AIR CONDITIONING DEMONSTRATION SYSTEM BASED ON DESICCANT EVAPORATIVE COOLING TECHNIQUE WITH SOLAR ENERGY INTEGRATION

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1 AIR CONDITIONING DEMONSTRATION SYSTEM BASED ON DESICCANT EVAPORATIVE COOLING TECHNIQUE WITH SOLAR ENERGY INTEGRATION J. Farinha Mendes, DER/INETI Estrada do Paço do Lumiar, 22, Lisboa, Portugal Tel: Fax: Rainer Rudischer, Uwe Franzke ILK Dresden - Bertolt Brecht Allee, 2, D-139 Dresden, Germany Tel: Fax: solar@ilkdresden.de Abstract The increase in comfort needs during the last years in Southern European countries lead to the spread out of conventional air conditioning units and to the consequent growing energy consumption tendency. Fortunately, these countries have also a lot of sunshine hours along the year and as cooling season matches with the better season of solar energy availability, there is a potential interest in solar cooling systems. Among the several systems that could take profit of solar energy, the desiccant evaporative cooling technique combined with sensible cooling was chosen for a Demonstration Unit, connected to a solar field of low concentration CPC collectors, which is mounted in Lisbon, at INETI installations. In this paper we present the design characteristics of the overall system and the results already obtained - system behaviour and energy balance. 1. INTRODUCTION Recent studies (POSAC, 199) show the growing energy consumption tendency in Southern European countries, due to an increase in comfort needs during the last years and resulting in the spread out of conventional air conditioning units. Fortunately, these countries have also a lot of sunshine hours along the year and as cooling season matches with the better season of solar energy availability, there is a potential interest in cooling systems that could take profit of solar energy. This combination will allow important energy savings and low environmental impact: i) decreasing emissions of CO 2, NO x, SO x etc., responsible for pollution and greenhouse effect, and ii) reducing the usage of refrigerants associated to depletion of ozone layer protecting the earth. If we think also on the possibility to combine the solar cooling systems with domestic solar water heating systems, then the system will be used all along the year, facilitating the investment recovery, and improving the economic competitiveness in comparison with conventional technologies. Several systems can take profit of solar energy and among them the desiccant evaporative cooling technique (DEC) combined with sensible cooling, is a good alternative because it can use thermal energy in the -7ºC range, which is very interesting for non evacuated low concentration CPC collectors or even for a good flat plate collector. A result of CODEC Project (CODEC, 1999) - realised in the framework of JOULE-THERMIE Program, coordinated by ILK Dresden from Germany and involving INETI from Portugal and INTA from Spain is a Solar Assisted Air Conditioning Unit of DEC type, to condition the air of one side of Building G basefloor, on INETI campus in Lisbon. The solar field is constituted by low concentration CPC type collectors, developed in the framework of this project: they differ from commercial CPC collectors on their absorber which is a volumetric absorber, based on a transparent glass tube filled with small recycled glass black balls and avoiding, by this way, the black paint. Fig. 1 Monthly electricity consumption in Portugal. This DEC unit is the first one installed in Portugal; thus the whole works like a Demonstration Unit for the solar

