New control technology steps up chilled-water efficiency
Fig. 1: Armstrong’s IPC 11550 integrated control system enables a chilled-water system to operate as a single integrated system.
Peter Thomsen maps the future for the energy efficiency of chilled-water plant and demonstrates how energy savings of 60% can be made over today’s best practices. Demand for highly-efficient chilled-water air-conditioning systems in the UK continues to grow, driven by the enduring popularity of architectural designs predominated by glass and by changes in planetary weather conditions. The question is how to come up with designs to satisfy the following requirements.
• Flexibility for future environmental changes.
• Allow for the integration of renewables.
• Provide for changes in building use.
• As efficient as possible for all scenarios with electrical use or traditional natural gas sources. The answer lies in several key new technological developments that elevate energy efficiency beyond today’s best practices. The challenge facing system designers has always been how to obtain maximum energy efficiency from three separate, independent PID feedback loops. With previous best practice a COP between 3.2 and 2.2 could generally be achieved as an annual average for water-cooled chilled-water plant with this approach. By utilising new control technology, however, which treats the system as a single integrated system, rather than three independent PID loops, the COP can be greatly improved. Ultra-efficient alternative technology can improve this annual average COP to greater than 7.0. Such a high COP compares extremely well with best-in-class performance offered by other modern technologies, offering an energy reduction of more than 30%. Compared to standard practice the level of energy saving is even higher — anywhere up to 60% energy reduction dependent on equipment selection. Using a price of 11p per kWh as a base rate for a 1000 kW chilled-water plant, that COP of 7.0 would lead to an annual saving of 158 MWh hours of electricity at a cost-saving of £17 380 compared with a COP of 3.2 for a plant operating 2117 h a year (12 h day) with an average capacity of 44%. There are three key factors involved in making these savings possible. First, the chilled-water plant must be an all-variable-speed system. Secondly, a fully integrated method of plant control is required capable of trading off energy savings and capacity between the individual components of the system to optimise efficiency of the entire system at all times. Last, the system must operate in ‘auto’ mode.
Fig. 2: Enabling a chiller to operate further down its performance curve at part load enables much higher operating efficiency to be achieved.
Hartman Loop Control is demand based, with sequencing following a natural curve operating under equal marginal performance principles. Armstrong achieves these aims with the incorporation of advanced control methodologies such as Hartman Loop. These achieve optimal power relationships across each system. Equipment loading in one device is traded off to pick up more load on another, thereby achieving the same net cooling capacity for a lower power input. Considering the ‘natural curve’ of a variable-speed centrifugal chiller (i.e. the locus of load points at various condenser and evaporator conditions at which the highest operating efficiency is achieved) shows how optimum performance is achieved. Sequencing on-line chillers to operate as close as possible to their natural curve delivers the most energy-efficient operation. In the same way, slowing (instead of shedding) cooling towers at low loads improves performance. At lower wet-bulb temperatures, the approach of cooling towers rises due to reduced moisture capacity of cooler air. When fans and pumps are slowed at lower loads, greater air and water volumes pass over larger surface areas per unit of energy expended. This improves part-load approach temperatures and provides the opportunity to operate on the lower areas of the curve for higher chiller efficiency. For part-load applications, we therefore move from the top right of the curve (where traditional control would have chillers operating) to the far more energy efficient lower part of the curve (Fig. 1). Armstrong’s IPC 11550 integrated control system (Fig. 2) is a fully-fabricated and pre-wired assembly and is available for a variety of system configurations and plant sizes to facilitate these energy-efficiency opportunities. Factory integrated plant solutions take this one step further by making energy savings a reality without the same level of system design input and on-site assembly. A key benefit is that lower carbon footprint is guaranteed at no extra upfront cost. Armstrong’s range of integrated plant solutions includes the IPP ultra-efficient chilled-water integrated plant package (Fig. 3). It is made off-site as a fully integrated operating plant offering, built-in as standard, energy-efficiency levels in advance of today’s best practice by harnessing advanced control methodologies such as Hartman Loop. New oil-free compressors incorporating magnetic bearings have less friction loss and far higher reliability than conventional models. As bearings can be digitally controlled, the energy consumption of the compressor, expansion valves and water chiller control can all be managed within a fully integrated approach. This high degree of compressor monitoring (of temperatures, pressures, motor current, motor power, shaft speed etc) can significantly improve overall efficiency of the chiller system, whilst the technical advantages of magnetic bearings over oil-based designs can reduce operating costs by around 42% over a 10-year life cycle with no structure-borne sound or vibration. As we move into the next era of high-efficiency air-conditioning, conventional technologies such as PID methodologies and traditional oil-based compressors will no longer meet the needs of an environmentally aware HVAC industry. The technologies that take their place, such as digital control, fully integrated plant solutions and magnetic bearing compressors, offer the ability not just to enhance energy efficiency, but to easily surpass the levels we currently consider to be acceptable. Peter Thomsen is marketing manager for strategic projects with Armstrong Pumps Inc.
Fig. 3: Armstrong’s range of integrated plant solutions include the IPP ultra-efficient chilled-water package.
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