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A Practical Guide to Free Cooling,Alternative Cooling, Night Cooling

and Low Energy Systems for AirConditioning Systemsby Mike Hardy

This article is provided by Ambthair Services . If you are interested in Ambthair's airconditioning consultancy, design and installation services, please contact us.

Contents

Prologueo Introductiono International Agreement and Commitment and the Kyoto Protocolo Emissions Trading and Permits

Free Cooling and Low Energy Systemso Free Cooling using Mixed Outside Air and Recirculation Systems and

Temperature and Enthalpy Control Additional Free Cooling using Dew Point Control and Direct Humidifiers Consideration of Legionnaires Disease and Humidifiers The Application in Practice of Dew Point Control and Direct Humidifiers

o 100% Outside Air Systems Outside Air Heat Recovery and Indirect Evaporative Humidifiers

o Desiccantso Solid Desiccant Systemso Liquid Desiccantso Thermal Energy Storage, Phase Change and Eutectic Chemicalso Alternative Methods of Cooling and Reducing Cooling Loads and Energy Costs

Night Cooling TermoDeck Passive Temperature Control Systems The Barra System Roofs, Roof Ponds and the Soil Liquid Lithium Chloride Absorption System Using Solar Energy

o Practical Choices for Free Cooling and Low Energy Systems Displacement vs. Mixed Flow Consideration of Minimum Outside Air Quantities and Odour Control

o The Units of Odour Intensityo ASHRAE Standard 62o Future Trends for Outside Air Quantities and Cooling Load Implications

Existing Buildings - the Retrospective Addition of Cooling New Buildings

Conclusion Acknowledgements References and Thanks

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Prologue

The discerning building owner would do well to take an interest in how the environment intheir building is achieved as the choices at arriving at what may or may not be a satisfactoryenvironment to the HVAC engineer are legion and all too often and regrettably have to betaken in isolation. The choices will probably have to be lived with throughout the life of thebuilding and as well as being one of the highest annual outgoing costs, further, the comfortof the building will probably be the single most important issue to the occupier effecting boththeir well being and health. The owners interest would empower the HVAC designer whowould explain the choices at the outset and would explain the trade off costs of compromisethat are often required in the building design process and construction.

Human comfort and low energy systems in buildings are not necessarily a contradiction interms, in fact it is a paradox that low energy systems more often than not provide better

comfort. It is also a paradox that in buildings in cities throughout the world, even intemperate climates, that vapour compression equipment is providing cooling throughout theyear even though the external temperature for the vast majority of time is lower than theinternal building ambient temperature. Any feedback about any of the information discussedin this article, such as that by Mr Dahliwal and contained herein, would be most appreciatedby the writer as of course would be any other feedback.

As the article is intended for and warmly welcomes those who may have no knowledge at allof the HVAC industry, the writer apologises in advance for what may be regarded by othersas obvious or oversimplification.

In t roduct ion

This article discusses practical methods of minimising the effect in buildings of, in particular,internal heat gains which have become more prevalent in recent years because of theinformation technology industry etc. and also discusses the issue of optimum comfort. Thethrust of the article is less concerned with rural buildings in temperate climates which evenwith quite high internal heat gains may only overheat for a few weeks a year but moreconcerned with urban buildings in temperate or hot climates. In his excellent book 'Passiveand Low Energy Cooling of Buildings’ the author Professor Baruch Givoni differentiatesbetween with what he describes as „tropical bioclimate architecture and other methods ofachieving cooling in buildings.

Tropical bioclimate architecture is more to do with mitigating the effect of heat by the use ofshading, colour, orientation of building, size of windows and structure of building and itseffective application is essential in any hot climate and, of course, many societies havepractised and have become extremely skilful in using this method of protecting themselvesfrom hot climates over many years. However this article is more related to methods ofintroducing cooling to buildings by passive or active ventilation or other means and of courseit is assumed that these methods should be used to augment effective tropical bioclimatearchitectural techniques.

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An American called Jacob Perkins living in London in the U.K. patented the first closedvapour compression refrigeration system in 1884. The refrigerants used were generallyflammable and/or toxic and generally not very satisfactory. The dramatic and acclaimedintroduction of chlorofluorocarbons (CFCs) in 1930 in the form of R12 at a meeting of the

American Chemical Society marked the beginning of modern refrigerants which would, intime, revolutionise technology throughout the world.

In 1974 Messrs. Rowland and Monila presented the hypothesis that chlorofluorocarbons andother gases were damaging the earth s atmosphere by the Greenhouse Effect which theyclaimed increased the atmospheres ability to absorb infra-red radiation therefore reducingthe rate at which the planet is able to shed energy into space and also by Ozone Depletionwhich stated that gases accumulating in the earth s atmosphere acted as a catalyst todestroy ozone.

In ternat ional Agreement and Com mitm ent and the KyotoProtocol

The international agreement reached at the Rio Earth Summit in 1992 resulted in TheFramework Convention on Climate Change and placed commitments on developedcountries to devise policies and measures to bring down greenhouse gas emission to their1990 levels by the year 2000 and also to provide financial assistance to developingcountries to meet their general commitments.

Some countries will meet those targets but others will not. Hence, international agreementwas reached that the next step had to involve a framework leading eventually to legallybinding targets. The world s industrialised nations signed the Kyoto Protocol on the 11thDecember 1997 agreeing to a collective cut in greenhouse gas emissions of 5.2% by 2008-2012. The emissions targets have as yet no legal „teeth although it is intended to try and

introduce legal enforceability at some stage, to date 176 countries have signed theagreement. The target will be achieved by cuts of 8% by Switzerland, most Central and EastEuropean states and the European Union, 7% by the US and 6% by Canada, Hungary andJapan, the countries of Poland, Russia, New Zealand and Ukraine are to stabilise theiremissions, while Norway may increase emissions by up to 1%, Australia by up to 8%, andIceland 10%.

