Category Archives: EuroSun2008-2

Materials and method

A chamber covering an area of 46.67 m2 with a North-South oriented central axis was used as the infrastructure. The East and West surrounding walls were 0.15 thick and 1m high — built with bricks and coated inside and outside with plaster — on which a glass with an aluminium frame is placed. The South wall, without glass, was built in the same way on a concrete foundation. The North wall is composed of a panel for the cooling system in the lower part and a glass surface in the upper part. The chamber structure also has:

• A low transparency cover made of semi-translucid glass fiber, under which a LTD long thermal life plastic cover was placed, with UV treatment to improve the chamber airtightness and to obtain an adequate cooling.

• Three iron box-like beds about 0.20m high, with an expanded metal grid base mounted on

0. 80m high legs. Inside the beds, a stony layer (0.05 m) and a pearlite layer (0.05 m) were

consecutively deposited. In two of the beds, used for the walnut production, a polyethylene pipe system of 0.0127 m in diameter was placed on the stony layer, separated at 0.10m one from the other, where water circulates at 22°C to heat the root zone. The pipes were covered with pearlite till the upper edge of the bed. One of the beds for the walnuts was prepared for micro grafting, so that apart from the basal heating system already described, an aerial polyethylene pipe system was added where water circulates at temperatures oscillating between 28°C and 31°C embracing the cicatrization area of the graft. On the third bed, prepared for carob rooting, a 0.80m wide and 0.60m high transparent plastic tunnel was built. It was set on an iron sheet and galvanized wire frame. The base heating of the carob stems was accomplished by the heat of the water in the pipes at 32°C, temperature above that required by the walnut plants.

• Mist system which allows the artificial creation of mist in the room keeping the relative humidity high at the level of the beds.

• Evaporating cooling system to diminish the temperature in the chamber. A straw savings panel was added to the North wall to maintain hydrocooling. The panel is 3.80 m wide, 1.00 m high and 0.20 m thick. Water is distributed from the upper part to maintain the straw humidity and the surplus water is taken by a channel for recirculation. Air is distributed by means of a 1.5 kW extractor fan placed on the opposite side (South).

Description of the sun — gas system

The system is made up of (Fig. 1):

• Two flat water collectors measuring 0.80 m x 2.96 m each.

• A hot water tank for house heating with a 110 l capacity, 5,75 W energy consumption, 235 l/h recuperation and a work capacity of 3,4 kg/cm2

• A 100 l water tank

• A 1/2 HP bomb for water circulation through the beds from the collectors to the hot water tank.

• Two 1” electrovalves and a 24 V-AC tension

• Two retention valves.

• “Thermostat” type sensor with a 30 — 90 °C range.

• A flow meter.

Sensors location

Three sensors were located inside the chamber at 2.00 m, 6.00 m y 8.00 m of the front NET wall along the central horizontal axis, at 1.50 m above the central bed. Sensors were put at the water inflow and outflow of each collector, and a thermostat type sensor was inside them. In the hot water tank, sensors were placed at the water inflow and outflow, and one inside so as to have the ignition reference. To measure the water volume entering the bed, a flow meter was installed. Radiation was measured inside and outside the chamber. In Fig. 2, the distribution circuit of the sensors and the water through the collectors, hot water tank and benches are shown.

To monitor the variables of the inner and outer environment, a computer equipped with Keithely 1600 and Pclab 812 acquisition cards was used. Sensors were used to measure air temperature type LM (semiconductor); Vaisala capacitive tips for humidity, Kipp & Zonen radiometers and LICOR 200SA pyranometers for solar inner and outer radiation, and a LICOR analogic lux-meter for the ilumination level.

image256 image257

Fig. 1. View of the propagation chamber and the sun — Fig.2. Diagram of the water distribution circuit gas system

Vegetal Material:

Walnut: young plants — six monts old — of native walnut were selected. They were obtained from seeds cultivated in greenhouses, and cv. Sunland walnut grafts 0.025 a 0.030 m long collected and kept in a cold chamber at 5°C.