2 field based on those CPC collectors, for the DEC unit and for the integration of both. By this reason the unit has a complete set of measuring devices (temperature and humidity sensors, flowmeters, etc.) installed on it, which permit to see the thermal behaviour of the overall system, to measure its efficiency and to follow its evolution. In point 2 we describe the operation principle of evaporative/desiccant air handling units, in point 3 we describe the system installed at INETI, in point 4 we present some preliminary results and in point the conclusions and possible developments for improving the unit efficiency.. 2. DESICCANT/EVAPORATIVE COOLING PROCESS Adiabatic cooling can be improved with evaporative/desiccant process, promoting the dehumidification of fresh air from outside, which is used for cooling in Summer as well as for heating in Winter (Loff, Pergamon Press). The conditioned air introduced into the rooms has no mixture with air coming out from the rooms, being continuously composed by 1% of new air. By this reason these systems are particularly adapted to condition the air of those places where there is special needs of hygiene like hospitals, clean rooms, etc. Two wheels promote energy transfer: i) a recovery wheel where the new air is pre-cooled by means of the air coming from the rooms which is also previously cooled by humidification and ii) a dehumidifier wheel with desiccant products, located at the handling unit entrance that promotes the dehumidification of the fresh air from outside. Recovery of the wheel desiccant properties is obtained at expenses of exhaust air, which is heated by means of collected solar energy and/or by means of an auxiliary heater. Fig.2 is a schematic representation of the operation principle of the Solar Assisted DEC unit and Fig.3 represents, for the cooling season, the state of the air along the DEC unit, in the h-x diagram for humid air. During this season the air introduced into the rooms removes the internal thermal load (computers, people, machines ) as well as the external thermal load (solar gains through the windows and heat through the walls and covers) of the rooms. Fig.2 Schematic representation of the Solar Assisted DEC unit operation (Henning, 1998). The unit works in the following way: the outside fresh air is adiabaticaly dehumidified (1-2) in a wheel with a desiccant product (silicagel or other), leaving it as hot/dry air. It follows the sensible cooling of this air when it passes through the recovery wheel (2-3), through which also passes the air coming from the rooms in contracurrent. Then the air is cooled again (3- ) by humidification (adiabatic cooling), before being introduced into the rooms. Because of the above referred thermal loads, the air is warmed inside the rooms and the internal water vapour increase its humidity contents (-6). At the same time a similar flowrate of air is being exhausted from the rooms, which is cooled by humidification after entering the machine (6-7), warmed in the recovery wheel (7-8), warmed again in an heat exchange (8-9) fed by an external heat source (solar system, gas or electric heater ), and conducted (9-1) to the exterior trough the drying wheel, to regenerate it desiccant property. Fig.3 Representation of air states along DEC unit, on the humid air T-x diagram, during Summer operation (Henning, 1998).

3 During heating season the fresh air from outside is warmed in contracurrent by the air coming from the rooms. The dehumidifier can be active (1-2) working as enthalpy exchanger (humidification regenerative of the new air) or it can be inactive saving electricity; in this case this wheel is by-passed. The recovery wheel (2-3) is active and the air is warmed again (3-4) in another water-air heat exchanger. This one is also fed by the solar system or by a conventional heat source. In case of necessity the humidifier (4-) can be active. The air is then introduced and cooled (6-1) in the rooms given its thermal losses through the walls, windows, etc. The heat of exhaust air is recovered when it passes through the recovery wheel (7-8) and eventually through the drying wheel (9-1) where it can transfer heat and humidity. During this season the heat exchanger (8-9) is inactive. According to this description only the two wheels constitute specialised equipment, produced by a small number of factories in the world. The major parts of the unit are regular equipment in air conditioning systems, which is a positive factor for dissemination of this technology. Table 1 gives an idea of the cooling and heating capacity, air flowrate and water consumption of these systems in function of the wheels size. 3. DESCRIPTION OF DEMONSTRATION UNIT INSTALLED AT INETI The DEC unit installed at INETI campus in Lisbon was sized to promote air conditioning of 11 office rooms of the basefloor of building G, where Renewable Energies Department is located. This building was designed for a different end user; so it was remodelled to the actual function but because of that some constraints to the size of the DEC unit and to the distribution air network, appeared since the first beginning of this project. Because of the high value of summer thermal load, the basic representation of Fig.2 had to be changed, to a new one including an heat pump that permit low temperature for the air entering the rooms while maintaining a reasonable flowrate of air that can be conducted through a network distribution system, which could be accommodated to the available space in the ceilings. Otherwise the size of the DEC unit, because of the size of the wheels, would be high enough to enable the application to this building. Fig. 4 is a schematic representation of the INETI DEC unit. This unit has the following main characteristics: Unit dimensions: 129 x 131 x 76 mm Air flowrate : m3/h (8 renovations/hour) Power of the ventilators : 2.2 KWe Power of the heat pump : 7. KWe Power of remaining equipment (wheels, pumps): 1 KWe. The DEC unit was sized for a cooling power value of 28.6 KW, which corresponds to cool the outside air in the worst summer conditions, since it enters the machine until it exits the machine towards the rooms. For that, the machine needs 38.2 kw thermal power, necessary to desiccant regeneration of the dehumidifier wheel, which is the sum of heat pump condenser thermal power and of heat exchanger. For this one, heat comes directly from solar system, from storage tank and/or from the gas heater, in accordance with DEC unit needs and solar energy availability. With the two above referred power values, is possible to calculate the COP of.7 for this unit. The solar field is constituted by 24 collectors prototype of the CPC type, with a total transparent area near to 4 m2, which were developed in the Diameter [ m ] Table 1 DEC unit capacity versus size of the wheels (ILK Dresden). Flowrate [ m3 / h ] Cooling power [ KW] Heating power [ KW ] Water consumption [ l / h ]

