Making use of free cooling

Traditional comfort air conditioning technology has been modified to control the operating environment in many hundreds of Internet and telecoms installations throughout Europe. More companies are taking advantage of free cooling to reduce their costs.

Historically, air conditioning was installed to provide acceptable conditions in occupied spaces where cooling requirements vary according to the time of day and the season.

As a result, the opportunities for free cooling were limited because the buildings were generally un-occupied throughout the night when external ambient temperatures offer maximum free cooling capability.

There were also significant seasonal variations in external ambients that would have offered additional savings at times of the year when the outside temperature was below the inside design temperature (see figure 1 and table 2). The story is different in un-occupied spaces which have a fairly constant cooling requirement – a situation which is commonly encountered in IT rich environments such as data centres, computer rooms and internet co-location centres.

A typical office building that is occupied throughout the day will have a mechanical cooling requirement whenever the internal temperature reaches the design set point, but when external daytime ambients are relatively high, the opportunities for free cooling are generally very limited especially in temperate climates.

However, when the cooling load shows little fluctuation – for example in large co-location centres – the design engineer can make maximum use of free cooling opportunities by taking advantage of the fluctuating external temperature.

Throughout the day, especially in summer, there will almost certainly be some requirement for mechanical cooling to maintain acceptable operating conditions within the building. But during the night, early morning and late evening there is generally sufficient difference between the outdoor and indoor temperatures to take advantage of free cooling.

In many regions of the UK, free cooling can contribute to the cooling requirement for as much as 62% of the year. A well designed system will therefore only depend on mechanical cooling alone for 38% of the year.

Chilled water in air conditioning
Air conditioning is provided by various types of plant, the most common being direct expansion (dx) systems which consist of an indoor unit connected via refrigerant pipework to an outdoor condensing unit. Refrigerant flows between the two units to bring about the heat transfer that is required to lower the indoor temperature. Split system comfort units are commonly configured along these lines. The systems are generally convenient to install, but have the disadvantage of relatively large volumes of refrigerant circulating within the building.

A chilled water system generally requires reduced refrigerant volume which is contained within the chiller unit usually positioned outside the building away from the occupants. Pipework specifications are also much less demanding than those relating to dx split systems. Taking a data centre as an example, with an almost constant year round cooling load, for an internal design temperature of 23°C, the chiller would typically be configured to supply chilled water at 7°C with return water at 12°C. Because the load is constant, unlike an office building, whenever the ambient falls below the return water temperature there is an opportunity to use free cooling. The lower the external ambient, the greater free cooling opportunity.

Since running costs are the major proportion of the life-time costs of a chiller, and far outweigh the capital cost of the system, and as energy costs rise, due to the effects of initiatives like the Climate Change Levy, end users are increasingly specifying the most energy efficient plant available.

The largest power draw on any dx cooling system is the compressor. Even slight reductions in compressor running time, operating capacity or operating conditions have significant effects on the annual power consumption of the unit.

The free cooling chiller will either reduce compressor run time, or when the compressor is required it will run in a part load condition, by making the best use of the varying outdoor ambient temperatures, ultimately reducing the annual power consumption.

Configuration
To achieve energy and cost savings, a free cooling water coil is fitted adjacent to the dx cooling coil. The chiller monitors external ambient temperature, and once this falls below the return water temperature, free cooling coils are brought into action to reduce the mechanical cooling requirement. The design allows a reduced value of dx cooling to support the free cooling, if free cooling cannot satisfy the requirement. In this situation the chiller is working in both mechanical and free cooling mode.

As the ambient temperature falls even further, the compressors are switched off and the cooling requirement is completely satisfied by free cooling, using the pump and condenser fans alone.

A conventional air-cooled chiller will generally be sized to a nominal duty at the maximum ambient temperature at which it is likely to operate, say 30°C in the UK. These conditions will hold up well in the summer when the chiller is operating at its design ambient.

However, at cooler times of the year, this type of chiller will still need to run its compressors if cooling is required. Energy savings can be made by reducing fan speeds or staging the compressors, but energy will still be wasted if the return water temperature is above that of the external ambient.

With the free cooling chiller the scenario is different. The free cooling chiller senses the differential between the return water temperature and the external ambient, and uses this to maximum advantage by reducing compressor running time.

Running costs
Table 1 compares the cooling requirements and running costs of a dx chiller and a free cooling chiller running 365 days per year, using power at 5.5p per kWh, situated in London.

With ever increasing energy costs, and the continued growth of the internet and e-commerce, the free cooling chiller has a role to play in the reduction of running costs and protection of the environment.

This is particularly true in situations where there is a year-round demand for cooling such as IT environments, internet and telecoms installations. These are some of the areas which can benefit most from the savings that are offered by free cooling technology.

The benefits of free cooling obviously vary according to geographical situation, industry type and local operating conditions, for example north versus south, continuous processes versus intermittent processes and city centre versus non-urban area.