J. P Pretorius, Dept. Mech. Eng. Univ. of Stellenbosch, South Africa D. G. Kroger, Dept. Mech. Eng. Univ. of Stellenbosch, South Africa J. D. Buys, Dept. Math. Univ. of Stellenbosch, South Africa T. W. Von Backstrom, Dept. Mech. Eng. Univ. of Stellenbosch, South Africa

The performance of large scale solar tower power plants are evaluated. The investi­gation focuses on a reference plant with a 4000 m diameter glass collector roof and a 1500 m high, 160 m diameter tower. A numerical simulation model solves the relevant draught and conservation equations using specified meteorological input data for a particular location in South Africa. It is shown that plant power output varies consid­erably with the time of day as well as monthly. The dependency of the power output on collector diameter and tower height is illustrated, while showing that greater power production can be facilitated by optimizing the solar collector roof shape and height.

1 Overview

A solar tower power plant, illustrated schematically in figure 1, consists of a transparent circular collector supported relatively low above the ground surface. Central to the collector is a high tower with one or more turbo-generators located at its base. Solar radiation penetrates the collector roof and heats the ground beneath, which in turn heats the adjacent air, causing it to rise through the central tower which drives the turbine and consequently generates electricity. Performance measurements from a 50kW prototype solar tower power plant plant built in Manzanares, Spain in 1982 proved that the solar tower concept is technically reliable ((Haaf etal., 1983; Haaf, 1984)) and potentially economically viable (Schlaich, 1994). The first relatively detailed published analyses of the performance of such a plant were presented by Gannon and Von Backstrom (2000) and Kroger and Buys (2001). More recent publications are by Gannon and Von Backstrom (2003) and Bernardes et al. (2003).

This study determines the performance characteristics of a reference solar tower power plant, as specified in the appendix of this document, for the meteorological conditions given in tables 1 and 2.

2 Analysis

2.1 Collector

Relevant conservation equations are derived for a defined elementary control volume in the collector of the solar tower power plant. Since changes in the dynamics of the air stream are relatively slow, transient terms in the conservation equations are found to be negligible.

2.1.1 Continuity

Assuming purely radial airflow in the collector, the steady state collector continuity equation is [15]

2.1.2 Momentum

The simplified steady state momentum equation applicable to a collector control volume is

There exists a developing flow region near the collector inlet which becomes fully developed fairly rapidly. This developing flow region was investigated by Kroger and Buys (1999), but is not considered in this study. For a fully developed flow assumption in the collector, according to Kroger and Buys (1999) the roof shear stress is given by тг = 0.02[(p°8v 1-8ц°-2)/(Н0-2)] while the ground shear stress is given by тд = 0.014875 pv2(eg/2H)0254[1.75 (ц/pvtg)°51+1].

The collector roof height at a specific radius is evaluated according to H = H2 (r2/r)b. Sup­ports are arranged under the collector roof at specified radial and tangential pitches. For an annular control volume (360°control volume between two collector radii), the support drag force per unit radial length is given by Fsupports = ^ FsrD/Ar where ^ FsrD is the sum of all the support drag forces in the specific control volume. The drag force enforced on the air by all the supports at a specific collector radius is given by FsrD = (CsD m2ds rb-1) / (4npP(H2 rbb).

2.1.3 Energy Collector roof

The steady state energy balance for a transparent collector roof is

aeb ihb + aed ihd + qgr = qra + qrs + qrh (3)

Approximating the collector roof as a horizontal surface, the total solar radiation incident on the roof is Ih = Ihb + Ihd. The respective beam and diffuse effective absorptivities are determined by aeb = (ay, b + a±, b)/2 and aed = (ay, d + a±, d)/2, where the respective parallel and perpendicular polarization components are evaluated according to a =

Ta)] / (1 — PTx). Relations for the roof interface reflectivity are given by Modest (1993) as

Ру = [tan2(0i — 02)] / [tan2(0i + 02)j and p± = [sin2(01 — 02)j / [sin2(01 + 02)j. According to Duffie and Beckman (1991), an equivalent incidence angle of 0! = 60° should be used for diffuse solar radiative calculations. The refractive angle 02 is determined using Snell’s law.

