Category Archives: EuroSun2008-7

USING A HEAT ABSORPTION SYSTEM OPERATING. WITH SOLAR ENERGY FOR COOLING IN THE STATE OF QATAR

Musbah Mahfoud and Ahmad Bin Marzook

Department of Mechanical Engineering Technology
College of the North Atlantic-Qatar

Abstract

Sun-light is an abundant and infinite source of energy that is currently being overlooked. The sate of Qatar, which is located close to the tropic of Cancer, is well positioned to exploit this potential. The country’s climate is sunny most of the year with temperatures ranging from 12oC to 58oC. In addition to being sunny, the climate is typically hot and humid. To avoid these extreme weather conditions, the population of Qatar depends on enclosed air-conditioned areas and other forms of refrigeration. Since the early 30’s, the country has spent millions of riyals on refrigeration and air conditioning to strive and survive in this climate.

Thus far, the main form of energy used in Qatar has been fossil fuels, a non-renewable energy source. Solar energy, an abundant renewable energy source, is available in the country, but has yet to be exploited. Solar energy has been used throughout the world in various applications such as cars, calculators and heating water, but its use as an energy provider for refrigeration has not been widely investigated. Some household items currently operated electronically can be replaced with solar powered alternatives. Thus, solar energy can reduce cost and save energy if exploited in Qatar.

The direct conversion of sunlight into electricity is a very elegant process that generates environmentally friendly, renewable energy. The purpose of this study was to present a method to attain cooling using solar thermal energy.

In terms of refrigeration, the idea was to investigate the possibility of developing a refrigeration system that depends fully on solar energy. The current project consists of the physical construction of a Solar Refrigeration System. The project involved analyzing, designing and assembling a solar system consisting of (along with other accessories) a laboratory refrigeration unit and a solar panel.

In terms of air conditioning, the objective was accomplished with a parabolic solar reflector and a heat absorption cooling system using ammonia, water, and hydrogen. It is shown that such a system can indeed produce significant cooling using solar thermal energy alone.

Introduction

A crucial response to the dangers of global warming is the worldwide utilization of solar energy. Solar thermal technology is modular, operated silently and is therefore suited to a broad range of applications and can contribute substantially to future energy needs. From a sustainability perspective, directly using solar energy is attractive because of its universal availability, low environmental impact, and low or no ongoing fuel cost. Solar cooling could be a useful technology in areas of the world where there is a demand for cooling, high insulation levels and no firm electricity supply to power conventional systems.

Solar systems are relevant to Qatar because:

o Ambient temperatures are high through out the year in most parts of the country. Higher ambient temperatures mean more energy is consumed in refrigeration. o Solar insulation is high in most parts of the country. This implies that we have more energy at our disposal.

o There is huge potential demand for refrigeration in rural areas, which are best suited for solar energy based refrigeration systems.

o There is a high potential for cost and energy savings.

Literature Review

Cooling using solar thermal heat was first patented by Shipman1 in 1936. The idea was not applied on a wide scale because the process was relatively inefficient and other forms of energy were cheap and readily available. Erickson and Donald2 have demonstrated recently that there has been a renewed interest in the process. Uli et al3 completed detailed studies on the performance of the bubble pump (the device that drives the process) last year. Erickson and Donald2 presented a large scale application of this process in their paper, which was published during the course of the research for the present work. They mentioned that heat absorption cooling is currently used to cool Cochise Community College in Arizona.

System Controller

For the development of standardized solar cooling systems it is indispensable to use a system controller for the complete system. The previous solar cooling demonstration and pilot projects are using several single controllers e. g. for the solar thermal system, for the chiller, for the recooler and for the cold or heat distribution, which are together cost intensive and are not always operating optimal together. The alternative was until now an expensive SPS controller which had to be programmed for each single case. Because of that the SolarNext has decided in the year 2007 to develop an own system controller for the whole system (Fig. 2.), which has an influence from the automotive sector and is cheap and system oriented.

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Fig. 2. chillii® Solar Cooling System scheme (source: SolarNext).

