Category Archives: Sonar-Collecttors

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.



100 MW

30 MW

5 MW


MFP Coll.


MFP Coll.


MFP Coll.

Vmfp /V*







ptMFP /pt*







Pmfp /P*







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


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.



Power; W


Pressure; Pa


Surface area; m2


Heat transfer rate; W




Temperature; K


Specific heat; J/kgK


Volume flow rate; m3/s


Solar irradiation; W/m2


Gravitational acceleration; m/s2



Height; m


Maximum fluid power




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 “



Date, Time-

fa) Surface temperature of the Rooftop (T^i

Spunking Sprinkling

Date, Time

Case 10


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.






















^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 —

□ $


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.







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.


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.

Experimental Study of Temperature Distributions inside Metallic Monoliths used as Volumetric Solar Absorbers

Silvia Palero, DER — Ciemat, Av. Complutense 22, 28040 Madrid, Spain.

Manuel Romero, DER — Ciemat, Av. Complutense 22, 28040 Madrid, Spain.

Claudio A. Estrada, Center for Energy Research, UNAM, PO 34, 62580 Morelos, Mexico Jose L. Castillo, Mathematical Physics and Fluids, UNED, PO 60141, 28080 Madrid, Spain. Rafael Monterreal, PSA Ciemat, PO 22, 04200 Tabernas (Almeria), Spain.

Jesus Fernandez-Reche, PSA-Ciemat, PO 22, 04200 Tabernas (Almeria), Spain.


Temperature distributions inside volumetric solar absorbers have been calculated with numerical models but there is not any experimental study focusing on that topic. With this purpose, several tests using metallic monoliths made of parallel ducts as volumetric solar absorbers have been perfomed at the CRS facility of the PSA. The radial air temperature distributions at the absorber exit have been measured for all the samples. Moreover, the axial wall temperature distribution, measured with thin thermocouples inserted inside the absorber channels, gives information about the volumetricity of the samples. The results show that the absorbers tested do not behave as pure volumetric axial absorbers because they present the maximal temperatures relatively close to the front surface and subsequently, the heat transfer from wall to air in the inner part of the absorber does not allow the homogenization of both temperatures.


Air-cooled volumetric solar receivers are considered a good option to absorb solar energy and to transfer the heat from the porous matrix to the air, because they reduce the re­radiation losses by decreasing the temperature of the absorber more external surface, thanks to the volumetric effect (Fricker, 1990). Models to simulate the heat transfer in the volumetric absorber have been developed, leading to theoretical porous material and air temperature distributions (Hoffschmidt,1996; Garcia-Casals, 2000). The computed radial temperature distribution at the absorber front surface shows a good agreement with the temperature distribution at this surface measured by an IR camera. However, the axial temperature distribution inside the absorber matrix has not been experimentally studied until now. The aim of this work has been to measure and to contrast axial and radial distribution of temperatures, as the preliminary step for further analysis of the influence of several parameters (like the absorber length/diameter ratio or the cell density) on the heat transfer in a monolithic metallic corrugated foil absorber and its comparison to numerical models of volumetric structures.


The liquid desiccant system is designed to serve as an open-cycle absorption system that can operate with low-grade solar heat. A schematic description of the final design version of the system is given in Figure 1. The system consists of six major components: an air dehumidifier or absorber, a solution regenerator or desorber, two water-to-solution heat exchangers, a solution-to-solution heat exchanger, and an air-to-air heat exchanger. Arabic numerals indicate working fluids state points at specific locations. Air flow is represented by thick solid lines, solution flow by thin solid lines and water flow by dashed lines.

The dehumidifier (absorber) consists of a packed tower and operates in an adiabatic mode. Ambient air at state 13 entering the bottom of the absorber packed section is brought into contact with a concentrated absorbent solution entering the unit at state 8. Water vapor is removed from the air stream by being absorbed into the solution stream. The dehumidified warm air leaving the absorber passes through the blower and leaves the system toward the air-conditioned space at state 14. The blower controls the flow of air, while raising its temperature slightly. Solution is pumped from the absorber pool at the bottom of the tower into the plate heat exchanger (state 7), where it is cooled by water from a cooling tower. The solution leaving the heat exchanger (state 8) then proceeds to the distributor at the top of the packing, from where it trickles down in counter-flow to the air stream and collects in the pool. A controlled solution stream is transferred from the absorber pool to the regenerator, as shown (state 11). The return (pumped) stream from the regenerator (10) goes directly into the pool.

As evident, the regenerator (desorber) device is very similar to the dehumidifier, and so are the flow system and associated components. The solution is heated in the liquid-to — liquid heat exchanger by solar-heated water (states 1-2). Ambient air is pre-heated in the air-to-air heat exchanger by recovering heat from the exhaust air leaving the desorber. After pre-heating, the air stream (state 15) enters the desorber where it serves to re­concentrate the solution (state 3). The exhaust air leaves the desorber, passing through the blower, then pre-heats the entering air stream and is rejected to the environment. The solution-to-solution heat exchanger facilitates pre-heating of the weak solution leaving the dehumidifier (states 11 to 12) and recovers heat from the hot strong solution leaving the regenerator (states 9 to 10).

