One method to create a surface that is evenly illuminated by solar radiation is to charac­terise the solar flux distribution about the focal region of the concentrating system. The easiest method to achieve this is by a theoretical simulation recreating all of the compo­nents of the optical systems particularly the reflected solar beam.

For this paper we used the mod­elling package described in [11].

The code recreates the terrestrial solar beam for any location and time of day [12], and provides a convenient infrastructure to model the optical components of a dish concentrator including the imper­fections in the mirrored surface and the effect that these imperfec­tions have on the reflected solar beam. The code traces a gener­ated sunshape through the optical components of the concentrator to any predefined quadric (or planar) surface. The output of the simu­lation is an intensity map on that given surface. The characterisics of this individual simulation are described in Table 1 and the code can be downloaded from <www. physics. usyd. edu. au/~buie/>

The focal region of our specific concentrator was divided up into 200 horizontal slices evenly spaced between the points 0.3 m above and below the focal point. Using the code in [11] the flux distribution on each of these slices was calculated (Figure 1a). Each of the slices were then concatenated (Figures 1b) to form a large block of data (or data cube), that completely characterises the flux about the focal region for planar surfaces.

Each individual point in the generated data cube represents a point in space about the focal region. The box bounding the cube is 0.3 x 0.3 x 0.6 m with the focal point as its centre. The data cube contains 400 x 400 x 200 points in the x, y,z directions respectively (z-direction represents up) resulting in a total of 3.2 x 107 points (250Mb). The value of each of those data points literally represents the amount of energy passing through the lower surface of a small cube surrounding that point’s position in space.

The test storage units are being tested on the Plataforma Solar de Almeria (PSA) in Spain, integrated in the parabolic trough test loop (Fig. 3). This test loop comprises 50 m LS-3 and 75 m Eurotrough parabolic troughs with a maximum thermal power of 480 kW using synthetic oil as heat transfer fluid. The storage units have been designed for a power of 350 kW which can be provided by the loop at an average insolation of 800 W/m2, assuming 34 kW of thermal losses of storage units and piping. Within this test loop, either two storage blocks of same material can be operated in series connection, or two blocks of different material can be operated in parallel.

Already before start up of the storage units, the LS-3 collectors have been damaged during maintenance works and had to be taken out of operation completely. Therefore for the whole project the power of the loop was reduced to 300 kW from the Eurotrough collectors. Fig. 4 shows flow rates for charging and discharging as well as the temperature in the core (TC9) of the storage module M1 (ceramic) at four different positions (P1 to P4) of the storage length. Cross section P1 is closest to the oil inlet during charging, so the temperature in cross section P1 is highest and in P4 lowest.

In this test the mass flow rates and oil inlet temperatures during charging and discharging have been kept constant.

An analysis of the power transferred in these cycles on 14.01.04 is shown in Fig. 5. Here the transferred power from the oil has been calculated from the measured mass flow, inlet and outlet temperatures just before and behind the distributors and collectors and specific heat as a function of temperature as given by the manufacturer. The power into and out of the storage module has been calculated from the heat capacities of the materials measured in the lab (see Table 1) and the change of the average storage temperature within a specific time. The average storage temperature is calculated from all thermocouples in all four cross sections P1 to P4.

Integration of these curves results in a total input of energy into the storage from the oil of 391 kWh, and a total output through the oil of 175 kWh during the two discharge phases. So net 216 kWh have been transferred to the storage. During this time the storage has experienced a net temperature raise from 122°C to 169°C, which is a capacity raise of 192 kWh. The difference between the net input from the oil and the capacity raise of the storage, 24 kWh is the net heat loss of the storage. This results in an average heat loss power of 5% of the input power.

Due to the lack of power from the LS-3 collector and unusually bad weather conditions during late summer and autumn of 2003 in the province of Almeria, the full test program could not be finished within the WESPE project. Up to now 325°C storage temperature has been reached. The essential cycle tests in the operating range of 340 — 390°C will be performed as soon as the LS-3 collector has been repaired (probably end of April 2004). Nevertheless test results so far let us expect a high suitability of the realized system for storage use. There was no degradation of heat transfer between heat exchanger and storage material, very high power levels could be realized during cooling already and also high temperature gradients between storage and oil have been handled without problems.

