Category Archives: Sonar-Collecttors

Comparison against advanced trough and tower technology

The CLFR/cavern 2010 proposal of 54% CF at US$1784 per kWe, offers costs well below 2020 estimates for both troughs at 56% CF (2225 — 3220 $/kWe) contained in a NREL report (NREL, 2003) which use Hitec salt storage at up to 500°C. The CLFR/cavern proposals at 68% and 81% offer costs (2118 and 2486 $/kWe) much below 2018 ‘base case’ solar tower plants at 73% (3591 $/kWe) and comparable to the revised Sunlab reference case of $2340 for the year 2018. It should be mentioned that the CLFR/cavern 2010 proposal is far from optimised; Tanner (2003) suggests cavern storage at 350°C would be cheaper, and a US nuclear turbines or modern Kalina cycle turbine operating at close to 300°C would offer a 10% efficiency increase, but this would have to be compared against turbine cost.


The potential cost advantage gained by low temperature operation derives from an unusual combination of large low cost low temperature turbines developed for the nuclear industry, and an inexpensive storage concept which suits that particular temperature range. Should both options be applicable, then this is likely to be the most

cost-effective and simple solar thermal electricity development path, using simple solar collector technology already being installed, and a proven turbine from the nuclear industry.

Cavern storage cannot be taken higher than about 360°C and still has some developmental uncertainty ahead of it, but two reports have now identified it as potentially the lowest cost storage concept. Recent discussions that the authors have had with geologists and mining companies suggest the concept is in the realm of current mining technology and can be widely applied; suitable rock structures are common. If suitable geological structures are not available, Caloria oil storage with a CLFR array is a low risk option available for a cost which is still below the trough collector systems. Environmentally, however, cavern storage would be safer than either molten salt or oil solutions.

The electricity wholesale cost for the unoptimised CLFR/cavern in 2010 (the earliest that one can be finished is about 2009) at 68% capacity factor, without the use of any Green support mechanisms, is comparable to the cost of some current conventional pulverised coal-fired (PC) generation in the USA. The cost advantage of coal appears at high capacity factor, but even at a coal CF of 90%, the advantage is only about US$5 per MWhe.

The CLFR/cavern approach is unoptimised and may benefit from slightly higher operational temperatures should a suitable turbine be available. Such turbines may be available in the USA or Europe. The coal fired plant referenced also has a larger turbine than the solar 240 MWe. According to NREL, 2003, a 400 MWe power block should be 25% cheaper per kWh delivered than a 240 MWe equivalent, which reduces cost by about US$3 per MWhe. Furthermore, David and Herzog (2003) suggest that pulverised coal plants could incur an additional cost of US$30 per MWhe for long term cost carbon sequestration.

This brief discussion needs extensive elaboration and more detailed work within the scope of a real project structure. The authors have begun site investigations for a 240 MWe plant of the type described, assisted by Australia’s largest utility.


The system under consideration is designed to air-condition a group of offices on the top floor of the Energy Research Center building at the Technion — Israel Institute of Technology (Haifa, Israel). The conditioned space consists of three offices, with one north­facing exterior wall each (including a window). The total floor area of the conditioned space is 35 m2. The walls and roof are made of 8” (20 cm) prefabricated low-weight concrete, not insulated, with cement plaster. Each office serves two people and their computers; hence total occupancy is 6 persons.

The city of Haifa is an ideal site to test such a system. Located on the Mediterranean coast at 33 degrees north latitude, it has the typical climate of Mediterranean cities. Outside summer conditions (typical for design) are 30oC and 70% relative humidity. Room design conditions have been selected at 24oC and 50% relative humidity.

A load calculation for the three typically staffed and equipped offices shows about 4.2 kW with a room sensible heat factor (RSHF) of 0.92. At 30 cfm (51 m3/hr) of fresh air per occupant (ASHRAE air quality recommendations), the additional fresh air-associated load is about 3.0 kW, most of which (2.4 kW) is latent. Thus, the total cooling capacity required is 7.2 kW, with a grand sensible heat factor (GSHF) of 0.62. The total supply air circulation needed (based on 12 air changes per hour) is 0.4 kg/sec (720 cfm). The desired conditions of the supply air are 14.7oC and 86% relative humidity.

The desiccant solution is regenerated by solar heat, supplied by flat-plate solar collectors of conventional design, of the type widely employed in Israel for domestic water heating, but with better than average quality to enable higher efficiency at high temperatures. The solar collectors and remaining parts of the system are located on the roof immediately above the top floor. Solar-heated water serves as the heat carrier. The option of heating the regenerated solution directly, by exposing it to the sun and to ambient air simultaneously, had been explored but found to be somewhat problematic. The advantages of the current option are simpler construction technology, simpler storage capability, dirt control and simpler ability for using an air-to-air heat exchanger for heat recovery. With the total latent heat load of 2.75 kW, the solar energy demand was calculated to be 4.77 kW. Assuming ten hours of continuous operation daily, and taking a small safety factor, the solar collector area was selected at 20 m2. Solution storage in the amount of 120 liters of LiCl solution at 43% concentration and a 1000 liter hot water tank added to the system make it possible to operate for a total of four hours with no insolation — a typical situation in the summer during the morning hours.