Supply Air 14



Solution Pump Drain

Water/Solution H. X. 7



U*—— Ms—

Cold Water From Cooling Tower




Figure 1: Schematic description of the liquid desiccant system

The above brief description of the system already reveals a number of advantages of this system over conventional absorption heat pump cycles: (1) The number of main components is reduced by one by transferring condensation of the refrigerant from a condenser to the environment. (2) Capital-intensive pressure-sealed units are avoided as the whole system operates at atmospheric pressure. (3) The amount of refrigerant (water) evaporated in the regenerator is independent of an evaporator, providing greater flexibility. (4) Efficient utilization of very low heat source temperatures is possible.

In the overall setup, the liquid desiccant system is connected in a flow arrangement allowing storage of concentrated solution and a capability to work in three different modes. The first is a manual mode used for testing individual components of the system. The other two modes are automatic, as may be selected by the user. One automatic mode is for full operation of the system (FOP) and the second is for regeneration only (REG). In the automatic FOP mode, all system components operate, including the solution storage circuit, if required. In this case, the absorber solution pump may supply the dehumidifier with solution from both the absorber pool and from the solution storage tank, in parallel. Thus, dehumidification can continue independent of regeneration. If the solar collectors cannot supply water at sufficiently high temperature, or if the concentration of the solution in the storage tank and/or the regenerator pool rises above a set limit, the regeneration side of the system will shut down for a certain time. In the REG mode, only the regenerator (desorber) side of the system operates. The system shuts down automatically when the concentration of the solution in the storage tank reaches a certain high value or when the temperature of the hot water drops below a certain limit. At the end of days of
high insolation, when a large amount of solar heat has been collected, the user can set the system to operate in the automatic REG mode before leaving the site.

Requirements for the absorption chiller

There are different types of solar thermal collectors. At present flat plate and vacuum tube collectors are most often used. The main differences are the efficiency and the cost. The efficiency of solar thermal collectors is a function of the fluid temperature inside the collec­tor (working temperature), the solar insolation and the ambient temperature. In Figure 1 the efficiency curves of some sample collectors are shown.

T — T [K]

m a

20 40 60 80 100 120

0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14

(T/g/Ig [(m2K)/W]

Figure 1: Efficiency of some solar collectors and working area for solar cooling applications

There are big differences between different collectors. Especially the efficiency of the cheaper flat plate collector decreases sharply with rising working temperatures of the col­lector. To reach a high efficiency of the complete system consisting of flat plate collectors and the absorption chiller it is important that the absorption chiller is able to work with rela­tively low driving (heating) temperatures.

Evaluation of Galleries

Direct sunlight enters to several spaces in the three museums. In two of these museums direct sunlight penetrates in galleries where artwork is displayed, that is the case of the Modern Art Museum and the Amon Carter Museum.

The museums were evaluated through site visits to the museums at different times of the year, interviews with curators and facility managers, photometric measurements (with and without electric lighting), photographic documentation of sunlight penetration using fish-eye photography with the SunPath program (Reference 4), and computer simulations using the ECOTECT and Desktop RADIANCE software to analyse the patterns of sunlight penetration and to calculate illuminance and luminance levels throughout the year. The following section describes the evaluation of each museum.

The Modern Art Museum

Large spans of glass illuminate the main entry lobby (north and south-facing), and the sculpture galleries (east-facing), both are two-story high spaces. Most of the galleries in the first floor are windowless and are mainly illuminated by electric light sources. The subtle transition between the highly illuminated east-facing galleries and darker windowless central galleries is successfully achieved through intermediate galleries and passages. Most of the galleries in the central area of the second floor use toplighting as the main source of illumination in combination with halogen lamps. The illuminance levels in these central galleries are well controlled through a series of internal and external louvers that intercepts most incoming direct and diffuse light rays. Light levels in these galleries do not exceed the 200-lux recommended for oil paintings. Along the west side of the second floor are located seven galleries that are illuminated mainly by side light windows with white interior screens (see Figure 2-right).

Sunlight penetration was observed throughout the year along the west-facing galleries, located on the second floor (see yellow areas of Figure 2-right). Illuminance value measured over an oil painting was between 2,000 and 4,000 lux (Figure 5), which are 10 to 20 times higher than the maximum recommended illuminance of 200 lux for oil paintings, as stated in the “IES Recommended Practice for Museum” (Reference 5).

Figure 6 is a fish-eye photo taken from an oil painting’s viewpoint with a sun path diagram, and shows the number of hours the painting is receiving direct sun on its surface. This painting is exposed to direct sunlight mainly during the late afternoon hours between 4:00 and 6:30 PM between the equinox and summer solstice. Many of these west-facing galleries cannot be used to exhibit light susceptible artwork. Several of these galleries had
remained empty since the opening of the museum, and few of them only display sculptures made of metal.

Figure 5: Direct sun over oil painting, at the Modern Art Museum’s west gallery, March 5, 2004, 6:30 PM.


Figure 6: Fish eye photo taken from painting’s viewpoint with sun path diagram, at the Modern Art Museum’s west gallery.