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MVHR makes sense
MVHR systems are becoming increasingly necessary, here Mike Farrer at Stadium Power highlights the need for the highest possible efficiency MVHR power sources to perform over a wide operating window
ustainability and the move towards ‘going green’ are now gaining more recognition than ever with active efforts to imple- ment greater energy efficiency across almost every sector. While opinions will always conflict, it’s a fact that the decision from global governments to adopt ‘greener’ and more sustainable practices now spans practically every industry at some level. With a target of making all new homes Zero Carbon by 2016, an impor- tant aspect of new builds is their efficiency in heating and/or cooling. Modern homes must now comply with legislation that requires them to be air tight, maximising their ability to lower heating costs through insulation and efficient windows/doors. Targets propose a roadmap of waypoints in improving insulation and heating systems, leading to the eventual Zero Carbon goal.
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This isn’t happening in isolation, of course; other aspects of legislation also dictate the restricted use or cessation of harmful components in building materials, coupled with a reduction in the contribution to CO2 emissions. On the face of it, an airtight building that is neither harbours or vents harmful materials, or contributes any CO2 to the environment would provide the ideal solution. However, this presents its own prob- lems, specifically in dealing with the affects of stale air and condensation. The solution to this is to vent stale air to the outside and replace it with fresh air. However, while not harmful to the envi- ronment, it is clearly wasteful in terms of the heat lost by venting warm and replacing it with cooler albeit fresher air. Mechanical Ventilation and Heat Recovery (MVHR) helps overcome this problem, by recovering the heat from
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the warmer internal air and using it to heat the fresh air from outside, before it is introduced in to the property. The performance of an MVHR instal- lation is dependent on a number of parameters, some of which are installa- tion-dependent — such as using rigid or flexible ducting — and some of which are determined purely by the equipment and components used, like the kind of fans employed, their power supplies and control systems.
Driving efficiency forward Predominantly, the components that make up the electronic subsystems are developed and assembled by a range of manufacturers, such as those listed in the database of performance data maintained by SAP (
www.sap-appendixq.org). This database of performance data describes key aspects of an MVHR system’s operational parameters meas- ured using SAP’s own criteria. These include (and are largely defined by) the power needed to (re)circulate a given volume of air based on the number of (wet) rooms in the dwelling. By example, the tested airflow for a kitchen is 13Litres/second and for wet rooms (bathrooms, for example) it’s 8L/s. The SAP Appendix Q specification measures this airflow in terms of electrical energy, or Watts. The result is a figure measured in Watts/Litre/ second, referred to as the Specific Fan Power (SFP). The current specification calls for fairly modest SFP figures; <1.5W/L/s and heat recovery effi- ciency > 70 percent.
The challenge these manufacturers face, however, is improving these already impressive performance figures under the variable airflow rates required by SAP Appendix Q; some- thing that is almost entirely dependent on the fan type used. Inevitably the
Mike Farrer is Technical Director at Stadium Power
requirements will get tougher. Fans are, generally speaking, relatively cheap. However, their cost is directly related to their efficiency, which in turn is dependent on their operating method. The cheapest and simplest fans run directly from an AC mains source but they are also the least efficient. Electronically Commutated Fans (EC-DC or ECM fans) take the AC mains source and convert it to a high voltage DC (direct current) which delivers a more efficient use of the power source, but significantly increases cost. Low voltage DC fans use a power supply to convert the AC mains voltage to a low voltage to operate the fan. These can be more efficient and cost effective. However, the low power and low flow rate requirements of Appendix Q can have a negative impact on the efficiency of Electrically Commuted Fans, which are normally designed to run most efficiently over a given flow rate range. The most efficient MVHR systems — those that can deliver low power con- sumption and high efficiency (<1.5W/L/s and >70 percent heat recovery) over a wide operating window — need a more efficient and typically bespoke power conversion stage. In response to this growing need power supply manufactur- ers such as Stadium Power are now developing power supplies that meet these requirements.
Figure 1:
MVHR & MEV range of power supplies from Stadium
Stadium Power
www.stadium-
electronics.com Enter 205
Given that a typical dwelling may have one kitchen and between one and, say, five wet rooms, and each of these rooms may have variable ventilation needs (perhaps controlled by external sensors such as humidity, temperature or occupancy), the load on the power supply could vary between 25 percent and 100 percent. For this reason, the company has designed a range of MVHR PSUs to deliver up to 92 percent efficiency at 25, 50, 75 and 100 percent load, achieving >87 percent between 10 and 100 percent loads. The company now offers six variants in a range developed specifically for MVHR and MEV applications; deliver- ing between 50W and 140W @ 24V DC with a ‘flat’ efficiency of >92 percent between 25 to 100 percent load. MVHR systems are becoming increasingly necessary as we near the Zero Carbon goals for dwellings by 2016. However, looking at system effi- ciency requires MVHR manufacturers to consider the environmental and financial impact of the variable power requirements such systems exhibit. In order to not only comply with the requirements of SAP Appendix Q but to also differentiate themselves amongst the competition, MVHR manufacturers should look closely at the PSU sub- components and strive to provide the highest possible efficiency over a wide operating window; parameters many ECM fans aren’t designed to deliver.
SEPTEMBER 2013 Electronics
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