Modest (1993) gives a relation for the transmissivity due to the absorptance as та = exp[-(Cetr)/cos 02]. The radiation heat flux from the ground to the collector roof is given by qgr = o(Tf — Tf / (1 /eg + 1 /er — 1). The convection heat flux from the collector roof to the ambient air is qra = hra(Tr — Ta), where hra is evaluated according to the very approxi­mate equation hra = 5.7 + 3.8vw (Duffie and Beckman, 1991). The radiative heat flux from the roof to the sky is qrs = erc(T4 — Tfy), where Tsky = 0.0552 T)5 according to Swin — bank (1963). The convection heat flux from the collector roof to the air in the collector is qrh = hrh(Tr — T), where hrh is determined using Gnielinski’s equation for fully developed turbulent flow: hrh = {[(f/8)(Re — 1000)Pr] / [1 + 12.7(f/8)1/2(Pr2/3 — 1)]}(k/dh). Gnielin — ski’s equation alone will tend to underestimate the convection heat transfer rate since it does not make provision for effects due to natural convection. For smooth surfaces the friction factor is given by f = (1.82 log10 Re — 1.64)-2 (Kroger, 2004). For rough surfaces, Haa — land (1983) recommends f = 0.3086 [log10(6.9/Re + (e/(3.75dh))1-11)]-2 for t/dh > 10-4 and f = 2.7778 {log10[(7.7/Re)3 + (e/(3.75dh))3-33]}-2 for cases where t/dh < 10-4.

Ground

The energy balance at the ground surface (z = 0) is

The respective beam and diffuse effective transmissivities are determined by Teb = (ту, b + т±,b)/2 and Ted = (ту, d + т±,d)/2, where the respective parallel and perpendicular polariza­tion components are evaluated according to т = [(1 — р)2та] / (1 — р2тО). From Duffie and Beckman (1991), the beam and diffuse transmittance-absorptance products of equation (4) are determined by (теag) = теag / [1 — (1 — ag)pd], where pd may be estimated using the equation by Duffie and Beckman (1991): pd = т^ — Ted. The convection heat flux from the ground surface to the air in the collector is qgh = hgh(Tg — T), where hgh is determined using the previously mentioned Gnielinski equation. Assuming constant ground properties, the energy balance below the ground surface (z > 0) is

At a considerable depth in the ground the temperature gradient becomes zero and the boundary condition dTg/dz « 0 is valid.

Collector air

When only regarding terms of significant order, the steady state energy balance for the air in a collector control volume becomes

qrh + qgh = pvH —{Cp T)

2.2 Tower

Relevant conservation equations are derived for a defined elementary control volume in the tower of the solar tower power plant.

2.2.1 Continuity

The tower steady state continuity equation is

d

(p, W )=0 (7)

2.2.2 Momentum

The simplified steady state momentum equation applicable to a tower control volume is

According to White (1999), the tower wall shear stress is determined by Tt = (ft ptvf)/8. The total bracing wheel drag force per unit tower height, based on the tower inlet dynamic pressure, is given by Fbw = [AtKbw (Ptiv%)/2 nbw ] / H.

2.2.3 Energy

When only regarding terms of significant order, the steady state energy balance for the air in the tower is

d / d

P, W g^(CPt + dZ (P,* gz) = 0

2.3 Power

The power generated by the turbine is P = ntgApturbVavg, where the pressure drop across the turbine is calculated by the draught equation:

Apturb = Ap — (Ар, + Apcoii + Apturb, І + Ap, + Apto + Apdyn) (10)

The driving force or potential that causes air to flow through the solar tower power plant is due to a pressure difference between a column of cold air outside and a column of hot air inside the tower. Assuming a dry adiabatic lapse rate (DALR) for the air outside and inside the tower, we find the drive potential from relations by Kroger (2004): Ap = pi{1 — [(1 — 0.00975 (Ht/T|)) / (1 — 0.00975 (Ht/T5))]3-5}, where the numbered subscripts refer to the positions in figure 1. Assuming a constant mass flow rate and constant specific heat capacity over the turbine, the temperature drop across the turbine can be expressed as T5 = T4 — (ApturbVavg )/(mCp).

