Category EuroSun2008-7

A Sensitivity Analysis Of A Desiccant Wheel

P. Bourdoukan1[16], E. Wurtz2, P. Joubert1 and M. Sperandio1

1 LEPTIAB, Universite de La Rochelle La Rochelle, Avenue Marillac, 17000 La Rochelle, France
2 Universite de Savoie, Campus Scientifique, 73376 Le Bourget du Lac, France
* Corresponding Author, paul. bourdoukan@univ-lr. fr

Abstract

Desiccant cooling powered by solar energy and using water as a refrigerant has a low environmental impact and appears as an important technique to reduce energy consumption in buildings. The cooling potential of the system is based on the performance of the desiccant wheel that removes humidity from outside air to increase the potential of the humidifier. In this paper a sensitivity analysis of the desiccant wheel dehumidification is performed using the design of experiments...

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Driving, heat rejection and chilling temperatures

The control system start the chiller only if a driving temperature above 72°C is available, and turns the machine off when it falls below 68°C (Fig. 5). The measured heat rejection temperatures were very favourable for the chiller: in 95% of the hours an inlet temperature to the machine between 18°C and 20°C was registered. These numbers show the effectiveness of the installed boreholes for the heat rejection in the present system. Chilled water temperatures (inlet temperature to the cooling coil) between 7°C and 14°C were measured. As a conclusion, the temperature lift defined as temperature difference between chilled water outlet and heat rejection temperature inlet to the machine was about 7.9K (Fig. 6).

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Desiccant wheel model. Model description

The heat and mass transfer model for the desiccant wheel used below is based on the analogy method with heat transfer that occurs in the sensible heat regenerator. It was first introduced by Banks [3] and Maclaine cross [4] then Jurinak [5] and Stabat [6] improved the model. The following assumptions are considered:

• The state properties of the air streams are spatially uniform at the desiccant wheel inlet

• The interstices of the porous medium are straight and parallel

• No leakage or carry-over of streams

• The interstitial air velocity and pressure are constant

• Heat and mass transfer between air and porous desiccant matrix is considered using lumped transfer coefficients

• Diffusion and dispersion in the fluid flow direction are neglected

• No radial variation of the flui...

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Solar fraction and electricity consumption

From the 282 operation hours in the cooling mode about 35.8% (101 hours) no solar operation was possible. On 148 hours (52%) a complete solar operation covering the whole hour was possible, on the remaining 33 hours the solar system could only provide a part of the driving heat. From an energetic point of view, 59.8% of the driving energy over the whole cooling operation period came from the solar system.

The electricity consumption over the whole cooling period was 10% of the cooling energy. This electricity consumption includes the electricity needs for the chiller itself, the heat rejection via the boreholes, the driving circuit and the cold distribution to the cooling coil in the air handling unit. But it does not include the pump of the solar loop.

image632

temperature lift: T_MT_in – T_NT...

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Experimental validation

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The experimental installation presented in section 2 is used to validate the above presented model. Different inlet conditions were considered with temperature varying form 25°C to 38°C and humidity ratio from 10 to 15 g/kg and for different regeneration temperature (55°C, 60°C 65°C, 70°C and 80°C).

The figure 2 [9] plots the temperature error versus the humidity ratio error for the most significant points. It shows the maximum predicted error 0.4 g/kg and root mean squared error (RMSE) is 0.22 g/kg. For the temperature and the maximum committed error in the outlet temperature is 1 °C for RMSE of 0.65 °C. It must be noted that the uncertainty in the measurement of the mean values is 0.7°C for the temperature and 0.3 g/kg...

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Description of the novel generator design

In this heat and mass transfer prototype, the desiccant solution and the air stream is brought into contact with each other in a cross flow configuration. The regenerator consists of a stack of polycarbonate (PC) twin wall plates. Each plate has an internal heating water circuit. The plates are covered with a wick fibre to facilitate intimate contact between the liquid desiccant and air. This is done to increase the exposure time over the plates and thereby enhance the desired mass transfer and heat exchange. The general design is shown in the left part of Fig. 2.

The advancement of the distribution system consists of:

• a separation between the liquid desiccant distributor and the contact area between air and desiccant...

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Operation day in the cooling mode

In Erro! A origem da referenda nao foi encontrada. an example of the operation during a normal summer day is shown. The diagram corresponds to June 13th, 2007. Cooling operation starts at 7:30 and ends at 14:30. In the morning till 11:30 the chiller is operated with heat from the heating net as the temperature in the solar storage was not sufficient to operate the machine. The shaded area between 11:30 and 14:30 shows the time when the system was operated with heat from the solar system.

Large temperature variations can be observed in the driving circuit...

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Sensitivity results and analysis

4.1 Results

Experiments have been conducted on the desiccant wheel with the parameters (e. g. outside temperature, outside humidity ratio, regeneration temperature and regeneration humidity ratio) varying in the range defined in the section 2. The complete DOE of 4 parameters operating between 2 levels needs 24=16 experiments. When a combination of the studied parameters was

difficult to achieve experimentally (3 experiments) the results of the model was used to complete the DOE. Table 1 below shows the results

Table 1. Dehumidification rate of the desiccant wheel for different operating conditions

Ti

w1

T8

W8

w1-w2

T1

w1

T8

w8

w1-w2

[°C]

[g/kg]

[°C]

[g/kg]

[g/kg]

[°C]

[g/kg]

[°C]

[g/kg]

[g/kg]

25

11

55

10

4.8

35

11

55

1...

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