Category Archives: Sonar-Collecttors
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
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.
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]
20 40 60 80 100 120
0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14
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.
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.
Of the 20.000 sq meters of hilly landscape, the location for the house was chosen in the middle of the south slope so that the top of the hill would protect it from cold winter winds. At the same time the view of the Farfa river Valley from the house would not be in anyway obstructed (Figure 3).
2. The design of the house
The house project was developed according to the following principles:
1. Advantage of the local renewable energy resources should be taken such as sun, vegetation, breezes and winds through bioclimatic strategies and solar technologies for
Fia.4 — Perspective of the house
heating and cooling naturally the building.
2. Traditional as well as innovative technologies are to be used so as to disseminate similar solutions in the area;
3. Technological and morphological solutions that could be applied in existing constructions are recommended;
4. Building materials with low environmental impact are to be used.
The house shape draws its inspiration from local architectural traditional buildings: very compact structures broken by terraces facing the valley (Figure 4). For administrative limitation required by the Regional Competition “Zero Emission Residential Buildings”4, the house was divided into two compact volumes (volume A and B) and organized around a Mediterranean patio with an olive tree in the centre (Figure 5). The olive tree is the most common tree of the area.
PIANTA DEL PIANO TERRA
Fig. 5 — Floor plan of the house
A terrace facing south and covered by a light canopy of deciduous climbing plants, essential to control solar radiation in the summer, links the two volumes. This solution allows taking advantage of the surrounding microclimate of the house, particularly during the summer.
A terrace, a garage, a technical room and a storage room are built under the terrace with a portion of their south wall above the ground. The solar thermal and photovoltaic technologies are integrated on this wall. This solution while respecting the natural land slopes minimizes the terrain movement (Figure 6).
6 — A-A Section Summer ventilation behaviour
The organization of the rooms follows bioclimatic principles so that the main rooms face
Only project of 95 sq meters houses will be financed.
south while the service spaces face north. The most important transparent surfaces face south so as to take advantage of sun radiation in winter as well as the beautiful panorama in all seasons (Figure 7). All rooms are daylighted.
The two volumes have windows in the four facades, in order to allow natural cross ventilation (Figure 6). Moreover, since part of the house is very high, openings in the upper part of the volume are designed in order to promote the “stack effect” during the summer when there is no wind.
An awareness of green building issues included the construction processes. In an effort to reduce wastage, minimise materials and a consciousness of cradle-to-grave costs, the following were implemented —
• with a significant amount of the building deconstructed prior to rebuilding, as much as possible of the existing structure was retained, including most of the western wall and a large section of the roofing structure. Where window openings were retained, existing window assemblies, where possible, were reglazed and painted to match new
windows, whilst other openings and the main stair were also retained as part of the new scheme;
wherever possible, demolition and packaging materials were separated for recycling, taking advantage of the services of the Council’s extensive Recycling Depot. This included separation of glass, concrete, timber, bricks and rubble, as well as paper and plastic packaging. These were then forwarded on as part of Council’s recycling program, such as rubble reused for fill, and timber sold off for use by local builders. A substantial amount of the original fitout was also sold off and removed for use by other builders and collectors;
asbestos was found on site within the original fixout, and removed with full OH&S compliance, and contained within a ‘tented’ area during demolition prior to removal for off-site burial;
implementation of Ultra-floor slab system reduced construction time as well as the amount of concrete and steel employed (and therefore costs and impact). The ribbed system, as wella s reducing concrete volume, also therefore allowed extra head room due to the thinner ability of slabs between floors. This was highly beneficial given the restriction of the existing poor height differences between floors; new voids on the upper level were a result of a design decision to reduce the contact between old and new slabs at first floor level, with ‘bridges’ connecting the slabs across at several locations. Minimal connection ensured less opportunity for movement and minimal cracking potential between the different slab qualities and ages. This also made construction easier, and was cost effective due to the reduced need for merging of the two slabs and therefore less concrete, as well as the added benefit of increased daylight penetration to the lower level;
Where possible, within the limitations of budget and the Council’s adaptability to change,
products were selected for their reduced impact on the environment as well as enhanced
health benefits. These included —
• external walls (all new walls, as well as existing walls made accessible during construction), were insulated with wool bulk insulation (R3.5 ceiling, R2 walls) in lieu of fibreglass, given the latter’s unresolved health issues;
• HDPE (High Density Polyethylene) was used instead of PVC for all drainage and plumbing, again in light of the relevant unresolved health issues of the chlorine content of standard PVC fittings;
• natural linoleum use for flooring in lieu of vinyl which normally contains PVC;
• timber selected only from plantation grown supply, without use of rainforest or imported timbers;
• the new Council Chamber furniture was locally crafted from old bridge timbers from the original Cowra Bridge;
Additionally, local and/or Australian-made products were given preference to imported
products and materials.
