Category Archives: Sonar-Collecttors

Solar furnace automation

Figure 5: CIEMAT-PSA solar furnace

The main objective of a solar furnace is to test samples of different materials by following prescheduled temperature profiles using the sun as energy source. Figure 5 shows the inside and outside views of the solar furnace of the PSA-CIEMAT.

Figure 6: Diagram of the solar furnace

As the primary energy source (solar radiation) cannot be manipulated, the energy entering the solar furnace is controlled and modulated using a shutter, which is the main control variable. The Solar Furnace Control System (SFCS) is composed of a data acquisition system to measure the sample temperature in different points, the solar radiation, shutter aperture, etc. (the heliostats automatically track the sun). Thus, the main objective of the SFCS is to control the temperature profile of the sample in spite of changes in solar radiation.

From the control viewpoint, the solar furnace is a system which presents various interesting characteristics which make the control problem a difficult task [11]:

• The characteristics of the samples are quite different depending on their nature (steel, cupper, etc.). Obtaining a fixed parameter controller to allow different samples to be controlled becomes a difficult task.

• The dynamic characteristics of each sample greatly depend on the temperature and introduce a high nonlinearity in the control system, which makes the behaviour of the controlled system change with the operating conditions.

• The control specifications are quite severe (rate of temperature increase, rate of temperature decrease, variable step changes, etc.) and have to be achieved with small errors.

• The system suffers from strong disturbances caused by solar radiation variations (slow variations due to the daily cycle or fast and strong variations due to passing clouds), which make the exact reproduction of the conditions of a determined test impossible.

• Limitations exist in the maximum temperature achievable by the materials and different constraints (nonlinearities) in the actuator (amplitude, slew rate, etc.).

Figure 7. Modelica model of the CIEMAT-PSA solar furnace

300 290









0 10 20 30 40 50

Figure 8. Simulation of the temperature profile of a sample

The control algorithms that are being developed take into account these aspects. So, the main control candidates are gain scheduling or adaptive control [11], to account for changing dynamics, predictive control to take into account amplitude and slew rate constraints, robust controllers to accommodate modelling uncertainties and fuzzy logic control to include experts knowledge (or a combination of these techniques, that have been successfully used in distributed solar collector fields [12]). Most of these techniques require dynamic models of the system. It is important in this case to use a modelling technology that allows reuse and easy modification, to avoid coding a new model each time a sample of a new material has to be tested. Again, the object oriented modelling language for physical systems Modelica [10] has been selected. These models are going to be used both for simulation and control purposes, to allow pre-evaluation of the developed control algorithms and a real-time implementation of the models, for instance, as prediction models within a model predictive control framework. Figure 7 shows an interface of the developed models and figure 8 an example of the evolution of the temperature profile in the sample under constant solar radiation conditions and after a step in the shutter aperture at the beginning of a test.

Figure 9. Screen shot of the solar furnace control system (courtesy of D. Lacasa)

Once the control strategies have been developed, it is also important that the SCADA (Supervisory Control and Data Acquisition) system allows easy integration of the controllers. The SCADA of the SFCS has been developed using Labview-DCS, that permits the easy integration of controllers and models developed using Matlab, Simulink, Dymola/Modelica, C++, etc. and can communicate with other systems using OPC (Ole for Process Control). Figure 9 shows a snapshot of the main screen of the SCADA system.

1. Conclusions

This work has presented the main activities and research lines that are being carried out within the scope of the specific collaboration agreement between the PSA-CIEMAT and the "Automatic Control, Electronics and Robotics” research group of the Universidad de Almerla. An overview of the decisions made in the selection of SCaDa systems, real-time distributed control systems, modeling and simulation environments has been included, as far as some ideas about the main control algorithms suitable for controlling this kind of plants.

2. Acknowledgements

This work has been performed within the scope of the specific collaboration agreement between the Plataforma Solar de Almerla and the Automatic Control, Electronics and Robotics (TEP197) research group of the Universidad de Almerla titled "Development of control systems and tools for thermosolar plants” and the projects financed by the MCYT DPI2001-2380-C02-02 and DPI2002-04375-C03. The authors would like to acknowledge many people who are involved in the mentioned agreement and projects, mainly Jose Domingo Alvarez (CRS systems) and David Lacasa (solar furnace).