Carob: stems from plants of the Central Valley of Catamarca were used for this work. They were 2 to 3 and 8 to 10 years old. Stems were taken from the basal part of the branch and were from 0.25 to 0.30 m in length and from 0.0025 to 0.0030 m in diameter. On each stem, 4 to 5 buds and leaves cut in halves were left to avoid greater perspiration. At the base of the stem, a bark scraping was made to favour the contact of the surface with the hormone solution.

First simulation results of a solar tower plant

Подпись: Fig. 5: Results of the solar concentrated power Pin, the power Prec and the gas temperature at the boiler inlet

Various specialized computer codes exist which are used for layout calculations and performance prediction of such solar power plant. For the solar field layout the software code WinDelsol is used, which allows the evaluation of the optimized heliostat arrangement and calculates the flux distribution on the receiver. Such results are used as input for the developed model under the MATLAB/Simulink environment. With the use of measured weather data (ambient temperature, direct normal irradiation) a realistic estimation of the annual plant performance is possible for a chosen location.

First simulation results with the model for the plant in Juelich for one day for the output power of the receiver Prec and the gas temperature at the boiler inlet are presented in Fig. 5. The chosen day was a clear summer day in Juelich with high solar concentrated radiation reaching the absorber (Fig. 5). The

temperature after the receiver rises at the beginning of the day and reaches the operation value. A control of the mass flow is been considered for a solar only operation without any storage.

5. Conclusion

Calculations of solar tower plants can be done with numerical procedures. The first results of the simulation analysis show that the created model library is a solid basis for the description of the components of the power block. The created component library will be developed further in order to describe the hybrid operation of the plant in Juelich. A future aim is to simulate the annual energy production of the solar tower plant with a gas turbine or a burner for different sizes and sides.

References

[1] Schwarzboezl P.: A TRNSYS Model Library for Solar Thermal Electric Components (STEC). Reference Manual. Release 3.0, November 2006.; available at: http://sel. me. wisc. edu/trnsys/trnlib/stec/stec. htm

[2] B. Hoffschmidt, G. Dibowski, M: Beuter, V. Fernandez, F. Tellez, et al.: TEST RESULTS OF A 3 MW SOLAR OPEN VOLUMETRIC RECEIVER. ISES Solar World Congress 2003, Goteburg, 14.-19. 06.2003, ISES, (2003)

[3] B. Hoffschmidt, P. Schwarzbozl, G. Koll, F. V. Quero: Design of the PS10 Solar Tower Power Plant. ISES Solar World Congress 2003, Goteburg, 14.-19. 06.2003, ISES, (2003)

[4] P. Schwarzboezl, R. Buck, C. Sugarmen, A. Ring, J. M. Crespo, P. Altwegg, J. Enrile Solar gas turbine systems: Design, cost and perspectives Solar Energy 80 (2006) 1231-1240

[5] K. Hennecke, P. Schwarzbozl, S. Alexopoulos, J. Gottsche, B. Hoffschmidt, M. Beuter, G. Koll, T. Hartz: SOLAR POWER TOWER JULICH The first test and demonstration plant for open volumetric receiver technology in Germany, Proceedings of the 14th Biennial CSP SolarPACES Symposium, Las Vegas, Nevada, 4-7 March 2008

[6] MATLAB/Simulink Manual, http://www. mathworks. com

[7] K. Hennecke, P. Schwarzbozl, B. Hoffschmidt, J. Gottsche, G. Koll, M. Beuter, T. Hartz (2007): The solar power tower Julich a solar thermal power plant for test and demonstration of air receiver. In: Goswami, Yogi; Zhao, Yuwen [Hrsg.]: 2007 ISES Solar World Congress, Beijing, Springer Verlag, S. 1749 — 1753, ISES Solar World Congress, Beijing, China, 2007-09-18 — 2007-09-21

A New Methodology for Optimum Design of Solar Power Plants. with Parabolic Trough Collectors