Small capacity water/lithium bromide absorption chiller. for solar cooling applications

Mathias Safarik1, Lutz Richter1, Carsten Heinrich2, Mike Otto3

1Institute of Air-conditioning and Refrigeration gGmbH, Bertolt-Brecht-Allee 20,
01309 Dresden, Germany; phone/fax +49 351 4081-684/635;

Email: mathias. safarik@ilkdresden. de; www. ilkdresden. de

2University of Applied Sciences Zittau/Gorlitz, Germany

3EAWEnergieanlagenbau GmbH, Oberes Tor 106, 98631 Westenfeld, Germany
phone/fax +49 36948 84-132/152;

Email: info@eaw-energieanlagenbau. de; www. eaw-energieanlagenbau. de


Using solar thermal collectors to provide hot water or heating is a well established technology. To reach a higher solar fraction in heating bigger collectors areas are needed. These relatively big collector areas generate excess heat in summer which cannot be used for heating at that time. Storage for the winter time is possible but costly.

Solar thermal powered absorption cooling offers a good possibility to use these ex­cess heat to provide cooling during the summer and to increase the efficiency of the whole system. Solar cooling is also a promising opportunity to cut electrical peak loads during the summer and to reduce fossil fuel consumption.

One of the constraints for a wider use of this technology in recent years was the unavailability of a suitable absorption chiller in the capacity range below 50 kW which is interesting for many applications.

A water/lithium bromide absorption chiller with a nominal capacity of 15 kW was developed. A special heat exchanger design is used to reach small differences be­tween external and internal temperatures. The chiller can be driven by hot water generated by solar thermal collectors or other heat sources. It was designed for low driving temperatures to allow the solar thermal collectors to work with a good effi­ciency. Design conditions are 90 °C hot water input, 32 °C cooling water input, 15 °C cold water output. At these conditions the chiller reaches a COP of 0,7.

Three test installations with various configurations and at different locations have been installed. Different collector types (flat plate and vacuum tube) and cooling towers (wet and dry) have been tested. To predict the performance of the whole so­lar cooling system with different peripheral equipment TRNSYS simulations were done. The effects of using different collectors, storages, cooling towers and cooling coils were evaluated. Measurement and simulation results for some applications at different locations are presented.


There is an increasing energy demand for cooling and climatisation in many parts of the world. The reasons are rising internal loads, the dynamic economic development of re­gions in warm and hot climates, growing standards of living and comfort needs as well as current trends in architecture (higher glass ratio in facades).

At present mostly electric driven compression chillers are used to satisfy the increasing cooling demand. Rising carbon dioxide emissions is one of the consequences. Because of the distinct distribution of the cooling demand over the day with a peak at noon and early afternoon there is also a peak in electric power consumption which impacts the grid and sometimes even leads to black outs.

It will be necessary to expand the capacity of the grid and to install new power plants to satisfy this power demand. Much of this capacity needed will not be used most of the year. One of the alternatives is solar thermal cooling with absorption chillers. Solar thermal col­lectors are widely used for hot water supply and heating in many parts of Europe. One problem is the distribution of insolation and heating demand over the year. Solar thermal plants for heating assistance produce a lot of excess heat in summer which cannot be used at this time. Storage for the winter time is possible but costly.

By using this excess heat for cooling the efficiency of the whole system can be increased. There is a high degree of congruence of the cooling demand and the insolation in many applications. Therefore only little storage capacity is needed.

In the small capacity range (below 35 kW) there was no suitable absorption chiller avail­able so far.

Figure 2: First (left) and second (right) floor plans of the Modern Museum of Art. . The Kimbell Art Museum

Figure 3: Floor plan of the Kimbell Art Museum.

The Kimbell Art Museum was designed by American architect Louis I. Kahn between 1966-1972. The museum has been widely acclaimed by its innovative use of natural light and subtle articulation of space and materials. The museum consists of six bays of 104’- long concrete cycloid shells, divided crosswise into three equal sections (Figure 3). The museum is illuminated by narrow skylights that admit natural light, which is then dispersed by perforated metal reflectors onto the underside of cycloid-shaped vaults and down the walls (Reference 2).

The Kimbell Art Museum introduced the novel transparent daylight reflector, which together with the cycloid-shaped roof system introduced a totally new quality of controlled ambient lighting in museums. The success of this innovation inspired a renewed interest in the use of daylighting in art museums and influenced art museum design thereafter.

Amon Carter Museum

The Amon Carter Museum was designed by American architect Philip Johnson in 1961. Initially the museum was conceived as a small memorial structure, its collection grew rapidly and needed additional space. The museum expanded its area in 1964, 1977 and finally in 2001. The current total area of gallery is 28,400 ft2 (2,638 m2). The collection of artwork consists of 240,000 objects of mainly paintings, sculpture, photography, and works on paper.

