Category Archives: Sonar-Collecttors

An advanced solar assisted sorption cycle for building. air-conditioning: the ECOS potential and performance assessment

Mario Motta, Hans-Martin Henning

Fraunhofer Institute for Solar Energy Systems (ISE)
Heidenhofstr. 2, 79110 Freiburg / Germany
mario@jse. fraunhofer. de, hansm@ise. fraunhofer. de

1 Introduction

In the past decade, growing environmental concerns and consistent effort in research and product development caused a rapid growth of active solar system’s market. In spite of a significant and growing market penetration rate, the main obstacle preventing the broad application of solar thermal collectors beyond their use in domestic hot water production has been the seasonal mismatch between heating demand and solar energy gains. A way to overcome the problem consists in exploiting solar thermal energy for air-conditioning of buildings during summer, i. e., sensible cooling and air dehumidification. The great advantage for this kind of application is that the seasonal cooling loads coincide with high solar radiation availability.

Buildings are one of the dominating energy consuming sectors in industrialized societies. In Europe about 30 % of primary energy consumption is due to services in buildings. During the last decades in most European countries the energy consumption for air conditioning purposes was increasing remarkably and it is expected that (i. e., in comparison to 1996 for small air-conditioners) the primary energy consumption increases by a factor of 4 in 2020 [1]. Moreover the concern for electricity peak demand increased recently, pushing decision makers to look at new technological solutions for air-conditioning. In this conditions the use of thermal energy, and in particular solar, for air-conditioning in buildings has gained a new interest.

Among the cooling technologies which raised increasing attention during the last fifteen years, there are desiccant and evaporative cooling systems. In desiccant and evaporative cooling (DEC) systems the potential of sorption materials is used for dehumidification of air in an open cycle. In this type of air conditioning systems the dehumidification effect is used for two purposes: to enhance the evaporative cooling potential at given environmental conditions and to control the humidity of ventilation air. However, a pure desiccant cycle using state-of-the-art technology is not able to provide desired supply air temperature and humidity states under all conditions. Particularly in hot-humid climates the desiccant cooling cycle has limitations. Therefore, employing standard technologies, a combination of a desiccant cooling air handling unit with a cold backup system is needed in those cases.

In this work a novel DEC concept is presented. The new system, indirect Evaporative COoled Sorptive heat exchanger (ECOS) is intended to overcome the thermodynamic limitations of standard DEC systems and provide a valuable option for air-conditioning applications without the need of a back-up system. Moreover the new concept can be implemented for small capacity plants (about 200 m3/h) overcoming a traditional restriction of standard DEC plants.

During the work reported on this paper a simplified mathematical model of the ECOS has been developed. The mathematical model has been then implemented in a software tool used to study the optimum system’s operation parameters. The performance of the sorptive cooled heat exchanger for typical air-conditioning applications has been investigated.

In particular the new system offers the possibility to use low temperature heat, e. g., heat from flat plate solar collectors, for air-conditioning, without the need of a conventional refrigeration system even under climatic conditions with high humidity values of the ambient air (e. g., Mediterranean or tropic climate).

Design and Planning Support for Solar Assisted Air­Conditioning: Guidelines and Tools

Edo Wiemken, Mario Motta, Carsten Hindenburg, Hans-Martin Henning Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany e-mail: Edo. Wiemken@ise. fraunhofer. de

Solar assisted air conditioning opens the opportunity to substitute partly con­sumption of primary energy partly by use of environmentally friendly solar energy. Although the components of such systems are in general commercially available, the composition of the entire system requires special attention in design, sizing of components and in the planning of the control strategy.

The support in the design and planning of these systems ranges from simple rules to advanced design tools, allowing a high degree in system modelling precision. A pre­requisite in nearly all situations is the determination of the load structure in the desired application. In this paper, a selection of rules, available guidelines and calculation tools, useful in the decision and planning process of a solar assisted air­conditioning system is presented and briefly discussed.

Luminance ratios between the window and the transition zone

In figure 3 the results of the luminance ratios between the window and the transition zone for the different variants is shown. The luminance value of the artificial sky luminance was not constant for the various measurements; therefore the average value of the artificial sky luminance measurements is plotted in figure 3. An other artefact that has been found is that the reference variant without a transition zone is shown to have a larger luminance in the empty transition zone than for the unobstructed artificial sky. The effect is most likely caused by a non-linearity of the simulated overcast sky. This effect can also be seen in some other variants where the second point on the transition zone has a higher luminance than expected, such as the variant with the dots on the glass or the variant with the coloured glass. Ignoring this effect, all non-empty variants have a gradual decrease in luminance values from the artificial sky to the wall.