Before designing the system we have studied the whole process to calculate actual heat load of each job (trolley).

2.1.Total Heat requirement of A trolley

I) Heat losses in the chemical tank water /chemical ( A )

a. Tank booth volume initial temp.

b. No. of Heated Tank ( with the temp. required )

c. Average Ambient Temp. into the shop

d. Evaporation losses at the end temp through the top open portion of the tank.

e. Losses through wall of insulation of the chemical tank.

Solar Flat Plate Collectors used for this system are of ANU Brand, ISI marked manufactured by Peenya Alloys Pvt. Ltd. Bangalore, India.

a. Actual Heat required per day — F

b. System efficiency ( considered ) — G

c. Type of collector

d. No. of fins ( riser )

e. Area of collector

f. Daily mean solar radiation ( Average )

g. Efficiency of the collector ( overall )

h. Average heat generated per collector No. of Solar Collectors required = ( F / G ) / H

Optical concentrator scheme is chosen in the form of a parabolic-cylindrical mirror reflector of the offset type, when only one branch of the parabola is used. Thus the problem of mirror shadowing by heat exchanger — thermo mechanical or thermo electrical converters, as in our case, disappears. Fig.2 illustrates offset type parabolic-cylindrical concentrator scheme.

■ n mmt/tnu/n Fig.2. Offset type parabolic-cylindrical concentrator scheme.

1. Mirror. 2. Rotary-supporting mechanism. 3. Thermo converters module. F. Focal point.

Another advantage of the offset scheme is that working mirror position has mainly vertical character even at high angles of solar position.

An important factor of comfort exploitation and thermo modules testing as well as keeping mirror surface safe is low position of the focal zone above the ground. As it was mentioned, surface area of the mirror equals to 10 -12 m2 and it is placed on the rotary­supporting mechanism with the possibility of synchronous tracking of the Sun moving along two coordinates; azimuth and elevation. The angle of the mirror opening is about 70° and the ratio of parabola focus distance to its diameter is equal to 0.35.

INNOVATION

The ISTC A-839 Project Proposal goal is the development, construction and testing of an acting prototype of low-power cost-effective automatic Sun tracking Solar Power Station. Project innovation aspects enclose in the use of ecologically clean technologies in the field of Solar energy conversion, based on a mirror concentrator with two new thermo converters with increased efficiency due to new technical solutions as well as cascade performance of thermo elements. Sun tracking system allows to receive during the day 30 % more energy compared with stationary immovable ones.

System simplicity and its compactness allow quick increase of the number of SPS on any non-prepared place, that leads to corresponding electricity power multiplying.

CONCLUSION

The works under this Project are being on a conceptual and experimental development stage. The major scientific-technical volume of the works consists of research and development of an offset type parabolic-cylindrical solar concentrator, thermal actuators based on the shape memory alloys in cascade performance and thermo electric generators also in cascade performance. As a result, the advantages of the Project are:

— Small area occupied by one SPS module;

— Almost Zero expenses and time for installation work;

— Possibility of manufacturing SPS models in stationary, handheld and mobile ( onboard of a small auto-trailer) variants;

— Lower expected prime cost of the SPS production — about $ 800 per 1 kW.

ACKNOWLEDGEMENT

The author is thankful to ISTC for support in participating at Eurosun 2004. Conception of the given paper is submitted to ISTC (International Science and Technology Center, www. istc. ru ) as a Project Proposal and is open for scientific collaboration or ISTC partnership program.

REFERENCES

1. "Materials with the Shape Memory Effect" Handbook, edited by V. A.. Likhachev.

The edition of Saint- Petersburg State University,1998, 374 pages.

2. V. M. Andreev, V. A. Grilikhes, V. D. Rumyantsev "Photovoltaic Conversion of Concentrated Sunlight” (monograph), John Willey & Sons, 1997, 294 pp.