The functional range of the chillii® System Controller (Fig. 3.) contains the control of different heat sources (e. g. solar heat, CHP waste heat, district heat, etc.), the back-up system (e. g. controllable oil/gas boiler or not controllable wood boiler or exhaust gas heat recovery), the storage management (heat and cold storage), the hot water, the chiller (e. g. chillii® STC8, PSC12, STC15, WFC18, EAW SE15, Yazaki WFC-SC10, etc.) and the re-cooling (e. g. wet, dry, and hybrid cooler) as well as heating and cooling circuits. The chillii® System Controller is the first system controller for thermal cooling and heating systems that controls many large hydraulic variables with one device. So the highest system efficiency is reached with the needed energy generation with priority in regenerative energy sources, optimized running of chillers as well as the re-cooling with speed control of the pumps and the re-cooling ventilator.

2.

Подпись: Fig. 3. chillii® System Controller (source: SolarNext).

Solar Cooling Systems

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During the last few years a few companies in the solar business have positioned on the market as system providers for solar cooling. In the small scale capacity range up to 30kW there is for example the company SolarNext with its chillii® Solar Cooling Kits and Systems respectively based on the chillii® STC8, chillii® PSC12, chillii® STC 15, chillii® WFC18 and absorption chillers from the company EAW and Yazaki. Further companies like e. g. Enus, Phonix, Schuco and Solution also offer solar cooling systems with the different chillers. The solar cooling systems basically contain solar thermal collectors with attachment, hot water storage, pump-sets, a chiller, a re-cooler, partly cold water storage and a control unit. Fig. 4. shows as an example the chillii® Cooling Kit 18, which can be supplemented by a solar package, a cold storage package, a cold distribution package, etc. The cooling kits are developed for the European market, whereas other re-coolers can be offered according to the country (e. g. in Spain a dry re-cooler).

Fig. 4. chillii® Cooling Kit 18 (source: SolarNext).

The average value of the specific collector surface of all until the year 2006 installed solar cooling systems in Europe is about 3 m2/kW. A value from 3.5 to 4.5 m2/kW can be considered as a reference value for thermal driven absorption and adsorption chillers. But these values are only rough reference values and can never replace the detailed design and simulation of a system. The specific total costs of installed solar cooling systems in Europe are so far between 5,000 and 8,000 EUR/kW. For 2008 system prices of 4,500 EUR/kW are reached, in the future 3,000 EUR/kW are expected.

First step : presentation of the method

This first step will present the method and the context it has been developed. This is due to the work of IEA Task 38 Subtask B4 [1] grouping several main actors of solar cooling field such as Fraunhofer ISE, Aiguasol and TECSOL. After a short presentation of the workgroup, the document is presenting the different steps of the method and the logic process : check list, decision scheme, technical calculations, economical calculations, results presentation.

Single-family house

For the single-family house, in Rome it is possible to achieve economic viability for a solar system with gas or electric backup when the solar fraction is between 40-80% — see Figure 3. Comparing with a conventional electric air-conditioning, this system leads to a reduction of 3.8 c€/kWh of produced energy. In situations where an electric backup is not possible (only gas backup), solar systems installed in Rome and Lisbon with solar fractions between 20-80% are economically interesting. For Berlin, profitability occurs just with the use of a flat-plate collector and solar fractions between 20-40%. A gas boiler as backup solution, instead of an electric compression chiller, allows a reduction in solar collector area between 0-20% for the same solar fraction. In the Mediterranean cities, the flat-plate collector compared to the vacuum tube collector, allows a reduction in the total cost of produced energy between 0-2.2 c€/kWh — see Figure 3. In Berlin, for an electric backup both collector technologies show similar results, but for gas backup the flat — plate collector presents better results. Vacuum tube technology has the advantage of allowing a reduction of collector area between 5-45% — see Figure 4.

Подпись: CD
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Fig. 3. Evolution of the total energy cost with solar fraction for electric (left) and gas (right) backup solution.

Fig. 4. Collector area needed according to annual solar fraction for the different building types.