Figure 3. Luminance distribution on the wall for the various variants given in Table 2.

Combining the results of figure 2 and figure 3, the luminance ratios between the artificial sky and the transition region, the luminance ratios between the transition region and the wall and the luminance ratios between the artificial sky and wall can be calculated. This has been done and the results are shown in table 3. Considering the required luminance contrast ratio of 1:20 between the artificial sky and the transition region and between the transition region and the wall, only the net curtain can fulfil this requirement.

empty

red

coloured

dots

lamellas

net

curtain

screen + 1 cm

profile

window / transition region

0.9

5.4

2.1

3.4

4.9

1.1

6.3

transition region / wall

158

70

119

21

13

38

48

window / wall

140

375

246

69

62

42

304

Table 3: luminance ratios in the black office of the VCE set-up.

In retrospect this should not come as a surprise. Curtains have been used extensively in the home environment and not only as a decorative addition to the window, but as a
luminance regulation, as we have shown, as well. And as a third positive point, the curtain can also be used as a privacy screen by applying the curtain over the whole window. However, for other functions of a space, such as offices, schools etc., a curtain is not thought to be suitable.

User strategies

The final distribution between the different forms of solar gains, and the thermal losses through the structure, are dependent on how the system is regulated between the two reflector modes. The complexity also increases due to more subjective response from the users due to thermal comfort and wish for daylight and view.

The system was initially designed for integration into a single family house, where most bright hours during weekdays are characterised by the absence of the inhabitants. A rough operating schedule is outlined: during morning hours, with low solar flux and high user activity, the reflectors can be opened to allow for daylight, view and direct passive heat gain. During solar peak hours, with family members being at work or at school, the reflectors can stand closed for maximal active performance. Late afternoons and evenings have similar characteristics like the mornings, thus the reflectors are likely to be opened. For avoiding view inside (i. e. allow for privacy) and thermal losses during dark hours, the reflectors should be mainly closed until the next morning. When integrated into larger areas, zoning of the system allows for combinations of closed and open modules during this cycle.

Another operating strategy could be automating the movement for the reflectors on response to the radiation intensity and the outdoor temperature. It could be programmed for closure at radiation levels too high for thermal or visual comfort, or at levels too low for any practical use, e. g. at night time. In combination with other “intelligent house” technologies, such as sensors indicating occupant absence, obvious opportunities for keeping the reflectors totally closed can be maximally used. For calculations on passive gains in Stockholm, Sweden (lat 59.31), the reflectors were considered closed at transmitted irradiance levels below 50 W/m2 and above 300 W/m2, and opened at intermediate levels, from March to October. Concerns have been taken to the solar shading effect of the reflectors by using the computer tool

Parasol. For November to February, the system was operated as a window with no thermal and power production and the reflectors were considered closed or opened, depending on the most beneficial thermal energy balance, for every hour. The energy balance was calculated for every hour of the year, according to Eq. (2):

W= I — U — AT

W is the net energy gain through the window, I is the transmitted irradiation, U is the heat transfer coefficient of the and AT is the temperature difference between indoors and outdoors.

The calculations indicate an annual positive net energy balance of 10 kWh/m2 for the winter season. For the warmer season, there is a loss of 14 kWh/m2 for the dark period with irradiance levels below 50 W/m2, and a passive gain of 214 kWh/m2 for the opened mode, at levels between 50 W/m2 and 300 W/m2. At levels above 300 W/m2, 245 kWh/m2 are available for the PV/T absorber.

