Energy Recovery – medium or high efficiency heat exchangers?
With emission rates and energy performance the watchwords of the commercial building industry, Emmanuel Guibal, Product and Market Development Director at Fläkt Woods, explains how differences in ventilation products can affect performance.
In the current era of sustainable and environmental construction, specifiers and building services designers are faced with a plethora of options to meet the ventilation requirements of a commercial building. And on the surface, heat and energy recovery ventilation products can look very much the same with two ventilation fans, a plate heat exchanger core and an additional heating device. But lifting the lid on these products exposes significant differences in key components, which can have a considerable bearing on a product’s performance, as well as the potential energy savings.
When it comes to heat/energy recovery, the plate heat exchanger core is the crucial component to ensure high efficiencies and peak performance. Now there are two main types of heat exchanger design – medium efficiency and high efficiency. Medium efficiency models utilise a ‘cross flow’ heat exchanger core (cell), which is generally square in section and transfers heat from one airflow to another as they pass each other inside the core. On paper, these products can return a thermal efficiency of up to 70%, but in our experience designers should not expect any more than 50-55% as a peak efficiency value - especially with varying air volumes and inlet temperatures.
Although there is still a requirement for medium efficiency heat exchangers in the industry, there has been a shift towards high efficiency models which already meet the 2015 ErP legislation. This has been achieved by using a ‘counter flow’ heat exchanger design that is perfect for applications with restricted space. Although ‘counter flow’ heat exchangers use similar design principles to ‘cross flow’ versions, they have longer parallel air paths to increase the contact time between airflows, which help deliver a greater heat transfer capacity. Consequently, peak thermal efficiencies of 90-95% can be frequently achieved, particularly as the technology is less sensitive to both air temperature and flow rate fluctuations.
The fundamental advantage of a ‘counter-flow’ system is evident when examining the entire building. Not only are the total building emissions reduced within the Simplified Building Energy Model (SBEM) calculations, but the improved efficiencies can contribute to a higher BREEAM rating. Plus, with higher temperatures delivered from the heat exchanger core, there is less need for an additional re-heater, saving on boiler load and associated infrastructure costs. To illustrate the advantages, an independent consultant study, undertaken by James Stubbings, took a typical school with eight teaching areas and compared the use of ventilation with two high efficiency recovery units, against ventilation with no heat recovery. The results indicated that energy savings of 83% and carbon emission reductions of 28 tonnes per annum can be achieved. Furthermore, the use of high efficiency heat exchangers instead of medium efficiency versions showed a variance of 31% and 10 tonnes per annum respectively - highlighting the substantial savings available from using a high efficiency unit.
But with these potential savings on offer, how can specifiers and building services designers be sure the quoted performance data can be achieved? One way is to look out for the ‘Eurovent Certification’ on the heat exchanger. This helps ensure the chosen product will operate in accordance with the design specifications, as well as correspond with the stated energy costs. This can also be combined with an illustration of the efficiency at the operating point, so specifiers can be confident when using the overall energy recovery performance in their calculations.
In response to this, Fläkt Woods has developed the highly efficient e3co-Crown, which combines the latest Eurovent certified ‘counter-flow’ heat exchanger technology - to deliver thermal efficiencies of 90-95% - with simple, integrated controls for a flexible ‘plug and play’ solution. And because we strongly believe in independent research to back-up our own data, the e3co-Crown has been tested by an independent consultant and revealed an impressive payback period of two and half years, as well as a major reduction in total building emissions of nearly 19%.
So for specifiers and building services designers, there are some key pieces of information to look out for when calculating a building’s ventilation requirements. Of course medium efficiency heat exchangers are one way of complying with Part L of the Building Regulations, however the long term performance and energy costs need to be considered. The more pragmatic solution is to use high efficiency products that are already future proofed for legislation and ErP compliant. After all, the payback for clients will be far quicker, the on-going energy efficiencies will make it a sound investment, and when it’s backed by independent research, it just makes good commercial sense.