The European Union have agreed amongst themselves that reductions will be:

Austria -10%

Belgium -7.5%

Denmark -21%

Finland 0%

France 0%

Germany -21%

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Greece +25%

Ireland +13%

Italy -6.5%

Luxembourg -28%

Netherlands -6%

Portugal +27%

Spain +15%

Sweden +4%

UK -12.5%

It was agreed the reduction in the three main greenhouse gases of carbon dioxide, nitrousoxide and methane would be measured against a baseline of 1990 emissions, whereassulphur hexafluoride, hydrofluorocarbons and perfluorocarbons would be set against abaseline of 1995 levels. The greenhouse gas target is now expressed as the total nationalemissions over the period of 2008 to 2012 to facilitate calculations of averages.

Nations may meet these binding targets by joint implementation, for instance a developedcountry may take credit for funding greenhouse cuts in another country. The Protocol alsoenvisages developed countries trading greenhouse gas emissions between themselves tothe effect that a country that has achieved, or even exceeded its own respective targetcould, hypothetically, exchange with another country which is struggling to achieve itsrespective target.

The current position of HCFCs (of which R22 is the most prevalent in air conditioning) inEurope is:

35% cut by 2002 60% by 2007 80% by 2010 95% by 2013 Phase out by 2015

The new draft is more radical and proposes the following:

10% cut by 2002 65% by 2003 70% by 2004 95% by 2008 Phase out by 2015

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From 2008 the use of virgin HCFCs shall be prohibited in the maintenance and servicing ofair conditioning equipment.

The fourth convention on Climate Change will be held in November in Buenos Aries andinformation on the Kyoto Protocol may be found on the United Nations climate changehomepage.

Emiss ion s Trading and Permits

What is most likely to affect both the building owner and the building service engineer as adirect result of Kyoto, is the prospect of Emission Permits. The US is the main proponent ofemissions trading, this based on its own successes with companies for whom sulphuremissions were a concern. Permits were allocated to companies for sulphur emissions; thisallocation being tradeable on the open market. The opening prices for Sulphur EmissionsPermits when first issued were high but rapidly fell, it is thought, for the two followingreasons. Firstly, abatement projects became more economical when the cost per ton abatedwas less than the market price of an Emissions Permit. Secondly, companies began torevise their thinking about the cost of abatement measures when they themselves needed tobuy an emissions permit.

Similarly, it is supposed that building owners would have a set of permits which cover greenhouse gas emissions from their buildings. The tradeable permit may perhaps only costmoney at the outset; very efficient companies may not need to have permits or alternativelyuse them to their financial advantage by selling them to other companies. What will beunfamiliar to all is that the permit may well be a legal document in the future. Whereas it hashitherto been considered by many countries in the past, that the price of energy would haveto become very high before building owners took serious notice of energy efficient design,the issuing of permits is likely to concentrate the building owners mind a great deal more.

Even if a permit system does not embrace building owners other countries might devise asystem that does. One country for example may wish to release onto the internationalemissions trading market the savings from replacing the district heating schemes of entirecities.

The Kyoto meeting set out an ambitious agenda to tackle global climate change and it isintended much of the detail needed to turn it into law will be negotiated at the Buenos Airesmeeting in November 1998. What is certain is that the building owners and developingcountries in particular would do well to address the issue now and have nothing to lose bythis but much to gain. For building service engineers throughout the world the KyotoProtocol provides a demanding and yet exciting challenge ahead.

Free Cooling and Low Energy Systems

Free cooling take place when the external ambient air enthalpy (the term used for acombination of sensible heat and moisture content) is less than the indoor air enthalpy andthe cool external air is transferred to the building envelope either directly or indirectly.

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Surprisingly this is not taken advantage of very widely in most countries of the world and ofcourse the efficacy of the external cool air would be increased when used in modernbuildings because of improved insulation values.

Free cooling may be used in conjunction with air systems and hydraulic systems. This articlediscusses the use of free cooling with air systems.

Air conditioning systems either provide supply air using

a. A mixture of outside air and recirculated air, orb. 100% outside air systems which are generally used in the cases of hospitals, densely

occupied areas such as theatres etc. or increasingly there is a preference for thesesystems as a matter of course in certain countries.

Free Cool ing u s ing Mixed Outs ide Air and Recircula t ionSys tems and Temp erature and Enthalpy Contro l

Free cooling may be used with mixed outside air and recirculation systems by the use ofmodulating dampers. Dampers are provided on the outside air intake ductwork, exhaust airductwork and the recirculation ductwork. In the event of cool outside air the quantity ofoutside air is increased and the quantity of recirculated air is reduced to provide the requiredsupply air temperature. In this way cooling by means of refrigeration equipment is avoidedaltogether at certain times of year and often at night times.

This system of free cooling is popular and uses thermostats to determine when the outsideair is cool and the proportion the outside air damper should be opened by. More accuratelythe proportion of outside air should be increased when the outside air enthalpy is lower thanthe room enthalpy. In reality temperature sensing is more popular because thermostats are

less costly and are less likely to drift out of calibration.

When the outside air temperature (alternatively enthalpy) is higher than the roomtemperature (alternatively enthalpy) in Summer the dampers will modulate to the minimumoutside air position to keep the load on the refrigeration equipment to a minimum.

Additional Free Cooling using Dew Point Control and DirectHumidifiers

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

method of sensible cooling is extremely effective but became less popular in the last decadeor more because of Legionnaires Disease. This form of cooling used to be provided by airwashers but the concern over Legionella meant that ultra-violet light and other forms ofbiocide control had to be used in conjunction with this equipment and added both capitalcosts and high maintenance costs. In reality the industry turned to steam injectionhumidifiers if humidification was required because there was no risk at all with respect toLegionella. However steam injection humidifiers do not provide any sensible cooling and infact provide a small amount of sensible heating and whilst they are valuable tools forhumidification are of no value for free cooling.