The power law model used so far contains an inconsistency: it assumes that m is independent of flow rate, but it then turns out that m is proportional to the collector efficiency, which is in fact a function of flow rate. This limits the power law method to cases where m varies only slightly with flow. The solution is to recognize that the analysis so far does not fully explore the potential of the simple collector model of Schlaich (1995). To find the potential fluid power, while recognizing that ncfe depends on V, we formulate the MFP Coll. model (where the added Coll. denotes the collector): multiply Eq. (12) by V: ^pcoll gHc a G ACOllV P = ptV = ЭР dV T pcoll g H( V pcollcp Acoll a G Acoll KlVpV a G AcollV pcollci T0 pcoll g H( V pcollcp + P Acoll (V pcollcp + P Acoll) -(n + 1)KlV» a G Acoll (V pcollcp + P Acoll ) a G AcollV pcollcp ) = 0 (24) . EFFECT OF VARIABLE COLLECTOR EFFICIENCY

(n + 1)KlV"(v Pcollcp +P Acoll)2)= 0

Figure 2: Plots of fluid power vs. volume flow for various modeling approaches (100 MW)

If n = 2, Eq. (24) is a fourth order polynomial for which an analytical solution procedure exists. Otherwise it has to be solved numerically for Vmfp Coll. It will be more instructive, however to compare power versus flow graphs for the constant m and variable collector efficiency approaches. Fig. 3 shows that for a 100 MW test case from Schlaich (1995), Vmfp pl and Vmfp Coll are quite similar, but both differ substantially from V*, the flow at which the turbine pressure drop is 2/3 of the pressure potential. It also shows that use of the 2/3 rule seriously over estimates the maximum flow at which the plant produces any power at all. This flow has a large effect on the turbine runaway speed. Table 2 summarizes similar comparisons for the data from Schlaich for several test cases.

Typically Vmfp pl and Vmfp Coll are between 67 and 62 % of V*, and the corresponding optimal turbine pressure drops are between 173 and 200 % of the values associated with V*. It is encouraging to see that the simple power law model predicts the maximum power flow within one percentage point compared to the MFP Coll. Model in all the test cases and is pessimistic in the prediction of the maximum fluid power value, and optimistic in the prediction of turbine pressure drop.

Nominal

power

100 MW

30 MW

5 MW

MFP PL

MFP Coll.

MFP PL

MFP Coll.

MFP PL

MFP Coll.

Vmfp /V*

0.659

0.669

0.640

0.643

0.617

0.616

ptMFP /pt*

1.808

1.730

1.890

1.823

1.996

1.920

Pmfp /P*

1.192

1.158

1.210

1.172

1.232

1.183

Table 2: Comparison of flow rate, turbine pressure drop and fluid power at MFP-condition of the power law model and the MFP Coll. model from Schlaich

CONCLUSIONS

The study developed two analyses for finding the optimal ratio of turbine pressure drop to available pressure drop in a solar chimney power plant for maximum fluid power. In the first part the system pressure potential is assumed to be proportional to Vm where V is the volume flow and m a negative exponent, and the system pressure loss is proportional to Vn where typically n = 2. Simple analytical solutions were found for the optimum ratio of pt/pp and for the flow associated with it. This ratio is not 2/3 as used in simplified analyses, but depends on the relationship between available pressure drop and volume flow, and on the relationship between system pressure loss and volume flow. The analysis shows that the optimum turbine pressure drop as fraction of the pressure potential is (n-m)/(n+1), which is equal to 2/3 only if m = 0 (i. e. constant pressure potential, independent of volume flow) and n = 2. Consideration of a basic collector model proposed by Schlaich led to the conclusion that the value of m is equal to the negative of the collector floor-to-exit heat transfer efficiency.

The basic collector model is sensitive the effect of volume flow on the collector efficiency. Its introduction into the analysis indicated that the power law model is conservative in its prediction of maximum fluid power produced by the plant, and in the magnitude of the flow reduction required to achieve this. It was shown that the constant pressure potential assumption may lead to appreciable under estimation of the performance of a solar chimney power plant, when compared to the model using a basic model for the solar collector. More important is that both analyses developed in the paper predict that maximum fluid power is available at much lower flow rate and much higher turbine pressure drop than the constant pressure potential assumption predicts. Thus, the constant pressure potential assumption may lead to overestimating the size of the flow passages in the plant, and designing a turbine with inadequate stall margin and excessive runaway speed margin. The derived equations may be useful in the initial estimation of plant performance, in plant performance analyses and in control algorithm design. The analyses may also serve to set up test cases for more comprehensive plant models.