5 Conclusions

By using an integrated solar system for combined heating and cooling, it is possible to save in total costs and CO2 emissions. This is particularly true for South-European locations, and economical results are more interesting when natural gas is used as system backup energy. Minimum costs depend on building type and location, but usually happen with annual solar fractions between 20 and 60%.

The single-family house and the hotel are the cases where the solar integrated system has a higher economic feasibility. Considering the present energy costs, Rome is the only city where it is possible to achieve a break-even situation. Compared to flat-plate collectors, vacuum tube collectors allow a reduction in collector area between 15 and 50%, although, due to their initial cost, flat-plate collectors lead to a higher economic viability. The best system in combination with solar is the electrical chiller; nevertheless, it is a gas boiler backup that leads to the best solar system efficiency.

An annual solar fraction of 60% can only represent a reduction between 35 and 45% of exploitation costs, because of significant maintenance and water consumption costs.

Although the exploitation cost of a solar assisted air-conditioning system is considerably lower when compared to a conventional system, the total cost (including investment, operation and maintenance costs) is actually high, even when extending the operation period as much as possible throughout the year. For solar cooling (and heating) to become more competitive, it is necessary that initial costs for absorption chillers and solar collectors are further reduced, considering the present costs of energy sources (gas, electricity).

References

[1] ESTIF, Solar Assisted Cooling — State of the Art. Report of project “Key Issues for Renewable Heat in Europe (K4RES-H)”, 2006.

[2] Solar Energy Laboratory, TRNSYS 16: A Transient System Simulation Program — Program Manual. University of Wisconsin-Madison, USA, 2004.

[4] IEA-SHC, TASK 38 — IEA Solar Heating and Cooling, 2007

[5] European Comission, Solar Air Conditioning in Europe, SACE, NNE5/2001/25, Evaluation Report, 2003

[6] European Comission, Solar Air Conditioning in Europe, SACE, NNE5/2001/25, Guidelines, 2003

[7] Henning H. S., Wiemken E., Solar Air Conditioning in Europe, SACE, NNE5/2001/25, Economic Study Report, 2003

[8] Henning H. S., Solar-assisted air-conditioning in buildings — a handbook for planners, second edition, Springer, 2007

Experimentation for pool 3: absorption process

Three absorption facilities are planned during this program to study small (less than 10kW) and large (more than 30kW) installations:

• Small installations will be tested on the LATEP and the SOLERA facilities.

• Large installation will be studied thanks to the RAFSOL installation worked out by the LPBS.

Подпись: Figure 2: The LATEP installation

The LATEP project installation is a 4.5 kW Rotartica machine with a drycooler, a cold and a hot storage as shown on the following layout.

The SOLERA project is a European project started in November 2007, for 4 years. The sub-group 2 “small scale solar air conditioning in southern France”, is composed of French companies. The demonstration installation will be built in the new building of CEA-INES at Chambery, France. The chiller will be the one produced by Rotartica, with a nominal cooling capacity of 4.5 kW. The solar collector will be flat plate collectors and the terminal units will be high energy efficiency console units. The re-cooling system will be a geothermal heat exchanger able to dissipate 15 kW for a inlet temperature of 25 °C. The flat plate collectors will be located on the roof of the Puma III building (Figure ) and the system will be use to cool a modular area of offices located at the first floor

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The main interest of the RAFSOL project is to propose a specific study of the absorption solar cooling processes in tropical climate. Moreover, this facility, designed to cool four classrooms of a university building operates without any backup system. The main components of the installation are proposed on the following figure:

1: absorption chiller (cooling capacity = 30 kW), 2: 36 solar collectors (total area = 90m2),

3: cooling tower (cooling capacity = 80 kW),

4: hot (V=1500L) and cold (V=1000L) storage, 5: cooling coils providing cooling in classrooms.

The absorption chiller

To model the absorption chiller we are going to use the operation curves of the chiller given by the manufacturer. Thanks to these curves and depending on the inlet temperature of the generator, we will be able to determine the frigorific power of the chiller, a generator power and cooling power.