The collector inlet pressure drop between the essentially stagnant air at 1 and the inlet at 2 (see figure 1) is Ap, = (pi — p2) = K (p2v|)/2 + (p2v|)/2. The pressure drop in the collector, Apcoli, caused by accelerating radial airflow, roof and ground friction and roof support drag forces are all incorporated in the collector momentum equation (equation (2)). The pressure drop over the turbine inlet is Apturb, i = p3 — p4) = Kturb, i (P4 V42)/2 + (p4 V42)/2 — (P3 V32)/2. The accelerating tower airflow, the inside tower wall friction and the internal bracing wheel drag forces result in a pressure drop over the tower height, Apt, and are incorporated in the tower momentum equation (equation (8)). The air exiting the tower experiences a pressure differential due to the shape of the tower outlet, and is expressed as Apto = Kto (p6vf)/2. Employing a relation by Kroger (2004), during relatively quiet (no significant ambient winds) periods, the tower outlet loss coefficient is approximated as Kto = -0.28Fr-1 + 0.04Fr-15,
where FrD is determined (see figure 1) by FrD = (m/A6)2 / [p6(P7 — p6)gdt]. The dynamic tower outlet loss is Apdyn = (p6vf)/2.

The analysis performed in this study does not differ significantly in approach to the paper by Bernardes et al. (2003). However, there are some notable differences: this study always assumes fully developed turbulent flow in the collector, takes into account the temperature change across the turbine and employs a quasi-steady state solution procedure. A draught equation calculates the pressure drop across the turbine for conditions determined during a specific iteration. At each time step, for the given environmental conditions, the mass flow rate through the system is optimized to produce a maximum plant power output. The study by Bernardes et al. (2003) selects a particular ratio of pressure drop over the turbine to total pressure difference (0.9) which governs the mass flow rate, pressures and temperatures throughout the plant. This paper does not specify such a ratio, but allows the optimized mass flow rate to govern the pressures and temperatures through the system.

The maximum temperature inside the pressure cooker is around 125° C, which corresponds to a pressure of 1.3 kg/cm2. The maximum thermal power available is about

2.3 kWTH with the system global efficiency of 60%. The thermal storage system based

FIG. 9 Front view of 360 mirrors and the oil storage container

STERILIZATION

As mentioned, the oven may be used for sterilization of medical instrumental because it is possible to make the 8 liter commercial pressure cooker reach more than 120°. FIG. 10 and 11 shows this. The sterilization cycle consists on 30 minutes at 120° C applied and then 15 minutes for dehydration followed by 15 minutes of cooling for the objects.

COOKING MEALS

This system may cook the Mexican rural people diet basis which consist on beans, corn and coffee; it means, due the high energy needs for cooking such meal, that its practically possible to cook anything that fits inside the commercial pressure cooker, FIG. 12 shows the fact. The capacity of the pressure cooker provides enough amount of meal for 8 adults a day. Unfortunately, it is still required to cook under sunlight when fried meals are desired.

At this time, it has been started a program whose purpose is to determine cooking times for several kind of typical Mexican food for different regions in the country, this will allow to have a list of recipes with its own solar cooking times.

Damien Buie[13], David Mills, Anne Gerd Imenes, Solar Energy Group, School of Physics, Building A28, University of Sydney, Australia 2006, and Philipp Schramek, Muhlbergstr.26, 82319 Starnberg, Germany

This paper presents a methodology to create a receiver surface in a concentrating system that will be illuminated with an homogenous solar radiation flux. By means of a theoretical simulation, a receiver surface in a paraboloidal dish concentrator was generated where variations in the illumination across the receiver surface was not greater than ±5%. The optical efficiency of the generated surface however, was not ideal, intercepting only 74% of the total reflected insolation. This poorer than ex­pected optical efficiency can be overcome with a more robust mathematical model for generating the receiver surface.