L. Gonzalez1*, E. Rojas1 and E. Zarza2

1 Ciemat — PSA, Avda. Complutense, 22, 28040 Madrid, Spain
3 Ciemat — PSA, Carretera de Senes, s/n, 04200 Tabernas, Almeria, Spain
lourdes. gonzalez@ciemat. es

Abstract

In the last years, it has reappeared the interest that there was in the Eighties of last century in solar power plants with parabolic-trough collectors. These plants allow generating electricity like in conventional power plants, but replacing the primary energy source by the Sun, a renewable energy source. Although the scene has changed from the conditions in the Eighties, now another favourable conditions exist that promote the construction of new systems, both in the United States and in Europe. In particular, in Spain a change in the legislation for electricity generation has encouraged the implementation of renewable energies among which the solar thermal energy is. As the solar field supposes one of the main investments when erecting a thermosolar power plant, it is necessary to have an effective tool for the determination of its dimensions. This paper presents a new methodology for designing optimum parabolic trough solar fields by finding the best value around a first estimation. This first estimation of the solar field is obtained from a simplified calculation of the performance of a parabolic power plant at the site of interest, assuming a fixed solar direct irradiance at the solstice when the Sun gets its maximum declination (summer solstice for northern hemisphere and winter solstice for southern hemisphere). The performance of the power plant along a real year (i. e. using beam solar irradiation and ambient temperature data from a typical meteorological year at that site) and varying the solar field size around the first estimation is analyzed to obtain the optimum solar field. Keywords: thermoelectric, parabolic troughs, solar field, design, optimization, dumping of energy

1. Introduction

The Kyoto Protocol has set the guidelines to be followed to reduce the changes caused by greenhouse gases. This requires promoting energy savings, but above all the use of new sources of cleaner energy (i. e. renewable energies) is necessary. Solar thermal energy is a good example. One of the more promising fields of application for solar thermal energy is electricity generation by means of solar thermal power plants, which have the potential to provide, at least, 5% of global energy demand in 2040, [1].

Solar power plant technology with parabolic trough collectors is one of the more mature technologies at present, [2], with 340 MWe connected to the electricity grid in Southern California (SEGS plants) and 64 MWe in Nevada (Nevada Solar One plant). In 2004, the Spanish Royal Decree 4361 and its reviewed version in 2007 (Royal Decree 661/2007) launched the major Spanish power market players to be among the first 500MW. Most of them are promoting parabolic trough plants with up to 50MW nominal power, like Andasol-1 and 2 in Andalusia or

the Iberdrola (Spanish electric utility company) and IDAE (Spanish Institute for Energy Savings a Diversification) thermosolar power plant in Puertollano, Ciudad Real.

A solar thermal power plant has the same components as a conventional power plant with the exception of the steam boiler, which is replaced by a solar system. This system is mainly composed by the solar field, the heat exchanger and in some cases by a thermal storage. The solar field is formed by a number of parallel rows of parabolic trough collectors connected in series.

The working fluid (thermal oil) circulates through the absorber pipes from the entrance to the exit of each row.

As the solar field supposes one of the main investments when erecting a thermosolar power plant, it is necessary to have an effective tool for the determination of its dimensions. In order to optimize the design of the solar field, the reported simulation tools [3-4] work with an economic criterion as the single figure of merit. It means that every time the specific economical situation to apply changes, the optimization has to be run again from the very beginning — or to leave the design like it was-. Considering the example of Spain, where the premium payment for the electricity produced by solar thermal has been reviewed 3 times in 5 years, an optimization tool based only on economical aspects may waste of lot of design engineers’ time. The optimization methodology presented in this paper establishes an energy-related criterion prior the economic one, limiting to a small amount of new simulations to run if the economical framework changes. The energy-related criterion is aimed at minimizing the waste of energy in summer while maximizing the annual electrical energy production. As the specific feature of the methodology proposed is this energetic optimization, this paper presents the energy-related optimization and not the economical optimization that would follow.

The optimization methodology includes two parts:

* A Pre-design of the solar field. With a simplified calculation process, a first estimation of the solar field size is obtained.

