the integrated heating project of solar pond was fulfilled in November, 2002. The main achievements are as follows:

3.1 Seawater solar pond

3.1.1 Membrane materials of high cost are not adopted and cement boards are used to protect slopes. The water level of the circumference of solar pond is raised to maintain the working water level of the solar pond.

3.1.2 Underground brine wells are digged far away from the coastline. The natural sea water is replaced by the water made up elaborately with the sea water. Brine solar pond is built suitable for the existence of the cultivation objects in sea water.

3.1.3 In the lower convective area of the solar pond, devices of hydrogen-increase and deodorization and system of forced water circulating are fixed to support the seawater solar pond to realize the healthy cultivation technology.

3.1.4 When the environmental temperature is -15"C, the lowest circulating temperature of the solar pond less than two meters is 10.0"C.

3.1.5 The seawater solar pond can meet the demands of the over-wintering temperature of some cultivation objects of warm water kind, such as mullets, basses, lefteye flounders, portunids, scylla serrata forskal, puffers, American snappers, penaeus japonicus and so on.

3.1.6 The pond is the first large-scaled underground brine solar pond that runs successfully in China. It meets the demands of the over-wintering of the cultivation objects.

3.1.7 The pond is the first one to combine brine solar pond, shallow-styled solar-pond and conservatory-modeled solar pond to work and to integrate heat. It is a typical model of satisfying over-wintering and cultivating young products successfully.

3.1.8 It paves the road for the national solar pond technology and for achieving commercialization and industrialization.

3.1.9 It provides a new technology-supporting system for the project of “building on-the-sea China.”

3.2 Shallow-styled solar pond.

3.2.1 The pond realizes streamlined production of bio-baits (mono-cellular algae) for the first time in China. It meets the demands of first-phase bait-supply in the process of raising river crabs.

3.2.2 The pond can provide cultivation water of 400m of 15"C to 25"C daily.

3.2.3 In May, the temperature of the pond sets a record, which is 7"C higher than the natural water temperature. It is practicable to rely on solar energy only for heat.

3.2.4 Shallow-styled solar pond 3000m underground is in operation successfully. It provides hot water of 3000m daily, which is 15 times more than that of the upground one.

3.2.5 Combining upground solar pond and conservatory-modeled solar pond can both improve the effect of heat-integration and make use of photosynthesis to produce mono-cellular algae to meet the demand of cultivation.

3.2.6 The cement-brick of the upground solar pond is low in cost. The underground solar pond is soil-structured. The heat-preserving ability of soil is great and can save up a large quantity of water, which satisfies the industrialized production.

3.2.7 The integrated heating of the shallow-styled solar pond underground and upground attempts successfully to supply heat to the aquatic industry in an industrialized way for the first time.

3.2.8 It opens up a new field for the industrialization of the high-efficiency shed agriculture.

3.3 Conservatory-modeled solar pond.

3.3.1 The solar pond is placed in the greenhouse. It is convenient for heat-integration, heat-preservation and over-wintering cultivation in spring and winter.

3.3.2 Farm-oriented dropless shed-membrane is low in cost, good in photopermeability and thus adopted to be lighting membrane.

3.3.3 Because the southern side and parabolic side select light at the same time, the temperature rises quickly in winter. The effect of heat-integration and heat-preserving is good and the temperature is 15-20"C higher than the natural one in ordinary circumstances.

3.3.4 Seawater solar pond is connected into the conservatory to provide convenience for fresh water from the seawater solar pond. It is also convenient to increase hydrogen and remove odor in the lower convective district.

3.3.5 Translucidus side of the greenhouse can endure the attack of force 11 wind and remain intact.

3.3.6 During spring and winter in 2001 and 2002, twelve cultivation objects in seawater of warm water kind

passed winter safely in the conservatory-modeled solar pond, including portunus trituberculatus, scylla serrata forskal, eriocheir sinensis, penaeus chinensis, penaeus japonicus, paralichthys olivaceus, scophthamus maximus, pagrus major, puffers, scapharca suberenata, venerupis variegata, sea snails and so on. The contribution percentage of the solar energy is 100%.

3.3.7 In the years of 2001 to 2002, the industrialized production of breeding river crabs is successfully carried out in the conservatory-modeled solar pond.

3.3.8 In May and June of 2002, the experiment of forced control and promoting growth of the young river crabs of later period was implemented in the last ten-day period of May. Commodity crabs of 75g to 100g each were raised that year.

3.3.9 Seawater solar pond, shallow-styled solar pond and conservatory-modeled solar pond are combined to work for two periods with water body of 500m, which creates the high-efficiency working technology in the district of saline and alkaline land and tidal-flat areas.

3.3.10 It provides experience for the agricultural and industrialized development in the saline and alkaline land and it also provides powerful technology-supporting system for the strategic structural adjustment of countryside and agriculture in the Northern coastal area of China.

The behaviour of the whole thermal solar system or its subsystems, respectively, was observed during different operating conditions (e. g. stagnation). In order to assess the durability and reliability, the quality and the suitability of the materials used as well as the way how they were processed was considered. Additionally the period of warranty for the most important components (collector, store and controller) was assessed.