Introduction

Homogeneous flux distributions in the focal region of concentrating solar power systems greatly improve both the performance and longevity of solar receiver modules. Photo­voltaic (PV) receivers for concentrating systems have limitations on both the thermal load and solar fluxes incident on their surface. Where the solar illumination on the PV cell’s surface is non-uniform the problem is exacerbated as it causes significant local heating and higher ohmic drops across the cell, the result of which is a decrease in efficiency [1,2, 3]. These local hotspots can also permanently damage expensive high concen­tration PV cells.

The flux distribution incident on a thermal absorbers can, in some cases, also benefit from an homogenous illumination profile. Hot spots can cause rapid degradation in se­lective surface coatings on thermal receivers and the effect of extreme fluxes within vol­umetric receivers are still being researched [4]. It is necessary to point out however, the performance gain achieved by even illumination over thermal receivers is of far less im­portance than the requirement for even illumination over photovoltaic receivers.

There exists a large number of methods to create an homogenous flux incident onto a re­ceiver, each with its own advantages and limitations. Ries [5, 6] among others described methods to directly tailor optical surfaces such that a flux distribution can be created in an imaging plane. Ries also illustrated that the shape of the flux distribution could be defined by correctly designing either the reflector or lens. This method is ideal for applications with a static illumination field, minimising optical losses by using only one concentration surface such as in trough or dish concentrators.

A second method of creating homogenous flux distributions, is to use secondary opti­cal components between the concentrator and the receiver modules, such as compound parabolic concentrators (CPC) or kaleidoscopic distributors [7]. This methods creates the desired flux distribution at the cost of optical efficiency, which is reduced due to the ad­ditional optical components in the system. These systems do however, have the advan­tage of a possible use in dynamic solar applications such as power towers, where the sun moves relative to the optical components. Combinations of both designing the concentrat­
ing component and secondary concentrators have also been successfully demonstrated [8] adopting both the pros and cons of each system.

Finally, homogenous fluxes have been generated by the micro-alignment of individual heliostat’s aiming points in large Fresnel concentrators [4]. This type of system is ideal in terms of optical considerations for large concentrators as it introduces no extra opti­cal surface. The system does however have the inherent problem of dealing with both the large amounts of residual errors that exist in the accuracy of all of the optical com­ponents, from the tracking accuracy of the heliostat to the calculated position of the sun, and the computer power required to optimise the flux distribution in real time. In spite of this, an homogenous flux distribution on a planar surface has been successfully demon­strated at the Plataforma Solar de Almeria (PSA-CIEMAT) [9].

An alternative approach to the above mentioned methods is to shape the receiver so that an even flux will fall on its surface [10]. By investigating the solar fluxes about the focal region and using a detailed optical model, this paper describes one method to create such surfaces for a single theoretical application. For this particular paper we chose to design an evenly illuminated surface for a simple case: a paraboloidal dish concentrator with an aperture and focal length of 3 m.

As comparison of both storage materials did not provide a clear preference for one material, both materials have been considered for the test storage units. Therefore two storage units have been designed and constructed. Dimensioning of the storage has been performed with the simulation environment “StorageTechThermo”, developed at DLR, where the storage is described by physical models, allowing also for dynamic simulation of the system. This model is described in detail in [3].

To allow for good comparison, both units are of same dimensions. The storage capacity for the ceramic storage unit is around 350 kWh storage capacity and for the high temperature concrete unit it is approx. 20% lower. Each of theses storage units is build up of two parallel storage modules.

The heat exchanger is composed of 36 tubes of high-temperature steel with nominal diameter of 21×2 mm. They are distributed in a square arrangement of 6 by 6 tubes with a distance of 80 mm. Collectors and distributors have been designed for best equal flow distribution in all tubes.

One storage module has the total dimensions of 0.48×0.48×23 m3. The thermal expansion of one module of castable ceramic with steel heat exchanger, when heated up from ambient to maximum operation temperature of 390°C, is 125 mm, for the concrete modules it’s 120 mm, so the modules will move approx. 60mm to each side. During cycling at operating temperature the temperature difference is only 40 K so the movement is only less than 10 mm to each side. To allow for this movement, the storage modules are bedded on two metal sheets as sliding planes.