The results of penetration for specular materials are depicted in Fig. 1. In all of them it is clearly shown that most of the absorption takes place in the first 5 to 20 mm of the duct, advancing an apparent lack of “volumetricity”. Parameters like reflectivity, pitch and view angle are influencing the profile of absorption and cumulated energy through the channel.
Fig. 1. Plots summarizing the profiles of absorbed energy through the specular channel for different values of view angle, channel pitch and reflectivity.
Specular reflectance model shows the maximum penetrability achievable within channels. Therefore a high reflectance material is strongly recommended since the flux distribution within duct is in these cases smoother. However in all cases higher incident flux values are reached within the first millimetres of the duct what will lead to the higher material temperature values closer to the channel entrance. This fact is by itself contrary to the volumetric philosophy.
Obviously the increment of channel pitch leads to a better penetration of photons, but this increment cannot be indefinite since we may presume that cumulated energy decreases because of higher radiation losses. View angles below 30° seem to be more adequate for volumetric receivers as well. High reflectivity materials lead to a change of penetration
profile and then a peak is registered a few mm inside the channel. On the contrary, when low reflectivities are used the maximum absorption is obtained at the front edge.
Fig. 2. Plots summarizing the profiles of absorbed energy through the channel for different values of reflectivity at diffuse and specular models.
In the case of the specular model the channels work as light traps, and therefore the cumulative energy through the channels increases to reach the total value of 1. When this value is not reached within the graph this is because the depth of the represented channel is not big enough.
In the case of the diffuse model part of the incident radiation is lost through the channel aperture (Fig. 2). The channel itself is of course a light trap increasing significantly the apparent absorptivity of the channel from the material absorptivity values (Fig. 3). The cumulated energy through the channels reaches an asymptotic value representing the apparent absorptivity of the duct. The diffuse model shows that apparent reflectivity is more sensitive to viewangle parameter than pitch length (Fig. 3 y Fig. 4).
The real case will be neither the specular model nor the diffuse model but something in between.
Ray tracing provides clues on internal behaviour of light, but only with a thermal analysis we may quantify some of the effects. Because of that we decided to analyze the influence of the previous photon penetration profiles on heat transfer figures.
Fig. 3. Apparent absorptivity of the channel as a function of material absorptivity and pitch length for the diffuse model
Fig. 4. Apparent absorptivity of the channel as a function of viewangle and pitch length for the diffuse model.
Standard desiccant systems are used to produce conditioned fresh air directly. Therefore, they can be used only if the air-conditioning system includes some equipment to remove the surplus internal loads by supplying conditioned ventilation air to the building. In the majority of cases this air-flow consists of ambient air, which needs to be cooled and dehumidified in order to meet the required supply air conditions. In desiccant cooling systems employing sorptive rotors, the components installed in the air-handling unit are activated according to the operation mode of the air-conditioning system.
These operation modes implement different physical processes for air treatment, depending on the load and the outdoor air conditions.
DEC systems are based on the physical principle of desiccant and evaporative cooling. Unsaturated air is able to take up water until a state of equilibrium, namely saturation has been achieved.
The evaporative cooling process uses the evaporation of liquid water to cool an air stream.