NOMENCLATURE

P

Power; W

p

Pressure; Pa

A

Surface area; m2

Q

Heat transfer rate; W

C

Coefficient

T

Temperature; K

cp

Specific heat; J/kgK

V

Volume flow rate; m3/s

G

Solar irradiation; W/m2

g

Gravitational acceleration; m/s2

Abbreviations

H

Height; m

MFP

Maximum fluid power

K

Coefficient

PL

Power law

The basic technology

1. Cabinet wall with 100 mm of insulation (made by Vestfrost)

2. Vaccine packages (in three baskets)

3. Integrated condenser

4. Lid (also 100 mm insulation)

5. Internal wall, insulated

6. Electric heating element, thermostat controlled by temperature in the bottom of the box

7. Evaporator (wire on tube) and ice packs

8. Self-acting damper

9. Compressor (made by Danfoss Compressors)

The main task has been to develop a new cooler, which fulfil the current WHO requirements for vaccine coolers with battery back up, as no standard exist for the battery less type. According to these guidelines the design temperature interval is 0 °C to + 8 °С. The vaccine must also be kept cool for four days without power, and this is the sizing

Fig. 1 Diagram of the first SolarChill prototype

criteria for the ice storage in the cooler. Computer simulation was done based on the most efficient mass-produced cabinets on the market. Those cabinets has 100 mm polyurethane insulation and are of the chest type.

The reason for choosing energy storage in ice was to avoid a lead battery for energy storage. Lead batteries tend to deteriorate, especially in hot climates, or they are misused for other purposes. This makes it necessary to install a new battery after a couple of years, and has in practise been an obstacle for the use of solar powered refrigerators. In addition to that some pollution of lead might be expected from the batteries.

Instead kerosene or gas powered absorption refrigerated coolers are widely used in areas with poor or no grid electricity. Absorption coolers are used for both vaccine storage and for household applications and obviously needs regular supply of fuel. Furthermore, they are difficult to adjust, which does often result in destructive freezing of the medicine.

So far, two generations of prototypes have been build and tested in climate chamber at the DTI and an advanced control were build with the purpose to control the temperature in the cooler and the speed of the DC-compressor in order to exploit maximum power from the solar panels.

Result of the experiment

Fig. 1 shows the meteorological conditions during the test period. On fair days from September 3 — 6, the amount of solar radiation exceeded 800W/m2 and the daytime highest temperature passed 30°C. On September 3, a 5mm rainfall was recorded between 16:00 and 17:00. Using the actual measured data of September 3 (the highest temp: 34.5°C) and September 4 after a rainfall, we examined the cooling effect for each Case.

Fig. 2(a) shows the change in the rooftop surface temperature Ti in Case 1~5. Although a cooling effect on the surface temperature of several degrees was observed immediately after sprinkling in Case 2~5, the effect was temporary. Fig. 3 shows the time change in the surface temperature immediately before and after sprinkling. The change was proportional to the solar radiation at the time of sprinkling in the order of Morning (sprinkling) < Evening < Noon. On the other hand, the duration of the continuous effect was longer in the order of Noon (sprinkling) < Evening < Morning. The duration is normally two hours or less with this kind of sprinkling. Thus, to expect a continuous effect during daytime, one needs to use water-permeable materials.

The decrease in the upper test piece temperature (Tii) can contribute to a reduction in the convective sensible heat on the atmosphere. Thus, by comparing the upper test piece temperature (Tii) to the rooftop surface temperature, we regarded the surface temperature difference as the cooling effect on the atmosphere. Fig.2(b) shows the change in the surface temperature and the upper test piece temperature in Case 6~9. First, the temperature difference ДТ was compared

Spnnkimg Spunking

Outdoor air temperature

Horizontal global solar radiation ”

Outdoor air temperature

Horizontal global solarradiation “

Precipitation

Precipitation

Date, Time-

fa) Surface temperature of the Rooftop (T^i

Spunking Sprinkling

Date, Time

Case 10

Case6

Outdoor air temperature ‘Precipitation

Horizontal global solarradration ~

(b) Top temperature of exeprimentalbodies (Tn)

Date, Time

(c) Bottom temperature of experimental bodies (Тш)

Fig.2 Temperature change of outdoor air, surface temperature of rooftop, top&bottom temperature of the test pieces

Fig.3 Lapse time of cooling effect after sprinkling.

