Trends in air conditioning

Projections for future use of building air conditioning show accelerating growth in the proportion of commercial floor area that is air conditioned, with corresponding growth in electricity consumption and power station carbon dioxide emissions.

The demand for air conditioning in UK buildings is growing rapidly in response to more intensive building use, higher occupant comfort demands, business and market pressures and the expectation of a warmer climate. Many new buildings are also being built with much higher levels of air tightness and thermal insulation, which when combined with more intensive occupancy levels results in many buildings being in cooling mode for much of the year.

A recent BRE paper¹ on projections for future use of building air conditioning showed accelerating growth with around 40% of commercial floor space expected to be air conditioned by 2020, compared to only 10% at the end of 1994. A major factor in the acceleration of the growth rate is that once a certain proportion of floor space is air conditioned then this tends to set the rental value and non air conditioned space generally has to be let at a discount. Once this point has been reached market pressures often dictate that new buildings are air conditioned.

Exploitation
Although we are seeing strong growth in the demand for air conditioning there are also increasing opportunities to exploit free cooling and for adopting simpler and more cost- effective technology. The incentives are not just environmental or in terms of reducing energy consumption and carbon dioxide emissions.

Over the last ten years a wide range of building use studies by BRE, the PROBE team and many others has shown that the performance of buildings with full air conditioning depends critically on good design, commissioning, maintenance and management.

In practice this is all too often not achieved and many buildings with complex services systems are unwieldy to operate and often under perform in terms of occupant satisfaction and operating costs. Many of the best performing buildings in PROBE and BRE studies are buildings with simpler passive or free cooling systems. Such buildings are also simpler to operate and maintain, an often overlooked factor when comparing the relative operating costs of alternative systems.

CIBSE² guidance makes it clear that designers should always carefully assess the need for cooling and where possible specify natural ventilation cooling system. The phase out of cfcs and hcfcs, and the fact that the hfc alternatives are unlikely to be acceptable in the long-term is yet another reason why there is so much interest in greener forms of air conditioning.

A recent Partners in Innovation research project involving a group of researchers, designers and manufacturers³ has investigated the opportunities that high performance building envelope technology has on heating and cooling system design.

A combination of numerical modelling studies and laboratory mock-up testing showed that in buildings designed to these higher specifications (not dissimilar to the proposed new Part L requirements) a separate perimeter heating system is no longer required. Simple displacement ventilation and chilled panel systems can be adequate for cooling perimeter zones close to areas of glazing protected by external shading.

High performance glazing systems reduce cold downdraught in winter such that heating is only required prior to occupancy. Preheat requirements are so low that chilled ceiling systems can double up as radiant heaters when fed with warm water from a change-over supply system. These findings are confirmed by BRE's own experience of testing perimeter chilled ceiling and displacement ventilation systems.

Chilled ceilings and displacement ventilation
Chilled ceilings, including chilled panels and chilled beams, and displacement ventilation are gaining in popularity. Although they offer many advantages over older types of ventilation and cooling system their main advantage in the context of free cooling is that they operate at much higher temperatures.

Chilled ceilings typically operate with chilled water at between 14°C and 17°C and displacement ventilation systems typically require supply air at around 19°C. These temperatures create an opportunity to employ free cooling systems for significant proportions of the year, particularly in buildings that have all year round cooling requirements.

An often quoted advantage of chilled ceiling and displacement ventilation systems is their simplicity and ease of control. In BRE's experience this does not necessarily extend to the design of systems for perimeter zones subject to solar gains. Because displacement ventilation and chilled ceiling cooling systems are essentially passive systems, driven by air buoyancy effects, they are particularly sensitive to location and proximity to other physical objects that may influence airflows.

In particular the design and geometry of perimeter ceilings, bulkheads, windows, window blinds and ceiling tiles can have large effects on the cooling performance and thermal comfort in the occupied space. It is very difficult to predict these effects so designers often commission environmental chamber based physical mock-up tests.

In recent tests carried out by BRE nine out of ten proposed systems required design modifications following initial tests. The benefit of this kind of testing is that the high cost and disruption of site remedial works and recommissioning are avoided.

Free cooling
High performance building envelopes coupled with displacement ventilation and chilled ceiling systems are creating a wide range of opportunities for free cooling.