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Consideration of Legionnaires’ Disease and Humidifiers

Legionella Pneumophila is ubiquitous in water and only believed to be a problem whengrowth and therefore colonisation takes place. This tends to happen in stagnant water andthe water temperature is typically between 20°C and 45°C. To be ingested into therespiratory system the water must be absorbed in an aerosol form.

Steam injection humidifiers became popular because all bacteria were pasteurised althoughthe benefit of sensible cooling using spray coils was lost. Chemical biocides (bacteria killingprocess) such as those that are chlorine and bromine based are suitable for cooling towers,where sprays are also used, but not suitable for supply air systems to occupied buildingsdue to their toxicity. The options remaining were therefore limited to biocides such as ultra-violet light (a further possible option is the use of chlorine dioxide which is soluble in waterand claimed by the manufacturers to be harmless when drunk).

Some of the new generation of water humidifiers address the problems brought about by theconcern for Legionella Pneumophila and also provide sensible cooling and may be used for

dew point control.

Evaporative Humidifiers

These humidifiers spray water on to a matrix and water is transmitted to the air streamthrough the saturated matrix or corrugation. Provided the air velocity through the matrix islimited to 1.75 m/s then the air velocity is sufficiently low to prevent aerosols forming andthey are thought to be safe to use without regular biocide treatment. The sump isgenerally sufficiently cold to prevent the colonisation of Legionella and when the sumpwater is not being used i.e. stagnant then the water should be drained from the sump.

Ultra-sonic Humidifiers

Ultra-sonic humidifiers use transducers (devices that use input energy to create adifferent form of output energy) vibrating at a high frequency in water to create fineaerosols. Manufacturers claim the ultrasonic shock acts as a biocide.

Pneumatic Humidifiers

These are generally connected directly to the water supply and compressed air is used topropel water through nozzles to create a fine aerosol. Close humidity control may beachieved by these humidifiers.

Of the three, the evaporative humidifier is likely to have the lowest capital costs and maygenerally be used without biocides for most applications.

The Application in Practice of Dew Point Control and Direct Humidifiers

The use of the humidifiers described above, known as adiabatic (constant enthalpy or heatenergy) humidifiers, have the effect of a useful reduction in dry bulb temperature whilstincreasing the air moisture content thus making no difference to the enthalpy of the air. The

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dew point of air is the temperature of saturated air i.e. air at 100% relative humidity and thedew point temperature of air at less than 100% relative humidity will always be lower thanthe dry bulb temperature and in this way therefore the saturation of air with water vapourlowers the dry bulb temperature by means of the process know as adiabatic cooling whichby definition is at constant enthalpy at 0°C feed water temperature. In reality and perhapssurprisingly the feed water temperature does not make a significant difference to theprocess. With feed water as high as 100°C the water is evaporated more quickly; this willcause a slight gain in enthalpy.

This simple but effective method of lowering the dry bulb temperature of the air has beenpractised for many years.

In Summer when the outdoor air enthalpy is higher than the indoor ambient enthalpy thenthe modulating dampers would modulate to minimum outside air and the humidifier wouldnot be used.

In Spring and Autumn when the outdoor enthalpy is lower than the indoor enthalpy but is

less than the required enthalpy for sufficient indoor cooling, the dampers are modulated tothe dew point of the supply air and the humidifier saturates the air to dew point. The air isthen reheated if required to the supply air temperature or may not require reheating if thecondition of the air after the humidifier is exactly as required to maintain room conditions.

In Winter, when the outside air enthalpy is low, then the modulating dampers wouldmodulate to minimum position to achieve the required dew point and the air reheated afterthe humidification process to achieve the required supply air condition. In very cold weathera preheater may be required.

The sensors required to achieve the above 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.4. The set point of (3) may be scheduled against ambient temperature or changes in

response to the extract air temperature by a dry bulb sensor in the return air duct.

The method of control described is simple but is wasteful of reheat because the intake airmust be cooled to saturation point to achieve the required dew point and then reheated tosupply air temperature. A more sophisticated control would be carried out using a BuildingEnergy Management System (BEMS system) controlling both moisture content andenthalpy. As close humidity control is required, a direct injection adiabatic humidifier wouldnormally be used with this form of control and installed after the heating coil, in contrast tothe control strategy described previously when the humidifier would normally be installedbefore the heating coil.

This method of control performs a series of logical tests at points around the system forvalues of enthalpy and moisture content to arrive at the most efficient control strategy and inthis way reduces the amount of reheat required.

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addition the power saving have exceeded 75% when compared with a mixed flow ventilationsystem capable of maintaining temp above 7°C above ambient.

This is our first job using displacement ventilation. We are now looking at the possibility ofapplying this scheme in textile spinning plant using a combination of Indirect/Direct evap.cooling and chilled water spray humidifier.

RegardsS.DHALIWAL

P.S. We are in the business of designing Indirect/direct evap systems for industrialapplicat ions.”

By the application of indirect and direct evaporative cooling and the use of displacementventilation the engineer was able to maintain a low level temperature of 7-8°C below theexternal ambient.

Desiccants

Evaporative humidification is an extremely useful technology particularly in hot dry climatesand 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 isconditional on the moisture content of the incoming air and as a result the use of desiccantwheels have become increasingly popular as they are able to increase the efficiency of theadiabatic 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 totransfer 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 lowenergy because in most climates evaporative cooling is sufficient at certain times of yearwithout the need for drying by the desiccants.

Sol id Des iccant Sys tems

Entering air to the system is dehumidified and heated by a rotating desiccant wheel. Thedrier air then passes through a sensible heat recovery wheel which reduces the dry bulbtemperature of the supply air without altering moisture content. The dry bulb temperature ofthe air is reduced still further by passing through an evaporative humidifier (called a directhumidifier when installed in the supply duct) and in to the air conditioned space. The

evaporative humidifier increases the moisture content of the air in proportion to reduction indry bulb temperature.

