A Practical Guide to Free Cooling, Alternative Cooling, Night Cooling and Low Energy Systems for Air Conditioning Systems
by Mike Hardy

Contents

Prologue

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

Human comfort and low energy systems in buildings are not necessarily a contradiction in terms, 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 in temperate climates, that vapour compression equipment is providing cooling throughout the year even though the external temperature for the vast majority of time is lower than the internal building ambient temperature. Any feedback about any of the information discussed in this article, such as that by Mr Dahliwal and contained herein, would be most appreciated by 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 all of the HVAC industry, the writer apologises in advance for what may be regarded by others as obvious or oversimplification.

Introduction

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 the information technology industry etc. and also discusses the issue of optimum comfort. The thrust of the article is less concerned with rural buildings in temperate climates which even with quite high internal heat gains may only overheat for a few weeks a year but more concerned with urban buildings in temperate or hot climates. In his excellent book 'Passive and Low Energy Cooling of Buildings’ the author Professor Baruch Givoni differentiates between with what he describes as ‘tropical bioclimate architecture’ and other methods of achieving cooling in buildings.

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

An American called Jacob Perkins living in London in the U.K. patented the first closed vapour compression refrigeration system in 1884. The refrigerants used were generally flammable and/or toxic and generally not very satisfactory. The dramatic and acclaimed introduction 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, in time, revolutionise technology throughout the world.

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

International Agreement and Commitment and the Kyoto Protocol

The international agreement reached at the Rio Earth Summit in 1992 resulted in The Framework Convention on Climate Change and placed commitments on developed countries to devise policies and measures to bring down greenhouse gas emission to their 1990 levels by the year 2000 and also to provide financial assistance to developing countries to meet their general commitments.

Some countries will meet those targets but others will not. Hence, international agreement was reached that the next step had to involve a framework leading eventually to legally binding targets. The world’s industrialised nations signed the Kyoto Protocol on the 11th December 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 the agreement. The target will be achieved by cuts of 8% by Switzerland, most Central and East European states and the European Union, 7% by the US and 6% by Canada, Hungary and Japan, the countries of Poland, Russia, New Zealand and Ukraine are to stabilise their emissions, while Norway may increase emissions by up to 1%, Australia by up to 8%, and Iceland 10%.

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

Austria-10%
Belgium-7.5%
Denmark-21%
Finland0%
France0%
Germany-21%
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, nitrous oxide and methane would be measured against a baseline of 1990 emissions, whereas sulphur hexafluoride, hydrofluorocarbons and perfluorocarbons would be set against a baseline of 1995 levels. The greenhouse gas target is now expressed as the total national emissions over the period of 2008 to 2012 to facilitate calculations of averages.

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

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

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

The fourth convention on Climate Change will be held in November in Buenos Aries and information on the Kyoto Protocol may be found on the United Nations climate change homepage.

Emissions Trading and Permits

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

Similarly, it is supposed that building owners would have a set of permits which cover green house gas emissions from their buildings. The tradeable permit may perhaps only cost money at the outset; very efficient companies may not need to have permits or alternatively use them to their financial advantage by selling them to other companies. What will be unfamiliar to all is that the permit may well be a legal document in the future. Whereas it has hitherto been considered by many countries in the past, that the price of energy would have to 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 a system that does. One country for example may wish to release onto the international emissions trading market the savings from replacing the district heating schemes of entire cities.

The Kyoto meeting set out an ambitious agenda to tackle global climate change and it is intended much of the detail needed to turn it into law will be negotiated at the Buenos Aires meeting in November 1998. What is certain is that the building owners and developing countries in particular would do well to address the issue now and have nothing to lose by this but much to gain. For building service engineers throughout the world the Kyoto Protocol 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 a combination of sensible heat and moisture content) is less than the indoor air enthalpy and the cool external air is transferred to the building envelope either directly or indirectly. Surprisingly this is not taken advantage of very widely in most countries of the world and of course the efficacy of the external cool air would be increased when used in modern buildings because of improved insulation values.

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

Air conditioning systems either provide supply air using

  1. A mixture of outside air and recirculated air, or
  2. 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 these systems as a matter of course in certain countries.

Free Cooling using Mixed Outside Air and Recirculation Systems and Temperature and Enthalpy Control

Free cooling may be used with mixed outside air and recirculation systems by the use of modulating dampers. Dampers are provided on the outside air intake ductwork, exhaust air ductwork and the recirculation ductwork. 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. In this way cooling by means of refrigeration equipment is avoided altogether at certain times of year and often at night times.