* Optimization itself of the solar field size. The number of collectors per row is kept as in the pre­design, but the number of rows in the solar field is optimized. The number of sizes or, in other words, the different numbers of rows, to simulate is limited by assuming that the waste of energy in summer (called “dumping of energy”) is below 3% of the annual production. The number of simulations is, then reduced to just 3 or 4 cases or sizes. Every simulation gives the thermo­electrical performance of the solar power plant along a year using typical meteorological data at that site. The simulation is based on a simplified physical model of a parabolic trough power plant. A brief example is also presented in the last section to illustrate this methodology.

Potential for future applications

In spite of the current small contribution to the worldwide installed solar thermal power, the potential for providing heat to industrial application is really relevant.

Looking at this new promising market, several studies on the potential for solar industrial process heat have been recently performed in different countries such as Austria, Spain, Portugal, Italy and Netherlands. The main results of the potential studies performed in several countries all over the world are summarised with the key outcomes categorized by:

industrial heat demand by temperature range;

most suitable industry branches and processes for solar thermal use;

potential of application for solar thermal technologies in industry for several countries and at the European level.

Drying woodchip characteristics

1.1. Dryer setup

The woodchip drying process has been assessed experimentally in the lab. The range of flow rates and heat inputs covers the values expected from the solar thermal system, thus the dryer works at low drying velocities and low temperatures.

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The dryer consisted of the supply unit and drying chamber where the woodchip was located. The supply unit consisted of the apparatus and instrumentation necessary to deliver the air flow and control its quality. The fan used was a PASPT 10W 12DC that blows a volume of air up to 280m3/h. The air temperature was regulated by an electrical resistor that provided heat up to 1200W.

The drying chamber was an isolated wooden box that contained the tray where the woodchip was held. The volume of the chamber was defined by the height, 65 cm and the area of the tray, a square area of 51 cm side. At the bottom of one of the sides, there was a square aperture (28 cm x 28 cm) that adapted the flow input getting into the box. Once the air crosses the tray of woodchip, the air was exhausted through a cardboard chimney located on the top of the drying chamber. The tray that holds the woodchip was made of aluminium sheet on the sides and a plastic mesh (3 mm square holes) at the base that allows the air pass through.

The woodchip treated for the study was shredded wood coming from Sitka Spruce trees grown in the Scottish forests. The woodchip employed had a normal distribution of sizes and shapes. The product was characterized by the length of the blade used to chip the wood; hence 2 cm was the characteristic dimension of the woodchips. The average of the initial moisture content in the sample, MC0, for all the tests was 53% MC.

Principle of Membrane Distillation

The principle setup of membrane distillation (MD) is based on a hydrophobic, microporous membrane as shown in figure 1. Due to the high surface tension of the polymeric membrane materials, liquid water is retained from entering the pores while molecular water in the vapor phase can pass through.

A hydrophobic membrane is characterized by the fact that the surrounding liquid water can not enter its pores (capillary depression). This effect depends on the relative intensity of cohesiveness between the liquid molecules and the adhesive power between liquid molecules and the membrane material. These forces are responsible for the contact angle © = 180 — © between the liquid surface and the membrane wall. In case of a non wet-able membrane, the contact angle is © > 90° and a convex meniscus as shown on the right hand side of figure 1 is formed. The hydrostatic pressure of the water columns on both sides of the membrane must remain less than the wetting pressure of the membrane in order to restrict liquid water from passing the pores.

Temperature and vapour pressure
profile across the membrane

Evaporator Condenser

channel channel

Fig 1: Principle of direct contact membrane distillation (DCMD)

Membranes for MD usually have a pore diameter of 0.1- 0.4 pm and are made from PTFE, PVDF or PP polymers. The driving force in a MD process is the vapor pressure difference across both

membrane interfaces. For direct contact membrane distillation (DCMD) , this vapor pressure difference arises due to a temperature gradient between a hot feed stream in the evaporator channel and a cooled permeate stream in the permeate channel.

