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.

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.

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

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.

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.

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.

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.

The integration of solar heat has a large potential in industrial applications, as the industrial sector covers about 28% of the total primary energy consumption for final uses in EU25. The recent study “ECOHEATCOOL” reports that about 30% of the total industrial heat demand is required at temperatures below 100 °C and 57% at temperatures below 400 °C [1]. As a matter of fact, in several industrial sectors, such as food, wine and beverage, transport equipment, machinery, textile, pulp and paper, the share of heat demand at low and medium temperature (below 250 °C) is about (or even above) 60% of the total figure [2].

100%

80%

60%

40%

20%

0%

In the framework of the IEA Task 33 SHIP a solar potential study was carried out that surveyed all data available for solar thermal potential studies for industrial applications. This study showed that the figures on temperature levels applied in different sectors that are obtained from industry statistics are fully confirmed by the outcomes of the estimates done in the reported potential studies for solar process heat [3]. The result of this study shows the potential of solar heat (based on potential studies of selected countries) for EU25. Solar process heat could cover 3,8% of the industrial heat demand, corresponding to 100 — 125 GWth.

For Styria, a detailed potential study for the industrial sector was carried out in 2006 in the framework of the Styrian Promise project. Based on a questionnaire and via telephone calls over 470 companies were contacted to gather energy demand data and required temperature levels for processes. Based on the information acquired a statistical analysis was done to calculate the total energy demand of all Styrian companies in the respective sectors. To calculate the solar potential the following criteria were taken into account:

Process technical potential (improvement of technologies for low temperature applications)

Solar technical potential (efficiency of solar technology, available roof area)

Ecological potential (CO2/SO2 emission limits)

Social potential (awareness of companies)

Economical potential (investment costs, funding, conventional fuel and biogenic fuel prices)

Quantitatively it was only possible to account for the process technical and the solar technical potential, as the other factors are underlying regulatory agreements that rely on the current political framework.

For the process technical potential the following figures were assumed: (a) 100% for the food sector (all processes are low temperature processes), (b) 20% for the metal sector (conservative assumption, and strong reduction as low temperature process steps (phosphating, pickling etc.) may only account to a part of the overall energy demand of metal companies) and (c) 31% for the paper industry (reduction due to short residence times and partly necessary steam applications).

The solar potential was fixed with 20% for the whole industrial sector [4].

The results show that the total potential in the industry sector amounts to approximately 0,618 PJ/a. This equals an installed collector-area of 480.000 m2. It has to be considered that the potential in the textile and chemical industry was not included due to missing data. The largest potential in industrial companies was found in the sectors of food (0,2 PJ/a) and paper (0,28 PJ/a).

For trade companies a similar approach was used, however the process technical potential was always set to 100% as only low temperature is used in the relevant trade companies. Among commercial enterprises sports facilities (0,31 PJ/a), garden markets (0,13 PJ/a) and hospitals (0,12 PJ/a) have the highest potentials. The total potential in the commercial sector amounts to 0,587 PJ/a, which equals an installed collector-area of approximately 460.000 m2.

In total, the potential study for Styria shows that by installing a collector-area of approximately 1 Mio. m2, 68.000 t of CO2 per year could be saved.

The box of the MMA can be oriented freely in two dimensions, i. e. azimuth and elevation angle. For each position of the MMA in a heliostat field, a specific combination of these angles yields the best performance on an annual basis.

As an example,

Figure 7 shows the influence of the box orientation on the annual performance for a heliostat that is located 71m north and 71m east of the tower. The annual reflection efficiency shows a maximum at an azimuth angle of 251.5° and an elevation angle of 36°. This in a reference system where an azimuth angle of 270° corresponds to the box oriented south (east = 0°), and an elevation angle of 0° indicates a non-tilted horizontal box.

In the following calculations, the heliostat under consideration is always oriented to achieve optimum annual reflection efficiency.

I. Canadas 1, D. Martinez1*, F. Tellez1, J. Rodriguez1, G. Mallol2

1 Plataforma Solar de Almeria-CIEMAT. P. O. Box 22; 04200-Tabemas; SPAIN 2

Instituto de Tecnologia Ceramica, Castellon (SPAIN)

* Corresponding Author: diego. martinez@psa. es

Abstract

The Spanish-fUnded ‘Solar PRO’ project is assessing the suitability of the ceramics manufacturing industrial process-heat applications.

An experimental setup has been erected and characterized in the Plataforma Solar de Almeria’s Solar Furnace. This setup is based on an open volumetric receiver, heating an air current up to 1100°C in a sample processing chamber.

In a first stage, this system has been optimized and characterized and further its suitability for ceramics manufacturing processes has been studied. Nevertheless, it’s a multi-process device able to work at any high-temperature industrial process within its temperature limits.

Keywords: solar; process heat, high temperature, solar furnace, volumetric receiver, ceramics manufacturing

1. Introduction

Solar thermal energy is the renewable energy which, because of its characteristics, must take on a relevant role in industry, as it provides, either directly or through transfer to a fluid or absorber material, the thermal energy necessary for many industrial processes, and can supply solar process heat at different temperatures.