4 Fig.4 Composition of DEC Unit installed at INETI. Fig. Schematic representation of primary and secondary circuits of solar system. (T T I 2 f amb f amb η = (.6 ±.2) (4.6 ±.) (.37 ±.8) (eq. 1) col ) (T T I col )

5 Fig. 6 Dimensional characteristics of the 1.46xCPC prototype, with volumetric absorber. framework of the CODEC project: CPC profile and glass volumetric absorber, filled with small black recycled glass balls (Fig.6). The 24 collectors, aligned E-W, are divided in twelve parallel rows, each one constituted by two collectors in series. The collected energy is transferred to the storage tank with 2 m3, or directly to the unit, through an external plate heat exchanger (Fig.). The auxiliary energy comes from a gas heater which maintains at constant temperature a small tank of 1 litres. The collector was tested according to ISO 968-Part 1, at INETI collector testing laboratory (LECS/INETI, Lisboa) and the result expressed in (eq.1) was obtained for its efficiency The whole system, DEC unit and solar system, is being followed continuously: sensors of temperature, humidity and pressure as well as flowmeters, pyranometer for the radiation and electric energy powermeters are installed and the corresponding values are being acquired and stored in the computer that woks with a data acquisition system. These values are used for the system control and can be visualised in real time, giving the state of the air along the DEC unit and of the fluid along primary and secondary circuits. 4. PRELIMINARY RESULTS The complete system evaluation will include indicators of system operation in different typical cases and computation of energy and efficiency values for the different subsystems as well as of the whole system like solar fraction or COP of the machine in the different forms it is presented in the literature. Confort and people satisfaction will also be evaluated for all seasons. The start-up of the machine occurred at the end of summer season and office rooms occupancy at the end of the year Both situations delayed to the 2 year the cooling season evaluation, and some adaptations and modifications during the Winter only permit us to present now some preliminary results of system operation. It is the case of solar field efficiency and solar fraction, which are strongly affected by the irregular operation and adjustments on hardware and control software of the machine, after the start-up. For the collector field efficiency, (Qcol/Qinc), during this winter and taking into account whole day operation, an accumulated value of 13% was found which is mainly affected by the very cold winter time of this year combined with the relatively high temperature of returning water from DEC machine (4ºC, average value). Some control problems in the solar circuit, also contributed to this low value, but its solution are pushing now the collector field efficiency to values around 2%, which match better with the measured efficiency curve of this collector, represented in eq. 1. For the next summer season it is expected a better efficiency, in accordance with design values. For the solar fraction, (Qcol/Qdec), a value of 38% can be indicated as the cumulative value of the same last three months. This confortable value is mainly due to the good solar exposition of the rooms, during afternoon, which strongly reduces the heating needs during second half of sunshine days. This can be seen in the graphs of Fig.7, where it can be seen a reduction in power needs of the machine, Qdec, along the day. This is a very important point because it means that in summer particular care is needed with solar exposition of those rooms, to avoid increase in cooling needs. The graphs of Fig. 7, were selected to show different situations representative of machine operation and their connection with relevant temperatures for people confort evaluation. It is the case of the temperature of the new fresh air from outside, Text, the air temperature at DEC unit exit, as it is introduced into the office rooms, Tdec, and the air temperature leaving the office rooms, Troom.