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.

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

Additional Free Cooling using Dew Point Control and Direct Humidifiers

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

Consideration of Legionnaires’ Disease and Humidifiers

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

Steam injection humidifiers became popular because all bacteria were pasteurised although the benefit of sensible cooling using spray coils was lost. Chemical biocides (bacteria killing process) 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 buildings due 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 water and claimed by the manufacturers to be harmless when drunk).

Some of the new generation of water humidifiers address the problems brought about by the concern 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 stream through the saturated matrix or corrugation. Provided the air velocity through the matrix is limited to 1.75 m/s then the air velocity is sufficiently low to prevent aerosols forming and they are thought to be safe to use without regular biocide treatment. The sump is generally sufficiently cold to prevent the colonisation of Legionella and when the sump water 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 a different form of output energy) vibrating at a high frequency in water to create fine aerosols. 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 to propel water through nozzles to create a fine aerosol. Close humidity control may be achieved by these humidifiers.

Of the three, the evaporative humidifier is likely to have the lowest capital costs and may generally 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 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 dew point of air is the temperature of saturated air i.e. air at 100% relative humidity and the dew point temperature of air at less than 100% relative humidity will always be lower than the dry bulb temperature and in this way therefore the saturation of air with water vapour lowers the dry bulb temperature by means of the process know as adiabatic cooling which by definition is at constant enthalpy at 0°C feed water temperature. In reality and perhaps surprisingly the feed water temperature does not make a significant difference to the process. With feed water as high as 100°C the water is evaporated more quickly; this will cause a slight gain in enthalpy.

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

In Summer when the outdoor air enthalpy is higher than the indoor ambient enthalpy then the modulating dampers would modulate to minimum outside air and the humidifier would not 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 to the dew point of the supply air and the humidifier saturates the air to dew point. The air is then reheated if required to the supply air temperature or may not require reheating if the condition 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 would modulate to minimum position to achieve the required dew point and the air reheated after the humidification process to achieve the required supply air condition. In very cold weather a 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 air must be cooled to saturation point to achieve the required dew point and then reheated to supply air temperature. A more sophisticated control would be carried out using a Building Energy Management System (BEMS system) controlling both moisture content and enthalpy. As close humidity control is required, a direct injection adiabatic humidifier would normally be used with this form of control and installed after the heating coil, in contrast to the control strategy described previously when the humidifier would normally be installed before the heating coil.

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

When heating is required the dampers will modulate to the required room moisture content and in this way reheat is reduced.

When cooling is required the dampers will modulate to the required room enthalpy and the air then adiabatically humidified to the required room supply air temperature.

For low enthalpy and moisture content conditions in Winter the moisture content will not be in the range of the damper modulation and in this event the air will require to be preheated before humidification.

100% Outside air systems

Outside Air Heat Recovery and Indirect Evaporative Humidifiers

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. Free cooling is obtained whenever the external ambient temperature exceeds the temperature in the exhaust duct after the evaporative humidifier. This system has the advantage of sensible heat only being transferred to the air stream and therefore moisture content is not increased. When the external ambient temperature is lower than the temperature after the evaporative humidifier then free cooling would be provided from outside air and modulating dampers used to allow the exhaust air to bypass the heat recovery unit. This system is a low design risk option and although it will not eliminate refrigeration entirely will considerably reduce the size of plant required.

An example of an application in Asia is as follows and Mr Dahliwal writes:

“I had read your article on displacement ventilation and found the scheme very useful for ventilating large buildings having high sensible heat loads. I have applied the scheme for ventilating a DG house, the data is reproduced here:

  • Heat dissipation: 400kW
  • Cooling source: Indirect/direct evaporative cooling
  • Air Quantity: 40,000 CFM (18,200 litres/s)
  • Ambient DB: 41°C
  • Supply Air: 21°C
  • Room condition: 32°C

Normally in India the DG houses were ventilated by using axial flow fans and accepted norm was to accept indoor temp. exceeding 50°C because the air quantities required to achieve lower temp become uneconomic. The mixed flow ventilation often provided air at 12 ft from the ground level. This arrangement caused the work level temp to be still higher.

