Here are some of the most relevant and factual excerpts from it...

Free Cooling from outside air and humidity manipulation

Free cooling take place when the external ambient air enthalpy (the term used for a combination of sensible heat and moisture content) is less than the indoor air enthalpy.

Free cooling may be used with mixed outside air and recirculation systems by the use of modulating dampers. In the event of cool outside air the quantity of outside air is increased and the quantity of recirculated air is reduced to provide the required supply air temperature.

This system of free cooling is popular and uses thermostats to determine when the outside air is cool and the proportion the outside air damper should be opened by. More accurately the proportion of outside air should be increased when the outside air enthalpy is lower than the room enthalpy. In reality temperature sensing is more popular because thermostats are less costly and are less likely to drift out of calibration.

Further sensible cooling (reduction in temperature as indicated on a thermometer) may be achieved by the use of humidifiers which spray water into the air steam to cool the air.

Adiabatic (constant enthalpy or heat energy) humidifiers, have the effect of a useful reduction in dry bulb temperature whilst increasing the air moisture content thus making no difference to the enthalpy of the air. The feed water temperature does not make a significant difference to the process. With warmer feed water as high as 100°C the water is evaporated more quickly; this will cause a slight gain in enthalpy.

The sensors required to achieve good control are:

1. Enthalpy or dry bulb sensor in the incoming outside air duct.
2. Dew point sensor after the humidifier with the set point at the required supply air moisture content.
3. Dry bulb sensor after the reheater to control the required re-heat.

A simple and yet effective way of providing free cooling is to provide a heat recovery unit between the supply and exhaust duct which is normal practice when using 100% outside air systems. An evaporative humidifier (known as an indirect humidifier when located in the return air duct) is then provided before the exhaust connection to the heat recovery unit.

Desiccants

Evaporative humidification is an extremely useful technology particularly in hot dry climates and may also be used to reduce the load on air conditioning equipment in damper climates. The limitation of evaporative humidifiers is that the reduction in dry bulb temperature is conditional on the moisture content of the incoming air and as a result the use of desiccant wheels have become increasingly popular as they are able to increase the efficiency of the adiabatic process by reducing the air moisture content prior to humidification.

The desiccant wheel dries the air and is usually used in conjunction with a thermal wheel to transfer sensible heat from the supply air duct. The system may not be considered as free cooling as heat is required to regenerate the desiccants but may be considered as low energy because in most climates evaporative cooling is sufficient at certain times of year without the need for drying by the desiccants.

Thermal Energy Storage, Phase Change and Eutectic Chemicals

When using desiccants for cooling, as the reheat required for the regeneration of the desiccants is by far the highest single energy cost, it makes sense to mitigate these costs as far as possible. Solar energy may be stored by the use of phase change material which raise the melting point and boiling point of a chemical solution using Eutectic salts. The thermal energy stored is released back in to the system when required and in this way provides free reheat at certain times.

Thermal energy storage may be used for storing either cool energy or heat energy and the appropriate phase change material selected for the particular application. Many countries offer cheap rate electricity costs at night to even electricity demand and therefore cool energy may be stored at night for use the following day with the resulting economy.

Alternative Methods of Cooling and Reducing Cooling Loads and Energy Costs

Night Cooling

The use of a building as a heat sink to absorb heat in occupied hours and then followed by night cooling has shown to be beneficial.

A much used technique is to have cool night air pass over a slab and in this way cool the slab at night. The warmer daytime air will be reduced in temperature when passing over the cooled slab and help to reduce the daytime peak load.

Consideration should be given to the slab at the design stage and in order to increase the exposed area of the slab a coffered or sinusoidal shape is advantageous. It has been demonstrated the effective rate of heat flow between the internal surface of a construction and the space temperature is the limiting factor to achieving heat storage rather than the thickness of the slab. (The maximum slab thickness required appears to be up to approximately 100mm.) In order to maximise the heat flow relationship it is concluded that mechanical ventilation should be used, the heatflow is further enhanced by ducting air through hollow cores in precast concrete slabs and the best heat flow was achieved by ducting supply air close to the slab surface beneath steel sheeting.