All four blocks have been built in parallel. For practical reasons, due to the different production methods, the two outer modules have been filled with the castable ceramic and the two middle modules with high temperature concrete as storage material. Fig. 1 shows the construction of the storage units. All four heat exchangers have been welded together on site. On this picture the first concrete module has just been casted and the worker in the background is compacting the material with a vibrator. The shuttering boards for the second concrete module are already fixed.

Fig. 1 Heat exchangers with collectors and distributors of the four storage modules, first high temperature concrete module just casted

The storage units are equipped with 208 thermocouples casted into the storage modules, 2 flow meters, 2 differential pressure sensors and 8 Pt-100 for measuring oil temperature. Fig. 2 shows all four storage modules after casting and curing, with all piping connected. The sensors are being connected to the three switch cases on the left side.

Fig. 2 Storage module without insulation

Mr. Prabhakar Wawge, Area Manager, Peenya Alloys Pvt. Ltd.

206, Patil Plaza, Parvati, Pune 411009 ( MS ) India

Introduction

In any industry, heating, cooling and compressed air the costliest part, which affects the production cost of any product. There are three types of indirect heat requirement or the requirement of heat can be divided in the three main categories.

1 low temp. 40 — 60 Deg

2 Medium temp. 80 — 150 deg.

3 High Temp applications — above 150

Solar Flat Collectors have been proven for the use of solar energy for medium temp. application in hotels, boiler feed water preheating, dairy for pasteurization and some other indirect heating applications. There is another neglected area of application of Solar Flat Plate collector is heat treatment for powder coating plants where heat requirement is bet 50 Deg C — 70 Deg C. In any automobile industry the aesthetic or look of the vehicle place a very important role as far as the sale is concern (after the mechanical performance). The aesthetic means the body and colour of the vehicle. To get a long lasting good quality color, the powder coating procedure plays a major role. Before powder coating there is requirement of different chemical treatment for the removal of rust, grease and other cleaning of the specific sheet metal body parts. The time duration and chemical composition is depends on the selection of body material. A proven method of a chemical treatment is seven / eight tank process.

The common system of heating chemicals is by way of electrical heaters, by diesel or other fuel fired boilers. This increases the cost of heat treatment process due the high cost of electricity ( for industries rate of electricity is 1.5 to 2 times than the domestic rate ) or oils. This can be replaced by Solar water heating system which can efficiently generate the temp of liquid upto 85 Deg C.

1.History

All automobile bodies are made up of CRCA ( cold rolled cold annealed ) sheets. To prevent from atmospheric reaction the manufacturer usesupplier of CRCA sheet use to send the sheets with oil coat. During pressing operations there are chances of getting different impurities on the finished surface which is to be treated before coating. Seven/Eight Tank Process: There are eight tanks having different chemicals to remove the oil & rusting and operations are performed is given below.

1. Emulsification: This is a hand operation and the emulsifier is applied to the surface of the job to reduce the stickiness of the oil so that the cold degreasing will be more effective. This is a floor operation.

2. Tank 1 Cold degreasing: This is cold operation and do not require heating this tank.

3. Tank 2 — Hot degreasing: In this operation the temp. of chemical is to be maintained is 55-60 Deg C. The dipping time of each job is 10 min.

4. Tank 3 — Water Rinse — to extract the released oil due the last treatment. This is cold operation.

5. Tank 4 — Water Rinse — to remove the oil remained in the last treatment. This is also cold operation.

6. Tank 5 — Activation — This is a cold operation and it has been performed to reduce the prospecting time.

7. Tank 6 — Phospheting — In this tank the phospheting chemical is to be maintain betn 35 — 40 Deg. C. The job is to be kept in this booth for 5-7 min.

8. Tank 7- Water Rinse — Cold operation booth and is to be done to remove phospheting chemical remained on the surface of the job.

9. Tank 8 — Passivation — In this tank the water is to be kept bet 65 — 70 Deg C. The operation time is of 1 min.

All the tanks are of 3000 ltrs. capacity and the three tanks which are having heated chemicals are insulated.