With a combination of Indirect/Direct evaporative cooling and displacement ventilation, we have been able to maintain the floor level temp almost 7 to 8°C below the ambient and in addition the power saving have exceeded 75% when compared with a mixed flow ventilation system capable of maintaining temp above 7°C above ambient.

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

Regards
S.DHALIWAL

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

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

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.

Solid Desiccant Systems

Entering air to the system is dehumidified and heated by a rotating desiccant wheel. The drier air then passes through a sensible heat recovery wheel which reduces the dry bulb temperature of the supply air without altering moisture content. The dry bulb temperature of the air is reduced still further by passing through an evaporative humidifier (called a direct humidifier 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 in dry bulb temperature.

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

The use of heat for the regeneration process is the highest energy requirement of the system. The elevated temperature of the return air duct removes moisture from the desiccants 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 return air may bypass the heater and desiccant wheel in order to minimise energy consumption.

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

Liquid Desiccants

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

The Genius 4000 has been developed in the USA by Albers Air Conditioning and uses liquid desiccants to dehumidify outside air or cool make up air without the need for conventional refrigerants. The liquid desiccant in this system is composed of 90% lithium bromide and 10% lithium chloride. According to Albers Air Conditioning, liquid desiccants do not deteriorate over time, unlike solid desiccants. However, sulphur contaminants in the air can combine with the desiccant to form lithium sulphate, which is not a desiccant. Ablers claim that, as there are about 80 litres in the machine, it would take a long time to reduce the desiccant to a significant level. In dirty conditions Albers states that it might prove necessary to change the desiccant as part of an annual service, although the regeneration process tends to remove most of the pollutants.

Performance figures, taken from the manufacturer’s tests at outside conditions of 28°C dry bulb and 50% relative humidity give 37 KW of gross cooling, air being supplied to the room at 11.4C. A room condition of 23°C dry bulb and 55% relative humidity was used in calculating the capacity and the cooling capacity would increase at higher temperatures and humidities. Primary energy use for the Genius is said to be typically 30% less than conventional vapour compression equipment and approximately halves the operating costs. The system would be best suited for systems with high outside air requirements and would have to be used in conjunction with other equipment for high internal sensible loads although this may not necessarily be the case if used in conjunction with a Termodeck (see later) system which does much to mitigate the effect of high internal sensible heat gains.

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. It is clear that during hot weather it would probably always be helpful for night time cooling to take place and the control strategy would be simple. The control strategy to be used for quite hot days is less obvious because the energy consumed by the running of a fan at night as against the energy saved by slab cooling may be marginal or counter-productive. In a predominantly hot climate the strategy for night cooling would be reasonably simple but in a temperate climate is less clear. On site tests carried out by the Building Service Research Association in the UK have suggested the 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 following criteria occur:

  • The indoor zone temperature is higher than the outside air temperature +2C
  • 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 periods of the building and once the criteria are not satisfied then night cooling would continue to operate for an additional two nights providing night cooling has taken place for the previous five nights.

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

When using a mechanically ventilated building for night cooling then the exhaust fan should be preferred to avoid temperature pick up from the supply fan (unless the supply fan motor is out of the air stream) and a minimum air velocity of 1.5 m/s. It may even be worth considering the installation of a night cooling bypass duct which would bypass the main plant to avoid pressure drop and minimise fan energy; in order that less energy is used then the 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 could cause condensation and this should be safe guarded against by the use of a moisture detector on the slab or other means.

It is estimated in the UK that internal temperatures can typically be held at 6-8°C below peak external summer time temperatures and in hotter climates particularly where there is a sharp contrast between Summer day temperatures and Summer night temperatures the reduction would 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 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.

With the TermoDeck systems the slab temperature is very close to the room temperature and makes it suitable for displacement ventilation as well as mixed flow ventilation (see later for comparison of displacement and mixed flow). In Summer the supply air fans at night bring in the cool air into the hollow slabs to cool the building and the warm outside air is cooled in the daytime.

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

The PROBE team conclude that of 12 recently constructed buildings in the UK this building had the highest occupant comfort scores and are also recorded the highest comfort scores recorded 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 for Sweden using TermoDeck between 30-50 kWh/msq./y, this building seems to be following the Swedish trend.