TermoDeck Passive Temperature Control Systems

The Swedish developed TermoDeck system uses the slab as both a structural component and also a means of ducting ventilation through the building through oval or round shaped holes within the concrete structure. Over 200 projects have been installed in Sweden and Norway and latterly Holland and Belgium.

The Barra System

This system was developed by Horazio Barra in Italy and originally used as a passive solar heating system. Floors of reinforced concrete are used with embedded channels utilising hollow concrete blocks. Outdoor air is blown through the channels and when originally used the hot air emerging from the insulated southern facing collecting wall served as thermal storage. The system may be modified and used as a cooling system as well. At night a fan blows ambient air through the channels and thus cold night energy will be stored within the ceiling mass.

During the daytime the cooled ceiling will absorb the heat from the interior space passively.

Roofs, Roof Ponds and the Soil

In his book 'Passive and Low Energy Cooling of Buildings' Professor Baruch Givoni describes methods of cooling using the above.

As roofs are usually insulated to minimise both heat loss and external heat gain it is not possible to take advantage of low nocturnal temperature unless the roof is designed in a certain way. A simple method of achieving radiant cooling is to use a heavy but highly conductive roof exposed to the sky at night which would be highly insulated in the day using operable insulation.

References

Building Services in the Greenhouse Spotlight - p36 - p37, June 1998 issue of Building Services Journal by Dr David Fisk

Night Cooling Control Strategies, BSRIA March 1996 by Messrs Martin and Fletcher

Probe Elizabeth Fry Building - p37 - p42, April 1998 issue of Building Services Journal by Mark Standeven, Robert Cohen, Bill Bordass and Adrian Leaman (The Probe Team)

Passive and Low Energy Cooling of Buildings - Professor Baruch Givoni and published by Van Nostrand Reinhold

Unglazed collector/ regenerator performance for a solar assisted open cycle absorption cooling system - by M.N.A. Hawlader, K.S. Novak and B. D. Wood of the Center for Energy System Research, College of Engineering and Applied Sciences, Arizona State University, Tempe. Published in Solar Energy Vol. 50 pp59 - 73 1993

Fresh air for sedentary occupants - pp55 - 56, Building Services Journal 1989 by Paul Appleby

Leaderer, BP and Cain WS (1983) Air quality in buildings during smoking and non smoking occupancy, ASHRAE Tran. 89 2A and 2B, pp601-623

Fanger PO, Lauridsen J, Bluyssen P and Clausen G (1988) Air pollution sources in offices and assembly halls, quantified by the olf unit. Energy and Buildings, 12 pp7 -19

Fanger PO, (1986) Body odour and carbon dioxide, minimum ventilation rates. IEA energy conservation in buildings and community systems programme. Annex 1X final report

Janssen JE (1988) Control of indoor air quality through ventilation. Proc. 5th Canadian Building and Construction Congress, Montreal, Quebec, Nov. 1998 NRC of Canada

Papers submitted at seminar on Desiccant and Solar Assisted Cooling in April 1998 and published by Gaia Research and as follows:

• Solar Air Conditioning Project - Sandy Halliday, Gaia Research
• Desiccant Cooling System Type Desicool - Operation Method, Performance, Some Experience to date - Dr Hans Hagberg, Munters Europe AB
• Liquid Desiccant Technology - Andrew Mongar, Albers
• Technical Trends in Solar Cooling - Options For Solar Air Conditioning - Ken Thompson, University of Warwick
• Cool Comfort in Buildings and the Impact of Climate Change - Dr David Arnold, Troupe Bywaters and Anders
• The Potential for Solar Powered Desiccant Cooling - Dr Clive Beggs : University of Leeds and Sandy Halliday, Gaia Research
• Solar Assisted Desiccant Air Conditioning - Simulation of Hot Water Production Plant - Sandy Halliday, Gaia Research and Dr Tariq Muneer, Napier University

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