6 Power [kw] : 12: 14: 16: 18: Qhp Tdec Troom Text Temperature [ºC] Power [ kw ] : 1: 12: 14: 16: 18: Qhp Text Tdec Troom Temperature [ ºC ] Power [ kw ] : 1: 12: 14: 16: Qhp Text Tdec Troom Temperature [ ºC ] Fig.7 Some representative operation states of DEC unit during cooling season.

7 Power [ kw ] Temperature [ºC] 6: 9: 12: 1: 18: 21: Qinc Qcol Qdec Troom Text Power [ kw ] Temperature [ºC] 6: 9: 12: 1: 18: 21: Qinc Qcol Qdec Troom Text Fig.8 Some representative operation states of DEC unit during heating season..as said before there is a heat pump assisting this demonstration unit which is the major power consumption component, becoming relevant all situations that apply for its assistance. In the graphs of Fig.6 it is represented its power consumption during the selected days, showing the time operation and its direct relation with temperature of treated air (Tdec). The selected days of Fig. 7, represent different situations of DEC unit operation. The first one is related with a non-automatic operation mode, where it was forced a relatively low temperature inside the office rooms, which obliged a continuous operation for the heat pump. In this case adiabatic cooling was insufficient to cover cooling needs. The other two graphs of Fig. 7 show situations of DEC unit operation in automatic mode. The office rooms confort temperature is calculated according to the outside air temperature and then the desiccant/evaporative cooling is assisted in an intermittent way by the heat pump along the day (second graph) or only during part of the day (third graph), when the capacity of the machine to keep the air temperature, in adiabatic mode, is insufficient. In Fig. 8 we present two graphs representing daily

8 operation of the machine in two selected days of the heating season. The graphs of Fig. 8, show representative power and temperature evolution values along the selected days, like the available solar energy incident on the collector field, Qinc, the collected power by the solar field, Qcol, and the power needed by the machine, Qdec, to keep rooms at confortable temperature level, Troom, when outside temperature is Text. The two graphs represent different climatological outside conditions as is expressed by the incident solar radiation power curve, Qinc, and by the outside external temperature, Text, of the air entering in the machine to be treated. In both cases the machine operation in automatic mode kept the room temperature around the confort value of 22ºC. accomplished. Regarding the solar field, these months of operation lead to failure of some collectors and as they are just prototypes that are not being produced in a commercial base, there is the need to make the replacements by commercial products. So we decided to change half of the initial solar field by the regular commercial CPC collectors that are being produced in Portugal. The two fields will be evaluated separately in continuous, after introducing the necessary flowmeters and software modifications. This system will be also the base of INETI participation in the works of TASK 2 of CH&CP of AIE, after signature by Portugal of the corresponding Implementing Agreement. 4. CONCLUSIONS. NEW DEVELOPMENTS The DEC unit and the solar system were installed and their start-up occurred at the end of the cooling season of 1999 without major problems which permitted to verify the different mode operation of the machine: automatic and manual mode. However, office rooms were only occupied at the end of year, when the machine was put in continuous operation. Only after that, some arrangements and need of small modifications were detected which were implemented during the last months and are still being implemented. This however delayed to the next year the complete seasonal evaluation of the system. The results already obtained are positive and lead us to preview a future correct behaviour of the machine. In the quality of first system installed in Portugal and given its location at INETI, the system is being visited by interested people, to whom the operation principles and results are being explained; by this way the demonstration feature of this unit is being REFERENCES Project POSAC : Contract RENA - CT94-17 (CEC DGXII). Final Report, 199. Project CODEC : Contract JOR3 - CT9-3 (EU DGXII. Final Report, George O.G.Lof Desiccant Systems in Solar Air Conditioning and Refrigeration Pergamon Press, Oxford England. H.M. Henning, T.Erpenbeck, C.Hindenburg, I.S.Santamaria The potential of solar energy use in desiccant cooling cycles Workshop on Solar Cooling, Task 2 of SH&CP of AIE, Palermo, September ILK Dresden, Bertolt-Brecht-Allee 2, D-139 Dresden. LECS/INETI -Laboratório de Ensaio de Colectores Solares, Estrada do Paço do Lumiar, 22, Lisboa.

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