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

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

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

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 although it is the opinion of Professor Givoni that practical and movable insulation 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 bags filled with water are placed above the steel decks and above them are movable insulation panels that are moved by a motor. In Winter the water bags are exposed to the sun in the day and covered by the insulation panels at night. In Summer when cooling is required the water bags are exposed and cooled at night and insulated during the daytime. As the cooled water bags are in direct contact with the metal deck the ceiling serves as a cooling element over the entire space. However it is the opinion of Professor Givoni that the availability of a simple and trouble free system of movable insulation is still in question.

In contrast to using indirect and direct evaporative cooling in conjunction with air being introduced into a building and as previously described it is possible to cool a roof by placing a cooled pond over it. The building is then cooled by conduction across the roof which lowers indoor air and radiant temperatures without increasing the indoor water vapour content.

Professor Givoni suggests the ceiling temperature in the case of a concrete roof over a well insulated building would be about 2°C above the water temperature. It is concluded that the water temperature of a shaded pond follows approximately the average wet bulb temperature. The suggested maximum wet bulb temperature for applications of this type of evaporative 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 or similar within a building and will provide useful cooling in hot climates. For a more detailed appraisal of this subject and calculations see Nick Pines website detailed at the end of this article. An indirect heat exchanger submerged in the water could be used and in this way cold water circulated to a closed loop system perhaps connected to mass produced fan coil units or other terminals. A submersible pump in the pond would be used to circulate water within the ‘open’ loop using a stainless steel fine mesh filter say of 20 micron to keep the heat exchanger clean, a submersed non-corrosive carbon fibre heat exchanger would best be used for separating 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 at best 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 cooling source stating that in temperate climates at a depth of 2 to 3 metres the natural temperature of the soil could be enough to serve as a cooling source but in hot climates the soil temperature is usually too high. However there are simple methods to lower the soil temperature such as covering the soil with mulch at least 10cm thick and in regions with dry climates irrigating it, or raising the building off the ground and allowing water that was provided 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 passive and active cooling.

Liquid Lithium Chloride Absorption System Using Solar Energy

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

“A black shingled roof was used as a collector/regenerator for the evaporation of water to obtain a strong solution of lithium chloride absorbent. In the house, water (the refrigerant) is sprayed into an evaporator and this was evacuated to a pressure of about 5mm of mercury where the water immediately flashes into vapour. Cold water pumped from the bottom of the evaporator then flowed through a fan coil unit which blew cold air into the area requiring cooling. (Fan coil units are usually used with chilled water in the HVAC industry). Water vapour from the evaporator flows over the absorber where it is absorbed by the concentrated absorbent (lithium chloride). The continuous absorption of water vapour maintained 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 the bottom of the absorber to be pumped over the roof for concentration.

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

The Authors demonstrated a regeneration efficiency of between 38 and 67% which corresponds 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 for very little energy costs.

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

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

Practical Choices for Free Cooling and Low Energy Systems

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

Displacement vs Mixed Flow

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

For a more detailed appraisal of the type of air distribution see the article titled 'Consideration of Displacement Ventilation vs Mixed Flow Ventilation for Building Owner/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 loads imposed on cooling plant for an air conditioned building and so is here considered in some detail.

There is no question that both the quantity and quality of outside air are crucial to the comfort of occupants in air conditioned buildings and the efficacy of the air conditioning of an entire building when related to comfort may be negated by the introduction of too little or poor quality outside air. Of course when designing low energy buildings there is a pressure to 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 since the inception of modern air conditioning and is as important now as it has ever been particularly as modern buildings are becoming more ‘tight’ and outdoor air quality, in particular in urban areas, poorer in quality.

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

Clearly if 100% of the air circulated through a building is from outside this would impose the maximum cooling load on cooling plant when the external ambient is hot and humid. Before useful cooling of the building is to be achieved the cooling plant has to reduce both the external dry bulb and moisture content to room conditions before the cooling plant provides useful indoor cooling and this will reflect both on the cooling plant size and the running costs. 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 during peak conditions and this would reflect in smaller cooling plant being required and lower running costs.

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 to appreciate 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 1936 provided the benchmark throughout most of the world for many years and this was based on work in American schools and determined the quantity of outside air needed to provide a satisfactory reduction of odours depending both on the number of people present and personal hygiene. This research determined that for the same volume of air that odours dispersed more rapidly when each occupied a greater volume of the room air. The guidelines for many years were therefore directly related to the quantity of outside air per occupant 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 person adopted by the UK and North America respectively were shown to be inadequate by laboratory based work of Leaderer, B P and Cain ‘Air quality in buildings during smoking and non-smoking occupancy'. The work demonstrated that the ventilation rates were inadequate when considering odour levels perceived by people when first entering an already occupied space and that the density of occupancy had no bearing on fresh air requirements. Minimum air quantities were then increased in both countries to 8 litres/second in the UK and to 7.5 litres/sec per person in North America.