3. Fedorov M. I., Gurieva E. A., Prokof’eva L. V., Zaitsev V. K. Prospects of various thermoelectric use in thermoelectric generators. Proceedings of the XIV International Conference on Thermoelectrics, 1995, St. Petersburg, pp.254-258.

Three types of systems are investigated in this work. A large solar water heating system suitable for a block of 10 flats or similar use, a solar heating system for a house and an industrial process heat system. A generic schematic diagram of the systems considered is shown in Fig. 1. The same basic system applies to all cases with the options concerning location of auxiliary and house heating as shown.

Fig. 1 Schematic diagram of the solar hot water and space heating system

1.1 Large hot water system

The system consists from a collector array, a storage tank, solar pump and auxiliary. A differential thermostat is used to compare the temperature at the exit of the collectors and the storage tank and give a signal to switch on the pump. The auxiliary energy considered in this case is diesel. The specifications of the system are shown in Table 1. Such a system can supply hot water to blocks of 10 flats or to any other similar size system.

1.2 Solar heating system

This is very similar to the hot water system with the difference that the hot water is supplied to the house radiators circuit. The specifications of the system are shown in Table

2.

Approximately 50 % of the production costs of a solar-thermal collector are related to the collector structure (figure 3; structure, glass cover, assembly cost). Furthermore, these components define the collector’s weight and its handling especially on the houses’ roofs. Since the collector manufacturers insist on the use of the expensive solar glass, reengineering activities have to concentrate on the collector structure.

Competitors Analysis / Types of Solar-Thermal Flat-Plate Collectors

Aluminium still is the dominating material of the standard frame structure designs (except for large-scale collectors where wood is still often in use for the collector structure).

Few companies (e. g. GREENoneTEC, St. Veit/A; thermolsolar, Landshut/D) offer collectors with deep-drawn aluminium troughs. The trough design, however, offers a considerable potential for cost reduction due to the minimum number of parts and thereby avoided processes of material handling, logistics and assembly.

Only Buderus, Wetzlar/D, currently uses plastic as structural material in a flat-plate collector (figure 4). The former ALLIGATOR Sunshine Technologies GmbH, Berlin/D, also made use of plastics for the structure of a roof-integrated collector (solar-thermal and photovoltaic) as seen in figure 5.

Dirk Kruger, Dirk Mangold*, Klaus Hennecke, Ralf Christmann, Jurgen Dersch, Eckhard
Lupfert and Klaus-Jurgen Riffelmann,

Solar Research, Institute of Technical Thermodynamics
Deutsches Zentrum fur Luft — und Raumfahrt e. V., 51170 Koln
Tel: (49) 02203 601 2661, Fax: (49) 02203 66900, E-mail: dirk. krueger@dlr. de
* Solar — und Warmetechnik Stuttgart (SWT)
ein Forschungsinstitut der Steinbeis-Stiftung, 70550 Stuttgart
Tel: (49) 0711 685 3279, Fax: (49) 0711 685 3242, E-mail: mangold@swt-stuttgart. de

The high exergy of the solar radiation allows to produce heat in thermal collectors, and also to generate electricity as in large solar thermal power plants or with photovoltaics in smaller applications. Solar district heating systems can be enhanced by small engines converting the valuable part of the energy at high temperature into electricity, while the remaining fraction at lower temperature is still used for heat production. This will improve the benefit for solar district heating. In this paper a solution for producing electricity and heat from one solar system, a parabolic trough collector field in conjunction with a steam engine is presented (Figure 1).

Applications

In the municipal sector various solar installations for district heating, sometimes with seasonal storage, have been erected for domestic hot water and heating purposes. Combining such kind of systems with a heat engine to produce electricity is the principle of a solar combined heat and power solution. Process heat applications needing heat up to 100°C in conjunction with electrical power are also appropriate. The concept is interesting for small heat and power applications in residential homes as well.