between T| in Case 1 and Tn in other Cases. Case 6, 8 showed a low surface temperature in the afternoon in comparison to Case 1. Meanwhile, Case 7, 9 showed a large decrease in the surface temperature towards the middle of the day. AT reached its maximum before and after 14:00 when the highest daytime temperature occurred, recording -5.3°C in Case 6, -12.9°C in Case 7, -5.2°C in Case 8 and — 9.1°C in Case 9. After sunset, the differences between Case 6~9 were very small. By 5:00 when the lowest daytime temperature occurred, AT were somewhere around -2.5°C in all Cases. Next, from the temperature difference AT in TII

between Case 6,7 and Case 8,9, the effects of water content and white-paint coating were examined. In the comparison of surface temperature between the cases of water-permeable tiles with and without water content, a maximum temperature difference of 8.9°C was recorded. Comparing the cases with or without a white-paint coating, the maximum temperature difference was 7.4°C. On September 4, there was no major difference in the surface temperature during daytime in Case 6,7. This was presumably because the amount of water content had been recovered to a great extent in both Cases because of the rainfall on the evening of September 3. Thus, it became clear that, when using water-permeable materials, the cooling effect for the next day and later could be expected not only from human-induced sprinkling but also from natural precipitation in a temperate climate region.

20

2Т,

Д

зт

о

4Т,

0

5Т,

6ТП

7Т„

8ТП

ОТ

о

6ТШ

О

7Тп.

8ТШ

д

етш

X

ютш

^G4 : Decrease of temperature on indoor is expected from the cooling effect of 3°C or more on the rooftop surface, mainly during the daytime

G3: Decrease of temperature on atmosphere is expected from the cooling effect of 8°С or more during the daytime, 4°С during the nighttime on the roof­top surface

.G2: Decrease of temperature on f ♦atmosphere is expected from vJJL* the cooling effect of 3-4°C throughout the day on the rooftop surface

_G1: Decrease of temperature on atmosphere and indoor is not ex­pected

16 —

□ $

12

S 8 — f,

<

CX /

The lower test piece temperature (TIM) in each Case is believed to contribute to the overall heat transfer into the room while, as a boundary condition for the rooftop slab, having a time delay due to the heat transmission of the rooftop slab. Therefore, by comparing the lower test piece temperature (Tm) to the rooftop surface temperature in Case1, we regarded the surface temperature difference as an index for the cooling effect on the indoor side. Fig. 2(c) shows the change in the surface temperature and the lower test piece temperature in Case 1 and Case 6~9. As a general trend, the daily change in Tm in each Case was observed to be 1~3 hours late in comparison to the change in the amount of solar radiation. Looking at Ti in Case 1 and AT, the temperature difference in Tiii in other Cases, the difference was almost none at night. Yet, a relatively large cooling effect was obtained towards the middle of the day in Case 6~10. AT reached its maximum around 14:00 when the daytime temperature became maximum, recording (a) -20.3°C in Case 7, (b) -13.8~-

15.1 °C in Case 6, 8 and 9 and (c) -9.5°C in Case 10. (a) In the Case 7 in which the greatest effect was observed, TIII was controlled in correspondence to the air temperature, blocking off most of the radiation heat. As with the change in TII in Fig.2(b), such effect lasted until September 4 because of natural precipitation. (b) The effects in Case 6, 8 and 9 are believed to stem mainly from the heat capacity and heat resistance of the test piece. In the case of the lower test piece temperature, the difference with or without a white-paint coating was about 1.3°C maximum, not a very large number. The reason was that the test was conducted under weak wind conditions that day; thus, the effect of removing heat from the radiation by the ventilation of perforated bricks turned out to be small. (c) In the case of artificial turf, though the cooling effect on the atmosphere could not obtained, that on the indoor side was expected.

12

,[°C]

16

20

A T

ghttir

A : Temperature difference during the daytime

A T lljim: Temperature difference during the nigthtime daytime: 2003/09/03 10:00-15:00 nighttime: 2003/09/04 22:00-2003/09/05 3:00

Fig.4 Cooing effects during the daytime and the nighttime.

In Fig.4, based on the discussion results in above, we obtained the difference between TI in Case 1 and TII, TIII in other Cases and then grouped the decreases in surface temperature by day and night. In G1 (Case 2~5), the decrease in the surface temperature was 1 °C or lower all day long; a cooling effect on the atmosphere and the indoor side cannot be expected much. In G2 (Case 6, 8) and G3 (Case 7, 9), the decrease in the surface temperature of 3

~4°C was observed all day; a reduction in the heat load in the atmosphere can be expected. Especially in G3, the surface temperature dropped by more than 8°C during daytime. In G4 (Case 6~10), a decrease of 8°C or higher in the daytime surface temperature was observed; a cooling effect on the indoor side can be expected.

