SOLAR DESSICANT COOLING EXAMPLE OF APPLICATION TO A LOW ENERGY BUILDING

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1 SOLAR DESSICANT COOLING EXAMPLE OF APPLICATION TO A LOW ENERGY BUILDING Thibaut Vitte, Monika Woloszyn, Jean Brau CETHIL, CNRS UMR 5008, UCBL, INSA, Lyon, France ABSTRACT This article presents the study of a solar air conditioning system based on "desiccant evaporative cooling (DEC) technology. Energy performance of the system used in an office building situated in Macon (France) is studied using numerical simulations in TRNSYS environment. The parameters of the low energy office building (envelope, glazing, solar protections, ventilation...) are optimized in order to diminish the needs for heating and cooling. After implementing the model of the desiccant wheel, numerical simulations are conducted in order to asses energy performance of solar DEC system. The results show substantial energy savings for summer period, with satisfactory thermal comfort of the occupants. Supplementary studies were also conducted to analyze the performance of different control strategies. Key Words : Desiccant cooling, Solar Energy, Moisture, Simulation, TRNSYS. 1.INTRODUCTION Growing demand for building cooling systems is one of the important issues of energy policy in the building domain. In order to limit the energy demand for cooling applications, it is necessary to develop technologies alternative to the classical compression chillers. System based on solar energy seem very promising, there are well adapted for cooling applications since high solar radiation is in general in phase with high cooling demand. This article exposes the results obtained for the study of a solar refreshing system by desiccation using solid adsorption, or "desiccant cooling" (DEC). In this technology water and its potential of phase change is used to cool the inlet air. 2.PRESENTATION OF THE SYSTEM Desiccant wheel is the heart of the DEC HVAC system (see fig. 1). The wheel is composed of a circular matrix of glass or aluminum fiber on which is deposited the desiccant material. Figure 1 : Principle of a desiccant wheel. Lower (process) stream: inlet air dried air. Upper (regeneration) stream: regeneration air moist air. The principle of the system is simple (see fig. 2 and 3): To maximize the effect of the water vaporization latent heat, the air is first dried out in the desiccant wheel. It is then cooled in an exchanger (in general a wheel exchanger), and then adiabatically cooled down in a humidifier. The desiccant wheel need to be regenerated using a hot air stream. In general the air extracted from the building is first cooled by an adiabatic humidifier, then used to cool down the inlet air in a wheel exchanger, heated to high temperatures - 1 -

2 (50 to 70 C, depending on the material), and then used to regenerate the desiccant material before being rejected to the outside. When the heating is done using air or water coming from a solar collector the term of "solar cooling" is used. In this case the use of primary energy is very limited. Rejected air Fresh air Solar air collectors Desiccant wheel Air-water heat exchanger Figure 2 : Diagram of an installation of desiccant cooling Figure 3 : Cycle's psychometric representation 3.MODELING THE DEC SYSTEM 3.1.Desiccant wheel Wheel exchanger humidifiers Return air Inlet air Internal loads The modeling of the desiccant wheel is a very important and not a straightforward task, since both air temperature and moisture content are modified by adsorption. Adsorption is an accumulation process of a substance on the interface of two phases (solid-gas, gas - liquid ) that finds its origin in the forces of molecular weak interactions, type VAN DER WALLS. The model used in this study establishes an analogy with a sensitive heat wheel exchanger (NTU «number of transfer units» theory). It is drawn from the thesis of P. Stabat (2003), and allows, in contrast to other models (Maclaine-Cross, 1974, Kays, 1984) to define the model using simply the manufacturers' data, without knowing the physical characteristics of the desiccating material. The algorithm was translated into FORTRAN in order to be used in the building energy simulation software TRNSYS 15. The principle of this algorithm is to use the equations defined by the «analogy» model twice. First time, they are used as inverse problem to calculate the two determining parameters of the model knowing the performance from manufacturers' data : UA the «conductance», that is the product of the heat exchange coefficient times the exchange surface, and MN, the product of the desiccant mass by the rotation speed of the wheel. Second time, to calculate the outlet conditions using the two parameters calculated before and air entrance characteristics. The algorithm was successfully tested on several series of data (Vitte and Novacq, 2004). The figure 4 shows the results given by DRI (Desiccant Rotors International compared to the values found by numerical simulation for two points of parameter initialization. It can be seen that the point used to calculate model parameters is very important, since the results of simulation are better for the values close to the initialization point. It is therefore recommended to use values close to operating conditions to initialize the model. 3.2.Heat exchanger As the flows are constant during the operation, the wheel exchanger was modeled using a constant effectiveness. According to the literature the value of 0,80 was used. The liquid-to-air exchanger used to heat the regeneration air is modeled using a standard type coming from the TRNSYS library. It is based on the NTU method. 3.3.Humidifiers The humidifiers are modeled assuming that the air flow enthalpy remains constant : while the air moisture content rises up, the lowering of its dry bulb temperature is obtained. The effectiveness of the humidifier in the process air stream is fixed to 70% and the effectiveness of the humidifier in the regeneration air stream to 90%. For water consumption, a loss of 10% for the dilution of the recuperation tank is assumed. 3.4.Solar system The modeling of the solar collectors is carried out using quadratic efficiency model from TRNSYS library. The parameters of the model : incidence angle modifiers, efficiency slope and efficiency curvature are given by the tests of thermal collectors. The values of these coefficients correspond to a flat plate collector of rather good performances - 2 -

