Is DC just about supply and demand?

Matching the variable demand with the supply should be the key factor in improving energy efficiency, writes Colin Bridges, business development director, Belimo Automation

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Colin Bridges, business development director, Belimo Automation.
Colin Bridges, business development director, Belimo Automation.

District Cooling (DC) is widely acknowledged, as the most efficient way to cost effectively deliver cooling to increasingly urbanised and fast growing cities like Dubai.

DC companies invest heavily to put in place large-scale production units, pumping stations and a comprehensive network of insulated pipes that distributes chilled water which provide us the cooling energy we need in our homes, workplaces, retail and recreation spaces. The economies of scale make this an energy efficient way to distribute the nearly 20% of total cooling used by Dubai in a year. Of all the types of cooling technologies available today, DC cooling provision is by far the lowest cost, due in no small measure to these economies of scale.
Dubai Supreme Council of Energy (DSCE) recognises this is only part of the story and have developed a Demand Side Management (DSM) strategy. The strategy is to drive 30% improvement in energy efficiency by 2030.
Reducing demand, therefore, seems a logical way forward in meeting this ambitious target. So the big question is, “how can we ensure buildings’ HVAC system takes only the cooling needed, no more and no less?”
Firstly, we should consider the demand. Is it truly what is required? We already have well designed and operated chilled water systems, don’t we?  Mostly yes, but these are designed to maintain our comfort on the very hottest days of the year, and as we know, that is certainly not all the year round.
Matching more precisely the variable demand with the supply should be the key factor in improving energy efficiency, and therefore, demand reduction. Managing the demand is managing the building cooling systems such that it reflects the needs of the occupants to remain comfortable whatever the outside temperature is. As we use chilled water to deliver our air conditioning, it would seem logical that we control the use of that chilled water effectively.

The three key areas here are: production, distribution and diffusion.

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Production: these are the chillers of the DC provider, generating huge quantities of chilled water and using large amounts of electrical energy, up to 75% of the cost of air conditioning, which consequently should be operated as efficiently as possible.                                                                                                                                   
Distribution: these are the main circulating pumps, sending water to the plate heat exchangers in the Energy Transfer Station, and the building chilled water pumps that then distribute it to the various cooling coils within the building. These can take as much as 25% of the cost of the air conditioning electrical energy. So any unnecessary over-pumping should be reduced to a minimum, if not eliminated altogether.                                                                                                                                         

Diffusion: these are the cooling coils that exchange the cooling energy in the water to the air that cools us, allowing us to be comfortable in the building. Here, flow and temperature control are critical.
Technologies exist that allow precise data harvesting by control devices such that they are able to respond to demand and performance variations quickly and effectively. To quote Lord Kelvin, “If you cannot measure it, you cannot improve it”. Well, if we decide to, we can! These smart devices are able to control flow by precise real-time monitoring and measurement, and then to intelligently control efficiency when they “see” the cooling delivered to the coils is not being fully utilised. Beneficially, they store this data, or share it through various platforms so that it can be used to analyse system trends, and help plan positive actions that benefit the system efficiency. Here the cloud may be used for data storage and retrieval. External analysis of this data may be reported back in the form of circuit Energy Performance Reports, direct to the system operator on a regular and continuous basis. This “continuous commissioning” makes data driven actions by the system operator, to enhance the energy efficiency of the building. Adjustments of design flow rates, or coil Delta T are just two possibilities. This enhanced visibility permits troubleshooting that pinpoints areas of interest and action. If, for example, design flow rates are not being reached and cooling is able to be delivered at lower flow rates, we have a good case to reduce the flow setting in the control valve. If the coil is oversized, we might decide that the Delta T may be increased, and the flow rate reduced, to make further efficiencies possible. In all cases, continuous and remote, (where required) monitoring permits simple changes to be made that can have a positive impact.
Control valves closing increases the differential pressure the Variable Speed Pump relies upon to turn down, saving distribution costs.
More efficient use of chilled water in diffusion, leads to less water being required to be produced and also means warmer water returning to the chillers, which is required if the chiller COP (coefficient of performance ) is to be maintained at an optimal level.

Getting the control right, with advanced coil control technology, means getting the diffusion right, which has a positive knock-on effect for the variable speed pumps. They slow down under light load (increasing differential pressure). This reduces pumping costs. And this positively affects the chillers, as we are now producing less chilled water to meet the cooling demand, and, the water sent through our network returns, having had its cooling energy extracted at the coil, at a higher temperature than otherwise.

Getting control right at the point of delivery will undoubtedly save energy right through the network, helping the efficiencies of chillers, pumps and coils.
Lord Kelvin knew a thing or two about the importance of being able to measure things, it’s good to be reminded!

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