The lower the relative humidity of the air, the higher is the potential for evaporative cooling. In this type of air conditioning systems the dehumidification effect is used for two purposes: to enhance the combined evaporative cooling potential at given environmental conditions and to control the humidity of ventilation air. However, a standard desiccant cycle is not able to provide desired supply air temperature and humidity states under all conditions. Particularly in hot — Figure 1-Psychometric chart representing humid climates the desiccant cooling cycle has the standard DEC pr°cess path
limitations. Therefore in those cases employing
standard components such as a rotary dehumidifier, a combination of a desiccant cooling air handling unit with a cold backup system is needed. The ECOS provides a valuable option for air-conditioning applications in hot-humid climates 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.
In the first planning phase of a solar assisted air conditioning system, the type of the building to be equipped with the air-conditioning system and the use of the building has to be closer analysed. High attention should be paid to architectural and technical measures which reduce the cooling load, e. g. by implementation of external shadings, night ventilation in combination with the thermal inertia of the building and by decreasing the internal loads as far as possible.
For further investigations, an annually load file should be prepared, containing at least hourly values of cooling and heating loads as well as humidification and dehumidification requirements. The load structure of a considered building area depends beside the physical properties of the building and the thermal and radiative gains on the usage of the building,
i. e. the frequency of occupation, occupation density and on the additional technical equipment in the rooms. The determination of a representative time series of loads is necessary since the correlation between solar radiation power and heating/cooling loads determines the energy demand from auxiliary sources as well as the utilisation of the solar thermal sub-system. Including these effects into the annual energy balance allows to assess primary energy savings, to choose appropriate sizes of the core components and thus to obtain reasonable economic figures of the planned installation. Several load calculation methods, e. g., according to the German standard VDI 2078 /1/, and building simulation programs like TRNSYS, ESP, etc., may be applied for this purpose.
A simplified decision scheme is presented in Figure 1 in order to support the pre-selection of air-conditioning technologies, which can be used in combination with solar thermal systems.
Figure 1: Simplified decision scheme of thermally driven air-conditioning systems. It is assumed that both, indoor temperature and humidity are to be controlled. The starting point is the assessment of the cooling loads and of the required air exchange rates. If the installation of a centralized air handling unit is feasible, the following basic decision is, whether or not the hygienic air change rate is sufficient to cover the cooling loads (latent + sensible). This will be typically the case in rooms with high ventilation rates, as required e. g. in lecture rooms. Next, the tightness of the building shell has to be considered for the decision, whether a supply/return air system makes sense or not. Depending on these building and load oriented tasks, the decision on the distribution medium is made: either a pure chilled water system, a pure air system or combined solutions are possible. Finally, the technology (third column) has to be selected. In case a chilled water supply is necessary, the lowest required temperature level of the chilled water is determined by the question
whether air dehumidification is realised by cooling the air below the dew point (conventional technique) or whether air dehumidification is realised by a desiccant process. In the latter case, the temperature of the chilled water may be higher since it has to cover sensible loads only. In extreme climates with high ambient air humidities, special configurations of the desiccant cooling cycle are required, when this technology will be employed.
Short cuts: DEC = Desiccant Cooling system, AHU = Air Handling Unit.
The scheme presented in Figure 1 can help to approach to the most appropriate technical
solution but does not cover the following questions:
— Necessity of a backup system for the cold/heat production or to allow solar autonomous operation of the solar assisted air-conditioning system;
— Type of the thermally driven chiller, e. g., absorption chiller or adsorption chiller;
— Flexibility in comfort conditions, e. g., to allow certain deviations from the desired air states;
— Economical issues;
— Availability of water for humidifiers in an air-handling unit or for cooling towers;
— Comfort habits for room installations: fan coils have lowest investment cost, but allow dehumidification only when connected to a drainage system; chilled ceilings and other gravity cooling systems require high investment cost, but provide high comfort.