Collector

Temperatures of 200°C to 400°C are desired in order to reach appropriate engine efficiency. Large parabolic trough collectors for solar power plants as the EuroTrough collector (Geyer et al (1), Geyer et al (2), Lupfert et al) can deliver heat at these temperatures efficiently, but they are not cost effective for smaller solar fields of up to several thousand square meters of aperture area. Existing medium sized parabolic trough collectors for process heat can deliver heat at 200°C and more, but their efficiency is fairly low at elevated temperature (Kruger et al). Assuming a small scale parabolic trough
collector would be enhanced by performance-improved features typical for solar power plant collectors as eg vacuum receivers and sufficient concentration quality, an annual yield described in Figure 2 could be reached according to simulations in TRNSYS. Especially a small collector, with a tilted north-south axis, could reach energy yields around 600 kWh/m2*a, when enhanced to an efficiency of the EuroTrough collector. Tracking from east to west provides a high output level for several hours on sunny days (Figure 3), as the collector is close to perpendicular irradiation all day.

Typical property of the solar heat is its discontinuous power and temperature level due to the day-night cycle and irradiance variations with weather conditions. This affects the selection of the appropriate heat engine.

Upper curve: High performance collector, tilt 35° to south

Centre curve: High performance collector, horizontal axis

Lower curve: Process heat collector of Industrial Solar Technology, horizontal axis

900 800 700 600 500 400 300 200 100 0

Engine

In principle, various types of engines can be used for conversion of solar heat to electricity: Steam turbines, ORC turbines, Stirling engines and steam engines. Steam turbines are
nowadays hardly available for the range below 500 kW electrical power. ORC turbines also start in the range of 500 kW electrical power. Stirling engines have been developed for small domestic CHP (Combined Heat and Power Production) and tested in combination with high temperature heat from parabolic dishes. The temperatures necessary exceed the temperature provided by parabolic trough collectors. New low temperature Stirling engines may be developed though. Steam engines are today only commercially available from the company Spilling, starting from 60 kW nominal electrical power with good part-load behaviour.

As the solar output of the collector varies with radiation and incident angle, a steam engine with its high part load efficiency is chosen for this study. Spilling produces a 120 kW machine, which can be operated by 210°C saturated steam.

Nominal thermal input power is 960 kW. According to supplier information, part load is possible down to 30%. Between 100% power down to 30% part load the gross electrical efficiency is almost constant at 12.5%. Parasitic power for pumps and assemblies amount to about 3 kW over full and part load. Outlet steam quality is wet steam at 110°C and 1.5 bar.

5 Watt-PV module is utilized for tracking solar oven concentrator system with 2.6 kWTH capacity and 250 Kg weight. The tracking system follows the Sun autonomously in altitude and azimuth using only 5 Watt-peak PV solar module as a tracking energy source. The tracking system is driven by means of two 12 DCV motors of 36 W each, and fed by electrolytic condenser with 78,000 pFd capacity charged properly by PV module. The PV based tracking system has two circuits in H Bridge configuration using N — and P-channel power MOSFET transistors. This electronic circuit commands DC motor rotation way, as a function of the optical sensors for altitude and azimuth position.

The proposed system must be designed based upon local technology and adopted to the needs. Simple design concept is one of the issues in this tracking system to reduce different troubles during its lifetime. The tracking system consists of electrolytic condenser storage, instead of conventional battery and its charge controller configuration. A couple of electrolytic condensers satisfy the total system energy needs. FIG.16 shows "H” configured basic electronic circuit for feeding two DC motors of 36W-each, one for solar altitude and the other for azimuth movements.

The design and construction of effective 2.6kWTH stand-alone solar concentrator oven tracking system was developed using 5 Watts-peak PV module. The objective is focused for simple and robust electronic tracking system for Mexican rural area application.

The generated power at PV module is coupled for charging electrolytic condenser. The maximum module voltage is 16V, and when the electrolytic condenser achieves 15V, the electronic circuit compare, and switches for discharging maximum of about 8.8 Joule of energy as (1/2 CV2), where C is the capacitance and voltage V provided by the module, the plot is shown in FIG. 17.

The energy delivered by the capacitors is conducted towards the selected DC motor according to the optical sensor decision. The DC motor has low internal resistance of ~2Q and considering PV module as a constant current source with about 340mA, the capacitor’s charging time lasts about 2 to 3.5 seconds depending of its charge state. The
electrolytic capacitor charging process for feeding low-resistance DC motor load is illustrated in FIG.17. The figure shows I-V and Voltage-Time curves for PV-module and capacitor charging operation with storage time.