Return air is generally passed through another evaporative humidifier where the return airtemperature is reduced and moisture content increased. This colder air is transferred via thesensible heat recovery wheel, previously mentioned to the supply air duct. The return airtemperature is then further increased by passing through a heater and the air temperatureelevated to a level sufficient to reactivate the desiccants in the desiccant wheel in the returnair section of the duct.

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The use of heat for the regeneration process is the highest energy requirement of thesystem. The elevated temperature of the return air duct removes moisture from thedesiccants and the wheel will then absorb moisture again.

Typical desiccant reactivation temperatures required in the return air duct for Europe are 70-80°C and air will be exhausted from the system at typically 40-50°C. A proportion of returnair may bypass the heater and desiccant wheel in order to minimise energy consumption.

Desiccant cooling is a potentially environmentally friendly technology for cooling buildingsparticularly if solar energy is used for the reactivation process. Research seems to indicatethat in Europe a hybrid system should be used to minimise reactivation costs. This wouldconsist of a desiccant ventilation system cooling the incoming air and using solar/gas forreactivation used in conjunction with low grade chilled water from towers or better stillpurpose constructed ponds serving ceiling chilled water coils providing sensible cooling andin this way avoid vapour compression equipment altogether. It would seem in a hot climatethat it is possible that all of the cooling could be provided by the desiccant system and solarenergy and this is likely to reduce capital costs, running costs and maintenance costs

considerably as well as avoid vapour compression equipment.

Liquid Des iccants

The desiccant system described previously uses solid desiccants i.e. silica gel or lithiumchloride; liquid desiccant systems use a liquid spray of desiccant solution such as lithiumbromide. The development of liquid desiccant systems compared to solid desiccant systemsis still in its infancy although it is claimed that liquid desiccant systems have the potentialadvantage of better efficiency as dehumidification and heat transfer take placesimultaneously rather than sequentially.

The Genius 4000 has been developed in the USA by Albers Air Conditioning and uses liquiddesiccants to dehumidify outside air or cool make up air without the need for conventionalrefrigerants. The liquid desiccant in this system is composed of 90% lithium bromide and10% lithium chloride. According to Albers Air Conditioning, liquid desiccants do notdeteriorate over time, unlike solid desiccants. However, sulphur contaminants in the air cancombine with the desiccant to form lithium sulphate, which is not a desiccant. Ablers claimthat, as there are about 80 litres in the machine, it would take a long time to reduce thedesiccant to a significant level. In dirty conditions Albers states that it might prove necessaryto change the desiccant as part of an annual service, although the regeneration processtends to remove most of the pollutants.

Performance figures, taken from the manufacturer s tests at outside conditions of 28°C drybulb and 50% relative humidity give 37 KW of gross cooling, air being supplied to the roomat 11.4C. A room condition of 23°C dry bulb and 55% relative humidity was used incalculating the capacity and the cooling capacity would increase at higher temperatures andhumidities. Primary energy use for the Genius is said to be typically 30% less thanconventional vapour compression equipment and approximately halves the operating costs.The system would be best suited for systems with high outside air requirements and wouldhave to be used in conjunction with other equipment for high internal sensible loads

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although this may not necessarily be the case if used in conjunction with a Termodeck (seelater) system which does much to mitigate the effect of high internal sensible heat gains.

Thermal Energy Sto rage, Phase Change and EutecticChemicals

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

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

Alternative Methods o f Cool ing and Reducing Coo l ing Loadsand Energy Cos ts

Night Cooling

The use of a building as a heat sink to absorb heat in occupied hours and then followed bynight 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 theslab at night. The warmer daytime air will be reduced in temperature when passing over thecooled slab and help to reduce the daytime peak load. It is clear that during hot weather itwould probably always be helpful for night time cooling to take place and the control strategywould be simple. The control strategy to be used for quite hot days is less obvious becausethe energy consumed by the running of a fan at night as against the energy saved by slabcooling may be marginal or counter-productive. In a predominantly hot climate the strategyfor night cooling would be reasonably simple but in a temperate climate is less clear. On sitetests carried out by the Building Service Research Association in the UK have suggestedthe following strategy:

That night time cooling should be initiated if any or a combination of the following occur:

Peak indoor zone temperature exceeds 23C Average zone indoor temperature exceeds 22C Average afternoon outdoor temperature is higher than 20C

They also conclude that night time cooling should continue to be used if all of the followingcriteria occur:

The indoor zone temperature is higher than the outside air temperature +2C

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The indoor zone temperature is higher than indoor heating set point. The outside air temperature is higher than 12C

The night cooling would be carried out seven days a weeks during the unoccupied periodsof the building and once the criteria are not satisfied then night cooling would continue tooperate for an additional two nights providing night cooling has taken place for the previousfive nights.

A minimum set point should be provided to prevent overcooling otherwise re-heating wouldbe required and therefore be counter-productive.

When using a mechanically ventilated building for night cooling then the exhaust fan shouldbe preferred to avoid temperature pick up from the supply fan (unless the supply fan motoris out of the air stream) and a minimum air velocity of 1.5 m/s. It may even be worthconsidering the installation of a night cooling bypass duct which would bypass the mainplant to avoid pressure drop and minimise fan energy; in order that less energy is used thenthe fan would need to be a variable speed fan or at least a two speed fan.

There is a possibility in certain climates that the temperature of the slab when cooled couldcause condensation and this should be safe guarded against by the use of a moisturedetector on the slab or other means.

It is estimated in the UK that internal temperatures can typically be held at 6-8°C below peakexternal summer time temperatures and in hotter climates particularly where there is a sharpcontrast between Summer day temperatures and Summer night temperatures the reductionwould be considerably more.