Additional constraints may arise from an architectural / planning point of view or from
— possibility to install the required air ducts in existing buildings
— question how the desired technical solution fits to the technical infrastructure of an existing building (e. g., if a cold water distribution system already exists this might lead to such a system because of economical considerations even if fig. 1 leads to a pure air system)
— available area to install the required solar collector field
— available space for technical equipment (e. g. thermally driven chillers, buffer storage, desiccant air handling unit)
From the results it is clear that low luminance ratios (of no more than 1:20) and a gradual decrease in luminance values can only be obtained when a. the material absorbs a certain amount (but not all) of the incoming light, b. the material reflects some of the incoming light onto the wall. This conclusion, however, is not complete. The conclusion should be written in terms of reduced transmission: low luminance ratios (of no more than 1:20) and a gradual decrease in luminance values can only be obtained when a. the material reduces the amount of the incoming light by absorption or reflection and b. the material reflects some of the incoming light onto the wall.
Figure 4: Effect of the size of the anidolic element on the luminance distribution on the wall
By substituting absorption with transmission, the origin of the main difference between the anidolic element described in earlier work and the materials described in this paper is clarified. This anidolic element was designed for optimal luminance ratios between the wall and the window. That this is indeed the case, has already been shown by van der Voorden et al. . Besides increasing the luminance ratios between the inner facade and the window, the anidolic element can also influence the size of the luminance ratio by applying a smaller or larger anidolic element, see figure 4. A larger element will receive more light and distribute thus more light onto the inner facade.
However, the anidolic element takes up a lot of space, especially the larger ones which have the lower luminance ratios. The size of the element perpendicular to the window is about half the size of the opening along the wall. And the anidolic element in its optimal position is situated in such a way that the end of the element is in the same position as the beginning of the wall next to the window. This means that the occupant of the building can still see the sky directly when the person’s main view is not perpendicular to the window but under a slight angle. In such a situation the visual comfort is immensely decreased. An overlap of the other materials like net curtain and screen in front of the window, prevent the occupant’s direct view of the sky. An overlap of the anidolic element with the wall can create a dark line on the wall next to the window when the focus line of the anidolic element is shifted due to a horizontal shift along the window of the element. If not the
element is moved sideways, but the length of the element is increased, then the bright focus line is obscured by the longer element, thus increasing the low luminance ratios. An overlap of the other materials like net curtain and screen + 1 cm, achieves a larger amount of light on the wall compared with the material which does not overlap, due to the diffuse property of the material and its possible internal reflections.
All above results were obtained in a black office with a white wall next to the window. In a grey or white office the luminance ratios are automatically lower due to the contribution of the internal reflections to the amount of light on the walls. For new buildings, it is therefore clear that the window posts must be painted as light as possible, as was already mentioned by . For luminance ratios that are still too high a transition region in the window by adding a pattern to the edges of the windowpane will already improve the visual comfort. If the luminance ratio between the window and the wall is even higher, a translucent, diffuse material positioned in front of the window and in front of a part of the wall will improve the visual comfort. This overlap will also prevent the occupant of the building from viewing the sky directly when the person’s main view is not perpendicular to the window. A coloured material can be applied, but this has the adverse effect that the view outside becomes coloured as well.
The luminance ratios for the investigated materials are always lower than the ones that can be achieved with an anidolic element, whether it is perforated or not, due to the focussing power of the specular element. However, the anidolic element takes up more space, and does not increase visual comfort for every position in a room.
1. Velds, M, Assessment of lighting quality in office rooms with daylighting systems, Ph. D. Thesis, 2000, Technical University Delft
2. Bokel, R. M.J., Heijmans B. N., Pel M., Voorden M. van der, Proceedings of the International Building Physics Conference, 2000, Eindhoven, The Netherlands.
3. Voorden M. van der, M. B.C. Aries, R. M.J. Bokel, Development of a contrast-reducing anidolic shaped element, Proceedings of PLEA conference 2002 (Toulouse).
4. Dutch NEN-norm 3078
5. Moore, F. Concepts and practice of architectural daylighting, Van Nostand Reinhold Company Inc., New York. 1985.
6. Osterhaus, W. K.E., S. E. Selkowitz, Background and conceptual plan for conducting post occupance evaluations of interior luminous environments, Energy and Environment Division, Lawrence Berkeley Laboratory, Berkeley, USA, 1992