Superposition of the Voltage-time axes indicates the energy charging process in the capacitor and its transference to the DC motor, using MPPES concept.

The energy stored in electrolytic condenser is discharged towards the DC motor by using MPPES (Maximum Power Point Energy Storage) concept. This is a DC/DC converter similar to MPPT (Maximum Power Point Tracking), feeding the load using maximum PV module power point. In the case of MPPES, the energy is temporally stored and discharged, repeating this cycle.

Respect azimuth and altitude mechanical traction, are driven by two independent pulleys connected to each DC motors through v-belts. The PV Module location on the solar oven is shown in FIG. 18.

This tracking configuration has some advantage, which prevents mechanical damage when is compared with the conventional “mechanical-gear” system. This tracking system driven by pulley and v-belts has great flexibility in movements but maintaining precise position.

The energy discharging process on DC motors for azimuth or altitude tracking is given by equation (a):

FIG 18 PV Module location at the top of the main structure of the solar oven

-V^exp-2tdt

Rmotor T

0

From theoretical calculation for voltage-current discharge cycle:

v(t) = Vm exp(-t/t) and i(t) = Vm / Rl exp(-t/x)

Solving equation (a) and using t = 0.156 sec, the stored energy is equal to the consumed energy. If the sun position displaces about 1.5° every 6 minutes, it is enough time for charging DC motor supply energy [4].

CONCLUSIONS

Most of the solar concentrator cooking systems does not posses an autonomous tracking system. We have demonstrated how 5 Watt-peak PV Module can track 2.6KwTH solar concentrator cooking system by means of electrolytic condenser storage system using two DC motors. This cooking system avoids deforestation, one of the mayor rural problems. The PV-based stand-alone tracking system has big energy factor-merit of about 520 times, due to the reduced electrical energy consumption for obtaining high thermal energy. This is thought as the first time that MPPES (Maximum Power Point Energy Storage) concept is used for a stand-alone Sun Tracker system[7]. In addition to the last facts, the Redundancy provided by the use of two detection elements increases the Sun Tracker’s efficiency.

Solar oven reliability is now in their evaluation stage and the total cost is about US$2,500 (by March 2004). The following paragraph describes obtained important fact.

From 60% to 65 % global thermal efficiency (output power available over the incident solar power) has been obtained. The prices of the produced energy is estimated to be US ф 3.0 per kWh as equivalent to electric energy and US$ 1.3 W-peak for an installed total system considering 5.2 peak-hour [2] locally available direct solar radiation resource. This is based on the 30-year estimated system lifetime.

They pay back time is estimated in 3.5 years considering energy price at US ф 15 per kWh (June 2003). This solar oven contributes reducing 2.87 Ton/year of firewood combustion, which means 5.32 Ton/year of CO2 emission to the atmosphere [5, 6].

The cost due the use of the oven is around US ф 30.2 per day, during the 30 years lifetime system, this cost represents US ф 3.8 per individual a day considering 8 people per solar oven.

As a protection issue to the Environment, the solar oven implies big benefits. This prototype can be promoted as a green bonus for CO2, that UNEP (United Nations Environment Program), the GEF (Global Environmental Facility) and the World Bank Institute provide as a result of the Kyoto Protocol to reduce greenhouse effect.

To determine the flux on an arbitrary surface two ele­ments are required, the en­ergy striking a region on a surface and the area of that individual region. The gener­ated data cube provides us with the first of these com­ponents, the energy. What is then required, is those energy values need to be compensate for the non­horizontal-planar surface en­ergy calculation and generate a curve that best fits all of these corrected data points.

Initially, from the data cube points in space representing 400 time the solar concentra­tion for planar surfaces below the focal point were extracted

an example of which can be seen in Figure 2. This provided a list of points in space of equal energy, and from this list a surface can be generated that best represents those points. For our simulations we chose to use only a quadric surface as described by [13]
and included for the readers convenience. This method describes the fitting of a quadric surface using the least squares method which has the general form

M2 + k2y2 + k3z2 + k4xy + k5yz + k6zx + k7x + k8y + k9z = 1, (1)

where x, y,z represent relative displacements about an orthogonal basis in three- dimensions and the nine coefficents ki(i = 1,2 9) define a unique quadric surface.

