Category Archives: Particle Image Velocimetry (PIV)

Isothermal storage systems using latent heat

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.

STAND ALONE TRACKING SYSTEM WITH SMALL PV MODULE

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.

Design of Hot Water Storage Tank

2.3. Design of Heat Exchanger

Heat exchanger is required to transfer the heat without mixing system hot water with the chemicals (liquid) available in the process tank. Following points have been considered while designing the heat exchanger.

a. Quality of the chemical (for selecting the material of H E)

b. Temp. rise required in the liquid.

c. Heat transfer co efficient of The H E material.

d. Time available to increase the temp to set level.

e. Volume of the chemical tank.

f. Evaporation losses during heating from the top portion.

g. Make up liquid volume

2.4. Control System & Piping layout designing

i) Control System & instrumentation: The system is designed such a way that max. Heat is to be collected even during the rainy season when sunshine is not with constant intensity. So we have used the differential controller system with the difference set point. As per the above calculation procedure we come to the requirement of 95 collectors for the process of 60 trolleys per day. To save the space for solar collector installation it has been decided to fabricate a SUPER STRUCTURE on the asbestos roof of the factory shed.

There are two Temp. sensors (PT100) provided to sense the hot water outlet temp of collector outlet and at the bottom of Hot Water storage tank. When the temp. of water at the collector outlet crosses the difference set, the pump gets started and forces the low
temp water of the storage tank to the collector and the process continues till the difference comes down below the set point the pump gets off and stops the circulation. Suitable capacity pump is used for circulation of water with mechanical seal to handle high temp. Liquid.

ii) Piping layout Designing ( Лв per the schematic diagram )

a. Solar Collector & Tank Piping (Solar Loop): Solar collectors are connected in series and parallel and cold-water inlet is connected at the bottom of the each row of the collectors. Hot water outlet has been taken from the top of the solar collector of each row. There is temp. sensor is fixed at solar collector outlet and bottom of the hot water storage storage tank. The pump is used at the inlet side of the collector, which sucks the water from the tank bottom (lowest temp. ) and pushes to the collector inlet. The size of the piping is design to circulate the required quantity of water within the average sunshine hours.

b. Chemical tank and hot water storage tank piping ( Process loop ) : The function of this loop is to carry the hottest water from the tank and circulate through the heat exchanger of the three tank where chemicals have to be heated. The rate of flow is decided by the calculation of heat exchanger and accordingly the pump capacity and pipe size has been decided. The complete piping is insulated by rock wool with aluminum cladding on it to prevent heat loss. The tube type heat exchanger is used so that it can occupy the minimum space with max heat exchange as well as it should be removable to clean the scaling formed due to the deposition of chemical on the tube. The on-off valves have been provided at the inlet of the heat exchanger of each tank to control the hot water flow as per requirement. This has been done manually and temp indicators with electronic buzzers are provided to indicate the temp and removal time of each batch. Compressed air is also circulated to mix the chemical uniformly & to get uniform temp. at every point in the tank.

Development of a MaReCo-Hybrid for Hammarby Sjostad, Stockholm

Anna Helgesson, Peter Krohn, and Bjorn Karlsson Vattenfall Utveckling AB, SE-814 26 ALVKARLEBY, Sweden,

Phone no: +46 26 83500, Fax no: +46 26 83670, e-mail: anna. helgesson@vattenfall. com

Introduction: The main disadvantage of using solar electricity is the high cost. By using concentrators to increase the radiation, some of the expensive absorber material can be replaced by cheaper reflecting material. One disadvantage with concentration is, however, that the cell temperature rises, resulting in a reduced electrical efficiency. The performance can be improved by cooling the cells, and this is the background for the development of the so-called MaReCo-hybrid. This is a specially developed concentrating collector giving both heat and electricity. The MaReCo-hybrid has been developed at the Swedish company Vattenfall Utveckling AB, and has now been delivered for installation in a newly build residential area in Stockholm. The paper will present the collector in more detail.

Introduction

Hammarby Sjostad is an old industrial area in Stockholm, Sweden. This area is now changed into a modern ecologically sustainable residential area. The company SBC Bostad has been planning to use solar energy for heating and electricity generation in one of its buildings. The use of solar energy will help to create a green image. The collector to be used is a so-called MaReCo-hybrid, developed at the Swedish company Vattenfall Utveckling AB. This construction gives both heat and electricity, and is therefore an interesting way of making an efficient use of the available roofing area. The utilisation of the heat can also be seen as a way to reduce the costs for the electricity generated.

During spring 2004, 30 m2 of the MaReCo-hybrid was delivered to Hammarby Sjostad. During the rest of the year, measurements will be made on the performance of the hybrid — system.