These figures were recommended particularly because of concern over the incidence of ventilation related problems in air condition buildings and based on the work of the aforementioned Leaderer et al. and also the work of P.O Fanger ‘Body odour and carbon dioxide, minimum ventilation rates’.

The work of P.O. Fanger, J. Lauridsen, P. Bluyssen and G Clausen 'Air pollution sources in offices and assembly halls, quantified by the Olf unit' addressed the question of ventilation rates and the relationship with a unit of odour intensity called the ‘Olf'. In this important work a large number of people were subjected to odour levels in a variety of naturally and mechanically ventilated buildings and air conditioned spaces. The research found that for every 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 the extent of soft furnishings and acoustically absorbent finishes.

The Units of Odour Intensity

The contribution to the industry of the work of Professor Fanger and his team in Denmark has been immense. Not only did Professor Fanger develop his well known ‘comfort equation', which was developed empirically and contains the many variables known to make up the comfort of people, but as well as other work also developed the concept of the units of odour - the olf and the decipol. The nature of indoor pollution is very difficult to assess because of the many indoor chemicals involved. Even for pollutants that may be detected because of their odour or irritation effect there is the issue of who should be the judge of what is acceptable. Within buildings pollutants come from many sources such as the building and furnishing materials and chemicals such as correction fluid or from equipment such as photocopiers.

Hundreds of volatile organic compounds have been identified in indoor air and most seem to be much lower than for occupational standards for industrial workers although little appears to be known of the effects of long term exposure. The sheer complexity of measuring indoor pollution raises the question of people themselves being used as the test instrument. The use of questionnaires given to building occupants for identifying and assessing problems with indoor environment has been established for some time. The approach of using a panel of assessors to judge air quality has been advocated by Professor Fanger and has given rise to the empirical units of the olf and the decipol. One olf is defined as the air pollution produced by one ‘standard' person (a standard person is also defined) and a decipol is defined as the perceived air pollution level in a space in which there is a source strength of one olf and which is ventilated at 10 litres/second with unpolluted air. The proposed European 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 the making has failed to be adopted by Europe as it was not endorsed by 71% of the 17 participating countries. The proposed standard is however a useful guide and has much valuable information although the assessment of outside air quantities based on the use of the olf and decipol is the subject of the greatest controversy.

Standards which deal explicitly with air quality for the general populace in Europe and the UK 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 for these).
  • UK Her Majesty’s Inspectorate of Pollution Technical Guidance Note 'Guidelines on discharge stack heights for polluting emissions’
  • 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 in 1989 and introduced the calculation of a dilution rate based on limiting concentrations for non-industrial exposure to contaminants. The Standard uses ambient air quality standards produced by the US Environmental Protection Agency as a basis for their definition of fresh air. This specifies exposure limits for common pollutants and if examination of records shows that these are exceeded then it is recommended that suitable air cleaning apparatus be installed. It also introduced a technique for determining the lead and lag time depending on the room volume per person and their fresh air allowance.

Since there is potential for a wide diversity in pollutant sources, contaminant types and population susceptibility to pollutants, it is stressed that compliance with Standard 62 does not ensure acceptable indoor air quality for everyone.

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

There appears to be a trend in Europe that the lead of the Scandinavians is being followed of a tendency to use 100% outside and exhaust air systems. Of course in a temperate climate this would mean that ‘free cooling’ would be provided for much of the year using very simple control. As heat exchanger efficiencies are being increased and simple and inexpensive methods of cooling are used then the ‘knee jerk’ reaction to minimise outside air may cease to be so important. The indoor air quality within a building using 100% outside air and exhaust air and a well filtered system would unquestionably provide optimum indoor air quality especially if used in conjunction with displacement ventilation removing contaminants etc. from the occupied zone.