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 beendemonstrated the effective rate of heat flow between the internal surface of a constructionand the space temperature is the limiting factor to achieving heat storage rather than thethickness of the slab. (The maximum slab thickness required appears to be up toapproximately 100mm.) In order to maximise the heat flow relationship it is concluded thatmechanical ventilation should be used, the heatflow is further enhanced by ducting airthrough hollow cores in precast concrete slabs and the best heat flow was achieved byducting 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 componentand also a means of ducting ventilation through the building through oval or round shapedholes within the concrete structure. Over 200 projects have been installed in Sweden andNorway and latterly Holland and Belgium.

With the TermoDeck systems the slab temperature is very close to the room temperatureand makes it suitable for displacement ventilation as well as mixed flow ventilation (see laterfor comparison of displacement and mixed flow). In Summer the supply air fans at night

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bring in the cool air into the hollow slabs to cool the building and the warm outside air iscooled in the daytime.

Two systems have recently been installed in the UK and the latest building The ElizabethFry Building uses mechanical ventilation with heating and no mechanical cooling at all.Mixed flow ventilation is used throughout the building except the Lecture Theatre wheredisplacement ventilation is used. The building has created much interest and is beingclosely monitored for energy consumption and occupant satisfaction by the PROBE team(independent organisation monitoring buildings after occupation). The slab temperature iskept at 22°C with a dea dband of 1/2°C for heating and 1.5°C for cooling. TermoDeck sinventor Loa Anderson predicts with computer modelling that at an external peak of 29°Cthe peak internal temperature should not rise above 26°C with a daily average roomtemperature around 22C.

The PROBE team conclude that of 12 recently constructed buildings in the UK this buildinghad the highest occupant comfort scores and are also recorded the highest comfort scoresrecorded by the independent survey specialists Building Use Studies. Typical energy

consumption for heating and ventilation for the UK are around 200 kWh/msq./y and forSweden using TermoDeck between 30-50 kWh/msq./y, this building seems to be followingthe Swedish trend.

The TermoDeck system has since been installed in hot climates such as Saudi Arabiawhere the slab tends to be kept at a temperature of 19C. The manufacturers claim that thecooling plant capacity and associated equipment is substantially reduced and the ElizabethFry research appears to confirm significantly reduced cooling loads and running costs.

The system may be used with mixed flow or displacement ventilation, night cooling, freecooling, desiccant cooling, packaged equipment cooling, DX equipment, chilled waterequipment and so is versatile.

As the room temperature and slab temperature are similar the room temperature tends to beuniform and therefore assists comfort.

The Barra System

This system was developed by Horazio Barra in Italy and originally used as a passive solarheating system. Floors of reinforced concrete are used with embedded channels utilisinghollow concrete blocks. Outdoor air is blown through the channels and when originally usedthe hot air emerging from the insulated southern facing collecting wall served as thermalstorage. The system may be modified and used as a cooling system as well. At night a fanblows ambient air through the channels and thus cold night energy will be stored within theceiling mass.

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

Roofs, Roof Ponds and the Soil

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

As roofs are usually insulated to minimise both heat loss and external heat gain it is notpossible to take advantage of low nocturnal temperature unless the roof is designed in acertain way. A simple method of achieving radiant cooling is to use a heavy but highlyconductive roof exposed to the sky at night which would be highly insulated in the day usingoperable insulation although it is the opinion of Professor Givoni that practical and movableinsulation is not available at present. Several buildings in the United States have used the„Skytherm' system in which the roof is made of structural steel deck plates. Plastic bagsfilled with water are placed above the steel decks and above them are movable insulationpanels that are moved by a motor. In Winter the water bags are exposed to the sun in theday and covered by the insulation panels at night. In Summer when cooling is required thewater bags are exposed and cooled at night and insulated during the daytime. As the cooledwater bags are in direct contact with the metal deck the ceiling serves as a cooling elementover the entire space. However it is the opinion of Professor Givoni that the availability of asimple and trouble free system of movable insulation is still in question.

In contrast to using indirect and direct evaporative cooling in conjunction with air beingintroduced into a building and as previously described it is possible to cool a roof by placinga cooled pond over it. The building is then cooled by conduction across the roof whichlowers indoor air and radiant temperatures without increasing the indoor water vapourcontent.

Professor Givoni suggests the ceiling temperature in the case of a concrete roof over a wellinsulated building would be about 2°C above the water temperature. It is concluded that thewater temperature of a shaded pond follows approximately the average wet bulbtemperature. The suggested maximum wet bulb temperature for applications of this type ofevaporative cooling in summer is 22-24°C and the dry bulb temperature not higher than 42-44C.

A shaded pond or lake adjacent to a building will provide cool water to fan coil units orsimilar within a building and will provide useful cooling in hot climates. For a more detailedappraisal of this subject and calculations see Nick Pines website detailed at the end of thisarticle. An indirect heat exchanger submerged in the water could be used and in this waycold water circulated to a closed loop system perhaps connected to mass produced fan coilunits or other terminals. A submersible pump in the pond would be used to circulate waterwithin the „open l oop using a stainless steel fine mesh filter say of 20 micron to keep theheat exchanger clean, a submersed non-corrosive carbon fibre heat exchanger would bestbe used for seperating the open and closed circuits. If the water were taken directly from the

pond to the fan coil, problems of corrosion and salt build up would probably make this atbest a temporary measure even if the water was well filtered.

Professor Givoni in his book suggests a number of methods of using the soil as a coolingsource stating that in temperate climates at a depth of 2 to 3 metres the natural temperatureof the soil could be enough to serve as a cooling source but in hot climates the soiltemperature is usually too high. However there are simple methods to lower the soiltemperature such as covering the soil with mulch at least 10cm thick and in regions with dry

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climates irrigating it, or raising the building off the ground and allowing water that wasprovided by either summer rains or irrigation to evaporate from the shaded soil surface.

Once cooled the reduced soil temperature may be used in a variety of ways for both passiveand active cooling.