Background

One barrier for an increased utilisation of the PV technology is the high costs and the low output. By using concentrators to increase the irradiation onto the solar cells, the use of PV can be made more cost competitive. In this way, some of the expensive solar cell area can be replaced by cheaper reflecting material. One disadvantage with concentration of the radiation onto the solar cells is, however, that the cell temperature rises, resulting in a reduced electrical efficiency. By cooling the cells with circulating water, the output from the PV-module can be improved. The heated cooling-water can then also be utilised for heating purposes, and this will result in an increased total energy output and a better performace/cost-ratio. A system giving both heat and electricity is called a hybrid system.

In this case, a special construction named “MaReCo” has been used. The abbreviation MaReCo stands for “Maximum Reflector Collector”, and the name refers to the purpose of the construction, which is to replace some of the expensive absorbing material with cheaper reflector material. The hybrid-MaReCo has been developed at the company Vattenfall Utveckling AB, and it consists of an asymmetric reflector trough and a specially designed hybrid-absorber. A large part of the development work has been made within the frames of two large Swedish RD&D-programmes concerning solar heating that have been
going on since 1996 (Helgesson et al, 2000 and 2004) and within a national R&D — programme concerning solar electricity managed by ELFORSK. The goal of the development of the MaReCo-hybrid has been that the installation cost shall not exceed SEK 2 000 per m2 glazed area, and that the construction shall give a yearly energy output of 50 kWh electricity and 200 kWh heat per m2 glazed area.

Concept Development

However, plastics are widely used in comparable applications (e. g. in buildings as doors and windows) and even in the highly sophisticated automobile industry under increased requirements concerning structural stability and quality of surface.

Despite the lack of flexibility, a trough was favoured to a frame design for cost reasons. In any case, it is first of all necessary to find the appropriate way of production: injection moulding or deep-drawing. Apparently, injection moulding offers much more freedom of design, however, the economically favourable solution counts. A commercially interesting alternative to the existing aluminium troughs means production costs of less than 25 € per trough (2 m2 collector, ready for assembly).

For that reason, a detailed cost simulation was carried out regarding several approaches of injection moulding. As was expected, caused by the necessary high investments, injection moulding is interesting only above 60.000 pieces p. a., yet provides opportunities to reduce the cost to about 20 € per trough. At the time of the project, such a high number of collectors of one single type was not produced by any of the big producers of solar — thermal collectors. Hence, for cost reasons, a deep-drawn trough design was selected, that leads to only moderate investments of tooling. The cost limit can be met using this production technology.

The following design work, as shown in figure 7 and figure 8, was accompanied by detailed material selection. Typical solar-thermal collector operation conditions have to be considered, such as

► life-time of at least 20 years,

► all-year unprotected weather exposure,

► temperature changes from -20 °C up to +80 °C,

► UV-exposure,

► salt water atmosphere, etc.

Thermoplastics such as ASA, especially when combined with polycarbonate and glass fibres, appear well suited and are already used in relevant applications, however, they are relatively expensive (figure 6).

Luran® S, produced by BASF AG, Ludwigshafen/D, for example is a thermoplastic based on ASA and is widely used in applications comparable to solar-thermal collectors:

► automotive construction (figure 6; also commercial and agricultural vehicles),

► electrical engineering (TV antennae parts, cable connection housings, weatherproof protective housings),

► sports and other outdoor uses (sailboats, surfboards, snowboards).

PFLEIDERER DACHZIEGEL GmbH, Winnenden/D, is even using this material for roof — integrated PV-systems as shown in figure 9.

Collector and Engine

A mean solar field temperature of 250°C is required to operate the engine due to the temperature drop in the steam generator, if pressurized water is used as heat transfer medium in the solar field. Alternatively the steam can be directly generated in the collectors, resulting in lower temperatures and less thermal losses (Hennecke et al, Zarza et al). At 250°C an efficiency of 65% at 800 W/m2 irradiation and an annual yield of about 450 kWh/m2*a can be expected from the collector including losses caused by heat capacities and in piping. A solar field of 1850 m2 aperture area would be necessary for the nominal load of the engine. Due to its size the field can only be erected with horizontally mounted collectors.

According to TRNSYS-based calculations the annual electrical output from the engine related to the solar aperture area is 47 kWh/m2*a. Heat delivery from the condenser of up to 100°C accumulates to 390 kWh/m2 per year. At times of low irradiation, when less than 30 % of the engine’s design heat input is delivered by the solar field, electricity production will be stopped.

