Exploiting relational control technology
Some approaches to control are much more effective at maximising energy efficiency than others. Andrew Harrop of Armstrong Fluid Technology describes how the EER of chiller plant can be doubled or, even, trebled — complete with a case study.
Building management systems (BMSs) can provide access to previously unparalleled amounts of information concerning the operations of HVAC equipment. Similarly, advances in variable-speed operation of equipment enable engineers to make the leap from capacity-based to demand-based operation. These provide myriad opportunities for saving energy, fuel costs and carbon emissions.
These capabilities, however, are too rarely utilised to the full. This is because information in isolation is useless without the correct control philosophy. For optimal carbon and energy efficiency to be achieved, advances such as BMS and variable-speed operation must be accompanied by an holistic energy-saving control strategy.
For example, the conventional approach in chilled-water systems is for the system to be managed as three independent control loops with PID feedback control. Variable-speed devices within these sub-systems are managed via capacity-based sequencing approaches which use on/off cycling to meet changes in demand. PID control (though well-established) is insufficiently flexible to continuously adapt to varying loads encountered in HVAC applications. This binary and anachronistic approach results in disappointing annual average EERs of between 3.2 and 2.2.
An innovative solution, which outperforms the older, tripartite feedback control methods, employs relational control technology such as Hartman Loop. In contrast to traditional systems, Hartman Loop controls variable-speed devices ‘holistically’, enabling them to maintain the optimal relationship between flow and pressure across varying load conditions. This relational control strategy ensures that individual variable-speed devices such as fans, pumps and chillers operate as closely as possible to their natural curves.
Another benefit of this approach is that optimal power relationships are achieved across each system (chiller, condenser pump and tower fan), with equipment loading in one device traded off to pick up more load on another, thereby achieving the same net cooling output for a lower power input.
Slowing (instead of shedding) towers at low loads improves performance as, 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. The result is a system EER of greater than 7.0 compared to previous averages of between 3.2 and 2.2.
Whilst Hartman Loop and the concept of relational control have been around for some time, it is only in recent years that system designers have been able to source ‘of- the-shelf’ solutions incorporating relational control. The arrival of these products has made it possible to exploit these new technologies without expensive and untried bespoke solutions.
System designers can now take three different routes.
When installing new chilled-water systems, an offsite-manufactured, fully-integrated chilled-water package such as the Armstrong IPP-CHW can be specified from the outset, which incorporates relational control technology.
Alternatively a chilled water integrated plant control system, such as the Armstrong IPC 11550, can be designed in as a component.
Lastly, since the launch in 2013 of the Armstrong Opti-Visor, it is also possible for optimisation of chilled water systems to be introduced on a retrofit basis. Opti-Visor is a relational control solution that provides an additional ‘bolt-on’ optimisation level for all-variable-speed systems. Designed to interface with the existing building automation system, it controls the key energy-using components of the chiller plant holistically. It is suitable for retrofit in buildings with chiller plant less than five years old, with greater than 10 500 MWh of operation per year (3500 kW cooling at 3000 run hours per year).
When retrofitted in a Government building in London recently, Opti-Visor achieved an immediate 42% reduction in energy usage from existing plant.
The site had already been extensively refurbished to consolidate civil servants from two separate offices into a single reconfigured building. The building cooling load was split during the refurbishment due to the segregation of the data-centre cooling requirement. The installed capacity of the plant remained the same, however, making it extremely inefficient because the equipment became oversized. An investment of around £200 000 in variable-speed drives was made to improve performance, but the actual energy efficiency of the site (as measured for its DEC rating) was still lower than anticipated.
An Armstrong Opti-Visor was integrated with the site’s BMS to provide relational control. The Opti-Visor receives data on plant from the BMS and calculates the optimal plant operating conditions (quantity of equipment to sequence, speed of pumps and fans) using Hartman Loop control methodology. The Opti-Visor then communicates the optimum operating parameters for the plant back to the BMS for the BMS plant automation module to implement. The results for this particular central government building have been significant carbon and savings, including a 5% reduction in total building carbon emissions and a reduction in running costs of £32 500 per annum (see graph).
In the 21st century, information is an ever present and precious commodity in the achievement of HVAC efficiency. Relational control technologies such as Hartman Loop and Opti-Visor will become increasingly important as engineers look to optimise performance. Their ability to harmonise the parts of a system and optimise the potential of BMS and variable speed applications, allows for faster response, better stability, the optimisation of thermodynamic effects at the equipment level and lower risk of equipment failure through cycling stress.
Andrew Harrop is building performance technical manager with Armstrong Fluid Technology