The input data to the fitting procedure is a set of 3D coordinates of the sample points (a series of (x, y,z) values). If there are m sample points, there are m( x, y, z) values. The­oretically, the nine unknown coefficients can be solved from a group of nine linear equa­tions of the form of Equation 1, each of which has one data point (xi, yi, zi) assigned to its corresponding variables, x, y and z. However, due to sampling errors such a result is not robust [13].

A more practical way to solve this quadric fitting problem is to find the least squares solu­tion of a group of m linear equations,

where,

generated by substituting in each of the m, points defining regions of constant illumina­tion,

X0 = [k1 k2 k3 k4 k5 k6 k7 k8 kg],

representing the coefficients and

bmx1 [1 1 … 1] .

Normally, the least squares solution does not satisfy all equations in the group, but it min­imises the value of the residual error

er = 11 AoXo_Bo |І2,

and this solution can be considered the optimal least squares solution of the problem.

The solution X0, can then be computed by the normal equation

Xo = (ATAo)’1ATb.

Having determined the coefficients k, the surface normal at each of the points is defined by the vector [14],

[2k1 Xi + k4yi + k5Zi + k7, 2k2Yi + k4Xi + kaZi + ke, 2k3Zi + k4Xi + k5yi + kg].

The initial data cube can now be rescaled by dividing each of the corresponding points by the dot product of this surface normal vector and the z-normal vector (representing the deviation of the flux from striking a non-horizontal-planar surface). The actual surface rep­resenting areas of uniform flux can then be generated by iteratively applying these meth­ods to optimise the surface area and energy combination until it falls within reasonable tolerances.

After the demonstration of the feasibility of direct steam generation in parabolic troughs [4] one focus of further research activities of this technology lies in the development of a suitable storage technology. The development of a cost effective DSG-storage concept (Fig. 7) is the aim of the recently launched DISTOR project funded by the European Community within the 6th Framework Programme on Research, Technological Development and Demonstration.

Regarding efficiency, a fundamental demand for thermal storage systems in power plants is the minimization of temperature differences between working fluid and storage medium. This requires isothermal storage systems for the DSG-process. An obvious solution is the application of latent heat storage materials. Fig. 8 shows the process in the T-s diagram: during the charging period, heat from the condensating steam is transferred to the melting storage material (Fig. 8, 2-3). During the discharge process, heat from the solidifying storage material is used to generate steam (Fig. 8, 6-7).

The selection of the latent heat storage material depends strongly on the saturation temperature resulting from the pressure in the steam cycle; the DSG-process with an operation range of 30-100 bar requires melting temperatures between 250°C and 300°C. Considering also economic aspects, candidate materials for latent heat storage systems are salts. Although this approach has often been suggested, only limited experience is available in this temperature range. Most problems result from the low thermal conductivity of salts, particularly in the solid phase.

Basically, there are two methods to overcome the problems resulting from the low thermal conductivity:

о reduce the specific resistance for heat conduction in the latent heat storage material

о reduce the average distance for heat conduction within the storage material Solutions based on both methods are investigated within the DISTOR project. The specific resistance for heat conduction can be reduced by embedding the storage material in a matrix made of a material with a high thermal conductivity, such as expanded graphite. This approach has been tested for low temperature applications and will be extended to the operation range of the DSG-process. This development aims at a composite material with an effective thermal conductivity in the range of 5-10W/(mK).

Reducing the average distance for heat transport in the storage material means an increase of the ratio of surface area to mass of storage material. By introducing an intermediate heat transfer medium between storage material and steam pipes the surface of the storage material can be extended while the mass of piping remains constant.

Within the DISTOR-project three basic storage concepts will be tested in laboratory scale. Based on the experiences gained with 10kW lab-units, one concept will be selected for the design of a 100kW storage unit that will be connected to the DISS test facility to assess the storage system under realistic operating conditions.