Apart from the above-described thermodynamics in stationary conditions, effects as heat capacity and controllability will play an important role. These can be best investigated in existing installations though.

Costs and Revenues

As there are no experiences about high-efficient, medium size parabolic trough collector field costs, assumptions are based on the EuroTrough technology and the PTC 1800 collector of the company Solitem. The EuroTrough collector cost is estimated to about 200 €/m2 (Geyer et al) in its first power plant project, whilst the PTC 1800 costs about 500 — 600 €/m2 (including peripheral equipment as pumps, control and installation) for a 400 m2 field size in its first installations. Basing on these numbers collector field costs of 400 €/m2 installed are assumed to be achievable. The solar field costs would amount to 740.000 €. The Spilling engine costs are 160.000 € including installation and control facilities. The investments amount to about 900.000 € in total.

While the electrical power can be fed to the grid, a typical application for the condensation heat could be domestic hot water distributed in district heating for about 500 housing units. For such an application the equivalent value of the thermal energy is about 0.05 €/kWh, amounting in income of 35.000 € per year for the solar heat. Under German regulations solar electricity from such an installation can be sold to the grid for 0.457 €/kWh,
amounting to 37.000 € income per year. This means that the electricity from the engine adds 50% of the revenues at only 20% of the investment costs.

A relation of 72,000 € savings and income per year and 900,000 € of investment cost can be regarded as acceptable for the first installations of such a system, but not in the long run though. So what are the potential future improvements in efficiency and cost?

Industrial Process Heat System

Such a system is suitable for supplying hot water or low temperature steam to various industrial applications (e. g. food industry). The system consists of an array of collectors, a circulating pump and a storage tank. It includes also the necessary controls and thermal relief valve, which relieves energy when storage tank temperature is above a preset value. The system is once through, thus the used hot water is replaced by mains water.

Mean monthly ground temperature values are used for the mains water temperature in simulations. When the temperature of the stored water is above the required process temperature, this is mixed with mains water to obtain the required temperature. If no water of adequate temperature is available in the storage tank its temperature is topped-up with an auxiliary heater before use. The system considered provides 1000 kg/hr of hot water at a temperature of 80°C (load). This is an average consumption of hot water for medium size food industries. The load is required for the first three quarters of each hour. The specifications of the system are shown in Table 3.

2. RESULTS

TRNSYS can give results in an annual, monthly, daily or hourly basis. Here mainly annual results are presented together with some typical monthly ones.

Simulations and results

An environment geometry where the center of the paraboloidal reflector defines the ori­gin of Euclidean space and the z-axis is aligned with the optical axis of the mirror, (forcing the focus to be at the position [0 0 3] and the base of the paraboloidal reflector to be at the point [0 0 0]) was generated. The flux distribution on 200 planes about the focal point was calculated and the initial data points of flux representing 400 times the solar insola­tion were isolated.

The correction of the data cube for non-horizontal surfaces took only five iterations and rapidly converged to high tolerances. The resulting surface designed for even illumina­tions can be seen in Figure 3a. The coefficients of the surface fit to Equation 1 are k1 = -3.915e-02, k2 = -3.915e-02, k3 = -1.208e-01, k4 = 2.755e-11, k5 = 8.652e-07, k6 = — 8.590e-07, ky = -3.184e-05, k8 = 3.183e-05 and kg = 6.951e-01.

Removing the near zero coefficients from the best fit surface generated above, the sur­face can be approximated by the general equation for this application by

x2 + y2 + a(z-b)2 = r2, (2)

which is an ellipsoid where a = k3/k1, b = kg/(2k3) and r2 = kg2/(4k1k3) + 1/k1. Again using

the code from [12] the flux distribution onto the surface defined in Equation 2 was gener­ated the results of which can be seen in Figure 3b. The variation in the intensity across the surface shape in no greater than ±5% from the desired value of 400 suns, which is within the accepted tolerances of common high-concentration photovoltaic cells.

The optical efficiency, defined here as the ratio of the amount of energy striking the re­ceiver surface to that of the total energy collected through the aperture of the paraboloidal concentrator (including a 100% reflection coefficient) was calculated at only 74% and de­creases for surfaces with higher solar concentration. This represents a value much below what is required to be commercially competitive. This short fall is created from the inabil­ity of the least squares approximation of the quadric surface to mold to the steep sides of the desired surface as seen in Figure 3a.

Discussion

It is clear that by applying the method described, a surface can be generated where upon a constant flux is incident. For the case of a paraboloidal dish concentrator where a con­stant illumination of 400 suns on its surface was desired, homogeneity was achieved with a tolerance of ±5% of value under simulation only The optical performance of such a generated surface was 74% of the available energy.