Liquid Lithium Chloride Absorption System Using Solar Energy

In a paper entitled 'Unglazed collector/regenerator performance for a solar assisted opencycle absorption cooling system' the following research was described:

“A black shingled roof was used as a collector/regenerator for the evaporation of water toobtain a strong solution of lithium chloride absorbent. In the house, water (the refrigerant) issprayed into an evaporator and this was evacuated to a pressure of about 5mm of mercurywhere the water immediately flashes into vapour. Cold water pumped from the bottom of theevaporator then flowed through a fan coil unit which blew cold air into the area requiringcooling. (Fan coil units are usually used with chilled water in the HVAC industry). Watervapour from the evaporator flows over the absorber where it is absorbed by theconcentrated absorbent (lithium chloride). The continuous absorption of water vapourmaintained a low pressure in the system and permitted flashing of water in the evaporator.The product of the absorption process which was a weak absorbent solution collected at thebottom of the absorber to be pumped over the roof for concentration.

The dilute lithium chloride solution was delivered to the collector surface through a sprayheader spanning the top of the roof and made from 2 inch PVC pipe fitted with 35 evenlyspaced nozzles. The concentrated solution collected at the bottom in a PVC rain gutter andreturned via gravity to a 425 gallon tank”.

The Authors demonstrated a regeneration efficiency of between 38 and 67% whichcorresponds to a cooling capacity range of from 31kW to 72kW (8.8-20 tons of refrigeration).This is about 3.5kW per 10 metres square or 1 ton per 100 foot squared of roof area.

The costs of the chemicals used in this research are insignificant.

These remarkable results indicate that at peak conditions a roof say of 10m x 10m(100square metres or approx. 1000square feet) could provide 35kW (10 tons) of cooling forvery little energy costs.

If the system was used to provide cold water to a heat exchanger which then had cold airblown through to say a passive Termodeck ventilation system in the building then during hotweather in many buildings it is possible that further cooling may not be required at all and, ofcourse, the running costs would be minimal.

This research should not be lost on building owners and building service designersparticularly in hot climates and it is quite possible that the research could further point theway to low energy design in the future.

Pract ica l Choices for Free Cool ing and Lo w Energy Sys tems

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Clearly the application of free cooling and low energy cooling to a building is an extremelycomplex choice.

Displacement vs Mixed Flow

If the building is high enough for a displacement effect to take place and has a high sensiblegain, then this will minimise the cooling capacity required, especially if the heat gain is athigh level where much of the heat may be exhausted at source. With a room of 2.3 metresheight and below, a displacement effect will not take place so it would be counter-productiveto use displacement terminals; in fact, a higher room volume of air would be required as thecooling supply air room temperatures would have to be higher than for high level mixed flowdistribution.

For a more detailed appraisal of the type of air distribution see the article titled'Consideration of Displacement Ventilation vs Mixed Flow Ventilation for BuildingOwner/Designers’ .

Consideration of Minimum Outside Air Quantities and Odour Control

The outside air sensible and latent cooling load may well be one of the largest single loadsimposed on cooling plant for an air conditioned building and so is here considered in somedetail.

There is no question that both the quantity and quality of outside air are crucial to thecomfort of occupants in air conditioned buildings and the efficacy of the air conditioning ofan entire building when related to comfort may be negated by the introduction of too little orpoor quality outside air. Of course when designing low energy buildings there is a pressureto minimise outdoor air in order to keep plant size small and running costs low. This subject

has therefore been the recipient of a huge amount of research throughout the world sincethe inception of modern air conditioning and is as important now as it has ever beenparticularly as modern buildings are becoming more „tight and outdoor air quality, inparticular in urban areas, poorer in quality.

The trend in Scandinavia and more recently in Canada are for low energy buildings - and yetusing 100% outside and exhaust air with heat recovery from the exhausted air, this may wellpoint the way for the future particularly in temperate climates. Even in urban areas with poorquality outdoor air the use of good carbon filters, electrostatic filters and/or HEPA filters willimprove the quality of the air immeasurably.

Clearly if 100% of the air circulated through a building is from outside this would impose themaximum cooling load on cooling plant when the external ambient is hot and humid. Beforeuseful cooling of the building is to be achieved the cooling plant has to reduce both theexternal dry bulb and moisture content to room conditions before the cooling plant providesuseful indoor cooling and this will reflect both on the cooling plant size and the runningcosts. Alternatively the other extreme is if the system recirculates 100% of the air and in a„tight' building the outside air would exert little influence on the building cooling loads duringpeak conditions and this would reflect in smaller cooling plant being required and lowerrunning costs.

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In reality a minority only of air conditioned buildings use either of these extremes.

It is instructive to look back briefly at a some of the history of outside air research in order toappreciate the present day situation.

Outdoor air is needed to meet the oxygen needs of occupants, the dilution of odours,contaminants and gases in particular carbon dioxide. The early work of Yaglou et al in 1936provided the benchmark throughout most of the world for many years and this was based onwork in American schools and determined the quantity of outside air needed to provide asatisfactory reduction of odours depending both on the number of people present andpersonal hygiene. This research determined that for the same volume of air that odoursdispersed more rapidly when each occupied a greater volume of the room air. Theguidelines for many years were therefore directly related to the quantity of outside air peroccupant and consideration was also given to the likely concentration of the occupants.

The minimum outdoor air quantities in the 1980s of 5 litres/s and 2.5 litres/s per personadopted by the UK and North America respectively were shown to be inadequate by

laboratory based work of Leaderer, B P and Cain „Air qua lity in buildings during smoking andnon-smoking occupancy'. The work demonstrated that the ventilation rates were inadequatewhen considering odour levels perceived by people when first entering an already occupiedspace and that the density of occupancy had no bearing on fresh air requirements. Minimumair quantities were then increased in both countries to 8 litres/second in the UK and to 7.5litres/sec per person in North America.

These figures were recommended particularly because of concern over the incidence ofventilation related problems in air condition buildings and based on the work of theaforementioned Leaderer et al. and also the work of P.O Fanger „Body odour and carbondioxide, minimum ventilation rates .