SolarChill 03-03-2004 < c о 3 О 6 5 4 3 2 1 0 0 100 200 300 400 500 600 700 800 900 1000 Irradiation W/m2 Fig.3 Current vs irradiance diagram with a 180 WpPV array(12V). The distinct curves represent the different RPM steps of the compressor . Field trial

In January-February 2004 9 coolers were shipped from Unicef in Copenhagen (3 to Senegal, 3 to Indonesia and 3 to Cuba) and they are expected to reach their destinations in March 2004 whereafter they will be installed and the field test will begin. One additional cooler has been installed at DTI for field test, which began in February 2004. Each unit is packed with 3×60 W solar PV panels and has data loggers integrated for evaluation of the operating conditions.

— Irrad —- T-ice


For the unit installed at DTI there are now sufficient data to conclude that the operation under real solar conditions ensures an inside temperature within the desired range (at an ambient temperature of 20°C). There have been sunny and less sunny periods, but from the figures below it can be seen that the temperature becomes rather stable after a period of freeze-in.

Fig.4 Decrease of ice pack temperature after installation. After 10 days the storage is totally charged (no free water left).

Design of the transmission / reflection device

The final concepts of the envelope and the screen are illustrated by the sequence of images given in Figure 3. In order to control precisely the illuminated sample area and thus minimize the parasitic reflections and blind zone, a quarter-circular frame supports a perforated sheet on which a motorized strip showing one circular aperture is unrolling, of diameter equal to the sample’s and facing the light source for any incident altitude angle 61. The sheet’s elliptic openings are of dimensions given by the apparent sample surface (accounting for inclination angle 0j) and are correspondingly positioned on the quarter circle arc.

(a) Strip hole over elliptic opening (b) Controlled illumination of sample

(c) Obstructing screen (d) Lifting of cover (e) Removal from path

Figure 3: Control of incident beam penetration and path through obstructing screen.

The projection screen concept relies on the removal of elliptic covers by a robotic mech­anism. The ellipses’ dimensions were again determined by the apparent sample area ac­counting for angle 0j, yet this time projected on a the screen surface, that is oblique to the sample plane with a tilt angle ©0 = 49.1°. The induced blind spot can thus be exactly reduced to the light beam’s area, which allows a minimal loss of information on the emerg­ing light distribution and negligible parasitic reflections around the sample area. Of course, a blind spot only appears for one of the six screen positions, except for normal incidence where the tip needs to be removed for all of them.

The optimal combination of altitude step Двг and sample diameter D is determined on one hand by the device’s geometry itself, and on the other hand by the minimal illuminated area required for advanced fenestration systems or coating materials characterizations; the mini­mal allowed sample diameter was thus found to be equal to 15 cm.

Once the concept’s applicability in practice was verified, the new components were designed and constructed.

Performance characterization

Solar efficiency (Eq. 6) and solar fraction (Eq. 7) will be used to characterize the thermal behaviour of the facades:




4 s — і

Where 7 stands for the incident solar radiation.

c_ QLOAD " ° S QLOAD + Qaux

Prototype description

A double skin envelope constituted by an external glass, an air channel and an indoor layer contanining the integral collector-accumulator has been experimentally and numeri­cally tested. The solar collector is formed by a glass layer, transparent insulation and the accumulation zone. This zone is a water tank, whose external surface is black-painted and its internal surface is covered by thermal insulation. Therefore, the facade external appear­ance does not present any particular feature, it looks like a completely glazed area, whereas

Figure 1: Schematic design of the implementation analyzed in a double skin facade, in this case for a space heating application

1- Outdoor glazing

2- Air channel

3- Double glazing

4- Blind in channel (optional)

5- Glass

6- Transparent insulation

7- Absorber

8- Accumulator

9- Insulation

10- Heat exchanger

the internal surface (facing indoor room), looks like an opaque conventional wall, it may presents a rebound if it is combined with a top glazed area, as may be observed in Figure 1. Air channel has been considered closed in the results shown in this paper. Geometry and thermo-physical properties considered are shown in Table 3, where the prototype is described from outdoors to indoors.

Table 3: Geometry and properties considered. Units in SI







Outdoor glass








Air channel


Integrated collector-accumulator:

Outdoor glass





Transparent insulation



Absorber surface




Water Accumulator





Internal wall: Accumulator wall






Insulation material





Inner wall






Total height h: 1.0 m, total width w: 0.262 m, total depth: 2 m.

