Free cooling in a mild climate

Free cooling in a mild climate — Brian Phillips.
Brian Phillips considers whether commonly held concerns about free-cooling systems in the UK have any validity.Free cooling is a familiar concept to most building specifiers and managers in the UK, but some are concerned that the British climate does not lend itself to the efficient use of this technology. However, the benefits of free- or air-cooling need not be left only to building operators in colder climates like Russia and Scandinavia. Modern techniques mean that payback times in the UK can be very short, and free-cooling provides a real solution to the problem of balancing comfort levels with energy efficiency. Free-cooling air-conditioning systems take advantage of the temperature difference between the outside air and the return water from the building to cool that water and return it at a lower temperature for chilling purposes. There are three popular misconceptions concerning free cooling systems and their application.
• The first misconception is that the free-cooling circuit has to be either on or off. • The second is that free-cooling is only economically viable with high-temperature chilled water and 24/7 operation.
• The third is that glycol is required in the chilled-water circuit. Traditionally, such systems were relatively simple, with the free-cooling circuit being either on or off, depending on the relative temperatures of the air and water. However, modern systems, such as the Aermec NRA/NSB range, can operate in a third mode. Using intelligent controls to continuously monitor the critical temperatures, the air-conditioning system can operate using only compressors, only free cooling, or a combination of the two. This third mode uses electronic expansion valves and sectionalised condenser coils to make best use of the free cooling available at smaller temperature differentials. Such a system leads to greater energy savings and, hence, much shorter payback periods than classic air-cooling equipment. As the ambient external temperature falls below that of the return water, the free-cooling coil is gradually introduced into the chilled-water circuit, thereby reducing the requirement for cooling by the compressors. With continuing temperature falls, the compressor is used less and less until eventually the free-cooling coil is left to handle the entire cooling load. Crucially in this scenario, the free cooling engages at a much higher ambient temperature than with traditional chillers. The reduced dependence on the compressors leads to significant energy savings. These savings vary according to internal building loads and the ambient conditions, but typically range from 20 to 60%. To explore the facts behind the second point (the belief that free-cooling is only economically viable with high-temperature chilled water and 24/7 operation), we have calculated the savings which could be made for a typical building in London. With a relatively mild micro-climate, London rarely experiences extremely low temperatures. But even in the middle of the city, free-cooling can be enormously beneficial. Let us consider a typical building that requires cooling all year round — one with a dedicated computer room, perhaps. With modern insulation standards and the extensive use of IT equipment, most buildings require near-constant chilling, whatever their principal use. If we assume that the computer room requires fixed cooling capacity of 132 kW with a supply water temperature of 7°C, we can calculate the energy savings which a modern free-cooling system would provide.
Fig. 1: Monthly energy consumption, based on a computer room installation in London, comparing the electrical consumption of a standard and free-cooling NRA chiller unit. It assumes that 8395 hours are worked each year. The standard chiller uses 259 167 kWh a year, costing £12 958 and achieving an average EER of 4.29. The chiller with free cooling using 170 823 kWh in a year, costing £8541 and achieving an average EER of 12.62.
Comparing the performance of a standard Aermec NRA 650 unit to a Aermec NRA 650 with free-cooling (Fig. 1), we see that the free-cooling system would save 88 344 kWh per year. With today’s seemingly ever-soaring energy costs, this energy saving equates to a payback period of less than seven months. Taking the same building load of 132 kW, but basing the free-cooling analysis on a Monday to Friday 12 hour per day operation, the payback period extends to two years — still a very attractive financial proposition. The third misconception concerns the use of glycol. The free-cooling water coil has to be protected against freezing in low ambient conditions, which traditionally means glycol. However, there are well documented objections to circulating glycol around a building. The Aermec ‘glycol free’ system, with its internal circuit comprising pump and intermediate plate heat exchanger, overcomes this particular problem and allows water to be circulated. An example of a modern city building that has found free-cooling to bring significant benefits is the recent redevelopment of 55 Baker Street by London & Regional Properties. This developer was committed to an environmentally friendly and energy-efficient approach from the outset, and the Aermec free cooling system has contributed to the award of BREEAM ‘Excellent’ status for this project. The development required a robust air-conditioning system which could provide cooling throughout a 100 000 m2, high-occupancy environment. It had to fit into the restricted available space and deliver superior efficiency levels within a low-noise environment. Aermec NSB Stepless air-cooled chillers with screw compressors were chosen as the best solution. They incorporate multiple high-efficiency scroll compressors which are more efficient at part load than at full load — the ‘third mode’ so necessary in the UK climate. Chillers with 16 compressors were installed using refrigerant R134a with infinitely variable capacity control from 10 to 100%. Together, these chillers provide over 9 MW of cooling at 32°C ambient temperature, with the potential of 4 MW of glycol-free free cooling. The free-cooling chillers deliver energy efficiency ratios up to 15 times greater than traditional compressor-only chillers. For maximum efficiency, the chillers are controlled by an Aermec multichiller sequence and load controller, which controls water temperature and regulates the sequencing and rotation of chillers and compressors according to demand. This high-profile development demonstrates that modern free-cooling systems should not be disregarded because of concerns about the mild British climate. Rather, they have the potential to make a significant contribution to increased energy efficiency ratings and lower energy bills for all types of commercial buildings across the UK. Brian Phillips is managing director of Aermec UK Ltd [formerly Aiax (AC) Ltd], UK distributor for Aermec Air Conditioning products.
Free-cooling chillers at 55 Baker Street in London are achieving EERs up to 15 times greater than traditional compressor-only chillers.
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