The work of P.O. Fanger, J. Lauridsen, P. Bluyssen and G Clausen 'Air pollution sources inoffices and assembly halls, quantified by the Olf unit' addressed the question of ventilationrates and the relationship with a unit of odour intensity called the „Olf'. In this important worka large number of people were subjected to odour levels in a variety of naturally andmechanically ventilated buildings and air conditioned spaces. The research found that forevery occupant and associated odour there may be another four to five odour equivalents(olfs) released from building materials, furnishings and the air handling system.

Auditoria are particularly thought to be prone to soiling of internal surfaces because of theextent of soft furnishings and acoustically absorbent finishes.

The Uni ts of Odou r In tens i ty

The contribution to the industry of the work of Professor Fanger and his team in Denmarkhas been immense. Not only did Professor Fanger develop his well known „comfortequation', which was developed empirically and contains the many variables known to makeup the comfort of people, but as well as other work also developed the concept of the unitsof odour - the olf and the decipol. The nature of indoor pollution is very difficult to assessbecause of the many indoor chemicals involved. Even for pollutants that may be detected

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because of their odour or irritation effect there is the issue of who should be the judge ofwhat is acceptable. Within buildings pollutants come from many sources such as thebuilding and furnishing materials and chemicals such as correction fluid or from equipmentsuch as photocopiers.

Hundreds of volatile organic compounds have been identified in indoor air and most seem tobe much lower than for occupational standards for industrial workers although little appearsto be known of the effects of long term exposure. The sheer complexity of measuring indoorpollution raises the question of people themselves being used as the test instrument. Theuse of questionnaires given to building occupants for identifying and assessing problemswith indoor environment has been established for some time. The approach of using a panelof assessors to judge air quality has been advocated by Professor Fanger and has givenrise to the empirical units of the olf and the decipol. One olf is defined as the air pollutionproduced by one „standard' person (a standard person is also defined) and a decipol isdefined as the perceived air pollution level in a space in which there is a source strength ofone olf and which is ventilated at 10 litres/second with unpolluted air. The proposedEuropean air quality standard 'prENV 1752 Ventilation for buildings: design criteria for the

indoor environment' often referred to as the „Fanger Standard and at least 7 years in themaking has failed to be adopted by Europe as it was not endorsed by 71% of the 17participating countries. The proposed standard is however a useful guide and has muchvaluable information although the assessment of outside air quantities based on the use ofthe olf and decipol is the subject of the greatest controversy.

Standards which deal explicitly with air quality for the general populace in Europe and theUK are:

World Health Organisation 'Air quality guidelines for Europe' UK DoE 'Expert Panel on Air Quality Standards’ (which has begun to publish reports

on the health effects of certain pollutants and recommending air quality standards forthese).

UK Her Majesty s Inspectorate of Pollution Technical Guidance Note 'Guidelines ondischarge stack heights for polluting emis sions’

EC Directive No 80/779/EE Official Journal of the European Communities, No.L229 (limit values for some pollutants).

ASHRAE Standard 62

ASHRAE Standard 62 'Ventilation for acceptable indoor air quality' was first published in1989 and introduced the calculation of a dilution rate based on limiting concentrations fornon-industrial exposure to contaminants. The Standard uses ambient air quality standardsproduced by the US Environmental Protection Agency as a basis for their definition of freshair. This specifies exposure limits for common pollutants and if examination of recordsshows that these are exceeded then it is recommended that suitable air cleaning apparatusbe installed. It also introduced a technique for determining the lead and lag time dependingon the room volume per person and their fresh air allowance.

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Since there is potential for a wide diversity in pollutant sources, contaminant types andpopulation susceptibility to pollutants, it is stressed that compliance with Standard 62 doesnot ensure acceptable indoor air quality for everyone.

The Standard is under constant review but major revisions to the Standard in 1996 whichwere to include the adoption of the olf and decipol unit of measurement were abandoned. Itis understood that the Standard will be progressively updated as firm data on odouremerges.

Future Trends for Ou ts ide Air Quant i t ies and Cool ing L oadImplicat ions

There appears to be a trend in Europe that the lead of the Scandinavians is being followedof a tendency to use 100% outside and exhaust air systems. Of course in a temperateclimate this would mean that „free cooling would be provided for much of the year usingvery simple control. As heat exchanger efficiencies are being increased and simple andinexpensive methods of cooling are used then the „knee jerk reaction to minimise outside airmay cease to be so important. The indoor air quality within a building using 100% outside airand exhaust air and a well filtered system would unquestionably provide optimum indoor airquality especially if used in conjuction with displacement ventilation removing contaminantsetc. from the occupied zone.

In hot climates too, the prospect of introducing 100% outside and exhausting 100% air atpeak conditions may also be less daunting using efficient heat exchangers. When supplying100% outside air at typical climate design conditions of say 47°C dry bulb and 24°C wet bulbwith return air of say 25°C dry bulb at 55% RH with the use of a thermal wheel and anindirect evaporator the incoming dry bulb temperature could be reduced to as little as a1.0°C increase above the return air temperature. (i.e. supply air dry bulb would be 26.0C -

the figures assume 94% efficiency for the humidifier and 75% efficiency for the thermalwheel.) This figure seems perhaps surprisingly, low, particularly as in hot climates theoutdoor air load is often calculated to be the largest single cooling requirement, especially ifthere is a large outside air requirement due to high occupancies. But as may be seen thesensible outside air load is almost reduced to zero.

If the air then passes through a cooling coil supplied with low grade cooling water at 15°Cand having a sensible to total ratio of 0.38 then the off coil would be 19°C dry bulb and18.89°C wet bulb. These temperatures are ideal for displacement ventilation and it ispossible that no further cooling would be required.

In a more humid climate say at 45°C dry bulb and 31°C wet bulb a similiar process could befollowed but desiccants used to reduce the wet bulb temperature of the incoming air.