The museum’s main entrance wall consists of an east-facing two-story curtain wall of dark tinted glass with bronze mullions, and with an arched portico in its front. The main entrance leads to a two-story exhibit hall of shell stone, brown teak and a floor of pink and grey granite (Reference 3). Beyond the main area, there are several windowless galleries. Later, a new atrium was included in the layout, which introduced natural light to the core of

Figure 4: First (left) and second (right) floor plans of the Amon Carter Museum.

the new building addition. Towards the south facade there is a gallery that receives natural light through a south-facing window. See Figure 4.

Very hot Very cold Climatic Profile cold comfort hot Methodology used in the designing of the house

All the steps of the “whole building design approach” have been applied to the design of the house including the following preliminary steps.



The climate microclimate

Very cold




Very hot






heat 6


cool 2

Fig. 1 — Climatic profile of Rieti

Since Casaprota lacks its own meteorological data, meteo values were taken from Rieti, a city at 400 meters a. s.l., located 40 km north.

As the climatic profile2 (figure 1) shows, the climate is temperate, with six months falling in the cold column, four in comfort column area and only two months hot column. [14]

Figure 2 shows that the average temperature in the winter is 5°C while in the summer is about 29°C, very near to the comfort values, with peaks of 35°C.

Average, average max and average min temperatures in Casaprota, Italy


Fig. 2 — Temperature values of Rieti

In this case the most important problem to face was to take care of the heating because it would be needed for six months of the year.

Temperature and humidity data of the site were also applied using the bioclimatic chart developed by Guillermo Gonzalo[15], to decide the design strategies to be applied: passive solar systems, thermal inertia, natural ventilation, an in some rare cases mechanical ventilation.

Solar Passive Design

To ensure the maximum benefit from the geothermal system, as well as generally reducing

running costs, passive measures were also employed to reduce the heating and cooling

loadings where possible. These included the following-

• the use of skylights and voids to maximise natural daylight penetration into the depths of the building, reducing the level of artificial lighting required; these voids were also a structural issue, with minimal contact between the existing old slab and the new upper floor slab via small ‘bridges’ (see later). The resulting voids allowed for natural light from the roof above to therefore penetrate into the ground floor below.

• glazing to roof lights using Danpalon tinted polycarbonate sheeting to reduce heat gain and loss due to its insulative qualities;

• low-e double glazing to windows and glazed doors, especially along the long street fagade, which faces due east, to ensure minimum solar gain in summer and heat loss in winter, and still providing natural lighting;

• the large glazed face of the upper-floor Council Chambers, with the removal of large areas of single glazing and the space frame awning, has similarly been replaced with low-e double glazing, assisting with solar control as well as the additional benefits of acoustic insulation;

• zoning of conditioned areas, along with air locks provided to main entry to reduce unnecessary heat exchange from air conditioned to external air;

• door seals to high-ventilation areas and external doors, with draught control also to the latter;

• energy efficient lighting, appropriately banked to allow individual switching;

• Verasol insulating blinds to external windows to improve heat loss retention or prevention as needed, but still allowing some daylight penetration;

Ray tracing and penetrability of solar photons

To assess the penetrability of light within channels of monolithic structures a ray tracing software has been used. The software has been created under Matlab® environment, using the Solar Concentrating Toolbox SCT developed by CIEMAT [7].

To take into account the sun shape and the beam quality of the concentrating system we have convolved the sun shape according to Abetti [8] with a Gaussian function representing the total optical error of the system. For the purpose of this analysis a total error of 3.2 mrad has been assumed in all the cases. The economy of computation time has been obtained via a hybridization of ray tracing techniques and the well known classical cone optics methodology. The reflected cone for each and every convolved ray has been generated randomly according to the Probability Density Function. The distribution of the reflected rays is a function of the reflected cone but also of the reflecting point at the concentrator surface. To accurately quantify both effects on the distribution we have created as many reflecting points on the reflector as rays representing the reflected cone. Then the total number of rays pointing to the target will be the number of reflecting points X number of rays per reflected cone, and therefore both effects would be represented adequately.

To predict the penetrability as a function of the view angle the study has been focused on parabolic dishes with different rim angles from 20° to 60°. Besides the influence of the view angle, the impact of the reflectivity and the specularity on penetrability has been a matter of this study as well. We have developed a specular reflectance model and a diffuse reflectance model to calculate the maximum and minimum penetrability achievable1.

Pitch length of individual ducts is also one of the key factors to be taken into account while designing. The study of the influence of the pitch length in penetrability is also essential. With SCT software it is possible to calculate the flux distribution by channels, it is possible to discern between close channels and also between walls inside a single channel. Although we have found differences in flux distribution between walls in the same channel and between channels according with the channel position from the center of the focal point, that results are not presented here because the aim of this analysis is finding general information about several design parameters and their influence on penetrability. Because of that, the results presented are mean values from the channels studied, placing their aperture at the focal plane. In total, as many as 250 cases (125 diffuse model and

The diffuse cases have been calculated from the case Reflectivity =0, applying shape factors.

125 specular model) were characterized by combination of five reflectivity values, from 0.1 to 0.9 each 0.2, five rim angles from 20° to 60°, and five pitch lengths from 1 to 5 mm. The results shown in this paper are those with view angles closer to typical designs in solar tower projects with heliostats fields.

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.








screen + 1 cm


window / transition region








transition region / wall








window / wall








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.