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Investigation of a Novel Dew Point Cooling Heat and Mass Exchanger for Air Conditioning of Buildings in Europe

Final Report Summary - DEW-COOL (Investigation of a Novel Dew Point Cooling Heat and Mass Exchanger for Air Conditioning of Buildings in Europe)

This Marie Curie project aimed to develop a novel dew point heat and mass exchanger and a dew point air conditioner. The new exchanger / air conditioner will lead to 15 to 35 % improvement in both wet bulb and dew point effectiveness compared to the conventional cross-flow exchanger. This innovation will minimise the need of fossil fuel related electricity for building air-conditioning, and will have obvious benefits in terms of reduced fossil fuel consumption and CO2 emission to the environment. The project contains the following three tasks:

1. Optimisation of the exchanger design, in terms of material, structure and geometric sizes.
2. Construction and laboratory testing of prototype exchangers, and finalisation of the exchanger design and computerised optimisation tool.
3. Economic, environmental and regional acceptance analyses.

Task 1 has been successfully completed which involved selection of material, optimisation of the exchanger geometry and sizes. In terms of material selection, several types of materials, namely metals, fibres, ceramics, zeolite and carbon, which have potential to be used as heat and mass transfer medium in indirect evaporative cooling systems, was investigated. It is found that the appropriate material would be a thin thermal conductive metal with wicks on its one side surface. Computer modelling for a perforation-less, polygonal counter flow exchanger has been carried out. The modelling results indicated that exchanger intake air velocity should be controlled to a value below 0.5-1.2 m/s; whilst the height of air passage (channel) should be about 6 mm or below and the length of the passage should be 200 time the height; in further the working-to-intake-air ratio should be around 0.4.

Task 2 has been successfully completed. A prototype dew point heat exchanger and an associated air conditioning unit was designed and constructed in lab based on the previous theoretical simulation and material feasibility studies. Testing was carried out to evaluate the performance of dew point exchanger / air conditioning prototype. Testing results were compared with the theoretical results derived from a verified computer model, and the difference between them were analysed and reasons for this analysed. As a result, an improved unit configuration suitable for commercialisation was designed and its performance was predicted using the verified computer model. Tests showed that system can achieve wet-bulb effectiveness of 55 % to 110 % and dew point effectiveness of 40 %-85 %.

Task 3 has been successfully completed involving feasibility, economic and environmental analyses of using dew point evaporative cooling in Europe buildings. It is concluded that the dew point system is suitable for most regions within EU countries, but unsuitable for some regions where the air is too humid to be dealt with using the system as it stands (for instance some regions in Italy such as Venice and Rome). Tap water has a suitable temperature to feed the system for cooling, with a water consumption rate in the range 2 to 2.5 litre/kWh output. The cooling output of the system ranges from 1.6 to 5.2 W per m3/h air flow rate, depending on the region where the system is applied. For a 100 m2 building space with 30 W/m2 cooling load, the required air volume flow rate varies with application location and is in the range from 700 to 1700 m3/h. This building space consumes 50 to 60 litres of water daily. Compared to the conventional mechanical compression cooling system, the dew point system has significant higher potential in saving energy bills. For a 8 kW rating dew point unit, the estimated payback period is around 1.05 - 1.8 years, and the life cycle cost saving is in the range EUR 2 500 to 4 700, depending upon the area the system is being used. Considering 5 million m2 of building space that the system is potentially in use within Europe region, the estimated carbon emission reduction will be 30 000 tons per annum.

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