Existing Buildings - the Retrospective Addition of Cooling

Generally the simplest, least disruptive and most used method of providing cooling to anexisting building is to use mass produced mixed flow direct expansion equipment but withmuch of this equipment there is little opportunity to provide free or low energy cooling.

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An option to consider would be to provide air handling equipment with modulating dampersfor free cooling using enthalpy or temperature control as described previously. Even themost temperate climates are likely to require further cooling at high external ambienttemperatures in Summer and the addition of indirect and direct evaporative humidifiersshould therefore be considered. If further cooling is required then desiccant systems withheat recovery and solar regeneration should be considered perhaps using phase changeequipment in hot countries. Research seems to indicate that in Europe in order to minimiseregeneration costs then the use of ceiling coils providing sensible cooling and using lowgrade pond water or if that is not practical, cooling water directly from towers.

New Buildings

There are many more choices and options when designing building services as part of thedesign team for a new buildings. The thrust of this article is, in particular, directed atbuildings with high internal sensible loads.

Clearly everything should be done to mitigate the effect of the high sensible load such as

designing the height of rooms so that displacement ventilation may be used and if possibleremove internal sensible gain at source or high level and it therefore does not become partof the cooling load. This would be particularly effective in buildings such as television studiosand displacement ventilation has the advantage of being able to achieve very low noiselevels as the outlet velocity from the terminals is so low. A suitably silenced displacementsystem could achieve down to Noise Criteria (NC) 10 and this has been achieved at the wellknown Air Studio in the UK, it is not practical to achieve such a low noise level with mixedflow ventilation as higher velocities are required for the throw of the air. In reality NC 10 is anextraordinary low sound criteria for an air conditioning system and most studios even withlive microphone applications are only designed to NC20 which is still a very low soundcriteria. ('Talking book studios are an exception because of the pause between words and alower sound criteria is therefore often required). For further information on air conditioningfor studios and sound control see 'Practical Ventilation and Air Conditioning Design forStudios, Control Rooms and Auditoria' .

The use of passive cooling could be considered at an early stage such as the use ofTermoDeck or the use of heat sinks such as roof ponds and the soil.

Air handling equipment would be likely to be provided with modulating dampers for freecooling using enthalpy or temperature control, indirect and direct humidifiers and desiccantsystems with solar regeneration in hot countries and all as described under the previousheadings. With high occupancy buildings such as Auditoriums or Concert Halls it is probablethat a 100% outside air will be required in which case it is essential to use heat recovery

equipment. There appears to be little doubt that low level displacement ventilation is the wayforward with high occupancy buildings such as these and the trend appears to be thepositioning of terminals at each seat for occupant adjustment and control - this design wasused in the recently refurbished Glyndebourne Opera House in the UK.

As stated previously the design should be used to augment the bioclimate architecturaldesign of the building.

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As fundamental decisions have to be taken for the efficacious application of low energydesign for new or refurbished buildings it is clearly essential that the building servicedesigner should be consulted from the outset.

Conclusion

An intrigueing question is, just why is it that refrigeration equipment quite often as big as ahouse and often constantly energized is used to cool a medium sized office block in atemperate climate let alone a hot climate. Willis Carrier the father of modern air conditioningindustry and whose name the largest manufacturer in the world still bares, used the simpleexpedient of passing air over ice to effectively cool a print works. The answer possibly is thatis simply the way the industry evolved with the simplicity of the application of modern and atthe time universally aclaimed „friendly refrigerants in much the same way as petrol drivenengines evolved rather than steam driven engines or the aeroplane rather than the airship.

Energy in the past and still today in some countries was just not an issue. But now in theHVAC industry, it has not been financial resources that has concentrated the mind but thecriticism of the use of energy and damage to the earths environment. Whatever your point ofview this has regardless set a challenge to the modern HVAC engineer and the status quo isunlikely to be maintained simply by finding of the „holy grail' of a totally benign „drop inreplacement for the ubiquitous HCFC known as R22. In many peoples view the industry ischanging forever and if it is that a hospital in a poor hot country through simple cost effectiveand simply maintainable engineering is the benefactor then that has to be wonderful. It is nodifferent to what engineers have had to address through the ages that a change ofcircumstances has been a pressure to re-think and innovate and that surely is the excitingchallenge to our industry.

Acknowledgements

Many thanks for information from Nick Pine of Nick Pine Associates, USA, Brian A. Rock Assoc. Prof. Architectural Eng. Dept., The University of Kansas, USA and Dan Mitchell ofMunters Northwest, USA via the sci.engr.heat-vent-ac/alt.hvac newsgroups which are„hosted by the indefatigable Paul Milligan - see these newsgroups for informative, positive,lively and altruistic discussion with friendly folk in the HVAC industry all over the world.

See Nick Pines informative website for solar heating and cogeneration design and thanks tohim for directing the writers attention to the innovative work of Messrs. Novak, Wood andHawlader and their research on the unglazed collector and absorption cooling and also theworks and books of Professor Baruch Givoni.

See Paul Milligans w ebsite for all manner of free HVAC software.

References and Thanks

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Building Services in the Greenhouse Spotlight - p36 - p37, June 1998 issue of BuildingServices 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 byMark Standeven, Robert Cohen, Bill Bordass and Adrian Leaman (The Probe Team)

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

Unglazed collector/ regenerator performance for a solar assisted open cycle absorptioncooling system - by M.N.A. Hawlader, K.S. Novak and B. D. Wood of the Center for EnergySystem 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 smokingoccupancy , ASHRAE Tran. 89 2A and 2B , pp601-623

Fanger PO, Lauridsen J, Bluyssen P and Clausen G (1988) Air pollution sources in officesand 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 energyconservation in buildings and community systems programme . Annex 1X final report

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

Papers submitted at seminar on Desiccant and Solar Assisted Cooling in April 1998 andpublished 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