2.2 Environmental aspects

The energy payback time was determined and the used materials as well as the packaging was assessed.

2.3 Safety aspects

The most important components as well as the whole system was investigated with respect to electrical safety and the risk of injury due to sharp edges, burning and scald. The documentation was checked with regard to notes dealing with safety aspects during the installation of the system. For systems with an integrated gas burner safety aspects related to gas and fire were considered additionally.

2.4 Handling

The way how the system has to be mounted, maintained and operated was assessed. Criteria of this assessment were e. g. the time required for the system installation as well as ergonomic aspects. Additionally it was examined if the corresponding work steps were described understandably, detailed and correctly in the documentation supplied with the system.

The procedure described above is applied for a CPC collector. The collector parameters gained during the collector test are summarised in Table 2.

The incident angle modifier K0b(0) of the investigated collector is calculated according to equation (6) /3/, the corresponding values of 0| and 0t are documented in Table 3.

The characteristics of the test sequence used for validation are summarised in Table 4, a graphical overview is given in Figures 2 and 3.

hour of the day [h]

Figure 4: Measured and calculated collector output during the test sequence used for validation together with the difference between measured and calculated collector output

4. Conclusions

The validation of collector parameters obtained from collector testing is desirable if the parameters are used for performance prediction using dynamic simulation. For this purpose an additional test sequence and a set of acceptance criteria have been defined taking typical every day operation into consideration. This method is able to detect parameter sets that are not fully able to mirror the thermal behaviour of a collector under dynamic conditions. The implementation of the method shows for the presented example very promising results. It is strongly recommended to introduce the presented method into the EN 12975 standard.

References

/1/ EN 12975-2:2001. Thermal solar systems and components — Solar collectors — Part 2: Test methods

/2/ EN 12977-2:2001. Thermal solar systems and components — Custom built systems — Part 2: Test methods

/3/ Mclntire W. R., Factored approximations for biaxial incident angle modifiers. Solar Energy 29, 315-322, 1982

Besides gathering data for later evaluation continuous monitoring is very useful. Errors are detected by self-analysis of the systems (plausibility checks of current measuring data, analysis of energy flow). Solar irradiance data measured by the system itself or data from the Internet are used for energy gain calculations. If there is a difference between calculated and real energy gains of the solar thermal system an error must have occurred: The Internet connection is used to send emails or SMS (short message service) for maintenance. Especially for solar thermal systems continuous monitoring is very important. Without monitoring system failures are imperceptible to the user because energy not gained by the solar thermal system is provided by conventional after-heating.

Conclusions

In future a great deal of homes will be equipped with computer and multimedia networks. Coming along with home networking more embedded systems with Internet connectivity will be used for controlling purposes. Bulk production will make the hardware affordable. Communication fees will be irrelevant. Remote administration and software updates of controller software via Internet will help to increase the efficiency of solar thermal systems. Breakdown durations of the systems will be minimized by self-analysis and automatic error message generation (email or SMS). With online visualisation and WAP-interface system control and error search is made very easy. Measuring data accessible via Internet enables external analysis of system performance to improve control algorithms. With more powerful controller hardware model based control algorithms will be feasible. They will help to increase energy efficiency.

Acknowledgement

We thank Projekttrager Julich (PTJ) and the Bundesministerium fur Wirtschaft und Arbeit (BMWA) for supporting this research work and RESOL (www. resol. de) for carrying out the embedded system.

At the time of this writing (March 2004), the prototype was under construction.

At the Congress (June 2004), the experimental set up and the preliminary results will be presented for the two configurations above, i. e., with direct energy delivery or with thermal storage through the heating of the aluminium mass.

References

Chi S. W. (1976). Heat Pipe Theory and Practice. Hemisphere Publishing Corporation. Washington.

Collares-Pereira M., Farinha Mendes J., Carbajal W. M. (2001). Primeros Ensayos de 2 cocinas solares con colector CPC — Tubo de Calor. Simposio Internacional de Fuentes Renovables de Energia, La Ceiba Honduras

Farinha Mendes J. (1988). Tubo de Calor para Aplicagao em Colector Solar do Tipo CPC de Concepgao Integrada. Lisboa.

Kobayachi, Yamamoto, Kuroki and Nagata (1984). Heat Transfer Performance of a Two — Phase Thermosyphons. Vth International Heat Pipe Conference, Tsukuba.

Nguyen C., Groll, Dang V. (1981). Entrainment or Flooding Limits in a Closed Two-Phase Thermosyphon. IVth International Heat Pipe Conference, London.

CPC Ao Sol 3000 www. aosol. pt

www. shell. pt (2002), Shell Thermia B

The identical design of the two systems does not guarantee equal thermal performance under identical flow rates, weather and boundary conditions. Therefore comparative measurements with equal flow rates in both collector loops were carried out. Results show that the thermal performance differs by less than 1 % between the two systems.