One can deduce from those analysis results: Case 6~10 are effective in reducing the air­conditioning load during daytime; Case 7, 9 are effective in inhibiting the heat island phenomenon during daytime; and Case 6~9 are effective in preventing tropical nights.

Advanced solar Facades with Integrated. Collectors-Accumulators for domestic hot water and. space heating applications

D. Faggembauu, M. Soria, A. Oliva and J. Cadafalch

Centre Tecndlogic de Transferencia de Calor (CTTC)

Lab. de Termotecnia i Energetica
Universitat Politecnica de Catalunya (UPC)
labtie@labtie. mmt. upc. es, www. cttc. upc. edu

Building facades have large glazed areas, which are normally combined with opaque zones. In double-skin facades, these opaque areas are located in the inner layer of the double-skin. It is possible to take advantage of these areas by means of integrated collector-storage systems, built in modular form and implemented in the facades. Collected and stored solar energy may be used to contribute to reduce the energy consumption needed to produce domestic hot water or space heating in a building. A numerical study has been carried out in this work by means of an own numerical code which allows the transient simulation of advanced facades. Numerical code was validated by comparison with experimental results obtained from prototypes tested in real outdoor conditions.

Introduction

Building envelopes must combine architectonic requirements with energetic performance in order to allow an optimal thermal behaviour of the building. The use of large transparent areas in the facades of singular buildings is becoming an extended practice in order to keep an uniform outdoor appearance and a transparent and modern image.

Transparent areas combined with opaque zones in multilayered facades is being a sub­ject of continuous development, in this field many researchers have analyzed its thermal optimisation [1], [5] and [3], in combination with transparent insulation [7] and phase change materials [7] and [8].

Facades present a potential utility not only for passive heating and cooling the building but also for allowing an extended energy collection and accumulation of solar energy. This work intends to contribute to this objective by means of the analysis of the thermal performance of liquid-based solar collector-accumulators, built in modular form and installed in the facades as part of the inner layer of a double-skin facade or directly as part of a single skin envelope.

Energy stored in water tanks at the facades may contribute to get a reduction of energy consumption applied to domestic hot water or space heating.

This work presents some conclusions of the results obtained in the framework of a CRAFT European project (ASFIC) [9]. A specific numerical simulation tool, called AGLA [6], for the simulation of the advanced facades has been used. Numerical work has considered a transient and one-dimensional behaviour. Numerical predictions have been compared with experimental measurements taken in real-scale test facade prototypes at outdoor con­ditions. Design aspects as collector areas, storage capacities, selective surfaces and the application of honeycomb type transparent insulation materials (TIM) were numerically in­vestigated for two climatic conditions: Mediterranean and Central-European climates. The results presented in this paper are restringed to the performance of a double skin envelope, with transparent insulation, for both climates. Extended situations were studied within the project.

A New Perspective for the Concept of the Discomfort Glare Index

Sebastian Golz, Fraunhofer-Institut fur Solare Energiesysteme ISE, Heidenhofstr. 2,
D-79110 Freiburg, Tel +49 (0)761/4588-5228; Fax: +49(0)761/4588-9217;
sebastian. goelz@ise. fraunhofer. de
Jan Wienold, Kerstin Schuler, Fraunhofer ISE, Heidenhofstr. 2,

D-79110 Freiburg, Tel +49 (0)761/4588-5133; Fax: +49(0)761/4588-9217
Jens Christoffersen

Danish Building and Urban Research, Energy and Indoor Climate Division,

PO Box 119, 2970 Hoersholm, DENMARK

Abstract: The prevention of glare caused by daylight is one of the most important issues for providing a comfortable working environment. A great deal of effort has been made to elaborate an index to describe conditions which lead to discomfort for a user (Discomfort Glare Index). Attempts to develop this Discomfort Glare Index have not yet succeeded in providing a satisfactory degree of congruence between physical lighting conditions and the comfort assessment of users in various trials of experiments. In existing research a number of factors have been found to influence the assessment of discomfort glare. Discomfort glare is compounded by other visual and aesthetic factors, such as the quality of the view from the window. The appearance of the window as well as the visual, aesthetic interior qualities of the room also affected discomfort glare. From these findings one must conclude that a pure physical paradigm to describe discomfort glare might be not sufficient in itself. In this paper, a psychological paradigm will be introduced and linked with the existing paradigm and state of research on discomfort glare. Implications for future research and on the elaboration of a more reliable glare index considering daylighting by integrating the new psychological paradigm will be outlined.

