Demand Control: Are We Saving Cash Or Carbon
I don’t think we need to rehash, restate or reframe the argument for energy savings. For decades the industry has been aware of the benefits of delivering all manner of energy-hungry services on-demand.
Lighting, ventilation, heating, cooling and even hot water have all been given the ‘demand control’ treatment over the years, but do we really use it wisely and do we really consider the overall cost?
Ventilation On Demand
In its simplest form, we have been delivering demand control for many decades by employing simple occupancy detection and delivering a trickle and boost arrangement for fan speed, which for the most part is sufficient. During unoccupied times, the ventilation in a condition space can be lowered to a trickle rate, boosting to deliver the design volume only when occupied. But does this go far enough when we consider larger or multiple occupancy spaces? Do we consider the amount of energy, for example, we waste at the extremities of a working day; particularly on winter mornings when the heating load is highest?
Take, as an example, a call centre office which houses 100 telephone operators who begin work at 9am. In this scenario the office systems will be firing up around 7am to bring the condition space up to a comfortable working temperature, the primary heating system will have a high demand to raise the room temperature for the start of the day.
Consider now the implications of two or three managers arriving early to prepare for the day-ahead before the rest of the workforce arrives. In this situation a PIR controlled ventilation system would start immediately delivering full design-duty fresh air which could be as much as 1 m3, based on 10 litres per second per person, despite the fact there are only 1 or two members of staff in the office. Aside from the obvious waste of fan energy, there are other energy leaks we often ignore.Consider the fact that the room load calculations probably include for 100 bodies generating heat: a valuable natural source of warmth currently absent from the condition space, 100 computers which are currently not running, and the simple fact that even the most efficient heat recovery system takes time to reach a comfortable operating temperature.
The simple introduction of CO2 sensor control, to directly link the ventilation rate to occupancy, yields a number of key benefits. Primarily, the energy consumption of the fans is massively reduced by delivering only the required amount of air to satisfy the current occupant demand, but the other lesser-considered energy drains are eliminated too. Running the fans more slowly at start up enables the heat exchangers to reach an effective temperature much more quickly; the initial work done by the primary heat source is protected and the condition space more closely resembles the heat load calculations.
Are We Frightened By “Design” Duties?
Consultants, occupiers and building designers are often quick to prescribe a “design duty”, a ventilation rate that is predetermined and is therefore required to satisfy a specification. But so often I find that the roots of this design duty calculation are forgotten. In a typical class room, office or meeting space this design duty will be calculated considering two key elements: the air change rate required to deliver a suitable air quality in terms of temperature and filtration, and a per-person summation of a fresh air requirement (often based on 10 litres per person).
I find myself confronted by a fear of this design duty almost daily, the challenge comes from whether it is appropriate, for the most part, to deliver less than the design volume. This dichotomy manifests itself at both the design and delivery phase of a project. Dare we design a system that for the majority of its working life will not deliver the “design duty”, and how do we commission a system designed to respond to the occupancy level of a space which at the commissioning stage will be largely unoccupied? Therefore do ever we prove that it will deliver its design condition at full occupancy?
The ‘Cause and Effect’ Contradiction
So we begin to accept that it is okay to rarely deliver the theoretical design duty, and whilst we provide a system that is capable of reaching these design conditions, I often pose the question ‘will it ever deliver this “maximum volume”?’
Consider again the call centre example which now employs the CO2 regulation method of demand control. In this scenario as the CO2 levels in the condition space begin to rise, so does the ventilation system fan speed. Extracting the stale air and delivering conditioned fresh air, this process quickly begins to lower the CO2 levels and drives back the vent rate towards its default minimum. It is this cycle of cause-and-effect that leads me to believe that, for the most part, the systems we design are actually oversized based mostly on a reluctance to accept diversity in design.
Does SBEM Really Care?
I have a further reservation when it comes to considering ‘design duty’, concerning the ‘duty point average’. When we submit calculations for SBEM assessment we almost always use the ‘design duty’ data points, the specific fan power, energy usage and recovery efficiency of systems running at the full design speed. But is this really an accurate representation of the building performance under “normal” conditions?
A ventilation system equipped with true demand control, with fan speeds linked to temperature or CO2, can deliver significant energy savings. For example, reducing the fan speed from 100% to 80% roughly halves the electrical consumption of the blowers before we even consider the savings in energy required to heat or cool that additional 20%. But do these significant savings ever get assessed in SBEM?
Software such as Nuaire’s Fan Selector can provide accurate data on the specific fan power and energy consumption at any fan running-speed. Therefore, using this “duty point average”, which is more accurate assessment of the expected day-to-day running speed under typical conditions, as the basis for SBEM analysis could provide a much more realistic representation of the building performance.
Cost Versus Carbon
There is one final contradiction when it comes to demand ventilation which is often overlooked, and that is the relationship between cost, carbon savings and the overall carbon impact. There are so many occasions where expensive, over-engineered and complex systems are implemented in the name of ‘energy reduction’ without any consideration for the balance between the cost and the potential savings. Let us briefly ignore the financial implications of a demand controlled solution and consider only the carbon footprint of the systems we design; it is often the case that the overall carbon impact of producing, shipping and installing an over engineered demand control solution far outweighs the potential savings it is designed to deliver.
Over the years I have seen so many solutions involving dampers, complex controls, extensive ductwork and countless hours of design resources committed to saving just a few watts per day, amounting to a few kilograms of carbon over the whole project lifecycle. Putting this aside and considering the capital costs and, more importantly, the potential return on investment, the same contradiction occurs. Sometimes many thousands of pounds are spent designing and implementing a system that will yield only a few hundred pounds worth of energy savings over its entire life span.
Finding a Balance
Does this mean that we should give up on Demand Control as an expensive waste of resources in an ever diminishing attempt to reduce our energy consumption? Of course the answer to this question is no. But, conversely, we should use this method where and whenever its savings outweigh its costs, either in financial or carbon terms. Finding a balance with the systems we design, by considering both the initial cost and life-cycle savings, and by aiming for a sensible return on investment whilst delivering a respectable carbon reduction, is the only way to truly implement demand control ventilation.
Solutions such as Nuaire’s Ecosmart controls, coupled with high-efficiency heat recovery products like XBC and Boxer Packaged Air Handling Units, make it so easy and cost-effective to implement demand control ventilation systems. The simple addition of a CO2 speed controller to a classroom, or a humidity sensor to a changing room area, can transform the energy consumption of the conditioned space in no time. The capital cost of such devices is low, and the return on investment is swift. The carbon footprint for delivering the solution is minimal and the carbon saving once implemented is significant. These systems are designed to work straight out of the box and require little or no setup, beyond choosing a set-point.
So my challenge to the industry is a simple one. Where it is appropriate, and where it yields the greatest benefit to the built environment, use these solutions where ever possible but avoid designing complex solutions that cost far more than they ever save. Remember there is still a place in the world for the humble PIR sensor and often this is all that is needed to achieve significant savings. When you are designing complex spaces with large swings in occupancy or usage characteristics, consider the key metric to identify demand and use this to drive control, keeping systems simple, proportional and cost-effective.
Nuaire can help select the right controls for the job, helping to develop simple cost-effective demand-led ventilation solutions that are proportional to the requirements, aiming to balance cost against the potential savings to deliver truly energy-conscious designs.
Written July 2016