In MD, it is assumed that mass transfer is based on convection and diffusion of water vapor through the microporous membrane. Finally, summarizing the constants of different equations describing the convection and diffusion process in a single transport coefficient C leads to the simplified equation: Nw = C ■ dp

Usually C is determined by experimental investigations and is in the order of 3■ 10-7 to 4■Ю6kg/m2sPa depending on membrane material and geometry.

As can be seen in the graph of figure 2, the pressure difference across the membrane dp is calculated for the temperature differences between the hot and cold membrane interface according

figure 1 AT10 = T1 — T0 from the gradient of vapor pressure curve dP at average temperature, dT

T = 0.5 ■ (T1 + T0). Respectively the mass transfer can be calculated according the following

Подпись: equation:Nw = СІР (71- T.)

dT

Подпись: dp

The gradient can be calculated using the Clausius-Clapeyron equation with sufficient

dT

Подпись: accuracy if AT10 < 10K :dp Ahv

dT RT2

Where Ahv is the latent heat needed for evaporation.

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Fig 2: Pressure difference across the membrane dp calculated for the temperature differences dT between the

hot and cold membrane interface

There are three steps of heat transfer in MD. The first is the heat transfer from the hot bulk stream in the evaporator channel to the membrane interface. It is calculated as a function of the heat transfer coefficient a1 and the temperature difference between the bulk stream and membrane

interface Th — Tx. The second step is the heat transfer through the membrane. It consists of three different mechanisms: first is the heat transfer of the latent heat of vaporization Ahv which is transported with the vapor flux Nw through the membrane. The second mechanism is the heat flux through membrane material and the third is the heat conduction through water vapor and air in the membrane pores. Heat conduction through the membrane is adversarial because this fraction of energy can not be utilized for evaporation and must be considered as heat loss.

The third step is the heat transfer from the cold membrane interface to the cold bulk stream in the condenser channel.

Today MD is not used in large scale desalination plants. There are, however, several advantages which make this a preferred technology for small plants especially in remote applications where low temperature waste heat or a solar thermal heat supply is available, facilitating the energy self­sufficient operation of the unit.

The main advantages of MD are:

• The operating temperature of the MD process is in the range of 60 to 80 °C. This is a temperature range where solar thermal flat plate collectors have a sufficient efficiency or waste heat from co­generation plants is available.

• The membranes used in MD are resistant against fouling and scaling.

• Chemical pre-treatment of the water supply is not necessary.

• Intermittent operation of the module is possible without heat storage.

Подпись: salinity ofThe system efficiency and the produced water quality are almost independent from the the raw water.

Some actual examples

Подпись: fig. 1, 2 Esslingen, Germany: FESTO AG, 1,330 m2 gross collector area for cooling and heating

Since 2004 Paradigma has been providing solar-thermal systems which are operated and connected to the conventional systems like the existing boiler. Since 2006 about 50 installations have been put into operation including the FESTO instalment, a 1,330 m2 CPC-ETC-field, the worldwide largest of its kind, providing heat for the largest adsorption chiller in the world and cooling 27,000 m2 of office space in summer and heating in winter (fig. 1, 2, 3). The interface to the heating system is a small and simple storage tank with only 17 m3 volume and without heat exchanger, so the existing huge hydronic net can be used as heat store as well.

The collector field does not contain any de-aerators, valves or other devices in the outdoor area. The planning of self-filling and self-de-aerating LSS requires extended mathematical hydraulic simulation means.

solar application cooling at 75-95 °C heating at 50-70 °C

collector area 1,330 m2 gross

Подпись:volume flow 30 m3/h

storage tank 17 m3

peak power 1.2 MW

max. continual power 0.65 MW

guaranteed yield 500 MWh per year

electrical energy req. 2.5 MWh per year

FESTO, hydraulic scheme, source (HfT-Stuttgart, FESTO AG)

According to the monitoring up to now this will probably be the first LSS that delivers more energy for the customer than the planners promised in advance.

Подпись: fig. 4 FESTO AG, temperatures and energy yield on February 24th 2008 On sunny winter days nearly 4.3 MWh were fed into the tank at temperatures of more than 80 °C (fig. 4). These are after all 3.2 kWh per square meter gross collector area — an outstanding result for a day in February. No flat plate collector could probably reach the same result on a sunny day, not even in August.