This was a specific requirement of solar water heating system for heating chemical of heat treatment tank chemical at least for one shift from M/s Chaphekar Suspensions Pvt. Ltd. Pune. The product to be treated was the trolley of Bajaj make goods tempo (5 wheeler) . We have designed the system to meet the requirement of 60 trolleys per day. (Between 9.30 am — 6.30 pm). Schematic Diagram 1

Air Vent

SCHEMATIC DIAGRAM OF SEVEN TANK PROCESS HEATING

BY SOLAR

In the focus of parabolic concentrator of SPS multi modules thermoelectric generators (TEG) will be installed. TEG the work of which is based on Seebeck effect present semiconductor thermo pairs and are aimed for direct transformation of heat energy into electric energy [2-3]. Solar energy, focused with the help of solar concentrators, may be heat sources for hot junctions of TEG.

The advantages of TEG are the following: a long term of work, high reliability, stability of parameters, noiselessness, stability to vibration.

The drawbacks of TEG are the following: low relative energetic indices: specific mass from 10 to 15 kg/kW, surface density of power — 10 kW/m2, volume density of power from 200 to 400 kW/m3 and low efficiency of energy transformation up to 8%.

TEG may be made as batteries of silicon-germanium thermoelectric elements (TEE), connected by matrix principle in succession in branches and branches may have parallel connections between themselves. TEE batteries are put into hermetic containers of flat or cylindrical forms and filled with inert gas to avoid oxidation and semiconductor aging. Construction of power electro outlet of TEG must provide simultaneously thermo density and electric isolation from container body, which are complicated technical problems. Cascade connection of TEG makes it possible to raise transformation efficiency up to 13 %. The quantity of electric energy generated by module is directly proportional to the quadrant of difference of temperatures on module junctions. That is why it is important to have maximum high temperature difference.

To reach these results it is necessary, firstly, to provide maximum heat quantity on the hot side of the module and, secondly, to make use of effective radiator on the cold side of the module, for example, with water cooling. Alongside with this, the temperature of hot junction should not be higher than the maximum temperature value allowed for given module. Load resistance should be chosen according to the module resistance. TEG will work in the regime of maximum power at equal resistances.

And TEG will work in the regime of maximum efficiency when the load resistance is approximately equal to 130 % of module resistance.

For practical purposes the following thermoelectric materials are used: alloys based on Bi2Te3 and Bi-Sb for intervals of low temperatures up to 300°C, alloys based on PbSb, PbTe, GeTe, AgSbTe2 for medium temperatures from 300°C to 600°C and alloys based on SnTe and GeSi for high temperatures over 600°C.

The TEG will be made in a special form to be installed in the focus of a parabolic concentrator. The calculated concentrator surface area is 10 m2 for the output electrical power of about 1 kW. One of the technical solution may be the performance of TEG in the form of a cylindrical construction (a quarter of its surface), placed into transparent vacuum glass packet and with internal injected cold water. At the system output direct current is generated.

The proper sizing of the components of a solar system is a complex problem, which includes both predictable (collector and other performance characteristics) and unpredictable (weather data) components. In this work the transient simulation program TRNSYS [4] is used.

The various systems investigated were simulated with TRNSYS using Typical Meteorological Year (TMY) data for Nicosia, Cyprus and Athens, Greece. TMY is defined as a year which sums up all the climatic information characterizing a period as long as the mean life of the system. Using this approach the long-term integrated system performance can be evaluated.

Petrakis et al. [5] have generated the TMY for Nicosia, Cyprus used in the present study from hourly measurements, of solar irradiance (global and diffuse) on horizontal surface, ambient temperature, wind speed and direction, and humidity ratio, for a seven-year period, from 1986 to 1992 using the Filkenstein — Schafer statistical method. The measurements were recorded by the Cyprus Meteorological Service at the Athalassa region, an area at the suburbs of the town of Nicosia. The TMY is considered as a representative year for the Cypriot environment. Using the same method Pissimanis et al.

[6] have generated the TMY for Athens, Greece.