Data of TIM: Solar reflectivity^.2, Thermal reflectivity= 0.1 Thermal extinction coefficient^ Щ}, Solar extinction coefficient= 2.0

In Table 3, a stands for the solar absorptivity, r is the solar transmissivity, є represents the thermal emissivity, p is the density, cp is the specific heat and Л is the thermal conductivity.

Implications for an integrated approach for the future research on the DGI

Integrated approaches of research on lighting conditions at work places have been conducted by Schierz & Krueger (1995) and Fleischer (2000). Schierz & Krueger acknowledge that the stimulus-response-systems is an existing and valid approach of perception in special cases where the perception of a visual object is not relying on an existing mental concept (schema). An example in this case is visual quality inspections of products where randomly located imperfections have to be detected. Another example is the classical visual test, when a randomly oriented visual sign has to be identified. Talking about glare at the work place means that people reside in a well known environment where determined cognitive structures prevail substantially.

This implies that the definition of lighting conditions which cause discomfort for people working in this setting has to incorporate the cognitive schemata concept to reach reliable parameters to identify Discomfort Glare.

The above cited results on research outcomes on DGI confirm the need for this integration of mental structures. Though the cognitive structures vary interpersonally a strong correlation among people in similar cultural background is expected. This permits the outline of a new approach for research and definition of Discomfort Glare parameters.

Combination of Solar Heat and Fossil Steam Power Plants: Process Dynamics

K. Roth, V. Scherer, T. Pockrandt, Ruhr-University of Bochum M. Eck; German Aerospace Center (DLR), Stuttgart

Introduction and purpose

In some European countries a specific percentage of the electricity has to be generated by renewable energy sources in order to reduce the emissions of greenhouse gases like CO2 and to promote renewable energy. The legislation in Italy, for example, requires 2 % nowadays, rising to 4.4 % in 2012. A promis­ing technique to meet these goals is to employ a hybrid power plant, where a renewable energy source is added to a conventional and fossil fueled power plant cycle. The great advantage of this concept is that it can be realized as a retrofit to existing power plants. Mainly solar thermic plants are favourable to be located in southern European countries, that are offering a high insolation. Such kind of power plants, for example with parabolic trough collectors heat­ing a synthetic oil which transfers the heat via heat exchangers to a conven­tional steam turbine cycle, are operating since many years and have proven their technological reliability. Direct solar steam generation is a possible im­provement of the parabolic trough power plants vaporizing water directly in the absorber pipelines and eliminating costly equipment like the heat exchanger and the oil pump. This steam generated by solar energy can be integrated into the water steam cycle of a fossil fueled power plant [1].

Figure 1: Schematic steam power plant with external heat source

A schematic process diagram is shown in figure 1. By this means less steam is consumed by the high pressure and low pressure preheaters. One or more steam extraction lines can be closed and the steam can be used for a very fast and dynamic additional power generation in the steam turbine. The power plant’s efficiency is improved simultaneously.

Since the integration of the transient heat sources will lead to substantial dy­namic energy shifts within the power plant’s water-steam-cycle, the occurring time dependent processes have to be studied on a numerical basis using a dy­namic simulation software. The description and analysis of these shifts to­gether with a cycle evaluation using an unsteady process simulation tool are the content of the current paper.

Simplified Model






^maPaC — Ca )

As an alternative to simplify the simulations and facilitate the implementation of absorbers and regenerators in simulation software packages as TRNSYS[8], a simplified model was developed. In this model, the liquid-desiccant stream is represented by one single node in the у direction. The temperature of the liquid-desiccant is assumed constant, at the same temperature of the wall. The air stream is also represented by only one node in the direction across the channel. The energy and species equations for the air stream are:

Nu =





The desiccant concentration is evaluated using a species balance for every step of the simulation, with the water mass transfer absorbed by the desiccant solution calculated through equation (22). The heat transfer coefficient is calculated using a correlation for Nusselt number for laminar flow in tubes with rectangular cross section [9]:




■ = PaCp




V Da J

The mass transfer coefficient is then determined using the Chilton-Colburn analogy:

It should be noted that the analogy and the correlation above are valid only for constant temperature and concentration along the interface desiccant-air. Although this is not the case, for sufficiently high desiccant flow rates the change in concentration is small. The assumption of constant concentration in the desiccant film is similar to the assumption of constant thickness, i. e., it is only possible to assume a constant film thickness if the desiccant flow rate is relatively high. It is assumed, in the simplified model, that the heat generated by the absorption of the water vapour into the desiccant solution is immediately removed, what effectively decouples the mass and heat transfer phenomena.