While the tolerances in homogeneity is acceptable for photovoltaic applications the op­tical performance is far below the ideal. This value however, could be greatly increased by changing the order of the surface fitting algorithm to either a cubic or even a quartic function. This gives greater flexibility in the surface fitting algorithm to mold to the desired shape. Alternatively greater optical efficiency can be gained by using combinations of quadric surfaces in regions or rapid change in structure (such as combinations of ellip­soids and tubular structures in our example).

This method is ideal for creating an evenly illuminated surface under static solar condi­tions, such as for paraboloidal dish collectors. However, the incoming solar radiation is a dynamic system. Variations in the shape of the receiver surface and/or surface orien­tation may be required to allow for variations in the terrestrial spatial energy distribution, tracking errors in concentrating components, degradation in the performance of optical components over time and the dynamic nature or Fresnel mirror concentrators such as power towers and central receiver designs. Creating a static surface to allow for all of these changes presents technical difficulties. The tolerances that each of these surfaces has for dynamic conditions needs to be determined.

This paper has presented a method to produce homogeneous solar flux distributions on a generated receiver surface. This work is complimentary to the that of [10], using differ­ent approaches to generated the receiver surfaces, for which both methods achieve high levels of homogeneity

Steam accumulators for buffer storage

The aim of buffer storage systems is the compensation of fast transients in solar radiation which usually result from passing clouds. These systems should protect the components of the power plant from the effects of sudden variations in thermal load. Characteristic features of buffer storage systems are short reaction times and high discharge rates, while the capacity is only in the range of 5-10 minutes. The function of energy storage for extended discharging periods will then be fulfilled by storage systems as previously proposed for oil and DSG parabolic trough plants.

Steam accumulators show the characteristic properties of buffer storage systems. Fig. 11 shows the basic scheme of a sliding pressure (Ruths-type) steam accumulator: pressurized, saturated water is used to store sensible thermal energy. During the discharge process, the pressure is decreased and saturated steam is generated using the sensible thermal energy from the liquid water volume.

Steam accumulators have been used for decades in process industry and power plants, applications cover a pressure range from a few bars up to 120 bar; characteristic storage capacity is 20-30 kWh/m3 [5].

There are different options for charging a steam accumulator; the energy in the storage volume can be increased by condensation of superheated steam or by feeding saturated liquid water into the steam accumulator. If a heat exchanger is integrated into the liquid water volume the steam accumulator can also be charged by a different fluid than water which might be at a lower pressure.

Fig. 12 Volume specific mass of saturated steam provided by steam accumulator for different initial pressures and pressure drops. Dashed line indicates example with initial pressure = 100bar and final pressure 55bar; steam accumulator delivers approx. 90kg saturated steam per m3 storage volume

The amount of saturated steam provided during the discharge process of the steam accumulator depends on the initial pressure and the extent of the pressure drop. Fig. 12 shows the volume-specific amount of saturated steam released during discharge for various initial pressures depending on end pressure.

A cost effective approach for integration of buffer storage capacity is the combination of the steam accumulator with other components of the power plant; Fig. 13 shows the simplified scheme of a parabolic trough power plant. The collector field is operated in the recirculation mode, i. e. wet steam from the collector field flows into a steam drum where the liquid phase
is separated from the gas phase. The volume of the steam drum can be used to store saturated water; by variation of the water level the energy content can be changed.

Steam accumulators can also be used for parabolic trough power plants using a thermal oil as a heat transfer medium in the solar collectors if the energy provided by the collector field is used in a steam process; here, the steam accumulator is integrated in the secondary loop. Fig. 14 shows a parabolic trough power plant with thermal oil in the solar collectors; the steam accumulator is used as a heat exchanger between oil loop and water/steam loop. Heat from the solar field is used to heat the liquid water volume of the steam accumulator indirectly. The energy content of the heat exchanger/steam accumulator is related to the water level

In a Ruths-type steam accumulator the steam pressure drops during discharge. For some applications, a storage system providing steam at constant pressure is advantageous. One option to avoid a pressure drop is the application of a separate flash evaporator (Fig. 15): the saturated liquid water taken from the steam accumulator is depressurized externally, cold water is fed into the storage vessel to keep the water level constant, mixing of hot and cold water must be minimized, thermal stress resulting from filling the pressure vessel with cold water must be considered.