Two types of flat plate collectors are considered, collectors painted with normal black and color paints with an emittance value of 0.9 and collectors with selective coatings with an emittance value of 0.1. The absorptance of the black collectors is 0.95 whereas the value for light color collectors is 0.85. This value applies to light-colored collectors irrespective of the actual color. The performance equations of the collectors considered are [1]:

TOC o "1-5" h z Type A — Black collector, a=0.95 and e=0.1: n=0.8319 — 4.2629 (ДП/G) (1)

Type B — Color collector, a=0.85 and e=0.1: n=0.7453 — 4.2648 (дп/g) (2)

Type C — Black collector, a=0.95 and e=0.9: n=0.7937 — 6.7128 (ДП/g) (3)

Type D — Color collector, a=0.85 and e=0.9: n=0.7109 — 6.7316 (дп/g) (4)

The questioning of the heating plumbers — the heating plumbers are in fact the most important customers for the solar technology companies — supplies considerable input to the specification of an optimum collector. Therefore, 30 plumbers in the region of Ingolstadt were contacted and personally interviewed using a list of 24 questions out of different areas of their business, mainly in order to find out deficits and optimisation potentials of solar-thermal collectors.

It became clear that solar-thermal systems still suffer from the high amount of labour required for the roof installation of the collectors. Apparently, % of the contacted plumbers need to visit the installation site twice with up to three workers.

More than 40 % of the plumbers considered the glazing and the fixation to the roof as the major sources for complaint and repair. In this respect, У of the questioned plumbers named the sealing of the collector itself. Remarkably, only % of the plumbers had to deal with leaking pipes.

Concerning the roof installation, the questioned plumbers named the aspects shown in figure 2. Still, collectors with sharp edges annoy the workers. The plumbers clearly ask for a shift of installation work from the roof down to the ground (first steps of installation). Additionally, they ask for improved devices for the transport of the collectors onto the roof. Number one of the plumbers’ troubles, however, was the high weight and the difficulties to handle those heavy and big collectors.

Hence, the heating plumbers clearly ask for improvements concerning the weight and the handling of the collectors.

Equations (1), (2), (4), (5), (6), (7), (8) and (9) are discretized using finite difference ap­proximations. These equations, together with equations (3) and (10) form the basis of the numerical solution procedure using standard Visual Basic 6.0 code. The performance of the solar tower power plant is evaluated by maximizing the power output by determining the optimum mass flow rate through the system.

3 Results

The maximum power output of the reference plant for the 21st day of selected months are shown in figure 2. The output varies considerably during the day with the peak output de­livered approximately an hour after solar noon. Due to the energy storage capability of the ground, some power is also produced at night. There are significant differences in the output during the summer and winter months. Figure 3 illustrates the temperature distributions in the ground at various times of the day on 21 June and 21 December.

Results show that the reference plant produces 298GWh/a, as is evident from figure 4. Kroger and Buys (2001) also present simulation results for the same reference plant specified in the appendix of this paper. Their model gives an annual plant power output of 341 GWh/a. The model by Kroger and Buys (2001) predicts a higher plant power output compared to this paper due to the following reasons: Kroger and Buys (2001) employ both a developing and fully developed flow region in the collector, the temperature drop across the turbine is not taken into account and most notably a different approach is used to maximize the plant power output.

The plant power output can be increased by altering the shape and inlet height of the collector roof, as shown in figure 4. For the reference plant, a maximum power output is reached with b = 1 and H2 = 3.1 m. The annual plant output is dramatically increased by enlarging the collector diameter or tower height, as indicated by figure 5. An optimal collector inlet height for each collector diameter is shown.

4 Conclusion

The performance of a solar tower power plant is evaluated. Comparative results to earlier studies are found and similar trends established. Numerical simulation results indicate that

the plant power output varies considerably during a typical day and seasonal changes in output are significant. Peak plant output is achieved shortly after midday, while the ground’s energy storage capacity facilitates power generation at night. It is shown that the output of a plant can be increased by optimizing the collector roof shape and inlet height. The power produced by the plant can also generally be increased by increasing the collector diameter or tower height.

SHAPE * MERGEFORMAT

5 Nomenclature

References

M. A. dos S. Bernardes, A. VoR, and G. Weinrebe. Thermal and technical analyses of solar chimneys. Solar Energy, 75(6):511-524, December 2003.