During the following tests both systems were exposed to the same weather and boundary conditions, the specific flow rates were set to 15 l/m2h (low-flow) and 40 l/m2h (high-flow). A total of 27 test days with different draw-off profiles and a high variability of solar irradia­tion have been measured and evaluated. In order to recognize correlations between the weather conditions and the differences of the solar gains of the two systems, the results have been evaluated day by day.

Results

The analysis of the test days shows an interesting effect: The difference between the daily solar gains of the two systems depends on how often the sunshine is interrupted by clouds. On clear and cloudless days the system gain under high-flow operation exceeded the gain of the low-flow system by up to 5.7 %, while for unsettled irradiation the thermal performance of the low-flow system was up to 4.6 % better. In the sum of all test days the system gain of the high-flow system is superior by only 0.5 %.

The decisive influence of irradiance characteristics becomes obvious when regarding two representative test days. On 26 June 2003 and 16 July 2003 the daily solar irradiation amounts to 7.59 kWh/m2d, but the irradiance characteristics is quite different. On 26 June 2003 the weather was changeable, whereas the 16 July 2003 was a clear and cloudless day. The daily results are shown in Figure 2 and 3.

1000 800

03 ^

ЧГ <D

600

І. з

400 200 0

4 6 8 10 12 14 16 18 20 t [h]

Figure 3: Irradiance, flow rates and switching cycles of the collector loop pumps on 16 July 2003. The clear and cloudless weather leads to a better performance under high-flow operation.

On 26 June 2003 the solar system gain of the low-flow operation exceeds the system gain of the high-flow operation by 1.4 %, whereas on the cloudless day (16 July 2003) the high — flow system shows a better thermal performance (2.3 %). Under intermittent sunshine the low-flow system benefits from a reduced number of switching cycles and from longer op­eration intervals. Periods of low irradiance are better utilized under low-flow operation.

For a meaningful evaluation of the different flow rates it is necessary to consider the en­ergy demand of the collector loop pumps. The operation time as well as the electrical power demand of the pumps are quite different for low-flow and high-flow operation. For a comparison of the primary energy savings the electrical power consumption are weighted by the primary energy factor 3. The savings of fossil fuel by the solar DHW system is weighted by the primary energy factor 1.1. Thus the primary energy savings Esav, prim turn out to:

= 1.1 • ^ — 3 • £

The calculation shows that every investigated day brings up higher primary energy savings of the sys­tem with low-flow rate (be­tween 2.2 % and 8.7 %). Totalized over the whole measurement period, the low-flow operation saves 5.2 % more primary energy than the high-flow operation. In consideration of the par­ticularly low efficiency of the collector loop pump under low-flow operation (low-flow: power level 1, 45 W / high — flow: power level 3, 90 W), this result is very surprising.

The differences of solar gain AQsOi and primary energy savings AEsav, prim for each test day are presented in Figure 4. The most important results are listed in Table 1.

Dynamic System Test (DST)

Besides the comparative analysis of single test days, the two systems were measured in a complete dynamic system test (DST) according to ISO/DIS 9459-5. With the system pa­rameters obtained from the dynamic fitting procedure a long term prediction for the loca­tion Wurzburg was calculated. The fractional solar gain under high-flow operation amounts to 43.0 %, whereas the low-flow operation achieves a fractional solar gain of 42.8 %. With regard to the high uncertainty of the DST procedure of 5 % the solar gain is equal for both flow rates.

Analysis of the Mean Collector Loop Temperatures

Due to higher collector outlet temperatures and a minor degree of collector efficiency un­der low-flow operation, a lower solar gain of the low-flow system was expected. A careful investigation of the collector loop temperatures showed that the return temperatures are significantly lower under low-flow operation. This leads, in interaction with the higher col­lector outlet temperatures of the low-flow system, to nearly equal mean collector loop tem­peratures and consequently to similar collector efficiencies for both flow rates. Thus the result of the comparative measurements — nearly equal solar gains under high-flow and low-flow operation — is explainable.

Two questions remain unsolved:

1. What is the reason for the lower return temperatures under low-flow operation?

2. Is it possible to enhance the performance of the high-flow system compared to the low-flow system by the use of a heat exchanger with a better heat transfer capability?

Recordings of the velocity field were taken in a frame of 0.127 x 0.159 m2 at each inlet as shown in Fig. 4. The duration between two succeeding illuminations of the particle field varied between 8 and 100 ms depending on the flow around the inlets. The time delay between succeeding velocity vector recordings was about 250 ms. The volume flow rate was about 2 l/min. The initial tank temperature was about 20 °C. The outlet flow was lead through the heating unit where an electrical heating element of 3 kW heated the water

similar to the behaviour of a low-flow solar collector when the pump starts to circulate the flow. The laser was focused to the centre and to the position 20 mm from the centre of each of the three inlets and exactly the same experiment was repeated for each of the 6 laser positions. The duration of the experiments was 50 minutes. The flaps are referred to as flap 1, flap 2 and flap 3 counted from the bottom of the tank. Figure 5 shows the tank temperatures, the inlet temperature and the temperatures in the pipe of each experiment with a time interval of 5 minutes. From the figure it can be seen that the test conditions were practically the same in the experiments. Also the figure shows that the thermal stratification is build up in a good way.