heat recovery

A novel ventilation system has been installed in buildings constructed for the New Campus of the University of Nottingham. Super-efficient mechanical ventilation has been used as part of an integrated environmental strategy and operates with fan input powers below 0.5 W. l-1.  s-1 of airflow. The complete plant was assembled from innovative low-pressure components and has exceptional performance. A key element of the design is that components of the system are bypassed when not in use. At the heart of the system is a low-velocity, high efficiency thermal wheel.

Air-to-air heat and moisture exchange between exhaust and supply airflows can substantially reduce HVAC costs. This paper outlines the design considerations that should be included when selecting a type of exchanger and shows how the performance of each recovery device can be determined. Energy wheels, which transfers both heat and water vapor, are given special consideration. The HV AC design for heat and moisture exchanger sizing is presented as a least life-cycle cost design problem.

Modern UK office buildings have a reputation of being energy profligate, largely due to the fan power requirements of commercial air conditioning. Most architects and HVAC designers only associate low-energy consumption with natural ventilation. However, the UK electricity utilities have peak maximum demands in winter, and buildings need to be designed for year-round lowenergy usage. Relatively few monitored studies of the total annual energy implications of natural and mechanical ventilation strategies operating in conjunction with fabric thermal storage have been published.

The impact of unintentional air flows on the performance of ventilation units with heat recovery is discussed on the basis of single room ventilation units. Assuming an external short circuit (outdoor) and internal (inside the ventilation unit) air leakages, which lead to internal short circuits, a model is developed and characteristic numbers for ventilation efficiency, efficiency of heating load reduction and effectiveness of electrical energy use are derived.

In 1998 the Dutch ventilation industry launched a new generation of domestic ventilation systems on the market with high efficiency heat recovery applying counter flow heat exchangers and DC fans. It is expected that these ventilation systems will play an important role in realising the goals of the Dutch national energy policy for reducing energy use in the built environment. Another important aspect is the contribution to a healthy indoor environment in dwellings with an extreme high energy efficiency, especially in relation to increasing air tightness and thermal insulation.

Infiltration has traditionally been assumed to affect the energy load of a building byan amount equal to the product of the infiltration flow rate and the sensible enthalpydifference between inside and outside. However, laboratory and simulation research hasindicated that heat transfer between the infiltrating air and walls may be substantial, reducingthe impact of infiltration.

High outdoor ventilation air requirements can lead to significant increases in building energy use, thermal discomfort, indoor air quality problems, and litigation. Engineers often avoid ground-source heat pumps because of the perception that there are no acceptable methods for conditioning the ventilation air. However, this difficulty is currently a problem with all types of heating and cooling systems. Decisions may be based on system performance at design conditions without regard to seasonal energy consumption.