The industrial processes that usually require the largest energy share are those that take place at high temperatures. For the future implantation of the solar thermal concentrating technology in high-temperature industrial processes, a strong boost for research is required and for each particular process, its technological feasibility must be demonstrated, adapting the design and production parameters.

The SolarPRO project, funded by Spanish Ministry for Education and Science, opens a new line of research, by demonstrating the technological feasibility of using solar thermal energy to supply high-temperature industrial processes other than electricity generation. The combined experience and knowledge from the many projects in central receiver technology and materials treatment in the Solar Furnace are made use of for that purpose.

The relatively small and very versatile Solar Furnace3,4,5 (figure 1) is used as a test bench, as it allows a broad range of experiments in which cost and conditions, control and monitoring can all be optimized.

The processes studied in this project are classified in two basic groups:

• Industrial production processes

• Waste treatment processes

2. Experimental

M N A Hawlader, Tobias Bestari Tjandra and Zakaria Mohd. Amin

Dept. of Mechanical Engineering,

National University of Singapore
9 Engineering Drive 1
Singapore 117576

Abstract

The Solar Assisted Heat Pump (SAHP) desalination, based on the Rankin cycle operates in low temperature and utilizes both solar and ambient energy. An experimental SAHP desalination system has been constructed at the National University of Singapore (NUS). The system consisted of two main sections: a solar assisted heat pump and a water distillation section. Experiments were carried out under the different metrological condition of Singapore and results showed that the system had a performance ratio close to 1.3. The heat pump has a Coefficient of Performance of about 10, with solar collector efficiencies of 80 and 60% for evaporator and liquid collectors, respectively. Economic analysis shows that to achieve a high production rate while maintaining a low investment cost, a system, without using liquid solar collector, is preferred. This system, at a production rate of 900 liter/day with an evaporator collector area of around 70 m[1] [2], will have a payback period of about 3.5 years.

Keywords: Desalination, heat pump, solar collector, evaporator collector, economic analyses, payback period.

applications at temperatures less than 100oC but the most promising source is the solar energy [2]. Experimental work on heat pump assisted water purification has been carried out in Mexico since 1981 [3], where electrically driven mechanical vapour compression pumps were first used. Absorption heat pumps were then tested in large-scale purposes and Siqueiros and Holland [3] found that the cost for desalination to produce potable water for cities was competitive to that of RO and ED.

Ozgener and Hepbasli [4] has performed energy and exergy analysis on solar assisted heat pump (SAHP) systems. Torres-Reyes and Cervantes [5] studied both theoretically and experimentally on a SAHP with direct expansion of the refrigerant within the solar collector and performed a thermodynamic optimization. The maximum exergy efficiency was determined by taking into account the typical parameters and performance coefficients.

The feasibility of a solar energy system is determined not only from its performance but also from an economic analysis, which must be carried out to evaluate its performance. Usually, solar energy systems require an initial high investment followed by a low maintenance and operation costs [6]. The economic Figure of merit used in the economic optimization is the payback period, as it shows how soon the initial investment can be returned by accumulated fuel savings [7].

At National University of Singapore (NUS), a direct expansion solar assisted heat pump (SAHP) system was designed and built,[8]. Studies performed on the system indicated the effectiveness of small-scale application. Modifications were made to incorporate the SAHP into a single effect MED desalination system and a series of experiments were performed. In this paper, experiments and economic analyses performed on a novel solar assisted heat pump desalination system is presented and discussed. [3] water tank. A thermostatic expansion valve regulates the refrigerant’s mass flow rate. After passing through the expansion valve, the refrigerant is divided into two branches, one through the evaporator-collector, and the other to a cooling coil located at the top of the desalination chamber to condense water vapors. These two streams are then mixed before entering the compressor.

In the desalination section, a commercial solar collector is used to preheat incoming feed water. An electrical heater is positioned at the outlet of this solar collector to provide auxiliary heating to ensure the feed water to maintain the desired temperature, when solar radiation is inadequate. The electrical heater will maintain the water temperature to be not less than 70°C. After passing through the electrical heater, feed water enters the desalination chamber. The chamber is evacuated to a pressure of 0.14 bar and at this pressure the corresponding saturation temperature for water is 52.6°C. Thus, feed water entering the chamber will undergo thermodynamic flashing. The remaining part of water that does not evaporate will flow down to the bottom of the chamber, where it will be heated further by the heat pump’s condenser coil, thus evaporating the water. Vapors generated from flashing and evaporation will be condensed at the top section of the chamber by a cooling coil of the heat pump. Distillate water produced will flow down to a collection tray.

In order to tune the heat exchanger model with the experimental data, the heat transfer coefficients need to be identified. For one exchanger, four heat transfer coefficients are defined: one for each zone on the refrigerant side, and one for the secondary fluid.

For given supply and exhaust temperature conditions, the condenser model predicts its pressure. Figure 7 shows that this pressure is predicted with a relative error of about 3%.

In the evaporator, the pressure is imposed by the expander and the feed pump. Given its supply temperature and the saturation pressure, the model predicts the heat flow and the exhaust temperature. Figure 8 shows that the heat flow is predicted with an error lower than 2%.