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,
& Solar Desiccant Systems
The Principles of Desiccant Cooling
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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Principles of Desiccant Cooling
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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of Operating Cycle
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
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.
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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information and results from the study please contact Gaia Research. The following
publications are available. Please send cheque with order to address:-
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.
Air Conditioning Technical Assessment & Demonstration: The Feasibility
Study, Gaia Research 1998 - £20.00.
and Solar Assisted Cooling - Notes from a Seminar at Warwick University
in 1998 with sponsorship from Munters - £20.00
Published papers include:
Potential for Solar Desiccant Cooling in the UK CIBSE Conference 2000
can be viewed and downloaded here as
a PDF file.
Solar Air Conditioning Feasible? Building Research & Information 1999
Vol. 27 (3) pp 149-164.
Theoretical Evaluation of Solar-Powered Desiccant Cooling in the UK, BSERT
Vol. 20 (3) 1999.
Powered/Gas Hybrid Air Conditioning CIBSE Conference 1998. can be viewed
and downloaded here as a PDF file.