3 14 Comparaison de deux points d'initialisation C C 50 hum. spec. sortie [g.kg] temp sortie [ C ] 4 2 hsent.=20g/kg hssort.=8.6g/kg tsortie = C hsent.=10g/kg hssort.=3.2g/kg tsortie = C hum. spec. entrée [g.kg] 0 Figure 4: Algorithm validation 4.APPLICATION 4.1. Description of the studied case The system is applied to a one floor high office building (of about 140 m²). The design parameters of the building were optimized in a previous study (Vitte et Novacq, 2004) in order to diminish the needs for heating and cooling. External walls are made out of concrete with external insulation, windows are equipped with high performance, low emissivity double glazing and with exterior solar shading devices. The building is occupied on a standard office schedule (9:00-18:00). Thermal loads represent 12 occupants and 10 computers. The DEC HVAC system is sized with 10 m² of solar collectors, and a ventilation rate of 4 vol/h in normal operating conditions and of 0.5 vol/h when the temperature of regeneration is too weak (inferior to 50 C). After a preliminary study the south orientation of the solar collectors was retained. The parameters of the DEC system are extracted from an installation suggestion of ROBATHERM ( Simulations are done for the whole year for the building located in Macon (France). The results concerning summer performance are presented in the following sections. TRNSYS v15 was used to perform the numerical simu lations. 4.2.Control Different control strategies were tested, they are listed in the table 1. CONTROLLED PARAMETERS OUTSIDE AIR ON if Text > X C & Hrel< X% Or Text > X C & E > X W/m² INDOOR AIR ON if Tint> X C & Hrel < X% REGENERATION ON if Tregen > X C ON if T collector > X C INLET AIR ON if Tsoufl < X C & Hrel < X C Table 1: Control strategies GOOD AND BAD POINTS Good :The wheel performances are a direct function of the entry conditions Bad : The user is not taken into account, irradiance sensor is expensive Good : Natural way of controlling indoor conditions. Bad : The wheel is not working in optimum conditions if there is no auxiliary regeneration. Good : The wheel works in very good conditions Bad : The user is not taken into account. Good : Useful to control thermal comfort. Bad : Internal loads are not taken into account

4 Some convergence problems were met when the control strategy included more than one parameter. The most effective strategy appears to be the one using the indoor temperature as target. Nevertheless some high values of relative humidity appear (>70%). It could be avoided with an additional control of moisture that would influence the operation of the humidifier of the process air stream. 4.3.Results The results for two summer weeks are presented in figure 5. During operation of the DEC system, the indoor conditions correspond to comfort conditions: temperature does not exceeds 27 C and indoor relative humidity is situated between 40% and 70%. Figure 6 shows the results for a typical day. From 8 am to 5 pm the indoor temperature is very correct (around 26 degrees), while if no cooling system is used it exceeds 30 C. Despite strong internal loads, the indoor temperature does not exceeds the outdoor temperature. Treated air is 4 to 10 C cooler that the outdoor temperature. An average difference of 6 C between outdoor and treated air is obtained and during more than 12 hours, it is less hot in the building than outside. The problem appears only after 5 pm, when the solar radiation is not sufficient to regenerate the wheel and the cooling stops. Figure 5. Main results obtained from June 27 to July 14 t 5.PERSPECTIVES AND FUTURE WORKS 5.1.Evaluation of the energy consumption The auxiliary consumptions of the installation were drawn from an installation suggestion of ROBATHERM, although certain data are not known. Hypotheses then were taken. The electric power of the system is estimated to Watt. The water consumption is estimated with a loss of 10% correspondent to the dilution in the recuperation tank during cleaning procedure