In hot climates too, the prospect of introducing 100% outside and exhausting 100% air at peak conditions may also be less daunting using efficient heat exchangers. When supplying 100% outside air at typical climate design conditions of say 47°C dry bulb and 24°C wet bulb with return air of say 25°C dry bulb at 55% RH with the use of a thermal wheel and an indirect evaporator the incoming dry bulb temperature could be reduced to as little as a 1.0°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 thermal wheel.) This figure seems perhaps surprisingly, low, particularly as in hot climates the outdoor air load is often calculated to be the largest single cooling requirement, especially if there is a large outside air requirement due to high occupancies. But as may be seen the sensible 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°C and having a sensible to total ratio of 0.38 then the off coil would be 19°C dry bulb and 18.89°C wet bulb. These temperatures are ideal for displacement ventilation and it is possible that no further cooling would be required.

In a more humid climate say at 45°C dry bulb and 31°C wet bulb a similar process could be followed 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 an existing building is to use mass produced mixed flow direct expansion equipment but with much of this equipment there is little opportunity to provide free or low energy cooling.

An option to consider would be to provide air handling equipment with modulating dampers for free cooling using enthalpy or temperature control as described previously. Even the most temperate climates are likely to require further cooling at high external ambient temperatures in Summer and the addition of indirect and direct evaporative humidifiers should therefore be considered. If further cooling is required then desiccant systems with heat recovery and solar regeneration should be considered perhaps using phase change equipment in hot countries. Research seems to indicate that in Europe in order to minimise regeneration costs then the use of ceiling coils providing sensible cooling and using low grade 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 the design team for a new buildings. The thrust of this article is, in particular, directed at buildings 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 possible remove internal sensible gain at source or high level and it therefore does not become part of the cooling load. This would be particularly effective in buildings such as television studios and displacement ventilation has the advantage of being able to achieve very low noise levels as the outlet velocity from the terminals is so low. A suitably silenced displacement system could achieve down to Noise Criteria (NC) 10 and this has been achieved at the well known Air Studio in the UK, it is not practical to achieve such a low noise level with mixed flow ventilation as higher velocities are required for the throw of the air. In reality NC 10 is an extraordinary low sound criteria for an air conditioning system and most studios even with live microphone applications are only designed to NC20 which is still a very low sound criteria. ('Talking book’ studios are an exception because of the pause between words and a lower sound criteria is therefore often required). For further information on air conditioning for studios and sound control see 'Practical Ventilation and Air Conditioning Design for Studios, Control Rooms and Auditoria'.

The use of passive cooling could be considered at an early stage such as the use of TermoDeck 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 free cooling using enthalpy or temperature control, indirect and direct humidifiers and desiccant systems with solar regeneration in hot countries and all as described under the previous headings. With high occupancy buildings such as Auditoriums or Concert Halls it is probable that 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 way forward with high occupancy buildings such as these and the trend appears to be the positioning of terminals at each seat for occupant adjustment and control - this design was used in the recently refurbished Glyndebourne Opera House in the UK.

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

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

Conclusion

An intriguing question is, just why is it that refrigeration equipment quite often as big as a house and often constantly energised is used to cool a medium sized office block in a temperate climate let alone a hot climate. Willis Carrier the father of modern air conditioning industry and whose name the largest manufacturer in the world still bares, used the simple expedient of passing air over ice to effectively cool a print works. The answer possibly is that is simply the way the industry evolved with the simplicity of the application of modern and at the time universally acclaimed ‘friendly’ refrigerants in much the same way as petrol driven engines 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 the HVAC industry, it has not been financial resources that has concentrated the mind but the criticism of the use of energy and damage to the earths environment. Whatever your point of view this has regardless set a challenge to the modern HVAC engineer and the status quo is unlikely to be maintained simply by finding of the ‘holy grail' of a totally benign ‘drop in’ replacement for the ubiquitous HCFC known as R22. In many peoples view the industry is changing forever and if it is that a hospital in a poor hot country through simple cost effective and simply maintainable engineering is the benefactor then that has to be wonderful. It is no different to what engineers have had to address through the ages that a change of circumstances has been a pressure to re-think and innovate and that surely is the exciting challenge 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 of Munters 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 to him for directing the writers attention to the innovative work of Messrs. Novak, Wood and Hawlader and their research on the unglazed collector and absorption cooling and also the works and books of Professor Baruch Givoni.

See Paul Milligans’ website for all manner of free HVAC software.

References and Thanks

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

Night Cooling Control Strategies, BSRIAMarch 1996 by Messrs Martin and Fletcher

Probe Elizabeth Fry Building - p37 - p42, April 1998 issue of Building Services Journalby 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