3.3. The cooling tower

To model the cooling tower, we considered a heat exchanger. The first fluid (1: the water), circulates through the exchanger, while the second fluid (2: the air), blows around and against the first flow. We chose to use the efficiency method and the formula is [8]:

with, ДТ^-Tjl-Tzl,

(m ■* Cp)mln = nrin(™a * C* ій£ * Cj, 1 — е[-в»а-ві

1 — C * e1

D

(ill* Cp)

<*,сии

Подпись: T = Подпись: N = Подпись: rnLi

= T. ‘ ^ …… ‘ " ‘ ^ ‘ T;: — T;: = "T.. ‘ ‘ T.: — T.; [8]

3.4. The Building

Here we present a simplified model of our classrooms. To do this we will consider a single area building and its main features are:

Length = 13m

Glass: Simple glazing (S = 25m2)

Students = 105

Width = 13m

Wall: reinforced concrete (thickness = 16 cm)

Lighting: 22 neon lights of 58W

Height = 3m

Infiltration air flight= 1 vol/h

Occupation : de 8h00 a 18h00

Подпись: First law of thermodynamic: [9]

[10]

We have now all equations needed to determine temperatures and powers throughout the day.

4. Results of the simulation

Подпись: Temperahne State 6:00 AM 9:00 AM 12:00 PM 3:00 PM 6:00 PM Tune (hour) Подпись: 6:00 AM 9:00 AM 12:00 PM 3:00 PM 6:00 PM Tune (hour) Solar Caught Generator Evaporator Cooling Tower Thermal COP

Our model needs to operate climatic data (sunshine and outside temperature). We chose those from the 2nd of June 2008. These data were derived from measurements made on our experimental platform by a weather station. Next, we must specify the time of occupation of the day: 10 AM to 12 PM and 1 PM to 5 PM. Then we have to set the initials temperatures inside the tow tanks (T_hot_tank = 66°C and T_cold_tank=20°C) as our installation on this day.

Подпись:Tout_captor

Tout_arai

T_cold_tauk —

Toutside —

occupation

Fig. 5 et 6: Evolution des temperatures et des puissances des principaux composants

We can see the evolutions of temperatures and powers on fig. 5 and 6. We notice that at 8 AM, the solar collectors start to produce hot water and the hot tank warms gradually. Students arrive at 10 AM (occupation = 1) and then leave at 12 PM. They come back at 1 PM and leave at 5 PM. The air temperature in the building increases with the arrival of students to reach its peak (30°C) 30 minutes later.

We see that the absorption chiller starts at 11 AM, and that the powers have been slowly declining throughout its operation (5 hours).

We can notice that the outlet temperature of the solar collectors does not exceed 90°C and the temperature of the hot tank reaches its peak at 11 AM and down gradually to 65°C at 4:15 PM, when the absorption chiller stops. Meanwhile the cold tank has reached its minimum value (about 8°C) at around 1 PM and the minimum outlet temperature of the evaporator is 6°C.

Investigation of the effect of dust

Подпись: Fig. 14 Solar Fraction of the system with different fractions of cleanliness Подпись: Fig. 15 Overall efficiency of the system with different fractions of cleanliness

Due to the conditions on the site some degrading of the collector reflectivity could occur due to the effect of dust exhausted by a close chimney. This effect is presented in figure 14 and figure 15 for a system with varying number of collectors, a 2 m3 storage vessel and mass flow in the HX of 3000 kg/hr.

In systems with a small collector field the effect of dust has bigger influence on the solar fraction as well as on the overall efficiency. Because a bigger amount of heat is dumped due to overheating in systems with bigger collector fields only part of the radiation lost by the effect of dust is “missing” in the system, these losses reduce the total system efficiency with about 1% .While for systems with smaller solar fields, which dump less energy are therefore more affected by the effect of dust, and these losses reduce the total system efficiency with about 5% . With respect to this effect the collector field should be designed in an appropriate way to gain the expected heat and frequent cleaning measurements should be done for the collectors to ensure high fraction of cleanliness.