In other installations an additional storage tank was expendable because the hydraulic system was able to absorb the solar input almost completely. This is very often the case in large industrial building complexes if the solar heat input can be absorbed permanently by the already existing instalment.

Подпись: fig. 5, 6 Grafschaft, sauna bath, 98 m2 collector area, support of a heating network

As a prototype a solar-thermal system for a large sauna bath has exactly been installed like an additional boiler (fig 5, 6). The solar heat is directly fed into the existing heating network (fig. 7).

solar application all-season support of an existing

Подпись: * heating network at about 65 °C

collector area 98 m2 gross

storage tank none

guaranteed yield about 50 MWh or 500 kWh/m2

respectively per annum

electrical energy req. 380 kWh per annum

fig. 7 direct supply of hydraulic networks

The sauna bath installation proves that this hydraulic principle is easily usable for the most district heating networks. The prospects for this principle are very promising because professional solar support for district heating could be a huge business field in the near future.

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Many of non-spectacular but well-running systems like multifamily buildings, condominiums, hospitals, schools and gyms, hotels, restaurants, convention centres, barracks, prisons etc deserves to be especially mentioned, but that is impossible. All of them work amazingly simple because the heating water is also used as collector heat transfer fluid (fig. 8)

fig. 8 — large-scale solar-thermal systems for domestic hot water and space heating

3. History

At the end of the 90’s the conventional technique for solar-thermal collectors led in a dead end road. The flat plate collectors achieved high stagnation temperatures but the real efficiency for temperature ranges from 60 °C to 150 °C was not exactly promising. The problem of stagnating glycol-filled collector fields was enlarged. State of the art is that large-scale glycol-filled collector arrays must be prevented from stagnation situations by all means. Standard solutions are huge storage tanks which indeed partially solve the problem of stagnating collectors but in reality reduce the cost effectiveness considerably.

Paradigma has been involved the research and development of collectors since 1988 and launched the most innovative flat plate collectors on the market many years. In 1997 Paradigma treaded a new path and turned to vacuum tube technology. Today Paradigma has an ETC market-share of

50 % in Europe. The tubes were improved considerably, fitted in collectors from Paradigma’s own production and complemented with self-produced, high-precision CPC mirrors. This vacuum tube system is combined with minimal heat losses and ensures a very comfortable technical application on the building site. The technology to overcome the most serious problems concerning the use of water in all-year-round pressurized solar systems, which is frost protection, has been developed. Only with very high-efficient collectors with lowest heat losses it makes sense to prevent the collector field from freezing with a minimum of low temperature heat of about 2 % of the yearly solar energy gain. Operating temperatures of 60 °C — 130 °C can be achieved with high efficiency. The high sophisticated feeding algorithm provides the thermal energy at a temperature level which is preset by the user. So the solar systems operate like conventional boilers, the only difference is, that the solar irradiation is not steady.

Analysis and discussion Thermal Results

The evaluation of the hygrothermal behavior of the chamber was carried out together with agronomic trials during winter. The heated beds and the pipes in contact with the grafted area contributed to raise the temperature inside the chamber. Fig. 3 shows three consecutive days measuring: it can be observed how the chamber reaches maximum temperatures (RT ins.) of 30°C and minimum values of 10°C, while the temperature outside (RT out.) was between 22°C and 6°C. Inside temperatures were maintained within the recommended limits. The three days had similar radiation levels with 730W/m2 at the most. The relative humidity during the first and third day was observed to be similar comparing the inside (RH. ins.) and outside (RH. out) values; whereas on the second day, a significant increase of outside humidity at night was observed when there is a 4°C difference between inside and outside temperature, being the latter at the lowest values this day.