J. A. Duffie and W. A. Beckman. Solar Engineering of Thermal Processes. John Wiley and Sons, Inc., New York, 2nd edition, 1991.

A. J. Gannon and T. W. Von Backstrom. Solar chimney cycle analysis with system loss and solar collector performance. Journal of Solar Energy Engineering, 122:133-137, 2000.

A. J. Gannon and T. W. Von Backstrom. Solar chimney turbine performance. Journal of Solar Energy Engineering, 125:101-106, 2003.

W. Haaf. Solar chimneys, part II: Preliminary test results from the Manzanares pilot plant. Interna­tional Journal of Solar Energy, 2:141-161, 1984.

W. Haaf, K. Friedrich, G. Mayr, and J. Schlaich. Solar chimneys, part I: Principle and construction of the pilot plant in Manzanares. International Journal of Solar Energy, 2:3-20, 1983.

S. E. Haaland. Simple and explicit formulas for the friction factor in turbulent pipe flow. Transactions of the ASME, Journal of Fluids Engineering, 185(3):89-90, 1983.

D. G. Kroger. Air-cooled Heat Exchangers and Cooling Towers. Pennwell Corp., Tulsa, Oklahoma,

2004.

D. G. Kroger and J. D. Buys. Radial flow boundary layer development analysis. South African Institu­tion of Mechanical Engineering, R & D Journal, 15:95-102, 1999.

D. G. Kroger and J. D. Buys. Performance evaluation of a solar chimney power plant. ISES 2001 Solar World Congress, Adelaide, South Australia, 2001.

M. F. Modest. Radiative Heat Transfer. McGraw Hill Inc., New York, 1993.

J. Schlaich. The Solar Chimney: Electricity from the Sun. Deutsche Verlags-Anstalt, Stuttgart, 1994.

W. C. Swinbank. Long wave radiation from clear skies, volume 89. Quart J. Royal Met. Soc., 1963.

F. M. White. Fluid Mechanics. McGraw-Hill, fourth edition, 1999.

A Appendix

For the purpose of comparison, a reference solar tower power plant and a typical operating environment is defined.

The solar radiation values and ambient air temperatures for the reference plant site are given in Table 1 and Table 2 respectively. These meteorological conditions apply to Sishen, South Africa, with latitude 27.67° South and longitude 23.00° East.

SHAPE * MERGEFORMAT

This section shows the two versions that have been proposed for the Sun detection.

The first version is shown in FIG. 13 and FIG 14. It consists in an array of eight trios of photodiodes located at the four quadrants formed by a "Greek cross”. This layers cause the photodiodes work in dark or light conditions that are handled by a microcontroller (AT89C2051) like on/off switches so it is possible to control the Altitude and Azimuth position for the 12V DC motors [8].

The design of the second version is based upon the detection of the Sun’s light intensity trough commercial photodiodes located over two small aluminum structures (detection turrets) in a certain way so that it will be possible to locate the Sun even when the oven is totally out of its focal condition, a view of both turrets is shown in FIG. 15 The analog signals provided by the photodiodes, which have proportional linearity to Sun intensity, are driven immediately into a microcontroller having an internal analog to digital converter of a 10 bit resolution. This proceeding in accordance with an algorithm, which considers the diverse positions of the detection photodiodes, will activate two H Bridge circuits to control two electric motors required for, and whose structure is already attached to the oven’s main structure.

The main contributions to be achieved by this tracker are:

a) The existence of two identical detection turrets where the photodiodes are mounted on; it is pretended to provide a redundancy: in the event that due to mechanical reasons one of these detection turrets is out of operation, it will be possible to use the other one to let the

sun tracking be still working. FIG. 15 Redundant Detection Turrets

b) The photodiodes location supplies the tracker three detection sorts: coarse, medium and fine; the first one helps to locate the Sun whenever there are unfavorable conditions, medium and fine detection work together to allow the oven reach its focal condition and once in such condition, it is possible to track the Sun continuously.

c) Due the Solar Oven has been thought to be used within the Tropics, the Sun Tracker must be capable of doing its job in certain Sun paths like its Zenith position and when it goes beyond it, these conditions do not exist outside the tropics.