6 Conclusion

The preliminary development of a solar assisted refrigeration system has been object of a simulation study with the aim to characterise the main components. The innovative concept is meant for process cooling of received fresh milk in a dairy factory in the city of Marrakech, Morocco. A portion of the process has been selected and the daily cooling load profile has been characterized: cooling power and the duration of the cooling process. The system components’ selection and system configuration was carried out to ensure the highest solar energy use considering the given cooling load profile. Moreover a parametric study on the main system components has been worked out through simulations.

Regarding the collectors’ field area, its increase resulted in a growth of both, the solar fraction and the dumped energy. The latter is due to defocusing of the collectors when the input energy exceeds the demand; which can be significant for large collectors’ areas. As a result for the given application 18 modules of parabolic through collectors were selected, in order to guarantee enough thermal energy with an acceptable amount of dumped energy.

The optimum size of the PCM storage, for the selected collector area, is 2 m3. Considering the flow rate, a middle value of 3000 kg/h was selected as the flow rate for the cold side of the system, aiming at a compromise between increase in solar fraction and electricity costs. Three percent of the solar fraction depends on the cleanliness of the collector field, and this was taken into consideration during the sizing process as well as for the planned frequent maintenance of the system. As result of this study, the solar refrigeration system, which will be installed at the diary in Marrakech (October 2008) will consist of: 18 modules of parabolic through collectors, 2m3 PCM storage, with cold water flow rate of 3000 kg/h.

References

[1] H. Henning, A. Haberle, A. Lodi, M. Motta; Solar cooling and refrigeration with high temperature lifts — thermodynamic background and technical solution; — Proc. of 61st National ATI Congress, ATI-IIR International Session ‘Solar Heating and Cooling’,14th September 2006, Perugia, Italy

[2] M. Motta, M. Aprile, H. Henning — High efficient solar assisted sorption system for air-conditioning of buildings; — Proc. of Eight international symposium gleisdorf solar, 6th — 8th September 2006, Gleisdorf, Austria.

[3] Duffie, J. A., and W. A. Beckman; Solar Engineering of Thermal Processes; J. Wiley and Sons, 1991

[4] Tess Documentation Library, chapter 10.Solar Library Technical Reference, type 536: LINEAR PARABOLIC CONCENTRATING SOLAR COLLECTOR. pp536.1-5

[5] Solar Energy Laboratory, University of Wisconsin-Madison, USA et. al.; TRNSYS 16 — a Transient System Simulation program; http://sel. me. wisc. edu/trnsys/user-resources/index. html (08.08.2008)

[6] Eams, I. W. and Adref, K. T. (2002) Freezing and melting of water in spherical enclosures of the type used in thermal (ice) storage systems. Applied Thermal Engineering. 22, pp. 733-745.

[7] M. Aprile, Simplified models implementation, MEDiterranean food and agro Industry applications of Solar COoling technologies (MEDISCO). STREP — FP6 — EU contract Project no. 032559.

[8] Aprile, M. (2006) — Simulation study of an innovative solar absorption cooling system — Master Thesis “Dalarna University” 2006.

Use of the model for online simulations and optimization

The simulation model of the installation will be used in future to perform online simulations in order to detect some potential errors/malfunctions of the system and alert the users. This process has to be automated in order to simulate at the end of each day the installation and compare the results with the measurements.

Additionally, the model will be used to study the possibility of varying the cycle of the adsorption chillers when the cooling demand of the building is not so high, in order to increase the COP and reduce the input heat in the chillers. Furthermore, the control strategy of turning on the 3 chillers according to the cooling demand will also be investigated.

3. Conclusions

This paper shows the results of dynamic simulations of a large solar adsorption cooling plant installed in the company FESTO AG nearby Stuttgart in Germany. Even if some works are still required to enhance the quality of the model, the results show an acceptable agreement between simulated values and measured ones. This simulation model will be used in future to perform online simulations for an automated alarm and fault detection system. The adsorption chiller model will be also used to study the effect of the variation of the cycle time of the chiller and to optimize the control strategy of turning on the 3 adsorption chillers (when should they be turned on).