Common forms of free cooling for chilled water systems include dry air coolers, air handling unit cooling coils, evaporative cooling towers and thermosyphon chillers. Thermosyphon chillers are often used in conjunction with evaporative condensers for efficient heat rejection. Lake, river and ground water is also used for free cooling. Rising ground water levels is becoming a major problem in many UK cities and so ground water cooling could have a double benefit if the water is rejected to surface drains or a river. This is already done at Portcullis House, the new Parliament building in Westminster. Barclaycard's Freshfields building near Northampton uses a large man made lake to cool chilled water serving the chilled beam system. This is so successful that the ammonia chillers are hardly ever used.

Simple displacement ventilation and a chilled panel system can be adequate for cooling

Of course free cooling is not actually free because it usually requires the expenditure of energy for fans or pumps. For example, systems that employ night time cooling to store 'coolness' in the building fabric, perhaps using the Thermodeck or Airdeck principles, require electric fans for air movement. Ground water cooling requires significant pumping power, and ambient air cooling using cooling towers or dry air coolers require pumps as well as fans. Thermosyphon chillers require fans in the evaporative condensers.

A supplier of dry air coolers⁴ recently quoted a fan energy consumption of 0·2 kW per 1 kW of cooling at 13°C outside air temperature to produce water at 16°C, decreasing to around 0·05 kW per 1 kW when the air temperature is below 10°C. This equates to coefficient of performances of ten and 20 respectively.

The design of real systems is always influenced by economic constraints and the designer must normally trade off potential energy savings against capital cost. Often there are also practical engineering constraints, such as the available roof top space and load capability that sometimes limits the size and number of dry air coolers.

BRE recently monitored energy consumption at a building which was specifically designed to exploit free cooling from a dry air cooler. The building was of recent construction and houses a call centre with operation between 0700h and 2400h as well as general offices. The tight and highly insulated fabric means that the building is in cooling mode for much of the year. Air conditioning is by active chilled beams with 100% fresh air ventilation by an ahu with heat recovery.

The specified control is that in cold weather chilled water is cooled using the ahu cooling coils which provide pre-heating of the air. When the ambient air temperature rises and the preheat coil is not needed, or the internal load exceeds the capability of the ahu cooling coil, a dry air cooler is used to reject heat. When the ambient temperature exceeds 15°C a chiller takes over cooling.

The results of BRE's monitoring are shown in Figure 1 which shows total air consumption against ambient air temperature. Clearly there is a problem with the switch over between the chiller and dry air cooler, but also it shows that the savings from the dry air cooler, for this particular installation, are not as great as one might expect. Unfortunately in this case it was not in BRE's remit to investigate possible causes or suggest corrective measures.

However, it was learnt that the contract maintenance has been recently handed over to a new company and it is quite possible that they do not understand the intended control strategy of the system. If a lesson can be learnt from this example it is that energy monitoring (and installation of appropriate sub-metering), and a clear understanding of the designer's original intentions, are necessary to maintain a buildings' state of tune.

Current research
BRE is currently involved in research aimed at developing displacement ventilation and chilled ceiling technologies to extend their applicability and increase opportunities for free cooling.

One project, which is sponsored by the DETR's Partners in Innovation programme, is investigating how to improve displacement ventilation technology so that it can meet higher cooling loads than is possible with existing products.

The cooling capacity of current displacement ventilation technology is limited, which restricts its use to buildings with relatively low heat gains. This limitation is due largely to the moderate air delivery temperature and low supply velocity, which in most other respects is an advantage because it lessens the risk of cold draught discomfort and reduces the operating time of refrigeration plant.

As a consequence many designers consider the cooling limit of displacement ventilation to be around 20 W/m² to 25 W/m², unless it is used with another cooling system such as chilled ceilings.

The advantages of using displacement ventilation alone is that it results in a very simple system that only requires a supply of fresh air, at a temperature that is high enough to make full fresh air cooling practical for much of the year.

Another new research project aims to investigate the complex interactions between perimeter building structural elements (for example windows and suspended ceiling components) with the performance of perimeter chilled beams. The work will be carried out using a combination of physical and numerical modelling techniques.

The aim is to produce detailed information on the relationships between the effect of the size and geometry of building's structural elements, including glazing, on the performance of perimeter chilled beams, and a design tool for designers to predict reliably the performance of perimeter chilled beams, reducing the design effort and risk of design failure.

Conclusions
The growing demand for air conditioning is a threat to the UK's greenhouse gas reduction targets. However, a great many opportunities exist to reduce the energy consumption of air conditioning through appropriate use of free cooling technology.

Growing use of displacement ventilation and chilled ceiling technology is also creating additional opportunities for free cooling, although the design of these systems for perimeter cooling needs care. Laboratory physical mock-up testing can help to greatly reduce the design risks.