Fresnel-Collectors in hybrid Solar Thermal Power Plants with high Solar Shares

Fig. 15 Constant pressure concept; external depressurization,

Saturated Steam

Fig. 16 Steam accumulator with integrated latent heat storage material.

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

Acknowledgement

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

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

Commission within the 5th Framework Programme on Research, Technological

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

The authors are responsible for the content of this publication.

References

[1] Tamme, R., Laing, D., Steinmann, W. D., Zunft, S., 2002, "Innovative Thermal Energy Storage Technology for Parabolic Trough Concentrating Solar Power Plants”, Proceedings EuroSun 2002, The 4th ISES Europe Solar Congress, Bologna, Italy

[2] Tamme, R., Steinmann, W. D., Laing, D., 2003, „High Temperature Thermal Energy Storage Technologies for Power Generation and Industrial Process Heat", Proceedings FUTURESTOCK 2003, 9th International Conference on Thermal Energy Storage, 1.-4. Sept. 2003, Warsaw, Poland.

[3] Tamme, R., Laing, D., Steinmann, W. D., 2004, „Advanced Thermal Energy Storage Technology for Parabolic Trough", ASME-J. of Solar Energy Engineering, Vol. 126, May 2004.

[4] Eck M., Zarza E., Eickhoff M., Rheinlander J., Valenzuela L.: Applied Research concerning the Direct Steam Generation in Parabolic Troughs, Solar Energy, Vol.

74 (2003) pp. 341-351

[5] Beckmann, G., Gilli, P. V. (1984): "Thermal Energy Storage", Springer Verlag

Hansjorg Lerchenmuller, Max Mertins, Gabriel Morin

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg e-mail: hansjoerg. lerchenmueller@ise. fraunhofer. de

Dr. Andreas Haberle

PSE GmbH, Solar Info Center, 79072 Freiburg, Germany e-mail:ah@pse. de

02

Dr. Stefan Bockamp, Dr. Markus Ewert, Matthias Fruth, Thomas Griestop E. ON Energie AG, Brienner Str. 40, 80333 Munich, Germany e-mail: markus. ewert@eon-energie. com

Dr. Jurgen Dersch

German Aerospace Centre (DLR), 51147 Cologne, Germany e-mail: juergen. dersch@dlr. de

Over the last few years Fresnel-Collectors have attracted a lot of attention within the solar thermal power sector. The main reason is comparatively low investment costs through simple components. The Fraunhofer Institute for Solar Energy Systems,

E. ON Energie AG and German Aerospace Centre (DLR) have carried out a feasibility study in order to assess the technology with respect to technical, economical and ecological aspects.

The mid to long term strategy of solar thermal electricity generation must aim at technical solutions with high solar shares. Thermal storage is not yet technically proven for direct steam generating systems. Therefore special configurations of hybrid operation are an interesting option from a technical and economical point of view. Full load hours of the power plant increase and allow for more stable plant operation. Based on Fresnel-Collectors, two different types of power plant configurations with low or zero CO2-emission are analysed in this paper:

• Hybrid operation of a solar field and a biomass vessel

• From the starting point of a Solar Only power plant, natural gas hybrid operation will be considered and the trade off between high solar share and low cost electricity production will be analysed in detail.

Calculations for this study were carried out in three steps:

• Thermodynamic calculations of the water/steam cycle were done with the commercial process simulation tool Ebsilon [1].

• Thermal and electrical yields were calculated with ColSim [2] for different solar field sizes and different options of hybridization. The simulations are based on the efficiencies of the power cycles — depending on ambient temperature and load — and hourly meteorological data for a site with a DNI of 2’247 kWh/(m2a) [3].

• Based on economic assumptions and on the results of the previous steps, calculations of levelised electricity costs (LEC) and profitability were carried out.