4. Acknowledgments

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This work would not have been possible without the collaboration of the company FESTO AG and the University of Applied Sciences Offenburg who have provided all the measurement data necessary for this study. This work has been supported by the 6th European Union Research Program’s Marie-Curie early stage research training network in “Advanced solar heating and cooling for buildings” — SOLNET. http://cms. uni-kassel. de/index. php? id=2142

References:

[1] Huber, K. “Detailmonitoring einer solarthermischen Anlage zur Unterstutzung des Kalteversorgung eines Buro — und Verwaltungsgebaudes”, 18. Symposium Thermische Solarenergie Staffelstein, 2008.

[2] Schumacher, J. “Digitale Simulation regenerativer elektrischer Energieversorgungssysteme”, Dissertation Universitat Oldenburg, 1991 www. insel. eu

[3] Dalibard, A., Pietruschka, D., Eicker. “Performance analysis and optimisation through system simulations of renewable driven adsorption chillers“. 2nd SAC, Tarragona, Spain, 2007.

[4] University of Applied Sciences Offenburg. Internal meeting project (July 2008)

[5] Saha, B. B. et al.: “Computer simulation of a silica gel-water adsorption refrigeration cycle—the influence of operating conditions of cooling output and COP”ASHRAE Transactions 13 (6) (1995) 348­355.

[6] K. C. Ng, H. T. Chua, C. Y. Chung, C. H. Loke, T. Kashiwagi, A. Akisawa, B. B. Saha. “Experimental investigation of the silica gel-water adsorption isotherm characteristics”, Applied Thermal Engineering, Volume 21, Issue 16, 2001, 1631-1642.

Solar desiccant cooling in other Australian climates

Simulations discussed above were repeated for Melbourne and Darwin climates. Mean monthly climate profiles for each of the three cities [8] are illustrated in Figure 6. Melbourne is a warm temperate climate, similar to Sydney, but with a colder winter and colder nights. Darwin is a tropical climate with wet and dry season differences dramatically influencing cloud cover.

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The impact of air flowrate on the frequency of high zone temperature events for the Melbourne and Darwin climates is illustrated in Figures 7 and 8 respectively

Figure 7 suggests that zone temperatures can be maintained below 26°C for all but 20 hours per year without the use of any backup fossil fuel heat source. It appears that the stand-alone solar desiccant cooling process can potentially provide an acceptable comfort airconditioning solution in the Melbourne climate.

Подпись: Fig. 9: Comparison of Melbourne summer thermal conditions (a) outside the building and (b) inside the building with two stage indirect/direct evaporative cooling (not solar desiccant cooling) and 6.18 air-changes/hr and (c) inside the occupied space with 0.167m2 collector area per m2 occupied space and 6.18 air-changes/hr.

The viability of the two processes (solar cooled and two stage evaporative cooled) in Melbourne is further illustrated in Figure 9, by plotting the simulated temperature and humidity in the occupied space at each half hour interval where temperature exceeds 22°C.

In Melbourne, the less complex evaporative cooling process achieves much of the same benefit as the solar desiccant cooling process. However, the solar desiccant step reduces the number of hours where temperature is above traditional airconditioning set-point temperatures (~23°C) and reduces humidity levels in the occupied space by around 5.5%

In contrast to the Melbourne climate, Figure 8 suggests that high temperature events can not be adequately prevented in Darwin by the stand-alone solar desiccant cooling process. Evaporative cooling appears to provide only limited assistance to the solar desiccant cooling process in the tropical Darwin climate. This is understandable because outdoor air starts off significantly warmer and more humid. Consequently, evaporative cooling is less able to achieve low temperatures consistent with desirable indoor air conditions.

Figure 10 illustrates the impact of target zone temperature and collector area on the number of hours per year that the zone temperature exceeds the target in Darwin.

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Target Zone Temperature (deg C)

Fig. 10: Influence of (i) target zone temperature and (ii) solar collector area, on the number of hours that the target temperature is exceeded in Darwin, with fixed desiccant cooled air flow of 3.71 airchanges per hour.