Rooting beds

Design of Solar Plants to Heat Industrial Halls

D. Jaehnig*and W. Weiss

AEE INTEC, AEE — Institute for Sustainable Technologies, Feldgasse 19, A-8200 Gleisdorf, Austria

* Corresponding Author, d. jaehnig@aee. at

Abstract

This paper presents a method to design solar thermal systems for space heating of factory buildings using underfloor heating systems. The method uses nomograms that were expanded upon using simulations of typical system configurations. There are nomograms for two different systems configurations that can be used to determine reasonable values for collector and storage tank size for a factory building with a known heat demand.

These nomograms help to decide whether it is necessary to use a storage tank or if it makes sense to use the thermal mass of the concrete floor slab as heat store. The advantage of using the concrete floor slab is that the costs for a (often times large) storage tank can be saved. Further aspects covered are the necessity of an insulation layer underneath the floor slab and specific collector yields that can be achieved with the different system configurations. Keywords: space heating, factory buildings, design guidelines

1. Introduction

Compared to other uses in buildings such as for domestic buildings, office and administrative buildings, factory halls are characterized by high room heights of 5 to 10 meters and relatively low room temperature requirements of 15 to 18 °С.

The heating demand in industrial halls is influenced by the insulation standard of the building and to a great extent by its use. High internal thermal loads, the frequent opening of hall doors, the supply of goods with a high storage mass, all of that influences the heating of halls quite considerably. Another important element is the fact that industrial halls mostly possess a very small storage mass — apart from the concrete flooring. On the other hand floor heating for industrial halls is becoming increasingly popular which once again favors the use of solar energy due to the low required flow temperatures.

To examine the influence of the different parameters on the heating of industrial halls, a reference hall was defined and different variants were compared via a simulation. Based on the parameter studies, design nomograms were prepared for typical cases in an analogous manner to the nomograms, which are used for solar thermal plants for hot water preparation and space heating.

Pre-Design of a Solar Field with Parabolic Trough Collectors

The first estimation of the solar field is given by a simplified calculation of the performance of the thermosolar power plant, for what some information is needed:

* the location of the power plant,

* power block characteristics and

* the characteristics of the solar system (geometry, collector type, etc.).

The site where the solar thermal power plant will be built determines the geographical latitude and longitude, both necessary to know the incidence angle of the Sun, q>, on the aperture area of the solar field to design at noon and on the solstice. Using these variables in the power plant model

and assuming representative beam solar irradiance, Ed, and ambient temperature, Tamb, data at solstice noon at site, one can calculate the thermal power delivered by the solar field at that moment.

The power block determines the conditions (temperature and pressure) of the heat transfer fluid (HTF) at the solar field inlet and outlet as well as the required thermal power. The inlet temperature is set by the temperature of the condensed water from the Rankine cycle. The outlet temperature will be limited by the maximum bulk temperature of the solar field working fluid. On the other hand, the nominal electric power of the steam turbine determines the required thermal power, taking into account the efficiency of the whole thermal cycle, including not only the turbine but also the heat exchanger and the internal electric consumption for running the plant (around 10 %).

Solar field characteristics include the technical specifications of the chosen collector. These specifications include both dimensions — like its length and aperture width, the inner and outer diameter of the absorber tube and the roughness of its steel pipe — and thermal performance parameters — like peak optical efficiency, t]op0, incidence angle modifier, K(q), and heat losses, Qloss. A soiling factor, Fe, is also included in order to have an estimation of the influence of maintenance strategy on the solar field performance.

The orientation of the collector axis depends, mainly, on the pattern of plant operation and it can be limited by the size and geometrical shape of the plot available for the plant. The most used options are the North-South and the East-West orientation: while the North-South orientation produces a larger amount of energy annually, the East-West orientation gives less difference in energy output between summer and winter days.

The pre-design of the solar field consists of the calculation of the number of collectors per row and the number of rows needed to meet the nominal conditions required by the steam turbine, taking into account the information mentioned in the previous paragraphs. The number of collectors per row is a function of the temperature increase between the input and output of each row for what the temperature increase in every collector of a row has to be calculated assuming a nominal mass flow that guarantees a good heat transfer coefficient in the absorber tubes. The number of rows depends on the thermal power provided by every row and the total thermal power to be given to the Rankine cycle.