Another option for constant pressure storage is the integration of phase change material (PCM) into the storage vessel partly replacing the liquid water (Fig. 16). Here, the thermal energy associated with the phase change between liquid and solid state is used for isothermal energy storage. PCMs usually exhibit a low thermal conductivity so layers of this material must be thin to ensure a sufficient heat transfer rate. One option to fulfill this demand is the encapsulation of PCM in small containers placed inside the liquid volume. Using PCM is not only attractive regarding the avoidance of thermo mechanical stresses resulting from temperature transients, the characteristic volume-specific storage capacity of PCMs is in the range of about 100kWh/m3. Compared to the corresponding value for water (20-30kWh/m3), the integration of PCM helps to increase the storage capacity of a given pressure vessel.

Although steam accumulators exhibit only a small storage capacity, the availability of these buffer storage systems can contribute to reduce the investment costs for storage capacity if they are combined with storage systems intended for longer periods of discharge. By reducing the requirements regarding response time and discharge rate the specific costs for storage systems with several hours of heat capacity can be reduced.

Acknowledgement

Part of the work presented in this paper has been funded by the German Federal

Environment Ministry under the contract code PARASOL/WESPE and part by the European

Commission within the 5th Framework Programme on Research, Technological

Development and Demonstration under contract no. ENK5-CT-2001-00540.

The authors are responsible for the content of this publication.

[1] Solar Energy Laboratory (LABSOLAR) — Florianopolis BSRN station.

[2] We would like to point out that the objective function is the collector gain on a daily (or monthly or yearly) basis. It is not absolutely necessary to be able to correctly describe the momentaneous collector performance in every timestep of operation.

[3] This process has been discussed in section 5.

[4] For collectors with a biaxial incident angle behaviour the incident angle in east west direction has to be considered

[5] the sum of the absolute values of the difference in calculated and measured power per time step divided by the sum of the measured power per time step must be less than 5% (equation 5).

[6] V. Weitbrecht, D. Lehmann, and A. Richter. Flow distribution in solar collectors with laminar flow conditions. Solar Energy, 73(6):433-441, 2002.

[7]eff = = г (equ — 2)

PTin ‘ Ac PTin ’n’ reff

The effective cross section for the flow in vertical direction was assumed as circular with the radius reff. To further improve the model accuracy, a constant, hoffset, was additionally taken into account:

[8] Presently a PhD candidate at Queen’s University, McLaughlin Hall, Kingston, ON, CANADA. K7L 3N6 Email — mesauita@me. queensu. ca

[9] Companhia Energetica de Minas Gerais, Av. Barbacena, 1.200, Belo Horizonte, MG, BRAZIL.30161-970

[10] 23456789 10

Solar irradiation on collector plane [kWh/(m2 d)]

[12]random order not according to the order in the presented diagrams and tables

[13] Corresponding author ph: +61(0)2 93515979 fax: +61(0)2 93517725 email: d. buie@physics. usyd. edu. au

[14] An air cooled condenser is used for both options. This will be not the case for the real plant. Since this investigation is a comparison between two technologies and not an investigation of a single option this difference is not that important.

[15]1 (PWH1-C

TRACKING SYSTEMS FOR SOLAR ENERGY. CONVERSION DEVICES

Ion VI§A, Prof. dr. eng., Transilvania University of Brasov, Romania, Department Product Design and Robotics, 2200 Brasov Bdul. Eroilor nr.29 e-mail: visaion@unitbv. ro tel.0040 268 419010

Mihai COM§IT, PhD. eng., Transilvania University of Brasov, Romania, Department Product Design and Robotics,2200 Brasov Bdul. Eroilor nr.29 e-mail:comsit@unitbv. ro tel.0040 268 419010

Introduction: The research on renewable energy systems, especially based on solar radiation conversion was mainly orientated on aspects related to the materials and processes directly involved. Already on the market, most of these systems must find now optimum mechanical design that would enhance the output by cutting losses or by using more efficient the solar radiation.

The main input data in designing systems for solar energy conversion is the solar radiation. A method to increase the performances of such a system is to orient the receiver (of the collector or PV panel) in order to follow the sun path on the sky.

The devices created to accomplish this function are called tracking systems. Orientation of the conversion systems in order to intercept the maximum amount of solar radiation that reaches the ground level, may increase the efficiency of the system from 25% up to 50% [3].

The available radiation at the ground level is called global solar radiation. Because of the atmosphere, the radiation may be transmitted, absorbed, scattered or reflected [10]. As a result of atmospheric effects three components of the solar radiation has to be considered: direct solar radiation, diffuse solar radiation and ground reflected radiation (Fig.1).

The most important component of that solar radiation, that determines the structure of the conversion systems, is the direct component of the solar radiation.

The aim of this paper is to identify accurate and efficient mechanical configurations suitable for tracking systems using a structural synthesis method based on Multi Body System theory.