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Desiccant & Solar Desiccant Systems


Gaia Research have been undertaking research to determine whether desiccant cooling technology using gas-solar generated low temperature heat for regeneration, might be reliable and cost effective in the UK in the near future.

Desiccant cooling is a new and potentially clean technology, which can be used to condition the internal environment of buildings and operates without the use of harmful refrigerants.

The research required monitoring of desiccant ventilation plant in the Midlands and in Scotland. The energy performance and control strategies of these two installed systems were analysed over a period of one year and the potential energy savings and reduction in greenhouse gas emissions assessed compared to alternatives. This provided valuable information on the application and performance of desiccant systems.

In addition an assessment was been made of what could be achieved if solar energy was utilised to drive the cycle.

A desiccant cooling model has been developed, and validated using data gathered from the two study buildings. The model predicts the running costs of desiccant systems. It has also been integrated with solar data to provide information on gas and gas/solar hybrid options based on real meteorological data and the actual performance of the two case study systems.

CD-Rom based software has been developed for general application. A limited-run edition disk plus a guidance note summarising the work on desiccant and solar desiccant systems is available

The research was part funded by the DETR through its Partners in Innovation Scheme,



Desiccant & Solar Desiccant Systems

Background
The Principles of Desiccant Cooling
Further Information
Publications


Background
Global environmental concerns, improving standards of ventilation and increasing concerns about indoor air quality have all contributed to a change in design thinking. If we are to seek development strategies compatible with economic and environmental sustainability then we must create systems which are largely self-sustaining and do not bring with them excessive operational complexity. Passive systems and renewable technologies are the ones to which we must aspire if we are to create a truly sustainable built environment.

Professional guidance is increasingly steering clients and consultants away from full air conditioning towards natural ventilation and mixed mode solutions in new and refurbishment projects. Nevertheless, inefficient lighting, increases in computer equipment, and architectural fashion mean that overheating is now the predominant design consideration for new offices in the UK. Older buildings often constrain natural solutions. The situation is likely to worsen if global warming scenarios are accurate.

Desiccant cooling systems are rapidly becoming established technology in most parts of the world, including the UK. This growth has been brought about by the contribution of refrigerants used in conventional cooling systems to the depletion of the ozone layer. Also, the contribution towards global warming of refrigerants and fossil fuels used to generate electricity to power the refrigeration systems is very significant.

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The Principles of Desiccant Cooling

Desiccant cooling is a new and potentially environmentally friendly technology, which can be used to condition the internal environment of buildings without the use of traditional refrigerants.

Unlike conventional air conditioning systems, which rely on electrical energy to drive the cooling cycle, desiccant cooling is an open heat driven cycle, which uses a desiccant wheel and thermal wheel in tandem to achieve both cooling and dehumidification. Because it is a heat driven cycle, there is the potential to utilise environmentally cleaner sources of energy such as gas, hot water, waste heat or any heat source, including solar thermal energy, able to elevate the air temperature to a level adequate for reactivation. Indirect benefits are associated with low humidity levels, including reduced corrosion and microbial growth. This makes it a very environmentally friendly technology choice if properly designed, sized and managed in use.

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Description of Operating Cycle

Cooling and Dehumidification Mode

The cooling/dehumidification process is illustrated by the psychrometric chart shown below. During the summer time warm, moist air at, for example 30oC and 13.7 g/kg moisture content is drawn through the desiccant wheel so that it comes off at, say 57oC and 6.3 g/kg moisture content. The supply air stream then passes through the thermal wheel where it is sensibly cooled to, say 28oC. The air then passes through a small DX or chilled water cooling coil and is sensibly cooled to the supply condition of, say 17oC and 6.3 g/kg moisture content. It should be noted that if humidity control is not required in the space, then an evaporative cooler could replace the cooling coil. In which case, air may be supplied to the room space at, say 18oC and 10.5 g/kg moisture content.

On the extract air side, air from the room at, for example, 25oC and 7.6 g/kg moisture content is first passed through an evaporative cooler so that it enters the thermal wheel at approximately 17.5oC and 10.8 g/kg moisture content. As the extract air stream passes through the thermal wheel, it is sensibly heated to approximately 47oC. The air stream is then heated up to approximately 80oC in order to regenerate the desiccant wheel. It should be noted that in order to save energy, approximately 20% of the extract air flow by-passes the regenerating coil and the desiccant wheel.





Heating Mode

During the winter time, much of the heat for the supply air stream comes from recovered heat from the thermal wheel. Should further sensible heating be required this can be achieved by either locating a heating coil in the supply air stream after the thermal wheel, or by using radiators within the room. In addition, an evaporative cooler on the supply side may be utilised in order to humidify the incoming air stream if required.

Dehumidification

In most comfort cooling applications any dehumidification that is achieved is considered secondary to the main task of sensible cooling the air. However, in applications where cool surfaces are involved, such as chilled ceilings or fan coil units, where it is important to prevent condensation occurring, dehumidification then becomes of paramount importance. In such applications the conventional approach is often to use DX cooling coils with a low apparatus dew point temperature (eg 5oC) to dehumidify the supply air. This results in an over-large cooling coil with a re-heat coil required to compensate for the over-cooling of air during dehumidification. In contrast to the DX cooling coil approach, the desiccant system avoids this problem, because it does not rely on cooling of the air to produce dehumidification.


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Further Information

Further information and results from the study please contact Gaia Research. The following publications are available. Please send cheque with order to address:-

  • Desiccant and Solar Desiccant Systems: Guidance Note and Solar Desiccant Calculator CD-ROM, 2000 - £20.00
    (The program is compatible with Windows 95/98/2000 or NT4.0 It is a limited run edition.)



  • Desiccant and Solar Desiccant Systems: Notes from a Seminar at CIBSE Building Services Engineering Centre, Gaia Research 2000 - £20.00.

  • Solar Air Conditioning Technical Assessment & Demonstration: The Feasibility Study, Gaia Research 1998 - £20.00.

  • Desiccant and Solar Assisted Cooling - Notes from a Seminar at Warwick University in 1998 with sponsorship from Munters - £20.00


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Published papers include:

  • The Potential for Solar Desiccant Cooling in the UK CIBSE Conference 2000 can be viewed and downloaded here as a PDF file.

  • Is Solar Air Conditioning Feasible? Building Research & Information 1999 Vol. 27 (3) pp 149-164.

  • A Theoretical Evaluation of Solar-Powered Desiccant Cooling in the UK, BSERT Vol. 20 (3) 1999.

  • Solar Powered/Gas Hybrid Air Conditioning CIBSE Conference 1998. can be viewed and downloaded here as a PDF file.

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