5 Table 2 :Consumptions results Number of hours of functioning Total consumption on this period 1054 hours 2318 kwh Water consumption 11.8 m 3 Specific consumption 84 L /m² & 16.6 kwh/m² Estimated energy consumption of 16.6 kwh/m² (see table 2) is in agreement with values of about 15 kwh/m² in summer given in the literature (see for example Ginestet et al. 2002). These results are to be compared with the consumption of a traditional airconditioning, from 30 to 70 kwh/m² for office buildings. The energy savings here are therefore more than 50%. 5.2.Contribution of the system for the heating period The installation can also be used to reheat the ventilated air in cold period thanks to the rotary heat exchanger. It would be also possible to use the desiccant wheel as a second sensible and latent heat exchanger in order to readjust the indoor humidity, that is often too low in the winter, and therefore contribute to better comfort in cold and dry conditions. Such simulations require the use of a different model for desiccant wheel, based on non regenerated enthalpic exchanger, as proposed by Spahier and Worek (2004) and Zhang and Niu (2002) The solar collectors could also be used to heat the rooms or to produce solar hot water and thus to optimize the realized investments. 5.3.Control system Some additional simulations to test different control strategies will also be conducted. Especially different combinations between passive and active cooling will be studied. The system can work simply in ventilation mode, then in indirect, direct evaporation and at last in DEC mode, and this while the air flow can vary from 2 to 6 vol/h, in order to minimize the auxiliaries' electric consumptions. 5.4.Impact of moisture buffering materials In this first study only the temperatures were taken into account and moisture buffering capacity of indoor materials was neglected. The impact of moisture buffering materials will be studied in the future, and it is anticipated that a correct choice of indoor materials would improve the performance of the system and avoid high indoor humidity peaks when the cooling demand is important. Figure 6 : Results on a typical day - 5 -

6 6.CONCLUSION Solar cooling system is still quite expensive and not as efficient as a classic installations. However its environmental impact is much lower than that of a classical system (less primary energy consumption, no use of refrigerants). This study showed that the DEC system can be effective in terms of energy consumption and of indoor comfort if applied to a building designed in order to minimize its cooling needs. In order to maximize energy savings and to increase thermal comfort of the occupants, the regulation of the system must be studied more in detail. An efficient control strategy, taking into account several refreshment methods, is needed to obtain a good global performance of the DEC system. In order to appreciate correctly the energy savings whole year operation should be taken into account, including both winter and summer performance. Bibliography GINESTET S., STABAT P., et MARCHIO D., Control strategies of open cycle desiccant cooling systems minimising energy consumption, esim, Montreal, Canada, KAYS W. Compact heat exchangers, 3 rd ed, New York McGraw-Hill, 1984 MACLANE-CROSS, I.L. A theory of combined heat and mass transfer in regenerators. PhD thesis, Dept of Mechanical Engineering, Monash University, Australia SPAHIER L.A., et WOREK W.M., Analysis of heat and mass transfer in porous sorbents used in rotary regenerators, Intern. Journ. Of Heat and Mass Transfert (Elsevier), vol.47, pp , (2004). STABAT P., Modélisation de composants de systèmes de climatisation mettant en œuvre l adsorption et l évaporation de l eau, Thèse de doctorat (Ecole des Mines), Paris, France, 252p, (2003). VITTE T., et NOVACQ A., Étude de la faisabilité d'un bâtiment autonome en énergie, Mémoire de Master (Institut National des Sciences Appliquées), Lyon, France, (2004). ZHANG L.Z., et NIU J.L., Performance comparisons of desiccant wheels for air dehumidification and enthalpy recovery, Applied Thermal Engineering (Elsevier), vol. 22, pp , (2002)