Lena Schnabel, Carsten Hindenburg, Torsten Geucke
Fraunhofer-Institut fur Solare Energiesysteme ISE, Heidenhofstr. 2, D-79110 Freiburg
email: lena. schnabel@ise. fraunhofer. de, carsten. hindenburg@ise. fraunhofer. de

1. Introduction

In the last seven years intensive research work towards the topic of solar desiccant cooling systems was conducted at Fraunhofer ISE. Due to the low driving temperatures the desiccant cooling technology is promising for cost effective application of solar thermal systems. For office buildings, especially for those with large window areas, there is a high timewise correlation between cooling loads in the building and the available solar irradiation. Therefore a feasibility study on a solar autonomous desiccant cooling system for a seminar room was conducted. Solar autonomous in this context means that the thermal driving energy for the cooling needs is by 100 % provided by the solar system. The promising simulation results led to the installation of a first pilot plant at the building of the chamber of trade and commerce in Freiburg. The plant was commissioned in June 2001. The nominal flow rate of the ventilation system is 10.200 m3/h and the collector array consists of 100 m2 of solar air collectors. The plant serves two rooms in the penthouse floor of the building with conditioned air. The large seminar room has a maximum capacity of 100 persons.

Within the last two years the collected monitoring data were evaluated in detail. In the paper the energy performance of the plant will be discussed. The analysis discusses the room comfort of the two last years, the electricity consumption, the collector and the cooling performance. In general the paper reports on a promising technology for the realisation of solar air conditioning.

A survey of contrast reducing materials. and methods

R. M.J. Bokel, C. I. Kranenburg, M. van der Voorden
Delft University of Technology, Faculty of Architecture,

Building Physics Group, PO Box 5043, 2600 Ga Delft, The Netherlands

1. Introduction

Large contrasts within the field of view are very unpleasant to the eye. This phenomenon is called discomfort glare. This discomfort glare is often perceived at the transition between a window and the inside of a facade. There are two possible ways to decrease this type of discomfort glare. The first is switching on an artificial lighting system in the room; the second is the application of some kind of daylighting element at the window. As the first diminishes the potential energy saving which can be reached by using daylight, the focus of our research is the investigation of materials and methods which diminishes the discomfort glare around a window.

The large contrast ratios between the window opening and the inside of the facade in the horizontal direction will not be reduced by using daylighting systems in the upper part of the window plane, which is the focus of most research [1]. A vertical element was therefore developed which decreases the luminance contrast ratio between the window and the inside of the facade [2,3]. At first a semi-circular vertical windowsill of a highly reflecting material was investigated. A more optimal shape, an anidolic shape, was also investigated and was shown to increase the amount of light on the inside of the facade and thus decrease the luminance contrast ratio between the window and the inside of the facade. However, the question remains whether cheaper and simpler methods would not work just as well to reduce the luminance contrast ratios between the window and the facade.

The domestic water heating

Up to this point, only room heating demand has been considered without taking into consideration the domestic hot water demand. The DHW demand is constant during the whole year, and in this case two persons would need 106 kWh/month that means 1273 kWh/year, almost half value of the room heating needed.

If this thermal energy load is added to the previous total heating demand, and is covered by the pellets/wood boiler, 147 kg of pellets is needed to cover the 100% demand, (27 Euro for the whole season), a small quantity compared to the Italian energy demand standards. This solution has two advantages: the usable solar energy is greater (since will be less solar energy wasted) and the pellets/wood boiler will be used for a longer time, decreasing the pay back time of the heating system.

Fig. 12 — Photovoltaic system scheme

4.5 The electricity demand

A grid connected photovoltaic system of 2,1 kWp provides the whole electricity demand (Figure 12).

21 PV panels of 100 Wp each organised in three strings are positioned in the upper surface on the south solar wall.