It is apparent that the addition of extra collector area will not easily produce acceptable comfort conditions in Darwin using the solar desiccant cooling process as described in Figure 1.

Alternative cycles/ component arrangements may need to be considered for tropical climates.

5. Conclusions

The hour by hour performance of a standalone, once-through desiccant cooling system for airconditioning a commercial office space, was examined using the TRNSYS computer simulation software. The study particularly focuses on the potential for designing and operating a desiccant cooling system without any thermal backup provided to mitigate for intermittent solar availability.

The study investigated the impact of manipulating (i) indirect evaporative cooler effectiveness, (ii) desiccant cooled air flow to the office space, and (iii) solar collector area, on the comfort conditions experienced in the office space. Differences between the performance of the solar desiccant cooling system in (i) the warm temperate climates of Melbourne and Sydney and (ii) the tropical climate of Darwin were also investigated.

When low humidity air is available, the effectiveness of the indirect evaporative cooler heat exchanger was shown to have a marked impact on the achievable temperature drop. Increasing collector area and air flowrate to the occupied space, were both shown to reduce the frequency of high temperature events in the occupied space. In the warm temperate climate of Melbourne (and to a lesser extent Sydney), high ventilation rates enabled comfort conditions to be maintained at or near acceptable levels in the occupied space, with-out the use of a backup thermal source. During extreme weather conditions, evaporative cooling appears to be the dominant mechanism for cooling the occupied space.

This synergy between evaporative cooling and solar desiccant cooling, observed in the warm

temperate climates, was not evident in the tropical Darwin climate. Further research is required to

model alternative cycles with more promise in tropical climates.

Nomenclature

П Solar collector efficiency

T Collector fluid inlet temperature (°С)

Tamb Ambient temperature (°С)

G Solar insolation (W/m2)

References

[1] White S. D., Kohlenbach P., and Bongs C., “Desiccant cooling system modelling and optimisation”, International Sorption Heat Pump Conference, Seoul, Korea, September 2008, in press

[2] Lam J. C., Hui S. C.M., and Chan A. L.S., “Regression analysis of high rise fully air-conditioned office buildings”, Energy and Buildings, 26, 1997, 189-197

[3] Beccali, M., Butera, F., Guanella, R., and Adhikari, R., “Simplified models for the performance evaluation of desiccant wheel dehumidification”, International Journal of Energy Research, 27, 2003, 17-19.

[4] TRNSYS v16.x TESS Libraries Version 2.0, Thermal Energy Systems Specialists, LLC, Madison, WI

[5] LTS-Collector Catalogue 2002, Institute fur Solartechnik SPF Rapperswil. BFE Bundersamt fur Energie, Bern, Switzerland

[6] H.-M. Henning, Solar Airconditioning and Refrigeration, Task 38 of the IEA Solar Heating and Cooling Programme, presentation to Sustainability Victoria, May, (2007)

[7] 2008 ASHrAe Handbook, HVAC Systems and Equipment, pgs 40.2-3

[8] Australian Bureau of Meteorology, http://www. bom. gov. au/climate/averages/

Control signal regulation

On-off Control

This kind of control is based on the comparison of two different signals: one is the variable we want to control Ts and the other is the setpoint we want to obtain Ti. It is evaluated the difference between them and in function of the value of this error is calculated the output. If it is in stop state, the output is not set to 1 until is reached a value of the difference higher than ATa. If it’s running and the error begins to decrease, it does not stop till this error reaches a value ATp. With this kind of control, is possible to reduce the number of oscillations of the system.

Control by means of PI (Proportional-Integral).

Once they have been established the conditions to start up the installation, using any of the previous strategies, the setpoint value can be sent to the pump, in which case there can be an on-off control system or on the other hand there can be a controller which receives the start signal and that varies the pump flow in order to maintain the temperature setpoints.

Although there are a huge amount of theories about the adjust of the parameters of the controller, note that the fine-tuning has been made in this case by means of trial and error, and so the values introduced on the system are the best of all obtained.

The PI controller has been used for the variable flow regulation of the pumps as well as for the three way valve if necessary (depends on the case).